It has been recently shown that clusters movement at the nanoscale can be induced by using "tapping mode" with amplitude feedback but the tip amplitude oscillation is intentionally raised by increasing the applied voltage on the driving piezo. The extra energy delivered to the system, composed by the cluster bonded on the surface, may eventually cause the cluster depinning.
For that reason it has been argued that the analysis of this process allows not only a controlled movement but also a measurement of the adhesion forces between cluster and surface.
On this contribution we will present a successful application of this method to obtain controlled manipulation of thiol-functionalised gold nanoparticles on silicon and to estimate the energy depinning threshold as a function of cluster characteristics.
Gold nanoparticles with sizes ranging between 1 to 100 nm were dispersed in a colloidal suspension which consists in an aqueous solution of tetrachloroauric(III)acid hydrate ([HAuCl4]·H2O), stabilized with citric acid trisodium. The sizes of the particles were controlled by temperature, addition of reducing agent (NaBH4) and modification of the concentration of tetrachloroauric(III)acid hydrate. Specific molecules, such as thiols according to self-assembling procedures in aqueous environment, were used to coat the particles with functional overlayers. Finally three different set of gold nanoparticles characterised by a nominal diameter of 12, 30 and 60 nm respectively were deposited on clean silicon wafers (Si(100) + native oxide).
Images and manipulation experiments were performed in air, at room temperature using the AFM microscope (Enviroscope, by VEECO ) in "tapping mode" with amplitude feedback.
The experiments show clearly that tip amplitude oscillation can be used in a controlled and reproducible way to image or to move nanoparticles on surfaces. Moreover we were able to measure a depinning threshold power which depends mainly on cluster size. However to obtain an energy scale independent from the particular AFM set-up a detailed calibration procedure has to be used. A proposed calibration procedure will be presented within this presentation.
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