2019/07/20 by Raju K. Biswas, Biswas, Raju K, Swapan K. Pati +1
Engineering · Materials Science · #Gas Sensing Nanomaterials and Sensors #Electronic and Structural Properties of Oxides #ZnO doping and properties
paper · pdf · doi:10.48550/arxiv.1907.08836
We report ab-initio calculations of the phonon dispersion relation on the\nbulk and single layer of SnO and PbO. We identify Raman active modes and\ninfrared active modes at the zone center \Γ point. In agreement with\nexperimental observations of Raman spectroscopy measurement, we find that A1g\nmode is higher in frequency than that of Eg mode. Moreover, the reason behind\nthe shift of A2u mode to higher frequency for monolayer of both SnO and PbO is\nrevealed from our calculations. We also find that long-range Coulomb\ninteraction enhances the dielectric constant and Born effective charges in bulk\nSnO and bulk PbO, compared to their monolayer. Here, we observe avoided\ncrossing or Landau degeneracy between longitudinal acoustics (LA) and low\nenergetic transverse optical (TO) modes in bulk form of both SnO and PbO.\nAdditionally, monolayer SnO also shows low energetic Raman modes (Eg and A1g)\nof same frequency as bulk. As a result, we notice avoided crossing between LA\nand TO modes in monolayer SnO. Interestingly, higher Born effective charge and\nlow dielectric constant enhances self-force constants and the interatomic force\nconstants (IFCs) between the M-O bonds. The enhanced force constants give rise\nto higher vibrational frequency of phonon modes for monolayer PbO. Our studies\nreveal that due to avoided crossing between two degenerate bands, the phonon\ndispersion near high symmetry X point lowers specific heat and vibrational\nentropy in bulk SnO, bulk PbO and only in monolayer SnO upto temperature 150 K.\nMoreover, the large mass difference between Pb and Oxygen atoms and absence of\ninterlayer van der Waal interactions give rise to high phonon vibration which\nreduces the occurrence of band crossing between two degenerate energy levels.\nThe absence of avoided crossing leads higher specific heat and vibrational\nentropy in monolayer PbO at low temperatures.\n