Direct patterning of gold nanoparticles using dip-pen nanolithography.

Various methods for the patterned assembly of metal nanoparticles have been developed in order to harness their unique electrical and optical properties for device applications. This paper discusses a method for direct writing of Au nanoparticles at nanoscale resolution using dip-pen nanolithography. First, a procedure was developed for increasing the loading of Au nanoparticles onto AFM tips to prolong patterning life. AFM tips were subsequently imaged by scanning electron microscopy to determine ink coverage and to gain insight into the deposition process. Next, surface interactions, relative humidity, and writing speed were controlled to determine an optimal range of conditions for deposition. Various ink-substrate combinations were studied to elucidate the dependence of deposition on interactions between Au nanoparticles and the substrate surface; inks consisted of positively and negatively charged particles, and substrates were SiO(2) surfaces modified as hydrophilic or hydrophobic and interacted electrostatically or covalently with Au nanoparticles. Results indicate that a highly hydrophilic surface is required for Au nanoparticle deposition, unless covalent binding can occur between the Au and substrate surface. The optimal range of relative humidity for patterning was found to be 40-60%, and Au nanoparticle deposition was not sensitive to writing speeds ranging from 0.01 to 2 microm/s.