Scanning probe microscopy (SPM) is a branch of microscopy that forms images of surfaces using a physical probe that scans the specimen. SPM was founded in 1981, with the invention of the scanning tunneling microscope, an instrument for imaging surfaces at the atomic level. The first successful scanning tunneling microscope experiment was done by Gerd Binnig and Heinrich Rohrer. The key to their success was using a feedback loop to regulate gap distance between the sample and the probe. Many scanning probe microscopes can image several interactions simultaneously. The manner of using these interactions to obtain an image is generally called a mode. The resolution varies somewhat from technique to technique, but some probe techniques reach a rather impressive atomic resolution. This is largely because piezoelectric actuators can execute motions with a precision and accuracy at the atomic level or better on electronic command. This family of techniques can be called "piezoelectric techniques". The other common denominator is that the data are typically obtained as a two-dimensional grid of data points, visualized in false color as a computer image.
Established types
AFM, atomic force microscopy Contact AFM Non-contact AFM Dynamic contact AFM Tapping AFM AFM-IR CFM, chemical force microscopy C-AFM, conductive atomic force microscopy EFM, electrostatic force microscopy KPFM, kelvin probe force microscopy MIM, microwave impedance microscopy MFM, magnetic force microscopy PFM, piezoresponse force microscopy PTMS, photothermal microspectroscopy/microscopy SCM, scanning capacitance microscopy SGM, scanning gate microscopy SQDM, scanning quantum dot microscopy SVM, scanning voltage microscopy FMM, force modulation microscopy TAFM, Tomographic AFM STM, scanning tunneling microscopy BEEM, ballistic electron emission microscopy ECSTM electrochemical scanning tunneling microscope SHPM, scanning Hall probe microscopy SPSM spin polarized scanning tunneling microscopy PSTM, photon scanning tunneling microscopy STP, scanning tunneling potentiometry SXSTM, synchrotron x-ray scanning tunneling microscopy SPE, Scanning Probe Electrochemistry SECM, scanning electrochemical microscopy SICM, scanning ion-conductance microscopy SVET, scanning vibrating electrode technique SKP, scanning Kelvin probe SECCM, scanning electrochemical cell microscopy FluidFM, fluidic force microscopy FOSPM, feature-oriented scanning probe microscopy MRFM, magnetic resonance force microscopy NSOM, near-field scanning optical microscopy (or SNOM, scanning near-field optical microscopy) nano-FTIR, broadband nanoscale SNOM-based spectroscopy SSM, scanning SQUID microscopy SSRM, scanning spreading resistance microscopy SThM, scanning thermal microscopy SSET scanning single-electron transistor microscopy STIM, scanning thermo-ionic microscopy CGM, charge gradient microscopy SRPM, scanning resistive probe microscopy
Image formation To form images, scanning probe microscopes raster scan the tip over the surface. At discrete points in the raster scan a value is recorded (which value depends on the type of SPM and the mode of operation, see below). These recorded values are displayed as a heat map to produce the final STM images, usually using a black and white or an orange color scale.
Constant interaction mode In constant interaction mode (often referred to as "in feedback"), a feedback loop is used to physically move the probe closer to or further from the surface (in the z axis) under study to maintain a constant interaction. This interaction depends on the type of SPM, for scanning tunneling microscopy the interaction is the tunnel current, for contact mode AFM or MFM it is the cantilever deflection, etc. The type of feedback loop used is usually a PI-loop, which is a PID-loop where the differential gain has been set to zero (as it amplifies noise). The z position of the tip (scanning plane is the xy-plane) is recorded periodically and displayed as a heat map. This is normally referred to as a topography image. In this mode a second image, known as the ″error signal" or "error image" is also taken, which is a heat map of the interaction which was fed back on. Under perfect operation this image would be a blank at a constant value which was set on the feedback loop. Under real operation the image shows noise and often some indication of the surface structure. The user can use this image to edit the feedback gains to minimise features in the error signal. If the gains are set incorrectly, many imaging artifacts are possible. If gains are too low features can appear smeared. If the gains are too high the feedback can become unstable and oscillate, producing striped features in the images which are not physical.
Constant height mode In constant height mode the probe is not moved in the z-axis during the raster scan. Instead the value of the interaction under study is recorded (i.e. the tunnel current for STM, or the cantilever oscillation amplitude for amplitude modulated non-contact AFM). This recorded information is displayed as a heat map, and is usually referred to as a constant height image. Constant height imaging is much more difficult than constant interaction imaging as the probe is much more likely to crash into the sample surface. Usually before performing constant height imaging one must image in constant interaction mode to check the surface has no large contaminants in the imaging region, to measure and correct for the sample tilt, and (especially for slow scans) to measure and correct for thermal drift of the sample. Piezoelectric creep can also be a problem, so the microscope often needs time to settle after large movements before constant height imaging can be performed. Constant height imaging can be advantageous for eliminating the possibility of feedback artifacts.
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