Vacancy-mediated and exchange diffusion in a Pb/Cu(111) surface alloy: concurrent diffusion on two length scales

Chemical transformation of Te into new ternary phase PbmCunTem+n nanorods and their surface atom diffusion and optical properties

The new phase of ternary Pb_mCu_nTe_m+n was first synthesized using sacrificial template Te. The diffusion of surface atoms was observed upon electron-beam irradiation and their optical band gaps show bigger values than that of PbTe.

Unidirectional thermal diffusion in bimetallic Cu@Au nanoparticles

Understanding the atomic diffusions at the nanoscale is important for controlling the synthesis and utilization of nanomaterials. Here, using in situ X-ray absorption spectroscopy coupled with theoretical calculations, we demonstrate a so far unexplored unidirectional diffusion from the Au shell to the Cu core in thermally alloying Cu@Au core@shell architecture of ca. 7.1 nm. The initial diffusion step at 423 K is found to be characterized by the formation of a diffusion layer composed of a Au-dilute substitutional CuAu-like intermetallic compound with short Cu-Au bond length (2.61 Å). The diffusion further happens by the migration of the Au atoms with large disorder into the interior Cu matrix at higher temperatures (453 and 553 K). These results suggest that the structural preference of a CuAu-like compound, along with the nanosized effect, plays a critical role in determining the atomic diffusion dynamics.

Nanoscale surface pattern evolution in heteroepitaxial bimetallic films

Nanoscale self-assembly dynamics of submonolayer bimetallic films was studied through simulation of a coarse-grained mesoscopic model. Simulations predict a phase transition sequence (hexagonal→stripe→inverse hexagonal) consistent with experimental observations of Pb/Cu(111) heteroepitaxial growth. Post-transition ordering dynamics of hexagonal and inverse hexagonal patterns was simulated and quantified in order to predict pattern quality and evolution mechanisms. Correlation length scaling laws and nanoscale evolution mechanisms were predicted through simulation of experimentally relevant length (≈1 μm(2)) and time scales, with findings supporting evidence of universal pattern behavior with other hexagonal systems. Results provide detailed dynamics and structure of this novel self-assembly process applicable to the design and optimization of functional bimetallic materials, such as bimetallic catalysts.

One-dimensional defect-mediated diffusion of Si adatoms on the Si(111)-(5 x 2)-Au surface

Using scanning tunneling microscopy, we determine that the one-dimensional diffusion of Si adatoms along the Si(111)-(5 x 2)-Au surface reconstruction occurs by a defect-mediated mechanism. Distinctive diffusion statistics, especially correlations between sequential adatom displacements, imply that the displacements are triggered by an interaction with a defect that is localized to the adatom. The defect is intrinsic and thermally activated. The measured diffusion statistics are modeled accurately by a Monte Carlo simulation. The measured adatom diffusion activation barrier is 1.24 +/- 0.08 eV.

Low-temperature ultrafast mobility in systems with long-range repulsive interactions: Pb/Si(111)

A realization of the numerous phases predicted in systems with long-range repulsive interactions was recently found in Pb/Si(111). Surprisingly, these numerous phases can be grown at low temperatures approximately 40 K over macroscopic distances. This unusual observation can be explained from theoretical calculations of the collective diffusion coefficient D(c) in systems with long-range repulsive interactions. Instead of a gradual dependence of D(c) on coverage, it was found that D(c) has sharp maxima at low temperatures for every stable phase (i.e., for every rational value of the coverage theta=p/q) in agreement with the experiment.

How Pb-overlayer islands move fast enough to self-assemble on Pb-Cu surface alloys

Low-energy electron microscopy reveals that two-dimensional, approximately 50 000 atom, Pb-overlayer and vacancy islands both have diffusion coefficients of 25.6+/-0.8 nm2/sec at 400 degrees C on Pb-Cu surface alloys. This high mobility, key to self-assembly in this system, results from the fast transport of Pb atoms on the surface alloy and of Cu through the Pb overlayer. A high Pb vacancy concentration, predicted by ab initio calculations, facilitates the latter.