M. A. Haque

Strain gradient effect in nanoscale thin films

Abstract Compared to uniform deformation, gradient-dominant deformation experiments at the micro-scale have consistently shown remarkable strengthening effect. It is proposed in the literature that pronounced strain gradient, on the order of the inverse of a characteristic length scale, adds to the materials strength. The literature contains several formulations that account for the strain gradient in the constitutive equation. However, the role of microstructures in these formulations remains to be investigated. Due to the difficulties in micro/nano scale experimentation, attempts to investigate this mechanism so far have been confined to specimens with dimensions more than 10 microns, or to nano-indentation experiments. In this study, we use MEMS-based testing techniques to explore the effect of strain gradient in 100, 150, 200 and 485 nm thick freestanding Aluminum specimens with average grain size of 50, 65, 80 and 212 nm respectively. Strain gradient plasticity analysis show that the characteristic length scale for Aluminum is about 4.5 μm, which is similar to the values for copper and nickel reported in the literature. Experimental results suggest that the strain gradient effect is fundamentally related to dislocation-based mechanisms, and is absent at extremely small length scales (

Mechanical behavior of 30–50 nm thick aluminum films under uniaxial tension

Abstract We present uniaxial tensile test results for 30–50 nm thick freestanding aluminum films. Young’s modulus and ductility were found to decrease monotonically with grain size. Reverse Hall–Petch behavior was observed with no appreciable room temperature creep. Non-linear elasticity with small irreversible deformation was observed for 50 nm thick specimens.

Microscale materials testing using MEMS actuators

Small size scale and high resolutions in force and displacement measurements make MEMS actuators appropriate for micromechanical testing. In this paper, for the first time, we present methodologies for uniaxial tensile and cantilever bending testing of both micrometer- and submicrometer-scale freestanding specimens using MEMS actuators. We also introduce dry fabrication processes for the specimens. The methodologies allow freestanding single or multilayered thin-film specimens to be fabricated separately from the MEMS actuators. For the uniaxial tension test, tensile forces are applied by lateral comb drive actuators capable of generating a total load of 383 /spl mu/N at 40 V with resolutions on the order of 3 nN. A similar actuator is used in the bending test, with load resolution of 58 nN and spring constant of 0.78 N/m. The tensile testing methodology is demonstrated with the testing of a 110-nm-thick freestanding aluminum specimen. The cantilever bending experiment is performed on a 100-nm-thick aluminum specimen. The experimental setups can be mounted in a SEM (and also in a TEM after modifications for tensile testing) for in situ observation of materials behavior under different environmental conditions. Remarkable strengthening is observed in all the specimens tested compared to their bulk counterparts in both tensile and bending experiments. Experimental results highlight the potential of MEMS actuators as a new tool for materials research.

Continuous ultra-thin MoS2 films grown by low-temperature physical vapor deposition

Uniform growth of pristine two dimensional (2D) materials over large areas at lower temperatures without sacrifice of their unique physical properties is a critical pre-requisite for seamless integration of next-generation van der Waals heterostructures into functional devices. This Letter describes a vapor phase growth technique for precisely controlled synthesis of continuous, uniform molecular layers of MoS2 on silicon dioxide and highly oriented pyrolitic graphite substrates of over several square centimeters at 350 °C. Synthesis of few-layer MoS2 in this ultra-high vacuum physical vapor deposition process yields materials with key optical and electronic properties identical to exfoliated layers. The films are composed of nano-scale domains with strong chemical binding between domain boundaries, allowing lift-off from the substrate and electronic transport measurements from contacts with separation on the order of centimeters.

The interfacial strength of carbon nanofiber epoxy composite using single fiber pullout experiments.

Carbon nanotubes and nanofibers are extensively researched as reinforcing agents in nanocomposites for their multifunctionality, light weight and high strength. However, it is the interface between the nanofiber and the matrix that dictates the overall properties of the nanocomposite. The current trend is to measure elastic properties of the bulk nanocomposite and then compare them with theoretical models to extract the information on the interfacial strength. The ideal experiment is single fiber pullout from the matrix because it directly measures the interfacial strength. However, the technique is difficult to apply to nanocomposites because of the small size of the fibers and the requirement for high resolution force and displacement sensing. We present an experimental technique for measuring the interfacial strength of nanofiber-reinforced composites using the single fiber pullout technique and demonstrate the technique for a carbon nanofiber-reinforced epoxy composite. The experiment is performed in situ in a scanning electron microscope and the interfacial strength for the epoxy composite was measured to be 170 MPa.

Notch insensitive fracture in nanoscale thin films

To study the effect of stress concentration at the nanoscale, we performed fracture experiments on single edge notched thin film specimens inside the transmission electron microscope. Even at about 4 GPa stress at the notch tip, the specimens failed far away from the notch at places with no apparent stress concentration. The in situ electron microscopy showed evidence of little or no plastic deformation at the notch tip. We propose that the apparent notch insensitivity arises from the breakdown of the classical fracture mechanics at the nanoscale, where materials fail by reaching a uniform rupture stress and not due to stress concentration.

Synthesis and elastic characterization of zinc oxide nanowires

Zinc oxide nanowires, nanobelts, and nanoneedles were synthesized using the vapor-liquid-solid technique. Youngs modulus of the nanowires was measured by performing cantilever bending experiments on individual nanowires in situ inside a scanning electron microscope. The nanowires tested had diameters in the range of 200-750 nm. The average Youngs modulus, measured to be 40 GPa, is about 30% of that reported at the bulk scale. The experimental results are discussed in light of the pronounced electromechanical coupling due to the piezoelectric nature of the material.

Mechanical testing of pyrolysed poly-furfuryl alcohol nanofibres

We present experimental results on the characterization of the mechanical properties of pyrolysed poly-furfuryl alcohol (PFA) nanofibres. Specifically, Youngs modulus and the fracture strain of the nanofibres were measured by performing uni-axial tensile experiments on individual nanofibres in situ in a scanning electron microscope (SEM) using a microfabricated tensile testing device. The nanofibres tested varied in diameter from 150 to 300 nm. Youngs modulus is observed to be within the 1.3–2 GPa range.

Doping dependence of electrical and thermal conductivity of nanoscale polyaniline thin films

We performed simultaneous characterization of electrical and thermal conductivity of 55?nm thick polyaniline (PANI) thin films doped with different levels of camphor sulfonic acids (CSAs). The effect of the doping level is more pronounced on electrical conductivity than on thermal conductivity of PANIs, thereby greatly affecting their ratio that determines the thermoelectric efficiency. At the 60% (the molar ratio of CSA to phenyl-N repeat unit of PANI) doping level, PANI exhibited the maximum electrical and thermal conductivity due to the formation of mostly delocalized structures. Whereas polarons are the charge carriers responsible for the electrical conduction, phonons are believed to play a dominant role in the heat conduction in nanoscale doped PANI thin films.

Influence of strain on thermal conductivity of silicon nitride thin films

We present a micro-electro-mechanical system-based experimental technique to measure thermal conductivity of freestanding ultra-thin films of amorphous silicon nitride (Si3N4) as a function of mechanical strain. Using a combination of infrared thermal micrography and multi-physics simulation, we measured thermal conductivity of 50 nm thick silicon nitride films to observe it decrease from 2.7 W (m K)?1?at zero strain to 0.34 W (m K)?1?at about 2.4% tensile strain. We propose that such strong strain?thermal conductivity coupling is due to strain effects on fraction?phonon interaction that decreases the dominant hopping mode conduction in the amorphous silicon nitride specimens.