Optical observation of single-electron charging effect in metallic particles

We have investigated the single-electron charging effect of small Au particles by scanning tunneling microscope (STM) light emission spectroscopy and scanning tunneling spectroscopy (STS) at 4.7 K. A double tunnel junction was formed by an Al film substrate, an oxide layer, a small Au particle, a vacuum gap, and the STM tip. The high-energy cutoff of the emitted light became less than the quantum-cutoff value ${\mathrm{eV}}_0$ when the Coulomb staircase appeared in the STS spectrum. Here e is the elementary charge and $V_0$ is the bias voltage applied across the double tunnel junction. The observed decrement of the high-energy cutoff from ${\mathrm{eV}}_0$ agrees with the value calculated with the circuit parameters of the double tunnel junction determined from the STS spectrum. This result demonstrates that the STM light emission spectroscopy provides an alternate means of investigating the single-electron charging effect complementary to the ordinary electrical measurements.