Scanning Tunneling Microscopy (STM)


Scanning Tunneling Microscopy (STM) is a non-optical, very high-resolution microscopy technique that is used for obtaining images of conductive surfaces at atomic scale level (~2 angstroms, i.e., 0.2 nanometer).  Just like the AFM, it is a type of scanning probe microscopy, employing an atomically sharp probe tip that is scanned over the sample surface in order to accomplish its imaging function.  The equipment used for STM is called a scanning tunneling microscope, which also goes by the acronym of 'STM'.  Aside from atomic level imaging, STM can also be used to alter the sample by manipulating individual atoms, initiating chemical reactions, and creating ions.


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STM operates on the basis of a phenomenon known as 'quantum mechanical tunneling.'  This phenomenon is characterized by the fact that a very small current will flow between a sharp metal probe tip and the surface of an electrically conductive material if: 1) a voltage is applied between the tip and the conductive material; and 2) the tip is positioned just a few nanometers away from the sample surface, i.e., no contact is made between the tip and the sample.


The current produced by tunneling is called 'tunneling current', which is on the order of one nanoamp for an applied voltage of 1 V. The amount of tunneling current produced is exponentially dependent on the distance between the tip and the sample surface. This sensitivity of the tunneling current to the tip's distance from the sample is utilized by the STM in its operation.


STM operates in constant current mode, i.e., the distance between the tip and the sample surface is kept constant in order to keep the tunneling current constant.  Since the topography of the sample surface changes, the ST microscope must move the probe tip according to how the sample topography varies in order to keep the tip-to-sample distance constant. 


The tip is mounted on a piezoelectric tube which controls the position of the tip in three dimensions relative to the sample. The piezo element that moves the tip towards or away from the sample surface is controlled by a feedback circuit that monitors the tunneling current in order to determine whether the tip is too close or too far from the surface.  This feedback circuit supplies the electrode of the piezo element with a control voltage that moves the tip in the right direction to keep the distance of the tip from the sample constant.


As the tip is scanned line by line over a small area of the sample surface in the x-y plane, topographic data based on the z-axis position of the tip  (which, in turn, is based on the tunneling current) is collected by the computer of the STM.  The image of the topography of the sample may then be reconstructed from the collected data. Under the right conditions, high-quality STM's can produce images with sufficient resolution to show individual atoms.  STM images are commonly presented in greyscale, with protrusions shown in white and depressions in black.


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STM is one of the most important tools for surface physics and chemistry studies.  With the ability to show the structure of the uppermost layer of atoms or molecules, STM can be used to reveal surface defects, display the morphology of various depositions, or measure the surface roughness of a wafer in the angstrom-range. STM may also be used in the study of conduction or charge transport mechanisms.


STM can also be used to move single atoms accurately, by pushing or dragging them with the tip at low temperatures. Electrons emitted by the tip can also be used to alter the sample. The ability of STM to serve as a tool for 'rearranging' atoms has made it an important tool in nanosciences.


STM does not need a vacuum in order to operate, although it is usually operated in an ultrahigh vacuum environment to avoid contamination or oxidation of sample surfaces when high-resolution imaging of metals or semiconductors is required. Surface oxidation reduces the conductivity of the sample's surface and affects the tunneling current, resulting in imaging problems.


Since STM operates on the flow of the tunneling current, it can not be used on non-conductive samples.  It may be possible to coat a non-conductive sample with a conductive layer such as gold to make it observable under an STM, but this coating step can mask hide certain features or degrade imaging resolution.  


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