Spectroscopic thickness and quality metrics for PtSe₂ layers produced by top-down and bottom-up techniques

Abstract

Thin films of noble-metal-based transition metal dichalcogenides, such as PtSe2, have attracted increasing attention due to their interesting layer-number dependent properties and application potential. While it is difficult to cleave bulk crystals down to mono- and few-layers, a range of growth techniques have been established producing material of varying quality and layer number. However, to date, no reliable high-throughput characterization to assess layer number exists. Here, we use top-down liquid phase exfoliation (LPE) coupled with centrifugation to produce PtSe2 nanosheets of varying sizes and thicknesses with a low degree of basal plane defectiveness. Measurement of the dimensions by statistical atomic force microscopy allows us to quantitatively link information contained in optical spectra to the dimensions. For LPE nanosheets we establish metrics for lateral size and layer number based on extinction spectroscopy. Further, we compare the Raman spectroscopic response of LPE nanosheets with micromechanically exfoliated PtSe2, as well as thin films produced by a range of bottom up techniques. We demonstrate that the Eg 1 peak position and the intensity ratio of the Eg 1/A1g 1 peaks can serve as a robust metric for layer number across all sample types.This will be of importance in future benchmarking of PtSe2 films.

Correlating Nanocrystalline Structure with Electronic Properties in 2D Platinum Diselenide

“…The E g peak positions shift to lower wave numbers for increasing film thicknesses (Figure 1b, bottom), in agreement with previous findings. [3,8,9,13,33,43,44] The shifts of the A 1g peak positions, however, do not display Table 1. Overview of the experimental results from the samples studied within this work.…”

“…[46,47] An E g FWHM of ≤ 7 cm −1 has been proposed as an indicator for high quality layered films. [44] Histograms of the FWHM of the E g peak were extracted from Raman area scans (Figure 1d). The FWHM varied between ≈ 4.5 and 8.5 cm −1 across the different samples, independent of the film thickness.…”

Correlation of Material Structure and Electronic Properties in 2D Platinum-Diselenide-based Devices

“…The Eg peak positions shift to lower wave numbers for increasing film thicknesses (Figure 1b, bottom), in agreement with previous findings. [3,8,9,13,33,43,44] The shifts of the A1g peak positions, however, do not display an obvious dependence on the film thickness (Figure 1b, top). Furthermore, the intensity ratio of the two peaks, I(A1g)/I(Eg), generally increases with increasing film thicknesses (Figure 1c), which is in line with experimental studies [8,9,43,44] and theoretical calculations.…”

“…It was previously suggested that the full width at half maximum (FWHM) of the Eg peak in PtSe2 is an indicator of the film quality, [33,44] similar to the FWHM of the 2D peak in graphene correlating to its charge carrier mobility. [46,47] An Eg FWHM of ≤ 7 cm -1 has been proposed as an indicator for high quality layered films.…”

“…[46,47] An Eg FWHM of ≤ 7 cm -1 has been proposed as an indicator for high quality layered films. [44] Histograms of the FWHM of the Eg peak were extracted from Raman area scans (Figure 1d).…”