Measurement of the Energy Gap of Au55 Nanoparticles Using Far-infrared Transmission Spectroscopy

We have carried out far-infrared (far-IR) transmission measurements on the ligand stabilized Au55 nanoparticles dispersed in Teflon at volume fractions ranging from 0.2% to 1%. Instead of a series of peaks which might arise from the electron transition between discrete energy levels, we have observed a broad far-IR absorption coefficient that follows with an onset at {\Delta} ~ 10 cm-1. Furthermore this frequency dependence is totally different from that of the expected absorption due to the induced electric dipole. The onset of the absorption reminiscent of an energy gap at ~ 10 cm-1 (~ 1. 2 meV) is surprisingly smaller than that of the expected for a metal sphere of 5.3 {\AA} in radius and independent of temperature. In this work we did not observe the level correlation effect on the far-IR absorption predicted for an ensemble of metal nanoparticles. It is concluded that Au55 nanoparticles behave like a three-dimensional (3D) semimetal with an energy gap {\Delta} ~ 10 cm-1 rather than a giant atom.