An atomic force microscope (AFM) works by scanning a sharp tip mounted on a flexible cantilever across a sample surface. It detects attractive and repulsive atomic forces in order to map the surface topography with sub-nanometre 3D resolution. The deflections of the cantilever on surface features are tracked by a laser, enabling the imaging of material properties. A typical AFM can operate in three modes: a) contact mode: the tip is in continuous contact with the sample, which is ideal for imaging hard surfaces; b) non-contact mode: the tip oscillates above the surface to avoid damaging soft samples, and c) intermittent contact mode (ICM): the tip lightly taps the surface to balance speed and sample protection.

How does it work? In a nutshell:

1. Scanning and sensing: a probe tip with an apex radius typically measuring less than 30 – 50 nm is brought extremely close to the surface of the sample. The van der Waals and electrostatic forces acting between the tip and the sample cause the cantilever to bend or deflect.

2. Detection: a laser beam reflects off the back of the cantilever onto a position-sensitive photodetector (PSPD). As the cantilever moves, the deflection of the laser beam allows the system to detect and track deflection and torsion of cantilever precisely.

3. Feedback mechanism: a piezoelectric scanner adjusts the vertical (z) position of the sample to maintain a constant interaction force between the tip and the sample during scanning in the x-y plane. This creates the sample’s topography map.

The SPOTLab system is additionally equipped with advanced, specialized AFM modes that enable the analysis of specific tip-sample interactions. The primary differences lie in the physical properties they measure and their operating modes:

KPFM (Kelvin probe force microscopy) enables mapping of the surface potential or contact potential difference between a conductive tip and the sample, relating directly to the sample’s local work function. It is used for mapping trapped charges and for analysing semiconductor and organic photovoltaic systems.

TFM (tip force microscopy) addresses the local mechanical properties of the sample, such as adhesion, stiffness, and friction.

PFM (piezoresponse force microscopy) addresses ferroelectric and piezoelectric properties by measuring the mechanical deformation (expansion or contraction) of the sample in response to an applied electric field. This allows domains in ferroelectric materials to be mapped, for instance.

c-AFM (conductive AFM) enables the local electrical conductivity or resistance of a sample to be mapped by measuring the current flow between a conductive tip and the sample. It is often used to map conductive pathways in polymers or semiconductor defects.

REFERENCES AND APPLICATIONS OF IR NANO-IMAGING AND -SPECTROSCOPY
[1] Polymers
[2] Inorganic materials
[3] 2D materials