arXiv subjects

D Murali

Publications and source records attributed to D Murali.

4 recordsLinked to original sources

arXiv2025

Thermoelectric properties of Lead halide Janus layers -- A theoretical investigation

Thermoelectric materials offer a promising route for efficient heat-to-power conversion. In search of materials functional at high and low operating temperatures, we investigate the thermoelectric properties of two-dimensional lead halide Janus layers (JLs) using density functional theory. The electronegativity difference between halides in JLs significantly modulates the electronic structure, particularly the strong Pb-F bonding in PbIF JLs leads to pronounced band curvature and a unique direct bandgap. Estimated through three-phonon interactions, the lattice thermal conductivity is intrinsically low, primarily due to acoustic phonon contributions and suppressed optical phonon transport. The thermoelectric coefficients are enhanced with carrier doping, resulting in figures of merit as high as 8.94 at room temperature and upto 36.31 at elevated temperatures. These findings establish two-dimensional lead halide Janus layers as exceptional candidates for thermoelectric conversion, and the insights into their elemental and electronic characteristics offer a valuable basis for the future design of high-performance lead-based thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2023

First Principles Investigation on Structural and Optoelectronic Properties of newly designed Janus Lead Halides PbXY (X, Y = F, Cl, Br, I )

Here, we design a novel class of Janus structures PbXY(X,Y = F, Cl, Br, I) and propose it for the solar mediated photocatalytic water splitting hydrogen production as well as for the photovoltaic solarcell applications. Charge analysis shows that covalent bonding for less electronegative atoms and ionic bonding for more electronegative atoms. Strong dual bonding like ionic one side and covalent other side is observed when heavy and lighter atoms are part of the same Janus structures. The as designed Janus structures show good dynamical stability through phonon calculations. Basic electronic structure using Generalised Gradient Approximation(GGA) reveal both direct and indirect nature of band gap with large tunability. Heyd-Scuseria-Ernzerhof (HSE) electronic structure calculations are performed and wider band gaps are predicted for these Janus structures. The calculated effective masses of charge carriers show robust charge carrier dynamics. The orbital resolved electronic density of states (DOS) shows that the conduction band edge is composed of pz orbital of Pb atom. The partialcharge density calculated at conduction band minimum (CBM) also support the result obtained from PDOS analysis. Breaking of centrosymmetry, polarization in the out of plane direction, the z-oriented orbitals of CBM all points that these materials are suitable for shift current generation. The calculated optical absorption spectra show that the Janus structures are suitable for visible light absorption. The calculated potential difference between the top and bottom layer show significant variation and maximum (1.02 eV) is observed for PbClF. Further, we show that combining both potential difference and HSE bandgap, valence band maximum(VBM) and CBM straddle the water redox potentials, thus making the Janus structures suitable for water redox reactions.

cond-mat.mtrl-sci↗

arXiv2023

Ab initio insights on the ultrafast strong-field dynamics of anatase TiO$_2$

Electron dynamics of anatase TiO$_2$ under the influence of ultrashort and intense laser field is studied using the real-time time-dependent density functional theory (TDDFT). Our findings demonstrate the effectiveness of TDDFT calculations in modeling the electron dynamics of solids during ultrashort laser excitation, providing valuable insights for designing and optimizing nonlinear photonic devices. We analyze the perturbative and non-perturbative responses of TiO$_2$ to 30 fs laser pulses at 400 and 800 nm wavelengths, elucidating the underlying mechanisms. At 400 nm, ionization via single photon absorption dominates, even at very low intensities. At 800 nm, we observe ionization through two-photon absorption within the intensity range of $1\times10^{10}$ to $9\times10^{12}$ W/cm$^2$, with a transition from multiphoton to tunneling ionization occurring at $9\times10^{12}$ W/cm$^2$. We observe a sudden increase in energy and the number of excited electrons beyond $1\times10^{13}$ W/cm$^2$, leading to their saturation and subsequent laser-induced damage. We estimate the damage threshold of TiO$_2$ for 800 nm to be 0.1 J/cm$^2$. In the perturbative regime, induced currents exhibit a phase shift proportional to the peak intensity of the laser pulse. This phase shift is attributed to the intensity-dependent changes in the number of free carriers, indicative of the optical Kerr effect. Leveraging the linear dependence of phase shift on peak intensities, we estimate the nonlinear refractive index ($n_2$) of TiO$_2$ to be $3.54\times10^{-11}$ cm$^2$/W.

physics.optics↗

arXiv2021

Quasiparticle electronic structure and optical response ($G_0W_0$+BSE) of anatase TiO$_2$ starting from modified HSE06 functionals

The quasiparticle electronic structure and optical excitation of anatase TiO$_2$ is determined within the framework of many-body perturbation theory (MBPT) by combining the $G_0W_0$ method and the Bethe-Salpeter Equation (BSE). A modified version of the HSE06 screened hybrid functional, that includes 20\% exact Fock exchange (HSE06(20)) as opposed to 25\% in the standard HSE06 functional, is used to set up the starting Hamiltonian for $G_0W_0$+BSE calculations. The HSE06(20) functional accurately predicts the ground state electronic band structure. BSE calculations based on data from $G_0W_0$+HSE06(20) yield direct optical excitation energies and oscillator strengths in excellent agreement with existing experiments and theoretical calculations characterizing direct excitation. In particular, an exciton binding energy of 229 $\pm$ 10 meV is obtained, in close agreement with experiments. The projections of excitonic states onto the quasiparticle band structure in a fatband representation shows that the lowest optical transition of anatase TiO$_2$ consists of excitons originating from the mixing of direct transitions within band pairs running parallel to the $\Gamma -Z $ direction in the tetragonal Brillouin zone. This implies a strong spatial localization of excitons in the $xy$ plane of the lattice. This investigation highlights the importance of a suitable non-interacting Hamiltonian for the MBPT based quasiparticle $G_0W_0$ and subsequent BSE calculations and suggests HSE06(20) as an optimal choice in the case of anatase TiO$_2$.

cond-mat.mtrl-sci↗