Visibility of dichalcogenide nanolayers

TL;DR

This paper establishes optimal optical imaging conditions for detecting and distinguishing ultrathin dichalcogenide nanolayers, such as MoS2, WSe2, and NbSe2, using simple contrast calculations and broad-band green illumination.

Contribution

It provides a simple optical model for contrast calculation and demonstrates effective detection and differentiation of mono-, bi-, and triple-layer dichalcogenides.

Findings

Single layers of MoS2, WSe2, NbSe2 can be optically detected on 90nm and 270nm SiO2.

Contrast measurements allow distinguishing between different layer numbers.

Optical detection is confirmed by AFM measurements.

Abstract

Dichalcogenides with the common formula MX2 are layered materials with electrical properties that range from semiconducting to superconducting. Here, we describe optimal imaging conditions for optical detection of ultrathin, two-dimensional dichalcogenide nanocrystals containing single, double and triple layers of MoS2, WSe2 and NbSe2. A simple optical model is used to calculate the contrast for nanolayers deposited on wafers with varying thickness of SiO2. The model is extended for imaging using the green channel of a video camera. Using AFM and optical imaging we confirm that single layers of MoS2, WSe2 and NbSe2 can be detected on 90nm and 270 nm SiO2 using optical means. By measuring contrast under broad-band green illumination we are also able to distinguish between nanostructures containing single, mono and triple layers of MoS2, WSe2 and NbSe2.