pubs.acs.org/Langmuir © 2009 American Chemical Society
Quantitative Friction-Force Measurements by Longitudinal Atomic Force Microscope Imaging Eric Karhu,† Mark Gooyers, and Jeffrey L. Hutter* Department of Physics and Astronomy, The University of Western Ontario, London, Ontario N6A 3K7, Canada. † Present address: Department of Physics and Atmospheric Science, Dalhousie University, Halifax, NS B3H 3J5, Canada. Received January 1, 2009. Revised Manuscript Received February 23, 2009 Since the first lateral force measurements by atomic force microscopy, one of the main obstacles to quantitative friction-force measurements has been the difficulty in measuring the torsional response of the probes. The influence of friction on images acquired in the usual longitudinal scanning direction has also long been recognized. However, in part due to its less favorable geometry, the longitudinal mode is not typically exploited for friction-force measurements. We show here that quantitative frictional-force measurements are possible in longitudinal imaging and provide several advantages over lateral-force imaging: for instance, topology and frictional effects are coupled in a well-defined way, and there is no need to estimate the torsional spring constant. More importantly, following frictional-force measurements by longitudinal imaging with traditional lateral-force imaging allows a convenient calibration that does not require additional equipment, cantilever preparation, or special samples.
Introduction In its simplest application, the atomic force microscope (AFM) relies on normal contact forces between a sample and a sharp tip attached to a weak cantilever spring to generate a topographical image of the sample. In this mode of operation, termed constant force mode or height imaging, the sample is moved vertically in response to changes in the cantilever deflection signal to maintain a constant deflection. Because of its exquisite sensitivity (the thermal noise limit is typically
