Quantification of nanomechanical properties of surfaces by higher harmonic monitoring in amplitude modulated AFM imaging.

The determination of nanomechanical properties is an intensive topic of study in several fields of nanophysics, from surface and materials science to biology. At the same time, amplitude modulation force microscopy is one of the most established techniques for nanoscale characterization. In this work, we combine these two topics and propose a method able to extract quantitative nanomechanical information from higher harmonic amplitude imaging in atomic force microscopy. With this method it is possible to discriminate between different materials in the stiffness range of 1-3 GPa, in our case thin films of PS-PMMA based block copolymers. We were able to obtain a critical lateral resolution of less than 20 nm and discriminate between materials with less than a 1 GPa difference in modulus. We show that within this stiffness range, reliable values of the Young's moduli can be obtained under usual imaging conditions and with standard dynamic AFM probes.

Identifying the nature of surface chemical modification for directed self-assembly of block copolymers.

In recent years, block copolymer lithography has emerged as a viable alternative technology for advanced lithography. In chemical-epitaxy-directed self-assembly, the interfacial energy between the substrate and each block copolymer domain plays a key role on the final ordering. Here, we focus on the experimental characterization of the chemical interactions that occur at the interface built between different chemical guiding patterns and the domains of the block copolymers. We have chosen hard X-ray high kinetic energy photoelectron spectroscopy as an exploration technique because it provides information on the electronic structure of buried interfaces. The outcome of the characterization sheds light onto key aspects of directed self-assembly: grafted brush layer, chemical pattern creation and brush/block co-polymer interface.

Functional dependence of resonant harmonics on nanomechanical parameters in dynamic mode atomic force microscopy.

We present a combined theoretical and experimental study of the dependence of resonant higher harmonics of rectangular cantilevers of an atomic force microscope (AFM) as a function of relevant parameters such as the cantilever force constant, tip radius and free oscillation amplitude as well as the stiffness of the sample's surface. The simulations reveal a universal functional dependence of the amplitude of the 6th harmonic (in resonance with the 2nd flexural mode) on these parameters, which can be expressed in terms of a gun-shaped function. This analytical expression can be regarded as a practical tool for extracting qualitative information from AFM measurements and it can be extended to any resonant harmonics. The experiments confirm the predicted dependence in the explored 3-45 N/m force constant range and 2-345 GPa sample's stiffness range. For force constants around 25 N/m, the amplitude of the 6th harmonic exhibits the largest sensitivity for ultrasharp tips (tip radius below 10 nm) and polymers (Young's modulus below 20 GPa).

Investigating organic multilayers by spectroscopic ellipsometry: specific and non-specific interactions of polyhistidine with NTA self-assembled monolayers.

BACKGROUND: A versatile strategy for protein-surface coupling in biochips exploits the affinity for polyhistidine of the nitrilotriacetic acid (NTA) group loaded with Ni(II). Methods based on optical reflectivity measurements such as spectroscopic ellipsometry (SE) allow for label-free, non-invasive monitoring of molecule adsorption/desorption at surfaces. RESULTS: This paper describes a SE study about the interaction of hexahistidine (His6) on gold substrates functionalized with a thiolate self-assembled monolayer bearing the NTA end group. By systematically applying the difference spectra method, which emphasizes the small changes of the ellipsometry spectral response upon the nanoscale thickening/thinning of the molecular film, we characterized different steps of the process such as the NTA-functionalization of Au, the adsorption of the His6 layer and its eventual displacement after reaction with competitive ligands. The films were investigated in liquid, and ex situ in ambient air. The SE investigation has been complemented by AFM measurements based on nanolithography methods (nanografting mode). CONCLUSION: Our approach to the SE data, exploiting the full spectroscopic potential of the method and basic optical models, was able to provide a picture of the variation of the film thickness along the process. The combination of Î´Δ i +1 ,i (λ), Î´Ψ i +1 ,i (λ) (layer-addition mode) and δΔ(†) i ', i +1(λ), δΨ(†) i ', i +1(λ) (layer-removal mode) difference spectra allowed us to clearly disentangle the adsorption of His6 on the Ni-free NTA layer, due to non specific interactions, from the formation of a neatly thicker His6 film induced by the Ni(II)-loading of the NTA SAM.

Spectroscopic ellipsometry meets AFM nanolithography: about hydration of bio-inert oligo(ethylene glycol)-terminated self assembled monolayers on gold.

For the first time, to our knowledge, spectroscopic ellipsometry (SE) has been combined with state-of-the-art AFM differential height measurements conducted after shaving nano-lithography of ultrathin, soft-matter films for thickness determination. We investigated self-assembled monolayers of SH-(CH2)11-EGn-OH molecules on gold, where EG is ethylene glycol units and n = 3 and 6, a prototypical non-fouling system. We performed SE measurements (245-1200 nm) focusing on the changes induced by the formation of the film (difference spectra). SE measurements, analysed by simple models, confirm the formation of the S-Au interface, transparency of the SAMs and provide a sharp picture of the ability of the EG functionality to protect the surface from unspecific adsorption of proteins. A quantitative assessment of the film thickness by SE was carried out ex situ, thanks to the optical contrast between the film and the ambient, and by AFM in liquid. The cross-check between SE and AFM height measurements combined with the comparison between in-liquid and ex situ SE measurements allowed obtaining non-perturbative information about the vertical density profile of the SAM. The in-liquid SE measurements indicate a refractive index matching between the aqueous medium and the outer part of the SAM, consistent with a disordered configuration of OEG and/or the penetration of water amid the OEG strands. A critical discussion provides a detailed insight into the subtle issues and pitfalls related to the thickness determination of soft-matter films to the monolayer limit.