Theorectical Investigation on Silicene and Bismuth Based Nano-structured Materials


Student thesis: Doctoral Thesis

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Awarding Institution
Award date21 Nov 2019


Two-dimensional silicene has aroused significant attention after the rapid research and development about graphene, owing to its similar structure and electronic properties to graphene. Outstanding performance of graphene as biosensors and gas sensor leads us to search for potential applications of silicene-based material in this area. The study of silicene-based material is therefore the main focus of this thesis.

First, based on density functional theory (DFT) calculations, the interaction between glycine and intrinsic and metal-doped (Ni, Cr, Fe and Mn) silicene was studied. The configurations of carboxylic acid group coordination and amine group coordination on the silicene were checked. The results showed that glycine on intrinsic silicene was physisorption whereas glycine was chemisorbed on Ni, Cr Fe and Mn-doped silicene. Comparatively, the glycine preferred to coordinate with the metal-doped silicene through metal-amine group coordination with relatively high binding energy. The analysis of density of states (DOS) showed obvious orbital hybridization even spin polarization, while the electron density showed electrons accumulating between the glycine and metal-doped silicene. These results suggest that the application of silicene-based materials for amino acids detection is feasible.

As a further exploration of silicene-based material as gas sensors, the adsorption characteristics of small gas molecules (CO2, CO, H2O, N2, NO, NH3, NO2 and O2) on trivacancy and Stone-Wales defected silicene were investigated by density functional theory calculations. The results revealed that CO, H2O and N2 were absorbed on trivacancy silicene in a physical way via van der Waals forces, whereas CO2, NO, NH3, NO2 and O2 were chemisorbed on trivancancy silicene via strong covalent (SisbndN or SisbndO) bonds. For Stone-Wales defected silicene, N2, H2O and CO2 were physisorbed whereas CO, O2, NO, NH3, and NO2 were chemisorbed. The chemisorption of gas molecules on trivancancy and Stone-Wales defected silicene widens the band gap. In particular, the NO2 chemisorption on trivacancy or Stone-Wales defective silicene showed significant hole doping. Additionally, O2 was found to be readily decomposed into two O atoms on trivacancy silicene due to the small energy barrier and large exothermic reaction heat. Our work offers a possible method to regulate the electronic properties of silicene for applications at the nanoscale.

Silicene quantum dots (SiQDs) have also shown great potential in multifarious optical and electronic applications. By theoretically investigating the effect of surface functionalization of oxygen-containing groups (-O-, -OH, -OCH3 and -COCH3) on the electronic and optical properties of SiQD by DFT and TDDFT methods, it was found that the absorption and emission properties of the systems changed markedly with the functionalized positions and concentration. Remarkably, given that the optical gap of the SiQD is just 1.29 eV, the optical gap difference between two SiQDs with epoxide isomers can be as large as 0.60 eV. The notably small optical gaps (0.71, 0.69 and 0.82 eV) in specific SiQD with epoxide configurations indicate that each kind of these chemical groups may greatly facilitate non-radiative decay in photo-excited SiQDs, and consequently cause diminished photoluminescence efficiency. All the optical characteristics of the selectively oxidized SiQDs were rationalized by electronic structure alterations upon SiQD surface functionalization. The functional group-concentration influence and Stokes shifting were also uncovered. This study sheds light on the role of O-containing groups in affecting the optical properties of SiQDs and related materials and provides a valuable reference for further experiments.

Finally, it is challenging to achieve high thermoelectric efficient materials due to the conflict between thermopower (Seebeck coefficient) and electrical conductivity. These parameters are the core factors defining the thermoelectric property of any material. Here isovalent substitution was used as a tool to decouple the interdependency of the Seebeck coefficient and the electrical properties of cerium doped bismuth selenide thermoelectric material. With this strategy, simultaneous increase of both electrical conductivity and Seebeck coefficient of the material was achieved by tuning the concentration of cerium doping, due to the formation of neutral impurities and consequently improvement of carrier mobility. Our theoretical calculation also reveals a downward shift of the valence band, and effective mass increase with increasing cerium concentration which influences the thermoelectric enhancement in the synthesised materials. An order of magnitude enhancement in the figure of merit was obtained due to isovalent substitution, thus paving a new avenue for enhancing the thermoelectric performance of materials.