Yongfeng Mei

Catalytic Microtubular Jet Engines Self‐Propelled by Accumulated Gas Bubbles

Strain-engineered microtubes with an inner catalytic surface serve as self-propelled microjet engines with speeds of up to approximately 2 mm s(-1) (approximately 50 body lengths per second). The motion of the microjets is caused by gas bubbles ejecting from one opening of the tube, and the velocity can be well approximated by the product of the bubble radius and the bubble ejection frequency. Trajectories of various different geometries are well visualized by long microbubble tails. If a magnetic layer is integrated into the wall of the microjet engine, we can control and localize the trajectories by applying external rotating magnetic fields. Fluid (i.e., fuel) pumping through the microtubes is revealed and directly clarifies the working principle of the catalytic microjet engines.

Dynamics of Biocatalytic Microengines Mediated by Variable Friction Control

We describe the motion of self-propelled hybrid microengines containing catalase enzyme covalently bound to the cavity of rolled-up microtubes. The high efficiency of these hybrid microengines allows them to move at a very low concentration of peroxide fuel. The dynamics of the catalytic engines is mediated by the generation of front-side bubbles, which increase the drag force and make them turn. The specific modification of the inner layer of microtubes with biomolecules can lead to other configurations to generate motion from different chemical fuels.

Naturally Rolled‐Up C/Si/C Trilayer Nanomembranes as Stable Anodes for Lithium‐Ion Batteries with Remarkable Cycling Performance

Lithium-ion batteries (LIBs) have attracted considerable interest because of their wide range of environmentally friendly applications, such as portable electronics, electric vehicles (EVs), and hybrid electric vehicles (HEVs). For the next generation of LIBs with high energy and high power density, improvements on currently used electrode materials are urgently needed. Among various anode materials, Si has been extensively studied owing to its highest theoretical capacity (4200 mAhg ), abundance in nature, low cost, and nontoxicity. However, Si-based anodes are notoriously plagued by poor capacity retention resulting from large volume changes during alloy/de-alloy processes (400%). The intrinsic strain generated during such expansion and contraction easily leads to electrode pulverization and capacity fading. Thus, it is a big challenge to achieve both excellent cyclability and enhanced capacity of Si-based anode materials. Significant efforts have been devoted to circumvent this issue caused by the volume change of silicon. Recently, a number of Si nanostructures, including nanoparticles, nanowires/nanorods, nanotubes, and porous nanostructures 22] as well as their composites, have been fabricated to achieve improved cycling performance. Among them, tubular structures, with extra interior space for electron and ion transport, as well as for accommodating volume changes, are one of the most attractive and promising configurations for LIBs. However, such anode materials are still far from commercialization, and new strategies for the synthesis of novel structures with superior cycling performance and stability are still much sought-after. Herein, we report a new tubular configuration made from naturally rolled-up C/Si/C trilayer nanomembranes, which exhibits a highly reversible capacity of approximately 2000 mAh g 1 at 50 mA g , and approximately 100 % capacity retention at 500 mA g 1 after 300 cycles. The sandwich-structured C/Si/C composites, with moderate kinetic properties toward Li ion and electron transport, are of the highest quality. The excellent cycling performance is related to the thin-film effect combined with carbon coating, which play a structural buffering role in minimizing the mechanical stress induced by the volume change of Si. The energy reduction in C/Si/C trilayer nanomembranes after rolling up into multi-winding microtubes results in a significantly reduced intrinsic strain, which can improve capacity and cycling performance. This synthetic process could be compatible with existing industrial sputtering deposition processes as well as roll-to-roll thin-film fabrication technology. The strategy for the self-release of C/Si/C trilayer nanomembranes using rolled-up nanotechnology to form multilayer C/Si/C microtubes is shown in Scheme 1. First, a sacrificial layer (red color, photoresist ARP 3510) was deposited on top of the Si substrates by spin-coating, then trilayer C/Si/C (10/40/10 nm, respectively) nanomembranes were sequentially deposited by radio frequency sputtering, during which the intrinsic strain caused by thermal expansion effects was generated. When the sacrificial layer was selectively under-

Strong blue emission from anodic alumina membranes with ordered nanopore array

We have investigated the photoluminescence (PL) from anodic alumina membranes with an ordered nanopore array formed on bulk Al foils. Most of the membranes fabricated by anodization in oxalic acid showed a strong PL peak in the blue. Due to an obvious asymmetry, the PL peak can be Gaussian divided into two bands around 405 and 455 nm, having a slight shift with the sample formed in different acid concentrations. The PL excitation (PLE) spectral examinations and analyses revealed that the two blue PL bands originate from optical transitions in two kinds of different oxygen-deficient defect centers, F (oxygen vacancy with two electrons) and F+ (oxygen vacancy with only one electron) centers. Their distributions were discussed on the basis of the observed PL and PLE behaviors. Our experimental results improve the understanding of the blue-emitting property of anodic alumina membranes.

Lab-in-a-Tube: Detection of Individual Mouse Cells for Analysis in Flexible Split-Wall Microtube Resonator Sensors

We report a method for the precise capturing of embryonic fibroblast mouse cells into rolled-up microtube resonators. The microtubes contain a nanometer-sized gap in their wall which defines a new type of optofluidic sensor, i.e., a flexible split-wall microtube resonator sensor, employed as a label-free fully integrative detection tool for individual cells. The sensor action works through peak sharpening and spectral shifts of whispering gallery modes within the microresonators under light illumination.

Self-Wound Composite Nanomembranes as Electrode Materials for Lithium Ion Batteries

www.MaterialsViews.com C O M M U Self-Wound Composite Nanomembranes as Electrode Materials for Lithium Ion Batteries N IC A T By Heng-Xing Ji , Xing-Long Wu , Li-Zhen Fan , Cornelia Krien , Irina Fiering , Yu-Guo Guo , * Yongfeng Mei , * and Oliver G. Schmidt IO N Bending and rolling is commonly employed in nature to release strain in fi lms to maintain structure stability. Recently, rolledup nanotechnology has proven to be an intriguing approach on the micro-/nanoscale for various promising future applications and concepts. [ 1–5 ] Nanomembranes composed of various functional stacks can self wind (or roll up) into micro/nanotubes upon detaching from a holding substrate by releasing intrinsic differential strain. The deposition and process methods for nanomembranes are compatible to industrial-level technologies like e-beam evaporation, sputtering deposition and atomic layer deposition, etc., which are demanded by advanced materials used for applications. Moreover, the intrinsic strain accommodated in multi-layer nanomembranes is effi ciently released by self winding and thus offers a minimization of the system energy. [ 6 ] Such tubular and strain-relaxed structures are liable to improve the materials tolerance against stress cracking and are therefore promising candidates for increasing the stability of energy storage devices such as lithium ion batteries. Lithium ion batteries are attractive for applications ranging from electric vehicles to microchips. [ 7–10 ] One of the big challenges is strain accommodation during electrode lithiation, which would prevent the electrodes in batteries from being pulverized which causes capacity fading. [ 10–12 ] For example, transition-metal oxides and lithium alloys are attractive anode materials owing to their high theoretical charge capacity, which is several times larger than existing graphite anodes. [ 13–16 ]

Combined surface plasmon and classical waveguiding through metamaterial fiber design.

A metamaterial integration for fiber optics, leading to a dual effect of surface plasmon and classical waveguiding, is presented along with experimental potentiality. We theoretically propose a metamaterial fiber in which, depending on the wavelength (from ultraviolet to infrared) and the particular metamaterial composition, one can transmit information through surface plasmon mediated or classical waveguidance. The metamaterial can be used as the core or cladding of a fiber which allows waveguidance through a subwavelength geometry.

Thinning and shaping solid films into functional and integrative nanomembranes.

Conventional solid films on certain substrates play a crucial role in various applications, for example in flat panel displays, silicon technology, and protective coatings. Recently, tremendous attention has been directed toward the thinning and shaping of solids into so-called nanomembranes, offering a unique and fantastic platform for research in nanoscience and nanotechnology. In this Review, a conceptual description of nanomembranes is introduced and a series of examples demonstrate their great potential for future applications. The thinning of nanomembranes indeed offers another strategy to fabricate nanomaterials, which can be integrated onto a chip and exhibit valuable properties (e.g. giant persistent photoconductivity and thermoelectric property). Furthermore, the stretching of nanomembranes enables a macroscale route for tuning the physical properties of the membranes at the nanoscale. The process by which nanomembranes release from a substrate presents several approaches to shaping nanomembranes into three-dimensional architectures, such as rolled-up tubes, wrinkles, and the resulting channels, which can provide fascinating applications in electronics, mechanics, fluidics, and photonics. Nanomembranes as a new type of nanomaterial promise to be an attractive direction for nanoresearch.

Sandwich-Stacked SnO2/Cu Hybrid Nanosheets as Multichannel Anodes for Lithium Ion Batteries

We have introduced a facile strategy to fabricate sandwich-stacked SnO2/Cu hybrid nanosheets as multichannel anodes for lithium-ion batteries applying rolled-up nanotechnology with the use of carbon black as intersheet spacer. By employing a direct self-rolling and compressing approach, a much higher effective volume efficiency is achieved as compared to rolled-up hollow tubes. Benefiting from the nanogaps formed between each neighboring sheet, electron transport and ion diffusion are facilitated and SnO2/Cu nanosheet overlapping is prevented. As a result, the sandwich-stacked SnO2/Cu hybrid nanosheets exhibit a high reversible capacity of 764 mAh g(-1) at 100 mA g(-1) and a stable cycling performance of ~75% capacity retention at 200 mA g(-1) after 150 cycles, as well as a superior rate capability of ~470 mAh g(-1) at 1 A g(-1). This synthesis approach presents a promising route to design multichannel anodes for high performance Li-ion batteries.

Rolled-up optical microcavities with subwavelength wall thicknesses for enhanced liquid sensing applications.

Microtubular optical microcavities from rolled-up ring resonators with subwavelength wall thicknesses have been fabricated by releasing prestressed SiO/SiO(2) bilayer nanomembranes from photoresist sacrificial layers. Whispering gallery modes are observed in the photoluminescence spectra from the rolled-up nanomembranes, and their spectral peak positions shift significantly when measurements are carried out in different surrounding liquids, thus indicating excellent sensing functionality of these optofluidic microcavities. Analytical calculations as well as finite-difference time-domain simulations are performed to investigate the light confinement in the optical microcavities numerically and to describe the experimental mode shifts very well. A maximum sensitivity of 425 nm/refractive index unit is achieved for the microtube ring resonators, which is caused by the pronounced propagation of the evanescent field in the surrounding media due to the subwavelength wall thickness design of the microcavity. Our optofluidic sensors show high potential for lab-on-a-chip applications, such as real-time bioanalytic systems.