Riyanka Karmakar

Scopus Publications

Multiple Carrier Generation at an Exceptionally Low Energy Threshold
Riyanka Karmakar, Pravrati Taank, Debjit Ghoshal, Pushpendra Yadav, Dipendranath Mandal, Megha Shrivastava, Amit Agarwal, Matthew C. Beard, Elisa M. Miller, K. V. Adarsh
Physical Review Letters, 2025
Multiple carrier generation (MCG), a process wherein two or more carriers are generated from a single high-energy absorbed photon, holds immense promise for quantum sensing, metrology, low-threshold lasers, and photovoltaics. Despite its potential, MCG has faced obstacles such as low efficiency and a high threshold photon energy at least twice the band gap (2E_{g}) of the semiconductor, limiting its application only to a class of materials with low E_{g}. Here, we present a new approach that overcomes this limitation by leveraging carrier-donor scattering to excite secondary electrons from donor states strategically positioned below the conduction band. Our method relies on strong Coulomb interaction, reduced dielectric screening, slow hot carrier cooling, and strictly follows the energy conservation rules. We experimentally demonstrated this idea in a model system of monolayer (1L) MoS_{2} by exploiting electron-donating chalcogen vacancy states. We observed an exceptionally low MCG threshold of ∼1.12E_{g} for the first time in 1L MoS_{2}. Remarkably, the quantum yield can be further increased to >3 by increasing the photon energy to 1.65E_{g}, representing a substantial advancement over existing methods. Our findings extend the horizon of MCG into next-generation high-performance optoelectronic devices with an on-demand operating spectral range spanning from infrared to ultraviolet.
Exciton many-body interactions and charge transfer in CsPbBr3 /graphene derivatives
Naresh Chandra Maurya, Riyanka Karmakar, Rajesh Kumar Yadav, Pravrati Taank, Santu K. Bera, Anirban Mondal, Dipendranath Mandal, Megha Shrivastava, Md. Nur Hasan, Tuhin Kumar Maji, Debjani Karmakar, K. V. Adarsh
Physical Review B, 2023
Charge separation and many-body interactions at the interface of the light-absorbing semiconductor and contact layer are of crucial importance to the photophysical properties and optoelectronic device performance. Here, we report the exciton many-body interactions and charge transfer dynamics at the interface of metal halide perovskite nanocrystals and graphene derivatives [graphene oxide (GO) and reduced GO (RGO)] using ultrafast transient absorption (TA) and time-resolved photoluminescence (PL) measurements. At the early timescales, the TA spectra of ${\mathrm{CsPbBr}}_{3}/\mathrm{GO}$ and ${\mathrm{CsPbBr}}_{3}/\mathrm{RGO}$ show an asymmetric derivative feature originating from the exciton many-body interactions. The band gap renormalization and binding energies of exciton and biexciton of ${\mathrm{CsPbBr}}_{3}$ nanocrystals are significantly reduced in ${\mathrm{CsPbBr}}_{3}/\mathrm{GO}$(RGO) due to the charge transfer and change in the dielectric environment, respectively. More specifically, the exciton (biexciton) binding energy of ${\mathrm{CsPbBr}}_{3}$ nanocrystals, originally $38\ifmmode\pm\else\textpm\fi{}2$ ($34\ifmmode\pm\else\textpm\fi{}1$) meV, decreases to $27\ifmmode\pm\else\textpm\fi{}1$ ($22\ifmmode\pm\else\textpm\fi{}1$) meV in ${\mathrm{CsPbBr}}_{3}/\mathrm{RGO}$ and $17\ifmmode\pm\else\textpm\fi{}1$ ($15\ifmmode\pm\else\textpm\fi{}1$) meV in ${\mathrm{CsPbBr}}_{3}/\mathrm{GO}$. Furthermore, we observe a reduction in the Auger recombination rate and exciton PL quenching in ${\mathrm{CsPbBr}}_{3}/\mathrm{GO}$ and ${\mathrm{CsPbBr}}_{3}/\mathrm{RGO}$, corroborating the charge transfer mechanism. Our systematic studies successfully describe photoexcited charge transfer from ${\mathrm{CsPbBr}}_{3}$ nanocrystals to GO (RGO) in $7.0\ifmmode\pm\else\textpm\fi{}0.4$ ($4.2\ifmmode\pm\else\textpm\fi{}0.1$) ps, which is one order of magnitude faster than the charge transfer for other acceptor materials such as metal oxide, fullerene, anthraquinone, 1-aminopyrene, and phenothiazine. Our results offer insights and guidance for perovskite-based high-performance optoelectronic devices.
Exciton recombination in few-layer MoS2 nanosheets: Role of free carriers and defects
Pravrati Taank, Riyanka Karmakar, K. V. Adarsh
Surface and Interface Analysis, 2023
A current obstacle to the development of extremely flexible and high‐performance optoelectronic devices using transition metal dichalcogenide nanosheets is the incomplete understanding of the exciton and free carrier recombination in the presence of defects. Here, taking liquid‐phase exfoliated few‐layer MoS 2 nanosheets as a model system, we demonstrate the exciton and free carrier recombination mechanism by employing pump energy and fluence‐dependent ultrafast transient absorption spectroscopy. We demonstrate that 3.10 eV pump excitation, much above the lowest energy A exciton of four to six layers of MoS 2 nanosheets (~1.84 eV), generates excitons and free carriers. The excitons decay quickly within ~3 ps due to the defect capture via the Shockley–Read–Hall mechanism. In contrast, free carriers show slower recombination (~1000 ps), which is an order of magnitude larger than the exciton recombination time. We verified this idea by exciting the sample with 1.94–2.22 eV pump excitations that predominantly generate excitons decaying within ~3 ps. Our systematic studies in few‐layer MoS 2 nanosheets reveal crucial information on the unexplored domain of excitons and free carriers recombination in the presence of defects for several optoelectronic applications.
Defect-mediated carrier dynamics and third-order nonlinear optical response of WS2 quantum dots
Riyanka Karmakar, Dipendranath Mandal, Megha Shrivastava, K. V. Adarsh
Optics Letters, 2022
In this Letter, we report the third-order absorptive and refractive nonlinear optical response of highly luminescent WS2 quantum dots (QDs) in the off-resonant femtosecond and nanosecond pulses, which is beneficial for optical limiting and quantum information processing. For 800 nm femtosecond excitation, QDs show two-photon absorption (β = (107 ± 2)×10−3 cm/GW) with positive nonlinearity originating from bound carriers. This picture changes significantly for 532 nm nanosecond excitation, where it shows reverse saturable absorption with negative nonlinearity primarily originating from the sequential absorption of two single photons through the shallow defects, creating free carriers. Our results provide a promising route toward low-dimensional optoelectronic devices.
An Insightful Picture of Multi-Particle Recombination in Few-Layer MoS2 Nanosheets
Pravrati Taank, Riyanka Karmakar, Rituraj Sharma, Rajesh Kumar Yadav, Megha Shrivastava, Naresh Chandra Maurya, Tuhin Kumar Maji, Debjani Karmakar, K. V. Adarsh
Journal of Physical Chemistry C, 2022
The electronic and optical properties of transition metal dichalcogenides are well understood. However, much less are known about the role of defects and free carriers in exciton recombination, which is of fundamental importance for optoelectronic applications. Here, we investigate the photoexcited carrier recombination mechanism in few-layer (4–6 L) MoS2 nanosheets by employing pump energy and fluence-dependent femtosecond transient absorption spectroscopy. We demonstrate that the multi-particle (excitons and free carriers) generated by 3.1 eV excitation well above the electronic bandgap exhibit distinct recombination times. For instance, free carriers slow down the recombination by orders of magnitude relative to excitons. In contrast, the recombination time of excitons generated upon near quasi-particle excitation (1.94–2.22 eV) drops to ∼3 ps, which is associated with fast exciton capture to defects. To understand the nature of defects, we have investigated different configurations of sulfur vacancies from density functional theory (DFT) and time-dependent DFT.
Third-Order Nonlinear Optical Response and Optical Limiting of WS2 Quantum Dots
Optics Infobase Conference Papers, 2022
Trapped carrier hopping and unusual bottleneck in coalescence dynamics of MoS2few layers
Riyanka Karmakar, Pravrati Taank, K. V. Adarsh
Aip Conference Proceedings, 2021
Unravelling the qualitative descriptions of ultrafast energy relaxation dynamics in transition metal dichalcogenides (TMDCs) is very important for the fundamental understanding of the nature of photoexcited carriers. Herein, we discuss the temporal evolution of B-exciton dynamics of colloidal MoS2 few layers by using femtosecond transient pump-probe absorption spectroscopy. We observe trapped carrier hopping and bimolecular recombination, which is portrayed as phonon-assisted coalescence through indirect bandgap at lower carrier-density and an unconventional bottleneck in decay kinetics above a critical density (⁓ 40 × 1013 cm-2). Also, we propose a new model describing the mechanistic origin of dynamical characterization of exciton relaxation with unprecedented carrier-densitydependent phenomena. This model is feasible to explain the physics behind the obscured nature of quenching of photoluminescence quantum yield of TMDCs and will provide powerful insight for fabricating defect-free optoelectronic devices in the near future.
Ultrafast Charge-Transfer Mediated Indirect-Excitons in CsPbBr3/MoS2Heterostructure
2021 Conference on Lasers and Electro Optics CLEO 2021 Proceedings, 2021
Ultrafast charge-transfer mediated indirect-excitons in CsPbBr3/MoS2 heterostructure
Optics Infobase Conference Papers, 2021
Unravelling the Role of Trap States on Carrier Dynamics of WS2 Quantum Dots
Conference Proceedings Lasers and Electro Optics Society Annual Meeting LEOS, 2020
Unravelling the role of trap states on carrier dynamics of WS2 quantum dots
Riyanka Karmakar, K.V. Adarsh
Optics Infobase Conference Papers, 2020