Engineering the Exciton Dynamics and Transport in Atomically Thin Organic and Inorganic Semiconductor Materials

Abstract

Two-dimensional (2D) semiconductor nanomaterials have recently gathered significant attention due to their remarkable optical and electrical properties. Quantum confinement as resultant of reduced dimensions results in exciting optoelectronic phenomenon, which have never been reported before. This makes them promising candidates for future optoelectronics and photonics device applications. Many body interactions between fundamental particles in 2D semiconductors are strongly enhanced compared with those in bulk semiconductors, because of the reduced dimensionality and thus reduced dielectric screening. These enhanced many body interactions lead to the formation of robust quasi-particles, such as excitons, trions and bi-excitons, which are extremely important for the optoelectronics device applications of 2D semiconductors, such as light emitting diodes, lasers and optical modulators, etc. This thesis focusses on experimentally demonstrating properties of these many body interactions/quasi-particles and engineering them for future device optoelectronic device applications. After the discovery of graphene, similar other inorganic 2D materials such as transition metal dichalcogenides (TMDCs) have been extensively researched. But the research is still in its early stages and there have been few attempts to study and develop organic 2D materials. First half of this thesis focusses on the interesting optical properties and exciton dynamics from a new class of organic 2D materials grown through low-temperature vapor deposition process. I observed an exciting phenomenon of superradiance/supertransport experimentally from organic 2D materials which has several potential applications in ultrafast excitonic circuits, OLEDs, high-efficiency solar cells, quantum circuits and exciton-polariton devices such as Bose-Einstein condensates at room temperature. The thesis also explores the dynamics and spatial transport mechanism of excitons and trions in inorganic TMDC materials using near-field and time-resolved PL imaging. In the thesis, I have demonstrated an external engineering control for the exciton transport in monolayer TMDCs using back gate voltage, substantiating their use for faster long-range exciton transport applications such as excitonic transistors, quantum logic gates. The thesis further demonstrates a 1D organic nanowire with exciting optical properties that has potential applications in organic display screens, OFETs and lasers. The exciton dynamics have been studied and reported at various temperatures to understand the nature of excitons as the dimensionality changes from 2D to 1D. The thesis reports the first demonstration of Shpol' skii spectra and zero phonon lines from a solid state system. This was achieved through high crystalline and an ordered growth of organic molecules over hBN adsorbent surfaces. The thesis also demonstrates a functional FET device made incorporating organic 1D wire to demonstrate external engineering control of the Shpol' skii spectra valuable for future nanowire based device applications. Finally, the thesis demonstrates a first of its kind 2D organic-inorganic hybrid semiconductor heterojunction, which has high efficiency for light to electricity conversion ratio. The organic 2D material reported earlier in the thesis were used to form a heterojunction with inorganic 2D TMDC material to report interesting optical properties. The organic and inorganic part are both atomically thin and that resulted in exciting optical properties that have not been demonstrated before. In sum, this thesis is focused on exploring the fundamental excitonic dynamics and properties in novel 2D/1D organic, inorganic atomically thin semiconductor materials for their applications in next generation miniaturized optoelectronic, electronic and photonics devices.