Atomic noncontact mode

Nonconjugated drying oils, 9 144—147, 150-151 Noncontact mode atomic force microscopy, 3 325-326 17 63... 

FIGURE 7.1 Atomic force microscopy image of prednisolone-loaded Compritol nanoparticles produced by cold homogenization. Imaging was performed by using the noncontact mode. The formulation is composed of 5% Compritol, 1% prednisolone, 2.5% poloxamer 188, and 92.5% water. (From zur Miihlen, A. and Mehnert, W., Pharmazie, 53, 552-55, 1998. With permission.)... 

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. 

The noncontact mode with a small oscillation amplitude (<5 nm) in vacuum, established for atomic resolution AFM [68], still remains to be applied to the TERS experiments, and this may provide an interesting opportunity for TERS since the tip-sample distance achieved in this case would be comparable to that of STM. Note that the force is rather a long-range interaction in ambient conditions, and a short tip-sample distance may only be achieved by detecting the force originating from the short-range interactions observable in vacuum. 

Atomic force microscopy (AFM) allows the topography of a sample to be scanned by using a very small tip made from silicon nitride. The tip is attached to a cantilever that is characterised by its spring constant, resonance frequency, and a quality factor. The sample rests on a piezoceramic tube which can be moved horizontally x,y motion) and vertically (z motion). Displacement of the cantilever is measured by the position of a laser beam reflected from the mirrored surface on the top side of the cantilever, whereby the reflected laser beam is detected by a photodetector. AFM can be operated in either contact or a noncontact mode. In contact mode the tip travels in close contact with the surface, whereas in noncontact mode the tip hovers 5-10 nm above the surface. 

Much progress has been made in understanding atomistic properties of surfaces by noncontact AFM [216]. In noncontact mode true atomic resolution was first obtained on Si(l 11)7 x 7 [217], on lnP(l 10) [218], and on NaCl [219]. Meanwhile, even subatomic features are observable by noncontact AFM [220]. In contact mode, atomic resolution is achievable but unlike with STM and noncontact AFM it is inconclusive if this resolution is real. True atomic resolution can be recognized by the correct imaging of lattice defects, for example, vacancies as depressions. Otherwise, apparent atomic resolution can arise from the corrugations of the tip s surface and the sample s surface being in phase. The image is then a superposition of many patches of the surface and vacancies cannot be seen. 

Figure 5.117. Atomic force microscopy in the noncontact mode of an uncleaned PS/ PEMA latex (A) and a cleaned PS/PEMA latex (B). (From O. L. Shaffer [581] unpublished.)...

NC-AFM noncontact mode in atomic force microscopy PA polyamide... 

Consequently, the lateral force between the tip and sample can be significantly reduced (Fig. 6.2B). Traditionally, contact mode typically could provide higher resolution, but recent advances in noncontact techniques have led to spatial resolution up to the atomic level in vacuums and liquids (Fukuma et al., 2005 Giessibl, 2003 Sugimoto et ah, 2007). Therefore, dynamic mode is preferred for soft and unstable samples. 

Fig. 2. Schematic diagrams of alternative force feedback modes. (A) Illustrates noncontact atomic force microscope feedback. (B) Shear-force feedback frequently used in NSOM imaging applications. From Paesler and Moyer (41), with permission.

Giessibl FJ, Tortonese M (1997) Self-oscillating mode for frequency modulation noncontact atomic force microscopy. Appl Phys Lett 70 2529... 

Atomic force microscopy (AFM) images are obtained by measuring the force created by the proximity of a sharp tip (mounted on a cantilever) to the surface sample. Different from STM, insulating tips and samples can be used in AFM. This technique allows three main image modes contact, noncontact, and intermittent contact [101]. 

Different modes of operation of AFM may be applied to the characterisation of biopolymers. As biopolymers differ in their rigidity (Young s modulus), their responses to the normal and sheer forces apphed by the AFM tip will also differ. As different modes of AFM are available, the user can forego resolution in order to minimise sample distortion (as in the case of tapping mode atomic force microscopy), or improve the resolution using noncontact or cryogenic AFM. 

---