Application Of Computational Modeling In Semiconductor And Heterogeneous Catalysts Problems

In the past decade, with the rapid development of computer hardware and quantum mechanics algorithm, computational modeling has become a powerful tool which can provide great insight in predicting properties of novel substances or designing substances and engineering processes before laboratory activities. To date, computational modeling are widely and successfully applied in both scientific field and industries such as drug design. In this thesis, we studied two industrial problems, the fullerene-functionalized films on important semiconductor surfaces and the structural and physicochemical properties of transition metal clusters.â… . Fullerene-Functionalized Films on Semiconductor SurfacesCurrently, the most important semiconductor materials in ultra-large scale integration (ULSI) industry include Silicon (Si) and Gallium Arsenide (GaAs). With a ultra-high packing density of physical units inside the chips, the parasitic resistance, parasitic capacitance and the wire cross will cause severe RC delay, power consumption and wire cross talk, which are the major factors limiting device performance. The critical solution to these problems is to design and develop novel inter-wire and inter-layer insulator materials with proper dielectric constant (low-k). As the characteristic size of microchips evolves into 65nm era or even smaller, the present dielectric films, such as SiOF, coated via plasma-enhanced chemical vapor deposition (PECVD) technology, is no longer practical because of both the technical instability and their relatively high dielectric constants. Fullerenes, however, are regarded as one of the most potential low-k materials due to their excellent thermal and mechanic stability. The porosity of fullerenes allows one to tune the dielectric values of the films in a wide range. Furthermore, their intrinsic curvature and highly delocalizedÏ€-orbitals enables the interaction between fullerenes and the dangling bonds or dimers of semiconductor surfaces. As a consequence, fullerenes can be tightly anchored onto the substrates and the well-ordered, wellcontrolled molecular deposition can be realized. The properties of the fullerene films are of course dependent on the surface substrates, the fullerene molecules and their interplays. In this thesis, we have conducted extensive computational study on the chemisorption of a series of fullerene molecule C_n (n=28, 32, 36, 40, 44, 48, 60) on the c(4×4) reconstructed GaAs(001) surface and the c(2×1) Si(001) surface. The main conclusions are listed below:1 On both the reconstructed GaAs(001)-c(4×4) and Si(001)-c(2×1) surfaces, strong and stable chemical covalent bonds are formed between the fullerenes and the substrate. Charge transfers from the substrate to fullerenes molecules. With the fullerene size increases,both the adsorption energies and the amount of charge transfer display a decreasing trend. During the adsorption process, the relaxation of either the substrate lattice and the fullerene molecules are observed and the relaxation extent is according to specific adsorption sites, fullerene orientation and the fullerene-substrate interplays;2 On the arsenic-rich c(4×4) reconstructed GaAs(001) surface, the most favorable adsorption configuration occurs at the trench sites when the fullerene molecules facing down to the substrate with a hexagon. The As atoms in the second layer of the surface with a dangling bond (due to unsaturated coordination number) play a critical role in anchoring fullerenes by forming covalent bonds. However, although the top layer -As=As- dimers are capable of interact with fullerenes via cycloaddition process, the adsorption strengths are obviously smaller than at the trench sites;3 In contrast with the GaAs surface, fullerenes molecules are able to interact much more stronger on the c(2×1) reconstructed Si(001) surface via favorable cycloaddition reactions between theÏ€-bonds of -C=C- and -Si=Si- dimers. The trench and double-dimer sites were identified to be almost equivalent for fullerenes anchoring, however, without strong fullerene orientation preference. The adsorption energies and charge transfer obey the same decreasing trend as what was found on GaAs surface. Generally, it is increasingly difficult for fullerenes to be accommodated in the trench channel as their sizes increase;4 The curvature of fullerenes near the adsorption sites dictates the local bonding strength between molecules and the surface; a large curvature always gives rise to a stronger bonding by relaxing the stress of the carbon atoms imposed by the quasi-sp² hybridization. The stability of the fullerene molecules in the gas phase and the structural symmetry also play very important roles to determine the adhesion strength, i.e. C₃₂ exhibits a much smaller adsorption on both the two surfaces comparing with the adjacent fullerenes mainly attributed to its unexpected stability which is almost comparable to C₆₀. A highly symmetric molecule can adopt an optimal orientation to effectively interact with surface yielding high adsorption energies, while the adsorption of asymmetric molecules is usually accompanied by ill fit and substantial distortion of the interface.5 Electronic structure analysis indicate that the fullerene-functionalize GaAs(001)-c(4×4) and Si(001)-c(2×1) surfaces will exhibit considerable metallic or semiconductor characteristics according to different fullerene species and the fullerene-substrate interaction. Consequently, it can be envisaged that one can tune the the surface properties by selecting appropriately sized fullerene molecules and to deposit them onto specific surfaces. These functionalized fullerene derivatives can further enrich the properties of fullerene films on semiconductor surfaces and provide a great opportunity for developing a rich variety of materials.â…¡. Structural and Physicochemical Properties of Transition Metal ClustersNano- and subnano-scale transition metal clusters are one of the most active and important subject in condensed-phase physics fields mainly due to their potential industrial application of serving as catalysts, semiconductors, sensors and magnetic memory storage. Understanding of the structural and physicochemical properties of those clusters are of fundamental importance for their realistic application. Among all the researches, the most attractive subject is about the catalytic performance of clusters. Generally, catalyst particles are well dispersed on support materials such as alumina, and the catalytic reactivity mainly originates from the surface defect sites (steps, kinks and corners). Since clusters own large specific surface area and rich defects, they provide a reasonable model for studying gas species dissociative chemisorption. Traditional reports always employed the model where just one or a few gas molecules reacting with the clusters were considered. However, in a real catalytic system, the catalyst surface are always closely surrounded by gas molecules which we named as the so-called "saturation state". In this thesis, we performed extensive computational study on the structural and physicochemical properties of small platinum (Pt) and palladium (Pd) clusters as well as the cluster-support interaction. Furthermore, we investigated the the catalytic hydrogenation reactions occur on Pt and Pd clusters under the hydrogen saturation state. Our main conclusions are listed below:1 The structures of Pt_n, and Pd_n (n=2-15) clusters: for a given size, computational search for energy minima on cluster potential energy surfaces always yields numerous isomers with degenerated energies. Although the topological configuration might be very different, these isomers can coexist thermodynamically. And kinetically, the estimated energy barriers of transforming structures from one one isomer to another, sampled by a few Pd isomers, were rather moderate, suggesting that these isomers may readily exchange their structures at ambient conditions. For both the Pt_nand Pd_n clusters, their ionization potential (IP), electron affinity (EA) and magnetic moment (μ) were found to be strongly sizeandstructural-dependent. Energetically the most favorable clusters tend to have higher IP, lower EA andμdue to their strong electron pairing;2 The cluster structural evolution from subnano/nano size to bulk includes three different growth patterns, the close-packed "irregular" structures, the icosahedral structures and fcc-like structures. At the subnanoscale, both Pt and Pd clusters essentially adopt a closepacked triangular growth pattern. Subsequently, at approximately n=19 for Pt and n=13 for Pd, the abrupt transition to the icosahedral structures occurs; at n=38 for Pt, the growth pattern transition from icosahedral to the fcc-like feature was observed. While for Pd, it is envisaged that the structural transition from icosahedral clusters to the fcc-like clusters will occur at a very large number;3 The calculate of a sing Pt atom, Pt_n (n=2-5) clusters and Pt monolayer adsorption on theα-Al₂O₃(0001) surface suggest that, in all cases, the Pt atoms can be stably anchored on the surface exhibiting as either cluster or dissociated fragments. The energetically the most favorable adsorption sites are identified to be the O₃ sites. Both the cluster shape distortion and the substrate lattice relaxation are observed, which decays as the cluster size increases. In particular, the n≥3 clusters prefer to interact with the substrate via their triangular faces to take advantage of the maximum interaction with the available O₃ sites. The driving force of the cluster anchoring largely arises from the charge transfer from Pt atoms to the O atoms of the substrate. Since the Pt-Pt interaction is stronger than Pt-O, metal clustering would be strongly preferred under high Pt loading, that's to say, the growth of metal films on theα-Al₂O₃(0001) surface is unlikely to be smooth and agglomeration could occur under certain conditions;4 The dissociation barriers of H₂ molecules on bare Pt_n and Pd_n (n=2-9, 13) clusters are considerably small. As the H coverage increases, the cluster structure expands, in particular, for the highly symmetric icosahedrons, structural rearrangement will occur. The clusterstructural change mainly due to the increasing hydrogen-metal bonding. A higher H loading pumps more electrons from the clusters which will definitely decrease the metalmetalbonding strength. Due to the strong overlap between the 1s-orbital of H atoms and the d-orbitals of metal atoms, the calculated band gaps of the metal hydrides gradually diminishand reach the minimum at full H saturation. With a large cluster size, the hydride systems even show a certain extent of metallic properties;5 The H₂ dissociative chemisorption energy (â–³E_CE) and the H desorption energy (â–³E_DE), a mathematical description to quantitatively or semi-quantitatively evaluate the catalytic performance for specific transition metal clusters, are strongly coverage-dependent. Although for different species, the most active adsorption sites and the H populating sequences are different, bothâ–³E_CE andâ–³E_DE of either close-packed and highlysymmetric icosahedral clusters decay with the increasing H loading. At full coverage, the thresholdâ–³E_CE,Pd^T andâ–³E_DE,Pd^T on close-packed Pt_n (4≤n≤9) clusters are identified to be in the range of 0.92-0.96 eV and 2.45-2.62 eV, respectively; while for close-packed Pd_n (4≤n≤9) clusters the two thresholds are 0.6-0.9 eV and 2.29-2.80 eV, respectively. Comparably, the data for icosahedral structures(â–³E_CE,Pd^T=0.90 eV,â–³E_DE,Pd^T=2.02 eV; â–³E_CE,Pd^T=0.73 eV,â–³E_dE,Pd^T=2.10 eV), are still neighboring the the narrow range found for small and sharp clusters despite the structural arrangement. Furthermore, the Pt:H and Pd:H ratio at full saturation was found to be 1:4 and 1:2, respectively. Consequently, it can be predicted that these important properties do not change significantly with respect to either cluster size or cluster shape but are dependent on the available surface atoms of metal which can be accessed by H atoms. The idea that investigating the key quantitiesat full saturation state provides a very powerful and useful model for studying other gas species, such as O₂, CO, and NO_x catalyzed by cluster to understand a real fuel-cell catalytic environment or catalyst poison. Such a model and the understanding on the underlying catalytic mechanism will be very helpful in design of novel catalysts for real industrial applications.