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Scanning probe microscopes produce high-detail 3D surface images and can measure physical properties of materials. The first, the scanning tunneling microscope (STM), was invented in the 1980s. Atomic force microscopy (AFM) can be used on all materials and has variations such as lateral force microscopy (LFM) and conductive atomic force microscopy (C-AFM). Other techniques include electrostatic force microscopy (EFM), scanning thermal microscopy (SThM), and magnetic force microscopes (MFM).
A scanning probe microscope is one of several microscopes that produce three-dimensional surface images in very high detail, including down to the atomic scale. Depending on the microscopy technique used, some of these microscopes can also measure the physical properties of a material, including electric current, conductivity, and magnetic fields. The first scanning probe microscope, called a scanning tunneling microscope (STM), was invented in the early 1980s. The inventors of the STM won the Nobel Prize in Physics a few years later. Since that time several other techniques have been invented, based on the same basic principles.
All scanning probe microscopy techniques involve a small sharp point scan of the surface of the material, as the data is digitally captured from the scan. The tip of the scanning probe must be smaller than the features on the surface being scanned in order to produce an accurate image. These tips need to be replaced every few days. They are usually mounted on cantilevers, and in many SPM techniques, the movement of the cantilever is measured to determine the surface height.
In scanning tunneling microscopy, an electric current is applied between the scanning tip and the surface to be scanned. This current is kept constant by adjusting the height of the tip, thus generating a topographic image of the surface. Alternatively, the tip height can be held constant while the variable current is measured to determine the surface height. Since this method uses electric current, it is only applicable to materials that are conductors or semiconductors.
Several types of scanning probe microscopes fall into the category of atomic force microscopy (AFM). Unlike scanning tunneling microscopy, AFM can be used on all types of materials, regardless of their conductivity. All types of AFM use an indirect measurement of the force between the scanning tip and the surface to produce the image. This is usually achieved through a measurement of the deflection of the cantilever. The various types of atomic force microscope include contact AFM, non-contact AFM, and intermittent contact AFM. Several considerations determine which type of atomic force microscopy is best for a particular application, including the sensitivity of the material and the size of the sample being scanned.
There are some variations on the basic types of atomic force microscopy. Lateral force microscopy (LFM) measures the twisting force at the scanning tip, which is useful for mapping surface friction. Scanning capacitance microscopy is used to measure sample capacitance while simultaneously producing an AFM topographical image. Conductive Atomic Force Microscopes (C-AFM) use a conductive tip just like STM does, thus producing an AFM topographical image and electrical current map. Force modulation microscopy (FMM) is used to measure the elastic properties of a material.
There are also other scanning probe microscope techniques for measuring properties other than the three-dimensional surface. Electrostatic force microscopes (EFM) are used to measure the electric charge on a surface. These are sometimes used to test microprocessor chips. Scanning thermal microscopy (SThM) collects thermal conductivity data and maps surface topography. Magnetic force microscopes (MFM) measure the magnetic field at the surface along with the topography.
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