Knowledge base: Atomic Force Microscope/Scanning Probe Microscope

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Types of Atomic Force Microscope


Atomic force microscopy (AFM)
Atomic force microscopy (AFM) or scanning force microscopy (SFM) 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. The precursor to the AFM, the scanning tunneling microscope, was developed by Gerd Binnig and Heinrich Rohrer in the early 1980s at IBM Research - Zurich, a development that earned them the Nobel Prize for Physics in 1986. Binnig, Quate and Gerber invented the first atomic force microscope (also abbreviated as AFM) in 1986. The first commercially available atomic force microscope was introduced in 1989. The AFM is one of the foremost tools for imaging, measuring, and manipulating matter at the nanoscale. The information is gathered by "feeling" the surface with a mechanical probe. Piezoelectric elements that facilitate tiny but accurate and precise movements on (electronic) command enable the very precise scanning. In some variations, electric potentials can also be scanned using conducting cantilevers. In newer more advanced versions, currents can even be passed through the tip to probe the electrical conductivity or transport of the underlying surface, but this is much more challenging with very few research groups reporting reliable data.

Scanning Probe Microscopy (SPM)
Scanning Probe Microscopy (SPM) 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 function of position. SPM was founded with the invention of the scanning tunneling microscope in 1981.
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. They owe this largely to the ability of piezoelectric actuators to execute motions with a precision and accuracy at the atomic level or better on electronic command. One could rightly call this family of techniques "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.

Scanning Tunneling Microscope (STM)
A scanning tunneling microscope (STM) is an instrument for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer (at IBM Zürich), the Nobel Prize in Physics in 1986. For an STM, good resolution is considered to be 0.1 nm lateral resolution and 0.01 nm depth resolution. With this resolution, individual atoms within materials are routinely imaged and manipulated. The STM can be used not only in ultra high vacuum but also in air, water, and various other liquid or gas ambients, and at temperatures ranging from near zero kelvin to a few hundred degrees Celsius.
The STM is based on the concept of quantum tunneling. When a conducting tip is brought very near to the surface to be examined, a bias (voltage difference) applied between the two can allow electrons to tunnel through the vacuum between them. The resulting tunneling current is a function of tip position, applied voltage, and the local density of states (LDOS) of the sample. Information is acquired by monitoring the current as the tip's position scans across the surface, and is usually displayed in image form. STM can be a challenging technique, as it requires extremely clean and stable surfaces, sharp tips, excellent vibration control, and sophisticated electronics.

Magnetic force microscope (MFM)
Magnetic force microscope (MFM) is a variety of atomic force microscope, where a sharp magnetized tip is scanning the magnetic sample; the tip-sample magnetic interactions are detected and used to reconstruct the magnetic structure of the sample surface. Many kinds of magnetic interactions are measured by MFM, including magnetic dipole–dipole interaction.

Electrostatic force microscopy (EFM)
Electrostatic force microscopy (EFM) is a type of dynamic non-contact atomic force microscopy where the electrostatic force is probed. ("Dynamic" here means that the cantilever is oscillating and does not make contact with the sample). This force arises due to the attraction or repulsion of separated charges. It is a long-ranged force and can be detected 100 nm from the sample. For example, consider a conductive cantilever tip and sample which are separated a distance z usually by a vacuum. A bias voltage between tip and sample is applied by an external battery forming a capacitor between the two. The capacitance of the system depends on the geometry of the tip and sample.

Conductive atomic force microscopy (C-AFM)
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.

Lateral Force Microscopy (LFM)
Lateral Force Microscopy (LFM) measures the deflection of the cantilever in the horizontal direction . The lateral deflection of the cantilever is a result of the force applied to the cantilever when it moves horizontally across the sample surface,and the magnitude of this deflection is determined by the frictional coefficient, the topography of the sample surface, the direction of the cantilever movement, and the cantilever’s lateral spring constant. Lateral Force Microscopy is very useful for studying a sample whose surface consists of inhomogeneous compounds. It is also used to enhance contrast at the edge of an abruptly changing slope of a sample surface, or at a boundary between different compounds.