Three-dimensional self-assembly using dipolar interaction.

Interaction between dipolar forces, such as permanent magnets, generally leads to the formation of one-dimensional chains and rings. We investigated whether it was possible to let dipoles self-assemble into three-dimensional structures by encapsulating them in a shell with a specific shape. We found that the condition for self-assembly of a three-dimensional crystal is satisfied when the energies of dipoles in the parallel and antiparallel states are equal. Our experiments show that the most regular structures are formed using cylinders and cuboids and not by spheroids. This simple design rule will help the self-assembly community to realize three-dimensional crystals from objects in the micrometer range, which opens up the way toward previously unknown metamaterials.

New type of microengine using internal combustion of hydrogen and oxygen.

Microsystems become part of everyday life but their application is restricted by lack of strong and fast motors (actuators) converting energy into motion. For example, widespread internal combustion engines cannot be scaled down because combustion reactions are quenched in a small space. Here we present an actuator with the dimensions 100 × 100 × 5 µm(3) that is using internal combustion of hydrogen and oxygen as part of its working cycle. Water electrolysis driven by short voltage pulses creates an extra pressure of 0.5-4 bar for a time of 100-400 µs in a chamber closed by a flexible membrane. When the pulses are switched off this pressure is released even faster allowing production of mechanical work in short cycles. We provide arguments that this unexpectedly fast pressure decrease is due to spontaneous combustion of the gases in the chamber. This actuator is the first step to truly microscopic combustion engines.

Wafer-scale fabrication of nanoapertures using corner lithography.

Several submicron probe technologies require the use of apertures to serve as electrical, optical or fluidic probes; for example, writing precisely using an atomic force microscope or near-field sensing of light reflecting from a biological surface. Controlling the size of such apertures below 100 nm is a challenge in fabrication. One way to accomplish this scale is to use high resolution tools such as deep UV or e-beam. However, these tools are wafer-scale and expensive, or only provide series fabrication. For this reason, in this study a versatile method adapted from conventional micromachining is investigated to fabricate protruding apertures on wafer-scale. This approach is called corner lithography and offers control of the size of the aperture with diameter less than 50 nm using a low-budget lithography tool. For example, by tuning the process parameters, an estimated mean size of 44.5 nm and an estimated standard deviation of 2.3 nm are found. The technique is demonstrated--based on a theoretical foundation including a statistical analysis--with the nanofabrication of apertures at the apexes of micromachined pyramids. Besides apertures, the technique enables the construction of wires, slits and dots into versatile three-dimensional structures.

Transient nanobubbles in short-time electrolysis.

Water electrolysis in a microsystem is observed and analyzed on a short-time scale of ∼10 µs. The very unusual properties of the process are stressed. An extremely high current density is observed because the process is not limited by the diffusion of electroactive species. The high current is accompanied by a high relative supersaturation, S > 1000, that results in homogeneous nucleation of bubbles. On the short-time scale only nanobubbles can be formed. These nanobubbles densely cover the electrodes and aggregate at a later time to microbubbles. The effect is significantly intensified with a small increase of temperature. Application of alternating polarity voltage pulses produces bubbles containing a mixture of hydrogen and oxygen. Spontaneous reaction between gases is observed for stoichiometric bubbles with sizes smaller than ∼150 nm. Such bubbles disintegrate violently affecting the surfaces of the electrodes.

Homoepitaxial branching: an unusual polymorph of zinc oxide derived from seeded solution growth.

The development of hydrothermal synthesis has greatly promoted bottom-up nanoscience for the rational growth of diverse zinc oxide (ZnO) nanostructures. In comparison with normal ZnO nanowires, ZnO nanostructures with a larger surface area, for instance, branched nanowires, are more attractive in the application fields of catalysis, sensing, dye-sensitized solar cells etc. So far the ZnO branched nanowires achieved by either one-step or multistep growth always present a boundary-governed nonepitaxial branch/stem interface. In this report, seeded growth of single-crystalline ZnO hexabranched nanostructures was achieved by selecting polyethylene glycol (PEG) as capping agent based on a low-temperature, laterally epitaxial solution growth strategy. We investigated the generality of this PEG-assisted growth process using different ZnO seed layers including continuous film, patterned dots, and vertically aligned nanowire arrays. It was revealed that PEG is a distinctive c-direction inhibitor responsible for the lateral growth and subsequent branching of ZnO due to its nonionic and nonacidic feature and weak reactivity in the solution system. All the obtained branched nanostructures are of single crystallinity in nature, which is methodologically determined by the homoepitaxial growth mode. This PEG-assisted process is versatile for diameter tuning and branch formation of ZnO nanowires by secondary growth. Our proof-of-concept experiments demonstrated that the ZnO hexabranched nanostructures presented superior photocatalytic efficiency for dye degradation relative to the normal ZnO nanowires.

3D nanofabrication of fluidic components by corner lithography.

A reproducible wafer-scale method to obtain 3D nanostructures is investigated. This method, called corner lithography, explores the conformal deposition and the subsequent timed isotropic etching of a thin film in a 3D shaped silicon template. The technique leaves a residue of the thin film in sharp concave corners which can be used as structural material or as an inversion mask in subsequent steps. The potential of corner lithography is studied by fabrication of functional 3D microfluidic components, in particular i) novel tips containing nano-apertures at or near the apex for AFM-based liquid deposition devices, and ii) a novel particle or cell trapping device using an array of nanowire frames. The use of these arrays of nanowire cages for capturing single primary bovine chondrocytes by a droplet seeding method is successfully demonstrated, and changes in phenotype are observed over time, while retaining them in a well-defined pattern and 3D microenvironment in a flat array.

Combustion of hydrogen-oxygen mixture in electrochemically generated nanobubbles.

Ignition of exothermic chemical reactions in small volumes is considered as difficult or impossible due to the large surface-to-volume ratio. Here observation of the spontaneous reaction is reported between hydrogen and oxygen in bubbles whose diameter is smaller than a threshold value around 150 nm. The effect is attributed to high Laplace pressure and to fast dynamics in nanobubbles and is the first indication on combustion in the nanoscale. In this study the bubbles were produced by water electrolysis using successive generation of H(2) and O(2) above the same electrode with short voltage pulses in the microsecond range. The process was observed in a microsystem at current densities >1000 A/cm(2) and relative supersaturations >1000.

Multi-silicon ridge nanofabrication by repeated edge lithography.

We present a multi-Si nanoridge fabrication scheme and its application in nanoimprint lithography (NIL). Triple Si nanoridges approximately 120 nm high and 40 nm wide separated by 40 nm spacing are fabricated and successfully applied as a stamp in nanoimprint lithography. The fabrication scheme, using a full-wet etching procedure in combination with repeated edge lithography, consists of hot H(3)PO(4) acid SiN(x) retraction etching, 20% KOH Si etching, 50% HF SiN(x) retraction etching and LOCal Oxidation of Silicon (LOCOS). Si nanoridges with smooth vertical sidewalls are fabricated by using Si 110 substrates and KOH etching. The presented technology utilizes a conventional photolithography technique, and the fabrication of multi-Si nanoridges on a full wafer scale has been demonstrated.

MEMS within a Swagelok: a new platform for microfluidic devices.

A novel packaging cum interfacing technique for microfluidic devices is reported. Unlike the conventional approach towards packaging in which the microsystem is first developed and finally packaged, a reverse approach is shown here that integrates the package with the microsystem either at the beginning or within the fabrication process. This new method employs standard glass tubes as substrates on which microfluidic components are fabricated. The tubular-substrate directly translates into a package and an interface, leading to 'plug-n-play' devices. Due to this approach, external handling forces on the microfluidic system are redirected towards the strong glass tube, and thus, improves device robustness. Maintaining the total size of the microsystem within the circumference of the glass tube enables this MEMS-on-tube assembly to be encapsulated within standard Swagelok connectors.