SPM and TOF-SIMS investigation of the physical and chemical modification induced by tip writing of self-assembled monolayers

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 ±5 V. When the bias absolute value was ≤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 CxHyO and CxHyN type peaks that increase with the tip bias, whereas an intensity reduction of the SiCxHy signals with respect to the unmodified regions was observed.