INVESTIGATION OF ELECTRONIC AND OPTICAL PROPERTIES OF MOLYBDENUM DISULFIDE MODULATED BY SURFACE FUNCTIONALIZATION LIN JIADAN (B.Sc. SICHUAN UNIVERSITY) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE (2014) Declaration I hereby declare that this thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. _______________ ___ LIN JIADAN Aug 2014 Dedicated to my beloved family, boyfriend and friends. ii Acknowledgement I take this opportunity to express my gratitude to the people who have been instrumental in the completion of this thesis. Foremost, I would like to express the deepest appreciation to my supervisor Prof. Chen Wei for his tremendous support and guidance. I am grateful to him for his stimulating suggestions and encouragement. In spite of his very busy schedule, he always gives me valuable and in-depth suggestions on my projects, reviews all my manuscripts carefully and offers detailed comments. What impressed me most is his enthusiasm, encouragement and faith in me throughout. Without his guidance, mentoring, persistent help and extensive knowledge, this thesis would not have been possible. I also would like to thank other group members in the surface and interface laboratory, Dr. Xie Lanfei, Mr. Han Cheng, Dr. Wang Yuzhang, Dr. Niu Tianchao, Dr. Wei Dacheng, Dr. Pan Feng, Dr. Zhang Jialin, Ms. Zhong Shu, Mr. Zhong Jianqiang, Dr. Mao Hongying, Mr. Wang Rui, Dr. Liu Yiyang, Dr. Rao Richuan and so many others, for their help and fruitful discussions during my experiments, for many happy days we spend together. I am also indebted to Dr. Li Hai from Nanyang Technological University, Dr. Lu JunPeng, Dr. Zheng Minrui, Dr. Hu Zhibin from Nanomaterials Research Lab, Dr. Jun You from Graphene Research Center, Mr. Zeng Shengwei, Dr. Wang Xiao from NanoCore, Dr. Mr. Han Sanyang from Chemistry Department, for supporting me during my study, for training me to use their equipments, for much fun working together. iii My sincere thanks to Prof. Andrew T. S. Wee, Prof. Zhang Hua, Prof. SOW Chorng Haur for their revision of my manuscript and allowing me to conduct experiment in their lab. The financial support from the National University of Singapore is gratefully acknowledged. Last but not the least I place a deep sense of gratitude to my family for loving, caring, and believing in me. Their love and guidance has better prepared me to face challenges in the future. Finally, special thanks to my loving and supportive boyfriend Mr. Di Kai, who has been a constant source of inspiration during my study. His tender love, companionship are appreciated throughout all the seasons of life. iv List of Publications 1. “Plasmonic enhancement of photocurrent in MoS2 field-effect-transistor” Lin JD, Li H, Zhang H, Chen Wei*, Appl. Phys. Lett. 102, 203109 (2013) 2. “Modulating electronic transport properties of MoS2 field effect transistor by surface overlayers” Lin JD, Zhong JQ, Zhong S, Li H, Zhang H, Chen Wei*, Appl. Phys. Lett. 103, 063109 (2013) 3. “Electron-Doping-Enhanced Trion Formation in Monolayer Molybdenum Disulfide Functionalized with Cesium Carbonate” Lin JD, Han C, Wang F, Wang R, Xiang D, Qin SQ, Zhang XA,Wang L, Zhang H, Wee ATS, Chen Wei*, ACS Nano 8, 5323 (2014) 4. “Manipulating the Electronic Properties of Graphene via Molecular Functionalization” (invited review article) Mao HY, Lu YH, Lin JD, Zhong S, Wee ATS, Chen Wei*, Prog. Surf. Sci. 88, 132-159 (2013) 5. “High work function anode interfacial layer via mild temperature thermal decomposition of C60F36 thin film on ITO” Mao HY, Wang R, Zhong JQ, Zhong S, Lin JD, Wang XZ, Chen ZK, Chen Wei*, J. Mater. Chem. C., 1, 1491-1499 (2013) 6. “Ionization Potential Dependent Air Exposure Effect on the MoO3/Organic Interface Electronic Structures” Zhong JQ, Mao HY, Wang R, Lin JD, Zhao YB, Zhang JL, Ma DG, Chen Wei* Organic Electronics 12, 2793-2800 (2012). 7. “The Role of Gap States on the Energy Level Alignment at the Organic Heterojunction Interfaces” (invited review article) Zhong S, Zhong JQ, Mao HY, Zhang JL, Lin JD, Chen Wei*, Phys. Chem. Chem. Phys., 14, 14127-14141 (2012). v 8. “Improving chemical vapor deposition graphene conductivity using molybdenum trioxide: An in-situ field effect transistor study ” Han C, Lin JD, Xiang D, Wang CC, Wang L and Wei Chen*, Appl. Phys. Lett. 103, 263117 (2013) 9. “Improved Photoelectrical Properties of MoS2 Films after Laser Micromachining” Lu JP, Lu JH, Liu HW, Liu B, Chan XK, Lin JD, Chen W, Loh KP, Sow Chorng Haur* ACS Nano 8, 6334 (2014) 10. “Ultrathin MnO2 Nanoflakes as an Efficient Catalyst for Oxygen Reduction Reaction” Wei C; Yu LH; Cui CL; Lin JD; Chen W; Mathews, N; Huo FW; Sritharan, T; Xu, Z* Chem. Commun 50, 7885 (2014) vi Table of Contents Declaration . i Acknowledgement iii List of Publications v Table of Contents .vii Summary . x List of Tables xii List of Figures . xiii List of Abbreviations .xxii Chapter Introduction 1.1 2D TMDCs: background and literature review 1.1.1 Synthesis of 2D TMDCs 1.1.2 Electronic, optical and vibrational properties of TMDCs . Electronic structure . Optical properties . 1.1.3 Device applications for 2D TMDCs 13 2D TMDCs FET . 13 Optoelectronics . 22 1.1.4 Surface functionalization of 2D TMDCs . 30 Substitutional doping 30 Charge transfer doping (Surface functionalization) . 32 1.2 Objective and scope of this thesis 39 Chapter Experimental techniques . 41 2.1 Preparation of MoS2 . 41 2.2 Experimental techniques for device fabrication and measurements. . 42 2.2.1 Electron beam lithography . 42 2.2.2 Electrical measurement 46 2.2.3 Optoelectronic measurement . 47 2.3 Experimental techniques for spectroscopic studies 48 2.3.1 Ultraviolet and X-ray photoemission spectroscopy . 48 vii 2.3.2 Raman spectroscopy and photoluminescence spectroscopy . 52 Chapter Modulating MoS2 electrical transport and optical properties by surface overlayers 54 3.1 Introduction 54 3.2 Experiments details 56 3.3 Modulating MoS2 FET electronic transport properties by C60 . 57 3.4 Modulating MoS2 electrical transport and optical properties by MoO3 . 61 3.5 Conclusion 66 Chapter Electron-Doping Enhanced Trion Formation in Monolayer Molybdenum Disulfide Functionalized with Cesium Carbonate 67 4.1 Introduction 67 4.2 sample preparation and device fabrication . 69 4.3 Modulating MoS2 FET electronic transport properties by Cs2CO3 surface functionalization . 71 4.4 Modulating optical properties of MoS2 by Cs2CO3 surface functionalization . 76 4.5 Air stability evaluation of Cs2CO3 doping effect . 78 4.6 Conclusion 81 Chapter Plasmonic enhancement of photocurrent in MoS2 field-effecttransistor . 82 5.1 Introduction 82 5.2 Experiments details 83 5.3 Photocurrent measurements of MoS2 FET with Au NPs. . 84 5.4 Simulations on Enhanced Light Intensity by Surface Plasma in Au Sphere . 89 5.5 Conclusion 91 Chapter Probing the interfacial interaction between monolayer MoS2 and Au nanoclusters 92 6.1 Introduction 92 6.2 Device fabrication 93 6.3 Investigate the initial growth mode of Au on MoS2 and its effect on transport properties of MoS2 FET 95 6.4 Probing the MoS2/Au interface interaction and the effect of Au deposition on the optical properties of MoS2 underneath 99 6.5 Conclusion 104 viii Chapter Conclusions and outlook . 105 7.1 Thesis summary 105 7.2 Future work 108 Bibliography . 110 ix (c) (b) (a) S2p Au4f Mo3d Au12 Å Intensity(a.u.) Au Å Au Å Au 0.5 Å MoS2 92 88 84 Binding Energy(eV) 80 168 166 164 162 160 158 Binding Energy(eV) 236 232 228 224 Binding Energy(eV) Figure 46 The XPS core level spectra of (a) Au 4f, (b) S 2p and (c) Mo 3d during the deposition of Au on bulk MoS2. We also investigated the influence of Au nanoclusters decoration on photoluminescence and vibration properties of single layer MoS2, as shown in Figure 6.6. Photoluminescence and Raman spectra were collected at room temperature on an Alpha 300 R microscopy system (laser wavelength 532 nm). The Raman peak of Si at 520 cm-1 was used as a reference to calibrate the spectrometer. Figure 6.6 a (blue line) displays a typical photoluminescence spectrum of single layer MoS2 with two prominent peaks located at around 1.86eV (A) and 2.01eV (B). These two peaks arise from the direct transitions at K point of the Brillouin zone between the highest spin-orbital split valence bands and lowest conduction bands219. After Au nanoclusters decoration, the PL intensity of single layer MoS2 was largely suppressed. As shown in Figure 6.3 a, Au decoration can slightly increase the electron charge carrier concentration. This can enhance the formation of the negatively charged trions and hence reduce the PL intensity, accompanied by a red shift of the peak A18, 30, 38-39. However, 101 we did not observe any obvious peak position shift of peak (A). Therefore, the slight electron doping in MoS2 is not the main reason to cause the PL intensity reduction. Because of the large electron affinity of Au, the photon-excitation induced electron in MoS2 can be trapped by Au nanoclusters, and hence prevent the charge recombination, thereby largely suppressing the photoluminescence in single layer MoS2. The Raman spectrum of the pristine single layer MoS2 (Figure 6.6 b) possesses two prominent peaks, arising from the out of plane Raman mode (A1g) at and the in plane mode (E12g), respectively37,136,193. The decoration of Au nanoclusters has a negligible influence to the E12g in plane mode, but induces a slight blue shift from to for the A1g out of plane mode. The decoration of Au nanoclusters on MoS2 can increase the effective mass along the out of plane direction, thereby stiffening the frequency of A1g phonon40. 102 Figure 47 (a) PL and (b) Raman spectra of 1L-MoS2 before and after Au nanoclusters decoration at room temperature. 103 6.5 Conclusion In summary, we have systematically investigated the interfacial interactions between single layer MoS2 and Au nanoclusters by in-situ MoS2 FET device evaluation. In-situ UPS/XPS measurements reveal that Au nanoclusters weakly interact with the single layer MoS2. The decoration of small amount of Au nanoclusters can screen the trapped charges in the single layer MoS2 and hence improve the electron mobility. As indicated by in-situ FET device characterization, electron concentration can slightly increase after the Au decoration. Moreover, the decorated Au nanoclusters can trap the photon-excitation induced electrons, and hence reduce the charge recombination, thereby largely suppressing the photoluminescence intensity. 104 Chapter Conclusions and outlook 7.1 Thesis summary This thesis explored unique electronic and optical properties of surfacemodified MoS2. We aim to study the molecular/MoS2 interface physics, and the interface engineered electrical transport and optical properties of single layer MoS2. After surface decoration with MoO3, C60, Cs2CO3 and some metal nanoparticles or films, many promising electronic and optical properties distinct from pristine MoS2 have been revealed. In-situ electrical transport measurements were performed device characterization as well as evaluation of doping effect from surface layers. UPS and XPS measurements were utilized to provide deep-insight information of interface physics. PL and Raman measurements were also employed to investigate the optical properties. In Chapter 3, in-situ bottom-gated MoS2 FETs device characterization and in-situ UPS/XPS measurements were combined to investigate the effect of surface modification layers of C60 and MoO3 on the electronic properties of single layer MoS2. It is found that C60 decoration keeps MoS2 FETs performance intact due to the very weak interfacial interactions, making C60 as an ideal capping layer for MoS2 devices. In contrast, decorating MoO3 on MoS2 induces significant charge transfer at the MoS2/MoO3 interface and largely depletes the electron charge carriers in MoS2 FET devices. The PL intensity of MoS2 was drastically enhanced by the adsorption of MoO3, which is explained by the suppression of the negative trions formation and hence the enhancement of charge recombination dominated PL process. 105 In contrast to adsorption of p-type dopants, we report effective and stable electron-doping of monolayer MoS2 by Cs2CO3 surface functionalization in Chapter 4. The electron charge carrier concentration in exfoliated monolayer MoS2 can be increased by about times after Cs2CO3 functionalization. The n-type doping effect was evaluated by in-situ transport measurements of MoS2 FETs, which is further corroborated by in-situ UPS/XPS, and Raman scattering measurements. The electron doping enhances the formation of negative trions in monolayer MoS2 under light irradiation, and significantly reduces the charge recombination of photo-excited electron-hole pairs. This results in large photoluminescence suppression and an obvious photocurrent enhancement in monolayer MoS2 FETs. MoS2 has attracted considerable attention for numerous applications in optoelectronics. For practical device applications, one of the primary challenges is to maximum light-MoS2 interactions. In Chapter 5, we demonstrate a plasmonic enhancement of photocurrent in MoS2 FETs decorated with Au NPs, with significantly enhanced photocurrent peaked at the plasmon resonant wavelength around 540 nm. Our findings offer a possibility to realize wavelength selectable photodetection in MoS2 based phototransistors. A better understanding of the interfacial interaction between monolayer MoS2 and Au is crucial for optimization of device performance, as Au is widely used as a material for surface modification as well as an electrodes material. Therefore, we investigated the in-situ evaluation of electrical transport properties during the growth of nanometer Au films on MoS2 FET, 106 where Au assembles into nanoclusters. In Chapter 6, we probe the Au/MoS2 interface by monitoring the device performance based on back-gated MoS2 FETs by gradually decorating Au nanoclusters on top. We observe mobility enhancement at small amount of Au nanoclusters, which declines hereafter, with slight electron transfer from the Au nanoclusters to the MoS2 channel. The weak dependence of electron concentration on Au nanoclusters thickness, combined with in-situ UPS/UPS measurements, reveals he nature of weak interaction at the Au/MoS2 interface. The absence of strong charge transfer from Au nanoclusters to MoS2 is further confirmed by the photoluminescence and Raman spectra of single layer MoS2 decorated by Au nanoclusters. The out-of-plane A1g phonon is stiffened after Au nanocluster decoration; whereas the in-plane E12g phonon remains essentially unchanged. These findings are useful for better understanding of the contact effects in MoS2 based devices. 107 7.2 Future work Complementary doping is essential for high-performance circuit applications based on MoS2 FETs. In Chapters and 4, we have achieved electron-doping and electron depletion in MoS2 with different type of dopants. However, a key unsolved issue is to realize effective p-type doping of MoS2 based FETs. One approach to realize p-type performance is to combine strong p-type dopants with channel materials, switching majority charge carrier type from electron to hole. Another approach is contact engineering through chemical doping at contact region. The chemical doping can adjust Schottky barrier at metal/semiconductor interface and modulate the work function of metal electrodes, facilitating charge carrier injection. This phenomenon may also be an interesting direction for future work. For optoelectronic applications, approaches for boosting light absorption and enhancing PL efficiencies need to be demonstrated. There are several methods to achieve maximum light absorption in MoS2. One of them is to utilize localized surface plasmons. In Chapter 5, we have studied the plasmonic enhancement of Au NPs on MoS2 surface. However, the effects of different arrangements of NPs, different sizes of NPs and even the composition of the NPs remain unknown. Therefore, future work on the plasominc enhancement of MoS2 with well controlled NPs arrangement and size distribution is required. Moreover, with the help of standard lithography processes, other well-designed plasmonic structures can be employed on MoS2. Wavelength selectivity could be achieved by applying plasmonic structures with different geometries. 108 In addition to applications in functional electronic circuits and optoelectronics, more work is needed to develop novel applications of surface modified 2D TMDCs, such as applications in sensing. Being layered semiconductors, members of the TMDCs family make natural partners for each other to form heterostructures. Combining the unique properties of different materials in hybrid heterostructures may realize new devices with numerous novel functionalities. 109 Bibliography 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman, and M. S. Strano, Nat. Nanotech. 7, 699 (2012). L. Li, Y. Yu, G. J. Ye, Q. Ge, X. Ou, H. Wu, D. Feng, X. H. Chen, and Y. Zhang, Nat. Nanotech. 9, 372 (2014). K. S. Novoselov, V. I. Falko, L. Colombo, P. R. Gellert, M. G. Schwab, and K. Kim, Nature 490, 192 (2012). A. K. Geim, Science 324, 1530 (2009). S. Balendhran, S. Walia, H. Nili, J. Z. Ou, S. Zhuiykov, R. B. Kaner, S. Sriram, M. Bhaskaran, and K. Kalantar-zadeh, Adv. Funct. Mater. 23, 3952 (2013). K. Watanabe, T. Taniguchi, and H. Kanda, Nat. Mater. 3, 404 (2004). G. C. Bond and S. F. Tahir, Appl. Catal. 71, (1991). A. K. Geim and K. S. Novoselov, Nat. Mater. 6, 183 (2007). J. H. Chen, C. Jang, S. D. Xiao, M. Ishigami, and M. S. Fuhrer, Nat. Nanotech. 3, 206 (2008). K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, Science 306, 666 (2004). F. Schwierz, Nat. Nanotech. 5, 487 (2010). F. N. Xia, D. B. Farmer, Y. M. Lin, and P. Avouris, Nano Lett. 10, 715 (2010). X. Dong, Y. Shi, Y. Zhao, D. Chen, J. Ye, Y. Yao, F.Gao, Z. Ni, T. Yu, Z. Shen et al., Phys. Rev. Lett. 102, (2009). W. J. Zhang, C. T. Lin, K. K. Liu, T. Tite, C. Y. Su, C. H. Chang, Y. H. Lee, C. W. Chu, K. H. Wei, J. L. Kuo et al., ACS Nano 5, 7517 (2011). S. Lebègue and O. Eriksson, Phys. Rev. B 79, (2009). R. Kasowski, Phys. Rev. Lett. 30, 1175 (1973). K. Mak, C. Lee, J. Hone, J. Shan, and T. Heinz, Phys. Rev. Lett. 105, (2010). R. S. Sundaram, M. Engel, A. Lombardo, R. Krupke, A. C. Ferrari, P. Avouris, and M. Steiner, Nano Lett. 13, 1416 (2013). H. R. Gutierrez, N. Perea-Lopez, A. L. Elias, A. Berkdemir, B. Wang, R. Lv, F. Lopez-Urias, V. H. Crespi, H. Terrones, and M. Terrones, Nano Lett. 13, 3447 (2012). K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos, and A. A. Firsov, Nature 438, 197 (2005). X. Huang, Z. Zeng, and H. Zhang, Chem. Soc. Rev. 42, 1934 (2013). H. Li, G. Lu, Y. Wang, Z. Yin, C. Cong, Q. He, L. Wang, F. Ding, T. Yu, and H. Zhang, Small 9, 1974 (2013). C. R. Dean, A. F. Young, I. Meric, C. Lee, L. Wang, S. Sorgenfrei, K. Watanabe, T. Taniguchi, P. Kim, K. L. Shepard et al., Nat. Nanotech. 5, 722 (2010). J. Kedzierski, P. L. Hsu, P. Healey, P. W. Wyatt, C. L. Keast, M. Sprinkle, C. Berger, and W. A. de Heer, IEEE Trans. Electron Dev. 55, 2078 (2008). M. Chhowalla, H. S. Shin, G. Eda, L. Li, K. P. Loh, and H. Zhang, Nat. Chem. 5, 263 (2013). Y. Hernandez, V. Nicolosi, M. Lotya, F. M. Blighe, Z. Sun, S. De, I. T. McGovern, B. Holland, M. Byrne, Y. K. G. 'Ko et al., Nat. Nanotech. 3, 563 (2008). K. Lee, H. Kim, M. Lotya, J. N. Coleman, G. Kim, and G. S. Duesberg, Adv. Mater. 23, 4178 (2011). D. Yang and R. F. Frindt, J. Phys. Chem. Solids 57, 1113 (1996). Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman, and M. S. Strano, Nat. Nanotech. 7, 699 (2012). Z. Zeng, Z. Yin, X. Huang, H. Li, Q. He, G. Lu, F. Boey, and H. Zhang, Angew. Chem. Int. Ed. 50, 11093 (2011). D. C. Wei and Y. Q. Liu, Adv. Mater. 22, 3225 (2010). X. Huang, Z. Yin, S. Wu, X. Qi, Q. He, Q. Zhang, Q. Yan, F. Boey, and H. Zhang, Small 7, 1876 (2011). A. Reina, X. T. Jia, J. Ho, D. Nezich, H. B. Son, V. Bulovic, M. S. Dresselhaus, and J. Kong, Nano Lett. 9, 30 (2009). K. Liu, W. Zhang, Y. Lee, Y. Lin, M. Chang, C. Su, C. Chang, H. Li, Y. Shi, H. Zhang et al., Nano Lett. 12, 1538 (2012). 110 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 Y. Lee, X. Zhang, W. Zhang, M. Chang, C. Lin, K. Chang, Y. Yu, J. T. Wang, C. Chang, L. Li et al., Adv. Mater. 24, 2320 (2012). A. L. Elías, N. Perea-López, A. Castro-Beltrán, A. Berkdemir, R. Lv, S. Feng, A.D. Long, T. Hayashi, Y. A. Kim, M. Endo et al., ACS Nano 7, 5235 (2013). C. Lee, H. Yan, L. E. Brus, T. F. Heinz, J. Hone, and S. Ryu, ACS Nano 4, 2695 (2010). J. McMenamin and W. Spicer, Phys. Rev. Lett. 29, 1501 (1972). K. S. Novoselov, Proc. Natl. Acad. Sci. U.S.A. 102, 10451 (2005). Y. Zhang, T. Chang, B. Zhou, Y. Cui, H. Yan, Z. Liu, F. Schmitt, J. Lee, R. Moore, Y. Chen et al., Nat. Nanotech. 9, 111 (2014). W. Zhao, Z. Ghorannevis, L. Chu, M. Toh, C. Kloc, P. Tan, and G. Eda, ACS Nano 7, 791 (2012). R. Coehoorn, C. Haas, and R. de Groot, Phys. Rev. B 35, 6203 (1987). R. Coehoorn, C. Haas, J. Dijkstra, C. Flipse, R. de Groot, and A. Wold, Phys. Rev. B 35, 6195 (1987). M. Traving, M. Boehme, L. Kipp, M. Skibowski, F. Starrost, E. E. Krasovskii, A. Perlov, and W. Schattke, Phys. Rev. B 55, 10392 (1997). H. Jiang, J. Phys. Chem. C 116, 7664 (2012). A. Kumar and P. K. Ahluwalia, EPJ B 85, (2012). S. Horzum, H. Sahin, S. Cahangirov, P. Cudazzo, A. Rubio, T. Serin, and F. Peeters, Phys. Rev. B 87, (2013). S. Tongay, J. Zhou, C. Ataca, K. Lo, T. S. Matthews, J. Li, J. C. Grossman, and J. Wu, Nano Lett. 12, 5576 (2012). T. Böker, R. Severin, A. Müller, C. Janowitz, R. Manzke, D. Voß, P. Krüger, A. Mazur, and J. Pollmann, Phys. Rev. B 64, 235305 (2001). A. Kumar and P. K. Ahluwalia, Physica B: Condens. Matter 407, 4627 (2012). A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C. Chim, G. Galli, and F. Wang, Nano Lett. 10, 1271 (2010). G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. Chen, and M. Chhowalla, Nano Lett 11, 5111 (2011). H. R. Gutierrez, N. Perea-Lopez, A. L. Elias, A. Berkdemir, B. Wang, R. Lv, F. Lopez-Urias, V. H. Crespi, H. Terrones, and M. Terrones, Nano Lett. 13, 3447 (2013). W. Zhao, R. M. Ribeiro, M. Toh, A. Carvalho, C. Kloc, A. H. Castro Neto, and G. Eda, Nano Lett. 13, 5627 (2013). Y. Chen, J. Xi, D. O. Dumcenco, Z. Liu, K. Suenaga, D. Wang, Z. Shuai, Y. Huang, and L. Xie, ACS Nano 7, 4610 (2013). P. Tonndorf, R. Schmidt, P. Böttger, X. Zhang, J. Börner, A. Liebig, M. Albrecht, C. Kloc, O. Gordan, D. R. T. Zahn et al., Opt. Express 21, 4908 (2013). K. Kośmider, J. W. González, and J. Fernández-Rossier, Phys. Rev. B 88, 035448 (2013). Z. Y. Zhu, Y. C. Cheng, and U. Schwingenschlögl, Phys. Rev. B 84, (2011). Q. Ji, Y. Zhang, T. Gao, Y. Zhang, D. Ma, M. Liu, Y. Chen, X. Qiao, P. H. Tan, M. Kan et al., Nano Lett. 13, 3870 (2013). K. F. Mak, K. He, C. Lee, G. H. Lee, J. Hone, T. F. Heinz, and J. Shan, Nat. Mater. 12, 207 (2013). A. K. M. Newaz, D. Prasai, J. I. Ziegler, D. Caudel, S. Robinson, R. F. Haglund Jr, and K. I. Bolotin, Solid State Commun. 155, 49 (2013). Aaron M. Jones, Hongyi Yu, Nirmal J. Ghimire, Sanfeng Wu, Grant Aivazian, Jason S. Ross, Bo Zhao, Jiaqiang Yan, David G. Mandrus, Di Xiao et al., Nat. Nanotech. 8, 634 (2013). S. Tongay, J. Suh, C. Ataca, W. Fan, A. Luce, J. S. Kang, J. Liu, C. Ko, R. Raghunathanan, J. Zhou et al., Sci. Rep. 3, 2657 (2013). H. Sahin, S. Tongay, S. Horzum, W. Fan, J. Zhou, J. Li, J. Wu, and F. Peeters, Phys. Rev. B 87, 165409 (2013). H. Terrones, E. Del Corro, S. Feng, J. M. Poumirol, D. Rhodes, D. Smirnov, N. R. Pradhan, Z. Lin, M. A. Nguyen, A. L. Elias et al., Sci. Rep. 4, 4215 (2014). S. Sahoo, A. P. S. Gaur, M. Ahmadi, M. J. F. Guinel, and R. S. Katiyar, J. Phys. Chem. C 117, 9042 (2013). 111 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 A. Berkdemir, H. R. Gutiérrez, A. R. Botello-Méndez, N. Perea-López, A. L. Elías, C. Chia, B. Wang, V. H. Crespi, F. López-Urías, J. Charlier et al., Sci. Rep. 3, 1755 (2013). W. Zhao, Z. Ghorannevis, K. K. Amara, J. R. Pang, M. Toh, X. Zhang, C. Kloc, P. H. Tan, and G. Eda, Nanoscale 5, 9677 (2013). D. Jariwala, V. K. Sangwan, L. J. Lauhon, T. J. Marks, and M. C. Hersam, ACS Nano 8, 1102 (2014). V. Podzorov, M. E. Gershenson, C. Kloc, R. Zeis, and E. Bucher, Appl. Phys. Lett. 84, 3301 (2004). W. S. Hwang, M. Remskar, R. Yan, V. Protasenko, K. Tahy, S. D. Chae, P. Zhao, A. Konar, H. Xing, A. Seabaugh et al., Appl. Phys. Lett. 101, 013107 (2012). S. Larentis, B. Fallahazad, and E. Tutuc, Appl. Phys. Lett. 101, 223104 (2012). Z. Y. Yin, H. Li, H. Li, L. Jiang, Y. M. Shi, Y. H. Sun, G. Lu, Q. Zhang, X. D. Chen, and H. Zhang, ACS Nano 6, 74 (2012). S. Ghatak, A. N. Pal, and A. Ghosh, ACS Nano 5, 7707 (2011). B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, and A. Kis, Nat Nanotech. 6, 147 (2011). Y. Yoon, K. Ganapathi, and S. Salahuddin, Nano Lett. 11, 3768 (2011). S. Walia, S. Balendhran, Y. Wang, R. A. Kadir, A. S. Zoolfakar, P. Atkin, J. Z. Ou, S. Sriram, K. Kalantar-zadeh, and M. Bhaskaran, Appl. Phys. Lett. 103, 232105 (2013). H. Qiu, L. Pan, Z. Yao, J. Li, Y. Shi, and X. Wang, Appl. Phys. Lett. 100, 123104 (2012). S. Das, H. Y. Chen, A. V. Penumatcha, and J. Appenzeller, Nano Lett. 13, 100 (2013). C. Gong, C. Huang, J. Miller, L. Cheng, Y. Hao, D. Cobden, J. Kim, R. S. Ruoff, R. M. Wallace, K. Cho et al., ACS Nano 7, 11350 (2013). W. Liu, J. Kang, D. Sarkar, Y. Khatami, D. Jena, and K. Banerjee, Nano Lett. 13, 1983 (2013). H. M. Li, D. Y. Lee, M. S. Choi, D. Qu, X. Liu, C. H. Ra, and W. J. Yoo, Sci. Rep. 4, 4041 (2014). W. Mönch, Appl. Phys. Lett. 72, 1899 (1998). H. Liu, M. Si, S. Najmaei, A. T. Neal, Y. Du, P. M. Ajayan, J. Lou, and P. D. Ye, Nano Lett. 13, 2640 (2013). H. Liu, M. Si, Y. Deng, A. T. Neal, Y. Du, S. Najmaei, P. M. Ajayan, J. Lou, and P. D. Ye, ACS Nano 8, 1031 (2013). D. Liu, Y. Guo, L. Fang, and J. Robertson, Appl. Phys. Lett. 103, 183113 (2013). S. Das, A. Prakash, R. Salazar, and J. Appenzeller, ACS Nano 8, 1681 (2014). J. R. Chen, P. M. Odenthal, A. G. Swartz, G. C. Floyd, H. Wen, K. Y. Luo, and R. K. Kawakami, Nano Lett. 13, 3106 (2013). A. Dankert, L. Langouche, M. V. Kamalakar, and S. P. Dash, ACS Nano 8, 476 (2013). B. Radisavljevic and A. Kis, Nat. Mater. 12, 815 (2013). W. Bao, X. Cai, D. Kim, K. Sridhara, and M. S. Fuhrer, Appl. Phys. Lett. 102, 042104 (2013). M. M. Perera, M. Lin, H. Chuang, B. P. Chamlagain, C. Wang, X. Tan, M. M. Cheng, D. Tománek, and Z. Zhou, ACS Nano 7, 4449 (2013). Y. Zhang, J. Ye, Y. Matsuhashi, and Y. Iwasa, Nano Lett. 12, 1136 (2012). D. Braga, I. G. Lezama, H. Berger, and A. F. Morpurgo, Nano Lett. 12, 5218 (2012). B. Radisavljevic, M. B. Whitwick, and A. Kis, ACS Nano 5, 9934 (2011). H. Wang, L. Yu, Y. H. Lee, Y. Shi, A. Hsu, M. L. Chin, L. J. Li, M. Dubey, J. Kong, and T. Palacios, Nano Lett. 12, 4674 (2012). J. Huang, J. Pu, C. Hsu, M. Chiu, Z. Juang, Y. Chang, W. Chang, Y. Iwasa, T. Takenobu, and L. Li, ACS Nano 8, 923 (2013). H. Wang, L. Yu, Y. Lee, W. Fang, A. Hsu, P. Herring, M. Chin, M. Dubey, L. Li, J. Kong et al., presented at the Electron Devices Meeting (IEDM), 2012 IEEE International, 2012 (unpublished). A. Ramasubramaniam, Phys. Rev. B 86, (2012). N. Perea-López, A. L. Elías, A. Berkdemir, A. Castro-Beltran, H. R. Gutiérrez, S. Feng, R. Lv, T. Hayashi, F. López-Urías, S. Ghosh et al., Adv. Funct. Mater. 23, 5511 (2013). 112 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 M. Fontana, T. Deppe, A. K. Boyd, M. Rinzan, A. Y. Liu, M. Paranjape, and P. Barbara, Sci. Rep. 3, 1634 (2013). J. Buck, J. Iwicki, K. Rossnagel, and L. Kipp, Phys. Rev. B 83, 075312 (2011). A. Schellenberger, R. Schlaf, C. Pettenkofer, and W. Jaegermann, Phys. Rev. B 45, 3538 (1992). M. Buscema, M. Barkelid, V. Zwiller, H. S. van der Zant, G. A. Steele, and A. Castellanos-Gomez, Nano Lett. 13, 358 (2013). C. Wu, D. Jariwala, V. K. Sangwan, T. J. Marks, M. C. Hersam, and L. J. Lauhon, J. Phys. Chem. Lett. 4, 2508 (2013). F. Bonaccorso, Z. Sun, T. Hasan, and A. C. Ferrari, Nat. Photon. 4, 611 (2010). F. N. Xia, T. Mueller, Y. M. Lin, A. Valdes-Garcia, and P. Avouris, Nat. Nanotech. 4, 839 (2009). F. N. Xia, T. Mueller, R. Golizadeh-Mojarad, M. Freitag, Y. M. Lin, J. Tsang, V. Perebeinos, and P. Avouris, Nano Lett. 9, 1039 (2009). W. Choi, M. Y. Cho, A. Konar, J. H. Lee, G. B. Cha, S. C. Hong, S. Kim, J. Kim, D. Jena, J. Joo et al., Adv. Mater. 24, 5832 (2012). O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic, and A. Kis, Nat. Nanotech. 8, 497 (2013). T. Georgiou, R. Jalil, B. D. Belle, L. Britnell, R. V. Gorbachev, S. V. Morozov, Y. Kim, A. Gholinia, S. J. Haigh, O. Makarovsky et al., Nat. Nanotech. 8, 100 (2013). G. Eda, T. Fujita, H. Yamaguchi, D. Voiry, M. Chen, and M. Chhowalla, ACS Nano 6, 7311 (2012). K. Kośmider and J. Fernández-Rossier, Phys. Rev. B 87, 075451 (2013). H. Terrones, F. Lopez-Urias, and M. Terrones, Sci. Rep. 3, 1549 (2013). D. Tsai, K. Liu, D. Lien, M. Tsai, C. Kang, C. Lin, L. Li, and J. He, ACS Nano 7, 3905 (2013). T. Dung-Sheng, L. Der-Hsien, T. Meng-Lin, S. Sheng-Han, C. Kuan-Ming, K. JrJian, Y. Yueh-Chung, L. Lain-Jong, and H. Jr-Hau, IEEE J. Sel. Top. Quant. Electron. 20, 30 (2014). K. Roy, M. Padmanabhan, S. Goswami, T. P. Sai, G. Ramalingam, S. Raghavan, and A. Ghosh, Nat. Nanotech. 8, 826 (2013). W. Zhang, C. Chuu, J. Huang, C. Chen, M. Tsai, Y. Chang, C. Liang, Y. Chen, Y. Chueh, J. He et al., Sci. Rep. 4, 3826 (2014). L. Britnell, R. M. Ribeiro, A. Eckmann, R. Jalil, B. D. Belle, A. Mishchenko, Y. J. Kim, R. V. Gorbachev, T. Georgiou, S. V. Morozov et al., Science 340, 1311 (2013). W. J.Yu, Y. Liu, H. Zhou, A. Yin, Z. Li, Y. Huang, and X. Duan, Nat. Nanotech. 8, 952 (2013). M. Bernardi, M. Palummo, and J. C. Grossman, Nano Lett. 13, 3664 (2013). L. Kai-Tak, C. Xi, and G. Jing, IEEE Electron Device Lett. 34, 1331 (2013). M. Li, D. Esseni, G. Snider, D. Jena, and H. G. Xing, J. Appl. Phys. 115, 074508 (2014). G. Eda and S. A. Maier, ACS Nano 7, 5660 (2013). A. Carvalho and A. H. Castro Neto, Phys. Rev. B 89, 081406 (2014). Q. Yue, S. Chang, S. Qin, and J. Li, Phys. Lett. A 377, 1362 (2013). K. Dolui, I. Rungger, C. D. Pemmaraju, and S. Sanvito, Phys. Rev. B 88, 075420 (2013). A. Klein, P. Dolatzoglou, M. Lux-Steiner, and E. Bucher, Sol. Energ. Mat. Sol. Cells 46, 175 (1997). S. Y. Hu, M. C. Cheng, K. K. Tiong, and Y. S. Huang, J. Phys.: Condens. Matter 17, 3575 (2005). M. R. Laskar, D. N. Nath, L. Ma, E. W. Lee, C. H. Lee, T. Kent, Z. Yang, R. Mishra, M. A. Roldan, J. Idrobo et al., Appl. Phys. Lett. 104, 092104 (2014). H. Komsa, J. Kotakoski, S. Kurasch, O. Lehtinen, U. Kaiser, and A. V. Krasheninnikov, Phys. Rev. Lett. 109, 035503 (2012). M. Chen, H. Nam, S. Wi, L. Ji, X. Ren, L. Bian, S. Lu, and X. Liang, Appl. Phys. Lett. 103, 142110 (2013). H. Nam, S. Wi, H. Rokni, M. Chen, G. Priessnitz, W. Lu, and X. Liang, ACS Nano 7, 5870 (2013). T. Kim, K. Cho, W. Park, J. Park, Y. Song, S. Hong, W. Hong, and T. Lee, ACS Nano 8, 2774 (2014). 113 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 H. Liu, Y. Du, A. T. Neal, M. Si, and P. D. Ye, Fellow, IEEE, IEEE Electron Device Lett. 34, 1328 (2013). S. Dey, H. S. Matte, S. N. Shirodkar, U. V. Waghmare, and C. N. Rao, Chem Asian J 8, 1780 (2013). F. K. Perkins, A. L. Friedman, E. Cobas, P. M. Campbell, G. G. Jernigan, and B. T. Jonker, Nano Lett. 13, 668 (2013). Y. Li, C. Xu, P. Hu, and L. Zhen, ACS Nano 7, 7795 (2013). S. Tongay, J. Zhou, C. Ataca, J. Liu, J. S. Kang, T. S. Matthews, L. You, J. Li, J. C. Grossman, and J. Wu, Nano Lett. 13, 2831 (2013). S. Mouri, Y. Miyauchi, and K. Matsuda, Nano Lett 13, 5944 (2013). S. Chuang, C. Battaglia, A. Azcatl, S. McDonnell, J. S. Kang, X. Yin, M. Tosun, R. Kapadia, H. Fang, R. M. Wallace et al., Nano Lett. 14, 1337 (2014). H. Fang, S. Chuang, T. C. Chang, K. Takei, T. Takahashi, and A. Javey, Nano Lett. 12, 3788 (2012). H. Fang, M. Tosun, G. Seol, T. C. Chang, K. Takei, J. Guo, and A. Javey, Nano Lett. 13, 1991 (2013). M. Tosun, S. Chuang, H. Fang, A. B. Sachid, M. Hettick, Y. Lin, Y. Zeng, and A. Javey, ACS Nano 8, 4948 (2014). J. Yun, Y. Noh, J. Yeo, Y. Go, S. Na, H. Jeong, J. Kim, S. Lee, S. Kim, H. Y. Koo et al., J. Mater. Chem. C 1, 3777 (2013). R. Bhandavat, L. David, and G. Singh, J. Phys. Chem. Lett. 3, 1523 (2012). H. Li, G. Lu, Z. Yin, Q. He, H. Li, Q. Zhang, and H. Zhang, Small 8, 682 (2012). http://en.wikipedia.org/wiki/Next-generation_lithography. D. R. S. Cumming, S. Thoms, S. P. Beaumont, and J. M. R. Weaver, Appl. Phys. Lett. 68, 322 (1996). The Nova NanoSEM User Operation Manual, (1st Edition 30 / 11 / 2007). Matteo Altissimo, Biomicrofluidics 4, 026503 (2010). http://emu.uct.ac.za/wp-content/uploads/2013/06/Nova-NanoSEM-Operationmanual.pdf. http://www.home.agilent.com/en/pd-1983602-pn-B2912A/precision-source-measureunit-2-ch-10-fa-210-v-3-a-dc-105-a-pulse?cc=SG&lc=eng. http://staff.science.nus.edu.sg/~chenwei/facilities.html. W.Chen, D. Qi, X. Gao, and A. T. S. Wee, Prog. Surf. Sci. 84, 279 (2009). M. Cardona, P.Y. Yu, Fundamentals of Semiconductors. Springer (Fourth Edition). http://en.wikipedia.org/wiki/Raman_scattering. http://en.wikipedia.org/wiki/Raman_spectroscopy. K. K.Ng, S. M.Sze, Physics of Semiconductor Devices. Wiley-interscience (Third Edition). D. Xiao, G. Liu, W. Feng, X. Xu, and W. Yao, Phys. Rev. Lett. 108, 196802 (2012). A. N. Grigorenko, M. Polini, and K. S. Novoselov, Nat. Photon. 6, 749 (2012). J. Lin, H. Li, H. Zhang, and W. Chen, Appl. Phys. Lett. 102, 203109 (2013). S. Z. Butler, S. M. Hollen, L. Cao, Y. Cui, J. A. Gupta, H. R. Gutiérrez, T. F. Heinz, S. S. Hong, J. Huang, A. F. Ismach et al., ACS Nano 7, 2898 (2013). R. S. Sundaram, M. Engel, A. Lombardo, R. Krupke, A. C. Ferrari, P. Avouris, and M. Steiner, Nano Lett. 13, 1416 (2013). H. S. Lee, S. Min, Y. Chang, M. K. Park, T. Nam, H. Kim, J. H. Kim, S. Ryu, and S. Im, Nano Lett. 12, 3695 (2012). J. Kibsgaard, Z. B. Chen, B. N. Reinecke, and T. F. Jaramillo, Nat. Mater. 11, 963 (2012). T. F. Jaramillo, K. P. Jorgensen, J. Bonde, J. H. Nielsen, S. Horch, and I. Chorkendorff, Science 317, 100 (2007). X. Zong, G. P. Wu, H. J. Yan, G. J. Ma, J. Y. Shi, F. Y. Wen, L. Wang, and C. Li, J. Phys. Chem. C 114, 1963 (2010). Q. Xiang, J. Yu, and M. Jaroniec, J. Am. Chem. Soc. 134, 6575 (2012). F.  Y Cheng, J. Chen, and X.  L Gou, Adv. Mater. 18, 2561 (2006). M. Shanmugam, T. Bansal, C. A. Durcan, and B. Yu, Appl. Phys. Lett. 100, 153901 (2012). W. J. Yu, Z. Li, H. Zhou, Y. Chen, Y. Wang, Y. Huang, and X. Duan, Nat. Mater. 12, 246 (2013). X. Dong, D. Fu, W. Fang, Y. Shi, P. Chen, and L. Li, Small 5, 1422 (2009). 114 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 L. Xie, X. Wang, H. Mao, R. Wang, M. Ding, Y. Wang, B. Özyilmaz, K. P. Loh, A. T. S. Wee, Ariando et al., Appl. Phys. Lett. 99, 012112 (2011). Z. Liu, A. A. Bol, and W. Haensch, Nano lett. 11, 523 (2010). T. J. Echtermeyer, L. Britnell, P. K. Jasnos, A. Lombardo, R. V. Gorbachev, A. N. Grigorenko, A. K. Geim, A. C. Ferrari, and K. S. Novoselov, Nat. Commun. 2, 458 (2011). W. Chen, S. Chen, D. C. Qi, X. Y. Gao, and A. T. S. Wee, J. Am. Chem. Soc. 129, 10418 (2007). H. L. Zhang, W. Chen, L. Chen, H. Huang, X. S. Wang, J. Yuhara, and A. T. S. Wee, Small 3, 2015 (2007). D. M. Guldi, B. M. Illescas, C. M. Atienza, M. Wielopolski, and N. Martin, Chem. Soc. Rev. 38, 1587 (2009). Y. Zhao, J. Chen, W. Chen, and D. Ma, J. Appl. Phys. 111, 043716 (2012). Z. Chen, I. Santoso, R. Wang, L. F. Xie, H. Y. Mao, H. Huang, Y. Z. Wang, X. Y. Gao, Z. K. Chen, D. Ma et al., Appl. Phys. Lett. 96, 213104 (2010). L. Chen, W. Chen, H. Huang, H.  L Zhang, J. Yuhara, and A.  T  S Wee, Adv. Mater. 20, 484 (2008). H. Y. Mao, R. Wang, H. Huang, Y. Z. Wang, X. Y. Gao, S. N. Bao, A. T. S. Wee, and W. Chen, J. Appl. Phys. 108, 053706 (2010). M. Irimia-Vladu, N. Marjanovic, A. Vlad, A. M. Ramil, G. Hernandez-Sosa, R. Schwoödiauer, S. Bauer, and N. S. Sariciftci, Adv. Mater. 20, 3887 (2008). http://en.wikipedia.org/wiki/Buckminsterfullerene. O. M. Hussain and K. S. Rao, Mater. Chem. Phys. 80, 638 (2003). M. B. Rahmani, S. H. Keshmiri, J. Yu, A. Z. Sadek, L. Al-Mashat, A. Moafi, K. Latham, Y. X. Li, W. Wlodarski, and K. Kalantar-zadeh, Sens. Actuators, B 145, 13 (2010). R. Tokarz-Sobieraj, K. Hermann, M. Witko, A. Blume, G. Mestl, and R. Schlögl, Surf. Sci. 489, 107 (2001). A. D. Sayede, T. Amriou, M. Pernisek, B. Khelifa, and C. Mathieu, Chem. Phys. 316, 72 (2005). D. O. Scanlon, G. W. Watson, D. J. Payne, G. R. Atkinson, R. G. Egdell, and D. S. L. Law, J. Phys. Chem. C 114, 4636 (2010). J. S. Ross, S. Wu, H. Yu, N. J. Ghimire, A. M. Jones, G. Aivazian, J. Yan, D. G. Mandrus, D. Xiao, W. Yao et al., Nat. Commun. 4, 1474 (2013). J. Lin, J. Zhong, S. Zhong, H. Li, H. Zhang, and W. Chen, Appl. Phys. Lett. 103, 063109 (2013). B. Chakraborty, A. Bera, D. V. S. Muthu, S. Bhowmick, U. V. Waghmare, and A. K. Sood, Phys. Rev. B 85, 161403 (2012). G. Li, C. W. Chu, V. Shrotriya, J. Huang, and Y. Yang, Appl. Phys. Lett. 88, 253503 (2006). C. Wu, C. Lin, Y. Chen, M. Chen, Y. Lu, and C. Wu, Appl. Phys. Lett. 88, 152104 (2006). H. Liao, L. Chen, Z. Xu, G. Li, and Y.Yang, Appl. Phys. Lett. 92, 173303 (2008). J. Huang, T. Watanabe, K. Ueno, and Y. Yang, Adv. Mater. 19, 739 (2007). J. Huang, Z. Xu, and Y. Yang, Adv. Funct. Mater. 17, 1966 (2007). J. Huang, G. Li, and Y. Yang, Adv. Mater. 20, 415 (2008). Z. Xu, L. Chen, G. Yang, C. Huang, J. Hou, Y. Wu, G. Li, C. Hsu, and Y. Yang, Adv. Funct. Mater. 19, 1227 (2009). Y. Liu, R. Cheng, L. Liao, H. Zhou, J. Bai, G. Liu, L. Liu, Y. Huang, and X. Duan, Nat. Commun. 2, 579 (2011). D. M. Schaadt, B. Feng, and E. T. Yu, Appl. Phys. Lett. 86, 063106 (2005). K. Nakayama, K. Tanabe, and H. A. Atwater, Appl. Phys. Lett. 93, 121904 (2008). M. M. Benameur, B. Radisavljevic, J. S. Héron, S. Sahoo, H. Berger, and A. Kis, Nanotechnology 22, 125706 (2011). G. Frens, Nature-Phys Sci 241, 20 (1973). S. J. Barrow, X. Wei, J. S. Baldauf, A. M. Funston, and P. Mulvaney, Nat. Commun. 3, 1275 (2012). P. K. Jain, W. Y. Huang, and M. A. El-Sayed, Nano Lett. 7, 2080 (2007). Y. Tian and T. Tatsuma, J. Am. Chem. Soc. 127, 7632 (2005). 115 209 210 211 212 213 214 215 216 217 218 219 A. Furube, L. Du, K. Hara, R. Katoh, and M. Tachiya, J. Am. Chem. Soc. 129, 14852 (2007). H. Li, Z. Yin, Q. He, H. Li, X. Huang, G. Lu, D. Wen, H. Fam, A. I. Y.Tok, Q. Zhang, and H. Zhang, Small 8, 63 (2012). J. Lince, D. Carré, and P. Fleischauer, Phys. Rev. B 36, 1647 (1987). Y. Shi, J. K. Huang, L. Jin, Y. T. Hsu, S. F. Yu, L. J. Li, and H. Y. Yang, Sci. Rep. 3, 1839 (2013). D. B. Farmer, R. Golizadeh-Mojarad, V. Perebeinos, Y. M. Lin, G. S. Tulevski, J. C. Tsang, and P. Avouris, Nano Lett. 9, 388 (2009). K. T. Chan, J. B. Neaton, and M. L. Cohen, Phys. Rev. B 77, 235430 (2008). T. S. Sreeprasad, P. Nguyen, N. Kim, and V. Berry, Nano Lett. 13, 4434 (2013). Z. Yin, B. Chen, M. Bosman, X. Cao, J. Chen, B. Zheng, and H. Zhang, Small (2014). X. Huang, Z. Zeng, S. Bao, M. Wang, X. Qi, Z. Fan, and H. Zhang, Mol. Ther. 4, 1444 (2013). X. Huang, S. Li, Y. Huang, S. Wu, X. Zhou, S. Li, C. L. Gan, F. Boey, C. A. Mirkin, and H. Zhang, Nat. Commun. 2, 292 (2011). K. F. Mak, K. He, C. Lee, G. H. Lee, J. Hone, T. F. Heinz, and J. Shan, Nat. Mater. 12, 207 (2013). 116 [...]... measurements, and Raman/photoluminescence (PL) measurements, together with various state -of- the-art surface analysis techniques We aim to investigate unique electronic and optical properties of surface- modified MoS2 and to further extend its potential applications We begin with an investigation of the effects of C60, molybdenum trioxide (MoO3), cesium carbonate (Cs2CO3) on the modulation of MoS2 properties. .. (d) and (e) reprinted from ref 36, with permission from American Chemical Society, Copyright 2013 3 Figure 1.2 (a) Side and top view of crystal structure of MX2 (b)-(e) Calculated electronic band structures of bulk MoS2, MoSe2, WSe2 and WS2, respectively (f)-(i) Calculated electronic band structures of monolayer MoS2, MoSe2, WSe2 and WS2, respectively (j)-(m) Second-derivative ARPES spectra of monolayer,... sheet of transition metal atoms sandwiched by two hexagonal planes of chalchogen atoms through strong covalent bonding The calculated indirect band gaps of bulk TMDCs (four examples: MoS2, MoSe2, WSe2 and WS2) are shown in Figures 1.2 b-e46 The conduction band minima locate between the Γ and K high symmetry points, and the valence band maxima at the Γ point The band structures depend on the number of. .. effects of the spin-orbit coupling and inversion symmetry-breaking lead to the clear spin-split bands only in the monolayer The band structure of 2D TMDCs, especially the transition of bandgap, have a significant impact on photonic and optoelectronics applications, which will be discussed in detail in subsections below 6 Figure 1.2 (a) Side and top view of crystal structure of MX2 (b)-(e) Calculated electronic. .. PL QY of thin layer for N= 1-6 Inset right: Representative optical image of mono -and few-layer MoS2 crystals on a silicon substrate with etched holes of 1.0 and 1.5 µm in diameter (b) PL spectra of monolayer and few-layer MoSe2 (c) PL spectra of monolayer and few-layer WSe2 (d) PL intensities for 1L, 2L, 3L, and bulk WS2 using the 488nm excitation laser line The positions for the excitons A and B as... (Figure 1.1d and 1.1e)36 For these methods, the thickness of the resulting layers depends on the initial precursors, and therefore, the number of outcome layers is not precisely controllable Therefore, more research efforts are needed to realize large area production of 2D TMDCs with controllable layer numbers 4 1.1.2 Electronic, optical and vibrational properties of TMDCs Electronic structure The band structures... the practical applications of MoS2-based devices Our results suggest that chemical doping via surface functionalization has great advantages in controlling the electronic and photoluminescence properties of single layer MoS2 Plasmonic metal nanostructures can be functionalized on 2D materials to further manipulate the optical and electronic properties We demonstrate that, by combining MoS2 with plasmonic...Summary This thesis aims to investigate the effect of surface functionalization on the properties of two-dimensional (2D) transition metal dichalcogenides (TMDCs), with particular emphasis on molybdenum disulfide (MoS2), a representative member of 2D TMDCs family Both electrical and optical properties are characterized by complementary methods, including in-situ bottom-gated MoS2 field-effect... monolayer and bilayer MoS2 samples in the photon energy range from 1.3 to 2.2 eV Inset left: PL QY of thin layer for N= 1-6 Inset right: Representative optical image of mono -and few-layer MoS2 crystals on a silicon substrate with etched holes of 1.0 and 1.5 µm in diameter (b) PL spectra of monolayer and few-layer MoSe2 (c) PL spectra of monolayer and few-layer WSe2 (d) PL intensities for 1L, 2L, 3L, and. .. physical contact between Au and MoS2 is established Here, we also systematically investigate the effect of the Au/MoS2 interface formation on the electronic and electrical transport properties of MoS2 through the combination of in-situ FET characterization and in-situ UPS/XPS measurements We found that Au atoms aggregate and form nanoclusters with average diameter of 25 nm on MoS2 and weakly interact with . INVESTIGATION OF ELECTRONIC AND OPTICAL PROPERTIES OF MOLYBDENUM DISULFIDE MODULATED BY SURFACE FUNCTIONALIZATION LIN JIADAN (B.Sc. SICHUAN. preparation and device fabrication 69 4.3 Modulating MoS 2 FET electronic transport properties by Cs 2 CO 3 surface functionalization 71 4.4 Modulating optical properties of MoS 2 by Cs 2 CO 3 surface. 2D TMDCs: background and literature review 2 1.1.1 Synthesis of 2D TMDCs 2 1.1.2 Electronic, optical and vibrational properties of TMDCs 5 Electronic structure 5 Optical properties 7 1.1.3