Potentiometry with the acoustic near field microscope : a new method for microscopy of surface potentials

We used a quartz tuning fork vibrating at 30 kHz both as an acoustic near field microscope and at the same time as a microscopic Kelvin probe. One leg of the tuning fork carried a small gold electrode serving as a conducting vibrating tip. By using this instrument and the method described here it is possible to measure simultaneously both the surface topography of the sample surface and the contact potential between tip and sample. The topography is observed by operating the instrument as an acoustic near Þeld microscope. The contact potential between the vibrating tip and the sample gives rise to a displacement current which is used here for the determination of the contact potential. In first applications of this method we demonstrate that the contact potential can be measured with a sensitivity of at least 100 mV and a local resolution of about 5 lm. It seems possible to use the microscopic method described here also for investigating local potentials at low temperatures and even in high magnetic fields. For example, the microscopic study of the Hall voltages in the quantum Hall effect might be an interesting application

Investigations of photoinduced tunneling current and local surface photovoltage by STM

Photoassisted scanning tunneling microscopy was used to simultaneously image topoyraphy, photoinduced tunneling current and local surface photovoltage on an nm-scale. A novel interrupted z-feedback technique is presented which overcomes the limitations of previously reported techniques, that were restricted to semiconductor surfaces with a high density of surface states. As an example, measurements on the van der Waals surface of WS2 are shown. This semiconductor surface is known to be free of intrinsic surface states. In the vicinity of monolayer steps an enhanced minority charge carrier recombination and a reduced photovoltage was observed