A direct growth of 2D materials heterostructures based on pulsed laser deposition of solid oxide precursors

Description

1. Introduction – Two-dimensional (2D) materials research emerged from discovery of graphene back in 2004 has pushed a deeper exploration of layered molecular crystals’ properties [1]. 2D layers of transition metal dichalcogenides (TMDs) are typical example of such crystals, which demonstrate chemical stability, and a wide range of unique physical properties. Among 2D TMDs MoS2, WS2, MoSe2, and WSe2 are the most promising candidates for various optoelectronics, as they exhibit visible-to-near infrared direct bandgap of 1-2 eV, and a strong light-matter coupling [2]. Additionally, 2D materials can be stacked as building blocks to construct vertical van der Waals heterostructures (vdWHs) with an arbitrary bandgap order and no stringent requirement of the lattice matching between the layers. However, before any far-reaching applications can be targeted, a method for the large-scale synthesis of vdWHs is needed. Here we suggest a novel approach towards vdWHs direct synthesis based on pulsed laser deposition (PLD).

2. Experimental – Inspired by the work of Xu et.al. [3], we have employed PLD-CVD two-step synthesis method to grow vertically stacked MoS2/WS2 vdWH. The method consists of: (1) oxide films deposition by PLD; (2) sulfurization in a sulfur-rich environment at high temperature. A key advantage of this method is a high precursor composition tunability. Raman and Photoluminescence spectroscopies, Raman mapping, XPS, AFM, SEM, optical microscopy, and STEM were used for MoS2/WS2 vdWHs characterization.

3. Results and Discussion –The oxides deposited by a non-equilibrium PLD process exhibit high oxygen vacancy concentration in both MoOx and WOx precursor films. The oxygen vacancies play an important role in the sulfurization process by facilitating MoS2 and WS2 lateral crystal growth upon sulfurization (see Image 1), which is beneficial for 2D MoS2/WS2 heterostructure creation (investigated using AFM in Image 1a,b and Raman spectroscopy in Image 1c). We have also investigated bilayer MoOx/WOx films high-temperature sulfurization as another path to achieve MoS2/WS2 heterostructure.

4. Conclusions – We have demonstrated a possibility of the vertical crystalline MoS2/WS2 heterostructure direct synthesis using PLD-based two-step process of epitaxial oxides sulfurization. Our study suggests a way towards fabrication of vdWHs without any individual layers transfer. The results reveal that by tuning oxide precursor growth conditions the properties of the heterostructure could be changed. These results may serve as a potential step towards dissimilar 2D TMDs heterolayers integration.

5. References
[1] Geim, A., Novoselov, K. The rise of graphene. Nature Mater 6, (2007), 183–191
[2] Jianghong Wu, Hui Ma, Peng Yin, et.al. Small Science Vol. 1, Iss. 4, Apr. (2021), 2000053
[3] Xiangming Xu et.al. 2D Materials. 6, (2019), 015030

Period	25 Jul 2022 → 27 Jul 2022
Event title	2nd International Conference on Materials & Nanomaterials
Event type	Conference
Conference number	2
Location	Rome, ItalyShow on map
Degree of Recognition	International