New routes for improving adhesion at the metal/α-Al2O3(0001) interface.

With the advent of new steel grades, galvanic protection by zinc coating faces a new paradigm. Indeed, enrichment in strengthening elements prone to oxidation, such as Al, Mn, and Si, leads to the formation of oxide films that are poorly wet by zinc. We study herein routes for the improvement of adhesion at the model Zn/α-Al2O3 interface by the addition of metals. As a first step, with the help of ab initio results on the adsorption characteristics of transition metal adatoms at α-alumina surfaces, we establish and rationalize clear trends in both the behavior of metal-alumina interaction strength and the relative thermodynamic stability of configurations with weakly and strongly bound metal adatoms. The reasons for the enhanced binding strength of transition metals, such as Cr, maintained regardless of the precise alumina termination and the surface charge state are pointed out. On these grounds, possible improvements of adhesion under realistic conditions are discussed. It is predicted that enrichment in transition metals, such as Cr, may produce strongly adhesive interfaces that lead to cohesive cleavage.

Temperature dependence of the radiative lifetimes in Ge and Si nanocrystals.

The effect of finite temperature on the optical properties of nanostructures has been a longstanding problem for their theoretical description and its omission presents serious limits on the validity of calculated spectra and radiative lifetimes. Most ab initio calculations have been carried out neglecting temperature effects altogether, although progress has been made recently. In the present work, the temperature dependence of the intrinsic radiative lifetimes of excited electron-hole pairs in Ge and Si nanocrystals due to classical temperature effects is calculated using ab initio molecular dynamics. Fully hydrogen-saturated Ge and Si nanocrystals without surface reconstructions show opposite behavior: the very short lifetimes in Ge increase with temperature, while the much longer ones in Si decrease. However, the temperature effect is found to be strongly dependent on the surface structure: surface reconstructions cause partial localization of the wave functions and override the difference between Si and Ge. As a consequence, the temperature dependence in reconstructed nanocrystals is strongly attenuated compared to the fully saturated nanocrystals. Our calculations are an important step towards predictive modeling of the optical properties of nanostructures.