Study of Structural and Optoelectronic Properties of Thin Films Made of a Few Layered WS2 Flakes

Anna Łapińska,Michał Kuźniewicz,Mariusz Zdrojek,Anna Dużyńska,Klaudia Żerańska-Chudek,Arkadiusz P Gertych,Anna Wróblewska,Jarosław Judek,Karolina Czerniak-Łosiewicz,Michał Świniarski

doi:10.3390/ma13235315

Abstract

We report a surfactant-free exfoliation method of WS2 flakes combined with a vacuum filtration method to fabricate thin (<50 nm) WS2 films, that can be transferred on any arbitrary substrate. Films are composed of thin (<4 nm) single flakes, forming a large size uniform film, verified by AFM and SEM. Using statistical phonons investigation, we demonstrate structural quality and uniformity of the film sample and we provide first-order temperature coefficient χ, which shows linear dependence over 300–450 K temperature range. Electrical measurements show film sheet resistance RS = 48 MΩ/□ and also reveal two energy band gaps related to the intrinsic architecture of the thin film. Finally, we show that optical transmission/absorption is rich above the bandgap exhibiting several excitonic resonances, and nearly feature-less below the bandgap.

Highlights

The group of 2D materials is wide and consists of materials with diverse properties
The starting point for thin WS2 films fabrication is the preparation of the bulk powder suspension and the exfoliation in2liquid phase down to individual
We have established the methodology for surfactant-free production of thin films made of 2D flakes and transferable on any arbitrary substrate, here using tungsten disulfide WS2 as an example

Summary

Introduction

The group of 2D materials is wide and consists of materials with diverse properties. That diversity enables 2D materials to be used in various applications like nanoelectronics, energy storage, or photonics [1,2,3,4]. Transition Metals Dichalcogenides (TMDs) are an example of such 2D materials, including tungsten disulfide (WS2 ), the most studied representative of TMDs. WS2 is a semiconductor with a tuneable direct-indirect bandgap varying from 1.3 to 2.1 eV, for bulk form and monolayer, respectively [5,6]. The bulk form of WS2 is popular in the industry due to its lubricanting properties, whereas the 2D counterpart of WS2 is known for e.g., interesting electronic and optical properties [7], including a strong Kerr effect, third-order non-linear optical response, or giant two-photon absorption [8]. The first step to achieve this is to produce a large quantity of flakes and form thin films out of it. For this purpose, liquid-phase exfoliation (LPE) technique can be used

Methods

Results

Conclusion