Room Temperature Dielectric Function of Low Dimensional TlMeX 2
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Room Temperature Dielectric Function of Low Dimensional TlMeX2 Nazim Mamedov, Kazuki Wakita, Seiji Akita and Yoshikazu Nakayama Department of Physics and Electronics, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 593-8531, Japan ABSTRACT Room temperature ellipsometric measurements of low dimensional TlMeX2 (Me=Ga, In; X=S,Se,Te) were performed in the photon energy range 1.5-6.5eV. Obtained ellipsometric data were then treated with allowance for optical anisotropy of the above materials. The components of dielectric function tensor were determined. The structures of the obtained dielectric function are interpreted in terms of the angle resolved photoemission and band structure results. The contribution of the atomic states of each atom (Tl, In, Se) in the band states in the point M of the Brillouin zone is traced back using LCAO analysis. An ultraviolet window in the joint density of electronic states of some of TlMeX2 is disclosed and substantiated to be a sign of ferroelectricity at room temperature. For the first time the nanorods of TlMeX2 are demonstrated. INTRODUCTION TlMeX2 (Me=Ga, In; X=S,Se,Te) incommensurate materials with quasi one dimensional (1D) and two dimensional (2D) lattice structure continue to attract much attention for their large thermoelectric power caused by the strong temperature-dependent shift of Fermi level [1], strongly non-linear electric properties [2], and record-breaking photo-induced memory effects [3,4]. With the aid of the advanced nanomanupulation techniques [5,6], preparation and examination of the TlMeX2 samples with subnanometer size are now in progress [7] and the above useful properties may soon be considered in relation to the application at the nanoscale. In this work we give experimental data and analyses on room temperature dielectric function obtained ellipsometrically on bulky TlMeX2. We also demonstrate the first patterns of nanorods of 1D-TlMeX2, which were prepared by pounding in a mortar, ball milling and ultrasonic dispersion [7]. The obtained results are discussed with references on electronic structure [1], [8-10] and in the light of possible consequences at the nanoscale. EXPERIMENTAL DETAILS Phase modulated ellipsometric technique for determination of the ellipsometric angles Ψ and ∆ , and the procedure of determination of the polarization degree of the light reflected from the samples were a common type as described previously elsewhere [8]. We used a Jobin-Yvon spectroscopic phase modulated ellipsometer UVISEL-9017TK for the studies in the photon energy range 1.5-6eV. The ellipsometeric angles Ψ and ∆ were obtained from the measurements on perfectly plane-parallel samples that were prepared by a cleaving from the ingots of Bridgeman-grown TlMeX2. The obtained ellipsometric data were then treated using two-phase model (substrate +air ambience) that was found to be almost perfect to the materials under consideration. At the same time, the uniaxial approach that was naturally justified for uniaxial 1D-TlMeX2 [
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