It is shown that the surface of an epitaxial graphene monolayer grown on Ru(0001) could be used as a quite efficient external mirror for He-atom microscopy, with a specular reflectivity of 20% of the incident beam. Furthermore, the system is stable up to 1150 K, and the He reflectivity remains almost unchanged after exposure to air. Additionally, the high reflectivity for H 2 molecules (11%) opens up the development of a H 2 microprobe suitable for lithography. The Debye temperature for this epitaxial graphene monolayer has been determined from a study of the temperature dependence of the He specular intensity as a function of incident parameters. A value of 1045 K has been obtained, which is much higher than the 590 K reported for graphite under similar conditions, and close to the value of 1287 K calculated for isolated graphene.
The reactivity of Cu monolayer (ML) and bilayer films grown on Ru(0001) towards O 2 and H 2 has been investigated. O 2 initial sticking coefficients were determined using the King and Wells method in the incident energy range 40-450 meV, and compared to the corresponding values measured on clean Ru(0001) and Cu(111) surfaces. A relative large O 2 sticking coefficient (∼0.5-0.8) was measured for 1 ML Cu and even 2 ML Cu/Ru(0001). At low incident energies, this is one order of magnitude larger than the value observed on Cu(111). In contrast, the corresponding reactivity to H 2 was near zero on both Cu monolayer and bilayer films, for incident energies up to 175 meV. Water adsorption on 2 ML Cu/Ru(0001) was found to behave quite differently than on the Ru(0001) and Cu(111) surfaces. Our study shows that Cu/Ru(0001) is a highly selective system, which presents a quite different chemical reactivity towards different species in the same range of collision energies.
The dissociative adsorption of molecular hydrogen on Pd(x)Ru(1-x)/Ru(0001) (0 ≤ x ≤ 1) has been investigated by means of He atom scattering, Density Functional Theory and quasi-classical trajectory calculations. Regardless of their surroundings, Pd atoms in the alloy are always less reactive than Ru ones. However, the reactivity of Ru atoms is enhanced by the presence of nearest neighbor Pd atoms. This environment-dependent reactivity of the Ru atoms in the alloy provides a sound explanation for the striking step-like dependence of the initial reactive sticking probability as a function of the Pd concentration observed in experiments. Moreover, we show that these environment-dependent effects on the reactivity of H2 on single atoms allow one to get around the usual constraint imposed by the Brønsted-Evans-Polanyi relationship between the reaction barrier and chemisorption energy.
We report a He and H 2 diffraction study of graphene-terminated Ru(0001) thin films grown epitaxially on c-axis sapphire. Even for samples exposed for several weeks to ambient conditions, brief annealing in ultrahigh vacuum restored extraordinarily high specular reflectivities for He and H 2 beams (23% and 7% of the incident beam, respectively). The quality of the angular distributions recorded with both probes exceeds the one obtained from in-situ prepared graphene on Ru(0001) single crystals. Our results for graphene-terminated Ru thin films represent a significant step toward ambient tolerant, high-reflectivity curved surface mirrors for He-atom microscopy. Electromagnetic and electrostatic lenses and mirrors allow the facile manipulation of beams of charged particles (electrons, ions). The focusing of neutral particle (atom, molecule) beams is significantly more challenging, but it can in principle be accomplished by either refractive (Fresnel) or reflective optics (surface mirrors). Efficient focusing of neutral He beams would establish a foundation for realizing scanning He atom microscopy.1,2 Using focused beams of neutral, low-energy ($50 meV) He atoms for imaging, this technique could avoid issues due to electrical charging and beam damage inherent in higher energy electron-or ion microscopy, and thus provide unprecedented insight into the nanoscale structure of biological materials, polymers, ceramics, and other insulators.While the focusing of He atom beams to $2 lm spots has been demonstrated using Fresnel zone plates 2-5 and surface mirrors, 1,6,7 both approaches entail challenges that need to be resolved before sub-lm focusing can be achieved. Free-standing zone plates for high-transmission focusing of neutral beams are fragile, require complex fabrication processes, and suffer from chromatic aberrations. Mirror opticsinherently achromatic-provide a possible alternative. The main obstacle to using atom-focusing mirrors has been the requirement for atomically smooth surfaces with high specular reflectivity, which can remain long-term stable in vacuum and are easily recovered following exposure to ambient conditions. Initial work has employed mirrors of H-passivated Si(111) with specular reflectivities of $1% and limited stability in ambient air.1 Quartz mirrors, appealing for their simplicity, also have low reflectivity except under grazing incidence.6 Quantum-stabilized Pb films on Si(111) 8 and Pb(H3 Â H3)R30/Si(111) surfaces 9 provide high specular He reflectivity ($15%) but are only stable in ultrahigh vacuum (UHV). We have recently shown that monolayer graphene (MLG) on Ru(0001) single crystals can provide an inert surface 10,11 with high reflectivity for thermal He and H 2 beams.12 However, a path toward using such surfaces in curved mirrors has not been demonstrated so far.Curved MLG/Ru focusing elements may be realized by using polycrystalline Ru thin films conformally coating a shaped amorphous substrate (e.g., fused silica), 10 but surface roughness could limit the achievable He spot sizes...
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