Atomic surface variation

In STM, image contrast is derived from spatial variations in current flowing between the proximal probe and the sample.126 Tunneling in an STM relies on the spatial overlap of the tip and sample electronic orbitals. Therefore, the tunneling current falls off very rapidly (on atomic length scales) as a function of distance between the tip and a particular sample feature such as an isolated atom. Tunneling current variations and information on surface chemistry are specifically derived from the associated atomic-scale variations in the density of states near the sample surface. 

The work that has to be done to remove an electron from a metal into vacuum with zero kinetic energy at zero K is termed the work function, and this is the same as the ionisation potential, but is larger than that of the free atom because of the space charge or surface dipole that exists at the surface, due to the asymmetry of electron density. Work function is greatest at planes having a high concentration of atoms (i.e. generally low-index planes), and decreases with step density at stepped surfaces. Variation of work function with crystal plane underlies the technique of Field-Emission Microscopy (FEM). 

The PUs microstructure can be also investigated by means of atomic force microscopy (AFM). Phase images obtained via AFM, enable visual representation of the PUs microphase separated morphology. AFM records the surface topography of materials by measuring attractive or repulsive forces between the probe and the sample. Vertical deflections caused by surface variations are monitored as a raster scan drugs a fine tip over the sample. A detailed description of different modes in AFM technology has been described in [195]. 

It was noted in connection with Eq. III-56 that molecular dynamics calculations can be made for a liquid mixture of rare gas-like atoms to obtain surface tension versus composition. The same calculation also gives the variation of density for each species across the interface [88], as illustrated in Fig. Ill-13b. The density profiles allow a calculation, of course, of the surface excess quantities. 

EXAFS Extended x-ray absorption fine structure [177, 178] Variation of x-ray absorption as a function of x-ray energy beyond an absorption edge the probability is affected by backscattering of the emitted electron from adjacent atoms Number and interatomic distance of surface atoms... 

Fig. XVIII-15. Oxygen atom diffusion on a W(IOO) surface (a) variation of the activation energy for diffusion with d and (b) variation of o- (From Ref. 136. Reprinted with kind permission of Elsevier Science-NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.)...

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. 

AFM measures the spatial distribution of the forces between an ultrafme tip and the sample. This distribution of these forces is also highly correlated with the atomic structure. STM is able to image many semiconductor and metal surfaces with atomic resolution. AFM is necessary for insulating materials, however, as electron conduction is required for STM in order to achieve tiumelling. Note that there are many modes of operation for these instruments, and many variations in use. In addition, there are other types of scaiming probe microscopies under development. 

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