Silicon oxide nanoparticles grown on graphite by co-deposition of the atomic constituents

Abstract

Nanoscale silicate dust particles are the most abundant refractory component observed in the interstellar medium and are thought to play a key role in catalysing the formation of complex organic molecules in the star forming regions of space. We present a method to synthesise a laboratory analogue of nanoscale silicate dust particles on highly oriented pyrolytic graphite (HOPG) substrates by co-deposition of the atomic constituents. The resulting nanoparticulate films are sufficiently thin and conducting to allow for surface science investigations, and are characterised here, in situ under UHV, using X-ray photoelectron spectroscopy, near-edge X-ray absorption atomic fine spectroscopy and scanning tunnelling microscopy, and, ex situ, using scanning electron microscopy. We compare SiO_x film growth with and without the use of atomic O beams during synthesis and conclude that exposure of the sample to atomic O leads to homogeneous films of interconnected nanoparticle networks. The networks cover the graphite substrate and demonstrate superior thermal stability, up to 1073 K, when compared to oxides produced without exposure to atomic O. In addition, control over the flux of atomic O during growth allows for control of the average oxidation state of the film produced. Photoelectron spectroscopy measurements demonstrate that fully oxidised films have an SiO₂ stoichiometry very close to bulk SiO₂ and scanning tunnelling microscopy images show the basic cluster building unit to have a radius of approximately 2.5 nm. The synthesis of SiO_x films with adjustable stoichiometry and suitable for surface science experiments that require conducting substrates will be of great interest to the astrochemistry community, and will allow for nanoscale-investigation of the chemical processes thought to be catalysed at the surface of dust grains in space.