Abstract
Scanning tunneling potentiometry allows for studying charge transport on the nanoscale to relate the local electrochemical potential to morphological features of thin films or two-dimensional materials. To resolve the influence of atomic-scale defects on the charge transport, sub-µV sensitivity for the electrochemical potential is required. Here, we present a complete analysis of the noise in scanning tunneling potentiometry for different modes of operation. We discuss the role of various noise sources in the measurements and technical issues for both dc and ac detection schemes. The influence of the feedback controller in the determination of the local electrochemical potential is taken into account. Furthermore, we present a software-based implementation of the potentiometry technique in both dc and ac modes in a commercial scanning tunneling microscopy setup with only the addition of a voltage-controlled current source. We directly compare the ac and dc modes on a model resistor circuit and on epitaxial graphene and draw conclusions on the advantages and disadvantages of each mode. The effects of sample heating and the occurrence of thermal voltages are discussed.
Highlights
The miniaturization of electrical devices in the last few decades1 has brought their dimensions down to the nm scale where mesoscopic transport properties start playing an important role.2 investigating nanoscale charge transport phenomena is a challenging task.3,4 Scanning tunneling potentiometry (STP) is a technique capable of measuring the local electrochemical potential (ECP) distribution with μV resolution in current-biased samples, providing direct insight into mesoscale electron conduction.5–11 The ECP is determined by zeroing the tunneling current at the position of the tip via a compensation voltage
We present here a straightforward implementation of STP in a commercial ultra-high vacuum scanning tunneling microscopy (STM) (RHK Pan-Scan), which can be adapted to any STM setup
If we assume a similar ratio between waiting time and averaging time as we have in our experiment, we conclude that our performance in terms of the signal-to-noise ratio is comparable to the most recent STP implementations8,11 where a resolution of around 10 μV was demonstrated
Summary
The miniaturization of electrical devices in the last few decades has brought their dimensions down to the nm scale where mesoscopic transport properties start playing an important role. investigating nanoscale charge transport phenomena is a challenging task. Scanning tunneling potentiometry (STP) is a technique capable of measuring the local electrochemical potential (ECP) distribution with μV resolution in current-biased samples, providing direct insight into mesoscale electron conduction. The ECP is determined by zeroing the tunneling current at the position of the tip via a compensation voltage. The STP technique maintains the functionality of scanning tunneling microscopy (STM) and, allows for acquiring the sample topography with Å resolution This offers a unique method to directly relate electrical transport properties as given by the local ECP to the morphological features of the sample, such as grain boundaries, defects, and changes in composition. We present a complete noise analysis of a general STP implementation using either a dc or an ac detection scheme to identify the different noise contributions We analyze both sample-and-hold and dual-feedback approaches and compare their advantages and disadvantages. We discuss the role of the feedback control loop in the noise performance, which was previously not considered We implement both dc and ac sample-and-hold measurement procedures into our setup and directly compare their performances using a resistor circuit and a current-biased epitaxial graphene sample. A high sensitivity will allow us to investigate the subtle modifications in the ECP due to the coherent wave nature of charge carriers at defects and charge and spin accumulations due to momentum and spin dependent scattering phenomena.
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