arXiv subjects

Sayan Bhattacharyya

Publications and source records attributed to Sayan Bhattacharyya.

6 recordsLinked to original sources

arXiv2025

Temperature and Pressure Dependent Vibrational Properties of Pristine and Doped Vacancy-Ordered Double Perovskite

Understanding lattice dynamics and structural transitions in vacancy-ordered double perovskites is crucial for developing lead-free optoelectronic materials, yet the role of dopants in modulatingthese properties remains poorly understood. We investigate the vibrational and optical properties of pristine and Antimony(Sb)-doped Cs$_2$TiCl$_6$ vacancy-ordered double perovskite through temperature-dependent Raman spectroscopy (4-273 K), high-pressure studies (0- \~30 GPa), ambient powder XRD, and photoluminescence measurements. Sb doping improves phase purity, reducing impurity-related Raman modes present in pristine samples. Most notably, Sb-doped samples exhibit an anomalous Raman mode M$_1$ appearing exclusively below 100 K at 314-319 cm$^{-1}$, accompanied by changes in the temperature coefficient $\chi$ and anharmonic constant $A$ across this threshold. This behavior is absent in pristine Cs$_2$TiCl$_6$. While these observations suggest possible structural changes at low temperature, the origin of the M$_1$ mode remains unclear and may arise from disorder-activated vibrations, symmetry breaking, or dopant-induced local distortions. Low-temperature structural characterization is needed to confirm the nature of this transition. Photoluminescence shows broad self-trapped exciton emission at 448 nm with broader FWHM in Sb-doped samples (164.73 nm) compared to Bi-doped samples (138.2 nm), consistent with enhanced structural disorder. High-pressure Raman measurements reveal continuous mode hardening to 30 GPa with no phase transitions. These results demonstrate that Sb doping modulates the vibrational properties of Cs$_2$TiCl$_6$, though further investigation is required to establish the underlying mechanisms.

cond-mat.mtrl-sci↗

arXiv2024

Emission enhancement and bandgap narrowing in $Cs_2TeBr_6$ under pressure

Pressure-induced emission enhancement and bandgap narrowing in vacancy-ordered halide double perovskite $Cs_2TeBr_6$ are extensively investigated through photoluminescence and absorption experiments. The below bandgap broad emission is attributed to self-trapped excitons recombination. The $Cs_2TeBr_6$ crystal, consisting of undistorted octahedra, exhibits substantial emission enhancement due to the lowering of the energy barrier between $^3P_1$ and self-trapped exciton states, as well as the suppression of nonradiative energy loss with increasing pressure. In the Raman measurements, the observed behavior of full width at half maximum of all Raman modes implies dominant electron-phonon interactions rather than anharmonic interactions between phonons. The pressure-dependent X-ray diffraction measurements reveal an anomalous behaviour in the normalized pressure as a function of the Eulerian strain.

cond-mat.mtrl-sci↗

arXiv2023

Phonon anharmonicity and soft-phonon mediated structural phase transition in $Cs_3Bi_2Br_9$

We have carried out temperature-dependent x-ray diffraction and Raman scattering experiments on powder $Cs_3Bi_2Br_9$. Trigonal to monoclinic structural transition at around 95 K is discussed and shown to be driven by the softening of the soft mode. We propose a model to describe the dynamics of the incomplete soft-mode. Raman scattering experiments demonstrate the origin of the soft mode to the rocking motions of Br atoms that participate to form $BiBr_6$ octahedra, which correlates the reported theoretical calculations. Some of the Raman mode frequencies exhibit anomalous temperature dependence due to strong anharmonic phonon-phonon coupling. Temperature-dependent x-ray diffraction analysis estimate the volume thermal expansion coefficient in trigonal phase to be $13.54\times10^{-5} K^{-1}$. In the trigonal phase, the broadening of the full width at half maximum (FWHM) with increase in temperature for $E_g$ and $A_{1g}$ modes is accompanied by decaying of one optical phonon into two acoustic phonons. The volume thermal expansion rather than anharmonic phonon-phonon interaction dominates the frequency shift for the Raman modes in trigonal phase. In the monoclinic phase, the strength of four phonon processes to the frequency shift and linewidth broadening is much smaller than that for three phonon processes for some of the modes. The observed temperature dependence of FWHM of certain Raman modes in both phases suggests unusual electron-phonon coupling in the crystal.

cond-mat.mtrl-sci↗

arXiv2022

Carrier thermalization and zero-point bandgap renormalization in halide perovskites from the Urbach tails of the emission spectrum

We develop techniques to study the temperature dependent localization, thermalization, and the effects of phonon scattering on the excitons in halide perovskites from the analysis of the emission spectra. The excitonic Urbach edge, when inferred from the low energy tails of the temperature dependent luminescence spectra, is shown to be sensitive to the electron distribution and thermalization. A method to observe the Urbach focus is devised for halide perovskites where the temperature dependence of the excitonic gap is anomalous. The value of the zero-point bandgap renormalization is inferred to be about 33 meV. This small value of the bandgap renormalization rules out the formation of small polarons and points to weak electron-phonon coupling. The experiments are performed on the nanosheets of the archetypal halide perovskite, CsPbBr$_3$.

cond-mat.mtrl-sci↗

arXiv2021

Pressure induced emission enhancement and bandgap narrowing: experimental investigations and first principles theoretical simulations on a model halide perovskite

We report high-pressure photoluminescence, Raman scattering, and x-ray diffraction measurements on a lead-free halide perovskite $Cs_3Sb_2Br_9$. At about 3 GPa, an electronic transition manifests itself through a broad minimum in linewidth, a maximum in the intensity of $E_g$, $A_{1g}$ Raman modes, and the unusual change in the $c/a$ ratio of the trigonal lattice. The large compressibility and observed Raman anomalies indicate to a soft material with strong electron-phonon coupling. The observed below bandgap broadband emission in the photoluminescence measurement indicates the recombination of self-trapped excitons. The initial blueshift of the photoluminescence peak reinforces itself to the redshift at around 3 GPa due to the change in the electronic landscape. A first order trigonal to a monoclinic structural transition is also seen at 8 GPa. The first-principles density functional theory (DFT) calculations reveal that the electronic transition is associated with direct-to-indirect bandgap transition due to changes in the hybridization of $Sb-5s$ and $Br-4p$ orbitals near the Fermi level in the valence band. The experimentally observed Raman modes are assigned to their symmetry using the density functional perturbation theory. In addition, the DFT calculations predict a 27.5\% reduction of the bandgap in the pressure range 0-8 GPa.

cond-mat.mtrl-sci↗

arXiv2019

Nonlinear optical responses of all-inorganic lead halide perovskite nanostructures by time-resolved beam-deflection technique

We have investigated nonlinear refraction in all-inorganic halide perovskites, the CsPbBr$_3$ and CsPbBr$_{1.5}$I$_{1.5}$ nanosheet and quantum dot colloids in toluene, by a novel beam deflection technique using near-resonant continuous wave lasers. The nonlinear refraction and its time-evolution measured here have originated from thermal lensing effect. Nonlinear behaviour of heat transport in terms of intensity dependent thermal diffusion rate has been observed. Effects of convective heat flow have been measured at high intensities. Quantum dots have higher nonlinear refraction as compared to the corresponding nanosheet samples, presumably due to reduced dimensionality. The effective values of nonlinear refractive index, estimated here for near-resonant excitations, exceed those reported in the literature for organic-inorganic hybrid perovskites in the nonresonant excitation regime, by several orders of magnitude.

cond-mat.mes-hall↗