Scanning Tunneling Microscope and Atomic Force Microscopy

Abstract:

Atomic Force Microscopy (AFM) is a development over Scanning Tunneling Microscopy (STM) with earlier technique only allowing conductors to be imaged. Atomic Force Microscopy has been able to image insulators also with atomic resolution by substituting tunneling current with an atomic contact force sensing arrangement, a delicate cantilever, which can image conductors and insulators alike via mechanical “touch” while running over surface atoms of the sample.

Since the sample surface contamination with foreign atoms and humidity can compromise the success of AFM, it is done in ultra-high vacuum environment with the surface adequately cleaned of impurities and prepared as thin-film.

The success of ATM as a cost-effective imaging tool with dramatically increased ease of use has seen its proliferation to numerous labs in universities and tech companies worldwide.

Article:

Imaging at atomic resolution had been an elusive goal until the introduction of scanning tunneling microscopy (STM) in 1981 by Binnig, Rohrer, Gerber and Weibel (1982). This novel approach based on the quantum mechanical concept of quantum tunneling whereby which an electron tunnels through the vacuum gap separating the biased conducting tip and conducting surface if the distance is very close i.e. atomic diameters ranges (typically 0.3-3Å).

The tunneling current being a function of separation distance, voltage difference and local density of states (LDOS) (a measure of available states per energy level in a quantum mechanical system such as an atom) fluctuates as probing tip passes over the sample surface and is converted into voltage which is mapped as imagery with the help of a computer software.

This modest instrument has provided a breakthrough in our ability to investigate and manipulate matter on atomic scale as for the first time individual surface atoms of flat surface were made visible in real time space. The invention of STM solved the most confounding problem of structure of Si (111)-(7x7) surface which was regarded as touchstone for applicability of emerging technology of STM. Takayanagi, Tanishiro, Takahashi and Takahashi (1985) complemented X-ray-crystallography with electron-scattering to STM and developed dimer-adatom-stacking fault (DAS) model for Si (111)-(7x7). Consequently, G. Binnig and H. Rohrer were awarded Nobel Prize in Physics in 1986 for their invention.

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Fig 1. Si (111)-(7x7) reconstruction imaged at 1000K, Kun and Bin - The Chinese University of Hong Kong, RHK Technology, Inc.

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Fig 2. Schematic of STR showing electron current tunneling through the potential barrier separating two electrodes (metallic or semiconductor) when distance is reduced to few atomic diameters.

Quantum physics is physics will come into play predominating when distance reach microscopic scales. Classical physics is macroscopic culmination of quantum physics happening at microscopic scale. Quantum mechanics is different from classical physics in the sense that a particle is not treated as a point object as in classical physics but a quantifiable “blob” which is accordance with Heisenberg uncertainty principle can be represented by fixed area two-dimensional objects on momentum-position plane. In quantum mechanics particle nature is discovered by measurement which collapses the assigned wave Ѱ(x,y,z,t) into one of the probabilities. Wave vector is change every time a measurement takes place. Hence all particles and their interaction can be considered as waves and properties associated with them. A wave continues to encompass an area where probability of finding the particle is high. When an act of measurement is done, the wave form representing the “blob” without shape or size collapses in to the particle form with observable shape and size which may go back to waveform if time is allowed. Thus act of measurement changes the wave function, collapsing it into an observable particle instantly of occurrence that fits a statistical model representing sufficiently large number of observations.

In classical physics the electron cannot penetrate a potential barrier Ф if its energy E is smaller. The quantum mechanical treatment predicts a different picture. It predicts the electron wave function will undergo exponential decaying while penetrating the barrier whilst being available on the other side. In STM a small bias voltage is applied so that due to electric field the tunneling of electron results in a tunneling current.

The height of the barrier can roughly be approximated by the average workfunction of the sample and tip.

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