Thermal stability of size-selected arrays of gold nanoparticles on (112¯0) sapphire - A thermal desorption spectroscopy study

Exploiting the self-organization of spherical gold salt loaded inverse micelles formed from diblock copolymer poly(styrene)-block-poly(2-vinylpyridine) (PS-b-P2VP) in toluene, arrays of Au nanoparticles were prepared exhibiting narrow size as well as interparticle distance distributions. Starting with such Au nanoparticles (average particle diameters ranging between 1 nm and 5 nm) deposited on top of (112¯0)-oriented sapphire substrates, thermal desorption spectroscopy (TDS) experiments were performed up to a maximum temperature of 1473 K at a standard rate of 5 K/s in order to test for size dependent vapor pressures as expected from Kelvin's equation. Opposite to the expected increase of the vapor pressure for decreasing particle diameters, a split desorption behavior is observed with two peaks. Analyzing the data according to the Polanyi-Wigner equation, from the peak at lower temperature a desorption energy and attempt frequency close to bulk Au is extracted while the peak at higher temperature delivers significantly larger values for both properties. The latter observation is attributed to a substrate-nanoparticle interaction, which becomes dominant for decreasing particle diameters.