SPM and TOF-SIMS investigation of the physical and chemical modification induced by tip writing of self-assembled monolayers B. Pignataro * , A. Licciardello, S. Cataldo, G. Marletta Laboratory for Molecular Surfaces and Nanotechnologies, Dipartimento di Scienze Chimiche, Universita ` degli Studi di Catania, V.le A. Doria 6, 95125 Catania, Italy Abstract The nanoelectrochemical modification of alkyl self-assembled monolayers (SAMs) obtained on hydrogenated silicon surfaces via radical- initiated reactions of 1-octadecene has been investigated. Scanning Probe Microscopy (SPM) showed that the modification of the organic layer occurs by applying either positive or negative biases to the tip at a threshold of about F 5 V. When the bias absolute value was V 6 V, the height of the monolayer was only faintly modified, whereas a consistent increase in tip/sample friction force was observed, in agreement with the formation of hydrophilic moieties at the organic surface. In addition to the increase of friction, bias absolute values larger than 6 V led to a significant raise of the surface height, the application of negative biases resulting in stronger effects. This suggests the occurrence of a concomitant growth of silicon oxide underneath the organic layer. Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) experiments, including chemical imaging and analysis of the retrospective spectra, were performed by writing patterns of some microns in size on the SAM. These experiments allowed to investigate the features of the chemical modification as a function of the applied bias. Positive spectra from the modified regions display the presence of C x H y O and C x H y N type peaks that increase with the tip bias, whereas an intensity reduction of the SiC x H y signals with respect to the unmodified regions was observed. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanopatterning; Tip writing; Self-assembled monolayers; TOF-SIMS; AFM; 1-Alkenes 1. Introduction The modification of silicon-based surfaces by means of organic and biological monolayers has attracted great atten- tion in the last few years, because of the wide scope of opportunities that it opens in several fields like molecular electronics, optoelectronics, biosensors, nonlinear optics and so on [1–4]. Among the different methods proposed, the self-assembled monolayers (SAMs) technology [4] allows to build up, easily and at low cost, monolayers stable at temperatures larger than 600–700 K and easy to be modified. In addition to the well-known SAMs of alkyltri- chlorosilane on silicon oxide [4], the strategy proposed by Linford et al. [5] allowed to prepare high quality alkyl SAMs on hydrogenated silicon surfaces via a Si–C mole- cule/surface linkage, i.e. without involving silicon oxide functions. The integration of the SAMs in nanotechnology, how- ever, requires in many cases the fabrication of laterally defined structures by employing a patterning procedure. On this respect, the atomic and molecular manipulation by Scanning Probe Microscopy (SPM) including ‘‘dip pen nanolitography’’, Scanning Tunnelling Microscopy-based lithography and Scanning Force Microscopy (SFM) nano- electrochemistry methods [6–11], seems to be very promis- ing. In particular, recently, spatially defined high-friction nanometric features have been successfully obtained on top of SAMs of different alkyltrichlorosilane on silicon oxide by locally inducing electrochemical modification via a biased conductive SFM tip [9,10]. Open questions concern the chemical nature of the probe-induced modification of the organic layer, due to the inherent difficulty in achieving a nanometric scale chemical characterization. Realistic hypotheses have been proposed by performing macroscale simulation experiments through the use of a tip-mimicking biased copper grid pressed against SAMs of n-octadecyltrichlorosilane and investigating them by IR spectroscopy [9]. 0928-4931/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0928-4931(02)00227-8 * Corresponding author. Tel.: +39-95-73-85073; fax: +39-95-53- 36422. E-mail address: bpignataro@unict.it (B. Pignataro). www.elsevier.com/locate/msec Materials Science and Engineering C 23 (2003) 7 – 12