Controlling photocurrent channels in scanning tunneling microscopy

We investigate photocurrents driven by femtosecond laser excitation of a (sub)-nanometer tunnel junction in an ultrahigh vacuum low-temperature scanning tunneling microscope (STM). The optically driven charge transfer is revealed by tip retraction curves showing a current contribution for exceptionally large tip-sample distances, evidencing a strongly reduced effective barrier height for photoexcited electrons at higher energies. Our measurements demonstrate that the magnitude of the photo-induced electron transport can be controlled by the laser power as well as the applied bias voltage. In contrast, the decay constant of the photocurrent is only weakly affected by these parameters. Stable STM operation with photoelectrons is demonstrated by acquiring constant current topographies. An effective non-equilibrium electron distribution as a consequence of multiphoton absorption is deduced by the analysis of the photocurrent using a one-dimensional potential barrier model

Local transport measurements in graphene on SiO2 using Kelvin probe force microscopy.

By using Kelvin Probe Force Microscopy with an additional applied electric field we investigate the local voltage drop in graphene on SiO2 under ambient conditions. We are able to quantify the variation of the local sheet resistance and to resolve localized voltage drops at line defects. Our data demonstrates that the resistance of line defects has been overestimated so far. Moreover, we show that wrinkles have the largest resistance, rho(Wrinkle) < 80 Omega mu m. Temperature-dependent measurements show that the local monolayer sheet resistance reflects the macroscopic increase in resistance with temperature while the defect resistance for folded wrinkles is best described by a temperature-independent model which we attribute to interlayer tunneling