Soap film inspired mechanical metamaterials approaching theoretical bound of stiffness across full density range.

Structural mechanical metamaterials, with their mass-efficient architectures and unprecedented mechanical properties, are in critical demand for high-performance applications. However, finding the optimal 3D geometries towards a particular property, such as reaching the stiffness upper bound, usually demands high volume of calculations or numerical optimizations. Here we generate structured mechanical metamaterials by imitating the natural occupation of periodic volume by inflated soap films. Our strategy of occupying volume between two periodic constant mean curvature (CMC) surfaces generates a series of mechanical metamaterials of varied relative densities ranging from 0 to 1. The mechanically isotropic ISO-CMC structures exhibit bulk moduli over 94% of the theoretical limit. Using finite element models, we reveal the fundamental mechanical behaviors of the structures that lead to ideal performances. These phenomena are found to be in close relation to the curvature-driven design of our metamaterial structures. These structures are compared to other reported mechanical metamaterials, such as closed-cell plate structures and triply periodic minimal surface (TPMS) structures. The unique curvature-driven thickening strategy of our method renders structures that outperform their peers in terms of bulk moduli and relative density coverage. The CMC structures present a new class of easily 3D printable, permeable and stiff mechanical metamaterials. The design methodology also could serve in future development of novel mechanical metamaterials powered by advanced computational tools.

Magnetically Aligned Ultrafine Cobalt Embedded 3D Porous Carbon Metamaterial by One-Step Ultrafast Laser Direct Writing.

Spatial manipulation of nanoparticles (NPs) in a controlled manner is critical for the fabrication of 3D hybrid materials with unique functions. However, traditional fabrication methods such as electron-beam lithography and stereolithography are usually costly and time-consuming, precluding their production on a large scale. Herein, for the first time the ultrafast laser direct writing is combined with external magnetic field (MF) to massively produce graphene-coated ultrafine cobalt nanoparticles supported on 3D porous carbon using metal-organic framework crystals as precursors (5 × 5 cm2 with 10 s). The MF-confined picosecond laser scribing not only reduces the metal ions rapidly but also aligns the NPs in ultrafine and evenly distributed order (from 7.82 ± 2.37 to 3.80 ± 0.84 nm). ≈400% increment of N-Q species within N compositionis also found as the result of the special MF-induced laser plasma plume. (). The importance of MF is further exmined by electrochemical water-splitting tests. Significant overpotential improvements of 90 and 150 mV for oxygen evolution reaction and hydrogen evolution reaction are observed, respectively, owing to the MF-induced alignment of the NPs and controlled elemental compositions. This work provides a general bottom-up approach for the synthesis of metamaterials with high outputs yet a simple setup.

Pulsed Laser Modulated Shock Transition from Liquid Metal Nanoparticles to Mechanically and Thermally Robust Solid-Liquid Patterns.

In this work, a general mechanism is discovered to form liquid-metal-based, stable and stretchable conductive patterns on rigid and soft substrates. It is discovered that pulsed laser irradiation of liquid metal nanoparticles (LMNPs) with tunable conditions can induce transformation to stable and stretchable solid-liquid (S-L) dual phases on various surfaces. Formation of this unique solid-liquid composite phase is the key to change the wetting behavior of the conductive patterns on various substrates and enables mechanically stable patterns on various substrates. Pulsed-laser-driven thermo-mechanical shock momentum is important for rupture and joining of the LMNPs, providing much better control than the traditional mechanical sintering. The solid nanophase forms a nanoporous matrix filled with and wetted by the LM, thereby providing a stabilization mechanism for the S-L composite patterned thin film. The mechanical and thermal reliability of the solid-liquid patterns is investigated. The S-L patterns can stretch up to 30% strain and cycle stably for 7000 cycles. It can be heated up to 177 °C with an input power of 0.58 W. The solid-liquid composite film provides great opportunity for various applications as a flexible conductor with unique mechanical and physics properties and further inspires design of LM devices for completely exposed applications.

Double-negative-index ceramic aerogels for thermal superinsulation.

Ceramic aerogels are attractive for thermal insulation but plagued by poor mechanical stability and degradation under thermal shock. In this study, we designed and synthesized hyperbolic architectured ceramic aerogels with nanolayered double-pane walls with a negative Poisson's ratio (-0.25) and a negative linear thermal expansion coefficient (-1.8 × 10-6 per °C). Our aerogels display robust mechanical and thermal stability and feature ultralow densities down to ~0.1 milligram per cubic centimeter, superelasticity up to 95%, and near-zero strength loss after sharp thermal shocks (275°C per second) or intense thermal stress at 1400°C, as well as ultralow thermal conductivity in vacuum [~2.4 milliwatts per meter-kelvin (mW/m·K)] and in air (~20 mW/m·K). This robust material system is ideal for thermal superinsulation under extreme conditions, such as those encountered by spacecraft.

Enhancement of osteoblast activity on nanostructured NiTi/hydroxyapatite coatings on additive manufactured NiTi metal implants by nanosecond pulsed laser sintering.

BACKGROUND: The osteoinductive behaviors of nitinol (NiTi)-based metal implants for bone regeneration are largely dependent on their surface composition and topology. Continuous-mode laser sintering often results in complete melting of the materials and aggregation of particles, which lack control of heat transfer, as well as microstructural changes during sintering of the nanocomposite materials. METHODS: In the current study, in situ direct laser deposition was used to additively manufacture three-dimensional NiTi structures from Ni and Ti powders. The mechanical property of NiTi has been shown to be similar to bone. Nanosecond pulsed laser sintering process was then utilized to generate a nanoporous composite surface with NiTi alloy and hydroxyapatite (HA) by ultrafast laser heating and cooling of Ni, Ti, and HA nanoparticles mixtures precoated on the 3D NiTi substrates; HA was added in order to improve the biocompatibility of the alloy. We then studied the underlying mechanism in the formation of NiTi/HA nanocomposite, and the synergistic effect of the sintered HA component and the nanoporous topology of the composite coating. In addition, we examined the activity of bone-forming osteoblasts on the NiTi/HA surfaces. For this, osteoblast cell morphology and various biomarkers were examined to evaluate cellular activity and function. RESULTS: We found that the nanoscale porosity delivered by nanosecond pulsed laser sintering and the HA component positively contributed to osteoblast differentiation, as indicated by an increase in the expression of collagen and alkaline phosphatase, both of which are necessary for osteoblast mineralization. In addition, we observed topological complexities which appeared to boost the activity of osteoblasts, including an increase in actin cytoskeletal structures and adhesion structures. CONCLUSION: These findings demonstrate that the pulsed laser sintering method is an effective tool to generate biocompatible coatings in complex alloy-composite material systems with desired composition and topology. Our findings also provide a better understanding of the osteoinductive behavior of the sintered nanocomposite coatings for use in orthopedic and bone regeneration applications.

Laser Sintering of Liquid Metal Nanoparticles for Scalable Manufacturing of Soft and Flexible Electronics.

Soft, flexible, and stretchable electronics are needed to transmit power and information, and track dynamic poses in next-generation wearables, soft robots, and biocompatible devices. Liquid metal has emerged as a promising material for these applications due to its high conductivity and liquid phase state at room temperature; however, surface oxidation of liquid metal gives it unique behaviors that are often incompatible with scalable manufacturing techniques. This paper reports a rapid and scalable approach to fabricate soft and flexible electronics composed of liquid metal. Compared to other liquid metal patterning approaches, this approach has the advantages of compatibility with a variety of substrates, ease of scalability, and efficiency through automated processes. Nonconductive liquid metal nanoparticle films are sintered into electrically conductive patterns by use of a focused laser beam to rupture and ablate particle oxide shells, and allow their liquid metal cores to escape and coalesce. The laser sintering phenomenon is investigated through comparison with focused ion beam sintering and by studying the effects of thermal propagation in sintered films. The effects of laser fluence, nanoparticle size, film thickness, and substrate material on resistance of the sintered films are evaluated. Several devices are fabricated to demonstrate the electrical stability of laser-patterned liquid metal traces under flexing, multilayer circuits, and intricately patterned circuits. This work merges the precision, consistency, and speed of laser manufacturing with the material benefits of liquid conductors on elastic substrates to demonstrate decisive progress toward commercial-scale manufacturing of soft electronics.

Straining effects in MoS2 monolayer on nanostructured substrates: temperature-dependent photoluminescence and exciton dynamics.

Strain-engineering of two-dimensional (2D) transition metal dichalcogenides (TMDs) has great potential to alter their electronic and optical properties. Thus far, experimental studies of the straining effects in 2D TMDs primarily focused on the static property measurements at room temperature. However, low-temperature and temperature-dependence studies are essential in understanding the underlying mechanisms of the unique properties of monolayer TMDs. Herein, the temperature-dependent dynamic properties of laser shock strain-engineered monolayer MoS2 were studied using temperature-dependent photoluminescence (PL) and pump-probe spectroscopy. Both the photoluminescence spectra and exciton dynamics exhibit the differences between the MoS2 monolayers transferred on the flat and nanostructured surfaces by laser shock strain engineering and display a strong temperature dependence. The laser-induced straining effect and temperature-dependent dynamic behavior of MoS2 were studied through molecular dynamics simulation. The observed behaviors can be explained by the thermally induced strain in the monolayer MoS2 due to the mismatching thermal expansion coefficients of the monolayer and the substrate, which are coupled by the van der Waals forces. The ultrafast pump-probe experiments were performed to investigate the effect of strain on the exciton dynamics upon optical excitation. The results from the pump-probe measurements indicate that the effects of strain extend beyond that of the static properties and profoundly influence the valley carrier dynamics. This report extends the understanding of the substrate-induced straining effect to temperature-dependent luminescence behaviors and dynamic behaviors of the TMD materials.

Ultrafast Laser-Shock-Induced Confined Metaphase Transformation for Direct Writing of Black Phosphorus Thin Films.

Few-layer black phosphorus (BP) has emerged as one of the most promising candidates for post-silicon electronic materials due to its outstanding electrical and optical properties. However, lack of large-scale BP thin films is still a major roadblock to further applications. The most widely used methods for obtaining BP thin films are mechanical exfoliation and liquid exfoliation. Herein, a method of directly synthesizing continuous BP thin films with the capability of patterning arbitrary shapes by employing ultrafast laser writing with confinement is reported. The physical mechanism of confined laser metaphase transformation is understood by molecular dynamics simulation. Ultrafast laser ablation of BP layer under confinement can induce transient nonequilibrium high-temperature and high-pressure conditions for a few picoseconds. Under optimized laser intensity, this process induces a metaphase transformation to form a crystalline BP thin film on the substrate. Raman spectroscopy, atomic force microscopy, and transmission electron microscopy techniques are utilized to characterize the morphology of the resulting BP thin films. Field-effect transistors are fabricated on the BP films to study their electrical properties. This unique approach offers a general methodology to mass produce large-scale patterned BP films with a one-step manufacturing process that has the potential to be applied to other 2D materials.

Graphene/PbS-Quantum Dots/Graphene Sandwich Structures Enabled by Laser Shock Imprinting for High Performance Photodetectors.

Quantum dots (QDs) integrated 2-dimensional (2D) materials have great potential for photodetector applications due to the excellent light absorption of QDs and ultrafast carrier transportation of 2D materials. However, there is a main issue that prevents efficient carrier transportation and ideal performance of photodetectors: the high interfacial resistance between 2D materials and QDs due to the bad contacts between 2D/0D interface, which makes sluggish carrier transfer from QDs to 2D materials. Here, a sandwich structure (graphene/PbS-QDs/graphene) with seamless 2D/0D contact was fabricated by laser shock imprinting, which opto-mechanically tunes the morphology of 2D materials to perfectly wrap on 0D materials and efficiently collect carriers from the PbS-QDs. It is found that this seamless integrated 2D/0D/2D structure significantly enhanced the carrier transmission, photoresponse gain (by 2×), response time (by 20×), and photoresponse speed (by 13×). The response time (∼30 ms) and Ip/ Id ratio (13.2) are both over 10× better than the reported hybrid graphene photodetectors. This is due to the tight contact and efficient gate-modulated carrier injection from PbS-QDs to graphene. The gate voltage dictates whether electrons or holes dominate the carrier injection from PbS-QDs to graphene.

Flyweight, Superelastic, Electrically Conductive, and Flame-Retardant 3D Multi-Nanolayer Graphene/Ceramic Metamaterial.

A ceramic/graphene metamaterial (GCM) with microstructure-derived superelasticity and structural robustness is achieved by designing hierarchical honeycomb microstructures, which are composited with two brittle constituents (graphene and ceramic) assembled in multi-nanolayer cellular walls. Attributed to the designed microstructure, well-interconnected scaffolds, chemically bonded interface, and coupled strengthening effect between the graphene framework and the nanolayers of the Al2 O3 ceramic (NAC), the GCM demonstrates a sequence of multifunctional properties simultaneously that have not been reported for ceramics and ceramics-matrix-composite structures, such as flyweight density, 80% reversible compressibility, high fatigue resistance, high electrical conductivity, and excellent thermal-insulation/flame-retardant performance simultaneously. The 3D well-ordered graphene aerogel templates are strongly coupled with the NAC by the chemically bonded interface, exhibiting mutual strengthening, compatible deformability, and a linearly dependent relationship between the density and Young's modulus. Considerable size effects of the ceramic nanolayers on the mechanical properties are revealed in these ceramic-based metamaterials. The designed hierarchical honeycomb graphene with a fourth dimensional control of the ceramic nanolayers on new ways to scalable fabrication of advanced multifunctional ceramic composites with controllable design suggest a great potential in applications of flexible conductors, shock/vibration absorbers, thermal shock barriers, thermal insulation/flame-retardant skins, and porous microwave-absorbing coatings.

Parallel Nanoshaping of Brittle Semiconductor Nanowires for Strained Electronics.

Semiconductor nanowires (SCNWs) provide a unique tunability of electro-optical property than their bulk counterparts (e.g., polycrystalline thin films) due to size effects. Nanoscale straining of SCNWs is desirable to enable new ways to tune the properties of SCNWs, such as electronic transport, band structure, and quantum properties. However, there are two bottlenecks to prevent the real applications of straining engineering of SCNWs: strainability and scalability. Unlike metallic nanowires which are highly flexible and mechanically robust for parallel shaping, SCNWs are brittle in nature and could easily break at strains slightly higher than their elastic limits. In addition, the ability to generate nanoshaping in large scale is limited with the current technologies, such as the straining of nanowires with sophisticated manipulators, nanocombing NWs with U-shaped trenches, or buckling NWs with prestretched elastic substrates, which are incompatible with semiconductor technology. Here we present a top-down fabrication methodology to achieve large scale nanoshaping of SCNWs in parallel with tunable elastic strains. This method utilizes nanosecond pulsed laser to generate shock pressure and conformably deform the SCNWs onto 3D-nanostructured silicon substrates in a scalable and ultrafast manner. A polymer dielectric nanolayer is integrated in the process for cushioning the high strain-rate deformation, suppressing the generation of dislocations or cracks, and providing self-preserving mechanism for elastic strain storage in SCNWs. The elastic strain limits have been studied as functions of laser intensity, dimensions of nanowires, and the geometry of nanomolds. As a result of 3D straining, the inhomogeneous elastic strains in GeNWs result in notable Raman peak shifts and broadening, which bring more tunability of the electrical-optical property in SCNWs than traditional strain engineering. We have achieved the first 3D nanostraining enhanced germanium field-effect transistors from GeNWs. Due to laser shock induced straining effect, a more than 2-fold hole mobility enhancement and a 120% transconductance enhancement are obtained from the fabricated back-gated field effect transistors. The presented nanoshaping of SCNWs provide new ways to manipulate nanomaterials with tunable electrical-optical properties and open up many opportunities for nanoelectronics, the nanoelectrical-mechanical system, and quantum devices.

Superplastic Formation of Metal Nanostructure Arrays with Ultrafine Gaps.

Laser shock compression of plasmonic nanoarrays results in ultrafine tunable line-gaps at sub-10 nm scale by collaborative superplastic flow. From molecular dynamics analysis, the metal nanostructures change from crystalline to liquid-like metals, expanding quickly but never fusing together, even when they are very close. This technique enables good tunability of surface plasmon resonances and significantly enhanced local fields.

Crystalline Nanojoining Silver Nanowire Percolated Networks on Flexible Substrate.

Optoelectronic performance of metal nanowire networks are dominated by junction microstructure and network configuration. Although metal nanowire printings, such as silver nanowires (AgNWs) or AgNWs/semiconductor oxide bilayer, have great potential to replace traditional ITO, efficient and selective nanoscale integration of nanowires is still challenging owing to high cross nanowire junction resistance. Herein, pulsed laser irradiation under controlled conditions is used to generate local crystalline nanojoining of AgNWs without affecting other regions of the network, resulting in significantly improved optoelectronic performance. The method, laser-induced plasmonic welding (LPW), can be applied to roll-to-roll printed AgNWs percolating networks on PET substrate. First principle simulations and experimental characterizations reveal the mechanism of crystalline nanojoining originated from thermal activated isolated metal atom flow over nanowire junctions. Molecular dynamic simulation results show an angle-dependent recrystallization process during LPW. The excellent optoelectronic performance of AgNW/PET has achieved Rs ∼ 5 Ω/sq at high transparency (91% @λ = 550 nm).

Three-dimensional printing of complex structures: man made or toward nature?

Current three-dimensional (3D) printing techniques enable the fabrication of complex multifunctional structures that are unimaginable in conventional manufacturing. In this Perspective, we outline recent progress in materials and manufacturing and propose challenges and opportunities for the future development of 3D printing of functional materials. The success of future 3D printing relies not only on multifunctional materials and printing techniques but also on smart design of complex systems. Engineers need to understand advanced materials, additive manufacturing, and, more importantly, creative design. Fortunately, we can learn from many structures that exist in nature and adapt them to engineered structures.

Ultrafast and scalable laser liquid synthesis of tin oxide nanotubes and its application in lithium ion batteries.

Laser-induced photo-chemical synthesis of SnO2 nanotubes has been demonstrated by employing a nanoporous polycarbonate membrane as a template. The SnO2 nanotube diameter can be controlled by the nanoporous template while the nanotube length can be tuned by laser parameters and reaction duration. The microstructure characterization of the nanotubes indicates that they consist of mesoporous structures with sub 5 nm size nanocrystals connected by the twinning structure. The application of SnO2 nanotubes as an anode material in lithium ion batteries has also been explored, and they exhibited high capacity and excellent cyclic stability. The laser based emerging technique for scalable production of crystalline metal oxide nanotubes in a matter of seconds is remarkable. The compliance of the laser based technique with the existing technologies would lead to mass production of novel nanomaterials that would be suitable for several emerging applications.