Ultramicroscopy 92 (2002) 67–76 Bias-induced forces in conducting atomic force microscopy and contact charging of organic monolayers X.D. Cui a , X. Zarate b , J. Tomfohr a , A. Primak b,c , A.L. Moore b , T.A. Moore b , D. Gust b , G. Harris c , O.F. Sankey a , S.M. Lindsay a, * a Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504, USA b Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1504, USA c Motorola Inc., 2100 East Elliot Road, AZ34/EL 704, Tempe, AZ 85284, USA Received 16 July 2001; received in revised form 14 November 2001; accepted 20 November 2001 Abstract Contactelectrification,asurfacepropertyofbulkdielectricmaterials,hasnowbeenobservedatthemolecularscale using conducting atomic force microscopy (AFM). Conducting AFM measures the electrical properties of an organic filmsandwichedbetweenaconductingprobeandaconductingsubstrate.Thispaperdescribesphysicalchangesinthe filmcausedbytheapplicationofabias.Contactoftheprobeleadstodirectmechanicalstressandtheappliedelectric field results in both Maxwell stresses and electrostriction. Additional forces arise from charge injection (contact charging). Electrostriction and contact charging act oppositely from the normal long-range Coulomb attraction and dominate when a charged tip touches an insulating film, causing the tip to deflect away from the film at high bias. A bias-induced repulsion observed in spin-coated PMMA films may be accounted for by either mechanism. In self-assembled monolayers, however, tunnel current signals show that the repulsion is dominated by contact charging. r 2002 Elsevier Science B.V. All rights reserved. 1. Introduction The scanning tunneling microscope (STM) has been widely used in molecular electronics research [1–8] but it suffers from the drawback that the contact force is not controlled independently. Just how hard the STM tip ‘touches’ an organic monolayer is still a matter of debate [9]. Modified STMs have been designed to monitor [10] or regulate [11] this contact force. A simpler ap- proach is to use a conducting atomic force microscope (AFM) in which a conventional force-sensing cantilever is coated with a conduct- inglayersothatelectronicandmechanicalproper- ties may be measured simultaneously [12–15]. In these experiments, a probe of end-radius on the order of a few nanometers is pressed against the film with a force of a few nanoNewtons, resultinginGPastressesinthefilm.Thesestresses, and their effect on the measured tunnel current, have been discussed at length elsewhere [13,16]. The question arises as to whether or not there are significant changes in contact force as the bias applied between a tip and a sample is varied,and,iftherearesuchvariations,thedegree to which they may influence the electronic *Corresponding author. Fax: +1-602-965-7954. 0304-3991/02/$-see front matter r 2002 Elsevier Science B.V. All rights reserved. PII:S0304-3991(02)00069-4