The design of stacks of layered materials in which adjacent layers interact by van der Waals forces has enabled the combination of various two-dimensional crystals with different electrical, optical ...and mechanical properties as well as the emergence of novel physical phenomena and device functionality. Here, we report photoinduced doping in van der Waals heterostructures consisting of graphene and boron nitride layers. It enables flexible and repeatable writing and erasing of charge doping in graphene with visible light. We demonstrate that this photoinduced doping maintains the high carrier mobility of the graphene/boron nitride heterostructure, thus resembling the modulation doping technique used in semiconductor heterojunctions, and can be used to generate spatially varying doping profiles such as p-n junctions. We show that this photoinduced doping arises from microscopically coupled optical and electrical responses of graphene/boron nitride heterostructures, including optical excitation of defect transitions in boron nitride, electrical transport in graphene, and charge transfer between boron nitride and graphene.
The design of stacks of layered materials in which adjacent layers interact by van der Waals forces1 has enabled the combination of various two-dimensional crystals with different electrical, optical ...and mechanical properties, and the emergence of novel physical phenomena and device functionality2-8. Here we report photo-induced doping in van der Waals heterostructures (VDHs) consisting of graphene and boron nitride layers. It enables flexible and repeatable writing and erasing of charge doping in graphene with visible light. We demonstrate that this photo-induced doping maintains the high carrier mobility of the graphene-boron nitride (G/BN) heterostructure, which resembles the modulation doping technique used in semiconductor heterojunctions, and can be used to generate spatially-varying doping profiles such as p-n junctions. We show that this photo-induced doping arises from microscopically coupled optical and electrical responses of G/BN heterostructures, which includes optical excitation of defect transitions in boron nitride, electrical transport in graphene, and charge transfer between boron nitride and graphene.
Mechanically interlocked molecules (MIMs) possess unique architectures and nontraditional degrees of freedom that arise from well-defined topologies that are achieved through precise mechanical ...bonding. Incorporation of MIMs into materials can thus provide an avenue to discover new and emergent macroscale properties. Here, the synthesis of a phenanthroline-based 2catenane crosslinker and its incorporation into polyacrylate organogels are described. Specifically, Cu(I) metalation and demetalation was used as a postgelation strategy to tune the mechanical properties of a gel by controlling the conformational motions of integrated MIMs. The organogels were prepared via thermally initiated free radical polymerization, and Cu(I) metal was added in MeOH to the pretreated, swollen gels. Demetalation of the gels was achieved by adding lithium cyanide and washing the gels. Changes in Young's and shear moduli, as well as tensile strength, were quantified through oscillatory shear rheology and tensile testing. The reported approach provides a general method for postgelation tuning of mechanical properties using metals and well-defined catenane topologies as part of a gel network architecture.
Shear‐thinning hydrogels represent an important class of injectable soft materials that are often used in a wide range of biomedical applications. Creation of new shear‐thinning materials often ...requires that factors such as viscosity, injection rate/force, and needle gauge be evaluated to achieve efficient delivery, while simultaneously protecting potentially sensitive cargo. Here, a new approach to establishing shear‐thinning hydrogels is reported where a host–guest cross‐linked network initially remains soluble in deionized water but is kinetically trapped as a viscous hydrogel once exposed to saltwater. The shear‐thinning properties of the hydrogel is then “switched on” in response to heating or exposure to visible light. These hydrogels consist of polynorbornene‐based bottlebrush copolymers with porphyrin‐ and oligoviologen‐containing side chains that are cross‐linked through the reversible formation of β‐cyclodextrin–adamantane inclusion complexes. The resultant viscous hydrogels display broad adhesive properties across polar and nonpolar substrates, mimicking that of natural mucous and thus making it easier to distribute onto a wide range of surfaces. Additional control over the hydrogel's mechanical properties (storage/loss moduli) and performance (adhesion) is achieved post‐injection using a low‐energy (blue light) photoinduced electron‐transfer process. This work envisions these injectable copolymers and multimodal hydrogels can serve as versatile next‐generation biomaterials capable of light‐based mechanical manipulation post‐injection.
A novel approach toward injectable, shear‐thinning hydrogels is reported, where bottlebrush polymers are cross‐linked into water‐soluble polymer networks through host–guest cross‐linking interactions. Injection of the pre‐gel solution into saltwater induces rapid gelation, affording viscous hydrogels with broad adhesive properties that can be converted into shear‐thinning materials in response to heating or through a novel visible‐light photoredox mechanism.
Catenanes are a well-known class of mechanically interlocked molecules that possess chain-like architectures and have been investigated for decades as molecular machines and switches. However, the ...synthesis of higher-order catenanes with multiple, linearly interlocked molecular rings has been greatly impeded by the generation of unwanted oligomeric byproducts and figure-of-eight topologies that compete with productive ring closings. Here, we report two general strategies for the synthesis of oligoncatenanes that rely on a molecular “zip-tie” strategy, where the “zip-tie” is a central core macrocycle precursor bearing two phenanthroline (phen) ligands to make odd-numbered oligoncatenanes, or a preformed asymmetric iron(II) complex consisting of two macrocycle precursors bearing phen and terpyridine ligands to make even-numbered oligoncatenanes. In either case, preformed macrocycles or 2catenanes are threaded onto the central “zip-tie” core using metal templation prior to ring-closing metathesis (RCM) reactions that generate several mechanical bonds in one pot. Using these synthetic strategies, a family of well-defined linear oligoncatenanes were synthesized, where n = 2, 3, 4, 5, or 6 interlocked molecular rings, and n = 6 represents the highest number of linearly interlocked rings reported to date for any isolated unimolecular oligoncatenane.
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