U.S. patent application number 12/812021 was filed with the patent office on 2010-12-23 for fabrication of conducting open nanoshells.
This patent application is currently assigned to IMEC. Invention is credited to Pol Van Dorpe, Willem Jozef Katharina Van Roy, Jian Ye.
Application Number | 20100323173 12/812021 |
Document ID | / |
Family ID | 40627229 |
Filed Date | 2010-12-23 |
United States Patent
Application |
20100323173 |
Kind Code |
A1 |
Van Roy; Willem Jozef Katharina ;
et al. |
December 23, 2010 |
FABRICATION OF CONDUCTING OPEN NANOSHELLS
Abstract
A method involving ion milling is demonstrated to fabricate
open-nanoshell suspensions and open-nanoshell monolayer structures.
Ion milling technology allows the open-nanoshell geometry and
upward orientation on substrates to be controlled. Substrates can
be fabricated covered with stable and dense open-nanoshell
monolayer structures, showing nanoaperture and nanotip geometry
with upward orientation, that can be used as substrates for
SERS-based biomolecule detection.
Inventors: |
Van Roy; Willem Jozef
Katharina; (Bierbeek, BE) ; Ye; Jian; (Leuven,
BE) ; Van Dorpe; Pol; (Spalbeek, BE) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET, FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Assignee: |
IMEC
Leuven
BE
|
Family ID: |
40627229 |
Appl. No.: |
12/812021 |
Filed: |
February 27, 2009 |
PCT Filed: |
February 27, 2009 |
PCT NO: |
PCT/EP09/52397 |
371 Date: |
July 7, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61032632 |
Feb 29, 2008 |
|
|
|
Current U.S.
Class: |
428/208 ; 216/13;
216/24; 977/958 |
Current CPC
Class: |
B82Y 30/00 20130101;
B22F 2001/0029 20130101; Y10S 977/958 20130101; G01N 21/658
20130101; Y10T 428/24909 20150115; B22F 1/025 20130101; B22F 1/0018
20130101 |
Class at
Publication: |
428/208 ; 216/24;
216/13; 977/958 |
International
Class: |
B32B 5/16 20060101
B32B005/16; B29D 11/00 20060101 B29D011/00; H01B 5/14 20060101
H01B005/14 |
Claims
1-23. (canceled)
24. A substrate having a layer thereon, the layer comprising
nanoparticles, each of the nanoparticles comprising a conductive
open shell, wherein substantially all of the nanoparticles have an
open part of their conductive open shell facing away from the
substrate.
25. The substrate having a layer thereon according to claim 24,
wherein the nanoparticles further comprise a dielectric core
partially surrounded by the conductive open shell.
26. The substrate having a layer thereon according to claim 25,
wherein the dielectric core comprises SiO.sub.2.
27. The substrate having a layer thereon according to claim 24,
wherein the substrate is flat and wherein substantially all of the
nanoparticles have edges of the open part of their conductive open
shell substantially in a plane making an angle of from 0.degree. to
45.degree. with a plane of the substrate.
28. The substrate having a layer thereon according to claim 24,
wherein the open part of the conductive open shells corresponds to
removal of from 5% to 45% of a surface area of the shell.
29. The substrate having a layer thereon according to claim 24,
wherein the conductive open shells comprise at least one material
selected from the group consisting of Au, Ag and Al.
30. The substrate having a layer thereon according to claim 24,
wherein the nanoparticles are immobilized on the substrate via a
functionalization layer present on the substrate.
31. The substrate having a layer thereon according to claim 24,
wherein the nanoparticles are not embedded in the substrate.
32. The substrate having a layer thereon according to claim 24,
wherein the substrate having a layer thereon comprises a component
of an imaging device.
33. The substrate having a layer thereon according to claim 24,
wherein the substrate having a layer thereon comprises a component
of an optical spectroscopy device.
34. The substrate having a layer thereon according to claim 24,
wherein the optical spectroscopy device is configured to employ
surface-enhanced Raman spectroscopy-based biomolecule
detection.
35. A method for fabricating a substrate having a layer thereon,
the method comprising: depositing a layer of nanoparticles on a
substrate surface, wherein the nanoparticles each comprise a
dielectric core and a complete conductive shell around the
dielectric core; and removing a part of each conductive shell at an
area of the nanoparticle facing away from the substrate
surface.
36. The method according to claim 35, further comprising coating
the substrate surface having nanoparticles thereon with a fluid
coating configured to form a solid matrix embedding the
nanoparticles, wherein coating is conducted between depositing and
removing, and wherein removing comprises, after the solid matrix is
formed, removing a part of the solid matrix at a surface thereof
facing away from the substrate, thereby removing a part of each
conductive shell.
37. The method according to claim 35, wherein the nanoparticles are
deposited in a fluid coating configured to form a solid matrix
embedding the nanoparticles, and wherein removing comprises, after
said solid matrix is formed, removing a part of the solid matrix at
a surface thereof facing away from the substrate, thereby removing
a part of each conductive shell.
38. The method according to claim 35, wherein removing is performed
via a directional removing technique, preferably a directional
etching technique.
39. The method according to claim 38, wherein the directional
removing technique is a directional etching technique.
40. The method according to claim 38, wherein the directional
removing technique is ion milling.
41. The method according to claim 35, further comprising chemically
functionalizing the substrate, wherein chemically functionalizing
is conducted prior to depositing.
42. The method according to claim 35, further comprising removing
the dielectric cores from the nanoparticles, each nanoparticle
comprising a conductive open shell.
43. A method for producing nanoparticles comprising a conductive
open shell, comprising: depositing a layer of nanoparticles on a
substrate surface, wherein the nanoparticles each comprise a
dielectric core and a complete conductive shell around the
dielectric core; and removing a part of each complete conductive
shell at an area of the nanoparticle away from the substrate
surface, thereby forming nanoparticles comprising a conductive open
shell.
44. The method according to claim 43, wherein removing is performed
via a directional removing technique, the method further comprising
removing the conductive open shells from the substrate.
45. The method according to claim 44, wherein the directional
removing technique is a directional etching technique.
46. The method according to claim 44, further comprising treating a
medical condition in a patient by thermotherapy using the removed
conductive open shells.
47. The method according to claim 44, further comprising conducting
biomedical imaging of a patient using the removed conductive open
shells.
48. The method according to claim 44, further comprising
fabricating a surface plasmon resonance biosensor incorporating the
removed conductive open shells.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is the national phase under 35 U.S.C.
.sctn.371 of prior PCT International Application No.
PCT/EP09/052397 which has an International Filing Date of Feb. 27,
2009, which designates the United States of America, and which
claims priority to U.S. Provisional Application No. 61/032,632
filed Feb. 29, 2008, the disclosures of which are hereby expressly
incorporated by reference in their entirety and are hereby
expressly made a portion of this application.
TECHNICAL FIELD OF THE INVENTION
[0002] The invention relates to the field of nanoparticles, in
particular nanoparticles for use in sensor, drug delivery or
imaging applications.
BACKGROUND OF THE INVENTION
[0003] Gold (Au) nanoshells are nanoparticles usually composed of a
dielectric core, typically silica, coated with an ultrathin Au
layer. These nanoparticles show interesting optical and chemical
properties for the applications of surface-enhanced Raman
spectroscopy (SERS) sensor, surface plasmon resonance (SPR) sensor,
drug delivery, biomedical imaging and cancer therapeutics among
others.
[0004] Reducing symmetry of Au nanoshells geometry shows
interesting properties. It is possible to excite different plasmon
modes in these particles when compared to standard particles. These
particles show angle dependent plasmon resonance. This unique
property may lead to a new class of optically active nanoparticles
that can be manipulated by applied static or frequency dependent
electric, magnetic, or optical fields. The particles enhance the
electric field intensity coming out of the particles when compared
to fully covered particles, i.e. particles whose symmetry has not
been reduced.
[0005] Several groups have developed and demonstrated
reduced-symmetrical nanoshells such as nano half-shells, nanocups,
nanomoons and nanoeggs for SERS applications. Reduced-symmetrical
nanoshells have been prepared before in various ways including
electron-beam evaporation (EBE) and electroless plating. By these
methods, the reduced-symmetrical structures of nanoshells, such as
nanoaperture or nanotip, are usually oriented randomly or with
their aperture downward, which obviously limits the molecular
binding to the electric field enhanced regions in SERS
applications. The Raman enhancement factors differ from place to
place on a substrate because of the random orientation of
reduced-symmetrical structures.
[0006] U.S. Pat. No. 6,660,381 describes a method for the
fabrication of composite particles containing metal shell layers,
i.e. a partial metal coverage. This fabrication method does not
allow good control of the orientation and the geometry of the
open-nanoshells.
[0007] WO2006135393 shows a method and a system for optimized
surface enhanced Raman scattering comprising a support with on top
nanoparticles having a shell surrounding a core. The local
electromagnetic field around complete shells is lower when compared
to the local electromagnetic field around the open-nanoshells.
[0008] WO2002059226 describes the fabrication of metal nanoshells
having partial coverage of a substrate.
[0009] JP2006198641 describes an ion-beam processing method for
forming nano-order convex portions.
[0010] Y. B. Zheng et al (J. of Non-crystalline Solids 352, 2006, p
2532-2535 describe the fabrication of ordered nanoring arrays for
nanoscale optical sensors. This fabrication method does not allow
good control of the geometry of the open-nanoshells.
[0011] J. Liu et al (Jap. Journ. Of Appl. Phys. 45 (22), 2006 p
L582-L584 describe the fabrication of 2-dimensional arrays of
hollow metal nanoshells on a substrate. This fabrication method
does not allow good control of the geometry of the
open-nanoshells.
[0012] M. A. Correa-Duarte et al (Advanced Materials 2005 (17), p
2014-2018) describe a method to fabricate asymmetric nanoshells
with/without a core. This fabrication method does not allow good
control of the orientation of the open-nanoshells.
[0013] Lu et al. (Nano Lett. 5 (1), 2005, p 119-124) present the
fabrication of Au moon structures for the enhancement of a local
electrical field at the edge area. This fabrication method does not
allow good control of the orientation of the open-nanoshells.
[0014] J. C. Love et al (Nano Lett. 2 (8) 2002, p 891-894) describe
the fabrication of silica core particles covered with Au on a
substrate. This fabrication method does not allow good control of
the orientation and the geometry of the open-nanoshells.
SUMMARY
[0015] It is an object of the present invention to present a method
to fabricate open nanoshells with a well-controlled geometry either
on a substrate or dispersed in a solvent. This method permits
fabricating substrates covered with open nanoshells with
well-controlled geometry and orientation. The above objective is
accomplished by a method and device according to the present
invention.
[0016] Over prior art materials using cores of polystyrene beads
with diameters of 100 nm or more, the present invention is more
versatile providing much smaller cores and a better control of the
apertures.
[0017] In a first aspect, the present invention relates to a
substrate covered with open nanoshells also called open shell
nanoparticles or nanoshell particles with reduced symmetry (i.e.
less symmetrical than spherical nanoparticles), said open shell
nanoparticles comprising a dielectric core and a conducting layer
partially surrounding said dielectric core, the uncovered part of
the dielectric core being located at a side essentially opposite to
the side of the substrate (i.e. at a side essentially opposite to
the side of the substrate adjacent to said open shell
nanoparticles).
[0018] In an embodiment of the first aspect, the uncovered part of
the dielectric core may be at a side essentially opposite to the
substrate, the edge of the conducting material being in a "cutting
plane" making an angle between 0.degree. and 45.degree. with the
plane of the substrate.
[0019] In an embodiment of the first aspect, the uncovered part of
the dielectric core may be between 45% and 5% of the total surface
area of the dielectric core.
[0020] In an embodiment of the first aspect, said dielectric core
may be a material comprising air or SiO.sub.2 and wherein said
conducting layer comprises at least one material selected from the
group consisting of Au, Ag, and Al. In SERS applications a higher
field enhancement is obtained with silver when used with a plasma,
but silver oxidises, whereas gold doe not. With air as the
dielectric core, the capacity of the open shell nanoparticles to
receive molecules increases, the contact area with the molecules
increases and higher fields become accessible.
[0021] In an embodiment of the first aspect, a functionalisation
layer for immobilising said open shell nanoparticles may be further
present on said substrate.
[0022] In an embodiment of the first aspect, the substrate may be
covered with at least one layer of open shell nanoparticles in an
optimal packing resulting in up to 90% of the surface area being
covered in the case of nanoparticles of identical size if viewed
perpendicularly to a planar substrate.
[0023] In a second aspect, the present invention relates to a
method for fabricating open nanoshells or open shell nanoparticles
or nanoparticles with reduced symmetry said method comprising the
steps of [0024] fabricating a plurality of dielectric cores; [0025]
depositing a conducting layer on said dielectric cores, thereby
creating nanoshells or nanoshell particles; [0026] selecting a
substrate; [0027] depositing said nanoshells on said substrate
thereby forming a layer of nanoshells on said substrate; [0028]
removing part of said conducting layer from said nanoshells at a
side essentially opposite of said side of the substrate adjacent to
said nanoshells thereby creating a layer of open nanoshells.
[0029] In an embodiment of the second aspect, removing part of said
conducting layer may be obtained by a directional etching
technique.
[0030] In an embodiment of the second aspect, said directional
etching technique may be ion milling.
[0031] In an embodiment of the second aspect, the method may
further comprise functionalisation of said substrate prior to
depositing said nanoshells on said substrate.
[0032] In an embodiment of the second aspect, the method may
further comprise removing said dielectric core from said open
nanoshells.
[0033] In an embodiment of the second aspect, the method may
further comprise removing said open nanoshells from said substrate,
for example by redispersing in solution.
[0034] In a third aspect, the present invention relates to a
substrate obtainable by a method according to the second
aspect.
[0035] In a fourth aspect, the present invention relates to the use
of a substrate according to the first or fourth aspect in an
imaging application, with an optical spectroscopy technique such as
SERS-based biomolecule detection being preferred.
[0036] Particular and preferred aspects of the invention are set
out in the accompanying independent and dependent claims. Features
from the dependent claims may be combined with features of the
independent claims and with features of other dependent claims as
appropriate and not merely as explicitly set out in the claims.
[0037] Although there has been constant improvement, change and
evolution of devices in this field, the present concepts are
believed to represent substantial new improvements, including
departures from prior practices, resulting in the provision of more
efficient, stable and reliable devices of this nature.
[0038] The above and other characteristics, features and advantages
of the present invention will become apparent from the following
detailed description, taken in conjunction with the accompanying
drawings, which illustrate, by way of example, the principles of
the invention. This description is given for the sake of example
only, without limiting the scope of the invention. The reference
figures quoted below refer to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1 shows a fabrication method for making open-nanoshells
(e.g. Au open nanoshells) suspensions and monolayer structures
according to embodiments of the present invention. The dimension of
open-nanoshell particle is shown in the figure. r is the diameter
of the core, R is the diameter of total particle, H is the height
of non-removed shell of the open-nanoshell.
[0040] FIG. 2 is a scheme of the simulation system used in finite
difference time domain (FDTD) calculations.
[0041] FIG. 3 shows finite difference time domain (FDTD)
simulations of optical extinction spectra and near-field
distribution pictures of nanoshells and open-nanoshells according
to embodiments of the present invention.
[0042] FIG. 4: normalized simulated optical extinction spectra (A)
and experimental optical extinction spectra (B) for an aqueous
suspension of Au nanoshells and Au open-nanoshells according to
embodiments of the present invention.
[0043] FIG. 5 shows optical extinction spectra of open-nanoshells,
according to embodiments of the present invention, made by ion
milling for different times nanoshells deposited on quartz in air
by drop-casting.
[0044] FIG. 6 shows TEM images of Au nanoshells (A: r/R/H=43/73/146
nm; C: r/R/H=115/130/260 nm) and Au open-nanoshells (B:
r/R/H=43/73/109 nm; D: r/R/H=115/130/195 nm) according to
embodiments of the present invention. Scale bars correspond to 100
nm.
[0045] FIG. 7 shows the line-profile of an AFM image (1
.mu.m.times.1 .mu.m) of Au open-nanoshells on a Si substrate
according to embodiments of the present invention.
[0046] FIG. 8 shows SEM images of Au nanoshells (r/R/H=43/73/146
nm) on an ITO glass (A) before and after ion milling for (B) 20,
(C) 40 and (D) 60 seconds according to embodiments of the present
invention.
[0047] FIG. 9 shows SEM images of monolayer structures of Au
nanoshells (r/R/H=115/130/195 nm) on a Si substrate (A) before and
(B, C) after ion milling for 40 seconds (B: top view image; C: side
view image) according to embodiments of the present invention.
[0048] FIG. 10 (A) shows normalized theoretical (top) and
experimental (bottom) optical extinction spectra of Au nanoshells
(a, c) and open-nanoshells (b, d) monolayer structures in air
according to embodiments of the present invention. (B) Theoretical
extinction peaks of Au nanoshells and open-nanoshells with
different ratios of particle radius and shell thickness in air. (C)
Local electric field intensity (log|E|.sup.2) around open-nanoshell
d at an extinction peak (1070 nm) in air.
[0049] FIG. 11 is a diagrammatic illustration of various
embodiments of the present invention.
[0050] FIG. 12 illustrates a method for obtaining a substrate
according to an embodiment of the present invention.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0051] The present invention will be described with respect to
particular embodiments and with reference to certain drawings but
the invention is not limited thereto but only by the claims. The
drawings described are only schematic and are non-limiting. In the
drawings, the size of some of the elements may be exaggerated and
not drawn on scale for illustrative purposes. The dimensions and
the relative dimensions do not correspond to actual reductions to
practice of the invention.
[0052] Furthermore, the terms first, second, third and the like in
the description and in the claims, are used for distinguishing
between similar elements and not necessarily for describing a
sequence, either temporally, spatially, in ranking or in any other
manner. It is to be understood that the terms so used are
interchangeable under appropriate circumstances and that the
embodiments of the invention described herein are capable of
operation in other sequences than described or illustrated
herein.
[0053] Moreover, the terms top, bottom, over, under and the like in
the description and the claims are used for descriptive purposes
and not necessarily for describing relative positions. It is to be
understood that the terms so used are interchangeable under
appropriate circumstances and that the embodiments of the invention
described herein are capable of operation in other orientations
than described or illustrated herein.
[0054] It is to be noticed that the term "comprising", used in the
claims, should not be interpreted as being restricted to the means
listed thereafter; it does not exclude other elements or steps. It
is thus to be interpreted as specifying the presence of the stated
features, integers, steps or components as referred to, but does
not preclude the presence or addition of one or more other
features, integers, steps or components, or groups thereof. Thus,
the scope of the expression "a device comprising means A and B"
should not be limited to devices consisting only of components A
and B. It means that with respect to the present invention, the
only relevant components of the device are A and B.
[0055] Reference throughout this specification to "one embodiment"
or "an embodiment" means that a particular feature, structure or
characteristic described in connection with the embodiment is
included in at least one embodiment of the present invention. Thus,
appearances of the phrases "in one embodiment" or "in an
embodiment" in various places throughout this specification are not
necessarily all referring to the same embodiment, but may.
Furthermore, the particular features, structures or characteristics
may be combined in any suitable manner, as would be apparent to one
of ordinary skill in the art from this disclosure, in one or more
embodiments.
[0056] Similarly it should be appreciated that in the description
of exemplary embodiments of the invention, various features of the
invention are sometimes grouped together in a single embodiment,
figure, or description thereof for the purpose of streamlining the
disclosure and aiding in the understanding of one or more of the
various inventive aspects. This method of disclosure, however, is
not to be interpreted as reflecting an intention that the claimed
invention requires more features than are expressly recited in each
claim. Rather, as the following claims reflect, inventive aspects
lie in less than all features of a single foregoing disclosed
embodiment. Thus, the claims following the detailed description are
hereby expressly incorporated into this detailed description, with
each claim standing on its own as a separate embodiment of this
invention.
[0057] Furthermore, while some embodiments described herein include
some but not other features included in other embodiments,
combinations of features of different embodiments are meant to be
within the scope of the invention, and form different embodiments,
as would be understood by those in the art. For example, in the
following claims, any of the claimed embodiments can be used in any
combination.
[0058] In the description provided herein, numerous specific
details are set forth. However, it is understood that embodiments
of the invention may be practiced without these specific details.
In other instances, well-known methods, structures and techniques
have not been shown in detail in order not to obscure an
understanding of this description.
[0059] The following terms are provided solely to aid in the
understanding of the invention.
[0060] As used herein and unless stated otherwise, the term
"nanoshell" or "nanoshell particle" relates to a nanoparticle with
a dielectric core (e.g. SiO2, polystyrene, . . . ) and complete
conducting layer (e.g. a metal such as, Au, Ag, Al, . . . , or a
semiconductor such as Si, GaAs, . . . ) around the core.
[0061] As used herein and unless stated otherwise, the term
"nanoshell particle with reduced symmetry", or "open nanoshell", or
"open shell nanoparticle" relates to a nanoparticle with a
dielectric core and non-complete or partial conducting layer around
the core.
[0062] As used herein and unless stated otherwise, the term
"nanoring" relates to a substrate supported ring or toroidal-shaped
nanostructure.
[0063] As used herein and unless provided otherwise, the term
"nanoaperture" relates to an aperture made in a open-nanoshell.
[0064] As used herein and unless provided otherwise, the term
"nanotip" relates to any sharp asperity at the edge of a
nanoaperture.
[0065] As used herein and unless provided otherwise, the term
"functionalisation molecule" relates to molecules able to attach to
both, the substrate surface and the nanoshell particles.
[0066] The invention will now be described by a detailed
description of several embodiments of the invention. It is clear
that other embodiments of the invention can be configured according
to the knowledge of persons skilled in the art without departing
from the true spirit or technical teaching of the invention, the
invention being limited only by the terms of the appended
claims.
DETAILED DESCRIPTION
[0067] In a first aspect, the present invention relates to a
substrate having a layer thereon, said layer comprising or
consisting of nanoparticles, said nanoparticles comprising an open
conductive shell. The nanoparticles may be monodisperse providing
greater reproducibility, or have a broad size distribution
providing wider resonances. The material of the substrate is not a
limiting feature of the present invention and can comprise
dielectric, semi-conducting or conducting materials. Examples of
dielectric materials suitable for the substrate comprise but are
not limited to glass, quartz, mica, Si.sub.3N.sub.4,
Al.sub.2O.sub.3 and polymers among others. Examples of
semi-conducting materials suitable for the substrate comprise but
are not limited to Si, Ge, GaAs, group IV semi-conducting
materials, group III-V semi-conducting materials, group II-VI
semi-conducting materials and chalcopyrite among others. Examples
of conductive materials suitable for the substrate comprise but are
not limited to metals (such as Au, Ag and Cu among others) or doped
semi-conductors such as ITO (e.g. on glass). Functionalised
materials (i.e. materials having a surface bearing a layer of
functionalisation molecules able to attach to both, the substrate
surface and the nanoparticles) such as functionalized Si, or other
materials are also suitable. In embodiments, the substrate may be a
planar substrate, a curved substrate or any other surface shape.
Preferably, the substrate comprises a planar surface.
[0068] In an embodiment of the present invention, the layer consist
of the open shell nanoparticles. In certain embodiments of the
present invention, the open shell nanoparticles are not embedded.
In other embodiments, a matrix linking the open shell nanoparticles
together is present, the layer comprising nanoparticles and a
matrix. The matrix can be any material able to act as a binder
between the nanoparticles. Preferably, the matrix is a polymer
matrix any polymer with binding properties to being suitable,
although translucent or transparent polymers are preferred. In yet
another embodiment, the layer comprises open shell nanoparticles
and functionalisation molecules at the interface with the substrate
surface. Examples of adhesion (i.e. functionalisation) molecules
comprise but are not limited to organosilanes, preferably
organosilanes comprising a thiol or dithio function such as
3-mercaptopropyl-trimethoxysilane (MPTMS) or
3-mercapto-propyl-triethoxysilane (MPTES), among others. The
functionalisation molecule may form a functionalisation layer on
the substrate and may immobilize the nanoparticles on said
substrate. In other words, to improve the stability and degree of
coverage of the substrate, nanoshells can be immobilized on the
substrate by using chemical functionalization such as a
mercaptosilane functionalization, for example on a mercaptosilane
functionalised ITO-coated glass or Si slide. The layer of open
nanoshells can consist in nanoshells or comprise nanoshells. If it
consists in nanoshells, the thickness of the layer is preferably
equivalent to the dimension of the nanoshells, i.e. the layer is
preferably a monolayer. If the layer comprises both nanoshells and
a matrix within which the nanoshells are embedded, the thickness of
the layer is also preferably equivalent to the dimension of the
nanoshells and preferably smaller than twice this dimension so that
a monolayer is formed. Open-nanoshells, for example Au open
nanoshells, can be loosely packed or densely packed to form a
monolayer with coverage on the substrate between 10% and 30%, or
between 30% and 60% or between 60% and 80%, in the best case more
than 80% and at most 90%. The coverage can be tuned from 10% to 80%
or more (and up to 90%) by the concentration of nanoshells
suspension. Part of the free space present between open-nanoshells
is due to spatial limit (i.e. due to the spherical geometry of the
particles theoretically limiting the coverage to
.pi. 12 ##EQU00001##
in the case of perfect spheres of identical size packed
hexagonally) and the additional part of the free spaces present
between open-nanoshells is possibly due to the incomplete
functionalization of the substrate (when the substrate is
functionalised). A higher degree of coverage can be obtained by
using bimodal or very broad size distributions of the
nanoparticles.
[0069] In a further embodiment of the present invention multilayers
of open shell nanoparticles on a substrate are provided. Such
multilayers can, for example, be built up layer by layer and may
provide field confinement effects and optical field
enhancement.
[0070] The open shell nanoparticles comprise a conductive open
shell, i.e. a conductive shell comprising an opening. They can have
any shapes such as spheroidal or cuboidal. Preferably, they are
substantially spherical. The conducting layer (i.e. the shell) can
be made of a metallic material (such as gold (Au), silver (Ag),
nickel (Ni), titanium (Ti), aluminum (Al), copper (Cu) or platinum
(Pt) amongst others), a semi-metallic material, or a (preferably
doped) semiconducting material (such as Si or GaAs amongst others)
or any other conducting material used in the field. Preferably, the
core and the shell are made of different materials and the core is
a dielectric and the shell a conductive material. Preferred
conductive materials are metals and doped semi-conductors. More
preferably, the conductive material is a metal wherein gold, silver
and aluminium are most preferred. The conducting layer can be made
of a single conducting material or can comprise different
conducting materials, for example selected from the list above. In
embodiments, the conductive open shell comprises at least on
material selected from the group consisting of Au, Ag and Al. Most
preferably, the shell is made of gold. Shells of various thickness
are suitable. The thickness of the conducting layer or nanoshell
layer can be from 5 nm to 100 nm, from 7 nm to 50 nm or from 10 nm
to 30 nm, or from 10 nm to 100 nm. The part of the core that is not
covered with conducting material (i.e. the uncovered part) can be
varied between 70% and 1% if a movable ion source is used in the
etching process, with between 60% and 5% being preferred, between
50% and 5% being particularly preferred, between 45% and 5% being
especially preferred, between 40% and 5% being particularly
especially preferred, between 30% and 5% being even more preferred,
between 20% and 5% being still more preferred, between 20% and 10%
of the total surface area of the core being yet still more
preferred. In the case of a hollow nanoshell (i.e. nanoshells whose
core has been removed), the part of the shell removed during
directional removal or etching can be varied between 70% and 1% if
a movable ion source is used in the etching process, with between
60% and 5%, being preferred, between 50% and 5% being particularly
preferred, between 45% and 5% being especially preferred, between
40% and 5% being particularly especially preferred, between 30% and
5% being even more preferred, between 20% and 5% being still more
preferred, and between 20% and 10% of the total original surface
area of the shell being yet still more preferred. In embodiments,
the surface area of the shell removed during the directional
etching step is from 5 to 45% of the surface area of the shell. The
open nanoshells may or may not have a dielectric core. The core is
preferably made of a dielectric material and can comprise e.g.
silicon dioxide (SiO.sub.2) (e.g. the core particles used to make
the nanoparticles can be silicon dioxide colloids), polymers such
as polystyrene, magnetic materials such as Fe.sub.2O.sub.3, or
other magnetic oxides. The core particles can be made of one
material or can comprise several materials that can for instance be
selected from the list above. When the core comprises more than one
material, it is possible that it comprises both, conductive and
dielectric materials if the outer surface of the core is
dielectric. For instance, the core could be made of a conductive
kernel coated with a dielectric coating. The important factor being
that at least the outer surface of the core is a dielectric.
Preferably, the core comprises (or consists of) SiO.sub.2, silica
having the additional advantage of being etchable. The shape of the
core is preferably the same as the shape of the shell. Nanoshells
of various core sizes are suitable. In an embodiment, the open
nanoshells comprise a dielectric core partially surrounded by a
conductive open shell.
[0071] The open nanoshells are oriented on the substrate in such a
way that a majority, preferably 90% or more, most preferably
substantially all of the open nanoshells have their opening
directed away from the substrate (i.e. their opening do not touch
the substrate, i.e. their opening is away from the substrate). In
embodiments, if an arrow would be traced from the center of the
particle to the center of the opening, this arrow would not point
to the substrate and would preferably point away from the
substrate. For instance, this arrow would make an angle of from 0
and 90.degree. with the substrate, preferably 45 to 90.degree.. In
other words, said nanoparticles have the center of their shell open
part at the half (or side) of the nanoparticles opposite to the
half (or side) of said nanoparticle adjacent to said substrate.
[0072] In embodiments, the substrate comprises a planar surface on
which the layer comprising nanoparticles is layered. The use of
directional etching techniques permits to obtain open nanoparticles
wherein the edges of the conductive open shell are approximately in
a plane. Preferably this plane is making an angle between
45.degree. and 90.degree. with the plane of the substrate planar
surface. Angles less than 90.degree. provide a tilting effect with
resulting improvement in turbulence during the wetting process with
improved wetting as a result. In other words the opening of the
open nanoshell is obtainable by etching said shell with a
directional etching source (i.e. a non-isotropic flux of etching
agent) making an angle of from 45 to 90.degree. with the substrate
planar surface. Expressed differently, when a core is present, the
part of the core that is not covered with conducting material (the
uncovered part) is at a side essentially opposite to the substrate.
Depending on the angle of the substrate with respect to the
directional etching (in case of ion milling the angle of the
substrate with respect to the ion beam), the orientation of the
open part of the nanoshells with respect to the substrate may
change. The angle between a plane ("cutting plane") comprising the
edge of the conducting material and the plane of the substrate can
be between 0.degree. and 90.degree., between 0.degree. and
60.degree., between 1.degree. and 60.degree., between 0.degree. and
50.degree., between 2.degree. and 50.degree., between 0.degree. and
45.degree., between 3.degree. and 45.degree., between 0.degree. and
40.degree., between 4.degree. and 40.degree., between 0.degree. and
30.degree., between 5.degree. and 30.degree., between 0.degree. and
20.degree., between 5.degree. and 10.degree..
[0073] FIG. 11 shows three substrates according to embodiments of
the present invention. On the left side of FIG. 11, an open
nanoshell (2) is represented laying on a substrate (1). The open
nanoshell comprises a conductive open shell (3) and a dielectric
core (5). The angle between a plane ("cutting plane") (6)
comprising the edge of the conducting material (3) and the plane of
the substrate (1) is 0.degree.. The substrate represented in the
centre of FIG. 11 has a conductive open nanoshell (2, 3) laying
thereon. This open nanoshell does not comprise a core (i.e. it is
hollow). The plane (6) comprising the edge of the conducting
material (3) makes an angle comprised between 0 and 45.degree. with
the substrate. The substrate represented on the right side of FIG.
11 has an open nanoshell (2, 3) thereon represented laying on a
substrate (1). The open nanoshell comprises a conductive open shell
(3) and no dielectric core (5). The opening (4) of the open
nanoshell is facing away from the substrate and the angle between
the plane ("cutting plane") (6) comprising the edge of the
conducting material (3) and the plane of the substrate (1) is
0.degree..
[0074] Substrates according to embodiments of the present invention
with their open nanoshells layer (preferably monolayer) structures
offer a stable and high density arrangement of open nanoshells on a
substrate for various sensing application such as for instance
surface-enhanced Raman scattering (SERS)-based biomolecules
detection. In other words, embodiments of the present invention
provide substrates topped with a layer of densely packed conductive
open nanoshells which is stable, i.e. well attached to the
substrate (when a chemical functionalisation is present between the
conductive open nanoshells and the substrate). In particular,
nanoshells with a small core size (<100 nm) and a thin shell
thickness (<10 nm) having a relatively small particle size and
having an optical response in near-infrared (NIR) region, are
advantageously used in several applications, such as the biomedical
imaging and thermotherapy for certain tumours.
[0075] In a second aspect, the present invention relates to a
method for fabricating a substrate according to the first aspect of
the present invention. In embodiments, the present invention
relates to a method for fabricating a monolayer structure
comprising conductive open nanoshells (such as e.g. gold
nanoshells) on a substrate.
[0076] In embodiments, nanoparticles comprising a dielectric core
and a conductive shell can first be prepared. Starting with
dielectric core particles, a conductive layer can be deposited
thereon. Deposition of the conducting layer can for example be done
by seeding followed by electroless plating, or polymerization, or
other chemical techniques. The surface roughness of the shells is
mainly determined by the deposition technique used (e.g. the
seeding and electroless plating process) to put the conducting
layer on the core. That way, dielectric nanoparticles covered with
a conducting layer can be prepared. Dimensions of open-nanoshells
can be controlled by tuning core sizes and shell thicknesses in a
broad range. The size of the core particles (i.e. the core size)
can be from 50 nm to 2000 nm, from 60 nm to 1500 nm, or from 80 nm
to 1000 nm and preferably from 80 nm to 400 nm. The thickness of
the conducting layer (i.e. the nanoshell layer) can be from 5 nm to
100 nm, from 7 nm to 50 nm or from 10 nm to 30 nm, or from 10 nm to
100 nm. In embodiments, this method may comprise the steps of
providing a layer as described in the first aspect on a substrate.
Before deposition, the substrate can be treated, for example by
cleaning with deionised water, piranha solution, UV ozone
treatment, ultrasonication or any other method known in the art. In
embodiments, prior to provide said layer on said substrate, the
substrate may be chemically functionalised. Functionalisation can
for instance be performed by applying an organosilane layer on the
substrate surface. Deposition of the nanoshell layer can be done by
drop-casting of nanoshells suspension, spin coating of nanoshells
suspension, immersing functionalized substrate into nanoshells
suspension, creating ordered monolayers, self-assembly, or other
techniques well known to the person skilled in the art. The layer
comprises or consists of nanoshell particles. Providing the layer
on the substrate result in the forming of a layer of nanoshell
particles on the substrate. In this aspect of the present
invention, the nanoshell particles of the provided layer comprise a
dielectric core and a conductive shell. In embodiments, the method
according to the second aspect of the present invention comprises
the step of depositing a layer on a substrate, said layer
comprising nanoparticles, thereby forming a layer of nanoparticles
on said substrate wherein said nanoparticles comprise a dielectric
core and a conductive shell. Once the layer is provided on the
substrate, part of the conductive shell, not in contact with the
substrate surface, is removed. Preferably, part of the conductive
shell is removed at the side of said nanoparticles opposite to the
side of the nanoparticles adjacent to the substrate, thereby
forming nanoparticles comprising a conductive open shell. In
embodiments, the step of removing part of the conductive shell may
be performed via a directional removing (e.g. directional etching
technique) such as e.g. ion milling. Directional removing (e.g.
etching) is advantageous as this allows removing (e.g. etching) the
material at a place away from the substrate (e.g. only at the top
side of the particles) thereby creating a layer of open nanoshells
on the substrate, whereby at least 50% of the open nanoshells,
preferably at least 90% of the open nanoshells, most preferably
substantially all open nanoshells have substantially the same
orientation. So, in embodiments, most or all open-nanoshells are
"facing up" with the open part at the side opposite to the
substrate. That way, open nanoshells layer (e.g. a monolayer
thereof) structures can be obtained on the substrate.
[0077] The etching rate is material dependent, and depends on the
etching technique, the system that is used and the operating
conditions of the system.
[0078] In embodiments, the method may be based on an ion-milling
process for fabricating the open nanoshells and the monolayer
structures comprising them. Although other methods such as resists
techniques, or directional techniques such as mechanical abrasion
e.g. scrubbing can also be used partly to remove the shell. An ion
milling process will result afterwards in open-nanoshells layer
(e.g. monolayer) structures on the substrate. Its intrinsic working
principle makes ion milling a very directional etching method. By
tuning the ion milling time, the amount of material removed from
the nanoshells can be varied, thereby tuning the geometry of the
open-nanoshell. Another advantage of ion milling is its excellent
repeatability (i.e. reproducibility). In addition, ion milling is a
facile, fast and clean technology to fabricate open-nanoshells.
[0079] The following parameters hold for the ion milling process.
The ion milling time depends on the parameters used in the ion
milling process (such as but not limits to gasses, ionisation
efficiency and accelerating voltage). During ion milling the base
pressure in the processing chamber can vary between
1.0.times.10.sup.-7 mTorr and 1.0.times.10.sup.-10 mTorr, or
between 1.0.times.10.sup.-8 mTorr and 1.0.times.10.sup.-9 mTor, or
even better below 8.0.times.10.sup.-8 mTorr. In the ion milling
process, typically a small amount of etching gas, for example Xe,
Ar, or others, is introduced in the deposition chamber. The flow
rate can vary between 0.5 sccm (Standard Cubic Centimeters per
Minute) and 10 sccm, or between 1 and 5 sccm, for example 2.4 sccm
for Xe. The gas is ionized by a filament under a large voltage,
thereby creating ions. The voltage can be from 50V to 3000V, or
from 100V to 1000V, or from 200V to 500V, typically 375 V. A large
electric field then accelerates the ions towards a grid under a
large negative bias. The bias voltage or accelerating voltage can
vary between 50V and 3000V, or between 100V and 1000V, or between
200V and 500V, typically 400V. Before hitting the sample, the ion
beam is neutralized by a cloud of electrons, generated by a plasma,
for example Ar plasma, Xe plasma, or other. The flow rate of the
gas used for creating the plasma can vary between 0.5 sccm and 10
sccm, or between 1 and 5 sccm, for example 2 sccm in the case of
Ar.
[0080] When the ions carry a large enough momentum they will knock
out atoms from the sample. That way nanotip and nanoaperture
structures can be fabricated on the open-nanoshells surface.
[0081] As mentioned before, the etch rate is material dependent,
and depends on the etching technique, the system that is used and
the operating conditions of the system. So the etching time, for
example the ion milling time, can be different for different shell
materials. The etch rate, for example in case of ion milling, can
vary between 2 nm/min and 40 nm/min, typically between 10 nm/min
and 35 nm/min. For a certain material the etch time, for example
the ion milling time, can be tuned such as to remove a certain
amount of material. The etch time, for example ion milling time,
can be from 1 s to 200 s, or from 10 to 100 s or from 20 to 60
seconds. By tuning the etching time the part of the core that is
not covered with conducting material (i.e. the uncovered part) can
be varied between 70% and 1%, between 60% and 5%, between 50% and
5%, between 45% and 5%, between 40% and 5%, between 30% and 5%,
between 20% and 5%, between 20% and 10% of the total surface of the
core.
[0082] Instead of ion milling, other techniques such as chemically
assisted ion beam etching (CAIBE), reactive ion etching (RIE), or
others with similar directional etching behaviour, can be used.
That way an etch chemistry can be chosen that etches the conducting
material much faster than the conducting core, such that the
conducting material can be etched highly selectively with respect
to the dielectic core, thereby etching the conducting material
while leaving the core unetched. An alternative directional
removing technique suitable when the layer of nanoparticles is
embedded in a matrix (e.g. a polymer matrix), is to mechanically
remove (e.g. via scrubbing in a Chemical Mechanical Polishing)
(CMP)-like process) an upper layer of the layer until the
dielectric core of the particles is reached. This results in
polymer layer comprising metallic spots or rings at its surface.
This embodiment is illustrated in FIG. 12. At the top of FIG. 12, a
substrate (1) having a layer (14), said layer comprising nanoshells
(13) embedded in a matrix, is represented. The bottom of FIG. 12,
shows the same substrate after that an upper layer of the layer
(14) has been removed creating open nanoshells and making metallic
spots or rings (15) apparent at the surface (14).
[0083] In embodiments, once part of the conductive open shell is
removed, the method may further comprise a step of removing the
dielectric core from the open shell nanoparticles. This results in
a layer of hollow open nanoshells. The chemistry used for removing
the core should preferably be chosen such that it can selectively
remove the core without affecting much the conducting shell.
Preferably, the chemistry used for removing the core should only
remove the core and leave the open conducting shell intact. In the
case of Au open-nanoshells with silica core for example, removing
the silica core can be done by using aqueous HF for suspensions or
vapor-phase HF for Au open-nanoshells monolayer structures. This
keeps the Au open-nanoshells intact.
[0084] Methods according to embodiments of the present invention
allow good control of the reduced-symmetrical geometry of the
nanoparticles and allows monolayer structures to be realised with
upward-oriented aperture on a substrate with good control and
reproducibility. This makes the fabricated open nanoshells (e.g. Au
open nanoshells) and substrate having a mono(layers) thereof
suitable for a range of applications, for example, as active
components in thermotherapy system and SPR biosensors. In
particular, these particles with the features of nanoaperture
optionally comprising nanotip structures can be good substrates for
optical spectroscopy techniques such as surface-enhanced Raman
scattering (SERS)-based molecule detection, or surface-enhanced
resonance Raman scattering (SERRS), surface-enhanced coherent
anti-Stokes Raman scattering (SECARS), surface-enhanced infrared
absorption (SEIRA), surface-enhanced fluorescence, surface-enhanced
hyper-Raman scattering (SEHRS).
[0085] Contrarily to the prior art, methods according to embodiment
of the present invention using controlled material removal such as
ion milling allow the open-nanoshells to have a feature of the
open-structure (i.e. the aperture) uniformly upward-oriented on the
substrate, implying an improved control of reduced-symmetrical
structure's orientation. An advantage of most of these directional
etching techniques, for example ion milling, is the reproducibility
of the ion milling process. Ion milling technology allows the
open-nanoshell geometry and upward orientation to be controlled.
The open-nanoshells monolayer structures, being stable and dense
structures, with nanoaperture and nanotip geometry, and having an
upward orientation are good substrates for SERS-based biomolecules
detection.
[0086] A benefit of the good control and reproducibility of these
directional etching techniques, for example milling technology, is
the fact that open-nanoshells with non-removed shell with different
heights can be fabricated by tuning the amount of time under
etching or ion milling. In other words, the height of the
non-removed shell of the open-nanoshells, not taking the core into
consideration, can be controlled by varying the directional
removing time. This offers the capability to control the
reduced-symmetrical geometry of open nanoshells (e.g. Au
open-nanoshells). That way, different SPR wavelengths can be
selected by making open-nanoshells with different heights of
non-removed shell, H i.e. the perpendicular distance between the
centre of the plane made by the edges of the remaining shell and
the furthest extremity of the shell as shown in FIG. 2. The height,
H, of the non-removed shell of the open-nanoshells is a parameter
for controlling their optical properties.
[0087] In a further aspect of the present invention, a method is
provided for producing open nanoshells involving the steps of the
method of the second aspect of the present invention. Where a
polymer matrix is not present, a last step of the method may
consist in removing the open nanoshells from the substrate e.g. by
dissolving away the optionally used functional layer or other means
by which the open nanoshells adhere to the substrate, thereby
obtaining unattached open nanoshells. If the removal step involved
the use of a liquid such as water or solvent to remove any optional
functional layer or other adhesive means, the free nanoshells so
obtained may be dispersed or suspended in said liquid. In that
case, the open-structure of the particles will be randomly
orientated. The relatively small size and near-infrared (NIR)
optical properties result in open-nanoshells suspensions suitable
for the biomedical imaging and thermotherapy.
[0088] In embodiments, the present invention provides a method of
fabricating Au (gold) open-nanoshells suspensions.
[0089] In embodiments, the present invention also provides for the
use of open nanoshells removed from the substrate in the treatment
of medical conditions by thermotherapy, in biomedical imaging and
as SPR biosensors.
[0090] FIG. 1 shows a method of fabrication of open-nanoshells
according to an embodiment of the present invention. First,
nanoshells (13) comprising a core (5) and a shell (3) can be
prepared. The dimensions of open-nanoshell particle are shown in
FIG. 1: "r" is the diameter of the core (5), "R" is the diameter of
total particle (and also the diameter of the shell (3)), "H" is the
height of non-removed part of the open-nanoshell (3). For this
purpose dielectric core particles (5) are selected.
[0091] On top of these core particles (5) (i.e. around said core
particles) a conducting layer (3) can be deposited (7) leading to
the formation of nanoshells (13).
[0092] In a next step, nanoshells (13) are deposited (e.g. directly
without the pre-deposition of a chemical functionalisation layer
(12) on the substrate (1) (path (8)) or after deposition of a
chemical functionalisation layer (12) (path (9)) (e.g. from a
suspension) onto a substrate (1). The substrate (1) can be
functionalised with e.g. a monolayer of organic adhesion molecules
(12) in order to immobilize the particles (13). To create
open-nanoshells (3+5), directional etching (10) of the conducting
layer (3) of the nanoshells (13), for example by ion milling, can
be done on the nanoshells (13) deposited on the substrate (1).
Since the electric field enhanced regions are mostly located on the
surface or the sharp edge of nanoaperture or nanotip structures,
the cores (5) of the open-nanoshells can be preserved.
[0093] However, it is also possible to remove the core (5).
Afterwards, the particles can be redispersed (11) (e.g.
ultrasonically) in a liquid medium, for example in water.
Example 1
[0094] First, Au nanoshells with various core sizes and shell
thicknesses were prepared by seeding and electroless plating from
silica colloids. Au nanoshells were synthesized according to a
modified method of Oldenburg et al [J. Chem. Phys. Lett. 1998, 288,
243-247]. Monodisperse silica nanoparticles were synthesized by the
hydrolysis of tetraethyl orthosilicate (TEOS) in basic solution via
the Stober process and functionalized with
(3-aminopropyl)triethoxysilane (APTES) in ethanol during 12 hours.
These functionalized silica particles were covered with dots of a
thin Au colloid (1-2 nm) prepared by the method of Duff et al
[Langmuir 1993, 9, 2301-2309]. A subsequent reduction of an aged
mixture of 1% chloroauric acid (HAuCl.sub.4) and potassium
carbonate (K.sub.2CO.sub.3) by a solution of formaldehyde
(CH.sub.2O) or hydroxylaminemono hydrochloride (H.sub.2NOH.HCl),
resulted in a complete Au shell coverage on the nanoparticle
surface. The resulting Au nanoshells were purified by repeated
centrifugation and washing with deionized water and finally
redispered in deionized water.
[0095] Au open-nanoshells monolayer structures were fabricated by
immobilizing Au nanoshells on a 3-mercaptopropyl-trimethoxysilane
(MPTMS) functionalized ITO glass or Si substrate to improve the
stability and coverage on the substrate. Scanning electron
microscopy (SEM) showed that most of Au open-nanoshells were
densely packed to form a monolayer with more than 80% coverage on
the substrate.
[0096] The nanoparticles in the water solution were deposited on
indium tin oxide (ITO)-coated glass or Si substrates. Prior to
their use, ITO-coated glasses or Si substrates were cleaned by a
piranha solution (1:3 (v/v) mixture of 30% by weight aqueous
H.sub.2O.sub.2 and concentrated (98.6 wt %) H.sub.2SO.sub.4),
rinsed well with deionized water and dried in a stream of N.sub.2.
Au open-nanoshells suspensions were typically prepared by
drop-casting aqueous suspensions of Au nanoshells on an ITO-coated
glass or Si substrate.
[0097] Au open-nanoshells suspensions and open-nanoshell monolayer
structures were fabricated by using an in-house made ion miller
system, using an energetic ion beam of Xe ions to bombard the
sample surface thereby etching away sample material. Therefore, Au
nanoshells on ITO glass or Si substrate were placed into the ion
miller system.
[0098] Different ion milling times were used with the following
parameters: 375 V beam voltage, 400 V accelerator voltage, 2.4 sccm
Xe flow rate, 2 sccm Ar flow rate and below 8.0.times.10.sup.-8
mTorr base pressure in the processing chamber. A small amount of Xe
gas (2.4 sccm Xe flow rate) is introduces into the chamber which
becomes ionized by a filament under a large voltage (375 V beam
voltage). A large electric field (400 V) then accelerates the Xe
ions towards a grid under a large negative bias (400 V accelerator
voltage). Before hitting the sample, the ion beam is neutralized by
a cloud of electrons generated by Ar plasma (2 sccm Ar flow rate
and below 8.0.times.10.sup.-8 mTorr base pressure). When the Xe
atoms carry a large enough momentum they will knock out atoms from
the sample. The etch rate is material dependent (e.g. 35 nm/min in
case of Au). An advantage is the fact that it is a very directional
etching method. When the beam of ions has a constant intensity, the
milling depth is controlled by the amount of time the sample is
being bombarded by the atoms.
[0099] To show the benefits of ion milling technology in the
fabrication of Au open-nanoshells, open-nanoshells with different
non-removed shell heights (FIG. 8) were fabricated by varying the
amount of time under ion milling. The ion milling time was varied
between 20 and 60 seconds with the parameters described above. This
offers the capability to control the reduced-symmetrical geometry
of Au open-nanoshells.
[0100] Afterwards, Au open-nanoshells were released from the slide
into an aqueous suspension by ultrasonication.
[0101] Different SPR wavelengths have been detected in Au
open-nanoshells with different non-removed shell heights as shown
in FIG. 5. The height of the non-removed shell of Au
open-nanoshells is a parameter to control their optical
properties.
[0102] All ion milling experiments here have been performed
repeatedly and the consistent results show excellent repeatability
of ion milling technology.
[0103] TEM and AFM measurements confirm the nanoaperture and
nanotip structures on the Au open-nanoshells obtained (FIGS. 6 and
7) allowing local electric field enhancement.
[0104] Transmission electron microscopy TEM images were recorded on
a 300 kV Philips CM30 instrument equipped with a field emission
source. A drop of the aqueous Au nanoshells or open-nanoshells
suspension was placed onto a carbon-coated copper grid (Holey
Carbon, 300 mesh Cu) to dry at room temperature for TEM imaging.
FIG. 6 shows TEM images of Au nanoshells (A: r/R/H=43/73/146 nm; C:
r/R/H=115/130/260 nm and Au open-nanoshells (B: r/R/H=43/73/109 nm;
D: r/R/H=115/130/195 nm). The dimensions of open-nanoshell particle
are shown in FIG. 1: "r" is the diameter of the core, "R" is the
diameter of total particle, and "H" is the height of the
non-removed shell of open-nanoshells. Scale bars correspond to 100
nm. TEM images FIGS. 6 A and C have shown Au nanoshells with
complete shells with different dimensions (core size and shell
thickness). FIGS. 6 B and D have shown Au open-nanoshells with
incomplete shells with different dimensions (core size and shell
thickness) and different orientations.
[0105] Scanning electron microscopy (SEM) images of Au nanoshells
deposited on an ITO-coated glass by drop-casting were taken using a
Philips XL30 FEG instrument operated at an accelerating voltage of
5 kV. SEM images have shown that Au open-nanoshells with different
dimensions and configurations can be prepared. Dimensions of
open-nanoshells can be controlled by tuning core sizes and shell
thicknesses in a broad range. The core size varied between 80 and
1000 nm and shell thickness can vary between 10 and 30 nm. In the
top and side views, nanotip and nanoaperture structures were
clearly observed on the open-nanoshells surface. The surface
roughness of the shells was determined by the applied electroless
plating procedure. All open-nanoshells were "facing up" with
respect to the open-structures (i.e. the shell opening is facing
away from the substrate).
[0106] FIG. 8 shows SEM of Au nanoshells (r/R/H=43/73/146 nm)
before ion milling (FIG. 8A) and after ion milling for 20 seconds
(FIG. 8B), 40 seconds (FIG. 8C) and 60 seconds (FIG. 8D). From FIG.
8, it has been shown that open-nanoshells with different
non-removed shell heights can be obtained by tuning the amount of
time under ion milling. This is a way to precisely control the
reduced-symmetrical geometry of Au open-nanoshells. Combining with
FIG. 5, it is demonstrated that the height of the non-removed shell
of Au open-nanoshells is a parameter to control their optical
properties.
[0107] FIG. 9 shows SEM images of monolayer structures of Au
nanoshells (r/R/H=115/130/195 nm) (A) before and (B, C) after ion
milling 40 seconds on a Si substrate (B: top view image; C: side
view image). The dimensions of open-nanoshell particle are shown in
FIG. 1: "r" is the diameter of the core, "R" is the diameter of
total particle, and "H" is the height of the non-removed shell of
the open-nanoshell. FIG. 9 (B, C) shows the open-nanoshells
monolayer structures when a mercaptosilane functionalization is
used. We observe the improvement of the stability and coverage of
the monolayer structure on the substrate. Most of Au
open-nanoshells were densely packed to form a monolayer with more
than 70% coverage on the substrate. Part of the free space present
between open-nanoshells is due to spatial limit (i.e. due to the
spherical geometry of the particles theoretically limiting the
coverage to
.pi. 12 ##EQU00002##
in the case of perfect spheres of identical size packed
hexagonally) and the additional part of the free spaces present
between open-nanoshells is possibly due to the incomplete
functionalization of the substrate.
[0108] A drop of the aqueous Au open-nanoshells suspension was cast
onto a Si substrate and then dried at room temperature for atomic
force microscopy (AFM) scanning. AFM images were acquired in the
tapping mode on a Dimension 3000/Nanoscope IV, VEECO, under ambient
conditions with the scan rate between 0.4 and 0.5 Hz. Si
cantilevers with a spring constant between 40 and 45 N/m were used
at resonance frequencies between 250 and 350 kHz. The free
amplitude peak is adjusted .about.1 V. All images post-processing
are performed using flatten order 1. FIG. 7 shows the rough surface
of Au layer in open-nanoshell structure and furthermore confirms
the nanoaperture and the nanotip structures on the Au
open-nanoshells, which allows local electro-magnetic field
enhancement. The line-profile In FIG. 7 of an AFM image (1
.mu.m.times.1 .mu.m) of Au open-nanoshells on a Si substrate shows
the surface morphology of the open Au nanoshells more clearly.
Example 2
Finite Difference Time Domain Simulations
[0109] Simulations of optical extinction spectra and near-field
distribution pictures were obtained based on the finite difference
time domain (FDTD) simulations or FDTD method using the program
FDTD Solutions (version 5.1) purchased from Lumerical Solutions,
Inc., (Vancouver, Canada). The simulations were performed with the
parallel FDTD option on a HP ProLiant DL145 G3 Server with 2
Dual-Core AMD Opteron 2000 processors at 2.8 GHz and 8 GB of RAM.
The FDTD method is based on the numerical solution of the Maxwell's
equations and can be used to obtain an adequate picture of the
electromagnetic near-field distribution around the structures with
arbitrary shapes.
[0110] The simulating system consists of a Au nanoshell or
open-nanoshell. FIG. 2 shows a schematic representation of the
simulating system used in FDTD calculations. In those simulations,
the Au nanoshell or open-nanoshell was placed in air. The particle
was illuminated with a total-field scattered-field (TFSF) source
[S. Taney et al, Laser Phys. Lett. 2006, 3, 594-598], which
propagates in the k=-Z direction. The direction of the electric
field E was perpendicular to k and parallel to the X direction. The
wavelength of incident light was varied from 400 nm to 1700 nm and
the amplitude was set as 1. A perfect matched layer (PML) was used
as radiation boundary condition. The simulation region is
800.times.800.times.800 nm.sup.3 with a grid size of 3 nm. The
whole simulation region was assumed in air. In these calculations,
the dimensions (r/R/H) of the Au nanoshell and open-nanoshell were
set as 115/130/260 nm and 115/130/195 nm, respectively. The
dispersion model for Au derived from the experimental data provided
by P. B. Johnson and R. W. Christy [Phys. Rev. B 1972, 6,
4370-4379] were used. The total complex-valued permittivity of the
Au .epsilon.(.omega.) is modelled by the combination of a Drude and
a Lorentz model, and hence results from the sum of three different
material modes .epsilon..sub.REAL(.omega.),
.epsilon..sub.L(.omega.) and .epsilon..sub.P(.omega.).
.epsilon..sub.REAL(.omega.) is the basic background permittivity
.epsilon..sub.REAL=6.8065. .epsilon..sub.L(.omega.) is the equation
based on a Lorentz model
L ( .omega. ) = LORENTZ .omega. 0 2 .omega. 0 2 - 2 .delta. 0
.omega. - .omega. 2 ##EQU00003##
with parameters .epsilon..sub.LORENTZ=1.6748,
.omega..sub.0=4.506608080759082.times.10.sup.15 Hz,
.delta..sub.0=6.820216162455338.times.10.sup.14 Hz, and
.epsilon..sub.P(.omega.) is the equation based on the Drude
model
P ( .omega. ) = .omega. P 2 .omega. v C + .omega. 2
##EQU00004##
with parameters .omega..sub.P=1.3538345417988594.times.10.sup.16
Hz, v.sub.C=1.068689183387936.times.10.sup.14 Hz. This fit provides
an accurate description of the dielectric data of Au in the
wavelength range from 400 nm to 1700 nm.
[0111] A full 3-dimensional FDTD calculation was executed in the
simulation. Cross sections (CS) of electrical field distribution
around a Au nanoshell (CS1 and CS2) and open-nanoshell (CS3 and
CS4) are shown in FIG. 3. CS1 and CS3 are in the plane of Y=0 nm,
CS2 is in the plane of Z=0 nm and CS4 is in the plane of Z=195 nm.
The shown field is normalized with respect to the incident field
amplitude. FIG. 3 shows X, Y, and Z components of the electric
field intensity and its summation (log|E|.sup.2) from the cross
sections (CS) of a Au (A, B) nanoshell (CS1 in the plane of Y=0 nm,
and CS2 in the plane of Z=0 nm) and (C, D) open-nanoshell (CS3 in
the plane of Y=0 nm, and CS4 in the plane of Z=195 nm). On bottom
of each panel is the maximum value of electric field intensity
(|E|.sup.2) in each plot (380 nm.times.380 nm). Full 3-dimensional
finite difference time domain (FDTD) calculation (see FIG. 10A)
showed the local electric field intensity distribution around Au
open-nanoshell in a plane vertically through the central axis. The
incident wavelength was in resonance with the first symmetric mode
(X=1279 nm). The local electric field was essentially similar to
that of Au nanoshell in a same size around outer shell regions, but
was substantially enhanced at the upper edge of the open-nanoshell
with a maximum enhancement factor of .about.19 with respect to the
incident light field. The field distribution in a plane
horizontally along the height of the non-removed shell of an
open-nanoshell (FIG. 10B) showed that the uniform enhancement was
maintained through the whole region of the edge surface. The
highest field regions were located at the inner and outer wall
edges with a maximum enhancement factor of .about.23, where a
plasmon resonance mode was similar to the case of nanoaperture
structures with a strong electromagnetic coupling between the inner
and outer wall edges, indicating a buildup of charge at two edges
which supports the field enhancement uniformly on the edge surface.
Even compared to Au nanoshells, the local electric field
enhancement around Au open-nanoshells was improved 4 times (FIG.
3), which points to Au open-nanoshells as a very promising
substrate for SERS-based biomolecules detection
[0112] The optical properties of the open-nanoshell structures and
the difference between the open-nanoshell and full shell particles
were assessed by full 3-dimensional finite difference time domain
(FDTD) calculations and spectrally resolved UV/VIS absorbance
measurements. FIG. 10A compares the simulated extinction spectra
(top) and the measured spectra (bottom) for two different nanoshell
sizes. The simulated and measured spectra show a fair agreement;
the remaining discrepancies are most likely attributable to the
polydispersed size and inherent rough shell surface of Au
open-nanoshells. The spectra furthermore indicate an interesting
trend: removing the top of the nanoshell results in a pronounced
red-shift of the plasmon resonance. This is corroborated by FIG.
10B, which shows the theoretical dependence of the extinction peak
on the ratio between the core size and the shell thickness, for
both open and closed nanoshells. The red-shift of the resonance is
consistent and becomes more pronounced as this ratio increases. The
resonance still has a dipolar character but is dominated by the
local charge-build-up at the edges of the open-nanoshell, as
indicated in FIG. 10C, which shows the electric field profile of
the nanostructure at resonance conditions. This charge build-up is
accompanied by a strong enhancement of the local electric field, as
compared to the local enhancement of closed nanoshells. This has
interesting prospects for surface enhanced Raman scattering (SERS),
which strongly depends on the local electromagnetic
enhancement.
Example 3
[0113] Optical properties of Au open-nanoshells suspensions and
monolayer structures have been studied experimentally (see below)
and theoretically with FDTD simulations see above (FIG. 4).
[0114] Optical extinction spectra of ITO glass with monolayer
structures of Au nanoshells or open-nanoshells were measured. All
experimental optical extinction spectra were measured using a
Shimadzu UV-1601PC spectrophotometer with a slit width of 2 nm and
a data interval of 0.5 nm. An ITO glass with monolayer structures
of Au nanoshells or open-nanoshells was orientated perpendicularly
to the incident light during the measurement in air. The aqueous
suspension of Au nanoshells and Au open-nanoshells were measured in
the cuvettes (Eppendorf UVette).
[0115] FIG. 4 shows normalized simulated optical extinction spectra
(A) and experimental optical extinction spectra (B) for an aqueous
suspension of (a, c) Au nanoshells and (b, d) Au open-nanoshells.
All simulated extinction spectra were calculated from FDTD
calculations. All particles dimensional parameters are shown in the
insets of (A).
[0116] FIG. 4 shows that with respect to Au nanoshells,
open-nanoshells display an apparent red-shift of plasmon resonance
in water or air, for example, from 675 nm for Au nanoshells
(r/R/H=43/73/146 nm) suspensions to 730 nm for Au open-nanoshells
(r/R/H=43/73/109 nm) suspensions (FIG. 4A), which can probably be
explained by the fact that the inner edge interacts strongly with
the outer edge in Au open-nanoshells. With the increasing of the
ratio of core size and shell thickness of the open-nanoshell, the
red-shift becomes more notable, since the interaction between the
inner and outer edges becomes stronger. Because of a surrounding
refractive index change and a random orientation, the extinction
spectra of open-nanoshells in water show a feature of red-shift and
broadening compare to those in air as expected.
[0117] A general agreement between the experimental and theoretical
optical extinction spectra was found and the discrepancies are most
likely attributable to the polydisperse size and inherent rough
shell surface of Au open-nanoshells.
[0118] Due to the difficulty to make Au nanoshells with a small
core size (<100 nm) and a thin shell thickness (<10 nm), Au
open-nanoshells are advantageous as their features of optical
response in near-infrared (NIR) region as well as their particle
size remaining relatively small lead to suitable applications, such
as the biomedical imaging and thermotherapy for certain tumour with
a size requirement.
[0119] It is to be understood that although preferred embodiments,
specific constructions and configurations, as well as materials,
have been discussed herein for devices according to the present
invention, various changes or modifications in form and detail may
be made without departing from the scope and spirit of this
invention. For example, any formulas given above are merely
representative of procedures that may be used. Functionality may be
added or deleted from the block diagrams and operations may be
interchanged among functional blocks. Steps may be added or deleted
to methods described within the scope of the present invention.
* * * * *