D. A. Dikin

Functionalized graphene sheets for polymer nanocomposites

Polymer-based composites were heralded in the 1960s as a new paradigm for materials. By dispersing strong, highly stiff fibres in a polymer matrix, high-performance lightweight composites could be developed and tailored to individual applications. Today we stand at a similar threshold in the realm of polymer nanocomposites with the promise of strong, durable, multifunctional materials with low nanofiller content. However, the cost of nanoparticles, their availability and the challenges that remain to achieve good dispersion pose significant obstacles to these goals. Here, we report the creation of polymer nanocomposites with functionalized graphene sheets, which overcome these obstacles and provide superb polymer-particle interactions. An unprecedented shift in glass transition temperature of over 40 degrees C is obtained for poly(acrylonitrile) at 1 wt% functionalized graphene sheet, and with only 0.05 wt% functionalized graphene sheet in poly(methyl methacrylate) there is an improvement of nearly 30 degrees C. Modulus, ultimate strength and thermal stability follow a similar trend, with values for functionalized graphene sheet- poly(methyl methacrylate) rivaling those for single-walled carbon nanotube-poly(methyl methacrylate) composites.

Mechanics of hydrogenated amorphous carbon deposits from electron-beam-induced deposition of a paraffin precursor

Many experiments on the mechanics of nanostructures require the creation of rigid clamps at specific locations. In this work, electron-beam-induced deposition (EBID) has been used to deposit carbon films that are similar to those that have recently been used for clamping nanostructures. The film deposition rate was accelerated by placing a paraffin source of hydrocarbon near the area where the EBID deposits were made. High-resolution transmission electron microscopy, electron-energy-loss spectroscopy, Raman spectroscopy, secondary-ion-mass spectrometry, and nanoindentation were used to characterize the chemical composition and the mechanics of the carbonaceous deposits. The typical EBID deposit was found to be hydrogenated amorphous carbon (a-C:H) having more sp2- than sp3-bonded carbon. Nanoindentation tests revealed a hardness of ∼4GPa and an elastic modulus of 30–60GPa, depending on the accelerating voltage. This reflects a relatively soft film, which is built out of precursor molecular ions impacting ...

Resonance vibration of amorphous SiO2 nanowires driven by mechanical or electrical field excitation

In this work, we have used the mechanical resonance method to determine the bending modulus of amorphous SiO2 nanowires and to study an electron charge trapping effect that occurs in these nanowires. For uniform amorphous nanowires having diameter ∼100 nm and length over 10 μm, the fit modulus values cluster near 47 GPa; this value is lower than the commonly accepted value of ∼72 GPa for fused silicon oxide (glass) fibers. For some SiO2 nanowires, we observed up to three closely spaced resonances that are a result of the nanowire anisotropy. We have compared the resonance vibration of nanowires driven by mechanical and also ac electrical field loading. All of the measurements were done inside the chamber of a scanning electron microscope where the nanowires were under bombardment of a flux of ∼3 keV energy electrons. By watching the interaction between the ac electrical field and exposed nanowire when driven at resonance frequency, we have observed significant charge trapping in the nanowires. The combina...

Patch clamp technique: Review of the current state of the art and potential contributions from nanoengineering

The patch clamp technique permits high-resolution recording of the ionic currents flowing through a cells plasma membrane. In different configurations, this technique has allowed experimenters to record and manipulate the currents that flow either through single ion channels or those that flow across the whole plasma membrane. Unfortunately, the conventional patch clamp method is laborious, requiring the careful fabrication of electrodes, skillful manipulation of the patch pipette towards a cell, and the clever design of electronics and apparatus to allow low-noise recordings. Advances in microfabrication offer promising technologies for high-throughput patch clamp recordings, particularly suitable for drug screening. This paper provides a review of the advances that have been made in the patch clamp technique over the years and considers where application of nanotechnology might provide significant contributions in the future.

Correction factors for 4-probe electrical measurements with finite size electrodes and material anisotropy: a finite element study

In four-probe (4-probe) electrical measurements, especially on highly resistive materials, it is not always possible to configure the electrodes such that the current density is uniform throughout the sample. Under such circumstances, simply considering the materials electrical resistivity to be proportional to the measured resistance with the proportionality constant given by the sample geometry can give an incorrect result. In this paper, a numerical finite element model is presented which can extract a materials true resistivity from co-linear 4-probe electrical measurements on highly resistive samples with large electrodes that extend across the sample width. The finite element model is used to investigate the influence of material anisotropy, the resistance of the sample–electrode interfaces and the relative electrode-to-sample size on the potential and current density distributions in the sample. A correction factor is introduced to account for the impact of these effects on the measured resistivity. In the limit of large interface resistance, excellent agreement is found with an analytical expression derived elsewhere (Esposito et al 2000 J. Appl. Phys. 88 2724–9). The approach presented here can be used to evaluate a variety of effects on co-linear 4-probe electrical measurements, can be extended to complex specimen geometries with arbitrary electrode arrangements and, additionally, could find use in the evaluation of data from 4-probe thermal conductivity measurements.

Three-dimensional representation of curved nanowires.

Nanostructures, such as nanowires, nanotubes and nanocoils, can be described in many cases as quasi one‐dimensional curved objects projecting in three‐dimensional space. A parallax method to construct the correct three‐dimensional geometry of such one‐dimensional nanostructures is presented. A series of scanning electron microscope images was acquired at different view angles, thus providing a set of image pairs that were used to generate three‐dimensional representations using a matlab program. An error analysis as a function of the view angle between the two images is presented and discussed. As an example application, the importance of knowing the true three‐dimensional shape of boron nanowires is demonstrated; without the nanowires correct length and diameter, mechanical resonance data cannot provide an accurate estimate of Youngs modulus.

Impedance characterization and modelling of an improved patch clamp device

The gold standard for studying the properties of ion channels is the patch clamp, a technique that measures pA currents passing through ion channels in an isolated patch of cell membrane. The authors sought to design an improved device for whole-cell patch clamp recordings using nanoscale pipettes which more readily form seals with the cell membrane and a movable metal electrode to decrease the access resistance, increase the signal bandwidth, and clear debris from the pipette tip. Electrical characterization of the instrument by impedance spectroscopy showed that advancing the metal electrode towards the pipette tip decreased the impedance distinctly in different frequency bands, the greatest decrease occurring between 1 and 100 kHz, and the smallest at 100 kHz. Finite element simulations of the electrode, incorporating the electrode/electrolyte interface, suggest that optimal signal transmission can be achieved with athick-walled pipette, nearly filled with alarge Ag/AgCl electrode, whose impedance can be reduced through surface modification.

Nanomanipulator Measurements of the Mechanics of Nanostructures and Nanocomposites

While a great deal of progress has been made in understanding the theoretical fundamentals of nanodevices and nanomaterials, to date the complexities of performing mechanical experiments at the nanoscale have limited the ability to compare theoretical and computational predictions with experimental data. In this chapter we will discuss some of the critical challenges that hamper mechanical experiments at the nanoscale, and present how these challenges have been, and are currently being, addressed by next-generation tools designed to perform nanoscale mechanical experiments. In particular, we will focus on how our work and understanding have evolved through our experience in this area. We will then present a number of examples of how our current nanomanipulator has been used to perform nanoscale mechanical testing of a variety of nanostructures, including: tensile loading of individual nanostructures, nanoscale pullout tests of nanostructures embedded in a matrix material, and forced vibrational resonance to measure the modulus of nanorods and nanotubes. We will conclude by discussing relevant near- and long-term goals and challenges of nanoscale mechanical experimentation.