Gaoshan Huang

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.

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.

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.

Dry‐Released Nanotubes and Nanoengines by Particle‐Assisted Rolling

Surface tension of self-assembled metal nanodroplets can be applied to overcome the deformation barriers of strain-engineered nanomembranes and produce extremely nanoscale tubes. Aggregated nanoparticles stress nanomembranes and subsequently integrate on the walls of rolled-up nanotubes, which can speed up the tubular engines owing to the enhanced electrocatalytic activity.

Material considerations and locomotive capability in catalytic tubular microengines

Driven by potential applications, such as cargo transportation (drug delivery) and biosensing, catalytic microengines have been shaped into tubular geometries with embedded catalytic and functional materials. The microengines harvest chemical energy from catalytic and biocatalytic reactions to realize autonomous locomotion at low Reynolds number, mimicking natural biomotors. The motion dynamics of these tubular microengines can be well-analyzed by a developed body-deformation model. The composition and morphology of the microengine play a key role in its overall performance and capabilities. This article highlights recent advances in the preparation of tubular microengines, related material considerations, and in their motion (speed and direction) control and functionalization towards a wide range of important practical nanoscale applications.

Optical properties of rolled-up tubular microcavities from shaped nanomembranes

Tubular optical microcavities have been fabricated by releasing prestressed SiO/SiO2 bilayer nanomembranes from polymer sacrificial layers, and their geometrical structure is well controlled by defining the shape of nanomembranes via photolithography. Optical measurements at room temperature demonstrate that resonant modes of microtubular cavities rolled up from circular shapes can be tuned in peak energy and relative intensity along the tube axes compared to those from square patterns. The resonant modes shift to higher energy with decreasing number of tube wall rotations and thickness, which fits well to finite-difference time-domain simulations. Polarization resolved measurements of the resonant modes indicate that their polarization axes are parallel to the tube axis, independent of the polarization of the excitation laser.Tubular optical microcavities have been fabricated by releasing prestressed SiO/SiO2 bilayer nanomembranes from polymer sacrificial layers, and their geometrical structure is well controlled by defining the shape of nanomembranes via photolithography. Optical measurements at room temperature demonstrate that resonant modes of microtubular cavities rolled up from circular shapes can be tuned in peak energy and relative intensity along the tube axes compared to those from square patterns. The resonant modes shift to higher energy with decreasing number of tube wall rotations and thickness, which fits well to finite-difference time-domain simulations. Polarization resolved measurements of the resonant modes indicate that their polarization axes are parallel to the tube axis, independent of the polarization of the excitation laser.

Optical resonance tuning and polarization of thin-walled tubular microcavities

We present experimental and finite-difference time-domain simulation results on the tunability of optical resonant modes of spiral microtube cavities, rolled-up from square patterned SiO/SiO(2) thin nanomembranes on glass substrates. The peak positions of resonant TM modes shift to lower energies by coating the microtube wall with Al(2)O(3) monolayers, which is well described by simulations. Moreover, a second group of tunable resonant modes appears beyond a certain critical thickness of the coated Al(2)O(3). The polarization of this group of modes is TE, as we find out by a detailed analysis of the polarization-dependent photoluminescence spectra.

Tubular oxide microcavity with high-index-contrast walls: Mie scattering theory and 3D confinement of resonant modes

Tubular oxide optical microcavities with thin walls (< 100 nm) have been fabricated by releasing pre-stressed Y2O3/ZrO2 bi-layered nanomembranes. Optical characterization demonstrates strong whispering gallery modes with a high quality-factor and fine structures in the visible range, which are due to their high-index-contrast property (high refractive index in thin walls). Moreover, the strong axial light confinement observed in rolled-up circular nanomembranes well agrees with our theoretical calculation by using Mie scattering theory. Novel material design and superior optical resonant properties in such self-rolled micro-tubular cavities promise many potential applications e.g. in optofluidic sensing and lasing.

Giant Persistent Photoconductivity in Rough Silicon Nanomembranes

This paper reports the observation of giant persistent photoconductivity from rough Si nanomembranes. When exposed to light, the current in p-type Si nanomembranes is enhanced by roughly 3 orders of magnitude in comparison with that in the dark and can persist for days at a high conductive state after the light is switched off. An applied gate voltage can tune the persistent photocurrent and accelerate the response to light. By analyzing the band structure of the devices and the surfaces through various coatings, we attribute the observed effect to hole-localized regions in Si nanomembranes due to the rough surfaces, where light can activate the confined holes.

Origin of the high p-doping in F intercalated graphene on SiC

The atomic and electronic structures of F intercalated epitaxial graphene on a SiC(0001) substrate are studied by first-principles calculations. A three-step fluorination process is proposed. First, F atoms are intercalated between the graphene and the SiC, which restores the Dirac point in the band structure. Second, saturation of the topmost Si dangling bonds introduces p-doping up to 0.37 eV. Third, F atoms bond covalently to the graphene to enhance the p-doping. Our model explains the highly p-doped state of graphene on SiC after fluorination [A. L. Walter et al., Appl. Phys. Lett. 98, 184102 (2011)].