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Register a new userDirect observation of double valence-band extrema and anisotropic effective masses of the thermoelectric material SnSe
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Synchrotron-based angle-resolved photoemission spectroscopy is used to determine the electronic structure of layered SnSe, which was recently turned out to be a potential thermoelectric material. We observe that the top of the valence band consists of two nearly independent hole bands, whose tops differ by ~20 meV in energy, indicating the necessity of a multivalley model to describe the thermoelectric properties. The estimated effective masses are anisotropic, with in-plane values of 0.16-0.39 m$_0$ and an out-of-plane value of 0.71 m$_0$, where m$_0$ is the rest electron mass. Information of the electronic structure is essential to further enhance the thermoelectric performance of hole-doped SnSe.
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IV-VI semiconductor SnSe has been known as the material with record high thermoelectric performance.The multiple close-to-degenerate valence bands in the electronic band structure has been one of the key factors contributing to the high power factor and thus figure of merit in the SnSe single crystal. To date, there have been primarily theoretical calculations of this particular electronic band structure. In this paper, however, using angle-resolved photoemission spectroscopy, we perform a systematic investigation of the electronic structure of SnSe. We directly observe three predicted hole bands with small energy differences between their band tops and relatively small in-plane effective masses, in good agreement with the ab initio calculations and critical for the enhancement of the Seebeck coefficient while keeping high electrical conductivity. Our results reveal the complete band structure of SnSe and help to provide a deeper understanding of the electronic origin of the excellent thermoelectric performances in SnSe.
The success of black phosphorus in fast electronic and photonic devices is hindered by its rapid degradation in presence of oxygen. Orthorhombic tin selenide is a representative of group IV-VI binary compounds that are robust, isoelectronic, and share the same structure with black phosphorus. We measured the band structure of SnSe and found highly anisotropic valence bands that form several valleys having fast dispersion within the layers and negligible dispersion across. This is exactly the band structure desired for efficient thermoelectric generation where SnSe has shown a great promise.
We analyze the influence of the Mg concentration on several important properties of the band structure of ZnMgO alloys in wurtzite structure using ab initio calculations. For this purpose, the band structure for finite concentrations is defined in terms of the Bloch spectral density, which can be calculated within the coherent potential approximation. We investigate the concentration dependence of the band gap and the crystal-field splitting of the valence bands. The effective electron and hole masses are determined by extending the effective mass model to finite concentrations. We compare our results with experimental results and other calculations.
By using angle-resolved photoemission spectroscopy (ARPES), the variation of the electronic structure of HfSe$_2$ has been studied as a function of sodium intercalation. We observe how this drives a band splitting of the p-orbital valence bands and a simultaneous reduction of the indirect band gap by values of up to 400 and 280 meV respectively. Our calculations indicate that such behaviour is driven by the band deformation potential, which is a result of our observed anisotropic strain induced by sodium intercalation. The applied uniaxial strain calculations based on density functional theory (DFT) agree strongly with the experimental ARPES data. These findings should assist in studying the physical relationship between doping and strain, as well as for large-scale two-dimensional straintronics.
Excellent thermoelectric performance in the out-of-layer n-doped SnSe has been observed experimentally (Chang et al., Science 360, 778-783 (2018)). However, a first-principles investigation of the dominant scattering mechanisms governing all thermoelectric transport properties is lacking. In the present work, by applying extensive first-principles calculations of electron-phonon coupling associated with the calculation of the scattering by ionized impurities, we investigate the reasons behind the superior figure of merit as well as the enhancement of zT above 600 K in n-doped out-of-layer SnSe, as compared to p-doped SnSe with similar carrier densities. For the n-doped case, the relaxation time is dominated by ionized impurity scattering and increases with temperature, a feature that maintains the power factor at high values at higher temperatures and simultaneously causes the carrier thermal conductivity at zero electric current (k_el) to decrease faster for higher temperatures, leading to an ultrahigh-zT = 3.1 at 807 K. We rationalize the roles played by k_el and k^0 (the thermal conductivity due to carrier transport under isoelectrochemical conditions) in the determination of zT. Our results show the ratio between k^0 and the lattice thermal conductivity indeed corresponds to the upper limit for zT, whereas the difference between calculated zT and the upper limit is proportional to k_el.
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