Atomic force acoustic microscopy: Influence of the lateral contact stiffness on the elastic measurements

Atomic force acoustic microscopy is a dynamic technique where the resonances of a cantilever, that has its tip in contact with the sample, are used to quantify local elastic properties of surfaces. Since the contact resonance frequencies (CRFs) monotonically increase with the tip-sample contact stiffness, they are used to evaluate the local elastic properties of the surfaces through a suitable contact mechanical model. The CRFs depends on both, normal and lateral contact stiffness, kN and kS respectively, where the last one is taken either as constant (kS<1), or as zero, leading to uncertainty in the estimation of the elastic properties of composite materials. In this work, resonance spectra for free and contact vibration were used in a finite element analysis of cantilevers to show the influence of kS in the resonance curves due to changes in the kS/kN ratio. These curves have regions for the different vibrational modes that are both, strongly and weakly dependent on kS, and they can be used in a selective manner to obtain a precise mapping of elastic properties.

Spontaneous adsorption of gold nanoparticles by polyelectrolyte thin films

Nanocomposed films constituted by gold nanoparticles immobilized onto polyelectrolytes were obtained and studied. To obtain the films, amino terminated silicon wafer surfaces were put in contact with aqueous solution of polyelectrolytes derived from Poly(maleic anhydride-alt-styrene) containing aryl and amine-alkyl groups in the side chains, in this condition the adsorption of macromolecules was achieved. The effects of the chemical nature of the side chains and ionic strength on the amounts of adsorbed polyelectrolytes were studied by ellipsometry. The adsorption of polyelectrolytes increases with increasing ionic strength in agreement with the screening-enhanced adsorption regime; the results are discussed considering the steric hindrance of the side chains and flexibility of the polymers. A spontaneous adsorption process of nanoparticles onto polyelectrolyte films took place when these last were immersed in a gold nanoparticles suspension. The adsorption amounts were qualitatively evaluated by SEM and AFM and these showed to be dependent on chemical structure of polyelectrolytes.