Nanometer-resolved quantification of mechanical response in nanoparticle-based composites.

Nanocomposites constitute an upcoming class of materials that has enormous potential within a broad range of areas, particularly with regard to mechanical applications. However, the tuning of material properties requires a full understanding of the mechanical response of the nanocomposite across all length scales. While characterization from the micro to macroscale is well established at this point, quantification of mechanical behavior at the nanoscale is still an unresolved challenge. With this background, the current work demonstrates the capabilities of quantitative contact resonance atomic force microscopy (CR-AFM) to localize and reliably characterize Ni nanoparticles that are embedded below the surface of thermally oxidized silicon thin films. Correlating these results with numerical simulations as well as high-resolution transmission electron microscopy measurements provides a comprehensive understanding of the subtle interplay between the structure and nanomechanical response of the composite.

Contractile cell forces deform macroscopic cantilevers and quantify biomaterial performance.

Cells require adhesion to survive, proliferate and migrate, as well as for wound healing and many other functions. The strength of contractile cell forces on an underlying surface is a highly relevant quantity to measure the affinity of cells to a rigid surface with and without coating. Here we show with experimental and theoretical studies that these forces create surface stresses that are sufficient to induce measurable bending of macroscopic cantilevers. Since contractile forces are linked to the formation of focal contacts, results give information on adhesion promoting qualities and allow a comparison of very diverse materials. In exemplary studies, in vitro fibroblast adhesion on the magnetic shape memory alloy Fe-Pd and on the l-lysine derived plasma-functionalized polymer PPLL was determined. We show that cells on Fe-Pd are able to induce surface stresses three times as high as on pure titanium cantilevers. A further increase was observed for PPLL, where the contractile forces are four times higher than on the titanium reference. In addition, we performed finite element simulations on the beam bending to back up the calculation of contractile forces from cantilever bending under non-homogenous surface stress. Our findings consolidate the role of contractile forces as a meaningful measure of biomaterial performance.

Influence of surface stresses on indentation response.

Surface stresses lead to an effective change in the elastic constants of thin films and at surfaces. The development of modern scanning probe techniques like contact resonance atomic force microscopy empowers the experimenter to measure at scales where these effects become increasingly relevant. In this paper we employ a computational multiscale approach where we compare density functional theory (DFT) and molecular dynamics simulations as tools to calculate the thin-film/surface elastic behavior for silicon and strontiumtitanate. From the surface elastic constants gained by DFT calculations we develop a continuum finite-element multilayer model to study the impact of surface stresses on indentation experiments. In general the stress field of an indenter and thus the impact of surface stresses on the indentation modulus depends on its contact radius and on its particular shape. We propose an analytical model that describes the behavior of the indentation modulus as a function of the contact radius. We show that this model fits well to simulation results gained for a spherical and a flat punch indenter. Our results demonstrate a surface-stress-induced reduction of the indentation modulus of about 5% for strontiumtitanate and 6% for silicon for a contact radius of [Formula: see text], irrespective of the indenter shape.

Nanoscale-resolved elasticity: contact mechanics for quantitative contact resonance atomic force microscopy.

Contact resonance atomic force microscopy (CR-AFM) constitutes a powerful approach for nanometer-resolved mechanical characterization of surfaces. Yet, absolute accuracy is frequently impaired by ad hoc assumptions on the dynamic AFM cantilever characteristics as well as contact model. Within the present study, we clarify the detailed interplay of stress fields and geometries for full quantitative understanding, employing combined experimental numerical studies for real AFM probes. Concerning contact description, a two-parameter ansatz is utilized that takes tip geometries and their corresponding indentation moduli into account. Parameter sets obtained upon experimental data fitting for different tip blunting states, are discussed in terms of model-specific artificiality versus real contact physics at the nanoscale. Unveiling the underlying physics in detail, these findings pave the way for accurate characterization of nanomechanical properties with highest resolution.

Plasma-assisted synthesis and high-resolution characterization of anisotropic elemental and bimetallic core-shell magnetic nanoparticles.

Magnetically anisotropic as well as magnetic core-shell nanoparticles (CS-NPs) with controllable properties are highly desirable in a broad range of applications. With this background, a setup for the synthesis of heterostructured magnetic core-shell nanoparticles, which relies on (optionally pulsed) DC plasma gas condensation has been developed. We demonstrate the synthesis of elemental nickel nanoparticles with highly tunable sizes and shapes and Ni@Cu CS-NPs with an average shell thickness of 10 nm as determined with scanning electron microscopy, high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy measurements. An analytical model that relies on classical kinetic gas theory is used to describe the deposition of Cu shell atoms on top of existing Ni cores. Its predictive power and possible implications for the growth of heterostructured NP in gas condensation processes are discussed.

Interfacing hard and living matter: plasma-assembled proteins on inorganic functional materials for enhanced coupling to cells and tissue.

When bringing functional hard materials to biomedical applications, control of interfaces with cells and tissue poses one of the largest challenges. Assembly of protein monomers within an inert-gas-plasma constitutes a novel approach to synthesize strongly adherent bioactive coatings that dramatically enhance coupling to living matter. As proof of concept this is demonstrated for a Fe-Pd ferromagnetic shape memory transducer that is functionalized with the amino-acid, l-lysine, by plasma-treatment, resulting in flexible, yet ultra-durable coatings. Containing high amounts of NH2 functional groups, they fulfill all requirements for strong adhesion of cells and tissues. This is confirmed by cell tests with living NIH/3T3 embryonic murine fibroblasts that demonstrate excellent biocompatibility, exceeding conventional poly-l-lysine coatings in terms of cells focal adhesion density. The physics and chemistry behind these scenarios are unraveled by employing ab initio computer calculations. This combined approach opens the way for plasma-assisted functionalization strategies for a broad class of metals and semiconductors with polypeptides.

Fe-Pd based ferromagnetic shape memory actuators for medical applications: Biocompatibility, effect of surface roughness and protein coatings.

Ferromagnetic shape memory (FMSM) alloys constitute an exciting new class of smart materials that can yield magnetically switchable strains of several percent at constant temperatures and frequencies from quasi-static up to some kilohertz. In addition to their FMSM properties, these alloys can still be operated as conventional shape memory materials and also exhibit related superelasticity, which are both important features for use in medical devices. In this study, extensive in vitro assessments demonstrate for the first time that vapor-deposited single crystalline Fe(70)Pd(30) thin films and roughness graded polycrystalline splats of the same stoichiometry exhibit excellent biocompatibility and even bioactivity in contact with different cell types-a prerequisite for medical applications. The present study shows that fibroblast and epithelial cell lines, as well as primary osteoblast cells, proliferate well on Fe-Pd. The number of focal contacts, important for strong tissue bonding, can be improved with different binding agents from the extracellular matrix. However, even without coating, there is clear evidence that cells on Fe-Pd substrates behave similarly to control experiments. Additionally, cytotoxic effects of polycrystalline surfaces with various roughness profiles can be excluded, giving another tunable parameter for applying Fe-Pd magnetically switchable membranes in, e.g., stents and valves.

Ion-irradiation-assisted phase selection in single crystalline Fe7Pd3 ferromagnetic shape memory alloy thin films: from fcc to bcc along the Nishiyama-Wassermann path.

When processing Fe-Pd ferromagnetic shape memory thin films, selection of the desired phases and their transformation temperatures constitutes one of the largest challenges from an application point of view. In the present contribution we demonstrate that irradiation with 1.8 MeV Kr(+) ions is the method of choice to achieve this goal: Single crystalline Fe(7)Pd(3) thin films that are grown with molecular beam epitaxy on MgO (001) substrates and subsequently irradiated with ions reveal a phase transformation along the whole phase transformation path ranging from fcc austenite to bcc martensite. While for 10(14) ions/cm(2) a fcc-fct phase transformation is observed, increasing the fluence to 5 × 10(14) ions/cm(2) and 5 × 10(15) ions/cm(2) leads to a phase transformation to the bcc phase. Pole figure measurements reveal an orientation relationship for the fcc-bcc phase transformation according to Nishiyama and Wassermann.

Tailoring substrates for long-term organotypic culture of adult neuronal tissue.

Organotypic tissue cultures are highly promising for performing in vivo type studies in vitro. Currently, however, very limited survival times of only a few days for adult tissue often severely limit their application. Here, superhydrophilic nanostructured substrates with ideal material properties ensure tissue adhesion, essential for organotypic culture, while migration of single cells out of the tissue is hampered. Tuning substrate properties, for the first time, adult neuronal tissue could be cultured for 14 days with no indications of degeneration.

In-plane mechanical response of TiO2 nanotube arrays - intrinsic properties and impact of adsorbates for sensor applications.

In-plane mechanical response of TiO2 nano-tube arrays is intrinsically characterized by very low elastic moduli and significant damping, while both are strongly affected by presence of adsorbates. These findings suggest use of TiO2 nanotube arrays as novel types of sensors, actuators as well as highly stretchable coatings in biomedical applications.

Activation energy of shear transformation zones: a key for understanding rheology of glasses and liquids.

Key manifestations of the glassy and liquid states, such as viscous flow and structural relaxation, occur spatial and temporal heterogeneously, within highly localized rare events, termed shear transformation zones. Characterization of these basic entities with respect to thermal activation and mechanical response is vital for understanding the rheology of glasses across length scales. This is achieved in classical molecular dynamics computer simulations on the model glass, CuTi, by determining the activation energy barrier and plastic yield strain of individual shear transformation zones as a function of size and external stress loading. Sizes of approximately equal to 140 atoms are identified to be especially energetically favorable with an activation energy barrier of approximately equal to 0.35 eV. Using these parameters, a rheology model is proposed to quantitatively explain viscosity.

Kinetic roughening of laser deposited polymer films: crossover from single particle character to continuous growth.

The crossover in kinetic roughening of thin films from a particle-character-dominated regime to continuous growth behavior has been observed in this work. This has been accomplished by atomic force microscopy investigations of pulsed laser deposited amorphous organic films with thicknesses ranging from several nanometers to more than 4 microm. The early-stage random-deposition-like processes end once a closed layer is formed, which grows without saturation on the characteristic length scales. In addition, the influence of oblique film deposition has been examined and interpreted.

Mechanisms of radiation-induced viscous flow: role of point defects.

Mechanisms of radiation-induced flow in amorphous solids have been investigated using molecular dynamics computer simulations. It is shown for a model glass system, CuTi, that the radiation-induced flow is independent of recoil energy between 100 eV and 10 keV when compared on the basis of defect production and that there is a threshold energy for flow of approximately 10 eV. Injection of interstitial- and vacancylike defects induces the same amount of flow as the recoil events, indicating that point-defect-like entities mediate the flow process, even at 10 K. Comparisons of these results with experiments and thermal spike models are made.

Surface smoothing of rough amorphous films by irradiation-induced viscous flow.

Surface roughening and smoothing reactions on vapor codeposited glassy Zr65Al7.5Cu27.5 films by 1.8 MeV Kr+ ion beam irradiation is investigated. Irradiation causes significant smoothing of initially rough surfaces, and nearly atomically smooth films can be achieved. Smooth surfaces roughen at high doses and long wavelengths. By a Fourier analysis, radiation-induced viscous flow is identified as the dominant surface relaxation mechanism. Two noise terms are identified, which operate on different length scales: One is due to sputtering and the other to thermal spikes. The irradiation-induced viscosity is compared with radiation-enhanced diffusion.

Model for intrinsic stress formation in amorphous thin films.

Metallic amorphous thin films evaporated on a substrate can be characterized by different growth regimes in dependence of the film thickness concerning surface morphology and intrinsic film stresses, independent of the details of the applied material systems. Here, a model is presented to link the surface topography and characteristic surface measures with the observed film stresses. This allows quantitative prediction of stresses in dependence of film preparation parameters for a tailored film production.