Directional Assembly of ZnO Nanowires via Three-Dimensional Laser Direct Writing.

The precise placement of semiconductor nanowires (NWs) into two- or three-dimensional (2D/3D) micro-/nanoarchitectures is a key for the construction of integrated functional devices. However, long-pending challenges still exist in high-resolution 3D assembly of semiconductor NWs. Here, we have achieved directional assembly of zinc oxide (ZnO) NWs into nearly arbitrary 3D architectures with high spatial resolution using two-photon polymerization. The NWs can regularly align in any desired direction along the laser scanning pathway. Through theoretical calculation and control experiments, we unveiled the laser-induced assembly mechanism and found that the nonoptical forces are the dominant factor leading to the directional assembly of ZnO NWs. A ZnO-NW-based polarization-resolved UV photodetector of excellent photoresponsivity was fabricated to demonstrate the potential application of the assembled ZnO NWs. This work is expected to promote the research on NW-based integrated devices such as photonic integrated circuits, sensors, and metamaterial with unprecedented controllability of the NW's placement in three dimensions.

3D Nanoprinting of Heterogeneous Metal Oxides with High Shape Fidelity.

3D nanoprinting can significantly enhance the performance of sensors, batteries, optoelectronic/microelectronic devices, etc. However, current 3D nanoprinting methods for metal oxides are suffering from three key issues including limited material applicability, serious shape distortion, and the difficulty of heterogeneous integration. This paper discovers a mechanism in which imidazole and acrylic acid synergistically coordinate with metal ions in water. Using the mechanism, this work develops a series of metal ion synergistic coordination water-soluble (MISCWS) resins for 3D nanoprinting of various metal oxides, including MnO2, Cr2O3, Co3O4, and ZnO, as well as heterogeneous structures of MnO2/NiO, Cr2O3/Al2O3, and ZnO/MgO. Besides, the synergistic coordination effect results in a 2.54-fold increase in inorganic mass fraction within the polymer, compared with previous works, which effectively mitigates the shape distortion of metal oxide microstructures. Based on this method, this work also demonstrates a 3D ZnO microsensor with a high sensitivity (1.113 million at 200 ppm NO2), surpassing the conventional 2D ZnO sensors by tenfold. The method yields high-fidelity 3D structures of heterogeneous metal oxides with nanoscale resolution, paving the way for applications such as sensing, micro-optics, energy storage, and microsystems.

3D Directional Assembly of Liquid Crystal Molecules.

The precise construction of hierarchically long-range ordered structures using molecules as fundamental building blocks can fully harness their anisotropy and potential. However, the 3D, high-precision, and single-step directional assembly of molecules is a long-pending challenge. Here, a 3D directional molecular assembly strategy via femtosecond laser direct writing (FsLDW) is proposed and the feasibility of this approach using liquid crystal (LC) molecules as an illustrative example is demonstrated. The physical mechanism for femtosecond (fs) laser-induced assembly of LC molecules is investigated, and precise 3D arbitrary assembly of LC molecules is achieved by defining the discretized laser scanning pathway. Additionally, an LC-based Fresnel zone plate array with polarization selection and colorization imaging functions is fabricated to further illustrate the potential of this method. This study not only introduces a 3D high-resolution alignment method for LC-based functional devices but also establishes a universal protocol for the precise 3D directional assembly of anisotropic molecules.

3D printing of nanowrinkled architectures via laser direct assembly.

Structural wrinkles in nature have been widely imitated to enhance the surface functionalities of objects, especially three-dimensional (3D) architectured wrinkles, holding promise for emerging applications in mechanical, electrical, and biological processes. However, the fabrication of user-defined 3D nanowrinkled architectures is a long-pending challenge. Here, we propose a bottom-up laser direct assembly strategy to fabricate multidimensional nanowrinkled architectures in a single-material one-step process. Through the introduction of laser-induced thermal transition into a 3D nanoprinting process for leading the point-by-point nanoscale wrinkling and the self-organization of wrinkle structures, we have demonstrated the program-controlled and on-demand fabrication of multidimensional nanowrinkled structures. Moreover, the precise control of wrinkle morphology with an optimal wavelength of 40 nanometers and the regulation of the dynamic transformation of wrinkled cellular microstructures via interfacial stress mismatch engineering have been achieved. This study provides a universal protocol for constructing nearly arbitrary nanowrinkled architectures and facilitates a new paradigm in nanostructure manufacturing.

High-Responsivity Multiband and Polarization-Sensitive Photodetector Based on the TiS3/MoS2 Heterojunction.

Two-dimensional (2D) material photodetectors have received considerable attention in optoelectronics as a result of their extraordinary properties, such as passivated surfaces, strong light-matter interactions, and broad spectral responses. However, single 2D material photodetectors still suffer from low responsivity, large dark current, and long response time as a result of their atomic-level thickness, large binding energy, and susceptibility to defects. Here, a transition metal trichalcogenide TiS3 with excellent photoelectric characteristics, including a direct bandgap (1.1 eV), high mobility, high air stability, and anisotropy, is selected to construct a type-II heterojunction with few-layer MoS2, aiming to improve the performance of 2D photodetectors. An ultrahigh photoresponsivity of the TiS3/MoS2 heterojunction of 48 666 A/W at 365 nm, 20 000 A/W at 625 nm, and 251 A/W at 850 nm is achieved under light-emitting diode illumination. The response time and dark current are 2 and 3 orders of magnitude lower than those of the current TiS3 photodetector with the highest photoresponsivity (2500 A/W), respectively. Furthermore, polarized four-wave mixing spectroscopy and polarized photocurrent measurements verify its polarization-sensitive characteristics. This work confirms the excellent potential of TiS3/MoS2 heterojunctions for air-stable, high-performance, polarization-sensitive, and multiband photodetectors, and the excellent type-II TiS3/MoS2 heterojunction system may accelerate the design and fabrication of other 2D functional devices.

An ultrathin memristor based on a two-dimensional WS2/MoS2 heterojunction.

Memristors are regarded as one of the key devices to break through the traditional Von Neumann computer architecture due to their capability of simulating the function of neural synapses. Among various memristive materials, two-dimensional (2D) materials are promising candidates to build advanced memristors with extremely high integration density and low power consumption. However, memristors based on 2D materials usually suffer from poor endurance and retention due to their vulnerability to material degradation during the formation/fusing processes of conductive filament channels within the switching media of 2D materials. Here, a new memristor architecture based on a WS2/MoS2 2D semiconducting heterojunction (metal/heterojunction/metal, MHM) is proposed, which is completely different from the conventional metal/insulator/metal (MIM) sandwich structure. Through the introduction of a type-II 2D heterojunction, a resistance switching mechanism based on band modulation rather than the conductive filaments can be realized to eliminate the material degradation during the set/reset processes. A prototype MHM memristor based on the WS2/MoS2 heterojunction is successfully developed with a large switching on/off ratio up to 104 and a clearly extended endurance over 120 switching cycles, showing the advantage of the 2D WS2/MoS2 heterojunction over the individual MoS2 or WS2 layers in memristive performance. The proposed method for the MHM-type 2D memristor has the potential to achieve a large-scale integrated memristor matrix with low power consumption and high integration density, which is promising for future artificial intelligence and brain-like computing systems.

Four-Dimensional Stimuli-Responsive Hydrogels Micro-Structured via Femtosecond Laser Additive Manufacturing.

Rapid fabricating and harnessing stimuli-responsive behaviors of microscale bio-compatible hydrogels are of great interest to the emerging micro-mechanics, drug delivery, artificial scaffolds, nano-robotics, and lab chips. Herein, we demonstrate a novel femtosecond laser additive manufacturing process with smart materials for soft interactive hydrogel micro-machines. Bio-compatible hyaluronic acid methacryloyl was polymerized with hydrophilic diacrylate into an absorbent hydrogel matrix under a tight topological control through a 532 nm green femtosecond laser beam. The proposed hetero-scanning strategy modifies the hierarchical polymeric degrees inside the hydrogel matrix, leading to a controllable surface tension mismatch. Strikingly, these programmable stimuli-responsive matrices mechanized hydrogels into robotic applications at the micro/nanoscale (<300 × 300 × 100 µm3). Reverse high-freedom shape mutations of diversified microstructures were created from simple initial shapes and identified without evident fatigue. We further confirmed the biocompatibility, cell adhesion, and tunable mechanics of the as-prepared hydrogels. Benefiting from the high-efficiency two-photon polymerization (TPP), nanometer feature size (<200 nm), and flexible digitalized modeling technique, many more micro/nanoscale hydrogel robots or machines have become obtainable in respect of future interdisciplinary applications.

Two-Step Freezing Polymerization Method for Efficient Synthesis of High-Performance Stimuli-Responsive Hydrogels.

The widespread use of stimuli-responsive hydrogels is closely related to their synthesis efficiency. However, the widely used thermal-responsive poly(N-isopropylacrylamide) (PNIPAM) hydrogels usually require a time-consuming synthesis process to produce (more than 12 h) and exhibit a relatively slow response speed in the field of cryo-polymerization. In this study, a sequence of thawing polymerization after freezing polymerization by a two-step method of free radical polymerization for the efficient synthesis of PNIPAM hydrogels (merely 2 h) with an excellent comprehensive performance is demonstrated. Results show that the overall performance of the as-synthesized PNIPAM hydrogels is at the top level among reported works despite the significantly reduced preparation time. Moreover, after incorporating multi-walled carbon nanotubes (MWNTs), the PNIPAM hydrogels exhibit a rapid near-infrared (NIR) light-response and programmable shape-morphing capability. It is believed that such a viable and time-saving synthetic method for producing PNIPAM hydrogels of high performance will lay a solid foundation for drug delivery and smart actuators.

Nanostructured electrically conductive hydrogels obtained via ultrafast laser processing and self-assembly.

Electrically conductive polymers have emerged as functional materials for future electronics due to their high electrical conductivity, real-time responsiveness, easy film-formation ability and desirable stretchability. However, the previously developed conductive polymer electronics are still limited to macroscopic hydrogels or films without complicated designs of fine features. Herein, a carbon nanotube-doped hydrophilic photoresist was ultrafast laser processed as an absorbent 3D scaffold to fabricate nanostructured electrically conductive hydrogels (NECHs) for the first time. Taking advantage of the intermolecular forces, we in situ interpenetrated π-conjugated poly(3,4-ethylenedioxythiophene) into NECHs by self-assembly to combine fine features (resolution down to 500 nm, at least two-order accuracy improvement than that in the case of standard 3D-printed electronics) and achieve a high electrical conductivity (0.1-42.5 S m-1), device-level mechanical properties and desirable tolerance to humid/acid environments. Consequently, several reliable, nanostructured, metal-free electrical circuits, alcohol micro-sensors, interdigital capacitors, and loop inductors have been experimentally identified and characterized. The NECHs successfully break current limitations by making better use of the two photon hydrogelation and highly conductive polymer. Optical clarity, conductivity, and extensibility of the NECHs promise their applications in micro energy storage devices, epidermal electronics, nanorobotics and electrical circuit boards for challenging conditions.