SPM Techniques (most of the information comes from online references)

Scanning Probe Microscopes (SPMs) are a family of instruments used for analyzing surface properties of materials, from the micron level downto the atomic scale.

 

All SPMs contain essentially a sample, a probe tip,  positioning systems, a feedback circuitry and a computer system, able to drive the scanner, to measure data and to convert data into an image.

 

All SPMs contain essentially the components depicted in the figure.

 

A piezoelectric scanner moves the sample under the tip or the tip over the sample in a raster pattern, for a fine positioning.

 

A coarse positioning system brings the tip into the proximity of the sample.

 

A feedback system controls the vertical position of the tip, trough a mechanism able to sense the vertical position of the tip.

Scanning Tunneling Microscopy

The Scanning Tunneling Microscope (STM) is the precursor of all scanning probe microscopes: it was the first instrument able to generate real-space images of conductive surfaces with atomic resolution (1981).
The basic principle is simple: a sharpened, conducting tip, polarized versus a sample, is brought within about 10┼ of the sample. Electrons from the sample begin to "tunnel" through the 10┼ gap into the tip or vice versa (depending on the sign of the bias voltage). The resulting tunneling current varies with tip-to-sample gap and this signal is used to create an STM image. The tunneling current is an exponential function of distance, giving STMs their sensitivity: these instruments can image the surface of the sample with sub-angstrom precision vertically and atomic resolution laterally.

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Atomic Force Microscopy

The Atomic Force Microscope (AFM) probes the surface of a sample  with a sharp tip, a couple of microns long and often less than 100┼ in diametetr, located at the free end of a cantilever that is 100-200Ám long Forces between the tip and the sample surface cause the cantilever to bend or deflect. The sample can be insulator, semiconductor as well as as electrical conductor. A detector measures the cantilever deflection as the tip is scanned over the sample, or the sample is scanned under the tip. The measured cantilever deflections allow a computer to generate a map of surface topography. Several forces contribute to the deflection of an AFM cantilever; the most typical is an interatomic force, called the van der Waals force. The dependence of the van der Waals force upon the distnce between the tip and the sample determines the operative modality of the AFM tecnique.

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Other SPM Techniques:

 

Most of the AFMs detect the position of the cantilever with optical techniques.
In the most common scheme a laser beam bounces off the back of the cantilever onto a position-sensitive photodetector (PSPD).
As the cantilever bends, the position of the laser beam on the detector shifts. The PSPD itself can measure displacements of light as small as 10┼. As a result, the system can detect sub-angstrom vertical movement of the cantilever tip.

According to the implemented SPM techniques, this detection method varies.

 

Magnetic Force Microscopy (MFM)

Lateral force Microscopy (LFM)

Force Modulation Microscopy (FMM)

Phase Detection Microscopy (PDM)

Electrostaic Force Microscopy (EFM)

Scanning Capacitance Microscopy(SCM)

 

SPMs as Surface Analysis and Modification Tools

SPMs are most commonly used as tools for generating images of a sample's surface. They can also be used, however, to measure material surface properties at a single x,y point and/or to modify it.
 

An STM can be used as a spectroscopy tool, probing the electronic properties of a material with atomic resolution.The study of the dependence of an STM signal upon the local electronic structure of the sample surface is known as scanning tunneling spectroscopy (STS).

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The AFM can measure force vs. distance curves, which provide information about the local elastic properties of a surface. A force vs. distance curve is a plot of the force on an AFM cantilever tip in the z direction as a function of the z position of the piezo scanner tube.

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In addition, either AFM or STM can be used to modify intentionally the sample's surface, by applying high forces with an AFM or high-field pulses with an STM. These techniques are known as Nanolithography.

                                                                                                           

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