arXiv subjects

San-Dong Guo

Publications and source records attributed to San-Dong Guo.

104 records · Page 6Linked to original sources

arXiv2016

Thermoelectric properties of orthorhombic group IV-VI monolayers from the first-principles calculations

Two-dimensional (2D) materials may have potential applications in thermoelectric devices. In this work, we systematically investigate the thermoelectric properties of orthorhombic group IV-VI monolayers $\mathrm{AB}$ (A=Ge and Sn; B=S and Se) by the first-principles calculations and semiclassical Boltzmann transport theory. The spin-orbit coupling (SOC) is included to investigate their electronic transport, which produces observable effects on power factor, especially for n-type doping. According to calculated $ZT$, the four monolayers exhibit diverse anisotropic thermoelectric properties, although they have similar hinge-like crystal structure. The GeS along zigzag and armchair directions shows the strongest anisotropy, while SnS and SnSe show mostly isotropic efficiency of thermoelectric conversion, which can be understood by the strength of anisotropy of their respective power factor, electronic and lattice thermal conductivities. Calculated results show that $ZT$ for different carriers of n- and p-type has little difference for GeS, SnS and SnSe. It is found that GeSe, SnS and SnSe show better thermoelectric performance compared to GeS in n-type doping, and SnS and SnSe exhibit higher efficiency of thermoelectric conversion in p-type doping. Compared to a lot of 2D materials, orthorhombic group IV-VI monolayers $\mathrm{AB}$ (A=Ge and Sn; B=S and Se) may possess better thermoelectric performance due to higher power factor and lower thermal conductivity. Our work would be beneficial to further experimental study.

cond-mat.mtrl-sci↗

arXiv2016

Biaxial strain tuned thermoelectric properties in monolayer $\mathrm{PtSe_2}$

Strain engineering is a very effective method to tune electronic, optical, topological and thermoelectric properties of materials. In this work, we systematically study biaxial strain dependence of electronic structures and thermoelectric properties (both electron and phonon parts) of monolayer $\mathrm{PtSe_2}$ with generalized gradient approximation (GGA) plus spin-orbit coupling (SOC) for electron part and GGA for phonon part. Calculated results show that compressive or tensile strain can induce conduction band minimum (CBM) or valence band maximum (VBM) transition, which produces important effects on Seebeck coefficient. It is found that compressive or tensile strain can induce significantly enhanced n- or p-type Seebeck coefficient at the critical strain of CBM or VBM transition, which can be explained by strain-induced band convergence. Another essential strain effect is that tensile strain can produce significantly reduced lattice thermal conductivity, and the room temperature lattice thermal conductivity at the strain of -4.02\% can decrease by about 60\% compared to unstrained one, which is very favorable for high $ZT$. To estimate efficiency of thermoelectric conversion, the figure of merit $ZT$ can be obtained by empirical scattering time $τ$. Calculated $ZT$ values show that strain indeed is a very effective strategy to achieve enhanced thermoelectric properties, especially for p-type doping. Tuning thermoelectric properties with strain also can be applied to other semiconducting transition-metal dichalcogenide monolayers $\mathrm{MX_2}$ (M=Zr, Hf, Mo, W and Pt; X=S, Se and Te).

cond-mat.mtrl-sci↗

arXiv2016

Potential thermoelectric material $\mathrm{Cs_2[PdCl_4]I_2}$: a first-principles study

The electronic structures and thermoelectric properties of $\mathrm{Cs_2[PdCl_4]I_2}$ are investigated by the first-principles calculations and semiclassical Boltzmann transport theory. Both electron and phonon transport are considered to attain the figure of merit $ZT$. A modified Becke and Johnson (mBJ) exchange potential, including spin-orbit coupling (SOC), is employed to investigate electronic part of $\mathrm{Cs_2[PdCl_4]I_2}$. It is found that SOC has obvious effect on valence bands, producing huge spin-orbital splitting, which leads to remarkable detrimental effect on p-type power factor. However, SOC has a negligible influence on conduction bands, so the n-type power factor hardly change. The temperature dependence of lattice thermal conductivity by assuming an inverse temperature dependence is attained from reported ultralow lattice thermal conductivity of 0.31 $\mathrm{W m^{-1} K^{-1}}$ at room temperature. Calculating scattering time $τ$ is challenging, but a hypothetical $τ$ can be adopted to estimate thermoelectric conversion efficiency. The maximal figure of merit $ZT$ is up to about 0.70 and 0.60 with scattering time $τ$=$10^{-14}$ s and $τ$=$10^{-15}$ s, respectively. These results make us believe that $\mathrm{Cs_2[PdCl_4]I_2}$ may be a potential thermoelectric material.

cond-mat.mtrl-sci↗

arXiv2016

Potential thermoelectric materials $\mathrm{CsMI_3}$ (M=Sn and Pb) in perovskite structures from the first-principles calculations

The thermoelectric properties of halide perovskites $\mathrm{CsMI_3}$ (M=Sn and Pb) are investigated from a combination of first-principles calculations and semiclassical Boltzmann transport theory by considering both the electron and phonon transport. The electronic part is performed using a modified Becke and Johnson (mBJ) exchange potential, including spin-orbit coupling (SOC), while the phonon part is computed using generalized gradient approximation (GGA). It is found that SOC has remarkable detrimental effect on n-type power factor, while has a negligible influence in p-type doping, which can be explained by considering SOC effect on conduction and valence bands. Calculated results show exceptionally low lattice thermal conductivities in $\mathrm{CsSnI_3}$ and $\mathrm{CsPbI_3}$, and the corresponding room-temperature lattice thermal conductivity is 0.54 $\mathrm{W m^{-1} K^{-1}}$ and 0.25 $\mathrm{W m^{-1} K^{-1}}$. At 1000 K, the maximal figure of merit $ZT$ is up to 0.63 and 0.64 for $\mathrm{CsSnI_3}$ and $\mathrm{CsPbI_3}$ with scattering time $τ$=$10^{-14}$ s, and the peak $ZT$ is 0.49 and 0.41 with $τ$=$10^{-15}$ s. These results make us believe that $\mathrm{CsMI_3}$ (M=Sn and Pb) in perovskite structures may be potential thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2016

Spin-orbital coupling effect on power factor in semiconducting transition-metal dichalcogenide monolayers

The electronic structures and thermoelectric properties of semiconducting transition-metal dichalcogenide monolayers $\mathrm{MX_2}$ (M=Zr, Hf, Mo, W and Pt; X=S, Se and Te) are investigated by combining first-principles and Boltzmann transport theory, including spin-orbital coupling (SOC). It is found that the gap decrease increases from S to Te in each cation group, when the SOC is opened. The spin-orbital splitting has the same trend with gap reducing. Calculated results show that SOC has noteworthy detrimental effect on p-type power factor, while has a negligible influence in n-type doping except W cation group, which can be understood by considering the effects of SOC on the valence and conduction bands. For $\mathrm{WX_2}$ (X=S, Se and Te), the SOC leads to observably enhanced power factor in n-type doping, which can be explained by SOC-induced band degeneracy, namely bands converge. Among all cation groups, Pt cation group shows the highest Seebeck coefficient, which leads to best power factor, if we assume scattering time is fixed. Calculated results show that $\mathrm{MS_2}$ (M=Zr, Hf, Mo, W and Pt) have best p-type power factor for all cation groups, and that $\mathrm{MSe_2}$ (M=Zr and Hf), $\mathrm{WS_2}$ and $\mathrm{MTe_2}$ (M=Mo and Pt) have more wonderful n-type power factor in respective cation group. Therefore, these results may be useful for further theoretical prediction or experimental search of excellent thermoelectric materials from semiconducting transition-metal dichalcogenide monolayers.

cond-mat.mtrl-sci↗

arXiv2016

Thermoelectric properties of half-Heusler $\mathrm{ZrNiPb}$ by using first principles calculations

We investigate electronic structures and thermoelectric properties of recent synthetic half-Heusler $\mathrm{ZrNiPb}$ by using generalized gradient approximation (GGA) and GGA plus spin-orbit coupling (GGA+SOC). Calculated results show that $\mathrm{ZrNiPb}$ is a indirect-gap semiconductor. Within the constant scattering time approximation, semi-classic transport coefficients are performed through solving Boltzmann transport equations. It is found that the SOC has more obvious influence on power factor in p-type doping than in n-type doping, leading to a detrimental effect in p-type doping. These can be explained by considering the SOC influences on the valence bands and conduction bands near the Fermi level. The lattice thermal conductivity as a function of temperature is calculated, and the corresponding lattice thermal conductivity is 14.5 $\mathrm{W m^{-1} K^{-1}}$ at room temperature. By comparing the experimental transport coefficients with calculated ones, the scattering time is attained for 0.333 $\times$ $10^{-14}$ s. Finally, the thermoelectric figure of merit $ZT$ can be attained, and the $ZT$ value can be as high as 0.30 at high temperature by choosing appropriate doping level. It is possible to reduce lattice thermal conductivity by point defects and boundaries, and make half-Heusler $\mathrm{ZrNiPb}$ become potential candidate for efficient thermoelectricity.

cond-mat.mtrl-sci↗

arXiv2016

Strain effect on power factor in monolayer $\mathrm{MoS_2}$

Biaxial strain dependence of electronic structures and thermoelectric properties of monolayer $\mathrm{MoS_2}$, including compressive and tensile strain, are investigated by using local-density approximation (LDA) plus spin-orbit coupling (SOC). Both LDA and LDA+SOC results show that $\mathrm{MoS_2}$ is a direct gap semiconductor with optimized lattice constants. It is found that SOC has important effect on power factor, which can enhance one in n-type doping, but has a obvious detrimental influence for p-type. Both compressive and tensile strain can induce direct-indirect gap transition, which produce remarkable influence on power factor. Calculated results show that strain can induce significantly enhanced power factor in n-type doping by compressive strain and in p-type doping by tensile strain at the critical strain of direct-indirect gap transition. These can be explained by strain-induced accidental degeneracies, which leads to improved Seebeck coefficient. Calculated results show that n-type doping can provide better power factor than p-type doping. These results make us believe that thermoelectric properties of monolayer $\mathrm{MoS_2}$ can be improved in n-type doping by compressive strain.

cond-mat.mtrl-sci↗

arXiv2016

Pressure enhanced thermoelectric properties in Mg2Sn

Pressure dependence of electronic structures and thermoelectric properties of $\mathrm{Mg_2Sn}$ are investigated by using a modified Becke and Johnson (mBJ) exchange potential, including spin-orbit coupling (SOC). The corresponding value of spin-orbit splitting at $Γ$ point is 0.47 eV, which is in good agreement with the experimental value 0.48 eV. With the pressure increasing, the energy band gap first increases, and then decreases. In certain doping range, the power factor for n-type has the same trend with energy band gap, when the pressure increases. Calculated results show that the pressure can lead to significantly enhanced power factor in n-type doping below the critical pressure, and the corresponding lattice thermal conductivity near the critical pressure shows the relatively small value. These results make us believe that thermoelectric properties of $\mathrm{Mg_2Sn}$ can be improved in n-type doping by pressure.

cond-mat.mtrl-sci↗

arXiv2015

Importance of spin-orbit coupling in power factor calculations for half-Heusler ANiB (A=Ti, Hf, Sc, Y; B=Sn, Sb, Bi)

We investigate the spin-orbit coupling (SOC) effects on the electronic structures and semi-classic transport coefficients of half-Heusler $\mathrm{ANiB}$ (A=Ti, Hf, Sc, Y; B=Sn, Sb, Bi) by using generalized gradient approximation (GGA). Calculated results show that SOC splits the valence bands at high symmetry $Γ$ point, and modifies the outline of $Γ$-centered valence bands, which has remarkable effects on the electron transport properties. Thermoelectric properties are performed through solving Boltzmann transport equations within the constant scattering time approximation. It is found that the compounds containing Sn atom have larger power factor in p-type doping than ones in n-type doping, and it is just the opposite for compounds containing Sb and Bi elements. The SOC has obvious detrimental influence on power factor in p-type doping, while has a negligible effect in n-type doping. These can be understood by considering the effects of SOC on the valence bands and conduction bands. The maximum power factors (MPF) are extracted in n-type and p-type doping with GGA and GGA+SOC, and the MPF at 300 K with SOC is predicted to be about 4.25\%$\sim$44.13\% smaller than that without SOC in the case of p-type doping for $\mathrm{ANiB}$ (A=Ti, Hf, Sc, Y; B=Sn, Sb, Bi). Therefore, it is crucial to consider SOC effects for theoretical analysis in the case of p-type doping in half-Heusler compounds composed of heavy elements.

cond-mat.mtrl-sci↗

arXiv2014

Re-t2g-splitting-driven semiconductor gaps in ferrimagnetic double perovskite Ca2MReO6 (M=Cr,Fe) from first principles

Motivated by the observation of nonmetallic nature in double perovskite Ca2CrReO6 and Ca2FeReO6 with high magnetic Curie temperatures of 360 and 522 K, we systematically investigate the structural, electronic, and magnetic properties of Ca2MReO6 (M=Cr,Fe) using the full-potential linear augmented plane wave (FP-LAPW) method within the density functional theory. Our full optimization confirms the stable ground-state structure with $P2_1/n$ symmetry. The modified Becke-Johnson (mBJ) exchange potential is used for investigating electronic structures. Our mBJ calculation shows that they are both ferrimagnetic semiconductors with semiconductor gaps of 0.38 eV and 0.05 eV, respectively, in contrast with wrong metallic phases from the generalized gradient approximation (GGA). The origin of semiconductor gap is due to the further distortion of ReO$_6$ octahedra caused by John-Teller effect, which drives the three partially-occupied Re $t_{2g}$ bands split into two fully-filled bands and one empty band in the minority-spin channel. With the spin-orbit coupling (SOC) taken into account, the Ca2MReO6 (M=Cr,Fe) shows high magneto-crystalline anisotropy (MCA) with the magnetic easy axis along pseudocubic [010] direction, and the total magnetic moments increase by 0.209$μ_B$ and 0.258$μ_B$ per formula unit, respectively, due to the strong SOC effect on Re ion. Although reducing to 0.31 and 0.03 eV, the semiconductor gaps remain open in spite of the SOC broadening of the Re $t_{2g}$-related bands. Therefore, our DFT investigation with mBJ has established the correct ferrimagnetic semiconductor ground state for the double perovskites Ca2MReO6 (M=Cr,Fe). This mechanism, different from that in double perovskite Sr2CrOsO6, can help understand physical properties of other similar compounds.

cond-mat.mtrl-sci↗

arXiv2010

Improved half-metallic ferromagnetism of transition-metal pnictides and chalcogenides calculated with a modified Becke-Johnson exchange potential

We use a density-functional-theory (DFT) approach with a modified Becke-Johnson exchange plus local density approximation (LDA) correlation potential (mBJLDA) [semi-local, orbital-independent, producing accurate semiconductor gaps. see F. Tran and P. Blaha, Phys. Rev. Lett. 102, 226401 (2009)] to investigate the electronic structures of zincblende transition-metal (TM) pnictides and chalcogenides akin to semiconductors. Our results show that this potential does not yield visible changes in wide TM d-t_{2g} bands near the Fermi level, but makes the occupied minority-spin p-bands lower by 0.25~0.35 eV and the empty (or nearly empty) minority-spin e_g bands across the Fermi level higher by 0.33~0.73 eV. Consequently, mBJLDA, having no atom-dependent parameters, makes zincblende MnAs become a truly half-metallic (HM) ferromagnet with a HM gap (the key parameter) 0.318eV, being consistent with experiment. For zincblende MnSb, CrAs, CrSb, CrSe, or CrTe, the HM gap is enhanced by 19~56% compared to LDA and generalized gradient approximation results. The improved HM ferromagnetism can be understood in terms of the mBJLDA-enhanced spin exchange splitting.

cond-mat.mtrl-sci↗

arXiv2010

Robust half-metallic ferromagnet of cubic VGe3Si4

To seek half-metallic ferromagnets compatible with the GeSi semiconductor, we use state-of-the-art density-functional-theory methods to study cubic $\mathrm{VGe_3Si_4}$. After optimizing its crystal structure with various magnetic orders considered, we find the ferromagnetic structure is the ground state phase and investigate its stability, and then study its spin-resolved electronic structure with an improved method. Our calculated results show that the cubic $\mathrm{VGe_3Si_4}$ is a half-metallic ferromagnet with half-metallic gap 350 meV, and it is structurally stable against deformations and magnetically robust against antiferromagnetic fluctuations. These results make us believe that the cubic $\mathrm{VGe_3Si_4}$ could be synthesized as good thin films soon and used in practical spintronic devices.

cond-mat.mtrl-sci↗

arXiv2009

Stable half-metallic ferromagnetism in nonstoichiometric cubic binary chromium chalcogenides

We find that three nonstoichiometric cubic binary chromium chalcogenides, namely Cr3S4, Cr3Se4, and Cr3Te4, are stable half-metallic ferromagnets with wide half-metallic gaps on the basis of systematic state-of-the-arts first-principles calculations. We optimize their structures, and then calculate their magnetic moments, electronic structures, formation heats, and elastic moduli and investigate their structural stability and robustness of ferromagnetism against antiferromagnetic fluctuations. Our calculated results show that the three sulvanite phases are structurally stable and ferromagnetically robust, and hence could be realized as epitaxial thin films. We attribute the structural and ferromagnetic stability and the better half-metallicity to their special effective Cr valence 2.667+. These findings will open doors for much more high-performance spintronic materials compatible with current semiconductor technology.

cond-mat.mtrl-sci↗

arXiv2009

Distorted magnetic orders and electronic structures of tetragonal FeSe from first-principles

We use the state-of-the-arts density-functional-theory method to study various magnetic orders and their effects on the electronic structures of the FeSe. Our calculated results show that, for the spins of the single Fe layer, the striped antiferromagnetic orders with distortion are more favorable in total energy than the checkerboard antiferromagnetic orders with tetragonal symmetry, which is consistent with known experimental data, and the inter-layer magnetic interaction is very weak. We investigate the electronic structures and magnetic property of the distorted phases. We also present our calculated spin coupling constants and discuss the reduction of the Fe magnetic moment by quantum many-body effects. These results are useful to understand the structural, magnetic, and electronic properties of FeSe, and may have some helpful implications to other FeAs-based materials.

cond-mat.mtrl-sci↗