Common to molecular electronics studies, nanoscale break junctions created through electromigration naturally produce electroluminescent arrays of individual gold nanoclusters spanning the electrodes. The arrival time stamping of detected photons from these electrically driven, room-temperature nanoclusters demonstrates that charge injection to the clusters is directly modulated by dynamic coupling to individual electrodes. These several-atom nanoclusters electroluminesce due to inelastic electron tunneling into cluster electronic energy levels. ac electrical excitation $(\sim 200\mathrm{MHz})$ with time-stamping of photon arrival times enables fast and local tracking of electrode-nanocluster coupling dynamics. The electrode-nanocluster coupling rate fluctuates by nearly an order of magnitude, and due to the asymmetry of the electromigration process, exhibits preferential charge injection from the anode. Directly reporting on nanoscale charge transport, time-tagged single-molecule electroluminescence reveals a significant mechanism for nanoscale charge transport in nanoscale gold break junctions and offers a direct readout of the electrode-molecule interactions that can be correlated with current flow.

• Received 17 February 2006

DOI:https://doi.org/10.1103/PhysRevB.74.064305