Carlos Cesar Bof Bufon

Light-Controlled Propulsion of Catalytic Microengines†

Control over the autonomous motion of artificial nano/ micromachines is essential for real biomedical and nanotechnological applications. Consequently, a complete nanomachine should be able to be turned on and off at will. Developments over the last few years on synthetic catalytic nano/microengines and motors have enabled the harvesting of chemical energy from local molecules and transforming it into an effective autonomous motion. Several impressive applications have recently reported the use of artificial micromachines for the detection of biomolecules with roving nanomotors, transport of animal cells in a fluid, and other microcargo delivery. Recently, the use of a light source has been implemented to propel microparticle-based motors generated by a selfdiffusiophoretic mechanism. Despite this interesting approach, the motion of the particles is limited by the dissolution of the materials and to the ultraviolet (UV) spectrum. Moreover, a reversible method to start and stop the propulsion of micromotors by a visible-light source remains a challenge. Here we report the tuning of the propulsion power of Ti/ Cr/Pt catalytic microengines (m-engines) through illumination of a solution by a white-light source. We show that light suppresses the generation of microbubbles, stopping the engines if they are fixed-to or self-propelled above a platinum-patterned surface. The m-engines are reactivated by dimming the light source that illuminates the fuel solution. The illumination of the solution with visible light in the presence of Pt diminishes the concentration of hydrogen peroxide fuel and degrades the surfactant, consequently reducing the motility of the microjets. Electrochemical measurements and analysis of the surface tension support our findings. We also study the influence of different wavelengths over the visible spectrum (500–750 nm) on the formation of microbubbles. Rolled-up Ti/Cr/Pt catalytic m-engines with diameters of 5–10 mm and a length of 50 mm were prepared as described previously elsewhere and in the Experimental Section. Microengines were immersed into solutions of aqueous H2O2 (2.5% v/v) as fuel and benzalkonium chloride (ADBAC) (0.5% v/v), as the surfactant, to determine the influence of white light on the mobility of the m-engines. At lower concentrations of both chemicals, the generation of microbubbles is significantly reduced. Thus, the motility of the catalytic m-engines is controlled by a small change in the fuel (H2O2 and/or surfactant) concentration. These conditions allow us to investigate a concentration range close to the metastable state, that is, where the probability of stopping the m-engines is high. Figure 1A

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.

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.

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.

Three-Dimensional Organic Conductive Networks Embedded in Paper for Flexible and Foldable Devices

The fabrication of three-dimensional (3D) polypyrrole conductive tracks through the porous structure of paper is demonstrated by the first time. We combined paper microfluidics and gas-phase pyrrole monomers to chemically synthesize polypyrrole-conducting channels embedded in-between the cellulose fibers. By using this method, foldable conductive structures can be created across the whole paper structure, allowing the electrical connection between both sides of the substrate. As a proof of concept, top-channel-top (TCT) and top-channel-bottom (TCB) conductive interconnections as well as all-organic paper-based touch buttons are demonstrated.

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.

Direct Drawing Method of Graphite onto Paper for High-Performance Flexible Electrochemical Sensors

A simple and fast fabrication method to create high-performance pencil-drawn electrochemical sensors is reported for the first time. The sluggish electron transfer observed on bare pencil-drawn surfaces was enhanced using two electrochemical steps: first oxidizing the surface and then reducing it in a subsequent step. The heterogeneous rate constant was found to be 5.1 × 10-3 cm s-1, which is the highest value reported so far for pencil-drawn surfaces. We mapped the origin of such performance by atomic force microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Our results suggest that the oxidation process leads to chemical and structural transformations on the electrode surface. As a proof-of-concept, we modified the pencil-drawn surface with Meldolas blue to electrocatalytically detect nicotinamide adenine dinucleotide (NADH). The electrochemical device exhibited the highest catalytic constant (1.7 × 105 L mol-1 s-1) and the lowest detection potential for NADH reported so far in paper-based electrodes.

Tuning resistive switching on single-pulse doped multilayer memristors

Short-period multilayers containing ultrathin atomic layers of Al embedded in titanium dioxide (TiO(2)) film-here called single-pulse doped multilayers-are fabricated by atomic layer deposition (ALD) growth methods. The approach explored here is to use Al atoms through single-pulsed deposition to locally modify the chemical environment of TiO(2) films, establishing a chemical control over the resistive switching properties of metal/oxide/metal devices. We show that this simple methodology can be employed to produce well-defined and controlled electrical characteristics on oxide thin films without compound segregation. The increase in volume of the embedded Al(2)O(3) plays a crucial role in tuning the conductance of devices, as well as the switching bias. The stacking of these oxide compounds and their use in electrical devices is investigated with respect to possible crystalline phases and local compound formation via chemical recombination. It is shown that our method can be used to produce compounds that cannot be synthesized a priori by direct ALD growth procedures but are of interest due to specific properties such as thermal or chemical stability, electrical resistivity or electric field polarization possibilities. The monolayer doping discussed here impacts considerably on the broadening of the spectrum of performance and technological applications of ALD-based memristors, allowing for additional degrees of freedom in the engineering of oxide devices.

Flexible and Foldable Fully-Printed Carbon Black Conductive Nanostructures on Paper for High-Performance Electronic, Electrochemical, and Wearable Devices

In this work, we demonstrate the first example of fully printed carbon nanomaterials on paper with unique features, aiming the fabrication of functional electronic and electrochemical devices. Bare and modified inks were prepared by combining carbon black and cellulose acetate to achieve high-performance conductive tracks with low sheet resistance. The carbon black tracks withstand extremely high folding cycles (>20 000 cycles), a new record-high with a response loss of less than 10%. The conductive tracks can also be used as 3D paper-based electrochemical cells with high heterogeneous rate constants, a feature that opens a myriad of electrochemical applications. As a relevant demonstrator, the conductive ink modified with Prussian-blue was electrochemically characterized proving to be very promising toward the detection of hydrogen peroxide at very low potentials. Moreover, carbon black circuits can be fully crumpled with negligible change in their electrical response. Fully printed motion and wearable sensors are additional examples where bioinspired microcracks are created on the conductive track. The wearable devices are capable of efficiently monitoring extremely low bending angles including human motions, fingers, and forearm. Here, to the best of our knowledge, the mechanical, electronic, and electrochemical performance of the proposed devices surpasses the most recent advances in paper-based devices.