Showing papers by "Mahesh Kumar published in 2016"

Effect of gamma irradiation on Schottky-contacted vertically aligned ZnO nanorod-based hydrogen sensor.

Abstract: We report the impact of gamma irradiation on the performance of a gold Schottky-contacted ZnO nanorod-based hydrogen sensor. RF-sputtered vertically aligned highly c-axis-oriented ZnO NRs were grown on Si(100) substrate. X-ray diffraction shows no significant change in crystal structure at low gamma doses from 1 to 5 kGy. As gamma irradiation doses increase to 10 kGy, the single crystalline ZnO structure converts to polycrystalline. The photoluminescence spectra also shows suppression of the near-band emission peak and the huge wide-band spectrum indicates the generation of structural defects at high gamma doses. At 1 kGy, the hydrogen sensor response was enhanced from 67% to 77% for 1% hydrogen in pure argon at a 150 °C operating temperature. However, at 10 kGy, the relative response decreases to 33.5%. High gamma irradiation causes displacement damage and defects in ZnO NRs, and as a result, degrades the sensor's performance as a result. Low gamma irradiation doses activate the ZnO NR surface through ionization, which enhances the sensor performance. The relative response of the hydrogen sensor was enhanced by ∼14.9% with respect to pristine ZnO using 1 kGy gamma ray treatment.

InN Quantum Dot Based Infra-Red Photodetectors.

Abstract: Self-assembled InN quantum dots (QDs) were grown on Si(111) substrate using plasma assisted molecular beam epitaxy (PA-MBE). Single-crystalline wurtzite structure of InN QDs was confirmed by X-ray diffraction. The dot densities were varied by varying the indium flux. Variation of dot density was confirmed by FESEM images. Interdigitated electrodes were fabricated using standard lithography steps to form metal-semiconductor-metal (MSM) photodetector devices. The devices show strong infrared response. It was found that the samples with higher density of InN QDs showed lower dark current and higher photo current. An explanation was provided for the observations and the experimental results were validated using Silvaco Atlas device simulator.

Luminescent GdVO4:Sm3+ quantum dots enhance power conversion efficiency of bulk heterojunction polymer solar cells by Förster resonance energy transfer

Abstract: In this work, we report enhanced power conversion efficiency (PCE) of bulk heterojunction polymer solar cells by Forster resonance energy transfer (FRET) from samarium-doped luminescent gadolinium orthovanadate (GdVO4:Sm3+) quantum dots (QDs) to polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7) polymer. The photoluminescence emission spectrum of GdVO4:Sm3+ QDs overlaps with the absorption spectrum of PTB7, leading to FRET from GdVO4:Sm3+ to PTB7, and significant enhancements in the charge-carrier density of excited and polaronic states of PTB7 are observed. This was confirmed by means of femtosecond transient absorption spectroscopy. The FRET from GdVO4:Sm3+ QDs to PTB7 led to a remarkable increase in the power conversion efficiency (PCE) of PTB7:GdVO4:Sm3+:PC71BM ([6,6]-phenyl-C71-butyric acid methyl ester) polymer solar cells. The PCE in optimized ternary blend PTB7:GdVO4:Sm3+:PC71BM (1:0.1:1.5) is increased to 8.8% from 7.2% in PTB7:PC71BM. This work demonstrates the potential of rare-earth based luminescent QDs in enhancing the PCE of polymer solar cells.

Improvement in the Sensing Response of Nano-Crystalline ZnO-Based Hydrogen Sensor: Effect of Swift Heavy Ion Irradiation

Abstract: Hydrogen gas sensing response of RF sputtered nano-crystalline ZnO thin film is improved by $\sim 34.73$ % using swift heavy ion irradiation technique. X-ray diffraction reveals single crystalline wurtzite structure of ZnO thin films. Full width half maximum (FWHM) of (0002) diffraction peak is decreased with increase in ion fluences. However, at very high fluence $1\times 10^{13}$ ions/cm2, crystallinity is degraded while FWHM of diffraction peak increases. The ZnO films were grown under highly compressive stress, which was relaxed with incremental ion fluences up to $1\times 10^{13}$ ions/cm2. Cathodoluminenscence spectroscopy shows intense predominant peak around $\sim 3.23$ eV, which indicates good optical quality of the films. As ion fluence increases, a blue shift appears in near band emission peak from 3.23 to 3.33 eV, which indicates that in-plane stress is decreased. Surface morphology shows generation of self-affine nanostructure and grain fragments as ion fluences were increased. Grain fragmentations had enhanced the surface reaction, and as a result, gas sensor’s relative response has improved. It was observed that the gas sensor’s sensitivity is strongly dependent on ion fluences and operating temperature. The sensitivity was enhanced from 66.68% to 89.84% with fluence variations from pristine to $1\times 10^{12}$ ions/cm2 at 175 °C operating temperature.

Modulation of Pb chemical state of epitaxial lead zirconate titanate thin films under high energy irradiation

Abstract: The chemical states of epitaxial PbZrxTi1-xO3 films were investigated by an X-ray photoelectron spectroscopy as a function of the gamma-ray doses. An anomalous behaviour was observed in Pb4f states, and a core level of Pb4f shifts towards a higher binding energy at 50 kGy and towards a lower binding energy at 200 kGy. The behaviour can be explained by a radiation induced reduction of PbO to metallic Pb. The metal-insulator-metal electrodes were fabricated by lithography, and the current-voltage characteristics were measured. A negative differential resistance (NDR) was observed in the leakage currents at room temperature. A higher current and disappearance of NDR characteristics were found in the 200 kGy irradiated samples, which further confirms the presence of metallic Pb.

Effect of Gamma Ray Irradiation on Epitaxial Pb(Zr,Ti)O 3 /SrRuO 3 Tunable Varactor Devices

Abstract: Epitaxial morphotropic PbZr0.52Ti0.48O3 (PZT) thin films were employed to enhance the dielectric tunability of microwave filter devices without compromising the device impedance matching and low bias voltage (<10 V) requirements. Epitaxial heterostructure of ferroelectric PZT(001)/SrRuO3 (SRO) were grown on single crystal SrTiO3 (001) substrates by pulsed laser deposition, and a platinum (Pt) electrode was deposited on top of the PZT film. The tunability of the Pt/PZT/SRO varactor devices are strongly dependent on bias voltage and exhibited good dielectric tunability of 55% at 100 kHz and 10 V. The capacitance (CP) of the heteroepitaxial varactor devices was 105 pF at 10 V applied bias with a corresponding small leakage current of 1 nA. The influence of gamma (γ-ray) irradiation on the intrinsic electrical properties of the epitaxial PZT varactor devices was investigated as a function of the irradiation dose from 0 kGy to 400 kGy, in terms of the capacitance–voltage (C–V) characteristics and loss tangent response. With enhancing γ-ray irradiation doses the ferroelectric capacitance was found to decrease accompanying degradation in the loss tangent values. The results indicate that tunability of the epitaxial PZT ferroelectric thin-film capacitors decreased with increasing gamma irradiation dose and degraded ∼25% at 400 kGy dose than unexposed devices. Possible reasons for the degradation behavior of dielectric properties and tunability due to radiation-induced defects has been discussed.

Effect of annealing on carrier transport properties of GaN-incorporated silicon

Abstract: GaN nanocrystals were formed in a silicon matrix by sequential implantation of Ga+ and N2+ ions followed by either Furnace Annealing (FA) or Rapid Thermal Annealing (RTA). The formation of Ga-rich clusters and Ga–N bonds was confirmed by X-ray photoelectron and photoluminescence spectroscopy. A Schottky top contact and ohmic bottom contact were fabricated and current–voltage (I–V) characteristics of the implanted samples were studied. Current values were found to be higher in RTA samples and lower in FA samples. It is observed that the height of the Schottky barrier strongly depends on the kind of annealing and is found to be 0.65 ± 0.02 eV for FA and 0.56 ± 0.02 eV for RTA compared to the value of 0.61 ± 0.02 eV for the pristine Si sample. A carrier transport mechanism is discussed based on experimental results for both kinds of annealing.

Childhood Somatoform Disorders and Its Associated Stressors

Abstract: Aim: To study the clinical and socio-demographic profile of somatoform disorders and its associated stressors among pediatric population in a rural setting. Settings and Design: This cross-sectional study was done from Jan 2015 to June 2015 at rural tertiary care teaching hospital. Methods and Materials: Fifty children presenting with physically unexplained symptoms during the study period were assessed. Complete physical and psychiatric assessment was done and diagnosis was made using DSM -IV TR criteria. Patient Health questionnaire-15 (PHQ-15) was used for recording presenting symptoms and severity. Data were analyzed with Statistical Package for social sciences (SPSS 17) and association between different variables was determined using Chi-square test. Results: Male: female ratio was 24:26. Forty-two (84%) of our patients were from rural background. Most common presenting symptom was fainting spells (28%) followed by pain in abdomen (20%). Stressors were elicited in 64% patients and most common stressor was strained interpersonal relations (20%) in family. Females presented with significantly more severe illness (p=0.024). Conclusions: Psychosocial stressors especially interpersonal relations are important factor which needs to be evaluated while encountering patients with somatoform disorder.

Electronic states in spherical GaN nanocrystals embedded in various dielectric matrices: The k ⋅ p-calculations

Abstract: Using the envelope-function approximation, the single-particle states of electrons and holes in spherical GaN nanocrystals embedded in different amorphous dielectric matrices (SiO2, Al2O3, HfO2 and Si3N4) have been calculated. Ground state energies of electrons and holes in GaN nanocrystals are determined using the isotropic approximation of the k ⋅ p -Hamiltonian. All the ground state energies are found to increase with lowering the nanocrystal size and are proportional to the R−n, where R is the nanocrystal radius, n =1.5-1.9 for electrons and 1.7-2.0 for holes. The optical gap of GaN nanocrystals changes from 3.8 to 5 eV for the nanocrystal radius ranging from 3 to 1 nm.

High Tolerance of BST Thin Film Based Varactor Under Neutron Irradiation

Abstract: We report neutron irradiation resistance of the barium strontium titanate (BST) thin-film-based varactor under different neutron fluences. BST thin films were deposited on sapphire substrate by RF magnetron sputtering and gold interdigital capacitor (IDC) electrodes were patterned with the photolithography technique. Capacitance-voltage ( $C$ – $V$ ) and leakage current characteristics were measured at several neutron fluences. A small change in capacitance is observed at different fluences. The changes are attributed to the radiation-induced defect dipoles and other charged defects, which serve as effective domain wall pinning sites. We supplemented this macroscopic $C$ – $V$ behavior of BST varactor with investigation through X-ray diffraction and atomic force microscopy for their structural and morphological changes, respectively. This paper shows that BST varactor can be used in neutron environment due to its high tolerance.

Performance Evaluation of Parabolic Solar Water Heater

Abstract: this paper was concerned with an experimental study of parabolic trough collector designed and manufactured. A parabolic trough solar collector uses GI sheet in the shape of a parabolic cylinder to reflect and concentrate sun radiations towards an absorber tube located at the focus line of the parabolic cylinder. The receiver absorbs the incoming radiations and transforms them into thermal energy, the latter being transported and collected by a fluid medium circulating within the absorber tube. The Designing and Fabrication of parabolic trough solar water heater for water heating was executed, the procedure employed includes design, construction and testing stages. The model which is made up of reflector surface, reflector support, absorber pipe and a stand with manual tracking arrangement was fabricated using locally sourced material for rural applications point of view. Performance evaluation of this system has been done during the months of February, at Gorakhpur 29° 27' 0" N , 75° 41' 0" E . Latitude and Longitude