Scanning Probe Microscopy (SPM)

Introduction

A pyramidal silicon nitride tip. Click for more information.

Above: A probe used for atomic force microscopy.

Below: How a probe tip scans over a sample (not to scale).

As the tip passes over bumps in the surface, the vertical deflection of the cantilever (delta-z) is measured.

Scanning probe microscopy covers several related technologies for imaging and measuring surfaces on a fine scale, down to the level of molecules and groups of atoms.

At the other end of the scale, a scan may cover a distance of over 100 micrometers in the x and y directions and 4 micrometers in the z direction. This is an enormous range. It can truly be said that the development of this technology is a major achievement, for it is having profound effects on many areas of science and engineering.

SPM technologies share the concept of scanning an extremely sharp tip (3-50 nm radius of curvature) across the object surface. The tip is mounted on a flexible cantilever, allowing the tip to follow the surface profile (see Figure).

When the tip moves in proximity to the investigated object, forces of interaction between the tip and the surface influence the movement of the cantilever. These movements are detected by selective sensors. Various interactions can be studied depending on the mechanics of the probe

Probe Techniques

The three most common scanning probe techniques are:

Atomic Force Microscopy (AFM) measures the interaction force between the tip and surface. The tip may be dragged across the surface, or may vibrate as it moves. The interaction force will depend on the nature of the sample, the probe tip and the distance between them.

Scanning Tunneling Microscopy (STM) measures a weak electrical current flowing between tip and sample as they are held a very distance apart.

Near-Field Scanning Optical Microscopy (NSOM) scans a very small light source very close to the sample. Detection of this light energy forms the image. NSOM can provide resolution below that of the conventional light microscope.

There are numerous variations on these techniques. AFM may operate in several modes which differ according to the force between the tip and surface:

Mode of Operation	Force of Interaction
contact mode	strong (repulsive) - constant force or constant distance
non-contact mode	weak (attractive) - vibrating probe
intermittent contact mode	strong (repulsive) - vibrating probe
lateral force mode	frictional forces exert a torque on the scanning cantilever
magnetic force	the magnetic field of the surface is imaged
thermal scanning	the distribution of thermal conductivity is imaged

In contact mode, the tip is usually maintained at a constant force by moving the cantilever up and down as it scans. In non-contact mode or intermittent contact mode (tapping mode^TM) the tip is driven up and down by an oscillator. Especially soft materials may be imaged by a magnetically-driven cantilever (MAC Mode^TM). In non-contact mode, the bottom-most point of each probe cycle is in the attractive region of the force-distance curve. In intermittent contact mode the bottom-most point is in the repulsive region. Variations in the measured oscillation amplitude and phase in relation to the driver frequency are indicators of the surface-probe interaction.

To image frictional force, the probe is dragged along the surface, resulting in a torque on the cantilever. To image the magnetic field of the surface, a magnetically-susceptible probe is used. In other variations, the electric charge distribution on the surface or the surface capacitance is imaged. For thermal scanning microscopy (TSM) the thermal conductivity of the surface with is probed with a resistive tip that acts as a tiny resistance thermometer.

In addition to these modes, many instruments are also designed to plot the phase difference between the measured modes, for example frictional force versus contact profile. This plot is called phase mode.

Several types of probes with different tips are used in scanning probe microscopy. Tip selection depends on the mode of operation and on the type of sample.

Applications

These techniques have the ability to operate on a scale from microns down to nanometers and can image clusters of individual atoms and molecules. STM relies on the electrical conductivity of the sample, so at least some features on the sample surface must be electrically conductive to some degree. AFM is used for studies of non-conductors and is the technique more commonly used for studies of macromolecules and biological specimens. AFM has been used for measurements on a wide variety of sample types, including:

Inorganic and Synthetic Materials
Surfaces	Nanostructures
Natural surface topography	Buckyballs and Nanotubes
Surface Chemistry	Surfaces of Polymers
Silicon wafers	Diffraction gratings
Data storage media	Integrated circuits
Ceramics

Biological Materials
Polymers and Polymer Matrix	Biological Structures
Natural resins and gums	Bacterial flagellae
Muscle proteins	Amyloid-beta
DNA	Chromosomes
Plant cell walls	Cell and membrane surfaces

Choice of Technique

The analytic mode is chosen based on the surface characteristics of interest and on the hardness or stickiness of the sample. Contact mode is most useful for hard surfaces. However, a tip in contact with a surface is subject to contamination from removable material on the surface. Excessive force in contact mode can also damage the surface or erode the sharpness of the probe tip. Near-contact mode has less tendency to deform a soft surface, but is more sensitive to environmental vibrations and to variations in the film of moisture that coats samples in a normal atmosphere. Moisture or other thin liquid films expert an attractive capillary force on the probe as it is withdrawn from the surface. In non-contact mode this force becomes significant.

Vibrating mode or intermittent contact modes are particularly suited for imaging soft biological specimen. However, biological samples are successfully imaged in the "harder" contact mode. Unfixed soft specimens are deformed in the z-dimension to a degree dependent on the imposed probe force, although spreading in the x-y plane may not be significant. Biological samples may be hardened to reduce probe-induced deformation by aldehyde fixation or frozen in a cryo-AFM. See the References for further information on technique and sample preparation.

Accuracy and Calibration

Instrumental Factors. The performance of a scanning probe instrument is limited by a number of factors. One of these is the resolution of the mechanical components used to move the tip and measure its position. The sharpness and stability of the probe tip determine the area of contact and the reproducibility of imaging. Obviously, environmental vibrations must be controlled to a high degree. In addition, most positioners depend on piezoelectric drive, which is subject to problems of non-linearity and to overshoot during rapid movements. The major manufacturers of SPM equipment have made substantial improvements in mechanical and electronic design. These improvements and advanced electronic calibration routines result in measurements that are more linear and accurate than the early models. Mark VanLandingham (University of Delaware) has published a discussion of instrumental uncertainties on the Web.

Accurately nanofabricated gratings are the basis for two and three-dimensional calibrations. Such calibration gratings and calibration software are commercially available.

Probe-Related Image Distortions. At very high magnifications and high-relief sample surfaces, the mode of imaging and the geometry of the probe tip can influence the scanned image. Knowledge of the probe geometry then becomes important for interpretation of the image.

To image individual atoms and molecules it is necessary for the tip-surface interaction to depend only on the nearest atom(s) of the tip. This occurs in scanning tunneling microscopy because the tunneling current passes only through the nearest atom of the tip. Tunneling current falls off very steeply with distance from the surface. In atomic force microscopy the tip-surface interaction forces fall off less steeply with distance. Thus an AFM probe responds to the average force of interaction for a number of tip atoms, depending on the sharpness of the tip. An AFM image does not show individual atoms, but rather an averaged surface. For ordered surfaces this will reflect the average unit cell.

Probe Deconvolution (Image Restoration). Imaging very sharp vertical surfaces (surfaces with high relief) is also influenced by the sharpness of the tip. Only a tip with sufficient sharpness can properly image a given z-gradient. Some gradients will be steeper or sharper than any tip can be expected to image without artifact. False images are generated that reflect the self-image of the tip surface, rather than the object surface. Mathematical methods of tip deconvolution can be employed for image restoration. The effectiveness of these methods will depend on the specific characteristics of the sample and the probe tip. A number of scientists have investigated this area. [ Follow link for Bibliography and Discussion ]

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