Theory of self-diffusion at and growth of Al(111)

First-principles calculations for understanding microstructures and mechanical properties of co-sputtered Al alloys

Recent experimental studies show that co-sputtering solutes with Al, together, can refine columnar grain size around few tens of nanometers and promote the formation and enhance the stability of planar defects such as stacking faults (SFs) and grain boundaries (GBs) in Al alloys. These crystal defects and fine columnar grains result in high strength, enhanced strain hardening and thermal stability of Al alloys. Using first-principles density-functional theory (DFT) calculations, we studied the role of eleven solutes in tailoring kinetics and energetics of adatoms and clusters on Al {111} surface, stable and unstable stacking fault energies, and kinetic energy barriers for the migration of defects. The calculations show that most solutes can effectively refine columnar grain size by decreasing the diffusivity of adatoms and surface clusters. These solutes do not necessarily decrease the stacking fault energy of Al alloys, but reduce the formation energy of faulted surface clusters and increase the energy barriers for the recovery of faulted surface clusters. Correspondingly, the formation of SFs is kinetically promoted during sputtering. Furthermore, solutes are segregated into the core of Shockley partial dislocations and play a pinning effect on SFs, SF arrays and twin boundaries, enhancing the thermal stability of these crystal defects. These findings provide insights into the design of high-strength Al alloys for high-temperature applications.

Temperature Dependence of AlF3 Protection on Far-UV Al Mirrors

More efficient and stable far ultraviolet (FUV) mirrors will enable future space observatories. Traditional FUV mirrors are based on MgF2-protected Al. AlF3 has been identified as a promising substitute for MgF2 to prevent Al oxidation. Hence, the reflectivity, stability, and morphology of AlF3-protected Al mirrors have been investigated as a function of deposition temperature of the AlF3 film. In this work, it is shown how AlF3 deposition temperature is an important parameter whose optimization ultimately yields valuable throughput enhancement and improved endurance to large storage periods. Al films were deposited at room temperature (RT) and AlF3 protective layers were deposited at temperatures ranging from RT to 350 °C. It was found that the optimum AlF3 deposition temperature was between 200 and 250 °C, yielding the largest FUV reflectance and a better stability of the mirrors, which had been stored in a desiccator environment. Increasing AlF3 deposition temperature resulted in an increase in film density, approaching bulk density at 250 °C. The morphology of Al and AlF3 films as a function of AlF3 deposition temperature was also investigated. The increase in the AlF3 deposition temperature resulted in a decrease of both Al and AlF3 surface roughness and in the growth of the grain width at the AlF3 outer surface. It also resulted in a trend for the prevalent (111) planes of Al nanocrystals to orient parallel to the coating surface.

Preferential Positioning, Stability, and Segregation of Dopants in Hexagonal Si Nanowires

We studied the physics of common p- and n-type dopants in hexagonal-diamond Si, a Si polymorph that can be synthesized in nanowire geometry without the need of extreme pressure conditions, by means of first-principles electronic structure calculations and compared our results with those for the well-known case of cubic-diamond nanowires. We showed that (i) as observed in recent experiments, at larger diameters (beyond the quantum confinement regime) p-type dopants prefer the hexagonal-diamond phase with respect to the cubic one as a consequence of the stronger degree of three-fold coordination of the former, while n-type dopants are at a first approximation indifferent to the polytype of the host lattice; (ii) in ultrathin nanowires, because of the lower symmetry with respect to bulk systems and the greater freedom of structural relaxation, the order is reversed and both types of dopant slightly favor substitution at cubic lattice sites; (iii) the difference in formation energies leads, particularly in thicker nanowires, to larger concentration differences in different polytypes, which can be relevant for cubic-hexagonal homojunctions; (iv) ultrasmall diameters exhibit, regardless of the crystal phase, a pronounced surface segregation tendency for p-type dopants. Overall these findings shed light on the role of crystal phase in the doping mechanism at the nanoscale and could have a great potential in view of the recent experimental works on group IV nanowires polytypes.

Towards Direct Synthesis of Alane: A Predicted Defect-Mediated Pathway Confirmed Experimentally

Alane (AlH3 ) is a unique energetic material that has not found a broad practical use for over 70 years because it is difficult to synthesize directly from its elements. Using density functional theory, we examine the defect-mediated formation of alane monomers on Al(111) in a two-step process: (1) dissociative adsorption of H2 and (2) alane formation, which are both endothermic on a clean surface. Only with Ti dopant to facilitate H2 dissociation and vacancies to provide Al adatoms, both processes become exothermic. In agreement, in situ scanning tunneling microscopy showed that during H2 exposure, alane monomers and clusters form primarily in the vicinity of Al vacancies and Ti atoms. Moreover, ball milling of the Al samples with Ti (providing necessary defects) showed a 10 % conversion of Al into AlH3 or closely related species at 344 bar H2 , indicating that the predicted pathway may lead to the direct synthesis of alane from elements at pressures much lower than the 10(4)  bar expected from bulk thermodynamics.

Selective catalytic burning of graphene by SiOx layer depletion

We report catalytic decomposition of few-layer graphene on an Au/SiOx/Si surface wherein oxygen is supplied by dissociation of the native SiOx layer at a relatively low temperature of 400 °C. The detailed chemical evolution of the graphene covered SiOx/Si surface with and without gold during the catalytic process is investigated using a spatially resolved photoelectron emission method. The oxygen atoms from the native SiOx layer activate the gold-mediated catalytic decomposition of the entire graphene layer, resulting in the formation of direct contact between the Au and the Si substrate. The notably low contact resistivity found in this system suggests that the catalytic depletion of a SiOx layer could realize a new way to micromanufacture high-quality electrical contact.

Effects of tensile strain on Ag(111) epitaxial growth by kinetic Monte Carlo simulations

The effects of surface strain on epitaxial growth are studied using the kinetic Monte Carlo (KMC) method. The strain dependences of the activation energy barrier and the attempt frequency of each elementary process are evaluated by the embedded atom method interatomic potential. KMC simulations of homoepitaxial growth on a Ag(111) surface with equibiaxial tensile strain are carried out and influences of the surface strain on the nucleation of islands and the surface morphology are investigated. The island density increases due to reduction of the adatom diffusion on the terrace. The averaged coordination number of atoms constituting islands decreases and the island shape is more dendritic. The tensile surface strain leads to an increase in the surface roughness at an early stage of the growth, but at high coverage the roughness is adversely lower for the strained surface.

Ripening Mechanisms in Ultrathin Metal Films

Results of recent experimental model studies on ripening of submonolayer films via Ostwald ripening and dynamic coalescence are described. The experiments have been performed on ensembles of Ag-adatom or vacancy islands on a Ag(111) surface using Scanning Tunneling Microscopy. For Ostwald ripening of adatom islands, deviations from the classical mean-field ripening behaviour are observed which show up as pronounced local correlations in island decay and growth rates. For ripening via dynamic coalescence which is studied for ensembles of vacancy islands, it is found that the increase of the average island size with time in the late-stage regime is correctly described by the classical binary collision model.

Interlayer diffusion of Au atoms in a heteroepitaxial system

In heteroepitaxy, thin-film growth proceeds in two-dimensional layer-by-layer, three-dimensional island, or layer-plus-island modes depending on the growth conditions. Interlayer mass transport plays a crucial role in determining the growth mode. We investigate interlayer diffusion of Au atoms from Au islands grown on Ir(111) by scanning tunneling microscopy (STM) and kinetic Monte Carlo (KMC) simulations. STM measurements reveal that the first Au layer on Ir(111) grows in a complete layer at 100 K, whereas the Au layer grows in a three-dimensional fashion from the second Au layer at this temperature. Annealing these surfaces to 300 K reduces the higher-layer islands, indicating that Au atoms undergo step-down diffusion. By measuring the density of the top-layer islands and comparing them with the KMC simulation results, the additional step-down diffusion barrier for Au atoms to descend from the Au islands is estimated to be 0.02 eV on the first Au layer and 0.04 eV on the second Au layer. The layer dependence of the additional step-down diffusion barrier is explained in terms of the lattice mismatch between Au and underlying layers.

Theory of defects in one-dimensional systems: application to Al-catalyzed Si nanowires

The energetic cost of creating a defect within a host material is given by the formation energy. Here we present a formulation allowing the calculation of formation energies in one-dimensional nanostructures which overcomes the difficulties involved in applying the bulk formalism and the possible passivation of the surface. We also develop a formula for the Madelung correction for general dielectric tensors. We apply this formalism to the technologically important case of Al-nanoparticle-catalyzed Si nanowires, obtaining Al concentrations significantly larger than in their bulk counterparts and predicting the fast consumption of the nanoparticles when the wires are grown on n-type substrates.

Diffusion mechanisms taking place at the early stages of cobalt deposition on Au(111)

In the present work a detailed atomic-level analysis of some of the main diffusion mechanisms which take place during cobalt adatom deposition are studied within atom dynamics (AD) and the nudged elastic band (NEB) method. Our computer simulations reveal a very fast exchange between Co and Au atoms when the deposit is a single cobalt adatom. However, when the nucleus size increases, a decrease in the exchange probability is observed. Activation energies for different transitions are obtained using AD in combination with the NEB method.

Steps of Al/Si nanocluster studied by scanning tunneling microscope Moiré method

The surface stress around the steps of Al/Si (111)-7x7 artificial nanocluster array has been studied by the scanning tunneling microscope (STM) Moiré method. The distributions of sigma(ij)(epsilon(ij)) near the selected step edge are precisely quantified by Moiré pattern. Observation by STM reveals the details of stress distribution that varies around steps and other defects of the surface. The experimental results show that the intrinsic surface stress tensors are quite different in the upper and lower terraces near a step, which provides indirect evidence that the exchange incorporation occurs for Al atoms on the Si (111) surface. The study also provides valuable data regarding control of the growth mode of artificial nanostructures by manipulating the growth conditions and kinetics.

Adatom ascending at step edges and faceting on fcc metal (110) surfaces

Using first-principles total-energy calculations, we show that an adatom can easily climb up at monatomic-layer-high steps on several representative fcc metal (110) surfaces via a place exchange mechanism. Inclusion of such novel adatom ascending processes in kinetic Monte Carlo simulations of Al(110) homoepitaxy as a prototypical model system can lead to the existence of an intriguing faceting instability, whose dynamical evolution and kinetic nature are explored in comparison with experimental observations.

Site determination and thermally assisted tunneling in homogenous nucleation

A combined low-temperature scanning tunneling microscopy and density functional theory study on the binding and diffusion of copper monomers, dimers, and trimers adsorbed on Cu(111) is presented. Whereas atoms in trimers are found in fcc sites only, monomers as well as atoms in dimers can occupy the fcc as well as the metastable hcp site. In fact the dimer fcc-hcp configuration is only 1.3 meV less favorable with respect to the fcc-fcc configuration. This enables a confined intracell dimer motion, which at temperatures below 5 K is dominated by thermally assisted tunneling.

Nature, strength, and consequences of indirect adsorbate interactions on metals

Atoms and molecules adsorbed on metals affect each other indirectly even over considerable distances. Via systematic density-functional calculations, we establish the nature and strength of such interactions, and explain for what adsorbate systems they critically affect important materials properties. This is verified in kinetic Monte Carlo simulations of epitaxial growth, which help rationalize a number of recent experimental reports on anomalously low diffusion prefactors.

Absolute values of surface and step free energies from equilibrium crystal shapes

It is shown that exact images of the three-dimensional equilibrium shape of crystallites (ECS), recorded at several temperatures between 0.3 and 0.8 of the melting temperature of a solid, can be evaluated to yield absolute values of the surface and step free energies versus temperature, in addition to the formation energy of kinks. The essential input for this novel approach is the temperature variation of the size of a facet on the ECS and of the separation between the Wulff point and that particular facet. This approach promises access to surface free energies over a large temperature range and for well-defined low-index surface orientations.

Dynamics of surface migration in the weak corrugation regime

We report a systematic study for metal-on-metal surface migration in the weak corrugation regime, i.e., with migration barriers falling below approximately 100 meV. The migration characteristics are elucidated by variable-temperature scanning tunneling microscopy observations in the 50-200 K temperature range, which are analyzed by means of nucleation theory. The results demonstrate that, upon entering the weak corrugation regime, the dynamics of the systems are characterized by increasingly reduced effective preexponential factors, while Arrhenius behavior prevails.