STM and AFM

STM and AFM are useful complements to SEM. since they can provide microstructural information at the sub-pm level of lateral resolution where SEM becomes resolution limited. With their use important information has been obtained about fiber microtopography and the effect of surface treatment (13). The topography of protective surface coatings can also be observed in Fig. 4 a 5x5 pm- area of a silicon carbide coating on a carbon fiber, recorded by AFM. is shown (14). 

The distribution of elements on a fiber sample can be obtained by using wavelength dispersive X-ray emission (WDS) in an electron microprobe. This approach has the advantage that it is possible to inve.stigate the locations of carbon, oxygen and other light elements which cannot be studied with the normal energy dispersive X-ray emission (EDAX) technique found on most SEM instruments, and has been applied in carbon fiber studies (15). 

Functional groups on carbon fiber surfaces can be analyzed by carrying out titration reactions with appropriate reagents [19], but the method is generally not very sensitive. 

An important probe of carbon surface functionality is mass spectrometry, in monitoring the nature of the gaseous phase produced from heated carbon fibers by temperature-programmed desorption (TPD). In the author s group TPD has been applied by heating the Fibers electrically while monitoring the temperature by optical pyrometry and the gaseous phase by mass spectrometry XPS data were recorded at the same time [8], 

ISS can also be used for studies of composites and their component fibers, e.g., carbon fiber surfaces 122.23]. ISS is an especially surface-specific technique and is good at detecting and quantifying light elements as in surface impurities (e.g.. sodium, potassium and calcium), as well as in the outer monolayers (e.g., carbon, oxygen and nitrogen). 

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Fig. XVin-3. AFM image of DNA strands on mica. Lower figure image obtained in the contact mode under water. The contrast shown covers height variations in the range of 0-2 nm. Upper figure observed profile along the line A-A of the lower figure. (From S. N. Magnov and M.-H. Whangbo, Surface Analysis with STM and AFM, VCH, New Yoric, 1996.)...

STM and AFM profiles distort the shape of a particle because the side of the tip rides up on the particle. This effect can be corrected for. Consider, say, a spherical gold particle on a smooth surface. The sphere may be truncated, that is, the center may be a distance q above the surface, where q < r, the radius of the sphere. Assume the tip to be a cone of cone angle a. The observed profile in the vertical plane containing the center of the sphere will be a rounded hump of base width 2d and height h. Calculate q and r for the case where a - 32° and d and h are 275 nm and 300 nm, respectively. Note Chapter XVI, Ref. 133a. Can you show how to obtain the relevent equation ... 

The most popular of the scanning probe tecimiques are STM and atomic force microscopy (AFM). STM and AFM provide images of the outemiost layer of a surface with atomic resolution. STM measures the spatial distribution of the surface electronic density by monitoring the tiumelling of electrons either from the sample to the tip or from the tip to the sample. This provides a map of the density of filled or empty electronic states, respectively. The variations in surface electron density are generally correlated with the atomic positions. 

One of the most important advances in electrochemistry in the last decade was tlie application of STM and AFM to structural problems at the electrified solid/liquid interface [108. 109]. Sonnenfield and Hansma [110] were the first to use STM to study a surface innnersed in a liquid, thus extending STM beyond the gas/solid interfaces without a significant loss in resolution. In situ local-probe investigations at solid/liquid interfaces can be perfomied under electrochemical conditions if both phases are electronic and ionic conducting and this... 

At their highest sensitivities, STM and AFM generate images that show how atoms are arranged on the surfaces they probe. At first, scientists used these tools to explore how atoms are arranged on surfaces. The example below shows individual atoms on the surface of nickel metal. 

Explain the principles of the scanning probe microscopies STM and AFM, and discuss the type of information these techniques provide. What are the major differences between the two ... 

Despite the enormous impact that scanning probe methods have had on our understanding of reactions at oxide surfaces, both STM and AFM suffer from the lack of chemical specificity. The application of STM-inelastic electron tunneling spectroscopy is a potential solution as it can be used to measure the vibrational spectrum of individual molecules at the surface [69, 70]. 

Magonov SN, Whangbo M-H (1996) Surface analysis with STM and AFM. VCH, New York... 

In addition, a bipotentiostat is used to control the tip potential with respect to the surface and independent of control of the surface potential with respect to the reference electrode. The tip potential E, is given by E, = Eg + E , where Eg is the bias potential that generates the tnnneling current between tip and surface, and E (a vital variable not typical of other applications of STM and AFM) is the potential of the surface relative to the reference electrode. 

STM has been used to study the molecules adsorbed on solid surfaces. The Langmuir-Blodgett (LB) films have been extensively investigated by both STM and AFM. 

These data showed that the isolectric point (IEP) of Si02 was around pH 2, as expected. The binding of phosphate ions to the chrome surface were also estimated as a function of pH and ionic strength (Choi et al 2004). Further, both STM and AFM have been used to investigate the corrosion mechanisms of metals exposed to the aqueous phase. Since both STM and AFM can operate under water, this provides a variety of possibilities. 

After some modification, the method of atomic charge superposition of Haneman and Heydock (1982), together with the analytic summation of Cabrera and Goodman (1972), can be applied to calculate STM and AFM images. We will discuss it in Chapter 6. 

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