Pacific Nanotechnology Inc.

Nanostructures

Sample Preparation for Imaging Nanostructures

There are two general types of nanostructured materials that may be scanned with an atomic force microscope. The first category, category I, are structures that are integrated or directly attached to a bulk structure. The second type, category II, are structures that must be attached to a substrate before imaging.

Category I
Category I samples can typically be imaged with an AFM without sample preparation. It is important that contamination resulting from preparation and handling be removed from the sample before scanning with the AFM. If there is residual contamination on the surface, the images of the sample will typically have streak marks.

Figure 2: AFM image of the nanostructure of a silicon nitride film measured in contact mode. The image size is 1.46 × 1.47 microns. Structures with only a few nanometers in horizontal dimensions are easily visualized in this AFM image.

Category II
Because an AFM scans a mechanical probe across the sample's surface, the structure being imaged must be directly attached to the surface i.e. the affinity of the nanostructure for the surface must be greater than the affinity of the nanostructure to the probe. When nanostructures do attach to the probe, the resulting images typically show reduced resolution. Also, if the nanostructure is not rigidly attached to a surface, streaking may occur.

Often nanostructures naturally attach to surfaces. For example, nanotubes naturally directly attach to many surfaces. They can be moved on the surface but only with considerable shear forces from the probe.

Figure 3: Single walled Carbon Nanotubes produced with a CVD process and an iron catalyst.

Figure 4: Contact mode AFM image of 14 nm diameter colloidal gold spheres on a mica substrate previously coated with a poly-L-Lysine film. Figure 4A shows three spheres on the surface. Figure 4B shows an SEM image of the AFM probe used for measuring the AFM image. Figure 4C is a line profile of the sphere; the measured diameter is approximately 13 nm.

In instances where there is no "natural" substrate, it often helps to use sticky substrates. Many of the common microscopy sample mounting supplies are appropriately covered through the use of double sided conductive carbon discs, sheets, and tapes. However, the unique needs of the AFM probe require some additional accommodation to expose a flat and stable surface. Examples of the two most recently successful techniques are described below. Supplies may be purchased at several microscopy supply venders. The ones used in the following example were purchased from SPI Supplies, Inc.