A high-reflectivity, ambient-stable graphene mirror for neutral atomic and molecular beams

Abstract

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...

Low-energy excitations of graphene on Ru(0 0 0 1)

“…The large scattering intensity from ruthenium RW is surprising, since Ru atoms are completely screened out by the graphene sheet, though compatible with the small surface charge density modulation induced by the exclusive motion of Ru substrate atoms. All this concurs to indicate a high flexibility of graphene at small wavevectors, which allows it to adapt to the instantaneous substrate profile, explaining the observed extraordinarily high specular reflectivities for He [15].…”

“…This system is especially interesting, since it is an excellent candidate to be used as a focusing mirror in scanning He atom microscopy [13]. A very high surface reflectivity to He atoms has been reported (23%) [14,15], which is surprising for a layer made of C atoms. Our current study provides a sound explanation for this fact, based on the low-energy phonon dynamics of this system.…”

Probing interlayer interactions between graphene and metal substrates by supersonic rare-gas atom scattering

“…It is found that the valley region of the moiré pattern on gr/Ru(0001) has a high effective mass reflecting stronger bonding of graphene to the substrate. This is attributable to the high reflectivity of He beam scattering reported for this system [35,36]. On the other hand, the effective mass of the hill region is found to be similar to that of HOPG, indicating that this region is well decoupled from the substrate and supported by van der Waals force.…”

“…However, the valley region seems to be tightly bound to the substrate. This fact contributes to the high reflectivity of the He beam, making this system attractive as a possibility for a mirror of a He atom microscope [35,36].…”

Zero-order filter for diffractive focusing of de Broglie matter waves

“…The use of graphene as a mirror coating gives an inert and very stable surface [24]. In addition, one-dimensional focusing with quantum reflection from a curved substrate has been demonstrated [25].…”