Raman spectroscopy of atomically thin HfX2 (X=S, Se)

Abstract

Abstract We investigated interlayer modes of few-layer HfX2 (X=S, Se) by using low-frequency micro-Raman spectroscopy with three excitation energies (1.96 eV, 2.33 eV, 2.54 eV) under vacuum condition (10−6 Torr). We observed interlayer modes in HfSe2 when the 2.54-eV excitation energy was used. The low-frequency Raman spectra reveal a series of shear and breathing modes (<50 cm−1) that are helpful for identifying the number of layers. The in-plane Eg and out-of-plane A1g modes of HfSe2 are located at ~150 cm−1 and ~200 cm−1, respectively. In HfS2, in-plane Eg and out-of-plane A1g optical phonons are observed at ~260 cm−1 and ~337 cm−1, respectively. The in-plane and out-of-plane force constants of atomically thin HfSe2 are obtained to be 1.87 ×1019 N/m3 and 6.55×1019 N/m3, respectively, by fitting the observed interlayer modes using the linear chain model. These results provide valuable information on materials parameters for device designs using atomically-thin layered HfX2 (X=S, Se).