B. A. Samuel

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.

Room temperature relaxation of freestanding nanocrystalline gold films

We present a novel experimental technique to study compound stress–strain relaxation of thin freestanding gold films at room temperature. The films were 125 nm thick with an average grain size of 50–60 nm. Both static experiments and time-dependent relaxation experiments were performed inside a field emission scanning electron microscope using a micro- electromechanical system (MEMS)-based test bed which is only 3 mm × 10 mm in size. The thin film specimens used in these experiments are co-fabricated with the micromechanical loading device and hence do not need to be externally aligned and gripped. We observed a significant effect of stress relaxation on Youngs modulus of gold thin films and modeled the relaxation behavior using a three-parameter standard anelastic solid. The freestanding nature of the thin film also provides us with information about the mechanical behavior of thin films without any substrate effects.

Synthesis and characterization of glassy carbon nanowires

The advent of carbon-based micro- and nanoelectromechanical systems has revived the interest in glassy carbon, whose properties are relatively unknown at lower dimensions. In this paper, electrical conductivity of individual glassy carbon nanowires was measured as a function of microstructure (controlled by heat treatment temperature) and ambient temperature. The semiconducting nanowires with average diameter of 150nm were synthesized from polyfurfuryl alcohol precursors and characterized using transmission electron and Raman microscopy. DC electrical measurements made at 90 K to 450 K show very strong dependence of temperature, following mixed modes of activation energy and hopping-based conduction.

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.

In-Situ Nanoscale Single Fiber Fragmentation Using Fluorescence Microscopy

We use fluorescence microscopy to perform real time single fiber fragmentation experiments on individual PolyFurfuryl Alcohol (PFA) nanofibers embedded within a Poly DiMethyl Siloxane (PDMS) matrix. By using fluorescent nanofibers in an optically transparent matrix, fragmentation of the nanoscale fibers (even less than 400 nm) can also be easily visualized using fluorescence emission from the nanowires. When the composite specimen was loaded to saturation strain the fragment lengths distribution was observed to follow a two parameter Weibull frequency distribution. In addition, we also present a digital image correlation based technique to obtain localized strain (and hence stress) data based on the use of fluorescent nanoscale spatial markers.© 2007 ASME

Mechanical Characterization of Polymer Nanowires Using MEMS

We present results on the mechanical properties of single freestanding poly-furfuryl alcohol (PFA) nanowires (aspect ratio > 50, diameters 100–300 nm) from experiments conducted using a MEMS-based uniaxial tensile testing device in-situ inside the SEM. The specimens tested were pyrolyzed PFA nanowires (pyrolyzed at 800° C).Copyright

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.

Experimental Study of Structure-Electrical Transport Correlation in Single Disordered Carbon Nanowires

We present experimental results characterizing the changes in electrical transport of single disordered carbon nanowires (diameter 150–250 nm) to the changes in microstructure within the nanowires induced by synthesis temperature. The material system studied is a nanoporous, semiconducting disordered carbon nanowire obtained from the pyrolysis of a polymeric precursor (polyfurfuryl alcohol). Unlike the other allotropes of carbon such as diamond, graphite (graphenes) and fullerenes (CNT, buckyballs), disordered carbons lack crystalline order and hence can exhibit a range of electronic properties, dependent on the degree of disorder and the local microstructure. Such disordered carbon nanowires are therefore materials whose electronic properties can be engineered to specifications if we understand the structure-property correlations. Using dark DC conductivity tests, measurements were performed from 300K to 450K. The charge transport behavior in the nanowires is found to follow an activation-energy based conduction at high temperatures. The conductivity for nanowires synthesized from 600°C to 2000°C is calculated and is linked to changes in the microstructure using data obtained from SEM, TEM and Raman spectroscopy. The electrical properties of the nanowire are shown to be linked intrinsically to the microstructure and the degree of disorder, which in turn can be controlled to a great extent just by controlling the pyrolysis temperature. This ability to tune the electrical property, specifically conductivity, and map it to the structural changes within the disordered material makes it a candidate material for use in active/passive electronic components, and as versatile transducers for sensors.Copyright

Thermo Electrical Characterization of Pyrolyzed Polyfurfuryl Alcohol Nanowires

We present experimental results on the electrical characteristics of carbonaceous nanowires using micro-fabricated two point electrodes. The nanowires (diameter varying from 150 nm to 250 nm, and tens of microns in length) are synthesized by the pyrolysis of polyfurfuryl alcohol (PFA) nanowire precursors. The microstructure of the pyrolyzed nanowire shows predominantly amorphous carbon structure with an increasing degree of graphitization reported in literature at higher pyrolysis temperatures. The effect of the nanowire surface temperature on its resistivity is also characterized. We observe temperature dependence in resistivity and decreasing resistance with increasing temperature which is evidence of semiconducting nature of these carbonaceous nanowires.© 2007 ASME

Mechanical Response of Single Biological Cells to Externally Applied Forces

We present an experimental technique capable of applying and measuring very small forces (nanoNewtons) on live biological cells. The technique is based on combing microfabricated molded elastomeric microneedles, and substrate deformation to actively apply loads on single cells. By using fluorescence and differential interference contrast imaging we can quantitatively study the response of biological cells to force exerted on it via the extra cellular matrix. Bovine Aortic Endothelial Cells were cultured on the micropillars and strained in-vitro to demonstrate the experimental technique.Copyright