arXiv subjects

Sandip Dhara

Publications and source records attributed to Sandip Dhara.

At least 19 recordsLinked to original sources

arXiv2025

Enhanced THz emission and exciton transfer in monolayer MoS2/GaAs heterostructures

For designing an efficient terahertz (THz) emitter, the defect density of the semiconductors is smartly increased to reduce carrier lifetime, which subsequently lowers the overall power output of the semiconductor. To overcome this fundamental trade-off, this study presents a novel approach, by integrating a direct band gap 2D semiconductor such as monolayer MoS2 (1L-MoS2) with a well-known THz emitter, low-temperature-grown gallium arsenide (GaAs). The fabricated hybrid 2D/3D vertical van der Waals heterostructure showed a 15% higher THz emission compared to bare GaAs due to phase-coherent addition of second-order non-linear susceptibility, and overall enhancement in the electric field of laser. The photoluminescence (PL) enhancement factor of 2.38 in heterostructures at GaAs emission energy of 1.42 eV. However, the substantial quenching of PL emission for 1L-MoS2 at the energy of 1.84 eV, is attributed to the Dexter-type exciton transfer mechanism at type-I band alignment. THz time-domain spectroscopy reveals a significant increase in optoelectronic properties, such as optical conductivity becoming doubled, and a 50% reduction in absorption coefficients. The study introduces a new route for fabricating large-area and compact mixed-dimensional van der Waals heterostructures, which can be used to enhance the efficiency of conventional semiconductor technologies and THz-based optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Evaluation of strain and charge-transfer doping in wet-polymeric transferred monolayer MoS2: implications for field effect transistors

Two-dimensional materials offer exceptional tunability of electronic and optical properties via strain and doping engineering. However, the unintentional introduction of polymeric residues during wet chemical 2D film transfer processes such as wet chemical etching and surface energy assisted methods remains critical, yet unexplored. This study systematically investigates the impact of such residues on the optical and electrical properties of monolayer MoS\textsubscript{2} using Raman and photoluminescence spectroscopy. We reveal that polymer residues in transferred films from wet chemical etching induce distinct strain and doping behaviors: PMMA-existed regions exhibit biaxial tensile strain and \textit{p}-type doping, while PMMA-free regions show compressive strain. In contrast, the surface energy assisted transfer method introduces compressive strain and \textit{n}-type doping in the transferred film due to residue interactions. Field-effect transistor measurements corroborate these findings, showing polymer residue-influenced modulation of charge transport. Notably, the surface energy assisted technique minimizes transfer-induced defects, highlighting its superiority for fabricating high-performance 2D optoelectronic devices. These results underscore the critical role of transfer methodologies in tailoring optoelectronic properties and provide practical insights for optimizing 2D material integration in advanced technologies.

cond-mat.mtrl-sci↗

arXiv2024

Fabrication of high electron mobility and high photoluminescence quantum yield nanoscrolled monolayer MoS2

We fabricated the 1D nanoscrolled monolayer MoS2 (1L-MoS2) with superior characteristics from 1L-MoS2 film in a facile route, using a suitable organic solvent with optimum surface tension, evaporation rate and dielectric constant, which facilitates the controlled scroll formation. These nanoscrolls behave as multilayers in morphology and monolayer electronically. The nanoscrolls exhibited a direct optical gap with enhanced photoluminescence quantum yield stemming from the weak interlayer coupling among constituent layers and were corroborated by low-frequency Raman measurements and Kelvin probe force microscopy measurements. Furthermore, enhanced photoluminescence emission after annealing uncovers the thermal stability of nanoscrolls. In addition, conducting atomic force microscopy results exhibit a significantly higher photocurrent in the nanoscrolled 1L-MoS2 compared to the 1L-MoS2. We also realized significantly improved field effect transistor device parameters in nanoscrolled 1L-MoS2 devices. In nanoscrolled devices, we report the highest mobility value of 2400 cm2V-1s-1 reported in any form of 1L-MoS2.

cond-mat.mtrl-sci↗

arXiv2023

Determination of Thermal Conductivity of phase pure 10H-SiC Thin Films by non-destructive Raman Thermometry

10 H SiC thin films are potential candidates for devices that can be used in high temperature and high radiation environment. Measurement of thermal conductivity of thin films by a non-invasive method is very useful for such device fabrication. Micro-Raman method serves as an important tool in this aspect and is known as Raman Thermometry. It utilises a steady-state heat transfer model in a semi-infinite half space and provides for an effective technique to measure thermal conductivity of films as a function of film thickness and laser spot size. This method has two limiting conditions i.e. thick film limit and thin film limit. The limiting conditions of this model was explored by simulating the model for different film thicknesses at constant laser spot size. 10H SiC films of three different thicknesses i.e. 104, 135 and 156 nm were chosen to validate the thin film limiting condition. It was found that the ideal thickness at which this method can be utilised for calculating thermal conductivity is 156 nm. Thermal conductivity of 156 nm film is found to be 102.385 $(Wm^{-1}K^{-1})$.

physics.app-ph↗

arXiv2021

Novel Observation of Piezoelectricity in VO2

VO2 is well known for its dual phase transitions; electrical as well as structural, at a single temperature of 340K. The low temperature structural phases of VO2 are different from its high temperature counterpart by means of structural symmetry. The strain induced modification of the structural distortion in VO2 is studied in details. A ferroelectric type distortion is observed, and therefore, the piezoelectric effect in the low temperature phases of VO2 is investigated, for the first time, using piezo-response force microscopy. The electronic behavior of piezoelectric materials can be tuned with the application of mechanical strain and strain is the only factor to separate the two low-temperature phases, namely, M1 and M2 in the phase diagram of VO2. The piezo-electric coefficient in the strained phase of VO2 was found as 11-12 pm/V making it eligible for piezotronic applications.

cond-mat.mtrl-sci↗

arXiv2021

Effect of Oxygen and Aluminium Incorporation on Local Structure of GaN Nanowires: Insight from Extended X-ray Absorption Fine Structure Analysis

A thorough investigation of local structure, influencing macroscopic properties of the solid is of potential interest. We investigated the local structure of GaN nanowires (NWs) with different native defect concentration synthesized by the chemical vapor deposition technique. Extended X-ray absorption fine structure (EXAFS) analysis and semi-empirical and the density functional theory (DFT) calculations were used to address the effect of dopant incorporation along with other defects on the co-ordination number and bond length values. The decrease of the bond length values along preferential crystal axes in the local tetrahedral structure of GaN emphasizes the preferred lattice site for oxygen doping. The preferential bond length contraction is corroborated by the simulations. We have also studied the impact on the local atomic configuration of GaN NWs with Al incorporation. AlxGa1-xN NWs are synthesized via novel ion beam techniques of ion beam mixing and post-irradiation diffusion process. The change in the local tetrahedral structure of GaN with Al incorporation is investigated by EXAFS analysis. The analysis provides a clear understanding of choosing a suitable process for ternary III-nitride random alloy formation. The local structure study with the EXAFS analysis is corroborated with the observed macroscopic properties studied using Raman spectroscopy.

cond-mat.mtrl-sci↗

arXiv2021

The comparative defect study on the polymeric transfer of MoS2 monolayers

The defect-free transfer of chemical vapour deposition (CVD) grown monolayer MoS2 is important for both fabrication of 2D devices and fundamental point of view for various studies where substrate effects need to be minimized. Among many transfer techniques, two well-known techniques that use the polymer as carriers are wet-transfer technique and the surface-energy-assisted transfer technique. In this work, we transferred a single CVD grown monolayer MoS2 by these two transfer methods on a similar substrate, and the intervention of strain and defects in the transfer process is probed by Raman and photoluminescence (PL) spectroscopy, respectively. We found that the conventional and commonly used wet transfer technique degraded the monolayer due to KOH contamination. In contrast, monolayers transferred using the surface-energy-assisted transfer method possess structural integrity and optical quality on a par with the as-grown MoS2 layers. As compared to the wet process a strain-free transfer was recorded in the surface-energy-assisted technique using Raman spectroscopic studies.

cond-mat.mtrl-sci↗

arXiv2020

Current progress in vanadium oxide nanostructures and its composites as supercapacitor electrodes

In recent years, vanadium oxides have gained immense attention in the field of energy storage devices due to their low-cost, layered structure and multi-valency despite their limited electrical conductivity and lower structural stability. In this brief review, we have tried to focus on electrochemical properties of the stoichiometric vanadium oxides along with VO_x composites. The morphology engineering, doping with heteroatom and formation of composites with carbon-based materials and/or conducting polymers in enhancing the supercapacitive performances of the vanadium oxides are discussed in details. Finally, the potentiality and challenges of vanadium oxides nanocomposites for supercapacitor applications are discussed.

physics.app-ph↗

arXiv2019

Role of polarized tip-enhanced Raman spectroscopy in the enhancement of interface optical phonon modes in AlGaN multi-quantum wells

Group III nitride based two-dimensional multi-quantum well (MQW) nanostructures find remarkable applications in the visible to ultraviolet light sources. The interface optical (IFO) phonon modes in a c-axis oriented superlattice of [Al0.35Ga0.65N (~1.75 nm)/Al0.55Ga0.45N (~2nm)]20 MQWs are observed using tip-enhanced Raman spectroscopic (TERS) studies. The near-field studies using TERS probe with an Au spherical nanoparticle of ~ 200 nm diameter were carried out at ambient conditions showing approximately two to three orders of enhancement in the Raman intensities. The interface phonon mode belonging to E1 symmetry [IFO(E1)] vibrating normal to the c-axis of MQWs appeared to be more prominent in the case of TERS measurement compared to that for the other interface phonon mode of A1 symmetry. The confined electric field of the polarized electro-magnetic excitation using TERS probe, parallel to the plane of the interface of MQW, is made responsible for the plasmonic enhancement of IFO(E1) phonon mode. The confinement was verified using finite-difference time-domain simulation.

cond-mat.mtrl-sci↗

arXiv2019

Site-substitution in GdMnO3 : effects on structural, electronic and magnetic properties

We report on detailed structural, electronic and magnetic studies of GdMn$_{1-x}$Cr$_x$O$_3$ for Cr doping levels 0 $\le$ $x$ $\le$ 1. In the solid solutions, the Jahn-Teller distortion associated with Mn$^{3+}$ ions gives rise to major changes in the ${bc}$-plane sub-lattice and also the effective orbital ordering in the ${ab}$-plane, which persist up to the compositions $x$ $\sim$ 0.35. These distinct features in the lattice and orbital degrees of freedom are also correlated with $bc$-plane anisotropy of the local Gd environment. A gradual evolution of electronic states with doping is also clearly seen in O $K$-edge x-ray absorption spectra. Evidence of magnetization reversal in field-cooled-cooling mode for $x$ $\ge$ 0.35 coinciding the Jahn-Teller crossover, suggests a close correlation between magnetic interaction and structural distortion. These observations indicate a strong entanglement between lattice, spin, electronic and orbital degrees of freedom. The nonmonotonic variation of remnant magnetization can be explained by doping induced modification of magnetic interactions. Density functional theory calculations are consistent with a layer-by-layer type doping with ferromagnetic (antiferomagnetic) coupling between Mn (Cr) ions for intermediate compound ($x$ = 0.5), which is distinct from that observed for the end members GMnO$_3$ and GdCrO$_3$.

cond-mat.mtrl-sci↗

arXiv2018

Surface enhanced Raman spectroscopy using 2D plasmons of InN nanostructures

We explored the surface enhanced Raman scattering (SERS) activity of the InN nanostructures, possessing surface electron accumulation (SEA), using the Rhodamine 6G (R6G) molecules. SERS enhancement is observed for the InN nanostructures which possess surface electron accumulation (SEA). In case of high temperature grown InN samples, a peak is observed in the low wave number (THz region) of Raman spectra of InN nanostructures originating from excitation of the two dimensional (2D) plasmons of the SEA. The enhancement factor of four orders was calculated with the assumption of monolayer coverage of analyte molecule. SERS enhancement of InN nanostructures is attributed to the 2D plasmonic nature of InN nanostructures invoking SEA, rather than the contributions from 3D surface plasmon resonance (SPR) and chemical interaction. The role of 2D plasmon excitation in SERS enhancement is corroborated by the near-field light-matter interaction studies using near-field scanning optical microscopy.

cond-mat.mtrl-sci↗

arXiv2018

Native defect assisted enhanced response to CH4 near room temperature by Al0.07Ga0.93N nanowires

Gas sensors at low operating temperature with high sensitivity are the demand for the group III nitrides owing to their high chemical and thermal stability. The CH4 sensing is realized for the Al0.07Ga0.93N nanowires (NWs) with an improved response over the GaN NWs at a low operating temperature of 50 °C, for the first time. Al0.07Ga0.93N NWs were synthesized via ion beam mixing process using inert gas ion irradiation on the bilayer of Al/GaN NWs. The sensing mechanism is explained with the help of native defects present in the system. The number of shallow acceptors created by Ga vacancy (V_Ga) is found to be higher in Al0.07Ga0.93N NWs than those in the as-grown GaN NWs. The role of O antisite defect (ON) for the formation of shallow V_Ga is inferred from photoluminescence spectroscopic analysis. These native defects strongly influence the gas sensing behaviour resulting in the enhanced and low temperature CH4 sensing.

cond-mat.mtrl-sci↗

arXiv2018

Effect of Scattering Efficiency in the Tip Enhanced Raman Spectroscopic Imaging of Nanostructures in the Sub Diffraction Limit

The experimental limitations in the signal enhancement and spatial resolution in spectroscopic imaging have been always a challenging task in the application of near-field spectroscopy for nanostructured materials in the sub-diffraction limit. In addition, the scattering efficiency also plays an important role in improving signal enhancement and contrast of the spectroscopic imaging of nanostructures by scattering of light. We report the effect of scattering efficiency in the Raman intensity enhancement, and contrast generation in near-field tip-enhanced Raman spectroscopic (TERS) imaging of one dimensional inorganic crystalline nanostructures of Si and AlN having a large variation in polarizability change. The Raman enhancement of pure covalently bonded Si nanowire (NW) is found to be two orders of higher in magnitude for the TERS imaging, as compared to that of AlN nanotip (NT) having a higher degree of ionic bonding, suggesting the importance of scattering efficiency of the materials in TERS imaging. The strong contrast generation due to higher signal enhancement in TERS imaging of Si NW also helped in achieving the better resolved spectroscopic images than that of the AlN NT. The study provides an insight into the role of scattering efficiency in the resolution of near-field spectroscopic images.

cond-mat.mtrl-sci↗

arXiv2018

Spectroscopically forbidden infra-red emission in Au-vertical graphene hybrid nanostructures

Implementation of Au nanoparticles (NPs) is a subject for frontier plasmonic research due to its fascinating optical properties. Herein, the present study deals with plasmonic assisted emission properties of Au NPs-vertical graphene (VG) hybrid nanostructures. The influence of effective polarizability of Au NPs on the surface enhanced Raman scattering and luminescence properties is investigated. In addition, a remarkable infra-red (IR) emission in the hybrid nanostructures is observed and interpreted on the basis of intra-band transitions in Au NPs. The flake-like nanoporous VG structure is invoked for the generation of additional confined photons to impart additional momentum and a gradient of confined excitation energy towards initiating the intra-band transitions of Au NPs. Integrating Au plasmonic materials in three-dimensional VG nanostructures enhances the light-matter interactions. The present study provides a new adaptable plasmonic assisted pathway for optoelectronic and sensing applications.

physics.app-ph↗

arXiv2018

Observation of surface plasmon polaritons in 2D electron gas of surface electron accumulation in InN nanostructures

Recently, heavily doped semiconductors are emerging as an alternate for low loss plasmonic materials. InN, belonging to the group III nitrides, possesses the unique property of surface electron accumulation (SEA) which provides two dimensional electron gas (2DEG) system. In this report, we demonstrated the surface plasmon properties of InN nanoparticles originating from SEA using the real space mapping of the surface plasmon fields for the first time. The SEA is confirmed by Raman studies which are further corroborated by photoluminescence and photoemission spectroscopic studies. The frequency of 2DEG corresponding to SEA is found to be in the THz region. The periodic fringes are observed in the near-field scanning optical microscopic images of InN nanostructures. The observed fringes are attributed to the interference of propagated and back reflected surface plasmon polaritons (SPPs). The observation of SPPs is solely attributed to the 2DEG corresponding to the SEA of InN. In addition, resonance kind of behavior with the enhancement of the near-field intensity is observed in the near-field images of InN nanostructures. Observation of SPPs indicates that InN with SEA can be a promising THz plasmonic material for the light confinement.

cond-mat.mtrl-sci↗

arXiv2018

Spectroscopic study of native defects in the semiconductor to metal phase transition in V2O5 nanostructure

Vanadium is a transition metal with multiple oxidation states and V2O5 is the most stable form among them. Besides catalysis, chemical sensing and photo-chromatic applications, V2O5 is also reported to exhibit a semiconductor to metal transition (SMT) at a temperature range of 530-560K. Even though, there are debates in using the term 'SMT' for V2O5, the metallic behavior above transition temperature and its origin are of great interests in the scientific community. In this study, V2O5 nanostructures were deposited on SiO2/Si substrate by vapour transport method using Au as catalyst. Temperature dependent electrical measurement confirms the SMT in V2O5 without any structural change. Temperature dependent photoluminescence analysis proves the appearance of oxygen vacancy related peaks due to reduction of V2O5 above the transition temperature, as also inferred from temperature dependent Raman spectroscopic studies. The newly evolved defect levels in the V2O5 electronic structure with increasing temperature is also understood from the downward shift of the bottom most split-off conduction bands due to breakdown of pdπ bonds leading to metallic behavior in V2O5 above the transition temperature.

cond-mat.mtrl-sci↗

arXiv2017

VO2 microcrystals as advanced smart window material at semiconductor to metal transition

Textured VO2(011) microcrystals are grown in the monoclinic, M1 phase which undergo a reversible first order semiconductor to metal transition (SMT) accompanied by a structural phase transition to rutile tetragonal, R phase. Around the phase transition, VO2 also experiences noticeable change in its optical and electrical properties. A change in color of the VO2 micro crystals from white to cyan around the transition temperature is observed, which is further understood by absorption of red light using temperature dependent ultraviolet-visible spectroscopic analysis and photoluminescence studies. The absorption of light in the red region is explained by the optical transition between Hubbard states, confirming the electronic correlation as the driving force for SMT in VO2. The thermochromism in VO2 has been studied for smart window applications so far in the IR region, which supports the opening of the band gap in semiconducting phase; whereas there is hardly any report in the management of visible light. The filtering of blue light along with reflection of infrared above the semiconductor to metal transition temperature make VO2 applicable as advanced smart windows for overall heat management of a closure.

cond-mat.mtrl-sci↗

arXiv2017

Local inversion symmetry breaking and spin-phonon coupling in perovskite GdCrO3

Our detailed temperature dependent synchrotron powder x-ray diffraction studies along with first-principles density functional perturbation theory calculations, enable us to shed light on the origin of ferroelectricity in GdCrO3. The actual lattice symmetry is found to be noncentrosymmetric orthorhombic Pna21 structure, sup- porting polar nature of the system. Polar distortion is driven by local symmetry breaking and by local distortions dominated by Gd off-centering. Our study reveals an intimate analogy between GdCrO3 and YCrO3. However, a distinctive difference exists that Gd is less displacive compared to Y, which results in an orthorhombic P na21 structure in GdCrO3 in contrast to monoclinic structure in YCrO3 and consequently, decreases its polar property. This is due to the subtle forces involving Gd-4f electrons either directly or indirectly. A strong magneto-electric coupling is revealed using Raman measurements based analysis in the system below Cr-ordering temperature, indicating their relevance to ferroelectric modulation.

cond-mat.str-el↗