Hezhu Shao

The electronic, optical, and thermodynamic properties of borophene from first-principles calculations

Borophene (a two-dimensional boron sheet) is a new type of two-dimensional material, which was recently grown successfully on single crystal Ag substrates. In this paper, we investigate the electronic structure and bonding characteristics of borophene by first-principles calculations. The band structure of borophene shows highly anisotropic metallic behaviour. The obtained optical properties of borophene exhibit strong anisotropy as well. Finally, the thermodynamic properties are investigated based on the phonon properties.

Enhanced thermoelectric performance in p-type polycrystalline SnSe benefiting from texture modulation

Tin selenide (SnSe) compound has attracted much attention due to its unprecedented high ZT (∼2.6) in single crystals. The polycrystalline SnSe materials were then prepared to improve the mechanical performance for large-scaled application. However, the ZT values of 0.3–0.8 were much lower due to their poor electrical properties. In the present study, the zone melting method is employed to prepare the polycrystalline SnSe samples, which show highly textured structures and strong anisotropic thermoelectric performance. A maximum power factor (S2σ) of 9.8 μW cm−1 K−2 was obtained in the polycrystalline samples, which is comparable with that of SnSe single crystals, resulting in a peak ZT of 0.92 at 873 K. The zone-melted ingot was then pulverized into powders and the bulk material was prepared by the spark plasma sintering (SPS) technique. As a result, the ZT value was enhanced to be over 1.0, owing to the slight reduction of lattice thermal conductivity and maintenance of electrical performance. The present investigation indicates that the TE performance of the SnSe compound can be significantly improved by the texture modulation.

Valence band engineering and thermoelectric performance optimization in SnTe by Mn-alloying via a zone-melting method

Tin telluride (SnTe) has recently attracted lots of interest due to its potential thermoelectric application as a lead-free rock-salt analogue of PbTe. However, pristine SnTe samples have high hole concentration due to the presence of intrinsic Sn vacancies, and shows a low Seebeck coefficient and high electrical thermal conductivity, resulting in poor thermoelectric performance. In this report, we show that zone-melted SnTe systems with additional Mn (1–7 mol%) can control the hole concentration by reducing the Sn vacancies, and modulate the electronic band structure by increasing the band gap and decreasing the energy separation between the light and heavy hole valence bands. Therefore, alloying with additional Mn enhances the contribution of the heavy hole valence band and significantly improves the Seebeck coefficient in SnMnxTe with the highest value of ∼270 μV K−1. A record power factor of 31.9 μW cm−1 K−2 has been obtained at 820 K. The maximum thermoelectric figure of merit ZT of ∼1.25 is found at 920 K for the high quality crystalline ingot of p-type SnMn0.07Te.

Low lattice thermal conductivity of stanene.

A fundamental understanding of phonon transport in stanene is crucial to predict the thermal performance in potential stanene-based devices. By combining first-principle calculation and phonon Boltzmann transport equation, we obtain the lattice thermal conductivity of stanene. A much lower thermal conductivity (11.6 W/mK) is observed in stanene, which indicates higher thermoelectric efficiency over other 2D materials. The contributions of acoustic and optical phonons to the lattice thermal conductivity are evaluated. Detailed analysis of phase space for three-phonon processes shows that phonon scattering channels LA + LA/TA/ZA ↔ TA/ZA are restricted, leading to the dominant contributions of high-group-velocity LA phonons to the thermal conductivity. The size dependence of thermal conductivity is investigated as well for the purpose of the design of thermoelectric nanostructures.

Optimization of thermoelectric properties in n-type SnSe doped with BiCl3

N-type SnSe compound has been synthesized through melting with spark plasma sintering. By doping BiCl3, the carrier concentration of SnSe is significantly increased, leading to a large enhancement of electrical conductivity. Meanwhile, the SnSe0.95-BiCl3 samples also exhibit higher Seebeck coefficient and lower lattice thermal conductivity, compared with polycrystalline SnSe. Consequently, a high power factor of similar to 5 mu W cm(-1) K-2 and a ZT of 0.7 have been achieved at 793 K. The synergistic roles of BiCl3 doping in SnSe provide many opportunities in the optimization of n-type SnSe materials

A first-principles study on the phonon transport in layered BiCuOSe

First-principles calculations are employed to investigate the phonon transport of BiCuOSe. Our calculations reproduce the lattice thermal conductivity of BiCuOSe. The calculated grüneisen parameter is 2.4 ~ 2.6 at room temperature, a fairly large value indicating a strong anharmonicity in BiCuOSe, which leads to its ultralow lattice thermal conductivity. The contribution to total thermal conductivity from high-frequency optical phonons, which are mostly contributed by the vibrations of O atoms, is larger than 1/3, remarkably different from the usual picture with very little contribution from high-frequency optical phonons. Our calculations show that both the high group velocities and low scattering processes involved make the high-frequency optical modes contribute considerably to the total lattice thermal conductivity. In addition, we show that the sound velocity and bulk modulus along a and c axes exhibit strong anisotropy, which results in the anisotropic thermal conductivity in BiCuOSe.

Stability and strength of atomically thin borophene from first principles calculations

ABSTRACT A new 2D material, borophene, has been grown successfully recently on single crystal Ag substrates. Three main structures have been proposed (, and striped borophene). However, the stability of three structures is still in debate. Using first principles calculations, we examine the dynamical, thermodynamical and mechanical stability of , and striped borophene. Free-standing and borophene is dynamically, thermodynamically and mechanically stable, while striped borophene is dynamically and thermodynamically unstable due to high stiffness along a direction. The origin of high stiffness and high instability in striped borophene along a direction can both be attributed to strong directional bonding. Our work shows a deep connection between stability and strength, and helps researchers to estimate accurately the mechanical performance of 2D materials. GRAPHICAL ABSTRACT IMPACT STATEMENT A benchmark for examining the relative stability of different structures of borophene is provided. Strong directional bonding in striped borophene leads to high stiffness and high brittleness.

Enhanced thermopower in rock-salt SnTe–CdTe from band convergence

SnCdxTe materials were synthesized by the zone-melting method for this thermoelectric performance study. The X-ray diffraction results show that the lattice parameter decreases with increasing x, following Vegards law of rock-salt structure SnTe and CdTe. Besides, the room temperature Seebeck coefficients of the SnCdxTe system are enhanced to >60 μV K−1, larger than those of Cd-doped SnTe synthesized by spark plasma sintering. A large power factor of ∼25 μW cm−1 K−1 is achieved in SnCd0.12Te at 820 K, which rivals those of high performance PbTe-based materials. As a result, the highest ZT of ∼1.03 at 820 K was achieved for SnCd0.12Te.

Phonon transport properties of two-dimensional group-IV materials from ab initio calculations

materials from ab initio Bo Peng1, Hao Zhang1,∗, Hezhu Shao2, Yuanfeng Xu1, Gang Ni1, Rongjun Zhang1 and Heyuan Zhu1 1Shanghai Ultra-precision Optical Manufacturing Engineering Research Center and Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education), Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China 2Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China

Theoretical understanding on band engineering of Mn-doped lead chalcogenides PbX (X = Te, Se, S)

Electronic structures of Mn-doped PbX (X = Te, Se, S) are investigated by first-principles calculations. It is found that the Mn-doping in PbTe enlarges the band gap and increases the valence bands degeneracy, showing good agreement with experimental measurements. This band adjustment is demonstrated to be from the anti-bonding of Te-p and Mn-d orbitals. Along the series of PbTe-PbSe-PbS, the band modification of Mn-doping undergoes a gradual transition from multiple valence bands type to resonant states type, owing to the downwards shifted anion-p orbitals. This work provides essential understandings on the band engineering of Mn-doped lead chalcogenides thermoelectric materials.