R. C. Cammarata

SURFACE AND INTERFACE STRESS EFFECTS IN THIN FILMS

Abstract Surface and interface stresses in solids are defined and their role in the thermodynamics of solids is presented. A discussion concerning the physical meaning of these quantities is given, along with a review of selected theoretical calculations and experimental measurements. It is shown that for a solid phase with one or more of its dimensions smaller than about 10 nm, the surface and interface stresses can be principal factors in determining the equilibrium structure and behavior of the solid. In particular, the effects of surface and interface stresses on thin films are reviered along with the related topic of surface reconstructions in metals.

In situ transmission electron microscopy studies of silicide‐mediated crystallization of amorphous silicon

The silicide‐mediated phase transformation of amorphous to crystalline silicon was observed in situ in the transmission electron microscope. Crystallization of nickel‐implanted amorphous silicon occurred at ∼500 °C. Nickel disilicide precipitates were observed to migrate through an amorphous Si film leaving a trail of crystalline Si. Growth occurred parallel to 〈111〉 directions. High resolution electron microscopy revealed an epitaxial NiSi2/Si(111) interface which was Type A. A diffusion‐controlled mechanism for the enhanced crystallization rate was determined.

Nanoindentation study of the mechanical properties of copper‐nickel multilayered thin films

The mechanical properties of multilayered Cu‐Ni thin films with bilayer thicknesses of 1.6–12 nm were investigated by a nanoindentation technique. Force‐displacement curves generated during loading and unloading of a diamond tip indenter were used to determine the hardness and elastic properties of the films. No enhancement in the elastic properties (supermodulus effect) was seen, but an enhancement in the hardness was observed. It is suggested that the enhancement, which displayed a Hall–Petch‐type behavior, can be understood as owing to dislocation pinning at the interfaces analogous to the mechanism of grain boundary hardening.

Simple model for interface stresses with application to misfit dislocation generation in epitaxial thin films

A simple model for the interfacial free energy of a semicoherent interface is used to develop expressions for interface stresses, which are surface thermodynamic quantities associated with solid–solid interfaces. An analysis of the thermodynamics of thin film epitaxy is presented that incorporates the effects of free surface and interface stresses, and an expression for the critical thickness for thin film epitaxy is obtained. Based on this analysis, the concept of effective pressures exerted by the thin film free surface and film–substrate interface is introduced. If it is assumed that misfit dislocations are generated at the film–substrate interface as a result of glide of threading dislocations, the thermodynamics and kinetics of stress relaxation can be discussed in terms of a balance of Peach–Koehler forces acting on the threading dislocations owing to the surface and interface pressures as well as to the coherency stress. An example is given that shows that, if the film has a relatively large surfac...

Controllable High-Speed Rotation of Nanowires

We report a versatile method for executing controllable high-speed rotation of nanowires by AC voltages applied to multiple electrodes. The rotation of the nanowires can be instantly switched on or off with precisely controlled rotation speed (to at least 1800 rpm), definite chirality, and total angle of rotation. We have determined the torque due to the fluidic drag force on nanowire of different lengths. We also demonstrate a micromotor using a rotating nanowires driving a dust particle into circular motion. This method has been used to rotate magnetic and nonmagnetic nanowires as well as carbon nanotubes.

Subcellular-resolution delivery of a cytokine through precisely manipulated nanowires

Precise delivery of molecular doses of biologically active chemicals to a pre-specified single cell among many, or a specific sub-cellular location, is still a largely unmet challenge hampering our understanding of cell biology. Overcoming this could allow unprecedented levels of cell manipulation and targeted intervention. Here, we show that gold nanowires conjugated with cytokine, such as tumour necrosis factor-alpha (TNFα), can be transported along any prescribed trajectory or orientation using electrophoretic and dielectrophoretic forces to a specific location with subcellular resolution. The nanowire, 6 μm long and 300 nm in diameter, delivered the cytokine and activated canonical nuclear factor-kappaB signaling in a single cell. Combined computational modeling and experimentation indicated that cell stimulation was highly localized to the nanowire vicinity. This targeted delivery method has profound implications for controlling signaling events on the single cell level.

Low-dimension structural properties and microindentation studies of ion-beam-sputtered multilayers of Ag/Al films

Artificially modulated silver/aluminum multilayer films with modulation wavelengths Λ between 1.35 and 21.3 nm, and total film thickness of 1 μm were prepared by ion-beam sputtering. The mechanical properties of these films were investigated by a low load muhardness indentation technique. The films displayed increased hardness as the modulation wavelength decreased. X-ray diffraction scans showed well-defined small angle peaks and satellite peaks for high angles indicating a coherent multilayer structure. The hardness enhancement as the modulation wavelength decreases may be partially attributed to the Ag2Al formation at the interfaces. The high quality of the well-defined layer structure is confirmed by the presence of interference maxima corresponding to the number of bilayers in the low angle diffraction data for films with total film thicknessesess than 33.0 nm.

Electrochemical Stability of Elemental Metal Nanoparticles

The corrosion behavior of nanometer-scale solids is important in applications ranging from sensing to catalysis. Here we present a general thermodynamic analysis of this for the case of elemental metals and use the analysis to demonstrate the construction of a particle-size-dependent potential-pH diagram for the case of platinum. We discuss the data set required for the construction of such diagrams in general and describe how some parameters are accessible via experiment while others can only be reliably determined from first-principles-based electronic structure calculations. In the case of Pt, our analysis predicts that particles of diameter less than approximately 4 nm dissolve via the direct electrochemical dissolution pathway, Pt --> Pt(2+) + 2e(-), while larger particles form an oxide. As an extension of previously published work by our group, electrochemical scanning tunneling microscopy is used to examine the stability of individual Pt-black particles with diameters ranging from 1 to 10 nm. Our experimental results confirm the thermodynamic predictions, suggesting that our analysis provides a general framework for the assessment of the electrochemical stability of nanoscale elemental metals.

Manipulation of nanowires in suspension by ac electric fields

Nanowires are potential building blocks for nanoscale devices. Manipulation of nanowires in suspension has been a formidable problem. Using ac electric fields applied to strategically designed microelectrodes, nanowires in suspension can be driven to align, to chain, to accelerate in directions parallel and perpendicular to its orientation, to concentrate onto designated places, and to disperse in a controlled manner with high efficiency despite an extremely low Reynolds number at the level of 10−5. The manipulation of nanowires can also be applied to other small elongated entities such as carbon nanotubes.

Processing and hardness of electrodeposited Ni/Al2O3 nanocomposites

Nanocomposite Ni/Al2O3 films have been produced by electrochemical deposition where 50 and 300 nm Al2O3 particles are dispersed in a nickel matrix. These films exhibit considerable enhancements in their hardness in comparison to pure nickel. The strengthening mechanism is explained in terms of an Orowan bowing hardening mechanism and, hence, related to the volume fraction of the reinforcing phase. These films may have application as strong coatings that retain many of the physical properties (e.g., optical, thermal, electrical) of the metal.