Probing the Electronic States and Impurity Effects in 2D Materials,2015.7.15
作者:Ning Wang, 发布日期:2015-07-15

2015.7.15 上午10




“Probing the Electronic States and Impurity Effects in 2D Materials”



Ning Wang 

Department of Physics, the Hong Kong University of Science and Technology, Hong Kong, China


Atomically thin two-dimensional (2D) crystalline materials such as graphene, black phosphorus and transition metal dichalcogenides exhibit novel electronic properties and tremendous potential for future technological applications. In this talk, I’ll introduce electronic property characterization techniques for graphene, atomically thin MoSand black phosphorus. Structural disorder inevitably exists in these 2D materials and has profound impacts on their extraordinary properties. Theoretically, some special point defects such as vacancies and metal adatoms can modify the band structures of these materials. I’ll present our current research work on detecting the impurity states in these 2D materials through transport and quantum capacitance measurements at cryogenic temperatures. One of the interesting disorder structures we observed is the resonant impurities induced by metal clusters on graphene. We found that the appearance of a robust resonant peak near the charge neutrality point and the splitting of the zero Landau level for Ag-adsorbed graphene were manifestations of the hybridization of electrons from graphene bands and the impurity bands. Graphene decorated with a high density of Ag resonant impurities displayed the unconventional phenomenon of negative quantum capacitance. We show that resonant impurities quench the kinetic energy and drive the electrons to the Coulomb energy dominated regime with negative electronic compressibility. 


Disorder also played an important role in property modification of other 2D materials such as MoSand black phosphorus. In atomically thin MoS2, the metal-insulator transition (MIT) is one of the remarkable electrical properties. The underlying mechanism and detailed transition process in MoS2, however, still remain largely unexplored. We built a new type of capacitor structure based on the vertical metal-insulator-semiconductor heterostructures using atomically thin MoSfor probing electron states. The vertical configuration offers the added advantage of eliminating the influence of large impedance at the band tails and allows the observation of fully excited electron states near the surface of MoS2 over a wide excitation frequency and temperature range. Different from the theory of electron–electron interactions which has been used in modelling the metal-insulator transition in MoS2, we suggested a percolation-type MIT in MoS2, driven by density inhomogeneity of electron states that describes the systems in which charge carriers are transported through percolating conductive channels in the disorder landscapes due to the poor screening effect at low carrier densities.



 Prof. Ning WANG (王宁) 

Department of Physics

The Hong Kong University of Science & Technology (HKUST)

Tel: (852)-23587489. Fax: (852)-23581652. 




 Dr. Ning Wang is Professorof Physics and Director of Materials Characterization & Preparation Facility (MCPF) at the Hong Kong University of Science and Technology. His research interests include fundamental issues of nano-structure formation, nano-fabrication technologies, physical properties of low-dimensional nanostructures, energy related nanomaterails and high-resolution transmission electron microscopy.

Professor Wang obtained his BSc (1985) and PhD (1990) degrees in materials physics from the University of Science and Technology, Beijing. In 1989, he received the Alexander von Humboldt Research Fellowship and worked at the Institute for Metal Physics, Goettingen University and the Fritz-Haber-Institute of the Max-Planck Society, Berlin, Germany. In 1993, he joined the Physics Department of the Hong Kong University of Science and Technology. During 1997-2000 he worked in the Department of Applied Physics and Materials Science, the City University of Hong Kong. 

Professor Wang has authored/co-authored over 200 peer-reviewed research papers in reputed international journals30 conference papers2 Book chapters5 US PatentsHe received Chien-Shiung Wu (吴健雄Physics Award, State Natural Science Award and Achievement in Asia Award.

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