Graphene Analogues Research Articles

Dual-conductive metal-organic framework@MXene heterogeneity stabilizes lithium-ion storage

Although a few pristine metal-organic frameworks (MOFs) of graphene analogue topology exhibit high intrinsic electrical conductivity, their use in lithium-ion batteries (LIBs) is still hampered by unfavorable Li+ adsorption energy (ΔEa). In this paper, an electroconductive ferrocene-based MOF@MXene heterostructure is built to provide stable anodes for Li+ storage. Charge density difference and planar average potential charge density show substantial redistribution of charges at the interfaces, transferring from MXene to MOF layers. Moreover, density functional theory (DFT) calculations reveal that the interaction between MXene and MOF significantly increases the ΔEa. As a result, the heterostructure anode exhibits high capacities and outstanding cycling stability with a capacity retention of 80% after 5000 cycles at 5 A g−1, outperforming mono-component MXene and MOF. Furthermore, the heterostructure anode is built into a full cell with a commercial NCM 532 cathode, delivering a high energy density of 611 Wh kg−1 and power density of 7600 W kg−1. The developed conductive MOF@MXene heterogeneity for improved LIB offers valuable insights into the design of advanced electrode materials for energy storage.

Asymptotics and Summation of the Effective Properties of Suspensions, Simple Liquids and Composites

We review the problem of summation for a very short truncation of a power series by means of special resummation techniques inspired by the field-theoretical renormalization group. Effective viscosity (EV) of active and passive suspensions is studied by means of a special algebraic renormalization approach applied to the first and second-order expansions in volume fractions of particles. EV of the 2D and 3D passive suspensions is analysed by means of various self-similar approximants such as iterated roots, exponential approximants, super-exponential approximants and root approximants. General formulae for all concentrations are derived. A brief introduction to the rheology of micro-swimmers is given. Microscopic expressions for the intrinsic viscosity of the active system of puller-like microswimmers are obtained. Special attention is given to the problem of the calculation of the critical indices and amplitudes of the EV and to the sedimentation rate in the vicinity of known critical points. Critical indices are calculated from the short truncation by means of minimal difference and minimal derivative conditions on the fixed points imposed directly on the critical properties. Accurate expressions are presented for the non-local diffusion coefficient of a simple liquid in the vicinity of a critical point. Extensions and corrections to the celebrated Kawasaki formula are discussed. We also discuss the effective conductivity for the classical analog of graphene and calculate the effective critical index for superconductivity dependent on the concentration of vacancies. Finally, we discuss the effective conductivity of a random 3D composite and calculate the superconductivity critical index of a random 3D composite.

A graphyne spoked wheel

<h2>Summary</h2> γ-Graphyne is an emerging carbon allotrope, and synthesis of large-size atomically precise γ-graphyne fragments would be crucial to understand its distinctive properties. Herein, a long-sought-after graphyne spoked wheel with 6-fold symmetry was synthesized mainly through cobalt(0)-catalyzed cyclotrimerization of a triyne intermediate followed by 12-fold intramolecular Stille coupling reactions. Its structure was unambiguously confirmed by X-ray crystallographic analysis, which revealed a closely stacked dimer structure. NMR and theoretical analysis disclosed that fusion of six anti-aromatic hexadehydro[12]annulene units led to a decrease in local aromaticity of all seven benzenoid rings (particularly, the central one), which is quite different from a graphene analog, hexa-<i>peri</i>-hexabenzocoronene (HBC). Its optical and electrochemical properties are also different from those of HBC, showing allowed HOMO → LUMO electronic transition and facile electrochemical reduction. This study provided a new method for the synthesis of even larger γ-graphyne fragments and demonstrated the unique electronic properties and packing structure of γ-graphyne as compared with those of graphene.

A novel chemical approach for synthesizing highly porous graphene analogue and its composite with Ag nanoparticles for efficient electrochemical oxygen reduction

Single-step phase transformation from amorphous to crystalline material avoiding drastic reaction condition is a novel approach in the synthetic and applied materials chemistry research. Both conjugated microporous polymers (CMPs) and graphene/graphene analogues are very demanding materials in the energy research. Depending upon the nature of the graphene material like amorphous, crystalline, functionalized, single-layered and multi-layered, its property and potential application can be largely tuned. In this perspective, 2D carbon allotropes like graphene and its analogues have attracted a huge attention due to their unique electrical, thermal, mechanical, and physico-chemical properties. On the other hand, CMPs are another class of porous nanomaterial synthesized through the polycondensation/coupling of reactive monomeric building units via bottom-up chemistry approach, where extended π-conjugation prevails in the entire organic framework. To the best of our knowledge, herein we first demonstrated a novel methodology for amorphous CMP to crystalline graphene analogue through single step chemical transformation avoiding drastic reaction conditions. For this purpose, we have synthesized a new pyrene-naphthalene based spherical amorphous conjugated microporous polymer, designated as pyrene-naphthalene CMP (PNC), which upon chemical treatment (oxidative C-C coupling reaction) transforms into a highly porous crystalline multilayer porous nanographene sheet-like material pyrene-naphthalene-graphene (PNG). The BET surface area has been drastically enhanced from 600 m2/g for the CMP to 1414 m2/g for this graphene analogue PNG. As analysed by scanning tunnelling spectroscopy (STS), the PNG material has showed atomic level graphitic images with almost zero bandgap, which suggested highly conductive nature for this crystalline graphene analogue. This material has been further explored in the electrochemical oxygen reduction reaction (ORR) by grafting silver nanoparticles (AgNPs) onto the interlayer nanosheets of PNG. The ORR results displayed an admirable half-wave potential (E1/2) and onset potential of 0.78 V and 0.984 V, respectively. The electrocatalyst achieved a very high current density of −0.53 mA cm−2 under the basic electrolyte medium (0.1 M KOH). An efficient ORR catalyst is very demanding towards the fabrication of metal-air batteries as well as fuel cells, which are promising for clean and green energy production.

Frequency-dependent surface wave suppression at the Dirac point of an acoustic graphene analog

Dirac points in the band structure of acoustic systems are essential features affording classical analogs of quantum condensed matter states. We show that measured dispersion curves near and at the Dirac point of an acoustic graphene analog can be suppressed by strong variations in the impedance boundary between free field and surface wave regimes under certain conditions. Increased Rayleigh scattering and diffractive excitation are shown to increase the dispersed surface wave pressure amplitude, circumventing the impedance-based wave suppression. The improved excitation and scattering conditions for observing acoustic Dirac points for two samples with two distinct operational frequency ranges are reported.

Theoretical Study on the High HER/OER Electrocatalytic Activities of 2D GeSi, SnSi, and SnGe Monolayers and Further Improvement by Imposing Biaxial Strain or Doping Heteroatoms.

Under the DFT calculations, two-dimensional (2D) GeSi, SnSi, and SnGe monolayers, considered as the structural analogues of famous graphene, are confirmed to be dynamically, mechanically and thermodynamically stable, and all of them can also possess good conductivity. Furthermore, we systematically investigate their electrocatalytic activities in overall water splitting. The SnSi monolayer can show good HER catalytic activity, while the SnGe monolayer can display remarkable OER catalytic activity. In particular, the GeSi monolayer can even exhibit excellent bifunctional HER/OER electrocatalytic activities. In addition, applying the biaxial strain or doping heteroatoms (especially P atom) can be regarded as the effective strategies to further improve the HER activities of these three 2D monolayers. The doped GeSi and SnSi systems can usually exhibit higher HER activity than the doped SnGe systems. The correlative catalytic mechanisms are also analyzed. This work could open up a new avenue for the development of non-noble-metal-based HER/OER electrocatalysts.

Mixed-Metal-Atom Markers Enable Simultaneous Imaging of Spatial Distribution in Two-Dimensional Heterogeneous Molecular Assembly by Scanning Transmission Electron Microscopy.

Atomic-scale observation by aberration-corrected scanning transmission electron microscopy (STEM) is essential for characterizing supramolecular assemblies with nonperiodic structures. Identifying the relative spatial arrangement in a mixture of molecular species in an assembly is crucial for understanding chemical reaction systems occurring in the assembly. Herein, we report the first direct observation of supramolecular assemblies comprising anionic clay mineral nanosheets and two types of cationic porphyrin complexes with Pt and Pd atom markers by annular dark-field STEM, enabling the simultaneous imaging of well-mixed spatial molecular distributions. The results expand the possibility of applying electron microscopy to self-assembly structures constructed via weak supramolecular interactions on relatively thick nanosheet materials and on one- to few-atom-thick graphene analogues, which will provide important guidelines for future material design.

Study on visible light photocatalytic performance of BiVO4 modified by graphene analogue boron nitride

In this study, a variety of boron nitride (BN) modified BiVO4 (BN–BiVO4) composites with visible-light response were prepared and used to degrade tetracyclines (TCs), including tetracycline (TC) and oxytetracycline (OTC). When treating the TCs solution under visible light irradiation, 4BN-BiVO4 displayed high photocatalytic performance (87.1% for TC and 86.2% for OTC), which were 3.6 and 2.3 times than that of BiVO4, respectively. Photoluminescence spectroscopy (PL) and transient photocurrent proved that the combination of BN and BiVO4 effectively promotes the efficient separation of photogenerated electrons and holes in the material, resulting in enhanced photocatalytic activity. Further, radical trapping experiments in combination with electron spin resonance (ESR) revealed that ·OH radicals and holes were the predominant reactive species. Ultimately, the possible photocatalytic mechanism for TCs degradation was proposed on the basis of the experiments and characterization analysis. This study offers a new promising approach for the design of photocatalysts with visible-light response for efficient TCs elimination.

Strain Tunable Quantum Spin Hall Insulating Phase in Halogen‐Decorated Double‐Layer Stanene

Group IV graphene analogues such as stanene, plumbene, etc. have drawn attention because of their large spin–orbit coupling and thus being promising candidates for topological insulator. However, fabrication of these ultrathin monolayers is a daunting task. To reduce the difficulties of the fabrication of 2D topological insulators, bilayer materials with sizable nontrivial bandgaps can be a viable option. Chemically decorated bilayer stanene is a novel structure which has not been studied before. Herein, the topological characteristics of halogen‐decorated bilayer of stanene are studied and their topological nontriviality is confirmed by calculating the Z2 invariant. As the topological nontriviality can be preserved for these bilayers, these materials will have advantages over their monolayer counterparts in terms of fabrication difficulties. Halogen‐decorated stanene bilayers (except Br) are topologically nontrivial and their nontrivial nature is tunable by external strain. Hybrid functional (HSE06) has been employed to calculate the band structures of the materials at various strains. Also, the band structures of their zigzag nanoribbons have been analyzed. Lastly, the formation energy, quantum molecular dynamics, and phonon dispersion confirm their electronic and structural stability. The proposed material may speed up the fabrication of 2D topological insulators aiming future spintronic and high‐speed electronic devices.

Emerging Graphene Derivatives and Analogues for Efficient Energy Electrocatalysis

AbstractThe demand for highly efficient conversion and storage of renewable energy sources calls for advanced electrocatalytic technologies. 2D graphene has long been investigated as a versatile material platform with flexible structure and tunable surface properties for the design of efficient electrocatalysts. In recent years, new types of graphene derivatives and analogues keep emerging and showing great potential for highly active and selective electrocatalytic applications. In this review, recent progresses on emerging graphene derivatives and analogues for electrocatalysis are summarized. This review starts from the introduction of the material requirements for efficient electrocatalysis and the advantages of 2D graphene derivatives and analogues. Then new advances on graphene derivatives for electrocatalysis are introduced, including twisted graphene superlattices, molecular‐level single‐atom catalysts, and graphene‐supported dual‐atom catalysts or atomic clusters. The next section deals with the emerging research directions of graphene analogues for electrocatalysis, including 2D metal‐free materials, metallenes, 2D transition metal borides (MBenes), and new design strategies of them. Finally, an overview on the development status of this field is provided, and perspectives on the future development of efficient electrocatalytic technologies based on 2D materials are proposed.

Phase transition of layer-stacked borophene under pressure

The 8-$Pmmn$ borophene, a boron analog of graphene, hosts tilted and anisotropic massless Dirac fermion quasiparticles owing to the presence of a distorted graphenelike sublattice. First-principles calculations show that stacked 8-$Pmmn$ borophene is transformed into fused three-dimensional borophene under pressure, being accompanied by partial bond breaking and bond reformation. Strikingly, fused 8-$Pmmn$ borophene inherits the Dirac band dispersion resulting in an unusual semimetal-semimetal transition. A simple tight-binding model derived from graphene qualitatively reveals the underlying physics due to the maximum preservation of the graphenelike substructure after the phase transition, which contrasts greatly to the transformation of graphite into diamond associated with the semimetal-insulator transition.

Twisted pillared phononic crystal plates

Recent discoveries in twisted heterostructure materials have opened research directions in classical wave systems. This Letter investigates a family of double-sided pillared phononic crystal plates as the elastodynamic analog of bilayer graphene, including twisted bilayer graphene. The phononic crystal plate design is first validated by studying the basic AA- and AB-stack configurations under weak interlayer coupling. A specific commensurate twist angle giving rise to the sublattice exchange even symmetry is then studied to examine the twist-modulated band structure. Finally, this study demonstrates that the same twist angle, in concert with an ultra-strong interlayer coupling, can collectively create valley-dependent edge states that have not been previously observed in electronic bilayer graphene.

Chlorine-based non-covalent graphene analog on Cu(111)

Advanced fabrication of specific graphene analogs on surfaces will facilitate the exploitation of unexplored physical properties that may enrich their potential applications in the future, and the quest for graphene analogs has expanded from covalent graphene analogs to non-covalent ones. Previously, artificial non-covalent molecular graphene has been assembled by atomic manipulation, which, however, is a technical challenge and extremely limits the creation of non-covalent graphene analogs over a large area. Herein, we achieve the fabrication of a chlorine(Cl)-based non-covalent graphene analog stabilized by copper(Cu) adatoms on Cu(111) through an easy-to-facilitate self-assembly approach, as demonstrated by the combination of scanning tunneling microscopy imaging and density functional theory calculations. Moreover, the Cu adatoms are found to uniformly distribute within such a non-covalent graphene analog, which is inaccessible for covalent ones and shows potential for stabilizing the non-covalent graphene analog as well as modulating its overall electronic properties. Such findings exemplify the construction of non-covalent graphene analogs with a large area by a more effective self-assembled approach in contrast to the previous atomic manipulation method.

Passivation of black phosphorus nanoflakes embedded in a silica glass matrix affords ambient saturable absorption stability enhancement.

Black phosphorus (BP) is a graphene analogue with ultrafast broadband nonlinear optical properties that make it a promising nanomaterial for saturable absorption. However, BP nanoflakes chemically degrade in ambient conditions. We developed air- and photo-stable BP nanoflakes via incorporation in inorganic-organic hybrid matrices. This realized passivation and materialization through a sol-gel method that produced high-quality, transparent bulk materials. Saturable absorption parameters of the passivated BP were maintained after five months in ambient storage and after 8000 300 µJ nanosecond laser shots. The nonlinear absorption coefficient was still 62% after 12 months in open air, which was higher than that for non-passivated BP after three days. The stability was attributed to dense silica-gel glasses that enveloped the BP, essentially eliminating oxygen and water penetration. The simplicity of this approach may stimulate potential applications for environmentally sensitive high-performance solid-state devices.

Acoustic analog of twisted bilayer graphene

The emergence of twistronics in bilayer graphene has inspired the creation of new phononic structures that translate quantum effects into macro systems. Here, we introduce an acoustic analog of twisted bilayer graphene that is built with 3D-printed star arrays confined in a two-dimensional acoustic waveguide. The lattices on the top and bottom of the waveguide are coupled by spoof surface acoustic waves. Like its quantum counterpart, the twisting angle of the structure influences wave propagation within the system and its resulting band structures. In analytical models, full-wave simulations, and experiments, we observe mode localization at magic twisting angle that hosts the flat bands. We also study other twisted bilayers with different twisting angles to compare with the magic-angle results. The observation of unusual twistronics effects in acoustic systems demonstrates the potential to identify new quantum materials with simplified acoustic models.

Ultralow thermal conductivity and anharmonic rattling in two-dimensional WB4 monolayer

Two-dimensional (2D) WB4 monolayer is a typical graphene analog with high electrical conductivity and structural stability. Yet, its thermal transport properties are not available. By using first-principles calculations and iteratively solving the linearized Boltzmann transport equation, we predict an ultralow in-plane lattice thermal conductivity (κlat) of 0.28 W/m K at T = 300 K. Such an ultralow κlat is attributed to WB4 monolayer's predominantly large phonon scattering rates and flat acoustic phonon dispersion caused by strong anharmonicity. By analyzing the vibrational patterns and bonding environment, we confirm the origin of the strong anharmonicity to be tungsten atom rattling inside the framework of two boron sheets. Such a mechanism fulfills the concept of phonon glass-electron crystal, making the WB4 monolayer an outstanding 2D thermoelectric material. The rich formation mechanism, including multiple interactions in the WB4 monolayer, provides us inspiration for searching for materials with ultralow κlat.

Borophene and Pristine Graphene 2D Sheets as Potential Surfaces for the Adsorption of Electron-Rich and Electron-Deficient π-Systems: A Comparative DFT Study.

The versatility of striped borophene (sB), β12 borophene (β12), and pristine graphene (GN) to adsorb π-systems was comparatively assessed using benzene (BNZ) and hexafluorobenzene (HFB) as electron-rich and electron-deficient aromatic π-systems, respectively. Using the density functional theory (DFT) method, the adsorption process of the π-systems on the investigated 2D sheets in the parallel configuration was observed to have proceeded more favorably than those in the vertical configuration. According to the observations of the Bader charge transfer analysis, the π-system∙∙∙sB complexes were generally recorded with the largest contributions of charge transfer, followed by the π-system∙∙∙β12 and ∙∙∙GN complexes. The band structures of the pure sheets signaled the metallic and semiconductor characters of the sB/β12 and GN surfaces, respectively. In the parallel configuration, the adsorption of both BNZ and HFB showed more valence and conduction bands compared to the adsorption in the vertical configuration, revealing the prominent preferentiality of the anterior configuration. The density-of-states (DOSs) results also affirmed that the adsorption process of the BNZ and HFB on the surface of the investigated 2D sheets increased their electrical properties. In all instances, the sB and β12 surfaces demonstrated higher adsorptivity towards the BNZ and HFB than the GN analog. The findings of this work could make a significant contribution to the deep understanding of the adsorption behavior of aromatic π-systems toward 2D nanomaterials, leading, in turn, to their development of a wide range of applications.

Optical conductivity and far-field radiation of silicene

Silicene is a silicon analogue of graphene. From symmetry considerations, we develop the matrix of Hamiltonian, including spin-orbit coupling in silicene. This work analyzes the heat and angular momentum radiation from silicene through the theory of far-field radiation and optical conductivity. We discuss the relation between the radiation and spin-orbit coupling parameters. Our results show the effects of different parameters, such as temperature and sublattice potential, on optical conductivity and radiation of silicene. In particular, the threshold frequency of optical conductivity causes the appearance of the second peak in energy radiation spectrum. We also find that the ratio of the perpendicular electric field to the intrinsic spin-orbit coupling is related to the strength of angular momentum radiation, which may promote the application of silicene.

Polarization‐ and Angle‐Dependent Plasmonic Synchronous Fluorescence Spectroscopy to Probe Molecular Vibrational Couplings on an Aluminum Nano‐Film

AbstractProbing light−matter interactions at the nanoscale is essential for elucidation of their fundamentals and construction of novel nano‐optical devices. In this work, it is proposed that polarization‐ and angle‐dependent plasmonic synchronous fluorescence spectroscopy could be powerful to resolve UV–vis plasmon mediated molecular couplings, even under vibrational sublevels. Taking studies of polycyclic aromatic hydrocarbons (PAHs, one general class of graphene analogs with emission range of 350–550 nm) as demonstration, a simple but efficient device of PAHs nanolayer (with thickness in tens to hundreds of nanometers) on an Al nano‐film is fabricated. By synchronously scanning excitation and emission wavelengths in a high resolution to depict polarization‐ and angle‐dependent spectrographic profiles, the vibrational transitions coupled to the Al plasmons could be identified, while these could not be attained by conventional fluorescence measurement. As a result, the featured molecular vibrational couplings for both surface plasmon‐ and plasmonic waveguide‐mediated PAHs–Al interactions have been successfully observed at room temperature. Even toward crowded and overlapped spectral components containing multiple vibrations, the transitions could be individually distinguished. These efforts will contribute to the further exploitation and construction of efficient light‐emitting devices via luminant molecules on a metal surface.

THE RISE OF BOROPHENE AND BOROPHENE NANORIBBONS: A POTENTIAL QUANTUM ELECTRONICS MATERIAL

Borophene, a boron analogue of graphene, has been considered as an innovative two-dimensional (2D) material for the scientific and research community. First 2D boron sheet was reported in 2015 by growing it on Ag substrate. Attributable to its exceptional structural, electronic, and spin properties, it has earned enormous exploration interests in the multifaceted fields of material science, nanotechnology and computational physics. Borophene is the newest rising 2D material, which demonstrates various diversified configurations that can be marked by anisotropic and polymorphism. These configurations enabled enhanced tailoring properties of borophene for various applications. This review provides brief discussion on various experimentally synthesized borophene and borophene nanoribbon (BNR), their structural, electronic, and magnetic properties variations along with the width effect. Based upon the type of edges, BNRs are categorized as line edge-BNR (LE-BNR) and zigzag-BNR (ZZ-BNR). Electronic properties of LE-BNR with vacancy defect show semiconductor behavior and ZZ-BNR indicates metallic property. It covers all the structural allotropes, their synthesis, stability and other crucial intrinsic properties, characterization and tailoring of physical properties. Hence, this comprehensive review reported here incorporates the crucial recent experimental and theoretical advances on borophene and BNRs till date. This work also covers recent modifications reported in borophene and BNR to alter their properties as per the demands of various application areas such as nanoelectronic devices, interconnects, biosensors, metal detectors and gas sensors.