Amit Pawbake

Highly Transparent Wafer-Scale Synthesis of Crystalline WS2 Nanoparticle Thin Film for Photodetector and Humidity-Sensing Applications.

In the present investigation, we report a one-step synthesis method of wafer-scale highly crystalline tungsten disulfide (WS2) nanoparticle thin film by using a modified hot wire chemical vapor deposition (HW-CVD) technique. The average size of WS2 nanoparticle is found to be 25-40 nm over an entire 4 in. wafer of quartz substrate. The low-angle XRD data of WS2 nanoparticle shows the highly crystalline nature of sample along with orientation (002) direction. Furthermore, Raman spectroscopy shows two prominent phonon vibration modes of E(1)2g and A1g at ∼356 and ∼420 cm(-1), respectively, indicating high purity of material. The TEM analysis shows good crystalline quality of sample. The synthesized WS2 nanoparticle thin film based device shows good response to humidity and good photosensitivity along with good long-term stability of the device. It was found that the resistance of the films decreases with increasing relative humidity (RH). The maximum humidity sensitivity of 469% along with response time of ∼12 s and recovery time of ∼13 s were observed for the WS2 thin film humidity sensor device. In the case of photodetection, the response time of ∼51 s and recovery time of ∼88 s were observed with sensitivity ∼137% under white light illumination. Our results open up several avenues to grow other transition metal dichalcogenide nanoparticle thin film for large-area nanoelectronics as well as industrial applications.

Temperature-Dependent Raman Spectroscopy of Titanium Trisulfide (TiS3) Nanoribbons and Nanosheets

Titanium trisulfide (TiS3) has recently attracted the interest of the 2D community because it presents a direct bandgap of ∼1.0 eV, shows remarkable photoresponse, and has a predicted carrier mobility up to 10000 cm(2) V(-1) s(-1). However, a study of the vibrational properties of TiS3, relevant to understanding the electron-phonon interaction that can be the main mechanism limiting the charge carrier mobility, is still lacking. In this work, we take the first steps to study the vibrational properties of TiS3 through temperature-dependent Raman spectroscopy measurements of TiS3 nanoribbons and nanosheets. Our investigation shows that all the Raman modes linearly soften (red shift) as the temperature increases from 88 to 570 K due to anharmonic vibrations of the lattice, which also includes contributions from the lattice thermal expansion. This softening with the temperature of the TiS3 modes is more pronounced than that observed in other 2D semiconductors, such as MoS2, MoSe2, WSe2, and black phosphorus (BP). This marked temperature dependence of the Raman spectra could be exploited to determine the temperature of TiS3 nanodevices by using Raman spectroscopy as a noninvasive and local thermal probe. Interestingly, the TiS3 nanosheets show a stronger temperature dependence of the Raman modes than the nanoribbons, which we attribute to lower interlayer coupling in the nanosheets.

Temperature dependent Raman spectroscopy of electrochemically exfoliated few layer black phosphorus nanosheets

The present investigation deals with temperature dependant Raman spectroscopy of electrochemically exfoliated few layer black phosphorus nanosheets. The temperature dependent study illustrates that softening of the A1g, B2g and A2g modes occurs as the temperature increases from 78 K to 573 K. The calculated temperature coefficients for the A1g, B2g and A2g modes were found to be −0.028 cm−1 K−1, −0.028 cm−1 K−1 and −0.018 cm−1 K−1 respectively. The observed phenomenon can be utilized for characterizing other emerging two-dimensional inorganic layered materials with atomic thickness.

Enhanced field emission behavior of layered MoSe2

Herein, we report one step facile chemical vapor deposition method for synthesis of single-layer MoSe2 nanosheets with average lateral dimension ~60 μm on 300 nm SiO2/Si and n-type silicon substrates and field emission investigation of MoSe2/Si at the base pressure of ~1 × 10−8 mbar. The morphological and structural analyses of the as-deposited single-layer MoSe2 nanosheets were carried out using an optical microscopy, Raman spectroscopy and atomic force microscopy. Furthermore, the values of turn-on and threshold fields required to extract an emission current densities of 1 and 10 μA cm−2, are found to be ~1.9 and ~2.3 V μm−1, respectively. Interestingly, the MoSe2 nanosheet emitter delivers maximum field emission current density of ~1.5 mA cm−2 at a relatively lower applied electric field of ~3.9 V μm−1. The long term operational current stability recorded at the preset values of 35 μA over 3 hr duration and is found to be very good. The observed results demonstrates that the layered MoSe2 nanosheet based field emitter can open up many opportunities for their potential application as an electron source in flat panel display, transmission electron microscope, and x-ray generation. Thus, the facile one step synthesis approach and robust nature of single-layer MoSe2 nanosheets emitter can provide prospects for the future development of practical electron sources.

High performance humidity sensor and photodetector based on SnSe nanorods

Tin selenide (SnSe) nanorods were synthesized using a one-step solvothermal route and their humidity sensing and photodetection performance at room temperature were investigated. The results depict that SnSe nanorod-based humidity and photosensors have good long-term stability, are highly sensitive and have fast response and recovery times. In the case of the humidity sensor it was observed that the resistance of the films decreased with increasing relative humidity (RH). The humidity sensing behaviors were investigated in the range 11–97% RH at room temperature. A response time of ~68 s and recovery time of ~149 s were observed for the humidity sensor. The photosensing behavior showed typical response /recovery times of ~3 s with highly reproducible behavior.

Effect of plasma treatment on multilayer graphene: X-ray photoelectron spectroscopy, surface morphology investigations and work function measurements

We report here the effect of plasma treatment on multilayer graphene samples as determined by X-ray photoelectron spectroscopy and surface morphology studies with atomic force microscopy, scanning electron microscopy and transmission electron microscopy. The plasma treatment was modified to introduce controlled levels of defects and functionalities to the graphene samples to give tunable properties. The elemental composition and structure were investigated by XPS and micro Raman spectroscopy. The XPS study showed that there was a slight variation in the sp2/sp3 hybridization ratio between the plasma-treated samples and the pristine sample. Kelvin probe measurements were carried out on all the multilayer graphene samples and indicated a slight variation in the work function of the graphene samples after plasma treatment.

Substrate temperature dependent studies on properties of chemical spray pyrolysis deposited CdS thin films for solar cell applications

Thin films of CdS have been prepared by chemical spray pyrolysis by spraying precursor solution directly onto soda lime glass (SLG) substrates. Influence of substrate temperature on structural, optical, morphological and electrical properties have been investigated by using various techniques such as low angle X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), UV–visible spectroscopy photoluminescence (PL) spectroscopy etc. Formation of CdS has been confirmed by low angle XRD, Raman spectroscopy and XPS analysis. XRD pattern showed that CdS films are polycrystalline, have hexagonal structure and prefer orientation of crystallites shifts from (101) to (002) with increase in substrate temperature. Raman spectroscopy revealed that exciton-phonon coupling depends on substrate temperature and hence on crystallite size. Optical band gap increased from 2.43 to 2.99 eV when substrate temperature increased from 325 to . Transmittance of the film also showed an increasing trend from to with increase in substrate temperature. Such high band gap and transmittance values of CdS films prepared at make it a useful window material in CdS/CdTe and CdS/Cu2S heterojunction solar cells.

Spatially branched CdS–Bi2S3 heteroarchitecture: single step hydrothermal synthesis approach with enhanced field emission performance and highly responsive broadband photodetection

This report explores the controlled hierarchical synthesis of CdS nanostructure branches on Bi2S3 nanorod cores via a facile single step hydrothermal route. Morphological and structural studies reveal the formation of CdS–Bi2S3 heteroarchitecture with excellent stoichiometry between the constituent elements. The growth of CdS over Bi2S3 strongly depends on optimization of the reaction conditions, especially low PVP concentration. Furthermore, the as-synthesized CdS–Bi2S3 heteroarchitecture demonstrates multifunctionality in field emission and photoresponse. Interestingly, the CdS–Bi2S3 heteroarchitecture shows enhanced field emission properties such as low turn-on field (∼1.8 V μm−1 for 10 μA cm2), high emission current density and better current stability in comparison to Bi2S3 and other nanostructures. The as-synthesized CdS–Bi2S3 heteroarchitecture exhibits considerable response and recovery times, ∼207 ms and 315 ms, respectively in comparison to bare Bi2S3 nanostructures (∼655 ms and 678 ms). The present results demonstrate CdS–Bi2S3 heteroarchitecture as a potential candidate for future optoelectronic device applications.

Synthesis of nanocrystalline silicon carbide thin films by HW-CVD using ethane carbon precursor for photo detector application

Hydrogenated nanocrystalline silicon carbide (nc-SiC:H) films were prepared by hot wire chemical vapor deposition (HW-CVD) method using ethane (C2H6) as a carbon precursor. The influence of deposition pressure on structural and optical properties was investigated. The formation of nc-SiC:H films was confirmed by low angle x-ray diffraction (XRD), Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy analysis. An inverse relation between deposition pressure and deposition rate was observed. Optical band gap values, ETauc and E04 increases with increase in deposition pressure. In fact, optical band gap values estimated from E04 method was found higher than the ETauc values calculated from Taucs plot. Finally, at optimized deposition pressure (450 mTorr), a photo detector having configuration glass/nc-SiC:H/Al have been fabricated and its photo response was studied. Further study is required to improve the quality of nc-SiC:H films to make use in photo detectors.