Pragati Kumar

Bio: Pragati Kumar is an academic researcher from University of Jammu. The author has contributed to research in topics: Thin film & Photoluminescence. The author has an hindex of 9, co-authored 25 publications receiving 273 citations. Previous affiliations of Pragati Kumar include University of Delhi & Bareilly College.

Nanotwinning and structural phase transition in CdS quantum dots

Abstract: Nanotwin structures are observed in high-resolution transmission electron microscopy studies of cubic phase CdS quantum dots in powder form by chemical co-precipitation method. The deposition of thin films of nanocrystalline CdS is carried out on silicon, glass, and TEM grids keeping the substrates at room temperature (RT) and 200°C by pulsed laser ablation. These films are then subjected to thermal annealing at different temperatures. Glancing angle X-ray diffraction results confirm structural phase transitions after thermal annealing of films deposited at RT and 200°C. The variation of average particle size and ratio of intensities in Raman peaks I2LO/I1LO with annealing temperature are studied. It is found that electron-phonon interaction is a function of temperature and particle size and is independent of the structure. Besides Raman modes LO, 2LO and 3LO of CdS at approximately 302, 603, and 903 cm−1 respectively, two extra Raman modes at approximately 390 and 690 cm−1 are studied for the first time. The green and orange emissions observed in photoluminescence are correlated with phase transition.

Nanotwinning in CdS quantum dots

Abstract: High resolution transmission electron microscopy, X-ray diffraction and photoluminescence measurements are carried out in order to study the defects in CdS quantum dots (QDs), synthesized in cubic phase by chemical co-precipitation method. The nanotwinning structures in CdS quantum dots (∼2.7 nm) are reported for the first time. Mostly CdS QDs are characterized by existence of nanotwin structures. The twinning structures are present together with stacking faults in some QDs while others exist with grain boundaries. Raman spectroscopy analysis shows intense and broad peaks corresponding to fundamental optical phonon mode (LO) and the first over tone mode (2LO) of CdS at 302 cm −1 and 605 cm −1 respectively. A noticeable shift is observed in Raman lines indicating the effect of phonon confinement. Fourier transform infrared spectroscopy analysis confirms the presence of Cd–S stretching bands at 661 cm −1 and 706 cm −1 . The photoluminescence spectrum shows emission in yellow and red regions of visible spectrum. The presence of stacking faults and other defects are explained on the basis of X-rays diffraction patterns and are correlated with photoluminescence spectrum. These nanotwinning and microstructural defects are responsible for different emissions from CdS QDs.

Gas sensing materials roadmap.

Abstract: Gas sensor technology is widely utilized in various areas ranging from home security, environment and air pollution, to industrial production. It also hold great promise in non-invasive exhaled breath detection and an essential device in future internet of things. The past decade has witnessed giant advance in both fundamental research and industrial development of gas sensors, yet current efforts are being explored to achieve better selectivity, higher sensitivity and lower power consumption. The sensing layer in gas sensors have attracted dominant attention in the past research. In addition to the conventional metal oxide semiconductors, emerging nanocomposites and graphene-like two-dimensional materials also have drawn considerable research interest. This inspires us to organize this comprehensive 2020 gas sensing materials roadmap to discuss the current status, state-of-the-art progress, and present and future challenges in various materials that is potentially useful for gas sensors.

Giant UV-sensitivity of ion beam irradiated nanocrystalline CdS thin films

Abstract: A highly sensitive UV-detector is devised for the first time from ion beam irradiated nanocrystalline CdS (nc-CdS) thin films. The UV-sensors are fabricated using pulsed laser deposited nc-CdS thin films on Si wafer and subsequent irradiation treatment. Swift heavy ion irradiation (SHII) of the nc-CdS thin films is carried out using 70 MeV 58Ni6+ ions. The sensors used in the present study are easy to fabricate and require inexpensive materials; they feature characteristics similar to those of UV sensors designed with complex structures and expensive procedures. Current–voltage (I–V) measurements reveal an enrichment in carrier concentration and improvement in conductivity after exposure to SHII. This giant conductivity may be attributed to an enhancement in sulfur vacancies as a consequence of SHII. The sensor exhibits improvements in the responsivity, photosensitivity, and efficiency as a function of the ion fluence and attains maximum values of ∼53 W A−1, 576.4% and 15.6 × 103% respectively for a film irradiated at a fluence of 1 × 1013 ions per cm2. The response time of the sensor reduces with the increase in ion fluence and reaches a minimum of a rise time of 165 ms and fall time of 65 ms at 3 V for these sensors. A possible mechanism involved in the SHII induced moderation of conductivity and consequently photosensitivity is explained on the basis of variation in the defect densities.

Recent Developments in One-Dimensional Inorganic Nanostructures for Photodetectors

Abstract: With large surface-to-volume ratios and Debye length comparable to their small sizes, one-dimensional inorganic nanostructures have extensively been investigated and widely used to fabricate high-performance nanoscale electronic and optoelectronic devices This feature article reviews the state-of-the-art research activities that focus on the one-dimensional inorganic nanostructures and their photodetector applications It begins with a survey of one-dimensional inorganic nanostructures and the fundamentals of photodetectors Some remarkable photoresponse characteristics are then presented, which are organized into sections covering several kinds of important nanostructures, such as ZnO, V 2 O 5 , ZnS, In 2 Se 3 , InSe, CdS, CdSe, ZnSe, Sb 2 Se 3 , ZrS 2 , Ag 2 S, and Zn x Cd 1-x Se Each section describes the corresponding photodetective properties in detail Finally, the article concludes with some perspectives and outlook on the future developments in the field

Pulsed Laser Deposited Dysprosium-Doped Gadolinium-Vanadate Thin Films for Noncontact, Self-Referencing Luminescence Thermometry.

Abstract: Lanthanide-doped vanadate thin films offer (i) a promising platform for luminescence-based noncontact temperature sensing; (ii) ratiometric/self-referencing absolute measurements; (iii) exceptional repeatability and reversibility for multirun uses and a long life cycle; (iv) 2% K(-1) maximum temperature sensitivity (among the highest recorded for inorganic nanothermometers); (v) a temperature resolution greater than 0.5 K; and (vi) the potential for high-resolution 2D temperature mapping.

The one phonon raman spectrum in microcrystalline silicon

Abstract: The red shift and the broadening of the Raman signal from microcrystalline silicon films is described in terms of a relaxation in the q-vector selection rule for the excitation of the Raman active optical phonons. The relationship between width and shift calculated from the known dispersion relation in c-Si is in good agreement with available data. An increase in the decay rate of the optical phonons predicted on the basis of the same model is confirmed experimentally.

Size Effects of Raman and Photoluminescence Spectra of CdS Nanobelts

Abstract: Different sizes of CdS nanobelts were synthesized at 800, 850, and 900 °C by the thermal evaporation of CdS powders on Au-coated silicon substrates and were used to study the size effects of Raman scattering and photoluminescent spectra. The Raman spectra of CdS nanobelts clearly exhibit first- and second-order longitudinal optical (LO) Raman peaks, surface phonon peaks, and multiphonon processes when excited using a wavelength of 532 nm. The mechanism of exciton–phonon coupling was observed to be mainly associated with the Frohlich interaction, and the coupling strength of the exciton–phonon increases with increasing lateral size. Compared with a larger CdS nanobelt, a narrower nanobelt exhibits a larger tensile strain. Recombination of free excitons (FX), excitons bound to neutral impurities (A0X), and donor–acceptor pairs (DAP) were identified from a low-temperature PL spectrum. At temperatures below ∼123 K, a red shift of the FX energy occurs with decreasing lateral size due to a larger uniaxial tensi...