Speaker: PRAGYA SHARMA . of Ph.D.(Engg)
Title: Engineering 2D Materials and Heterostructures for Advanced Gas Sensing and Molecular Memory Devices
Date/Time: Mar 24 / 15:00:00
Venue: Lecture Hall 1, Instrumentation and Applied Physics
Research Supervisor: Sanjiv Sambandan
Abstract: The increasing need for monitoring industrial and environmental health has spurred the search for advanced sensing materials to create cutting-edge gas sensing platforms. The inherent attributes of 2D materials, such as their high surface-area-to-volume ratio, tunable electronic properties, and strong adsorption capabilities, render them prime candidates for gas sensing applications. The ability to fabricate van der Waals heterostructures by integrating distinct 2D materials further expands the possibilities for tailoring sensor characteristics and achieving enhanced performance. The capacity to fine-tune the electronic properties of these materials through techniques like doping and functionalization further enhances their selectivity towards specific gases of interest. Unravelling the mechanisms behind these interactions will provide crucial knowledge for optimizing FET-based gas sensors and their potential for molecular memory applications. This thesis aims to explore the gas interaction mechanisms in the 2D materials and their heterostructures and develop strategies to enhance sensitivity and selectivity, and investigate novel functionalities like molecular memory. 2D Bi2S3 and MoSe2, including their strong adsorption capabilities, gate tunable conductivity and tunable bandgaps, make them promising materials for advancing gas sensing and molecular memory technologies. The ability to functionalize these materials further expands their potential for creating highly sensitive, selective, and efficient devices for various applications. Bi2S3 has emerged as a promising material for the detection of low-concentration NO2 at room temperature, attributed to their inherent physicochemical properties. Bi2S3 a member of metal chalcogenide possess direct bandgap (1.3-1.7 eV) which enhances charge carrier mobility, facilitating efficient electron transfer during gas adsorption. By utilizing its optical properties, the material gas sensing capabilities can be adjusted under visible light conditions. Additionally, the high surface area of Bi2S3 nanostructures provides numerous active adsorption sites, further improving its gas sensing performance. A Bi2S3-graphene heterostructure based FET is investigated for NO2 sensing at room temperature. The device exhibits a rectifying behavior, and its response to NO2 is studied under both dark and UV illu mination conditions. The results reveal that UV light enhances the adsorption and desorption of gas molecules, leading to a significant reduction in response time from 326.18s (dark) to 294.4s (UV) for 8 ppm NO2 exposure. Furthermore, the percentage recovery impro ves from 60.6% (dark) to 79.36% (UV). The ON current of the device decreases significantly from 437 nA to 143 nA upon exposure to 15 ppm NO2, highlighting its sensitivity. The interaction of gold-functionalized MoSe2 with H2S gas is also investigated. Few layer MoSe2 is functionalized with gold nanoparticles. The device characteristics are studied before and after the incorporation of nanoparticles. The shift in threshold voltage of the device suggests interfacial electron transfer between gold and MoSe2 owing to work function differences. The study demonstrates the ability to sense H2S concentrations as low as 100 ppb with a response of 16.18%. The incorporation of gold nanoparticles enhances the sensitivity of the device, the catalytic effect by the gold nanoparticles provides enhanced sensitivity for H2S. Further the MoSe2-Au device gate-tuned gas molecule sensitivity is studied. For varying back gate bias applied the MoSe2-Au heterojunction shows maximum sensitivity for gate bias of -10V. Moreover, the device is selective to H2S amongst other gases like NO2, and NH3. The incorporation of gold nanoparticles, has emerged as a strategic approach to enhance the gas sensing capabilities of MoSe2. This paves the way for the development of next-generation H2S sensors.
Furthermore, the thesis delves into the hysteresis phenomenon observed in MoSe2 FETs under different gas atmospheres. The research examines the impact of various factors, such as gate voltage sweep range and sweep rate, on the hysteresis window. The findings highlight the potential of utilizing the hysteresis effect in MoSe2 FETs for molecular memory applications, where the memory state can be written by gas injection (NO2 exposure) and erased by applying a gate pulse. The study further investigates the effect of different gate pulse heights and widths on the desorption of gas molecules and the resulting hysteresis behavior, demonstrating the potential for tunable and erasable memory devices.