We elucidate on the low mobility and charge traps of the chemically reduced
graphene oxide (RGO) sheets by measuring and analyzing temperature dependent
current-voltage characteristics. The RGO sheets were assembled between source
and drain electrodes via dielectrophoresis. At low bias voltage the conduction
is Ohmic while at high bias voltage and low temperatures the conduction becomes
space charge limited with an exponential distribution of traps. We estimate an
average trap density of 1.75x10^16 cm^-3. Quantitative information about charge
traps will help develop optimization strategies of passivating defects in order
to fabricate high quality solution processed graphene devices.Comment: 6 pages, 3 figures, 1 tabl

Chunder, A.

Joung, Daeha

Khondaker, Saiful I.

Zhai, Lei

Applied Physics Letters

English

arXiv

Daeha Joung

A. Chunder

Lei Zhai

Saiful I. Khondaker

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Space charge limited conduction with exponential trap distribution in reduced graphene oxide sheets

We elucidate on the low mobility and charge traps of the chemically reduced graphene oxide (RGO) sheets by measuring and analyzing temperature dependent current-voltage characteristics. The RGO sheets were assembled between source and drain electrodes via dielectrophoresis. At low bias voltage the conduction is Ohmic while at high bias voltage and low temperatures the conduction becomes space charge limited with an exponential distribution of traps. We estimate an average trap density of 1.75 x 10(16) cm(-3). Quantitative information about charge traps will help develop optimization strategies of passivating defects in order to fabricate high quality solution processed graphene devices

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University of Central Florida STARS Faculty Bibliography 2010s Faculty Bibliography 1-1-2010 Space charge limited conduction with exponential trap distribution in reduced graphene oxide sheets Daeha Joung University of Central Florida A. Chunder University of Central Florida Lei Zhai University of Central Florida Saiful I. Khondaker University of Central Florida Find similar works at: https://stars.library.ucf.edu/facultybib2010 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 2010s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Joung, Daeha; Chunder, A.; Zhai, Lei; and Khondaker, Saiful I., "Space charge limited conduction with exponential trap distribution in reduced graphene oxide sheets" (2010). Faculty Bibliography 2010s. 318. https://stars.library.ucf.edu/facultybib2010/318 Appl. Phys. Lett. 97, 093105 (2010); https://doi.org/10.1063/1.3484956 97, 093105© 2010 American Institute of Physics.Space charge limited conduction withexponential trap distribution in reducedgraphene oxide sheetsCite as: Appl. Phys. Lett. 97, 093105 (2010); https://doi.org/10.1063/1.3484956Submitted: 10 June 2010 . Accepted: 31 July 2010 . Published Online: 01 September 2010Daeha Joung, A. Chunder, Lei Zhai, and Saiful I. KhondakerARTICLES YOU MAY BE INTERESTED INSpace-Charge-Limited Currents in Organic CrystalsJournal of Applied Physics 33, 205 (1962); https://doi.org/10.1063/1.1728487Trap density in conducting organic semiconductors determined from temperaturedependence of  characteristicsJournal of Applied Physics 94, 1283 (2003); https://doi.org/10.1063/1.1582552Space-charge limited conduction with traps in poly(phenylene vinylene) light emittingdiodesJournal of Applied Physics 82, 6326 (1997); https://doi.org/10.1063/1.366523Space charge limited conduction with exponential trap distributionin reduced graphene oxide sheetsDaeha Joung,1,2 A. Chunder,1,3 Lei Zhai,1,3 and Saiful I. Khondaker1,2,4,a1Nanoscience Technology Center, University of Central Florida, Orlando, Florida 32826, USA2Department of Physics, University of Central Florida, Orlando, Florida 32826, USA3Department of Chemistry, University of Central Florida, Orlando, Florida 32826, USA4School of Electrical Engineering and Computer Science, University of Central Florida, Orlando,Florida 32826, USAReceived 10 June 2010; accepted 31 July 2010; published online 1 September 2010We elucidate on the low mobility and charge traps of the chemically reduced graphene oxide RGOsheets by measuring and analyzing temperature dependent current-voltage characteristics. The RGOsheets were assembled between source and drain electrodes via dielectrophoresis. At low biasvoltage the conduction is Ohmic while at high bias voltage and low temperatures the conductionbecomes space charge limited with an exponential distribution of traps. We estimate an average trapdensity of 1.751016 cm−3. Quantitative information about charge traps will help developoptimization strategies of passivating defects in order to fabricate high quality solution processedgraphene devices. © 2010 American Institute of Physics. doi:10.1063/1.3484956Nanostructures of graphene oxide GO have created alot of attention as it provides a pathway to produce largequantities of graphene sheets in solution at low cost.1–13 Theeasy processibility of GO and compatibility with varioussubstrates including plastics makes them an attractive candi-date for high yield manufacturing of graphene based elec-tronic devices. However, GO is an electrically insulating ma-terial consisting of a large number of C–O bonds. Removalof C–O bonds by chemical and/or thermal reduction tech-nique produces reduced graphene oxide RGO sheets andallows one to restore the electrical properties. RGO sheetshave been used for field effect transistors FETs,3–9 chemicalsensors,10organic solar cells,11 as well as transparent elec-trodes in photovoltaic devices.12,13 However, the electricalconductivity and field effect mobility values for RGO sheetsare much inferior to that of pristine graphene. This has beenattributed to a large amount of disorder present in the RGOsheets. Disorder in RGO sheets have mostly been character-ized with temperature dependence of resistivity where vari-able range hopping and activated hopping have beenobserved.6–8 Such a study does not provide detailed informa-tion about the nature and density of charge traps in the sys-tem. A detailed knowledge of charge conduction and quanti-tative information about charge traps is important for furtheroptimization of reduction techniques and synthetic strategiesfor increasing conductivity and mobility of RGO towardpristine graphene. Similar strategies have been successfullyused for passivating defects in silicon to fabricate high qual-ity complementary metal oxide semiconductor circuits.In this paper we present measurements and analysis oftemperature dependent current density J-voltage V char-acteristics to elucidate on the nature and density of chargetraps in the RGO sheets. The GO sheets suspended in waterwere reduced chemically in the solution and then assembledbetween prefabricated gold source and drain electrodes viaac dielectrophoresis DEP. We show that the J-V character-istic of the RGO devices measured at different temperatures295 to 77 K follow power law behavior, JVm. At low bias,the conduction is Ohmic with m=1, while at higher biasvoltages m increases from 2 to 3 with reducing temperaturesignifying space charge limited conduction SCLC with atransition from trap free TF-SCLC regime at room tem-perature to exponentially distributed trap EDT-SCLC re-gime at low temperatures. We estimate an average trap den-sity of 1.751016 cm−3 for our samples.RGO sheets were synthesized through a chemical reduc-tion of GO in solution prepared by modified Hummersmethod.14 Oxidized graphite in water was ultrasonicated toachieve GO monolayer sheets followed by centrifugation for30 min at 3000 rpm to remove any unexfoliated oxidizedgraphite. The pH of GO dispersion in water 0.1 mg/mLwas adjusted to 11 using a 5% ammonia aqueous solution.15 L of hydrazine solution 35% in N,N-dimethylformamide was then added and the mixture washeated at 95–100 °C for 1 h. X-ray photoelectron spectros-copy spectra show effective reduction in GO see Ref. 15.Devices were fabricated on heavily doped silicon Si sub-strates capped with a thermally grown 250 nm thick SiO2layer. Source and drain electrode patterns with a channellength of 500 nm and width of 500 nm were defined byelectron beam lithography and thermal deposition of 5 nm Crand 20 nm Au followed by lift off.The RGO sheets were assembled between the source anddrain electrodes via DEP. Figure 1a shows a cartoon of theDEP setup. Details of the assembly were presented in ourprevious publication.9 In brief, a small drop of RGO solutionwas placed onto the electrodes pattern. An ac voltage of ap-proximately 3VP-P at 1 MHz was applied for 20–30 s afterwhich the solution droplet was blown off by nitrogen gas.Figure 1b shows tapping-mode atomic force microscopeAFM image of a representative devices with the heightanalysis shown in Fig. 1c. It can be seen that the thicknessvaries from 2 to 8 nm in the channel indicating that up toeight layers of RGO sheets have been assembled. The de-vices were then annealed in Ar /H2 gas at 200 °C for anaAuthor to whom correspondence should be addressed. Electronic mail:saiful@mail.ucf.edu.APPLIED PHYSICS LETTERS 97, 093105 20100003-6951/2010/979/093105/3/$30.00 © 2010 American Institute of Physics97, 093105-1hour, and loaded into a variable temperature cryostat afterbonding. Temperature dependent electronic transport mea-surements from 295 to 77 K were performed using a Kei-thley 2400 source-meter, and a current preamplifier DL1211 capable of measuring subpicoampere signal interfacedwith LABVIEW program. A total of nine devices were inves-tigated.Figure 1d shows the room temperature transfer charac-teristics of a representative RGO device device A where thecurrent I is plotted as a function of gate voltage Vg mea-sured in vacuum. The gate voltage was scanned from 40 to+40 V with a fixed source-drain voltage of V=0.5 V. Thedevice shows an ambipolar FET behavior while. The roomtemperature hole mobility and the carrier concentration werecalculated as 0.71 cm2 /V s and 121012 cm−2, respec-tively. The FET properties are similar to other studies re-ported previously.3–9 However, recent report suggests that themobility can be significantly higher when the size of RGO isbig and reduction is done at very high temperature.16,17Figures 2a and 2b show the I-V characteristics atvarious temperatures measured from 5 to 5 V with Vg=0for device A. The magnitude of the applied bias was limitedto 5 V to avoid electrical breakdown of the RGO devices.The I-V curves are highly symmetric at all temperatures. Inaddition, the I-V curve became increasingly nonlinear withdecreasing temperature. The nonlinearity in I-V curves indisordered semiconductor systems has been explained usingi Schottky barrier SB between metal electrode and semi-conductor, ii Fowler–Nordheim FN tunneling, and iiiSCLC. In general, the I-V curve of a metal-semiconductor-metal junction should be highly asymmetric if the SB isdominated. However, our I-V curves are highly symmetricsee Fig. 2b giving evidence that charge transport is notdominated by SB. Additionally, the attempt to fit our datawith the SB was not successful not shown here. FN tunnel-ing can be ruled out as in FN model,18 the I-V curves areindependent of temperature while our I-V curves are highlysensitive to temperature.We now discuss SCLC mechanism. SCLC occurs in lowmobility semiconductors when injected charge density ex-ceeds the intrinsic free carrier density of the material.Analysis of the J-V characteristics using the SCLC modelis one of the experimental methods for the detection ofcharge trap states in disordered semiconductors. In the ab-sence of any trap states or when trap states do not dominatethe transport, the J-V characteristics are described using,J=90rV2 /8d3 where 0 is the permittivity of freespace, r the dielectric constant of the RGO,  is thecharge carrier mobility, and d is the spacing betweenelectrodes.19 However, in presence of trap states that areexponentially distributed in energy, the J-V relationship isgiven by J= Nv /ql−12l+1 / l+11/l0 /Ntl / l+1lVl+1 /d2l+1 where, l is an exponent and should begreater than 1, q is the electronic charge, Nv is effectivedensity of states, and Nt is the trap density.20 By plottingJ and V in log-log scale one can determine the value ofexponent m. For trap free TF-SCLC regime, m=2 whilefor exponentially distributed trap EDT-SCLC regime m= l+12.Fig. 2c shows J versus V curves at different tempera-ture in a log-log scale for sample A. The dotted symbols arethe experimental data points and the solid lines are a fit toJVm. At low voltages, m equals to 1 at all temperaturessignifying Ohmic conduction. However, at higher voltages,m deviates from 1. Vth, the onset voltage where log J-log Vcurves begin to inflect to higher slope region, shifts to lowervoltage with decreasing temperature. For VVth, m=2 atroom temperature implying TF-SCLC regime. However, asthe temperature is reduced, the value of m for VVth isincreased to 2.3 at 200 K, 2.6 at 150 K, 2.8 at 100 K, and 3at 77 K. The J-V curves for higher values of V are moreclearly shown in Fig. 2d. For these temperatures, the con-duction is governed by traps that are exponentially distrib-uted in energy EDT-SCLC.A transition from TF-SCLC regime at room temperatureto EDT-SCLC regime at low temperature can occur for thefollowing reason: At room temperature, the free career den-sity is higher than the trap density. However, as the tempera-ture is reduced, free career density is also reduced and thetrap states started to dominate by localizing charge carriersand we enter EDT-SCLC regime with m2.FIG. 1. Color online a Cartoon of DEP assembly setup. b Tapping-mode AFM of a RGO device assembled via DEP. Scale bar represents 500nm. c Height profile of the AFM image shown in Fig. 1b. d Roomtemperature transfer characteristic of device A. FIG. 2. Color online a Current-Voltage I-V characteristics at differenttemperatures for device A. b I-V curves at different temperatures plotted insemi log scale to clearly show highly symmetric nature of the curves. cCurrent density J vs V plotted in a log-log scale for different temperatures.The symbols are the experimental points while the solid lines are fits toJVm. Two regions are separated by Vth. For VVth, m=1 and the conduc-tion is Ohmic. VVth the conduction is due to SCLC with exponentiallydistributed trap states. d Expanded view of the SCLC regime. For clarity,J at 77 K was divided by 1.2.093105-2 Joung et al. Appl. Phys. Lett. 97, 093105 2010More quantitative information about traps can be ob-tained by extrapolating J-V curves in voltage. If charge trapsare distributed in energy, they will be gradually filled withincreasing electric field at all temperatures and at a certaincritical voltage, all traps will be filled. According to Kumaret al.21 this critical voltage is independent of temperature andis given by Vc=qNtd2 /2r0. By extrapolating the J-V curveswe can obtain values for Vc and calculate trap density Nt. InFig. 3, we extrapolate the log J and log V characteristic athigher bias voltages for all temperatures except room tem-perature. The Vc=10 V was found for device A. In order tocalculate the trap density from Vc, we need to know thedielectric constant r of chemically RGO. The value of rcan be calculated by using; r=n2−k2, where n and k arerefractive index and extinction coefficient of RGO film, re-spectively. Using the values of n and k in the thermally re-duced GO,22 we calculate r=3.5. Using this value, we ob-tain trap density Nt=1.541016 cm−3 for A.Similar EDT-SCLC regime at low temperatures was ob-served in all nine samples that we have measured. In Ref. 15Table I, we summarize all measured RGO devices wherewe tabulate room temperature hole mobility, Vc and Nt val-ues. All measured devices show similar trap density. Theaverage trap density was about 1.751016 cm−3.An exponential distribution of trap in energies is ex-pected for the traps originating from surface defects andstructural disorders.23 When GO is produced via oxidation,some C–C bonds can get broken creating vacancies.24 Dur-ing the reduction process, carbon can also get removed fromthe GO backbone.25 Although, this has been theoreticallyshown for thermal reduction cases, however, such study forchemical reduction cases are yet to be done. In addition, linedefects such as wrinkles and folding of RGO sheet also cre-ate defects.2 Because of the presence of the large number oftrap states, charge in the gate would induce charge in thetraps rather than free charge carrier in the semiconductingRGO sheets causing a low mobility in these sheets. Ourstudy suggests that a large number of trap states exists inRGO and call for more theoretical and experimental investi-gations to understand how the trap states are created and howcan they be minimized.In conclusion, we elucidated on the low mobility anddensity of charge traps in RGO sheets by measuring andanalyzing temperature dependent current-voltage characteris-tics. The GO sheets suspended in water were reduced chemi-cally in the solution and then assembled between prefabri-cated gold source and drain electrodes via ac DEP. We showthat at low bias voltage the conduction is Ohmic while athigh bias voltage the conduction becomes SCLC. At roomtemperature, the conduction is governed by trap free SCLCwhile at a lower temperature it is dominated by traps that areexponentially distributed in energy. We estimated an averagetrap density of 1.751016 cm−3. Quantitative studies ofcharge traps using SCLC presented in this work will facili-tate further development of strategies for the chemical modi-fication of RGO surfaces to passivate traps and therebyimprove the carrier transport by providing quantifying evi-dences of the developed strategies of passivating defects inorder to fabricate high quality solution processed graphenedevices.This work has been partially supported by U.S. NSFunder Grant No. ECCS 0748091 to S.I.K. and under GrantNo. DMR 0746499 to L.Z.1S. Stankovich, D. A. Dikin, G. H. B. Dommett, K. M. Kohlhaas, E. J.Zimney, E. A. Stach, R. D. Piner, S. T. Nguyen, and R. S. Ruoff, NatureLondon 442, 282 2006.2H. C. Schniepp, J. L. Li, M. J. McAllister, H. Sai, M. Herrera-Alonso, D.H. Adamson, R. K. Prud’homme, R. Car, D. A. Saville, and I. A. Aksay,J. Phys. Chem. B 110, 8535 2006.3S. Gilje, S. Han, M. Wang, K. L. Wang, and R. B. Kaner, Nano Lett. 7,3394 2007.4I. Jung, D. A. Dikin, R. D. Piner, and R. S. Ruoff, Nano Lett. 8, 42832008.5G. Eda, G. Fanchini, and M. Chhowalla, Nat. Nanotechnol. 3, 270 2008.6C. Gómez-Navarro, R. T. Weitz, A. M. Bittner, M. Scolari, A. Mews, M.Burghard, and K. Kern, Nano Lett. 7, 3499 2007.7A. B. Kaiser, C. Gómez-Navarro, R. S. Sundaram, M. Burghard, and K.Kern, Nano Lett. 9, 1787 2009.8G. Eda, C. Mattevi, H. Yamaguchi, H. Kim, and M. Chhowalla, J. Phys.Chem. C 113, 15768 2009.9D. Joung, A. Chunder, L. Zhai, and S. I. Khondaker, Nanotechnology 21,165202 2010.10G. Lu, L. E. Ocola, and J. Chen, Appl. Phys. Lett. 94, 083111 2009.11Q. Liu, Z. Liu, X. Zhang, N. Zhang, L. Yang, S. Yin, and Y. Chen, Appl.Phys. Lett. 92, 223303 2008.12X. Wang, L. Zhi, and K. Mullen, Nano Lett. 8, 323 2008.13J. Wu, H. A. Becerril, Z. Bao, Z. Liu, Y. Chen, and P. Peumans, Appl.Phys. Lett. 92, 263302 2008.14M. Hirata, T. Gotou, S. Horiuchi, M. Fujiwara, and M. Ohba, Carbon 42,2929 2004.15See supplementary material at http://dx.doi.org/10.1063/1.3484956 forsupplementary materials containing XPS data of GO and RGO to checkeffectiveness of reduction process, and summary of all measured RGOdevices.16S. Wang, P. K. Ang, Z. Wang, A. L. L. Tang, J. T. L. Thong, and K. P. Loh,Nano Lett. 10, 92 2010.17H. Yamaguchi, G. Eda, C. Mattevi, H. Kim, and M. Chhowalla, ACSNano 4, 524 2010.18S. M. Sze, Physics of Semiconductor Devices, 2nd ed. Wiley, New York,1981.19N. F. Mott and R. W. Gurney, Electronic Processes in Ionic Crystal Do-ver, New York, 1964.20P. Mark and W. Helfrich, J. Appl. Phys. 33, 205 1962.21V. Kumar, S. C. Jain, A. K. Kapoor, J. Poortmans, and R. Mertens, J.Appl. Phys. 94, 1283 2003.22I. Jung, M. Vaupel, M. Pelton, R. Piner, D. A. Dikin, S. Stankovich, J. An,and R. S. Ruoff, J. Phys. Chem. C 112, 8499 2008.23S. Baidyaroy and P. Mark, Surf. Sci. 30, 53 1972.24W. Zhang, V. Carravetta, Z. Li, Y. Luo, and J. Yang, J. Chem. Phys. 131,244505 2009.25A. Bagri, C. Mattevi, M. Acik, Y. J. Chabal, M. Chhowalla, and V. B.Shenoy, Nat. Chem. 2, 581 2010.FIG. 3. Color online Extrapolation of the J-V curves at low temperaturesplotted in log-log scale for device A. From this plot, we determine thetemperature independent crossover voltage Vc=10 V.093105-3 Joung et al. Appl. Phys. Lett. 97, 093105 2010

Space charge limited conduction with exponential trap distribution in
  reduced graphene oxide sheets

Space charge limited conduction with exponential trap distribution in reduced graphene oxide sheets

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