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Polycrystalline gold thin films with thickness of 250nm, 500nm and 1000nm were deposited on Si substrates by means of both E-Beam evaporation and sputtering techniques. High-resolution SEM, including electron-backscattered diffraction (EBSD), was employed to provide a crystallographic analysis including grain orientation maps of the studied films. The Membrane Deflection Experiment (MDE) was employed to perform the microscale tensile testing. The Young's modulus of gold films deposited by E-Beam evaporation was measured consistently in the range of 55-62 GPa while it increased to 68-72 GPa for sputtered films. Plastic yielding of the e-beam and sputtered films was contrasted due to varying microstructure of each deposition technique, which appears to assert a measure of control on the deformation mechanics.
The in-situ SEM observation of real-time hillock evolution in pure Al thin films on glass substrate during isothermal annealing at 194°C was analyzed quantitatively to understand the compressive stress relaxation mechanism by focusing on the effect of Mo interlayer between Al film and glass substrate. There is a good correlation between the hillock-induced stress relaxation and the measured stress relaxation by wafer curvature method. It is also clearly shown that the existence of Mo interlayer plays an important role in hillock formation probably due to the large difference in interfacial diffusivity of Al films.
The advancement of imprint lithography as a method for fabricating nanostructures is impeded by a lack of effective tools for characterizing mechanical properties and geometry at the nanoscale. In this report, we describe the development of methods for determining elastic moduli and cross sectional dimensions of imprinted nanolines from Brillouin light scattering (BLS) measurements using finite-element (FE) and Farnell-Adler models for the vibrational modes. An array of parallel nanoimprinted lines of polymethyl methacrylate (PMMA) with widths of ∼65 nm and heights of ∼140 nm served as a model specimen. Several acoustic modes were observed with BLS in the low-gigahertz frequency range, and the forms of the vibrational displacements were identified through correlation with calculations using measured bulk-PMMA moduli and density as input. The acoustic modes include several flexural, Rayleigh-like, and Sezawa-like modes. Fitting of Farnell-Adler calculations to the measured dispersion curves was explored as a means of extracting elastic moduli and nanoline dimensions from the data. Some values obtained from this inversion analysis were unrealistic, which suggests that geometric approximations in the model introduce significant systematic errors. In forward calculations, the frequencies determined with the FE method are found to more closely match measured frequencies. This suggests that the FE approach may be more accurate for inversion analysis. Initial estimates of uncertainties in the FE calculations support this conclusion.
This research develops Electrostatic Actuated NAno Tensile testing devices (EANATs) to evaluate mechanical and electrical properties of carbon nanowires fabricated by focus ion beam- assisted chemical vapor deposition (FIB-CVD). This research carried out nanoscale uniaxial tensile tests for 90 nm- to 150 nm-diametric carbon nanowires using EANATs. Young's modulus of cabon nanowires averaged 58 GPa, which was close to that of hydrogenated diamond-like carbon films. On average, fracture stress and strain of carbon nanowires reached values of 4.2 GPa and 0.08, respectively. This research also measured I-V characteristics of 100 nm-diametric carbon nanowires under tensile loading to reveal the piezo resistivity of nanowires. The piezoresistive effect of carbon nanowire was observed. The tensile load was about 0.75 GPa at maximum value of the resistance change.
This paper presents a bilayer model to account for surface effects on the wrinkling of ultrathin polymer films. Assuming a surface layer of finite thickness, effects of surface properties on the critical strain, the equilibrium wavelength, and the wrinkle amplitude are discussed in comparison with conventional analysis. Experimental measurements of wrinkling in polymer films with thickness ranging from 200 nm to 5 nm are conducted. The bilayer model provides a consistent understanding of the experiments that deviate from conventional analysis for thickness less than 30 nm. A set of empirical surface properties is deduced from the experimental data.
A new device is presented for the combinatorial analysis of complex nano-scale material systems. The parallel nano-differential scanning calorimeter (PnDSC) is a micro-machined array of calorimetric cells. This new approach to combinatorial calorimetry should expedite the analysis of nano-scale material thermal properties. A power compensation differential scanning calorimetry measurement, not yet performed on a device of this type, is described. A NiTi specific heat measurement demonstrates the scanning calorimetry capability of the PnDSC.
Using molecular dynamics simulation of nanocrystalline (nc) samples with grain size of 10 nm, a reverse martensitic transformation from hexagonal close-packed (hcp) to face-centered cubic (fcc) structure is observed in nc-cobalt and nc-zirconium undergoing plastic deformation. In nc-cobalt hcp-to-fcc transformation is prevalent and deformation twinning is rarely observed. The transformation mechanism involves the motion of Shockley partial dislocation 1/3<1100> in every other (0001)hcp /(111)fcc plane. In nc-zirconium the hcp-to-fcc transformation competes with the deformation twinning. From the simulation results, it is suggested that the interaction among partials should be considered to understand the deformation mechanisms of hcp nc metals.
The glass transition temperature in thin film depends strongly on film thickness and interaction with the substrate and it is normally a priori not clear which way it deviates from the bulk value. This causes new challenge in the technological advancement of smaller and smaller electronic devices. In this study molecular dynamics simulations of a low-molecular weight organic glass former, ortho-terphenyl, are carried out in bulk and freestanding films. The main motivation is to provide insight into the confinement effect without interface interactions. Based on earlier models of ortho-terphenyl we developed an atomistic model for bulk simulations. The model reproduces the literature data from simulations as well as experiments. After characterizing the bulk model we form a freestanding film. This film gives us the opportunity to study the dynamical heterogeneity near the glass transition by in-plane mobility and reorientation dynamics. We also develop a structurally coarse-grained model for this glass former based on our atomistic model to study bigger system for a longer period of time.
Exposure of poly(dimethylsiloxane) (PDMS) to oxygen plasma creates a thin, stiff surface-modified layer that reaches a submicron depth. Due to a significant modulus mismatch between the stiff surface-modified layer and the compliant bulk PDMS the surface-modified layer forms intricate patterns of surface buckles when under compressive stress and nano-cracks when under tensile stress. It is desirable to be able to design patterns of nano-cracks, or at least to have an understanding of them. Among the properties necessary to do this are the thickness and elastic modulus of the surface-modified layer. Due to the very small length scale of the surface-modified layer, it is a significant challenge to measure these properties. In this proceedings paper, a two-step method is described for determining the thickness and elastic modulus of the surface-modified layer using the atomic force microscope (AFM). First, nanoindentation is performed from which the bending stiffness of the surface-modified layer is calculated. Second, the surface-modified layer thickness is determined by using phase imaging on the cross-section of oxidized PDMS to map the region of the relatively stiffer surface-modified layer.
The interface between a highly-crosslinked polymer film and a thin silicon nitride layer can be regulated using adhesion promoting molecules. This work compares the effects of both indirect polymer/inorganic interface chemistry modification by blending organosilane adhesion promoting molecules into the polymer layer, and direct modification by confining the organosilane molecules to the substrate surface. Of particular interest are the effects of these modifications on the occurrence of an anomalous subcritical debonding phenomenon previously observed for the unmodified interface. While significantly different adhesion values were measured, the influence of the blended organosilanes was limited to moderating moisture diffusion through the polymer layer, which correllates with moderated near-threshold growth rates. Conversely, nanoscale confinement of the adhesion promoting molecules did not result in expected universal increases in adhesion energy but did inhibit anomalous near-threshold behavior.
We report a study on the plasma-enhanced chemical vapor deposition of silicon carbonitride, as well as the resonant behavior of nanomachined SiCN structures. Films with thicknesses of 1 um, and 200 nm were deposited at varying gas ratios using ammonia (NH3), nitrogen (N2), and diethylsilane (DES) as precursors. X-ray photoelectron spectroscopy revealed high nitrogen and low carbon content in films deposited at high NH3:DES gas flow ratios. Selected samples annealed at varying temperatures experienced shifts in stress towards tensile of Δσ = 235 MPa, 432 MPa, 724 MPa, and 1140 MPa, at annealing temperatures of T = 400 °C, 500 °C, 600 °C, and 700 °C respectively. Infrared spectroscopy reported a loss of incorporated hydrogen as a mechanism of stress modulation. Resonant assaying of cantilevers fabricated from 200 nm-thick SiCN yielded root-modulus-over-density values of √(E/ρ) = 6.95 × 103 m/s and √(E/ρ) = 8.35 × 103 m/s, comparable to those of silicon.
A continuum model is used for the description of the mechanical response of bilayer lipid membranes (BLMs) subjected to hydrostatic pressure. The model is formulated under the assumption that the BLMs are Smectic A liquid crystals. The mean orientation of the amphiphilic molecules is postulated to be perpendicular to the lipid layers and each layer is idealized as a two dimensional liquid. The permeation process governs the motion of the molecules through the smectic layers. The approach taken in this study is based on the seminal works of Helfrich  and de Gennes  on Smectic A liquid crystals. The failure process of the BLMs, which is observed in the experimental studies, is considered to be due to extrusion of the BLMs through the pores of the polycarbonate filters.
Thin films of Ge-Si with a duplex nanocrystalline structure were fabricated by magnetron co-sputtering and nanoindentations were made on these films. Transmission electron microscopy and Raman spectroscopy were used to analyze the deformed microstructures in the residual indentations. Amorphization and diamond-cubic (dc) to non-diamond-cubic (non-dc) phase transformation were observed and considered as the major micromechanisms in the deformation of the Ge-Si duplex nanocrystals.
Carbon nanotubes are attractive for switching applications since electrostatically-actuated CNT switches have low actuation voltages and power requirements, while allowing GHz switching speeds that stem from the inherently high elastic modulus and low mass of the CNT. Our first NEM structure, the air-bridge switch, consists of suspended single-walled nanotubes (SWNTs) that lie above a sputtered Nb electrode. Electrical measurements of these air-bridge devices show well-defined ON and OFF states as a dc bias of a few volts is applied. The switches were measured to have switching times down to a few nanoseconds. Our second NEM structure, the vertical CNT switch, consists of nanotubes grown perpendicular to the substrate. Vertical multi-walled nanotubes (MWNTs) are grown directly on a heavily doped Si substrate, from 200 − 300 nm wide, ∼ 1 μm deep nano-pockets, with Nb metal electrodes to result in the formation of a vertical single-pole-double-throw CNT switch architecture.
It has been observed in experiments that charging of nanometer-sized porous material can lead to expansion or contraction of this material. This effect can be explained by a change in surface stress as a function of surface electron charge density. Here, we employ ab initio density functional calculations using a mixed-basis pseudopotential approach to study the change in surface stresses, f, as a function of surface charge density, q for Au thin films with (111) and (100) surfaces. The derivative of the surface stress with respect to the charge, ôf/ôq, at equilibrium is related to and can be evaluated from ôμ/ôe of an uncharged slab, where μ is the chemical potential of the slab and e the tangential strain. The responses of the (111) and (100) surfaces to charging are evaluated in this way as −1.86 V and −0.90 V, respectively. The calculated values compare well to experimental observations (−0.9 V).
The strain distributions and of reflection high energy electron diffraction (RHEED) patterns of uncapped pyramidal shape InAs Stranski-Krastanov quantum dots fabricated on GaAs(001) substrate are investigated theoretically. The three dimensional strain anisotropy is computed with an atomistic elasticity approach, using inter-atomic Keating potentials and the strain energy is minimized using the conjugate gradient numerical method. RHEED images are predicted in the framework of the kinematical theory, by taking into account the refraction of the electron beam at the quantum dot/vacuum interface. Clear correlation between RHEED image features and quantum dot structural properties is established. The study stresses the potential of RHEED for future experimental real-time (during growth) detections and deciphering of strain anisotropies in quantum dots.
Carbon nanotubes and nanowires are important materials for new nanotechnology devices and sensors. Future optoelectronic devices can be made from assemblies of nanostructured materials. One difficulty in preparing these assemblies from nanotubes is the lack of site-specific points of contact and the subsequent compliance of the linkage between nanoparticles. Using molecular mechanics, semiempirical and dynamics calculations, we have modeled the assembly process of two-dimensional and three-dimensional structures of carbon nanotubes. The linkers between the nanotubes consist of novel metallodendrimers. These dendrimers have multiple binding sites with chemically specified chirality. Most importantly, they are mechanically rigid. This enables the multidimensional constraints and geometry, required for advanced electronic and optoelectronic devices.
The mechanical properties of thin metal films as compared to their bulk counterparts have been in the focus of materials science in the recent years. Owing to their technological importance, almost only metals with a face centered cubic structure like copper and aluminum have attracted scientific interest. Thin films made of bcc metals, on the other hand, have been largely neglected.
However, from a scientific point of view, the mechanical properties of bcc metals are of special interest. As an example, the yield stress of bcc metals is strongly temperature dependent for low temperatures, while it shows a behavior similar to fcc metals for higher temperatures. This is often referred to as the brittle to ductile transition (BDT). Despite intense research the underlying mechanisms leading to this phenomenon are still not understood in full detail. A major problem is the understanding of dislocation dynamics on the microscopic scale, which is different from that of fcc metals because of the special symmetry of the crystal system.
A first step is to verify in thin films that for temperatures above the BDT the thermomechanical behavior of bcc metals resembles that of fcc metals. As a model system we chose iron with a BDT temperature slightly above room temperature. We deposited iron by means of an MBE system on sapphire substrates. The so-produced epitaxial thin iron films with thicknesses above 50 nm were then thermally cycled from room temperature to 540°C in a vacuum substrate curvature apparatus to test their thermomechanical behavior.
We will present the results of substrate curvature measurements performed with iron films of different thicknesses and discuss similarities and differences to results obtained for metals with fcc crystal structures.
A semi-continuum model is constructed to study the size effects on the mechanical properties of face-cubic-center crystal structure nanofilms. Unlike the classical continuum theory, the current model directly takes the discrete nature in the thickness direction into consideration. In-plane and out-plane Poisson's ratios as well as in-plane Young's modulus are investigated with this model. It is found that the values of the Young's modulus and Poisson's ratio depend on the film thickness and approach the respective bulk values asymptotically.
The micromechanical behavior of randomly-oriented epoxy/SWNT composites has been investigated by using Raman spectroscopy. In particular, the shift of the nanotube Raman G' band with composite strain has been monitored. It has been demontrated that there is a strong interface and good stress transfer between the epoxy matrix and SWNTs up to about 0.4% strain but at higher matrix strains the interface appears to break down. By comparing the data in a cyclic deformation experiment, the detailed behavior of carbon nanotube composites can be determined, such as the efficiency of stress transfer and interface breakdown.