Metasurfaces, planer artificial materials composed of subwavelength unit cells, have shown superior abilities to manipulate the wavefronts of electromagnetic waves. In the last few years, ...metasurfaces have been a burgeoning field of research, with a large variety of functional devices, including planar lenses, beam deflectors, polarization converters, and metaholograms, being demonstrated. Up to date, the majority of metasurfaces cannot be tuned postfabrication. Yet, the dynamic control of optical properties of metasurfaces is highly desirable for a plethora of applications including free space optical communications, holographic displays, and depth sensing. Recently, much effort has been made to exploit active materials, whose optical properties can be controlled under external stimuli, for the dynamic control of metasurfaces. The tunability enabled by active materials can be attributed to various mechanisms, including but not limited to thermo‐optic effects, free‐carrier effects, and phase transitions. This short review summarizes the recent progress on tunable metasurfaces based on various approaches and analyzes their respective advantages and challenges to be confronted with. A number of potential future directions are also discussed at the end.
Various mechanisms to achieve dynamic control of metasurfaces enabled by active materials are discussed. Recent works on tunable metasurfaces based on thermo‐optic effects, free carrier effects, and phase transitions are summarized. Their respective advantages and challenges are analyzed and an outlook on the potential development of tunable metasurfaces is presented.
Actuators that can convert environmental stimuli into mechanical work are widely used in intelligent systems, robots, and micromechanics. To produce robust and sensitive actuators of different ...scales, efforts are devoted to developing effective actuating schemes and functional materials for actuator design. Carbon‐based nanomaterials have emerged as preferred candidates for different actuating systems because of their low cost, ease of processing, mechanical strength, and excellent physical/chemical properties. Especially, due to their excellent photothermal activity, which includes both optical absorption and thermal conductivities, carbon‐based materials have shown great potential for use in photothermal actuators. Herein, the recent advances in photothermal actuators based on various carbon allotropes, including graphite, carbon nanotubes, amorphous carbon, graphene and its derivatives, are reviewed. Different photothermal actuating schemes, including photothermal effect–induced expansion, desorption, phase change, surface tension gradient creation, and actuation under magnetic levitation, are summarized, and the light‐to‐heat and heat‐to‐work conversion mechanisms are discussed. Carbon‐based photothermal actuators that feature high light‐to‐work conversion efficiency, mechanical robustness, and noncontact manipulation hold great promise for future autonomous systems.
This review highlights the recent advances in carbon‐based photothermal actuators. Physical properties and light‐to‐heat conversion mechanisms of various carbon‐based functional materials are summarized. Photothermal actuating schemes such as photothermal expansion, desorption, phase change, surface tension effect, and magnetic susceptibility are reviewed. The current challenges and future perspectives of this field are also discussed.
Compared with thin‐film morphology, 1D perovskite structures such as micro/nanowires with fewer grain boundaries and lower defect density are very suitable for high‐performance photodetectors with ...higher stability. Although the stability of perovskite microwire‐based photodetectors has been substantially enhanced in comparison with that of photodetectors based on thin‐film morphology, practical applications require further improvements to the stability before implementation. In this study, a template‐assisted method is developed to prepare methylammonium lead bromide (MAPbBr3) micro/nanowire structures, which are encapsulated in situ by a protective hydrophobic molecular layer. The combination of the protective layer, high crystalline quality, and highly ordered microstructures significantly improve the stability of the MAPbBr3 single‐crystal microwire arrays. Consequently, these MAPbBr3 single‐crystal microwire‐array‐based photodetectors exhibit significant long‐term stability, maintaining 96% of the initial photocurrent after 1 year without further encapsulation. The lifetime of such photodetectors is hence approximately four times longer than that of the most stable previously reported perovskite micro/nanowire‐based photodetector; this is thought to be the most stable perovskite photodetector reported thus far. Furthermore, this work should contribute further toward the realization of perovskite 1D structures with long‐term stability.
MAPbBr3 single‐crystal microwire arrays with designable shape, controllable size, and high crystalline quality are fabricated. The combination of the hydrophobic molecular protective layer, high crystalline quality, and highly ordered microstructures, improve the intrinsic stability of MAPbBr3 single‐crystal microwire arrays. Thus, a flexible photodetector with long‐term performance stability of more than 1 year is achieved.
Three-dimensional (3D) laser nanoprinting allows maskless manufacturing of diverse nanostructures with nanoscale resolution. However, 3D manufacturing of inorganic nanostructures typically requires ...nanomaterial-polymer composites and is limited by a photopolymerization mechanism, resulting in a reduction of material purity and degradation of intrinsic properties. We developed a polymerization-independent, laser direct writing technique called photoexcitation-induced chemical bonding. Without any additives, the holes excited inside semiconductor quantum dots are transferred to the nanocrystal surface and improve their chemical reactivity, leading to interparticle chemical bonding. As a proof of concept, we printed arbitrary 3D quantum dot architectures at a resolution beyond the diffraction limit. Our strategy will enable the manufacturing of free-form quantum dot optoelectronic devices such as light-emitting devices or photodetectors.
Photoprinting nanoparticles
Nanoparticle assembly often requires tailored selection of the ligands so that they can selectively bond, as with complementary DNA strands. Alternately, they can be linked together at specified locations using photopolymerization to connect ligands at desired places. However, this process adds to the complexity of making the nanoparticles and is limited by the fidelity of the ligand attachment. Liu
et al
. show that light can be used to desorb surface thiolate ligands from cadmium selenide/zinc sulfide core shell quantum dots (see the Perspective by Pan and Talapin). The resulting trapped holes drive bonding between the particles through the remaining surface ligands. The authors reveal photoprinting of arbitrary three-dimensional architectures at a resolution beyond the diffraction limit and for a range of nanocrystals. Printing can be optically selected based on the size and/or bandgap of the quantum dots. —MSL
Photoexcitation-induced chemical bonding enables high-resolution three-dimensional printing of semiconductor quantum dots.
Reported here is a bioinspired fabrication of superhydrophobic graphene surfaces by means of two‐beam laser interference (TBLI) treatment of graphene oxide (GO) films. Microscale grating‐like ...structures with tunable periods and additional nanoscale roughness are readily created on graphene films due to laser induced ablation effect. Synchronously, abundant hydrophilic oxygen‐containing groups (OCGs) on GO sheets can be drastically removed after TBLI treatment, which lower its surface energy significantly. The synergistic effect of micro‐nanostructuring and the OCGs removal endows the resultant graphene films with unique superhydrophobicity. Additionally, dual TBLI treatment with 90° rotation is implemented to fabricate superhydrophobic graphene films with two‐dimensional grating‐like structures that can effectively avoid the anisotropic hydrophobicity originated from the grooved structures. Moreover, the superhydrophobic graphene films become conductive due to the laser reduction effect. Unique optical characteristics including transmission diffraction and brilliant structural color are also observed due to the presence of periodic microstructures. As a mask‐free, chemical‐free, and cost‐effective method, the TBLI processing of GO may open up a new way to biomimetic graphene surfaces, and thus hold great promise for the development of novel graphene‐based microdevices.
A bioinspired fabrication of superhydrophobic graphene surfaces by means of laser holographic treatment of graphene oxide (GO) films is presented. Microscale grating‐like structures with nanoscale roughness are created on graphene films, and hydrophilic oxygen groups on GO sheets are drastically removed. The synergistic effect endows the resultant graphene films with unique superhydrophobicity and unique optical properties that mimic butterfly wings.
The optical manipulation of tiny objects is significant to understand and to explore the unknown in the microworld, which has found many applications in materials science and life science. Physically ...speaking, these technologies arise from direct or indirect optomechanical coupling to convert incident optical energy to mechanical energy of target objects, while their efficiency and functionalities are determined by the coupling behavior. Traditional optical tweezers stem from direct light-to-matter momentum transfer, and the generation of an optical gradient force requires high optical power and rigorous optics. As a comparison, the opto-thermophoretic manipulation techniques proposed recently originate from high-efficiency opto-thermomechanical coupling and feature low optical power. Through rational design of the light-generated temperature gradient and exploring the mechanical response of diverse targets to the temperature gradient, a variety of opto-thermophoretic techniques were developed, which exhibit broad applicability to a wide range of target objects from colloid materials to biological cells to biomolecules. In this review, we will discuss the underlying mechanism of thermophoresis in different liquid environments, the cutting-edge technological innovation, and their applications in colloidal science and life science. We also provide a brief outlook on the existing challenges and anticipate their future development.
Direct light‐to‐work conversion enables remote actuation through a non‐contact manner, among which the photothermal Marangoni effect is significant for developing light‐driven robots because of the ...diversity of applicable photothermal materials and light sources, as well as the high energy conversion efficiency. However, the lack of nanotechnologies that enable flexible integration of advanced photothermal materials with actuators of complex configurations significantly restricts their practical applications. In this paper, laser‐induced graphene (LIG) tape is reported as stick‐on photothermal labels for developing light‐driven actuators based on the Marangoni effect. With the help of direct laser writing technology, graphene patterns with superior photothermal properties are prepared on the PI tape. The patterned LIG tape can be stuck on any desired objects and generates an asymmetric photothermal field under light irradiation, forming a photothermal Marangoni actuator. Additionally, the PI tape with LIG patterns can be folded into 3D origami actuators that permit photothermal Marangoni actuation including both translation and rotation. The graphene‐based photothermal Marangoni actuators feature biocompatibility, which is confirmed by MDA‐MB‐231 cells proliferation experiments. Owing to the excellent photothermal property of LIG patterns, the as‐produced photothermal actuators can be manipulated by a variety of light sources, holding great promise for developing light‐driven soft robots.
Laser‐induced graphene (LIG) tape as stick‐on photothermal labels for developing light‐driven actuators based on Marangoni effect is reported. The LIG embedded PI tape can be either stuck on any desired objects or folded into 3D origami, forming a photothermal Marangoni actuator. The LIG tapes hold great promise for the facile development of light‐driven soft robots.
Natural musculoskeletal systems have been widely recognized as an advanced robotic model for designing robust yet flexible microbots. However, the development of artificial musculoskeletal systems at ...micro-nanoscale currently remains a big challenge, since it requires precise assembly of two or more materials of distinct properties into complex 3D micro/nanostructures. In this study, we report femtosecond laser programmed artificial musculoskeletal systems for prototyping 3D microbots, using relatively stiff SU-8 as the skeleton and pH-responsive protein (bovine serum albumin, BSA) as the smart muscle. To realize the programmable integration of the two materials into a 3D configuration, a successive on-chip two-photon polymerization (TPP) strategy that enables structuring two photosensitive materials sequentially within a predesigned configuration was proposed. As a proof-of-concept, we demonstrate a pH-responsive spider microbot and a 3D smart micro-gripper that enables controllable grabbing and releasing. Our strategy provides a universal protocol for directly printing 3D microbots composed of multiple materials.
Recent years have seen a considerable growth of research interests in developing novel technologies that permit designable manufacture and controllable manipulation of actuators. Among various ...fabrication and driving strategies, light has emerged as an enabler to reach this end, contributing to the development of actuators. Several accessible light‐mediated manufacturing technologies, such as ultraviolet (UV) lithography and direct laser writing (DLW), are summarized. A series of light‐driven strategies including optical trapping, photochemical actuation, and photothermal actuation for controllable manipulation of actuators is introduced. Current challenges and future perspectives of this field are discussed. To generalize, light holds great promise for the development of actuators.
Recent advances in light‐mediated manufacture and manipulation of actuators are highlighted. Several optical fabrication technologies, including UV lithography and direct laser writing, and various photo‐driven strategies, such as optical trapping, and photochemical and photothermal actuation are reviewed. Emerging trends and future perspectives of light‐enabled actuators are discussed.
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