Bruce W. Alphenaar

The magnetic coupling of a piezoelectric cantilever for enhanced energy harvesting efficiency

It is shown that the energy harvesting capabilities of a piezoelectric cantilever can be enhanced through coupling to a static magnetic field. A permanent magnet is fixed to the end of a piezoelectric cantilever, causing it to experience a non-linear force as it moves with respect to a stationary magnet. The magnetically coupled cantilever responds to vibration over a much broader frequency range than a standard cantilever, and exhibits non-periodic or chaotic motion. While the off-resonance response is substantially increased compared to that of a standard cantilever, no reduction in the response at the resonant frequency is observed, as long as a symmetric magnetic force is applied. The magnetically coupled cantilever motion is analyzed using a simple driven harmonic oscillator model with a non-linear magnetic force term. The results show that magnetic coupling can be used to increase the amount of power scavenged from environments containing multi-mode, or random vibration sources.

Displacement current detection of photoconduction in carbon nanotubes

Using a capacitive photocurrent measurement technique, we demonstrate the ability of both semiconducting and metallic single wall nanotubes to function as photodetectors over a wide spectral range. We observe clear peaks in the photo induced displacement current of a nanotube-plated capacitor that correspond directly to the semiconducting and metallic transitions in the nanotube absorbance spectrum. The signal increases substantially as the carrier drift velocity is raised with applied bias. A large increase in the photocurrent observed below temperatures of 100K suggests that the nanotube hot carrier relaxation rate decreases substantially at low temperatures.

Viscous damping of microresonators for gas composition analysis

The damping effect of various gas environments on a silicon, lateral microresonator implemented with piezoresistive detection is investigated in this study. The resonant frequency of the cantilever shifts due to viscous damping by an amount that is directly determined by the molar mass of the gas, thereby providing a method to determine the composition of the gas environment. In addition, the microresonator demonstrates the ability to perform CO2 composition analysis using this nonreaction based detection method. The advantages of this gas analysis method are that it is simple, repeatable, reversible and not limited to reactive gases.

Electrical contacts to ultrananocrystalline diamond

The contact behavior of various metals on n-type nitrogen-doped ultrananocrystalline diamond (UNCD) thin films has been investigated. The influences of the following parameters on the current-voltage characteristics of the contacts are presented: (1) electronegativity and work function of various metals, (2) an oxidizing acid surface cleaning step, and (3) oxide formation at the film/contact interface. Near-ideal ohmic contacts are formed in every case, while Schottky barrier contacts prove more elusive. These results counter most work discussed to date on thin diamond films, and are discussed in the context of the unique grain-boundary conductivity mechanism of the nitrogen-doped UNCD.

Adsorption of oxygen molecules on individual single-wall carbon nanotubes

Our study of the adsorption of oxygen molecules on individual semiconductiong single-walled carbon nanotubes at ambient conditions reveals that the adsorption is physisorption, the resistance without O2 increases by approximately two orders of magnitude as compared to that with O2, and the sensitive response is due to the pinning of the Fermi level near the top of the valence band of the tube, resulting from impurity states of O2 appearing above the valence band.

Enhancement of Energy Harvested from a Random Vibration Source by Magnetic Coupling of a Piezoelectric Cantilever

It is demonstrated experimentally that the energy harvested from a random noise source by a piezoelectric cantilever can be substantially enhanced by introducing a magnetic coupling force. The coupled cantilever responds to a 1/f vibration spectrum (‘pink noise’) with chaotic motion that on average has larger amplitude than the non-chaotic motion of an uncoupled cantilever. A 50% increase in output voltage was observed in the coupled cantilever compared to the uncoupled cantilever. Calculations show that the magnetic force transforms the quadratic spring potential of the cantilever into a double valley of two potential wells. Fluctuations between the two potential minima increase the amplitude of the cantilever motion over a range of vibration frequencies.

Observation of the triplet exciton in EuS-coated single-walled nanotubes.

Photon absorption by carbon nanotubes creates bound electron-hole pairs called excitons, which can exist in spin-polarized triplet or spin-unpolarized singlet configurations. Triplet excitons are optically inactive owing to the weak spin-orbit coupling in nanotubes. This prevents the optical injection of electron spin into nanotubes for spintronic applications and limits the efficiency of photocurrent generation. Here, we show that it is possible to optically excite the triplet exciton by using a ferromagnetic semiconductor as a spin filter to mix the singlet and triplet excitons. The triplet contribution to the photocurrent is detected, representing the first direct evidence of the triplet exciton in carbon nanotubes.

Gated spin transport through an individual single wall carbon nanotube

Hysteretic switching in the magnetoresistance of short-channel, ferromagnetically contacted individual single wall carbon nanotubes is observed, providing strong evidence for nanotube spin transport. By varying the voltage on a capacitively coupled gate, the magnetoresistance can be reproducibly modified between +10% and −15%. The results are explained in terms of wave vector matching of the spin polarized electron states at the ferromagnetic ∕ nanotube interfaces.

Evaluation of the direct and indirect response of blood leukocytes to carbon nanotubes (CNTs)

UNLABELLED Carbon nanotubes (CNTs) possess unique structural and functional properties and are readily internalized by various mammalian cells, making them highly attractive as a tool for gene and drug delivery. However, prior to use in vivo as carriers for therapeutics, their toxicity and potential to elicit an immune response need to be understood. To evaluate the acute response of blood leukocytes to CNTs in vitro, we recreated two specific events: (a) a direct-exposure event that may occur due to presence of CNTs in circulation and (b) presentation of CNTs to blood leukocytes via antigen presenting cells. The potential for activation of different leukocyte subpopulations was then evaluated by profiling various early activation markers using flow cytometry. To ensure relevance to gene and drug delivery, these experiments utilized single-walled CNTs (SWCNTs) functionalized with single-stranded (ss)-DNA fragments consisting of guanine-thymine (GT) repeat sequences, which have potential to serve as a backbone for transport of biomolecules and also as a surfactant to prevent aggregation. Results from this study demonstrate that ssDNA-functionalized SWCNTs does not elicit an acute immune response from blood leukocytes through either direct or indirect interactions as verified by the expression of early leukocyte activation markers. FROM THE CLINICAL EDITOR Carbon nanotubes offer a possible option for targeted gene and drug delivery, but first their toxicity and potential to elicit an immune response need to be understood. The authors of this study demonstrate that ssDNA-functionalized SWCNTs do not elicit an acute immune response from blood leukocytes as verified by the expression of early leukocyte activation markers.

A new technique for reversible permeabilization of live cells for intracellular delivery of quantum dots.

A major challenge with the use of quantum dots (QDs) for cellular imaging and biomolecular delivery is the attainment of QDs freely dispersed inside the cells. Conventional methods such as endocytosis, lipids based delivery and electroporation are associated with delivery of QDs in vesicles and/or as aggregates that are not monodispersed. In this study, we demonstrate a new technique for reversible permeabilization of cells to enable the introduction of freely dispersed QDs within the cytoplasm. Our approach combines osmosis driven fluid transport into cells achieved by creating a hypotonic environment and reversible permeabilization using low concentrations of cell permeabilization agents like Saponin. Our results confirm that highly efficient endocytosis-free intracellular delivery of QDs can be accomplished using this method. The best results were obtained when the cells were treated with 50 μg ml⁻¹ Saponin in a hypotonic buffer at a 3:2 physiological buffer:DI water ratio for 5 min at 4 °C.