Near-Field Forces

Like the STM, the atomic force microscope (AFM) uses a very sharp tip to probe and map sample topography. The AFM detects near-field forces between the tip and sample, instead of detecting the tunneling current. Knowledge of the near-field forces acting between tip and sample is necessary for us to understand the AFM working principles. There are several types of the near-field forces which are briefly described as follows. 

Betzig E, Finn P L and Weiner J S 1992 Combined shear force and near-field scanning optical microscopy/4pp/. Phys. Lett. 60 2484... 

A wide variety of measurements can now be made on single molecules, including electrical (e.g. scanning tunnelling microscopy), magnetic (e.g. spin resonance), force (e.g. atomic force microscopy), optical (e.g. near-field and far-field fluorescence microscopies) and hybrid teclmiques. This contribution addresses only Arose teclmiques tliat are at least partially optical. Single-particle electrical and force measurements are discussed in tire sections on scanning probe microscopies (B1.19) and surface forces apparatus (B1.20). 

Sugiura, T., Okada, T., Inouye, Y Nakamura, O. and Kawata, S. (1997) Gold-bead scanning near-field optical microscope with laser-force position control. Opt. Lett., 22, 1663-1665. 

J.N. Seiber, J.E. Woodrow, R.I. Kiieger, and T. Dinoff, Determination of Ambient MITC Residues in Indoor and Outdoor Air in Townships near Fields Treated with Metam Sodium, Final Report to the Metam Sodium Task Force, University of Nevada, Reno, NV and University of California, Riverside, CA (1999). 

In the present review, first we will describe how to fabricate artificial photosynthetic reaction center in nanometer scales by making use of phase separation in mixed monolayers of hydrocarbon (HC) and fluorocarbon (FC) amphiphiles [2,5,20-26] as shown in Fig. 2b [3]. The phase separated structures were studied by SPMs such as AFM, SSPM, and scanning near-field optical/atomic force microscopy (SNOAM) [27-33] as well as a conventional local surface analysis by SIMS [3,5], The model anionic and cationic HC amphiphilic... 

Atomic force microscopy (AFM) is a variant of STM and was introduced in 1986 by Binnig et al. (11). AFM belongs to a family of near-field microscopies and is capable of imaging a wide variety of specimens surface down to an atomic scale. The technique employs a probe (pyramidal tip) mounted at the end of a sensitive but rigid cantilever (see Fig. 2). The probe is drawn across the specimen under very light mechanical loading (1). Measurements of the probe s interaction with the sample s surface are accomplished with a laser beam reflected from the cantilever. 

Scanning Near Field Atomic Force MIcroscopo... 

Clancy, C. M. R. Krogmeier, J. R. Pawlak, A. Rozanowska, M. Sarna, T Dunn, R. C. Simon, J. D. Atomic Force Microscopy and Near-Field Scanning Optical Microscopy Measurements of Single Human Retinal Lipofuscin Granules. J. Phys. Chem. B 2000, 104, 12098-12101. 

Mechanistic investigations of gas-solid and solid-solid reactions as well as their proper engineering require identifiable crystal surfaces for atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM) [1,3, 13-15] in combination with X-ray diffraction data, which are the basis of crystal packing analyses [1,3,16-18]. 

Steric elution mode occurs when the particles are greater than 1 jm. Such large particles have negligible diffusion and they accumulate near the accumulation wall. The mean layer thickness is indeed directly proportional to D and inversely proportional to the field force F (see Equation 12.3). The condition is depicted in Figure 12.4b. The particles will reach the surface of the accumulation wall and stop. The particles of a given size will form a layer with the particle centers elevated by one radius above the wall the greater the particle dimension, the deeper the penetration into the center of the parabolic flow profile, and hence, larger particles will be displaced more rapidly by the channel flow than smaller ones. This behavior is exactly the inverse of the normal elution mode and it is referred to as inverted elution order. The above-described mechanism is, however, an oversimplified model since the particles most likely do not come into contact with the surface of the accumulation wall since, in proximity of the wall, other forces appear—of hydrodynamic nature, that is, related to the flow—which lift the particles and exert opposition to the particle s close approach to the wall. 

In section IID, we introduced the utilization of chemical enhancement effect for higher sensitivity in TERS. Here, it should be pointed out that in addition to electromagnetic enhancement and chemical enhancement effects, physical deformation induced by tip-applied force showed extra enhancement effect in TERS on carbon materials such as SWNTs and fullerene molecules (Yano et al. 2005, 2006 Verma et al. 2006). This tip-pressurized effect is a unique feature of TERS and not observable in SERS. Since the spatial resolution of TERS with tip-pressurized effect is determined by the size of the very end of the metallic tip that has direct contact with the molecules, this is a very promising approach to improve the spatial resolution of the near-field microscope. 

The electromagnetic field of the composite photon contains advanced and retarded components, without any causality breach. The forcefield of doublet is described by the symmetric Maxwell s equations [87b]. Three different regions appear in the forcefield inner, near-field, and far-field. Longitudinal components of force are always present. However, in the far field, they dissapear for practical purposes. In this sense, the equations developed contain the standard set as a limiting case. 

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