Binnig & Rohrer: Paper 2 (Physics Review Letters 49 (1982), 57-61):The next publication, by the same four authors, combined the tunneling effect with scanning. STM was now tried out on surfaces of CaIrSn4 and Au samples, and they argued that the STM may be applied to two areas: 1) surface studies generally, incorporating space-resolved tunneling spectroscopy, microscopy of adsorbed molecules, and crystal growth; and 2) to fundamental aspects of tunneling. This second area highlights the fact that for the next two years Binnig and Rohrer struggled to achieve a more sophisticated understanding of the tunneling current than the one-dimensional potential barrier calculation of quantum physics textbooks. Rather, the work functions (energy levels) of tip and sample atoms somehow determined the current, and as our interview with them makes clear, this is in fact how they initially thought about the problem. At this stage, Binnig and Rohrer identified such a contribution to the current as a disturbing factor that could be isolated by using high-frequency modulation of the gap length during scanning. (Why these samples? Gold was commonly used in surface science, but we need to know more about the status of these materials in 1982. Can you help?)How to convince others of the correctness of STM measurements? One way
was to compare information gleaned on the surface under investigation
with that from other instruments and tools. Binnig, Rohrer, Gerber, and
Weibel first used low-energy electron diffraction (LEED) experiments for
this purpose. Of course, LEED does not actually measure the same kind
of thing as STM; for example while diffraction experiments show results
of interactions at the atomic level they necessarily average over these
interactions. And diffraction also requires that you infer the material
structure from a scattering pattern. By contrast, the claim for the STM
was that it revealed individual atomic structures and in real-space (no
inference, no Fourier transformation). Hence, there is a limit to the
evidential support that the STM could gain from LEED, but the comparison
could at least show that STM was not pure imagination. One might wonder
what the actual set-up was that allowed simultaneous investigation with
STM and LEED? One presumable keeps the sample still, while utilizing STM
and LEED one after the other. Sounds straightforward, but was it? The
authors did not test their results against sample analyses with other
techniques. They merely stated that the STM had greater resolution than
the scanning electron microscope (orders of magnitude better), and did
not destroy the sample as field ionization microscopy would do.
This page was last updated on 15 May 2001 by Arne Hessenbruch. |
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