Platinum-based transition metal dichalcogenides have been gaining renewed interest because of the development of a new method to synthesize thin film structures. Here, using first-principles calculation, we explore the electronic properties of PtX 2 (X = S, Se, and Te) with respect to film thickness. For bulk and layered structures (1 to 10 layers), octahedral 1T is the most stable. Surprisingly, we also find that the 3R structure has comparable stability relative to the 1T, implying possible synthesis of 3R. For a bulk 1T structure, PtS 2 is semiconducting with an indirect band gap of 0.25 eV, while PtSe 2 and PtTe 2 are both semi-metallic. Still, all their corresponding monolayers exhibit an indirect semiconducting phase with band gaps of 1.68, 1.18, and 0.40 eV for PtS 2 , PtSe 2 , and PtTe 2 , respectively. For the band properties, we observe that all these materials manifest decreasing/closing of indirect band gap with increasing thickness, a consequence of quantum confinement and interlayer interaction. Moreover, we discover that controlling the thickness and applying strain can manipulate van Hove singularity resulting to high density of states at the maximum valence band. Our results exhibit the sensitivity and tunability of electronic properties of PtX 2 , paving a new path for future potential applications.

A large gap two-dimensional (2D) topological insulator (TI), also known as a quantum spin Hall (QSH) insulator, is highly desirable for low-power-consuming electronic devices owing to its spin-polarized backscattering-free edge conducting channels. Although many freestanding films have been predicted to harbor the QSH phase, band topology of a film can be modified substantially when it is placed or grown on a substrate, making the materials realization of a 2D TI challenging. Here we report a first-principles study of possible QSH phases in 75 binary combinations of group III (B, Al, Ga, In, and Tl) and group V (N, P, As, Sb, and Bi) elements in the 2D buckled honeycomb structure, including hydrogenation on one or both sides of the films to simulate substrate effects. A total of six compounds (GaBi, InBi, TlBi, TlAs, TlSb, and TlN) are identified to be nontrivial in unhydrogenated case; whereas for hydrogenated case, only four (GaBi, InBi, TlBi, and TlSb) remains nontrivial. The band gap is found to be as large as 855 meV for the hydrogenated TlBi film, making this class of III-V materials suitable for room temperature applications. TlBi remains topologically nontrivial with a large band gap at various hydrogen coverages, indicating the robustness of its band topology against bonding effects of substrates.

Deposition at room temperature of Ga on Si(100) produces single-atom-wide metal rows orthogonal to the Si-dimer rows. Detailed analysis using scanning tunneling microscopy reveals a monotonically decreasing size (i.e., length) distribution for these rows. This is unexpected for homogeneous nucleation without desorption, conditions which are operative in this system. Kinetic Monte Carlo simulation of an appropriate atomistic model indicates that this behavior is primarily a consequence of the feature that the capture of diffusing atoms is greatly inhibited in the Ga∕Si(100) system. The modeling also determines activation barriers for anisotropic terrace diffusion, and recovers the experimental distribution of metal rows. In addition, we analyze a variety of other generic deposition models and determine that the propensity for a large population of small islands in general reflects an enhanced nucleation rate relative to the aggregation rate. Disciplines Chemistry CommentsThis article is from Physical Review B 72 (2005) Deposition at room temperature of Ga on Si͑100͒ produces single-atom-wide metal rows orthogonal to the Si-dimer rows. Detailed analysis using scanning tunneling microscopy reveals a monotonically decreasing size ͑i.e., length͒ distribution for these rows. This is unexpected for homogeneous nucleation without desorption, conditions which are operative in this system. Kinetic Monte Carlo simulation of an appropriate atomistic model indicates that this behavior is primarily a consequence of the feature that the capture of diffusing atoms is greatly inhibited in the Ga/ Si͑100͒ system. The modeling also determines activation barriers for anisotropic terrace diffusion, and recovers the experimental distribution of metal rows. In addition, we analyze a variety of other generic deposition models and determine that the propensity for a large population of small islands in general reflects an enhanced nucleation rate relative to the aggregation rate.

The structures of AgSi(n) (n=1-13) clusters are investigated using first-principles calculations. Our studies suggest that AgSi(n) clusters with n=7 and 10 are relatively stable isomers and that these clusters prefer to be exohedral rather than endohedral. Moreover, doping leaves the inner core structure of the clusters largely intact. Additionally, the plot of fragmentation energies as a function of silicon atoms shows that the AgSi(n) are favored to dissociate into one Ag atom and Si(n) clusters. Alternative pathways exist for n>7 (except n=11) in which the Ag-Si cluster dissociates into a stable Si(7) and a smaller fragment AgSi(n-7). The AgSi(11) cluster dissociates into a stable Si(10) and a small fragment AgSi. Lastly, our analysis indicates that doping of Ag atom significantly decreases the gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital for n>7.