Dat Do

Physics of bandgap formation in Cu–Sb–Se based novel thermoelectrics: the role of Sb valency and Cu d levels

In this paper we discuss the results of ab initio electronic structure calculations for Cu(3)SbSe(4) (Se4) and Cu(3)SbSe(3) (Se3), two narrow bandgap semiconductors of thermoelectric interest. We find that Sb is trivalent in both the compounds, in contrast to a simple nominal valence (ionic) picture which suggests that Sb should be 5 + in Se4. The gap formation in Se4 is quite subtle, with hybridization between Sb 5s and the neighboring Se 4s, 4p orbitals, position of Cu d states, and non-local exchange interaction, each playing significant roles. Thermopower calculations show that Se4 is a better p-type system. Our theoretical results for Se4 agree very well with recent experimental results obtained by Skoug et al (2011 Sci. Adv. Mater. 3 602).

Bonds, bands, and band gaps in tetrahedrally bonded ternary compounds: The role of group V lone pairs

Abstract An interesting class of tetrahedrally coordinated ternary compounds has attracted considerable interest because of their potential as good thermoelectrics. These compounds, denoted as I 3 –V–VI 4 , contain three monovalent-I (Cu, Ag), one nominally pentavalent-V (P, As, Sb, Bi), and four hexavalent-VI (S, Se, Te) atoms; and can be visualized as ternary derivatives of the II–VI zincblende or wurtzite semiconductors, obtained by starting from four unit cells of (II–VI) and replacing four type II atoms by three type I and one type V atoms. We find that nominally pentavalent-V atoms are effectively trivalent and their lone ( ns 2 ) pairs play an active role in opening up a gap. The lowest conduction band is a strongly hybridized anti-bonding combination of the lone pair and chalcogen (VI) p -states. The magnitude of the gap is sensitive to the nature of the exchange interaction (local vs non-local) and the V–VI distance. We also find that the electronic structure near the gap can be reproduced extremely well within a local theory if one can manipulate the position of the filled d bands of Cu and Ag by an effectively large U .

Spin splitting in 2D monochalcogenide semiconductors.

We report ab initio calculations of the spin splitting of the uppermost valence band (UVB) and the lowermost conduction band (LCB) in bulk and atomically thin GaS, GaSe, GaTe, and InSe. These layered monochalcogenides appear in four major polytypes depending on the stacking order, except for the monoclinic GaTe. Bulk and few-layer ε-and γ -type, and odd-number β-type GaS, GaSe, and InSe crystals are noncentrosymmetric. The spin splittings of the UVB and the LCB near the Γ-point in the Brillouin zone are finite, but still smaller than those in a zinc-blende semiconductor such as GaAs. On the other hand, the spin splitting is zero in centrosymmetric bulk and even-number few-layer β-type GaS, GaSe, and InSe, owing to the constraint of spatial inversion symmetry. By contrast, GaTe exhibits zero spin splitting because it is centrosymmetric down to a single layer. In these monochalcogenide semiconductors, the separation of the non-degenerate conduction and valence bands from adjacent bands results in the suppression of Elliot-Yafet spin relaxation mechanism. Therefore, the electron- and hole-spin relaxation times in these systems with zero or minimal spin splittings are expected to exceed those in GaAs when the D’yakonov-Perel’ spin relaxation mechanism is also suppressed.

Crystal Growth and Characterization of the Narrow-Band-Gap Semiconductors OsPn2 (Pn = P, As, Sb)

Using metal fluxes, crystals of the binary osmium dipnictides OsPn2 (Pn = P, As, Sb) have been grown for the first time. Single-crystal X-ray diffraction confirms that these compounds crystallize in the marcasite structure type with orthorhombic space group Pnnm. The structure is a three-dimensional framework of corner- and edge-sharing OsPn6 octahedra, as well as [Pn2(4-)] anions. Raman spectroscopy shows the presence of P-P single bonds, consistent with the presence of [Pn2(-4)] anions and formally Os(4+) cations. Optical-band-gap and high-temperature electrical resistivity measurements indicate that these materials are narrow-band-gap semiconductors. The experimentally determined Seebeck coefficients reveal that nominally undoped OsP2 and OsSb2 are n-type semiconductors, whereas OsAs2 is p-type. Electronic band structure using density functional theory calculations shows that these compounds are indirect narrow-band-gap semiconductors. The bonding p orbitals associated with the Pn2 dimer are below the Fermi energy, and the corresponding antibonding states are above, consistent with a Pn-Pn single bond. Thermopower calculations using Boltzmann transport theory and constant relaxation time approximation show that these materials are potentially good thermoelectrics, in agreement with experiment.

Chemical Ordering Rather Than Random Alloying in SbAs

Daniel P. Shoemaker, Thomas C. Chasapis, Dat Do, Melanie C. Francisco, Duck Young Chung, S. D. Mahanti, Anna Llobet, and Mercouri G. Kanatzidis ∗ Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA Lujan Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, NM 87545, USA

Electronic structure and thermoelectric properties of monolayers of group V atoms

Quantum confinement effects in narrow band gap semiconductors tend to enhance their thermoelectric properties. In the extreme limit of 2 dimensional (2D) confinement, the observation of massless Dirac particles associated with low energy excitations of honeycomb (HC) lattices of group IV atoms (C, Si, Ge, Sn) near the Dirac points (DPs) and spin-orbit interaction (SOI) induced gaps at the DPs have led to extensive study of the physical properties of these 2D monolayer systems. In this paper we discuss the electronic structure of 2D HC lattices of group V atoms (As, Sb, Bi) with a focus on their thermoelectric properties, particularly their thermopower. We find DPs at the K points of the Brillouin zone for both planar (flat) and puckered sheets. Unlike the group IV systems the Fermi energy in group V systems lies above the DPs. The flat sheets are metallic but undergo structural distortions to form puckered sheets that are semiconducting. SOI profoundly alters their band structure, opens up gaps at the DPs, and in binary systems BiSb and SbAs gives large Rashba-type spin splitting. Monolayers of group V atoms show excellent thermoelectric properties, particularly in the hole-doped regime.Quantum confinement effects in narrow band gap semiconductors tend to enhance their thermoelectric properties. In the extreme limit of 2 dimensional (2D) confinement, the observation of massless Dirac particles associated with low energy excitations of honeycomb (HC) lattices of group IV atoms (C, Si, Ge, Sn) near the Dirac points (DPs) and spin-orbit interaction (SOI) induced gaps at the DPs have led to extensive study of the physical properties of these 2D monolayer systems. In this paper we discuss the electronic structure of 2D HC lattices of group V atoms (As, Sb, Bi) with a focus on their thermoelectric properties, particularly their thermopower. We find DPs at the K points of the Brillouin zone for both planar (flat) and puckered sheets. Unlike the group IV systems the Fermi energy in group V systems lies above the DPs. The flat sheets are metallic but undergo structural distortions to form puckered sheets that are semiconducting. SOI profoundly alters their band structure, opens up gaps at the DPs...