Conductive atomic force microscopy (C-AFM) is a variation of atomic force microscopy (AFM) and scanning tunneling microscopy (STM), which uses electrical current to construct the surface profile of the studied sample. The current is flowing through the metal-coated tip of the microscope and the conducting sample. Usual AFM topography, obtained by vibrating the tip, is acquired simultaneously with the current. This enables to correlate a spatial feature on the sample with its conductivity, and distinguishes C-AFM from STM where only current is recorded. A C-AFM microscope uses conventional silicon tips coated with a metal or metallic alloy, such as Pt-Ir alloy.

The C-AFM can be operated in the imaging mode and spectroscopic mode.

Imaging mode

In the conventional imaging mode, vibrating tip is scanned over a small sample area (typically square micrometres); a negative voltage bias is applied to the sample, and the electrons tunneling from the sample to the tip are being collected. This polarity is chosen for several reasons:

• The electron barrier in this case is the conduction band onset at the Si/oxide interface, which is better known than the tip/oxide interface.
• The emission area for substrate injection is homogeneous and depends mostly on the tip/sample contact area. On the contrary, the emission area in the case of tip injection depends on the shape of the tip.
• During the measurement, the tip is in contact with the sample, and many studies materials are hydrophilic. Therefore, the tip drags along water and other contaminants adsorbed at the sample surface. The applied voltage induces a high electrical field between the tip and the substrate. This field ionizes water, producing the OH^-. If a negative voltage is applied to the tip, the OH^- ions are attracted to the surface of the sample; they oxidize it thereby permanently blocking the current flow. If a positive voltage is applied to the tip, the OH^- ions are dragged to the tip, oxidizing it and breaking the electrical circuit. However, whereas the studied sample may be unique, the tips are disposable and easy to replace, but after replacement, it is difficult to relocate exactly the same area. The tip degradation, as well as image quality, also depend on the scanning parameters.

Spectroscopic mode

In the spectroscopic mode, the tip is stationary, while the voltage is being swept. This allows recording conventional current–voltage characteristic from tiny areas of the sample, and thereby to extract information on the local electronic properties, such as local density of states

Density of states

In solid-state and condensed matter physics, the density of states of a system describes the number of states per interval of energy at each energy level that are available to be occupied by electrons. Unlike isolated systems, like atoms or molecules in gas phase, the density distributions are not...

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See also

• Atomic force microscope
  Atomic force microscope
  Atomic force microscopy or scanning force microscopy is a very high-resolution type of scanning probe microscopy, with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit...
  
  
• Microscopy
  Microscopy
  Microscopy is the technical field of using microscopes to view samples and objects that cannot be seen with the unaided eye...
  
  
• Scanning probe microscopy
  Scanning probe microscopy
  Scanning Probe Microscopy is a branch of microscopy that forms images of surfaces using a physical probe that scans the specimen. An image of the surface is obtained by mechanically moving the probe in a raster scan of the specimen, line by line, and recording the probe-surface interaction as a...
  
  
• Scanning tunneling spectroscopy
  Scanning tunneling spectroscopy
  Scanning Tunneling Spectroscopy is an extension of Scanning Tunneling Microscopy which is used to provide information about the density of electrons in a sample as a function of their energy....
  
  
• Spin polarized scanning tunneling microscopy
  Spin polarized scanning tunneling microscopy
  Spin polarized scanning tunneling microscopy is a specialized application of scanning tunneling microscopy that can provide detailed information of magnetic phenomena on the single-atom scale additional to the atomic topology gained with STM...