Bablu Mukherjee

NIR Schottky Photodetectors Based on Individual Single-Crystalline GeSe Nanosheet

We have synthesized high-quality, micrometer-sized, single-crystal GeSe nanosheets using vapor transport and deposition techniques. Photoresponse is investigated based on mechanically exfoliated GeSe nanosheet combined with Au contacts under a global laser irradiation scheme. The nonlinearship, asymmetric, and unsaturated characteristics of the I-V curves reveal that two uneven back-to-back Schottky contacts are formed. First-principles calculations indicate that the occurrence of defects-induced in-gap defective states, which are responsible for the slow decay of the current in the OFF state and for the weak light intensity dependence of photocurrent. The Schottky photodetector exhibits a marked photoresponse to NIR light illumination (maximum photoconductive gain ∼5.3 × 10(2) % at 4 V) at a wavelength of 808 nm. The significant photoresponse and good responsitivity (∼3.5 A W(-1)) suggests its potential applications as photodetectors.

Complex electrical permittivity of the monolayer molybdenum disulfide (MoS 2 ) in near UV and visible

Temperature and Fermi energy dependent exciton eigenenergies of monolayer molybdenum disulfide (MoS2) are calculated using an atomistic model. These exciton eigen-energies are used as the resonance frequencies of a hybrid Lorentz-Drude-Gaussian model, in which oscillation strengths and damping coefficients are obtained from the experimental results for the differential transmission and reflection spectra of monolayer MoS2 coated quartz and silicon substrates, respectively. Numerical results compared to experimental results found in the literature reveal that the developed permittivity model can successfully represent the monolayer MoS2 under different biasing conditions at different temperatures for the design and simulation of MoS2 based opto-electronic devices.

Electrical and photoresponse properties of Co3O4 nanowires

Electrical and photocurrent characteristics of single Co3O4 nanowire devices were studied systematically. Current-voltage characteristics’ measurements and impedance spectroscopy of single Co3O4 nanowire devices were performed and analysed using possible mechanism. Photoresponses of individual nanowires were obtained by global irradiation of laser beams with photon energies above band gap and at sub-band gap of the nanowires. The magnitude of photocurrent and its response time revealed that defect level excitations significantly contribute to the photoresponse of Co3O4 nanowires. In addition, the electrically Ohmic nature of the nanowire/Pt contact and p-type conductivity of Co3O4 nanowire is extracted from the current-voltage characteristics and spatially resolved photocurrent measurements.

Stepped-surfaced GeSe2 nanobelts with high-gain photoconductivity

Single crystalline stepped-surfaced GeSe2 nanobelts (NBs) were synthesized by vapor transport and deposition method with the presence of Au catalyst. The NBs were grown via VLS mechanism with catalytic Au–Ge–Se alloy droplet formed at the tip of the NBs. The dynamic reshaping of the catalyst particle leads to formation of steps along the NB. Photodetectors comprising individually isolated NBs were fabricated to study their photodetection properties. The photoresponsivity of the devices was investigated at four different excitation wavelengths of 405 nm, 532 nm, 808 nm and 1064 nm. High photoresponsivity of ∼1040 A W−1 and a photoconductive gain of ∼121 800% was achieved at a wavelength of 1064 nm, suggesting that the excitation to defect-related energy states near or below the mid band-gap energy plays a major role in the generation of photocurrent in these highly stepped NB devices.

Photocurrent characteristics of individual GeSe2 nanobelt with Schottky effects

Single crystal GeSe2 nanobelts (NBs) were successfully grown using chemical vapor deposition techniques. The morphology and structure of the nanostructures were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffractometry, and Raman spectroscopy. Electronic transport properties, photoconductive characteristics, and temperature-dependent electronic characteristics were examined on devices made of individual GeSe2 nanobelt. Localized photoconductivity study shows that the large photoresponse of the device primarily occurs at the metal-NB contact regions. In addition, the electrically Schottky nature of nanobelt-Au contact and p-type conductivity nature of GeSe2 nanobelt are extracted from the current-voltage characteristics and spatially resolved photocurrent measurements. The high sensitivity and quick photoresponse in the visible wavelength range indicate potential applications of individual GeSe2 nanobelt devices in realizing optoelectronic switches.

K-Enriched WO3 Nanobundles: High Electrical Conductivity and Photocurrent with Controlled Polarity

Potassium ions are successfully intercalated into WO3 nanobundles with the integrity of the pseudo-orthorhombic structure remaining intact. The nanobundles display a 5-fold increase in the electrical conductivity. It changes from a value of 10(-4) Sm(-1) for pure WO3 to 40 Sm(-1) upon potassium intercalation. The electrical conductivity also increases by ~200 times as temperature increases from 23 to 200 °C whereby analysis shows a thermal activation energy of ~1 eV. Density functional theory calculations show that K ions cause the reduction of the surrounding W atoms and lead to an increase in the electron population in the conduction band. Hence, the conductivity of the K-WO3 nanobundles is greatly enhanced. The calculated band structure also shows a gap of 1 eV that is consistent with the measured thermal activation energy. Upon illumination of focused laser beam, individual and isolated nanobundle displays significant photon induced current (9 nA) without external bias at low laser power (2 mW); the amplitude and polarity of photocurrent could be controlled by location of laser spot.

Raman analysis of gold on WSe2 single crystal film

Synthesis and characterization of high-quality single-crystal tungsten diselenide (WSe2) films on a highly insulating substrate is presented. We demonstrate for the first time that the presence of gold (Au) nanoparticles in the basal plane of a WSe2 film can enhance its Raman scattering intensity. The experimentally observed enhancement ratio in the Raman signal correlates well with the simulated electric field intensity using both three-dimensional electromagnetic software and theoretical calculation considering layered medium coupled-dipole approximation (LM-CDA). This work serves as a guideline for the use of Au nanoparticles on WSe2 single-crystal thin films for surface enhanced Raman scattering (SERS) applications in the future.Synthesis and characterization of high-quality single-crystal WSe2 films on highly-insulating substrate is presented. We demonstrate for the first time that the presence of gold nanoparticles in the basal plane of a WSe2 film can enhance its Raman scattering intensity. The experimentally observed enhancement ratio in the Raman signal correlates well with the simulated electric field intensity using a three-dimensional electromagnetic software and theoretical calculation. This work provides guidelines for the use of two-dimensional WSe2 films as a SERS substrate.

Visibility of atomically-thin layered materials buried in silicon dioxide.

Recently, the coating of thin oxide or nitride film on top of crystals of atomically-thin layered material (ATLM) has been introduced, which benefits optical and electrical properties of the materials and shields them from environmental contact, and has important implications for optoelectronics applications of layered materials. By calculating the reflection contrast, we show the possibility of using an additional oxide film on top of ATLM with good average optical color contrast in broad- and narrow-band wavelength ranges. Our work presents a more comprehensive map of optical color contrast of various ATLMs including graphene, MoS2, MoSe2, WS2, and WSe2 when kept in a sandwich structure between two thin SiO2 films on a Si substrate. The average color contrasts of ATLMs with varying thicknesses of SiO2 films at three different wavelength ranges (i.e. broadband range, range for green filtering and range for red filtering) have been discussed with a summary of optimized thicknesses of the top and bottom oxide films in order to achieve the highest color contrast from the sandwich structures.

Reversible hysteresis inversion in MoS2 field effect transistors

The origin of threshold voltage instability with gate voltage in MoS2 transistors is poorly understood but critical for device reliability and performance. Reversibility of the temperature dependence of hysteresis and its inversion with temperature in MoS2 transistors has not been demonstrated. In this work, we delineate two independent mechanisms responsible for thermally assisted hysteresis inversion in gate transfer characteristics of contact resistance-independent multilayer MoS2 transistors. Variable temperature hysteresis measurements were performed on gated four-terminal van der Pauw and two-terminal devices of MoS2 on SiO2. Additional hysteresis measurements on suspended (~100 nm air gap between MoS2 and SiO2) transistors and under different ambient conditions (vacuum/nitrogen) were used to further isolate the mechanisms. Clockwise hysteresis at room temperature (300 K) that decreases with increasing temperature is shown to result from intrinsic defects/traps in MoS2. At higher temperatures a second, independent mechanism of charge trapping and de-trapping between the oxide and p+ Si gate leads to hysteresis collapse at ~350 K and anti-clockwise hysteresis (inversion) for temperatures >350 K. The intrinsic-oxide trap model has been corroborated through device simulations. Further, pulsed current–voltage (I–V) measurements were carried out to extract the trap time constants at different temperatures. Non-volatile memory and temperature sensor applications exploiting temperature dependent hysteresis inversion and its reversibility in MoS2 transistors have also been demonstrated.MoS 2 devices: variable temperature measurements unveil reversible hysteresis mechanismsDefects and traps in MoS2 van der Pauw devices give rise to a hysteresis inversion mechanism which is reversible with temperature. A team led by Saurabh Lodha at the Indian Institute of Technology Bombay performed variable temperature hysteresis measurements on four- and two-terminal MoS2 devices, both suspended and supported on a SiO2 substrate. The onset of a clockwise hysteresis at room temperature was attributed to intrinsic MoS2 defects, whereas an additional mechanism resulting in an anticlockwise hysteresis was observed at higher temperature, and attributed to extrinsic charge trapping and de-trapping between the oxide and the silicon gate. By leveraging the temperature dependence of the hysteresis in MoS2, the authors developed a non-volatile memory and a temperature sensor.

Direct laser micropatterning of GeSe2 nanostructures film with controlled optoelectrical properties

We demonstrate that a direct focused laser beam irradiation is able to achieve localized modification on GeSe2 nanostructures (NSs) film. Using a scanning focused laser beam setup, micropatterns on GeSe2 NSs film are created directly on the substrate. Controlled structural and chemical changes of the NSs are achieved by varying laser power and the treatment environment. The laser modified GeSe2 NSs exhibit distinct optical, electrical and optoelectrical properties. Detailed characterization is carried out and the possible mechanisms for the laser induced changes are discussed. The laser modified NSs film shows superior photoconductivity properties as compared to the pristine nanostructure film. The construction of micropatterns with improved functionality could prove to be useful in miniature optoelectrical devices.