IMAGING IN THE ATOMIC FORCE MICROSCOPE

At lower gas pressures, most VPSEM use a passive scintillator as the imaging detector. This is an efficient detector of backscattered electrons in the energy range used, and there are typically more backscattered than secondary electrons. Most VPSEM/ESEM images are taken at low magnification where the secondary electron resolution is that of the SE2, that is, the same as for the backscattered electrons. The secondary electron image will therefore be very similar to the backscattered electron image. 

Charging of the specimen is suppressed in the VPSEM, as the space above the specimen is full of ionized gas—charged particles of low energy. In particular, as insulating specimens normally charge negatively at these incident 

It is the combination of high surrounding gas pressure and no requirement for conductive coating that makes these instruments uniquely useful. Experiments that demand its use include the dynamic observation of reactions involving a solid and a gas or liquid, such as oxidation or other corrosion, as well as observation of wet specimens. However, if specimen charging is the only problem, a low voltage SEM may be a more practical solution. 

---

Argaman, M., Golan, R., Thomson, N. H., and Hansma, H. G. 1997. Phase imaging of moving DNA molecules and DNA molecules replicated in the atomic force microscope. Nucleic Acids Res 25, 4379-4384. 

Hansma, H.G. and Hoh, J.H. (1994) Biomolecular imaging with the atomic force microscope. Annual Reviews in Biophysics and Biomolecular Structure 23, 115-139. 

The force microscope, in general, has several modes of operation. In the repulsive-force or contact mode, the force is of the order of 1-10 eV/A, or 10 -10 newton, and individual atoms can be imaged. In the attractive-force or noncontact mode, the van der Waals force, the exchange force, the electrostatic force, or magnetic force is detected. The latter does not provide atomic resolution, but important information about the surface is obtained. Those modes comprise different fields in force microscopy, such as electric force microscopy and magnetic force microscopy (Sarid, 1991). Owing to the limited space, we will concentrate on atomic force microscopy, which is STM s next of kin. 

Manne, S. Butt, H. J. Gould, S. A. C. and Hansma, P. K. (1990). Imaging metal atoms in air and water using the atomic force microscope. Appl. Phys. Lett. 56, 1758-1759. 

The atomic force microscope (AFM) has been used to investigate LB film quality and other properties and to obtain sizes and distributions of MC produced within LB films. For example, in an image of a three-layer CuAr film on mica, large pits were evident (9). In a cross-section analysis a stepped drop of 7.5 nm to the substrate was consistent with three monolayers of an M-Ar film ( 2.5 nm per layer). The thickness of LB films can also be obtained in good-quality films by excavating down to the substrate by the AFM tip in contact mode (45,80). 

There are of course many other similarities and differences, and some of them are listed in Table 5.1 without further explanations. In general, STM is very versatile and flexible. Especially with the development of the atomic force microscope (AFM), materials of poor electrical conductivity can also be imaged. There is the potential of many important applications. A critically important factor in STM and AFM is the characterization of the probing tip, which can of course be done with the FIM. FIM, with its ability to field evaporate surface atoms and surface layers one by one, and the capability of single atom chemical analysis with the atom-probe FIM (APFIM), also finds many applications, especially in chemical analysis of materials on a sub-nanometer scale. It should be possible to develop an STM-FIM-APFIM system where the sample to be scanned in STM is itself an FIM tip so that the sample can either be thermally treated or be field evaporated to reach into the bulk or to reach to an interface inside the sample. After the emitter surface is scanned for its atomic structure, it can be mass analyzed in the atom-probe for one atomic layer,... 

The resolution of the atomic force microscope depends on the radius of curvature of the tip and its chemical condition. Solid crystal surfaces can often be imaged with atomic resolution. At this point, however, we need to specify what Atomic resolution is. Periodicities of atomic spacing are, in fact, reproduced. To resolve atomic defects is much more difficult and usually it is not achieved with the atomic force microscope. When it comes to steps and defects the scanning tunneling microscope has a higher resolution. On soft, deformable samples, e.g. on many biological materials, the resolution is reduced due to mechanical deformation. Practically, a real resolution of a few nm is achieved. 

---