Mussel‐Inspired Two‐Dimensional Freestanding Alkyl‐Polydopamine Janus Nanosheets

Mussel‐inspired two‐dimensional freestanding, alkyl‐polydopamine (alkyl‐PDA) Janus nanosheets, with a well‐controlled nanometer thickness and a lateral size of up to micrometers, have been developed. A self‐assembled octadecylamine (ODA) bilayer is used as the reactive template for the dopamine polymerization, resulting in the formation of well‐defined nanosheets. The alkyl‐PDA nanosheets show an amphiphilic nature with hydrophilic PDA and hydrophobic alkyl chains on opposing sides. The nanosheets can be used to functionalize many substrates and is dependent on the configuration of surface of the nanosheets. The nanosheets are quite stable, as the morphology is preserved after carbonization at 900 °C. Post‐modification of the nanosheets can be easily achieved because of the reactive nature of PDA. This work will provide a new strategic approach for fabricating polymeric Janus nanosheets, which can find applications for surface modifications, catalyst supports, and guided self‐assembly.     

Photoswitchable Dissipative Two‐Dimensional Colloidal Crystals

Control over particle interactions and organization at fluid interfaces is of great importance both for fundamental studies and practical applications. Rendering these systems stimulus‐responsive is thus a desired challenge both for investigating dynamic phenomena and realizing reconfigurable materials. Here, we describe the first reversible photocontrol of two‐dimensional colloidal crystallization at the air/water interface, where millimeter‐sized assemblies of microparticles can be actuated through the dynamic adsorption/desorption behavior of a photosensitive surfactant added to the suspension. This allows us to dynamically switch the particle organization between a highly crystalline (under light) and a disordered (in the dark) phase with a fast response time (crystallization in ≈10 s, disassembly in ≈1 min). These results evidence a new kind of dissipative system where the crystalline state can be maintained only upon energy supply.     

A Two‐Dimensional Lamellar Membrane: MXene Nanosheet Stacks

Two‐dimensional (2D) materials are promising candidates for advanced water purification membranes. A new kind of lamellar membrane is based on a stack of 2D MXene nanosheets. Starting from compact Ti₃AlC₂, delaminated nanosheets of the composition Ti₃C₂T_x with the functional groups T (O, OH, and/or F) can be produced by etching and ultrasonication and stapled on a porous support by vacuum filtration. The MXene membrane supported on anodic aluminum oxide (AAO) substrate shows excellent water permeance (more than 1000 L m⁻² h⁻¹ bar⁻¹) and favorable rejection rate (over 90 %) for molecules with sizes larger than 2.5 nm. The water permeance through the MXene membrane is much higher than that of the most membranes with similar rejections. Long‐time operation also reveals the outstanding stability of the MXene membrane for water purification.     

Oriented Two‐Dimensional Porous Organic Cage Crystals

The formation of two‐dimensional (2D) oriented porous organic cage crystals (consisting of imine‐based tetrahedral molecules) on various substrates (such as silicon wafers and glass) by solution‐processing is reported. Insight into the crystallinity, preferred orientation, and cage crystal growth was obtained by experimental and computational techniques. For the first time, structural defects in porous molecular materials were observed directly and the defect concentration could be correlated with crystal growth rate. These oriented crystals suggest potential for future applications, such as solution‐processable molecular crystalline 2D membranes for molecular separations.     

A Generalized Strategy for the Synthesis of Large‐Size Ultrathin Two‐Dimensional Metal Oxide Nanosheets

Two‐dimensional (2D) nanomaterials show unique electrical, mechanical, and catalytic performance owing to their ultrahigh surface‐to‐volume ratio and quantum confinement effects. However, ways to simply synthesize 2D metal oxide nanosheets through a general and facile method is still a big challenge. Herein, we report a generalized and facile strategy to synthesize large‐size ultrathin 2D metal oxide nanosheets by using graphene oxide (GO) as a template in a wet‐chemical system. Notably, the novel strategy mainly relies on accurately controlling the balance between heterogeneous growth and nucleation of metal oxides on the surface of GO, which is independent on the individual character of the metal elements. Therefore, ultrathin nanosheets of various metal oxides, including those from both main‐group and transition elements, can be synthesized with large size. The ultrathin 2D metal oxide nanosheets also show controllable thickness and unique surface chemical state.     

On‐Surface Evolution of meso‐Isomerism in Two‐Dimensional Supramolecular Assemblies

Chiral structures created through the adsorption of molecules onto achiral surfaces play pivotal roles in many fields of science and engineering. Here, we present a systematic study of a novel chiral phenomenon on a surface in terms of organizational chirality, that is, meso‐isomerism, through coverage‐driven hierarchical polymorphic transitions of supramolecular assemblies of highly symmetric π‐conjugated molecules. Four coverage‐dependent phases of dehydrobenzo[12]annulene were uniformly fabricated on Ag(111), exhibiting unique chiral characteristics from the single‐molecule level to two‐dimensional supramolecular assemblies. All coverage‐driven phase transitions stem from adsorption‐induced pseudo‐diastereomerism, and our observation of a lemniscate‐type (∞) supramolecular configuration clearly reveals a drastic chiral phase transition from an enantiomeric chiral domain to a meso‐isomeric achiral domain. These findings provide new insights into controlling two‐dimensional chiral architectures on surfaces.     

Cryogenic Exfoliation of Non‐layered Magnesium into Two‐Dimensional Crystals

Physical exfoliation of layered precursors is one of the most prevailing techniques to prepare two‐dimensional (2D) crystals, which, however, is considered to be intrinsically inapplicable to non‐layered bulks. Now, plane cleavage differentiation is identified in metallic magnesium at cryogenic temperature (CT), and a cryogenic exfoliation strategy of non‐layered magnesium into 2D crystals is developed. The cleavage anisotropy of the Mg lattice in response to the external mechanical stress originates from the CT‐induced specific inactivation of basal slip, which results in the basal cleavage perpendicular to c axis. The exfoliated novel 2D Mg crystals exhibit remarkable localized surface plasmon resonances, holding great promise for the applications in harvesting and converting solar energy. Beyond creating a new member for the burgeoning 2D family, this study may provide a useful tool for the physical exfoliations of various non‐layered materials.