Topography imaging at the nanoscale is one of the main scopes of AFM. Analysis of chemical composition and structure of various blends, block-copolymers, fibres, crystallites and single molecules is primarily based on visualization of topography data. Different processes like crystallizing/melting, wetting/dewetting, diffusion, adsorption, self-assembly and chemical reactions can also be visualized using topography scans.

Basically, there are three different AFM modes used for topography imaging, which are Non-contact mode, Contact mode and Tapping mode.

Tapping mode

Tapping mode is widespread for its ability of non-destructive high-resolution imaging of soft and fragile samples in ambient conditions. The success of the experiment depends not only on a correctly chosen probe but to a large extent on optimal imaging routine. A minimal tip-sample force, a properly adjusted feedback and an appropriate scanning rate are a combination of important parameters for most precise topographic imaging. The minimization of the tip-force in tapping mode is achieved by choosing set-point amplitude (A_sp) as close as possible to the amplitude of free-oscillating probe (A₀).

Due to a number of instrumental imperfections (stability of laser, variations in mechanical coupling between the probe and its holder, thermal drift, etc) it is rather difficult to insure high stability of the amplitude and to conduct imaging say at A_sp = 0.99A₀. Due to these imperfections one can observe a slow change of the imaging contrast during scan. Thermal drift is one of the important factors influencing imaging in amplitude modulation modes, such as tapping mode. High quality factors of the probe places time restrictions for feedback mechanism and imaging in tapping mode should be conducted relatively slow with scanning rate not much exceeding 1 Hz. These rates are recommended for imaging areas below 1 µm X 1 µm, and the rates are smaller for scanning larger areas.

In many cases, one can use moderately stiff cantilevers of the DP14 series (~1 N/m) and General Purpose tips with radius ~10nm, which provide good-quality images for a broad variety of samples. The choice of the probe tip is determined by the resolution needed in the experiment. When imaging conditions are optimized than sequential images of the same area will be almost identical.

It might be also useful at the end of the studies of the particular area to make a final scan on a large area that includes the location where multiple scans were made. In some cases you might find that imaging of the smaller area left a "window" seen in the large scan. This will be a strong indication that tip-force should be minimized further either by choosing smaller A₀ (still keeping A_sp close to A₀) or by taking softer probes (DP18/GP or even softer DP09/GP).

Tapping mode in liquid requires cantilevers with a relatively low resonant frequency. It is generally recommended to use uncoated cantilevers in liquids or probes with overall chemically stable Cr-Au coating having tip radius about 50 nm.

Contact mode

Contact mode is a high-speed AFM technique, which is mainly used to image hard surfaces when the presence of lateral forces is not expected to modify the morphological features. Conventional Contact mode cantilever for using in air has a normal spring constant of 0.9 N/m and Aluminum coating on the backside. The choice of resolution is limited to GP and LS tips.

Though the backside coating is stable in air and some fluids like water and ethanol, it dissolves readily in caustic alkali solutions to give aluminum hydroxide. Uncoated probes or probes with overall chemically inert Cr-Au coating having larger tip radius about 50 nm are recommended for using in aggressive liquid media.

Noncontact mode

In non-contact mode the tip-sample interaction is minimized. Ultrahigh vacuum conditions are essential in this mode as small amplitudes of oscillation and small tip-sample distance are used to enhance the sensitivity to short-range tipsample forces. High spring constant of 20..100 N/m is required to avoid tip sticking to surface at small amplitudes. Cantilevers with higher resonant frequency are preferable as they are faster and less noisy, use low-frequency probes only if your system does not support probes with short lever arms.