The total energy, the electronic properties, phase transitions, and elastic properties of Cu2ZnSnS4(CZTS) in the three structures are investigated by first-principles calculations based on density functional theory. Results show that the total energies of stannite (ST) and primitive-mixed CuAu (PMCA) structures are higher than that of kesterite-type (KS), and the KS is the ground state structure. Relationships between enthalpy and pressure of the KS, ST and PMCA structure of CZTS are also investigated at 0 K, since the pressure can have profound impacts on the electronic structure, possible phase transitions and structure stability. And results also show that KS structure is always the most stable; ST is the second; and the PMCA structure is the most unstable; phase transitions of three structures could not occur in high pressure. The high ratios of shear modulus to bulk modulus (G/B) indicate that CZTS compounds in three types have ductile behaviors. The Poisson ratios for the three structures are from 0.27 to 0.31, which again proves that all structures of CZTS have better plasticity. The results can increase more hints about further research directions, and these effects can play an important role in future experimental preparation technology and theoretical work of CZTS materials.
In the present work we report on the Single-crystal samples synthesis of type-VIII clathrates Ba8Ga16−xCuxSn30(x = 0, 0.75, 1, 1.5, 2) by the Sn-flux method and the effects of Cu doping Ba8Ga16Sn30have been investigated using x-ray photoemission spectroscopy (XPS). The structural and electronic properties have been studied by first-principles method based on the density-functional theory. We found that the Cu doping induced rearrangement of host-cage configuration, and consequently change of the electron transport properties. It was found that the binding energies for the Ba8Ga16−xCuxSn30series decrease with increasing Cu substitution, resulting in a decrease of stability. Calculations indicate that these alloys are all indirect gap semiconductors and the band gap increases with increasing Cu content. Theoretical results indicate that Cu doping Ba8Ga16−xCuxSn30should bep-type semiconductors, which does not agree with the previous experimental results.
Thermoelectric (TE) devices are increasingly being seen as having the potential to make important contributions to reducing greenhouse gas emissions and providing cleaner forms of energy. A number of articles have been devoted to the thermoelectric properties of materials. From the search for novel and effective thermoelectric materials the clathrate structures has emerged as one of the most promising candidates for achieving very high thermoelectric figure of merit: ZT= α2σT/κ, where α, T, σ and κ are the Seebeck coefficient, absolute temperature, electrical conductivity, and total thermal conductivity, respectively [1]. For the past decade, caged clathrate compounds of group IV elements have attracted much attention because they would possess a low kL value as the theoretical minimum one, which results from rattling of atoms filled in their cages [2-3]. There are the type-I, type-III, and type-VIII structures in thermoelectric clathrates, but most compounds adopt type-I structure (space group No.223; Pm-3n). A large number of the type-I clathrates with the chemical formula of II8III16IV30 (II=Ba, Sr, Eu, III=Al, Ga, In, and IV= Si, Ge, Sn) have been synthesized and studied intensively [5-11], which results in relatively high ZT values such as 0.7 at 700 K for Ba8Ga16Ge30 and 0.87 at 870 K for Ba8Ga16Si30 [3]. Among type-I clathrates, a single-crystal n-type Ba8Ga16Ge30 grown using the Czochralski method with a ZT of 1.35 at 900 K is one of the most promising results [12].
In this paper, we present the characterization of Ge-induced crystallization of amorphous Si (a-Si) films deposited by magnetron sputtering. The film structures of a-Si films were characterized by Raman spectroscopy, Atomic Force microscope (AFM), and field emission scanning electron microscope (FESEM). The result show that 60% of a-si film with a layer of 400 nm Ge buried is crystallized at growth temperature of 800 °C. The surface roughness and average surface grain size obtained by AFM is 2.39 nm and 60 nm for the crystallized film, respectively. The films growth at temperature of 500°C and 650 °C shows a PL spectrum band from 1.6 eV to 1.8 eV, and the PL peak shifts to lower energy as the growth temperature increased. As for the film grown at 800 °C, the PL spectrum is nearly extinguished. The crystallization of a-Si film induced by buried Ge might be a useful technology to develop high quality poly-Si film without annealing.
First-principles plane wave pseudopotential method based on the density functional theory is used to study of the evolution rule of the optical properties and electronic structure for ZnO tetragonal phase under different pressure. The results show that the chemical bond between zinc and oxygen is shortened with the increase of the external pressure, while the increase of orbital hybridization between the zinc atom and the oxygen atom. The calculations also show that the band structure of ZnO tetragonal phase changes with the increase of the pressure, and the band gap is broadened, which causes that the optical absorption margin and the optical absorption peak move to the direction of high energy.
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