Surface Chemistry Involved in Epitaxy of Graphene on 3C-SiC(111)/Si(111)

Surface chemistry involved in the epitaxy of graphene by sublimating Si atoms from the surface of epitaxial 3C-SiC(111) thin films on Si(111) has been studied. The change in the surface composition during graphene epitaxy is monitored by in situ temperature-programmed desorption spectroscopy using deuterium as a probe (D2-TPD) and complementarily by ex situ Raman and C1s core-level spectroscopies. The surface of the 3C-SiC(111)/Si(111) is Si-terminated before the graphitization, and it becomes C-terminated via the formation of C-rich (6√3 × 6√3)R30° reconstruction as the graphitization proceeds, in a similar manner as the epitaxy of graphene on Si-terminated 6H-SiC(0001) proceeds

In Situ SR-XPS Observation of Ni-Assisted Low-Temperature Formation of Epitaxial Graphene on 3C-SiC/Si

Low-temperature (~1073 K) formation of graphene was performed on Si substrates by using an ultrathin (2 nm) Ni layer deposited on a 3C-SiC thin film heteroepitaxially grown on a Si substrate. Angle-resolved, synchrotron-radiation X-ray photoemission spectroscopy (SR-XPS) results show that the stacking order is, from the surface to the bulk, Ni carbides(Ni(3)C/NiC(x))/graphene/Ni/Ni silicides (Ni(2)Si/NiSi)/3C-SiC/Si. In situ SR-XPS during the graphitization annealing clarified that graphene is formed during the cooling stage. We conclude that Ni silicide and Ni carbide formation play an essential role in the formation of graphene

Solving the Task Variant Allocation Problem in Distributed Robotics

We consider the problem of assigning software processes (or tasks) to hardware processors in distributed robotics environments. We introduce the notion of a task variant, which supports the adaptation of software to specific hardware configurations. Task variants facilitate the trade-off of functional quality versus the requisite capacity and type of target execution processors. We formalise the problem of assigning task variants to processors as a mathematical model that incorporates typical constraints found in robotics applications; the model is a constrained form of a multi-objective, multi-dimensional, multiple-choice knapsack problem. We propose and evaluate three different solution methods to the problem: constraint programming, a constructive greedy heuristic and a local search metaheuristic. Furthermore, we demonstrate the use of task variants in a real instance of a distributed interactive multi-agent navigation system, showing that our best solution method (constraint programming) improves the system’s quality of service, as compared to the local search metaheuristic, the greedy heuristic and a randomised solution, by an average of 16, 31 and 56% respectively

Sample collection from asteroid (162173) Ryugu by Hayabusa2: Implications for surface evolution

International audienc

Passive and Active Oxidation of Si(100) by Atomic Oxygen: A Theoretical Study of Possible Reaction Mechanisms

Reaction mechanisms for oxidation of the Si(100) surface by atomic oxygen were studied with high-level quantum mechanical methods in combination with a hybrid QM/MM (Quantum mechanics/Molecular Mechanics) method. Consistent with previous experimental and theoretical results, three structures, “back-bond”, “on-dimer”, and “dimer-bridge”, are found to be the most stable initial surface products for O adsorption (and in the formation of SiO2 films, i.e., passive oxidation). All of these structures have significant diradical character. In particular, the “dimer-bridge” is a singlet diradical. Although the ground state of the separated reactants, O+Si(100), is a triplet, once the O atom makes a chemical bond with the surface, the singlet potential energy surface is the ground state. With mild activation energy, these three surface products can be interconverted, illustrating the possibility of the thermal redistribution among the initial surface products. Two channels for SiO desorption (leading to etching, i.e., active oxidation) have been found, both of which start from the back-bond structure. These are referred to as the silicon-first (SF) and oxygen-first (OF) mechanisms. Both mechanisms require an 89.8 kcal/mol desorption barrier, in good agreement with the experimental estimates of 80−90 kcal/mol. “Secondary etching” channels occurring after initial etching may account for other lower experimental desorption barriers. The calculated 52.2 kcal/mol desorption barrier for one such secondary etching channel suggests that the great variation in reported experimental barriers for active oxidation may be due to these different active oxidation channels

Collisional history of Ryugu's parent body from bright surface boulders

International audienc