AVS 66th International Symposium & Exhibition
    2D Materials Thursday Sessions
       Session 2D+EM+MI+NS+QS+SS-ThM

Paper 2D+EM+MI+NS+QS+SS-ThM10
Chemical Migration and Dipole Formation at TMD/TI Interfaces

Thursday, October 24, 2019, 11:00 am, Room A216

Session: Dopants, Defects, and Interfaces in 2D Materials
Presenter: Brenton Noesges, The Ohio State University
Authors: B.A. Noesges, The Ohio State University
T. Zhu, The Ohio State University
D. O'Hara, University of California, Riverside
R. Kawakami, The Ohio State University
L.J. Brillson, The Ohio State University
Correspondent: Click to Email
Proximity effects at the interface between two materials can induce physical properties not present in either material alone. Topological insulators (TIs) such as Bi2Se3 with non-trivial surface states are sensitive to interface proximity effects where overlayers and adsorbates can act as a dopant source, chemically interact with the TI surface, or couple across the TI surface states leading to novel quantum phases. Transition metal dichalcogenides (TMDs), a class of 2D van der Waals materials, are a promising candidate to control this interface given the shared general hexagonal symmetry and wide range of TMD properties. However, the interface between TMDs and Bi2Se3 can be more complex than the ideal van der Waals interface. Chemical species exchange like metal cation exchange and selenium migration from substrate to growing film can impact the structure and properties of either layer. Self-assembly mechanisms have also been observed where complete metal monolayers form inside the Bi2Se3 quintuple layer [1]. We used x-ray photoelectron spectroscopy (XPS) connected in vacuo via UHV suitcase to a molecular beam epitaxy (MBE) system to investigate chemical interaction at the interface between selenide TMDs and Bi2Se3. Air-free transferring is crucial to minimize contamination at the interface and prevent oxidation in the air-sensitive TMDs. We compare the effects of ultrathin pure Mn metal overlayers and monolayer MnSex on Bi2Se3 to pristine Bi2Se3. In the case of pure Mn metal on Bi2Se3, Bi core levels exhibit a 1.7 eV shift toward lower binding energies while the Mn core levels also show signs of Mn-Se bonding. These core level changes indicate that, in the absence of excess Se during growth, Mn pulls Se from the substrate leaving behind Bi2 bilayers near the surface. Depositing a monolayer of MnSex produces very different results than the pure metal case. Bi2Se3 core levels measured below the monolayer MnSex film exhibit a rigid 0.8 eV chemical shift toward higher binding energies indicative of surface/interface dipole formation. The presence of this dipole is likely due to growth of primarily α-MnSe instead of the 1T-MnSe2 2D phase [2]. Scanning tunneling microscopy (STM) height maps and spectroscopy data provide further evidence of majority α-MnSe formation. XPS core level analysis combined with controlled depositions, air-free transfers and surface analysis can provide a consistent explanation of chemical diffusion and dipole formation at a TMD/TI interface. This work is supported by NSF MRSEC under award number DMR-1420451.

[1] J. A. Hagmann et al., New J. Phys. 19, 085002 (2017).

[2] D.J. O’Hara et al. Nano Lett., 18(5), 3125-3131 (2018).