Quantitative analysis of the functionalization of gallium phosphide with organic azides

Gallium phosphide (GaP) surfaces were functionalized with two different molecules that contain an azide moiety at their terminus. Compound 4-azidophenacyl bromide (4AB) is an aryl azide with a bromine group at its opposite terminus that provides easy identification of the molecule's presence on the surface with x-ray photoelectron spectroscopy (XPS). O-(2-aminoethyl)-O'-(2-azidoethyl)pentaethylene glycol (AAP) is a small poly(ethylene glycol) molecule with an amine group at its opposite terminus. Atomic force microscopy was used to identify the uniformity of the clean and functionalized GaP surfaces. Water contact angle revealed a more hydrophobic surface with AAP functionalization (33°) and even more hydrophobic (53°) with the 4AB functionalized surface compared to a clean surface (16°). XPS confirmed the presence of each of the organic azides on the surface. XPS was further used to calculate the adlayer thickness of each functionalization. This analysis revealed an adlayer thickness of about 8 Å for the 4AB functionalized surfaces compared to 1 Å for the AAP adlayer, which led to the conclusion that AAP functionalization only provided partial coverage. A stability study using 4AB-functionalized surfaces showed good stability in saline solutions with varying concentrations of hydrogen peroxide. Finally, inductively coupled plasma mass spectrometry was used to evaluate the gallium concentration in the stability solutions. While the functionalization with the organic azides did not provide complete suppression of gallium leaching, both of the azides decreased the leaching by 10-50%.