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Scanning Tunneling Microscopy (STM)

The Scanning Tunneling Microscope (STM) works well only with (electrically) conducting and semi conducting samples. The STM, a Primary Imaging Mode in SPM, was the first modern SPM, invented in 1982, winning the Nobel Prize in Physics for the inventors. The papers in which the STM invention and its first applications were revealed are the most widely cited papers in subsequent SPM-related publications.3 Despite the limitation that it does not work with electrically insulating samples, the STM has provided the highest resolution images in three dimensions on more samples than any other SPM method.

In STM, the probe is a sharp metal tip, typically made of either tungsten, or a platinum-iridium (Pt-Ir) alloy. The detector signal is the tunneling current I between the sample and the tip (typically 100’s pA to several nA) when a bias voltage V (typically 10‘s or 100‘s mV) is applied between the two. The magnitude of this current is extremely sensitive to, varying exponentially with, the small gap &delta that separates the nearest atoms between the tip and the sample surface.

This sensitivity, together with sub-Angstrom positioning precision and control of the instrument’s Z-actuator render the STM the highest-resolution SPM in all 3 dimensions. Atomic-scale resolution in STM images, in X, Y, and in Z, are routinely possible on atomically smooth surfaces of metals and semiconductors if the instrument‘s noise floor is low enough.

When the STM is operated in feedback mode, the tunneling current is the input signal for the feedback system. The output signal usually drives the Z-actuator, (same as in Contact mode AFM). In general, though, this type of image is not always a straightforward representation of the atomic scale topography because, the energy sates into and out of which the tunneling current .ows across the tip-sample separation depend on numerous parameters, including the strength and the polarity of the bias voltage.

STM’s reliance on the electric current limits its usefulness to samples that can sustain a large enough tunneling current that can stand out of the noise signal in the detector electronics. Although low-current STM has extended the use of STM to less conducting samples, in general insulators cannot be imaged with STM, or even with the more sensitive low-current STM.

3 G. Binning, H.Rohrer, C. Gerber, E. Weibel, “Surface studies by scanning tunneling microscopy,” Physical Review Letters, 49, pp. 57-61, 1982.

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