Crystallographic and optical properties of wide bandgap photovoltaic semiconductor system, Cu(Al,In)Se2

We characterized the optical and electronic properties of chalcopyrite-type Cu(Al,In)Se2, which is a candidate for wide-bandgap solar cell materials. The bandgap energy was determined from diffuse reflectance spectra. The band gap energy increased from 1.00 eV for CuInSe2 to 2.61 eV for CuAlSe2 with an increase in the Al content. The ionization energy corresponding to the energy levels of the valence band maximum (VBM) was determined using photoemission yield spectroscopy (PYS). The VBM level of the Cu(Al,In)Se2 system stayed relatively constant, but the conduction band minimum (CBM) level increased with increasing Al content. To analyze the local structures of Cu and In atoms in Cu(Al,In)Se2, Cu and In K-edge X-ray absorption fine structure (XAFS) spectra were measured at SPring-8. We discuss the crystallographic characteristics of Cu(Al,In)Se2 based on the results of the XAFS analyses and a comparison of the phase diagrams of the Cu2Se-Al2Se3, Cu2Se-In2Se3, and Cu2Se-Ga2Se3 systems.

Velocity Map Imaging VUV Angle-Resolved Photoelectron Spectroscopy of Isolated Silver Sulfide Nanoparticles

The angle-resolved photoelectron spectroscopy of isolated silver sulfide nanoparticles was carried out by using velocity map imaging technique at the DESIRS beamline of SOLEIL synchrotron facility. The reported spectroscopy results were obtained after interaction of the synchrotron radiation with a polydisperse aerosol produced from aqueous dispersion of silver sulfide particles, approximately 16 nm in diameter. The photoelectron and UV-Vis-NIR absorption spectra were used to estimate the maximum energy of the valance- and the minimum energy of the conduction-band of the nanoparticles. With respect to the vacuum level, the obtained values were found to be 5.5±0.1 eV and 4.5±0.1 eV for the valence band maximum and conduction band minimum, respectively. The dependence of the asymmetry parameter on the electron energy along the silver sulfide valence band showed an onset of inelastic scattering at ~1 eV electron kinetic energy.

Janus MSiGeN4 (M = Zr and Hf) monolayers derived from centrosymmetric β-MA2Z4: A first-principles study

A two-dimensional (2D) MA2Z4 family with and phases has been attracting tremendous interest, the MoSi2N4 and WSi2N4 of which have been successfully fabricated ( Science 369, 670 (2020)). Janus monolayers have been achieved in many 2D families, so it is interesting to construct a Janus monolayer from the MA2Z4 family. In this work, Janus MSiGeN4 (M = Zr and Hf) monolayers are predicted from -MA2Z4, which exhibit dynamic, mechanical and thermal stabilities. It is found that they are indirect band-gap semiconductors by using generalized gradient approximation (GGA) plus spin-orbit coupling (SOC). With biaxial strain from 0.90 to 1.10, the energy band gap shows a nonmonotonic behavior due to a change of conduction band minimum (CBM). A semiconductor to metal transition can be induced by both compressive and tensile strains, and the phase transformation point is about 0.96 for compressive strain and 1.10 for tensile strain. The tensile strain can change the positions of CBM and valence band maximum (VBM), and can also induce the weak Rashba-type spin splitting near CBM. For MSiGeN4 (M = Zr and Hf) monolayers, both an in-plane and out-of-plane piezoelectric response can be produced, when a uniaxial strain in the basal plane is applied, which reveals the potential as piezoelectric 2D materials. The high absorption coefficients in the visible light region suggest that MSiGeN4 (M = Zr and Hf) monolayers have potential photocatalytic applications. Our works provide an idea to achieve a Janus structure from the MA2Z4 family, and can hopefully inspire further research exploring Janus MA2Z4 monolayers.

Effect of Substitutional Point Defect of Gold (Au) in Indium (In) Site of Double Halide Perovskite (Cs2InSbCl6)

Lead (Pb) free (non-toxic) perovskite solar cells materials have attracted great interest in the commercialization of the photovoltaic devices. In this work, density functional theory (DFT) and linear response time-dependent within density functional theory (TDDFT) are used to simulate and investigate the effect of gold (Au) dopedPb-free double halide perovskite A2BB?X6(A = Cs; B = In, Au; B? = Sb; X = Cl) on the structural, electronic, and optical properties for perovskite solar cell application. On the structural properties, bond length and bulk modulus calculations show that the doped compound is more likely to resist deformation than the undoped compound. The calculated band structure for both materials (doped and undoped) reveals the presence of the Valence Band Maximum (VBM) and the Conduction Band Minimum (CBM) at around the same symmetry point which indicates a direct band gap nature (at ???? point). The band gap value for the initial compound (= 0.99 eV) agrees with published theoretical values. For the gold doped compound, the value of the band gap increased to a value of 1.25eV. The result of the optical properties shows that the Au-doped material has higher absorption coefficient, lower reflectivity and higher optical conductivity when compared with the initial, as such demonstrates better properties as a candidate for solar cell applications and in other optoelectronic devices.

Influence of Graphite Layer on Electronic Properties of MgO/6H-SiC(0001) Interface

This paper concerns research on magnesium oxide layers in terms of their potential use as a gate material for SiC MOSFET structures. The two basic systems of MgO/SiC(0001) and MgO/graphite/SiC(0001) were deeply investigated in situ under ultrahigh vacuum (UHV). In both cases, the MgO layers were obtained by a reactive evaporation method. Graphite layers terminating the SiC(0001) surface were formed by thermal annealing in UHV. The physicochemical properties of the deposited MgO layers and the systems formed with their participation were determined using X-ray and UV photoelectron spectroscopy (XPS, UPS). The results confirmed the formation of MgO compounds. Energy level diagrams were constructed for both systems. The valence band maximum of MgO layers was embedded deeper on the graphitized surface than on the SiC(0001).

Induced Magnetism in Non-metal Doped CdS Monolayer

The structural, electronic, and magnetic properties of CdS monolayer doped with non-metallic (NM) atoms B, C and N are studied based on ab initio density functional theory calculations within the generalized gradient approximation as revised for solids by Perdew, Burke and Ernzerhof (PBE-sol). The total magnetic moments per supercell of B, C and N-doped CdS monolayer is found to be ~1.0 µB, ~2.0 µB and ~1.0 µB respectively. As the electronegativity of the dopant increases, the local magnetic moment tends to localize and 2p-states of the dopants gradually move towards the valence band maximum of the host CdS. Our study also confirmed that the introduction of impurity atom by substitution of S atom results in half-metallic magnetism. Our investigation concludes that doping of NM element is an efficient way of altering the magnetic and electronic properties in CdS monolayer.

Mixed-Metal Cu-Zn Thiocyanate Coordination Polymers with Melting Behavior, Glass Transition, and Tunable Electronic Properties

<p>The solid-state mechanochemical reactions under ambient conditions of CuSCN and Zn(SCN)₂ resulted in two novel materials: partially Zn-substituted <i>α</i>-CuSCN and a new phase Cu_xZn_y(SCN)_x+2y. The reactions take place at the labile S-terminal, and both products show melting and glass transition behaviors. The optical band gap and solid-state ionization potential can be adjusted systematically by adjusting the Cu:Zn ratio. Density functional theory calculations also reveal that the Zn-substituted CuSCN structure features a complementary electronic structure of Cu 3<i>d</i> states at the valence band maximum (VBM) and Zn 4<i>s</i> states at the conduction band minimum (CBM). This work shows a new route to develop semiconductors based on coordination polymers which are becoming technologically relevant for electronic and optoelectronic applications.</p>

Mixed-Metal Cu-Zn Thiocyanate Coordination Polymers with Melting Behavior, Glass Transition, and Tunable Electronic Properties

<p>The solid-state mechanochemical reactions under ambient conditions of CuSCN and Zn(SCN)₂ resulted in two novel materials: partially Zn-substituted <i>α</i>-CuSCN and a new phase Cu_xZn_y(SCN)_x+2y. The reactions take place at the labile S-terminal, and both products show melting and glass transition behaviors. The optical band gap and solid-state ionization potential can be adjusted systematically by adjusting the Cu:Zn ratio. Density functional theory calculations also reveal that the Zn-substituted CuSCN structure features a complementary electronic structure of Cu 3<i>d</i> states at the valence band maximum (VBM) and Zn 4<i>s</i> states at the conduction band minimum (CBM). This work shows a new route to develop semiconductors based on coordination polymers which are becoming technologically relevant for electronic and optoelectronic applications.</p>

Band Structure and Thermoelectric Properties of Ni(x)Zn(1-x)Fe2O4

Thermoelectric materials has made a great potential in sustainable energy industries, which enable the energy conversion from heat to electricity. The band structure and thermoelectric properties of Ni(x)Zn(1-x)Fe2O4 have been investigated. The bulk pellets were prepared from analytical grade ZnO, NiO and Fe2O3 powder using solid-state method. It was possible to obtain high thermoelectric properties of Ni(x)Zn(1-x)Fe2O4 by controlling the ratios of dopants and the sintering temperature. XRD analysis showed that the fabricated samples have a single phase formation of cubic spinel structure. The thermoelectric properties of Ni(x)Zn(1-x)Fe2O4 pellets improved with increasing Ni. The electrical conductivity of Ni(x)Zn(1-x)Fe2O4 pellets decreased with increasing Ni content. The electrical conductivity of Ni(x)Zn(1-x)Fe2O4 (x = 0.0) is (0.515 x10-3 Scm-1). The band structure shows that ZnxCu1-xFe2O4 is an indirect band gap material with the valence band maximum (VBM) at M and conduction band minimum (CBM) at A. The band gap of Ni(x)Zn(1-x)Fe2O4 increased with increasing Ni content. The increasing band gap correlated with the lower electrical conductivity. The thermal conductivity of Ni(x)Zn(1-x)Fe2O4 pellets decreased with increasing Ni content. The presence of Ni served to decrease thermal conductivity by 8 Wm-1K-1 over pure samples. The magnitude of the Seebeck coefficient for Ni(x)Zn(1-x)Fe2O4 pellets increased with increasing amounts of Ni. The figure of merit for Ni(x)Zn(1-x)Fe2O4 pellets and thin films was improved by increasing Ni due to its high Seebeck coefficient and low thermal conductivity.