Se Substitution Research Articles

Defect-Controlled Electronic Structure and Phase Stability in Thermoelectric Skutterudite CoSb3

Controlling extrinsic defects to tune the carrier concentration of electrons or holes is a crucial point with regard to the engineering application of thermoelectric semiconductors. To understand the defect-controlled electronic structure in thermoelectric materials, we apply density functional theory (DFT) to investigate the defect chemistry of dopants M (M = O, S, Se, or Te) in CoSb3. DFT predicts that the breakage of Sb4 rings induced by these dopants produces the unexpected (n- or p-type) conductivity behavior in CoSb3. For example, energetically dominant O interstitials (Oi) chemically break Sb4 rings and form O–4Sb five-membered rings, leading to the charge neutral behavior of Oi. While S interstitials (Si) collapse Te–3Sb four-membered rings within Te doped CoSb3 leading to p-type conduction behavior, Se substitution on Sb (SeSb) breaks the Se–Te–2Sb four-membered ring, resulting in a charge neutral behavior of the SeSb+TeSb complex defect. Furthermore, the solubility limits of M dopants (M = S, Se...

Electronic coupling between a FeSe monolayer film and SrTiO3 substrate

Several experimental groups have reported superconductivity in single unit cell layers of FeSe on ${\mathrm{SrTiO}}_{3}$ and a few other substrates, with critical temperature ${T}_{c}$ reports ranging up to 100 K, and a variety of theoretical work has been done. Here we examine more closely the interaction of a single FeSe layer with a ${\mathrm{TiO}}_{2}$ terminated ${\mathrm{SrTiO}}_{3}$(001) (STO) substrate. Several situations are analyzed: the underlying ideal interface, the effect of $z$-polarized longitudinal optic $[\mathrm{LO}(\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{z}$)] phonons in STO, electron doping of the STO substrate, substitution of Se by the bordering chalcogenides S and Te, and doping by adsorption of K on the FeSe surface. These results complement earlier studies of O and Se vacancies. The O ${p}_{y},{p}_{y}$ surface band of STO persists at the interface, and by sharing holes with the hole pocket of FeSe it plays a part in the behavior around the interface, initially by determining the Fe Fermi level lineup with the STO band gap. The $\mathrm{LO}(\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{z})$ mode causes strong band shifts around the interface but the strength of coupling to the Fe bands cannot be obtained with our methods. Adsorption of 25% K (one K per four Fe) fills the small O interface hole pocket and donates the rest of the electrons to the Fe hole pockets, filling them.

FeTe1−xSex monolayer films: towards the realization of high-temperature connate topological superconductivity

We performed angle-resolved photoemission spectroscopy studies on a series of FeTe1−xSex monolayer films grown on SrTiO3. The superconductivity of the films is robust and rather insensitive to the variations of the band position and effective mass caused by the substitution of Se by Te. However, the band gap between the electron- and hole-like bands at the Brillouin zone center decreases towards band inversion and parity exchange, which drive the system to a nontrivial topological state predicted by theoretical calculations. Our results provide a clear experimental indication that the FeTe1−xSex monolayer materials are high-temperature connate topological superconductors in which band topology and superconductivity are integrated intrinsically.

Anticorrelation between polar lattice instability and superconductivity in the Weyl semimetal candidate MoTe2

The relation between the polar structural instability and superconductivity in a Weyl semimetal candidate MoTe2 has been clarified by finely controlled physical and chemical pressure. The physical pressure as well as the chemical pressure, i.e., the Se substitution for Te, enhances the superconducting transition temperature Tc at around the critical pressure where the polar structure transition disappears. From the heat capacity and thermopower measurements, we ascribe the significant enhancement of Tc at the critical pressure to a subtle modification of the phonon dispersion or the semimetallic band structure upon the polar-to-nonpolar transition. On the other hand, the physical pressure, which strongly reduces the interlayer distance, is more effective on the suppression of the polar structural transition and the enhancement of Tc as compared with the chemical pressure, which emphasizes the importance of the interlayer coupling on the structural and superconducting instability in MoTe2.

Role of the local structure in superconductivity of LaO0.5F0.5BiS2−xSex system

We have studied the local structure of LaO0.5F0.5BiS2−xSex by Bi L1-edge extended x-ray absorption fine structure (EXAFS). We find a significant effect of Se substitution on the local atomic correlations with a gradual elongation of average in-plane Bi-S bondlength. The associated mean square relative displacement, measuring average local distortions in the BiS2 plane, hardly shows any change for small Se substitution, but decreases significantly for . The Se substitution appears to suppress the local distortions within the BiS2 plane that may optimize in-plane orbital hybridization and hence the superconductivity. The results suggest that the local structure of the BiS2-layer is one of the key ingredients to control the physical properties of the BiS2-based dichalcogenides.

Thermoelectric and transport properties of N-type Bi2−x Sb x Te3−y Se y solid solutions

Bi2−x Sb x Te3−y Se y (x = 0.1, 0.2 and y = 0.15, 0.3) solid solutions were prepared using encapsulated melting and hot pressing. The lattice constants decreased with increases in the Sb and the Se contents, which revealed the successful formation of solid solutions. The relative densities of the hot-pressed specimens were 95 - 98%. All specimens exhibited n-type conduction at temperatures from 323 K to 523 K, and the electrical conductivity slightly decreased with increasing temperature. With an increase in the Se content, the Seebeck coefficient increased while the electrical and the thermal conductivities decreased; thus, the dimensionless figure of merit could be improved. The maximum dimensionless figure of merit ZT max = 0.89 was obtained at 423 K for Bi1.8Sb0.2Te2.7Se0.3. An increase in the Sb content resulted in a decrease in the lattice thermal conductivity because of an increase in alloy scattering, but its effect on the electrical properties was not superior to the effect of Se substitution. Therefore, Sb substitution could effectively control the thermal properties while Se substitution could effectively control the electrical properties.

Thermoelectric properties of isoelectronically substituted bismuth compounds: a computational study

This work focuses on using isoelectronic substitution to modify electronic density of states (DOS) of bismuth (Bi) compounds for thermoelectric property modification. The calculations include first-principle density functional theory (DFT) and analytical calculations based on Mott formula. The thermoelectric materials selected in the present study are Bi compounds, i.e. Bi2Te3, Bi2Se3, Bi2Se2Te, Bi2Te2Se, Bi2O2Te, and Bi2O2Se. The results reveal that isoelectronic substitution of Se and Te atoms with O atoms (Bi2O2Te and Bi2O2Se) introduces changes in DOS around the valence band maximum and the conduction band minimum, exhibiting the figure of merit (ZT) approaching the values 0.004 and 0.03 at room temperature, for Bi2O2Se and Bi2O2Te, respectively. Though the ZT of these oxide compounds are inferior to conventional thermoelectric materials, it is known that they show better stability and less toxicity which could be the alternative materials.

Evolution of Eu valence and superconductivity in layered Eu0.5La0.5FBiS2−xSex system

We have studied the effect of Se substitution on Eu valence in a layered ${\mathrm{Eu}}_{0.5}{\mathrm{La}}_{0.5}{\mathrm{FBiS}}_{2\ensuremath{-}x}{\mathrm{Se}}_{x}$ superconductor using a combined analysis of x-ray absorption near-edge structure (XANES) and x-ray photoelectron spectroscopy (XPS) measurements. Eu ${L}_{3}$-edge XANES spectra reveal that Eu is in the mixed valence state with coexisting ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$. The average Eu valence decreases sharply from $\ensuremath{\sim}2.3$ for $x=0.0$ to $\ensuremath{\sim}2.1$ for $x=0.4$. Consistently, Eu $3d$ XPS shows a clear decrease in the average valence by Se substitution. Bi $4f$ XPS indicates that effective charge carriers in the ${\mathrm{BiCh}}_{2}$ (Ch = S, Se) layers are slightly increased by Se substitution. On the basis of the present results it has been discussed that the metallic character induced by Se substitution in ${\mathrm{Eu}}_{0.5}{\mathrm{La}}_{0.5}{\mathrm{FBiS}}_{2\ensuremath{-}x}{\mathrm{Se}}_{x}$ is likely to be due to increased in-plane orbital overlap driven by reduced in-plane disorder that affects the carrier mobility.

Effect of Isovalent Substitution on the Electronic Structure and Thermoelectric Properties of the Solid Solution α-As2Te3-xSex (0 ≤ x ≤ 1.5).

We report on the influence of Se substitution on the electronic band structure and thermoelectric properties (5-523 K) of the solid solution α-As2Te3-xSex (0 ≤ x ≤ 1.5). All of the polycrystalline compounds α-As2Te3-xSex crystallize isostructurally in the monoclinic space group C2/m (No. 12, Z = 4). Regardless of the Se content, chemical analyses performed by scanning electron microscopy and electron probe microanalysis indicate a good chemical homogeneity, with only minute amounts of secondary phases for some compositions. In agreement with electronic band structure calculations, neutron powder diffraction suggests that Se does not randomly substitute for Te but exhibits a site preference. These theoretical calculations further predict a monotonic increase in the band gap energy with the Se content, which is confirmed experimentally by absorption spectroscopy measurements. Increasing x up to x = 1.5 leaves unchanged both the p-type character and semiconducting nature of α-As2Te3. The electrical resistivity and thermopower gradually increase with x as a result of the progressive increase in the band gap energy. Despite the fact that α-As2Te3 exhibits very low lattice thermal conductivity κL, the substitution of Se for Te further lowers κL to 0.35 W m-1 K-1 at 300 K. The compositional dependence of the lattice thermal conductivity closely follows classical models of phonon alloy scattering, indicating that this decrease is due to enhanced point-defect scattering.

Synthesis, structures and electronic properties of Co(III) complexes with 2-quinolinecarboxaldehyde thio- and selenosemicarbazone: A combined experimental and theoretical study

Cobalt(III) complexes derived from thio- and selenosemicarbazone ligands have been studied to elucidate the nature and consequences of S to Se substitution on their possible biological activity. Solid state structures of cobalt(III) complexes with bis-tridentate coordinated 2-quinolinecarboxaldehyde thio- and selenosemicarbazone were determined by single crystal X-ray diffraction analysis. The complexes were also characterized by spectroscopic methods and cyclic voltammetry. Electronic properties of the complexes were studied using DFT and TD?DFT methods. Finally, evident in vitro antioxidant activity of the complexes was demonstrated.

Bulk Superconductivity Induced by In-Plane Chemical Pressure Effect in Eu0.5La0.5FBiS2−xSex

We have investigated Se substitution effect to superconductivity of an optimally-doped BiS2-based superconductor Eu0.5La0.5FBiS2. Eu0.5La0.5FBiS2-xSex samples with x = 0-1 were synthesized. With increasing x, in-plane chemical pressure is enhanced. For x = 0.6, 0.8, and 1, superconducting transitions with a large shielding volume fraction are observed in magnetic susceptibility measurements, and the highest Tc is 3.8 K for x = 0.8. From low-temperature electrical resistivity measurements, a zero-resistivity state is observed for all the samples, and the highest Tc is observed for x = 0.8. With increasing Se concentration, characteristics of electrical resistivity changes from semiconducting-like to metallic, suggesting that the emergence of bulk superconductivity is linked with the enhanced metallicity. A superconductivity phase diagram of the Eu0.5La0.5FBiS2-xSex superconductor is established. Temperature dependences of electrical resistivity show an anomalous two-step transition under high magnetic fields. Hence, the resistivity data are analyzed with assuming in-plane anisotropy of upper critical field.

Impact of tellurium on glass transition and crystallization in the Ge-Se-Te-Bi system

Samples with compositions of Ge20Se67−xTexBi13 (x=0, 5, 10, 15 and 20mol%) were prepared and characterized by XRD, Raman spectroscopy and DSC. XRD patterns and Raman spectra present consistent evidences of glass transition and crystallization in association with the variation of the structure network due to the substitution of Te for Se. Precipitation of thermoelectric (TE) crystals experiences an evolution from Bi2Se2Te (BST) to Bi2SeTe2 (BTS) phases with the increasing Te content. Based on DSC data performed at five heating rates, crystallization kinetics is analyzed by using Kissinger's relation, Ozawa model and Augis-Bennett approximation. Different methods show the identical evaluations of characteristic activation energies (Ec), while additional parameters of frequency factor (K0) and Avrami exponent (n) are available with AB method. In particular, calculated Ec and K0 demonstrate that with the substitution of Se by Te the crystallization process of the target TE BST crystal phase is much enhanced while that of the undesired GeSe2 phase is slightly hindered.

Positron Annihilation Study of Ternary Sb2Te3−x Se x for Its Tuning Electrical and Thermal Properties

Atomic scale point defects play important roles in tuning the carrier concentration and ultimately influencing electrical and thermal properties. Herein, we fabricated the ternary Sb2Te3−xSex alloys to study the intimate relationship of internal point defects and thermoelectric performance. The Se substitution of Te atoms in the Sb2Te3 lattice decreased the electrical conductivity from 2.2 × 105 S/m to 6.4 × 104 S/m owing to the reduced holes concentration. The declined point defects, including antisite defects and vacancies in materials, gave rise to the decrease in carrier concentration. The Seebeck coefficient of the ternary Sb2Te3−xSex exhibited an increase with doping of Se atoms. Simultaneously, the thermal conductivity behaved a fallihg trend as well as increasing Se content. As a result, the ZT value reached the maximum from the corresponding Sb2Te2.9Se0.1 pellet. Positron annihilation measurement revealed that the average positron lifetime showed a monotonic decrease with Se addition, demonstrating the reduced point defects, which was in agreement with the thermoelectric performance.

Superconductivity in the Narrow Gap Semiconductor RbBi11/3Te6.

Superconductivity was discovered in the layered compound RbBi11/3Te6, featuring Bi vacancies and a narrow band gap of 0.25(2) eV at room temperature. A sharp superconducting transition at ∼3.2 K was observed in polycrystalline ingots. The superconducting volume fraction of oriented single crystals is almost 100%, confirming bulk superconductivity. Systematic Se and Sb substitutions in RbBi11/3-ySbySexTe6-x revealed a dependence of the superconducting transition on composition that can increase the Tc up to ∼10%. The RbBi11/3Te6 system is the first member of the new homologous series Rb[Bi2n+11/3Te3n+6] with infinite Bi2Te3-like layers. The large degree of chemical tunability of the electronic structure of the homology via doping and/or substitution gives rise to a new family of superconductors.

Relation between the structural phase transition and superconductivity inCuxIrTe2−ySey

We have investigated the relation between structural transition with Ir-dimer formation and superconductivity in ${\mathrm{IrTe}}_{2}$ by combining the Se substitution and the Cu intercalation. Regardless of the structural transition temperature (${T}_{\mathrm{s}}$), which increases with increasing the Se content ($y$) in ${\mathrm{IrTe}}_{2\ensuremath{-}y}{\mathrm{Se}}_{y}$, superconductivity emerges robustly by the Cu intercalation. As the Cu content $x$ in ${\mathrm{Cu}}_{x}{\mathrm{IrTe}}_{2\ensuremath{-}y}{\mathrm{Se}}_{y}$ increases, ${T}_{\mathrm{s}}$ tends to decrease, followed by the emergence of superconductivity with showing the highest critical temperature (${T}_{\mathrm{c}}$) at the optimum Cu concentration (${x}_{\mathrm{opt}}$) close to the structural phase boundary. Based on the transport and thermodynamic properties, the electron-phonon coupling constant is found to be enhanced near the structural phase boundary, which suggests an essential role of the structural instability for the superconductivity in doped ${\mathrm{IrTe}}_{2}$. With increasing $y$ from 0 to 0.5 in ${\mathrm{Cu}}_{x}{\mathrm{IrTe}}_{2\ensuremath{-}y}{\mathrm{Se}}_{y}, {T}_{\mathrm{s}}$ at $x=0$ increases by about 80%, whereas ${T}_{\mathrm{c}}$ at $x={x}_{\mathrm{opt}}$ decreases by about 20%. This can be understood by the weakening of the interlayer hybridization upon the Se substitution, resulting in the weak but negative correlation between ${T}_{\mathrm{s}}$ and ${T}_{\mathrm{c}}$ through $y$.

Phase relations and superconductivity in the Fe7(Se1−yTey)8 system: Effect of phase coexistence

The effect of the Te for Se substitution in the Fe7Se8 compound having a layered NiAs-type structure has been studied by means of X-ray diffraction, scanning electron microscopy, thermal expansion, electrical resistivity and magnetization measurements. The growth of Te tellurium content in Fe7(Se1−yTey)8 above y ∼ 0.15 is found to result in the phase segregation; besides the NiAs-type phase the superconducting tetragonal PbO-type phase appears in these samples. It has been observed that the crystal structure and composition of the hexagonal Fe7(Se,Te)8 phase (P63/mmc) transforms with increasing Te content to the CdI2-type phase (P3¯m1) and then to the monoclinic Fe3(Se,Te)4-type phase (I12/m1). The concentration intervals in which these NiAs-type phases coexist with the tetragonal phase have been determined. At y ≥ 0.4, the Fe7(Se1−yTey)8 samples show bulk superconductivity with higher superconducting transition temperatures (Tc up to 17.8 K) compared with single-phase samples of Fe(Se,Te). Such an enhancement is suggested to result from the interactions between the superconducting and non-superconducting phases coexisting in Fe7(Se1−yTey)8.

Interactions between temozolomide and guanine and its S and Se-substituted analogues

Temozolomide was paired with guanine, 6-selenoguanine, and 6-thioguanine, as well as the SH tautomer of the latter. The potential energy surface of each heterodimer was searched for all minima, using Dispersion-Corrected Density Functional Theory and MP2 methods. Among the dozens of minima, three categories were observed. Stacked geometries place the aromatic systems of the two molecules parallel to one another, while the two systems are roughly perpendicular to one another in a second category. Also found are coplanar structures held together by H-bonds. Dispersion proves to be a dominating attractive force for the stacked structures, less so for perpendicular, and smallest for the coplanar dimers. Geometries and energetics are relatively insensitive to S and Se substitution, but tautomerization reverses relative stabilities of different geometries.

Extremely Low Thermal Conductivity in Thermoelectric Ge0.55Pb0.45Te Solid Solutions via Se Substitution

In this contribution, we report that an extremely low lattice thermal conductivity can be achieved in a Ge0.55Pb0.45Te-based thermoelectric system through gradually replacing Te by Se. It was revealed that substitution of Se promotes the solid solubility of Pb in GeTe, hence leading to a reduction of PbTe precipitation. The PbTe precipitation was found to completely disappear when 60% of the Te was replaced by Se. In the optimized composition alloy (x = 0.5), the substantial percentage of solute Pb and Se atoms in the GeTe-based phase, together with the twinning microstructures in the GeTe-based matrix phase, enhances phonon scattering sufficiently, leading to extremely low thermal conductivities (0.67 and 0.45 W m–1 K–1 at at 300 and 723 K, respectively), which are the lowest ever reported. The maximum ZT of 1.55 at 723 K was obtained in the alloy Ge0.55Pb0.45Te0.5Se0.5, which is 8 times higher than that of its Se-free counterpart Ge0.55Pb0.45Te.

Thermoelectric Properties of Bi2Te3−y Se y :I m Prepared by Mechanical Alloying and Hot Pressing

Bi2Te3−ySey:Im (y = 0.15–0.6 and m = 0.0025–0.01) solid solutions were prepared by mechanical alloying and hot pressing. The lattice constants that were measured from x-ray diffraction patterns decreased linearly with increasing Se content, but they were not changed remarkably by I doping. The average relative densities of the hot-pressed specimens are higher than 97%. All of the specimens exhibited n-type conductions in the measured temperature range from 323 K to 523 K, and their electrical conductivity decreased slightly with increasing temperature, indicating degenerate semiconductor behaviors. The electrical conductivity decreased with increasing Se content, whereas it was increased by I doping, and this is in contrast with the Seebeck coefficient; this resulted from the changes of the electron concentrations due to the Se substitution and the I doping. The thermal conductivity decreased with increasing Se content, and this is the result of both the decreased electronic thermal conductivity due to the decreased carrier concentration and the decreased lattice thermal conductivity due to the increased alloy scattering. The maximum dimensionless figure of merit for Bi2Te2.4Se0.6, ZTmax = 0.84 at 473 K, is due to its low thermal conductivity and high Seebeck coefficient.

Effect of five-membered ring and heteroatom substitution on charge transport properties of perylene discotic derivatives: A theoretical approach

Density functional theory calculations were carried out to investigate the evolvement of charge transport properties of a set of new discotic systems as a function of ring and heteroatom (B, Si, S, and Se) substitution on the basic structure of perylene. The replacement of six-membered rings by five-membered rings in the reference compound has shown a prominent effect on the electron reorganization energy that decreases ∼0.2 eV from perylene to the new carbon five-membered ring derivative. Heteroatom substitution with boron also revealed to lower the LUMO energy level and increase the electron affinity, therefore lowering the electron injection barrier compared to perylene. Since the rate of the charge transfer between two molecules in columnar discotic systems is strongly dependent on the orientation of the stacked cores, the total energy and transfer integral of a dimer as a disc is rotated with respect to the other along the stacking axis have been predicted. Aimed at obtaining a more realistic approach to the bulk structure, the molecular geometry of clusters made up of five discs was fully optimized, and charge transfer rate and mobilities were estimated for charge transport along a one dimensional pathway. Heteroatom substitution with selenium yields electron transfer integral values ∼0.3 eV with a relative disc orientation of 25°, which is the preferred angle according to the dimer energy profile. All the results indicate that the tetraselenium-substituted derivative, not synthetized so far, could be a promising candidate among those studied in this work for the fabrication of n-type semiconductors based on columnar discotic liquid crystals materials.