Optical Spectroscopy Investigation of the Structural and Electrical

Article pubs.acs.org/JPCC

Optical Spectroscopy Investigation of the Structural and Electrical Evolution of Controllably Oxidized Graphene by a Solution Method Shengnan Wang,†,‡ Rui Wang,†,§ Xinfeng Liu,† Xiaowei Wang,† Dongdong Zhang,†,§ Yanjun Guo,† and Xiaohui Qiu*,† †

National Center for Nanoscience and Technology, Zhongguancun, Beijing 100190, China Department of Physics, Tsinghua University, Beijing, 100084, China § Academy of Advanced Interdisciplinary Studies, Peking University, Beijing, 100871, China ‡

S Supporting Information *

ABSTRACT: A precisely controlled chemical modification of exfoliated graphene on a substrate was achieved by solution-phase oxidation. The structural and electrical evolution of graphene induced by oxygen-related defects was investigated using micro-Raman and photoluminescence spectroscopy. The sp2-hybrid carbon network in monolayer graphene was found to gradually decrease with increasing degree of oxidation. The size of the graphene quantum dots was finally reduced to about 1 nm, which exhibited an energy band gap of 2 eV. The double-layer graphene showed a symmetry breaking induced by the defects. The process of solution modification may provide a facile method to tailor the electrical properties of graphene on a chip for constructing carbon-based nanoelectronics.

■

INTRODUCTION The challenge remains to modify the electronic structure of graphene to enhance its current on−off ratio in the application of logic circuits.1−3 A promising way is to fabricate ultranarrow graphene by cutting graphene into nanoribbons or unzipping carbon nanotubes using physical and chemical methods.4,5 Theoretical studies have predicted that a band gap of ∼1 eV will open when the graphene nanoribbons are narrower than 2 nm, which has been verified experimentally.6,7 Alternatively, as a single layer of sp2-hybridized carbon atoms, graphene possesses the electronic properties that can be modulated by means of physical or chemical adsorption of different reactive species.8−10 For example, the atomic hydrogen treatment transformed the graphene to graphane, an insulating material compared to the zero-band gap of pristine graphene.8,9 Exposing graphene to a xenon difluoride (XeF2) atmosphere produced fluorographene, which is a high-quality insulator with an optical gap of 3 eV.10 Oxidation is a versatile approach to tailoring the electronic properties of graphene. Oxidant solution and oxygen etching are the two commonly used methods to incorporate various functional groups (i.e., carboxyl, hydroxyl, and epoxy groups) and structural disorders (i.e., atom vacancy, site or line defects, planar distortion) into the carbon lattice, which is expected to extend the applications of graphene.11−15 The morphology and electronic properties of graphene treated by controlled oxygen etching have been widely investigated by various techniques, including atomic force microscopy (AFM), Raman spectroscopy, and electrical transport measurement.16−18 It was demonstrated that graphene can be etched into micro/ © 2012 American Chemical Society

nanostructures and strongly p-doped under an oxygen atmosphere at high temperature.17 Recently, photoluminescence (PL) has been excited in a extensively oxidized singlelayer graphene (SLG) by oxygen plasma, which generated highly dense oxygen-related defects that eventually transformed graphene into separated carbon quantum dots with a band-gap opening.19 The solution-phase oxidation, such as modified Hummer’s methods, was widely employed for large-scale production of graphene oxide (GO) suspended in aqueous solution, which is the precursor of graphene-based materials.20,21 The atomic structure and surface chemistry of GO have been elucidated by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and nuclear magnetic resonant spectroscopy (NMRS).11,14,15 Recent studies demonstrated that topological, chemical, and electronic properties of the oxidizing products can be intentionally altered by modifying the oxidative condition.22,23 A further elucidation of the oxygen-related defects in graphene is important for exploring a new approach to tuning the energy band structure of graphene via defect engineering. In this paper, we report a solution-phase modification procedure capable of precisely tailoring the structural and electronic properties of graphene deposited on a SiO2 substrate. The low concentration oxidant solution of potassium permanganate (KMnO4) and sulfuric acid (H2SO4) allows a controlled oxidation of graphene with a highly precise reaction Received: December 17, 2011 Revised: April 17, 2012 Published: April 26, 2012 10702

dx.doi.org/10.1021/jp212184n | J. Phys. Chem. C 2012, 116, 10702−10707

The Journal of Physical Chemistry C

Article

rate by optimizing the concentration and pH value. It enables us to perform a time-dependent study on a given graphene sample subjected to oxidation. Micro-Raman and photoluminescence spectroscopy was employed to in situ investigate the structural and energetics variations of graphene tuned by the oxygen-related defects. We observed that the sp2-carbon network in SLG was progressively destroyed by the immigrated oxygen species and was finally surrounded by an sp3-hybrid C− O matrix at extended oxidation times. The size of graphene quantum dots was eventually reduced to approximately 1−3 nm, which had an energy gap of ∼2 eV verified by photoluminescence emission. Comparing with SLG, bilayer graphene (BLG) and few-layer graphene (FLG) are more chemically inert. A symmetry breaking as a result of the top surface oxidation was noticed in BLG by micro-Raman spectroscopy, demonstrating a layer-dependent modification of graphene by a time-controlled oxidation. The underlying mechanism of defects engineering on graphene’s structural and electronic properties was discussed.

■

EXPERIMENTAL METHODS Preparation of Graphene and Graphene Oxide. The highly n-doped Si substrates coated with 300 nm SiO2 were cleaned by the oxygen plasma before depositing the graphene samples. SLGs connected with thin-film graphene (TFG) sheets were prepared from micromechanical cleavage of natural graphite flakes (Alfa Aesar) using Scotch transparent tape (3M). The positions of the exfoliated graphene flakes on the substrate were precisely located under an optical microscope (Olympus) by means of reference marks prefabricated on the wafers. The thickness and structural quality of pristine graphene samples were characterized by micro-Raman microscopy (Renishaw inVia). The on-chip oxidation of graphene was implemented as described below. The silicon wafer with pristine SLG and multilayer graphene (MLG) samples was immersed in an acidic KMnO4 solution (volume ratio: 50% H2SO4/0.015 M KMnO4 = 1:1) at room temperature for a period of 30−900 s. After taking it out from the reactant solution, the wafer was rinsed copiously with deionized (DI) water for a few minutes in order to fully remove the unreacted KMnO4 on the graphene surface. The wafers were dried with N2 flow immediately after taking them out of the DI water. After each step of the oxidation treatment, the graphene sample was characterized by Raman and PL spectroscopy. Raman Spectroscopy. The Raman spectra were acquired with a Renishaw inVia Raman spectroscope at a laser wavelength of 514 nm. The laser power was set below 1.1 mW to avoid the heating effect. A 100× objective lens with N.A. = 0.95 was used to focus the laser beam to a region of about 0.6 μm. The full width at half-maximum (fwhm), intensity, and position of the Raman peak were obtained by fitting the Raman spectra with the Voigt function. The acquisition time per pixel in the Raman mapping image (Figure 1) was 30 s. The step size was 500 nm. PL Characterization. A confocal microscope (Olympus) with a 50× objective lens with N.A. = 0.8 was employed to focus a cw laser (wavelength at 514 nm) on the graphene sample. The PL spectra were collected with a monochromator (SP2300i) and a liquid-N2-cooled charge-coupled device (CCD) (Princeton Instrument, Spec10). The acquisition time per pixel of the PL mapping image was 0.5 s. The step size was 500 nm.

Figure 1. (a) Optical micrograph of a graphene sample with different layers. The area in the red square is 5 × 5 μm2 in size. (b) I(D) Raman mapping of the area marked by the red square in (a) after oxidation. (c) Raman spectra acquired at the different layers in the graphene sample (a). The dotted and solid lines correspond to the samples before and after oxidation, respectively. The oxidation time is 180 s.

■

RESULTS AND DISSCUSION The graphene samples on the SiO2 substrate were located by their optical contrast, as shown in Figure 1a. The different layer numbers of graphene were identified from the micro-Raman spectra (Figure 1c). The symmetry of the 2D band (∼2680 cm−1) and the relatively intensity of the G band (∼1580 cm−1) are strongly dependent on the thickness of graphene films.24 For the pristine graphene samples, the D band (∼1350 cm−1), D′ band (∼1620 cm−1), and D+D′ band (∼2940 cm−1) associated with intrinsic defects were barely detectable, indicating the high quality of the newly peeled graphene samples. Figure 1b shows the D band intensity (I(D)) mapping of the chemically modified graphene after 180 s oxidation in KMnO4/H2SO4 solution. The defect-related bands became noticeable in SLG. The previous theoretical studies of the Raman spectrum have demonstrated that the D band is activated by local basal plane derivations (includes defects and disorders) associated with the sp3 phase in carbon materials.25−27 A comparison of the Raman spectra of graphene before and after the oxidation treatment further revealed a clear upshift of the G peak in SLG, suggesting a downshift of the Fermi level induced by the adsorbed oxygen atoms.28 A distinct behavior was noticed in BLG and FLG. We did not observe the defect-related bands in the oxidized BLG and FLG, indicating that they are more chemically inert compared with SLG. The layer-dependent chemical reactivity of graphene has also been proposed by other groups for modified graphene with πconjugated molecules, hydrogen, and oxygen species.17,29−31 A comparison result using the O2-plasma etching is shown in the Supporting Information (Figure S1). Figure 2a shows the evolution of graphene’s Raman spectra with increasing oxidation time. A significant growth of D and D′ peaks, accompanied with the appearance of a combination phonon mode (D+D′), was observed at the first 180 s period. As the oxidation proceeded, the Raman spectra of SLG showed 10703



Article

band over the G band, I(D)/I(G),33 which first increased and then decreased with the oxidation time, as shown in Figure 3. A similar effect has also been found for the graphene treated by the oxygen and hydrogen plasma.19,34 According to the Raman theories, the D band activated by defects arises from a breathing mode of the aromatic rings. As discussed in ref 26, the value of I(D)/I(G) is quantitatively related to the sp2-carbon clustering. The Tuinstra and Koening relation (TK) shows that the I(D)/ I(G) is inversely proportional to the carbon cluster size (La) in the case of micro- and nanocrystalline graphite

I(D) C(λ) = I(G) La where C (514 nm) is ∼4.4 nm.25 The appearance of the D band indicates the presence of the carbon aromatic rings. As the oxidation continues, the rings begin to break. The decrease of I(D) corresponds to the reduced cluster size, which is described by the formula below

Figure 2. Raman characterization of SLG as a function of the oxidation time. (a) Evolution of Raman spectra of SLG samples with increasing oxidation time. (b) Statistical data of the G peak shift, 2D peak shift, and I(2D)/I(G) from (a).

I(D) = C′(λ)La 2 I(G)

a systematic variation in both the intensity and the position of each band until 900 s, when two small humps and a broad PL background were observed. (The Raman results on SLG treated with different concentrations of KMnO4/H2 SO4 solution are presented in Figure S2, Supporting Information.) These experimental results agree well with the previous work on oxygen plasma treated graphene.18,19 The defect-related Raman bands of graphene are indicative of the generation of sp3-hybrized carbon.26,27 It has been reported that the sp3carbon defects in the oxidized graphene mainly involved the five- and six-membered lactol rings; ester of tertiary alcohol; and epoxy, hydroxyl, carboxyl, and ketone functionalities, as confirmed by NMR and XPS studies.11,22 (Our XPS experiment on chemical vapor deposition (CVD)-grown graphene treated by the same chemical method is shown in Figure S3, Supporting Information.) The fitting parameters to the main Raman bands of SLG are summarized in Figures2b and 3. The

where C′ (514 nm) is ∼0.55 nm−2.26,27 On the basis of this model, we deduce La based on the dependence of the oxidation time, as plotted out in Figure 3. The carbon cluster size was decreased to ∼1.1 nm after 900 s oxidation and did not change much for a longer treatment time. The high coverage of oxygen-related defects also induced PL in SLG. Figure 4a shows the optical image of a graphene sample with SLG and MLG after 540 s oxidation. The PL background at ∼605 nm (2 eV) started to appear with the remarkable Raman peaks of the D band (∼554 nm) and the G band (∼566 nm), as shown in Figure 4b. A longer oxidation time led to a rapid increase in PL intensity, implying an increase of defect density and a band-gap opening in the defective graphene. We interpret that the PL arises from the quantum confinement effect of small π-electron carbon islands with the average size of ∼1 nm deduced from the Raman study. The oxidized sites act as the barriers that limit the delocalization of the π electrons between sp2-carbon clusters. Given the massless electrons in graphene, we estimate that the energy band gap is about 2 eV for the islands with the size of ∼1 nm,19 which is in the range of the PL emission energy from 1.8 to 2.3 eV. It should be noted that a PL at around 4 eV was reported for chemically derived GO in solution and was attributed to the formation of a few aromatic rings under the aggressive oxidation condition.35 The spatial D band mapping of the oxidized sample (Figure 4c) and the corresponding intensity mapping of PL (Figure 4d) match well with each other, further indicating that the PL emission was originated from the defects or disorder in graphene. The absence of PL on BLG and MLG could be explained by the top surface oxidation mechanism. The underlying layers were protected from oxidation, as suggested by Raman studies on controlled modified graphene.36,37 As discussed above, the BLG and FLG are more inert compared with the SLG. No defect-related bands appeared on both BLG and FLG even with a long oxidation time (Figures 1c and 4c). However, the oxidation-induced change in the line shape of BLG’s G band was still observed (Figure 5). The G band eventually split into two modes of the G+ and G− bands at around 1590 and 1580 cm−1, respectively. As the oxidation time increased, the intensity ratio of these two bands (I(G−)/I(G+)) also increased. The splitting of the G band was previously

Figure 3. Evolution of the I(D)/I(G) acquired on SLG with different oxidation durations concluded from Figure 2a.

upshift of the peak positions (both G and 2D bands) and the decreasing intensity ratio of the 2D band over the G band (I(2D)/I(G)) depended on the oxidation time. It has been reported that the line shape of the G and 2D bands is related to the electron and hole concentration of graphene under the gating electric field.32 We thus conclude that the shift of the G and 2D bands and decreased I(2D)/I(G) are induced by the oxygen species attached on the graphene surface that act as electron acceptors. An important parameter to describe the disorders or defects in graphene is the intensity ratio of the D 10704



Article

Figure 4. (a) Optical micrograph of a graphene sample with a single layer and multilayers. (b) PL spectra were taken at the graphene sample in (a) after 540 s (solid) and 900 s (dotted) oxidation. The Raman peak is distinct from the PL background. (c) The I(D) Raman mapping for sample (a). (d) The PL mapping for sample (a). The D band mapping and the PL mapping images were acquired at 554 and 605 nm, respectively.

Table 1. Statistical Data for the Evolution of the G Band in Bilayer Graphene with Different Oxidation Times Raman shift (cm−1) G+ Raman shift (cm−1) G− intensity ratio G−/G+

Figure 5. Evolution of the Raman G band of the BLG sample as a function of the oxidation time. Two Lorentzian curves are used to fit the G band from 60 to 900 s. The blue lines connecting peaks are guides for the eye.

60 s

90 s

180 s

360 s

540 s

720 s

900 s

1590

1590

1592

1592

1591

1591

1592

1580

1580

1582

1582

1582

1582

1582

0.18

0.22

0.62

1.06

1.11

1.18

1.37

bands.40 The broken inversion symmetry caused by the oxygen defects in this experiment is equal to applying a gate voltage of −50 V on the bilayer graphene.40,41 Figure 6 shows that the blue shift of the G band after oxidation decreases rapidly with an increasing thickness of graphene. The upshift of the G band is only around 2 cm−1 for bulk graphite, which can be ignored as measurement error. This result demonstrates that the holedoping effect by oxygen atoms becomes weaker with the increasing layer number of graphene, suggesting the top surface modification of the graphene sample.17

observed in the electric field gated and fluorinated bilayer graphene. The asymmetrical G band was interpreted as the inversion symmetry breaking induced by the gate field effect and top surface modification.37−40 In the case of oxidized BLG, we believe that the majority of oxygen atoms were located on the top layer of graphene. Dipole moments were formed due to the charge transfer between the oxygen-related species and graphene. We propose that the nonequivalence between the top and bottom layers is induced by the static potential of dipoles, which breaks the symmetry in BLG. The broken symmetry of BLG makes the antisymmetric mode (G−) active in the Raman spectra, explaining the two components appearing in the G band. Table 1 shows the statistical analysis of the splitting G band induced by oxidation from Figure 5. We estimate a downshift of the Fermi level on bilayer graphene around 150 meV from the separation between the G+ and G−

■

CONCLUSIONS Defect-induced modulation of structural and electronic properties of graphene was achieved using a method of solution oxidation. The density of defects in graphene can be precisely controlled by adjusting the oxidation time. Raman and PL spectra suggested that the monolayer graphene is more chemically active compared with BLG and FLG. The oxygenrelated defects introduced the sp3-hybrid C−O matrix into the sp2-carbon network of SLG, significantly affecting the local 10705



Article

(2) Ohta, T.; Bostwick, A.; Seyller, T.; Horn, K.; Rotenberg, E. Science 2006, 313, 951−954. (3) Guinea, F.; Katsnelson, M. I.; Geim, A. K. Nat. Phys. 2009, 6, 30− 33. (4) Jiao, L.; Zhang, L.; Wang, X.; Diankov, G.; Dai, H. Nature 2009, 458, 877−880. (5) Kosynkin, D. V.; Higginbotham, A. L.; Sinitskii, A.; Lomeda, J. R.; Dimiev, A.; Price, B. K.; Tour, J. M. Nature 2009, 458, 872−877. (6) Han, M. Y.; Ö zyilmaz, B.; Zhang, Y.; Kim, P. Phys. Rev. Lett. 2007, 98, 206805. (7) Son, Y. M.; Cohen, M. L.; Louie, S. G. Phys. Rev. Lett. 2006, 97, 216803. (8) Elias, D. C.; Nair, R. R.; Mohiuddin, T. M. G.; Morozov, S. V.; Blake, P.; Halsall, M. P.; Ferrari, A. C.; Boukhvalov, D. W.; Katsnelson, M. I.; Geim, A. K.; et al. Science 2009, 323, 610−613. (9) Jaiswal, M.; Lim, C. H. Y. X.; Bao, Q.; Toh, C. T.; Loh, K. P.; Ö zyilmaz, B. ACS Nano 2011, 5, 888−896. (10) Nair, R. R.; Ren, W.; Jalil, R.; Riaz, I.; Kravet, V. G.; Britnell, L.; Blake, P.; Schedin, F.; Mayorov, A. S.; Yuan, S.; et al. Small 2010, 6, 2877−2884. (11) Gao, W.; Alemany, L. B.; Ci, L. J.; Ajayan, P. M. Nat. Chem. 2009, 1, 403−408. (12) Lerf, A.; He, H. Y.; Forster, M.; Klinowski, J. J. Phys. Chem. B 1998, 102, 4477−4482. (13) Szabó, T.; Berkesi, O.; Forgó, P.; Josepovits, K.; Sanakis, Y.; Petridis, D.; Dékány, I. Chem. Mater. 2006, 18, 2740−2749. (14) Mkhoyan, K. A.; Contryman, A. W.; Silcox, J.; Stewart, D. A.; Eda, G.; Mattevi, C.; Miller, S.; Chhowalla, M. Nano Lett. 2009, 9, 1058−1063. (15) Erickson, K.; Erni, R.; Lee, Z.; Alem, N.; Gannett, W.; Zettl, A. Adv. Mater. 2010, 22, 4467−4472. (16) Tao, H.; Moser, J.; Alzina, F.; Wang, Q.; Sotomayor-Torres, C. M. J. Phys. Chem. C 2011, 115, 18257−18260. (17) Liu, L.; Ryu, S.; Tomasik, M. R.; Stolyarova, E.; Jung, N.; Hybertsen, M. S.; Steigerwald, M. L.; Brus, L. E.; Flynn, G. W. Nano Lett. 2008, 8, 1965−1970. (18) Kim, D. C.; Jeon, D. Y.; Chung, H. J.; Woo, Y. S.; Shin, J. K.; Seo, S. Nanotechnology 2009, 20, 375703. (19) Gokus, T.; Nair, R. R.; Bonetti, A.; Böhmler, M.; Lombardo, A.; Novoselov, K. S.; Geim, A. K.; Ferrari, A. C.; Hartschuh, A. ACS Nano 2009, 3, 3963−3968. (20) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339. (21) Schniepp, H. C.; Li, J.; McAllister, M. J.; Sai, H.; HerreraAlonso, M.; Adamson, D. H.; Prud’homme, P. K.; Car, R.; Saville, D. A.; Aksay, I. A. J. Phys. Chem. B 2006, 110, 8535−8539. (22) Marcano, D. C.; Kosynkin, D. V.; Berlin, J. M.; Sinitskii, A.; Sun, Z.; Slesarev, A.; Alemany, L. B.; Lu, W.; Tour, J. M. ACS Nano 2010, 4, 4806−4814. (23) Pan, S.; Aksay, I. A. ACS Nano 2011, 5, 4073−4083. (24) Ferrari, A. C.; Meyer, J. C.; Scadaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Phys. Rev. Lett. 2006, 97, 187401. (25) Tuinstra, R.; Koening, J. L. J. Chem. Phys. 1970, 53, 1126−1130. (26) Ferrari, A. C.; Robertson, J. Phys. Rev. B 2000, 61, 14095. (27) Ferrari, A. C.; Robertson, J. Phys. Rev. B 2001, 64, 075414. (28) Wang, R.; Wang, S.; Zhang, D.; Li, Z.; Fang, Y.; Qiu, X. ACS Nano 2011, 5, 408−412. (29) Sharma, R.; Baik, J. H.; Perera, C. J.; Strano, M. S. Nano Lett. 2010, 10, 398−405. (30) Koehler, F. M.; Jacobsen, A.; Ensslin, K.; Stampfer, C.; Stark, W. J. Small 2010, 6, 1125−1130. (31) Ryu, S.; Han, M. Y.; Maultzsch, J.; Heinz, T. F.; Kim, P.; Steigerwald, M. L.; Brus, L. Nano Lett. 2008, 8, 4597−4602. (32) Das, A.; Pisana, S.; Chakraborty, B.; Piscanec, S.; Saha, S. K.; Waghmare, U. V.; Novoselov, K. S.; Krishnamurthy, H. R.; Geim, A. K.; Ferrari, A. C.; et al. Nat. Nanotechnol. 2008, 3, 210−215. (33) Luo, Z.; Yu, T.; Ni, Z.; Lim, S.; Hu, H.; Shang, J.; Liu, L.; Shen, Z.; Lin, J. J. Phys. Chem. C 2011, 115, 1422−1427.

Figure 6. Variation of the Raman G band of the graphene sample as a function of the layer numbers: (a) Raman spectra, (b) statistical data from (a). The oxidation time is 90 s.

electronic structure. As the size of the sp2-carbon quantum dots was reduced to 1 nm, the SLG showed an optical gap of approximately 2 eV. The oxygen-related defects also induced a broken inversion symmetry on BLG, resulting in a splitting of the G band in Raman spectra. The defect-induced nonequivalence between the top and bottom layers of BLG caused a 150 meV downshifting of the Fermi level. Our study provides a facile approach to modify the graphene on a chip and yields complementary information about the defect engineering on the band structure of graphene.

■

ASSOCIATED CONTENT

S Supporting Information *

Additional information on the Raman comparison of modified graphene by solution-phase oxidation and oxygen plasma, Raman comparison of oxidized graphene treated by different concentrations of KMnO4/H2SO4 solution, and XPS and Raman analysis of CVD-grown graphene treated by solutionphase oxidation. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 86-10-82545583. Fax: 8610-62656765. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Prof. Jin Zhang and Prof. Hailin Peng at Peking University for providing CVD-grown graphene. Financial support by the National Science Foundation of China (Nos. 20973046, 60911130231) and the Ministry of Science and Technology of China (No. 2010DFA54310) is acknowledged.

■

REFERENCES

(1) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183−191. 10706



Article

(34) Luo, Z.; Yu, T.; Kim, K.; Ni, Z.; You, Y.; Lim, S.; Shen, Z.; Wang, S.; Lin, J. ACS Nano 2009, 3, 1781−1788. (35) Eda, G.; Lin, Y.; Mattevi, C.; Yamaguchi, H.; Chen, H.; Chen, I.; Chen, C.; Chhowalla, M. Adv. Mater. 2010, 22, 505−509. (36) Nourbakhsh, A.; Cantoro, M.; Klekachev, A. V.; Pourtois, G.; Vosch, T.; Hofkens, J.; van der Veen, M. H.; Heyns, M. M.; Gendt, S. D.; Sels, B. F. J. Phys. Chem. C 2011, 115, 16619−16624. (37) Bruna, M.; Borini, S. Phys. Rev. B 2010, 81, 125421. (38) Yan, J.; Villarson, T.; Henriksen, E. A.; Kim, P.; Pinczuk, A. Phys. Rev. B 2009, 80, 241417. (39) Yan, J.; Henriksen, E. A.; Kim, P.; Pinczuk, A. Phys. Rev. Lett. 2008, 101, 136804. (40) Malard, L. M.; Elias, D. C.; Alves, E. S.; Pimenta, M. A. Phys. Rev. Lett. 2008, 101, 257401. (41) Oostinga, J. B.; Heersche, H. B.; Liu, X.; Morpurgo, A. F.; Vandersypen, L. M. K. Nat. Mater. 2008, 7, 151−157.

10707


Optical Spectroscopy Investigation of the Structural and Electrical

Recommend Documents