Scanning Tunneling Atomic Force and Magnetic Force Microscopies

The scanning tunneling microscope (STM) is an analytical technique based on the quantum mechanical phenomenon called tunneling, by which a high potential barrier does not eliminate the possibility of finding a particle in a region of high potential or even beyond a region of high potential. A probe tip typically made out of tungsten is attached to a piezodrive, which is a system of very sensitive piezocrystals that expand or contract in reaction to an applied voltage. By using the piezo to position the tip within a few angstroms

Figure 17. 57Fe Mossbauer spectrum at room temperature of the Fe(B) nanocapsules. The solid squares are the experimental data. The solid line is for the fitting results. The sub-spectra for Fe3O4(B), Fe3B, FeB, a-Fe (and/or a-Fe(B) solid solution) and y-Fe phases are represented as different lines, respectively. After [122], Z. D. Zhang et al., Phys. Rev. B 64, 024404 (2001). © 2001, American Physical Society.

Figure 17. 57Fe Mossbauer spectrum at room temperature of the Fe(B) nanocapsules. The solid squares are the experimental data. The solid line is for the fitting results. The sub-spectra for Fe3O4(B), Fe3B, FeB, a-Fe (and/or a-Fe(B) solid solution) and y-Fe phases are represented as different lines, respectively. After [122], Z. D. Zhang et al., Phys. Rev. B 64, 024404 (2001). © 2001, American Physical Society.

of the sample, the electron wavefunctions in the tip and the sample overlap, leading to a tunneling current flow when a bias voltage is applied between the tip and the sample. A computer in the STM measures the current flow between the metal tip and the sample, which are very close together. The tunneling current is amplified and fed into the computer while processing a negative feedback loop to keep the current constant. The potential barrier is a function of distance between the two surfaces and so is the current. If the current increases, the computer can move the tip farther away from the sample, thus increasing the potential barrier, decreasing the probability of an electron jumping from the tip to the sample, and thus decreasing the current. If the current is too low, the computer will do just the opposite, moving the tip closer to the sample. By keeping track of the movements of the tip, a realistic picture of the electron density of a surface can be created. The STM can resolve local electronic structure at an atomic scale on every kind of conducting solid surface. This electron density plot can then in turn be interpreted as the general arrangement or positioning of atoms on a conductive surface. By tunneling current out of a single atom on the tip, the sensitivity of the instrument can be such that single atom layers on a surface can be measured. One major area of STM research currently is to study self-assembled monolayers, which is a single layer of molecules aggregating on a surface.

The atomic force microscope (AFM), also termed scanning force microscope (SFM), is a newly developed for observing nanoscale topography and other properties of a surface soon after the discovery of the STM. The principles on how the AFM works are very simple. An atomically sharp tip is scanned over a surface with feedback mechanisms that enable the piezoelectric scanners to maintain the tip at a constant force (to obtain height information) or height (to obtain force information) above the sample surface. Tips are typically made from Si3N4 or Si and extended down from the end of a cantilever. The nanoscope AFM head employs an optical detection system in which the tip is attached to the underside of a reflective cantilever. A diode laser is focused onto the back of a reflective cantilever. As the tip scans the surface of the sample, moving up and down with the contour of the surface, the laser beam is deflected off the attached cantilever into a dual element photodiode. The photode-tector measures the difference in light intensities between the upper and lower photodetectors and then converts to voltage. Feedback from the photodiode difference signal, through software control from the computer, enables the tip to maintain either a constant force or constant height above the sample. In the constant force mode, the piezoelectric transducer monitors real time height deviation. In the constant height mode, the deflection force on the sample is recorded. The latter mode of operation requires calibration parameters of the scanning tip to be inserted in the sensitivity of the AFM head during force calibration of the microscope. By using an AFM one can not only image the surface in atomic resolution but also measure the force at nano-Newton scale. The force between the tip and the sample surface is very small, usually less than 10-9 N. The detection system does not measure force directly, which senses the deflection of the microcantilever. The detecting systems for monitoring the deflection fall into several categories, such as the tunneling current, interferometer, beam bounce, diode laser, etc. According to the interaction of the tip and the sample surface, an AFM can be classified as repulsive contact mode and attractive noncontact mode. In its repulsive contact mode, the instrument lightly touches a tip at the end of a cantilever to the sample. As a raster scan drags the tip over the sample, some sort of detection apparatus measures the vertical deflection of the cantilever, which indicates the local sample height. Thus, in the contact mode the AFM measures hard-sphere repulsion forces between the tip and sample. In the noncontact mode, the AFM derives topographic images from measurements of attractive forces and the tip does not touch the sample. The AFM can achieve a resolution of 10 pm and can image samples in air and under liquids. The AFM is being applied to studies of phenomena such as abrasion, adhesion, cleaning, corrosion, etching, friction, lubrication, plating, and polishing. Figure 18 shows an AFM image of a layer-by-layer polyelectrolyte assembled capsule [562].

A magnetic force microscope (MFM) images the spatial variation of magnetic forces on a sample surface. The mechanism of the MFM is similar to that of the AFM, but for the MFM, the tip is coated with a ferromagnetic thin film. The alternating voltage on the probe tip of the AFM is replaced with an alternating magnetic field. The tip is used to probe the magnetic stray field above the sample surface, which is mounted on a small cantilever, translating the force into a deflection that can be measured. The system operates in noncontact mode, detecting changes in the resonant frequency of the cantilever induced by the magnetic field's dependence on tip-to-sample separation. The MFM can be used to image naturally occurring and deliberately written domain structures in magnetic materials. An image taken with a magnetic tip contains information about both the topography and the magnetic properties of a surface. Which effect dominates depends upon the distance of the tip from the surface, because the interatomic magnetic force

Figure 18. AFM image of a layer-by-layer polyelectrolyte assembled capsule. Reprinted with permission from [562], S. Leporatti et al., Lang-muir 16, 4059 (2000). © 2000, American Chemical Society.

persists for greater tip-to-sample separations than the van der Waals force. If the tip is close to the surface, in the region where a standard noncontact AFM is operated, the image will be predominantly topographic. If the separation between the tip and the sample increases, magnetic effects become apparent. Collecting a series of images at different tip heights is one way to separate magnetic from topographic effects.

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