A. V. Desai

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.

Sliding of zinc oxide nanowires on silicon substrate

Adhesion and friction forces between zinc oxide nanowires and silicon substrate were studied in situ inside a scanning electron microscope. A procedure for measuring these forces from the bending profiles of the nanowires was developed and the van der Waals and friction forces were found to be about 81.05pN and 7.7nN, respectively. The pronounced friction was explained using nanoscale adhesion-friction coupling mechanisms. Immediate implication of the findings is on the accuracy of nanomechanical characterization using bending experiments.

Design and fabrication of a direction sensitive MEMS shear stress sensor with high spatial and temporal resolution

Shear stress at the fluid–wall interface is one of the most frequently studied parameters in fluid dynamics. It is also a parameter of very small magnitude and calls for high resolution force sensors. Macroscopic sensors compromise dynamic bandwidth for the required high resolution and therefore cannot resolve shear stress data in space and/or time, which is very important for fundamental understanding in non-laminar fluid dynamics. We exploit the linear reduction in stiffness accompanied by cubic reduction in mass by miniaturization to design and fabricate a novel micro-electro-mechanical sensor (MEMS) for direct measurement of shear stress along and across the direction of fluid flow, with 0.01 Pa resolution and 50 kHz bandwidth along the flow. The mechanical component of the sensor is a floating beam element and capacitive comb drives supported by an in-plane torsional spring. A resonant RLC circuit, capable of sub-femtofarad capacitive sensing, is used to sense the displacement in the floating beam under shear. Fabrication of the sensor is demonstrated using silicon-on wafer (SOI) technology. The small overall size of the sensor, wide range of measurement, large bandwidth and high spatial and temporal resolution will make it useful in a wide variety of civil and military applications such as aerospace, automotive, marine and biomedical.

Influence of electromechanical boundary conditions on elasticity of zinc oxide nanowires

Uniaxial tensile experiments were performed on single crystal zinc oxide nanowires with a custom microfabricated tool. The measured Young’s modulus is about 30%—40% of the bulk value for specimens with 200–400nm in diameter, which cannot be explained with classical elasticity formulations. We discuss this anomaly in light of the enhanced electromechanical coupling due to static mechanical and isolated electrical boundary conditions that can significantly contribute to the softening of the material, irrespective of the length scale.

Test Bed for Mechanical Characterization of Nanowires

Nanowires are one-dimensional solids that are deemed to be the building-block materials for next-generation sensors and actuators. Owing to their unique length scale, they exhibit superior mechanical properties and other length-scale-dependent phenomena. Most of these are challenging to explore, owing to the difficulties in specimen preparation, manipulation, and the requirement of high-resolution force and displacement sensing. To address these issues, a micromechanical device for uniaxial mechanical testing of single nanowires and nanotubes is used here. The device has 10 nN force and 1 nm displacement resolution and its small size (2 ×1 mm) allows for in situ experimentation inside analytical chambers, such as the electron microscopes. A microscale pick-and-place technique is presented as a generic specimen preparation and manipulation method for testing single nanowires. Preliminary results on zinc oxide nanowires show the Youngs modulus and fracture strain to be about 76 GPa and 8 per cent respectively.

Fracture toughness characterization of advanced coatings

We present an experimental technique for the measurement of the fracture toughness of advanced surface coatings in situ in a scanning electron microscope. The technique is demonstrated on titanium–titanium nitride multi-layer thin films. Titanium–titanium nitride multi-layers are part of a new class of advanced erosion-resistant coatings with optimized toughness and hardness for performance and life respectively, and the fracture properties of these surface coatings determine their effectiveness. Thin film specimens were prepared using a lift-out technique in focused ion beam-scanning electron microscope. The fracture toughness of a 300 nm thick specimen perpendicular to the multi-layer thickness was measured to be 2.90 ± 0.3 MPa m+1/2. The fracture characterization technique can be applied for a wide variety of surface coatings, and thin films in general.

Microscale application of column theory for high resolution force and displacement sensing

We present the design, fabrication, and experimental validation of a device which exploits the amplification of displacement and attenuation of structural stiffness in the post-buckling deformation of slender columns to obtain pico-Newton force and nanometer displacement resolution, even under an optical microscope. The extremely small size, purely mechanical sensing scheme and vacuum compatibility of the instrument makes it compatible with existing visualization tools of nanotechnology. The instrument has a wide variety of potential applications ranging from electro-mechanical characterization of one dimensional solids to single biological cells.

Mechanical properties of ZnO nanowires

Semiconductor nanowires such as zinc oxide nanowires are projected to be the next generation materials for nanoscale sensors and actuators. They also serve as ideal systems for studying material behavior at the small scale. In this paper, we report experimental results on the mechanical properties of zinc oxide nanowires. We have designed a MEMS (microelectromechanical systems) test-bed for mechanical characterization of nanowires and use a microscale version of pick-and-place as a generic specimen preparation and manipulation technique. We performed experiments on zinc oxide nanowires inside a scanning electron microscope (SEM) and estimated the Youngs modulus to be approximately 21 GPa and the fracture strain to vary from 5% to 15%. We attribute the difference in mechanical properties of the nanowires from bulk properties to several factors such as lower number of defects, charge redistribution at the atomic scale and surface effects.

A novel MEMS device for high resolution force and displacement measurement

We present the design and fabrication of a MEMS device for high resolution force and displacement measurements. Quantitative and qualitative measurements can be performed in-situ in SEM, TEM or STM, where the small chamber size makes it challenging to integrate conventional force-displacement sensors. The device exploits the amplification of displacement and attenuation of structural stiffness in the post-buckling region of slender silicon beams to obtain pico-Newton force and nanometer displacement resolution. The specimen deformation can be read in optical microscopes, thus avoiding complex displacement sensing mechanisms. The device can be used for characterization of carbon nanotube-polymer interfaces, nanoscale thin films and mechanical testing of single biological cells.

Relaxation in Nanoscale Freestanding Gold Films Using MEMS

We present experimental results to describe the stress relaxation behavior of thin (125 nm) freestanding gold films at room temperature. The experiments were performed inside a field emission scanning microscope using a MEMS-based test bed which is only 3mm × 10mm in size. The effect of stress relaxation on the young’s modulus of gold thin films is observed. The thin film specimen used in the experiment is co-fabricated with the micromechanical loading device and hence eliminates problems of alignment and gripping. Freestanding thin films provide us with information about the mechanical behavior of thin films in the absence of substrate effects.© 2005 ASME