Deposition and distribution of gold nanoparticles in a coffee-stain ring on the HOPG terraces

Abstract

Cetyltrimethylammonium bromide (CTAB)-stabilized low-aspect ratio gold nanorods have been synthesized using seed-mediated wet chemical approach. Freshly cleaved highly oriented pyrolytic graphite (HOPG) surface is used as a substrate for the deposition of nanoparticles via the droplet drying technique. Owing to the presence of CTAB molecules in suspension, nanoparticles on the non-wetting HOPG surface get deposited in the form of a coffee-stain ring. Such deposits at various positions of the coffee-stain ring have been characterized by scanning helium ion microscopy (HIM) and atomic force microscopy (AFM). Besides the assembly of nanoparticles, AFM is specially employed to investigate the deposition of CTAB molecules. The deposition and distribution of nanoparticles are studied and quantified at various positions of the coffee-stain ring. On flat HOPG terraces, HIM shows merely the deposits of the gold nanostructures whereas AFM reveals both nanoparticles and CTAB deposits over the surface. This research study primarily focused on the nanoparticle deposits within the coffee-stain ring formed over the non-wetting HOPG terraces.