Experimental Investigation of Low Frequency Electromagnetic Wave Interaction with Metallic Nanoparticles

The interaction of electromagnetic radiation with small metallic particles has been studied in greater detail during the last decade. It is well known that the noble metallic nanoparticles, like gold and silver exhibit remarkable optical properties, viz, strong colors exhibited by these nanoparticles. These particles acquire a characteristic color due to plasmon resonance. Plasmon resonance occurs due to coherent oscillation of the conduction band electrons induced by the incident electromagnetic field at optical frequencies. Optical techniques are conventionally used to detect the surface plasmon resonance modes in metallic nanoparticles with nanometer resolution. In these techniques, the electric field around the nanoparticle is usually sensed and imaged. Moreover, these techniques are used at optical frequencies. The imaging of magnetic field around the nanoparticles at high frequencies is very complicated. The imaging of magnetic field around nanoparticles at low frequency electromagnetic radiation has not been reported so far in the literature, to the knowledge of the authors. In this paper, we report a new methodology to image magnetic moments of metallic nanoparticles in low frequency electromagnetic fields. To accomplish this, a traditional atomic force microscope (AFM) is externally modified to detect near-field magnetic fields of the nanoparticles. Samples of silver and platinum nanoparticles are kept in the electromagnetic field excited at frequencies in the range of 30-100 kHz. The magnetic field around the nanoparticles is then detected by a magnetic AFM tip-cantilever positioned at a distance of few nanometers. The output of the cantilever is used to separate topography and magnetic field image using external electronic instrumentation. Magnetic field images are obtained at different frequencies and the effect of size, shape and frequency on the magnetic field distribution is studied. The results of the magnetic field distribution are analyzed in view of the possibility of using the methodology for sensing application.