Stephen T. Purcell

Giant osmotic energy conversion measured in a single transmembrane boron nitride nanotube

New models of fluid transport are expected to emerge from the confinement of liquids at the nanoscale, with potential applications in ultrafiltration, desalination and energy conversion. Nevertheless, advancing our fundamental understanding of fluid transport on the smallest scales requires mass and ion dynamics to be ultimately characterized across an individual channel to avoid averaging over many pores. A major challenge for nanofluidics thus lies in building distinct and well-controlled nanochannels, amenable to the systematic exploration of their properties. Here we describe the fabrication and use of a hierarchical nanofluidic device made of a boron nitride nanotube that pierces an ultrathin membrane and connects two fluid reservoirs. Such a transmembrane geometry allows the detailed study of fluidic transport through a single nanotube under diverse forces, including electric fields, pressure drops and chemical gradients. Using this device, we discover very large, osmotically induced electric currents generated by salinity gradients, exceeding by two orders of magnitude their pressure-driven counterpart. We show that this result originates in the anomalously high surface charge carried by the nanotube’s internal surface in water at large pH, which we independently quantify in conductance measurements. The nano-assembly route using nanostructures as building blocks opens the way to studying fluid, ionic and molecule transport on the nanoscale, and may lead to biomimetic functionalities. Our results furthermore suggest that boron nitride nanotubes could be used as membranes for osmotic power harvesting under salinity gradients.

Contact angle measurements on superhydrophobic carbon nanotube forests: Effect of fluid pressure

In this paper the effect of pressure on the contact angle of a water drop on superhydrophobic carbon nanotube (CNT) forests is studied. Superhydrophobic CNT forests are obtained from a new and simple functionalization strategy, based on the gold-thiol affinity. Using a specifically devised experimental setup, we then show that these surfaces are able to withstand high excess pressures (larger than 10 kPa) without transiting toward a roughness-invaded state, therefore preserving their low adhesion properties. Together with the relatively low technical cost of the process, this robustness vs. pressure makes such surfaces very appealing for practical integration into microfluidic systems.

Digital and FM Demodulation of a Doubly Clamped Single-Walled Carbon-Nanotube Oscillator: Towards a Nanotube Cell Phone

Electromechanical resonators are a key element in radio-frequency telecommunication devices and thus new resonator concepts from nanotechnology can readily find important industrial opportunities. Here, the successful experimental realization of AM, FM, and digital demodulation with suspended single-walled carbon-nanotube resonators in a field-effect transistor configuration is reported. The crucial role played by the electromechanical resonance in demodulation is clearly demonstrated. The FM technique is shown to lead to the suppression of unwanted background signals and the reduction of noise for a better detection of the mechanical motion of nanotubes. The digital data-transfer rate of standard cell-phone technology is within the reach of these devices.

High Q factor for mechanical resonances of batch-fabricated SiC nanowires

The authors present here the measurements of high mechanical Q factors for singly clamped, batch-fabricated SiC nanowires measured by field emission (FE) in ultrahigh vacuum. The resonances of two nanowires, glued to the ends of tungsten support tips, were electrostatically excited and detected by the variation in the FE microscopy (FEM) images. Low amplitude oscillations were measured by numerical analysis of the FEM image blurring during frequency scans through the resonances. This avoided the artificial broadening of the resonances by nonlinear effects. A room temperature Q factor of 159 000 was achieved after high temperature in situ cleaning.

Evidence for Poole-Frenkel conduction in individual SiC nanowires by field emission transport measurements

In this paper we examine carrier transport mechanisms in individual Silicon Carbide nanowires (NWs) by an original use of field emission (FE). Total energy distributions were measured as a function of temperature and extraction voltage allowing us to determine the voltage drops along the NWs and thus the temperature-dependent current-voltage (I-V-T) characteristics. The measurements were analyzed using different transport mechanisms of which only the Poole–Frenkel model gives an excellent fit. The dielectric constant was estimated for several samples at ɛ~10 in excellent agreement with the bulk value. The characteristic trap energies, Ea, were determined from the I-V-T data to be ∼0.3 eV. In general this work shows how FE can be used for transport measurements on individual semiconducting NWs.

Atomic-size metal ion sources: principles and use

The controlled emission of ions by field desorption from nanotips is used to develop an atomic-size ion source. The source has been shown to emit Au and Ag ions from layers pre-deposited on W base tips with two characteristics important for applications: (1) the ion emission can be controlled such that it originates exclusively from the atomic-scale apex of a single nanotip positioned precisely on the main axis of the base tip and (2) the nanotip emits stable metallic ion currents in the pA range for at least tens of hours. Emission patterns, energy distributions and emitted ion currents versus time, voltage and temperature have been measured. These data permit a qualitative understanding of the source characteristics by considering their relation to the three essential source elements: the axial nanotip, the supply function and ionization by field desorption. Computer simulations are performed to explore the use of these sources to create nanometre-scale structures when coupled to electrostatic lenses or in a near-field configuration as well as the interactions of the ions in the range of 100-1000 eV with a Si substrate.

Current Saturation in Field Emission from H-Passivated Si Nanowires

This paper explores the field emission (FE) properties of highly crystalline Si nanowires (NWs) with controlled surface passivation. The NWs were batch-grown by the vapor-liquid-solid process using Au catalysts with no intentional doping. The FE current-voltage characteristics showed quasi-ideal current saturation that resembles those predicted by the basic theory for emission from semiconductors, even at room temperature. In the saturation region, the currents were extremely sensitive to temperature and also increased linearly with voltage drop along the nanowire. The latter permits the estimation of the doping concentration and the carrier lifetime, which is limited by surface recombination. The conductivity could be tuned over 2 orders of magnitude by in situ hydrogen passivation/desorption cycles. This work highlights the role of dangling bonds in surface leakage currents and demonstrates the use of hydrogen passivation for optimizing the FE characteristics of Si NWs.

Simple modeling of self-oscillations in nanoelectromechanical systems

We present here a simple analytical model for self-oscillations in nanoelectromechanical systems. We show that a field emission self-oscillator can be described by a lumped electrical circuit and that this approach is generalizable to other electromechanical oscillator devices. The analytical model is supported by dynamical simulations where the electrostatic parameters are obtained by finite element computations.

High frequency top-down junction-less silicon nanowire resonators

We report here the first realization of top-down silicon nanowires (SiNW) transduced by both junction-less field-effect transistor (FET) and the piezoresistive (PZR) effect. The suspended SiNWs are among the smallest top-down SiNWs reported to date, featuring widths down to ~20 nm. This has been achieved thanks to a 200 mm-wafer-scale, VLSI process fully amenable to monolithic CMOS co-integration. Thanks to the very small dimensions, the conductance of the silicon nanowire can be controlled by a nearby electrostatic gate. Both the junction-less FET and the previously demonstrated PZR transduction have been performed with the same SiNW. These self-transducing schemes have shown similar signal-to-background ratios, and the PZR transduction has exhibited a relatively higher output signal. Allan deviation (σA) of the same SiNW has been measured with both schemes, and we obtain σ(A) ~ 20 ppm for the FET detection and σ(A) ~ 3 ppm for the PZR detection at room temperature and low pressure. Orders of magnitude improvements are expected from tighter electrostatic control via changes in geometry and doping level, as well as from CMOS integration. The compact, simple topology of these elementary SiNW resonators opens up new paths towards ultra-dense arrays for gas and mass sensing, time keeping or logic switching systems on the SiNW-CMOS platform.

Realization of an axially aligned Au-ion source of atomic size

Nanotips are high-brightness electron sources with a principal characteristic source-size of one atom. We explore here nanotips as a type of atom-size source of Au ions. Conditions were found for a reproducible fabrication of a single Au nanotip on the principal axis of a W-base tip that emitted a beam of Au exclusively from its apex. The measured source characteristics were (i) the atomic-size source; (ii) currents: 10−14–1012 A; (iii) stability: 5 h; (iv) beam opening angle: 3.2°; and (v) energy dispersion <1 eV. First focusing experiments showed crossover and that the entire emitted beam was focused.