Photoacoustic and modulated reflectance studies of indirect and direct band gap in van der Waals crystals

Szymon J. Zelewski,Robert Kudrawiec

doi:10.1038/s41598-017-15763-1

Szymon J. Zelewski, Robert Kudrawiec

Open Access

https://doi.org/10.1038/s41598-017-15763-1

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Abstract

Photoacoustic (PA) and modulated reflectance (MR) spectroscopy have been applied to study the indirect and direct band gap for van der Waals (vdW) crystals: dichalcogenides (MoS2, MoSe2, MoTe2, HfS2, HfSe2, WS2, WSe2, ReS2, ReSe2, SnS2 and SnSe2) and monochalcogenides (GaS, GaSe, InSe, GeS, and GeSe). It is shown that the indirect band gap can be determined by PA technique while the direct band gap can be probed by MR spectroscopy which is not sensitive to indirect optical transitions. By measuring PA and MR spectra for a given compound and comparing them with each other it is easy to conclude about the band gap character in the investigated compound and the energy difference between indirect and direct band gap. In this work such measurements, comparisons, and analyses have been performed and chemical trends in variation of indirect and direct band gap with the change in atom sizes have been discussed for proper sets of vdW crystals. It is shown that both indirect and direct band gap in vdW crystals follow the well-known chemical trends in semiconductor compounds.

Highlights

IntroductionVan der Waals (vdW) crystals are known for a long time[1,2,3,4,5,6,7,8,9,10,11,12,13,14] but in recent years some of them have gained significant interest because of unique mechanical and optical properties of samples obtained by exfoliation of bulk material to atomically thin layers[15,16,17,18,19,20,21,22,23,24,25]
We show that by comparing PA and modulated reflectance (MR) spectra it is possible to conclude about the band gap character in van der Waals crystals
It has been shown that PA spectroscopy is a powerful tool to study the indirect band gap in van der Waals crystals

Summary

Introduction

Van der Waals (vdW) crystals are known for a long time[1,2,3,4,5,6,7,8,9,10,11,12,13,14] but in recent years some of them have gained significant interest because of unique mechanical and optical properties of samples obtained by exfoliation of bulk material to atomically thin layers[15,16,17,18,19,20,21,22,23,24,25]. This is an important advantage of PA spectroscopy over regular absorption measurements (combined transmission and reflectance measurements) where the determination of the absorption edge for indirect semiconductors becomes difficult if the sample thickness is small and/or irregular, i.e., the case of exfoliated van der Waals crystals. Very important advantage of this method is that it can be applied even if the investigated samples are very small or the incident light is scattered due to sample surface irregularities, since only the absorbed light fraction causes the signal generation We recently utilized this advantage to study GaAsBi nanowires[38] but it is worth highlighting in the case of van der Waals crystals. We show that by comparing PA and MR spectra it is possible to conclude about the band gap character in van der Waals crystals

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