Quantitative Nanotribology by AFM:  A Novel Universal Calibration Platform

Abstract

The quantitative determination of friction forces by atomic force microscopy (AFM) in nanotribology requires the conversion of the output voltage signal of the sector area-sensitive photodiode to force using (a) the torsional spring constant of the cantilever and (b) the lateral sensitivity of the photodiode. Many existing methods provide calibration factors with large errors and suffer from poor reproducibility. We report on the fabrication, validation, and application of a new, universally applicable standard specimen that enables one to accurately calibrate all types of AFM cantilevers and tips for quantitative friction force measurements. The Si(100) calibration standard, which exhibits 30 and 50 mum wide notches with tilt angles theta between 20 degrees and 35 degrees with respect to the wafer surface, was fabricated by focused ion beam (FIB) milling. The quantification of friction forces obtained on this universal standard specimen using a direct method (the improved wedge calibration method, as introduced by Ogletree, Carpick, and Salmeron Rev. Sci. Instrum. 1996, 67, 3298-3306), which yields (a) and (b) simultaneously, was critically tested for various types of Si3N4 integrated cantilever-tip assemblies. The error in the calibration factors obtained was found to be ca. 5%, which is a significant improvement compared to errors of 30-50% observed for the often applied two-step calibration procedures of cantilever lateral force constant and photodiode sensitivity. As demonstrated for oxidized Si(100), thin films of poly(methyl methacrylate) (PMMA), and micropatterned self-assembled monolayers (SAMs) on gold, the calibration of various V-shaped and single beam cantilevers based on the application of the new universal standard in conjunction with the direct wedge method proposed allows one to conveniently perform quantitative nanotribological measurements for a wide range of materials and applications.

Reactive Microcontact Printing on Block Copolymer Films: Exploiting Chemistry in Microcontacts for Sub‐micrometer Patterning of Biomolecules

“…Atomic Force Microscopy: The contact-mode AFM measurements were carried out with a NanoScope III multimode AFM (Digital Instruments/Veeco, Santa Barbara, CA) using silicon nitride tips/cantilevers in ambient atmosphere, as described earlier [26].…”

Tribological changes in the articular cartilage of a human femoral head with avascular necrosis

“…However, more recently, some investigators have reported that the so-called wedge method has found wide acceptance for the calibration of the torsional stiffness of AFM cantilevers. [23][24][25][26] Regarding the relationship between the surface roughness and the frictional coefficient, some investigators have reported significant positive correlations between the two on the surfaces of middle-finger 27 and footwear materials, olefins, 30 and plate bricks. 31 However, no significant relationship has been reported between the surface roughness and frictional behavior with regard to the surfaces of a selfassembled monolayer, silicon ruler, and metal evaporated tape through AFM measurements (torsional resonance mode) as well as on the surfaces of Cu, Al, Zn, and Sn through pin-on-plate measurements.…”

Novel method for measuring nanofriction by atomic force microscope

“…Further, it has been reported 24,26 that the torsional spring constant ␣ may increase with load; our method avoids this concern.…”

“…Because of this, several methods have been developed for measuring or calculating the cantilever spring constants. [12][13][14][15][17][18][19][20][21][22][23][24][25][26][27] Usually, the cantilever is calibrated for the specific equipment used, 7,9,19,[28][29][30][31][32][33] but the accuracy obtained for spring constant values frequently is low. Due to the uncertainty in material properties and cantilever thickness, 26 the calculated spring constants k can have large relative errors of 40%-45% and 30%-35% ͑assuming uniform thickness͒ for V-shaped and single beam cantilevers, respectively.…”