Chemically Mediated Grain Growth in Nanotextured Au, Au/Cu Thin Films: Novel Substrates for the Formation of Self-Assembled Monolayers

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Abstract

We describe a solution-based procedure that affects the bulk recrystallization of Au and Au/Cu alloy thin films. Nanocrystalline Au or Au/Cu films of 1800−2000 Å in thickness were deposited on glass substrates by DC-plasma-source sputtering. A 5 Å thick Ti layer was used to promote the adhesion of these films to the substrate. The films were sequentially treated with piranha solution (3:1 concentrated H2SO4/30% H2O2), followed by immersion in 3:1:16 HCl/HNO3/H2O. Atomic force microscopy (AFM) revealed that this treatment results in a coalescence of the fine-scale metal grains into larger, often irregular shaped domains. Commercially acquired films grown by thermal evaporation behaved in a similar manner but displayed more extensive grain growth. X-ray diffraction (XRD) rocking angle measurements made of the Au(222) peak show a reduction in the fwhm of approximately 50−80% in the treated Au films, indicating that the oxidative treatment induces a significant bulk recrystallization of the metal. The dynamics suggest that the recrystallization is related to the preferential dissolution of Au and/or impurities present at grain boundaries. This dissolution leads to their unpinning and subsequent merger into larger grains. This hypothesis was tested by cosputtering copper with the Au to form a dilute Cu/Au thin film. AFM data confirmed that the inclusion of this diluent significantly enhances grain growth and decreases surface rms roughness. The bulk recystallization effect diminishes, however, with either an increase in the adhesion layer thickness or copper content above a finite limit. To test their surface qualities, treated Au films were used as substrates for the growth of self-assembled monolayers (SAMs) of hexadecanethiol. We found that the resulting SAMs had exceptional barrier properties, being extremely impermeable to aqueous redox moieties as measured by cyclic voltammetry.

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