Effect of strain on electronic and thermoelectric properties of few layers to bulk MoS2_{2}

The sensitive dependence of electronic and thermoelectric properties of MoS$_2$ on the applied strain opens up a variety of applications in the emerging area of straintronics. Using first principles based density functional theory calculations, we show that the band gap of few layers of MoS$_2$ can be tuned by applying i) normal compressive (NC), ii) biaxial compressive (BC), and iii) biaxial tensile (BT) strain. A reversible semiconductor to metal transition (S-M transition) is observed under all three types of strain. In the case of NC strain, the threshold strain at which S-M transition occurs increases with increasing number of layers and becomes maximum for the bulk. On the other hand, the threshold strain for S-M transition in both BC and BT strain decreases with the increase in number of layers. The difference in the mechanisms for the S-M transition is explained for different types of applied strain. Furthermore, the effect of strain type and number of layers on the transport properties are also studied using Botzmann transport theory. We optimize the transport properties as a function of number of layers and applied strain. 3L- and 2L-MoS$_2$ emerge as the most efficient thermoelectric material under NC and BT strain, respectively. The calculated thermopower is large and comparable to some of the best thermoelectric materials. A comparison between the feasibility of these three types of strain is also discussed.Comment: 18 pages, 7 figure

Strain-induced electronic phase transition and strong enhancement of thermopower of TiS2

Using first principles density functional theory calculations, we show a semimetal to semiconducting electronic phase transition for bulk TiS 2 by applying uniform biaxial tensile strain. This electronic phase transition is triggered by charge transfer from Ti to S, which eventually reduces the overlap between Ti-(d) and S-(p) orbitals. The electronic transport calculations show a large anisotropy in electrical conductivity and thermopower, which is due to the difference in the effective masses along the in-plane and out of plane directions. Strain induced opening of band gap together with changes in dispersion of bands lead to three-fold enhancement in thermopower for both p- and n-type TiS2 . We further demonstrate that the uniform tensile strain, which enhances the thermoelectric performance, can be achieved by doping TiS2 with larger iso-electronic elements such as Zr or Hf at Ti sites.Comment: 8 pages, 6 figure

Thermoelectric properties of β{\beta}-FeSi2_{\text2}

We investigate the thermoelectric properties of ${\beta}$-FeSi$_{\text2}$ using first principles electronic structure and Boltzmann transport calculations. We report a high thermopower for both \textit{p}- and \textit{n}-type ${\beta}$-FeSi$_{\text2}$ over a wide range of carrier concentration and in addition find the performance for \textit{n}-type to be higher than for the \textit{p}-type. Our results indicate that, depending upon temperature, a doping level of 3$\times10{^{20}}$ - 2$\times10{^{21}}$ cm${^{-3}}$ may optimize the thermoelectric performance

Effect of Hydrostatic Pressure on Lone Pair Activity and Phonon Transport in Bi2_2O2_2S

Dibismuth dioxychalcogenides, Bi$_2$O$_2$Ch (Ch = S, Se, Te) are emerging class of materials for next generation electronics and thermoelectrics with an ultrahigh carrier mobility and excellent air stability. Among these, Bi$_2$O$_2$S is fascinating because of stereochemically active 6$s^2$ lone pair of Bi$^{3+}$ cation, heterogeneous bonding and high mass contrast of constituent elements. In this work, we systematically investigate the effect of hydrostatic pressure and its implications on lattice dynamics and phonon transport properties of Bi$_2$O$_2$S by employing first principles calculations along with the Boltzmann transport theory. The ambient $Pnmn$ phase exhibits a low average lattice thermal conductivity ($\kappa_l$) of 1.71 W-m/K at 300 K. We also find that Bi$_2$O$_2$S undergoes a structural phase transition from low symmetry ($Pnmn$) to a high symmetry ($I4/mmm$) structure around 4 GPa due to the Bi$^{3+}$ cation centering. Upon compression the lone pair activity of Bi$^{3+}$ cation is suppressed, which increases $\kappa_l$ by nearly 3 times to 4.92 W-m/K at 5 GPa for $I4/mmm$ phase. The calculated phonon lifetimes and Gr\"uneisen parameters show that anharmonicity reduces with increasing pressure due to further suppression of lone pair, strengthening of intra and inter molecular interactions, which raises the average room temperature $\kappa_l$ to 12.82 W-m/K at 20 GPa. Overall, the present study provides a comprehensive understanding of hydrostatic pressure effects on stereochemical activity of the Bi$^{3+}$ cation lone pair and its consequences on phonon transport properties of Bi$_2$O$_2$S.Comment: 19 pages, 7 figures and supporting informatio

Semiconductor to metal transition in bilayer phosphorene under normal compressive strain

Phosphorene, a two-dimensional (2D) analog of black phosphorous, has been a subject of immense interest recently, due to its high carrier mobilities and a tunable bandgap. So far, tunability has been predicted to be obtained with very high compressive/tensile in-plane strains, and vertical electric field, which are difficult to achieve experimentally. Here, we show using density functional theory based calculations the possibility of tuning electronic properties by applying normal compressive strain in bilayer phosphorene. A complete and fully reversible semiconductor to metal transition has been observed at $\sim13.35\%$ strain, which can be easily realized experimentally. Furthermore, a direct to indirect bandgap transition has also been observed at $\sim3\%$ strain, which is a signature of unique band-gap modulation pattern in this material. The absence of negative frequencies in phonon spectra as a function of strain demonstrates the structural integrity of the sheets at relatively higher strain range. The carrier mobilities and effective masses also do not change significantly as a function of strain, keeping the transport properties nearly unchanged. This inherent ease of tunability of electronic properties without affecting the excellent transport properties of phosphorene sheets is expected to pave way for further fundamental research leading to phosphorene-based multi-physics devices.Comment: 12 pages, 5 figure

Flattening conduction and valence bands for interlayer excitons in a moir\'e MoS2_2/WSe2_2 heterobilayer

We explore the flatness of conduction and valence bands of interlayer excitons in MoS$_2$/WSe$_2$ van der Waals heterobilayers, tuned by interlayer twist angle, pressure, and external electric field. We employ an efficient continuum model where the moir\'e pattern from lattice mismatch and/or twisting is represented by an equivalent mesoscopic periodic potential. We demonstrate that the mismatch moir\'e potential is too weak to produce significant flattening. Moreover, we draw attention to the fact that the quasi-particle effective masses around the $\Gamma$-point and the band flattening are \textit{reduced} with twisting. As an alternative approach, we show (i) that reducing the interlayer distance by uniform vertical pressure can significantly increase the effective mass of the moir\'e hole, and (ii) that the moir\'e depth and its band flattening effects are strongly enhanced by accessible electric gating fields perpendicular to the heterobilayer, with resulting electron and hole effective masses increased by more than an order of magnitude leading to record-flat bands. These findings impose boundaries on the commonly generalized benefits of moir\'e twistronics, while also revealing alternate feasible routes to achieve truly flat electron and hole bands to carry us to strongly correlated excitonic phenomena on demand