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Graphene-On-Silicon Schottky Junction Solar Cells

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By Xinming Li , Hongwei Zhu , * Kunlin Wang , * Anyuan Cao , Jinquan Wei , Chunyan Li , Yi Jia , Zhen Li , Xiao Li , and Dehai Wu

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Graphene, a single atomic layer of carbon hexagons, has stimu-lated a lot of research interest owing to its unique structure and fascinating properties. [ 1 ] Graphene has been produced in the form of ultrathin sheets consisting of one or a few atomic layers by chemical vapor deposition (CVD) [ 2–4 ] or solution processing [ 5 , 6 ] and can be transferred to various substrates. The two-dimensionality and structural fl atness make graphene sheets ideal candidates for thin-fi lm devices and combination with other semiconductor materials such as silicon. These fi lms typically show sheet resist-ances on the order of several hundred ohm per square at about 80% optical transparency. [ 7 ] With modifi cation on the electronic properties and improvement of processing techniques, graphene fi lms show potential for use in conductive, fl exible electrodes, as an alternative for indium tin oxide (ITO).

Graphene applications are just starting, and current investi-gations are on a number of areas such as fi llers for composites, nanoelectronics, and transparent electrodes. [ 8 ] For applications related to solar cells, graphene microsheets were dispersed into conjugated polymers to improve exciton dissociation and charge transport. [ 9–11 ] Also, solution-processed thin fi lms were used as conductive and transparent electrodes for organic [ 12 ] and dye-sensitized [ 13 ] solar cells, although the cell effi ciency is still lower than those with ITO and fl uorine tin oxide (FTO) electrodes. Compared with carbon nanotube fi lms that have been exten-sively studied, graphene fi lms may have several advantages. A continuous single-layer graphene fi lm could retain high conduc-tivity at very low (atomic) thickness, and avoid contact resistance that occurs in a carbon nanotube fi lm between interconnected nanotube bundles. In addition, graphene fi lms have minimum porosity and, in small areas, can provide an extremely fl at sur-face for molecule assembly and device integration. There are many opportunities in utilizing distinct properties of graphene and exploring novel applications.

Bulk heterojunction structures based on carbon materials have attracted a great deal of interest for both scientifi c fundamentals and potential applications in various new optoelectronic devices,

© 2010 WILEY-VCH Verlag GAdv. Mater. 2010, 22, 2743–2748

DOI: 10.1002/adma.200904383

[∗] Dr. X. M. Li, Prof. H. W. Zhu, Prof. K. L. Wang, Prof. J. Q. Wei, Ms. C. Y. Li, Dr. Y. Jia, Dr. Z. Li, Dr. X. Li, Prof. D. H. WuKey Laboratory for Advanced Manufacturing by Material Processing Technology and Department of Mechanical EngineeringTsinghua UniversityBeijing 100084 (P. R. China)E-mail: hongweizhu@tsinghua.edu.cn; wangkl@tsinghua.edu.cnProf. A. Y. CaoDepartment of Advanced Materials and NanotechnologyCollege of EngineeringPeking UniversityBeijing 100871 (P. R. China)

e.g., photovoltaic solar cells. The earliest successful carbon-based semiconductor to partially replace silicon in the solar cell is diamond-like amorphous fi lm (a-C). The successful formation of p-type a-C/n-type silicon (n-Si) junctions represented a fi rst step toward further realizing practical carbon-based solar cells. [ 14–16 ] However, a-C is mainly a unipolar semiconductor. It is diffi cult to process dopants by diffusion, to remove defects by annealing, because of its extreme bond energy and large diffusion ener-gies. These problems have delayed applications of this material. Recently, carbon nanotubes (CNTs)/Si heterojunctions and their photovoltaic properties have been intensively investigated. [ 17–20 ] Basically, a bulk CNTs/Si junction combines two types of het-erostructures (Schottky and p–n junction) as both metallic and semiconducting nanotubes generally coexist in as-grown mate-rials. However, fi lms composed of CNT networks exhibit a lot of interspace between bundles, which is advantageous to the trans-parency but sacrifi ces the conductivity of the fi lm. Moreover, the highest measured conductivities for CNT fi lms reported so far are approaching a limiting value that is regulated by the junc-tion resistances formed between tubes/bundles. [ 21 ]

Here, we deposited graphene sheets (GS) on n-Si wafer to make solar cells with effi ciencies up to 1.5%. We show that GS fi lms can be combined with Si to form Schottky junctions and effi cient solar cells. As illustrated in Figure 1 A , a fi lm con-sisting of thin-layer GS was coated conformally onto a patterned Si/SiO 2 substrate with a 0.1 ≈ 0.5 cm 2 Si window and predepos-ited Au line contacts around. Details on GS synthesis, cell con-fi guration, and assembly are summarized in the Experimental Section. Scanning electron microscopy (SEM) characterization on the GS fi lm reveals the following three key features: First, the surface of GS fi lm is smooth, with many wrinkles as also observed in previously reported graphene sheets ( Figure 1B ). Second, the GS fi lm is coated on the Si substrate conformally, and continuous across the patterned steps between the Au line and the SiO 2 area ( Figure 1C ) and between SiO 2 and Si ( Figure 1D ). The surface coverage by GS is 100%, compared with porous carbon nanotube fi lms. Third, the fi lm consists of multiple layers of GS sheets that are overlapped and inter-connected, which ensures a conducting pathway even if there are cracks formed in one of the layers ( Figure S1 ). The multi-layer structure is expected to provide a higher carrier mobility based on a recent study on graphene layers decoupled from bulk graphite. [ 22 ] Transmission electron microscopy (TEM) and Raman characterization show that most of GS sheets are composed of mono-layer, bi-layer, and few-layer graphene ( Figure S1, S3) . Individual sheets usually have a size of tens to hundreds of square micrometers ( Figure 1E ).

Recently, we have shown that semiconducting CNTs can form heterojunctions (p–n) with n-Si. [ 17 , 18 ] Here, our calculations indicate that GS fi lms form a Schottky junction with Si, which

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Figure 1. Characterizations of the GS/n-Si Schottky junction. A) Schematic illustration of the device confi guration. Bottom-left inset: cross-sectional view, photogenerated holes (h + ) and electrons (e − ) are driven into the GS and n-Si, respectively, by the built-in electric fi eld. Bottom-right inset: photograph of a GS/n-Si Schottky cell with a 0.1 cm 2 junction area. B) A SEM top-view image of a GS/n-Si junction. Scale bar: 2 μm. C) GS across the Au/SiO 2 and D) SiO 2 /Si steps. Scale bars: 2 μm and 1 μm. The insets show the corresponding optical images. Scale bars: 10 μm. E) TEM image of graphene. Scale bar: 500 nm.

is favorable for producing a relatively large built-in fi led ( V 0 , 0.55–0.75 V) and charge separation ( Figure 2 A ). A space–charge region accompanied by V 0 is formed in the n-Si near the GS/n-Si interface. This indicates that the GS fi lm not only serves as a transparent electrode for light illumination, but also an active layer for electron–hole separation and hole transport.

© 2010 WILEY-VCH Verlag Gm

Dark-current–voltage ( I– V ) curves obtained from the GS/n-Si cells ( Figure 2B ) exhibit rectifying characteristics, and demonstrate that the GS/n-Si heterostructures behave as well-defi ned diodes with a rectifi cation ratio of 10 4 ∼ 10 6 . A fi t to a ln( I )– V curve recorded in forward bias from a typical GS/n-Si junction (left inset of Figure 2B ) is nearly linear in the range of 0.1 ≈ 0.4 V and yields

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Figure 2. Photovoltaic characterizations of GS/n-Si solar cells. A) Energy diagram of the forward-biased GS/n-Si Schottky junction upon illumination. Φ G (4.8 ≈ 5.0 eV), Φ n-Si (4.25 eV) is the work function of GS and n-Si, respectively. V 0 is the built-in potential. Φ b is the barrier height. χ is the electron affi nity of silicon (4.05 eV). E g is the bandgap of silicon (1.12 eV) and E F is the energy of the Fermi level. V bias is the applied voltage. The depth of the Fermi level below the Si conduction band edge ( E C – E F ) is ≈0.25 eV for the n-Si used in this work. B) Semilogarhythmic-scale dark I– V curves of two GS/n-Si cells of different junction areas. The ideality factor ( n ) and the series resistance ( R s ) of the 0.1 cm 2 cell extrapolated from the linear regimes in the insets are 1.57 and 10.5 Ω, respectively. The shunt resistance is up to 45 MΩ which is estimated from reverse bias I– V sweep. C) Light J– V curves of the cells illuminated with simulated AM 1.5 Global light. D) Light-intensity-dependent J sc , V oc , FF, and η plots of a 0.1 cm 2 GS/n-Si cell.

Vacuum level

ΦG

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EF

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a diode ideality factor ( n ) of 1.57. The reverse leakage current is nearly proportional to the area of the contact as the leakage current is restricted to the volume of n-Si directly under the GS/n-Si Schottky contact. As shown in Figure 2C , the 0.1 cm 2 Schottky diode has a slightly larger turn-on voltage thanks to the higher barrier height ( Φ b ) of ≈0.78 eV estimated based on Equation 1 (see the Experimental Section), which is in agreement with the difference between Φ G and χ ( Φ G − χ = 4.8 − 4.05 = 0.75 eV).

The photovoltaic properties of the GS/n-Si solar cells were characterized under air mass 1.5 (AM 1.5) illumination. The pho-togenerated carriers are separated by the built-in fi eld, and holes are diffused to the junction where they are swept to the GS side. The two-dimensional, highly conductive graphene will reduce or eliminate lateral potential drop along the GS, thereby ena-bling uniform carrier separation and collection. Light current-density–voltage ( J – V ) data recorded from typical GS/n-Si cells yield down-shift curves with an open-circuit voltage ( V oc ) of 0.42 ∼ 0.48 V, a short-circuit current density ( J sc ) of 4 ≈ 6.5 mA cm –2 and a fi ll factor (FF) of 45 ≈ 56%, which corresponds to an

© 2010 WILEY-VCH Verlag GmAdv. Mater. 2010, 22, 2743–2748

overall solar energy conversion effi ciency ( η ) of 1.0 ≈ 1.7%. As shown in Figure 2C , the 0.1 cm 2 and 0.5 cm 2 devices are characterized by a η of 1.65% and 1.34%, respectively. These photovoltaic values have been easily reproducible and were repeatable for measurements made over an approximately two-month period, thus exhibiting excellent stability of our GS/n-Si cells. The average η of the unoptimized cells is ≈1.5%, which is still lower than that for reported a-C/Si, CNTs/Si devices, but might be improved through balancing the conductivity and transparency of GS and improving the GS/n-Si interface. It was also found that GS essentially acts as an antirefl ection coating and reduces refl ection by ≈70% in the visible region and ≈80% in the near-IR region (see the Supporting Information). The overall effi ciency could also be further increased with improvements in V oc by means of, for example, surface passivation of silicon.

In Figure 2D , the performance parameters of the GS/n-Si cell ( J sc , V oc , FF, and η ) are plotted as functions of the incident light intensity. J sc depends linearly on the light-intensity incident on the cell ( Figure 2D ), consistent with a systematic increase

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Figure 4. A) Light J – V curves of two GS/n-Si cells in series and parallel connections. The J– V characteristics of individual cells are also plotted for comparison. The corresponding circuit connection schematics are plotted in the insets. B) GS/n-Si OR and AND logic gates powered by two cells in series (top) and parallel (bottom). The insets show the circuit schematics for the logic gates when the input state is “1 0”.

Figure 3. IPCE spectra versus photon energy ( E ) for three GS/n-Si cells. The inset shows a differential IPCE spectrum.

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in photogenerated carriers. A similar trend was observed for V oc . FF and η fall off monotonically with an increase in the light intensity and approach 0.69 and 2.2%, respectively, under AM1.5 condition at 15 mW cm −2 .

The normalized incident-photon-to-electron conversion effi ciencies (IPCE, defi ned as the number of charge carriers collected per incident photon) of the GS/n-Si devices were meas-ured in the visible and near-infrared region (photon energy: 1 ≈ 4 eV). Figure 3 displays typical IPCE spectra for three cells, which show similar photoresponse. At low energies, the IPCE spectra signifi cantly overlap but diverge as the incident photon energy increases past 1.5 eV and enters the visible region. This can be attributed to the difference in visible transmittance of GS used in these cells. The fi rst-order derivative of IPCE shown in the inset yields a sharp peak (1.22 eV) with two shoulder peaks (1.19 eV, 1.12 eV), which represent the fastest photon-to-electron conversion and can be assigned to the bandgap of silicon.

The GS/n-Si photovoltaic cells were assembled into a solar array in series and in parallel to multiply the output voltage or (and) current, i.e., the output power. J – V data recorded from two illuminated GS/n-Si cells ( Figure 4 A ) show that intercon-nection of the two cells in series and parallel yields V oc and J sc values, respectively, which equal to the sum of the values of the two base cells. The corresponding time-resolved on–off pho-toresponse of the interconnected GS/n-Si cells are plotted in Figure 4B . Notably, the interconnected cells can perform a self-powered logical operation on two inputs and produce a logic output of voltage ( V out ) or current ( I out ) when the cell is open-circuited or short-circuited. The input state is defi ned as “1” (high) or “0” (low) when the corresponding cell is illuminated or not. The output voltages (open-circuited) V out both show OR logic dependence on the input states for the interconnections in series and parallel, while the output current (short-circuited) I out shows AND logic for series connection and OR logic for parallel connection. This result shows the potential for devel-oping light-controlled sensors and switches especially that rely on remote activation and triggering.

It is worth mentioning that there are a few drawbacks to the present GS/n-Si solar cells that deserve future work. J – V data recorded using GS of different thicknesses exhibited essentially

© 2010 WILEY-VCH Verlag G

the different photovoltaic response. The J sc and V oc are smaller for cells with thicker GS because more carrier recombination occurs in the thicker fi lms, which also makes it diffi cult to achieve enough light absorption, thus indicating that the trade-off between the conductivity and transparency of GS needs to be further optimized. The depletion width in the device was estimated to be 0.5 ≈ 0.7 μm, which is still small and needs to be appropriately increased to improve IPCE. Further enhanced uniformity of GS is also necessary to provide better hole-transport, easy exciton separation, and suppression of charge-carrier recombination.

The fabrication of Schottky junctions has the merits of low cost and simplicity. Although devices with partial replacements are still far lower in effi ciency than pure silicon cells, this simple concept of Schottky junctions made of GS and n-Si, with an improved understanding of electronic coupling, surface passi-vation, doping, and junction formation, will lead to much more effi cient and stable graphene/graphite-based solar cells in the future. It is expected that after complete development, carbon-based photovoltaic cells may become more practicable than

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silicon-based cells by making full use of the above-mentioned advantageous characteristics and, unlike silicon, the material is highly environmentally friendly.

Experimental Section GS Synthesis and Characterizations : GS were synthesized by a

chemical vapor deposition method using nickel fi lms [ 2 , 3 ] or foils [ 23 ] as substrates. The growth parameters were carefully controlled to ensure uniform graphene precipitation. Ni was gradually heated to 1000 °C in an Ar/H 2 (200/100 mL min −1 ) fl ow for 1 h. Then methane (ethanol) was introduced with a feeding speed of 20 mL min −1 (20 μL min −1 ) in an Ar/H 2 (800/200 mL min − 1 ) fl ow for 10 ≈ 20 min. Carbon decomposed from methane (ethanol) was fi rst dissolved into Ni. After carburization, Ni was withdrawn from the heating zone and cooled down to room temperature at a fast cooling rate of 10 ≈ 20 °C s − 1 . The growth is based on a non-equilibrium surface segregation process by controlled cooling as previously reported. [ 2 , 3 , 23 ] During the cool-down period, the carbon solubility in Ni decreases and carbon segregates at the surface of Ni substrate to form GS layers.

Freestanding GS were obtained by detaching the GS from Ni in an acid solution (e.g., HNO 3 ) [ 23 ] or FeCl 3 solution, [ 3 ] followed by rinsing with deionized water. The raw GS fi lms were then immersed into a H 2 O 2 solution (30%) for over 24 h to remove the amorphous carbon impurities and obtain a hydrophilic surface. The treated GS samples were then washed with deionized water. The resulting GS fi lms can be collected easily from the water surface with an arbitrary substrate. The microstructure of GS was investigated by TEM and SEM. As shown in Figure S1 , Supporting Information, the GS consists of mono-layer, bi-layer, and most few-layer graphenes. The GS has a multi-layered structure to ensure conductive channels even cracks are present on the top layer. The GS have nominal thicknesses ( d n ) varied between 10 ≈ 100 nm. d n was measured when considering the surface roughness of GS due to the presence of wrinkles, and the value is larger than the real thickness of GS. Figure S3A , Supporting Information, shows the transmission spectra of GS of different d n .

Raman spectra were obtained with a Renishaw 2000 Raman system with a 633 nm (1.96 eV) laser. Figure S2 , Supporting Information, shows the Raman spectra of as-grown graphenes of different layers. Monolayer graphene is identifi ed unambiguously from the high I 2D / I G ratio (≈4.5) and single peak 2D band with a full width at half maximum (FWHM) of ≈29 cm − 1 . The G-band is located at a normal position (1581.6 cm − 1 ). The relatively small blueshift for the 2D band (≈10 cm − 1 comparing with 2650 cm −1 ) further proves the weak graphene–Ni interaction, revealing that the strain effect [ 24 ] caused by Ni grain deformation during high-temperature carburization is relaxed thanks to the wrinkle formation on Ni surface.

Cell Assembly : n-Si (100) wafers (doping density: 1.5 ≈ 3 × 10 15 cm − 3 ) with a 300-nm SiO 2 layer were patterned by photolithography and wet-etching of oxide (by hydrofl uoric acid solution) to prepare square windows (0.1 ≈ 0.5 cm 2 ) where n-Si was exposed. The front and back contacts were produced using sputtered Au on the SiO 2 and Ti/Pd/Ag on the back side of the n-Si. GS was then transferred to the top of the patterned wafer and naturally dried to achieve a conformal coating with the Au layer and the underlying n-Si. In this confi guration, GS serves as the semitransparent upper electrode and the antirefl ection layer. Figure S3B , Supporting Information, shows the comparison of the refl ection spectra of n-Si and GS/n-Si.

Schottky Junction Characterizations : Forward bias is defi ned as positive voltage applied to the GS. The current–voltage data were recorded using a Keithley 2601 SourceMeter.

It is well known that any semiconductor can form a Schottky junction with a certain metal if the difference between their work functions is large enough, and the carrier density of the semiconductor is moderate. The non-linear I– V characteristic of the Schottky junction can be expressed by the thermoionic emission model: [ 25 ]

© 2010 WILEY-VCH Verlag GmAdv. Mater. 2010, 22, 2743–2748

I I eV

nkTT

kT⎛⎝⎛⎛⎝⎝

⎞⎠⎞⎞⎠⎠

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1

(1)

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⎛⎝⎜⎛⎛⎝⎝

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( )V IR− s

(2)

W

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= ⎛⎝⎜⎛⎛⎝⎝

⎞⎠⎟⎞⎞⎠⎠

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D

/

(3)

where A is the contact area, A ∗ is the effective Richardson constant [≈252 A cm − 2 K −2 for n-type silicon], T is the absolute temperature, e is electronic charge, Φ b is the barrier height, k is the Boltzman constant, n is the diode ideality factor. I s is the reversed saturated current. R s is the series resistance. W is the depletion width. N D is the doping density (1.5 ≈ 3 × 10 15 cm − 3 ). ε is the dielectric permittivity ( ε = 11.9 ε 0 for silicon, ε 0 = 8.854 × 10 − 14 F cm − 1 ).

The reverse leakage current ( I s ) is 0.05 ≈ 0.5 μA for a 0.1 cm 2 cell. Based on Equation 1 , the barrier height ( Φ b ) is estimated to be 0.75 ≈ 0.8 eV, which is in agreement with the difference between Φ G and χ ( Φ G − χ = 4.8 − 4.05 = 0.75 eV), as illustrated in the thermal equilibrium energy band diagram of the GS/n-Si junction in the dark ( Figure S4A , Supporting Information). The series resistance ( R s ) can be determined from Equation 2 to be 9 ∼ 12 Ω in the range of 0.1 ≈ 2.0 V. Reverse bias measurements show that the GS/n-Si diode breaks down at relatively low reverse-bias voltage (about −30 V). Estimated from Equation 3 , the junctions have a depletion layer of 0.5 ≈ 0.7 μm, which was established in the n-Si.

Light Characteristics of GS/n-Si Cells : The devices were tested with a solar simulator (Thermo Oriel 91192-1000) under AM 1.5 condition. The photocurrent action spectra of the GS/n-Si solar cells were measured with CEP-25/CH. Figure S4B , Supporting Information, shows the energy band diagrams of a short-circuited and an open-circuited Schottky junction upon illumination. When the GS/n-Si cell is short-circuited, the extracted photogenerated carriers can transit through the external circuit, generating a short-circuit current. When the GS/n-Si cell is open-circuited, the separation of photogenerated electrons and holes will produce an open-circuit voltage V oc . The quasi-Fermi levels of GS and n-Si are separated with an energy offset of e V oc , corresponding to applying a forward bias ( V oc ) to the Schottky junction. The photocurrent is opposite to the forward-biased current of the device. At V = V oc , these two currents will cancel each other and result in a zero net current.

Acknowledgements This work is supported by the National Science Foundation of China (#50972067) and the Program for New Century Excellent Talents in University, State Education Ministry of China (#NCET-08-0322).

Supporting Information Supporting Information is available online from Wiley InterScience or from the author.

Received: December 22, 2009 Revised: February 2, 2010

Published online: April 9, 2010

[1] K. S. Novoselov , A. K. Geim , S. V. Morozov , D. Jiang , Y. Zhang , S. V. Dubonos , I. V. Grigorieva , A. A. Firsov , Science 2004 , 306 , 666 .

[2] A. Reina , X. Jia , J. Ho , D. Nezich , H. Son , V. Bulovic , M. S. Dresselhaus , J. Kong , Nano Lett. 2009 , 9 , 30 .

[3] K. S. Kim , Y. Zhao , H. Jang , S. Y. Lee , J. M. Kim , K. S. Kim , J. Ahn , P. Kim , J. Choi , B. H. Hong , Nature 2009 , 457 , 706 .

2747bH & Co. KGaA, Weinheim

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[4] X. S. Li , W. W. Cai , J. H. An , S. Kim , J. Nah , D. X. Yang , R. Piner , A. Velamakanni , I. Jung , E. Tutuc , S. K. Banerjee , L. Colombo , R. S. Ruoff , Science 2009 , 324 , 1312 .

[5] X. Li , G. Zhang , X. Bai , X. Sun , X. Wang , E. Wang , H. Dai , Nat. Nanotechnol. 2008 , 3 , 538 .

[6] Y. Hernandez , V. Nicolosi , M. Lotya , F. M. Blighe , Z. Sun , S. De , I. T. McGovern , B. Holland , M. Byrne , Y. K. Gun’Ko , J. J. Boland , P. Niraj , G. Duesberg , S. Krishnamurthy , R. Goodhue , J. Hutchinson , V. Scardaci , A. C. Ferrari , J. N. Coleman , Nat. Nanotechnol. 2008 , 3 , 563 .

[7] X. S. Li , Y. W. Zhu , W. W. Cai , M. Borysiak , B. Y. Han , D. Chen , R. D. Piner , L. Colombo , R. S. Ruoff , Nano Lett. 2009 , 9 , 4359 .

[8] A. K. Geim , Science 2009 , 324 , 1530 . [9] Q. Liu , Z. Liu , X. Zhang , N. Zhang , L. Yang , S. Yin , Y. Chen , Appl.

Phys. Lett. 2008 , 92 , 223303 . [10] W. J. Hong , Y. X. Xu , G. W. Lu , C. Li , G. Q. Shi , Electrochem. Commun.

2008 , 10 , 1555 . [11] Z. F. Liu , Q. Liu , Y. Huang , Y. F. Ma , S. G. Yin , X. Y. Zhang , W. Sun ,

Y. S. Chen , Adv. Mater. 2008 , 20 , 3924 . [12] J. B. Wu , H. A. Becerril , Z. Bao , Z. Liu , Y. Chen , P. Peumans , Appl.

Phys. Lett. 2008 , 92 , 263302 . [13] X. Wang , L. J. Zhi , K. Mullen , Nano Lett. 2008 , 8 , 323 . [14] H. A. Yu , Y. Kaneko , S. Yoshimura , S. Otani , Appl. Phys. Lett. 1996 ,

68 , 547 .

© 2010 WILEY-VCH Verlag Gm

[15] K. Mukhopadhyay , I. Mukhopadhyay , M. Sharon , T. Soga , M. Umeno , Carbon 1997 , 35 , 863 .

[16] Z. Q. Ma , B. X. Liu , Sol. Energy Mater. Sol. Cells 2001 , 69 , 339 . [17] J. Q. Wei , Y. Jia , Q. K. Shu , Z. Y. Gu , K. L. Wang , D. M. Zhuang , G.

Zhang , Z. C. Wang , J. B. Luo , A. Y. Cao , D. H. Wu , Nano Lett. 2007 , 7 , 2317 .

[18] Y. Jia , J. Q. Wei , K. L. Wang , A. Y. Cao , Q. K. Shu , X. C. Gui , Y. Q. Zhu , D. M. Zhuang , G. Zhang , B. B. Ma , L. D. Wang , W. J. Liu , Z. C. Wang , J. B. Luo , D. H. Wu , Adv. Mater. 2008 , 20 , 4594 .

[19] A. Arena , N. Donato , G. Saitta , S. Galvagno , C. Milone , A. Pistone , Microelectronics J. 2008 , 39 , 1659 .

[20] Z. R. Li , V. P. Kunets , V. Saini , Y. Xu , E. Dervishi , G. J. Salamo , A. R. Biris , A. S. Biris , ACS Nano 2009 , 3 , 1407 .

[21] L. F. C. Pereira , C. G. Rocha , A. Latgé , J. N. Coleman , M. S. Ferreira , Appl. Phys. Lett. 2009 , 95 , 123106 .

[22] P. Neugebauer , M. Orlita , C. Faugeras , A. L. Barra , M. Potemski , Phys. Rev. Lett. 2009 , 103 , 136403 .

[23] Q. K. Yu , J. Lian , S. Siriponglert , H. Li , Y. P. Chen , S. S. Pei , Appl. Phys. Lett. 2008 , 93 , 113103 .

[24] Z. H. Ni , W. Chen , X. F. Fan , J. L. Kuo , T. Yu , A. T. S. Wee , Z. X. Shen , Phys. Rev. B 2008 , 77 , 115416 .

[25] S. M. Sze , K. K. Ng , Physics of semiconductor devices (3rd edition) , Wiley , Hoboken 2007 .

bH & Co. KGaA, Weinheim Adv. Mater. 2010, 22, 2743–2748