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Title: Quantum Chemical Studies of Niobium and Vanadium-doped Gold Clusters
Other Titles: Kwantum Chemische Studies van Niobium en Vanadium-gedopeerde Goud Clusters
Authors: Nhat, Pham Vu; S0209478
Issue Date: 20-Dec-2012
Abstract: Our main interest in this doctoral study was to cope with the microscopic details of metallic structures having dimension of nanometers by using quantum chemical methods. We mainly concentrated on the structural, electronic and energetic properties of two transition metal clusters, which would provide a fundamental understanding for their possible technological applications. We also investigated the effect of doping to certain properties of cluster systems. Most of the calculations were performed in the framework of density function theory in conjunction with the pseudo-potential basis sets.In the following section we describe very briefly how the thesis has been evolved.Chapter 1 is a concise overview on the metal clusters studied. In this chapter we have introduced the importance of such nano-particles in terms of both scientific and technological viewpoints. Recent achievements in the study of transition metal clusters have also been discussed. We in addition mentioned to the general trends resulting from the presence of the d electrons, the lanthanoid contraction, and the relativistic effects, which have strong effects on many properties of d-element clusters. More significantly, this emphasized our motivations to get involved with rather difficult targets for theoretical investigations. Indeed, we find that while experimental observations become more and more available, relatively little quantitative information has been taken to understand and interpret the observed findings.Chapter 2 is a critical analysis about the performance of current DFT methods and potential applications of various available functionals in transition metal chemistry. Albeit DFT calculations have become an integral part for analyzing electronic structures and related properties of transition metal compounds, each functional naturally has its own advantages and shortcomings for specific systems. This analysis has been served as a first but useful guidance in choosing suitable functionals for certain parameters of a given system. To some extent, functionals with low amount of HF exchange usually perform better than fully hybrid methods in calculating the molecular properties of compounds containing only transition metal elements. However, we recognize that no functional is actually suitable for all structural and energetic parameters. Hence, it is crucial to examine carefully the applicability of different computational options for every new type of compounds in the studies by a systematic comparison of the corresponding computed results to either experimental data or high-level MO methods.In the Chapter 3 we systematically investigate the structural evolution, vibrational signatures and energetics of Nbn clusters (n = 2 − 20), in both neutral and charged states. Several basic energetics of clusters including electron affinities, ionization energies, binding energy per atom, and stepwise dissociation energies are computed and compared with the measured values. The vibrational (IR) spectra of systems smaller than Nb13 are investigated in concert with the observed far-IR features. Though the experimental IR spectra for larger clusters have not been published so far, they are also presented as predictions that may allow the optimal structures to be verified when the relevant spectroscopic information is available. Based on these theoretical and experimental results, we then draw some general and important points for Nbn clusters with n ranging from 2 to 20.Another project of this doctoral study was a comprehensive investigation on the growth behavior, energetics and CO affinity of vanadium-doped gold clusters AunV with n = 1 – 20. In this chapter, we have examined in detail how the pure gold systems are perturbed due to the dopant atom. It is clearly seen that the structural evolution and related properties of gold clusters are strongly modified due to the introduction of vanadium atom. Such a replacement is found to enhance considerably the thermodynamic stability of odd-numbered pure clusters but significantly reduce that of even-numbered systems. The CO adsorption is also affected in such a way that it is generally reinforced when the dopant is introduced. Deeper insights into the electronic, geometric and energetic properties of transition metal clusters are very important for the continuing development of relevant nanomaterials and nanostructure technologies. That renders evident the need for detailed theoretical understanding of such systems. The results reported in the present work could thus be helpful for future experimental and theoretical studies of metal clusters. However, several open issues still remain unresolved and some perspectives would motivate further investigations.Although DFT methods provide us with reasonable information on the ground state properties of these species, they still demand a substantial computational cost. Hence, up to now, the study is limited to systems with a small number of atoms, and few calculations of excited state properties could be performed. In the coming years, we hope to see breakthrough developments both in terms of methodology and computational facility in such a way that the investigations on larger clusters of metals and semiconductors will become less challenging and more reliable. We are also looking forward to the appearance of spectroscopic data from various experiments in order to confirm our predictions. Then further studies on larger systems could be carried out in concert.The applications of transition metal clusters in colloidal chemistry, catalysis, medical science, electronic industries, and so on, are also interesting and promising perspectives. Therefore, theoretical investigations on optoelectronic and magnetic behaviors, aggregate properties, and interactions of these species with industrial gases (H2, CO, NO, CH4, ...) and with organic and biomolecular compounds (from simple alcohols, amino acids, to DNA bases, ...) … are truly necessaryand could open a wide avenue for quantum chemical theory to play its essential role, namely to predict with confidence the chemical phenomena.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Quantum Chemistry and Physical Chemistry Section

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