K. Lakshmi Ganapathi

Bio: K. Lakshmi Ganapathi is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Plasmon & Conductance. The author has an hindex of 3, co-authored 5 publications receiving 21 citations.

Probing defect states in few-layer MoS 2 by conductance fluctuation spectroscopy

Abstract: Despite the concerted effort of several research groups, a detailed experimental account of defect dynamics in high-quality single-and few-layer transition-metal dichalcogenides remains elusive. In this paper we report an experimental study of the temperature dependence of conductance and conductance fluctuations on few-layer MoS2 exfoliated on hexagonal boron nitride and covered by a capping layer of high-kappa dielectric HfO2. The presence of the high-kappa dielectric made the device extremely stable against environmental degradation as well as resistant to changes in device characteristics upon repeated thermal cycling, enabling us to obtain reproducible data on the same device over a timescale of more than 1 year. Our device architecture helped bring down the conductance fluctuations of the MoS2 channel by orders of magnitude compared to previous reports. The extremely low noise levels in our devices made it possible to detect the generation-recombination noise arising from charge fluctuation between the sulfur-vacancy levels in the band gap and energy levels at the conductance band edge. Our work establishes conduction fluctuation spectroscopy as a viable route to quantitatively probe in-gap defect levels in low-dimensional semiconductors.

Near Infrared Random Lasing in Multilayer MoS2

Abstract: We demonstrated room temperature near infrared (NIR) region random lasing (RL) (800–950 nm), with a threshold of nearly 500 μW, in ∼200 nm thick MoS2/Au nanoparticles (NPs)/ZnO heterostructures usi...

Plasmon induced brightening of dark exciton in monolayer WSe2 for quantum optoelectronics

Abstract: In the present work, we report plasmon induced brightening of dark excitons (XD) in Au nanoparticle (Au-NP) coated monolayer (1L) WSe2. We observed one order enhancement in photoluminescence (PL) intensity and surface enhanced Raman scattering in Au-NP/1L-WSe2 at room temperature (RT). Temperature dependent PL measurements showed enhanced PL emission from RT down to 100 K in contrast to reduced PL emission which is generally observed for pristine 1L-WSe2. We attribute this effect to the out-of-plane electric field induced by the scattering from Au-NPs, which results in the out-of-plane dipole moment and spin-flip of conduction band electrons in Au-NP/1L-WSe2, making XD bright. Our approach provides a facile way to harness excitonic properties in low-dimensional semiconductors, offering simple strategies for quantum optoelectronics.

ZnO/Au/ZnO Configuration for High Performance Multiband UV Photo-Detection

Abstract: A scalable configuration has been employed to advance the visible blind nature of ZnO-based UV detectors. The device with Au nanoparticles sandwiched between two ZnO layers, i.e., a ZnO/Au/ZnO configuration was found to be suitable for multiband, i.e., UV-A, B, C detection. The device has shown high UV to visible rejection ratio of 1.32 × 10 3 and UV to NIR rejection ratio of 8.79 × 10 3 with a photo-responsivity value of 10.64 AW -1 (at λ ex = 315 nm). The values for photo-detectivity, linear dynamic range, and external quantum efficiency were calculated to be ~7.51 × 10 12 cmHz 1/2 W -1 , 78 dB and ~4170%, respectively. It was perceived that insertion of Au plays a noteworthy role in the tuning of photo-responsivity, UV to a visible rejection ratio as well as to make ZnO suitable for multiband UV detection.

Intercalated water mediated electromechanical response of graphene oxide films on flexible substrates.

Abstract: The confinement of water between sub-nanometer bounding walls of layered two-dimensional materials has generated tremendous interest. Here, we examined the influence of confined water on the mechanical and electromechanical response of graphene oxide films, prepared with variable oxidative states, casted on polydimethylsiloxane (PDMS) substrates. These films were subjected to uniaxial strain under controlled humid environments (5 to 90 % RH), while dc transport studies were performed in tandem. Straining resulted in the formation of quasi-periodic linear crack arrays. The extent of water intercalation determined the density of cracks formed in the system thereby, governing the electrical conductance of the films under strain. The crack density at 5 % strain, varied from 0 to 3.5 cracks/mm for hydrated films and 8 to 22 cracks/mm for dry films, across films with different high oxidative states. Correspondingly, the overall change in the electrical conductance at 5 % strain was observed to be ~ 5 to 20 folds for hydrated films and ~ 20 to 35 folds for the dry films. The results were modeled with a decrease in the in-plane elastic modulus of the film upon water intercalation, which was attributed to the variation in the nature of hydrogen bonding network in graphene oxide lamellae.

Annual Review of Physical Chemistry

Abstract: This volume of the Annual Reviews of Physical Chemistry (ARPC67) once again brings out the sheer breadth of contemporary physical chemistry research. The so-called ‘middle kingdom’ is indeed rather vast – as is eminently clear from the various articles that range from processes happening inside a cell to tracking the motion of electrons. ARPC67 has a good balance between theory and experiments, applied and fundamental, representing frontline research on gas, condensed and solid phase systems.

2D Materials Enabled Next-Generation Integrated Optoelectronics: from Fabrication to Applications

Abstract: 2D materials, such as graphene, black phosphorous and transition metal dichalcogenides, have gained persistent attention in the past few years thanks to their unique properties for optoelectronics. More importantly, introducing 2D materials into silicon photonic devices will greatly promote the performance of optoelectronic devices, including improvement of response speed, reduction of energy consumption, and simplification of fabrication process. Moreover, 2D materials meet the requirements of complementary metal-oxide-semiconductor compatible silicon photonic manufacturing. A comprehensive overview and evaluation of state-of-the-art 2D photonic integrated devices for telecommunication applications is provided, including light sources, optical modulators, and photodetectors. Optimized by unique structures such as photonic crystal waveguide, slot waveguide, and microring resonator, these 2D material-based photonic devices can be further improved in light-matter interactions, providing a powerful design for silicon photonic integrated circuits.

Tunneling-current-induced local excitonic luminescence in p-doped WSe2 monolayers.

Abstract: We have studied the excitonic properties of exfoliated tungsten diselenide (WSe2) monolayers transferred to gold substrates using the tunneling current in a Scanning Tunneling Microscope (STM) operated in air to excite the light emission locally. In obtained spectra, emission energies are independent of the applied bias voltage and resemble photoluminescence (PL) results, indicating that, in both cases, the light emission is due to neutral and charged exciton recombination. Interestingly, the electron injection rate, that is, the tunneling current, can be used to control the ratio of charged to neutral exciton emission. The obtained quantum yield in the transition metal dichalcogenide (TMD) is ∼5 × 10−7 photons per electron. The proposed excitation mechanism is the direct injection of carriers into the conduction band. The monolayer WSe2 presents bright and dark defects spotted by STM images performed under UHV. STS confirms the sample as p-doped, possibly as a net result of the observed defects. The presence of an interfacial water layer decouples the monolayer from the gold support and allows excitonic emission from the WSe2 monolayer. The creation of a water layer is an inherent feature of the sample transferring process due to the ubiquitous air moisture. Consequently, vacuum thermal annealing, which removes the water layer, quenches excitonic luminescence from the TMD. The tunneling current can locally displace water molecules leading to excitonic emission quenching and to plasmonic emission due to the gold substrate. The present findings extend the use and the understanding of STM induced light emission (STM-LE) on semiconducting TMDs to probe exciton emission and dynamics with high spatial resolution.

Plasmonic Nanocavity Induced Coupling and Boost of Dark Excitons in Monolayer WSe2 at Room Temperature.

Abstract: Spin-forbidden excitons in monolayer transition metal dichalcogenides are optically inactive at room temperature. Probing and manipulating these dark excitons are essential for understanding exciton spin relaxation and valley coherence of these 2D materials. Here, we show that the coupling of dark excitons to a metal nanoparticle-on-mirror cavity leads to plasmon-induced resonant emission with the intensity comparable to that of the spin-allowed bright excitons. A three-state quantum model combined with full-wave electrodynamic calculations reveals that the radiative decay rate of the dark excitons can be enhanced by nearly 6 orders of magnitude through the Purcell effect, therefore compensating its intrinsic nature of weak radiation. Our nanocavity approach provides a useful paradigm for understanding the room-temperature dynamics of dark excitons, potentially paving the road for employing dark exciton in quantum computing and nanoscale optoelectronics.