NSLS-II Seminar

It is clear that complete understanding of macroscopic properties of materials is impossible without a thorough knowledge of behavior at the smallest length scales. While the past 25 years have witnessed major advances in a variety of techniques that probe the nanoscale properties of matter, electrical transport measurements - the heart of condensed matter research - have lagged behind. This talk will describe a scanning tunneling potentiometry (STP) system constructed by us to simultaneously map the transport-related electrochemical potential distribution of a biased sample along with its surface topography.
Combining a novel sample biasing technique with a continuous current-nulling feedback scheme pushes the noise performance of the measurement to its fundamental limit - the Johnson noise of the STM tunnel junction. The resulting 130 nV voltage sensitivity allows us to spatially resolve local potentials at scales down to 2 nm, while maintaining atomic scale STM imaging, all at scan sizes of up to 15 microns. A mm-range two-dimensional coarse positioning stage and the ability to operate from liquid helium to room temperature with a fast turn-around time greatly expand the versatility of the instrument. Use of carefully selected model materials, combined with excellent topographic and voltage resolution has allowed us to distinguish measurement artifacts caused by surface roughness from true potentiometric features, a major problem in previous STP studies. The measurements demonstrate that STP can produce physically meaningful results for homogeneous transport as well as non-uni
form conduction dominated by material microstructures. The results establish scanning tunneling potentiometry as a useful tool for physics and materials science.