A simple yet highly reproducible method to suppress contamination of graphene
at low temperature inside the cryostat is presented. The method consists of
applying a current of several mA through the graphene device, which is here
typically a few $\mu$m wide. This ultra-high current density is shown to remove
contamination adsorbed on the surface. This method is well suited for quantum
electron transport studies of undoped graphene devices, and its utility is
demonstrated here by measuring the anomalous quantum Hall effect.Comment: Accepted for publication in Applied Physics Letter

American Physical Society

Bachtold, Adrian

Barreiro Megino, Amelia

Moser, Joel

English

arXiv

J. Moser

A. Barreiro

A. Bachtold

Crossref

Current-induced cleaning of graphene

3 pages, 4 figures.-- PACS nrs.: 81.05.Uw, 81.65.Cf, 68.35.Dv, 68.43.Mn, 73.43.Fj.-- ArXiv pre-print available at: http://arxiv.org/abs/0709.0607A simple yet highly reproducible method to suppress contamination of graphene at low temperature inside the cryostat is presented. The method consists of applying a current of several milliamperes through the graphene device, which is here typically a few microns wide. This ultrahigh current density is shown to remove contamination adsorbed on the surface. This method is well suited for quantum electron transport studies of undoped graphene devices, and its utility is demonstrated here by measuring the anomalous quantum Hall effect.This work was supported by an EURYI grant and FP6-IST-021285-2.Peer reviewe

Barreiro, Amelia

Digital.CSIC

Current-induced cleaning of grapheneJ. Moser,a A. Barreiro, and A. BachtoldbCIN2, Universitat Autonoma de Barcelona, E-08193 Bellaterra, Spainand CNM-CSIC, Universitat Autonoma de Barcelona, E-08193 Bellaterra, SpainReceived 2 August 2007; accepted 4 September 2007; published online 19 October 2007A simple yet highly reproducible method to suppress contamination of graphene at low temperatureinside the cryostat is presented. The method consists of applying a current of several milliamperesthrough the graphene device, which is here typically a few microns wide. This ultrahigh currentdensity is shown to remove contamination adsorbed on the surface. This method is well suited forquantum electron transport studies of undoped graphene devices, and its utility is demonstrated hereby measuring the anomalous quantum Hall effect. © 2007 American Institute of Physics.DOI: 10.1063/1.2789673The discovery of graphene, a one-atom thick layer ofgraphite, has generated considerable interest by opening av-enues in condensed matter physics.1 Graphene is an appeal-ing system for basic research due to its unusual electronicproperties; namely, charge carriers behave like chiral mass-less Dirac particles.2–5 The material also holds promise fortechnological applications, such as in nanoelectronics;6 inaddition, graphene has been shown to be an outstanding gassensor able to detect minute amounts of moleculardopants.7,8 Yet, what makes graphene an enticing material isalso the source of great technological difficulties: being inessence a surface, graphene proves to be extremely sensitiveto contamination.Recently, different approaches have been put forward toaddress this problem. Annealing at several hundreds of Cel-sius in ultrahigh vacuum or in Ar/H2 environment has beenshown to remove contamination.9,10 Unfortunately, however,such annealing processes are difficult to implement in a cry-ostat for low-temperature measurements, and transferringsamples from the annealing chamber to the cryostat is notconvenient, since exposure to air reintroduces contamination.In this letter, a simple yet highly reproducible method tosuppress contamination at low temperature inside the cry-ostat is presented. The method is based on the application ofa large current through the graphene device generating sev-eral tens of milliwatts dissipation over a few m2 large sur-face. Remarkably, graphene can sustain such extreme condi-tions while the adsorbed contamination gets removed. Weemploy atomic force microscopy AFM to show how adsor-bates, as well as intentionally deposited CdSe particles, meltaway. These cleaned graphene devices allow us to measurethe anomalous quantum Hall effect.We start by briefly describing our fabrication technique.Graphene flakes are deposited by mechanical exfoliation ofkish graphite Toshiba Ceramics on degenerately doped sili-con substrates coated with 270 nm of thermal silicon oxide.We use standard wafer protection tape as it leaves little ad-hesive residue on substrates. Cr/Au electrodes are fabricatedon top of the samples by electron beam lithography followedby lift-off in acetone and dicholoroethane. Two-point dctransport measurements are conducted both at room tempera-ture in air for AFM imaging and in a He4 cryostat.Graphene is able to carry very high electrical currentswithout sustaining damage. Applying a source-drain bias Vof a few volts across the sample induces a large current flowI of a few milliamperes, as shown in Fig. 1. Taking thesample width of 4 m and a thickness of 0.35 nm, this trans-lates into an extremely large current density J of a few108 A/cm2. For comparison, J is only a few times larger incarbon nanotubes,11,12 and for both materials, J is severalorders of magnitude larger than in present-day interconnects.The effects of a large electrical current passing through amesoscopic, conducting device include electromigration13,14and Joule heating. We expect that the large power P20 mW dissipated over a small area of a few m2 signifi-cantly increases the sample temperature T. In order to showthis, we monitor the four-point resistance R of a gold stripR60  at room T fabricated on the substrate 30 maway from the graphene sample. Having initially calibratedR against a commercial thermometer in thermal equilibrium,we estimate that the temperature rises by 25 K at the goldstrip when a power of 34 mW is dissipated in the graphenesample at 4.2 K. However, an estimate of the graphene tem-perature is difficult to obtain, since most of the heat is evacu-ated through the Au electrodes that are thermally anchored toaElectronic mail: moser.joel@gmail.combElectronic mail: adrian.bachtold@cnm.esFIG. 1. Color online current I left axis and current density J right axisas a function of source-drain bias V applied across the graphene sample inhelium atmosphere at T=76 K. Graphene width w=4 m and interelectrodedistance L=1 m. Inset: schematic of the device.APPLIED PHYSICS LETTERS 91, 163513 20070003-6951/2007/9116/163513/3/$23.00 © 2007 American Institute of Physics91, 163513-1Downloaded 22 Oct 2007 to 158.109.49.216. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspthe liquid helium bath see below. Further insight is gainedby depositing CdSe nanoparticles with a mean diameter of5 nm on a graphene sample and applying a large current.Figure 2 presents topographic AFM scans of the sample be-fore Fig. 2a and after Fig. 2b the current flow. Figure2b shows that most nanoparticles have vanished. Owing tohigh temperatures generated by the large Joule heating,nanoparticles might have undergone various phase transi-tions: they might have melted and formed a thin film over thesample, or they might also have evaporated or sublimated. Inaddition, each of these mechanisms may be assisted byelectromigration.15 A rough estimate for the temperaturereached is given by the melting temperature of 5 nm diam-eter CdSe particles, which is expected to be 600 °C.16 Fu-ture work should help identify the mechanism by whichnanoparticles disappear and allow for a better estimate of thegraphene temperature. Eventually, note that the presence ofthe remaining particles close to the electrodes suggests thatthese act as heat sinks Fig. 2b.In the following, we demonstrate how this effect can beharnessed to remove contaminants from graphene samples.AFM imaging reveals the influence of the large electricalcurrent on the morphology of the sample surface. Figures3a–3c display AFM scans for the signal amplitude at vari-ous stages of the cleaning process. Typically, freshly depos-ited graphene sheets have an average roughness Ra0.1 nm. Right after fabrication Fig. 3a, Ra0.4 nm ismeasured, suggesting the presence of contamination on topof the graphene. Figure 3b captures an intermediate stateobtained by ramping V to 3 V. Contaminants migrate andcluster up. As V is further increased, the sample area halfwaybetween source and drain becomes almost as smooth as thesubstrate Fig. 3c, for V=4.6 V and the roughness is downto Ra0.1 nm, as before fabrication. Figure 3c also showsthat some contamination has accumulated at the corner of thedevice such a migration is not observed with CdSe par-ticles.This contamination removal has drastic effects on thesample doping. Electric field effect can be used to dopegraphene either with electrons or holes, resulting in a mini-mum conductivity at the charge neutrality point. The blackcurve in Fig. 3d represents the low-bias conductance G as afunction of a backgate bias Vbg applied to the silicon sub-strate, for the same sample as in Fig. 1 but prior to any largecurrent flow. Measurements are performed under helium at-mosphere at T=76 K in a cryostat. G decreases continuouslyas a function of Vbg, which indicates that the conductanceminimum is beyond the voltage range and that doping isimportant. After high-bias treatment we observe a well-defined conductance minimum Gmin close to zero Vbg thatdenotes the reduction of doping.The strong shift of Gmin to zero Vbg, along with the re-covered smoothness of the sample, indicates that unavoid-able fabrication residues act as dopants that are removedupon passing a large electrical current through the sample.These residues may consist in part of nondissolved polym-ethyl methacrylate PMMA.9 Note that in air, this cleaningprocess tends to be counterbalanced by the doping effect ofother molecular adsorbates e.g., oxygen, as a result ofwhich Gmin is found to slowly drift in time back to large,positive Vbg values.Special care has to be taken to preserve the integrity ofthe device upon applying large source-drain biases; we pro-ceed as follows. Setting Vbg=0, we slowly raise V. We regu-larly hold V constant to monitor the time dependence of thecurrent I. If I stays constant, we keep on increasing V. This isrepeated until I continuously decreases as a function of timeand saturates see Fig. 3e for a fixed V=5 V, sample ofFigs. 1 and 3d. The continuous decay of I in time at fixedV and Vbg=0 signals a smooth shift of Gmin to zero Vbg. Notethat an acceleration of the current decay usually precedes therupture of graphene.FIG. 3. Color online a–c AFM signal amplitude scans captured atvarious stages of the current-induced cleaning process in air. d Two-pointconductance G as a function of backgate bias Vbg before and after the ap-plication of a large current in helium atmosphere and at T=76 K. e Timedependence of current I at fixed bias V=5 V.FIG. 2. Color online CdSe nanoparticles are deposited on a graphenesample, and imaged by AFM before Fig. 2a and after Fig. 2b applyinga large electrical current between source and drain. Both figures are topo-graphic scans. Figure 2b indicates that most nanoparticles have vanishedfrom the area located halfway between the source and the drain. We imagedthe entire sample and verified that particles did not form clusters and did notmigrate to the edges.163513-2 Moser, Barreiro, and Bachtold Appl. Phys. Lett. 91, 163513 2007Downloaded 22 Oct 2007 to 158.109.49.216. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspRemarkably, this simple cleaning technique allows us toobserve the anomalous, half-integer quantum Hall effect2,3that is the signature of a clean, single graphene layer. Ingraphene, the nth Landau level LL energy reads En=sgnnvF2enB, where n is the electron or hole LL indexand vF106 m s−1 is the Fermi velocity. Edge channels havea degeneracy g=4 that accounts for both spin and sublatticedegeneracies. The presence of electron and hole edge chan-nels at the neutrality point LL index n=0 reduces the num-ber of current-carrying modes by half, yielding g=2 seeschematic of Fig. 4. The two-point Landauer conductancereads G=ge2 /h, where the sum is carried over all the lev-els, and is therefore quantized in 4e2 /hn+1/2 as Vbg isswept.17 Figure 4 shows a clear quantization at 1 /24e2 /h,whereas the other plateaus are not as well defined the sameis observed for five other samples. The robustness of thefirst plateau is explained by the large energy gaps E±1−E01200 K at B=9 T. Importantly, the plateau is visible onlyafter the current-induced cleaning process. This indicates thatour process enhances the mobility of the sample and/or im-proves the homogeneity of the doping over the device area.To conclude, we have demonstrated a simple, yet highlyreproducible technique to obtain clean graphene devices outof initially highly contaminated samples. Our method takesadvantage of the large electrical current density thatgraphene can sustain to remove contaminants from thesample surface. Compared to other methods, the reportedtechnique has the advantage to be operative at low T inside acryostat, without having to cycle the cryostat to room T. Inaddition, we avoid contamination in air that would naturallyoccur upon transferring the device from the annealing cham-ber to the cryostat.We thank P. Jarillo-Herrero for showing us the mechan-cal exfoliation technique. This work was supported by anEURYI grant and FP6-IST-021285-2.1For a review, see: A. Castro Neto, F. Guinea, and N. M. Peres, “Drawingconclusions from graphene,” Phys. World, November 2006. A. K. Geimand K. S. Novoselov, Nat. Mater. 6, 183 2007; M. I. Katsnelson, Mater.Today 10, 20 2007.2K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson,I. V. Grigorieva, S. V. Dubonos, and A. A. Firsov, Nature London 438,197 2005.3Y. Zhang, Y.-W. Tan, H. L. Stormer, and P. Kim, Nature London 438,201 2005.4V. P. Gusynin and S. G. Sharapov, Phys. Rev. Lett. 95, 146801 2005.5N. M. R. Peres, F. Guinea, and A. H. Castro Neto, Phys. Rev. B 73,125411 2006.6C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud, D. Mayou, T. Li,J. Hass, A. N. Marchenkov, E. H. Conrad, P. N. First, and W. A. de Heer,Science 312, 1191 2006.7F. Schedin, A. K. Geim, S. V. Morozov, E. H. Hill, P. Blake, M. I. Katsnel-son, and K. S. Novoselov, Nat. Mater. 6, 652 2007.8E. H. Hwang, S. Adam, S. Das Sarma, and A. K. Geim, e-printarXiv:cond-mat/0610834v1.9M. Ishigami, J. H. Chen, W. G. Cullen, M. S. Fuhrer, and E. D. Williams,Nano Lett. 7, 1643 2007.10E. Stolyarova, K. T. Rim, S. Ryu, J. Maultzsch, P. Kim, L. E. Brus, T. F.Heinz, M. S. Hybertsen, and G. W. Flynn, Proc. Natl. Acad. Sci. U.S.A.104, 9209 2007.11Z. Yao, C. L. Kane, and C. Dekker, Phys. Rev. Lett. 84, 2941 2000.12B. Bourlon, D. C. Glattli, B. Plaçais, J. M. Berroir, C. Miko, L. Forró, andA. Bachtold, Phys. Rev. Lett. 92, 026804 2004.13R. E. Hummel, Int. Mater. Rev. 39, 97 1994.14H. Park, A. K. L. Lim, J. Park, A. P. Alivisatos, and P. L. McEuen, Appl.Phys. Lett. 75, 301 1999.15B. C. Regan, S. Aloni, R. O. Ritchie, U. Dahmen, and A. Zettl, NatureLondon 428, 924 2004.16The melting temperature Tm of 4 nm diameter CdS particles has beenmeasured to be 1180 K A. N. Goldstein, C. M. Echer, and A. P. Alivisa-tos, Science 256, 1425 1992. Tm of CdSe particles can be roughlyestimated using Tm /Tmbulk=1−C /d, where d is the particle diameter andTmbulk is the bulk material melting temperature 1510 K for CdSe and2020 K for CdS Z. Zhang, M. Zhao, and Q. Jiang, Semicond. Sci.Technol. 16, L33 2001. C is material dependent; yet, here, we take thesame value for CdSe and CdS.17H. B. Heersche, P. Jarillo-Herrero, J. B. Oostinga, L. M. K. Vandersypen,and A. F. Morpurgo, Nature London 446, 56 2007.FIG. 4. Color online Left Two-point conductance GVbg at B=9 T andT=4.2 K revealing the anomalous quantum Hall effect. Right Edge chan-nels in ideally clean, single layer graphene. Electron and holelike channelscoexist close to the neutrality point, yielding a channel degeneracy g=2. They axis is along the sample width. Levels are bent at the edges due to hard-wall confinement.163513-3 Moser, Barreiro, and Bachtold Appl. Phys. Lett. 91, 163513 2007Downloaded 22 Oct 2007 to 158.109.49.216. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Applied Physics Letters

A simple yet highly reproducible method to suppress contamination of graphene at low temperature inside the cryostat is presented. The method consists of applying a current of several milliamperes through the graphene device, which is here typically a few microns wide. This ultrahigh current density is shown to remove contamination adsorbed on the surface. This method is well suited for quantum electron transport studies of undoped graphene devices, and its utility is demonstrated here by measuring the anomalous quantum Hall effect

Diposit Digital de Documents de la UAB

Current-induced cleaning of grapheneJ. Moser, A. Barreiro, and A. Bachtold  Citation: Applied Physics Letters 91, 163513 (2007); doi: 10.1063/1.2789673 View online: http://dx.doi.org/10.1063/1.2789673 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/91/16?ver=pdfcov Published by the AIP Publishing                      This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:158.109.223.71 On: Thu, 20 Feb 2014 09:30:52Current-induced cleaning of grapheneJ. Moser,a A. Barreiro, and A. BachtoldbCIN2, Universitat Autonoma de Barcelona, E-08193 Bellaterra, Spainand CNM-CSIC, Universitat Autonoma de Barcelona, E-08193 Bellaterra, SpainReceived 2 August 2007; accepted 4 September 2007; published online 19 October 2007A simple yet highly reproducible method to suppress contamination of graphene at low temperatureinside the cryostat is presented. The method consists of applying a current of several milliamperesthrough the graphene device, which is here typically a few microns wide. This ultrahigh currentdensity is shown to remove contamination adsorbed on the surface. This method is well suited forquantum electron transport studies of undoped graphene devices, and its utility is demonstrated hereby measuring the anomalous quantum Hall effect. © 2007 American Institute of Physics.DOI: 10.1063/1.2789673The discovery of graphene, a one-atom thick layer ofgraphite, has generated considerable interest by opening av-enues in condensed matter physics.1 Graphene is an appeal-ing system for basic research due to its unusual electronicproperties; namely, charge carriers behave like chiral mass-less Dirac particles.2–5 The material also holds promise fortechnological applications, such as in nanoelectronics;6 inaddition, graphene has been shown to be an outstanding gassensor able to detect minute amounts of moleculardopants.7,8 Yet, what makes graphene an enticing material isalso the source of great technological difficulties: being inessence a surface, graphene proves to be extremely sensitiveto contamination.Recently, different approaches have been put forward toaddress this problem. Annealing at several hundreds of Cel-sius in ultrahigh vacuum or in Ar/H2 environment has beenshown to remove contamination.9,10 Unfortunately, however,such annealing processes are difficult to implement in a cry-ostat for low-temperature measurements, and transferringsamples from the annealing chamber to the cryostat is notconvenient, since exposure to air reintroduces contamination.In this letter, a simple yet highly reproducible method tosuppress contamination at low temperature inside the cry-ostat is presented. The method is based on the application ofa large current through the graphene device generating sev-eral tens of milliwatts dissipation over a few m2 large sur-face. Remarkably, graphene can sustain such extreme condi-tions while the adsorbed contamination gets removed. Weemploy atomic force microscopy AFM to show how adsor-bates, as well as intentionally deposited CdSe particles, meltaway. These cleaned graphene devices allow us to measurethe anomalous quantum Hall effect.We start by briefly describing our fabrication technique.Graphene flakes are deposited by mechanical exfoliation ofkish graphite Toshiba Ceramics on degenerately doped sili-con substrates coated with 270 nm of thermal silicon oxide.We use standard wafer protection tape as it leaves little ad-hesive residue on substrates. Cr/Au electrodes are fabricatedon top of the samples by electron beam lithography followedby lift-off in acetone and dicholoroethane. Two-point dctransport measurements are conducted both at room tempera-ture in air for AFM imaging and in a He4 cryostat.Graphene is able to carry very high electrical currentswithout sustaining damage. Applying a source-drain bias Vof a few volts across the sample induces a large current flowI of a few milliamperes, as shown in Fig. 1. Taking thesample width of 4 m and a thickness of 0.35 nm, this trans-lates into an extremely large current density J of a few108 A/cm2. For comparison, J is only a few times larger incarbon nanotubes,11,12 and for both materials, J is severalorders of magnitude larger than in present-day interconnects.The effects of a large electrical current passing through amesoscopic, conducting device include electromigration13,14and Joule heating. We expect that the large power P20 mW dissipated over a small area of a few m2 signifi-cantly increases the sample temperature T. In order to showthis, we monitor the four-point resistance R of a gold stripR60  at room T fabricated on the substrate 30 maway from the graphene sample. Having initially calibratedR against a commercial thermometer in thermal equilibrium,we estimate that the temperature rises by 25 K at the goldstrip when a power of 34 mW is dissipated in the graphenesample at 4.2 K. However, an estimate of the graphene tem-perature is difficult to obtain, since most of the heat is evacu-ated through the Au electrodes that are thermally anchored toaElectronic mail: moser.joel@gmail.combElectronic mail: adrian.bachtold@cnm.esFIG. 1. Color online current I left axis and current density J right axisas a function of source-drain bias V applied across the graphene sample inhelium atmosphere at T=76 K. Graphene width w=4 m and interelectrodedistance L=1 m. Inset: schematic of the device.APPLIED PHYSICS LETTERS 91, 163513 20070003-6951/2007/9116/163513/3/$23.00 © 2007 American Institute of Physics91, 163513-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:158.109.223.71 On: Thu, 20 Feb 2014 09:30:52the liquid helium bath see below. Further insight is gainedby depositing CdSe nanoparticles with a mean diameter of5 nm on a graphene sample and applying a large current.Figure 2 presents topographic AFM scans of the sample be-fore Fig. 2a and after Fig. 2b the current flow. Figure2b shows that most nanoparticles have vanished. Owing tohigh temperatures generated by the large Joule heating,nanoparticles might have undergone various phase transi-tions: they might have melted and formed a thin film over thesample, or they might also have evaporated or sublimated. Inaddition, each of these mechanisms may be assisted byelectromigration.15 A rough estimate for the temperaturereached is given by the melting temperature of 5 nm diam-eter CdSe particles, which is expected to be 600 °C.16 Fu-ture work should help identify the mechanism by whichnanoparticles disappear and allow for a better estimate of thegraphene temperature. Eventually, note that the presence ofthe remaining particles close to the electrodes suggests thatthese act as heat sinks Fig. 2b.In the following, we demonstrate how this effect can beharnessed to remove contaminants from graphene samples.AFM imaging reveals the influence of the large electricalcurrent on the morphology of the sample surface. Figures3a–3c display AFM scans for the signal amplitude at vari-ous stages of the cleaning process. Typically, freshly depos-ited graphene sheets have an average roughness Ra0.1 nm. Right after fabrication Fig. 3a, Ra0.4 nm ismeasured, suggesting the presence of contamination on topof the graphene. Figure 3b captures an intermediate stateobtained by ramping V to 3 V. Contaminants migrate andcluster up. As V is further increased, the sample area halfwaybetween source and drain becomes almost as smooth as thesubstrate Fig. 3c, for V=4.6 V and the roughness is downto Ra0.1 nm, as before fabrication. Figure 3c also showsthat some contamination has accumulated at the corner of thedevice such a migration is not observed with CdSe par-ticles.This contamination removal has drastic effects on thesample doping. Electric field effect can be used to dopegraphene either with electrons or holes, resulting in a mini-mum conductivity at the charge neutrality point. The blackcurve in Fig. 3d represents the low-bias conductance G as afunction of a backgate bias Vbg applied to the silicon sub-strate, for the same sample as in Fig. 1 but prior to any largecurrent flow. Measurements are performed under helium at-mosphere at T=76 K in a cryostat. G decreases continuouslyas a function of Vbg, which indicates that the conductanceminimum is beyond the voltage range and that doping isimportant. After high-bias treatment we observe a well-defined conductance minimum Gmin close to zero Vbg thatdenotes the reduction of doping.The strong shift of Gmin to zero Vbg, along with the re-covered smoothness of the sample, indicates that unavoid-able fabrication residues act as dopants that are removedupon passing a large electrical current through the sample.These residues may consist in part of nondissolved polym-ethyl methacrylate PMMA.9 Note that in air, this cleaningprocess tends to be counterbalanced by the doping effect ofother molecular adsorbates e.g., oxygen, as a result ofwhich Gmin is found to slowly drift in time back to large,positive Vbg values.Special care has to be taken to preserve the integrity ofthe device upon applying large source-drain biases; we pro-ceed as follows. Setting Vbg=0, we slowly raise V. We regu-larly hold V constant to monitor the time dependence of thecurrent I. If I stays constant, we keep on increasing V. This isrepeated until I continuously decreases as a function of timeand saturates see Fig. 3e for a fixed V=5 V, sample ofFigs. 1 and 3d. The continuous decay of I in time at fixedV and Vbg=0 signals a smooth shift of Gmin to zero Vbg. Notethat an acceleration of the current decay usually precedes therupture of graphene.FIG. 3. Color online a–c AFM signal amplitude scans captured atvarious stages of the current-induced cleaning process in air. d Two-pointconductance G as a function of backgate bias Vbg before and after the ap-plication of a large current in helium atmosphere and at T=76 K. e Timedependence of current I at fixed bias V=5 V.FIG. 2. Color online CdSe nanoparticles are deposited on a graphenesample, and imaged by AFM before Fig. 2a and after Fig. 2b applyinga large electrical current between source and drain. Both figures are topo-graphic scans. Figure 2b indicates that most nanoparticles have vanishedfrom the area located halfway between the source and the drain. We imagedthe entire sample and verified that particles did not form clusters and did notmigrate to the edges.163513-2 Moser, Barreiro, and Bachtold Appl. Phys. Lett. 91, 163513 2007 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:158.109.223.71 On: Thu, 20 Feb 2014 09:30:52Remarkably, this simple cleaning technique allows us toobserve the anomalous, half-integer quantum Hall effect2,3that is the signature of a clean, single graphene layer. Ingraphene, the nth Landau level LL energy reads En=sgnnvF2enB, where n is the electron or hole LL indexand vF106 m s−1 is the Fermi velocity. Edge channels havea degeneracy g=4 that accounts for both spin and sublatticedegeneracies. The presence of electron and hole edge chan-nels at the neutrality point LL index n=0 reduces the num-ber of current-carrying modes by half, yielding g=2 seeschematic of Fig. 4. The two-point Landauer conductancereads G=ge2 /h, where the sum is carried over all the lev-els, and is therefore quantized in 4e2 /hn+1/2 as Vbg isswept.17 Figure 4 shows a clear quantization at 1 /24e2 /h,whereas the other plateaus are not as well defined the sameis observed for five other samples. The robustness of thefirst plateau is explained by the large energy gaps E±1−E01200 K at B=9 T. Importantly, the plateau is visible onlyafter the current-induced cleaning process. This indicates thatour process enhances the mobility of the sample and/or im-proves the homogeneity of the doping over the device area.To conclude, we have demonstrated a simple, yet highlyreproducible technique to obtain clean graphene devices outof initially highly contaminated samples. Our method takesadvantage of the large electrical current density thatgraphene can sustain to remove contaminants from thesample surface. Compared to other methods, the reportedtechnique has the advantage to be operative at low T inside acryostat, without having to cycle the cryostat to room T. Inaddition, we avoid contamination in air that would naturallyoccur upon transferring the device from the annealing cham-ber to the cryostat.We thank P. Jarillo-Herrero for showing us the mechan-cal exfoliation technique. This work was supported by anEURYI grant and FP6-IST-021285-2.1For a review, see: A. Castro Neto, F. Guinea, and N. M. Peres, “Drawingconclusions from graphene,” Phys. World, November 2006. A. K. Geimand K. S. Novoselov, Nat. Mater. 6, 183 2007; M. I. Katsnelson, Mater.Today 10, 20 2007.2K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson,I. V. Grigorieva, S. V. Dubonos, and A. A. Firsov, Nature London 438,197 2005.3Y. Zhang, Y.-W. Tan, H. L. Stormer, and P. Kim, Nature London 438,201 2005.4V. P. Gusynin and S. G. Sharapov, Phys. Rev. Lett. 95, 146801 2005.5N. M. R. Peres, F. Guinea, and A. H. Castro Neto, Phys. Rev. B 73,125411 2006.6C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud, D. Mayou, T. Li,J. Hass, A. N. Marchenkov, E. H. Conrad, P. N. First, and W. A. de Heer,Science 312, 1191 2006.7F. Schedin, A. K. Geim, S. V. Morozov, E. H. Hill, P. Blake, M. I. Katsnel-son, and K. S. Novoselov, Nat. Mater. 6, 652 2007.8E. H. Hwang, S. Adam, S. Das Sarma, and A. K. Geim, e-printarXiv:cond-mat/0610834v1.9M. Ishigami, J. H. Chen, W. G. Cullen, M. S. Fuhrer, and E. D. Williams,Nano Lett. 7, 1643 2007.10E. Stolyarova, K. T. Rim, S. Ryu, J. Maultzsch, P. Kim, L. E. Brus, T. F.Heinz, M. S. Hybertsen, and G. W. Flynn, Proc. Natl. Acad. Sci. U.S.A.104, 9209 2007.11Z. Yao, C. L. Kane, and C. Dekker, Phys. Rev. Lett. 84, 2941 2000.12B. Bourlon, D. C. Glattli, B. Plaçais, J. M. Berroir, C. Miko, L. Forró, andA. Bachtold, Phys. Rev. Lett. 92, 026804 2004.13R. E. Hummel, Int. Mater. Rev. 39, 97 1994.14H. Park, A. K. L. Lim, J. Park, A. P. Alivisatos, and P. L. McEuen, Appl.Phys. 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Electron and holelike channelscoexist close to the neutrality point, yielding a channel degeneracy g=2. They axis is along the sample width. Levels are bent at the edges due to hard-wall confinement.163513-3 Moser, Barreiro, and Bachtold Appl. Phys. Lett. 91, 163513 2007 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:158.109.223.71 On: Thu, 20 Feb 2014 09:30:52

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