Lateral Force Microscopy (LFM)

The principle of Lateral Force Microscopy (LFM) is very similar to contact mode AFM. Whereas in contact mode we measure the deflection of the cantilever in the vertical direction to gather sample surface information, we measure the deflection of the cantilever in the horizontal direction in LFM.

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.

Magnetic Force Microscopy (MFM)

The Magnetic Force Microscope (MFM) is a variety of Atomic Force Microscope, where a sharp magnetized tip scans a 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. MFM scanning often uses non-contact AFM mode.

Electric 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-range force and can be detected by 100 nm or more height 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, C, between those two.

The capacitance of the system depends on the geometry of the tip and sample. The total energy stored in that capacitor is U = ½ C⋅ΔV2. The work done by the battery to maintain a constant voltage, ΔV, between the capacitor plates (tip and sample) is -2U. By definition, taking the negative gradient of the total energy Utotal = -U gives the force.

The cantilever deflects when it scans over static charges. EFM images contain information about electric properties such as the surface potential and charge distribution of a sample surface. EFM maps locally charged domains on the sample surface, similar to how MFM plots the magnetic domains of the sample surface.

Force Spectroscopy

Atomic Force Microscope (AFM) Spectroscopy is an AFM based technique to measure, and sometimes control the polarity and strength of the interaction between the AFM tip and the sample. Although the tip-sample interaction may be studied in terms of the energy, the quantity that is measured first is always the tip-sample force, and thus the nomenclature: force spectroscopy. Unlike imaging, force spectroscopy is performed mostly when the servo feedback loop is deactivated. In force spectroscopy, the cantilever-tip assembly acts as a force sensor.

Force spectroscopy is widely used in air, liquids, and different controlled environment. Force spectroscopy provides the necessary sensitivity to characterize biomolecular interactions such as the unfolding forces of single proteins or forces of a single chemical bond.

Force Modulation Microscopy (FMM)

Force Modulation Microscopy (FMM) is an extension of AFM imaging that operates in contact Atomic Force Microscopy mode and is used to detect variations in the mechanical properties of the sample surface such as surface elasticity, adhesion, and friction.

In FMM mode, the AFM tip is scanned in contact with the sample surface, and the Z feedback loop maintains a constant cantilever deflection as in constant-force mode AFM. In addition, a periodic signal known as the ‘driving signal’ is applied to the bimorph piezo and vibrates either the tip or the sample. The resulting tip motion is converted to an electrical signal. This electrical signal is separated into AC and DC components for analysis.

The DC signal represents tip deflection as in contact AFM. The Z feedback loop uses this signal to maintain a constant force between the tip and the sample to generate a topographic image.

The AC signal contains the tip response due to oscillation. The amplitude of the AC signal (called ‘FMM Amplitude’) is sensitive to the elastic properties of the sample surface. A hard surface will deflect the oscillation, resulting in a large amplitude response.

On the other hand, a soft surface will absorb the oscillation, resulting in a small amplitude response. The FMM image, which is a measure of the sample’s elastic properties, is generated from variations in the FMM amplitude.

Force Kelvin Probe Force Microscopy (KPFM)

Kelvin Probe Force Microscopy (KPFM) is a tool that enables nanometer-scale imaging of the surface potential on a broad range of materials. KPFM measurements require an understanding of both the details of the instruments and the physics of the measurements to obtain optimal results.

Kelvin probe force microscopy, or KPFM, was introduced as a tool to measure the local contact potential difference between a conducting atomic force microscopy (AFM) tip and the sample, thereby mapping the work function or surface potential of the sample with high spatial resolution. KPFM has been used extensively as a unique method to characterize the nano-scale electronic/electrical properties of metal/semiconductor surfaces and semiconductor devices. Recently, KPFM has also been used to study the electrical properties of organic materials/devices and biological materials.

Conductive Atomic Force Microscopy (CAFM)

Conductive Atomic Force Microscopy (C-AFM) is a mode of Atomic Force Microscopy in which a conductive tip is scanned in contact with the sample surface, while a voltage is applied between the tip and the sample, generating a current image. At the same time, a topographic image is also generated. Both, the current and the topographic images are taken from the same area of the sample, which allows the identification of features on the surface conducting more or less current.

After acquiring a topographic image, the tip may be moved to a specific desired location.The voltage is then ramped while the current is measured to generate local current versus voltage ( I-V ) curves.

Several kinds of conductive tips can be used in C-AFM, but the most successful are the conductive diamond-coated silicon tips. Besides having a good conductivity, the diamond layer is resistant to wear. The main advantage of C-AFM over standard electrical measurement techniques is the high spatial resolution.

For example, C-AFM measurements on polycrystalline thin films have been able to identify differences in the conductivity between grain boundaries and the interior of the grains. Also, C-AFM has been shown to be suitable to identify conducting paths in solar cells and to locate microshunts.

Researchers map variations of electrical conductivity for a range of studies and processes, including electrical defect characterization and investigation of conductive polymers, semiconductors, nanotubes, and even certain organic materials.

Piezoresponse Force Microscopy (PFM)

Piezoresponse Force Microscopy (PFM) can be employed to study multi- domain structures and domain distributions of the ferroelectric thin film using AFM technique. The local polarization on the film can be generated by applying dc voltage between the electrode of AFM and the sample.

PFM can be employed to study multi- domain structures and domain distributions of the ferroelectric thin film using AFM technique. The local polarization on the film can be generated by applying dc voltage between the electrode of AFM and the sample.

The polarized ferroelectric film is then characterized by AFM using a two- pass method. In the first pass, the surface morphology can be recorded in contact mode with a fixed set point. In the second pass, a piezoelectric image can be obtained under the piezo- response mode, during which the sample surface is scanned by applying ac voltage between the AFM tip and sample at sample displacement.

Piezoelectric induced images of various sample displacement corresponding to the different stress exerted by the tip on the sample surface, can be recorded and analyzed by force- sample displacement.