Inspired by natural autonomous systems that demonstrate controllable shape, appearance, and actuation under external stimuli, a facile preparation of moisture responsive graphene-based smart actuators by unilateral UV irradiation of graphene oxide (GO) papers is reported. UV irradiation of GO is found to be an effective protocol to trigger the reduction of GO; however, due to the limited light transmittance and thermal relaxation, thick GO paper cannot be fully reduced. Consequently, by tuning the photoreduction gradient, anisotropic GO/reduced GO (RGO) bilayer structure can be easily prepared toward actuation application. To get better control over the responsive properties, GO/RGO bilayer paper with a certain curvature and RGO patterns are successfully prepared for actuator design. As representative examples, smart humidity-driven graphene actuators that mimic the cilia of respiratory tract and tendril climber plant are successfully developed for controllable objects transport.

Surface-enhanced Raman spectroscopy (SERS) has been combined with microfluidic Lab-on-a-Chip (LoC) systems for sensitive optofluidic detection for more than a decade. However, most microfluidic SERS devices still suffer from analyte contamination and signal irreproducibility. In recent years, both the microfluidics and SERS communities have developed their own solutions that are complementary to each other; their combination even has potential for commercialization. In this review, the recent advances in both fields are summarized with regard to the development of reliable multifunctional SERS-enabled LoC systems and their broad applications. Starting from SERS fundamentals, reproducible SERS substrates and dynamic microfluidic trapping are discussed. Based on their combination, on-chip applications beyond SERS are presented, and insight can be gained into the commercialization of portable SERS chips.

Recently great progress has been achieved in highly effective hybrid solar cells fabricated using aqueous materials. The state-of-the-art energy conversion efficiency has been close to 5% with high photocurrent. However, charge separation and transport mechanism in the aqueous-processed hybrid solar cells are rarely reported and are usually assumed to be similar to oil-phase processed systems; that is, self-assembly polymers are mainly responsible for charge separation and carrier transport. To date, this assumption has prohibited further improvement of the conversion efficiency in aqueous-processed hybrid systems by adopting any appropriate technique routes. Here, ultrafast carrier dynamics in these hybrid solar cells consisting of poly(p-phenylenevinylene) (PPV)-based aqueous polymers and water-solution CdTe nanocrystals (NCs) are investigated in detail. Self-charge separation in grown CdTe NC partly capped CdS shell layers after anneal treatment is unambiguously identified. Different from their oil-soluble counterparts, these core/shell nanocrystals do not have the restrictions of quantum confinement and surface ligands, form effective charge transport networks, and play a dominant role in the charge separation and carrier transport processes. These findings provide a greater understanding on the fundamental photophysics in aqueous-processed hybrid systems.

Organic single crystals have attracted great attention because of their advantages of high charge-carrier mobility, high chemical purity, and potential for flexible optoelectronic devices. However, their intrinsic properties of sensitive to organic solvent and fragile result in a difficulty in the fabrication of the organic crystal-based devices. In this work, a simple and non-destructive technique of template stripping is employed to fabricate single-crystal-based organic light-emitting devices (OLEDs). Efficient and uniform carrier injection induced by an improved contact between crystals and both top and bottom electrodes is realized, so that a homogeneous and bright electroluminescence (EL) are obtained. Highly polarized EL and even white emission is also observed. Moreover, the crystal-based OLEDs exhibit good flexibility, and keep stable EL under a small bending radius and after repeated bending. It is expectable that this technique would support broad applications of the organic single crystals in the crystal-based optoelectronic devices.

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.

In view of mass-production and solution-processing capabilities, graphene oxides (GOs), generally prepared by chemical oxidation of graphite and subsequent exfoliation in aqueous solution, are widely used as an effective route to graphene-like materials. However, the oxygen-containing groups (OCGs) on the graphene sheets make GO insulating, which significantly restricts its applications, especially for electronics. Therefore, reduction methods that are used to remove OCGs become critical. In recent years, in addition to thermal and chemical reduction, photoreduction has emerged as an appealing alternative because photoreduction does not rely on either high temperature or toxic chemicals. In this progress report, the recent development of the photoreduction of GOs and their unique properties are highlighted, as well as their related applications. Photoreduction strategies including photothermal reduction, catalytic/catalyst-free photochemical reduction, and solid state/in-solution laser reduction are summarized. Moreover, photoreduction of GO permits exquisite control over film conductivities, residual oxygen contents, porosity, and surface wettability, which lead to various functionalities towards a wide range of applications, such as field-effect-transistors (FETs), flexible electrodes, sensors, supercapacitors, Li-ion batteries, photovoltaic devices, and photocatalysis. It is anticipated that, with the rapid progress of photoreduction methodology, GOs would be more competitive in the graphene-oriented applications.

The small field-of-view (FOV) limits the range of vision in various detecting/imaging devices from biological microscopes to commercial cameras and military radar. To date, imaging with FOV over 90° has been realized with fish-eye lenses, catadioptric lens, and rotating cameras. However, these devices suffer from inherent imaging distortion and require multiple bulky elements. Inspired by compound eyes found in nature, here a small-size (84 μm), distortion-free, wide-FOV imaging system is presented via an advanced 3D artificial eye architecture. The 3D artificial eye structure is accomplished by exploiting an effective optical strategy — high-speed voxel-modulation laser scanning (HVLS). The eye features a hexagonal shape, high fill factor (FF) (100%), large numerical aperture (NA) (0.4), ultralow surface roughness (2.5 nm) and aspherical profile, which provides high uniformity optics (error < ±6%) and constant resolution (FWHM = 1.7 ± 0.1 μm) in all directions. Quantitative measurement shows the eye reduces imaging distortion by two/three times under 30°/45° incidence, compared with a single lens. The distortion-free FOV can be controlled from 30° to 90°.

The reduction of graphene oxide can be used as a simple way to produce graphene on a large scale. However, the numerous edges produced by the oxidation of graphite seriously degrade the quality of the graphene and its carrier transport property. In this work, the reduction of oxygen-passivated graphene edges and the subsequent linking of separated graphene sheets by calcium are investigated by using first-principles calculations. The calculations show that calcium can effectively remove the oxygen groups from two adjacent edges. The joining point of the edges serves as the starting point of the reduction and facilitates the reaction. Once the oxygen groups are removed, the crack is sutured. If the joining point is lacking, it becomes difficult to zip the separated fragments. A general electron-reduction model and a random atom-reduction model are suggested for these two situations. The present study sheds light on the reduction of graphene-oxide edges by using reactive metals to give large-sized graphene through a simple chemical reaction.

Graphene quantum dots (GQDs) have recently emerged as a promising type of low-toxicity, high-biocompatibility, and chemically inert fluorescence probe with a high resistance to photobleaching. They are a prospective substitution for organic materials in light-emitting devices (LED), enabling the predicted concept of much brighter and more robust carbon LED (CLED). However, the mechanism of GQD emission remains an open problem despite extensive studies conducted so far, which is becoming the greatest obstacle in the route of technical improvement of GQD quantum efficiency. This problem is solved by the combined usage of femtosecond transient absorption spectroscopy and femtosecond time-resolved fluorescence dynamics measured by a fluorescence upconversion technique, as well as a nanosecond time-correlated single-photon counting technique. A fluorescence emission-associated dark intrinsic state due to the quantum confinement of in-plane functional groups is found in green-fluorescence graphene quantum dots by the ultrafast dynamics study, and the two characteristic fluorescence peaks that appear in all samples are attributed to independent molecule-like states. This finding establishes the correlation between the quantum confinement effect and molecule-like emission in the unique green-fluorescence graphene quantum dots, and may lead to innovative technologies of GQD fluorescence enhancement, as well as its broad industrial application.

Over the past two decades, microfluidics, represented by lab-on-a-chip (LoC) systems, have been developed because of their unique advantages of low reactant consumption, environmental friendliness, high safety, high efficiency, high sensitivity, portability, and easy handling of reactants. The distinguishing features of microfluidics have made the on-chip reactor a highly efficient platform for general chemical experiments, especially catalysis. In this paper, through a brief review of the recent work on microfluidic catalysis, we highlight the importance of on-chip catalytic microreactors. New approaches to the fabrication of on-chip catalytic microreactors and their integration with multifunctional components are briefly introduced. Finally, the current challenges and future perspectives of this up-and-coming field are discussed based on our own opinions. It is believed that, with the progress of interdisciplinary cooperation, microfluidics and catalysis could be complementary sciences; catalysts may play a very important role in LoC systems, and on-chip catalytic microreactors could be a highly efficient experimental platform for modern catalysis research.

Organic crystals have great potential for the applications in laser devices. This article presents an effective approach for fabrication of distributed feedback single crystal lasers. With the laser interference ablation method, high quality grating structures have been fabricated on the organic single-crystalline thin film materials. The relationship between the depth, periodicity, and laser fluence is discussed. The optical properties, such as photoluminescence, and diffractive properties are studied in detail. With the appropriate period, strong laser emission has been observed from these devices. Distributed feedback lasing is demonstrated from the laser interference ablated organic single crystals for the first time.

Initial nanointerfacial electron transfer dynamics are studied in dye-sensitized solar cells (DSSCs) in which the free energy and kinetics vary over a broad range. Surprisingly, it is found that the decay profiles, reflecting the electron transfer behavior, show a universal shape despite the different kinds of dye and semiconductor nanocrystalline films, even across different device types. This renews intuitive knowledge about the electron injection process in DSSCs. In order to quantitatively comprehend the universal behavior, a static inhomogeneous electronic coupling model with a Gaussian distribution of local injection energetics is proposed in which only the electron injection rate is a variant. It is confirmed that this model can be extended to CdSe quantum dot-sensitized films. These unambiguous results indicate exactly the same physical distribution in electron injection process of different sensitization films, providing limited simple and important parameters describing the electron injection process including electronic coupling constant and reorganization energy. The results provide insight into photoconversion physics and the design of optimal metal-free organic dye-sensitized photovoltaic devices by molecular engineering.

Rice leaves with anisotropic sliding properties have the ability to directionally control the movement of water microdroplets. However, the realization of artificial anisotropic sliding biosurfaces has remained challenging. It is found, by a systematic investigation, that the height of 200-μm-width groove arrays on rice leaves reaches up to 45 μm, far greater than the smaller microgrooves that are widely adopted for the study of anisotropic wetting. A new model based on three-level microstructures (macro/micro/nano) is developed to interpret the anisotropic sliding behavior. Moreover, artificial rice leaves with different macrogrooves are demonstrated by combining micro/nanostructures and macrogrooves, which are prepared by photolithography, PDMS imprinting, and micro/nanostructure coating. Sliding-angle measurement further prove that the third-level macrogroove arrays are the determining factor for anisotropic sliding. Finally, a new testing method, curvature-assisted droplet oscillation (CADO), is developed to quantitatively reveal the anisotropic dynamic behavior of biomimetic rice-leaf-like surfaces.

E, E-1, 4-bis[4′-(N,N-dibutylamino)styryl]-2,5-dimethoxy-benzene (DBASDMB) organic crystals with high crystalline quality, large size and excellent optical properties are prepared. The linear and nonlinear properties in the crystal are comparatively studied. The relaxation dynamics pumped by two-photon are very similar with that pumped by one-photon. The crystal exhibits very strong two-photon excited fluorescence and amplified spontaneous emission. Efficient two-photon absorption, reasonably high fluorescent quantum efficiency, and high crystal quality together with stimulated emission make organic crystals ideal for the application in frequency upconversion and other optoelectronic fields.