Crystallization of a Two‐Dimensional Hydrogen‐Bonded Molecular Assembly: Evolution of the Local Structure Resolved by Atomic Force Microscopy

Structures of the aromatic N‐heterocyclic hexaazatriphenylene (HAT) molecular synthon obtained by surface‐assisted self‐assembly were analyzed with sub‐Å resolution by means of noncontact atomic force microscopy (nc‐AFM), both in the kinetically trapped amorphous state and in the thermodynamically stable crystalline phase. These results reveal how the crystallization governs the length scale of the network order for non‐flexible molecular species without affecting the local bonding schemes. The capability of nc‐AFM to accurately resolve structural relaxations will be highly relevant for the characterization of vitreous two‐dimensional supramolecular materials.     

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.     

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.     

HPCO—A Phosphorus‐Containing Analogue of Isocyanic Acid

We describe the isolation and spectroscopic characterization of the heavier phosphorus‐containing analogue of isocyanic acid (HPCO), and its isotopologue (DPCO). This fundamental small molecule, which has been postulated to exist in interstellar space, has thus far only been observed at low gas phase concentrations or in inert gas matrices. In this report we describe its synthesis, spectroscopic properties, and reactivity in solution.     

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.     

Phase Transformation Behavior of a Two‐Dimensional Zeolite

Understanding the molecular‐level mechanisms of phase transformation in solids is of fundamental interest for functional materials such as zeolites. Two‐dimensional (2D) zeolites, when used as shape‐selective catalysts, can offer improved access to the catalytically active sites and a shortened diffusion length in comparison with their 3D analogues. However, few materials are known to maintain both their intralayer microporosity and structure during calcination for organic structure‐directing agent (SDA) removal. Herein we report that PST‐9, a new 2D zeolite which has been synthesized via the multiple inorganic cation approach and fulfills the requirements for true layered zeolites, can be transformed into the small‐pore zeolite EU‐12 under its crystallization conditions through the single‐layer folding process, but not through the traditional dissolution/recrystallization route. We also show that zeolite crystal growth pathway can differ according to the type of organic SDAs employed.     

A Dynamic Hydrogen‐Bonded Azo‐Macrocycle for Precisely Photo‐Controlled Molecular Encapsulation and Release

A light‐responsive system constructed from hydrogen‐bonded azo‐macrocycles demonstrates precisely controlled propensity in molecular encapsulation and release process. A significant decrease in the size of the cavity is observed in the course of the E→Z photoisomerization based on the results from DFT calculations and traveling wave ion mobility mass spectrometry. These macrocyclic hosts exhibit a rare 2:1 host–guest stoichiometry and guest‐dependent slow or fast exchange on the NMR timescale. With the slow host–guest exchange and switchable shape change of the cavity, quantitative release and capture of bipyridinium guests is achieved with the maximum release of 68 %. This work underscores the importance of slow host–guest exchange on realizing accurate release of organic cations in a stepwise manner under light irradiation. The light‐responsive system established here could advance further design of novel photoresponsive molecular switches and mechanically interlocked molecules.     

A Metastable Crystalline Phase in Two‐Dimensional Metallic Oxide Nanoplates

A simple method was adopted in which ultrathin cerium oxide nanoplates (<1.4 nm) were synthesized to increase the surface atomic content, allowing transformation from a face‐centered cubic (fcc) phase to a body‐centered tetragonal (bct) phase. Three types of cerium oxide nanoparticles of different thicknesses (1.2 nm ultrathin nanoplates, 2.2 nm nanoplates, and 5.4 nm nanocubes) were examined using transmission electron microscopy and X‐ray diffraction. The metastable bct phase was observed only in ultrathin nanoplates. Thermodynamic energy analysis confirmed that the surface energy of the ultrathin nanoplates is the cause of the remarkable stabilization of the metastable bct phase. The mechanism of surface energy regulation can be expanded to other metallic oxides, thus providing a new means for manipulating and stabilizing novel materials under ambient conditions that otherwise would not be recovered.     

CO2‐Assisted Fabrication of Two‐Dimensional Amorphous Molybdenum Oxide Nanosheets for Enhanced Plasmon Resonances

As a remarkable class of plasmonic materials, two dimensional (2D) semiconductor compounds have attracted attention owing to their controlled manipulation of plasmon resonances in the visible light spectrum, which outperforms conventional noble metals. However, tuning of plasmonic resonances for 2D semiconductors remains challenging. Herein, we design a novel method to obtain amorphous molybdenum oxide (MoO₃) nanosheets, in which it combines the oxidation of MoS₂ and subsequent supercritical CO₂‐treatment, which is a crucial step for the achievement of amorphous structure of MoO₃. Upon illumination, hydrogen‐doped MoO₃ exhibits tuned surface plasmon resonances in the visible and near‐IR regions. Moreover, a unique behavior of the amorphous MoO₃ nanosheets has been found in an optical biosensing system; there is an optimum plasmon resonance after incubation with different BSA concentrations, suggesting a tunable plasmonic device in the near future.     

Two‐Dimensional Semiconductors Grown by Chemical Vapor Transport

Developing controlled approaches for synthesizing high‐quality two‐dimensional (2D) semiconductors is essential for their practical applications in novel electronics. The application of chemical vapor transport (CVT), an old single‐crystal growth technique, has been extended from growing 3D crystals to synthesizing 2D atomic layers by tuning the growth kinetics. Both single crystalline individual flakes and continuous films of 1 L MoS₂ were successfully obtained with CVT approach at low growth temperatures of 300–600 °C. The obtained 1 L MoS₂ exhibits high crystallinity and comparable mobility to mechanically exfoliated samples, as confirmed by both atomic resolution microscopic imaging and electrical transport measurements. Besides MoS₂, this method was also used in the growth of 2D WS₂, MoSe₂, Mo_x W_1−x S₂ alloys, and ReS₂, thus opening up a new way for the controlled synthesis of various 2D semiconductors.