First-Principles Study Of Structures And Electronic Properties Of Silicon-based And Boron-based Clusters

Clusters are an intermediate state of matter between bulk materials and atoms or molecules,which are technologically important for catalysis,optical and magnetic applications,crystal growth,environment and energy,and designing novel cluster-assembled materials.In the past few decades,great efforts have been made to understand their properties which depend in general on size and shape,while for multicomponent clusters the arrangement of atoms could be quite different from what is known in bulk.Accordingly,clusters offer great opportunities to develop novel materials with desired properties.However,it is generally difficult to determine the atomic structure of clusters experimentally.Therefore,theoretical developments and calculations with predictive capabilities such as those based on density functional theory(DFT)are generally necessary to determine the most stable atomic structures and explore the physical and chemical properties of these finite systems.As the most important elementary semiconductor,silicon has attracted lots of attention due in part to it is the backbone of modern microelectronics industry.In this respect,the continuously increasing miniaturization of silicon-based transistors requires a deeper understanding of fundamental physical and chemical properties of silicon clusters and nanostructures.Since silicon clusters favor sp³-like covalent bonding,surface reconstruction reducing the number of dangling bonds is usually strong and can lead to distinctive cluster structures with electronic properties very different from those of the bulk diamond phase.Medium-sized silicon clusters are particularly interesting since they represent the key intermediates in the transition of silicon from molecular to bulk states.We systematically evaluated the DFT functionals for the calculation of the energetics of silicon clusters.The HSE06 functional with aug-cc-pVDZ basis set is found to show the best performance.Based on the benchmark calculations,we carry out a combined study of DFT calculations and experimental photoelectron spectroscopy on anionic TimSin-clusters with m=1-2,n=14-20.Satisfactory agreement is found between the experimental photoelectron spectra and the theoretical ones.Structural evolution from cage structures to quasi-fullerene has been revealed.Among these clusters,we have identified two superatomic clusters,i.e.,Ti₁Si₁₆ and Ti₂Si₁₅,which possess an enhanced stability with a magic number of 68 electrons filling closed electronic shell of 1S²1P⁶1D¹⁰1F¹⁴2S²2P⁶1G¹⁸2D¹⁰.Furthermore,we revisit large-gap Si₁₆ clusters encapsulating group-? metal atoms(Zr and Hf).These theoretical and experimental results establish a comprehensive picture of the structure,stability and electronic properties of the low-energy isomers of Ti/Zr/Hf-doped Si clusters as a function of cluster size and help identify the building blocks for novel self-assembled silicon-based nanomaterials.Moreover,multi-non-metal elements doping silicon clusters,B₃Si_n^-clusters with n=410,have been investigated for the first time using a combined DFT calculations and experimental photoelectron spectroscopy.Satisfactory agreement is found between theory and experiment.Most of the lowest-energy structures of B₃Si_n^-(n=4-10)clusters can be derived by using the squashed pentagonal bipyramid structure of B₃Si₄ as the major building unit.Among these clusters,we find that B₃Si₆^-and B₃Si₉^-clusters have relatively high stability and enhanced chemical inertness.In particular,the B₃Si₉^-cluster with high symmetry(C_3v)stands out as an interesting superatom cluster with a magic number of 40 skeletal electrons and a closed-shell electronic configuration of 1S²1P⁶1D¹⁰2S²2P⁶1F¹⁴ for superatom orbitals.As the neighboring element to carbon,boron holds unique promise as a complement of carbon but with its own features.With three valence electrons,boron is electron deficient compared with carbon;thus it prefers to form multicenter bonds and stronger covalent bonds.Despite a long history of boron research,many properties of boron have not been fully understood.Stimulated by the early theoretical prediction of B₈₀ fullerene and the experimental finding of the B₄₀ cage,the structures of medium-sized boron clusters have attracted intensive research interest during the last decade,but a complete picture of their size-dependent structural evolution remains a puzzle.Using a genetic algorithm combined with density-functional theory calculations,we have performed a systematic global search for the low-lying structures of neutral Bn clusters with n=31-50.Diverse structural patterns,including tubular,quasi-planar,cage,core-shell,and bilayer,are demonstrated for the ground-state Bn clusters;for certain cluster sizes,unprecedented geometries are predicted for the first time.Their stabilities at finite temperatures are evaluated,and the competition mechanism between various patterns is elucidated.Chemical bonding analysis reveals that the availability of localized ? bonds and delocalized ? bonds in the Bn clusters play a key role in their structural stability.Our results provide important insights into the bonding pattern and growth behavior of medium-sized boron clusters,which lay the foundation for experimental design and synthesis of boron nanostructures.The present thesis provides a theoretical guidance for deep understanding the growth patterns and electronic properties of silicon-and boron-based clusters.More importantly,the investigation of the superatomic clusters and the structural motif is beneficial to theoretical design or synthesis of novel cluster-assembled materials.