Impressively Printing Patterns of Gold and Silver Nanoparticles.

The optical and chemical properties of gold and silver nanoparticles make them useful for many applications, including surface enhanced spectroscopy-based biosensors, photostable colorants, enhanced photovoltaics, and nanoscale optical elements. We report a simple technique to generate patterns of gold and silver nanoparticles with controlled shape and shape-dependent optical properties using metal stamps to impress them onto a glass substrate or flexible polymers. The pressure flattens the nanoparticles, converting initially spherical nanoparticles into discs with reduced height and increased diameter. This deformation causes their localized surface plasmon resonance wavelength to red-shift. Nanoparticles were characterized by electron microscopy, atomic force microscopy, and dark field optical scattering spectroscopy. The deformed nanoparticle patterns had a lateral resolution limited by the nanoparticle diameter (single particles are partly flattened only where they contact the stamp). The method also (i) transfers the stamp's topography, with smooth stamps generating flattened nanoparticles with uniform height, and small changes in stamp height are evident in the nanoparticle height and scattering wavelength, and (ii) allows facile removal of undeformed nanoparticles using scotch tape, and patterns of deformed nanoparticles can be transferred to a thin polymer-film. The patterning process is simple and inexpensive. It can be performed by hand for demonstrations or artistic applications, with controlled force for plasmonics research, and potentially automated on reel-to-reel presses for large scale production.

Tuning Localized Surface Plasmon Resonance Wavelengths of Silver Nanoparticles by Mechanical Deformation.

We describe a simple technique to alter the shape of silver nanoparticles (AgNPs) by rolling a glass tube over them to mechanically compress them. The resulting shape change in turn induces a red-shift in the localized surface plasmon resonance (LSPR) scattering spectrum and exposes new surface area. The flattened particles were characterized by optical and electron microscopy, single nanoparticle scattering spectroscopy, and surface enhanced Raman spectroscopy (SERS). AFM and SEM images show that the AgNPs deform into discs; increasing the applied load from 0 to 100 N increases the AgNP diameter and decreases the height. This deformation caused a dramatic red shift in the nanoparticle scattering spectrum and also generated new surface area to which thiolated molecules could attach as evident from SERS measurements. The simple technique employed here requires no lithographic templates and has potential for rapid, reproducible, inexpensive and scalable tuning of nanoparticle shape, surface area, and resonance while preserving particle volume.