Molecular dynamics simulations in conjunction with MEAM potential models have been used to study the melting and freezing behavior and structural properties of both supported and unsupported Au nanoclusters within a size range of 2 to 5 nm. In contrast to results from previous simulations regarding the melting of free Au nanoclusters, we observed a structural transformation from the initial FCC configuration to an icosahedral structure at elevated temperatures followed by a transition to a quasimolten state in the vicinity of the melting point. During the freezing of Au liquid clusters, the quasimolten state reappeared in the vicinity of the freezing point, playing the role of a transitional region between the liquid and solid phases. In essence, the melting and freezing processes involved the same structural changes which may suggest that the formation of icosahedral structures at high temperatures is intrinsic to the thermodynamics of the clusters, rather than reflecting a kinetic phenomenon. When Au nanoclusters were deposited on a silica surface, they transformed into icosahedral structures at high temperatures, slightly deformed due to stress arising from the Au-silica interface. Unlike free Au nanoclusters, an icosahedral solid-liquid coexistence state was found in the vicinity of the melting point, where the cluster consisted of coexisting solid and liquid fractions but retained an icosahedral shape at all times. These results demonstrated that the structural stability in the structures of small Au nanoclusters can be enhanced through interaction with the substrate. Supported Au nanoclusters demonstrated a structural transformation from decahedral to icosahedral motifs during Au island growth, in contrast to the predictions of the minimum-energy growth sequence: icosahedral structures appear first at very small cluster sizes, followed by decahedral structures, and finally FCC structures recovered at very large cluster sizes. The simulations also showed that island shapes are strongly influenced by the substrate, more specifically, the structural characteristic of a Au island is not only a function of size, but also depends on the contact area with the surface, which is controlled by the wetting of the cluster to the substrate.

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Using first-principles density-functional theory calculations, we have investigated the structural, electronic, and dielectric properties, as well as the O vacancy formation in amorphous HfO2. The structural properties of the generated amorphous models were analyzed via the pair correlation functions and the distribution of the atomic coordination number. The PBE0 hybrid density functional was employed for the analysis of the electronic properties and the charge transition levels of the O vacancy in amorphous HfO2. The dielectric and vibrational properties of the generated models were analyzed using the linear response method based on the density functional perturbation theory. According to the generated structural models, the density of a-HfO2 was 8.63 g/cm3, and the average coordination numbers of O and Hf atom were 3.06 and 6.10, respectively. The electronic band gap of a-HfO2 was predicted to be 5.94 eV, and the static dielectric constants were calculated to be ∼ 22, both in good agreements with the experimental measurements. The computed formation energy of a neutral O vacancy in a-HfO2 was 6.50 eV on average, which is lower than that in m-HfO2 by 0.2–0.3 eV but remains higher than that in a-SiO2. Unlike in m-HfO2, the highest occupied defect levels of the negatively charged O vacancies in a-HfO2 may lie within the band-gap region of silicon. In addition, O vacancies in the charge state q =− 2 may appear as a stable state as the electron chemical potential lies within the electronic band gap, and thus, some of the O vacancies can possess the negative-U property in a-HfO2.

New findings on the lithiation mechanisms and the achievable Li capacity limits of various types of functional groups on the basal plane and those terminating the edge sites of graphene nanomaterials based on first-principles density functional theory calculations.

A comparative study of the hydrogen spillover phenomenon on pristine graphene and anatase (101)-supported Pt 4 catalysts has been carried out by using density functional theory with Hubbard correction (DFT + U) and dispersion correction (Grimme-D3). The adsorption of the H 2 molecule causes no dissociation on graphene but dissociation with nearly zero adsorption energy on anatase (101). This emphasizes the need for a metal catalyst for H 2 dissociation to aid the stronger chemisorption of hydrogen atoms or protons on the substrate. The metal−support interaction is different for both substrates as Pt 4 shows p-type doping for graphene and n-type doping for anatase (101) surfaces with binding energies of −2.16 and −5.82 eV, respectively. The differing nature of H 2 adsorption and metal−support interactions lead to different hydrogen spillover phenomena for the two supports. Hydrogen spillover is unlikely to occur on Pt 4 /graphene even at high hydrogen coverage (24H atoms per Pt 4 ) but has a tendency to take place on anatase (101) at medium hydrogen coverage (10H atoms per Pt 4 ) from the perspectives of both thermodynamics and kinetics.