Wobbe, MCC; (2016) Computational insights into excited state properties of MgO and other nanoparticles with the rocksalt structure. Doctoral thesis , UCL (University College London).

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Abstract

This work investigates the applicability of (time-dependent) Density Functional Theory, (TD)DFT, for the description of the excited state properties of MgO, and other inorganic nanoparticles exhibiting the rocksalt structure. Firstly, the excitation behaviour of MgO nanoparticles is studied and it is shown that the commonly used exchange-correlation (xc)-functional B3LYP severely underestimates the lowest vertical excitation energies (LVEE) in these systems. It is demonstrated that TD-B3LYP erroneously stabilises those excitations with charge-transfer (CT) character. The use of different approximations to the exchange-correlation functional, such as the long-range corrected xc-functional CAM-B3LYP or the hybrid xc-functional BHLYP, significantly improves the energetic description of excited states with CT character. Different inorganic nanoparticles exhibiting the rocksalt structure, such as those of the alkaline earth chalcogenides CaO, SrO, BaO, MgS and MgSe as well as CdO and PbS, are investigated and found to behave similarly to MgO in that their localised excited states are poorly described with the TD-B3LYP xc-functional. It is demonstrated that the concept of CT character is not binary. Nanoparticles of both the CdO and PbS materials have delocalised excited electrons and a high dielectric constant: in these systems the excited state appears to be suitably described using TD-B3LYP. The lowest excited states of a series of MgO nanoparticles are optimised and it is found that, despite providing a different description of the excitation character, all investigated xc-functionals optimise to a similar lowest lying excited singlet state. The larger particles studied exhibit an excited electron localised on a magnesium corner site and the corresponding hole localised on an oxygen corner site. The magnesium and oxygen corners lie along the same edge of a given particle. TD-B3LYP is found to underestimate the experimentally observed photoluminescence peak, reminiscent of the results obtained for the vertical excitation energies. In contrast to the LVEE results, TD-BHLYP and TD- CAM-B3LYP also underestimate the experimental photoluminescence peak. The lowest excited states of a series of MgO nanoparticles are optimised and it is found that, despite providing a different description of the excitation character, all investigated xc-functionals optimise to a similar lowest lying excited singlet state. The larger particles studied exhibit an excited electron localised on a magnesium corner site and the corresponding hole localised on an oxygen corner site. The magnesium and oxygen corners lie along the same edge of a given particle. TD-B3LYP is found to underestimate the experimentally observed photoluminescence peak, reminiscent of the results obtained for the vertical excitation energies. In contrast to the LVEE results, TD-BHLYP and TD-CAM-B3LYP also underestimate the experimental photoluminescence peak. Finally, doped MgO nanoparticles are studied. The dopants include alkaline earth elements Be, Ca, Sr and Ba as well as group 12 elements Zn and Cd. It is found that the experimental absorption spectra are not replicated by the computational model of just one dopant present in the system and it is suggested that the experimental absorption peaks attributed to the dopants in the system appear to result from dopant clusters on the particle or that the dopants are not evenly distributed throughout the particle. Furthermore, the preliminary photoluminescence results show that the exciton behaves differently in doped nanoparticles when compared to pure MgO nanoparticles in that the electron and hole remain more localised, yielding higher photoluminescence energies, in-line with experimental observations.

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