Data acquisition and control system for molecule and atom-resolved tunneling spectroscopy

Abstract

A high-speed data acquisition and control system for atom-resolved tunneling spectroscopy based on an 80386 computer and a commercially available data acquisition card is descibed. An analog proportional-integral feedback controller with sample and hold is used to control the scanning tunneling microscope. Constant tip-sample separation during data collection is maintained by interrupting the feedback loop. Assembly language routines taking advantage of the direct memory access capabilities of the data acquisition card are used to minimize the data collection time. A current-to-voltage amplifier based on measuring the voltage drop across a resistor in series with the tunnel junction is used to amplify the tunnel current. This amplifier has a bandwidth of 250 kHz at a gain of 109 V/A. The complete system is capable of obtaining an entire tunneling spectrum in 1.5 ms. The system can also simultaneously collect 128 spectroscopic images, a topographic image, and spatially resolved tunneling spectra. Similar techniques can also be used to obtain local barrier height measurements and barrier height images. Scanning tunneling microscopy and spectroscopy data for C60 adsorbed on Au(111) are presented as examples of data collected using this system.