- Learn About Probes - MICROSCOPY SUPPLIES

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Learn about Probes

An AFM probe is a sharp tip placed at the free end of a cantilever that is attached to a probe holder. Typical dimensions are on the micrometer scale with the tip radius being a few nanometers to tens of nanometers.

The cantilever is attached to a rectangular chip on the millimeter scale that enables to user to grab the probe and place it in the probe holder so it can be used on an Atomic Force Microscope. AFM tips are made using processes pioneered by the semiconductor industry. Known today as MEMS technology, most probes are made of silicon and silicon nitride.

The tip on the end of the cantilever can be different lengths, sharp or blunt and be coated with different materials to enable different scanning properties including longer wear, material bonding and magnetics.

In addition to different coatings on the tip, the cantilevers are engineered to different specifications so as to interact better with the sample they are scanning and the AFM mode they are using. For example, different frequencies enable faster or slower scanning and the spring constant can enable softer or harder interaction with the sample surface. Probes are considered consumables as they wear out over time.

How to Choose a Probe:

With all of the probes available, how do you choose the right one?

There isn't a one-size-fits-all solution for choosing the right AFM probe. How do you determine which one is right for your experiment? Even for a skilled user, choosing the right probe can be daunting. From hours of unproductive work, to sample contamination or damage, employing an inappropriate probe can prove disastrous.

Our team of applications scientists have scanned many types of materials and bio samples using different modes, conditions and probes. This webinar aims to make you an expert at selecting the right probe to match your AFM experiment, no matter what your skill level may be.

Learn About Modes

Contact Mode can be used to image sample topography by keeping the tip in contact with the sample at a constant force. The cantilever is raised or lowered as needed during the scan to keep the cantilever deflection constant.

One of the most popular AFM imaging modes, tapping mode can be used to measure topography on a variety of samples. The cantilever is driven at or near resonance and its amplitude is maintained constant during the scan.

See Tapping mode probes

Video-rate AFMs like the Cypher VRS1250 use tapping mode imaging with smaller, faster cantilevers to achieve speeds up to 45 frames per second, enabling them to capture the details of nanoscale dynamic events.

In Bimodal Dual AC, the cantilever is driven simulatanously at two separate resonance modes. This technique often provides enhanced and even unique contrast related to material properties.

Contact Resonance provides quantitative imaging of elasticand loss modulus. The cantilever is scanned in contact while simultaneously being excited at the tip-sample contact resonance. This resonance is tracked with various techniques (e.g. DART or Band Excitation).

Kelvin Probe Microscopy (KPFM) is a two-pass imaging mode where a conductive tip images the surface in tapping mode in the first pass, and then lifts above the surface by a constant height to acquire a quntitative.

Electrostatic Force Microscopy (EFM) is a two-pass imaging mode where the sample's longer-range electrostatic forces are qualitatively imaged in a secondary pass after an initial surface pass. Variations in trapped charge, potentiol, or the sample's conductivity or permittivity can be imaged.

Fast Force Mapping mode measures force-distance curves at high speed (up to 2500 Hz) while capturing every curve in the image. Both real-time and offline analysis models can be applied to calculate modulus, adhesion, and other properties from the acquired force curves.

During a force curve measurement, the cantilever is ramped toward and then away from the sample surface while the forces that it experiences are recorded. Phenomena such as protein unfolding, adhesion, and sample viscoelasticity can be studied.

AM-FM is an imaging mode used for viscoelastic mapping. The cantilever is driven simultaneously at two separate resonance modes. For the higher mode, changes in frequency are releated to sample stiffness and elasticity, whereas changes in amplitude are related to sample dissipation and loss.

Scanning Capacitance Microscopy (SCM) is a nanoelectrical imaging technique that uses a microwave radio frequency (RF) signal to map electric charge carrier locations, dopant levels, and dopant types (p-type vs. n-type) in semiconductors and other samples.

Lateral force microscopy (LFM) can be used to study nanotribology and operates in contact mode with the tip scanning orthogonal to the long axis of the cantilever. Tortional bending of the cantilever will result in a change in the lateral signal.

Magnetic force miscrosopy (MFM) is a two-pass imaging mode where a magnetized tip images the surface in the first pass, and then lifts above the siurface by a constant height to image the longer-range magnetic forces in a second pass.

Conductive AFM (ORCA) scans in contact mode while measuring any current flowing through the sample ino the conductive tip. Additionally, this mode allows for localized I-V measurements which can be made at specific user-defined points.

Scanning Tunneling Microscopy (STM) is an AFM imaging mode based on the concept of quantum tunneling. When the tip is brought very near to the surface to be examined, a bias voltage applied between the two allows electrons to tunnel through the vacuum separating them. The resulting tunneling current is a function of the tip position, applied voltage, and the local density of states (LDOS) of the sample. Information is acquired by monitoring the current as the tip scans across the surface, and is usually displayed in image form.

Nanoscale Time Dependent Dielectric Breakdown is a point measuremnet mode that is used to study dielectric breakdown at the nanoscale. A high voltage bias ramp is sent to the tip until the breakdown event in the sample occurs.

Nanolithography is a capability in which the tip is used to scratch or write on the sample surface. Geometric shapes, freehand drawings, and imported images can be used to define the lithography.

A Force Map is simply an array of force curves. Quantities like sample height, sample modulus, and tip-sample adhesion can be extracted from this array of force curves, in addition to a number of other preset and even custom calculated quantities.

Scanning Thermal Microscopy (SThM) is a mode used to measure the temperature of the sample. For isothermal samples, SThM can alternatively be used to map variations in the sample's thermal conductivity.

Electrochemical Strain Microscopy (ESM) is used to study electrochemical processes and ionic transport in solids. The cantilever is driven at its contact resonance through the application of an AC bias, inducing ionic transport which in turn causes surface deformation in the appropriate sample, thereby driving the cantilever.

Fast Current Mapping is a mode that collects current continuously during an array of force curves taken at high speed. Current maps of the sample can be generated with the benefit of minimal tip-wear since the tip is not scanning the surface.

Much like conventional tapping mode, this mode can be used to measure topography on a variety of samples. Instead of the fundamental frequency, the cantilever is driven at the resonant frequency of the 2nd, 3rd, 4th, or sometimes even higher eigenmodes (not to be confused with higher harmonics). Each higher eigenmode has an increasingly higher sensitivity and increasingly higher stiffness, allowing for effective imaging at sub-nm amplitudes that can make molecular and atomic scale imaging easier on certain samples.

PFM is used to image the electromechanical response of a material. An AC bias applied to the tip induces a mechanical response from appropriate samples. The cantilever can either be driven and maintained at its contact resonance or excited at a fixed frequency.

This technique maps the sample's loss tangent, which is the ratio of the energy dissipated to the energy stored as the cantilever taps the surface.

In Force Modulation, the cantilever is driven below resonance while the tip is in contact with the sample. The amplitude response of the cantilever is indicative of the sample stiffness.

This mode uses tapping mode to track the topography of a sample in liquid (typically electrolyte solution) in a 3-electrode electrochemical cell. The AFM probe is not part of the electronic circuit, only an observer of topography. An external potentiostat monitors or drives an electrochemical reaction on the sample surface, and the resulting images can help reveal surface conformational changes as a function of applied bias or time.

In TMnI, a point measurement is taken by landing on the sample surface, heating the sample with the probe and measuring deflection. With TMnI, users are able to understand the glass transition temperature (Tg) and melting temperature (Tm) at a specific point on their samples (typically polymers).