Carbon Monolayer Research Articles

Novel adjustable monolayer carbon nitride membranes for high-performance saline water desalination

In this study, via molecular dynamic simulations, we showed that the latest described graphene-like carbon nitride membranes, such as g-C4N3, g-C6N6, and g-C3N4 single-layers, can be used as high-performance membranes for water desalination. In addition to having inherent nanopores and extraordinary mechanical properties, the carbon nitride membranes have high water permeability and strong ion rejection (IR) capability. The important point about carbon nitride membranes is that the open or closed state of the pores can be changed by applying tensile stress and creating a positive strain on the membrane. The effect of the imposed pressure, the tensile strain, the ion concentration, and the effective pore size of the membranes are reported. It is demonstrated that, with the applied tensile strain of 12%, the g-C6N6 membrane is the best purification membrane, with a water permeability of 54.16 l cm−2 d−1 MPa−1 and the IR of 100%. Its water permeability is one order of magnitude greater than other one-atom-thick membranes.

Two-dimensional blue-phase CX (X = S, Se) monolayers with high carrier mobility and tunable photocatalytic water splitting capability

Photocatalytic water splitting utilizing solar energy is considered as one of the most ideal strategies for solving the energy and environmental issues. Recently, two-dimensional (2D) materials with an intrinsic dipole show great chance to achieve excellent photocatalytic performance. In this work, blue-phase monolayer carbon monochalcogenides (CX, X = S, Se) are constructed and systematically studied as photocatalysts for water splitting by performing first-principles calculations based on density functional theory. After confirming the great dynamical, thermal, and mechanical stability of CX monolayers, we observe that they possess moderate band gaps (2.41 eV for CS and 2.46 eV for CSe) and high carrier mobility (3.23 × 104 cm2 V−1 s−1 for CS and 4.27 × 103 cm2 V−1 s−1 for CSe), comparable to those of many recently reported 2D photocatalysts. Moreover, these two monolayer materials are found to have large intrinsic dipole (0.43 D for CS and 0.51 D for CSe), thus the build-in internal electric field can be self-introduced, which can effectively drive the separation of photongenerated carriers. More importantly, the well-aligned band edge as well as rather pronounced optical absorption in the visible-light and ultraviolet regions further ensure that our proposed CX monolayers can be used as high efficient photocatalysts for water splitting. Additionally, the effects of external strain on the electronic, optical and photocatalytic properties of CX monolayers are also evaluated. These theoretical predictions will stimulate further work to open up the energy-related applications of CX monolayers.

Atomic-Level Electronic Properties of Carbon Nitride Monolayers.

Heteroatom-doped carbon-based materials are of significance for clean energy conversion and storage because of their fascinating electronic properties, low cost, high durability, and environmental friendliness. Atomically precise fabrication of carbon-based materials with well-defined heteroatom-dopant positions and atomic-scale understanding of their atomic-level electronic properties is a challenge. Herein, we demonstrate the bottom-up on-surface synthesis of 1D and 2D monolayer carbon nitride nanostructures with precise control of the nitrogen-atom doping sites and pore sizes. We also observe an electronic band offset at the C-N heterojunction. Using high-resolution scanning tunneling microscopy, the atomic structure of the as-prepared carbon nitride nanoporous monolayers are revealed, indicating successful and precise control of the structures and N atom doping sites. Furthermore, corroborated by theoretical calculations, scanning tunneling spectroscopy measurements reveal a valence band shift of 140 meV that results in an electric field of 2.9 × 108 V m-1 at the C-N heterojunction, indicating efficient separation of the electron-hole pair at the N doping site. Our finding offers direct atomic-level insights into the local electronic structure of the heteroatom-doped carbon-based materials.

Confining Ultrathin 2D Superlattices in Mesoporous Hollow Spheres Renders Ultrafast and High‐Capacity Na‐Ion Storage

AbstractSodium‐ion batteries have attracted ever‐increasing attention in view of the natural abundance of sodium resources. Sluggish sodiation kinetics, nevertheless, remain a tough challenge, in terms of achieving high rate capability and high energy density. Herein, a sheet‐in‐sphere nanoconfiguration of 2D titania–carbon superlattices vertically aligned inside of mesoporous TiO2@C hollow nanospheres is constructed. In such a design, the ultrathin 2D superlattices consist of ordered alternating monolayers of titania and carbon, enabling interpenetrating pathways for rapid transport of electrons and Na+ ions as well as a 2D heterointerface for Na+ storage. Kinetics analysis discloses that the combination of 2D heterointerface and mesoporosity results an intercalation pseudocapacitive charge storage mechanism, which triggers ultrafast sodiation kinetics. In situ transmission electron microscope imaging and in situ synchrotron X‐ray diffraction techniques elucidate that the sheet‐in‐sphere architecture can maintain robust mechanical and crystallographic structural stability, resulting an extraordinary high rate capability, remarkable stable cycling with a low capacity fading ratio of 0.04% per cycle over 500 cycles at 0.2 C, and exceptionally long‐term cyclability up to 20 000 cycles at 50 C. This study offers a method for the realization of a high power density and long‐term cyclability battery by designing of a hierarchical nanoarchitecture.

Monolayer carbon nanoshells by pyrolysis of organics

Monolayer carbon materials, including monolayer graphene, carbon nanohorn, single-wall nanotube, monolayer giant fullerenes and monolayer amorphous carbon, have been attracting great interest due to their unique physicochemical properties, and new methods for facile and large scale preparation of monolayer carbons are challenging and of great importance. Herein, N-P-doped monolayer carbon foam were prepared by a direct pyrolysis method. The aerogel-like carbon foam was composed of connected and/or fused monolayer carbon nanoshells with the size of 2−6 nm, and the hierarchically porous structure gave rise to an extremely large surface area of 2570 m2 g−1. The carbon foam was preliminarily tested as catalyst support and negative electrode material for lithium ion battery and exhibited good performance. This study would inspire further study on formation mechanism of monolayer carbon, as well as modification and functionalization of the monolayer carbon nanoshells for application in catalysis and energy conversion and storage.

Oxidized Si terminated diamond and its MOSFET operation with SiO2 gate insulator

During selective epitaxial growth of diamond through SiO2 masks, silicon terminations were formed on a diamond surface by replacing oxygen terminations under the masks. The high temperature of selective growth and its reductive atmosphere possibly allowed Si atoms in SiO2 to interact with the diamond surface, resulting in silicon terminated diamond (C–Si diamond) composed of a monolayer or thin multi-layers of carbon and silicon bonds on diamond. Diamond metal oxide semiconductor field effect transistors (MOSFETs), with a C–Si diamond channel and selectively grown undoped or heavily boron-doped (p+) source/drain (S/D) layers, have been fabricated. Both the MOSFETs with undoped and p+ S/D exhibited enhancement mode (normally off) FET characteristics. The drain current (IDS) of the undoped device reached −17 mA/mm with threshold voltage (VT) −19 V; the p+ device attained a high IDS −165 mA/mm with a VT of −6 V being one of the best normally off diamond FETs. Transmission electron microscopy and energy dispersive x-ray spectroscopy confirmed the presence of C–Si diamond under the SiO2 masking area. The field effect mobility and interface state density at the C–Si/SiO2 (220 nm)/Al2O3 (100 nm) MOS capacitor are 102 cm2 V−1 s−1 and 4.6 × 1012 cm−2 eV−1, respectively. The MOSFET operation of C–Si diamond provides an alternative approach for diamond.

Achieving superior high-capacity K-ion batteries with the C57 carbon monolayer anode by first-principles calculations

K-ion batteries (KIBs) have attracted much attention due to their low cost, high operating voltage and fast ionic conductivity in the electrolyte. However, the development of KIBs lags because the ion size and mass of K+are larger than Li+and Na+. Benefitting from the small molar weight of carbon and its structure diversity, the two-dimensional carbon allotropes can be considered as a potential electrode material for batteries with large-radius ions. In the paper, we report a metallic carbon allotrope (C57 monolayer) as an excellent candidate for the anode material of KIBs. The low diffusion barrier and small OCV values indicate that the C57 monolayer has excellent mobility properties and a large driving force for K-ion diffusion in the charge/discharge process. Most remarkably, the maximum capacity up to 1117 mA h g−1is three times as high as the capacity of graphite-based commercial electrode. Based on our results, the C57 monolayer is ideal for an excellent anode material for KIBs.

Temperature-Programmed Growth of Quasi-Free-Standing N-Doped Graphene Single Crystals from Acetonitrile Molecules

This paper presents an original technique for temperature-programmed growth of N-doped substrate-sized graphene single crystals on Ni(111). The process includes acetonitrile adsorption at about −10°C, flash heating of the sample to 140°C, and annealing at 400°C to form a continuous graphene-structure epitaxial carbon monolayer. The intercalation of gold under the carbon layer on Ni(111) helps create a quasi-free-standing N-doped graphene single crystal. Scanning tunneling microscopy together with density functional theory calculations have been used to determine the structure of nitrogen centers in the graphene. One finding is that nitrogen can be present in the graphene lattice as individual atoms or two- and three-atom clusters. The nitrogen content in graphene can be from 0.2 to 0.6%.

Continuous Zachariasen carbon monolayers under tensile deformation: Insights from molecular dynamics simulations

From a series of molecular dynamics simulations, we show how the mechanical behaviour of amorphous carbon monolayers can be altered as a function of the underlying network topology. We control both the ring size as well as the ring neighbourhood distribution based on a Monte Carlo dual bond-switching algorithm. This leads to non-planar carbon monolayers whose out-of-plane displacements depend on the level of amorphousness. Subsequent tensile deformation reveals that increasing the network heterogeneity decreases both the tensile strength as well as the stiffness in the elastic range. Furthermore, we investigate the statistics of the elementary plastic events in the amorphous carbon monolayers and report on the power-law distribution of the stress-drops associated with the plastic deformation. The results of this study provide insights into the role of structural disorder of a carbon monolayer and can be used to modify its physical properties.

Tunable Graphene Systems for Water Desalination

AbstractThe desalination technologies of sea water are becoming of importance in providing sustainable fresh water. Graphene, as a fashion of two‐dimensional carbon monolayer, has emerged as a promising candidate to build up the material platform for water desalination. In this review, we summarize the development of tunable graphene systems applied in three kinds of water desalination techniques, including capacitive deionization, solar steam generation, and nanofiltration. The current achievements in this field have been highlighted with discussion of the challenges for desalination technologies.

Selective C-C Coupling by Spatially Confined Dimeric Metal Centers.

SummaryDirect conversion of carbon dioxide (CO2) to high-energy fuels and high-value chemicals is a fascinating sustainable strategy. For most of the current electrocatalysts for CO2 reduction, however, multi-carbon products are inhibited by large overpotentials and low selectivity. Herein, we exploit dispersed 3d transition metal dimers as spatially confined dual reaction centers for selective reduction of CO2 to liquid fuels. Various nitrogenated holey carbon monolayers are shown to be promising templates to stabilize these metal dimers and dictate their electronic structures, allowing precise control of the catalytic activity and product selectivity. By comprehensive first-principles calculations, we screen the suitable transition metal dimers that universally have high activity for ethanol (C2H5OH). Furthermore, remarkable selectivity for C2H5OH against other C1 and C2 products is found for Fe2 dimer anchored on C2N monolayer. The role of electronic coupling between the metal dimer and the carbon substrates is thoroughly elucidated.

Monolayer carbon-encapsulated Mo-doped Ni nanoparticles anchored on single-wall carbon nanotube film for total water splitting

Electrochemical water splitting is regarded as a sustainable and cost-effective route for the production of hydrogen. However, the high-cost and poor stability of traditional rare-earth metal-based electrocatalysts make it difficult to yield hydrogen economically. Here, we report an efficient and durable film electrocatalyst of N-doped monolayer carbon encapsulated Mo-doped ultrafine Ni nanoparticles anchored on single-wall carbon nanotube network (NMoNi/SWCNT) for total water splitting in an alkaline solution. The single layer carbon prevents oxidation of encapsulated Ni and Mo species and facilitates desired electronic structure modulation to achieve a high catalytic activity. Hence, the freestanding NMoNi/SWCNT film catalyst shows low overpotentials of 255 mV and 130 mV to attain a current density of 10 mA cm−2 for oxygen evolution reaction and hydrogen evolution reaction, respectively, with a good stability. More importantly, the NMoNi/SWCNT film only requires a potential of 1.6 V to reach a current density of 20 mA cm−2 when employed as both anode and cathode for a total water splitting.

Coulomb drag between a carbon nanotube and monolayer graphene

We study Coulomb drag in a system consisting of a carbon nanotube (CNT) and monolayer graphene. Within the Fermi liquid theory we calculate the drag resistivity and find that the dimensional mismatch of the system components leads to a dependence of the drag rate on the carrier density, temperature, and spacing, which is substantially different from what is known for graphene double layers. Due to the competing effects of forward and backward scattering, we identify new features of the drag dependence on the electron density, which allows us to control their relative contribution to the drag resistivity.

Synthesizing single-layer diamond

Thin Films The carbon allotropes of diamond and graphene have different types of bonding that lead to their exceptional properties. Bakharev et al. pull off the impressive trick of making a monolayer carbon film that is diamond-like in its bonding. The authors accomplish this by attaching fluorine atoms to the carbon film, creating “F-diamane.” Diamane is a long-sought-after, but challenging to make, material that should have useful properties. F-diamane may find use in a variety of applications, from microelectronics as a semiconductor to a seed material for growing single-crystal diamond films. Nat. Nanotechol. 15 , 59 (2020).

DFT-based modelling of carbon adsorption on the AlN surfaces and influence of point defects on the stability of diamond/AlN interfaces

We discuss the adsorption process of carbon atoms on the AlN{0001} surfaces based on first principles calculations in the framework of density functional theory. Some stable adsorption sites are found on the Al- and N-terminated surfaces that in consequence allow the formation of flat carbon monolayer. Next, by means of the experimentally justified model of the diamond/AlN heterojunction with the growth direction [111] we demonstrate how to improve the energetical stability of the diamond/AlN interfaces by means of certain substitutional defects introduced intentionally to the topmost AlN substrate's layer.

Graphene oxide nanosheets modulate spinal glutamatergic transmission and modify locomotor behaviour in an in vivo zebrafish model.

Graphene oxide (GO), an oxidised form of graphene, is widely used for biomedical applications, due to its dispersibility in water and simple surface chemistry tunability. In particular, small (less than 500 nm in lateral dimension) and thin (1-3 carbon monolayers) graphene oxide nanosheets (s-GO) have been shown to selectively inhibit glutamatergic transmission in neuronal cultures in vitro and in brain explants obtained from animals injected with the nanomaterial. This raises the exciting prospect that s-GO can be developed as a platform for novel nervous system therapeutics. It has not yet been investigated whether the interference of the nanomaterial with neurotransmission may have a downstream outcome in modulation of behaviour depending specifically on the activation of those synapses. To address this problem we use early stage zebrafish as an in vivo model to study the impact of s-GO on nervous system function. Microinjection of s-GO into the embryonic zebrafish spinal cord selectively reduces the excitatory synaptic transmission of the spinal network, monitored in vivo through patch clamp recordings, without affecting spinal cell survival. This effect is accompanied by a perturbation in the swimming activity of larvae, which is the locomotor behaviour generated by the neuronal network of the spinal cord. Such results indicate that the impact of s-GO on glutamate based neuronal transmission is preserved in vivo and can induce changes in animal behaviour. These findings pave the way for use of s-GO as a modulator of nervous system function.

First-principles exploration of superconductivity in MXenes.

MXenes are an emerging class of two-dimensional materials, which in their thinnest limit consist of a monolayer of carbon or nitrogen (X) sandwiched between two transition metal (M) layers. We have systematically searched for superconductivity among MXenes for a range of transition metal elements, based on a full first-principles characterization in combination with the Eliashberg formalism. Thus, we identified six superconducting MXenes: three carbides (Mo2C, W2C and Sc2C) and three nitrides (Mo2N, W2N and Ta2N). The highest critical temperature of ∼16 K is found in Mo2N, for which a successful synthesis method has been established [Urbankowski et al., Nanoscale, 2017, 9, 17722-17730]. Moreover, W2N presents a novel case of competing superconducting and charge density wave phases.

A systematic investigation of the catalytic performances of monolayer carbon nitride nanosheets C1−xNx

Graphitic carbon nitrides (CNs) are potential candidate materials for the electro-catalytic industry due to their unique physical and chemical properties. However, to date, a full understanding of the electro-catalytic properties of CNs is still lacking. Herein, by using density functional theory calculations, we systematically investigate the catalytic performances in the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), N2 reduction reaction (NRR), and CO2 reduction reaction (CO2RR) of monolayer graphitic carbon nitrides (C1-xNx), C3N (x = 1/4), C2N (x = 1/3), and g-C3N4 (x = 4/7). We also evaluated the NRR activity of B doped C1-xNx, and the CO2RR activity of Cu and Pd modified C1-xNx. The cohesive energy and ab initio molecular dynamics (AIMD) results show that C3N, C2N, and g-C3N4 are stable at room temperature. The C3N-C1 site is predicted to deliver the best HER catalytic performance with a reaction Gibbs free energy (ΔGH*) of -0.03 eV (close to the ideal value (0 eV)). Among the studied C1-xNx materials, the C3N-C2 site is predicted to possess a favorable ηOER of 0.82 V for OER. Pure C3N, C2N, and g-C3N4 are not suitable for NRR and CO2RR. Due to the strong hybridization between the N 2p orbital and the B 2p orbital, the NRR performances of B doped BN-C2N, BN-C3N, and BN-g-C3N4 are greatly enhanced, with corresponding overpotential ηNRR of 0.57 V, 0.70 V, and 0.72 V, respectively. The transition metals Cu and Pd can enhance the CO2RR activity of C3N, C2N, and g-C3N4. The limiting potentials UL of pure C3N, C2N, and g-C3N4 are 0.96 V, 0.86 V, and 2.37 V, respectively, while these values are 0.63 V, 0.68 V, and 0.77 V with Cu or Pd modification. This work provides deep understanding of the catalytic properties of monolayer C1-xNx and guidance for synthesizing higher activity catalysts in the future.

Flat Band and Hole-induced Ferromagnetism in a Novel Carbon Monolayer

In recent experiments, superconductivity and correlated insulating states were observed in twisted bilayer graphene (TBG) with small magic angles, which highlights the importance of the flat bands near Fermi energy. However, the moiré pattern of TBG consists of more than ten thousand carbon atoms that is not easy to handle with conventional methods. By density functional theory calculations, we obtain a flat band at EF in a novel carbon monolayer coined as cyclicgraphdiyne with the unit cell of eighteen atoms. By doping holes into cyclicgraphdiyne to make the flat band partially occupied, we find that cyclicgraphdiyne with 1/8, 1/4, 3/8 and 1/2 hole doping concentration shows ferromagnetism (half-metal) while the case without doping is nonmagnetic, indicating a hole-induced nonmagnetic-ferromagnetic transition. The calculated conductivity of cyclicgraphdiyne with 1/8, 1/4 and 3/8 hole doping concentration is much higher than that without doping or with 1/2 hole doping. These results make cyclicgraphdiyne really attractive. By studying several carbon monolayers, we find that a perfect flat band may occur in the lattices with both separated or corner-connected triangular motifs with only including nearest-neighboring hopping of electrons, and the dispersion of flat band can be tuned by next-nearest-neighboring hopping. Our results shed insightful light on the formation of flat band in TBG. The present study also poses an alternative way to manipulate magnetism through doping flat band in carbon materials.

Electrically tunable graphene-based metamaterials: A brief review

Metamaterials (MMs) represent a group of exciting artificial materials that interact with electromagnetic waves in unnatural ways. The motivation behind MM research arises not only from fundamental interest in their unique physical properties but also from the desire of creating smarter materials for advanced technological applications. Despite an abundance of studies on numerous shapes, sizes and operating frequencies, the use of conventional metal-dielectric components makes the post-fabrication physical properties of MMs unalterable. Therefore, the integration of other nonlinear materials is necessary for exploring the functional limits of MMs. In this regard, a mono-layer of carbon, the so-called graphene, with its unique electrical conductivity is identified as a promising candidate. This review discusses the recent progress on tunable graphene-based THz MMs for perfect absorption and electromagnetically-induced transparency effects. A short overview of prospect challenges and tendencies is also given for future development of graphene-integrated MMs towards upcoming smart meta-devices.