Two‐Dimensional Noble‐Metal Chalcogenides and Phosphochalcogenides

Noble‐metal chalcogenides, dichalcogenides, and phosphochalcogenides are an emerging class of two‐dimensional materials. Quantum confinement (number of layers) and defect engineering enables their properties to be tuned over a broad range, including metal‐to‐semiconductor transitions, magnetic ordering, and topological surface states. They possess various polytypes, often of similar formation energy, which can be accessed by selective synthesis approaches. They excel in mechanical, optical, and chemical sensing applications, and feature long‐term air and moisture stability. In this Minireview, we summarize the recent progress in the field of noble‐metal chalcogenides and phosphochalcogenides and highlight the structural complexity and its impact on applications.     

Two‐, One‐, and Zero‐Dimensional Elemental Nanostructures Based on Ge9‐Clusters

We investigated the structural principles of novel germanium modifications derived by oxidative coupling of Zintl‐type [Ge₉]^4?clusters in various ways. The structures, stabilities, and electronic properties of the predicted {²_∞[Ge₉]_n} sheet, {¹_∞[Ge₉]_n} nanotubes, and fullerene‐like {Ge₉}_n cages were studied by using quantum chemical methods. The polyhedral {Ge₉}_n cages are energetically comparable with bulk‐like nanostructures of the same size, in good agreement with previous experimental findings. Three‐dimensional structures derived from the structures of lower dimensionality are expected to shed light on the structural characteristics of the existing mesoporous Ge materials that possess promising optoelectronic properties. Furthermore, 3D networks derived from the polyhedral {Ge₉}_n cages lead to structures that are closely related to the well‐known LTA zeolite framework, suggesting further possibilities for deriving novel mesoporous modifications of germanium. Raman and IR spectra and simulated X‐ray diffraction patterns of the predicted materials are given to facilitate comparisons with experimental results. The studied novel germanium modifications are semiconducting, and several structure types possess noticeably larger band gaps than bulk α‐Ge.     

Two‐Dimensional Boron Hydride Sheets: High Stability,Massless Dirac Fermions,and Excellent Mechanical Properties

Two‐dimensional (2D) boron sheets have been successfully synthesized in recent experiments, however, some important issues remain, including the dynamical instability, high energy, and the active surface of the sheets. In an attempt to stabilize 2D boron layers, we have used density functional theory and global minimum search with the particle‐swarm optimization method to predict four stable 2D boron hydride layers, namely the C2/m, Pbcm, Cmmm, and Pmmn sheets. The vibrational normal mode calculations reveal all these structures are dynamically stable, indicating potential for successful experimental synthesis. The calculated Young's modulus indicates a high mechanical strength for the C2/m and Pbcm phases. Most importantly, the C2/m, Pbcm, and Pmmn structures exhibit Dirac cones with massless Dirac fermions and the Fermi velocities for the Pbcm and Cmmm structures are even higher than that of graphene. The Cmmm phase is reported as the first discovery of Dirac ring material among boron‐based 2D structures. The unique electronic structure of the 2D boron hydride sheets makes them ideal for nanoelectronics applications.     

Phase Engineering of Two‐Dimensional Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDs) possess a large number of two‐dimensional (2D) materials with novel physical and chemical properties and hold great potential applications in electronic devices, optical devices as well as catalysts. TMDs usually have poly‐phases, such as 2H, 3R and 1T. Chemical and physical properties, including electrical conductivity, superconductivity, magnetism and catalytic activity, are different for different phases of TMDs. Therefore, great efforts have been made to obtain a specific pure phase of 2D TMD materials. Here, we review the recent phase engineering research for 2D TMDs, including ion insertion, alloying, temperature, defects, strain and electric field.     

Functionalization of Liquid‐Exfoliated Two‐Dimensional 2H‐MoS2

Layered two‐dimensional (2D) inorganic transition‐metal dichalchogenides (TMDs) have attracted great interest as a result of their potential application in optoelectronics, catalysis, and medicine. However, methods to functionalize and process such 2D TMDs remain scarce. We have established a facile route towards functionalized layered MoS₂. We found that the reaction of liquid‐exfoliated 2D MoS₂, with M(OAc)₂ salts (M=Ni, Cu, Zn; OAc=acetate) yielded functionalized MoS₂–M(OAc)₂ materials. Importantly, this method furnished the 2H‐polytype of MoS₂ which is a semiconductor. X‐ray photoelectron spectroscopy (XPS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFT–IR), and thermogravimetric analysis (TGA) provide strong evidence for the coordination of MoS₂ surface sulfur atoms to the M(OAc)₂ salt. Interestingly, functionalization of 2H‐MoS₂ allows for its dispersion/processing in more conventional laboratory solvents.     

Prediction of Boron–Boron Triple‐Bond Polymers Stabilized by Janus‐Type Bis(N‐heterocyclic) Carbenes

A class of polymeric compounds containing boron–boron triple bonds stabilized by N‐heterocyclic biscarbenes is proposed. Since a triply bonded B₂ is related to its third excited state, the predicted macromolecule would be composed by several units of an electronically excited first‐row homonuclear dimer. Moreover, it is shown that the replacement of biscarbene with N₂ or CO as spacers could change the bonding profile of the boron–boron units to a cumulene‐like structure. Based on these results, different types of diboryne polymers are proposed, which could lead to an unprecedented set of boron materials with distinct physical properties. The novel diboryne macromolecules could be synthesized by the reaction of Janus‐type biscarbenes with tetrabromodiborane, B₂Br₄, and sodium naphthalenide, [Na(C₁₀H₈)], similarly to Braunschweig’s work on the room temperature stable boron–boron triple bond compounds (Science, 2012 , 336, 1420).     

Boron Atoms as Spin Carriers in Two‐ and Three‐Dimensional Systems

To “B” or not to “B” : The unusual bonding of boron in organoboranes or oligoboron clusters is not only apparent in diamagnetic molecules but also in paramagnetic systems, including mixed‐valent species and oligoborane/carborane cluster radicals. The picture shows the singly occupied molecular orbital of the radical ion [C₄B₈R₄H₈]^.?, determined by DFT calculations.

     

Structurally Tunable 3‐Cyanoformazanate Boron Difluoride Dyes

The straightforward synthesis of a series of 3‐cyanoformazanate boron difluoride dyes is reported. Phenyl, 4‐methoxyphenyl and 4‐cyanophenyl N‐substituted derivatives were isolated and characterized by single‐crystal X‐ray crystallography, cyclic voltammetry, and UV/Vis spectroscopy. The compounds were demonstrated to possess tunable, substituent‐dependent absorption, emission, and electrochemical properties, which were rationalized through electronic structure calculations.     

Simultaneously Probing Two Ultrafast Condensed‐Phase Molecular Symmetry Breaking Events by Two‐Dimensional Infrared Spectroscopy

In condensed phases, a highly symmetric gas‐phase molecule lowers its symmetry under perturbation of the solvent, which is vital to a variety of structural chemistry related processes. However, the dynamical aspects of solvent‐mediated symmetry‐breaking events remain largely unknown. Herein, direct evidence for two types of solvent‐mediated symmetry‐breaking events that coexist on the picosecond timescale in a highly symmetric anion, namely, hexacyanocobaltate, is presented: 1) an equilibrium symmetry‐breaking event in which a solvent‐bound species having lowered symmetry undergoes a population exchange reaction with the symmetry‐retaining species; 2) a dynamic symmetry‐breaking event that is composed of many dynamic population‐exchange reactions under fluctuating solvent interactions. Ultrafast two‐dimensional infrared spectroscopy is used to simultaneously observe and dynamically characterize these two events. This work opens a new window into molecular symmetry and structural dynamics under equilibrium and non‐equilibrium conditions.     

Experimental and Theoretical Studies of Quadrupolar Oligothiophene‐Cored Chromophores Containing Dimesitylboryl Moieties as π‐Accepting End‐Groups: Syntheses,Structures, Fluorescence,and One‐ and Two‐Photon Absorption

Quadrupolar oligothiophene chromophores composed of four to five thiophene rings with two terminal (E)‐dimesitylborylvinyl groups ( 4 V – 5 V ), and five thiophene rings with two terminal aryldimesitylboryl groups ( 5 B ), as well as an analogue of 5 V with a central EDOT ring ( 5 VE ), have been synthesized via Pd‐catalyzed cross‐coupling reactions in high yields (66–89 %). Crystal structures of 4 V , 5 B , bithiophene 2 V , and five thiophene‐derived intermediates are reported. Chromophores 4 V , 5 V , 5 B and 5 VE have photoluminescence quantum yields of 0.26–0.29, which are higher than those of the shorter analogues 1 V – 3 V (0.01–0.20), and short fluorescence lifetimes (0.50–1.05 ns). Two‐photon absorption (TPA) spectra have been measured for 2 V – 5 V , 5 B and 5 VE in the range 750–920 nm. The measured TPA cross‐sections for the series 2 V – 5 V increase steadily with length up to a maximum of 1930 GM. We compare the TPA properties of 2 V – 5 V with the related compounds 5 B and 5 VE , giving insight into the structure–property relationship for this class of chromophore. DFT and TD‐DFT results, including calculated TPA spectra, complement the experimental findings and contribute to their interpretation. A comparison to other related thiophene and dimesitylboryl compounds indicates that our design strategy is promising for the synthesis of efficient dyes for two‐photon‐excited fluorescence applications.     

Modelling the Two‐Dimensional Polymerization of 1,4‐Benzene Diboronic Acid on a Ag Surface

Modelling of the two‐dimensional polymerization of 1,4‐benzene diboronic acid molecules on the Ag(111) surface, which leads to the formation of a covalent organic framework, is reported. An estimation of free enthalpy is given that takes into account the constraints induced by the molecular adsorption on the surface. The various thermodynamic functions, enthalpies, entropies, and free enthalpies, are obtained from DFT calculations. The entropic effect of the surface plays an important role in the polymerization free energy. A germination threshold is obtained.