DATA ACQUISITION AND CONTROL-SYSTEM FOR MOLECULE AND ATOM-RESOLVED TUNNELING SPECTROSCOPY

A high-speed data acquisition and control system for atom-resolved tun neling spectroscopy based on an 80386 computer and a commercially avai lable data acquisition card is descibed. An analog proportional-integr al feedback controller with sample and hold is used to control the sca nning 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 capabil ities of the data acquisition card are used to minimize the data colle ction time. A current-to-voltage amplifier based on measuring the volt age drop across a resistor in series with the tunnel junction is used to amplify the tunnel current. This amplifier has a bandwidth of 250 k Hz at a gain of 10(9) V/A. The complete system is capable of obtaining an entire tunneling spectrum in 1.5 ms. The system can also simultane ously collect 128 spectroscopic images, a topographic image, and spati ally resolved tunneling spectra. Similar techniques can also be used t o obtain local barrier height measurements and barrier height images. Scanning tunneling microscopy and spectroscopy data for C60 adsorbed o n Au (111 are presented as examples of data collected using this syste m.