Investigation of electronic and magnetic properties of pristine and functionalized graphene

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Investigation of electronic and magnetic properties of pristine and functionalized graphene

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INVESTIGATION OF ELECTRONIC AND MAGNETIC PROPERTIES OF PRISTINE AND FUNCTIONALIZED GRAPHENE XIE LANFEI (B. Sc, SHANDONG UNIV) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE (2011) ACKNOWLEDGEMENTS As I sit down to start writing the acknowledgement, I am reminded that the completion of this thesis represents the closing of one memorable phase of my life. At this moment, I would like to thank all those people who made this thesis possible and an enjoyable experience for the last four years. First and foremost, I would like to express my deepest gratitude to my supervisor Dr. Chen Wei who has given his constant support, help and guidance. Without his patient guidance, help and encouragement, it is impossible for me to obtain the necessary research skills in such a short time and finish this thesis in four years. A big thank to my supervisors Prof. Andrew T. S. Wee. Prof. Wee always gives me valuable and in-depth suggestions on experiments. Despite his busy schedule as the dean of Science faculty, he reviewed and revised all my manuscripts and thesis word by word with greatest diligence. I also want to thank Dr. Özyilmaz Barbaros who has taught me all the basic technology for device fabrication and brought me to the world of graphene. A special thanks to Dr. Vitor Manuel, Pereira who reviewed my thesis word by word and gave in-depth comments. My sincere thanks to Dr. Xu Xiangfan, Dr. Huang Han, Dr. Qi Dongchen, Dr. Iman Santosoi, Dr. Chu Xinjun, Dr. Poon Siewwai and Dr. Mao Hongying who has helped me during my studies. I also thank my colleges Mr. Wang i   Xiao, Mr. Wang Rui, Mr. Wong Siewliang, Mr. Zeng Minggang, Mr. Wang Yuzhan, Mr. Toh Chee Tat, Mr. Jayakumar Balakrishnan, Mr. Alexandre Pachoud, Mr. Luo Zhiqiang, Miss Wang Yingying and many other lab mates who have worked together. Life out of lab was also memorable. The friendships I made while at NUS I will cherish a lifetime. I want to thank Miss Zhang Kaiwen, Mr. Xu Xiangfan, Mr. Chu Xinjun, Miss Diao Yingying, Mr. Yao Guanggan, Miss Wangqian and so many other friends. We have shared so many wonderful memories during the past four years. The financial support from the National University of Singapore is gratefully acknowledged. Last but not least, I thank my mother who has born me and given me a chance to enjoy all the moments in life, thank my father who has influenced me always and brought me to the wonder world of physics, and thank Mr. little horse whose love completes my world. ii     LIST OF PUBLICATIONS  Anomalous spectral features of a neutral bilayer graphene L. F. Xie, C. -M. Cheng, H. O. Moser, W. Chen, A. T. S. Wee, A. H. Castro Neto, K.-D. Tsuei, B. Özyilmaz Submitting (2011)  Room temperature ferromagnetism in partially hydrogenated epitaxial graphene L. F. Xie, X. Wang, J. Lu, Z. H. Ni, Z. Q. Luo, H. Y. Mao, R. Wang,Y. Y. Wang, H. Huang,D. C. Qi, R. Liu, T. Yu, Z. X. Shen, T. Wu, H. Y. Peng,B. Özyilmaz, K. P. Loh, A. T. S. Wee, Ariando, W. Chen Applied Physics Letters, 98, 193113 (2011)  Surface transfer hole doping of epitaxial graphene using MoO3 thin film Z. Y. Chen, I. Santoso, R. Wang, L. F. Xie, H. Y. Mao, H. Huang, Y. Z. Wang, X. Y. Gao, Z. K. Chen, D. G. Ma, A. T. S. Wee, W. Chen Applied Physics Letters, 96, 213104 (2010)  Electrical measurement of non-destructively p-type doped graphene using molybdenum trioxide L. F. Xie, X. Wang, H. Y. Mao, R. Wang, M. Z. Ding, Y. Wang, B. Özyilmaz, K. P. Loh, A. T. S. Wee, Ariando, W. Chen Applied Physics Letters, 99, 012112 (2011)   LIST OF PATENTS  Fabrication of room temperature ferromagnetic graphene by surface modification with high work function metal oxides W. Chen, L. F. Xie, X. Wang, J. T. Sun, Ariando, A. T. S. Wee US Provisional Application No.: 61/404,975, Filing Date: 12 October 2010 iii   TABLE OF CONTENTS   Chapter Introduction 1.1 Carbon in two dimensions: background and literature review . 1.1.1 Carbon family and history of graphene 1.1.2 Electronic properties of graphene . Hall effect in classical physics . 1.2 Objective and scope of this thesis . 12   Chapter Experimental techniques . 14 2.1 Preparation of graphene 14 2.1.1 Micromechanical exfoliation 14 2.1.2 Thermal decomposition of SiC . 15 2.1.3 Chemical vapor deposition . 16 2.2 Experimental techniques for spectroscopic studies 17 2.2.1 Ultraviolet photoemission spectroscopy and X-ray photoemission spectroscopy 17 2.2.2 Near-Edge X-ray Absorption Fine Structure measurements . 21 2.2.3 Angle Resolved Photoemission Spectroscopy 23 2.2.4 Electron Energy Loss Spectroscopy . 25 2.2.5 Raman Spectroscopy . 26 2.3 Experimental techniques for electronic and magnetic studies 29   Chapter ARPES studies on mechanically exfoliated bilayer graphene 36 3.1 ARPES studies on mechanically exfoliated graphene on Si substrate with native oxide . 38 3.1.1 Sample preparation . 38 3.1.2 ARPES experimental details . 40 3.1.3 Results and discussions . 43 3.2 Approaches for ARPES measurements on mechanically exfoliated graphene with SiO2/Si substrate 51 3.3 Summary . 55   Chapter Ferromagnetism observed in partially hydrogenated graphene 58 4.1 Sample preparation and experimental procedures . 60 4.2 Magnetism studies by SQUID measurements . 62 4.3 Origin of magnetism observed in partially HEpG 65 4.3.1 NEXAFS and HREELS investigations . 65 4.3.2 Discussion on origins of magnetism observed in partially HEpG 69 4.4 Summary . 72   iv   Chapter Surface modification of epitaxial graphene by MoO3 thin film 73 5.1 Surface transfer hole doping of epitaxial graphene using MoO3 thin film 75 5.1.1 High work function transition metal oxide MoO3 75 5.1.2 Sample preparation and experimental procedures 76 5.1.3 PES and ARPES studies of MoO3 doped epitaxial graphene . 78 5.2 Summary . 85   Chapter Fabrication and electrical characterization of p-type doped graphene using MoO3 86 6.1 Electrical Measurements of Non-destructively p-type Doped Graphene using MoO3 . 86 6.1.1 Sample preparation and experimental procedures 86 6.1.2 Quantum Hall and magneto resistance measurements of MoO3 doped epitaxial graphene . 89 6.1.3 UPS studies on air exposure effect . 93 6. Summary 95   Chapter Conclusions and outlook . 97 7.1 Thesis summary 97 7.2 Future work . 100   References . 102   v   Summary This thesis presents experimental investigations on a promising two dimensional carbon material – graphene and its chemical derivatives. Both the band structure and their magnetic/electronic properties are characterized by complementary techniques, including angle-resolved photoemission spectroscopy (ARPES), superconducting quantum interference device (SQUID) and physical property measurement system (PPMS) measurements, as well as a wide range of surface analytical techniques. The first part of this thesis aims for a comprehensive understanding of the many-body interaction mechanisms which perturb the bare graphene band structure. The second part of the thesis is devoted to chemically modified graphene via hydrogen plasma treatment and surface modification with high work function metal oxide – molybdenum tri-oxide (MoO3). The band structure of exfoliated bilayer graphene was characterized by ARPES measurements on charge neutral bilayer graphene on a highly doped Si substrate. Full band mapping of pristine bilayer graphene was acquired revealing the absence of a band-gap between the π and π* bands. In such undoped and gapless exfoliated bilayer graphene, the marginal-Fermi liquid quasi-particle behavior was observed where the self energy varies linearly with the binding energy. Chemical modification of graphene in this thesis refers to partial hydrogenation and surface modification with a thin film of MoO3. vi   Ferromagnetism was detected in epitaxial graphene by SQUID measurements after partial hydrogenation. The origin of this hydrogenation induced ferromagnetism was systematically investigated by high resolution electron energy loss spectroscopy measurements and near-edge X-ray absorption fine structure studies. The ferromagnetism was suggested to be induced by the formation of unpaired electrons, together with the remnant delocalized π bonding network existing in the partially hydrogenated epitaxial graphene. Effective surface hole doping of epitaxial graphene using a high work function transition MoO3 thin film was demonstrated by photoemission spectroscopy investigations. The large work function difference between MoO3 and epitaxial graphene drives the spontaneous electron transfer from graphene to the MoO3 thin film upon deposition, resulting in a hole accumulation layer in graphene. As revealed by ARPES, this effective surface transfer p-type doping of epitaxial graphene resulted in a Fermi level shift to 0.38 eV below the graphene Dirac point. The hole doping effect of the MoO3 thin film was confirmed by electrical transport measurements on Hall-bar patterned, mechanically exfoliated, graphene devices. According to PPMS measurements, MoO3 modified graphene retains its high charge carrier mobility, facilitating the observation of the quantum Hall effect. By performing an in-situ ultraviolet photoelectron spectroscopy study, we also found that air exposure of MoO3 modified graphene significantly reduces the doping efficiency. vii   LIST OF TABLES Table 3.1 | Tight binding parameters (in eV) from the present and previous experimental works. . 45   Table 6.1| Computed charge carrier densities and charge carrier mobilities of the graphene device, before and after modification with MoO3 ultrathin film, at K and 300 K. . 90 viii   LIST OF FIGURES Figure 1.1 | Crystal and band structure of graphene. a. Two equivalent sublattices in graphene crystal structure; b. Illustration of valence and conduction band in single layer graphene.6 . Figure 1.2 | Energy-momentum dispersion spectrum for single layer graphene (A), bilayer graphene (B) and tri-layer graphene (C)30.   Figure 1.3 | Spatial density fluctuations and electron/hole puddles39. a. Color map of the spatial density variations in the graphene flake when the average carrier density is zero. b, Histogram of the density distribution in a.   Figure 1.4 | Illustration of classical Hall effect under a transverse magnetic field   Figure 1.5 | Quantum Hall effect in monolayer graphene (a) and bilayer graphene (b).6 11   Figure 2.1 | Optical contrast of exfoliated graphene with different layers50. . 15   Figure 2.2 | LEED (a) and corresponding STM image (b) of epitaxial graphene30. 16   Figure 2.3 | Example of a typical PES spectrum showing the various energy levels. The inset displays the schematic of photoelectron emission process in a PES experiment.56 18   Figure 2.4 | Schematic diagram of UPS and XPS57. 20   Figure 2.5 | X-ray absorption spectrum including both NEXAFS (low energy region) and EXAFS (high energy region)59. . 22   Figure 2.6 | Layout of ARPES measurements. Top right: A cartoon of the photoemission process and experimental setup of ARPES experiments. Left: The geometry of the electron detector showing the energy filtering process. Bottom right: Sample ARPES spectra in energy-momentum space62. . 24   Figure 2.7 | A schematic representation of the reflection EELS experiment. ix   method, it might be possible to achieve higher hole concentration in MoO3 modified graphene devices. 96   Chapter Conclusions and outlook 7.1 Thesis summary This thesis explored the band structure, electronic and magnetic properties of graphene and surface-modified graphene. Two main types of graphene - ExG and EpG grown on silicon carbide - were investigated to understand their intrinsic properties. After hydrogenation or surface modification with high work function MoO3 film, various promising electronic and magnetic properties distinct from pristine graphene have been revealed. We performed ARPES measurements on charge-neutral bilayer graphene isolated by mechanical cleavage method on highly doped Si substrate with native oxide, and obtained the first full band mapping of pristine bilayer graphene. The clear bilayer dispersion and three fold symmetry of the energy contour indicates the high quality of our data as well as the similarity between ExG and EpG grown thermally on SiC. The main difference between ExG and EpG is the absence of a band-gap between the π and π* bands in free-standing ExG. We observed, in undoped and gapless ExBLG, and for the first time in a graphene system, the marginal-Fermi liquid quasi-particle behavior, where the self-energy behaves linearly with binding energy. These findings are of considerable significance to the interpretation and understanding of many-body interactions between graphene quasiparticles. Another significant contribution from this work is that we have developed a sample preparation 97   method to solve the charging problem during photoemission process on ExG, which allows more convenient ARPES measurements of the intrinsic band structure of small samples of exfoliated graphene. In Chapter 4, an experimental study of partially hydrogenated epitaxial graphene was presented. SQUID measurements were utilized to study the magnetic properties of pristine and hydrogenated graphene. We found that after partial hydrogenation, graphene exhibits ferromagnetism as deduced from the FC and ZFC divergence and a clear hysteresis loop in the magnetization plot. HREELS and NEXAFS were utilized to study the structure of hydrogen attachment on graphene and the origin of its ferromagnetism. HREELS measurements revealed a C-H stretching peak, indicating the successful attachment of hydrogen atoms to graphene. The observed ferromagnetism is suggested to be induced by the formation of unpaired electrons, together with the remnant delocalized π bonding network in the partially hydrogenated EpG. Utilizing this controllable hydrogenation method, we can turn graphene into a robust room-temperature ferromagnetic semiconductor, and open the possibility of making highly tunable graphene-based spintronic devices. In Chapter 5, we investigated the effective surface hole doping of epitaxial graphene using a high work function MoO3 thin film. The deposition of MoO3 on ExG induces significant electron transfer from the graphene layer to the MoO3 film, resulting in a hole accumulation layer in graphene. As 98   revealed by synchrotron-based high resolution PES studies, after deposition of 0.8 nm MoO3, the work function of EpG increased by 1.4 eV and the graphene-related C 1s peak shifts toward lower binding energy by 0.7 eV. This effective surface transfer p-type doping of EpG results in the Fermi level moving to 0.38 eV below the graphene Dirac point. In comparison to other surface transfer dopants such as F4-TCNQ molecular film or metallic Au, Bi film, MoO3 possesses excellent chemical stability in harsh environments such as in air or solution, and therefore has great potential in graphene based nanoelectronic devices fabricated using standard lithography processes. Chapter extends this work from epitaxial graphene to ExG and CVD grown large area graphene to confirm the doping effect of MoO3 thin films by electrical transport measurements. Hall-bar patterned devices were fabricated on ExG and the electrical transport measurements were conducted in a PPMS with varying magnetic field. We observed that p-type doping via MoO3 modification of graphene can lead to the downward shift of Fermi level towards the valence band; at the same time, MoO3 modified graphene retains its high charge carrier mobility, facilitating the observation of the quantum Hall effect. By performing an in-situ ultraviolet photoelectron spectroscopy study, we also found that air exposure of MoO3 modified graphene can largely reduce the doping efficiency. Our results suggest that by developing a suitable encapsulation method, it is possible to achieve higher hole concentration in MoO3 modified graphene devices. 99   7.2 Future work In the first part of the thesis, we have studied the band structure and many body interactions in charge neutral ExBLG on native oxide Si substrate. However, there are also many unanswered questions to address regarding the band structure studies of pristine graphene. In addition to the ARPES measurements on bilayer graphene, we can future work on single layer graphene which is equally interesting. To perform such ARPES studies on small ExG samples, we need to solve the low efficiency of locating the sample on a native oxide Si substrate. One approach of surrounding the sample by metal is suggested in the thesis which could solve this problem. Studying the band structure of pristine graphene is essential to study the many body effects in this two dimensional crystal and, until now, this area is still less investigated as previous ARPES measurements were performed on graphene with supported substrates. We believe that, by detailed analysis of band structure studies utilizing ARPES, many unsolved questions would be answered, and other new and interesting issues would also arise. In the second part of this thesis, we have discussed hydrogenated graphene as well as surface modification of graphene with a strong electron donor MoO3. There are also various different methods of chemical modification, such as gas plasma treatment or doping with other molecules. However, in addition to modifying the intrinsic physical and electronic 100   properties of graphene, much work is needed to develop applications from chemically modified graphene, such as applications in gas sensors, optoelectronic devices, energy storage, and in the field of medicine/biology. One proposed future work related to the applications of chemically modified graphene is to utilize their ferromagnetic properties for spintronic devices. For example, hydrogenated graphene exhibits ferromagnetism compared with pristine graphene. Pristine graphene possesses long spin relaxation times and weak spin-orbit coupling, making it an excellent spin transport layer. Hence, it may be possible in the future to fabricate all-graphene spin transistors, opening up a new field of flexible spintronic devices. With the help of standard lithography processes, selected areas of the graphene surface may be chemically modified to allow the fabrication of spintronic devices. There are also several problems to address these two kinds of chemical medications to graphene: First, though partially hydrogenated graphene exhibit magnetization loop by SQUID measurements, yet no butterfly loop of magneto resistance is detected by PPMS. One possible reason may be due to the small magnetization from the thin film. Last but not least, as mentioned in Chapter 6, air exposure can significantly reduce the doping level of the MoO3 modified graphene sheet. However, devices have to be operated at ambient conditions for commercial or industry use. Hence, developing a suitable encapsulation method to achieve higher hole concentration in MoO3 modified graphene devices is required for applications. 101   References 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). C. Berger, Z. Song, T. Li, X. Li, A.Y. Ogbazghi, R. Feng, Z. Dai, A. N. Marchenkov, E. H. Conrad, P. N. First, and W. 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Lett. 96, 133308 (2010).  111   [...]... the center of three pairs of oxygen atoms.165 76   Figure 5.3 | Height and phase images of 10 µm × 10 µm AFM measurements on a pristine graphene; b graphene with 0.5 nm of MoO3; c graphene with 5 nm of MoO3; d graphene with 10 nm of MoO3 77   Figure 5.4 | Raman spectrum of pristine EpG (a), graphene with 0.5 nm deposition of MoO3 thin film (b), and graphene with 10 nm deposition of MoO3... leveraged by the outstanding properties of graphene and its chemical derivatives To realize its electronic applications, it is important to understand its band structure, and the effects of many-body interactions in particular While the electronic properties of graphene have been widely investigated, fewer studies have been devoted to its magnetic properties In this thesis, pure graphene and graphene with... continuous but also of remarkably high crystal quality1,10,11,24,25 This has sparked a flurry of experimental activity making graphene one of the hottest topics in physics in recent years, and a graphene “gold rush” has started since then 1.1.2 Electronic properties of graphene The discovery of both single-layer graphene (SLG) and bilayer graphene (BLG) has revolutionized the physics of low dimensional... simply stacking graphene sheets15 All of these carbon materials have been used in many applications much earlier before graphene emerged, yet many of their electronic and magnetic properties originate from the properties of graphene Indeed, graphene has been theoretically studied to describe other carbon-based materials for around sixty years16-18 before it became a reality The isolation of graphene is... scope of this thesis In the first part of this thesis, the main focus is to explore the band structure of graphene using angle-resolved photoemission spectroscopy The many-body interactions in bilayer ExG will be analyzed as an emphasis The effect of various chemical modifications on the electronic, magnetic, and transport properties of graphene will be investigated in detail in the second part of this... observed b, Raman spectrum of measured sample The relative height of the 2D and G peaks shows the bilayer graphene property 40   Figure 3.2 | Schematic of momentum space cut of ARPES measurements: the angle to the Γ-K-M direction is 8.5° and 9.5° for 54 eV and 83 eV, respectively 41   Figure 3.3 | Band dispersion of bilayer exfoliated graphene a, False color plot of EDCs vs k|| at 54 eV photon... conduction bands, as shown in Fig.1b Because the two sublattices give different contributions in the quasi-particles’ make up, a pseudo-spin8 is defined for the relative contribution of the A and B sublattices12,28 Figure 1.1 | Crystal and band structure of graphene a Two equivalent sublattices in graphene crystal structure; b Illustration of valence and conduction band in single layer graphene (reprinted... momentum, and m is the “rest” mass) In contrast to single layer graphene, the absence of a gap in BLG is entirely due to an accidental degeneracy Thus, a perpendicular electric field can be used to further lift the degeneracy between the two layers and hence open an energy gap29,36 The origins of those outstanding properties of both single layer graphene and bilayer graphene can be explored from graphene s... transfer between graphene and MoO3 80   Figure 5.7 | Synchrotron PES core level spectra during the deposition of MoO3 on EpG: a C 1s, b Si 2p, and c Mo 3d All spectra are measured with photon energy of 350 eV 81   Figure 5.8 | Dispersion of π-bands for a as grown graphene on 4H–SiC (0001) and b after deposition of 0.2 nm MoO3, as measured by ARPES with photon energy of 60 eV and at room... coated graphene with SiN membrane 54   Figure 4.1 | Raman spectra of EpG before and after hydrogenation 62 xi     Figure 4.2 | Magnetization with background of clean EpG (a) partially HEpG (b) and HSiC (c) 63   Figure 4.3 | (a) ZFC and FC data showing the temperature variation of magnetization of HEpG; (b) Magnetic hysteresis (in unit of Bohr magnetons per benzene ring) of HEpG . INVESTIGATION OF ELECTRONIC AND MAGNETIC PROPERTIES OF PRISTINE AND FUNCTIONALIZED GRAPHENE XIE LANFEI (B. Sc, SHANDONG UNIV) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF. bilayer graphene on a highly doped Si substrate. Full band mapping of pristine bilayer graphene was acquired revealing the absence of a band-gap between the π and π * bands. In such undoped and. by the outstanding properties of graphene and its chemical derivatives. To realize its electronic applications, it is important to understand its band structure, and the effects of many-body

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