Faculty Publications
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Item Observation of electrical threshold switching behavior and thermal crystallization in bulk Se86-xTe14Snx chalcogenide glasses(Elsevier Ltd, 2023) Joshi, S.; Udayashankar, N.K.Selenium-rich chalcogenides have gained popularity as materials for selector devices due to their unique Ovonic Threshold Switching behavior. Bulk Se86-xTe14Snx glassy alloys (0 ≤ x ≤ 6) were prepared through the traditional melt quenching method. The samples with Sn atomic percentage (x) = 3 to 6 are found to exhibit a rapid and reversible transition between a highly resistive and conductive state affected by an electric field. A remarkable decrement in threshold voltage (Vth) from 453 V to 62 V has been observed with increase in the Tin content. Differential scanning calorimetry (DSC) analysis was carried out to understand the variation of Glass transition temperature (Tg), Crystallization temperature (Tc), and other important glass stability parameters and their compositional dependence. Se82Te14Sn4 sample was found to be thermally most stable with Herby's parameter value (HR) of 0.3860 and a maximum number of switching cycles at room temperature. X-Ray diffraction patterns of annealed samples were compared with pristine glass to study the multi-phasic Se–Te–Sn alloy. Further, the threshold voltage (Vth) and the number of threshold switching cycles are found to decrease with an increase in temperature till crystallization on-set temperature (Toc). The temperature-dependent conductivity studies showed an abrupt increase in the conductivity of the samples as the temperature crossed the crystallization onset temperature. © 2023 Elsevier Ltd and Techna Group S.r.l.Item Peculiarities of Electrical Switching and Phase Transition Dynamics in Bismuth-Infused Se-Te Chalcogenide Glasses: From Bulk to Thin Film Devices(American Chemical Society, 2024) Joshi, S.; Rodney, J.D.; Udayashankar, N.K.Herein, the electrical switching behavior of both bulk and thin film forms of Se86-xTe14Bix (0 ≤ x ≤ 4) chalcogenide glasses was investigated. The melt-quench-derived glasses were found to be amorphous, and the switching behavior exhibited a threshold-type response below a certain current limit (Ith) for bismuth (Bi)-doped bulk samples. Interestingly, as current levels surpassed this threshold, a noteworthy change occurred in the switching behavior, converting it into a memory-type response. The threshold voltage (Vth) exhibited a decreasing trend from ∼228 V to ∼36 V with an increasing Bi content, and differential scanning calorimetry (DSC) was utilized to study the phase transition phenomena and thermal stability of the amorphous glasses. These DSC results unequivocally confirmed that the transition from amorphous to crystalline phase occurred readily and at lower temperatures in the Se82Te14Bi4 composition. Furthermore, annealing studies were carried out to gain insight into the phase transformations that occur when the material makes the transition from an amorphous to a crystalline state. Subsequently, the same melt-quench-derived glasses were deposited as a thin film using physical vapor deposition (PVD) into a three-layered Al/Se-Te-Bi/Al device, and the memory switching voltage experienced a remarkable drop to 2.88 V compared to the bulk material. This exploration sheds light on the captivating electrical switching behavior of Se86-xTe14Bix chalcogenide glasses and holds promise for potential applications spanning the realm of emerging electronics and phase change material (PCM) devices. © 2024 American Chemical Society.Item Defect-engineered single crystal Bi2Te3 via Sb and Se doping for enhanced thermoelectric performance(Springer, 2025) Puthran, S.; Hegde, G.S.; Prabhu, A.N.; Wang, Y.-L.; Kuo, Y.K.; Joshi, S.; Udayashankar, N.K.; Nayak, R.The limitation of the single crystal melt growth method to tune the microstructure of the materials in a controlled way and the need for enhancing the thermoelectric properties of single crystal grown Bismuth telluride (Bi2Te3), through defect and microstructural engineering, has motivated this work. In this work, we address this limitation through a controlled doping strategy using antimony (Sb) and selenium (Se) to introduce targeted defects and microstructural modifications within single-crystalline Bi2Te3. Sb and Se substitutions create atomic scale strain, point defects, and micro-grain structures, enhancing phonon scattering without significantly disrupting the crystalline order. The resulting defect-engineered single crystals exhibit improved thermoelectric performance, with a notable reduction in lattice thermal conductivity and retention of excellent electrical properties. The co-doped compositions, Bi2Te2.7Se0.3 and (Bi0.98Sb0.02)2Te2.7Se0.3, exhibited significantly enhanced thermoelectric performance, with Seebeck coefficients reaching ~ 253 ?V/K and ? 211 ?V/K, respectively, over the 10–400 K range. The power factor improved remarkably, showing a ~ 30-fold increase for Bi2Te2.7Se0.3 and ~ 20-fold for the Sb-doped variant, while the figure of merit (ZT) improved by ~ 28.5 and ~ 14 times, respectively. Further, a flexible thermoelectric device fabricated from these optimized materials generated output power of 2.7 nW and 3.35 nW at ambient temperature. The non-monotonic variation of the Seebeck coefficient with Sb content, showing an optimal enhancement at x = 0.04, highlights the delicate balance between carrier concentration and band structure modification, emphasizing moderate Sb substitution achieves the most favorable conditions for thermoelectric performance. Our results present a scalable strategy for bridging the performance gap between pristine single crystals and heavily nanostructured thermoelectrics, opening new avenues for high-efficiency energy harvesting devices. © The Author(s) 2025.Item Optimizing thermoelectric properties of Bi2Te3 via Sb and Se Co-doping: experimental insights and finite elemental simulations using COMSOL(Springer, 2025) Puthran, S.; Prabhu, A.N.; Kamble, M.; Babu, P.D.; Joshi, S.; Udayashankar, N.K.In this study, we investigated the impact of antimony (Sb) and selenium (Se) co-dopants on the thermoelectric properties of bismuth telluride (Bi2Te3). Our findings reveal that Sb doping significantly enhances the electrical conductivity of the material, increasing it by a factor of 2.83 for (Bi0.98Sb0.02)2Te2.7Se0.3, primarily due to an increase in carrier concentration. The electrical resistivity of pristine Bi2Te3 at 300 K is 2.79 × 10?4 ?·m, which decreases substantially to 0.006 × 10?4 ?·m at 303 K with Sb doping at x = 0.02. Additionally, (Bi0.96Sb0.04)2Te2.7Se0.3 composition achieves the highest power factor of 9.744 × 10?5 W/m·K2 at 300 K, a 3-times improvement over the pristine Bi2Te3 (3.143 × 10?5 W/m·K2). The ZT value of Bi2Te2.7Se0.3 is 3.5 times higher than that of the pristine material at 350 K. COMSOL simulations support the experimental findings, revealing a maximum temperature gradient of 35 °C (hot end: 20 °C, cold end: ? 15 °C) for the (Bi0.98Sb0.02)2Te2.7Se0.3 module with comparable p-type and n-type parameters. The increased temperature gradient in the COMSOL simulation correlates with the improved thermoelectric performance observed experimentally, indicating that co-doping Bi2Te3 with Sb and Se effectively enhances its thermoelectric properties. © The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2025.