Atomic force measurement

Long and short-range forces at the molecular level between surfaces have been directly measured using the surface forces microbalance (SFM) first developed by Israelachvili and Tabor and Klein. The SFM utilizes the simultaneous measurement of separation distance and surface force. Electrostatic repulsion forces and van der Walls forces between surfaces in liquids are discussed by Israelachvili and McGuiggan (1988). 

The applications of the SFM include force measurement between surfaces in liquid and vapor, adhesion between similar or dissimilar materials, contact deformation, wetting and capillary condensation, viscosity in thin films, forces between surfactant and polymer-coated surfaces, and surface chemistry. Fluid-electrolyte interactions between conductive surfaces can also be measured [Smith, et. al., 1988]. A typical microforce of 10 nN can be detected over separation distances to a resolution of 0.1 nm with optical interoferometry between reflective surfaces. With electrostatic forces, relatively large separation are measured 1-100 nm, whereas, short range forces such as van der Waals forces take place over distances of less than 3.0 nm. Ultrasmooth and electrically conductive surfaces can be formed by the deposition of a metal film (40 nm thickness) such as Pt on a smooth substrate of mica [Smith, et. al., 1988]. The separation distance between the two surfaces is controlled by a 

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Stoica I, Barzic AI, Hulubei C. The impact of rubbing fabric t)rpe on surface roughness and tribological properties of some semi-alicyclic pol3dmides evaluated from atomic force measurements, send for publication in Appl. Surf. Sci. 2012... 

AFM Atomic force microscopy [9, 47, 99] Force measured by cantilever deflection as probe scans the surface Surface structure... 

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. 

The force between two adjacent surfaces can be measured directly with the surface force apparatus (SEA), as described in section BT20 [96]. The SEA can be employed in solution to provide an in situ detennination of the forces. Although this instmment does not directly involve an atomically resolved measurement, it has provided considerable msight mto the microscopic origins of surface friction and the effects of electrolytes and lubricants [97]. 

Carpick R W, Agrait N, Ogletree D F and Salmeron M 1996 Measurement of interfacial shear (friction) with an ultrahigh vacuum atomic force microscope J. Vac. Sc/. Technol. B 14 1289... 

Ducker W A, Senden T J and Pashley R M 1991 Direct measurement of colloidal forces using an atomic force microscope Nature 353 239... 

Thundat T, Zheng X-Y, Chen G Y, Sharp S L, Warmack R J and Schowalter L J 1993 Characterization of atomic force microscope tips by adhesion force measurements App/. Phys. Lett. 63 2150... 

Meyer G and Amer N M 1990 Simultaneous measurement of lateral and normal forces with an optical-beam-deflection atomic force microscope Appl. Phys. Lett. 57 2089... 

Overney R M, Meyer E, Frommer J, Brodbeck D, Luthi R, Flowald L, Guntherodt Fl-J, Fu]ihara M, Takano FI and Gotoh Y 1992 Friction measurements of phase separated thin films with a modified atomic force microscope Nature... 

Jarvis S P and Tokumoto FI 1997 Measurement and interpretation of forces in the atomic force microscope Probe Microscopy 1 65... 

Kelly T W ef a/1998 Direct force measurements at polymer brush surfaces by atomic force microscopy Macromoiecuies 31 4297-300... 

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). 

The atomic force microscope (ATM) provides one approach to the measurement of friction in well defined systems. The ATM allows measurement of friction between a surface and a tip with a radius of the order of 5-10 nm figure C2.9.3 a)). It is the tme realization of a single asperity contact with a flat surface which, in its ultimate fonn, would measure friction between a single atom and a surface. The ATM allows friction measurements on surfaces that are well defined in tenns of both composition and stmcture. It is limited by the fact that the characteristics of the tip itself are often poorly understood. It is very difficult to detennine the radius, stmcture and composition of the tip however, these limitations are being resolved. The AFM has already allowed the spatial resolution of friction forces that exlribit atomic periodicity and chemical specificity [3, K), 13]. 

Figure C2.9.3 Schematic diagrams of the interfaces reaiized by (a) tire atomic force microscope, (b) tire surface forces apparatus and (c) tire quartz crystai microbaiance for achieving fundamentai measurements of friction in weii defined systems.

Advances have been made in directly measuring the forces between two surfaces using freshly cleaved mica surfaces mounted on supports (15), and silica spheres in place of the sharp tip of an atomic force microscopy probe (16). These measurements can be directly related to theoretical models of surface forces. 

All this being said, perhaps the most definitive study of the relative roles of electrostatic and van der Waals forces was performed by Gady et al. [86,101,102]. In their studies, they attached a spherical polystyrene particle, having a radius between 3 and 6 p.m, to the cantilever of an atomic force microscope. They then conducted three distinct measurements that allowed them to distinguish between electrostatic and van der Waals forces that attracted the particle to various conducting, smooth substrates. 

Burnham, N.A. and Colton, R.J., Measuring the nanomechanical properties and surface forces of materials using an atomic force microscope. J. Vac. Sci. Technol. A Vac. Surf. Films, 7(4), 2906-2913 (1989). 

Weisenhom, A.L., Maivald, P, Butt, H.J. and Hansma, P.K., Measuring adhesion, allrac-lion, and repulsion between surfaces in liquids with an atomic-force microscope. P/ry.v. Rev. B Condens. Matter, 45(19), 11226-11232 (1992). 

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