Aaron Shojinaga with Jie Shan

AC  Conductivity of Silver Nanoparticles

(started in fall 2009 and continuing into fall 2010)

The electrical conductivity for bulk metal is described by the well-known Drude model. As the size of the metal is reduced to the nanometer scale however, the energy levels become discrete, rather than continuous. The average spacing between adjacent energy levels in a metal nanoparticle is called the Kubo gap, and is related to the Fermi energy of the metal and the size of the nanoparticle. For instance, in a silver nanoparticle of 3-nm diameter containing ~103 atoms, the Kubo gap is around 5-10 meV. Therefore, at room temperature when the thermal energy is greater than this gap, the electrical conductivity will be the same as in bulk metal. As the temperature is lowered however, the Kubo gap becomes significant and the nanoparticle becomes an insulator. Although the DC properties of this metal-to-insulator transition are well understood, the experimental observations and theoretical description for AC conductivity are much less comprehensive. In this project I will investigate the AC conductivity of silver nanoparticles in an interesting frequency range that corresponds with the Kubo gap of the nanoparticles. Conductivity will be measured using terahertz (1 THz ~ 4.2 meV) time-domain spectroscopy based on a mode-locked laser. The frequency dependence of the complex conductivity will be investigated as a function of the sample temperature to understand the metal-to-insulator transition.