Dominic J. Thurmer

Self-Assembled Ultra-Compact Energy Storage Elements Based on Hybrid Nanomembranes

Self-assembly methods combined with standard top-down approaches are demonstrated to be suitable for fabricating three-dimensional ultracompact hybrid organic/inorganic electronic devices based on rolled-up nanomembranes. Capacitors that are self-wound and manufactured in parallel are almost 2 orders of magnitude smaller than their planar counterparts and exhibit capacitances per footprint area of around 200 microF/cm(2). This value significantly exceeds that which was previously reported for metal-insulator-metal capacitors based on Al(2)O(3), and the obtained specific energy (approximately 0.55 Wh/kg) would allow their usage as ultracompact supercapacitors. By incorporating organic monolayers into the inorganic nanomembrane structure we can precisely control the electronic characteristics of the devices. The adaptation of the process for creating ultracompact batteries, coils and transformers is an attractive opportunity for reducing the size of energy storage elements, filters, and signal converters. These devices can be employed as implantable electronic circuits or new approaches for energy-harvesting applications. Furthermore, the incorporation of functional organic molecules gives rise to novel devices with almost limitless chemical and biological functionalities.

On-chip Si/SiOx microtube refractometer

The authors fabricate rolled up microtubes consisting of Si/SiOx on Si substrate and analyze the possibility to use them as a refractometric sensor. An aqueous sugar solution is inserted into the microtube, which leads to a change in refractive index and, as a result, to a detectable spectral shift of the whispering gallery modes. Experimental results can fit well with finite-difference time-domain simulations, which are used to determine the sensitivity of this tube refractometer. The ratio of spectral sensitivity to channel cross-sectional area of the refractometer is particularly striking and allows analysis of fluid volumes in the range of femtoliters. A comparative discussion with other existing refractometer schemes concludes this work.

Rolled-up nanomembranes as compact 3D architectures for field effect transistors and fluidic sensing applications.

We fabricate inorganic thin film transistors with bending radii of less than 5 μm maintaining their high electronic performance with on-off ratios of more than 10(5) and subthreshold swings of 160 mV/dec. The fabrication technology relies on the roll-up of highly strained semiconducting nanomembranes, which compacts planar transistors into three-dimensional tubular architectures opening intriguing potential for microfluidic applications. Our technique probes the ultimate limit for the bending radius of high performance thin film transistors.

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.

Hybrid organic/inorganic molecular heterojunctions based on strained nanomembranes.

In this work, we combine self-assembly and top-down methods to create hybrid junctions consisting of single organic molecular monolayers sandwiched between metal and/or single-crystalline semiconductor nanomembrane based electrodes. The fabrication process is fully integrative and produces a yield loss of less than 5% on-chip. The nanomembrane-based electrodes guarantee a soft yet robust contact to the molecules where the presence of pinholes and other defects becomes almost irrelevant. We also pioneer the fabrication and characterization of semiconductor/molecule/semiconductor tunneling heterojunctions which exhibit a double transition from direct tunneling to field emission and back to direct tunneling, a phenomenon which has not been reported previously.

Nanomembrane-Based Mesoscopic Superconducting Hybrid Junctions

A new method for combining top-down and bottom-up approaches to create superconductor-normal metal-superconductor niobium-based Josephson junctions is presented. Using a rolled-up semiconductor nanomembrane as scaffolding, we are able to create mesoscopic gold filament proximity junctions. These are created by electromigration of gold filaments after inducing an electric field mediated breakdown in the semiconductor nanomembrane, which can generate nanometer sized structures merely using conventional optical lithography techniques. We find that the created point contact junctions exhibit large critical currents of a few milliamps at 4.2 K and an I(c)R(n) product placing their characteristic frequency in the terahertz region. These nanometer-sized filament devices can be further optimized and integrated on a chip for their use in superconductor hybrid electronics circuits.

Magnetically capped rolled-up nanomembranes.

Modifying the curvature in magnetic nanostructures is a novel and elegant way toward tailoring physical phenomena at the nanoscale, allowing one to overcome limitations apparent in planar counterparts. Here, we address curvature-driven changes of static magnetic properties in cylindrically curved magnetic segments with different radii of curvature. The curved architectures are prepared by capping nonmagnetic micrometer- and nanometer-sized rolled-up membranes with a soft-magnetic 20 nm thick permalloy (Ni(80)Fe(20)) film. A quantitative comparison between the magnetization reversal processes in caps with different diameters is given. The phase diagrams of magnetic equilibrium domain patterns (diameter versus length) are generated. For this, joint experimental, including X-ray magnetic circular dichroism photoelectron emission microscopy (XMCD-PEEM), and theoretical studies are carried out. The anisotropic magnetostatic interaction in cylindrically curved architectures originating from the thickness gradient reduces substantially the magnetostatic interaction between closely packed curved nanowires. This feature is beneficial for racetrack memory devices, since a much higher areal density might be achieved than possible with planar counterparts.

Surface acoustic wave mediated dielectrophoretic alignment of rolled-up microtubes in microfluidic systems

The alignment behavior of solution dispersed rolled-up microtubes by surface acoustic waves (SAW) is demonstrated. In contrast to the random alignment of rolled-up insulated silicon oxide tubes, metallic chromium tubes can be effectively aligned and assembled into “tube-chains” parallel to the SAW propagation direction. The experiments suggest that the tube orientation is mainly determined by the dielectrophoresis (DEP) forces acting on the tubes. The DEP forces arise from the induced dipole moment of the tubes in the SAW generated piezoelectric field on the LiNbO3 substrate.

Three-dimensional chemical sensors based on rolled-up hybrid nanomembranes

Moving towards the realization of ultra-compact diagnostic systems, we demonstrate the design, realization and characterization of rolled-up nanomembrane-based chemical sensing elements operating at room temperature. The tube-shaped devices with a final diameter of ∼15 μm rely on a fabrication process which combines top-down and bottom-up approaches and is compatible with standard processing technologies. Arrays of sensors are created in parallel on-a-chip, consequently, the integration of such elements into lab-in-a-tube devices as sensing units certainly seems feasible. The sensing properties of the devices are created by the selective incorporation of thin organic active layers in the inner wall of the microtubes. While the sensitivity towards volatile organic compounds is observed to be similar to previously reported sensors, indicating that the integration of the organic layer is efficiently achieved, the occupied footprint area of the tube-shaped devices is at least one order of magnitude smaller than its planar counterpart. This particular feature makes this procedure an attractive pathway to condense sensing elements for ultra-compact devices.

Temperature-dependent Raman investigation of rolled up InGaAs/GaAs microtubes

Large arrays of multifunctional rolled-up semiconductors can be mass-produced with precisely controlled size and composition, making them of great technological interest for micro- and nano-scale device fabrication. The microtube behavior at different temperatures is a key factor towards further engineering their functionality, as well as for characterizing strain, defects, and temperature-dependent properties of the structures. For this purpose, we probe optical phonons of GaAs/InGaAs rolled-up microtubes using Raman spectroscopy on defect-rich (faulty) and defect-free microtubes. The microtubes are fabricated by selectively etching an AlAs sacrificial layer in order to release the strained InGaAs/GaAs bilayer, all grown by molecular beam epitaxy. Pristine microtubes show homogeneity of the GaAs and InGaAs peak positions and intensities along the tube, which indicates a defect-free rolling up process, while for a cone-like microtube, a downward shift of the GaAs LO phonon peak along the cone is observed. Formation of other type of defects, including partially unfolded microtubes, can also be related to a high Raman intensity of the TO phonon in GaAs. We argue that the appearance of the TO phonon mode is a consequence of further relaxation of the selection rules due to the defects on the tubes, which makes this phonon useful for failure detection/prediction in such rolled up systems. In order to systematically characterize the temperature stability of the rolled up microtubes, Raman spectra were acquired as a function of sample temperature up to 300°C. The reversibility of the changes in the Raman spectra of the tubes within this temperature range is demonstrated.