Atomic Force Microscope (AFM)

Atomic force microscopy (AFM) is a scanning probe technique that utilizes a tip to obtain topographic information on nano and sometimes atomic scales. The sharp tip is attached to a cantilever, where the change in cantilever position can be tracked using a laser reflected off the back of the cantilever.

Two forms of AFM are typically employed, contact and non-contact modes. In contact mode, the tip is in contact with the sample. As the tip or sample is scanned, the tip will be deflected due to changes in surface topography, which is recorded with respect to position.

In non-contact mode, the tip oscillates at a high frequency a few nanometers above the surface. Due to Van der Waals’ forces, the oscillation of the tip is dampened near the surface. Using fast feedback loops, the amplitude of oscillations can be held constant while scanning. The change in height to hold the cantilever at a constant amplitude is recorded and a topographical image is obtained. Non-contact mode is a highly desired method of imaging because the sample and tip are not damaged during imaging, allowing for high reproducibility.

Bruker ICON

Figure 1: Bruker Dimension ICON

The ICON employs a third technique developed by Bruker called “ScanAsyst” which applies a force curve at each pixel in order to obtain a hardness map of the surface.  Most importantly, this method auto images, allowing a user to only set the size and location in order to get publishable images.

These AFM methods can be coupled with other techniques to measure topography in liquids, force spectroscopy, electrical and other properties of thin films.

 

 

Asylum Research Cypher

Figure 2: Asylum Research Cypher

The Cypher AFM is a high resolution instrument for imaging in inert atmosphere and liquids.  It utilizes a new technique, called “blue drive”.  Asylum Research’s blueDrive Photothermal Excitation option for atomic force microscopes (AFMs) makes tapping mode techniques simpler, more stable, and more quantitative. Tapping mode is by far the dominant choice in the world of AFM, measuring not just topography, but also mechanical, electrical, and magnetic properties. Typically, piezoacoustic excitation is used to drive the cantilever oscillation. Though piezo drive is favored for design simplicity, the response of the cantilever is often far from ideal. Asylum’s blueDrive excitation mechanism produces an almost perfect response by directly exciting the cantilever photothermally. This provides significant performance and ease of use benefits for all tapping mode techniques.

 

Molecular Vista Photoinduced Force Microscope (PiFM)

Figure 3. Molecular Vista Photoinduced Force Microscope (PiFM)

Photo-induced force microscopy (PiFM) detects photo-induced molecular polarizability of feature sizes down to the molecular level by mechanical detection of the force gradient of the interaction between the optically driven molecular dipole and its mirror image in a metal coated AFM tip.  Thus PiFM not only excites the sample with near-field but also detects the response in near-field, a feature truly unique among tip-enhanced optical microscopy techniques. The near-field detection allows PiFM to be significantly easier and more robust to operate than the techniques relying on far-field detection. Another major advantage is the absence of far-field background signal, leading to excellent signal-to-noise ratio even with low excitation power and from extremely thin samples (as thin as ~ 1 nm thick). Yet another advantage is that PiFM relies on non-contact or light tapping AFM mode, which (1) prevents even the softest samples from damage and (2) achieves higher spatial resolution than AFM topography due to the steeper functional dependence of dipole-dipole force on the tip-sample distance.

 

 

Videos

AFM Imaging of DNA and Protein Complexes in Liquid