Glossary

The atomic force microscope (AFM) or scanning force microscope (SFM) is a very high-resolution type of scanning probe microscope, with demonstrated resolution 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, a development that earned them the Nobel Prize for Physics in 1986. Binnig, Quate and Gerber invented the first AFM in 1986. The AFM is one of the foremost tools for imaging, measuring and manipulating matter at the nanoscale. The term 'microscope' in the name is actually a misnomer because it implies looking, while in fact the information is gathered by "feeling" the surface with a mechanical probe.

The AFM has several advantages over the scanning electron microscope (SEM). Unlike the electron microscope which provides a two-dimensional projection or a two-dimensional image of a sample, the AFM provides a true three-dimensional surface profile. Additionally, samples viewed by AFM do not require any special treatments (such as metal/carbon coatings) that would irreversibly change or damage the sample. While an electron microscope needs an expensive vacuum environment for proper operation, most AFM modes can work perfectly well in ambient air or even a liquid environment. This makes it possible to study biological macromolecules and even living organisms. In principle, AFM can provide higher resolution than SEM. It has been shown to give true atomic resolution in ultra-high vacuum (UHV). UHV AFM is comparable in resolution to Scanning Tunneling Microscopy and Transmission Electron Microscopy.

The AFM consists of a microscale cantilever with a sharp tip (probe) at its end that is used to scan the specimen surface. The cantilever is typically silicon or silicon nitride with a tip radius of curvature on the order of nanometers.

Cantilever: The fragile, flexible part of the probe which is attached to the substrate. The deflection of the cantilever is the basis for most imaging.

The resonance frequency of the cantilever is measured using the TappingMode cantilever tune and Equation (4) is then used to calculate the spring constant based on the cantilever size and material properties, which are specified in the documentation. 

The value of the cantilever spring constant is based on its dimensions and material properties. “Static deflection measurements” where the spring constant is determined by loading the cantilever with a known static force, and “Dynamic deflection measurements” where the resonance behavior of the cantilever is related back to its spring constant.

The Cell Biology Kit extends the capabilities of BioScope II. BioScope II is capable of analyses of a variety of specimens and for diverse properties but is specially designed to address the special needs of biological research. The cell biology kit provides two enhancements important to some biological studies and useful in some studies of non-biological specimens. Perfusion Cell and Heater Stage are the main accessories of the Veeco Cell biology kit.

In contact AFM, the tip is in perpetual contact with the sample. The tip is attached to the end of a cantilever with a low spring constant, lower than the effective spring constant holding the atoms of most solid samples together. As the scanner gently traces the tip across the sample (or the sample under the tip), the contact force causes the cantilever to bend and the Z-feedback loop works to maintain a constant cantilever deflection.

Adjusting detector offsets

In AFM technology, if the cantilever tip were scanned at a constant height, there would be a risk that the tip would collide with the surface of the sample, causing damage. Hence, in most cases a feedback mechanism is employed to adjust the tip-to-sample distance to maintain a constant force between the tip and the sample.

Force-Distance Measurements are performed to study attractive and repulsive forces on a tip as it approaches and retracts from the sample surface. Commonly applied to investigating fundamental force interactions, nano-scale adhesive and elastic response, binding forces, colloidal studies, and chemical sensing.

The atomic force microscope (AFM) offers extraordinarily high resolution in force measurement applications, routinely yielding useful data down to the thermal noise floor of the cantilever, typically about 10pN. This along with the ease with which it is applied to many biological systems has made it a popular tool for studying such things as the specific interactions between biomolecules, the forces required to stretch polymeric molecules, and the forces that stabilize proteins. These sorts of applications have come to be collectively referred to as “force spectroscopy” applications.

Force Volume produces a two-dimensional array of force-distance measurements over a specified area to display images of force variations and topography along with individual force curves at any point.

Integral and proportional gain settings are adjusted to produce feedback to produce optimum images. Feedback is generally more sensitive to the Integral gain setting than to the Proportional gain setting. Gain settings are adjusted while observing the scope trace. Optimum gain settings (for most applications) produce trace and retrace images that mirror each other and are representative of the sample.

LP Deflection is a digital, one-pole, lowpass filter to remove high-frequency noise from Realtime data. The filter operates on the collected digital data regardless of the scan direction. The parameter specifies the cutoff frequency of the filter at which higher frequency signals begin to be attenuated. Therefore, lower values will result in less deflection noise and may provide more stable operation.

Phase imaging is a secondary imaging mode derived from TappingMode that goes beyond topographical data to detect variations in composition, adhesion, friction, viscoelasticity, and other properties, including electric and magnetic.

When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hooke's law. Depending on the situation, forces that are measured in AFM include mechanical contact force, Van der Waals forces, capillary forces, chemical bonding, electrostatic forces, magnetic forces (see Magnetic force microscope (MFM)), Casimir forces, solvation forces etc. As well as force, additional quantities may simultaneously be measured through the use of specialised types of probe (see Scanning probe microscopy#SThM, photothermal microspectroscopy, etc.). Typically, the deflection is measured using a laser spot reflected from the top of the cantilever into an array of photodiodes.

The MultiMode PicoForce scanning probe microscope system brings unprecedented accuracy and flexibility to molecular biology and nanoscale materials research. With its innovative force-measurement features, proven SPM technology, and high-speed fifth-generation NanoScope V controller, the MultiMode PicoForce is ideally suited for a broad variety of studies, from protein unfolding and antigen-antibody binding to membrane elasticity. The system's handheld PicoAngler tool allows users to manually explore tip-sample interactions with incredible ease. This innovative tool is particularly useful for single-molecule force spectroscopy, providing highly sensitive approach and retraction of the cantilever tip.

Piezoelectricity is the ability of some crystals to generate an electric potential in response to applied mechanical stress. This may take the form of a separation of electric charge across the crystal lattice. If the material is not short-circuited, the applied charge induces a voltage across the material. The piezoelectric effect is reversible in that materials exhibiting the direct piezoelectric effect (the production of electricity when stress is applied) also exhibit the converse piezoelectric effect (the production of stress and/or strain when an electric field is applied). When working with AFM, the sample is mounted on a piezoelectric tube, that can move the sample in the z direction for maintaining a constant force, and the x and y directions for scanning the sample. Alternatively a 'tripod' configuration of three piezo crystals may be employed, with each responsible for scanning in the x,y and z directions. This eliminates some of the distortion effects seen with a tube scanner. The resulting map of the area s = f(x,y) represents the topography of the sample.

The AFM can be operated in a number of modes, depending on the application. In general, possible imaging modes are divided into static (also called Contact) modes and a variety of dynamic modes.

The resolution of the microscopes is not limited by diffraction, but only by the size of the probe-sample interaction volume (i.e., point spread function), which can be as small as a few picometres. The interaction can be used to modify the sample to create small structures (nanolithography).

Scanning probe microscopy (SPM) is a new 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.

TappingMode AFM, the most commonly used of all AFM modes, is a patented technique (Veeco Instruments) that maps topography by lightly tapping the surface with an oscillating probe tip. The cantilever’s oscillation amplitude changes with sample surface topography, and the topography image is obtained by monitoring these changes and closing the z feedback loop to minimize them.

A micro sharp feature at the end of the cantilever. The interaction between the tip and sample causes cantilever deflection.