Prevention of Water Permeation by Strong Adhesion Between Graphene and SiO 2 Substrate

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Prevention of Water Permeation by Strong Adhesion Between Graphene and SiO 2 Substrate

Wonsuk Jung , Joonkyu Park , Taeshik Yoon , Taek-Soo Kim , Soohyun Kim ,*

and Chang-Soo Han *

Graphene exists as a two-dimensional sheet of carbon atoms

in a hexagonal lattice structure; it exhibits quantum Hall

effects at room temperature on SiO 2 substrates. [ 1,2 ] Graphene

with massless Dirac fermions can be fabricated on this sub-

strate and is widely used in nanoelectronics devices, such as

fi eld-effect transistors (FETs), [ 3–5 ] infrared nanoscopes, [ 6 ] and

sensors. [ 7,8 ] In devices such as these, graphene deposited by

chemical vapor deposition (CVD) and then transferred onto

the SiO 2 substrate using a polymer fi lm is typically used. [ 9–11 ]

However, the use of a polymer for transfer of the graphene

to the SiO 2 substrate can present several challenges, such as

residue, [ 12,13 ] ripples, [ 14–16 ] charge inhomogeneity caused by

electron–hole puddles, [ 17 ] and weak substrate adhesion by

only Van der Waals forces.

In particular, weak adhesion of graphene to the target

substrate may reduce the reliability of the fabricated devices.

For example, graphene can easily detach from the substrate

during wet chemical processes, such as the development of

a photoresist when fabricating graphene transistors and

nanoscale ribbon structures. [ 18 ] Delamination of the gra-

phene might occur when graphene is used as the electrode in

a fuel cell. [ 19,20 ] Moreover, graphene can be readily wrinkled

and folded when exposed to a high-humidity environment. [ 21 ]

Recently, several studies of graphene adhesion to sev-

eral substrates have been reported. [ 22–27,48,49 ] Among them

are several studies in which adhesion energy of graphene

was measured using deformed graphene on corrugated SiO 2

substrates or nanoparticles, for which the reported adhesion

energies measured using atomic force microscopy (AFM)

were approximately 0.07 Jm −2 [ 25 ] or 0.15–0.45 Jm −2 , respec-

tively. [ 22,24,28 ] Furthermore, the adhesion energy between

monolayer graphene and Cu was found to be 0.72 Jm −2 using

a double cantilever beam (DCB) test. [ 29 ] However, the goal

of these studies was the measurement of adhesion energy, not

methods to improve the adhesion.

In our study, we developed a thermo-electrostatic

bonding (TEB) method to improve adhesion between gra-

phene and SiO 2 substrate. This newly developed bonding

method, which are based on annealing and electrostatic force

under pressing process, allows it to better conform to the

morphology of the target substrate and increases effective

contact area of graphene to the substrate [51–55] . Eventually,

TEB method enhances the adhesion of graphene to the sub-

strate. We evaluated the adhesion of graphene to SiO 2 using

DCB fracture mechanics tests and AFM, and we investigated

the extent to which adhesion energy could be improved using

this approach. In addition, we evaluated water permeation

of the graphene in a high-humidity environment. We found

that bonded graphene (BG) samples that were bonded using

the TEB method did not exhibit water permeation and main-

tained their initial fl atness; in contrast, the properties of non-

bonded graphene (NBG) samples were signifi cantly altered

after exposure to high humidity. We also characterized the

properties of BG and NBG samples before and after expo-

sure to high humidity using resistance measurements, AFM,

and Raman spectroscopy.

We used a TEB process to bond monolayer graphene

to SiO 2 /Si substrates. We used monolayer graphene depos-

ited by CVD on a Cu fi lm, because a large area of uniform

monolayer graphene is needed to properly analyze frac-

ture mechanics using DCB tests and to directly measure

the graphene adhesion energy. Monolayer graphene was

transferred to large-area SiO 2 /Si substrates. A wet transfer

method was used for graphene transfer to the SiO 2 /Si sub-

strates using polymethyl methacrylate (PMMA) and a Cu

etchant, ammonium persulfate (APS-100). SiO 2 thin fi lms of

300 nm thickness on Si substrates were thermally deposited

at 1000 °C to obtain a fl at surface with less than 1 nm rms

roughness. The newly developed TEB process is depicted

in Figure 1 a. The SiO 2 substrate onto which graphene was

transferred was aligned between two Cu electrodes (Jigs).

A Ni substrate, which could not be ionized, was placed on

the graphene to generate a uniform electrical fi eld on the

graphene surface, as shown in Figure 1 a. Next, pressure and

heat of about 3 kgf/cm 2 and 380 °C, respectively, were applied

under vacuum of 50 mTorr. We then applied a voltage of

100 V for 20 min; a safety factor of 1.5 was used to avoid elec-

trical breakdown of SiO 2 at 150 V / 300 nm. The voltage was

applied with the anode and cathode connected to the top of

the Cu electrode on the Ni substrate and bottom of the Si DOI: 10.1002/smll.201302729

Graphene

W. Jung, T. Yoon, Prof. T.-S. Kim, Prof. S. Kim Dept. Mechanical Engineering KAIST , Daejeon , 305–701 , South Korea E-mail: [email protected]

J. Park, Prof. C.-S. Han School of Mechanical Engineering Seoul , 136–713 , South Korea E-mail: [email protected]

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W. Jung et al.

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substrate. The resulting electrical fi eld generated completely

conformal contact of the graphene to the SiO 2 substrates.

This electric fi eld is smaller than the threshold energy for for-

mation of C-O bonds between graphene and SiO 2 from their

original composition, such as C–C or C=C structures.

We used Raman spectra to evaluate the properties of gra-

phene before and after the TEB process. Raman spectroscopy

is useful for characterization of graphene properties, [ 30 ] such

as doping, [ 31 ] defects, and the number of graphene layers. [ 32 ]

We obtained Raman spectra at an excitation wavelength

of 514.5 nm using a High-Resolution Dispersive Raman

Microscope (ARAMIS, Horiba Jobin Yvon). As depicted in

Figure 1 b, we observed no change in the Raman signal of the

CVD-grown NBG (C-NBG) after the TEB process (CVD

grown BG, C-BG). The black line in Figure 1 b corresponds

to the Raman spectrum of a graphene-free area of the SiO 2

substrate, i.e., to SiO 2 only. The red and blue lines corre-

spond to Raman spectra of C-NBG and C-BG. As shown in

Figure 1 b, the graphene exhibits a monolayer structure on the

SiO 2 /Si substrate; [ 33 ] the intensity of the 2D peak at 2695 cm −1

is about twice the intensity of the G peak at 1592 cm −1 for

both C-NBG and C-BG. The D peaks, indicating defects,

maintained similarly low intensities for both C-NBG

and C-BG, respectively. A detailed mapping of intensity

ratios I D /I G and I 2D /I G after bonding is shown in Figure 1 c.

The width and height of the mapping area are 20 × 20 μ m 2 .

The I D /I G intensity ratios were less than 0.13 throughout the

whole area, which indicates that there were no signifi cant

defects formed after bonding. The I 2D /I G intensity ratios were

uniformly approximately 1.75. These mapping results indicate

that no damage was caused by the TEB process and that the

properties of the graphene were not signifi cantly changed.

Moreover, the shifting of G and 2D Raman peaks from

1580 cm −1 and 2680 cm −1 after bonding indicates that there

was no change in the graphene before and after bonding.

After the TEB process, we evaluated the adhesion of the

graphene based on AFM scans of the gap distance between

the monolayer graphene and the SiO 2 substrates. [ 18 ] The

reported distance between graphene and SiO 2 is approxi-

mately 1 nm, [ 24,27,28 ] which is greater than the thickness of

typical monolayer graphene, 0.34 nm. This larger thickness

indicates that there are weak van der Waals interactions

between SiO 2 and graphene. Moreover, PMMA residue on

the graphene surface after wet transfer could increase the dis-

tance between the graphene and the substrate. In our study,

the distance between the graphene and the substrate before

and after bonding was directly measured by AFM. The meas-

ured nominal height of graphene was 1.81 nm, based on the

histogram before bonding, as shown in Figures 2 a and b. This

value is much greater than the thickness of typical monolayer

graphene, 0.34 nm. After TEB, however, the distance at the

same location on the substrate was reduced to 0.91 nm, as

shown in Figures 2 c and d. These results suggest that effects

of annealing and surface charge at the interface generated

by electrostatic force of TEB process induce conformal con-

tact of graphene to the substrate, [ 51–55 ] which increases the

van der Waals force, [ 56 ] and fi nally, TEB method enhances the

adhesion energy of graphene to the substrate.

There AFM-measured adhesion energies of graphene

on nanoparticles and corrugated SiO 2 substrates are about

0.15 ∼ 0.3 Jm −2 and 0.07 Jm −2 , respectively. [ 22–27 ] For measure-

ment of graphene adhesion, several indirect methods have

been proposed; however, these methods are not appropriate

for evaluating large-area graphene. [ 22,24,25 ] In this study, we

Figure 1. Thermo-electrostatic bonding of monolayer graphene. (a) Schematic illustration of the newly developed bonding method. Monolayer graphene transferred to a SiO 2 substrate is conformably attached to the substrate by thermo-electrostatic force. The Ni substrate generates a uniform electric fi eld on the graphene surface. (b) The Raman spectra of the graphene on a SiO 2 substrate before and after bonding. (c) The Raman mappings of the graphene having a width and height of 20 μ m, showing I D /I G and I 2D /I G intensity ratios after bonding. The right two fi gures indicate shifting of G and 2D Raman peaks from 1580 cm −1 and 2680 cm −1 , respectively, after bonding.

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used a DCB fracture mechanics test to directly measure the

adhesion energy of the graphene. The graphene layer could

be directly delaminated by tensile loading; this fracture mode

is a pure mode I, based on tensile load only. A micromechan-

ical test system (Delaminator Adhesion Test System, DTS

Company) was used for the DCB test. The bottom beam was

composed of a graphene/SiO 2 /Si substrate; the upper beam

was a silicon substrate having the same Young’s modulus as

the bottom Si substrate to induce symmetric deformation

during the DCB tests. An adhesive, such as epoxy, is gener-

ally used to affi x the bottom and upper beams for the DCB

test. However, this epoxy could induce inaccurate measure-

ments by permeation of epoxy through cracks in the gra-

phene and SiO 2 substrates. Even large, high-quality graphene

sheets may contain some cracks generated from the wet

transfer or CVD growing processes. Therefore, permeation of

epoxy through the cracks is possible, which would reinforce

the adhesion between the graphene and each substrate and

thus dramatically increase the adhesion energy measured by

the DCB test. Therefore, we modifi ed the conventional DCB

test, deploying instead an aluminum thin fi lm deposited on

the graphene surface, as shown in Figure 3 a. We deposited

an aluminum layer of 50 nm on the graphene surface using

electron beam evaporator before affi xing the upper substrate.

Next, the upper Si substrate was affi xed to the graphene

surface using an epoxy adhesive of about 1 um. These thin

layers of aluminum and epoxy, which have

exceedingly smaller bending stiffness than

the silicon layer of the specimen, don’t

affect symmetric beam deformation as

shown in Table S1. After these two beams

were epoxy-bonded, the loading taps were

attached to each substrate for pulling of

the specimen in the micromechanical test

system. The DCB test procedures were

monitored as shown in Figure 3 b. First,

the specimen was tensile-loaded until the

loading exceeded the adhesion energy

between the graphene and the substrate.

If the load overcame the adhesion energy

of graphene, crack growth occurred at the

interface. This point in crack growth is

defi ned as the critical load, and the crack

begins to propagate. The unloading mode

is held until a certain point is reached, and

then the motion is reversed to increase

the load, thereby initiating crack growth.

Multiple loading/crack growth/unloading

cycles were repeated to accurately

measure the graphene adhesion energy.

After several cycles, we calculated the

adhesion energy based on crack length

and critical load, which were obtained

from the slopes of the curves as shown

in equations S1, 2. The measured adhe-

sion energy was approximately 1.81 Jm −2

and is plotted in the inset of Figure 3 b.

This value is higher than observed for the

C-NBG samples, as shown in supplemen-

tary data, Figure S1a. The C-NBG samples exhibited adhe-

sion energy of only about 0.65 Jm −2 . These results indicate

that the adhesion energy of graphene after the TEB process

increased by a factor of about three. We also investigated the

position of the delaminated graphene after the DCB test.

After the DCB test, NBG samples were delaminated from

the SiO 2 surface and attached to the aluminum deposition

layer, which was affi xed to the upper Si substrate using epoxy.

Figures S1b,c present the Raman spectra and a conceptual

image of the position of the graphene after the DCB test.

We obtained Raman spectra at an excitation wavelength of

514.5 nm. Although no peaks for graphene were observed in

the Raman spectra on the bottom SiO 2 side, Raman peaks

for graphene were detected on the aluminum side, which was

affi xed to the upper substrate. Therefore, in C-NBG sam-

ples, the cracks from the DCB tests were propagated at the

interface between graphene and the bottom SiO 2 substrate,

and graphene was detached from the bottom SiO 2 substrate

and transferred to the upper substrate. In contrast, in BG

samples, a large area of graphene remained on the SiO 2 sub-

strate; in this case, graphene was not delaminated after the

DCB test, as shown in Figures 3 c and d. We also evaluated

the Raman spectra on the aluminum side of the upper sub-

strate and the SiO 2 of bottom substrate, respectively. Raman

spectral peaks for graphene were observed for the SiO 2 side,

but no graphene peaks were observed on the aluminum

Figure 2. Tapping-mode AFM images of the wet transferred monolayer graphene before and after bonding on the SiO 2 substrate. AFM image of monolayer graphene that was transferred onto a SiO 2 substrate by the wet transfer method before bonding [(a) and (b)] and after bonding [(c) and (d)]. The dashed line of boxes in the left of (a) and (c) denote SiO 2 surface wihout graphene. The scale bars are 1 μ m in (a) and (c). Height histograms of graphene from the SiO 2 substrate indicate a 1.81-nm separation before bonding and a 0.91-nm separation after bonding. The Gaussian distribution curve centered at zero (gray line) was obtained on the SiO 2 surface; blue lines indicate graphene, and the black and blue square lines correspond to the original low data.

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side. Moreover, the micro opticalscope image indicates that

a large area of BG remained intact on the SiO 2 bottom

substrate, and only some of the areas were detached to the

upper substrate, as shown in Figure 3 d. These results suggest

that the real adhesion energy of C-BG to the bottom SiO 2

substrate is greater than the measured value. However, this

measured value is composed of two adhesion energies from

interfaces betwen C-BG and SiO 2 or aluminum. Therefore,

we analyzed the correlation between ratio of graphene area

on SiO 2 surface and adhesion energy at each crack length

as shown in Figure S6 and Table S3 to obtain more accurate

value of adhesion energy between C-BG and SiO 2 . Areas

of C-BG, remained on SiO 2 surface after DCB test, were

obtained from the image anlysis by optical microscope. After

that, we analyzed two areas between crack length 2 and 3 as

well as 4 and 5 to calculate adhesion energy of each interface

of C-BG/SiO 2 and C-BG/aluminum.The ratio of C-BG and

SiO 2 area in the fi rst area between crack line 2 and 3 was

obtaining 0.1902 and 0.8098, repectively. The other side, the

second area between crack line 4 and 5, showed smaller ratio

of C-BG area,0.0244 as shown in Table S3. Adhesion energy

for interfaces of C-BG/SiO 2 and C-BG/Aluminum could be

obtained by solving the simultaneous equations based on

the ratios of area and the measured total adhesion energy at

DCB test. The calculated adhesion energies was 2.032 Jm −2

for C-BG/SiO 2 interface and 1.721 Jm −2 for C-BG/aluminum

interface, respectively. Therfore, our TEB process induced

conformal contact of graphene to the substrate, which rein-

forced the graphene adhesion energy.

High-humidity environments induce electrical insta-

bility in devices, especially when devices are exposed to

high humidity for a long time. In this study, we measured

the resistance changes after 160 minutes for four sam-

ples at the same time: two C-BG and two C-NBG samples.

Figure 4 a presents the electrical stability of the samples

when both the temperature and the relative humidity (RH)

were simultaneously changed from 24 °C to 60 °C and from

3% to 90% RH, respectively. Unlike the C-BG samples, the

resistances of C-NBG samples were highly unstable when

the environmental conditions were changed. Generally, the

electrical resistance of graphene decreases as the tempera-

ture increases. [ 34 ] In our samples, however, the resistances

increased as the temperature was increased. This phenom-

enon is closely related to the humidity change; as we previ-

ously reported, graphene resistance increases upon water

adsorption. [ 35 ] As shown in Figure S2, the resistances of

C-NBG samples increased greatly as the RH increased, while

those of C-BG samples exhibited only gradual and small

changes. It was, however, diffi cult to determine the exact

infl uence of RH on the resistances, because the RH fl uctuates

as the temperature changes. Thus, we calculated the current

vapor pressure of water to rule out the effects of temperature

change on the humidity change. Some infl uence was apparent

when the RH changed rapidly, so we noted only the gradient

of the current water vapor pressure change. The gradient

of the current vapor pressure of water was approximately

32.68 hPa/min, deduced from the Goff-Gratch equation at

the moment at which the largest resistance change occurred

(see Figure S2c). This analysis demonstrated that the resist-

ance fl uctuated greatly when the gradient of the current

vapor pressure of water was at least 32.68 hPa/min. To dif-

ferentiate the humidity change effect from the temperature

change effect for large fl uctuations, we evaluated each effect

separately, saturated at each set point, 60 °C and 90% RH, for

Figure 3. Adhesion energy measurement of BGs using the DCB test. (a) Schematic illustration of the DCB test. Deposition of a 50-nm aluminum thin fi lm on the wet transferred graphene prevents the permeation of epoxy into the interface between graphene and the SiO 2 substrate. The specimen can be directly delaminated using a micromechanical test system (Delaminator Adhesion Test System, DTS Company). (b) Multiple loading/crack growth/unloading cycles in the DCB tests are used to determine crack lengths and adhesion energy. Some of the measured crack lengths, a, at each cycle and the measured adhesion energies are expressed. Photographs of each side of the samples after the DCB test are shown in the inset images. (c) Raman spectra of BGs after the DCB test on the bottom of the SiO 2 and on the upper aluminum side. (d) Micro optical scope image of graphene on the bottom of the SiO 2 substrate after the DCB test. A large area of BG remained on the SiO 2 substrate after the test.

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10 minutes, as shown in Figure 4 b. No changes were observed

in either C-BG or C-NBG samples as the temperature

increased, but drastic resistance increases in C-NBG sam-

ples were observed when the gradient of the current vapor

pressure of water was approximately 462.15 hPa/min (see

Figure S2d). We concluded that the humidity change was the

dominant factor in resistance change. These results may have

been due to water molecules that permeated the graphene/

substrate interface and to local defects generated by the cur-

rent fl ow under the high-humidity conditions. Figure 4 c pre-

sents the Raman I D /I G and I 2D /I G ratios after the humidity

test. The average I D /I G peak ratio, which refl ects the amount

of defects in the sample, is about 0.4 for the C-BG samples,

while that of C-NBG samples is about 0.6, which indicates the

presence of more defects in C-NBG samples. The average I 2D /I G

peak ratios are approximately 2.6 and 1.5 for C-BG and

C-NBG samples, respectively. The lower I 2D /I G ratio of the C-NBG

samples indicate more intense p-type doping in the C-NBG

samples compared to the C-BG samples, because the 2D peak

intensity diminishes as doping increases as a result of water

permeation. The extent of doping is in good agreement with

the experimentally found p-doping caused by shifted Fermi

energy [ 36 ] and is also supported by the average blue-shifted

G peak values, which are approximately 5 cm −1 and 10 cm −1

for C-BG and C-NBG samples, respectively, as depicted in

Figure 4 d. [ 36 ] However, although the shifted 2D peak values

observed in this study are similar to those reported in the

literature, [ 36 ] they occur in the opposite direction; [ 37 ] further

research is thus required.

Topographic images of the surface obtained by AFM

(multi-mode, Veeco) were obtained to determine whether

water molecules permeate the space between graphene

and the substrate after exposure to a humid environment.

Exfoliated graphene samples were used for these observa-

tions in order to maximize the water permeation effect by

decreasing the size of graphene regardless of the presence of

polymer used in the graphene transfer process which might

disturb the water permeation under the sheet of graphene.

For these tests, we prepared exfoliated graphene samples

on SiO 2 substrates rather than using CVD-grown samples,

and we subjected half of the samples to the TEB process to

prepare both exfoliated non-bonded and bonded graphene

(E-NBG and E-BG) samples.

Typically, wrinkles, folded areas, and water bubbles on the

underside of the graphene layer of NBG samples can lead to

signifi cant water permeation when the samples are exposed

to 90% RH at room temperature for several days. [ 21 ] We

placed the E-NBG and E-BG samples together in a thermo-

hygrostat and performed the “8585” test (85 °C and 85%

RH) for about 50 hours. Figures 5 a and d present the AFM

topographic images of the E-NBG and E-BG samples before

the test, respectively. As shown in the inset of each Figure, the

Raman I 2D /I G peak ratio is about 2, which indicates the pres-

ence of monolayer graphene samples. In these two Figures,

the areas with blue and yellow dotted contour lines indicate

multi- and monolayer graphene, respectively.

Wrinkles and trapped water bubbles in E-NBG caused

by water permeation are depicted in Figure 5 b. The wrin-

kles are located in the multilayer graphene region, which is

indicated by a red arrow, and the trapped water bubbles are

agglomerated under the monolayer region, which is indi-

cated by a white arrow. The black and red profi le lines in

Figure 4. Electrical stability and Raman spectrascopy analysis of CVD graphene exposed to high humidity conditions. (a) A real time resistance measurement in log scale for the electrical stability of C-NBG and C-BG samples. The temperature and RH were set from 24 °C to 60 °C and from 3% to 90% RH. (b) The same experiment describes in (a), but the temperature and RH were separately saturated. (c) The I D /I G and I 2D /I G values for C-NBG and C-BG samples after the test. The D to G ratio was 0.4 for C-BG samples. (d) The average 2D to G peak ratios were approximately 2.6 and 1.5 for C-BG and C-NBG samples, respectively.

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Figures 5 a and b, respectively, are described in Figure 5 c;

they indicate that the thickness of E-NBG doubled (blue

arrows), compared to the thickness before the test ( ∼ 1nm),

with an approximate 2-nm water bubble height inside

(green arrows). The observed thickness of monolayer gra-

phene (purple arrows) before the test is higher ( ∼ 1 nm) than

reported values, [ 13 ] either because of the reason explained

above or the difference between the free amplitude of the

probes. [ 38 ] The thickness of E-NBG samples more than dou-

bled, even when their thickness before the test was less than

1 nm (see Figure S3). On the other hand, the thickness of

monolayers in the E-BG samples remained unchanged, as

shown in Figure 5 f. These results indicate that the adhe-

sion energy created using thermo-electric bonding of gra-

phene to the SiO 2 substrate is suffi ciently strong to protect

graphene from water permeation, whereas the reported

adhesion energy in NBG is too weak to prevent water

permeation. [ 21 ]

When water molecules trapped under the graphene layer,

it leads to p-type doping of the graphene [ 37,39,40 ] by a charge

transfer from water to graphene. This doping gives rise to

the more carrier concentration in graphene, and non-adia-

batically pushes the Fermi surface up or down depending

on the type of carrier in electronic band. Here, the changed

Fermi surface moves the Kohn anomaly, which is the origin

of the phonon wave vector softening, away from its initial

state near at Γ point in phonon dispersion for un-doped gra-

phene. [50] This absence of Kohn anomaly leads to stiffening

of G peak Raman signal which is accompanied with peak

shifts. [ 36,41 ] Raman spectra for several samples (6 E-NBGs

and 5 E-BGs; inVia Raman microscope, Renishaw) at an

excitation wavelength of 514.5 nm are shown in Figures 6 a

and b, respectively. These spectra were obtained from the

same samples that were subjected to the 8585 test and the

AFM measurements described above. Pairs of spectra of the

same color in Figures 6 a and b correspond to samples before

(dashed) and after (solid) trapping of water in the 8585 test.

The red arrows in both Figures indicated the 2D peak change,

or blue shift, after the test. To investigate the extent of blue

shift, we used the G and 2D peak positions of the spectra

obtained before the test as a reference. Comparison of the

peak positions before and after the test indicated blue shifts

in E-NBG samples of ∼ 15 cm −1 for the 2D peaks and ∼ 2 cm −1

for the G peaks. These results refl ect p-type doping caused by

trapped water, as shown on the left sides of Figures 6 c and d.

In contrast, as shown on the right sides of Figures 6 c and d,

the 2D peak shifts in the E-BG samples is almost zero, and

the G peaks are red-shifted by a minimal amount ( ∼ 2 cm −1 ),

which indicates that water permeation rarely occurred. Here,

the original positions of the 2D peaks of E-BG samples

( ∼ 2690 cm −1 ) are initially blue-shifted compared to those of

the E-NBG samples ( ∼ 2680 cm −1 ). However, these values are

either still within a reasonable regime ( ∼ 2700 cm −1 ), [ 42–45 ] or

the samples are slightly p-doped as a result of compressive

strain. [ 46,47 ] Full width at half maximum (FWHM) values for

the G peaks, as shown in Figure 6 f, remain nearly constant in

both E-NBG and E-BG samples after the test. FWHM values

for the 2D peaks are shown in Figure 6 e. Because of broad-

ening and softening in 2D peaks when the carrier density

increases upon doping, FWHM values of 2D peaks increased

Figure 5. Tapping mode AFM images of water permeation exfoliated graphene. (a) and (d) AFM topographic images of E-NBG and E-BG samples, respectively, taken before the 8585 test with a Raman spectrum of monolayer graphene in each inset. The purple arrows indicate the initial thickness of the E-NBG samples before the test ( ∼ 1 nm). The areas with blue and yellow dotted contour lines indicate multi- and monolayer graphene, respectively. (b) and (e) AFM topographic images after the test. Blue arrows indicate for the thickness of graphene after the test; thickness was about 4 nm for E-NBG sample. Green arrows in (b) indicate the thickness ( ∼ 2 nm) of water bubbles under the graphene layer. The red and white arrows indicate regions with winkles and trapped water bubbles. (c) and (f) Thickness changes in E-NBG and E-BG samples, respectively. Black and red lines correspond to results before and after the test, respectively.

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as doping increases. These results are in accordance with the

experimental results. [ 36 ]

In summary, we developed a TEB method for increasing

adhesion and durability of monolayer graphene by inducing

conformal contact of graphene to the target substrate. We

evaluated the graphene properties before and after this TEB

process by analysis of the Raman spectra. We used a DCB

test to confi rm that the adhesion energy in C-BG samples

was higher than that in C-NBG samples. The adhesion energy

in C-BG samples was 1.81 Jm −2 , whereas the adhesion energy

of C-NBG samples was 0.65 Jm −2 . We also performed AFM

measurements for both E-NBG and E-BG samples before

and after exposing the samples to high-humidity conditions

(8585 test). The thickness of E-BG samples was unchanged,

while that of E-NBG samples changed signifi cantly as a

result of water permeation-induced wrinkles and bubbles.

Three Raman spectroscopy analyses – blue shifts, ratios, and

FWHM peak values – were performed for both E-NBG and

E-BG samples. In E-NBG samples, the G and 2D peaks were

blue-shifted by ∼ 15 cm −1 and ∼ 2 cm −1 , respectively, while

in E-BG samples, signifi cant blue shifts were not observed.

The blue-shift values for the E-NBG sam-

ples were consistent with an I 2D /I G peak

ratio of ∼ 1.7 after the test. Moreover, the

FWHM values for the 2D peak in the

E-NBG samples increased after the test,

indicating broadening and softening as a

result of trapped water doping. Further-

more, in electrical stability tests at high

humidity, E-NBG samples exhibited signif-

icant instability along with a change in the

current vapor pressure of water, while the

E-BG samples exhibited small resistance

changes. Our proposed TEB method will

be valuable for improving the durability of

graphene and nanomaterial devices.

Experimental Section

Growth Method of Graphene : Monolayer graphene was grown on copper (Cu) foil (99.8%, Alfa Aesar, item No.13382) by a thermal CVD system. The Cu foil with 25 μ m thick was synthesized on the foil at 1000 °C by introducing CH 4 :H 2 (105:6 sccm) for 20 min at 0.45 Torr after annealing in a quartz tube at 1000 °C with a H 2 fl ow of 6 sccm for 30 min at low pressure. After the growth of monolayer graphene, the furnace was cooled to room temperature.

Graphene Wet Transfer Process : After the synthesis, the mono layer graphene on one side of the copper foil was removed by the oxygen plasma treatment with 25 sccm for 30 s. The other side was spin-coated with PMMA to support the graphene during wet transfer process. This specimen was fl oated on the surface of ammonium persulfate (APS-100)

for several hours to etch copper foil. This fl oating graphene/PMMA layer was transferred to SiO 2 substrate and then, PMMA layer was removed by chloroform.

Electrical Stability Test in High Humidity : We used a thermo-hygrostat (TH-PE 025, Jeio Tech) for the high humidity environment. For this experiment, we used the CVD grown graphene samples of which size is 0.25 cm 2 in square shape were prepared on SiO 2 substrate. They were connected with two contacts of silver paste at the corners of sample in order for measuring a real time resistance change by Keithely 2400’s source and measure unit (SMU) system. The samples were placed in the chamber in which environment condition was 60 °C and 90% RH (6090 test).

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Figure 6. Changes in Raman spectra of exfoliated graphene due to trapped water. (a) and (b) Raman spectra for E-NBG and E-BG samples respectively. The red arrows indicate 2D peak blue shifts. (c) Histogram of 2D peak blue shifts for E-NBG and E-BG samples, for which the average values are approximately 15 cm −1 and nearly zero, respectively. (d) Histogram of average G peak blue shifts for E-NBG samples ( ∼ 2 cm −1 ) and for E-BGs ( ∼ 2 cm −1 ) in the direction of the red shift, respectively. (e) Histogram of average 2D peak FWHM values. Signifi cant changes in the E-NBG samples were observed, while minimal changes were observed in E-BG samples (f) Histogram of average 2D peak FWHM values; minimal changes were observed.

small 2013, DOI: 10.1002/smll.201302729

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communications


Acknowledgements

This work was supported by Global Frontier Research Center for Advanced Soft Electronics and Nano Material Fundamental Research from MSIP in Korea.

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Received: August 20, 2013Revised: October 10, 2013Published online:

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Prevention of Water Permeation by Strong Adhesion Between Graphene and SiO 2 Substrate