Charge transport dynamics through molecular scale materials is of common interest to both scientific and engineering disciplines. Putting molecules on nanoscale break junctions is the most straightforward setup to study charge transport dynamics through single molecules. Electromigration process can provide a simple and easy method of forming metallic oxide nanogap junctions. By using silver oxide thin films to form such nanogap junctions, silver nanoclusters (Ag2~Ag8) are also formed in-situ within the junctions. Formed silver nanoclusters strongly and stably electroluminesce under DC, AC, and customized pulse train excitation. By detecting extremely sensitive feedback, i.e. photons, two interesting behaviors of single molecule charge transport dynamics were revealed: 1) asymmetric charge transport and 2) discrete energy level tunneling. The discrete energy level tunneling of field emitted electrons yields negative differential resistance (NDR). Combined with photoconductivity and optical reduction of silver oxide to form silver nanoclusters, junction-asymmetry and NDR can be very useful in both electronic and optoelectronic applications such as on-demand electronics fabrication, single photon sources, and nanoscale photon detectors.