Probing the surface forces of monolayer films with an atomic-force microscope

Abstract

Using an atomic-force microscope* (AFM), we have studied the attractive and adhesive forces between a cantilever tip and sample surfaces as a function of sample surface energy. The measured forces systematically increased with surface energy. The AFM is very sensitive; changes in the surface forces (i.e., attraction and adhesion) of monolayer-covered samples could be clearly discerned when only the surface group of the monolayer film was changed from -CH3 to -CF3.PACS numbers: 68.35.Md, 62.20.PnThe atomic-force microscope 1 (AFM) has been used primarily to image the surfaces of insulating materials 2 " 6 with nanometer-scale resolution. The principal component of an AFM is a small cantilever which measures the force between a tip attached to the cantilever and the surface of interest. The force is determined by multiplying the measured cantilever deflection by the calibrated spring constant of the cantilever. Recording the deflection of the cantilever as a function of sample position generates a force map, or image, of the surface.Recently, the AFM has also been used to investigate the mechanical properties of materials including atomicscale friction, 7 elasticity, 8 and surface forces. 8,9 The dependence of the AFM surface-force measurement on tip-sample geometry and materials properties has not been studied systematically until now. In this study, cantilever tip size and shape are measured with a scanning electron microscope before and after use, and the composition, structure, and cleanliness of the sample surfaces are characterized using infrared reflectionabsorption spectroscopy. Sample surface energies are determined by contact angle measurements. We measure the attractive and adhesive forces between a tungsten tip and a variety of surfaces, and find the forces detected by AFM increase with sample surface energy. The attractive force results are compared to van der Waals forces calculated for a sphere approaching a flat surface. 10 Derjaguin-Muller-Toporov (DMT) adhesion theory 11,12 is used to analyze the adhesive forces. We estimate the contact area between tip and sample at zero applied load, and discuss the implications for tribology and imaging.Our instrument employs a "double cross" cantilever which constrains the motion of the tip to the z direction (normal to the sample surface). The cantilever has an effective spring constant of 50 ±10 N/m and its deflection is measured with a tunneling microscope. The tunneling microscope was operated in the constantcurrent mode, well below its maximum slew rate. Therefore, the force between the tunneling tip and cantilever was constant, and was ignored in our calculations. De-tails of the instrumentation are described elsewhere. The AFM measurements were done in a glove box under dry nitrogen. The partial pressure of water in the dry box was determined by dew-point measurements to be less than 1 /im of mercury. This is more than 3 orders of magnitude lower than the humidity required for nanometer capillary condensation. 10 As a result the measured attractive forc...

Direct measurement of hydrogen bonding in DNA nucleotide bases by atomic force microscopy.

“…When the tip pulls away from the surface, hysteresis is observed. The hysteresis is explained because of adhesive bonding between tip and sample (33). The measured adhesion forces between 5 and 17 nN are in good agreement with previously measured values (13).…”

Flame-Formed Carbon Nanoparticles: Morphology, Interaction Forces, and Hamaker Constant from AFM

“…This occurs in all the force-distance curves acquired and is evidenced by the presence of a hysteresis as shown in Figure 1. The larger F adh is due to the formation of adhesive bonds between tip and particle surface during contact in addition to a possible deformation of the particle around the tip, which may be responsible for an increase in contact area (Burnham et al 1990;Weisenhorn et al 1992). In force-distance measurements, the presence of hysteresis can also be attributed to the interactions of capillary fluids covering both the tip and the sample due to ambient humidity (Hashemi et al 2008).…”

Measuring nanomechanical properties of a dynamic contact using an indenter probe and quartz crystal microbalance

“…Numerous experimental characterizations of the physical properties of nanoscale contacts have been performed by means of nanoindentation, [1][2][3][4] atomic force microscopy [5][6][7][8] and scanning tunneling microscopy ͑STM͒. 9,10 More recently, it has become recognized that a probe tip-quartz crystal microbalance ͑QCM͒ [11][12][13][14][15][16][17][18][19][20] or closely related geometries 21 could also have enormous potential as a sensitive probe of interfacial physical properties.…”