THREE-DIMENSIONAL GRAPHENE/GRAPHITE STRUCTURE FOR ULTRA-SENSITIVE
BIOSENSOR
BY
JONGHYUN CHOI
THESIS
Submitted in partial fulfillment of the requirements
for the degree of Master of Science in Mechanical Engineering
in the Graduate College of the
University of Illinois at Urbana-Champaign, 2014
Urbana, Illinois
Adviser:
Professor SungWoo Nam
ii
ABSTRACT
Graphene has been attracting significant research interests for post-silicon electronics due
to its unique properties such as extraordinarily high carrier mobility, mechanical
robustness/flexibility and biocompatibility. Furthermore, bio-sensing capabilities of graphene-
based field-effect transistors interfaced with cells/tissues have been widely investigated by
several research groups. However, the reported sensor devices based on graphene have been
planar structures, which present substantial challenges for three dimensional (3D) intimate
interfacing with biological systems and simultaneous extra- and intracellular sensing of action
potentials. Here, a novel approach of graphene transfer is reported to provide intimate and
conformal interfacing of biological systems with underlying sensing platforms.
Polydimethylsiloxane (PDMS), which is widely accepted as biocompatible material, was used as
the substrate material. Toluene was exploited to pre-swell the substrate before the transfer, to
reduce the suspension of graphene and consequentially minimize the damage of graphene. The
large area, conformal transfer of graphene was characterized with Raman spectroscopy and
scanning electron microscope (SEM), demonstrating that the continuous monolayer graphene
was on top of 3D features without significant damages. Furthermore, we expand our discussion
to the fabrication of graphene-based field-effect sensors and 3D heterostructure consisting of
graphene/graphite foams.
iii
This thesis is dedicated to my beloved family for their limitless love and spiritual support.
Particularly, I express my sincere appreciation to JeongSong Cultural Foundation, Office of
International Cooperation at Hanyang University, since I would not even be able to start my life
in the U.S.A. without their financial supports.
iv
ACKNOWLEDGMENTS
I would like to thank my advisor, Professor SungWoo Nam, for guiding and supporting
my research here at the University of Illinois at Urbana-Champaign. I am also grateful to
Professor Won Il Park at Hanyang University in South Korea for the research collaboration. I
also express my appreciation to all my colleagues: Jaehoon Bang, Mike Cai Wang, SungGyu Chun,
Ali Ashraf, Hoejoon Kim, Juyoung Leem and Keong Yong Han.
Last but not the least, I also thank my financial sponsors, JeongSong Cultural Foundation,
and Office of International Cooperation at Hanyang University, for allowing me the opportunity of
starting the graduate study in the United States.
v
TABLE OF CONTENTS
CHAPTER 1 INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CHAPTER 2 SYNTHESIS, FABRICATION AND TRANSFER OF GRAPHENE
ONTO THREE-DIMENSIONAL SUBSTRATES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CHAPTER 3 MATERIALS CHARACTERIZATIONS OF GRAPHENE ON
THREE-DIMENSIONAL SUBSTRATES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CHAPTER 4 FABRICATION OF GRAPHENE FIELD-EFFECT SENSOR
PLATFORM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CHAPTER 5 THREE-DIMENSIONAL GRAPHENE/GRAPHITE FOAM. . . . . . . .
APPENDIX A SUPPLEMENTARY INFORMATION . . . . . . . . . . . . . . . . . . . . . . . . .
APPENDIX B EXPERIMENTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APPENDIX C MATERIALS / EQUIPMENT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REFERENCES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1
CHAPTER 1
INTRODUCTION
Graphene, a two-dimensional (2D) honeycomb lattice of close packed sp
2-bonded carbon
atoms, is a basic building block for graphitic materials, including 0D fullerenes, 1D nanotubes as
well as 3D graphite [1-4]. Graphene has drawn significant attention due to its unique physical
properties such as extremely high carrier mobility (200,000 cm2 V
-1 s
-1), massless relativistic
carriers, and thermal conductivity, all of which make graphene distinguished candidate for post-
silicon electronics [5-8]. So far, graphene has been investigated for potential applications such as
photovoltaic devices, logic transistors, as well as supercapacitors [9-13]. The chemical inertness
and biocompatibility of graphene paves a way as a promising material for bio-sensing, where the
interaction between target cells/tissues and graphene perturbs the carrier transport of graphene
and be monitored by measuring conductivity change [14-17].
Initial attempts to obtain graphene include mechanical exfoliation, where graphene layers
were exfoliated from bulk graphite and transferred onto a SiO2 on a silicon wafer, called scotch
tape method [1-4]. In recent, one of the common methods for large-area, high-quality graphene
synthesis is the chemical vapor deposition (CVD), using gaseous precursors and catalyst layers;
monolayer graphene was synthesized by CVD with methane and hydrogen gases on Cu/Ni
catalyst layers [18-22]. In addition, a large monolayer graphene synthesis up to 30 inches has
been demonstrated through a roll-to-roll continuous process [23], and wafer-scale growth of
single-crystal monolayer graphene was demonstrated using a hydrogen-terminated germanium
buffer layer [24]. Furthermore, a novel synthesis technique to create an all-carbon structure was
investigated, using heterogeneous catalyst structure containing Cu, Ni, and Co [22]. Due to the
2
difference in carbon solubility of each metal as well as different growth mechanism, graphene
was synthesized on Cu layers by absorption of carbon atoms, whereas thick graphite was grown
by segregation/precipitation of the dissolved carbon atoms on Ni/Co layers [21-22, 25-26].
Graphene biosensors have potential advantages compared to 1D nano-materials (e.g.
nanowires or nanotubes) in terms of large sensing area per unit volume, since all the atoms in a
single-layer graphene function as surface atoms [27]. Furthermore, graphene has inherently low
electrical noise owing to the high-quality crystal lattice, enabling the enhanced screening of
charge fluctuation compared to 1D nano-materials [5, 28-30]. Several research groups have
reported graphene bioelectronics interfaced with living cells/tissues [15, 25, 31-39]. Cohen-Karni
et al. investigated graphene field-effect transistors (FETs) as well as combined graphene &
nanowire FETs for nanoscale bioelectronics with cells/tissues [15]. Embryonic chicken
cardiomyocytes were interfaced with graphene FETs, which demonstrated well-defined
extracellular signals with signal-to-noise ratio of above 4. The conductance amplitude could be
controlled by modulating the water gate potential (Vwg), and the variation of Vwg across the Dirac
point demonstrated ambipolar behavior which is consistent with the semimetallic characteristics
of graphene [3, 6, 20-21]. The peak-to-peak width was proportional to the area of graphene,
implying the average signal from different points across the membrane of the cells. 1D silicon
nanowire FETs (SW-FETs) incorporated with graphene FETs further characterized the temporal
resolution and multiplexed measurements. Moreover, Daly et al. presented a label-free graphene-
based biosensor array, where the composition of the nutritive components in culturing medium
was monitored [35]. FET arrays with multiple sensors were fabricated to provide parallel
measurements and improve the statistical confidence. Escherichia coli were adhered onto
graphene FETs, which showed an accurate positioning with each sensor. The charge transport
3
responses of graphene were varied with the concentration of the lysogeny broth (LB) medium,
where clear shifts in conductivity were detected in the liquid environment. However, plausible
contaminations in multiple lithography processes or unwanted doping from substrates degraded
the performance of the devices. Furthermore, An et al. reported graphene based liquid-ion gated
FETs that have high sensitivity as well as selectivity for Hg [39]. Graphene based aptasensor,
which exploited an aptamer (30-amine-TTC TTT CTT CCC CTT GTT TGT-C10 carboxylic
acid-50), was fabricated to demonstrate the ability of Hg detection. The response time was below
1 second, and a strong field-induced response was substantialized through the binding between
Hg2+
ions and the aptamer. Consequently, Hg2+
ions with extremely low concentration could be
detected, with 2-3 orders of magnitude more sensitive in electrical response than previously
reported Hg sensors. Excellent selectivity was further demonstrated toward Hg2+
ions in mixed
solution containing other non-target metal ions. The aptasensor was transferred on flexible
polyethylene naphthalate (PEN) substrate, and demonstrated superior mechanical durability and
flexibility in terms of bending/relaxing.
Such graphene biosensors reported so far, however, are based on planar sensing channel
design, which is challenging for intracellular recording and for physically intimate interfacing.
Intracellular recording provides more accurate measurement of electrophysiology [40], but
typical intracellular electrodes such as patch-clamp or metal microelectrodes suffer from
mechanical invasiveness to cells, low spatial resolution, and limited size of probe due to the
device impedance [41-44]. Significant progress utilizing nanotechnology has addressed these
issues; nano-electrodes based on nanopillars or nanowires, which possess small size as well as
high surface to volume ratio, do not suffer from invasiveness issue while surpassing the
sensitivity/resolution of conventional patch-clamp or microelectrodes [45-48]. In this work, 3D
4
graphene with a sharp tip was fabricated to potentially provide capabilities for extracellular and
intracellular recoding. Furthermore, the 3D interfacing with biological systems and the
mechanical flexibility of graphene provide a promising bio-sensing platform that conforms to the
multi-dimensionality and mechanical characteristics of the biological systems, which is
challenging in conventional rigid and planar sensing devices. Figure 1.1 illustrates the proposed
3D sensing device based on graphene channel. 3D polydimethylsiloxane (PDMS) which exhibits
biocompatibility [49] was used as a substrate material to form 3D graphene structures through
conventional wet transfer. Furthermore, the actual fabrication/testing of sensor devices were
performed based on conventional photolithography and thermal evaporation. The 3D fabrication
of graphene presented here will provide unique capabilities to form conformal/flexible interface,
as well as record intra- and extracellular action potentials in electrogenic cells in future.
Figure 1.1 Schematic illustration of proposed 3D sensing device. Compared to typical sensors
that utilized planar graphene as sensing channels, this 3D structure is conformal to the multi-
dimensionality of the target cells/tissues and enables simultaneous intra- and extracellular
electrophysiological sensing.
5
CHAPTER 2
SYNTHESIS, FABRICATION AND TRANSFER OF GRAPHENE
ONTO THREE-DIMENSIONAL SUBSTRATES
Graphene was synthesized by CVD (Figure B.1). Compared to mechanical exfoliation or
carbon nanotubes slicing, this method produces a large-area and high-quality graphene, although
additional efforts are required to obtain a perfect single-crystalline graphene [18-22].
Commercial Cu foil was used as a catalyst layer to synthesize monolayer graphene. Before the
synthesis, pre-treatment of a Cu foil was performed using HCl solution to remove CuOx layer
and contaminants, which resulted in a lower D-band (~1,350cm-1
) (defect characteristic peak of
graphene) in Raman spectroscopy [50]. Cu etchant such as Na2(SO4)2 or FeCl3 was also tried as
solutions for pre-treatment, but these chemicals caused pores and contamination on the foils.
Figure 2.1 illustrates the fabrication procedure to get a monolayer graphene for the 3D
transfer. Thermal evaporation of a 30 nm-thick Au layer was performed to provide a supporting
layer to prevent a breakage of graphene as the graphene layer gets suspended near the sharp 3D
features and conform. Here, the thickness of the Au layer was optimized to 30 nm because
thinner layers could not provide a full coverage of Au onto graphene, whereas thicker layers
could attenuate the ductility of Au layer which is crucial for conformity of graphene with
substrates. Sputtering of any materials on graphene must be prohibited since plasma that
generated in sputter physically/chemically interacts with and damages graphene. Poly(methyl
methacrylate) (PMMA) was also attempted as the transfer layer, but it could not provide a
conformal transfer due to the brittleness of PMMA, which resulted in the collapsing of graphene
after the etching. In accordance with this, direct transfer of graphene (without any transfer layer)
only works when no significant suspension of graphene is expected. Au has been widely used for
6
various applications including graphene transfer, not only for its ductility, but it is also
chemically stable to corrosion/oxidation and has chemical selectivity that is important in the
following etching processes of catalysts [51-53]. Before transferring the graphene onto sharp 3D
substrates, the graphene on the backside of Cu foil should be completely removed to obtain clear
monolayer graphene after the transfer without perturbation from the backside graphene. Plasma
assisted dry etching has been commonly used to etch graphene and pattern nanostructures, where
highly reactive radicals dissociated from gas (e.g. O2) chemically etches the graphene [54-56].
O2 plasma operating at 300 W was used here to etch the backside graphene with PMMA
passivation layer on the topside. The plasma etching conditions such as the gas pressure, power,
or etching time should be carefully controlled since the upside PR passivation could be
simultaneously etched, which would affect the electronic structure of underlying graphene. Here,
we utilized bilayer PMMA to minimize the pores in PMMA layer and prevent damages of
graphene on top-side [57]. Figure A.1 shows the Raman spectra of the as-grown graphene,
backside of the Cu foil, and graphene transferred on SiO2/Si substrate, respectively, confirming
monolayer graphene was successfully obtained. The PMMA and underlying catalyst metal were
removed afterward, using acetone and Na2(SO4)2 solution, respectively. The thin Au/Graphene
film floating on deionized (DI) water then became ready to be transferred onto various 3D
substrates.
7
Figure 2.1 Fabrication procedure for monolayer graphene transfer. (a) graphene is synthesized
via typical CVD process on a 25µm-thick Cu foil. (b) Thin Au layer is deposited by thermal
evaporation. (c) PMMA is spin-coated to provide a protection layer for backside-graphene
etching. (d) backside-graphene was removed by reactive ion etching using a O2 plasma. (e)
PMMA protection layer is removed by wet-etching using acetone, followed by catalyst etching
using a Na2(SO4)2 solution. (f) Au (30nm)/Graphene layer is ready to transferred onto a 3D sharp
substrate.
3D PDMS, which was used as substrate material, was prepared using silicon mold that
could generate multiple PDMS substrates. Figure 2.2 describes the fabrication procedure for the
mold. First, 100nm-thick silicon nitride deposition was performed with STS Plasma-enhanced
CVD, which is used later as the etch-mask for KOH wet-etching. A mixed frequency recipe was
used to minimize the thermal stress of the resulting silicon nitride film. SiH4 and NH3 were used
8
as gaseous precursors for the deposition. Patterns for pyramid structures are created via typical
photolithography process. Square patterns were used to get pyramid shapes, which will be
described later in the procedure. Silicon nitride was subsequently etched with reactive ion
etching, with photoresist (PR) as an etch mask. Wet-etching was also tried, but in terms of the
uniformity of the patterns created, isotropic dry etching was found to be better for this fabrication.
Photoresist was then removed in acetone and followed by piranha solution to remove all the
resist cleanly. Now, to etch the underlying silicon substrate to get pyramid structures, anisotropic
(chemical) etching was applied to perform the directional etching of 54.7º to the [100] silicon
with KOH solution at 75ºC. The etch rate was approximately 20µm / hour, so depending on
feature sizes, the time required for the complete etching should vary. However, once the pyramid
shapes are created, the etching is complete and no additional etching occurs from that point.
Therefore, we could get pyramid shapes with different sizes by one-time fabrication without any
discrepant features. The silicon nitride layer was subsequently removed by HF solution. We
could confirm that the silicon nitride was removed by observing the changes in color or the
surface characteristics; the surface changes from hydrophilic to hydrophobic. Finally, thin Teflon
deposition was performed to make the surface of the mold hydrophobic, since a thin natural
silicon oxide layer is usually grown on bare silicon substrates, which makes the silicon substrate
hydrophilic. Otherwise, it becomes challenging for hydrophobic PDMS to be detached well from
hydrophilic surface of the mold.
9
Figure 2.2 Fabrication procedure for the silicon mold. (a) Thin silicon nitride layer is deposited
with plasma-enhanced CVD process. (b) Square patterns are created via typical photolithography
process. (c) Silicon nitride is etched with the PR as the etch-mask. (d) PR is removed with
acetone and piranha cleaning. (e) Silicon layer is etched by the KOH solution, with the direction
of 54.7º. (f) Silicon nitride is removed by HF solution.
PDMS (liquid) was poured onto the mold substrate with the curing agent to create 3D
solid-phase PDMS. To remove all the bubbles in PDMS that generated by pouring and mixing
processes, desiccation was performed at a low pressure of 5 mTorr. Subsequently the mixed
PDMS was heated at 80ºC on hotplate to promote the crosslinking/hardening.
We utilized pre-swollen PDMS in the initial graphene transfer and then allow it to re-
shrink after the transfer, to obtain a conformal graphene coating. PDMS swelling has been
widely investigated since controlling the swelling is critical in a number of applications
10
including micro-reactors for organic reactions [58-60]. It has been found that nonpolar solvents
such as toluene, hydrocarbons, and dichloromethane generate a large ratio of swelling PDMS.
Thin Au film with 30nm thickness used here is critical to prevent graphene from collapsing
during the shrinkage. Figure 2.3 shows an example of PDMS pre-swelling, with the mixing ratio
of 10:1. Depending on the mixing ratio, swelling time, or the selection of the swelling liquid, the
strain of the PDMS could be varied (Figure A.2). Here, toluene was used here to get the saturated
engineering strain of 30 %. Note that the 3D PDMS cannot be used directly for the graphene
transfer. Due to the mechanical characteristics of the substrate, graphene is supposed to suspend
near the 3D sharp features (Figure 2.4 (b)), not creating conformal coating to the substrate. This
is very important because suspended graphene not only fails to conform to its underlying
substrate with 100 percent coverage, but the trapped water at the interface generates capillary
force, leading the collapsing of graphene.
Figure 2.3 PDMS pre-swelling process. (a) PDMS is cut with a size of 3cm × 3cm without
swelling. (b) Pre-swollen PDMS for 3D graphene transfer. The saturated swollen size is 3.8cm ×
3.8cm, with the engineering strain (εE) of 26.7%. The mixing ratio of the liquid PDMS to the
curing agent was 10:1.
11
Fabricated Au/graphene film as shown in Figure 2.1 was then transferred onto the pre-
swollen PDMS substrate, and subsequently the 3D substrate was shrunken again to recover the
initial geometry (Figure 2.4). The structural difference between/after the swelling was
investigated, and there was no significant bucking and mechanical deformation on the PDMS
structures. The top of the pyramid and the planar area played roles as anchoring points to the
Au/graphene film, and as the substrate was shrunken the suspended part near the pyramid
features simultaneously created a conformal shape to the substrate, with some minor crumples on
the periphery. Finally, the thin Au film was etched off with conventional wet-etching method
using KI/I2 solution.
Figure 2.4 Transfer procedure of graphene (fabricated at Figure 2.1) to create 3D features. (a)
12
PDMS substrate is pre-swollen before the transfer. (b) Graphene is transferred, but due to the
sharp 3D underlying features the graphene is not conformal to the substrate. (c) The pre-swollen
substrate is shrunk again to generate conformal graphene coating to the substrate, with some
minor crumpling near the 3D features. (d) Au layer that deposited for the transfer is removed
with KI/I2 solution.
The Au etching process, which is the final step of the fabrication, is critical to avoid any
damages/delamination of graphene on 3D substrates. Note that for hydrophilic surfaces, because
of the surface characteristics and the adhesion energy difference of graphene to the Au and
PDMS, the conventional wet-etching approach causes a complete delamination of graphene.
When the substrate is hydrophilic, instead of conventional wet-etching, we utilized vapor-phase
etching, with KI/I2 solution in sealed chamber to generate high pressure vapor of the etchant
(Figure A.3). It is not only less aggressive to the structure, but also, regardless of the surface
characteristics, the delamination of graphene could be avoided. Controlling the chamber
temperature, making a good seal could reduce the time required to complete the etching process.
Additional solution etching should be applied to clean the Au residue at the final step.
Hydrophilic surfaces are often desired to obtain conformal transfer/coating of thin films
on a substrate. Figure 2.6 shows two antithetical images of thin film transfer on hydrophobic
(Figure 2.6 (a)) and hydrophilic surface. The hydrophobic substrate shows the transferred thin
film is not conformal but suspended because of the 3D features on the substrate, whereas the
hydrophilic one demonstrates a good conformity. Alcohols such as butanol, hexagonal were also
tried for the conformal transfer owing to their low surface tension, but as the size of underlying
features increase, the low surface tension could not play a significant role. Here, we used
hydrophobic substrate and utilized pre-swelling and re-shrunk, but in case of rigid substrate such
as silicon that is not capable for mechanical deformation, the surface interaction between the
13
substrate and graphene, the characteristics of transfer solutions should be carefully considered to
avoid the suspended structures and collapsing of graphene.
Figure 2.5 Thin film (30nm-thickness Au) transferred on 3D PDMS substrates that have
different surface characteristics. (a) The film transferred onto hydrophobic substrate, creating
suspended film structure onto the substrate. (b) Same film transferred onto the substrate that was
processed with O2 plasma surface-treatment. The surface became hydrophilic due to the
hydroxyl (OH-) groups, resulting a transfer with a good conformity.
14
CHAPTER 3
MATERIALS CHARACTERIZATIONS OF GRAPHENE ON THREE-
DIMENSIONAL SUBSTRATES
Figure 3.1 shows the optical microscope images of transferred films on PDMS pyramid
arrays. Figure 3.1 (a) demonstrates the Au/graphene film transferred on a PDMS film, with
crumples generated as predicted earlier. This figure corresponds to the Figure 2.5 (b), implying
the Au/graphene film became conformal to the underlying features due to the shrinkage (see also
Figure A.4). Figure 3.1 (b) further exhibits the graphene/PDMS structure after the Au supporting
layer was etched with KI/I2 solution. Note that the PDMS substrate should be kept relatively
hydrophobic to provide stronger adhesion of graphene to PDMS and prevent any delamination
during the Au etching step.
Figure 3.1 (a) 30nm Au/graphene transferred onto pre-swollen PDMS by toluene. (b) After
etching off the Au layer through the vapor-phase etching. The height, width of the pyramids are
10 um. Black/dark squares are the top images of PDMS pyramids. Scale bars: 30um.
15
To further demonstrate that this method is generically applicable for even larger 3D
features, transfer onto larger pyramids with step heights of 10µm were tried. Figure 3.2 shows
the scanning electron microscope (SEM) images along with the corresponding Raman spectra.
The SEM images (Figure 3.2 (a, c, e)) were taken tilted 25º to clearly show the 3D feature, and
exhibited no remarkable electron charging that comes from non-conductive surfaces,
demonstrating that the graphene is mostly continuous around the sharp features as well as at the
side walls and flat areas. Crumples were generated due to the shrinkage, which could be
significantly reduced by careful post-treatment with acetone/IPA liquid. The low surface-tension
nature of these liquids help the graphene to reduce its crumpling, with a slight swelling effect of
acetone to the PDMS (~4 %). Since the main purpose of this structure is for biological
applications, the Au residue should be carefully removed by rinsing the substrate with the Au
etchant/DI water repeatedly. Figure 3.2 (b, d, f) further demonstrates the material coated on the
PDMS is graphene, with distinct 2D (~2,680cm-1
) and G (~1,590cm-1
) peaks [20, 61]. The peaks
at ~1,400cm-1
and ~1,250cm-1
were observed from PDMS substrates, and used as reference
peaks to compare the relative Raman intensity of graphene peaks to that of PDMS. Here, a slight
blue-shift of 2D band was observed at the top of pyramids with the amount of ~5cm-1
, indicating
strains were generated in graphene around the pyramids. The Raman intensity of the 3D
graphene was 1.3 – 1.5 times higher than that of planar graphene, plausibly due to the increased
focal volume of laser at the top, compared to the planar area with the focal size of around ~1 µm.
To further test how the thin Au layer influences the conformal transfer, direct transfer of
graphene without any transfer layer was also tried as control experiments. Figure A.5-6 shows
that for the direct transfer, graphene was damaged due to the underlying features, generating a
16
significant electron charging during the SEM imaging and obsolesced intensity of Raman
spectrum.
Figure 3.2 Characterizations of 3D conformal graphene to the underlying nonplanar PDMS
substrates. (a, c, e) SEM images (25º tilited) of monolayer graphene transferred onto pyramid
arrays. Graphene was transferred onto sharp features without significant damages. Small
17
crumples were found owing to the re-shrunk, along with grain boundaries of graphene. Scale
bars: 30µm. (b, d, f) Raman spectra corresponding to each SEM images, demonstrating clear 2D
(~2,680cm-1
) and G (~1,590cm-1
) at the top of the pyramids (red) and planar area (blue),
respectively.
To further demonstrate the reliability of the proposed transfer mechanism, high
magnitude SEM images were taken with four different feature sizes. Figure 3.3 shows the
graphene was continuously transferred without significant damages at the top of pyramids.
Figure 3.3 High magnitude SEM images of graphene transferred onto 3D features with the
height/width of (a) 10µm, (b) 20µm, (c) 30µm, and (d) 50µm. Scale bars are 5µm, 10µm, 15µm,
and 25µm, respectively.
18
In summary, we fabricated an integrated structure by transferring graphene on a
nonplanar pyramidal substrate. The biocompatibility of graphene and PDMS could provide an
excellent platform for bio-sensing material with mechanical robustness and flexibility [49]. The
3D structure resembles the multidimensionality of cell/tissues/ which could be exploited as
intimate and conformal interfacing with biological systems [25-26]. Toluene was used to control
the degree of swelling effect, to enable conformal coating of graphene without significant
damage. Thin Au film was critical for graphene to maintain the mechanical robustness and be
protected from contamination, along with a good ductility which is also crucial for the conformal
transfer [51-53]. The continuous transfer of graphene without significant damages with a sharp
underlying features were confirmed by SEM and Raman spectroscopy, showing no electron
charging and clear 2D & G bands. A slight blue-shift of 2D band in Raman spectroscopy implies
the strain generated in graphene on top of pyramids. Future research should be focused on the
device fabrication (FETs) and biological sensing experiments. The 3D conformal coupling of
living cell/tissues with underlying sensing material perturbs the carrier transport of monolayer
graphene, which subsequently could be monitored by measuring the changes in conductivity.
19
CHAPTER 4
FABRICATION OF GRAPHENE FIELD-EFFECT SENSOR PLATFORM
Graphene-based FET devices were fabricated on a SiO2/Si substrate (Figure 4.1). First,
four alignment marks were created for further patterning of graphene/Au electrodes. Second,
standard graphene transfer using a PMMA, and photolithography was performed to pattern the
nine graphene channels (Figure 4.1 (a)). Similarly as previously mentioned, the backside
graphene on Cu foil should be removed beforehand to prevent the bilayer transfer or graphene
crumples. RIE was used to pattern the graphene with SPR 220 photoresist as the protective mask.
The photoresist should be carefully removed after the patterning to avoid unexpected
doping/contamination on graphene. The Au electrodes were deposited via thermal evaporation
with a thickness of 60 nm, with a thin layer (2 nm) of Cr to promote the adhesion of Au to the
SiO2/Si substrate. Finally, a biocompatible layer of SU-8 was coated to provide a passivation
layer for the sensing experiment. The final device in Figure 4.1 (a) was passivated with SU-8,
with nine openings where graphene channels will be in contact with water. Figure 4.1 (b) shows
a typical water-gate response recorded from the device fabricated at Figure 4.1 (a), with the
Dirac point of ~0.18 V. Furthermore, we integrated the graphene-based FETs array with a PCB-
chip, to realize fully-integrated graphene sensor platform (Figure 4.2). Single molecules could be
bound to the ultra-thin sensor platform with one-atomic thickness and high surface to volume
ratio, providing high resolution and sensitivity. The fabricated graphene-based FETs could
perform as a biological/chemical sensing materials, as target materials on sensing channels
perturb the carrier transport of underlying graphene. [14-17].
20
Figure 4.1 (a) Optical microscope image of the microdevice with SU-8 passivation layer. The
each source electrode (right) contains nine graphene channels & Au electrodes (left) with four
sources in total, providing a multiplexed sensing platform. (b) current vs. water-gate voltage
relation, with the Dirac point of 0.18V.
Figure 4.2 Graphene-based FETs array integrated with PCB-chip.
21
CHAPTER 5
THREE-DIMENSIONAL GRAPHENE/GRAPHITE FOAM
In this chapter, we expand our discussions to all-carbon 3D structures, so calle
graphene/graphite foams. Park et al. investigated monolithic graphene/graphite synthesis using
heterogeneous catalyst substrate and CVD process [22]. This approach reduced complicated
fabrication steps such as lithography, annealing, etching which are necessary for silicon-based
electronics. We adopted this method for the 3D graphene/graphite structure, using 3D foam of a
Cu / Co catalyst structure.
Graphene foam has been attracting significant research interest due to the extremely low
density and flexibility, and was investigated from several groups for the anode and cathode
materials for the applications such as lithium ion battery [62-67]. As the thickness of graphite
synthesized from Co could be modulated via controlling the thickness of Co layer, we fabricated
graphene/graphite foam, where the thickness of graphite will be gradually increased according to
the thickness of underlying Co layer. The ultimate research goal of this structure is the
investigation on the mechanical/electrical properties of the all-carbon structure with graded
density.
Figure 5.1 shows the foams that consist of Cu and Co heterostructure. The pore size of
the Cu foam was around 600 µm, and Co was deposited onto the Cu foam. Sputtering was used
here to get more conformal coating of Co; sputtering is more appropriate for 3D conformal
coating whereas evaporation is more proper for planar coating. Before the deposition, the Cu
foam was annealed for 20 minutes at 400ºC to increase the grain size and remove the native
oxide layer.
22
Figure 5.1 Optical images of Cu/Co foam before (left) and after (right) the synthesis. Top
figures are the front side of the foam, whereas bottoms are the backside. The patterned
deposition was conducted with aluminum foil, to achieve half-Co and half-Cu foam structure.
The CVD recipe for the monolithic synthesis is shown in Figure B.2. In contrast to the
synthesis onto a catalyst foil, no annealing step was applied to minimize unnecessary Co
diffusion to Cu foam structure. Figure 5.2 shows the Raman spectra, demonstrating the graphene
/ graphite foam were synthesized on the 3D catalyst structure. Here, the Raman intensity for the
graphite foam was much larger, confirming synthesis of graphite (multilayer graphene will add
to higher Raman intensity). The numerical data further confirms that graphene / graphite
heterostructure, with broader FWHM and a slight blue-shift of 2D band in graphite. Figure 5.3 (a)
23
shows the image of as-synthesized graphene/graphite foam on the Cu/Co foam, which is
suspended on the etchant solution for the further processing.
Figure 5.2 Raman spectra for two separated areas, demonstrating graphite and graphene
synthesized on Co and Cu foams, respectively. (c) Numerical data calculated from the Raman
spectra.
To demonstrate the flexibility and low density of the all-carbon structure, the catalyst
structure was subsequently etched to obtain the pure graphene / graphite foam. PMMA was used
to support the graphene/graphite foam during the etching, as the structure easily collapsed down
due to its own weight [66-68]. The as-synthesized graphene/graphite was immersed in PMMA
for 5 minutes, as the conventional spin-coating could not provide an enough thickness for the
supporting layer. After immersing for 5 minutes, it was taken out and baked for 1.5 minutes at
190ºC to evaporate solvents. Figure 5.3 (a) shows the etching process to remove the underlying
Cu/Co foam. Subsequently, the graphene/graphite foam was immersed in acetone to remove the
coated PMMA, followed by replacing the acetone with IPA to completely clean the structure.
Figure 5.3 (b) demonstrates the graphene/graphite foam suspended in IPA without significant
24
damage/collapsing. The color difference further demonstrates the monolithic synthesis of
graphene/graphite and the graded density of carbon structures.
Figure 5.3 (a) Solution etching process with PMMA coating. Na2(SO4)2 solution was used here
to etch the Cu / Co foam. (b) The graphene / graphite foam suspending on DI water.
To further investigate the microstructure, SEM images were obtained from several
locations of the graphene/graphite structure. Figure 5.4 demonstrates the porous structure of
graphene/graphite foams. Future research will be performed to reduce the surface defects and
collapsing, and mechanical/electrical characterization with gradually increasing the thickness of
the graphite.
25
Figure 5.4 SEM images of graphene/graphite foam from several different locations,
demonstrating the porous structure. Scale bar: 100µm.
26
APPENDIX A
SUPPLEMENTARY INFORMATION
Figure A.1 (a) Raman spectra for graphene (red) transferred onto SiO2 / Si substrate, (blue) as-
synthesized on Cu foil, and (black) backside of the Cu foil after the O2 plasma treatment. (b)
Raman spectrum demonstrating no resonant breathing mode (RBM), indicating no CNTs were
synthesized. (c) Numerical data from the Raman spectrum in (a), confirming the backside
etching resulted in monolayer graphene transferred onto SiO2/Si.
27
Figure A.2 The relationship between the engineering strain (εE) versus swelling time (s). Three
types of PDMS with different mixing ratio (PDMS liquid : curing agent) were tested. The
engineering strain drastically increases at the beginning stage and enters the saturation region
after 10 minutes. The time required for the saturation was longer in case of higher mixing ratio
(35:1) sample since the more amount of toluene was supposed to be absorbed to the less cross-
linked PDMS. The shrinkage/crumples could be carefully controlled with desired value of
engineering strain obtained here.
28
Figure A.3 Vapor-phase etching utilizing etchant vapor that sealed in small chamber with
Au/graphene/PDMS. The time required for the complete etching is varied depending on the
thickness of Au film, temperature, etc. Bilayer Parafilms was used to ensure the better sealing.
For a 30 nm-thickness Au with ambient pressure, the etching takes about a week in average.
29
Figure A.4 Optical microscope images of PDMS pyramid arrays with thin Au (35 nm) /
graphene transferred on top. The height/width of pyramids: (a) 20 µm, (b) 30 µm, (c) 40 µm, (d)
50 µm. Scale bars: 100µm.
30
Figure A.5 Comparison of Au-assisted graphene transfer (a,b) to the direct transfer (c,d, without
any supporting layer). (a) Graphene transferred with Au supporting layer with followed etching
of Au showed a conformal coating of graphene on the underlying substrate. (b) Raman spectra
showed enhanced intensity for 3D graphene due to the increase focal volume of laser at the top
of pyramid. (c) Graphene transferred without any supporting layer, showing a significant
electron charging-up due to non-conductive PDMS which was not coated with graphene that is
conductive. (d) Raman spectra further demonstrates the graphene at the top was significantly
damaged, by decreased intensity of 2D band (~2,680cm-1
) compared to the reference PDMS
peaks and nearby planar graphene. Scale bar: 10µm.
31
Figure A.6 Low magnitude SEM image (25º tilted) of direct-transferred graphene on pyramid
arrays. Electron charging was observed (black horizontal line as well as on the pyramids),
indicating the graphene was severely damaged. Scale bar: 20µm.
32
APPENDIX B
EXPERIMENTS
1. Graphene Synthesis
Graphene was synthesized with typical CVD process using CH4, H2 as gaseous
precursors (Figure B.1). The synthesis was conducted onto Cu foil as catalyst layer. The flow
rates were set as 50 sccm and 100 sccm for H2 and CH4, respectively. Rapid cooling was
performed after the synthesis, for 5 minutes at 600ºC. Consequently, the CVD chamber was
cooled down with Ar flow to purge the residual flammable gases.
Figure B.1 The synthesis protocol of graphene with low pressure CVD (LP CVD). After 45
minutes of heating, additional 15 more minutes was supplemented for the temperature at the
33
center to reach the desired temperature. The synthesis was performed for 2 minutes with 20
minutes of annealing. Cu foil with thickness of 25µm was used as catalyst layer.
Figure B.2 further shows the synthesis protocol for the graphene/graphite foams. All
procedures are same except for the annealing step.
Figure B.2 The recipe for low pressure CVD (LP CVD) graphene / graphite foam synthesis.
After 45 minutes of heating, additional 15 more minutes was supplemented for the temperature
at the center to reach the synthesis temperature. The synthesis was performed for 2 minutes
without any annealing step.
34
2. Inverse Pyramid (MOLD) Fabrication
1.1 Silicon Nitride Deposition
Equipment: STS PECVD - MNTL
Recipe: MF (Mixed-frequency)
Thickness: ~100 nm
1.2 Spin Photoresist (NR7-1500P)
Equipment: Spinner 3.2.1
Recipe: dehydration bake
Spin SPR 220– Spin Recipe #3
Softbake: hotplate 60 °C, 2 min + 110 °C, 1min with Al ring
Thickness = ~ 4 µm
1.3 Photolithography of Mask #1 (2.7 um Tip Structures)
Equipment: EV420 (MMS)
Recipe: Exposure
Mode: Hard contact (6 µm separation)
Time: 12sec
Development: 5:1 400K for 90 sec
Rinse with DI Water and dry w/ N-gun
1.4 Hard Bake
Equipment: Hot Plate
Recipe: 120 °C for 10 min with an Al ring
1.5 SiNx Etch
Equipment: PlasmaLab Freon RIE
Recipe: Program 3 (CF4 - Freon 14)
Time : 2 min (30 more sec if SiNx is not fully etched)
Estimated Etch Rate : 70 nm/sec
1.6 PR Wafer clean (Acetone)
Equipment: Solvent Bench
Recipe: Acetone, Time = 10 min
Rinse with DI water and dry with N-gun
1.7 PR Piranha Clean
Equipment: Acid Bench
Recipe: Piranha Solution (H2SO4:H2O2 : 70%:30%)
Temperature: 120oC, Time = 10 min
Rinse with DI water and dry with N-gun
1.8 KOH Etch
Equipment: Base Hood - MNMS
Recipe: 45% Potassium Hydroxide
35
Temperature: ~ 60C
Estimated Etch Rate = ~20 µm/hr
(use SEM to observe the etch rate)
1.9 Isotropic SiNx Wet Etch
Equipment: Wet Bench
Recipe: BOE dip around 3.5 min
Note: Surface will change from hydrophilic to hydrophobic
1.10 SEM
Equipment: SEM
Recipe: Measure tip curvature (if tips are blunt, go back to 1.8)
2. Device Fabrication
6.1 SPR 220
Equipment: Spinner (MNTL)
Recipe: dehydration bake
Spin SPR 220 at 500 rpm for 5sec (acceleration 250 rpm/sec)
and 4000 rpm for 40 sec (acceleration 1000rpm/sec)
Softbake: hotplate 110°C for 90 sec
6.2 Photolithography of Mask #2 (alignment markers) Equipment: Karl Suss Mask Aligner
Recipe: Exposure
Hard contact mode
Expose: 13.5 sec
Development: AZ 400K solution (AZ 400K: DI=1:5), around 45 sec
Rinse with DI water and dry with nitrogen gun
Hardbake: hotplate 110°C for 60 sec
6.3 Au deposition Equipment: Kurt J. Lesker Thermal Evaporator (Nano 36)
Recipe: low deposition rate
Thickness: 60 nm
Time: 1 hour
6.4 Metal Liftoff Equipment: Wet Bench
Recipe: Soak the substrate in Acetone bath at room temperature
Time: 1 hour
6.5 SPR 220 Spin-Coating
Equipment: Spinner (MNTL)
36
Recipe: dehydration bake
Spin SPR 220 at 500 rpm for 5sec (acceleration 250 rpm/sec)
and 4000 rpm for 40 sec (acceleration 1000 rpm/sec)
Softbake: hotplate 110°C for 90 sec
6.6 Photolithography of Mask #3 (graphene channel) Equipment: Karl Suss Mask Aligner
Recipe: Exposure
Hard contact mode
Expose: 13.5sec
Development: AZ 400K solution (AZ 400K: DI=1:5), around 45sec
Rinse with DI water and dry with nitrogen gun
Hardbake: hotplate 110°C for 60sec
6.7 Graphene patterning Equipment: TI Planer Plasma System
Recipe: O2 plasma with 300 W
Time: 2 mins 30 sec
6.8 SPR 220 Equipment: Spinner (MNTL)
Recipe: dehydration bake
Spin SPR 220 at 500 rpm for 5 sec (acceleration 250 rpm/sec)
and 4000 rpm for 40 sec (acceleration 1000 rpm/sec)
Softbake: hotplate 110°C for 90 sec
6.10 Photolithography of Mask #4 (Au electrodes) Equipment: Karl Suss Mask Aligner
Recipe: Exposure
Hard contact mode
Expose: 13.5 sec
Development: AZ 400K solution (AZ 400K: DI=1:5), around 45 sec
Rinse with DI water and dry with nitrogen gun
Hardbake: hotplate 110°C for 60 sec
6.11 Au deposition Equipment: Kurt J. Lesker Thermal Evaporator (Nano 36)
Recipe: high deposition rate
Thickness: 100 nm
Time: 1.5 hour
6.12 Metal Liftoff Equipment: Wet Bench
Recipe: Soak the substrate in Acetone bath at room temperature
Time: 1 hour
37
6.13 SU-8 2002 Equipment: Spinner
Recipe: dehydration bake
Spin SU-8 2005 at 500 rpm for 5 sec (acceleration 250 rpm/sec)
and 4000 rpm for 40 sec (acceleration 1000 rpm/sec)
Softbake: hotplate 65°C for 1 min and 95°C for 1 min
6.14 Photolithography of Mask #5 (SU-8 passivation) Equipment: Karl Suss Mask Aligner
Recipe: Exposure
Hard contact mode
Expose: 9.5 sec
Postbake: hotplate 65°C for 1 min and 95°C for 1 min
Development: SU-8 developer around 90 sec
Rinse with IPA and dry with nitrogen gun
Hardbake: hotplate 65°C for 1 min and 180°C for 5 mins
Cooling: natural cooling on hot plate
6.15 Device final check Equipment: Keithley 2614b
Recipe: N/A
I-V, backgate I-V, and leakage current check
38
APPENDIX C
MATERIALS / EQUIPMENT
1. Materials
A. Cu foil: Copper foil, 0.025mm (0.001in) thick, annealed, coated, 99.8% (metals basis)
(Alfa Aesar).
B. Cu Etchant: Sodium persulfate (reagent grade, ≥98%) (216232-500G) (Sigma
Aldrich).
C. Au Etchant: Gold Etchant GE-8148 (Transene Company).
D. Au Source: GOLD PELLETS, Au, 99.999% PURE, 1/8" DIAMETER X 1/8" LONG
(Kurt J. Lesker).
E. Photoresist: 950 PMMA C 2 (MicroChem Corp), SPR 220 (MicroChem Corp).
F. SiO2/Si Substrate: 3" N/Ph (100) 1-10 ohm-cm 380um SSP Prime with 285nm of
Oxide (Nova Electronics).
G. Polydimethylsiloxane (PDMS): DC 184 SYLGARD 0.5KG 1.1LB KIT (Krayden,
INC.)
2. Experimental Equipment
A. Graphene Chemical Vapor Deposition: Thermal CVD System RMR2000 (Rocky
Mountain Vacuum Tech).
B. Thermal Evaporator: Kurt J. Lesker Nano 36.
C. Sputter: AJA 8-gun DC Metal Sputtering System.
39
D. Reactive Ion Etching: TI Planar O2 Plasma Etcher (For backside graphene etching and
descum), PlasmaLab Freon/O2 Reactive Ion Etcher System (PDMS mold fabrication).
E. Plasma Enhanced Chemical Vapor Deposition: STS Mixed-Frequency Nitride PECVD
System.
F. Plasmatherm Deep Reactive Ion Etching (For the Teflon deposition).
3. Characterization Equipment
A. Optical microscopy: Carl Zeiss Microscopy.
B. Raman spectroscopy: Renishaw Raman/PL Micro-spectroscopy System. Excitation
laser wavelength: 633nm.
C. Scanning electron microscope: Hitachi S-4800 Field Emission Scanning Electron
Microscope.
D. Probe Station: Karl Suss Probe Systems PM8, Keithley 2614b.
40
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