Probing local currents in semiconductors with single molecules

The narrow zero-phonon lines of single molecules at low temperatures can be used as sensitive probes for slow dynamical processes in solids at nanometer scales. Here we propose to probe electric conduction in semiconductors. In poorly conducting samples of ZnO, we found that only a small fraction of the molecules react to electric currents, but these reactions can be very strong, leading to line broadening or to changes in the fluorescence autocorrelation function. Even for moderate applied voltages., we found a few "hot spots" pointing to a strong spatial concentration of joule heating in areas less than 100 nm in size. A single molecule can therefore act as a nanothermometer. In more conducting samples of indium-tin oxide, we found even more surprising effects. For most molecules, we observed large shifts of the molecular lines under static voltages. The shift does not arise from a conventional Stark effect and cannot be attributed to lattice heating because the lines do not broaden, even for the highest voltages we used. We propose that the shift is caused by a change of polarizability of the semiconductor on application of a current, possibly related to hot carriers. When we applied ac currents to the sample, we observed clear resonant structures at very low frequencies, between 100 Hz and a few MHz. The resonance spectra were completely different for different molecules in the same laser spot of less than 1 micrometer in radius. We also observed autooscillations of the molecular transition frequency when a dc voltage was applied to the semiconductor film, with a clear threshold and oscillation frequencies lower than 100 kHz. The interpretation of these effects is still open, but we think that the molecules are very close to the semiconductor surface, making image effects quite strong. The surprising resonating systems we discovered could be related to recharging waves, whose existence was predicted theoretically some 30 years ago in compensated semiconductors.