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In this talk I will present our recent progress in scanning tunneling microscopy and spectroscopy investigations of emerging 2D materials beyond graphene, including topological insulators (TI) and transition metal dichalcogenides (TMDs).
The scientific interests of TI originate from its spin chiral topological surface state (TSS). However, it has been very difficult to create a topological insulator with true insulating bulk. For example, in Bi2Se3, spontaneous formations of surface point defects leads to strong surface band bending, resulting simultaneous existence of conduction from bulk and surface states. I will introduce a novel method which allows us to map out the 3D potential landscape. Through spatial correlation we further identify the culprits responsible for lateral potential fluctuations as well as the surface band bending [1]. I will then show the result of a quaternary compound, Bi1Sb1Te1Se2, which shows robust intrinsic insulating bulk [2].
In the second part of the talk I will discuss our recent progress in probing the electronic of TMDs. More specifically, we show that not only the quasi-particle band gaps [3] but also the critical point energy locations and their origins in the Brillouin Zone (BZ) can be revealed using this comprehensive form of STS [4]. By using this new method, we unravel the systematic trend of the critical point energies for TMDs due to atomic orbital couplings, spin-orbital coupling and the interlayer coupling. Moreover, by combining the micro-beam X-ray photoelectron spectroscopy (micro-XPS) and STS, we determine the band offsets in planar heterostructures formed between dissimilar single layer TMDs (MoS2, WSe2, and WS2). We show that both commutativity and transitivity of heterojunction band offset hold within the experimental uncertainty [5].
[1] Chris Mann et al., "Mapping the 3D surface potential in Bi2Se3", Nature Communications Volume: 4 Article Number: 2277 (2013)
[2] Yang Yu et al, “Observation of topological surface state quantum Hall effect in an intrinsic three-dimensional topological insulator” Nature Physics. DOI: 10.1038/NPHYS3140
[3] Chendong Zhang et al, “Direct Imaging of Band Profile in Single Layer MoS2 on Graphite: Quasiparticle Energy Gap, Metallic Edge States, and Edge Band Bending,” Nano Letters 14, 2443 (2014).
[4] Chendong Zhang et al., “Measuring Critical Point Energies in Transition Metal Dichalcogenides”, arXiv:1412.8487
[5] M. H. Chiu et al., "Determination of band alignment in transition metal dichalcogenides heterojunctions". arXiv:1406.5137 | 
