Fabrication and characterization of multi-functional nonspherical particles via mechanical deformation

Nanoparticles bridge the gap between bulk materials and molecules. They possess exciting but contrasting mechanical, chemical, magnetic and optical properties when compared to their bulk counterparts due to their shape and size. When the size of noble metal nanoparticles is reduced to less than the wavelength of light, the particles intensely absorb and scatter light at wavelengths that depend on the particle size, shape, and local dielectric environment due to Localized Surface Plasmon Resonance (LSPR) modes. Recently, plasmonic particles have been used in a wide variety of sensor and optical device applications including immunoassays and surface enhanced Raman spectroscopy substrates. Controlling nanoparticle shape and resonance is essential for tuning these nanoparticles for a given application. Thus, fabrication of particles with different sizes and morphologies has been under research for a long time. The most prevalent methods for this purpose have been chemical synthesis and nano lithographic techniques. Mechanical deformation is an alternative method to control particle shape. This approach has been largely ignored since Faraday's pioneering work on converting gold particles into films by beating. Here we report a simple, but effective technique to control particle shape and LSPR wavelength via physical deformation of metal nanoparticles (∼50–100nm diameter). Particle size and shape is characterized using both electron microscopy and atomic force microscopy, while LSPR red-shifts are observed with dark-field spectroscopy. Controlling the shape and size of deformed particles requires quantification of force applied. A spring loaded instrument has been designed for this purpose and force applied on the particles and consequent deformation has been studied. This deformation method has also been applied to polystyrene, magnetic and hybrid micro and nano spheres. The processing technique employed here has potential for rapid and inexpensive tuning of nanoparticle shape and resonance while preserving particle volume. Thus, we establish a proof-of-principle depicting the validity of mechanical deformation as a means of fabricating non spherical particles of different functionalities.