Sandesh R. Jadkar

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.

Efficient dye-sensitized solar cells based on hierarchical rutile TiO2 microspheres

Dye-sensitized solar cells (DSSCs) are fabricated based on rutile TiO2 microspheres that are synthesized by a hydrothermal route. We found that, with increasing deposition time, semi microspheres get converted into microspheres. Our results show that the TiO2-based cells exhibit a noticeable improvement in the overall efficiency: maximum 3.81% versus 0.67% for the reference cell made of a rutile TiO2 semispherical nanocrystalline film. This extraordinary result is attributed to the effective light trapping and dye loading resulting in the highest efficiency 3.81%.

The role of hydrogen dilution of silane and phosphorus doping on hydrogenated microcrystalline silicon (μc-Si:H) films prepared by hot-wire chemical vapor deposition (HW-CVD) technique

Abstract The electrical, structural and optical properties of undoped and phosphorus doped μc-Si:H films prepared by a HW-CVD technique have been studied. The hydrogen (H 2 ) dilution of silane has been varied carefully to produce undoped μc-Si:H films. The amorphous-to-microcrystalline transition was observed for a hydrogen dilution ratio >0.75. The phosphorus doped μc-Si:H films were deposited by varying the phosphine (PH 3 ) gas flow rate. The structural properties of these films have been investigated by Raman spectroscopy, low angle X-ray diffraction spectroscopy and Fourier transform infrared vibrational spectroscopy. Electrical characterization has been carried out by dark conductivity and charge carrier activation energy measurements. The phosphorus doped μc-Si:H films showed that the addition of PH 3 to the source gases promotes the growth of crystallinity. The increase in crystallite size and crystalline volume fraction with the addition of PH 3 to the source gases indicates that it enhances the crystallization of the μc-Si:H film. Low angle XRD studies shows that the PH 3 doped μc-Si:H does not show any preferential orientation crystallites. For optimized deposition conditions PH 3 doped μc-Si:H films with high dark conductivity (0.4 S/cm), low activation energy (0.03 eV) and high band gap (1.82 eV) were obtained with a high deposition rate (13 A/s). However, for these optimized conditions, the hydrogen content was relatively large (8.3 at.%).

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.

Dramatic Enhancement in Photoresponse of β-In2S3 through Suppression of Dark Conductivity by Synthetic Control of Defect-Induced Carrier Compensation

We report on the synthesis of dense and faceted indium sulfide (β-In2S3) nano-octahedron films on fluorine-doped tin oxide-coated glass by the hydrothermal method and their photoresponse properties in a flip chip device configuration. We have examined the temporal evolution of the phase constitution, morphology, and optoelectronic properties for films obtained after growth interruption at specific intervals. It is noted that, initially, an In(OH)3 film forms, which is gradually transformed to the β-In2S3 phase over time. In the case of the film wherein most, but not all, of In(OH)3 is consumed, an exceptionally large photoresponse (light to dark current ratio) of ∼10(4) and response time(s) (rise/fall) of ∼88/280 ms are realized. This superior performance is attributed to nearly complete carrier compensation achievable in the system under high pressure growth leading to dramatic reduction of dark conductivity. It is argued that the temporally growth-controlled equilibrium between quasi-In interstitials and cation vacancies dictates the optoelectronic properties.

Narrow band gap, high photosensitivity a-SiGe:H films prepared by hot wire chemical vapor deposition (HW-CVD) method

Abstract In this letter, we report narrow band gap (1.39–1.53 eV) a-SiGe:H films with high photosensitivity (∼10 4 –10 5 ) are grown successfully by HW-CVD using a mixture of (GeH 4 +SiH 4 ) at low flow rates and without hydrogen dilution with higher deposition rates (>10 A/s). These films are characterized by Raman spectroscopy, FTIR spectroscopy and UV–Visible spectroscopy. The band gap of a-SiGe:H films can be narrowed by increasing the germane gas fraction without apparent degradation in their electronic properties. The low hydrogen content in a-SiGe:H films indicates that the growth of a-SiGe:H films is mainly from the atomic species (Si, Ge and H) evaporated from the hot filament.

Hydrogenated Nanocrystalline Silicon Thin Films Prepared by Hot-Wire Method with Varied Process Pressure

Hydrogenated nanocrystalline silicon films were prepared by hot-wire method at low substrate temperature (200∘C) without hydrogen dilution of silane (SiH4). A variety of techniques, including Raman spectroscopy, low angle X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, atomic force microscopy (AFM), and UV-visible (UV-Vis) spectroscopy, were used to characterize these films for structural and optical properties. Films are grown at reasonably high deposition rates (>15 Å/s), which are very much appreciated for the fabrication of cost effective devices. Different crystalline fractions (from 2.5% to 63%) and crystallite size (3.6–6.0 nm) can be achieved by controlling the process pressure. It is observed that with increase in process pressure, the hydrogen bonding in the films shifts from Si–H to Si–H2 and complexes. The band gaps of the films are found in the range 1.83–2.11 eV, whereas the hydrogen content remains <9 at.% over the entire range of process pressure studied. The ease of depositing films with tunable band gap is useful for fabrication of tandem solar cells. A correlation between structural and optical properties has been found and discussed in detail.

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.