arXiv subjects

San-Dong Guo

Publications and source records attributed to San-Dong Guo.

At least 91 records · Page 5Linked to original sources

arXiv2018

Isolated highly localized bands in $\mathrm{YbI_2}$ monolayer caused by $4f$ orbitals

The novel electronic structures can induce unique physical properties in two-dimensional (2D) materials. In this work, we report isolated highly localized bands in $\mathrm{YbI_2}$ monolayer by the first-principle calculations within generalized gradient approximation (GGA) plus spin-orbit coupling (SOC). It is found that $\mathrm{YbI_2}$ monolayer is an indirect-gap semiconductor using both GGA and GGA+SOC. The calculations reveal that Yb-$4f$ orbitals constitute isolated highly localized bands below the Fermi level at the absence of SOC, and the bands are split into the $j = 7/2$ and $j = 5/2$ states with SOC. The isolated highly localized bands can lead to very large Seebeck coefficient and very low electrical conductivity in p-type doping by producing very large effective mass of the carrier. It is proved that isolated highly localized bands have very strong stability again strain, which is very important for practical application. When the onsite Coulomb interaction is added to the Yb-$4f$ orbitals, isolated highly localized bands persist, and only their relative positions in the gap change. These findings open a new window to search for novel electronic structures in 2D materials.

cond-mat.mtrl-sci↗

arXiv2018

Biaxial strain tuned electronic structures and power factor in Janus Transition Metal Dichalchogenide monolayers

Tuning physical properties of transition metal dichalcogenide (TMD) monolayers by strain engineering have most widely studied, and recently Janus TMD monolayer MoSSe has been synthesized. In this work, we systematically study biaxial strain dependence of electronic structures and transport properties of Janus TMD MXY (M = Mo or W, X/Y = S, Se, or Te) monolayer by using generalized gradient approximation (GGA) plus spin-orbit coupling (SOC). It is found that SOC has a noteworthy detrimental influence on power factor in p-type MoSSe, WSSe, n-type WSTe, p-type MoSeTe and WSeTe, and has a negligible influence on one in n-type MoSSe, MoSTe, p-type WSTe and n-type MoSeTe. These can be understood by considering SOC effects on their valence and conduction bands. For all six monolayers, the energy band gap firstly increases, and then decreases, when strain changes from compressive one to tensile one. It is found that strain can tune strength of bands convergence of both valence and conduction bands by changing the numbers and relative position of valence band extrema (VBE) or conduction band extrema (CBE), which can produce very important effects on their electronic transport properties. By applying appropriate compressive or tensile strain, both n- or p-type Seebeck coefficient can be enhanced by strain-induced band convergence, and then the power factor can be improved. Our works further enrich studies on strain dependence of electronic structures and transport properties of new-style TMD monolayers, and motivate farther experimental works.

cond-mat.mtrl-sci↗

arXiv2018

Nonmonotonic strain dependence of lattice thermal conductivity in monolayer SiC: a first-principles study

An increasing number of two-dimensional (2D) materials have already been achieved experimentally or predicted theoretically, which have potential applications in nano- and opto-electronics. Various applications for electronic devices are closely related to their thermal transport properties. In this work, the strain dependence of phonon transport in monolayer SiC with a perfect planar hexagonal honeycomb structure is investigated by solving the linearized phonon Boltzmann equation. It is found that room-temperature lattice thermal conductivity ($κ_L$) of monolayer SiC is two orders of magnitude lower than that of graphene. The low $κ_L$ is due to small group velocities and short phonon lifetimes, which can also be explained by polarized covalent bond due to large charge transfer from Si to C atoms. In considered strain range, it is proved that the SiC monolayer is mechanically and dynamically stable. With increased tensile strain, the $κ_L$ of SiC monolayer shows an unusual nonmonotonic up-and-down behavior, which is due to the competition between the change of phonon group velocities and phonon lifetimes of low frequency phonon modes. At low strains ($<$8\%), the phonon lifetimes enhancement induces the increased $κ_L$, while at high strains ($>$8\%) the reduction of group velocities as well as the decrease of the phonon lifetimes are the major mechanism responsible for decreased $κ_L$. Our works further enrich studies on phonon transports of 2D materials with a perfect planar hexagonal honeycomb structure, and motivate farther experimental studies.

cond-mat.mtrl-sci↗

arXiv2018

Biaxial tensile strain tuned up-and-down behavior on lattice thermal conductivity in $β$-AsP monolayer

Various two-dimensional (2D) materials with graphene-like buckled structure emerge, and the $β$-phase AsP monolayer has been recently proposed to be thermodynamically stable from first-principles calculations. The studies of thermal transport are very useful for these 2D materials-based nano-electronics devices. Motivated by this, a comparative study of strain-dependent phonon transport of AsP monolayer is performed by solving the linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). It is found that the lattice thermal conductivity ($κ_L$) of AsP monolayer is very close to one of As monolayer with similar buckled structure, which is due to neutralization between the reduce of phonon lifetimes and group velocity enhancement from As to AsP monolayer. The corresponding room-temperature sheet thermal conductance of AsP monolayer is 152.5 $\mathrm{W K^{-1}}$. It is noted that the increasing tensile strain can harden long wavelength out-of-plane (ZA) acoustic mode, and soften the in-plane longitudinal acoustic (LA) and transversal acoustic (TA) modes. Calculated results show that $κ_L$ of AsP monolayer presents a nonmonotonic up-and-down behavior with increased strain. The unusual strain dependence is due to the competition among reduce of phonon group velocities, improved phonon lifetimes of ZA mode and nonmonotonic up-and-down phonon lifetimes of TA/LA mode. It is found that acoustic branches dominate the $κ_L$ in considered strain range, and the contribution from ZA branch increases with increased strain, while it is opposite for TA/LA branch. By analyzing cumulative $κ_L$ with respect to phonon mean free path (MFP), tensile strain can modulate effectively size effects on $κ_L$ in AsP monolayer.

cond-mat.mtrl-sci↗

arXiv2018

Phonon transport of Janus monolayer MoSSe: a first-principles study

Transition Metal Dichalcogenide (TMD) monolayers have most widely studied due to their unique physical properties. Recently, Janus TMD Monolayer MoSSe with sandwiched S-Mo-Se structure has been synthesized by replacing the top S atomic layer in $\mathrm{MoS_2}$ with Se atoms. In this work, we systematically investigate the phonon transport and lattice thermal conductivity ($κ_L$) of MoSSe monolayer by first-principles calculations and linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). Calculated results show that the $κ_L$ of MoSSe monolayer is very lower than that of $\mathrm{MoS_2}$ monolayer, and higher than that of $\mathrm{MoSe_2}$ monolayer. The corresponding sheet thermal conductance of MoSSe monolayer is 342.50 $\mathrm{W K^{-1}}$ at room temperature. These can be understood by phonon group velocities and lifetimes. Compared with $\mathrm{MoS_2}$ monolayer, the smaller group velocities and shorter phonon lifetimes of MoSSe monolayer give rise to lower $κ_L$. The larger group velocities for MoSSe than $\mathrm{MoSe_2}$ monolayer is main reason of higher $κ_L$. The elastic properties of $\mathrm{MoS_2}$, MoSSe and $\mathrm{MoSe_2}$ monolayers are also calculated, and the order of Young's modulus is identical with that of $κ_L$. Calculated results show that isotope scattering leads to 5.8\% reduce of $κ_L$. The size effects on the $κ_L$ are also considered, which is usually used in the device implementation. When the characteristic length of MoSSe monolayer is about 110 nm, the $κ_L$ reduces to half. These results may offer perspectives on thermal management of MoSSe monolayer for applications of thermoelectrics, thermal circuits and nanoelectronics, and motivate further theoretical or experimental efforts to investigate thermal transports of Janus TMD monolayers.

cond-mat.mtrl-sci↗

arXiv2017

Predicted Janus monolayer ZrSSe with enhanced n-type thermoelectric properties compared with monolayer $\mathrm{ZrS_2}$

In analogy to transition-metal dichalcogenide (TMD) monolayers, which have wide applications in photoelectricity, piezoelectricity and thermoelectricity, Janus MoSSe monolayer has been successfully synthesized by substituting the top Se atomic layer in $\mathrm{MoSe_2}$ by S atoms. In this work, Janus monolayer ZrSSe is proposed by ab initio calculations. For the electron part, the generalized gradient approximation (GGA) plus spin-orbit coupling (SOC) is used as exchange-correlation potential, while GGA for lattice part. Calculated results show that the ZrSSe monolayer is dynamically and mechanically stable, which exhibits mechanical flexibility due to small Young's modulus. It is found that ZrSSe monolayer is an indirect-gap semiconductors with band gap of 0.60 eV. The electronic and phonon transports of ZrSSe monolayer are investigated by semiclassical Boltzmann transport theory. In n-type doping, the $ZT_e$ between ZrSSe and $\mathrm{ZrS_2}$ monolayers is almost the same due to similar outlines of conduction bands. The p-type $ZT_e$ of ZrSSe monolayer is lower than that of $\mathrm{ZrS_2}$ monolayer, which is due to larger spin-orbit splitting for ZrSSe than $\mathrm{ZrS_2}$ monolayer. The room-temperature sheet thermal conductance is 33.6 $\mathrm{W K^{-1}}$ for ZrSSe monolayer, which is lower than 47.8 $\mathrm{W K^{-1}}$ of $\mathrm{ZrS_2}$ monolayer. Compared to $\mathrm{ZrS_2}$ monolayer, the low sheet thermal conductance of ZrSSe monolayer is mainly due to small group velocities and short phonon lifetimes of ZA mode. Considering their $ZT_e$ and lattice thermal conductivities, the ZrSSe monolayer may have better n-type thermoelectric performance than $\mathrm{ZrS_2}$ monolayer. These results can stimulate further experimental works to synthesize ZrSSe monolayer.

cond-mat.mtrl-sci↗

arXiv2017

Soft phonon modes driven huge difference on lattice thermal conductivity between topological semimetal WC and WN

Topological semimetals are currently attracting increasing interest due to their potential applications in topological qubits and low-power electronics, which are closely related to their thermal transport properties. In this work, by solving the Boltzmann transport equation based on first-principles calculations, we systematically investigate the phonon transport properties of topological semimetal WC and WN. The predicted room-temperature lattice thermal conductivities of WC (WN) along a and c directions are 1140.64 (7.47) $\mathrm{W m^{-1} K^{-1}}$ and 1214.69 (5.39) $\mathrm{W m^{-1} K^{-1}}$. Considering the similar crystal structure of WC and WN, it is quite interesting to find that the thermal conductivity of WC is more than two orders of magnitude higher than that of WN. It is found that, different from WN, the large acoustic-optical (a-o) gap prohibits the acoustic+acoustic$\rightarrow$optical (aao) scattering, which gives rise to very long phonon lifetimes, leading to ultrahigh lattice thermal conductivity in WC. For WN, the lack of a-o gap is due to soft phonon modes in optical branches, which can provide more scattering channels for aao scattering, producing very short phonon lifetimes. Further deep insight can be attained from their different electronic structures. Distinctly different from that in WC, the density of states (DOS) of WN at the Fermi level becomes very sharp, which leads to destabilization of WN, producing soft phonon modes. It is found that the small shear modulus $G$ and $C_{44}$ limit the stability of WN, compared with WC. Our works provide valuable information for phonon transports in WC and WN, and motivate further experimental works to study their lattice thermal conductivities.

cond-mat.mtrl-sci↗

arXiv2017

Ultrahigh lattice thermal conductivity in topological semimetal TaN caused by large acoustic-optical gap

Topological semimetal may have potential applications like topological qubits, spintronics and quantum computations. Efficient heat dissipation is a key factor for the reliability and stability of topological semimetal-based nano-electronics devices, which is closely related to high thermal conductivity. In this work, the elastic properties and lattice thermal conductivity of TaN are investigated by first-principles calculations and the linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). According to the calculated bulk modulus, shear modulus and $C_{44}$, TaN can be regarded as a potential incompressible and hard material. The room-temperature lattice thermal conductivity is predicted to be 838.62 $\mathrm{W m^{-1} K^{-1}}$ along a axis and 1080.40 $\mathrm{W m^{-1} K^{-1}}$ along c axis, showing very strong anisotropy. It is found that the lattice thermal conductivity of TaN is several tens of times higher than one of other topological semimetal, such as TaAs, MoP and ZrTe, which is due to very longer phonon lifetimes for TaN than other topological semimetal. The very different atomic masses of Ta and N atoms lead to a very large acoustic-optical band gap, and then prohibits the scattering between acoustic and optical phonon modes, which gives rise to very long phonon lifetimes. Based on mass difference factor, the WC and WN can be regarded as potential candidates with ultrahigh lattice thermal conductivity. Calculated results show that isotope scattering has little effect on lattice thermal conductivity, and that phonon with mean free path(MFP) larger than 20 (80) $\mathrm{μm}$ at 300 K has little contribution to the total lattice thermal conductivity. This work implies that TaN-based nano-electronics devices may be more stable and reliable due to efficient heat dissipation.

cond-mat.mtrl-sci↗

arXiv2017

Elastic and transport properties of topological semimetal ZrTe

Topological semimetal may have substantial applications in electronics, spintronics and quantum computation. Recently, ZrTe is predicted as a new type of topological semimetal due to coexistence of Weyl fermion and massless triply degenerate nodal points. In this work, the elastic and transport properties of ZrTe are investigated by combining the first-principles calculations and semiclassical Boltzmann transport theory. Calculated elastic constants prove mechanical stability of ZrTe, and the bulk modulus, shear modulus, Young's modulus and Possion's ratio also are calculated. It is found that spin-orbit coupling (SOC) has slightly enhanced effects on Seebeck coefficient, which along a(b) and c directions for pristine ZrTe at 300 K is 46.26 $μ$V/K and 80.20 $μ$V/K, respectively. By comparing the experimental electrical conductivity of ZrTe (300 K) with calculated value, the scattering time is determined for 1.59 $\times$ $10^{-14}$ s. The predicted room-temperature electronic thermal conductivity along a(b) and c directions is 2.37 $\mathrm{W m^{-1} K^{-1}}$ and 2.90 $\mathrm{W m^{-1} K^{-1}}$, respectively. The room-temperature lattice thermal conductivity is predicted as 17.56 $\mathrm{W m^{-1} K^{-1}}$ and 43.08 $\mathrm{W m^{-1} K^{-1}}$ along a(b) and c directions, showing very strong anisotropy. Calculated results show that isotope scattering produces observable effect on lattice thermal conductivity. It is noted that average room-temperature lattice thermal conductivity of ZrTe is slightly higher than that of isostructural MoP, which is due to larger phonon lifetimes and smaller Gr$\mathrm{\ddot{u}}$neisen parameters. Finally, the total thermal conductivity as a function of temperature is predicted for pristine ZrTe.

cond-mat.mtrl-sci↗

arXiv2017

Lower lattice thermal conductivity in SbAs than As or Sb monolayer: a first-principles study

Phonon transports of group-VA elements (As, Sb, Bi) monolayer semiconductors have been widely investigated in theory, and Sb monolayer (antimonene) of them has recently been synthesized. In this work, phonon transport of SbAs monolayer is investigated from a combination of first-principles calculations and the linearized phonon Boltzmann equation. It is found that the lattice thermal conductivity of SbAs monolayer is lower than ones of both As and Sb monolayers, and the corresponding sheet thermal conductance is 28.8 $\mathrm{W K^{-1}}$ at room temperature. Calculated results show that group velocities of SbAs monolayer are between ones of As and Sb onolayers, but phonon lifetimes of SbAs are smaller than ones of both As and Sb monolayers. Hence, low lattice thermal conductivity in SbAs monolayer is attributed to very small phonon lifetimes. Unexpectedly, the ZA branch has very little contribution to the total thermal conductivity, only 2.4\%, which is obviously different from ones of As and Sb monolayers with very large contribution. This can be explained by very small phonon lifetimes for ZA branch of SbAs monolayer. The large charge transfer from Sb to As atoms leads strongly polarized covalent bond, being different from As or Sb monolayer. The strongly polarized covalent bond of SbAs monolayer can induce stronger phonon anharmonicity than As or Sb monolayer, leading to lower lattice thermal conductivity. It is found that isotope scattering produces neglectful effect, and the lattice thermal conductivity with the characteristic length smaller than 30 nm can reach a decrease of about 47\%. These results may offer perspectives on tuning lattice thermal conductivity by mixture of multi-elements for applications of thermal management and thermoelectricity, and motivate further experimental efforts to synthesize monolayer SbAs.

cond-mat.mtrl-sci↗

arXiv2017

Phonon transport of three-fold degeneracy topological semimetal MoP

Recently, three-component new fermions in topological semimetal MoP are experimentally observed, which may have potential applications like topological qubits, low-power electronics and spintronics. These are closely related to thermal transport properties of MoP. In this work, the phonon transport of MoP is investigated by solving the linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). The calculated room-temperature lattice thermal conductivity is 18.41 $\mathrm{W m^{-1} K^{-1}}$ and 34.71 $\mathrm{W m^{-1} K^{-1}}$ along the in- and cross-plane directions, exhibiting very strong anisotropy. The isotope and size effects on the lattice thermal conductivity are also considered. It is found that isotope scattering produces little effect, and phonon has little contribution to the lattice thermal conductivity, when phonon mean free path(MFP) is larger than 0.15 $\mathrm{μm}$ at 300 K. It is noted that average room-temperature lattice thermal conductivity of MoP is lower than that of representative Weyl semimetal TaAs, which is due to smaller group velocities and larger Gr$\mathrm{\ddot{u}}$neisen parameters. Our works provide valuable informations for the thermal management of MoP-based nano-electronics devices, and motivate further experimental works to study thermal transport of MoP.

cond-mat.mtrl-sci↗

arXiv2017

Monolayer enhanced thermoelectric properties compared with bulk for BiTeBr

It is believed that nanostructuring is an effective way to achieve excellent thermoelectric performance. In the work, by combining the first-principles calculations and semiclassical Boltzmann transport theory, we investigate the thermoelectric properties of bulk and monolayer BiTeBr including both the electron and phonon transports. The generalized gradient approximation (GGA) plus spin-orbit coupling (SOC) is employed for the electron part, and GGA for the phonon part. It is found that SOC has important effects on electronic transport coefficients because of SOC-induced obvious influences on energy band structures. In p-type doping, monolayer has larger Seebeck coefficient than bulk in wide doping range, which is beneficial to excellent thermoelectric performance. The calculated average lattice thermal conductivity of bulk is 1.71 $\mathrm{W m^{-1} K^{-1}}$ at room temperature, which is close to experimental value 1.3 $\mathrm{W m^{-1} K^{-1}}$. Calculated results show that monolayer has better $ZT_e$ and lower lattice thermal conductivity than bulk, which suggests that monolayer has better thermoelectric performance than bulk. The lower lattice thermal conductivity in monolayer than bulk is due to shorter phonon lifetimes. By comparing the experimental electrical conductivity of bulk with calculated value, the scattering time is determined for 3.3 $\times$ $10^{-14}$ s. Based on electron and phonon transport coefficients, the thermoelectric figure of merit $ZT$ of bulk and monolayer are calculated. It is found that monolayer has higher peak $ZT$ than bulk, and the peak $ZT$ of monolayer can be as high as 0.55 in n-type doping and 0.75 in p-type doping at room temperature. These results imply that monolayer BiTeBr may be a potential two-dimensional (2D) thermoelectric material, which can stimulate further experimental works to synthesize monolayer BiTeBr.

cond-mat.mtrl-sci↗

arXiv2017

Strain effects on phonon transport in antimonene from a first-principles study

Strain engineering is a very effective method to continuously tune the electronic, topological, optical and thermoelectric properties of materials. In this work, strain-dependent phonon transport of recently-fabricated antimonene (Sb monolayer) under biaxial strain is investigated from a combination of first-principles calculations and the linearized phonon Boltzmann equation. It is found that the ZA dispersion of antimonene with strain less than -1\% gives imaginary frequencies, which suggests that compressive strain can induce structural instability. Experimentally, it is possible to enhance structural stability by tensile strain. Calculated results show that lattice thermal conductivity increases with strain changing from -1\% to 6\%, and lattice thermal conductivity at 6\% strain is 5.6 times larger than that at -1\% strain at room temperature. It is interesting that lattice thermal conductivity is in inverse proportion to buckling parameter $h$ in considered strain range. Such a strain dependence of lattice thermal conductivity is attributed to enhanced phonon lifetimes caused by increased strain, while group velocities have a decreased effect on lattice thermal conductivity with increasing strain. It is found that acoustic branches dominate the lattice thermal conductivity over the full strain range. The cumulative room-temperature lattice thermal conductivity at -1\% strain converges to maximum with phonon mean free path (MFP) at 50 nm, while one at 6\% strain becomes as large as 44 $\mathrm{μm}$, which suggests that strain can give rise to very strong size effects on lattice thermal conductivity in antimonene. These results may provide guidance on fabrication techniques of antimonene, and offer perspectives on tuning lattice thermal conductivity by size and strain for applications of thermal management and thermoelectricity.

cond-mat.mtrl-sci↗

arXiv2017

Potential 2D thermoelectric materials ATeI (A=Sb and Bi) monolayers from a first-principles study

Lots of two-dimensional (2D) materials have been predicted theoretically, and further confirmed in experiment, which have wide applications in nanoscale electronic, optoelectronic and thermoelectric devices. Here, the thermoelectric properties of ATeI (A=Sb and Bi) monolayers are systematically investigated, based on semiclassical Boltzmann transport theory. It is found that spin-orbit coupling (SOC) has important effects on electronic transport coefficients in p-type doping, but neglectful influences on n-type ones. The room-temperature sheet thermal conductance is 14.2 $\mathrm{W K^{-1}}$ for SbTeI and 12.6 $\mathrm{W K^{-1}}$ for BiTeI, which are lower than one of most well-known 2D materials, such as transition-metal dichalcogenide, group IV-VI, group-VA and group-IV monolayers. By analyzing group velocities and phonon lifetimes, the very low sheet thermal conductance of ATeI (A=Sb and Bi) monolayers is mainly due to small group velocities. It is found that the high-frequency optical branches contribute significantly to the total thermal conductivity, being obviously different from usual picture with little contribution from optical branches. According to cumulative lattice thermal conductivity with respect to phonon mean free path (MFP), it is difficulty to further reduce lattice thermal conductivity by nanostructures. Finally, possible thermoelectric figure of merit $ZT$ of ATeI (A=Sb and Bi) monolayers are calculated. It is found that the p-type doping has more excellent thermoelectric properties than n-type doping, and at room temperature, the peak $ZT$ can reach 1.11 for SbTeI and 0.87 for BiTeI, respectively. These results make us believe that ATeI (A=Sb and Bi) monolayers may be potential 2D thermoelectric materials, and can stimulate further experimental works to synthesize these monolayers.

cond-mat.mtrl-sci↗

arXiv2017

Phonon transport in $\mathrm{Na_2He}$ at high pressure from a first-principles study

Phonon transport of recently-fabricated $\mathrm{Na_2He}$ at high pressure is investigated from a combination of first-principles calculations and the linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). The calculated room-temperature lattice thermal conductivity is 149.19 $\mathrm{W m^{-1} K^{-1}}$, which is very close to one of Si. It is found that low-frequency optical modes comprise 16\% of the lattice thermal conductivity, while high-frequency optical modes have negligible contribution. The high lattice thermal conductivity is due to large group velocities, small Gr$\mathrm{\ddot{u}}$neisen parameters, and long phonon lifetimes. The size effects on lattice thermal conductivity are considered by cumulative thermal conductivity with respect to phonon mean free path(MFP). To significantly reduce the lattice thermal conductivity, the characteristic length smaller than 100 nm is required, and can reach a decrease of 36\%. These results may be useful to understand thermal transport processes that occur inside giant planets.

cond-mat.mtrl-sci↗

arXiv2017

Thermoelectric properties of $β$-As, Sb and Bi monolayers

Monolayer semiconductors of group-VA elements (As, Sb, Bi) with graphenelike buckled structure offer a potential to achieve nanoscale electronic, optoelectronic and thermoelectric devices. Motivated by recently-fabricated Sb monolayer, we systematically investigate the thermoelectric properties of $β$-As, Sb and Bi monolayers by combining the first-principles calculations and semiclassical Boltzmann transport theory. The generalized gradient approximation (GGA) plus spin-orbit coupling (SOC) is adopted for the electron part, and GGA is employed for the phonon part. It is found that SOC has important influences on their electronic structures, especially for Bi monolayer, which can induce observable SOC effects on electronic transport coefficients. More specifically, SOC not only has detrimental influences on electronic transport coefficients, but also produces enhanced effects. The calculated lattice thermal conductivity decreases gradually from As to Bi monolayer, and the corresponding room-temperature sheet thermal conductance is 161.10 $\mathrm{W K^{-1}}$, 46.62 $\mathrm{W K^{-1}}$ and 16.02 $\mathrm{W K^{-1}}$, which can be converted into common lattice thermal conductivity by dividing by the thickness of 2D material. The sheet thermal conductance of Bi monolayer is lower than one of other 2D materials, such as semiconducting transition-metal dichalcogenide monolayers and orthorhombic group IV-VI monolayers. A series of scattering time is employed to estimate the thermoelectric figure of merit $ZT$. It is found that the n-type doping has more excellent thermoelectric properties than p-type doping for As and Bi monolayer, while the comparative $ZT$ between n- and p-type doping is observed in Bi monolayer. These results can stimulate further experimental works to open the new field for thermoelectric devices based on monolayer of group-VA elements.

cond-mat.mtrl-sci↗

arXiv2016

Small compressive strain induced semiconductor-metal transition and tensile strain enhanced thermoelectric properties in monolayer $\mathrm{PtTe_2}$

Biaxial strain effects on electronic structures and thermoelectric properties of monolayer $\mathrm{PtTe_2}$ are investigated by using generalized gradient approximation (GGA) plus spin-orbit coupling (SOC) for the electron part and GGA for the phonon part. Calculated results show that small compressive strain (about -3\%) can induce semiconductor-to-metal transition, which can easily be achieved in experiment. The conduction bands convergence is observed for unstrained $\mathrm{PtTe_2}$, which can be removed by both compressive and tensile strains. Tensile strain can give rise to valence bands convergence by changing the position of valence band maximum (VBM), which can induce enhanced Seebeck coefficient, being favorable for high power factor. It is found that tensile strain can also reduce lattice thermal conductivity, which at the strain of 4\% can decrease by about 19\% compared to unstrained one at room temperature. By considering tensile strain effects on $ZT_e$ and lattice thermal conductivity, tensile strain indeed can improve p-type efficiency of thermoelectric conversion. Our results demonstrate the potential of strain engineering in $\mathrm{PtTe_2}$ for applications in electronics and thermoelectricity.

cond-mat.mtrl-sci↗

arXiv2016

Thermoelectric properties of topological insulator $\mathrm{BaSn_2}$

Recently, $\mathrm{BaSn_2}$ is predicted to be a strong topological insulator by the first-principle calculations. It is well known that topological insulator has a close connection to thermoelectric material, such as $\mathrm{Bi_2Te_3}$ family. In this work, we investigate thermoelectric properties of $\mathrm{BaSn_2}$ by the first-principles combined with Boltzmann transport theory. The electronic part is carried out by a modified Becke and Johnson (mBJ) exchange potential, including spin-orbit coupling (SOC), while the phonon part is performed using generalized gradient approximation (GGA). It is found that the electronic transport coefficients between the in-plane and cross-plane directions show the strong anisotropy, while lattice lattice thermal conductivities show an almost isotropy. Calculated results show a very low lattice thermal conductivity for $\mathrm{BaSn_2}$, and the corresponding average lattice thermal conductivity at room temperature is 1.69 $\mathrm{W m^{-1} K^{-1}}$, which is comparable or lower than those of lead chalcogenides and bismuth-tellurium systems as classic thermoelectric materials. Due to the complicated scattering mechanism, calculating scattering time $τ$ is challenging. By using a empirical $τ$=$10^{-14}$ s, the n-type figure of merit $ZT$ is greater than 0.40 in wide temperature range. Experimentally, it is possible to attain better thermoelectric performance, or to enhance one by strain or tuning size parameter. This work indicates that $\mathrm{BaSn_2}$ may be a potential thermoelectric material, which can stimulate further theoretical and experimental works.

cond-mat.mtrl-sci↗