Poonam Sehrawat

Growth of monolayer WS₂ of domain size beyond few microns is a challenge even today; and it is still restricted to traditional exfoliation techniques, with no control over the dimension. Here, we present the synthesis of mono- to few layer WS₂ film of centimeter² size on graphene-oxide (GO) coated Si/SiO₂ substrate using the chemical vapor deposition CVD technique. Although the individual size of WS₂ crystallites is found smaller, the joining of grain boundaries due to sp²-bonded carbon nanostructures (~3–6 nm) in GO to reduced graphene-oxide (RGO) transformed film, facilitates the expansion of domain size in continuous fashion resulting in full coverage of the substrate. Another factor, equally important for expanding the domain boundary, is surface roughness of RGO film. This is confirmed by conducting WS₂ growth on Si wafer marked with few scratches on polished surface. Interestingly, WS₂ growth was observed in and around the rough surface irrespective of whether polished or unpolished. More the roughness is, better the yield in crystalline WS₂ flakes. Raman mapping ascertains the uniform mono-to-few layer growth over the entire substrate, and it is reaffirmed by photoluminescence, AFM and HRTEM. This study may open up a new approach for growth of large area WS₂ film for device application. We have also demonstrated the potential of the developed film for photodetector application, where the cycling response of the detector is highly repetitive with negligible drift.

Presently, the negative electrodes of lithium-ion batteries (LIBs) are constituted by carbon-based materials, which exhibit a limited specific capacity 372 mAh g⁻¹ associated with the cycle in the composition between C and LiC₆. Therefore, many efforts are currently made towards the technological development of nanostructured graphene materials because of their extraordinary mechanical, electrical, and electrochemical properties. Recent progress on advanced hybrids based on graphene oxide (GO) and reduced graphene oxide (rGO) has demonstrated the synergistic effects between graphene and an electroactive material (silicon, germanium, metal oxides (MO_x)) as electrode for electrochemical devices. In this review, attention is focused on advanced materials based on GO and rGO and their composites used as anode materials for lithium-ion batteries.

In this report, we present highly sensitive Vis-NIR optical detector by utilizing the extraordinary optical and electrical properties of reduce graphene oxide. Graphene oxide (GO) was produced via Hummer's technique. Uniform GO film was deposited on a quartz substrate by the spin coating technique. During the oxidation process, a large number of functional groups (epoxy, carbonyl, carboxy and hydroxyl) attached to the surface and at the edges of the GO. Due to which GO becomes electrically insulator. To regain its electrical behavior, GO was thermally reduced at 250 °C for 1 h in inert atmosphere (Argon). The microstructure of the developed film was studied before and after reduction using TEM, SEM, Raman, and XRD analyses. The fabricated sensors were thoroughly investigated for different illumination wavelengths of light (i.e., 635, 785, and 1064 nm). The photo-illumination induced sensor response as change in current was measured while the incident laser power density was maintained at 1.4 mW/mm². As expected, there exists a linear relation between the illumination power density and photo response of the device. The maximum photoresponsivity of 1.2 mA/W was observed when illuminated with 635 nm wavelength laser at 1.4 mW/mm² power density. The fabricated sensors show excellent reproducibility of the photoresponse at different laser wavelengths without any significant drift at room temperature.

Present study reports the temperature sensing performance of Molybdenum Diselenide (MoSe₂) based thermistor. MoSe₂ has been hydrothermally synthesized and the thermistor is fabricated by drop cast method. Powder X-ray diffraction (XRD), Raman and scanning electron microscope (SEM) analysis have been used for characterizations of the prepared material. The temperature dependent sensor response shows good stability in the temperature range of 173–373K. Temperature coefficient of resistance (TCR) of the thermistor is found to be∼-0.51%/K in the temperature range of 273– 373K and it enhances to∼-1.75%/K in low temperature range (273–173K). The response and recovery time are found to be 88s and 94s in the temperature range of 273-373K respectively. This study gives a direction for the development of simple, economical and reproducible thermistor from MoSe₂.

The controlled modification of graphene’s electronic band structure poses serious challenges. In the present work, we study the effect of sp ² cluster size variation on the electronic band gap and photoconductive properties of reduced graphene oxide (RGO). This is achieved by performing reversible functionalization of RGO with oxygen species. The reversible functionalization of RGO results in its partial transformation to graphene oxide (GO) so that the size of the sp ² clusters within the sp ³ matrix varies, thereby affecting the π-π* band structure and photoconductive properties. The study reveals: (1) incremental creation/elimination of oxygenated surface bonds’ related energy states within the π-π^* band; (2) customized tuning of the sp ²/sp ³ ratio; (3) the presence/absence of oxygenated states impacts the optical transition processes both from band-to-band and oxygenated states; and (4) the incremental addition/depletion of surface states in a tunable manner directly influences the carrier transport in the photoconductive device. Experiments show a two-stage transformation of RGO electronic properties with changing oxygen functionalities: oxidation (Stage I) and decomposition or erosion (Stage II). Sp ² cluster size variation induced bandgap change was analyzed by Raman and photoluminescence studies, indicating the possibility for photodetection in a specific band encompassing NIR to UV, depending on the sp ²/sp ³ ratio. Energy-dispersive x-ray spectroscopy and Fourier transform infrared studies confirm the surface oxygenation/de-oxygenation during plasma treatment, and XRD confirms partial transformation of RGO to GO and its amorphization at higher plasma exposure times. In addition, the photodetector performance is optimized in terms of carrier generation-recombination and carrier-lattice scattering. Thus, manipulating better photoconductive response is possible through suitable handling of the parameters involved in the plasma treatment process. This is the first study on the influence of the sp ²/sp ³ ratio-induced lattice structure evolution on photodetection.

We report the fabrication and human trial of a novel wearable temperature sensor based on WS2-QDs/RGO; which performs instant measurement like thermometer in a wide temperature range: 77K-398 K, in both static- and instant mode. The device is simple, scalable, flexible and cost-effective, where nanoscience and technology played a vital role behind its concept and realization. The WS2-QDs/RGO heterostructure is developed by decorating WS2-QDs on pre-RGO coated cotton textile. In static mode, the crucial parameters such as temperature coefficient of resistance (TCR) and thermal hysteresis (Hth) were analyzed in depth to get the intricate mechanism behind the working of a temperature sensor; and check its worthiness to be a better candidate in the field of temperature sensor. Temperature sensing data at both high- and low temperatures are very much encouraging; and endorses its viability. Human trial is conducted to make reliable and hassle free temperature monitoring like thermometer where the sensor device is found capable to measure accurate body temperature with exceptional resolution i.e. the minimum change in temperature the device can measure is ~0.06K in addition to, fast response- and recovery time ~1.4 s and 1.7 s respectively. In every sense, the developed sensor has exhibited highest degree of superiority vis-a-vis its counterpart commercial thermometer used in healthcare. Besides, the device has passed through all deformation test successfully and proved its mettle. This sensor device proved its flexibility and stability under various mechanical deformation(s), showing its promising potential for future generation wearable health monitoring devices. To the best of our knowledge, this is the first report on WS2 in general, and WS2-QDs, in specific, based temperature sensing device and its operational demonstration as of now.

We document the fabrication and investigations of a novel photodetector based on a WS₂ quantum dots and reduced graphene oxide (RGO) (WS₂-QDs/RGO) heterostructure. The proposed photodetector is simple, scalable, cost-effective, and flexible and works in an ambient environment. An enhanced photodetection efficiency is observed due to the superior electronic properties of WS₂-QDs and excellent electrical as well as thermal properties of the carrier transportation medium, RGO. For device fabrication, GO and WS₂-QDs were separately synthesized via different chemistry followed by decorating WS₂-QDs on RGO coated cotton textile. Characterization studies confirm the transformation of exfoliated WS₂-2D flakes into WS₂-0D quantum dots and graphene oxide (GO) to RGO. The optimized photodetection performance of WS₂-QDs/RGO demonstrates its photoresponsivity of 5.22 mA W^–1 at 1.4 mW mm^–2 power density of a 405 nm illumination source. Other sensor parameters such as photosensitivity (∼20.2%), resolution (∼0.031 mW mm^–2 μA^–1), response time (1.57 s), recovery time (1.83 s), and specific detectivity (∼1.6 × 10⁶ jones) are found for WS₂-QDs/RGO sensor, and a few of these parameters are comparable and even superior to some of the devices as reported. Photosensing mechanism is explained in terms of charge transfer caused by appropriate band alignment across the interface between WS₂-QDs and RGO, where dimensionality and quantum confinement of nanostructures synergistically enhance the overall performance of the heterostructure. The device flexibility is examined through bending, stretching, and twisting experiments and successfully demonstrated its potentiality. Sensor performance even after being soaked in water and subsequent drying shows the possibility of reuse. The attributes of flexibility, high sensitivity and responsivity, superior resolution, and cost-effectiveness of our novel flexible photodetector indicate its promising potential for flexible and wearable optical detectors operating in UV band. Although negative photoconductance of the WS₂-QDs/RGO sensor is a major cause for not allowing the sensor to show its best performance, a trade-off is made with improved device design to qualify the expectations of being a competitive device, and this has been demonstrated with experimental facts.