Atomic-scale imaging and spectroscopy using scanning tunneling microscopy

Citation

Youngquist, Michael George (1993) Atomic-scale imaging and spectroscopy using scanning tunneling microscopy. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/0d6k-be86. https://resolver.caltech.edu/CaltechTHESIS:01112013-110414331

Abstract

Advances in scanning tunneling microscopy (STM) instrumentation and applications are presented. An ultrahigh vacuum (UHV) scanning tunneling microscope incorporating computer-controlled two-dimensional sample translation and in vacuo tip and sample transfer was developed. Its performance is documented through large-area and atomic-resolution imaging of highly stepped Si(111) 7x7 reconstructed surfaces and physisorbed clusters on graphite. An STM with automated approach and intra-Dewär spring suspension was developed for operation in cryogenic liquids. A high performance digital signal processor (DSP) based control system was constructed, and software with advanced spectroscopic imaging and data processing capabilities was developed.

The feasibility of individual-molecule vibrational spectroscopy via STM-detected inelastic electron tunneling is assessed. In preliminary experiments, a low-temperature STM was used for energy gap and phonon spectroscopy of superconducting Pb films. The first STM observation of phonon density of states effects in a superconductor is reported.

A systematic UHV STM imaging and spectroscopy study of 2H-MoS_2 was conducted. Atom-resolved images from three distinct imaging modes are presented. Occasional appearance of negative differential resistance (NOR) in I vs. V measurements is traced to changing tip electronic structure rather than localized surface states. Other potential NOR mechanisms are discussed including electron trap charging and resonant tunneling through a double-barrier quantum well structure arising from layer separation in the MoS_2 crystal.

DNA was imaged at atomic resolution with a UHV STM. Images show double-helical structure, base pairs, and atomic-scale substructure. Experimental STM profiles have atom-for-atom correlation with the A-DNA van der Waals surface. This work demonstrates the potential of the STM for characterization of large biomolecular structures.

Impurity-pinned steps on silicon and gold surfaces were imaged by STM. Pinned gold steps have short linear coherence lengths and form step loops at impurities by an Orowan-type bypassing mechanism. Step loops were not observed at Si(111) pinning sites; step contours seem to be correlated with the degree of order in the Si surface reconstruction.

Thesis Files