Spectroscopic size and thickness metrics for liquid-exfoliated h-BN

TL;DR

This study demonstrates how optical and Raman spectra of liquid-exfoliated h-BN can be used to determine nanosheet size and thickness, despite the spectra's simplicity, through detailed analysis and theoretical modeling.

Contribution

It introduces spectroscopic metrics for sizing and thickness estimation of h-BN nanosheets, supported by experimental data and ab initio calculations.

Findings

Optical extinction coefficient varies with nanosheet size due to scattering.

Absorbance peak energy shows weak dependence on nanosheet thickness.

Raman linewidth varies slightly with nanosheet thickness.

Abstract

For many 2D materials, optical and Raman spectra are richly structured, and convey information on a range of parameters including nanosheet size and defect content. By contrast, the equivalent spectra for h-BN are relatively simple, with both the absorption and Raman spectra consisting of a single feature each, disclosing relatively little information. Here, the ability to size-select liquid-exfoliated h-BN nanosheets has allowed us to comprehensively study the dependence of h-BN optical spectra on nanosheet dimensions. We find the optical extinction coefficient spectrum to vary systematically with nanosheet lateral size due to the presence of light scattering. Conversely, once light scattering has been decoupled to give the optical absorbance spectra, we find the size dependence to be mostly removed save for a weak but well-defined variation in energy of peak absorbance with nanosheet thickness. This finding is corroborated by our ab initio GW and Bethe-Salpeter equation calculations, which include electron correlations and quasiparticle self-consistency (QSGW). In addition, while we find the position of the sole h-BN Raman line to be invariant with nanosheet dimensions, the linewidth appears to vary weakly with nanosheet thickness. These size-dependent spectroscopic properties can be used as metrics to estimate nanosheet thickness from spectroscopic data.