Vibrational and electrical properties of sputtered films of the copper telluride system are presented. Despite of its technological importance in photovoltaics, the fundamental properties of copper tellurides are poorly understood. Films were deposited at 200 °C by rf sputtering from targets containing mixtures of copper and tellurium powders at nominal concentrations of Cu1.25Te, Cu1.5Te, Cu1.75Te and Cu2Te. Remarkably for the copper telluride system, it was possible to obtain single-phase vulcanite (CuTe) from the Cu1.25Te target. Two-phase mixtures of rickardite (Cu7Te5) and weissite (Cu2−xTe) were achieved for other cases. Raman spectra were obtained using two laser lines: 633 and 488 nm. Density functional theory was employed to calculate the phonon dispersion curves and density of states for vulcanite. The Raman bands were in good correspondence with the calculated frequencies. In general, the Raman spectra consisted of high-intensity totally symmetric modes superimposed on monotonically decaying signals. These were explained in terms of three contributing phenomena: convolution of vibrational normal modes, phonon-coupled charge density fluctuations and time-varying local-field contributions to the electric susceptibility. Studies on the conductivity, mobility and carrier concentration were carried out by the Van der Pauw method. Micro/nano scale surface potential studies were performed through Kelvin probe force microscopy mapping.
Layered-structure materials are currently relevant given their quasi-2D nature. Knowledge of their physical properties is currently of major interest. Niobium ditelluride possesses a monoclinic layered-structure with a distortion in the tellurium planes. This structural complexity has hindered the determination of its fundamental physical properties. In this work, NbTe2 crystals were used to elucidate its structural, compositional, electronic and vibrational properties. These findings have been compared with calculations based on density functional theory. The chemical composition and elemental distribution at the nanoscale were obtained through atom probe tomography. Ultraviolet photoelectron spectroscopy allowed the first determination of the work function of NbTe2. Its high value, 5.32 eV, and chemical stability allow foreseeing applications such as contact in optoelectronics. Raman spectra were obtained using different excitation laser lines: 488, 633, and 785 nm. The vibrational frequencies were in agreement with those determined through density functional theory. It was possible to detect a theoretically-predicted, low-frequency, low-intensity Raman active mode not previously observed. The dispersion curves and electronic band structure were calculated, along with their corresponding density of states. The electrical properties, as well as a pseudo-gap in the density of states around the Fermi energy are characteristics proper of a semi metal.
Titanium disulfide is a quasi-2D transition-metal dichalcogenide relevant for various potential applications. To exploit its technological capabilities, it is important to determine its fundamental properties including the lattice dynamics. The TiS 2 phonon dispersion curves available to date do not reproduce properly the experimental data for several reasons: i) all available experimental data are not taken into consideration, thus poor theory-experiment agreements have been obtained; ii) the (unknown) frequency of the infrared mode A 2u is erroneously assumed; and iii) such incorrect assignment has propagated in the literature, particularly in phonon dispersion calculations. It is presented here a thorough density functional theory analysis to determine the phonon dispersion curves of TiS 2 , accounting for the frequencies of all experimental phonon data available to date. These include the frequencies of nine zone-edge Raman active modes of Ag-intercalated TiS 2 , in addition to infrared, Raman, and neutron scattering data. An incorrect frequency assignment of the A 2u mode in the literature is thoroughly discussed. Moreover, results of attenuated total reflection terahertz spectroscopy applied to TiS 2 are provided. A self-intercalation paradigm is presented to give a rationale for the temperature dependence of the poorly understood Raman features observed in pristine TiS 2 at frequencies above the A 1g mode.
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