S. G. Mayr

Amorphous thin-film growth: Theory compared with experiment

Experimental results on amorphous ZrAlCu thin-film growth and the dynamics of the surface morphology as predicted from a minimal nonlinear stochastic deposition equation are analysed and compared. Key points of this study are: i) an estimation procedure for coefficients entering into the growth equation and ii) a detailed analysis and interpretation of the time evolution of the correlation length and the surface roughness. The results corroborate the usefulness of the deposition equation as a tool for studying amorphous growth processes.

Dewetting of Ni and NiAg solid thin films and formation of nanowires on ripple patterned substrates

The solid state dewetting process of thin Ni films during thermal annealing on smooth and ripple patterned silicon dioxide was investigated. The ripple pattern was created by ion etching under oblique incidence and has a corrugation wavelength of about 40nm. Scanning electron microscopy images show a faster decrease of Ni surface coverage for the templated dewetting on the ripple pattern with increasing temperature indicating an additional driving force of curvature induced diffusion. The ridges act as diffusion barriers trapping Ni in the valleys. Finding adequate parameters for film thickness and annealing temperature leads to the formation of nanorods or nanowires. The kinetics of dewetting was analyzed by means of in situ electrical resistance measurements, whereas Ni surface diffusion on Ni(111) planes was determined to be the kinetically limiting process with an activation energy of 0.32±0.02eV. The addition of Ag to the Ni film switches the dewetting mechanism from the common capillarity driven gro...

Biocompatibility of single crystalline Fe70Pd30 ferromagnetic shape memory films

Controllable by an external magnetic field, ferromagnetic shape memory materials reveal a high potential for actuators in biomedical applications. Simulated body fluid (SBF) and cell tests were performed to assess the biocompatibility of Fe70Pd30 ferromagnetic shape memory thin films as grown on MgO substrates. Calcium-phosphate aggregates were detected on the film surface after soaking in SBF. Biocompatibility tests with NIH 3T3 fibroblasts revealed adhesion and proliferation on the film surface but morphological modifications with a reduced cell size became evident as well as changes in cell viability for continuous and noncontinuous FePd films. The results are compared to FePd on SiO2.

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.

Epitaxial growth and stress relaxation of vapor-deposited Fe–Pd magnetic shape memory films

To achieve maximum performance in microscale magnetic shape memory actuation devices epitaxial films several hundred nanometers thick are needed. Epitaxial films were grown on hot MgO substrates (500 °C and above) by e-beam evaporation. Structural properties and stress relaxation mechanisms were investigated by high-resolution transmission electron microscopy, in situ substrate curvature measurements and classical molecular dynamics (MD) simulations. The high misfit stress incorporated during Vollmer–Weber growth at the beginning was relaxed by partial or perfect dislocations depending on the substrate temperature. This relaxation allowed the avoidance of a stress-induced breakdown of epitaxy and no thickness limit for epitaxy was found. For substrate temperatures of 690 °C or above, the films grew in the fcc austenite phase. Below this temperature, iron precipitates were formed. MD simulations showed how these precipitates influence the movements of partial dislocations, and can thereby explain the higher stress level observed in the experiments in the initial stage of growth for these films.

Film Lift–Off from MgO: Freestanding Single Crystalline Fe–Pd Films Suitable for Magnetic Shape Memory Actuation – and Beyond

Magnetic shape memory (MSM) alloys are an exciting new class of smart materials, which reveal reversible shape changes by reorientation of martensite variants in external magnetic fi elds, as fi rst reported in Ni-Mn-Ga by Ullakko et al. [ 1 , 2 ] Yielding strains as high as 10% in bulk Ni-Mn-Ga [ 3 ] with switching frequencies up to several kHz, they are generally considered complementary to piezos or classical (viz. thermal) shape memory actuators – especially when requiring constant temperature, as e.g. in biomedical applications. Considering the latter, primarily the biotoxicity of Ni renders Ni-Mn-Ga lethal to cells within minutes of exposure, and thus unusable, as shown in our recent investigations. [ 4 ] Here, Fe–Pd based MSM alloys, [ 5 , 6 ]

Effect of microgrooved surface topography on osteoblast maturation and protein adsorption

Microgrooved surfaces have been used extensively to influence cell contact guidance. Guiding cell growth, extracellular matrix deposition, and mineralization is important for bone implant longevity. In this study, we investigated the osteoblast response to microgrooved metallic surfaces in serum-supplemented medium. Groove spacing was comparable with the spread osteoblast size. Focal adhesions were observed to confine to the intervening ridge/groove boundaries. Osteoblasts bridged over the grooves and were unable to conform to the concave shape of the underlying grooves. Microgrooved surfaces induced higher osteoblast proliferation and metabolic activity after 14 days in osteogenic medium compared with as-received surfaces, resulting in higher mineralization and alignment of cell-secreted collagen after 28 days. To establish whether preferential cell attachment at the ridge/groove boundaries was influenced by the adhesion proteins contained in the serum-supplemented media, fluorescently labeled fibronectin was adsorbed onto the microgrooved substrates at low concentrations, mimicking the concentrations found in blood serum. Fibronectin was found to selectively adsorb onto the ridge/groove boundaries, the osteoblast focal adhesion sites, suggesting that protein adsorption may have influenced the cell attachment pattern.

Mechanisms of stress generation and relaxation during pulsed laser deposition of epitaxial Fe?Pd magnetic shape memory alloy films on MgO

Mechanical stress generation during epitaxial growth of Fe–Pd thin films on MgO from pulsed laser deposition is a key parameter for the suitability in shape memory applications. By employing in situ substrate curvature measurements, we determine the stress states as a function of film thickness and composition. Depending on composition, different stress states are observed during initial film growth, which can be attributed to different misfits. Compressive stress generation by atomic peening is observed in the later stages of growth. Comparison with ex situ x-ray based strain measurements allows integral and local stress to be distinguished and yields heterogeneities of the stress state between coherent and incoherent regions. In combination with cross-sectional TEM measurements the relevant stress relaxation mechanism is identified to be stress-induced martensite formation with (111) twinning.

Nanoscale mechanical surface properties of single crystalline martensitic Ni–Mn–Ga ferromagnetic shape memory alloys

Located beyond the resolution limit of nanoindentation, contact resonance atomic force microscopy (CR-AFM) is employed for nano-mechanical surface characterization of single crystalline 14M modulated martensitic Ni–Mn–Ga (NMG) thin films grown by magnetron sputter deposition on (001) MgO substrates. Comparing experimental indentation moduli—obtained with CR-AFM—with theoretical predictions based on density functional theory (DFT) indicates the central role of pseudo plasticity and inter-martensitic phase transitions. Spatially highly resolved mechanical imaging enables the visualization of twin boundaries and allows for the assessment of their impact on mechanical behavior at the nanoscale. The CR-AFM technique is also briefly reviewed. Its advantages and drawbacks are carefully addressed.

Tailoring the surface morphology of amorphous thin films by appropriately chosen deposition conditions

Amorphous thin film growth on a substrate by cocondensation is dominated by growth instabilities arising from self shadowing and surface diffusion, and these lead to pronounced three dimensional growth. It is possible to influence structure formation profoundly through systematic variation of the deposition characteristics of the particles, i.e., the deposition energy, the deposition angle, and the angle distribution. Experimentally, this can be achieved by varying the deposition technique from vapor deposition to sputtering, and changing the deposition angle with or without simultaneous rotation of the substrate. While roughening can be enhanced by oblique particle incidence, sputtered amorphous films have smooth surfaces. This behavior can be understood in terms of shadowing and energy transfer effects as illustrated by Monte Carlo and continuum growth models.