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Two-dimensional (2D) MoS2 exhibits excellent optical, electrical, and mechanical properties, suggesting its great potential in the development of optoelectronic devices. Given its low electron affinity, MoS2 tends to form a Schottky contact with metal electrodes. Therefore, a back-to-back diode structure is usually formed in symmetric metal-MoS2-metal devices, and a weak rectifying effect is observed in such devices. Recent research progress in our group found that the self-aligned MoS2-Mo5N6 lateral heterostructure effectively lowers the barrier height and leads to an Ohmic-like contact between 2D MoS2 and Mo5N6, Hereby, we investigate the fabrication and performance improvement of a MoS2/Mo5N6 Schottky diode by eliminating one of the Schottky contacts in MoS2 devices through a Mo5N6-MoS2-Au asymmetric device structure. Our results show that by protecting the interface of MoS2 and Mo5N6, the resistance at the same voltage is significantly reduced. Compared with the MoS2/Mo5N6 Schottky diode without interface protection, the quality factor of the device with protected interface is lower, indicating that the device with protected interface is closer to an ideal Schottky diode.

In the development of next-generation electronic devices and photovoltaic (PV) solar cells to replace the conventional silicon technology, it is necessary that these devices are lightweight, economical and highly efficient. Novel nano-scale materials play a vital role in this technological transition. Particularly, low-dimensional semiconductors are of intense research interests owing to their unique electronic and optical properties. The reduced dimensionality compared with bulk materials results in lower dielectric screening and strong electron-electron interactions. Importantly, given the devices typically operate at room temperature, the scattering of electrons by phonons and phonon-assisted optical transitions are significant factors to be considered and controlled. In this dissertation, we apply first-principles simulations to understand and quantify exciton-phonon interactions in low-dimensional materials. For organic one-dimensional (1D) and inorganic monolayer two-dimensional (2D) materials, we utilize density functional theory (DFT) and many-body perturbation theory (MBPT) to describe the electronic properties, and a combination of MBPT and recently developed special displacement method (SDM) to describe optical properties under electron-phonon interactions. For both classes of materials, electron-phonon interactions are expected to be significant but are not well-understood. For a 1D π-stacked array of perylene diimide molecules, we demonstrate that phonon-assisted transitions lower the optical gap by 0.5 eV but do not result in significant modification of the exciton wavefunction, indicating that intermolecular coupling survives the presence of phonons. Hence, we expect that electron mobility and exciton delocalization will be maintained at room temperature. For 2D germanium selenide (GeSe), we explore the phonon-induced renormalization of the exciton wavefunctions, excitation energies, and oscillator strengths. We determine that the onset of optical absorption is red-shifted by 0.1 eV due to phonon-induced renormalization and that the 2D Wannier exciton is distorted but not localized. Both optical and acoustic phonons were found to couple to the excited-state, with the strongest coupling with optical phonons at ~ 100 cm−1 and indication of phonon-assisted inter-valley scattering of electrons. We also determined that the exciton-phonon coupling is similar between the bulk and the monolayer. Based on the lessons learned in this study, we have initiated a high-throughput framework to study electron-phonon couplings for a series of 2D materials extracted from a recently-developed database. In summary, by applying state-of-the-art first-principles theory, we extract fundamental physical properties related to electron-phonnon interactions. Such an understanding will allow for the design of materials with tailored properties for new nanoelectronic and optical devices.

π-stacked organic materials are tunable light absorbers with many potential applications in optoelectronics such as light emitting diodes, solar cells, and photocatalysts. Their optical properties are highly dependent on the nature and energy of electron-hole pairs or excitons formed upon light absorption, which are in turn determined by the intra- and inter-molecular electronic and vibrational excitations. In this dissertation, first principles methods such as density functional theory (DFT), time-dependent DFT (TDDFT), and a recently developed time-resolved non-adiabatic dynamics approach are used to understand excitons and their interactions with atomic vibrations. Perylene diimide (PDI) molecules are studied as a model system to gain physical insight about these phenomena. This class of materials is highly suitable for solar energy conversion because of the strong optical absorbance, efficient energy transfer, and chemical tunability. TDDFT, including vibronic effects, was applied to macromolecular DNA-based surrogates composed of one to three stacked PDI molecules, in order to understand the influence of electronic coupling to vibrational modes on the exciton. This approach is validated by comparison to experimental measurements and it was determined that intra- and inter-molecular interactions result in distinct vibrational, electronic, and optical properties. Additionally, exciton dynamics within these macromolecules is studied, simulating the internal energy decay from a high to lower energy excitonic state due to coupling of the excitation with atomic vibrations. It is shown that stacking leads to enhanced energy decay because of decreased energy spacing between states. Additionally, a new approach is presented to identify the vibrational modes that assist energy transfer, revealing that interactions between stacked molecules modulate the normal modes that couple to the exciton. Lastly, by studying the dynamics of the transition density, it is demonstrated that stacking impacts the localization of the exciton, a key feature of interest for solar energy conversion. For the dimer, the exciton quickly localizes and oscillates between two monomers, while the trimer can host long-time delocalization of the exciton. In summary, by applying first-principles theory, the coupling of inter-/intra-molecular electronic and vibrational excitations and their effects on energy transfer is identified. These findings provide fundamental understanding of the atomic-scale process associated with energy conversion, and provide insight towards rational design of new optoelectronic organic assemblies. / 2023-08-26T00:00:00Z

Two-dimensional (2D) non-van der Waals (vdW) transition metal carbides and nitrides (TMCs and TMNs) have emerged as new members of the 2D materials family with various applications. Their high conductivity and excellent chemical inertness are promising in electrical applications, especially when combined with other 2D semiconductors. Here, this dissertation will mainly focus on developing synthesis methods for 2D non-vdW TMCs and TMNs, investigating their crystal and domain structures, and exploring their application as conductive electrodes in 2D electronic devices. A series of synthesis methods for 2D non-vdW TMCs and TMNs will be demonstrated, including liquid metal growth, chemical conversion, and plasma-assisted conversion. Polarized Raman spectroscopy and electron microscopy are applied to investigate the domain structures and formation mechanisms of 2D non-vdW TMCs and TMNs, as well as their heterostructures. Electrical properties are examined for 2D non-vdW TMCs and TMNs and their heterostructures, showcasing their application as electrode materials in 2D electronics.

Mixed powders, particularly when the ingredients have different size, density and shape, tend to segregate during processing and handling, a process that has obvious technological and economic implications. Despite the well recognized consequences of powder segregation, this phenomenon, in food powders has not been systematically studied. Although food powders can almost invariably be regarded as cohesive, selective segregation of certain fine ingredients is not uncommon. This research was designed to investigate the segregation mechanism of food powders with special emphasis on the role of interparticle surface affinity between food powders of different chemical species in regulating the segregation intensity. Since established quantitative segregation criteria for food powders are non-existent, the project also dealt with the development and testing of such criteria and the selection of appropriate experimental procedures for their determination. In order to quantify and monitor segregation intensity, binary mixtures of crystalline, proteinaceous and starchy food powders were subjected to vertical vibration by a specially designed reciprocating device enabling the control of the vibration frequency and to horizontal vibration using a commercial type of shaker. Also, the composition distribution in a container filled with the mixtures was sampled at various locations. Binary mixtures were analyzed by Scanning Electron Microscopy before and after exposure to various levels of relative humidity. Based on their microstructure, they were classified as: random, ordered, partially random, and partially ordered randomized. Also studied were the bulk density and compressibility of binary mixtures, the effect of mixing order on food powder mixture structure and properties (e.g. color) and the migration of fine powders to the surface of particulates of different species as a result of selective affinity.

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