Date of Award
Doctor of Philosophy
Material Science and Engineering
Anupama B. Kaul
Two-dimensional (2D) materials encompass a diverse array of properties from the metallic-like character of graphene to the semiconducting nature of many of the transition metal dichalcogenides (TMDCs) such as tungsten diselenide (WSe2). These materials exhibit intriguing interactions with incoming radiation which makes them interesting for electronics and optoelectronics, especially given the high electron mobility and the ballistic nature of the electronic transport under certain conditions. In particular, the semiconducting TMDCs, such as WSe2, express an indirect-to-direct optical transition with scaling from the bulk to monolayers. Despite the enhanced optical absorption characteristics of monolayer WSe2 stemming from its direct band gap, the optical absorption is nonetheless weakened to some extent, given the ultra-thin nature of the monolayer structures, which compromises the performance of photodetectors, by often reducing photoresponsivity and detectivity. On the other hand, zero-dimensional (0D) materials, such as fullerenes, endohedrals, gold nanoparticles and quantum dots often show superb light absorption characteristics over a wide range of incoming wavelengths, despite exhibiting poorer electronic transport characteristics. In this work, we have developed techniques to fabricate two-terminal structures of graphene and WSe2, where these materials have been integrated with 0D structures for the realization of hybrid 0D-2D molecular assemblies that synergistically leverage the positive attributes of each for optoelectronics. A combination of mechanical exfoliation and chemical vapor deposition was used to realize few-layer graphene and monolayer WSe2 on SiO2/Si substrates, respectively, where metal electrodes were deposited as electrical contacts. The hybrid molecular assemblies were characterized using Raman, Photoluminescence and Optical Absorption Spectroscopy, and Scanning Electron and Atomic Force Microscopy. Electronic and optoelectronic measurements were conducted before and after attachment of the 0D ensembles over a wide range of temperatures to explore the photo-induced charge transport in the structures. Such measurements have demonstrated the enhancement in the optoelectronic properties for the hybrid assemblies. The high photocurrent is attributed to the doping enhancement arising in 2D materials with the adsorption of fullerenes and endohedrals. For graphene-based hybrid structures, the high gain originates from the continuous hole doping in graphene. The holes in graphene move faster and can circulate multiple times through the circuit, giving a large photocurrent. In case of WSe2-based hybrid structures, the doping depends upon the PL intensity and the Coulomb interactions between the three-body system consisting of the excitons, positive trions, and negative trions. The PL intensity of monolayer WSe2 is enhanced drastically because of the reduction of the positive trion and enhancement of the neutral exciton formations through electron transfer from the 0D material. The measured responsivity for the graphene-fullerene hybrid was found to be 10 times higher compared to graphene-TMDC based hybrid structures; for WSe2-Sc3N@C80 hybrids, the responsivity was 102 times high compared to the bare WSe2 structure over a broad wavelength range from 400-1100 nm when Sc3N@C80 molecules were deposited on its surface. This work serves as a pivotal stepping-stone for future studies to explore the immense possibilities arising from 0D-2D hybrid structures for photodetector applications.
Received from ProQuest
Chugh, Srishti, "Photo-Induced Charge Transport In Graphene And Semiconducting Wse2 Integrated With Zero-Dimensional Materials For Enhancing Optoelectronic Device Characteristics" (2018). Open Access Theses & Dissertations. 1411.