A tag-and-count approach for quantifying surface silanol densities on fused silica based on atomic layer deposition and high-sensitivity low-energy ion scattering

Abstract

Surface silanols (SiOH) are important moieties on glass surfaces. Here we present a tag-and-count approach for determining surface silanol densities, which consists of tagging surface silanols with Zn via atomic layer deposition (ALD) followed by detection of the zinc by high sensitivity-low energy ion scattering (HS-LEIS). Shards of fused silica were hydroxylated with aqueous hydrofluoric acid (HF) and then heated to 200, 500, 700, or 900 °C. These heat treatments increasingly condense and remove surface silanols. The samples then underwent one ALD cycle with dimethylzinc (DMZ) or diethylzinc (DEZ) followed by water. As expected, fused silica surfaces heated to higher temperatures showed lower Zn coverages. When fused silica surfaces treated at 200 °C were exposed to DMZ for two different dose times, the same sub-monolayer quantity of Zn was obtained by X-ray photoelectron spectroscopy (XPS). Surface cleaning/preparation immediately before HS-LEIS, including atomic oxygen treatment and annealing, played a critical role in these efforts. Surfaces treated with DMZ generally showed slightly higher Zn signals by LEIS. Using this methodology, a value of 4.59 OH/nm2 was found for fully hydroxylated fused silica. Both this result and those obtained at 500, 700, and 900 °C are in very good agreement with literature values.