Science news — 02/03/2018

Growth of quasi-freestanding transition metal disulphide monolayers on van der Waals substrates

Together with our colleagues Borna Pielić and Marko Kralj, group of researches from Germany has recently published a paper on the growth of single-layer disulfides on vdW substrates in ultra-high vacuum. Growth of Different TMDCs require only a small change of parameters in the synthesis procedure, making this method attractive for wide use.

Molecular beam epitaxy of quasi-freestanding transition metal disulphide monolayers on van der Waals substrates: a growth study

Joshua Hall, Borna Pielić, Clifford Murray, Wouter Jolie, Tobias Wekking, Carsten Busse, Marko Kralj, Thomas Michely

2D Materials 5, 025005 (2018).

doi: 10.1088/2053-1583/aaa1c5

Thanks to the discovery of graphene, there has been an enormous increase of interest in the scientific community for other two-dimensional van der Waals (vdW) materials. Transition metal dichalcogenides (TMDC) are a special group of vdW materials corresponding to the formula MX2 (M=transition metal, X=chalcogen atom), with semiconductors such as MoS2, WS2, MoSe2 and WSe2 in the center of attention due to their 2D form and technologically interesting electronic band gap. Going from bulk to monolayer leads to dramatic modifications of their electronic structure, resulting in the indirect band gap becoming direct, making them desired candidates for optoelectronics. On the other hand, metallic TMDCs display a range of electronic instabilities such as superconductivity, charge-density waves or Mott states. It important to understand whether these properties exist in the single-layer limit.

(a) Upper panel: STM topograph after growth of MoS2 islands on Gr/Ir(1 1 1), resulting in a coverage of 0.3 ML. Arrows on the lefthandside highlight mirror twin boundaries in MoS2 islands. The [1 0 -1] Ir substrate direction is indicated by a black arrow on the righthandside. Lower panel: height profile along the blue line in the upper panel. (b) Atomically resolved topograph of an MoS2 island. (c) Section of a corresponding 80 eV LEED pattern. The first order reflections of MoS2, Ir(1 1 1) and Gr are indicated.

(a) Upper panel: STM topograph after growth of MoS2 islands on Gr/Ir(1 1 1), resulting in a coverage of 0.3 ML. Arrows on the lefthandside highlight mirror twin boundaries in MoS2 islands. The [1 0 -1] Ir substrate direction is indicated by a black arrow on the righthandside. Lower panel: height profile along the blue line in the upper panel. (b) Atomically resolved topograph of an MoS2 island. (c) Section of a corresponding 80 eV LEED pattern. The first order reflections of MoS2, Ir(1 1 1) and Gr are indicated.

In order to retain superior intrinsic properties and also to enable their investigation, it is necessary to grow pure crystal phases which interact only minimally with their substrate. Together with our colleagues Borna Pielić and Marko Kralj, a group of researches from universities in Köln, Münster, and Siegen in Germany has recently published a paper on the growth of single-layer disulfides on vdW substrates in ultra-high vacuum. They have developed a unique synthesis procedure that combines standard molecular beam epitaxy (MBE) and pyrite heating as a new alternative precursor for sulfur. As a result, pure, well-oriented and almost defect-free single layer islands of MoS2, WS2 and TaS2 have been obtained. By using a Scanning Tunneling Microscopy (STM) and Low Energy Electron Diffraction (LEED), growth mechanisms were systematically explored and optimized with respect to annealing. Growth of Different TMDCs require only a small change of parameters in the synthesis procedure, making this method attractive for wide use.

An additional advantage of the described method is its invariance to the selection of the vdW substrate, as has been concluded from the synthesis of disulphides on graphene and as well as hexegonal boron nitride. Furthermore, by using Scanning Tunneling Spectroscopy (STS) the authors have measured the value of electronic band gap for single-layer MoS2 (2.55 eV) and, as a consequence of low sample-substrate interaction, found it possible to manipulate and move single layer islands using the scanning probe tip.

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