Rolled-up Micro Research Articles

Development and applications of rolled-up oxide micro/nanotubes

Micro/nanotubes fabricated via self-rolled-up nanotechnology have wide potential applications in materials science, chemistry and biology due to their simple fabrication methods and splendid physical and chemical properties. Oxides, with great stability, crystal structure and optical properties, especially unique semiconductor properties, magnetic and electrical properties of some oxides, become common materials of rolled-up micro-/nanotubes. This review introduces the principles and methods of self-rolled-up micro/nanotubes fabrications, and the applications in optical resonant cavities, sensors, cell study, anodes of Li ion batteries, self-propel engines.

Phonon Spectrum Engineering in Rolled-up Micro- and Nano-Architectures

We report on a possibility of efficient engineering of the acoustic phonon energy spectrum in multishell tubular structures produced by a novel high-tech method of self-organization of micro- and nano-architectures. The strain-driven roll-up procedure paved the way for novel classes of metamaterials such as single semiconductor radial micro- and nano-crystals and multi-layer spiral micro- and nano-superlattices. The acoustic phonon dispersion is determined by solving the equations of elastodynamics for InAs and GaAs material systems. It is shown that the number of shells is an important control parameter of the phonon dispersion together with the structure dimensions and acoustic impedance mismatch between the superlattice layers. The obtained results suggest that rolled up nano-architectures are promising for thermoelectric applications owing to a possibility of significant reduction of the thermal conductivity without degradation of the electronic transport.

Improvement of the light extraction efficiency of GaN-based LEDs using rolled-up nanotube arrays.

In this paper, we have investigated the effect of rolled-up nanotubes on the light extraction efficiency of GaN-based LEDs using two-dimensional finite element method simulation. The light extraction involves two successive steps, including the coupling from the light source to the tube and the subsequent emission from the tube to the air. Significantly enhanced light extraction efficiency is observed for both TE and TM waves by optimizing the nanotube geometry and dimension as well as the separation between the nanotube and light source. We have further shown that densely packed nanotube arrays can be integrated with GaN-based LEDs to achieve unequivocal improvement of light extraction efficiency over a large surface area. With recent advances in rolled-up micro- and nanotubes, it is expected that this study can offer a potentially flexible, low cost approach to enhance the light extraction of various LED devices.

Rolled-Up Nanotech: Illumination-Controlled Hydrofluoric Acid Etching of AlAs Sacrificial Layers.

The effect of illumination on the hydrofluoric acid etching of AlAs sacrificial layers with systematically varied thicknesses in order to release and roll up InGaAs/GaAs bilayers was studied. For thicknesses of AlAs below 10 nm, there were two etching regimes for the area under illumination: one at low illumination intensities, in which the etching and releasing proceeds as expected and one at higher intensities in which the etching and any releasing are completely suppressed. The “etch suppression” area is well defined by the illumination spot, a feature that can be used to create heterogeneously etched regions with a high degree of control, shown here on patterned samples. Together with the studied self-limitation effect, the technique offers a way to determine the position of rolled-up micro- and nanotubes independently from the predefined lithographic pattern.

Process integration of microtubes for fluidic applications

Three-dimensional InGaAs∕GaAs microtubes are integrated by photolithography into a microfluidic device. The integration process, made possible due to advances in fabricating long, homogeneous rolled-up microtubes, is described in detail. Liquid filling and emptying of individual microtubes, and the final microfluidic device are investigated by video microscopy. The authors find an agreement for their channels with the Washburn equation [Phys. Rev. 17, 273 (1921)] for filling using a modified capillary pressure fit to experimental conditions. Emptying of a vacuum pumped microfluidic device also qualitatively agrees with theory. The results suggest rolled-up micro- and nanotubes as possible systems to provide fully integrative fluid analysis on a chip.

Rolled-up micro- and nanotubes from single-material thin films

The authors fabricate well-positioned and size-scalable semiconductor micro- and nanotubes from single-material layers. The tubes form when a partially strain-relaxed film, grown at low substrate temperatures, is released from the substrate by selective underetching. The layer rolls downwards or upwards depending on whether it is initially tensile or compressively strained. They create silicon and indium-gallium-arsenide tubes with diameters accurately tunable by varying the layer thickness. They draw a simple model to describe the mechanism responsible for the tube formation from a single-material thin film. Moreover, the tube diameters are shown to scale with strain and layer thickness.