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

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We investigated interlayer modes of few-layer HfX 2 (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 HfSe 2 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 E g and out-of-plane A 1g modes of HfSe 2 are located at ∼150 cm −1 and ∼200 cm −1 , respectively. In HfS 2 , in-plane E g and out-of-plane A 1g 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 HfSe 2 are obtained to be 1.87 × 10 19 N m −3 and 6.55 × 10 19 N m −3 , 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 HfX 2 (X = S, Se).