Dielectrophoretic assembly and atomic force microscopymodification of reduced graphene oxideYu Zhang, Lianqing Liu, Ning Xi, Yuechao Wang, Zaili Dong et al. Citation: J. Appl. Phys. 110, 114515 (2011); doi: 10.1063/1.3665212 View online: http://dx.doi.org/10.1063/1.3665212 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v110/i11 Published by the American Institute of Physics. Related ArticlesElectrical conductivity in Li2O2 and its role in determining capacity limitations in non-aqueous Li-O2 batteries J. Chem. Phys. 135, 214704 (2011) Refinement of the theory for extracting cell dielectric properties from dielectrophoresis and electrorotationexperiments Biomicrofluidics 5, 044109 (2011) Current-controlled negative differential resistance due to Joule heating in TiO2 Appl. Phys. Lett. 99, 202104 (2011) Quantitative characterization of acid concentration and temperature dependent self-ordering conditions of anodicporous alumina AIP Advances 1, 042113 (2011) Direct solar energy conversion and storage through coupling between photoelectrochemical and ferroelectriceffects AIP Advances 1, 042104 (2011) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

Dielectrophoretic assembly and atomic force microscopy modificationof reduced graphene oxide

Yu Zhang,1,2 Lianqing Liu,1,a) Ning Xi,1,3,a) Yuechao Wang,1 Zaili Dong,1

and Uchechukwu C. Wejinya1,4

1State Key Laboratory of Robotics, Shenyang Institute of Automation Chinese Academy of Sciences,Shenyang 110016, China2Graduate School of Chinese Academy of Sciences, Beijing 100001, China3Department of Electrical and Computer Engineering, Michigan State University, East Lansing,Michigan 48824, USA4Department of Mechanical Engineering, University of Arkansas, Fayetteville, Arkansas 72701, USA

(Received 19 June 2011; accepted 29 October 2011; published online 8 December 2011)

A simple and controllable method is developed to experimentally study the effects of defects on

reduced graphene oxide (RGO) sheets for nanoelectronics application. First, a deterministic

technique is developed to assemble a single layer graphene oxide sheet onto the gaps of

microelectrodes by optimizing the dielectrophoretic parameters (10 Vpp at 1 MHz for 5 s). This is

followed by the utilization of atomic force microscopy–based mechanical cutting method to form

line defects on RGO sheets. Based on these two procedures, the experimental studies of the effects of

line defects on RGO are investigated, which provides an alternative approach to study the influence

of defects on graphene. The electric transport measurement results show that the electrical

performance of the defected RGO devices generally decrease due to Anderson localization, which

supports the theoretical studies of the influence of defects on the electrical properties of RGO.VC 2011 American Institute of Physics. [doi:10.1063/1.3665212]

I. INTRODUCTION

Graphene, a stable two-dimensional structure, has

attracted tremendous attention worldwide,1,2 taking advant-

age of its unique electrical properties and high crystal quality.

It is being predicted to have numerous potential applications

and may bring revolutionary influence to the next generation

of electronic devices. Although graphene-based FETs,3,4 gas

sensors,5 solar cells,6 etc. have been successfully demon-

strated with excellent properties, the structural defects

induced during fabrication in experiment will significantly

affect the electronic properties of these devices. Conse-

quently, the study of these defects in graphene becomes

extremely important. Previous research on defects in gra-

phene has focused on theoretical studies9–15 of defects, such

as Stone-Wales defects, vacancy defects, heptagon defects,

and pentagon defects. To date, there are three mechanisms to

induce defects on graphene experimentally:16 (1) crystal

growth;17,18 (2) irradiation with energetic particles, such as

electrons19–23 or ions;24–27 and (3) chemical treatment.28 The

main shortcoming of these three methods is that they are not

controllable and, hence, are not able to induce defects at a

specific location. Therefore, it becomes absolutely necessary

to develop a simple and controllable process to experimen-

tally study the effects of defects on graphene for nanoelec-

tronics development and application. Since one route to

obtain graphene is chemical reduction of graphene oxide

(GO) sheets, an alternative approach to study the effects of

defects on graphene can be realized in the following three

steps: The first step is that GO sheets can be deposited

between microelectrodes with high yield and the process is

controllable. Then, GO sheets are reduced to graphene by wet

chemical reduction. Finally, defects are induced in reduced

graphene oxide (RGO) by an AFM-based mechanical cutting

method, which is also highly controllable.

Among the assembling methods in nanotechnology,

dielectrophoresis (DEP) has emerged as a powerful technique

for the selective deposition or directed movement of micro-

and nanoscale entities in non-uniform electric fields. DEP

has been applied to the assembly of nanoparticles,29 carbon

nanotubes,30 DNA,31 and cells.32 Recently, graphene and GO

sheets have been successfully assembled by DEP.33–36 How-

ever, these works focused specifically on the assembly and

not on the layer control. In this paper, GO sheets can be

assembled onto the gap of the microelectrode, with layer con-

trol depending on different dielectrophoretic deposition

parameters. A single-layer GO sheet can be assembled by

optimizing the parameters (10 Vpp at 1 MHz for 5 s).

For the introduction of line defects, several methods

have been proposed, such as catalytic cutting technique,37–39

scanning probe microscope (SPM)-based electric field tailor-

ing technique,40,41 and energy beam cutting method.42,43 The

drawback to these methods is that they all require compli-

cated experimental conditions. In this paper, an AFM-based

mechanical cutting method is developed to introduce line

defects in RGO. This method is highly controllable and sim-

ple to be performed.

a)Authors to whom correspondence should be addressed. Electronic

addresses: lqliu@sia.cn and xin@egr.msu.edu.

0021-8979/2011/110(11)/114515/6/$30.00 VC 2011 American Institute of Physics110, 114515-1

JOURNAL OF APPLIED PHYSICS 110, 114515 (2011)

II. EXPERIMENTS

A. Dielectrophoretic assembly and reduction ofgraphene oxide

Stable aqueous dispersions of GO were obtained from

the Institute of Metal Research (IMR), Chinese Academy of

Sciences. It was synthesized by a modified Hummers

method.44 The original aqueous GO solution was diluted by

1:1 and then sonicated for 4 min at 59 Hz in the Ultrasonic

Oscillator (Shanghai KeDao Limited Company of Ultrasonic

Instrument, SK5210LHC). A droplet (2 ll) of the diluted

solution was dispensed on the freshly cleaved mica. The

prepared sample was dried at room temperature and

then imaged using an AFM (Veeco, Dimension 3100).

Figure 1(a) shows a tapping-mode AFM image of the GO

sheets along with their height analysis. More than 90% of

the sheets were monolayer GO, the height of which is

�1 nm, as shown in Fig. 1(b). This height is similar to mono-

layer GO sheets’ heights reported in previous studies.45,46

Figure 2 depicts a schematic of the DEP setup. The

electrodes of the present study were fabricated by standard

photolithography and lift-off techniques. The gold electrodes

have a thickness of 40 nm and a gap of 1-2.5 lm between

them, as depicted in Fig. 3.

FIG. 1. (Color online) Height images of GO on mica in an AFM tapping mode. (a) GO sheets imaging sonicated for 4 min. Scan size: 70 lm� 70 lm. Data

scale: 10 nm. (b) Height analysis trace, showing the thickness of the GO (white line indicating) in (a) is 1.15 6 0.1 nm.

FIG. 2. (Color online) Schematic of the DEP setup.

FIG. 3. (Color online) Electrode chip schematic.

FIG. 4. (Color online) Schematic of graphene cutting by an AFM scratching

technique.

114515-2 Zhang et al. J. Appl. Phys. 110, 114515 (2011)

A small droplet (0.5 ll)) of the aqueous GO solution

was placed onto a chip, and a sinusoidal potential difference

(5-10 Vpp at 1 MHz) was applied to the bias electrode by a

function generator (Escort, EGC-3230). After 10-40 s, the

generator was switched off and the droplet dried at room

temperature. Then, the chips were characterized using an

AFM. Lastly, the GO sheets were reduced by chemical

reduction. The chips with GO sheets’ bridged gaps were

dipped in 55% hydroiodic (HI) acid for 3-5 min at 100 �C,47

which is performed by the Institute of Metal Research, Chi-

nese Academy of Sciences.

B. Formation of line defects by AFM

The line defects were induced by AFM scratching tech-

niques. The tip is scanned under strong loading forces to

FIG. 5. (Color online) Height images of GO sheets on mica after cutting in AFM tapping mode with their height analysis. (a) Scan size is 11.3 lm� 11.3 lm

and data scale is 30 nm. The thickness is around 1.135 nm. (b) Scan size is 6.9 lm� 6.9 lm and the data scale is 20 nm. The thickness of GO sheet is 1.360 nm.

(c) Size is 16.6 lm� 16.6 lm, data scale is 30 nm. The thickness is about 1.315 nm.

FIG. 6. (Color online) AFM images of three GO devices of various thickness. (a) An AFM image, including height trace, displaying the sheet thickness from

12 to 25 nm. Scan size is 17.4 lm� 17.4 lm and data scale is 150 nm. (b) The thickness is around 5.0 nm, the scan size is 5.5 lm� 5.5 lm, and the data scale

is 250 nm. (c) The height image, scan size is 4.0 lm� 4.0 lm and the data scale is 150 nm. The thickness is 1.3 6 0.3 nm, indicating single layer GO sheet in

the channel. (d) The phase image of (c).

114515-3 Zhang et al. J. Appl. Phys. 110, 114515 (2011)

FIG. 7. (Color online) Current-voltage measurement of the junction before and after reduction. (a) An AFM image of GO junction before reduction, including

height trace, displaying the sheet thickness in the channel is around 25 nm. The scan size is 17.4 lm� 17.4 lm and the data scale is 150 nm. (b) An AFM image

of RGO junction with the height trace. The thickness is down to 5.0 nm. The scan size is 17.4 lm� 17.4 lm and the data scale is 150 nm. (c) Current-voltage

characteristics of the GO sheet before and after reduction, exhibiting an electrical resistance from R ¼ 161 MX (shown by the dash-dot line) down to R¼ kX(indicated by the solid line).

FIG. 8. (Color online) Electrical transport measurement on the RGO sheet before and after cutting. (a) An AFM image of RGO sheet after cutting. The white

circle displays the position of the cutting. The scan size is 10.0 lm� 10.0 lm and the data scale is 150 nm. (b) Current-voltage characteristics of the RGO sheet

before and after cutting. The resistance before cutting is 397.4 kX, which is denoted by the dash-dot line. After cutting, the resistance is up to 1.9730 MX,

which is denoted by the solid line. (c) and (d) Additional result. The scan size is 16.0 lm� 16.0 lm and the data scale is 150 nm. The resistance is 61.2 kXbefore cutting, which is denoted by the dash-dot line. The resistance decreases to 194.5 kX after cutting, which is denoted by the solid line.

114515-4 Zhang et al. J. Appl. Phys. 110, 114515 (2011)

remove the substrate or resist. Normal antimony-doped tips

(type: MPP-11100-10 with a radius of 8-12 nm) with a

normal spring constant 40 N/m (Veeco Company) is used to

perform the experiment. The normal length, width, and

thickness of the cantilever are 125 lm, 35 lm, and 3.75 lm,

respectively. GO sheets were cut automatically using Nano-

Man software of the AFM (in contact mode under feedback

on) by setting different deflection setpoints, which adjusts

the cutting force, as shown in Fig. 4. Figure 5 shows forma-

tion of line defects in GO sheets by an AFM nanocutting

technique.

III. EXPERIMENTAL RESULTS

Figure 6 depicts the tapping-mode AFM images of GO

sheets deposited by DEP along with their height analysis.

Firstly, an AC voltage of approximately 5 Vpp at 1 MHz was

applied to the electrode pair for 20 s. The result shows that

the thickness varies from 12 to 25 nm in the channel

(Fig. 6(a)). This is followed by an AC voltage of 10 Vpp

at 1 MHz applied for 10 s. The result shown in Fig. 6(b) indi-

cates that the thickness is down to 5 nm by AFM analysis.

When the time decreased to 5 s, the thickness of the sheet is

1.3 6 0.3 nm, indicating a single layer GO sheet assembled

in the channel, as shown in Fig. 6(c). It can be seen that it is

possible to deposit a single-layer GO sheet by optimizing

the DEP deposition parameters (10 Vpp at 1 MHz for 5 s).

Figure 6(d) is the phase image of Fig. 6(c), from which a

nanochannel can be clearly seen. This nanochannel would be

used to study quantum computing.

After assembly, GO sheets’ bridged gaps were reduced

by chemical reduction. The current-voltage (I-V) measure-

ments at room temperature and ambient condition are

presented in Fig. 7. The height of the GO sheet decreases

from 25 nm to 16 nm after chemical reduction, as shown in

Fig. 7(a) and Fig. 7(b). Because the chip is dipped in HI

acids for 3-5 min, some GO sheets beside electrodes have

been soaked away. The GO sheets are electrically insulating.

After reduction, GO sheets change from insulating to con-

ducting. The total electrical resistance, consisting of the

sheet and contact resistance, decrease from R ¼ 161 MX to

R ¼ 32 kX (Fig. 7(c)). Good ohmic contacts are verified by

the linear relationship.

The effects of line defects on the electrical properties of

the RGO junction have been investigated. After reduction,

line defects were induced by AFM-based mechanical cutting.

Then, current-voltage measurements were performed. It is

found that all RGO devices show electrical performance deg-

radation after defects induced (see Fig. 8), which is consist-

ent with theoretical studies.48–50 In Figs. 8(a) and 8(b), when

a horizontal line defect was formed in the RGO sheet, the re-

sistance increased from 397.4 kX to 1.9730 MX. When a ver-

tical line defect was introduced, the resistance increases

from 61.2 kX to 194.5 kX, as shown in Fig. 8(c) and

Fig. 8(d), respectively. By using the optimizing DEP param-

eters, the influence of line defect on single layer RGO has

been investigated too. Figure 9(a) displays the height image

of the defected single layer RGO with its height analysis.

From the height analysis, it can be seen that the thickness of

the RGO is around 0.879 nm, indicating that a single layer

GO sheet has been assembled in the channel. The phase

image (Fig. 9(b)) shows more clear details. After inducing a

line defect, the electrical transport measurement is per-

formed immediately. The resistance of the defected single

layer RGO also increases (from 137.7 kX to 291.5 kX), as

shown in Fig. 9(c).

FIG. 9. (Color online) Electrical transport measurement on single layer RGO sheet before and after cutting. (a) An AFM image of single layer RGO sheet after

cutting, including height trace, indicating the thickness is around 0.879 nm. The white circle displays the position of the cutting. The scan size is

16.1 lm� 16.1 lm and the data scale is 80 nm. (b) The phase image of (a). The scan size is 16.1 lm� 16.1 lm and the scale bar is 0� 50 degrees. (c) Current-

voltage characteristics of single layer RGO sheet before and after cutting. The resistance before cutting is 137.7 kX, which is denoted by the dash-dot line. Af-

ter cutting, the resistance is up to 291.5 kX, which is denoted by the solid line.

114515-5 Zhang et al. J. Appl. Phys. 110, 114515 (2011)

IV. CONCLUSIONS

In conclusion, a DEP-based deterministic approach to

assembling GO sheets onto the gaps of microelectrodes with

controllable layers is presented. Single layer GO sheet as-

sembly is demonstrated with optimized dielectrophoretic

deposition parameters of 10 Vpp at 1 MHz for 5 s. This

results in high quality and controllable assembling of GO-

based nanodevices that have the potential for real advance-

ments in GO-related developments. In addition, an AFM-

based mechanical cutting method is developed to form

defects in RGO sheets. The amazing advantage of this

method is that it can realize massive fabrication through the

parallel multi-tip technology, which makes it possible to fab-

ricate defects in RGO at low cost and high efficiency. This

process will ultimately provide opportunities and means for

large scale nanomanufacturing of RGO-based nanodevices.

Furthermore, the effect of line defects on a RGO sheet has

been experimentally investigated based on these two meth-

ods, which shows that all of the resistance increases with the

introduction of defects. This is the first experimental result

and procedure to show the influence of introduced defect on

the electronic properties of RGO and provide solid proof to

the theoretical studies.

ACKNOWLEDGMENTS

The authors acknowledge the support of the National

High Technology Research and Development Program of

China (Grant No. 2009AA03Z316), National Natural Science

Foundation of China (Project Nos. 60904095, 51050110445,

and 61175103), and the CAS FEA International Partnership

Program for Creative Research Teams. Additionally the

authors would like to thank Jinping Zhao and Zhongshuai Wu

for helpful discussions and support.

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