Nanostructured carbon materials prepared by hole-mask colloidal lithography

Licentiate thesis, 2007

Research and development of nanofabrication methods can be motivated both for manufacturing of commercially available products like micro electronic components and for development of model systems for fundamental and applied science. The fabrication process developed during this work, hole-mask colloidal lithography (HCL), is primarily oriented towards the latter two, specifically for research in the fields of catalysis, nanoparticle plasmons, and bio- and fundamental physics. Design of structured samples with precise control over size and shape of the nanostructures are crucial components in all of these fields. The thesis describes the hole-mask colloidal lithography (HCL) technique, the general principles of HCL and the technical and functional differences from standard colloidal lithograpy (CL). The technique is illustrated with examples giving details on how to fabricate features with diameters from ~40 to 400 nm and with different shapes and mutual orientations. Some of the demonstrated geometries are discs, ellipses, cones, particle pairs and particles buried into a TiO2 surface. The nanostructuring of carbon materials using the HCL technique is described in detail. Nanostructured carbon surfaces are relevant as model systems to study the optical properties of naturally occurring nanocarbon structures like aerosols and interstellar dust. The applied fabrication process utilizes oxygen plasma to etch the part of the carbon surfaces, not protected by the HCL mask. Analysis of the structure size and shape resulting from the applied process parameters gives information about the materials durability in reactive oxygen atmospheres, which is valuable for applications where carbon materials are exposed to similar environments. The HCL technique is used to create etch-masks subsequently used to nanostructure carbon surfaces via oxygen RIE. HOPG and GC surfaces are patterned in parallel using identical fabrication processes. Careful characterization of the resulting size and shape of the carbon nanostructures, using SEM and AFM, revealed a significant difference in response to oxygen plasma treatments for the two materials. On HOPG lateral etching under the etch mask is effectively suppressed thus resulting in practically no undercut while GC is subject to severe etching under the masks. The etch rate in the forward direction was found to be more than three times higher for GC than HOPG (0.65 and 0.19 nm/s respectively). The HOPG nanostructuring process was also followed with spectrophotometry, revealing decreased reflectance as a result of the evolution of nanostructures. Part of the change in reflectance is due to the presence of the etch mask, which consists of gold nanodiscs, but the major part is attributed to the carbon nanostructures.