engleski

Van der Waals epitaxy of two-dimensional transition metal disulphides

Thinning of transition metal dichalcogenides (TMDs) to monolayer variants is followed by tuning their electronic properties, which is interesting from the perspective of application and fundamental research. In order to prepare 2D TMDs and investigate their intrinsic properties, van derWaals (vdW) molecular beam epitaxy (MBE) under ultra-high vacuum (UHV) is used. In this thesis, synthesis of atomically thin MoS_2, WS_2 and TaS_2 is studied, as well as their heterostructures (HSs). The samples were primarily characterized by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). Prior to TMD preparation, we study adsorption of S on Ir(111) and S intercalation under graphene on Ir(111), which is used as a vdW substrate. A two-step vdW MBE is used for preparing MoS2 monolayer islands. Optimization of the growth parameters is done to obtain a phase pure system. The preparation method showed invariance to a change of a vdW substrate (e.g. to hexagonal boron nitride) or selection of TMD (WS_2, TaS_2). Then, a self-intercalation is provoked to modify the vdW interaction between the graphene and MoS_2. Weakening of the interaction is found for S-c(4×2) intercalation, while strengthening for intercalation of Mo. Moreover, a well oriented semi-closed layer of MoS_2 is found to coexist with Mo intercalation. Growth of TMD HSs is achieved sequentially by applying the two- step vdW MBE growth. Such an approach is firstly demonstrated for the HSs between MoS_2 and WS_2, which were simultaneously realized in lateral and vertical form. It is shown that similar HSs can be formed between MoS_2 and TaS_2, as well as for more exotic one, with WS_2 as a third building unit. The electronic structure of MoS_2 and WS2, and their HSs is studied by means of scanning tunneling spectroscopy (STS) and density functional theory (DFT). STS line scans are used to examine the one-dimensional (1D) interface of lateral HS, as well as a metallic edge and grain boundary in the vertical HS. In particular, we find sharp 1D interfaces with narrow depletion regions exhibiting strong built-in electric fields, which is relevant for electron-hole recombination processes and optoelectronics applications.

transition metal dichalcogenides ; MoS2 ; van der Waals interaction ; molecuar beam epitaxy ; scanning tunneling microscopy and spectroscopy ; graphene on Ir(111) ; selfintercalation ; lateral and vertical heterostructures ; one-dimensional interfaces

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