Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocat...

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers with permanent porosity and highly ordered structures. Unlike other polymers, a significant feature of COFs is that they are structurally predesignable, synthetically controllable, and functionally manageable. In principle, the topological design diagram offers geometric guidance for the structural tiling of extended porous polygons, and the polycondensation reactions provide synthetic ways to construct the predesigned primary and high-order structures. Progress over the past decade in the chemistry of these two aspects undoubtedly established the base of the COF field. By virtue of the availability of organic units and the diversity of topologies and linkages, COFs have emerged as a new field of organic materials that offer a powerful molecular platform for complex structural design and tailor-made functional development. Here we target a comprehensive review of the COF field, provide a historic overview of the chemistry of the COF field, survey the advances in the topology design and synthetic reactions, illustrate the structural features and diversities, scrutinize the development and potential of various functions through elucidating structure-function correlations based on interactions with photons, electrons, holes, spins, ions, and molecules, discuss the key fundamental and challenging issues that need to be addressed, and predict the future directions from chemistry, physics, and materials perspectives.

Covalent Organic Frameworks: Design, Synthesis, and Functions.

Although current high-energy-density lithium-ion batteries (LIBs) have taken over the commercial rechargeable battery market, increasing concerns about limited lithium resources, high cost, and insecurity of organic electrolyte scale-up limit their further development. Rechargeable aqueous zinc-ion batteries (ZIBs), an alternative battery chemistry, have paved the way not only for realizing environmentally benign and safe energy storage devices but also for reducing the manufacturing costs of next-generation batteries. This Review underscores recent advances in aqueous ZIBs; these include the design of a highly reversible Zn anode, optimization of the electrolyte, and a wide range of cathode materials and their energy storage mechanisms. We also present recent advanced techniques that aim at overcoming the current issues in aqueous ZIB systems. This Review on the future perspectives and research directions will provide a guide for future aqueous ZIB study.

Recent Advances in Aqueous Zinc-Ion Batteries

Covalent organic frameworks (COFs) are a class of crystalline porous polymer that allows the atomically precise integration of organic units into extended structures with periodic skeletons and ordered nanopores. One important feature of COFs is that they are designable; that is, the geometry and dimensions of the building blocks can be controlled to direct the topological evolution of structural periodicity. The diversity of building blocks and covalent linkage topology schemes make COFs an emerging materials platform for structural control and functional design. Indeed, COF architectures offer confined molecular spaces for the interplay of photons, excitons, electrons, holes, ions and guest molecules, thereby exhibiting unique properties and functions. In this Review, we summarize the major progress in the field of COFs and recent achievements in developing new design principles and synthetic strategies. We highlight cutting-edge functional designs and identify fundamental issues that need to be addressed in conjunction with future research directions from chemistry, physics and materials perspectives. Covalent organic frameworks are crystalline porous polymers with precisely ordered polygon architectures. In this Review we summarize recent advances in the design principles and synthetic reactions, highlight the current status in structural construction and functionality design, and predict challenging issues and future directions.

Covalent organic frameworks: a materials platform for structural and functional designs

The discovery of graphene and other two-dimensional (2D) materials together with recent advances in exfoliation techniques have set the foundations for the manufacturing of single layered sheets from any layered 3D material. The family of 2D materials encompasses a wide selection of compositions including almost all the elements of the periodic table. This derives into a rich variety of electronic properties including metals, semimetals, insulators and semiconductors with direct and indirect band gaps ranging from ultraviolet to infrared throughout the visible range. Thus, they have the potential to play a fundamental role in the future of nanoelectronics, optoelectronics and the assembly of novel ultrathin and flexible devices. We categorize the 2D materials according to their structure, composition and electronic properties. In this review we distinguish atomically thin materials (graphene, silicene, germanene, and their saturated forms; hexagonal boron nitride; silicon carbide), rare earth, semimetals, transition metal chalcogenides and halides, and finally synthetic organic 2D materials, exemplified by 2D covalent organic frameworks. Our exhaustive data collection presented in this Atlas demonstrates the large diversity of electronic properties, including band gaps and electron mobilities. The key points of modern computational approaches applied to 2D materials are presented with special emphasis to cover their range of application, peculiarities and pitfalls.

https://www.rsc.org/suppdata/cs/c4/c4cs00102h/c4cs00102h1.pdf

An atlas of two-dimensional materials

Co3O4 anode materials exhibit poor conductivity and a large volume change, rendering controlling of their nanostructure essential to optimize their lithium storage performance. Carbon-doped Co3O4 hollow nanofibers (C-doped Co3O4 HNFs), for the first time are synthesized using bifunctional polymeric nanofibers as template and carbon source. Compared with undoped Co3O4 HNFs and solid Co3O4 NFs, C-doped Co3O4 HNFs feature a remarkably high specific capacity, excellent cycling stability, and superior rate capacity as anode materials for lithium-ion batteries. The superior performance of C-doped Co3O4 HNFs electrodes can be attributed to their structural features, which confer enhanced electron transportation and Li+ ion diffusion due to C-doping, and tolerance for volume change due to the 1D hollow structure. Density functional theory calculations provide a good explanation of the observed enhanced conductivity in C-doped Co3O4 HNFs.

Template‐Based Engineering of Carbon‐Doped Co3O4 Hollow Nanofibers as Anode Materials for Lithium‐Ion Batteries

The rational design of oxygen evolution reaction (OER) catalysts from the perspective of electronic structure is highly desirable to optimize electrocatalytic activity. Monometallic phosphides such...

Tailoring the d-Band Centers Endows (NixFe1–x)2P Nanosheets with Efficient Oxygen Evolution Catalysis

We demonstrate the strategies and principles for the performance improvement of layered semiconductor based photodetectors using multilayer indium selenide (InSe) as the model material. It is discovered that multiple reflection interference at the interfaces in the phototransistor device leads to a thickness-dependent photo-response, which provides a guideline to improve the performance of layered semiconductor based phototransistors. The responsivity and detectivity of InSe nanosheet phototransistor can be adjustable using applied gate voltage. Our InSe nanosheet phototransistor exhibits ultrahigh responsivity and detectivity. An ultrahigh external photo-responsivity of ∼104 A W−1 can be achieved from broad spectra ranging from UV to near infrared wavelength using our InSe nanosheet photodetectors. The detectivity of multilayer InSe devices is ∼1012 to 1013 Jones, which surpasses that of the currently exploited InGaAs photodetectors (1011 to 1012 Jones). This research shows that multilayer InSe nanosheets are promising materials for high performance photodetectors.

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Ultrahigh photo-responsivity and detectivity in multilayer InSe nanosheets phototransistors with broadband response†

We performed a co-condensation reaction between aromatic aldehyde and aromatic diamine monomers on a highly oriented pyrolytic graphite surface either at a solid/liquid interface at room temperature or in low vacuum with moderate heating. With this simple and moderate methodology, we have obtained surface-confined 2D covalent organic frameworks (COFs) with few defects and almost entire surface coverage. The single crystalline domain can extend to more than 1 μm2. By varying the backbone length of aromatic diamines the pore size of 2D surface COFs is tunable from ∼1.7 to 3.5 nm. In addition, the nature of the surface COF can be modified by introducing functional groups into the aromatic amine precursor, which has been demonstrated by introducing methyl groups to the backbone of the diamine. Formation of small portions of bilayers was observed by both scanning tunneling microscopy (STM) and AFM, which clearly reveals an eclipsed stacking manner.

Xin Zhou

Papers

Template‐Based Engineering of Carbon‐Doped Co3O4 Hollow Nanofibers as Anode Materials for Lithium‐Ion Batteries

Tailoring the d-Band Centers Endows (NixFe1–x)2P Nanosheets with Efficient Oxygen Evolution Catalysis

Ultrahigh photo-responsivity and detectivity in multilayer InSe nanosheets phototransistors with broadband response†

Surface-confined crystalline two-dimensional covalent organic frameworks via on-surface Schiff-base coupling.

A bismuth rich hollow Bi4O5Br2 photocatalyst enables dramatic CO2 reduction activity