U.S. patent application number 12/515232 was filed with the patent office on 2010-03-04 for micropatterning of conductive graphite particles using microcontact printing.
This patent application is currently assigned to Board of Trustees of Michigan State University. Invention is credited to Lawrence T. Drzal, Troy R. Hendricks, Ilsoon Lee, Jue Lu.
Application Number | 20100052995 12/515232 |
Document ID | / |
Family ID | 39204683 |
Filed Date | 2010-03-04 |
United States Patent
Application |
20100052995 |
Kind Code |
A1 |
Lee; Ilsoon ; et
al. |
March 4, 2010 |
MICROPATTERNING OF CONDUCTIVE GRAPHITE PARTICLES USING MICROCONTACT
PRINTING
Abstract
Methods involve a combination of polyelectrolyte multilayer
(PEM) coating or silane self assembly on a substrate; microcontact
printing; and conductive graphite particles, especially size
controlled highly conductive exfoliated graphite nanoplatelets. The
conductive graphite particles are coated with a charged polymer
such as sulfonated polystyrene. The graphite particles are
patterned using microcontact printing and intact pattern transfer
on a substrate that has an oppositely-charged surface. The method
allows for conductive organic patterning on both flat and curved
surfaces and can be used in microelectronic device fabrication.
Inventors: |
Lee; Ilsoon; (Okemos,
MI) ; Drzal; Lawrence T.; (Okemos, MI) ; Lu;
Jue; (Okemos, MI) ; Hendricks; Troy R.;
(Lansing, MI) |
Correspondence
Address: |
HARNESS, DICKEY & PIERCE, P.L.C.
P.O. BOX 828
BLOOMFIELD HILLS
MI
48303
US
|
Assignee: |
Board of Trustees of Michigan State
University
East Lansing
MI
|
Family ID: |
39204683 |
Appl. No.: |
12/515232 |
Filed: |
November 15, 2007 |
PCT Filed: |
November 15, 2007 |
PCT NO: |
PCT/US07/23985 |
371 Date: |
May 15, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60859297 |
Nov 15, 2006 |
|
|
|
Current U.S.
Class: |
343/700MS ;
106/31.92; 174/254; 174/388; 252/511; 264/105; 427/98.4 |
Current CPC
Class: |
H05K 3/1275 20130101;
Y10T 428/30 20150115; H05K 2203/105 20130101; H05K 2201/0323
20130101; B82Y 40/00 20130101; H05K 1/0259 20130101; B82Y 10/00
20130101; G03F 7/0002 20130101; H05K 2203/09 20130101; H05K
2203/0108 20130101 |
Class at
Publication: |
343/700MS ;
427/98.4; 264/105; 252/511; 106/31.92; 174/254; 174/388 |
International
Class: |
H01Q 1/38 20060101
H01Q001/38; B05D 5/12 20060101 B05D005/12; B28B 3/00 20060101
B28B003/00; C09D 11/10 20060101 C09D011/10; H01B 1/24 20060101
H01B001/24; H05K 1/00 20060101 H05K001/00; H05K 9/00 20060101
H05K009/00 |
Goverment Interests
SPONSORSHIP
[0002] Subject matter described herein was developed in part with
research funding provided by the United States Government under
Grants from the Air Force Office of Scientific Research and The
National Science Foundation (CTS-0609164). The U.S. Government may
have certain rights to the invention.
Claims
1. A method of depositing conductive materials onto substrates,
comprising providing a substrate having a charged surface layer;
microcontact printing an oppositely-charged conductive material
onto the charged surface layer in a pattern that covers less than
100% of the surface with charged material.
2. A method according to claim 1, wherein the substrate comprises a
polyelectrolyte membrane (PEM).
3. A method according to claim 1, wherein microcontact printing
comprises intact pattern transfer of a layer by layer (LBL)
assembled multilayer comprising alternating layers of a) graphite
particles coated with a first polyelectrolyte and b) a second
electrolyte of opposite charge to the first polyelectrolyte.
4. A method according to claim 1, wherein the conductive material
comprises exfoliated graphite nanoplatelets.
5. A method according to claim 3, wherein graphite is coated with a
polyanion and the charged surface layer is cationic.
6. A method according to claim 3, wherein the graphite is coated
with a polycation and the charged surface layer is anionic.
7. A method of fine patterning conductive material onto a surface
for electronic applications, the method comprising exposing high
surface area graphite particles to a solution of a charged polymer
to form a charged conductive material; applying an ink comprising
the charged conductive material to the surface of a stamp; and
bringing the surface of the stamp into contact with a
polyelectrolyte multilayer thin film, wherein the polyelectrolyte
multilayer thin film has an outer surface of charge opposite the
charge conductive material; wherein the surface area of the
graphite particles is 50 m.sup.2/g or higher.
8. A method according to claim 7, wherein the surface area is 100
m.sup.2/g or higher.
9. A method according to claim 7, wherein applying the ink
comprises layer by layer assembly of alternating layers of a) an
ink comprising the charged conductive material and b) a
polyelectrolyte having a charge opposite of the charged conductive
material.
10. A method according to claim 9, wherein the aspect ratio of the
graphite particles is 1000 or higher.
11. A method according to claim 7, wherein the graphite particles
are exfoliated graphite nanoplatelets (x-GnP).
12. A method according to claim 11, wherein the charged conductive
material is coated with a polyanion.
13. A method according to claim 11, wherein the charged conductive
material is coated with a polycation.
14. A method of depositing a pattern of conductive material onto a
surface, comprising coating exfoliated graphite nanoplatelets with
a charged polymer; forming an ink comprising the coated platelets;
applying the ink to a microstamp; and transferring the coated
platelets onto a surface by microcontact printing with the
microstamp.
15. A method according to claim 14, wherein the exfoliated graphite
nanoplatelets are coated with a polyanion.
16. A method according to claim 15, wherein the polyanion is
sulfonated polystyrene.
17. A method according to claim 14, wherein the nanoplatelets are
coated with a polycation.
18. A method according to claim 17, wherein the polycation is
polydiallyldimethylammonium chloride.
19. A method according to claim 14, wherein the surface comprises
the outer charged layer of a polyelectrolyte multilayer film.
20. A method according to claim 19, wherein the polyelectrolyte
multilayer film comprises 10 or more bilayers of alternating
polyanion and polycation.
21. A method according to claim 14, wherein applying the ink to the
microstamp comprises layer by layer assembly of alternating layers
of a) an ink comprising the charged conductive material and b) a
polyelectrolyte having a charge opposite of the charged conductive
material.
22. A method according to claim 14, wherein transferring comprises
intact pattern transfer of a LBL layer by layer assembled
multilayer comprising alternating layers of a) graphite particles
coated with a first polyelectrolyte and b) a second electrolyte of
opposite charge to the first polyelectrolyte
23. An RFID antenna comprising exfoliated graphite nanoplatelets
conductive particles in a pattern laid down by the method of claim
14.
24. A conductive circuit comprising exfoliated graphite
nanoplatelets deposited on a PEM thin film.
25. A circuit according to claim 24, in the form of an antenna.
26. A circuit according to claim 24, in the form of an RFID
antenna.
27. A circuit according to claim 24, in the form of an
electromagnetic interference shielding material.
28. Exfoliated graphite nanoplatelets coated with a charged
polymer.
29. Exfoliated graphite nanoplatelets according to claim 28, coated
with a polyanion.
30. Exfoliated graphite nanoplatelets according to claim 28, coated
with a polycation.
31. Exfoliated graphite nanoplatelets according to claim 28, having
an aspect ratio of 1000 or greater.
32. An ink comprising water and exfoliated graphite nanoplatelets
according to claim 28.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/859,297, filed on Nov. 15, 2006. The disclosure
of the above application is incorporated herein by reference.
INTRODUCTION
[0003] Deposition of conductive materials onto substrates in small
scale or micropatterns is a key to a wide variety of electronic
applications. Many such industrial applications are experiencing
high growth and include radio frequency identification (RFID)
antenna fabrication, and the fabrication of electromagnetic
interference (EMI) shielding materials.
[0004] Photolithography to form metal patterns has been a standard
method for forming electronic circuits in electronic devices.
Photolithography is an expensive step in the fabrication process.
In addition, the materials that can be used in the process are
somewhat limited and the method generally requires flat substrates.
On the other hand, many fast growing areas of electronic
technology, such as the RFID and EMI applications discussed above,
require low cost fabrication techniques to deposit conductive
materials on a variety of flexible surfaces using a wide range of
materials.
SUMMARY
[0005] Methods of creating thin films with conductive organic
patterns are described that can be formed on any surface for
electronic device applications. In one aspect, the methods involve
a combination of polyelectrolyte multilayer (PEM) coating or silane
self assembly on a substrate; microcontact printing; and conductive
graphite particles, especially size controlled highly conductive
exfoliated graphite nanoplatelets, to create the thin films. In one
aspect, the conductive graphite particles are coated with a charged
polymer such as sulfonated polystyrene. The polymer coated graphite
particle, which has an effective negative charge, is then patterned
using microcontact printing on a substrate that has a surface
charged oppositely to the graphite particle. In various
embodiments, the charged surface on the substrate is provided by a
polyelectrolyte multilayer or a self assembled monolayer. The
method allows for conductive organic patterning on both flat and
curved surfaces which can be used in microelectronic device
fabrication. To illustrate, the PEMs can be deposited on a wide
variety of surfaces. Using microcontact printing, negatively
charged graphite patterns are transferred from a stamp to the
positively charged outer surface of the multilayers. After rinsing,
conductive patterns remain strongly bound on the surface.
DRAWINGS
[0006] The drawings described herein are for illustration purposes
only and are not intended to limit the scope of the present
disclosure in any way.
[0007] FIG. 1 shows micrographs of conductive particles on a
substrate in a micropattern.
[0008] FIG. 2 shows the surface resistivity of multilayer films
DESCRIPTION
[0009] In one embodiment, the invention provides a method of
depositing conductive materials onto substrates. The method
involves providing a substrate having a charged surface layer and
microcontact printing an oppositely charged conductive material
onto the charged surface layer. Advantage is taken of the
microcontact printing to print a conductive material and pattern
that covers less than 100% of the surface with charged material. In
various embodiments, the substrate is a polyelectrolyte multilayer
(PEM) film and/or the conductive material comprises graphite. In a
particular embodiment, the conductive material is made of
exfoliated graphite nanoplatelets (x-GnP).
[0010] As noted, the conductive material is charged oppositely to
the charge on the charged surface layer. In various embodiments,
graphite is coated with a polyanion, while the charged surface onto
which it is microcontact printed is cationic. In another
embodiment, graphite is coated with a polycation and the charged
surface layer is anionic.
[0011] In a further embodiment, the invention provides a method of
fine patterning conductive material onto a surface for electronic
applications. The method involves exposing high surface area
graphite particles to a solution of a charged polymer to form a
charged conductive material. In this step, the charged polymer
attaches to the outside surface of the graphite particle. Next, the
method involves applying an ink containing the charged conductive
material to the surface of a stamp such as a microstamp. Then, the
surface of the stamp is brought into contact with a PEM thin film,
in which the PEM thin film has an outer surface with a charge
opposite to that on the charged conductive material. The high
surface area graphite particles have a surface area of 50 m.sup.2/g
or greater, 75 m.sup.2/g or greater, and 100 m.sup.2/g or greater,
in non-limiting embodiments.
[0012] In addition to high surface areas, the graphite particles
are characterized in some embodiments by high aspect ratios. Thus
in various embodiments, the aspect ratio of the graphite particles
is 100 or higher. In a further embodiment, the aspect ratio is 1000
or higher. In various embodiments, the graphite particles comprise
exfoliated graphite nanoplatelets that are coated with a polyanion
or a polycation to provide a charged conductive material.
[0013] In another embodiment, the invention provides a method of
depositing a pattern of conductive material on a surface. The
method involves coating exfoliated nanoplatelets with a charged
polymer and then forming an ink comprising the coated
nanoplatelets. The ink is then applied to a microstamp and the
coated nanoplatelets on the microstamp are transferred onto the
surface by microcontact printing. In various embodiments, the
surface has an outside layer with a charge opposite to that of the
charged polymer coated on the exfoliated graphite
nanoplatelets.
[0014] In various embodiments, applying an ink to the surface of a
microstamp in the methods described herein involves layer by layer
assembly of alternating layers of a) an ink comprising a charged
conductive material (such as polyelectrolyte coated graphite
particles) and b) a polyelectrolyte having a charge opposite of the
charged conductive material. When such an ink applying step is
used, microcontact printing or transferring the ink to a charged
surface involves intact pattern transfer of the layer by layer
assembled multilayer. The multilayer comprises alternating layers
of a) graphite particles coated with a first polyelectrolyte and b)
a second polyelectrolyte of opposite charge to the first
polyelectrolyte.
[0015] In various embodiments, the nanoplatelets are coated with a
polyanion, such as sulfonated polystyrene in an non-limiting
example. In other embodiments, the nanoplatelets are coated with a
polycation such as, without limitation, poly(diallyldimethyl
ammonium) chloride (PDAC). In preferred embodiments, the surface
onto which the coated platelets are transferred is a charged layer,
such as the outer charged layer of a PEM film. Suitable PEM films
include those having 10 or more bilayers of alternating polyanion
and polycation.
[0016] In another embodiment, the invention provides exfoliated
graphite nanoplatelets coated with a polyelectrolyte, such as a
polyanion or a polycation. In various embodiments, the coated
nanoplatelets have high surface areas and are characterized by a
length to thickness aspect ratio of 1000 or higher and in
particular 10,000 or higher.
[0017] The invention further provides a conductive circuit
comprising exfoliated nanoplatelets deposited on a PEM thin film.
In various embodiments, the nanoplatelets are deposited on the film
in the form of an antenna such as an RFID antenna, or in the form
of electromagnetic shielding materials. Thus, in various
embodiments, the invention provides an RFID antenna comprising
exfoliated graphite nanoplatelet conductive particles in a pattern
on a surface laid down by any of the methods described herein.
[0018] In various embodiments, inks containing slurries or
suspensions of graphite particles with a surface charge are applied
onto microcontact stamps and then microprinted onto charged
substrates. The inks contain graphite particles that, in a
preferred embodiment, are coated with a charged polymer. The
charged polymer contains either a negative or a positive charge.
Non-limiting examples of charged polymers include sulfonated
polystyrene, which carries a negative charge.
[0019] The graphite particles in various embodiments are graphite
flakes, graphite nanotubes, exfoliated graphite nanoplatelets,
expanded graphite, and the like, as well as mixtures. In preferred
embodiments, the graphite particles are nanosized in that at least
one dimension of the particle is less than 1 .mu.m. Preferably, the
graphite particles have a high surface area, which is believed to
contribute to the adhesion of the polymer-coated particles on the
substrate. In various embodiments, the graphite particles have a
surface area of 50 m.sup.2/g or higher. For example, the particles
may have a surface area of 75 m.sup.2/g or higher or 100 m.sup.2/g
or higher. In various embodiments, the particles have an aspect
ratio of 100 or higher. In other embodiments, the particles have an
aspect ratio of 1000 or higher or of 10,000 or higher. As the
thickness of the particles decreases, the aspect ratio increases,
given that the lateral diameter of the particle or platelet remains
the same, and in general the surface area also increases. The upper
limit of surface area, for example, approaches 2700 m.sup.2/g for a
theoretical or hypothetical single graphene sheet or monolayer
graphite. The theoretical aspect ratio of such a structure is also
very high. Practically available surface areas and aspect ratio are
lower than the theoretical maxima.
[0020] In a preferred embodiment, the graphite particles comprise
exfoliated nanoplatelets. Such nanoplatelets are described for
example in U.S. Patent Publication 2006/0241237, published Oct. 26,
2006 by Drzal et al., the disclosure of which is hereby
incorporated by reference.
[0021] Graphite is a layered material. Individual molecular layers
are held together with weak Van der Waals forces and are capable of
intercalation with organic or inorganic molecules and eventual
expansion. These nanosized expanded graphite platelet materials are
very large platelets having large diameters and are very thin in
thickness. The graphite structure is stiff in bending. Graphite is
a very good thermal and electrical conductor.
[0022] An expanded graphite is one that has been heated to separate
individual platelets of graphite. An exfoliated graphite is a form
of expanded graphite where the individual platelets are separated
by heating with or without an agent such as a polymer or polymer
component.
[0023] Typical starting materials to make exfoliated graphite
nanoplatelets are natural graphite flakes intercalated with
oxidizing agents, such as a sulfuric acid-based intercalated
graphite obtained from UCAR international Inc. These intercalated
graphite flakes are expanded, for example by exposure to microwave
energy, typically at 2.45 GHz frequency for a few seconds to a few
minutes in an oven, whereby they undergo significant expansion (for
example, about 500 times). Pulverization using an ultrasonic
processor follows to break down the worm-like structure resulting
in individual graphite nanoplatelets that are less than 10 nm thick
and have a diameter of about 15 .mu.m. Their diameter can be
further reduced by using a vibratory mill for a suitable time such
as 72 hours, resulting in graphite nanoplatelets with the same
thickness and diameter less than 1 .mu.m.
[0024] The use of microwave (MW) energy or radiofrequency (RF)
induction heating provides a fast and economical method to produce
expanded graphite nanoflakes, graphite nanosheets, or graphite
nanoparticles. In various embodiments, a graphite nanoplatelet is
generally 10 nm or less in thickness and can range in size from 1
micron to 500 microns. "Nanoparticles" is used herein as a generic
term for nanoflakes, nanosheets, nanoplatelets, and the like.
Nanoflakes is a description of a starting material that is
exfoliated into nanoplatelets. The microwave or RF methods are
especially useful in large-scale production and are very
cost-effective.
[0025] Chemically intercalated graphite flakes are expanded by
application of the RF or microwave energy. The expansion occurs
rapidly. Heating for 3-5 minutes removes the expanding chemical.
The graphite absorbs the RF or microwave energy very quickly
without being limited by convection and conduction heat transfer
mechanisms. The intercalant heats up past the boiling point and
causes the graphite to expand to many times its original volume.
The process can be performed continuously by using a commercially
available induction or microwave system with conveyors.
[0026] In various embodiments, the combination of RF or microwave
expansion and appropriate grinding technique, such as planetary
ball milling (and vibratory ball milling), produces nanoplatelet
graphite flakes with a high aspect ratio efficiently. In general,
such procedures reduce the "particle diameter", i.e. the x-y
dimensions, but do not change the thickness. The pulverized
graphite has an aspect ratio of 100, 1000 or 10,000 or higher. The
surface area of the pulverized graphite is 50 m.sup.2/g, 75
m.sup.2/g, or 100 m.sup.2/g or higher. Microwave or RF expansion
and pulverization of the crystalline graphite to produce suitable
graphite flakes enables control of the size distribution of
graphite flakes more efficiently.
[0027] In various embodiments, nanoplatelets are prepared by
expanding unexpanded intercalated graphite in the presence of a
gaseous atmosphere with a chemical that expands upon heating to
produce expanded graphite. The platelets generally are made of
finely divided expanded graphite consisting essentially of
platelets that are less than 300 .mu.m in length and normally less
than 0.1 .mu.m (100 nm) in thickness. In various embodiments, the
platelets are less than 200 .mu.m in length, and/or less than 20 nm
or less than 15 nm in thickness. The platelets are characterized by
an aspect ratio (length over thickness of 100 or more, 1000 or
more, up to about 10,000-100,000.
[0028] A precursor graphite used to make the exfoliated graphite
nanoplatelets includes a graphite flake that has been treated with
a fuming oxy acid and then heated to form the expanded graphite
particles. Sulfuric acid is a non-limiting example of a suitable
oxy acid.
[0029] In a preferred embodiment, the graphite particles are
combined with a charged polymer. The charged polymer has a positive
or negative charge and is applied onto the graphite particles to
provide charge on the particle that will enable it to bind to the
charged outer surface of the substrate. In a non-limiting example,
the charged polymer is applied to the graphite particle by
suspending the particles in a solution of the polymer and providing
energy to the suspension by means of ultrasound, heat or other
means. In a preferred embodiment, a suspension of the graphite
particle in a solution of the charged polymer is ultrasonicated to
apply the charged polymer to the graphite. An ink is then formed
that contains a slurry or suspension of the polymer coated graphite
particles.
[0030] In a subsequent step, the inks are applied to the surface of
a stamp. The ink can be applied to a stamp by soaking the surface
of the stamp in the ink for a suitable time. Depending on the
concentration of the particles in the slurry, the nature of the
charged polymer on the surface of the graphite particles, and other
factors, the soaking time can be varied as desired to provide
suitable build up of conductive particles on the stamp.
[0031] Stamps for microcontact printing are well known. In a
preferred embodiment, the stamp is made of a polydimethylsiloxane
(PDMS) material. Kits are commercially available for producing the
stamps. In a preferred embodiment, an elastomeric stamp is made by
curing PDMS on a microfabricated silicon master, which acts as a
mold, to allow the surface topology of the stamp to form a negative
replica of the master. To illustrate, PDMS stamps are made by
pouring a 10:1 solution of elastomer and initiator over a prepared
silicon master. In various embodiments, the silicon master is
pretreated with fluorosilanes to facilitate the removal of the PDMS
stamps from the masters. In a preferred embodiment, PDMS stamps are
plasma treated to render the surface hydrophilic.
[0032] In various embodiments, it is preferred to provide a
continuous layer of graphite particles on the stamp so that when it
is later transferred to the charged surface of a substrate, the
resulting conductive pattern exhibits suitable electronic
performance. It has been observed that continuous patterns of
graphite particles can be built up by increasing the soaking time
and/or by sequential dipping of the stamp in the ink, wherein the
total contact between the stamp and the ink is sufficient to form a
continuous pattern of graphite particles. Thus, in various
embodiments, soaking is carried out for a matter of minutes such as
from 1-60 minutes, preferably from 1-20 minutes. Alternatively, the
total soak or exposure time is divided into two or more dipping
steps wherein the stamp is exposed to the ink for a time, then
removed and the process repeated.
[0033] Thus, in various embodiments the ink is applied to the stamp
by exposing the ink to the surface of the stamp for at least 1-2
minutes and preferably for at least 5 minutes. Alternatively, the
stamp is dipped into the ink and soaked for a time, then the stamp
is removed and the cycle is repeated, for example three times.
Alternatively, solutions slurries or suspensions containing the
graphite particles can be sprayed, brushed, or rolled on to the
stamp surface. In general, the ink may be applied to the stamp in
any known process.
[0034] In various embodiments, ink is applied to a stamp which
contains a layer of polyelectrolyte having a charge opposite to the
charged polymer coating the graphite particle. For example, a layer
of polyelectrolyte alone (for example positively charged) is first
absorbed onto the stamp. The stamp containing the polyelectrolyte
is then dipped in or exposed to the ink containing the polymer
coated charged graphite particles or nanoplatelets.
[0035] In a preferred embodiment, conductive multilayered
polymer/exfoliated graphite nanoplatelet (xGnP.TM.) nanocomposite
films are created using layer by layer (LBL) assembly of films
containing xGnP and the intact pattern transfer (IPT) of these
films to a substrate. IPT is known from J. Park, L. D. Fouche, P.
T. Hammond, Advanced Materials 2005, 17, 2575; J. Park, P. T.
Hammond, Advanced Materials 2004, 16, 520; N. Kohli, R. M. Worden,
I. Lee, Chemical Communications 2005, 316; and N. Kohli, R. M.
Worden, I. Lee, Macromolecular Bioscience 2007, 7, 789, the
disclosures of which are incorporated by reference.
[0036] In a non-limiting embodiment, multilayered graphite is
exfoliated followed by milling to create size controlled xGnP as in
K. Kalaitzidou, H. Fukushima, L. T. Drzal, Carbon 2007, 45, 1446
(incorporated by reference herein). The xGnP are then coated with a
charged polymer to form a stable aqueous solution. The charged
polymer can be cationic or anionic. The solution is then used for
electrostatic LBL assembly, with an oppositely charged
polyelectrolyte as the counter ion, onto the surface of an
uncharged hydrophobic elastomeric stamp. LBL assembly proceeds by
alternatingly exposing the stamp and the built up layers to first
an ink comprising xGnP coated with a charged polymer and then to a
polyelectrolyte of charge opposite that of the polymer coating the
nanoplatelets. Once the film is formed, it is placed in direct
contact with a substrate of the opposite charge to directly
transfer the multilayer film. If enough layers of xGnP are adsorbed
to the stamp, the LBL film becomes conductive. In various
embodiments, four layers and six layers (or more) have been found
sufficient to provide a film that transfers efficiently to a
surface. Before LBL assembly on the stamp, the elastomeric stamp is
coated with a layer of polyelectrolyte using relatively weak
hydrophobic interactions between the stamp and film. When the stamp
is removed from the substrate the strong electrostatic interactions
between the oppositely charged films on the stamp and substrate
hold the multilayer film on the substrate surface.
[0037] The substrate that will receive the graphite ink or the
impact pattern transfer is also prepared with a changed surface. In
one embodiment, the charged outer surface is anionic (an example is
the polyanion outer layer of a PEM film as described herein).
Polyelectrolyte multilayers are prepared layer-by-layer. In one
exemplary process, they are prepared by sequentially immersing a
substrate, such as silica, glass, or plastic slide, in alternating
positively and negatively charged polyelectrolyte solutions in a
cyclic procedure. A wide range of negatively charged and positively
charged polymers is suitable for making the layered materials.
Suitable polymers are water soluble and sufficiently charged (by
virtue of the chemical structure and/or the pH state of the
solutions) to form a stable electrostatic assembly of electrically
charged polymers. Sulfonated polymers such as sulfonated
polystyrene are commonly used as the negatively charged
polyelectrolyte. Quaternary nitrogen-containing polymers such as
poly (diallyldimethylammonium chloride) (PDAC) are commonly used as
the positively charged electrolyte.
[0038] Other methods of layer-by-layer formation of PEM films are
by spin casting, solution casting, and spray assembly of the
alternating polyelectrolytes to build up a desired number of
layers. These methods apply both to build up of layers on the
substrate, and to deposition of inks containing charged conductive
material on the microstamp.
[0039] Polyelectrolytes include positively and negatively charged
polymers, and are also divided among "strong" and "weak"
polyelectrolytes depending on whether the charged groups do or do
not maintain their charge over a wide pH range. For example, a
sulfonated polymer is considered a strong polyelectrolyte because
it is negatively charged over a wide pH range; an acrylic acid
polymer is considered a weak polyelectrolyte because it is
protonated below a pH of about 4 but contains a negative charge at
higher pH. Strong polyelectrolytes include sulfonated polystyrene
(SPS) and poly (diallyldimethyl ammonium chloride) (PDAC). Weak
polyelectrolytes include polyacrylics such as polyacrylic acid, as
well as positively charged polyelectrolytes such as poly (allyl
amine) and branched and linear polyethyleneimines as their
respective ammonium salts.
[0040] Assembly of the PEM's is well known; an exemplary process is
illustrated in Decher, Science vol. 277, page 1232 (1997), the
disclosure of which is incorporated by reference. The method can be
conveniently automated with robots and the like. In one embodiment,
a polycation is first applied to a substrate followed by a rinse
step. Then the substrate is dipped into a negatively charged
polyelectrolyte solution for deposition of the polyanion, followed
again by a rinse step. The procedure is repeated as desired until a
number of layers is built up. In another embodiment, a polyanion is
applied first to the substrate, followed by a polycation. In either
case, alternating polycation and polyanion are applied to build up
bilayers. A bilayer consists of a layer of polycation and a layer
of polyanion. Thus for example, 10 bilayers contain 20 layers,
while 10.5 bilayers contain 21 layers. With an integer number of
bilayers, the top surface of the PEM has the same charge as the
substrate. With a half bi-layer (e.g. 10.5 illustrated) the top
surface of the PEM is oppositely charged to the substrate. Thus,
PEM's can be built having either a negative or a positive charge
"on top".
[0041] Instead of PEMs on the substrates, the substrates can
contain self assembled monolayers (SAM) such as are provided by
various silane materials that contain charged functional groups. In
a non-limiting embodiment, a self-assembled monolayer is prepared
on a substrate by exposing the substrate to a silane material
containing a quaternary ammonium group. The self assembled
monolayer in this case provides a positive charge that attracts the
negative charge on the graphite particle to provide a conductive
material on the substrate in the pattern provided by the stamp.
EXAMPLES
[0042] Poly(diallyldimethylammonium chloride) (PDAC, Mw
.about.70,000), sulfonated poly(styrene), sodium salt (SPS, Mw
.about.150,000), 6-carboxyfluorescein (6-CF) and nitrocellulose
membranes (0.22 .mu.m pore diameter) were purchased from
Sigma-Aldrich (Milwaukee, Wis.) and used as received.
Poly(dimethylsiloxane) (PDMS) was created using a Sylgard 184 kit
from Dow Corning (Midland, Mich.). Poly(allylamine hydrochloride)
(PAH, Mw 60,000) was obtained from Polysciences, Inc. (Warrington,
Pa.). Graphite Intercalate Compounds (GIC) were purchased from UCAR
Inc. Purified deionized (DI) water was obtained from a Barnstead
Nanopure Diamond (Barnstead International, Dubuque, Iowa,
resistivity>18.2 M.OMEGA.-cm) and used exclusively for all
experiments. All polymer solution concentrations were based on the
repeat unit of the polymer.
Example 1
xGnP Preparation
[0043] xGnP Preparation: xGnP are created using a process developed
in the Composite Materials and Structures Centre and MSU. See H.
Fukushima, Ph.D. Dissertation, Chemical Engineering & Materials
Science, Michigan State University, (East Lansing, Mich.), 2003 and
J. Lu, L. T. Drzal, R. M. Worden, I. Lee, Chemistry of Materials
2007, In Press [DOI: 10.1021/cm702133u]. GIC, which are acid
intercalated graphite about 300 .mu.m in size are expanded using
microwave radiation. The microwaves cause the intercalated acids to
evaporate quickly and expand the multilayered graphite. The
expanded graphite is then ultrasonicated using a tip sonicator (see
e.g. G. H. Chen, W. G. Weng, D. J. Wu, C. L. Wu, J. R. Lu, P. P.
Wang, X. F. Chen, Carbon 2004, 42, 753). This creates xGnP of about
15 .mu.m in diameter and 5-10 nm in thickness. The size of the xGnP
is then reduced to a 0.5-1 .mu.m diameter using ball milling, as
disclosed in D. Cho, S. Lee, G. M. Yang, H. Fukushima, L. T. Drzal,
Macromolecular Materials and Engineering 2005, 290, 179; K.
Kalaitzidou, H. Fukushima, L. T. Drzal, Carbon 2007, 45, 1446; and
K. Kalaitzidou, H. Fukushima, L. T. Drzal, Composites Science and
Technology 2007, 67, 2045.
Example 2
Coated Graphite Particles and Inks
[0044] xGnP used in the following experiments is 5-10 nm in
thickness with a 0.5-1 .mu.m diameter. 0.1 g of xGnP is added to a
100 mL aqueous solution containing 0.01 M SPS and 0.1 M NaCl. The
solution is then tip sonicated with a Virsonic 100 (SP Industries
Inc, Warminster, Pa.) for 30 min, agitated with stirring for 24 h
and filtered using nitrocellulose membranes. The SPS coated xGnP
are then redispersed in 100 mL of DI water and tip sonicated for 20
min. After sonication, any undispersed nanoplatelets on the surface
of the solution are removed using wax paper. This xGnP-SPS solution
(ink) is then used for LBL assembly.
Example 3
Film Preparation on PDMS
[0045] PDMS stamps are fabricated by pouring a degassed 10:1
prepolymer/initiator mixture over a patterned silicon wafer (Keck
Microfabrication Facility, Michigan State University) and curing
overnight in an oven at 60.degree. C. The stamps are then placed in
a 0.05 M solution of PAH at a pH of 7.5 or 10 for 20 min. The
samples are then alternately dipped by hand into the xGnP-SPS
solution of Example 2 and a solution containing 0.01 M PDAC and 0.1
M NaCl. The polymer dipping time is 20 min which is followed by two
5 min washing steps in water before the next polymer layer is
adsorbed. Typically 4 or 6 bilayers are built up on the stamp,
which has a positively charged outer PDAC surface.
Example 4
Substrate Film Preparation
[0046] A Carl Zeiss slide stainer (Richard-Allan Scientific,
Kalamazoo, Mich.) is used to perform LBL assembly on glass
microscope slides. Glass slides are prepared by bath sonication
(Branson Ultrasonics Corporation, Danbury, Conn.) for 20 min in an
aqueous Alconox (Alconox Inc., New York, N.Y.) solution followed by
bath sonication for 10 min in DI water. The slides are then dried
with nitrogen and oxygen plasma cleaned in a Harrick Plasma cleaner
(Harrick Scientific Corporation, Broadway Ossining, N.Y.) for 10
min at a pressure of 125 mTorr. After plasma cleaning, LBL assembly
of PDAC and SPS is performed using 20 minute immersion times
followed by two 5 min washing steps to form 10 bilayers with a
negatively charged SPS outer surface denoted as
(PDAC/SPS).sub.10.
Example 5
Intact Pattern Transfer
[0047] The coated stamps are removed from the washing solution and
gently dried with nitrogen. The composite film formed on a PDMS
stamp with an outer PDAC surface is misted with DI water from a
spray bottle and placed in conformal contact with the outer SPS
surface on the PEM coated substrate. After one hour of contact time
the stamp is removed and the xGnP containing film transferred to
the surface due to the strong electrostatic forces.
Example 6
Characterization
[0048] Optical and fluorescent microscope images are obtained using
a digital camera mounted on a Nikon Eclipse ME 600 or ME 400
respectively. 6-CF is dissolved in 0.1 M NaOH and used to
selectively bind to positively charged regions on the surface. SEM
images are obtained using a JOEL 6400V microscope equipped with a
LaB.sub.6 filament and operated at 8 keV. AFM images are obtained
in tapping mode using a Nanoscope IV multimode scope (Digital
Instruments). The conductivity of the samples is measured using
Impedance Spectroscopy. Two copper tape electrodes are attached to
the surface of the film and the resistance (R) is measured using a
0.1 V potential. The resistances are normalized by the dimensions
of the films and reported as a surface resistivity
(.OMEGA..sub.s=R.times.L/D) where L is the length between the
electrodes and D is the width of the electrode (see J. G. Smith, J.
W. Connell, D. M. Delozier, P. T. Lillehei, K. A. Watson, Y. Lin,
B. Zhou, Y. P. Sun, Polymer 2004, 45, 825 and L. Y. Ji, E. T. Kang,
K. G. Neoh, K. L. Tan, Langmuir 2002, 18, 9035).
Example 7
[0049] In a typical process, multilayered graphite is expanded
using acid intercalation, followed by exfoliation using
ultrasonication (see for example D. Cho, S. Lee, G. M. Yang, H.
Fukushima, L. T. Drzal, Macromolecular Materials and Engineering
2005, 290, 179; G. H. Chen, W. G. Weng, D. J. Wu, C. L. Wu, J. R.
Lu, P. P. Wang, X. F. Chen, Carbon 2004, 42, 753; and X. S. Du, M.
Xiao, Y. Z. Meng, Journal of Polymer Science Part B-Polymer Physics
2004, 42, 1972, the disclosures of which are incorporated by
reference) and finally milled to create xGnP nanoplatelets (few
layer graphene particles with a thickness of 1-10 nm and a 100-1000
nm diameter).
[0050] Since graphite is naturally hydrophobic, the xGnP need to be
further modified with a polyelectrolyte to be used in LBL assembly
from aqueous solutions. Illustrating for the case of an anionic
polyelectrolyte, ultrasonication is used to disperse the xGnP in a
solution containing a negatively charged polymer (e.g. sulfonated
polystyrene, or SPS). The SPS coats the xGnP through interactions
between the sp.sup.2 hybridized graphitic surface and the aromatic
rings of the polymer (see S. Stankovich, R. D. Piner, X. Q. Chen,
N. Q. Wu, S. T. Nguyen, R. S. Ruoff, Journal of Materials Chemistry
2006, 16, 155). This coating facilitates the formation of a stable
aqueous solution by preventing agglomeration of the SPS coated xGnP
(xGnP-SPS) through like charge repulsion. This charge also enables
the xGnP-SPS to be used for electrostatic LBL assembly. Zeta
potential measurements confirm the negative charge of the
xGnP-SPS.
[0051] Before LBL assembly, topographically patterned uncharged
hydrophobic poly(dimethylsiloxane) (PDMS) stamps are coated with
poly(allylamine hydrochloride) (PAH) using hydrophobic interactions
at a pH of 7.5 or 10. The PDMS substrates are then placed into a
solution containing xGnP-SPS, where the xGnP-SPS electrostatically
deposits onto the PAH coated surface. After washing, the sample is
then placed into a solution containing a positively charged
polyelectrolyte, such as poly(diallyidimethylammonium chloride)
(PDAC) where a second layer deposits on the surface. Repeated
immersion into the xGnP-SPS and PDAC solutions creates multilayer
films denoted as PAH/(xGnP-SPS/PDAC).sub.n where n is the number of
bilayers deposited on the PAH coated surface. The build up of
multilayered films is confirmed by the gradually increasing
darkness of the deposited film. The positive electrolyte (PDAC in
the example) is typically the final layer adsorbed.
[0052] After the film is fabricated on the stamp, it is removed
from solution, gently blown dry and placed in contact with a
negatively charged PEM coated substrate. Contact times are
typically one hour. Preferably the film is completely dry before
the stamp is removed from the substrate.
[0053] FIG. 1 shows optical (FIG. 1a) and scanning electron
microscope (SEM) images of the transferred patterns of the
PAH/(xGnP-SPS/PDAC).sub.4 nanocomposite films using the IPT method.
Films containing four and six bilayers could be patterned on areas
as large as the stamp (1.5 cm.times.3 cm) using this technique. In
the SEM image, FIG. 1b, the xGnP are observed in multilayer stacks
created by the LBL process. Additionally, the xGnP packed densely
enough on the surface to conduct electrical current. Atomic Force
Microscope (AFM) analysis shows that the transferred four and six
bilayer films have a thickness of 85 nm and 120 nm,
respectively.
Example 8
[0054] Glass slides are coated with (PDAC/SPS).sub.10.5 bilayers.
0.1 g of exfoliated graphite nanoplatelets (approximately 1 .mu.m
in diameter with a 1-10 nm thickness) is dispersed in 100 ml 10 mM
SPS solution with 0.1 M NaCl by sonication at 20 W for 30 minutes
and then incubated for 24 hrs. The resulting SPS-coated graphite
nanoparticles are filtered using a membrane with pore size of 0.22
.mu.m and washed three times with DI water. SPS coated
nanoparticles are then redispersed into 100 ml DI water by
sonication at 10-15 W for 10 minutes. Plasma treated PDMS stamps
are then soaked in the nanoparticle suspension/solution for 20
minutes. After soaking, the stamps are dried with nitrogen and
placed in conformal contact with the PEM surface of the glass slide
substrate.
Example 9
[0055] Alternatively, glass slides are coated with
(PDAC/SPS).sub.10 bilayers. Exfoliated graphite nanoplatelets are
coated with PDAC or PEI (polyethyleneimine) the same way as with
SPS in Example 2. The nanoparticles have a positively charged
surface. The pH value of a suspension of PEI-coated graphite
nanoparticles is adjusted to 5-6. Before patterning, polymer coated
nanoparticles are sonicated in a Branson sonication bath for 20
minutes. Plasma treated PDMS stamps are then soaked in the
nanoparticle suspension/solution for 20 minutes. After soaking, the
stamps are dried with nitrogen and placed in conformal contact with
the PEM surface of the glass slide substrate
Example 10
Surface Resistance of Films
[0056] FIG. 2 shows the surface resistance of the films. The
electrical properties of the films were determined by measuring the
surface resistance. Films of xGnP-SPS were created on PAH adhesion
layers at a pH of 7.5 or 10. However, the films formed onto the
layers formed at 7.5 had a higher resistance than films formed on
adhesion layers adsorbed at a pH of 10. FIG. 2 displays the results
of measurements of xGnP-SPS films with an adhesion layer formed at
a pH of 10. Uncoated PDMS substrates and two bilayers on PDMS were
found to be nonconductive. (Samples on PDMS were measured without
the final layer of PDAC.) However, when four bilayers are used, the
percolation threshold is surpassed and the samples are found to be
conductive. Six bilayer films on PDMS show the lowest surface
resistivity of 5.8.times.10.sup.4 Ohm/sqr. This resistance is
comparable to previous reports by Kotov et al. which used
exfoliated graphite created by oxidation to create the film,
followed by a reduction step to render the film conductive, see N.
A. Kotov, I. Dekany, J. H. Fendler, Advanced Materials 1996, 8,
637. However, our process does not include oxidation or reduction
steps. In addition, we use a smaller number of layers, six instead
of ten, to achieve nearly the same resistance. The addition of more
bilayers could be used to further reduce the surface resistivity of
the film.
[0057] Four and six bilayer films were transferred from PDMS to PEM
coated glass slides. The surface resistance of these transferred
films increased slightly. The increase in resistance is caused by
the presence of the dense 4-5 nm PAH layer which covers the
xGnP-SPS layer. We attempted to lower the resistance of the
transferred films by removing the layer of PAH. Soaking the samples
in a high pH solution to cause charge screening and removal of the
PAH was unsuccessful. Additionally plasma treatment did not remove
the PAH layer. Heating the samples to 275.degree. C. for 3 hours to
burn out the polymers also did not lower the measured resistance of
the transferred six bilayer films. Films with surface resistivities
in this range are potentially useful for controlling electrostatic
charge dissipation and preventing damage to surrounding electronic
equipment, as described for example in J. G. Smith, J. W. Connell,
D. M. Delozier, P. T. Lillehei, K. A. Watson, Y. Lin, B. Zhou, Y.
P. Sun, Polymer 2004, 45, 825 and L. Y. Ji, E. T. Kang, K. G. Neoh,
K. L. Tan, Langmuir 2002, 18, 9035.
* * * * *