U.S. patent application number 14/410961 was filed with the patent office on 2015-11-12 for three-dimensional crystalline, homogenous, and hybrid nanostructures fabricated by electric field directed assembly of nanoelements.
The applicant listed for this patent is Northeastern University. Invention is credited to Ahmed Busnaina, TaeHoon Kim, Sivasubramanian Somu, Cihan Yilmaz.
Application Number | 20150322589 14/410961 |
Document ID | / |
Family ID | 49784049 |
Filed Date | 2015-11-12 |
United States Patent
Application |
20150322589 |
Kind Code |
A1 |
Busnaina; Ahmed ; et
al. |
November 12, 2015 |
Three-Dimensional Crystalline, Homogenous, and Hybrid
Nanostructures Fabricated by Electric Field Directed Assembly of
Nanoelements
Abstract
A variety of homogeneous or layered hybrid nanostructures are
fabricated by electric field-directed assembly of nanoelements. The
nanoelements and the fabricated nanostructures can be conducting,
semi-conducting, or insulating, or any combination thereof. Factors
for enhancing the assembly process are identified, including
optimization of the electric field and combined dielectrophoretic
and electrophoretic forces to drive assembly. The fabrication
methods are rapid and scalable. The resulting nano structures have
electrical and optical properties that render them highly useful in
nanoscale electronics, optics, and biosensors.
Inventors: |
Busnaina; Ahmed; (Needham,
MA) ; Yilmaz; Cihan; (Boston, MA) ; Kim;
TaeHoon; (Revere, MA) ; Somu; Sivasubramanian;
(Natick, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Northeastern University |
Boston |
MA |
US |
|
|
Family ID: |
49784049 |
Appl. No.: |
14/410961 |
Filed: |
July 1, 2013 |
PCT Filed: |
July 1, 2013 |
PCT NO: |
PCT/US13/48948 |
371 Date: |
December 23, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61666181 |
Jun 29, 2012 |
|
|
|
Current U.S.
Class: |
204/477 ;
219/148; 228/196 |
Current CPC
Class: |
C25D 13/02 20130101;
H01L 21/76885 20130101; H01L 21/76879 20130101; C25D 3/48 20130101;
H01L 2221/1094 20130101; B23K 31/00 20130101; B81C 1/00111
20130101; H01L 21/2885 20130101; C25D 13/18 20130101; C25D 13/22
20130101; B81B 2207/056 20130101; B81B 2203/0361 20130101; C25D
13/12 20130101; B81C 2201/0187 20130101; C25D 5/02 20130101 |
International
Class: |
C25D 13/18 20060101
C25D013/18; B23K 31/00 20060101 B23K031/00; C25D 13/22 20060101
C25D013/22; C25D 13/02 20060101 C25D013/02; C25D 13/12 20060101
C25D013/12 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] The invention was developed with financial support from
Grant No. 0832785 from the National Science Foundation. The U.S.
Government has certain rights in the invention.
Claims
1. A method of fabricating a hybrid nanostructure by electric field
directed assembly of nanoelements, the method comprising the steps
of: providing a nanosubstrate comprising a base layer, a conductive
layer deposited onto the base layer, and an insulating layer
deposited onto the conductive layer, the insulating layer
comprising a via, the via forming a void in the insulating layer
and defining a pathway through the insulating layer that exposes
the conductive layer; contacting the nanosubstrate with an aqueous
suspension of first nanoelements; applying an electric field
between the conducting layer and an electrode in the suspension for
a period of time sufficient for migration of first nanoelements
from the suspension into the via and their assembly in the via,
wherein the electric field consists of the sum of a DC offset
voltage and an AC voltage; and repeating, after the assembly, the
contacting and the applying steps, using an aqueous suspension of
second nanoelements different from the first nanoelements, thereby
obtaining a hybrid nanostructure.
2. The method according to claim 1, wherein the first and second
nanoelements differ in electrical conductivity.
3. The method according to claim 2, wherein the first and/or second
nanoelements are electrically conducting, semi-conducting, or
insulating.
4. The method according to claim 3, wherein the first nanoelements
are conducting and the second nanoelements are semi-conducting or
insulating.
5. The method according to claim 3, wherein the first nanoelements
are semi-conducting and the second nanoelements are conducting or
insulating.
6. The method according to claim 3, wherein the first nanoelements
are insulating and the second nanoelements are conducting or
semi-conducting.
7. The method according to claim 3, wherein the electric field used
to assemble either semi-conducting or insulating nanoelements is
greater than the electric field used to assemble conducting
nanoelements.
8. The method according to claim 3, wherein the electric field
produces a dielectrophoretic force that acts on the nanoelements,
and a greater dielectrophoretic force is used to assemble either
semi-conducting or insulating nanoelements than the
dielectrophoretic force used to assembly conducting
nanoelements.
9. The method according to claim 1, wherein the nanosubstrate
comprises a plurality of vias, and a plurality of hybrid
nanostructures are formed.
10. The method according to claim 1, wherein the period of time for
migration and assembly is adjusted to obtain a desired height or
configuration of the nanostructure.
11. The method according to claim 1, wherein the nanostructure
possesses a geometry selected from the group consisting of
nanopillars, nanoboxes and nanorings.
12. The method according to claim 1, wherein the first and/or
second nanoelements are selected from the group consisting of
nanotubes, nanoparticles, and nanowires.
13. The method according to claim 12, wherein the first and/or
second nanoelements are nanoparticles and the nanoparticles are
selected from the group consisting of conducting, semiconducting
and insulating nanoparticles.
14. The method according to claim 12, wherein the nanoelements are
nanotubes and the nanotubes are selected from the group consisting
of single-walled metallic nanotubes, single-walled carbon
nanotubes, semiconducting single walled carbon nanotubes, and
multi-walled carbon-nanotubes.
15. The method according to claim 13 wherein the nanoparticles are
insulating nanoparticles comprising a polymer.
16. The method according to claim 15 wherein the polymer is
selected from the group consisting of polystyrene, polystyrene
latex, poly(methyl methacrylate, poly(lactic-co-glycolic acid),
polycaprolactone, polyethylene glycol (PEG), and functionalized PEG
lipids.
17. The method according to claim 13 wherein the nanoparticles are
semiconducting nanoparticles comprising silicon (Si), gallium
arsenide (GaAs), zinc oxide (ZnO), cadmium selenide (CdSe), cadmium
selenide-zinc sulfide (CdSe--ZnS), cadmium telluride (Cd--Te),
cadmium sulfide (CdS), lead selenide (PbSe), lead telluride (PbTe),
or lead sulfide (PbS).
18. The method of claim 1 further comprising fusing the first
and/or second nanoelements to form a solid mass by either heating
the assembled nanoelements or by applying a large DC electric
potential.
19. The method of claim 1, wherein the AC voltage is about 12 volts
peak to peak (v.sub.pp).
20. The method of claim 1, wherein the AC frequency is from about
10 to about 50 kilohertz.
21. The method of claim 1, wherein the DC offset voltage is about 2
volts and the steady-state current is about 50 .mu.A.
22. A method of fabricating a nanostructure having a diameter at
least about 200 nm by electric field directed assembly of
nanoparticles, the method comprising the steps of: providing a
nanosubstrate comprising a base layer, a conductive layer deposited
onto the base layer, and an insulating layer deposited onto the
conductive layer, the insulating layer comprising a via having a
diameter of at least about 200 nm, the via forming a void in the
insulating layer and defining a pathway through the insulating
layer that exposes the conductive layer; contacting the
nanosubstrate with an aqueous suspension of the nanoparticles,
wherein the nanoparticles have a diameter of about 20-100 nm;
applying an electric field between the conducting layer and an
electrode in the suspension for a period of time sufficient for
migration of nanoparticles from the suspension into the via and
their assembly in the via, wherein the electric field consists of
the sum of a DC offset voltage and an AC voltage thereby producing
an incomplete nanostructure; and fusing the assembled nanoparticles
by heating the incomplete nanostructure by applying an external
heat source or applying a DC voltage between the conducting layer
and the electrode, thereby obtaining the nanostructure.
23. The method of claim 22, wherein the nanosubstrate comprises a
plurality of vias, and a plurality of nanostructures are
formed.
24. The method according to claim 22, wherein the DC voltage is
from about 5 V to about 30 V.
25. The method according to claim 22, wherein the temperature is
about from about 100.degree. C. to about 400.degree. C.
26. The method according to claim 22, wherein the nanoparticles are
gold nanoparticles.
27. The method according to claim 22, wherein the AC voltage is
about 12 V.sub.pp, and the DC offset voltage is about 2V.
28. The method according to claim 22, wherein the period of time
for migration and assembly is adjusted to obtain a desired height
or configuration of the nanostructure.
29. The method according to claim 22, wherein the nanoparticles are
conducting, semiconducting, or insulating nanoparticles.
30. The method according to claim 22, wherein the nanoparticles are
insulating nanoparticles and the period of time for migration and
assembly is in the range of about 30-180 seconds.
31. A hybrid nanostructure comprising a first portion and a second
portion, the first and second portions differing in electrical
conductivity.
32. The hybrid nanostructure according to claim 31, wherein one of
said first and second portions comprises a conducting material and
the other comprises a semiconducting or insulating material.
33. The hybrid nanostructure according to claim 31, wherein one of
said first and second portions comprises a semiconducting material
and the other comprises a conducting or insulating material.
34. The hybrid nanostructure according to claim 31, wherein one of
said first and second portions comprises an insulating material and
the other comprises a conducting or semiconducting material.
35. The hybrid nanostructure according to claim 31, wherein the
nanostructure has a geometry selected from the group consisting of
a nanopillar, a nanoring, and a nanobox.
36. The hybrid nanostructure according to claim 31, wherein the
diameter of the nanostructure is at least about 200 nm.
37. A nanostructure array, comprising a plurality of nanostructures
according to claim 31 arranged in an array.
38. The nanostructure array according to claim 37, wherein each
nanostructure has a geometry selected from the group consisting of
a nanopillar, a nanoring, and a nanobox.
39. The nanostructure array according to claim 37, wherein each
nanostructure is a nanopillar comprising a conducting material.
40. The nanostructure array according to claim 39, wherein the
nanopillar comprises gold and is polycrystalline.
41. The nanostructure array according to claim 40, wherein the
nanopillars further display an electrical resistivity equivalent to
electroplated gold and supports plasmon resonance.
42. A hybrid nanostructure comprising a plurality of nanoelements,
produced by electric field directed assembly of the nanoelements
comprising the steps of: providing a nanosubstrate comprising a
base layer, a conductive layer deposited onto the base layer, and
an insulating layer deposited onto the conductive layer, the
insulating layer comprising a via, the via forming a void in the
insulating layer and defining a pathway through the insulating
layer that exposes the conductive layer; contacting the
nanosubstrate with an aqueous suspension of first nanoelements;
applying an electric field between the conducting layer and an
electrode in the suspension for a period of time for migration and
assembly of first nanoelements from the suspension into the via,
wherein the electric field consists of the sum of a DC offset
voltage and an AC voltage; and repeating, after the assembly, the
contacting and the applying steps, using an aqueous suspension of
second nanoelements, thereby obtaining a hybrid nanostructure.
43. The hybrid nanostructure according to claim 42, wherein the
period of time for migration and assembly is adjusted to obtain a
desired height or configuration of the nanostructure.
44. The hybrid nanostructure according to claim 42, wherein the
nanostructure possesses a geometry selected from the group
consisting of nanopillars, nanoboxes and nanorings.
45. The hybrid nanostructure according to claim 42 wherein the
first and/or second nanoelements are selected from the group
consisting of nanotubes, nanoparticles, and nanowires.
46. The hybrid nanostructure according to claim 45, wherein the
nanoelements are nanoparticles and the nanoparticles are selected
from the group consisting of conducting, semiconducting and
insulating nanoparticles.
47. The hybrid nanostructure according to claim 45 wherein the
nanoelements are nanotubes and the nanotubes are selected from the
group consisting of single-walled metallic nanotubes, single-walled
carbon nanotubes, semiconducting single walled carbon nanotubes,
and multi-walled carbon-nanotubes.
48. A nanostructure having a cross-sectional size of at least about
200 nm, fabricated by electric field directed assembly of
nanoparticles, comprising the steps of: providing a nanosubstrate
comprising a base layer, a conductive layer deposited onto the base
layer, and an insulating layer deposited onto the conductive layer,
the insulating layer comprising a via having a cross-sectional size
of at least about 200 nm, the via forming a void in the insulating
layer and defining a pathway through the insulating layer that
exposes the conductive layer; contacting the nanosubstrate with an
aqueous suspension of the nanoparticles, wherein the nanoparticles
have a diameter of about 20-100 nm; applying an electric field
between the conducting layer and an electrode in the suspension for
a period of time sufficient for migration of nanoelements from the
suspension into the via and for assembly in the via, wherein the
electric field consists of the sum of a DC offset voltage and an AC
voltage, thereby producing an incomplete nanostructure; and fusing
the assembled nanoparticles by heating the incomplete nanostructure
using an external heat source or applying a DC voltage, thereby
obtaining the nanostructure.
49. The nanostructure according to claim 48, wherein the
nanosubstrate comprises a plurality of vias, and a plurality of
nanostructures are formed.
50. A method of converting an incomplete nanostructure formed by
electric field directed assembly of nanoelements into a complete
nanostructure, the incomplete nanostructure comprising unfused
nanoelements, the method comprising: heating the incomplete
nanostructure; whereby the unfused nanoelements are fused to form
the complete nanostructure.
51. The method of claim 50, wherein the incomplete nanostructure is
heated to about 250.degree. C.
52. The method of claim 50, further comprising: applying a DC
voltage across the incomplete nanostructure.
53. The method of claim 52, wherein the DC voltage is about
30V.
54. A method of converting an incomplete nanostructure formed by
electric field directed assembly of nanoelements into a complete
nanostructure, the incomplete nanostructure comprising unfused
nanoelements, the method comprising: applying a DC voltage across
the incomplete nanostructure; whereby the unfused nanoelements are
fused to form the complete nanostructure.
55. The method of claim 54, wherein the DC voltage is about
30V.
56. The method of claim 54, further comprising: heating the
incomplete nanostructure.
57. The method of claim 56, wherein the incomplete nanostructure is
heated to about 250.degree. C.
58. The method of claim 1, wherein the applied electric field
attains a magnitude of at least 2 MV/m within the via.
59. The method of claim 1, wherein a higher dielctrophoretic force
is applied on the second nanoelements than on the first
nanoelements.
60. The method of claim 1, wherein a higher concentration is used
for the second nanoelements than for the first nanoelements.
61. A method of increasing the rate, extent, or completeness of
formation of a nanostructure by electric field directed assembly of
nanoelements, the method comprising a step selected from the group
consisting of: increasing the electric field used for assembly;
decreasing the frequency of an AC component of the electric field
used for assembly; increasing a dielectrophoretic force acting on
the nanoelements; increasing an electrophoretic force acting on the
nanoelements; increasing a nanoelement concentration used for
assembly; decreasing a dimension of a via into which the
nanoelements are assembled; decreasing a density of vias into which
the nanoelements are assembled; and increasing a dimension of the
nanoelements.
62. A method of fabricating a homogeneous electrically insulating
nanostructure by electric field directed assembly of nanoelements,
the method comprising the steps of: providing a nanosubstrate
comprising a base layer, a conductive layer deposited onto the base
layer, and an insulating layer deposited onto the conductive layer,
the insulating layer comprising a via, the via forming a void in
the insulating layer and defining a pathway through the insulating
layer that exposes the conductive layer; contacting the
nanosubstrate with an aqueous suspension of electrically insulating
nanoelements; and applying an electric field between the conducting
layer and an electrode in the suspension for a period of time
sufficient for migration of the nanoelements from the suspension
into the via and their assembly in the via, wherein the electric
field consists of the sum of a DC offset voltage and an AC voltage;
thereby obtaining a homogeneous electrically insulating
nanostructure.
63. The method of claim 62, wherein the electrically insulating
nanoelements comprise an inorganic oxide or an organic polymer.
64. The method of claim 63, wherein the electrically insulating
nanoelements are selected from silica, alumina, titania,
polystyrene, polystyrene latex, poly(methyl methacrylate,
poly(lactic-co-glycolic acid), polycaprolactone, polyethylene
glycol (PEG), and functionalized PEG lipids.
65. A homogeneous electrically insulating nanostructure obtainable
by the method of claim 62.
66. The method according to claim 1 wherein the first and second
nanoelements are conducting, and the first and second nanoelements
are not the same.
67. The method according to claim 1 wherein the first and second
nanoelements are insulating, and the first and second nanoelements
are not the same.
68. The method according to claim 1 wherein the first and second
nanoelements are semiconducting, and the first and second
nanoelements are not the same.
69. A nanoantenna comprising an array of hybrid nanostructures
fabricated according to claim 1.
70. The nanoantenna according to claim 69, wherein the first and
second nanoelements differ in electrical conductivity.
71. The nanoantenna according to claim 69, wherein the first and/or
second nanoelements are electrically conducting, semi-conducting,
or insulating.
70. The nanoantenna according to claim 67, wherein the first
nanoelements are conducting and the second nanoelements are
semi-conducting or insulating.
71. The nanoantenna according to claim 67, wherein the first
nanoelements are semi-conducting and the second nanoelements are
conducting or insulating.
72. The nanoantenna according to claim 67, wherein the first
nanoelements are insulating and the second nanoelements are
conducting or semi-conducting.
73. The nanoantenna according to claim 67, wherein the first and
second nanoelements are conducting, and the first and second
nanoelements are not the same.
74. The nanoantenna according to claim 67, wherein the first and
second nanoelements are insulating, and the first and second
nanoelements are not the same.
75. The nanoantenna according to claim 67, wherein the first and
second nanoelements are semiconducting, and the first and second
nanoelements are not the same.
72. The nanoantenna according to claim 69, wherein the first
nanoelements are conducting and the second nanoelements are
semi-conducting or insulating.
73. The nanoantenna according to claim 69, wherein the first
nanoelements are semi-conducting and the second nanoelements are
conducting or insulating.
74. The nanoantenna according to claim 69, wherein the first
nanoelements are insulating and the second nanoelements are
conducting or semi-conducting.
75. The nanoantenna according to claim 69, wherein the first and
second nanoelements are conducting, and the first and second
nanoelements are not the same.
76. The nanoantenna according to claim 69, wherein the first and
second nanoelements are insulating, and the first and second
nanoelements are not the same.
77. The nanoantenna according to claim 69, wherein the first and
second nanoelements are semiconducting, and the first and second
nanoelements are not the same.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of provisional
application Ser. No. 61/666,181 filed Jun. 29, 2012 and entitled
"NANOSCALE INTERCONNECTS AND COMPOSITE THREE-DIMENSIONAL
NANOSTRUCTURES FABRICATED BY ELECTRICAL FIELD DIRECTED ASSEMBLY OF
NANOELEMENTS", which is hereby incorporated herein by reference in
its entirety.
BACKGROUND
[0003] The precise assembly of nanoscale materials in a desired
location and orientation on surfaces makes it possible to fabricate
various types of novel structures and devices (Baughman et al.,
Science, vol. 297, pp. 787-792, 2002; Daniel and Astruc Chem. Rev.,
vol. 104, pp. 293-346, 2004; Huang et al., Adv. Mater., vol. 21,
pp. 4880-4910, 2009). Nanoparticles are model nanoscale building
blocks due to their zero-dimensional geometry and unique
size-dependent properties (Murray et al., Annu. Rev. Mater. Sci.,
vol. 30, pp. 545-610, 2000). For example, functionalized gold
nanoparticles can be used to fabricate sensitive biosensors (Liu
and Lu, J. Amer. Chem. Soc., vol. 125, pp. 6642-6643, 2003).
Closely packed metallic nanoparticle arrays show significant
electromagnetic field enhancement (Yan et al., Amer. Chem. Soc,
(ACS) Nano, vol. 3, pp. 1190-1202, 2009), and silica nanoparticles
in predefined arrays enable novel optical devices (Colodrero et
al., Langmuir, vol. 24, pp. 44304434, 2008). Bottom-up
nanoparticle-directed assembly processes can be used to fabricate
these nanoscale structures. For example, 1-D organization of
individual metallic nanoparticles has been used to fabricate 1-D
metal nanowires (Bhatt, Langmuir, vol. 20, pp. 467476, 2004). These
structures show unique electrical and optical properties (Hermanson
et al., Science, vol. 294, pp. 1082-1085, 2001; Maier et al., Nat.
Mater., vol. 2, pp. 229-232, 2003), and provide solutions for the
technological and fundamental challenges faced by the conventional
top-down fabrication processes in the sub-100 nm regime ((Lu and
Lieber, Nat. Mater., vol. 6, pp. 841-850, 2007).
[0004] Techniques have been developed to integrate nanoparticles
directly onto surfaces for various applications (Cui et al., Nano
Lett., vol. 4, pp. 1093-1098, 2004; Cha et al., Langmuir, vol. 25,
pp. 1 137 5-1 1382, 2009; Maury et al., Adv. Mater., vol. 17, pp.
2718-2723, 2005; Xiong et al., Appl. Phys. Lett., vol. 89, pp.
193108-1-193108-3, 2006), and dielectrophoresis (DEP) (Pohl, H. A.
Dielectrophoresis. Cambridge, Mass.: Cambridge Univ. Press, 1978)
has been used to manipulate nanoparticles onto electronic devices.
Nanoscale interconnects have been fabricated using interparticle
chain formation (Bhatt and Velev, Langmuir, vol. 20, pp. 467476,
2004). Electrical characterization of produced in-plane (2-D)
(Colodrero et al., Langmuir, vol. 24, pp. 44304434, 2008) and
intraplane (3-D) (Khanduja et al., Appl. Phys. Lett., vol. 90, pp.
0831 05-1-0831 05-3, 2007) interconnects has been achieved. In
these assembly processes, the two electrodes required for applying
an electric field are usually on the same substrate or very close
to each other (order of microns), making the fabrication techniques
unsuitable for specific applications such as interconnects in
complementary metal-oxide-semiconductor (CMOS) based devices
(International Technology Roadmap For Semiconductors 2007 Edition
Interconnecf. ITRS, 2007, Available: http://www.itrs
.net/Links/2007 ITRS/Home 2007 hhm) and various types of
electromagnetic-field-enhancement sensors).
[0005] A recent study demonstrates the fabrication of gold nanorods
in a porous alumina template using an AC electric field between two
electrodes (Lee et al., Sens. Actuators B, vol. 136, pp. 320-325,
2009.). However, the process provided no control over the length of
the nanorods, impeding the potential use of the rods in sensors and
CMOS interconnect applications. Carbon nanotubes are considered as
a potential candidate material for interconnect applications in
gigascale systems (Naeemi et al., 2007, Electron. Devices IEEE
Trans. 54: 26-37; Nihei et al., 2005, Proc. IEEE Int. Interconnect
Tech. Conf., pp. 234-36. Piscataway: IEEE; Vajtai et al., 2003,
Nanotechnol. IEEE Trans. 2:355-61; Nieuwoudt and Massoud, 2006,
Nanotechnol. IEEE Trans. 5:758-65) due to their resistance to
electromigration and larger mean free path compared to that of
metals. For vertical and lateral interconnects researchers have
used high temperature chemical vapor deposition methods to grow
carbon nanotubes on pre-patterned substrates. However, these high
temperature processes are not compatible with CMOS technology
(Maury et al., Adv. Mater., vol. 17, pp. 2718-Z723, 2005; Xiong et
al., Appl. Phys. Lett., vol. 89, pp. 193108-1-193108-3, 2006). For
lateral interconnects researchers have used post-growth assembly
techniques such as dielectrophoresis in which only an AC field or
an electrical field gradient is used. For local vertical
interconnects, the methods that have been carried out are not
highly scalable.
[0006] Previously available directed assembly techniques have not
demonstrated the ability to make solid homogeneous or hybrid
crystalline nanostructures with nanoscale precision, and also do
not enable assembly of larger diameter (e.g., 200 nm or above)
nanostructures with nanoscale precision, or the assembly of
nonconducting nanoparticles. Thus, there remains a need to develop
methods for rapid and precise assembly and fusion of nanoparticles
for producing homogenous and hybrid crystalline 3-D nanostructures
using externally applied electric field.
SUMMARY OF THE INVENTION
[0007] Methods for fabricating nanostructures by electric field
directed assembly of nanoelements, and nanostructures fabricated by
nanoelement assembly are provided herein.
[0008] As used herein a "nanostructure" refers to a structured
material object having one or more outside dimensions, or all of
its outside dimensions in the range from 1-999 nm, or less than 500
nm, or less than 400 nm, or less than 300 nm. As used herein, a
"nanopillar" is an approximately cylindrical nanostructure. The
"diameter" of a nanostructure refers generally to the maximum width
of a the structure in cross-section, even if the cross-section is
not circular.
[0009] An embodiment of the invention is a method of fabricating a
hybrid nanostructure by electric field directed assembly of
nanoelements. The method includes the steps of: (a) providing a
nanosubstrate including a base layer, a conductive layer deposited
onto the base layer, and an insulating layer deposited onto the
conductive layer, the insulating layer comprising a via, the via
forming a void in the insulating layer and defining a pathway
through the insulating layer that exposes the conductive layer; (b)
contacting the nanosubstrate with an aqueous suspension of first
nanoelements; (c) applying an electric field between the conducting
layer and an electrode in the suspension for a period of time
sufficient for migration of first nanoelements from the suspension
into the via and their assembly in the via, wherein the electric
field consists of the sum of a DC offset voltage and an AC voltage;
and (d) repeating, after the assembly, the contacting and the
applying steps, using an aqueous suspension of second nanoelements
different from the first nanoelements, thereby obtaining a hybrid
nanostructure.
[0010] In related embodiments, the first and second nanoelements
differ in electrical conductivity. In other embodiments the first
and/or second nanoelements are electrically conducting,
semi-conducting, or insulating. For example, the first nanoelements
are conducting and the second nanoelements are semi-conducting or
insulating. Alternatively, the first nanoelements are
semi-conducting and the second nanoelements are conducting or
insulating. In another embodiment, the first nanoelements are
insulating and the second nanoelements are conducting or
semi-conducting.
[0011] In various related embodiments, the first and second
nanoelements are conducting, and the first and second nanoelements
are not the same. In other related embodiments, the first and
second nanoelements are insulating, and the first and second
nanoelements are not the same. In yet other related embodiments,
the first and second nanoelements are semiconducting, and the first
and second nanoelements are not the same.
[0012] A related embodiment is a nanoantenna comprising an array of
hybrid nanostructures fabricated according to an above method of
the invention. In related embodiments, the first and second
nanoelements differ in electrical conductivity. In other related
embodiments, the first and/or second nanoelements are electrically
conducting, semi-conducting, or insulating. Related embodiments
include those in which the first nanoelements are conducting and
the second nanoelements are semi-conducting or insulating.
According to other related embodiments, the first nanoelements are
semi-conducting and the second nanoelements are conducting or
insulating. In yet other related embodiments, the first
nanoelements are insulating and the second nanoelements are
conducting or semi-conducting. Related embodiments include
nanoantenna such that: the first and second nanoelements are
conducting, and the first and second nanoelements are not the same;
or the first and second nanoelements are insulating, and the first
and second nanoelements are not the same; or the first and second
nanoelements are semiconducting, and the first and second
nanoelements are not the same.
[0013] According to other embodiments, the electric field used to
assemble either semi-conducting or insulating nanoelements is
greater than the electric field used to assemble conducting
nanoelements.
[0014] In related embodiments of the method, the applied electric
field attains a magnitude of at least about 1.5 MV/m, or at least
about 1.75 MV/m, or at least about 2 MV/m, or at least about 2.25
MV/m, or at least about 2.5 MV/m within the via. In other related
embodiments, a higher dielectrophoretic force is applied on the
second nanoelements than on the first nanoelements. According to
other related embodiments, a higher concentration is used for the
second nanoelements than for the first nanoelements.
[0015] In various embodiments, the electric field produces a
dielectrophoretic force that acts on the nanoelements, and a
greater dielectrophoretic force is used to assemble either
semi-conducting or insulating nanoelements than the
dielectrophoretic force used to assembly conducting
nanoelements.
[0016] In related embodiments, the nanosubstrate comprises a
plurality of vias, and a plurality of hybrid nanostructures are
formed.
[0017] According to an aspect of the invention the time for
migration and assembly is adjusted to obtain a desired height or
configuration of the nanostructure.
[0018] Related embodiments include nanostructure possessing
geometry selected from the group consisting of nanopillars,
nanoboxes and nanorings.
[0019] In various embodiments, the first and/or second nanoelements
are selected from the group consisting of nanotubes, nanoparticles,
and nanowires. For example, the first and/or second nanoelements
are nanoparticles and the nanoparticles are selected from the group
consisting of conducting, semiconducting and insulating
nanoparticles. In related embodiments, the nanoelements are
nanotubes and the nanotubes are selected from the group consisting
of single-walled metallic nanotubes, single-walled carbon
nanotubes, semiconducting single walled carbon nanotubes, and
multi-walled carbon-nanotubes. In other embodiments the
nanoparticles are insulating nanoparticles including a polymer. For
example, the polymer is polystyrene latex (PSL), polystyrene (PS),
poly(methyl methacrylate) PMMA, poly(lactic-co-glycolic acid)
(PLGA), polycaprolactone (PCL), polyethylene glycol (PEG) and
functionalized PEG lipids such as DSPE-PEG.
[0020] In related embodiments, the nanoparticles are semiconducting
nanoparticles including silicon (Si), gallium arsenide (GaAs), zinc
oxide (ZnO), cadmium selenide (CdSe), cadmium selenide-zinc sulfide
(CdSe--ZnS), cadmium telluride (Cd--Te), cadmium sulfide (CdS),
lead selenide (PbSe), lead telluride (PbTe), and lead sulfide
(PbS).
[0021] According to another embodiment of the invention, the method
further includes fusing the first and/or second nanoelements to
form a solid mass by either heating the assembled nanoelements or
by applying a large DC electric potential.
[0022] In related embodiments the AC voltage is about 12 volts peak
to peak (v.sub.pp). In other related embodiments the AC frequency
is about 10-50 kilohertz. In yet other embodiments the DC offset
voltage is about 2 volts and the steady-state current is about 50
nA.
[0023] Another embodiment of the invention is a method of
fabricating a nanostructure having a diameter at least about 200 nm
by electric field directed assembly of nanoparticles, the method
including the steps of: providing a nanosubstrate including a base
layer, a conductive layer deposited onto the base layer, and an
insulating layer deposited onto the conductive layer, the
insulating layer including a via having a diameter of at least
about 200 nm, the via forming a void in the insulating layer and
defining a pathway through the insulating layer that exposes the
conductive layer; contacting the nanosubstrate with an aqueous
suspension of the nanoparticles, such that the nanoparticles have a
diameter of about 20-100 nm; applying an electric field between the
conducting layer and an electrode in the suspension for a period of
time sufficient for migration of nanoparticles from the suspension
into the via and their assembly in the via, such that the electric
field consists of the sum of a DC offset voltage and an AC voltage
thereby producing an incomplete nanostructure; and fusing the
assembled nanoparticles by heating the incomplete nanostructure by
applying an external heat source or applying a DC voltage between
the conducting layer and the electrode, thereby obtaining the
nanostructure. In various embodiments the range of DC voltage used
is about 5-30 V. For example, the DC voltage is about 30 V. For
example, the temperature is about 250.degree. C.
[0024] In related embodiments, the nanosubstrate includes a
plurality of vias, and a plurality of nanostructures is formed.
[0025] In various embodiments, the nanoparticles are gold
nanoparticles. In other related embodiments the AC voltage is about
12 V.sub.pp, and the DC offset voltage is about 2V.
[0026] According to certain embodiments of the invention, the
period of time for migration and assembly is adjusted to obtain a
desired height or configuration of the nanostructure.
[0027] Related embodiments of the invention of a method of
fabricating a nanostructure having a diameter at least about 200 nm
by electric field directed assembly of nanoparticles include those
in which the nanoparticles are conducting, semiconducting, or
insulating nanoparticles. In other related embodiments the
nanoparticles are insulating nanoparticles and the period of time
for migration and assembly is in the range of about 30-180
seconds.
[0028] Embodiments of the invention includes a hybrid nanostructure
having a first portion and a second portion, the first and second
portions differing in electrical conductivity. In related
embodiments, one of said first and second portions includes a
conducting material and the other includes a semiconducting or
insulating material. In related embodiments, the hybrid
nanostructure is such that one of said first and second portions
comprises a semiconducting material and the other comprises a
conducting or insulating material. According to other embodiments,
one of said first and second portions comprises an insulating
material and the other comprises a conducting or semiconducting
material. Further, the embodiments includes nanostructures having a
geometry selected from the group consisting of a nanopillar, a
nanoring, and a nanobox. In various embodiments of the invention
the size of the nanostructure is at least about 200 nm.
[0029] Yet another embodiment of the invention is a nanostructure
array, including a plurality of nanostructures arranged in an
array. For example, each nanostructure has a geometry selected from
the group consisting of a nanopillar, a nanoring, and a nanobox.
For example, each nanostructure is a nanopillar comprising a
conducting material. For example, the nanopillar includes gold and
is polycrystalline. In related embodiments, the nanopillars further
display an electrical resistivity equivalent to electroplated gold
and supports plasmon resonance.
[0030] Embodiments of the invention includes a hybrid nanostructure
comprising a plurality of nanoelements, produced by electric field
directed assembly of the nanoelements including the steps of:
providing a nanosubstrate including a base layer, a conductive
layer deposited onto the base layer, and an insulating layer
deposited onto the conductive layer, the insulating layer including
a via, the via forming a void in the insulating layer and defining
a pathway through the insulating layer that exposes the conductive
layer; contacting the nanosubstrate with an aqueous suspension of
first nanoelements; applying an electric field between the
conducting layer and an electrode in the suspension for a period of
time for migration and assembly of first nanoelements from the
suspension into the via, such that the electric field consists of
the sum of a DC offset voltage and an AC voltage; and repeating,
after the assembly, the contacting and the applying steps, using an
aqueous suspension of second nanoelements, thereby obtaining a
hybrid nanostructure.
[0031] In related embodiments, the period of time for migration and
assembly is adjusted to obtain a desired height or configuration of
the nanostructure.
[0032] In various embodiments the nanostructure possesses a
geometry selected from the group consisting of nanopillars,
nanoboxes and nanorings.
[0033] According to related embodiments of the invention, the first
and/or second nanoelements are selected from the group consisting
of nanotubes, nanoparticles, and nanowires. For example, the
nanoelements are nanoparticles, and the nanoparticles are selected
from the group consisting of conducting, semiconducting and
insulating nanoparticles. In related embodiments, the nanoelements
are nanotubes and the nanotubes are selected from the group
consisting of single-walled metallic nanotubes, single-walled
carbon nanotubes, semiconducting single walled carbon nanotubes,
and multi-walled carbon-nanotubes.
[0034] Yet another embodiment of the invention is a nanostructure
having a cross-sectional size of at least about 200 nm, fabricated
by electric field directed assembly of nanoparticles, comprising
the steps of: providing a nanosubstrate including a base layer, a
conductive layer deposited onto the base layer, and an insulating
layer deposited onto the conductive layer, the insulating layer
including a via having a cross-sectional size of at least about 200
nm, the via forming a void in the insulating layer and defining a
pathway through the insulating layer that exposes the conductive
layer; contacting the nanosubstrate with an aqueous suspension of
the nanoparticles, such that the nanoparticles have a diameter of
about 20-100 nm; applying an electric field between the conducting
layer and an electrode in the suspension for a period of time
sufficient for migration of nanoelements from the suspension into
the via and for assembly in the via, such that the electric field
consists of the sum of a DC offset voltage and an AC voltage,
thereby producing an incomplete nanostructure; and fusing the
assembled nanoparticles by heating the incomplete nanostructure
using an external heat source or applying a DC voltage, thereby
obtaining the nanostructure.
[0035] In related embodiments of the invention the nanosubstrate
includes a plurality of vias, and a plurality of nanostructures are
formed.
[0036] Embodiments of the invention includes a method of converting
an incomplete nanostructure formed by electric field directed
assembly of nanoelements into a complete nanostructure, the
incomplete nanostructure including unfused nanoelements, the method
including: heating the incomplete nanostructure; whereby the
unfused nanoelements are fused to form the complete nanostructure.
In related embodiments, the incomplete nanostructure is heated to
about 250.degree. C. In other related embodiments, the method
further includes applying a DC voltage across the incomplete
nanostructure. For example, the DC voltage is about 30V.
[0037] In another embodiment, the invention is a method of
converting an incomplete nanostructure formed by electric field
directed assembly of nanoelements into a complete nanostructure,
the incomplete nanostructure comprising unfused nanoelements, the
method including: applying a DC voltage across the incomplete
nanostructure; whereby the unfused nanoelements are fused to form
the complete nanostructure. For example, the DC voltage is about
30V. In related embodiments, the method further comprises heating
the incomplete nanostructure. For example, the incomplete
nanostructure is heated to about 250.degree. C.
[0038] Another embodiment of the invention is a method of
increasing the rate, extent, or completeness of formation of a
nanostructure by electric field directed assembly of nanoelements,
the method including a step selected from the group consisting of:
increasing the electric field used for assembly; decreasing the
frequency of an AC component of the electric field used for
assembly; increasing a dielectrophoretic force acting on the
nanoelements; increasing an electrophoretic force acting on the
nanoelements; increasing a nanoelement concentration used for
assembly; decreasing a dimension of a via into which the
nanoelements are assembled; decreasing a density of vias into which
the nanoelements are assembled; and increasing a dimension of the
nanoelements.
[0039] Further, an embodiment of the invention is a method of
fabricating a homogeneous electrically insulating nanostructure by
electric field directed assembly of nanoelements, the method
including the steps of: providing a nanosubstrate comprising a base
layer, a conductive layer deposited onto the base layer, and an
insulating layer deposited onto the conductive layer, the
insulating layer comprising a via, the via forming a void in the
insulating layer and defining a pathway through the insulating
layer that exposes the conductive layer; contacting the
nanosubstrate with an aqueous suspension of electrically insulating
nanoelements; and applying an electric field between the conducting
layer and an electrode in the suspension for a period of time
sufficient for migration of the nanoelements from the suspension
into the via and their assembly in the via, such that the electric
field consists of the sum of a DC offset voltage and an AC voltage;
thereby obtaining a homogeneous electrically insulating
nanostructure. A related embodiment of the invention is a
homogeneous electrically insulating nanostructure obtainable by the
method.
[0040] In other related embodiments, the electrically insulating
nanoelements includes an inorganic oxide or an organic polymer.
According to yet other related embodiments, the electrically
insulating nanoelements are selected from selected from silica,
alumina, titania, polystyrene, polystyrene latex, poly(methyl
methacrylate, poly(lactic-co-glycolic acid), polycaprolactone,
polyethylene glycol (PEG), and functionalized PEG lipids.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] FIG. 1 panels A-E are schematic diagrams showing fabrication
of 3-D nanostructures through electric field directed assembly of
nanoparticles (NPs). FIG. 1 panel A shows migration and assembly of
nanoparticles from an aqueous suspension of NPs into vias located
within a layer of patterned dielectric or insulator material
deposited on a conducting surface. The conducting surface functions
as an electrode. A counter electrode is placed at a distance of
about 5 mm from the conducting surface and an electric field is
applied between the two electrodes. FIG. 1 panel B shows
nanopillars formed by fusion of Type A nanoparticles following
their assembly in the vias. FIG. 1 panel C shows the Type A
nanostructures after removal of the dielectric material that formed
the vias. FIGS. 1D-E show production of hybrid nanostructures using
sequential directed assembly of different types of NPs. In this
process, nanopillars B are assembled from NP B and are layered
above nanopillars A, assembled from NP A. FIG. 1 panel D shows an
array of nanopillars A that have been overlayered with nanopillars
B. FIG. 1 panel E shows an array of hybrid 3-D nanostructures
obtained upon removal of the insulator layer, each nanostructure
having nanopillar B layered over nanopillar A.
[0042] FIG. 2 shows SEM micrographs, diagrams of cross-sectional
views of vias, and graphs correlating simulated electric field
intensity inside a via with aspect ratio of the via or with the
spacing between the via and adjacent vias to show the effect of via
geometry and inter via spacing on nanostructure formation. The vias
in panels A-F have a height of 150 nm. FIG. 2 panel A is a SEM
micrograph of a complete nanopillar formed in a 50 nm wide via
through assembly and fusion of 5 nm gold NPs under an electric
field of 12 V.sub.pp AC voltage with a frequency of 50 kHz applied
for 90 seconds. FIG. 2 panel B is a SEM micrograph of a complete
nanopillar formed in a 100 nm wide via through assembly and fusion
of 5 nm gold NPs under the same conditions of applied electric as
in FIG. 2 panel A. FIG. 2 panel C is a SEM micrograph of an
incomplete nanopillar formed in a 200 nm wide via through assembly
and fusion of 5 nm gold NPs under the same conditions of applied
electric field as in FIG. 2 panel A. FIG. 2 panel D is a
cross-sectional view of a 50 nm wide via showing simulated static
electric field contours. In FIG. 2 panels D-F, the darker regions
within a via have greater electric field intensity, and bars in the
upper left corner represent magnitude of electric field
intensities. FIG. 2 panel E is a cross-sectional view of a 100 nm
wide via showing simulated static electric field contours. The
intensity of the electric field is lower compared to that in the 50
nm wide via shown in panel D. FIG. 2 panel F is a cross-sectional
view of a 200 nm wide via showing simulated static electric field
contours. The intensity of the electric field is much lower
compared to that in the 50 nm wide via shown in panel D. FIG. 2
panel G presents a set of curves showing variation in the electric
field intensity inside the center of a via having a defined
diameter, as a function of the aspect ratio of the via. FIG. 2
panel F presents a set of curves showing variation in the electric
field intensity inside a via, having the specified diameters, as a
function of spacing between adjacent vias.
[0043] FIG. 3 panels A-K show SEM micrographs and atomic force
microscopy images (AFM) of nanostructures fabricated using
electrical field directed assembly. FIG. 3 panel A is a low
magnification SEM micrograph of a gold nanopillar array. A close-up
view of a region of the array is shown to the right. FIG. 3 panel B
is a SEM micrograph (upper half) of a gold nanopillar with an
aspect ratio of 1. The lower half is an AFM image of the
nanopillar. FIG. 3 panel C is a SEM micrograph (upper half) of a
gold nanopillar with an aspect ratio of 3. The lower half is an AFM
image of the nanopillar. FIG. 3 panel D is a SEM micrograph (upper
half) of a gold nanopillar with an aspect ratio of 6. The lower
half is an AFM image of the nanopillar. FIG. 3 panel E is a SEM
micrograph of a nanopillar made of copper. FIG. 3 panel F is a SEM
micrograph of a nanopillar made of fluorescent polystyrene latex
(PSL). FIG. 3 panel G is a SEM micrograph of a nanopillar made of
fluorescent silica. FIG. 3 panel H is a SEM micrograph of a hybrid
nanopillar made of gold layered over with fluorescent PSL. The
inset shows a fluorescence image of an array of the hybrid
nanopillars. FIG. 3 panel I is a SEM micrograph of a hybrid
nanopillar made of gold layered over with fluorescent silica. The
inset shows a fluorescence image of an array having the hybrid
nanopillars. FIG. 3 panel J is a SEM micrograph of a hybrid
nanopillar made of a gold surface layered over with a fluorescent
PSL nanopillar layer, which is further layered over with a gold
nanopillar layer. A fluorescence image of an array having these
hybrid nanopillars is shown in the inset. FIG. 3 panel K is a set
of images which includes: a high-angle SEM image of a 3-D nanobox
(lower left), a high-angle SEM image of a 3-D nanoring (lower
right), a top-down SEM image of a nanobox (upper left), and a
top-down SEM image of a nanoring (upper right).
[0044] FIG. 4 panels A-C are transmission electron microscopy (TEM)
micrographs and a graph comparing resistance measurements of
nanopillars fabricated using electrical field directed assembly of
nanoparticles to that produced using electroplating. FIG. 4 panel A
is a bright field TEM micrograph (left) of a nanopillar made using
electric field directed assembly of nanoparticles. An enlarged view
is presented to the right. The inset shows a diffraction pattern of
the pillar. FIG. 4 panel B is a high resolution TEM image that
shows the existence of two grains within the 30 nm.times.30 nm area
of a single nanopillar. Arrows indicate two different lattice
directions, one corresponding to each grain. FIG. 4 panel C is a
graph showing resistance measurements of ten nanopillars from two
chips (I and II) fabricated by electric field directed assembly of
nanoparticles, and that of ten nanopillars from two chips (I' and
II') produced using electroplating. The inset to the right shows a
high-angle SEM micrograph of gold nanopillars in the insulating
layer (PMMA) during the electrical characterization. The panel in
the upper right is a SEM image of a nanopillar array, and the inset
shows a probe positioned near a nanopillar in the array.
[0045] FIG. 5 panels A-E are a SEM micrograph and graphs showing
intensity enhancement of incident light by a nanopillar, refractive
index sensitivity, and shifts in surface plasmonic resonance due to
accumulation of biomass. FIG. 5 panel A is a SEM micrograph of a
nanopillar fabricated using electrical field directed assembly.
FIG. 5 panel B is a graph showing near-field intensity enhancement
distribution of incident light calculated at the top surface of the
nanopillar. The X and Y axes represent distances in perpendicular
directions from the center of the top surface of the nanopillar.
The intensity enhancement of incident light is proportional to the
brightness of the field. FIG. 5 panel C is a graph showing
near-field intensity enhancement distribution of incident light
calculated through the cross section of the nanopillar. The X axis
represents distance from the center of a cross sectional plane of
the nanopillar. The Y axis represents distance from the base of the
nanopillar. Zero represents the bottom of the gold nanopillar where
the nanopillar meets the gold electrode. The intensity enhancement
of incident light is proportional to the brightness of the field.
FIG. 5 panel D is a graph showing variation in reflection (Y axis)
as a function of wavelength (X axis). The plasmonic resonance shows
a strong shift as the refractive index (n) of the bulk solution in
the interface with the nanopillar is changed from air to deionized
water (n=1.333), acetone (n=1.356), or isopropyl alcohol (n=1.377).
FIG. 5 panel E shows the variation in reflection (Y axis) as a
function of wavelength (X axis) in air versus in the presence of a
monolayer of Protein A/G (A/G) or a bilayer of Protein A/G bound to
immunoglobulin (A/G-IgG). A robust shift in plasmonic resonance was
observed with accumulation of biomass on the sensor platform. The
inset at right depicts antigen-antibody complexes bound to a
nanopillar.
[0046] FIG. 6 panels A-C are diagrams showing contours of electric
field magnitude and potential. FIG. 6 panel A shows electric field
magnitude contours near and far from three vias for an applied
voltage of 12 V.sub.pp. The field intensities were similar for the
three vias (diameter of each via is 50 nm); scale: highest,
2.526.times.10.sup.6 V/m; lowest, 1.095.times.10.sup.5 V/m. FIG. 6
panel B shows contours of electric potential near a via surface;
scale: highest, 8.417V; lowest, 8.001V. FIG. 6 panel C shows that
electric field magnitude arises from the potential drop shown in
panel B. Simulation time: 0.1 second.
[0047] FIG. 7 panels A-C show electric field simulations in 400 nm
diameter vias of different aspect ratios under an applied voltage
of 12 V.sub.pp. The aspect ratios of the vias were 0.75 (panel A),
0.375 (panel B), and 0.125 (panel C). The scale on the left of each
panel correlates with the magnitude of electric field intensity
inside the via shown.
[0048] FIG. 8 panels A-D show electric field simulations showing
electric field magnitude as function of spacing in vias that are
100 nm in diameter and 150 nm high. In each panel the bars at the
upper left corner represent the electric field intensity
magnitudes. The darker regions within a via have greater electric
field intensity. The vias are separated by 50 nm (panel A), 100 nm
(panel B), 200 nm (panel C) and 500 nm (panel D).
[0049] FIG. 9 panels A-D are electron micrographs showing results
of nanostructure assembly processes performed at different applied
voltages for a duration of 90 seconds. FIG. 9 panel A shows the
result of assembly at an applied voltage of 20 V.sub.pp, and a
frequency of 50 kHz. The arrows highlight some of the regions in
the via array that featured particle agglomeration. Inset shows a
high-angle SEM image of an assembly area indicating high assembly
rate in the vias. FIG. 9 panel B shows the result of assembly at an
applied voltage of 6 V.sub.pp, and a frequency of 50 kHz. FIG. 9
panel C shows the result of assembly performed at a voltage of 12
V.sub.pp, and a frequency of 100 kHz. FIG. 9 panel D shows the
result of assembly performed at a voltage of 12 V.sub.pp, and
frequency of 10 kHz. Inset shows a higher-magnification SEM image
of the assembly area after PMMA removal, revealing particle
over-deposition on the gold surface.
[0050] FIG. 10 is a SEM image of a nanopillar array with
nanopillars of 150 nm height and 200 nm diameter, separated by 500
nm.
[0051] FIG. 11 is a set of SEM micrographs of nanopillars assembled
by varying the time of assembly of NP to control nanopillar height.
FIG. 11 panel A shows nanopillar height of 30 nm with an assembly
time of 15 seconds. FIG. 11 panel B shows nanopillar height of 85
nm with an assembly time of 45 seconds. FIG. 11 panel C shows
nanopillar height of 110 nm with an assembly time of 60 seconds.
FIG. 11 panels D shows a nanopillar with a mushroom-like flat-cap
geometry that resulted when the assembly time was increased to 180
seconds using a via depth of 150 nm.
[0052] FIG. 12 panels A-D are a set of SEM micrographs and
fluorescence images of homogeneous and hybrid nanopillars. The
exposure time for each of the fluorescence images was 2 min. FIG.
12 panel A is a SEM micrograph (upper half) and a fluorescence
image (lower half) of a homogeneous gold nanopillar. FIG. 12 panel
B is a SEM micrograph (upper half) and a fluorescence image (lower
half) a gold-fluorescent silica hybrid nanopillar. FIG. 12 panel C
is a SEM micrograph (upper half) and a fluorescence image (lower
half) a gold nanopillar. FIG. 12 panel D is a SEM micrograph (upper
half) and a fluorescence image (lower half) a gold-fluorescent PSL
hybrid nanopillar.
[0053] FIG. 13 panels A-B are diagrams showing simulations of
electric field magnitude inside two 50 nm diameter, 150 nm high
vias after the first segment of the hybrid nanopillars was formed.
Electric field intensity in the via after formation of a gold
nanopillar is shown in panel A, and after formation of a PSL
nanopillar is shown in panel B. In each panel the bars at the upper
left corner represent the magnitude of electric field
intensities.
[0054] FIG. 14 panels A-B are SEM micrographs (upper panels) and
fluorescence images (lower panels) of fluorescent PSL (panel A),
and fluorescent PSL-gold (panel B) 3-D heterostructures. The
exposure time for each fluorescence image was 2 min.
[0055] FIG. 15 panels A-B are diagrams and SEM images of square and
circular patterned nanostructures manufactured using
dielectrophoretic NP assembly. FIG. 15 panel A shows a top-down
view of electric field simulations for a square pattern. Inset
shows a cross-sectional view from a portion of the pattern. The
darker regions within the via has greater electric field intensity.
Bars in the upper left corner represent magnitude of electric field
intensities. FIG. 15 panel B shows top-down (upper) and tilted
(lower) SEM images of 3-D square (left) and circular patterns
(right).
[0056] FIG. 16 shows SEM micrographs of a lamella obtained after
performing focused ion beam (FIB) thinning under low (panel A) and
high (panel B) magnification.
[0057] FIG. 17 shows bright-field TEM images and corresponding
electron diffraction patterns (insets) of two different
nanopillars, revealing the polycrystalline nature of the
structures.
[0058] FIG. 18 panels A-D are SEM micrographs of 3-D gold
nanopillars obtained with two different processes, electroplating
and electric field-directed nanoparticle assembly. FIG. 18 panel A
shows a nanostructure obtained by electroplating for 90 seconds.
FIG. 18 panel B shows an array of nanostructures obtained by
electroplating for 240 seconds. Button mushroom-like geometries
(inset) were obtained after the electroplating process. FIG. 18
panel C shows a nanostructure obtained by directed assembly for 90
seconds. FIG. 18 panel D shows an array of nanostructures obtained
by directed assembly for 240 seconds. Flat-cap mushroom-like
geometry (inset) was obtained after the directed assembly
process.
[0059] FIG. 19 panels A-C are SEM images of gold nanopillars, and a
graph correlating electric current with applied voltage.
[0060] FIG. 19 panel A shows a high-angle SEM image of gold
nanopillars in PMMA vias. The assembly time was adjusted so that
the nanopillar was slightly higher than the PMMA layer to achieve
good contact with the nanopillars, as shown in the inset. FIG. 19
panel B is a SEM image of a tungsten probe gently contacting a
150-nm-thick PMMA surface. A second probe was positioned on the
gold layer beneath the PMMA. FIG. 19 panel C is a graph of
electrical current plotted as a function of the applied voltage for
the probe/PMMA configuration in panel B. The PMMA broke down when
the applied voltage was greater than 1.5 V (which is much larger
than the voltages applied during electrical characterization of the
nanopillars).
[0061] FIG. 20 panels A-E are a set of SEM images of nanopillars
and probes, and graphs of electric current measured as a function
of voltage. FIG. 20 panel A shows a configuration in which the
probe was placed near a 50-nm-diameter NP-based gold nanopillar,
but without making contact. No current is observed in this
configuration. FIG. 20 panel B shows a configuration in which the
probe barely touched the top section of the pillar surface. The
resistance was high because of the loose contact between the
nanopillar and the probe. FIG. 20 panel C shows the same
configuration shown in panel B after the electrical characteristics
had been measured. The probe has penetrated into the gold
nanopillar, resulting in low resistance. The values of resistance
were three orders of magnitude lower than those observed in the
configuration in panel B. FIG. 20 panel D shows a NP-based
nanopillar fabricated using longer assembly time. The resistance
value was further improved by increasing the contact area. FIG. 20
panel E shows an electroplated nanopillar fabricated using a longer
assembly time. The probe was positioned on the pillar as shown in
the inset. The resistance was very low because of the large contact
area.
[0062] FIG. 21 is a set of graphs showing current measured as a
function of voltage for sets of NP-based gold nanopillars (NP;
panel A) and electroplated gold nanopillars (EP; panel B).
Nanopillars on two different chips were measured for each type.
[0063] FIG. 22 shows a simulation of near-field intensity
enhancement with a gold nanopillar standing on the dielectric
substrate, glass. A value of (|E|.sup.2/|E.sub.in|.sup.2) of only
as high as 250 was achieved. The scale to the right shows magnitude
of fold enhancement of near-field intensity, and correlates with
the fold enhancement shown in the FIG. The corresponding parameters
are Diameter=200 nm, Height=400 nm and Pitch=600 nm, where pitch is
the distance between the centers of the two adjacent vias.
[0064] FIG. 23 panels A-C show simulation of electric field
magnitude and SEM micrographs of nanopillars. FIG. 23 panel A shows
simulation of electric field magnitude inside a 200 nm diameter,
400 nm high via. Bars in the upper left corner represent magnitude
of electric field intensities. FIG. 23 panel B shows a tilted SEM
image of a nanopillar array after 50 nm gold NPs were assembled in
the vias having the geometry shown in panel A. FIG. 23 panel C
shows a tilted SEM image of the nanopillar array of panel B
following heat treatment.
DETAILED DESCRIPTION OF THE INVENTION
[0065] Directed assembly of nanoparticles (NPs) has been shown to
be a promising approach for building functional nanomaterials and
nanostructures for many applications such as electronics
(Hermanson, Science 294, 1082-1086, 2001), optics (Liberman et al.,
Adv. Mater. 22, 4298-4302, 2010), and biosensing (Zayats et al., J.
Am. Chem. Soc. 125, 16006, 2003). NPs have been assembled into one,
two and three-dimensional nanostructures by utilizing electric
(Hermanson, et al., Science 294, 1082-1086 2001; Zayats et al., J.
Am. Chem. Soc. 125, 16006, 2003; Lee et al., Sens. Actuators B 136,
320-325 (2009); Lee et al., Nano Lett. 11, 119-124, 2011) magnetic
(Erb et al., Nature 457, 999-1002, 2009) and fluidic forces (Tobias
et al., Nature Nanotech. 2, 570-576, 2007).
[0066] However, fabrication of solid, crystalline, homogenous or
hybrid, well-defined nanostructures with nanoscale precision has
not been demonstrated, largely due to the difficulties in
controlling the assembly and fusion of different types of NPs. NPs
can differ in composition, functionalization, size and the media in
which NPs are suspended. Depending on these parameters, the forces
driving the assembly and fusion of NPs differ from particle to
particle affecting the nanostructure formation process. For
example, larger size NPs have a higher melting temperature compared
to smaller ones (Ko et al., Nano Lett. 7, 1869-1877, 2007) making
them difficult to fuse into solid structures. Similarly, depending
on the properties of the suspension medium, NPs possess different
surface conditions such as surface charge and energy, which can
also affect forces driving the NPs to surfaces (Min et al., Nature
Mater. 7, 527-538, 2008).
[0067] Described herein are methods to precisely control the
assembly of various types of nanoparticles for the fabrication of
3-D homogenous nanostructures made of metals, oxides, polymers, or
hybrid nanostructures made of combinations of different materials,
each in a separate layer, including metal-polymer, metal-oxide and
metal-polymer-metal heterostructures. The methods involve directing
colloidal NPs, using dielectrophoresis (DEP) (Pohl, H. A.,
Dielectrophoresis, Cambridge Univ. Press, Cambridge, Mass. 1978)
toward a template. The template is a substrate having a conductive
film coated by any insulator it dielectric material, e.g.
poly(methyl methacrylate) (PMMA), that features nanoscale patterns
such as vias, as shown in FIG. 1A, from which the insulator has
been removed.
[0068] To perform the methods an AC electric field is applied
between the template, through the exposed conductive film in the
vias, and a counter electrode positioned opposite the template
(e.g., approximately 5 mm removed from the template) in the NP
suspension. The electric field creates a dielectrophoretic force on
the NPs, causing them to move toward the vias where the magnitude
of electric field is highest (FIG. 6). The NPs under the effect of
the dielectrophoretic force, also experience a pearl-chaining force
(Xiong et al., Appl. Phys. Lett. 91, 063101-1-063101-3, 2007),
which plays an important role in assembling the NPs into the vias.
The chaining force arises from the interaction of induced dipoles
between the NPs. This interaction plays an important role in
attaching NPs to the already-assembled NPs at the bottom surface of
the vias. In addition, due to the ionic atmosphere, the surface
charge of the particles creates an additional induced dipole moment
(Bhatt et al., Langmuir 20, 467-476 2004). As the NPs assemble, the
applied electric field induces their fusion, forming arrays of
solid nanostructures (FIG. 1B). The fusion of small colloidal
chains into structures such as wires under an applied electric
field arises from localized joule heating, induced by the applied
AC voltage at the NP junctions (Barsotti et al, Small 3, 488-499
(2007).
[0069] Following the assembly and fusion process, the insulator
layer can be removed (optionally, as needed), to obtain
free-standing 3-D nanostructures such as nanopillars, as shown in
FIG. 1C. Sequential assembly of different types of NPs can also be
carried out so as to fabricate hybrid nanopillars (FIGS. 1, A, D
and E).
[0070] FIG. 2 shows the effect of template geometry (i.e., geometry
of the vias) on nanopillar formation. Assembly and fusion of 5 nm
gold NPs in 50 nm and 100 nm diameter vias under the application of
12 V.sub.pp AC voltage with a frequency of 50 kHz for 90 seconds is
demonstrated in FIGS. 2 A and B. Since gold NPs (or any conductive
NPs, such as metallic NPs) are highly polarizable, they are
attracted toward the vias, where the electric field intensity is
high, under a DEP force of 4.51.times.10.sup.-14 N.
[0071] The DEP force acting on a spherical particle is given
by,
F.sub.DEP=2.pi..di-elect
cons..sub.mRe[K(w)]a.sup.3.gradient.|E|.sup.2 (1)
where .di-elect cons..sub.m is the dielectric constant of medium, a
is the particle radius, w is the angular frequency, and
.gradient.|E|.sup.2 is the gradient of the electric field (Jones,
T. B., Electromechanics of Particles, Cambridge University Press,
Cambridge, 1995). The direction of the force is determined by the
sign of the real part of the Clausius-Mossotti factor, Re[K(w)],
shown in equation
Re [ K ( w ) ] = p - m p + 2 m + 3 ( m .sigma. p - p .sigma. m )
.tau. M W ( .sigma. p + 2 .sigma. m ) 2 ( 1 + .omega. 2 .tau. M W 2
) , ( 2 ) ##EQU00001##
where .di-elect cons..sub.p is the dielectric constant of the
particle and .sigma..sub.p and .sigma..sub.m is are the
conductivities of the particle and medium, respectively.
.tau..sub.MW is the Maxwell-Wagner charge relaxation time that
indicates decay of a dipolar distribution of charge on the surface
of a spherical particle, and is given by
.tau. M W = p + 2 m .sigma. p + 2 .sigma. m ( 3 ) ##EQU00002##
[0072] If the sign of the real part of the Clausius-Mossotti factor
is greater than zero, i.e., Re[K(w)]>0, the process is called
"positive dielectrophoresis" and the particles are attracted to
regions where the field intensity is high. For Re[K(w)]<0, the
process is called "negative dielectrophoresis" and the particles
are repelled from regions of high field intensity. Metallic and
other highly polarizable particles yield Re[K(w)].apprxeq.1 in
aqueous suspensions; these particles are always attracted toward
regions of high field intensity (Gierhart et al. Langmuir, 23,
12450-12456, 2007; Velev and Bhatt, Soft Matter, 2, 738-750,
2006).
[0073] To estimate the value of .gradient.|E.sub.rms|.sup.2 near
the via, values of E.sub.rms obtained from the simulation were
used. Considering only the change in electric field in the y
direction, shown in FIG. 1C, the magnitude of the electric field
close to the via surface is given approximately by
E .fwdarw. rm s .apprxeq. V rm s y y ^ ( 4 ) ##EQU00003##
where V.sub.rms is the rms AC voltage applied to the vias, y is the
distance from the bottom of the via, and y is a unit vector
pointing in the y direction. From this equation the gradient of the
electric field squared is calculated,
.gradient. .fwdarw. ( E rm s 2 ) .apprxeq. - 2 ( V rm s 2 y 3 ) y ^
( 5 ) ##EQU00004##
Combining equations (4) and (5),
.gradient. ( E r m s 2 ) .apprxeq. - 2 ( E .fwdarw. r m s 2 y ) y ^
( 6 ) ##EQU00005##
[0074] From the simulation results, the electric field magnitude
was calculated to be 2.50.times.10.sup.6 V/m at p1 (80 nm from the
bottom of the via) and 2.35.times.10.sup.5 V/m at p2 (230 nm from
the bottom of the via). The electric field gradient between these
points was estimated by ({right arrow over
(.gradient.)}(E.sub.rms)) to be 8.26.times.10.sup.19
V.sup.2/m.sup.3 using equation (6). By using this value in equation
(1), the DEP force on the 5 nm gold nanoparticles (NPs) was
calculated to be to be 4.51.times.10.sup.-14 N.
[0075] Electric field simulations of 50 nm diameter and 150 nm deep
vias under an applied voltage of 12 V.sub.pp are shown in FIG. 6.
The rms voltages used were obtained by multiplying the applied
voltage by a factor of 0.707. FIG. 6B shows contours of electric
potential inside the via surface; scale: red, 8.417V; dark blue,
8.001V. The applied rms potential of 8.48 V decreased sharply from
the bottom of the via towards the top. The electric field magnitude
resulting from the rms potential is shown in FIG. 6B. The electric
field decreased rapidly (p1=2.50.times.10.sup.6 V/m;
p2=2.35.times.10.sup.5 V/m) within 100 nm above the top of the via,
resulting in a high field gradient in this region.
[0076] The actual DEP force experienced by a particle is expected
to be greater than the calculated value because the surface charge
of the particle would also interact with the AC electric field,
creating an additional induced dipole moment in the ionic
atmosphere (Bhatt and Velev, Langmuir 20, 467-476, 2004). The
motion of the particles during the dielectrophoresis is also
influenced by the Brownian force and other forces such as drag
force and electrohydrodynamic forces (Bhatt et al., Langmuir 21,
6603-6612, 2005). The fusion of small single colloidal chains into
wires under an applied electric field has been reported previously
Bernard et al., Nanotechnology 18, 235202, 2007); it arises mainly
from localized joule heating, induced by the applied AC voltage, at
the NP junctions (Tsong et al., Phys. Rev B 44, 13703-13710,
1991).
[0077] It was observed that filling vias with larger diameter
(about 200 nm or more), using the same experimental conditions as
used to fill sub 100 nm diameter vias, resulted in partial NP
assembly in the via and an incomplete nanopillar formation (FIG.
2C). The incomplete nanopillar formation is a function of the
electric field intensity and distribution in the vias. To fully
understand this phenomenon a 3-D computational fluid dynamics
simulation was used to model the electric field near the via. The
simulation results showed that the intensity of the electric field
in the vias varies depending on geometrical parameters such as
diameter, aspect ratio and spacing. FIGS. 2D-F display static
electric field contours in a via for three different via
geometries. For a given height and via spacing the decrease in
electric field intensity in larger-diameter vias results from a
geometry-induced edge effect, namely the amplification of the
electric field at the edges of the via (Bhatt et al., Langmuir 21,
6603-6612, 2005). The electric field strength of any via is always
higher at the edges compared to the center. This effect is not
noticeable in small vias because the edges are closer to each
other. However, as a via's diameter becomes sufficiently large, the
center part of the via is not affected by the amplification of the
electric field at the edges, leading to lower field intensities at
the center. Therefore, NPs are exposed to smaller DEP forces at the
center of a large via, resulting in only partial particle assembly
and incomplete fusion at the edges.
[0078] The DEP force near large diameter vias was estimated by
simulation of electric field magnitudes. For a via that is 200 nm
in diameter and 150 nm deep, the electric field magnitude was
calculated to be to be 1.56.times.10.sup.6 V/m at p1 (80 nm from
the bottom of the via) and 5.58.times.10.sup.5 V/m at p2 (230 nm
from the bottom of the via). Under these conditions, using equation
(1), the DEP force on 5 nm gold NPs was calculated to be
1.54.times.10.sup.-14 N.
[0079] Simulation results further showed that the magnitude of the
DEP force on a 5 nm particle that is at a distance of 80 nm from
the bottom of a 200 nm diameter via is 1.54.times.10.sup.-14 N,
which is 3 times smaller than the force at the same distance from a
50 nm diameter via. The simulation results thus explain the
incomplete nanopillar assembly shown in FIG. 2C.
[0080] The electric field intensity changes also as a function of
the aspect ratio of the vias (FIG. 2G). Diameter remaining
constant, as the aspect ratio of a via is decreased, the electric
field intensity increases because the via has a thinner insulator
since it is shorter. However, at very small aspect ratios, the edge
of the insulating layer (PMMA) did not produce sufficient electric
fields relative to those of the higher aspect ratio structures
because the insulating layer was very thin. As a result, the
geometrical edge effect was less significant, resulting in a low
electric field in the via (FIG. 7). The simulations also revealed
that, for a given diameter and the same number of vias, the
electric field intensity in the closely spaced vias was small
compared to ones separated by a larger distance. (FIG. 2H). When
the vias were close to each other, the effect of the insulating
layer between them was very small. Therefore, the electric field
contours near these vias interacted with each other, resembling the
case of a single large via with a low electric field (FIG. 8). For
via spacing greater than 500 nm, the electric field interactions
between the vias decreased significantly, resulting in higher field
intensities in the vias.
[0081] The electric field intensity is influenced also by the
aspect ratio and the spacing between the vias. As shown in FIG. 7,
the electric field in the middle of a via initially increased upon
decreasing the aspect ratio. Further reduction of the aspect ratio
resulted, however, in a decreased electric field, due to reduced
geometrical edge effect. In addition, the electric field in the
middle of a via increases upon increasing the spacing between the
vias. As shown in FIG. 8A, when the dielectric between the vias is
thin, the electric field contours interact with each other,
resulting in low electric field in the vias. For the case of 50 nm
spacing, the magnitude of the electric field in a 200 nm diameter
via was close in magnitude to that in a 200 nm diameter via that is
separated from other vias by a distance of 500 nm or more.
[0082] Amplitude and frequency of the applied voltage plays a key
role in obtaining successful nanopillar formation for different via
geometries. It was observed that 50 nm diameter pillars are
typically obtained with an applied voltage of 12 V.sub.pp and a
frequency of 50 kHz, for 90 seconds. Lower voltages (.about.6
V.sub.pp) reduces DEP, and chaining forces decrease, resulting in
partially assembled and partially fused particles in the vias (FIG.
9B). Application of very high voltages (.about.20 V.sub.pp)
significantly increased the assembly rate but could result in over
assembly of the particles and agglomeration on the PMMA surface
(FIG. 9A).
[0083] NP assembly in the vias could be controlled also by varying
the frequency of the applied field. At higher frequencies
(.about.100 kHz), the counterions on the particles could not follow
the rapidly oscillating electric field, which decreased the
particle-electric field interaction (Hermanson et al., Science 294,
1082-1086 (2001) and the assembly rate (FIG. 9C). At low
frequencies (.about.10 kHz), the assembly rate became very high,
also resulting in undesirable particle over assembly and
agglomeration in localized regions mostly near the edges of the via
array (FIG. 9D).
[0084] Therefore, fabrication of large diameter (between 50-200 nm)
nanostructure arrays requires voltages to be slightly higher than
12 V.sub.pp, for example, about 14-16 V.sub.pp (see FIG. 2F showing
poor assembly at 12 V.sub.pp).
[0085] Further, with a feature diameter larger than 200 nm, the
electric field near vias becomes too low for nanopillar formation
using only AC voltage. For successful particle assembly and
chaining to occur in a via it is necessary that the DEP force be
above a certain threshold, in addition to there being a sufficient
particle concentration near the vias. For very large diameters
(i.e., larger than 200 nm), the electric field was higher at the
edges of the vias, causing NPs to form chains only at these
locations. Since the DEP force was effective only close to via
patterns (i.e., a few hundred nm from a via; Morgan, H. &
Green, N. G. AC Electrokinetics: colloids and nanoparticles
Research Studies Press Ltd. Baldock, Hertfordshire, England, 2003),
NPs far from the via (i.e., further distant than a few hundred nm
out into the NP suspension) do not contribute to particle chaining,
leading to incomplete assembly in the vias. To overcome the
incomplete assembly a constant DC offset voltage can be applied in
addition to the AC voltage. The DC offset voltage creates an
additional electrophoretic (EP) force on the NPs to drive the
nanoparticles closer to the vias, thereby effectively increasing
the concentration of the particles in the vicinity of the vias. The
magnitude of electrophoretic force on 5 nm gold NPs under a DC
voltage of 2V is calculated to be 1.78.times.10.sup.-12 N, which is
almost 2 orders of magnitude higher than the DEP force.
[0086] The DEP force on 5 nm gold NPs near a 200 nm diameter, 150
nm deep via is 1.54.times.10.sup.-14 N, which is 3 times lower than
the force near a 50 nm diameter via with a same depth. In this
case, AC voltage alone was not sufficient to fill these vias.
Applying an electrophoretic force on the NPs using a DC offset
voltage resulted in assembly of the NPs in these vias.
Electrophoretic directed assembly of nanoparticles into
nanotrenches is governed by the transport of charged particles
under externally applied uniform electric field. The induced charge
on the particles depends on the particle zeta potential according
to Debye-Huckel approximation of the
Derjaguin-Landau-Verwey-Overbeek theory,
q=4.pi.R.di-elect cons..sub.r.di-elect
cons..sub.0(1+.kappa.R).zeta. (7)
where R is the radius of a colloidal particle, .di-elect
cons..sub.r is permittivity of suspension, .di-elect cons..sub.0 is
permittivity of vacuum, .kappa. is inverse Debye length, and .zeta.
is the zeta potential on the particles. Debye length is calculated
by,
.kappa. - 1 = r 0 k B T 2 N A e 2 I ( 8 ) ##EQU00006##
where I is the ionic strength of the electrolyte, k.sub.B is the
Boltzmann constant, T is the absolute temperature in kelvins,
N.sub.A is the Avogadro number, e is the elementary charge.
[0087] The zeta potential of 5 nm gold NPs was measured to be 48 mV
in the particle suspension with a conductivity of 100 .mu.S/cm. At
these conditions, the charge on these particles is calculated to be
3.18.times.10.sup.-18 C. The electrophoretic force then can be
calculated using,
F.sub.EP=q*E (9)
where E is the static electric field intensity at a particular
distance from the vias. The electrophoretic force, due to a 2 V DC
offset, on the particles located 230 nm away from the bottom of the
vias was calculated to be 1.78.times.10.sup.-12 N.
[0088] The additional DC offset voltage resulted in a uniform
nanopillar formation in 200 nm diameter vias (FIG. 10).
[0089] FIG. 10 shows successful formation of 200 nm diameter, 150
nm deep vias using 12V.sub.pp voltage with a 2V DC offset at 50 kHz
frequency. The assembly time was 90 seconds.
[0090] Nanopillar arrays, including those having nanopillars with
diameter as small as 25 nm, and made of different material types,
were fabricated (FIG. 3 A-D). Specific assembly parameters are
needed for fabrication of nanopillars of a particular dimension and
made of a particular material. These parameters are summarized in
Table 1.
TABLE-US-00001 TABLE 1 Summary of the parameters affecting the 3-D
nanostructure fabrication process. 3-D nanostructure Range type
Parameter Via diameter <200 nm Via diameter .gtoreq.200 nm
Conductor Voltage 12-16 V.sub.pp 16-20 V.sub.pp (Au, Cu, W, Al,
etc.) Frequency 30-70 kHz 30-70 kHz DC voltage -- 2 V Assembly time
30-90 s 90-600 s Particle size .ltoreq.10 nm >50 nm Particle
concentration ~10.sup.13 ml.sup.-4 10.sup.13-10.sup.14 ml.sup.-2
Post heat treatment -- ~250.degree. C. Insulator/ Voltage 12-16
V.sub.pp 16-20 V.sub.pp semiconductor Frequency 30-70 kHz 34-70 kHz
(SiO.sub.2, PSL, Si, CdSe, etc.) DC voltage -- 2 V Assembly time
30-90 s 90-600 s Particle size .ltoreq.30 nm >50 nm Particle
concentration 10.sup.13-10.sup.14 ml.sup.-3 10.sup.13-10.sup.18
ml.sup.-3 Post heat treatment polymers (--) polymers
(>250.degree. C.) oxides (>250.degree. C.) oxides
(>400.degree. C. semiconductors (>250.degree. C.)
semiconductors (>250.degree. C.) Hybrid Type Range (all
diameters) Hybrid Insulator/semiconductor same for fabricating
insulating/semiconducting on a conductor structure Conductor on an
insulator/ 12-16 V.sub.pp 30 kHz, 2 V DC, 120-180 s,
10.sup.18-10.sup.16 ml.sup.-1 semiconductor structure for particles
>10 nm ~250.degree. C. post heat treetment is needed
[0091] Desired pillar density on a template with nanopillars of
identical pillar dimensions was achieved by adjusting the spacing
between the vias (FIG. 3A). High-magnification SEM images (FIGS. 3
B-D) show that the aspect ratios of the pillars could be adjusted
by controlling the diameter and depth of the vias. The
corresponding atomic force microscopy images (FIGS. 3B-D lower
half) indicate that all fabricated nanopillars have smooth side
walls. In addition to gold, nanopillars made of copper or other
metals can be fabricated on a gold or another conductive surface
using similar assembly parameters used for fabrication of the gold
nanopillars (FIG. 3E).
[0092] The directed assembly approach described here is compatible
with conductors, and also with other types of inorganic or organic
insulating NPs such as polymers and oxides (e.g, silica, alumina,
titania). FIG. 3F shows a SEM image of a 50 nm diameter
polystyrene-latex (PSL) nanopillar fabricated by directly
assembling 22 nm fluorescent PSL particles into the vias. Although
PSL particles have lower bulk polarizability than the medium, they
experience positive DEP at tens of kHz, because the conductance of
the ionic layer near the particle surface becomes more dominant
compared to intrinsic conductance of the particle (Ermolina et al.
J. Colloid Interface Sci. 285, 419-428, 2005). The ionic layer
creates an additional dipole moment, contributing to the DEP force.
Therefore, solution properties such as pH and ionic conductivity
play an important role in the assembly of these particles.
[0093] For an insulating particle such as PSL, bulk conductivity,
.sigma..sub.b.apprxeq.b. Therefore, both conductivity and
dielectric constant of the particle becomes lower than medium
(.sigma..sub.p<.sigma..sub.m and .di-elect
cons..sub.p<.di-elect cons..sub.m). In this case, equation (2)
yields Re[K(w)]=-0.5. Since this value is below 0, one can expect
negative dielectrophoresis. However, results obtained using methods
herein showed positive dielectrophoresis under these conditions.
Although insulating particles have low intrinsic conductivity,
surface conductance component produced by the movement of
counterions dominates at low frequencies (Jones, T. B.,
Electromechanics of Particles, Cambridge University Press,
Cambridge, 1995; et al., Langmuir, 23, 12450-12456, 2007).
[0094] The total conductivity of a solid sphere particle is given
by
.sigma. p = .sigma. b + 2 K s R ( 10 ) ##EQU00007##
where .sigma..sub.b is the bulk conductivity and
2 K s R ##EQU00008##
is the surface conductivity. K.sub.s is a general surface
conductance (typically 1 nS for latex particles 1) and R is the
particle radius. The magnitude of
2 K s R ##EQU00009##
was calculated to be 1.82.times.10.sup.-1 S/m for 22 nm PSL
particles. This value is higher than the conductivity of the
solution, which is 1.times.10.sup.-2 S/m. Hence, new Re[K(w)] is
calculated to be 0.851, resulting in positive dielectrophoresis.
Under these conditions, the DEP force on the 22 nm PSL particles
was calculated to be 3.25.times.10.sup.-12 S/m.
[0095] The zeta potential of 22 nm fluorescent PSL particles was
measured to be about -53 mV at pH 11. At this pH, the calculated
DEP force was 3.25.times.10.sup.-12 N. Due to this large DEP force,
the PSL particles assembled into vias under AC voltage without
requiring a DC offset. Similar to metallic particles, the PSL
particles also were fused in the via due to the localized joule
heating. The fusion process did not impair the fluorescence
properties of particles, and the resulting nanopillars continued to
be fluorescent (FIG. 12).
[0096] Fabrication of silica nanopillars (FIG. 3G) required
modification of the assembly process described above. Due to
increased melting temperature associated with larger size, and
other intrinsic properties, silica NPs did not entirely fuse in the
vias. However, either heat treatment or application of a large DC
electric potential following the assembly process can be utilized
to form fused nanopillars.
[0097] NP assembly also was controlled by altering the assembly
time. The assembly process was timed to achieve a preferred
nanopillar height in the via (FIG. 11A-C). When the assembly
process was continued after the vias had become completely full,
the NP chains developed over the via surface, resulting in a
flat-cap mushroom-like geometry (FIG. 11D). The ability to adjust
the nanopillar height through variation in assembly time is
important for applications where fabrication of structures with a
very high aspect ratio (e.g., 1-D nanopillar arrays) is desired
(Xia et al., Adv. Mater. 15, 353-389, 2003).
[0098] Methods described above were used to fabricate nanoscale
hybrid structures, viz., gold-fluorescent PSL (FIG. 3H) and
gold-fluorescent silica layered nanopillars (FIG. 3I). Fabrication
of the hybrid structures required precise control of nanopillar
height within the via. To achieve a desired nanopillar height the
assembly rate of gold NPs was estimated by varying the assembly
time (other parameters were kept constant at 12V.sub.pp and 50
kHz). The height of nanopillars increased with time in a linear
manner at a rate of 2 nm/second (FIG. 11). A preferred height for
the gold segment of hybrid nanopillar was obtained by adjusting the
assembly time. Following the formation of gold segment fluorescent
PSL or silica NPs were assembled and fused into the remaining part
of the via. The resulting hybrid nanostructure is clearly shown by
the fluorescent microscopy images in the insets of FIGS. 3H and I,
and FIG. 12. The above nanostructures were created using a
conductive base, and utilizing electric field directed assembly to
assemble polymer or silica particles on top of gold
nanopillars.
[0099] The electric field directed assembly approach described here
is useful to also assemble metals on top of an insulator or a
semiconductor to obtain multi-segmented 3-D structures, which are
useful in many applications such as optics (Wadell et al., Nano
Lett. 12, 4784-4790, 2012), electronics (Wu et al., Nature 430,
61-65, 2004), energy (Fan et al., Nat. Mater. 8, 648-653, 2009) and
biomedicine (Salem et al. Nature Mater. 2, 668-671, 2003). Hybrid
nanopillar formation was observed by SEM and optical microscopy.
FIGS. 12A and C shows SEM images of gold nanopillars before the
fabrication of hybrid nanopillars. The bare gold nanopillars did
not show any fluorescence signal under the optical microscopy.
FIGS. 12B and D show the SEM images of hybrid nanopillars after the
assembly of fluorescent silica and fluorescent PSL NPs
respectively. The gold portion of these hybrid nanopillars was
fabricated using the same experimental parameters as in FIGS. 12A
and C. Optical images showed green and red signal from the via
arrays for gold-fluorescent silica and gold-fluorescent PSL
nanostructures respectively.
[0100] 5 nm gold NPs were assembled and fused simultaneously on top
of the fabricated dielectric fluorescent PSL nanopillars with no
post heat treatment (FIG. 3J). Simulated electric field in the vias
that already have gold nanopillars were compared to those that have
PSL nanopillars as the first segment (FIG. 13). The resulting
electric field intensity was significantly lower in the via that
has PSL nanopillar since PSL is a dielectric material. Therefore,
gold NPs were exposed to extremely low DEP (almost 5 orders
magnitude lower) and EP (more than 3 orders of magnitude lower)
forces compared to forces on PSL NPs assembling on gold
nanopillars. Because of the lower DEP and EP forces, the uniform
fabrication of hybrid nanopillar with PSL nanopillar as the first
segment and gold nanopillar as the second segment, high NP
concentrations (2 times higher concentration compared to if the
gold NPs were assembled first) were used. For vias with diameter
less than 200 nm, typically the concentration used was 10.sup.13
particles/ml, and for vias with diameter greater than 200 nm, the
typical concentration was in the range of 10.sup.13-10.sup.14
particles/ml. Following the gold NP assembly, the fluorescence
signal from the PSL nanopillars decreased significantly, indicating
the formation of gold segments on top of the PSL nanopillar (FIG.
3J inset and FIG. 14).
[0101] Electric field simulations of gold and PSL nanopillars
within the first 50 nm of the via are shown in FIGS. 13A and B
respectively. It was observed that the via with PSL nanopillars
showed significantly lower electric field intensity compared to the
via with gold nanopillars. As a PSL particle approaches the via
having a gold nanopillar (FIG. 13A) it is exposed to a DEP force of
5.91.times.10.sup.-11 N. On the other hand, the DEP force on a 5 nm
gold particles approaching the via having a PSL nanopillar (FIG.
13B) was estimated to be 5.26.times.10.sup.-16 N. In addition,
calculations showed that the EP force on 5 nm gold nanoparticles at
a distance of 230 nm from the bottom of the via in the case of a
PSL nanopillar (FIG. 13 B) was 4.78.times.10.sup.-13 N, which is
more than 3 orders of magnitude lower compared to EP force on PSL
particles in the case of a gold nanopillar (EP force of
9.0.times.10.sup.-1.degree. N; FIG. 13A).
[0102] Complex 3-D nanostructures having a cross-sectional profile
of a hollow geometric figure (e.g., a circle, ellipse, square,
rectangles, triangles, and the like), such as nanorings and
nanoboxes, with a wall thickness as small as 25 nm, can be
fabricated using modifications of the electric field directed
assembly approach described above (FIG. 3K). The nanobox geometries
have a larger exposed area and lower electric field than those of
the 50 nm diameter via geometries shown in FIG. 1 and FIG. 15. To
fabricate these structures, a higher voltage (16 V.sub.pp) and a
lower frequency (30 kHz) were applied for 90 seconds, compared to
the voltage (12 V.sub.pp) and frequency (50 kHz) used respectively
to assemble 5 nm gold particles. Ring shaped nanostructures are
useful for spintronics applications, such as magnetoresistive
random access memory (MRAM) (Chappert et al., Nature Mater. 6,
813-823, 2007). The ring and box shape nanostructures were
fabricated without using complex or multiple fabrication steps.
[0103] Material characteristics of the gold nanopillars
manufactured using the electric field directed assembly approach
described above were determined using transmission electron
microscopy (TEM). Gold nanopillars were placed on a copper TEM grid
using a conventional lamella lift-out process (FIG. 16). The
bright-field images in FIGS. 16A and B, and the small-area electron
diffraction (SAED) pattern in the insets indicate that the gold NPs
were completely fused during the assembly process, transforming
them into polycrystalline nanopillars lacking air voids or gaps
(FIG. 17). The bright-field image in FIG. 4A also indicates that
the gold NPs were completely fused during the assembly process,
transforming them into homogeneous nanopillars without any voids or
gaps. The SAED pattern obtained from the entire nanopillar shown in
the inset to FIG. 4A revealed the polycrystalline nature of the
nanopillars. Notably, only two grains, each having its lattice
oriented in one direction, were observed over the 30 nm.times.30 nm
area of the nanopillar (FIG. 4B). Since the nominal diameter of an
individual NP used was 5 nm, it was inferred that large number of
NPs fused into a single grain. The formation of a
single-crystalline material might have resulted from the
recrystallization of multiple NPs during the fusion process (Tang
et al. Science 297, 237-240, 2002). Based on the observations it is
envisioned here that it is possible to manufacture
single-crystalline nanopillars through further tuning of the
assembly parameters.
[0104] Electrical characteristics of the nanostructures produced
using the methods described above show that the nanostructures are
useful as nanoelectronics. The electrical characteristics of the
nanostructures were compared with those of 3-D nanopillars
fabricated by a conventional electroplating process using an
SEM-based in situ Zyvex S-100 nanomanipulator (FIGS. 18-21).
[0105] Electroplated nanopillars were produced by applying a DC
voltage between the template and the counter electrode through a
gold electrolyte solution. The gold layer under the PMMA served as
a seed layer during the electroplating process. Because of
electrochemical reactions at the metal-electrolyte interface, the
gold atoms nucleated on the seed layer and grew vertically in the
vias. As in the directed NP assembly, the dimensions of the
electroplated nanopillars were controlled by the diameter and
height of the vias. For a constant current density Faraday's
formula was used to determine the electroplating rate (metal height
per minute).
[0106] The electroplating rate was estimated using Faraday's law,
with deposition stopped when the desired pillar height was
achieved. The plating rate is given by
R = D * A n F .rho. ( 11 ) ##EQU00010##
where R is the plating rate (cm/sec), A is the molecular weight of
then metal (g/mol), n is the valence of the dissolved metal in
solution (equivalents/mol), F is Faraday's constant (C/equivalent),
p is the density of the metal (g/cm.sup.3), and D is the current
density (A/cm.sup.2).
[0107] The electroplating rate of gold at a current density of 2.5
mA/cm.sup.2 was estimated to be 79.3 nm/min Results showed that
electroplating of the gold solution produced approximately 150 nm
high nanopillars in the via, thus verifying the calculated
electroplating rate. FIGS. 18A and C show that the nanopillar
formation rate for the directed assembly process was similar to
that for the electroplating method. The plating process was stopped
as desired metal height in the vias was reached. When plating was
continued after the vias had become completely full, gold
deposition occurred over the PMMA surface, forming button
mushroom-like nanostructures (FIG. 18B), which are different than
those from the NP assembly process (FIG. 18 D).
[0108] Both the NP-based and electroplated gold nanopillars yielded
comparable resistance (FIG. 4C). Based on the resistance
measurements, the lowest resistivity (calculated) for 20 different
NP-based pillars was 1.96.times.10.sup.-7 .OMEGA.m, which is only
an order of magnitude higher than the bulk resistivity of gold
(2.44.times.10.sup.-8 .OMEGA.m; FIG. 21). The lowest resistivity
value is lower compared to some of the previously obtained values
of resistivity for gold nanowires having similar dimensions (Chen
et al., Nanotechnology 16, 1112-1115, 2005).
[0109] Further, flexibility in choice of material makes the method
described above superior to electroplating. Because the formation
of the nanopillars in this process is governed by physical assembly
followed by fusion of NPs on the surface, and not by the chemical
nucleation as in electroplating, the method offers the advantage
that any conducting, semiconducting, or insulating materials can be
directly fabricated on surfaces without requiring an intermediate
seed layer or chemical additives. It is envisioned that this
advantage together with the scalability of the process described
here, would make possible development of seedless copper
interconnect technology (Park et al., J. Electrochem. Soc. 157,
D609-D613, 2010) and fabrication of very-small-diameter (<16 nm)
interconnects (Reid et al. Solid State Technol. 53, 14-17, 2010),
which currently pose challenges in CMOS (complementary
metal-oxide-semiconductor) based manufacturing.
[0110] Nanostructure fabrication methods described above can be
used to fabricate high quality plasmonic nanostructures for optical
device applications. In a plasmonic nanostructure, surface plasmons
localize, enhancing light at a metal/dielectric interface and
leading to strong light/matter interactions (Aydin et al. Nature
Commun. 2, 1-7, 2011). Advances in plasmonics require the ability
to pattern high quality metals and hybrid materials at nanoscale
dimensions. In recent years several new fabrication approaches have
been proposed to exploit plasmons for a wide range of applications
(Lu, Science 337, 450-453, 2012; Boltasseva, J. Opt. A: Pure Appl.
Opt. 11, 114001, 2009). Biosensing is one application of plasmonics
(Khademhosseinieh et al., Appl. Phys. Lett. 97, 221107, 2010;
Yanik, Nano Letters, 10 (12), 4962-4969 2010). Biosensing platforms
utilize plasmonic resonances that show variations due to change in
the refractive index of their surrounding medium. To achieve a
reliable biosensor with low limit of detection, narrower resonances
and high overlap between optical fields of the plasmonic mode and
the interacting biomolecules are needed. A plasmonic metamaterial
structure based on randomly positioned nanopillar arrays was
recently shown to be suitable for ultrasensitive biosensing by
Kabashin et al., Nat. Mater. 8, 867-871, 2009. A periodic
nanopillar system was analyzed numerically to further improve
biosensing performance (Cetin et al., Appl. Phys. Lett. 98, 111110,
2011). As shown in FIG. 5A, the optimized nanopillars of the
present invention have radius and height of about 100 nm and 400
nm, respectively. Nanostructures with such high aspect ratio are
challenging to make using lift-off based electron-beam lithography
techniques. FIGS. 5 B and C show near-field intensity enhancement
(|E|.sup.2/|E.sub.in|.sup.2) distribution at the top surface and
through cross section, respectively. Numerical calculations were
performed, which indicated that nanopillar arrays enhanced the
incident light intensity up to 10,000 times. Further, these
enhanced local fields extend deep into the medium, making them
easily accessible to monitor changes in their surrounding medium,
and strongly amplifying the sensitivity of nanopillar based
plasmonic nanosensors to determine local refractive index
changes.
[0111] An example of a well-defined periodic nanopillar array
prepared using the fabrication method of the present invention is
as follows. It was observed that, for the fabrication of large
diameter and deep structures over a large area (over 0.2 mm.sup.2
area), further modification of the assembly conditions were
necessary due to very low electric field in the vias. Instead of 5
nm gold particles, 50 nm particles were used. Larger particles
experienced larger DEP forces, which increased the assembly rate in
the vias. Further, a DC offset in addition to the AC electric field
during the assembly process was used. As a result, the force on the
particles was increased, and a uniform particle assembly was
obtained over the via array (Yilmaz, C. et al., IEEE Trans
Nanotechnol. 9, 653-658, 2010). Further, the assembly time was
increased to 10 min. Moreover, it was observed that large particles
were not fused during the assembly. Therefore, heat treatment at
250.degree. C. on a hot plate was employed to fuse the large
particles. FIG. 23 shows that the electric field intensity inside a
200 nm diameter and 400 nm high via, which produces a very large
exposed area, is very low. It was observed that although the 50 nm
NPs filled the vias, they did not fuse completely. However,
particles were fused by applying heat from an external heat source
following the assembly process. Compared to the melting temperature
of bulk materials, the NPs start to melt at much lower temperatures
(Ko et al. Nano Lett. 7, 1869-1877, 2007). FIG. 23C shows the fused
gold NPs after heat treatment at 250.degree. C. on a hot plate. The
unfused NPs can also be fused by applying a large DC voltage (ca.
30V) between the template and the counter electrode following
assembly.
[0112] The nanostructures obtained using methods described herein
demonstrated high optical quality, supporting strong plasmonic
resonances with line-widths as narrow as 13 nm. The resonance was
observed to shift strongly with different refractive indices of
bulk solutions including DI-water n.sub.DI=1.333, acetone
n.sub.acetone=1.356 and IPA n.sub.IPA=1.377. As shown in FIG. 5D, a
refractive index sensitivity as large as 571 nm/RIU was observed.
Due to spectrally narrow resonances the experimental figure of
merit achieved was as large as 44.
[0113] Nanopillar structures were also found suitable for
surface-based biosensing as demonstrated by the detection of
monolayers of Protein A/G (a fusion of Protein A and Protein G
having the immunoglobulin binding domains of both), and bilayers of
immunoglobulin (IgG) bound to Protein A/G. As shown in FIG. 5E, due
to accumulation of biomass on the sensor platform, the plasmonic
resonance was observed to shift robustly by 4 nm and 14 nm after
addition of Protein A/G and IgG, respectively.
Examples
Example 1
Particle and Template Preparation
[0114] Aqueous gold NPs (nominal diameter: 5 nm) were purchased
from British Biocell International. Aqueous copper NP suspension
(nominal diameter: 10 nm) was purchased from Meliorium Technologies
(Rochester, N.Y.). Aqueous fluorescent polystyrene-latex (PSL) NP
suspension (Fluoro-Max Red, nominal diameter: 22 nm) was purchased
from Thermo Scientific (Waltham, Mass.). Aqueous polystyrene
fluorescent silica NP suspension (fluorescent-green, nominal
diameter: 30 nm) was purchased from Kisker-biotech (Steinfurt,
Germany). The template depicted in FIG. 1 was prepared by
sputtering Cr/Au (2 nm/120 nm) onto a SiO.sub.2/Si (470 nm/380
.mu.m) wafer followed by dicing the wafer into 12 mm.times.12 mm
chips. The Cr/Au chips were cleaned with piranha solution
(H.sub.2SO.sub.4/H.sub.2O.sub.2, 2:1) and spin-coated with PMMA.
Nanoscale patterns were fabricated using conventional electron beam
lithography and developed subsequently with methyl isobutyl ketone
and isopropyl alcohol (MIBK/IPA, 1:3).
Example 2
Nanoparticle-Based Nanopillar Manufacturing
[0115] The template prepared as described in Example 1 and a
counter electrode (Cr/Au sputtered gold) were connected to a
function/arbitrary waveform generator (Agilent 33220A) and
submerged into a NP suspension. Following the application of a
sinusoidal AC electric field with or without a DC offset, the
template and the planar counter electrode were removed from the
suspension using a dip coater (KSV NIMA) at a controlled speed (85
mm/min) Finally, the PMMA layer on the template was removed using
acetone for metallic and silica nanopillars, or ethanol for PSL
nanopillars.
Example 3
Nanopillar Electroplating Process
[0116] Techni-Gold 25 ES RTU (ready-to-use) solution was purchased
from Technic, Inc. (Pawtucket, R.I.). The solution included
sulfuric acid, ethylenediamine, sodium gold sulfite and sodium
sulfite. The temperature of the solution was held at 60.degree. C.
The patterned template and a counter electrode were submerged into
the electroplating solution. In contrast to the directed assembly,
a platinized titanium mesh was used as the counter electrode. The
solution was heated at a set temperature of 60.degree. C., while
stirring with a magnetic stirrer at a set rate of 75 rpm. A DC
voltage was applied between the template and the counter electrode
using a Keithley 2400 source meter. The magnitude of the applied
voltage and duration of deposition were altered to control the
electroplating rate.
Example 4
Electrical Characterization
[0117] Electrical properties of the nanopillars produced according
to Example 2 were measured using an in situ NanoManipulator, Zyvex
S-100 (Richardson, Tex.). Three tungsten probes with a
20-nm-diameter tip were used to form electrical contact. The
manipulator probes were connected to an Agilent 4156C source
measure unit (Fort Worth, Tex.). Initially, two of the probes were
contacted to the bottom gold layer to measure resistance of the
thin film. Once good contact was achieved between the two probes
and the gold layer (resistance: ca. 10 n), a third probe was
applied to a nanopillar. The resistance of the nanopillar was
measured by activating one of the probes on the gold layer and the
probe on the nanopillar.
Example 5
TEM Sample Preparation and Characterization
[0118] High-resolution TEM and selected-area diffraction were
employed to characterize the manufactured nanopillars. A Zeiss
Auriga focused ion beam/scanning electron microscopy (FIB-SEM)
workstation was employed for preparation of high-resolution TEM
samples. To prepare a TEM specimen from a bulk wafer presenting the
fabricated nanopillars, an array of nanopillars was coated with a
carbon protection layer having a thickness of up to 1.5 .mu.m. The
carbon protection layer was formed using electron beam-assisted
deposition for approximately the first 100 nm followed by ion
beam-assisted deposition up to 1.5 .mu.m. A lamella that contained
the chosen array was cut out by focused ion beam (FIB) milling. The
lamella was then lifted out in situ and welded onto an Omniprobe Mo
TEM grid using an Omniprobe Autoprobe 300 (Ted Pella, Inc., Redding
Calif.) mounted on the roof of the FIB/SEM chamber. Once affixed to
the TEM grid, the lamella was further thinned down to approximately
100 nm using a 30-keV Ga ion beam. Final polishing and cleaning was
performed using a 2-keV ion beam to minimize the ion beam damage to
the nanopillars, resulting in lamellae having a final thickness of
approximately 50-60 nm.
[0119] High-resolution TEM imaging and diffraction were performed
using a 200-kV Zeiss Libra 200 field emission energy-filtering
transmission electron microscope (FEG EF-TEM; Carl Zeiss
Microscopy, Thornwood, N.Y.). All bright-field TEM images were
acquired at 200 keV. SAED was achieved using a 10-.mu.m condenser
aperture, selectively illuminating the area of interest on the
specimen.
Example 6
Fluorescence Images
[0120] A Nikon Optiphot 200 fluorescence microscope with a
Micropublisher 5.0 cooled RTV camera was utilized to acquire
optical images. Two different filters B2-A and G2-A (Nikon Inc.)
were used for fluorescent silica and fluorescent PSL particles,
respectively.
Example 7
Electric Field Simulation
[0121] The magnitudes of the electrical potential and electric
field counters near the patterns were simulated using commercial
3-D finite-volume modeling software (FLOW-3D). In the simulations,
the root-mean-square (RMS) value of the utilized voltage was
applied to the gold layer beneath the vias. The calculated local
electric field near the via was used to determine the electric
field gradient, which was then used to calculate the DEP force on
the particles.
Example 8
FDTD Simulations
[0122] Three-dimensional finite-difference time-domain (3D-FDTD)
simulations were carried out to numerically analyze the far- and
near-field responses of the NP system. The permittivity of gold was
taken from the Handbook of Optical Constant of Solids; E. D. Palik,
ed. Academic Press, Orlando, Fla. 1985. In FDTD simulations,
periodic boundary conditions were applied along the x- and
y-directions and Perfectly Matched Layer (PML) boundary condition
was applied along the direction of the illumination source, z. The
mesh size was chosen to be 2 nm along the x-, y- and
z-directions.
Example 9
Detection of Protein Bound to Nanopillar Surface
[0123] Nanopillars produced using methods described in Example 2
were used for detection of bound protein by surface plasmon
resonance. Nanopillars were immersed in ethanol for 30 minutes to
remove any organic contamination on the surface. A protein
monolayer was formed by applying Protein A/G, a recombinant fusion
protein that consists of binding domains of both Protein A and
Protein G, on the nanopillar chip surface. A 1 mg/ml solution of
Protein A/G was used, and incubation was carried out for 1 hour.
Protein A/G attaches to the gold surface of the nanopillar by
physisorption. After incubation nanopillars were rinsed with PBS
(phosphate buffered saline) to remove unbound protein.
[0124] A protein bilayer was formed by applying a 1 mg/ml solution
of immunoglobulin G (IgG) on the chip surface bound to protein A/G,
and incubating for 1 hour. IgG was immobilized on protein A/G
monolayer due to the high affinity of protein A/G to the Fc regions
of IgG. Unbound IgG was removed by rinsing washing with PBS.
[0125] A resonance shift of 4 nm was observed due to accumulation
of biomass on the sensor platform due to the binding of protein A/G
to the platform (FIG. 5 E). Further, a resonance shift of 14 nm was
observed due to binding of IgG to Protein A/G bound to the
nanopillars (FIG. 5 E).
Example 10
Spectral Measurements
[0126] Spectral measurements were performed using a Nikon
Eclipse-Ti microscope coupled to a SpectraPro 500i spectrometer.
Normally incident light was used to excite surface plasmons on
nanopillars. Reflected data were then normalized using a thick gold
standard.
Example 11
Electrical Characterization of Nanopillars
[0127] A SEM-based in situ nanomanipulator (Zyvex S-100) was used
to compare the electrical characteristics of the electroplated and
NP-based nanopillars. The electrical measurements were performed
using two tungsten probes having a tip diameter of 20 nm; one of
the probes contacted the nanopillars while the other contacted the
thin gold layer under the PMMA (FIG. 19 A). The pillars broke
readily when the measurements were performed after the PMMA layer
had been removed. Therefore, the PMMA layer was not removed during
the I-V measurements, i.e. encapsulated nanopillars were monitored.
The PMMA layer also served as a dielectric barrier during the
measurements, preventing any possible current leakage between the
probe and the underlying gold surface (FIGS. 19 B and C).
[0128] The quality of the contact between the probe and the
nanopillar was an important parameter affecting the reliability of
the measurements. Achieving optimal contact (slight penetration
into pillars) between the probe and a 50-nm-diameter nanopillar was
observed to be difficult with a small pillar diameter. Large
variations in resistance (from tens of ohms to hundreds of
kiloohms) was observed for small size nanopillars depending on the
quality of the contact (FIG. 20 A-C). The over-deposited
nanostructures shown in FIGS. 20 D and E consistently produced
lower values of resistance (hundreds of ohms or lower), indicating
that a larger contact area, due to probe penetration, improved the
reliability of the measurements.
[0129] The measurement was carried out for two chips each having
several hundred nanopillars. From each chip ten nanopillars were
randomly chosen and tested (FIG. 20). The resistance of bottom gold
surface was measured as 10.OMEGA. using two probes. Similar
resistance value was obtained when we changed the distance between
the probes was changed. Hence, the obtained resistance of 10.OMEGA.
was assumed to be due to the contact between the probe and the
metal. Therefore, the amount of 10.OMEGA. was subtracted from the
measured resistance while calculating the resistivity of
nanopillars.
[0130] Based on the measurements, the lowest resistivity
(calculated) for 20 different NP-based pillars was
1.96.times.10.sup.-7 .OMEGA.m. This value is lower compared to some
of the previously obtained values of resistivity for gold nanowires
having similar dimensions (Chen, et al. Nanotechnology 16,
1112-1115, 2005; Calleja, M. Appl. Phys. Lett. 79, 2471-2473,
2001). The results obtained are particularly significant in view of
reports that the resistivity of gold at nanoscale dimensions is
higher than its bulk resistivity (Maissel, L. I., in Handbook of
Thin Film Technology, edited by L. I. Maissel and R. Gland,
McGraw-Hill, New York, reissue 1983). Increased metal resistivity
in nanoscale structures might also be due to electron scattering
from grain boundaries and interfaces.
Example 12
Comparison of the NP System Fabricated on a Metal and Dielectric
Substrate
[0131] Numerical simulations were performed to estimate near-field
intensity enhancement (|E|.sup.2/|E.sub.in|.sup.2) distribution at
the top surface and through the cross-section for an unpolarized
light source normal to the plane of nanopillars (FIG. 4 C). The
simulations indicated that the nanopillar antenna arrays can
enhance the intensity of the incident light up to 10.000 times.
This value is much larger than that achieved with the NP system of
identical dimensions fabricated on a dielectric substrate. FIG. 22
shows that a platform consisting of nanopillars fabricated on a
glass substrate yields near-field intensity enhancement only as
large as 250.
[0132] More importantly, for the NP system fabricated on a metal
layer, the large local electromagnetic fields are mainly
concentrated at the top surface of the nanopillars and extend deep
into the surrounding medium. This is in sharp contrast with the
nanopillar system fabricated on a dielectric substrate where most
of the field is inaccessible since the field is concentrated under
the supporting substrate. This result demonstrates that the NP
system standing on a gold substrate fabricated by the manufacturing
process described in methods herein is more advantageous compared
to typical particle based systems fabricated on a dielectric
substrate, such as rod or pillar configurations achieved through
conventional lift-off based nanofabrication processes.
[0133] As used herein, "consisting essentially of" does not exclude
materials or steps that do not materially affect the basic and
novel characteristics of the claim. Any recitation herein of the
term "comprising", particularly in a description of components of a
composition or in a description of elements of a device, can be
exchanged with "consisting essentially of" or "consisting of".
[0134] While the present invention has been described in
conjunction with certain preferred embodiments, one of ordinary
skill, after reading the foregoing specification, will be able to
effect various changes, substitutions of equivalents, and other
alterations to the compositions and methods set forth herein.
* * * * *
References