Guest Editorial—TTA Special Section on Terahertz Functional Devices

In the past decades, terahertz technology has a great development and steady improvement, which has kept discovering and developing a series of potential applications in terahertz sensing, imaging, spectroscopy, security, and communication. After the recent technical breakthroughs in reliable sources and sensitive detectors, terahertz functional devices, such as waveguides, switches, filters, splitters, isolators, modulators and sensors, are indispensable for the construction of compact application systems and have become a worldwide supreme issue in research.

Functional microfluidics:theory,microfabrication,and applications

Microfluidic devices are composed of microchannels with a diameter ranging from ten to a few hundred micrometers.Thus,quite a small(10-9–10-18l)amount of liquid can be manipulated by such a precise system.In the past three decades,significant progress in materials science,microfabrication,and various applications has boosted the development of promising functional microfluidic devices.In this review,the recent progress on novel microfluidic devices with various functions and applications is presented.First,the theory and numerical methods for studying the performance of microfluidic devices are briefly introduced.Then,materials and fabrication methods of functional microfluidic devices are summarized.Next,the recent significant advances in applications of microfluidic devices are highlighted,including heat sinks,clean water production,chemical reactions,sensors,biomedicine,capillaric circuits,wearable electronic devices,and microrobotics.Finally,perspectives on the challenges and future developments of functional microfluidic devices are presented.This review aims to inspire researchers from various fields engineering,materials,chemistry,mathematics,physics,and more—to collaborate and drive forward the development and applications of functional microfluidic devices,specifically for achieving carbon neutrality.

Assemblies and composites of gold nanostructures for functional devices

Gold has been one of the most vastly used noble metals due to its unique properties.In modern manufacturing,gold is extensively used in electronics industry as electrical connectors due to the high conductivity and corrosion resistance.With advancements in gold chemistry and nanofabrication technologies,gold materials can be tailored down to the dimension of nanoscale,which enables various novel properties.However,it is still a huge challenge to realize practical functional devices by rational utilizing these gold nanostructures-based materials.Herein,the recent developments in the design and fabrication of various functional devices based on assemblies and composites of gold nanostructures are summarized.Starting from the plasmonic effect,optical colorimetric sensors,optoelectronics and surface-enhanced Raman spectroscopy sensors are introduced.Followed by conductive devices with novel properties,flexible transparent conductors,stretchable electronics,wearable and implantable devices are discussed.Both bottom-up and top-down approaches to prepare assemblies and composites are covered.In addition,the challenges and future developments in the field are also addressed.It is believed that further developments in gold nanostructures-based materials will greatly contribute to the nextgeneration biosensors,optoelectronics,wearable and implantable electronics.

Current advances and future perspectives of additive manufacturing for functional polymeric materials and devices

Three-dimensional(3D)printing has received extensive attention due to its unique multidimensional functionality and customizability and has been recognized as one of the most revolutionary manufacturing technologies.Functional 3D printed products represent an important orientation for next-generationmanufacturing and attract a great spotlight for the application in sensors,actuators,robots,electronics,and medical devices.However,the lack of functions of printing polymeric materials dramatically limits the development of functional 3D printing.Different from traditional processing,the physical properties,such as geometry and rheological behavior,of the polymericmaterialsmust match the printing process,making the selection of printable materials limited.More importantly,challenges in large-scale production of such materials further stifle the development of functional 3D printing industry.In this review,we aim to outline recent advances in polymeric materials and methodologies for the functional 3D printing technology.The reports are classified based on functionalities,including electronic conductive,thermally conductive,electromagnetic interference shielding,energy storage,and energy harvesting materials.This study attempts to provide a comprehensive overview of the challenges and opportunities for 3D printing functional polymeric materials/devices,also seeks to enlighten the orientation of future research in this field.

New functions of VCSEL-based optical devices Invited Paper

We have seen a lot of unique features off vertical cavity surface emitting lasers （VCSELs）, such as low power consumption, wafer-level testing, small packaging capability, and so on. The market of VCSELs has been growing up rapidly in recent years and they are now the key devices in local area networks using multi-mode optical fibers. In addition, new functions on VCSELs have been demonstrated. In this paper, the recent advances of VCSEL photonics will be reviewed which include the wavelength engineering and the athermal operation based on microelectro mechanical system （MEMS） technologies. Also, this paper explores the potential and challenges for new functions of VCSELs, including high-speed control of optical phase, slow light devices, plasmonic VCSELs, and so on.

Molecular-scale electronics： From device fabrication to functionality

By wiring molecules into circuits, ＂molecular electronics＂ aims at studying electronic properties of single molecules and their ensembles, on this basis exploiting their intrinsic functionalities, and eventually applying them as building blocks of electronic components for future electronic devices. Herein, fabricating reliable solid-state molecular devices and developing synthetic molecules endowed with desirable electronic properties, have been two major tasks since the dawn of molecular electronics. This review focuses on recent advances and efforts regarding the main challenges in this field, highlighting fabrication of nanogap electrodes for single-molecule junctions, and self-assembled-monolayers （SAMs） for functional devices. The prospect of molecular-scale electronics is also discussed.

ecent progress in the theoretical design of two-dimensional ferroelectric

Two-dimensional(2D)ferroelectrics(FEs),which maintain stable electric polarization in ultrathin films,are a promising class of materials for the development of various miniature functional devices.In recent years,several 2D FEs with unique properties have been successfully fabricated through experiments.They have been found to exhibit some unique properties either by themselves or when they are coupled with other functional materials(e.g.,ferromagnetic materials,materials with 5 d electrons,etc.).As a result,several new types of 2D FE functional devices have been developed,exhibiting excellent performance.As a type of newly discovered 2D functional material,the number of 2D FEs and the exploration of their properties are still limited and this calls for further theoretical predictions.This review summarizes recent progress in the theoretical predictions of 2D FE materials and provides strategies for the rational design of 2D FE materials.The aim of this review is to provide guidelines for the design of 2D FE materials and related functional devices.

Single-chain and monolayered conjugated polymers for molecular electronics

Exploring the charge transport properties and electronic functions of molecules is of primary interest in the area of molecular electronics.Conjugated polymers（CPs） represent an attractive class of molecular candidates,benefiting from their outstanding optoelectronic properties.However,they have been less studied compared with the small-molecule family,mainly due to the difficulties in incorporating CPs into molecular junctions.In this review,we present a summary on how to fabricate CP-based singlechain and monolayered junctions,then discuss the transport behaviors of CPs in different junction architectures and finally introduce the potential applications of CPs in molecular-scale electronic devices.Although the research on CP-based molecular electronics is still at the initial stage,it is widely accepted that（1） CP chains are able to mediate long-range charge transport if their molecular electronic structures are properly designed,which makes them potential molecular wires,and（2） the intrinsic optoelectronic properties of CPs and the possibility of incorporating desirable functionalities by synthetic strategies imply the potential of employing tailor-made polymeric components as alternatives to small molecules for future molecular-scale electronics.

Advanced unconventional techniques for sub-100 nm nanopatterning

Patterned nanostructures with ultrasmall features endow functional devices with unique nanoconfinement and performance enhancements.The increasing demand for miniaturization has stimulated the development of sub-100 nm nanopatterning techniques.Beyond conventional lithography—which is limited by unavoidable factors—advanced patterning techniques have been reported to produce nanoscale features down to molecular or even atomic scale.In this review,unconventional techniques for sub-100 nm nanopatterning are discussed,in particular the principles by which to achieve the desired patterns(among other important issues).Such techniques can be classified into three categories:template-replica,template-induced,and template-free techniques.Moreover,multi-dimensional nanostructures consist of various building materials,the unique properties of which are summarized.Finally,the remaining challenges and opportunities for large-scale patterning,the improvement of device perfor-mance,the multi-dimensional nanostructures of biocompatible materials,molecular-scale patterning,and the carbon footprint requirements for future nanofabrication processes are discussed.

Anisotropy of two-dimensional ReS_(2)and advances in its device application

Two-dimensional(2D)transition metal dichalcogenides(TMDCs)have attracted growing interest regarding their potential applications in next-generation electronic and optoelectronic devices.Owing to their atomic thickness and tunable bandgap,they exhibit unique mechanical,electrical,and optical properties.As a specific member of the TMDC family,rhenium disulfide(ReS_(2))has stimulated intensive interest due to its anisotropic crystal structure,weak inter-layer coupling,and anisotropic electrical and optical properties.In this review,we summarize the distinct crystal structure and intrinsic anisotropic properties of ReS_(2),followed by an introduction to its synthesis methods.The current applications of ReS_(2)and its heterojunctions are presented based on its anisotropic properties.This review not only provides a timely summary of the current applications of ReS_(2)and its heterojunctions,but also inspires new approaches to develop other innovative devices based on 2D materials with a low lattice symmetry.

Colorful liquid metal printed electronics

Liquid metal based printed electronics was a newly emerging frontier in recent years. However, restricted by the single silver-white appearance of the liquid metal(LM), the colors of currently available printed electronics were rather limited. Here,a new conceptual LM based colorful printed electronics was proposed where electrical wires and circuits with numerous colors can be made via a straightforward, efficient and accurate printing procedure. Firstly, the LM was printed on the substrate to construct a conductive wire. Then it was frozen to a solid. Subsequently, colorful pigments were coated on the originally printed liquid metal conductive wires, which finally were packaged with PDMS. Such multicolored conductive wire exhibits excellent conductivity, and good temperature resistance(do not fade at high temperature). Further, the adhesion mechanism of the mineral pigments on the liquid metal layer was disclosed. And the pigment layer was discovered to well protect the LM from the outside environments, and enhance the durability of the LM conductive wire at the same time. These multicolored liquid metal wires take an aesthetic appearance, excellent printability, flexibility, large conductivity and stable performance, which would significantly enhance the sense of beauty and experience when compared to the conventional printed electronics.

UV-Vis-NIR Light-deformable Shape-memory Polyurethane Doped with Liquid-crystal Mixture and GO towards Biomimetic Applications

Nature has been inspiring material researchers to fabricate biomimetic functional devices for various applications, and shape-memory polymer materials(SMPMs) have received tremendous attention since the promising intelligent materials possess more advantages over others for the fabrication of biomimetic functional devices. As is well-known, SMPMs can be stimulated by heat, electricity, magnetism, pH, solvent and light. From the viewpoint of practical applications, ultraviolet(UV)-visible(Vis)-near infrared(NIR) light-responsive SMPMs are undoubtedly more advantageous. However, up to now, UV-Vis-NIR light-deformable SMPMs by combining photothermal and photochemical effects are still rarely reported. Here we designed a UV-Vis-NIR light-deformable SMP composite film via incorporating a liquid crystal(LC) mixture and graphene oxide(GO) into a shape-memory polyurethane matrix. The elongated composite films exhibited interesting photomechanical bending deformations with different light-triggered mechanisms,(1) photochemically induced LC phase transition upon UV exposure,(2) photochemically and photothermally induced LC phase transition upon visible-light irradiation,(3) photothermally triggered LC phase transition and partial stress relaxation upon low-intensity NIR exposure. All the deformed objects could recover to their original shapes by high-intensity NIR irradiation.Moreover, the biomimetic circadian rhythms of acacia leaves and the biomimetic bending/spreading of fingers were successfully achieved, which could blaze a way in the field of biomimetic functional devices due to the excellent light-deformable and shape-memory properties of the SMP composite films.

Enabling novel device functions with black phosphorus/MoS2 van der Waals heterostructures

Van der Waals heterostructure,which consists of various twodimensional(2D)layered materials stacked along the direction perpendicular to their 2D plane,has emerged as a promising material system for device applications in recent years[1].Due to the weak van der Waals interlayer force,2D layered materials with very different lattice constants can be stacked without being constrained by the traditional‘‘lattice-match"condition to form

Design,fabrication and characterization of dual-channel real space transfer transistor

In this paper,using aδ-doping dual-channel structure and GaAs substrate,a real space transfer transistor(RSTT)is designed and fabricated successfully.It has the standardΛ-shaped negative resistance I-V characteristics as well as a level and smooth valley region that the conventional RSTT has.The negative resistance parameters can be varied by changing gate voltage(VGS).For example,the PVCR varies from 2.1 to 10.6 while VGS changes from 0.6 V to 1.0 V.The transconductance for IP(ΔIP=ΔVGS)is 0.3 mS.The parameters of VP,VV and threshold gate voltage(VT)for negative resistance characteristics arising are all smaller than the value reported in the literature.Therefore,this device is suitable for low dissipation power application.