Atomic Force Microscopy

NREL uses atomic force microscopy (AFM) to analyze materials.

How It Works

AFM operates in several modes. In contact mode, a tip that is attached to a cantilever is scanned over the sample surface, while the force between tip and sample is measured.

While the tip is scanned laterally, the force is kept constant by moving the cantilever/tip assembly up and down, so that the deflection of the cantilever is kept constant. The vertical movement of the cantilever/tip assembly is recorded and used to generate an image of the topography of the sample.

For very flat samples, the vertical position of one edge of the cantilever is kept fixed, while the edge with the tip is allowed to move vertically as the sample topography changes. This vertical movement is monitored and used to generate the topographic image.

In another mode, the cantilever is oscillated close to its resonant frequency, while the amplitude of the oscillation is measured. As the cantilever/tip assembly approaches the surface, the interaction changes the resonant frequency, which in turns changes the oscillation amplitude. The change in the amplitude is the interaction that will be probed and generate the image.

In all these modes, the position of the cantilever is measured with the help of a laser, which reflects on the top of the cantilever just above the tip to a set of photodiodes. Because it uses the force as interaction, AFM can generate high magnifications (up to atomic resolution) of almost any type of sample, from insulators to conductors. Because the interaction force is generally small (less than the bonding force between atoms), even soft samples, such as biological materials, can be analyzed.

Several other modes are possible with AFM. For instance, in contact mode, friction between the tip and the sample may twist the cantilever, causing a lateral movement of the laser on the photodiodes, which generates an image in lateral force mode. This is used to distinguish different phases on the sample that have similar topography. Some of the special modes of analysis that we use include conductive AFM, scanning kelvin probe microscopy, and scanning capacitance microscopy.

Capability Examples

Atomic force microscopy measures material properties such as height, optical absorption, or magnetism using a probe close to the sample; this black, gray, and white image of a sample semiconductor device on glass shows microscopic grains as separate clusters in the material.

High-resolution AFM image of Zn₂SnO₄/glass, where grains smaller than 100 Å can be observed.

Left: High-resolution image obtained using atomic force microscopy; the image is of a device sample made of gallium phosphide on silicon and appears as yellow and red rice-like clusters. Right: An image equivalent to the one at left obtained using atomic force microscopy; this image was obtained with the feedback signal on and appears as a flat-looking orange surface marked by many small jagged yellow lines.

Left: AFM image of a GaP/Si sample. Right: By measuring the cantilever vertical displacement, with the feedback signal on, an image equivalent to the derivative of the AFM image is obtained. This image is particularly useful to detect sudden variations of the topography, such as steps on the top of the terraces.

High-resolution, three-dimensional image of sample gallium phosphide on silicon device; the image was obtained using atomic force microscopy and features several densely packed yellow and red elevated areas that appear three-dimensional.

Three dimensional AFM image of a GaP/Si sample. This is a real 3-dimensional representation of the data, which can be rotated to reveal features not observed in a given orientation.

Left: Topographic image of sample gallium phosphide on silicon device; the image features gray and white areas apparently overlapping on a black background, and it was obtained using atomic force microscopy. Right: Two linescans of data obtained using atomic force microscopy; the linescans refer to sections of a gallium phosphide on silicon device sample shown at left.

Topography and linescans of a GaP/Si sample. Because of the digital character of AFM data, linescans can be generated from any part of the image, and it is also possible to measure the angle between features. In the present case, the angle between the sections marked by the red arrows is 23.5°.

High-resolution gray, black, and white image of a sample semiconductor device made of cadmium sulfide and tin oxide on a glass substrate; the image appears as small clusters of densely packed round shapes and suggests the data, such as surface area, that can be obtained using atomic force microscopy.

Other parameters that can be automatically obtained from AFM analysis include peak-to-valley distance, roughness and image surface area.

High-resolution image of sample carbon nanotube; the image was obtained using atomic force microscopy and features dense, dendritic tangles of white and gray on a black background. Several of the tubes are entangled.

A second high-resolution image of carbon nanotubes.

Another high-resolution image of carbon nanotubes. The image is similar to those above but appears three-dimensional.

AFM images of carbon nanotubes. The image on the top bottom shows the entanglement of several nanotubes.

Contact

Acting Group Manager-Analytical Microscopy and Imaging

Chun.Sheng.Jiang@nrel.gov
303-384-6687