Optical-Based Thickness Measurement of MoO3 Nanosheets

Abstract

Considering that two-dimensional (2D) molybdenum trioxide has acquired more attention in the last few years, it is relevant to speed up thickness identification of this material. We provide two fast and non-destructive methods to evaluate the thickness of MoO₃ flakes on SiO₂/Si substrates. First, by means of quantitative analysis of the apparent color of the flakes in optical microscopy images, one can make a first approximation of the thickness with an uncertainty of ±3 nm. The second method is based on the fit of optical contrast spectra, acquired with micro-reflectance measurements, to a Fresnel law-based model that provides an accurate measurement of the flake thickness with ±2 nm of uncertainty.

Keywords: 2D materials; MoO3; complex oxides; optical microscopy; thickness determination.

Figure 1

Thickness dependent apparent color of MoO₃ flakes. (a) AFM measurements of the exfoliated flakes with different thickness placed on a 297 nm SiO₂/Si substrate. (b) Optical images of the flakes and a colorbar with the apparent color of flakes with thickness from 5 nm up to 93 nm (from 7 to ~130 layers). Scale bars: 10 µm.

Figure 2

Thickness dependent apparent color of MoO₃ flakes. (a) AFM measurements of the exfoliated flakes with different thickness placed on an 88 nm SiO₂/Si substrate. (b) Optical images of the flakes and a colorbar with the apparent color of flakes with thickness up to 88 nm. Scale bars: 10 µm.

Figure 3

Thickness dependent apparent color of MoO₃ flakes on SiO₂/Si substrates with different oxide capping layer thickness.

Figure 4

(a) Optical model used to calculate the MoO₃ optical contrast. (b) Optical contrast spectra measured on MoO₃ flakes, on a 297 nm SiO₂/Si substrate, of different thickness. (c) Calculated optical contrast (solid lines) using the Fresnel law-based model [52]. The shaded area accounts for an uncertainty of ±1 nm in the thickness of the flake [53].

Figure 5

(a) Experimental optical contrast (black) and comparison with the calculated ones (color lines) of MoO₃ flakes, with thicknesses from 10 to 18 nm, placed onto a substrate of 297 nm of SiO₂. Inset of figure: minimum square value for different values of thickness of the flakes. (b) Comparison of thickness measured with AFM and the Fresnel model. Experimental data represented as red dots and line in black has a slope of 1.