Multiscale modeling of interaction of alane clusters on Al(111) surfaces: A reactive force field and infrared absorption spectroscopy approach

We have used reactive force field (ReaxFF) to investigate the mechanism of interaction of alanes on Al(111) surface. Our simulations show that, on the Al(111) surface, alanes oligomerize into larger alanes. In addition, from our simulations, adsorption of atomic hydrogen on Al(111) surface leads to the formation of alanes via H-induced etching of aluminum atoms from the surface. The alanes then agglomerate at the step edges forming stringlike conformations. The identification of these stringlike intermediates as a precursor to the bulk hydride phase allows us to explain the loss of resolution in surface IR experiments with increasing hydrogen coverage on single crystal Al(111) surface. This is in excellent agreement with the experimental works of Go et al. [ E. Go, K. Thuermer, and J. E. Reutt-Robey, Surf. Sci. 437, 377 (1999) ]. The mobility of alanes molecules has been studied using molecular dynamics and it is found that the migration energy barrier of Al_(2)H_6 is 2.99 kcal/mol while the prefactor is D_0 = 2.82 × 10^(−3) cm^2/s. We further investigated the interaction between an alane and an aluminum vacancy using classical molecular dynamics simulations. We found that a vacancy acts as a trap for alane, and eventually fractionates/annihilates it. These results show that ReaxFF can be used, in conjunction with ab initio methods, to study complex reactions on surfaces at both ambient and elevated temperature conditions

Interfacial Chemistry in Al/CuO Reactive Nanomaterial and Its Role in Exothermic Reaction.

Interface layers between reactive and energetic materials in nanolaminates or nanoenergetic materials are believed to play a crucial role in the properties of nanoenergetic systems. Typically, in the case of Metastable Interstitial Composite nanolaminates, the interface layer between the metal and oxide controls the onset reaction temperature, reaction kinetics, and stability at low temperature. So far, the formation of these interfacial layers is not well understood for lack of in situ characterization, leading to a poor control of important properties. We have combined in situ infrared spectroscopy and ex situ X-ray photoelectron spectroscopy, differential scanning calorimetry, and high resolution transmission electron microscopy, in conjunction with firstprinciples calculations to identify the stable configurations that can occur at the interface and determine the kinetic barriers for their formation. We find that (i) an interface layer formed during physical deposition of aluminum is composed of a mixture of Cu, O, and Al through Al penetration into CuO and constitutes a poor diffusion barrier (i.e., with spurious exothermic reactions at lower temperature), and in contrast, (ii) atomic layer deposition (ALD) of alumina layers using trimethylaluminum (TMA)produces a conformal coating that effectively prevents Al diffusion even for ultrathin layer thicknesses (∼0.5 nm), resulting in better stability at low temperature and reduced reactivity. Importantly, the initial reaction of TMA with CuO leads to the extraction of oxygen from CuO to form an amorphous interfacial layer that is an important component for superior protection properties of the interface and is responsible for the high system stability. Thus, while Al e-beam evaporation and ALD growth of an alumina layer on CuO both lead to CuO reduction, the mechanism for oxygen removal is different, directly affecting the resistance to Al diffusion. This work reveals that it is the nature of the monolayer interface between CuO and alumina/Al rather than the thickness of the alumina layer that controls the kinetics of Al diffusion, underscoring the importance of the chemical bonding at the interface in these energetic materials

Basic Mechanisms of Al Interaction with the ZnO Surface

Deposition of Al on ZnO is used for a number of electronic and catalytic devices as well as for nanoenergetic materials. The interface structure and chemical composition often control the performance of devices. In this study, in situ infrared spectroscopy, X-ray photoemission spectroscopy, and low energy ion scattering are combined to investigate the initial stage of interface formation between Al and ZnO. We find that (a) the interface is highly inhomogeneous with discontinuous Al patches, leaving ∼10% of the ZnO surface uncovered even after deposition of an equivalent of 11 nm-thick Al film; (b) upon Al deposition, Al reduces ZnO by forming Al₂O₃ and releasing Zn to the surface, and this process continues as more Al is deposited; (c) the reduced surface Zn atoms readily desorb at 150 °C; and (d) at higher temperature (>600 °C) all Al is oxidized as a result of mass transport. Deposition of a thin Al₂O₃ layer on ZnO prior to Al deposition effectively prevents Al penetration and Zn release, requiring higher temperatures to oxidize Al

Digermane Deposition on Si(100) and Ge(100): from Adsorption Mechanism to Epitaxial Growth

Controlled fabrication of nanometer-scale devices such as quantum dots and nanowires requires an understanding of the initial chemisorption mechanisms involved in epitaxial growth. Vapor phase epitaxy can provide controlled deposition when using precursors that are not reactive with the H-terminated surfaces at ambient temperatures. For instance, digermane (Ge₂H₆) has potential as such a precursor for Ge ALE on Si(100) surfaces at moderate temperatures; yet, its adsorption configuration and subsequent decomposition pathways are not well understood. In situ Fourier transform infrared spectroscopy and first principles calculations reveal that Ge₂H₆ chemisorbs through a β-hydride elimination mechanism, forming Ge₂H₅ and H on both Si(100)-(2 × 1) and Ge(100)-(2 × 1) surfaces, instead of the previously proposed Ge–Ge bond breaking mechanism, and subsequently decomposes into an ad-dimer. The resulting coverage of Ge after a saturation exposure is estimated to be about 0.3 monolayers. Interestingly, the decomposition of adsorbed Ge₂H₅ on Si(100) is faster than Si₂H₅ on Ge(100) at 173 K. The desorption temperature of hydrogen on Si(100) is shown to depend on the Ge coverage, falling from 698 K for ∼1/4 ML Ge on Si(100) to 573 K for a nearly full Ge coverage, consistent with H desorption on Ge(100). Furthermore, hydrogen is observed to migrate from Ge to Si, prior to desorption. This property opens the door for selective growth of Ge on patterned H-terminated Si surfaces

Enhancing the Reactivity of Al/CuO Nanolaminates by Cu Incorporation at the Interfaces

International audienceIn situ deposition of a thin (∼5 nm) layer of copper between Al and CuO layers is shown to increase the overall nanolaminate material reactivity. A combination of transmission electron microscopy imaging, in situ infrared spectroscopy, low energy ion scattering measurements, and first-principles calculations reveals that copper spontaneously diffuses into aluminum layers (substantially less in CuO layers). The formation of an interfacial Al:Cu alloy with melting temperature lower than pure Al metal is responsible for the enhanced reactivity, opening a route to controlling the stochiometry of the aluminum layer and increasing the reactivity of the nanoenergetic multilayer systems in general