RippleFree Graphene Sheets on Alkanethiol SelfAssembled Monolayers Provided from Unzipped MultiWalled Carbon Nanotubes

The honeycomb structure for the graphene monolayer and “three-for-six” triangular pattern for the multi-layer graphene sheets on C8S SAMs were clearly observed without ripples by the hig

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Delivered by Publishing Technology to: jacob busch IP: 129.15.118.74 On: Thu, 30 Jul 2015 17:14:25 Copyright: American Scientific Publishers

Printed in the United States of America

Article Journal of Nanoscience and Nanotechnology

Vol 15, 1203–1208, 2015

www.aspbs.com/jnn

Ripple-Free Graphene Sheets on Alkanethiol

Self-Assembled Monolayers Provided from

Unzipped Multi-Walled Carbon Nanotubes

Hyunmo Koo1, Nam-Suk Lee2, Hoon-Kyu Shin2, Jaegeun Noh3,

Yutaka Majima4, and Gyoujin Cho1 ∗

1Department of Printed Electronics of World Class University Program, Sunchon National University,

Jeonnam 540-742, Korea

2National Center for Nanomaterials Technology, Pohang University of Science and Technology,

Pohang 790-784, Korea

3Department of Chemistry, Hanyang University, Seoul 133-791, Korea

4Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan

Octanethiol (C8S, CH3(CH2)7SH) self-assembled monolayers/Au(111) were utilized as an inert

sur-face to provide ripple-free graphene oxide layers provided from chemically unzipped multi-walled

carbon nanotubes (MWCNTs) The resulting graphene oxide monolayers were characterized with

atomic resolution by UHV-STM The honeycomb structure for the graphene monolayer and

“three-for-six” triangular pattern for the multi-layer graphene sheets on C8S SAMs were clearly observed

without ripples by the high-resolution UHV-STM These results provide new insight into the

prepa-ration of ripple-free graphene monolayers

Microscopy

1 INTRODUCTION

Graphene is a very promising material for the next

generation in microelectronics, especially for ﬂexible

electronics,1–3 because it has high mechanical

flexibil-ity and excellent conductivflexibil-ity and mobilflexibil-ity properties

However, those great potentials of graphene in

elec-tronic devices have been attenuated due to the

inﬂu-ence of the ambient environment, primarily the substrate

For example, the carrier mobility of graphene fabricated

on SiO2 of Si will be deteriorated by the charge

trap-ping in oxide Furthermore, due to the inﬂuence from

the substrate, such as surface induced ripples, also

gen-erate electron–hole puddles,4 5 the suppression of weak

localization,6 decreased carrier mobility7 and enhanced

chemical reactivity.8–10

Therefore, in order to eliminate the substrate induced

perturbation and ripples on the graphenes, suspended

graphene11 and atomically ﬂat graphene12 on mica have

been reported However, those methods still somewhat

∗ Author to whom correspondence should be addressed.

apart from practically fabricating ripple-free graphene monolayer, called “ultra-ﬂat graphene monolayer,” on sub-strates with minimizing any possible interaction induced from the substrates Here, we have developed self-assembled monolayers (SAMs)13of normal alkyl thiol on gold surface as a general substrate to provide ripple-free graphene while any interaction with the substrates can

be avoided due to inert alkyl group of SAMs By using SAMs, ripple-free and environmentally stable graphene monolayer should be prepared on the any gold evaporated substrate with high reliability using a simple and low cost process

The fabrication method for the ripple-free graphene monolayer with minimizing the interaction from substrates should be simple and costless while the reliability always should be guaranteed so that the method may be used as

a general one to fabricate the ultra-ﬂat graphene mono-layer on any substrates As a consequence of developing the general method, we are employing unzipping chemical reaction of multi-walled carbon nanotubes (MWCNTs) to produce slightly oxidized graphene sheets14 15 and SAMs

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as an inert substrate By unzipping the MWCNTs, the

slightly oxidized graphene sheets (SOG) can be mass

pro-duced, and the dispersed solution of the SOG can be

sim-ply fabricated onto the SAMs on gold evaporated substrate

This method will be used in the near future as a general

costless method to directly fabricate graphene monolayers

on any gold evaporated substrate Furthermore, by

exploit-ing SAMs, not only the direct interaction of the substrate

to the graphene oxide monolayer can be minimized but

also variations in the surface structure of SAMs will be use

tailor the electrical properties of the graphene monolayers

for using in various nanoscale applications.13 16 17

In this work, we would like to demonstrate a

gen-eral method for fabricating SOG monolayers without

any ripples or deformations using octanethiol (C8S,

CH3(CH2)7SH) SAMs on substrates We prepared

mono-and multi-layer of SOG on C8S SAMs by simple

drop-casting of the SOG dispersed solution prepared by

chemically unzipped MWCNTs The resulting SOG layers

were investigated using ultrahigh vacuum scanning

tunnel-ing microscopy (UHV-STM) to elucidate the ultra-ﬂat and

ripple-free SOG monolayer with the honeycomb structure

and the “three-for-six” triangular structure for the

multi-layers without any ripples

2 EXPERIMENTAL DETAILS

Unzipping reaction of MWCNTs was carried out using

exactly same procedures in the report from Tour’s group15

except using MWCNTs provided from Hanhwa Nanotech

Co (CM-95, Korea) with 10–15 nm of mean diameter The

dispersed ethanol solution of SOG sheets was prepared

with 20 mL of ethanol and 8 mg of SOG sheets through

sonication for 30 min The resulting solution was ﬁltered

using 0.2m ﬁlter before drop casting on the SAMs on

Au(111) substrate using a micro-syringe

The Au(111) substrates were fabricated by vacuum

deposition of gold onto freshly cleaved mica sheets

pre-baked at 300C with a base pressure of 10−7to 10−8Torr

After deposition, the substrates were annealed at 350 C

in the same vacuum chamber for 2 h The substrate was

brieﬂy ﬂame-annealed and quenched in ethanol to form an

atomically ﬂat Au(111) surface

The Au(111) substrate was immersed in a 1 mM

solu-tion of octanethiol (C8S) (Sigma Aldrich, USA) in ethanol

at 50C for 12 h and the samples were rinsed with ethanol

two times and dried in a pure N2ﬂow Au(111) substrate

is not only widely used for the bottom electrode in

elec-tronic devices, but it is also used for the STM

character-ization The resulting partially oxidized graphene sheets

casted substrate was dried at room temperature for 10 min

in a desiccator Then, the sample was introduced into the

UHV-STM (Omicron, Germany)

Scanning tunneling microscopy (STM) image

obser-vations were carried out using a Micro-STM (Omicron,

Germany) with the tungsten tip prepared by the

electrochemical etching method All STM images were obtained in ultrahigh vacuum (39 × 10−11 torr) using a constant current mode at room temperature Bias voltages ranging from− 500 to 500 mV were used to get images while the tunneling currents between the tip and the sam-ple were kept in the range from 20 to 500 pA

3 RESULTS AND DISCUSSION

Figure 1(a) shows TEM images of as-prepared few-layer SOG from the unzipping reaction of MWCNTs Figure 1(b) is the high-resolution TEM image of the red box in Figure 1(a) Hexagonal structures of the graphene are conﬁrmed Fourier transform of image (Fig 1(c)) shows the typical hexagonally arranged lattice of carbon

in graphene

To further characterize unzipped SOG, X-ray photoe-mission spectroscopy (XPS; Fig 1(d)) was used to conﬁrm the degree of oxidation on unzipped SOG by checking the intensity of C–O–C, C O and COOH signals in the broad ranges of 286 eV to 289 eV Based on XPS results, the resulting graphene sheets would be less densely oxidized because the intensity of C–O–C, C O and COOH peaks (Fig 1(d)) is a lot weaker than fully oxidized graphene sheets where ratio of C–C peak is almost same to those of C–O–C (including C O) peaks.15

Figure 2 shows the STM images of the C8S SAMs, which were used for the buffer layer between the SOG monolayer and the Au(111) substrate

STM images of C8S SAMs in Figure 2(a) show the typical features of a gold surface covered by alkanethiol SAMs such as etch pits and domain boundaries The depth

of the pits was 0.25 nm, which is close to the monatomic step height of the Au(111) surface, and therefore, these pits were assigned to the vacancy islands (VIs) on the gold sur-face The molecularly resolved STM image in Figure 2(b) presents the well-known (√

3×√3)R30 structure and

c(4 × 2) super lattice showing a difference in

molecu-lar brightness in the unit cell and clear domain bound-aries The high resolution STM image (7 nm× 7 nm) of the Au(111) surface showed the (√

3×√3)R30structure, which is known to be a typical structure of C8S SAMs,

as shown in Figure 2(c) The (√

3×√3)R30 unit cell is indicated by the rhombus with black lines Each side of the rhombus was determined to be 2.88 Å×√3= 499 Å The

molecularly resolved STM image in Figure 2(c) clearly shows well-ordered C8S SAMs with a densely packed structure and an atomically uniform surface

Figure 3 show STM images of the unzipped SOG on C8S SAMs In Figure 3(a), point A and B indicate the STM images (scan size: 100 nm× 100 nm) of the mono-and the bi-layer of unzipped SOG The measured height difference between the area A and B is≈ 0.35 nm which

is well matched with the value of the graphene mono-layer but not from graphene oxide monomono-layer.19 Further-more, the SOG from the other spots also show the same

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(c)

295 290 285 280 0.0

0.2 0.4 0.6 0.8 1.0

Binding energy (eV)

(d)

C-C

C-O C=O COOH

Figure 1 (a) TEM image of few-layer SOG sheets on the carbon coated copper grid (800× 800 nm 2 ) (b) High-resolution TEM image of the SOG sheet (c) Fourier transform image of the hexagonal type which is indicating the graphene lattice (d) XPS analysis of unzipped SOG sheets.

thickness of 0.35 nm Those results could be proved

from XPS results on unzipped SOG which may be

par-tially oxidized only at the edges Even though exfoliated

graphenes on a solid substrate are known to have ripples

due to the Van der Waals interaction with the substrate,20–22

the unzipped SOG on C8S SAMs/Au(111) are

charac-terized as the mono- and bi-layers SOG without ripples

Figure 2 STM image (V b = 05 V, I = 500 pA) of the C8S SAMs The STM image in (a) shows the typical features of a gold surface covered with

an alkanethiol SAM such as etch pits and domain boundaries Both depths of the pits are around 0.25 nm, which is close to the monatomic step height

of the Au(111) surface High-resolved STM image in (b) presents the well-knownc(4 × 2) superlattice with the difference in molecular brightness in

the unit cell and clear domain boundaries The high resolution STM image (7 nm ×7 nm) of the Au(111) surface shows the (√3 ×√3)R30 structure,

as shown in (c), which is known to be another typical structure of C8S SAMs The ( √

3 ×√3)R30 unit cell is indicated by the rhombus with black lines Each side of the rhombus measures 2.88 Å×√3= 499 Å.

over the STM scan areas which ranged over 100 nm by

100 nm

Figures 3(b) and (c) are high-resolution STM images

of a mono- and bi-layer SOG shown in Figure 3(a) Figure 3(b) shows a constant current STM image of the monolayer SOG on the area A in Figure 3(a) As shown

in Figure 3(b), the honeycomb has the inter-carbon length

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A

B

(a)

100 nm

(b)

(c)

7 nm

a b

(d)

Graphene sheet

Height (nm)

OT SAMs

2.5 nm

3 nm

0.000 0.002 0.004 0.006 0.008

–0.08 –0.04 0.00 0.04 0.08

Figure 3 Images for STM topographs; (a) SOG bilayer (100 nm× 100 nm) on C8S SAMs/Au(111) measured with values of V b = 05 V and

I = 40 pA The black line shows the cross-sectional proﬁle in inset The measured height of SOG monolayer is ≈ 0.35 nm, consistent with the

measured height of the SOG bilayers in (a) (b) Constant current STM image (V b = −05 V, I = 60 pA) of the SOG monolayer measured at the

area A in (a) As shown in (b), the honeycomb has the side-length of 0.14 nm, corresponding to the size of one hexagonal carbon ring (c) STM image (V b = −05 V, I = 60 pA) of the bilayer SOG measured at the area B in (a) Illustrated hexagon with three circles is superimposed to highlight

the graphene honeycomb lattice The insets in each ﬁgure show the 7 nm × 7 nm STM images of the (b) and (c), respectively (d) Histogram of the height values acquired from the STM images of graphene and C8S SAMs (red and blue squares represent the SOG monolayer and the C8S SAMs, respectively) The histograms are well-described by Gaussian distributions with standard deviations of 0.037 and 0.035 nm for SOG monolayer and C8S SAMs, respectively.

of 0.14 nm, corresponding to the size of one

hexago-nal carbon ring Figure 3(c) shows a STM image of the

bilayer SOG measured on the area B in Figure 3(a) The

illustrated hexagon with three circles is superimposed to

highlight the graphene honeycomb lattice and the three

carbon atoms imaged as protrusions The insets show the

7 nm× 7 nm STM images of the Figures 3(b) and (c),

respectively

Our experimental results in the range of 7× 7 nm2area

show that the SOG has no ripples on the C8S SAMs which

is used as the buffer layer between the SOG monolayer

and Au(111) substrate The height variation of 0.05 nm,

which is a very small value compared with the

previously-reported values of 0.19 nm,210.5 nm,20and 0.78 nm,23was

conﬁrmed by the section analysis as presented on the STM

images in Figure 3 The height variation is also comparable

with the histograms based on STM images of 7 nm×7 nm

of a SOG monolayer including 512× 512 measurement

points each The apparent height histogram of SOG mono-layer and C8S SAMs was shown in Figure 3(d) The histogram of SOG monolayer (red) is well-described by Gaussian distribution with a full width at half maximum (FWHM) of 0.037 nm, while C8S SAMs (blue) exhibits a FWHM of only 0.035 nm The Gaussian ﬁts of the SOG and the C8S SAMs coincide with each other in the error range of± 0.002 nm This means that the C8S SAMs are well-formed with the densely packed structure and uni-form at the atomic level Consequently, SOG monolayers are deposited onto the substrate without ripples Previ-ous reports show that the height variation of the graphene monolayer on a SiO2 substrate is 0.19 to 0.78 nm.20 21 23

On the other hand, our experimental results show that the height variation of the SOG monolayer on the C8S SAMs decreased to 0.037 nm By the effect of the C8S SAMs, ripple-free ultra-ﬂat SOG monolayers are formed in the range of 7× 7 nm2area

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Figure 4 STM images (V b = −05 V, I = 20 pA) of the SOG monolayer without ripples on the C8S SAMs and the edge structure (a) The STM

image of the SOG monolayer shows the honeycomb structure in which the section indicated by an arrowhead mark shows a slightly overlapped section

at the edge in the SOG monolayer (b) The enlarged image of the white dot square presented in (a) (c) The STM image at the boundary between the SOG monolayer and the SOG bilayer The image enlarged at the black dot square is presented in (d) As illustrated in the STM image, it can be veriﬁed that the edge structure of the SOG bilayer is a zigzag pattern.

The edge structures of unzipped SOG monolayer have

also been studied Figure 4 shows the STM images of

the SOG monolayer and their edge structure without

rip-ples on the C8S SAMs Figure 4(a) represents the

hon-eycomb structure of the SOG monolayer at the upper

and lower area Here, the section indicated by an

arrow-head mark shows a slightly overlapped image of the

edge of the SOG monolayer In this study, we

deter-mined SOG monolayer using the cross-sectional analysis

(below 0.1 nm height) Figure 4(b) shows the magniﬁed

STM image of the edge structure of the SOG

mono-layer of the white dot square in Figure 4(a) As in the

pre-reported studies,13 the SOG unzipped with the

longi-tudinal direction of the CNTs shows a zigzag structure

at the edge Here, the edge structure shows an

arm-chair as shown in Figure 4(b) Figure 4(c) shows the

STM image of the honeycomb structure of the

mono-layer and “three-for-six” triangular pattern of the bimono-layer

SOG at the center and left lower area, respectively The

enlarged image of the black dot square is presented in

Figure 4(d) As illustrated in the STM image, it can be ver-iﬁed that the edge structure of the SOG bilayer is a zigzag pattern

4 CONCLUSION

We demonstrated that ripple-free SOG monolayers were prepared from chemically unzipped SOG on C8S SAMs and characterized by using TEM and STM We observed the honeycomb and “three-for-six” triangular patterns at

a mono- and bi-layer of SOG on C8S SAMs using high resolution STM (scan size: 7 nm× 7 nm) The mea-sured height variation of SOG monolayer on C8S SAMs

is 0.037 nm, which is much smaller than those previ-ously reported Because of the C8S SAMs, as the buffer layer between the SOG monolayer and Au(111) substrate, the additional deformation would be minimized Conse-quently, for the development of future graphene-based nano-electronic devices, those attained results would pro-vide valuable information to understand the variations in

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electrical properties originated from the ripple formation

in the near future

Acknowledgment: This work was supported by the

Ministry Of Trade, Industry and Energy (MOTIE) through

the project of GTFAM Regional Innovation Center (RIC)

This work is also supported by a Grant-in-Aid for

Sci-entiﬁc Research on Innovative Areas (No 20108011,

pi-space) from MEXT, Japan; the Global Centers of

Excel-lence (COE) Program of “Photonics Integration-Core

Elec-tronics,” MEXT; and Collaborative Research Project of

Materials and Structures Laboratory, Tokyo Institute of

Technology

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Received: 13 June 2013 Accepted: 1 November 2013

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