Theoretical models of photo-induced processes at surfaces of oxide nano-particles.
Doctoral thesis, UCL (University College London).
In this Thesis I have studied the mechanisms of photo-induced processes at surfaces of nano-crystalline oxides, such as MgO and CaO. To model the photon-induced processes in MgO and CaO nano-particles, I have employed an embedded cluster approach, which combines a quantum-mechanical treatment of a site of interest with a self-consistent classical description of the remaining part of the system. The main results of this work can be summarised as follows: 1) G-tensor, absorption spectra, and formation energies of excitons, electron and hole in MgO and CaO surfaces are calculated and compared with experimental results. It is demonstrated that low coordinated sites such as corner sites are active centre for selective absorption and emission of light with sub bulk band gap energy. By calculating adiabatic energy surfaces for the ground and several excited states, mechanisms of photo induced process in corner sites are proposed. In particular, theoretical estimates of absorption spectra and luminescence energy for these sites are provided for the first time. Results are in good agreement with experiments. Adsorption of CI and F ion by MgO corners and terrace has been also studied. Results indicate that CI and F have shown similar behaviour, trapping an electron and producing an hole centre in the neighbour O ion on the MgO surface. Absorption spect5ra and g tensor estimates have been provided for experimental comparisons. 2) Recent experimental data clearly demonstrate that using both femto- and nano-second laser pulses with photon energies tuned to excite particular surface features at the MgO and CaO nano-cluster surfaces, one can achieve a very effective desorption of hyper-thermal atomic O and Mg species. Two main mechanisms are suggested: the double excitation process and trapping hole or electron and subsequent excitation. From the precursor excitation, trapped electron, or hole in corner ions, a further excitation can neutralize the charge of ion terminated corner and lead to desorption of atom. The adiabatic potential energy surfaces for this excited state is such that atom prefers to leave the surfaces with the maximum kinetic energy of several tenths on an eV. The surface site relaxes with the formation of an F+ or V- centre at the corner site previously occupied by the desorbed atom. Results are in good agreement with the experimental photon energies and kinetic energies of desorbed species. 3) Photo induced desorption processes can be iterated from the formed corner vacancies (mechanisms of secondary processes of desorption). Results of calculations show that subsequent excitations in Mg corner vacancy can lead to further desorption of O atom. To summarise, I have provided mechanisms that can explain both production of electron hole centres and photo induced desorption of species. Estimates of hyperthermal kinetic energy of desorbed species have been given.
|Title:||Theoretical models of photo-induced processes at surfaces of oxide nano-particles|
|Additional information:||Authorisation for digitisation not received|
|UCL classification:||UCL > School of BEAMS > Faculty of Maths and Physical Sciences > Physics and Astronomy|
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