Tubular/helical architecture construction based on rolled-up AlN nanomembranes and resonance as optical microcavity

Aluminum nitride(AlN)has attracted a great amount of interest due to the fact that these group III–V semiconductors present direct band gap behavior and are compatible with current micro-electro-mechanical systems.In this work,three dimensional(3D)AlN architectures including tubes and helices were constructed by rolling up AlN nanomembranes grown on a silicon-on-insulator wafer via magnetron sputtering.The properties of the AlN membrane were characterized through transmission electron microscopy and X-ray diffraction.The thickness of AlN nanomembranes could be tuned via the RIE thinning method,and thus micro-tubes with different diameters were fabricated.The intrinsic strain in AlN membranes was investigated via micro-Raman spectroscopy,which agrees well with theory prediction.Whispering gallery mode was observed in AlN tubular optical microcavity in photoluminescence spectrum.A postprocess involving atomic layer deposition and R6G immersion were employed on as-fabricated AlN tubes to promote the Q-factor.The AlN tubular micro-resonators could offer a novel design route for Si-based integrated light sources.In addition,the rolled-up technology paves a new way for AlN 3D structure fabrication,which is promising for AlN application in MEMS and photonics fields.

Preface to the Special Issue on Flexible Materials and Structures for Bioengineering,Sensing,and Energy Applications

Human body with curved and soft interfaces requests advanced flexible materials and structures for the interaction with organs and signal collection from targets in applications such as bioengineering and diagnostic devices.Among them,it is highly demanded to achieve creative design in flexible materials and structures with great stretchable capability for required applications.To this end,both inorganic and organic materials could be adopted and designed with assembly and self-assembly methods for flexible electronics and electrodes.Soft or flexible materials and structures inspired by nature can lead to highly conformal contacts between devices and the human body.These approaches hold great potential for applications in flexible electronics,medical imaging technology and portable disease diagnostics.Novel strategy on related sensors/actuator and energy storage/generation devices could overcome certain limitations on flexible materials engineering and thus advance the field as well.All these methods would deliver a profound impact to our future intelligent society.

ess Enhanced photothermoelectric conversion in selfrolled tellurium photodetector with geometryinduced energy localization

Photodetection has attracted significant attention for information transmission.While the implementation relies primarily on the photonic detectors,they are predominantly constrained by the intrinsic bandgap of active materials.On the other hand,photothermoelectric(PTE)detectors have garnered substantial research interest for their promising capabilities in broadband detection,owing to the self-driven photovoltages induced by the temperature differences.To get higher performances,it is crucial to localize light and heat energies for efficient conversion.However,there is limited research on the energy conversion in PTE detectors at micro/nano scale.In this study,we have achieved a twoorder-of-magnitude enhancement in photovoltage responsivity in the self-rolled tubular tellurium(Te)photodetector with PTE effect.Under illumination,the tubular device demonstrates a maximum photovoltage responsivity of 252.13 VW^(-1)and a large detectivity of 1.48×10^(11)Jones.We disclose the mechanism of the PTE conversion in the tubular structure with the assistance of theoretical simulation.In addition,the device exhibits excellent performances in wide-angle and polarization-dependent detection.This work presents an approach to remarkably improve the performance of photodetector by concentrating light and corresponding heat generated,and the proposed self-rolled devices thus hold remarkable promises for next-generation on-chip photodetection.

2D-material-integrated whispering-gallery-mode microcavity

Optical microcavities, which support whispering gallery modes, have attracted tremendous attention in both fundamental research and potential applications. The emerging of two-dimensional materials offers a feasible solution to improve the performance of traditional microcavity-based optical devices. Besides, the integration of two-dimensional materials with microcavities will benefit the research of heterogeneous materials on novel devices in photonics and optoelectronics, which is dominated by the strongly enhanced light–matter interaction.This review focuses on the research of heterogeneous two-dimensional-material whispering-gallery-mode microcavities, opening a myriad of lab-on-chip applications, such as optomechanics, quantum photonics, comb generation, and low-threshold microlasing.

Spectrum projection with a bandgap-gradient perovskite cell for colour perception

Optoelectronic devices for light or spectral signal detection are desired for use in a wide range of applications,including sensing,imaging,optical communications,and in situ characterization.However,existing photodetectors indicate only light intensities,whereas multiphotosensor spectrometers require at least a chip-level assembly and can generate redundant signals for applications that do not need detailed spectral information.Inspired by human visual and psychological light perceptions,the compression of spectral information into representative intensities and colours may simplify spectrum processing at the device level.Here,we propose a concept of spectrum projection using a bandgap-gradient semiconductor cell for intensity and colour perception.Bandgap-gradient perovskites,prepared by a halide-exchanging method via dipping in a solution,are developed as the photoactive layer of the cell.The fabricated cell produces two output signals:one shows linear responses to both photon energy and flux,while the other depends on only photon flux.Thus,by combining the two signals,the single device can project the monochromatic and broadband spectra into the total photon fluxes and average photon energies(i.e.,intensities and hues),which are in good agreement with those obtained from a commercial photodetector and spectrometer.Under changing illumination in real time,the prepared device can instantaneously provide intensity and hue results.In addition,the flexibility and chemical/bio-sensing of the device via colour comparison are demonstrated.Therefore,this work shows a human visual-like method of spectrum projection and colour perception based on a single device,providing a paradigm for high-efficiency spectrum-processing applications.

Conductive resilient graphene aerogel via magnesio- thermic reduction of graphene oxide assemblies

Graphene aerogels are desirable for energy storage and conversion, as catalysis supports, and as adsorbents for environmental remediation. To produce graphene aerogels with low density, while maintaining high electrical conductivity and strong mechanic performance, we synthesized graphene aerogels by the magnesiothermic reduction of a freeze-dried graphene oxide （GO） self-assembly and subsequent etching of the formed MgO in acid solution. The reduced graphene oxide （rGO） aerogel samples exhibited densities as low as 1.1 mg·cm^-3. The rGO aerogel was very resilient, exhibiting full recoveryeven after being compressed by strains of up to 80%; its elastic modulus （E） scaled with density （p） as E-p^2. The rGO aerogels also exhibited high conductivities （e.g., 27.7 S·m^-1 at 3.6 mg·cm^-3） and outperformed many rGO aerogels fabricated by other reduction processes. Such outstanding properties were ascribed to the microstructures inherited from the freeze-dried GO self-assembly and the magnesiothermic reduction process.

Requirement and Development of Hydrogel Micromotors towards Biomedical Applications

With controllable size,biocompatibility,porosity,injectability,responsivity,diffusion time,reaction,separation,permeation,and release of molecular species,hydrogel microparticles achieve multiple advantages over bulk hydrogels for specific biomedical procedures.Moreover,so far studies mostly concentrate on local responses of hydrogels to chemical and/or external stimuli,which significantly limit the scope of their applications.Tetherless micromotors are autonomous microdevices capable of converting local chemical energy or the energy of external fields into motive forces for self-propelled or externally powered/controlled motion.If hydrogels can be integrated with micromotors,their applicability can be significantly extended and can lead to fully controllable responsive chemomechanical biomicromachines.However,to achieve these challenging goals,biocompatibility,biodegradability,and motive mechanisms of hydrogel micromotors need to be simultaneously integrated.This review summarizes recent achievements in the field of micromotors and hydrogels and proposes next steps required for the development of hydrogel micromotors,which become increasingly important for in vivo and in vitro bioapplications.

Atomic layer deposition-induced integration of N-doped carbon particles on carbon foam for flexible supercapacitor

Flexible devices have attracted abundant attention in energy storage systems.In this paper,we presented a novel approach for fabricating flexible supercapacitor based on metal organic frameworks-derived material.In this approach,a uniform zeolitic imidazolate frameworks-8 layer with a high mass loading was deposited on a flexible carbon foam(CF)skeleton efficiently by the induction of a uniform ZnO nanomembrane prepared via an atomic layer deposition technique.A flexible N-doped carbon particle-carbon foam(N-CP-CF)composite with a hierarchically porous structure and a large specific surface area(i.e.,538 m^(2) g^(-1))was obtained in a subsequent pyrolysis process.The resultant materials have the excellent electrochemical performance(i.e.,a high specific capacitance of 300 F g^(-1) and a high energy density of 20.8 W h kg^(-1)).The N-CP-CF composite can provide a stable capacitance(i.e.,250 F g^(-1))and an energy density(i.e.,17.36 W h kg^(-1))under large deformation(25% of original thickness).This work could propose a promising strategy in fabrication of flexible electrode with a large potential towards energy storage applications in the future.

Shape Memory Alloy Helical Microrobots with Transformable Capability towards Vascular Occlusion Treatment

Practical implementation of minimally invasive biomedical applications has been a long-sought goal for microrobots.In this field,most previous studies only demonstrate microrobots with locomotion ability or performing a single task,unable to be functionalized effectively.Here,we propose a biocompatible shape memory alloy helical microrobot with regulative structure transformation,making it possible to adjust its motion behavior and mechanical properties precisely.Especially,towards vascular occlusion problem,these microrobots reveal a fundamental solution strategy in the mechanical capability using shape memory effect.Such shape-transformable microrobots can not only manipulate thrust and torque by structure to enhance the unclogging efficiency as a microdriler but also utilize the high work energy to apply the expandable helical tail as a selfpropulsive stent.The strategy takes advantage of untethered manipulation to operate microsurgery without unnecessary damage.This study opens a route to functionalize microrobots via accurate tuning in structures,motions,and mechanical properties.

Biodegradable germanium electronics for integrated biosensing of physiological signals

Transient electronics that can disappear or degrade via physical disintegration or chemical reaction over a pre-defined operational period provide essential for their applications in implantable bioelectronics due to the complete elimination of the second surgical extraction.However,the dissolution of commonly utilized bioresorbable materials often accompanies hydrogen production,which may cause potential or irreparable harm to the human body.This paper introduces germanium nanomembrane-based bioresorbable electronic sensors,where the chemical dissolution of all utilized materials in biofluidic theoretically have no gaseous products.In particular,the superior electronic transport of germanium enables the demonstrated bioresorbable electronic sensors to successfully distinguish the crosstalk of different physiological signals,such as temperature and strain,suggesting the significant prospect for the construction of dual or multi-parameter biosensors.Systematical studies reveal the gauge factor and temperature coefficient of resistance comparable to otherwise similar devices with gaseous products during their dissolution.

Strain effect on intersubband transitions in rolled-up quantum well infrared photodetectors

Pre-strained nanomembranes with four embedded quantum wells（QWs） are rolled up into threedimensional（3D） tubular QW infrared photodetectors（QWIPs）,which are based on the QW intersubband transition（ISBT）.A redshift of ～0.42 meV in photocurrent response spectra is observed and attributed to two strain contributions due to the rolling of the pre-strained nanomembranes.One is the overall strain that mainly leads to a redshift of ～0.5 meV,and the other is the strain gradient which results in a very tiny variation.The blue shift of the photocurrent response spectra with the external bias are also observed as quantum-confined Stark effect（QCSE）in the ISBT.

Hierarchically nanostructured nitrogen-doped porous carbon multilayer confining Fe particles for high performance hydrogen evolution

Precise assembly of active component with sophisticated confinement in electrocatalyst are promising to increase the active site exposure for enhanced hydrogen evolution reaction(HER).Here,PCN-333 films with mesopores are firstly assembled on titanium carbide MXene with the assistance of atomic layer deposited oxide nanomembrane.With the whereafter pyrolysis process,the composite is converted to Ndoped porous carbon multi-layer containing Fe nanoparticles.The strong confinement of Fe active particle in carbon as well as great contact between metal and carbon effectively enhance active site exposure.Furthermore,this multi-layer porous structure provides high specific surface area and plentiful mesopores for electrolyte penetration.Due to the structural advantage,the composite can be well functioned in both acid and alkaline electrolytes with excellent HER performance,e.g.,low overpotential/Tafel slope.The present work may have great potential in developing high efficiency transition-metal based electrocatalysts.

Artificial neuron synapse transistor based on silicon nanomembrane on plastic substrate

Silicon nanomembrane（SiNM）transistors gated by chitosan membrane were fabricated on plastic substrate to mimic synapse behaviors.The device has both a bottom proton gate（BG）and multiple side gates（SG）.Electrical transfer properties of BG show hysteresis curves different from those of typical SiO2 gate dielectric.Synaptic behaviors and functions by linear accumulation and release of protons have been mimicked on this device：excitatory post-synaptic current（EPSC）and paired pulse facilitation behavior of biological synapses were mimicked and the paired-pulse facilitation index could be effectively tuned by the spike interval applied on the BG.Synaptic behaviors and functions,including short-term memory and long-term memory,were also experimentally demonstrated in BG mode.Meanwhile,spiking logic operation and logic modulation were realized in SG mode.

Printable inorganic nanomaterials for flexible transparent electrodes:from synthesis to application

Printed and flexible electronics are definitely promising cutting-edge electronic technologies of the future. They offer a wide-variety of applications such as flexible circuits, flexible displays, flexible solar cells, skinlike pressure sensors, and radio frequency identification tags in our daily life. As the most-fundamental component of electronics, electrodes are made of conductive materials that play a key role in flexible and printed electronic devices. In this review, various inorganic conductive materials and strategies for obtaining highly conductive and uniform electrodes are demonstrated. Applications of printed electrodes fabricated via these strategies are also described. Nevertheless, there are a number of challenges yet to overcome to optimize the processing and performance of printed electrodes.

Recent advances in microsystem approaches for mechanical characterization of soft biological tissues

Microsystem technologies for evaluating the mechanical properties of soft biological tissues offer various capabilities relevant to medical research and clinical diagnosis of pathophysiologic conditions.Recent progress includes(1)the development of tissue-compliant designs that provide minimally invasive interfaces to soft,dynamic biological surfaces and(2)improvements in options for assessments of elastic moduli at spatial scales from cellular resolution to macroscopic areas and across depths from superficial levels to deep geometries.This review summarizes a collection of these technologies,with an emphasis on operational principles,fabrication methods,device designs,integration schemes,and measurement features.The core content begins with a discussion of platforms ranging from penetrating filamentary probes and shape-conformal sheets to stretchable arrays of ultrasonic transducers.Subsequent sections examine different techniques based on planar microelectromechanical system(MEMS)approaches for biocompatible interfaces to targets that span scales from individual cells to organs.One highlighted example includes miniature electromechanical devices that allow depth profiling of soft tissue biomechanics across a wide range of thicknesses.The clinical utility of these technologies is in monitoring changes in tissue properties and in targeting/identifying diseased tissues with distinct variations in modulus.The results suggest future opportunities in engineered systems for biomechanical sensing,spanning a broad scope of applications with relevance to many aspects of health care and biology research.

Tubular micro/nanoengines： boost motility in a tiny world

Since 20th century,the development of rocket science has put forward to the dream of humankind that one day we can reach other planets except for observing them through telescope.The rockets get rid of gravity by a powerful engine,consuming chemical fuel to generate a vast thrust.Inspired by rockets and motors in macroscale,researchers on nanoscience and nanotechnology are able to construct engines which can carry and transport cargos

Preface to the Special Issue on Flexible and Wearable Electronics： from Materials to Applications

Electronic systems that can cover large areas on flexible/stretchable substrateshave received increasing attention in the past several years because they enable new classes of applications that lie outside those easily addressed with wafer-based microelectronics.Some attractive examples include flexible displays,flexible solar cells,electronic textiles,sensory skins,detectors,active antennas,etc.The field expends very fast and great developments have been obtained

Rolling origami with smart materials

Three-dimensional (3D) functional devices have become an interesting topic meeting the challenge of device miniaturization and high integration in electron devices and microelectromechanical systems. Based on this exciting situation, origami in micro/nanoscale combining art and advanced science was widely utilized to transform two-dimensional (2D) sheets into 3D microstructures for various applications, such as micro-grippers [1](Fig. 1a).