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

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 (∼10⁻⁶Torr). 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 E_g and out-of-plane A_1g modes of HfSe₂ are located at ∼150 cm⁻¹ and ∼200 cm⁻¹, respectively. In HfS₂, 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 HfSe₂ are obtained to be 1.87 × 10¹⁹N m⁻³ and 6.55 × 10¹⁹N 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 HfX₂ (X = S, Se).