Abstract

ABSTRACT Frequency modulation (FM) and amplitude modulation (AM) atomic force microscopy have been used to imageself-organised assemblies of octanethiol-passivated Au nanoparticles adsorbed on SiO 2 /Si(111) samples (wherethe oxide is either 200 nm or 2 nm thick). Imaging at negative frequency shifts - i.e. in the attractive forceregime - in FM mode in ultrahigh vacuum we measure nanoparticle heights which are over 50 % larger thanthose measured using conventional (repulsive modeŽ) tapping mode imaging in air. A similar dierence innanoparticle height is observed for attractive mode imaging in air. For nanoparticles adsorbed on 200 nm thickoxide layers, force-distance ( F (z)) spectra (measured in FM mode) comprise both a van der Waals componentwith the conventional power law ( 1z 2 ) dependence and a strong electrostatic force which is best “tted using alogarithmic function of the form ln ( 1z ).Keywords: Nanoparticles; self-organised; atomic force microscopy; force spectroscopy 1. INTRODUCTION

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