Carbon Monolayer Research Articles

Structural analysis of an iron-assisted carbon monolayer for delivery of 2-thiouracil

An idea of employing an iron-assisted carbon (FeC) monolayer for delivery of 2-thiouracil (2TU) was examined in this work by analyzing structural features for singular and bimolecular models. Density functional theory (DFT) calculations were performed for optimizing the structures and evaluating molecular and atomic descriptors for analyzing the models systems. Two bimolecular models were obtained assigning by S-FeC and O-FeC models, in which each of S and O atom of 2TU was relaxed towards the Fe region of FeC surface in the mentioned models, respectively. The results indicated that both models were achievable with slightly more favorability for formation of S-FeC model. The obtained molecular orbital properties revealed the dominant role of FeC monolayer for managing future interactions of attached 2TU, which is indeed a major role for employing nanomaterials for targeted drug delivery purposes. In addition to energies and molecular orbital features, atomic quadrupole coupling constants indicated the benefit of employing FeC monolayer for drug delivery of 2TU.

Coulomb Drag between a Carbon Nanotube and Monolayer Graphene.

We have measured Coulomb drag between an individual single-walled carbon nanotube (SWNT) as a one-dimensional (1D) conductor and the two-dimensional (2D) conductor monolayer graphene, separated by a few-atom-thick boron nitride layer. The graphene carrier density is tuned across the charge neutrality point (CNP) by a gate, while the SWNT remains degenerate. At high temperatures, the drag resistance changes sign across the CNP, as expected for momentum transfer from drive to drag layer, and exhibits layer exchange Onsager reciprocity. We find that layer reciprocity is broken near the graphene CNP at low temperatures due to nonlinear drag response associated with temperature dependent drag and thermoelectric effects. The drag resistance shows power-law dependences on temperature and carrier density characteristic of 1D Fermi liquid-2D Dirac fluid drag. The 2D drag signal at high temperatures decays with distance from the 1D source slower than expected for a diffusive current distribution, suggesting additional interaction effects in the graphene in the hydrodynamic transport regime.

DFT-Based Studies on Carbon Adsorption on the wz-GaN Surfaces and the Influence of Point Defects on the Stability of the Diamond-GaN Interfaces.

Integration of diamond with GaN-based high-electron-mobility transistors improves thermal management, influencing the reliability, performance, and lifetime of GaN-based devices. The current GaN-on-diamond integration technology requires precise interface engineering and appropriate interfacial layers. In this respect, we performed first principles calculation on the stability of diamond–GaN interfaces in the framework of density functional theory. Initially, some stable adsorption sites of C atoms were found on the Ga- and N-terminated surfaces that enabled the creation of a flat carbon monolayer. Following this, a model of diamond–GaN heterojunction with the growth direction [111] was constructed based on carbon adsorption results on GaN{0001} surfaces. Finally, we demonstrate the ways of improving the energetic stability of diamond–GaN interfaces by means of certain reconstructions induced by substitutional dopants present in the topmost GaN substrate’s layer.

New effect of strong oscillation and anisotropy of electrical conductance in graphene films with vertically aligned carbon nanotubes and monolayer pillared graphene films

For the first time, a strong anisotropy of electrical conductance is established in graphene films with regular holes, graphene films with vertically aligned armchair carbon nanotubes (CNTs) seamlessly bonded to graphene, and monolayer pillared graphene films. Electrical conductance along the armchair direction is 3–7 times larger than that along the zigzag direction. Also, a prominent oscillation in electrical conductance along the armchair direction is first discovered. This effect is clearly manifested when the CNT length is increased by atomic layers. In this case, the electrical conductance oscillation can reach 80–90% and the oscillation frequency is equal to three.

Enhancement of light-induced resistance effect in the nanostructure of Ag/graphene based on the n-type silicon

The direct coupling of material properties across a nanoscale interface is a promising route to achieve the functionality unavailable in bulk materials. Graphene is a kind of sp2 hybridized carbon monolayer and has been investigated in many applications due to its high charge-carrier mobility. In this paper, a type of enhanced light-induced resistance effect (LRE) is observed in the structure of Ag/graphene/n-type Si. This effect features a remarkable linear resistance change with a sensitivity of 4.39 kΩ/mm when a laser moves along the surface of the structure. With the optimal thickness of the Ag film, the resistance change ratio of LRE can reach 472%, which is significantly higher than the Ag/Si control sample (6.4%), showing an obvious graphene-induced enhancement. Photocarriers' diffusion and recombination at the heterojunction interface are crucial for the enhancement. These findings offer an effective way to study the carrier dynamics at the heterojunction interface and will be useful in the development of graphene-based optoelectronic devices, such as laser-controlled variable resistors, laser-induced diodes, and storage devices.

Improving textural properties of magnesium-based metal-organic framework for gas adsorption by carbon doping

A magnesium-based metal-organic framework (MOF), Mg-MOF-74, has been denoted as a landmark for CO2 capture due to its highly porous structure and strong metal adsorptive sites. However, the gas uptake would be improved if particle texture could favor diffusion into the micropores and accessibility to the adsorption sites. For that reason, multi-wall carbon nanotubes (CNT) or monolayer graphene oxide (GO) was introduced into the MOF material at 0.3 wt% via in-situ synthesis method as an attempt to improve gas adsorption capacities. Mg-MOF-74, [email protected] and [email protected] were synthesized under solvothermal reaction and characterized by PXRD, FTIR, FESEM, TGA and N2@77 K and CO2@298 K adsorption/desorption analysis. The morphology of the Mg-MOF-74 composites showed that the particles were composed of agglomerated crystallites of which the crystal structure was well-preserved with the adjunction of carbon doping agents. The BET specific surface area and the micropore volume of the Mg-MOF-74 pristine material respectively attained 1370 m2/g and 0,4 cm3/g whilst these values were increased up to 1660–1720 m2/g and ̴ 0.49 cm3/g for carbon composites. These variations were attributed to better activation of carbon-MOF composites thanks to more regular arrangement of the crystallites composing the agglomerates, facilitating the evaporation of the organic solvent during the thermal treatment applied in the last stage of the synthesis. The introduction of a small fraction of CNT and GO in the reactant mixture also improves the crystallization process, leading to average larger particle sizes. Consequently, the uptake of CO2 at 1 bar and 25 °C was shown to be improved by 18 mol% and 23 mol% for [email protected] and [email protected], respectively, compared with the pristine Mg-MOF-74. This current work demonstrates that by doping Mg-MOF-74 synthesis with carbonaceous agents, although at very low concentration levels, both morphological and textural properties are modified leading to larger gas adsorption capacities.

Computational assessment of Stone-Wales defects on the elastic modulus and vibration response of graphene sheets

In this work we assess the influence of Stone-Wales defects on the elastic modulus and vibration responses of single-layered graphene sheets. To this end, the atomic position of the C-atoms within the defective layers are inserted as initial conditions into finite element calculations used to estimate the mechanical properties and natural frequencies of carbon monolayers. Moreover, we consider two sets of initial atomic positions: the harmonic solution predicted by discrete dislocation theory and its fully-relaxed configuration, which is a novelty in this type of study. Thus, the mechanical properties of the beam elements are depicted based on the equivalence between the interatomic potential energy of the atomic model and the strain energy of the equivalent continuum model. Furthermore, based on the results, we propose an extension of the classical beam model that accurately defines the mechanical properties of the elements in terms of the actual length of each atomic bond. The finite element model is implemented in the commercial software Abaqus/Standard. The results obtained are in reasonable agreement with previous data presented by other authors. In this work we have specifically ascertained the importance of using a non-linear model to predict the Young’s modulus and vibration responses of defective graphene sheets.

Interaction and dynamics of two nanodroplets separated by monolayer graphene

Understanding the physical mechanism of wetting transparency of monolayer graphene, or how atoms or molecules located on opposite sides of the atomically thin carbon monolayer transmit molecular interactions (such as van der Waals and electrostatic forces) is of significant importance for many extraordinary applications from scientific and practical perspectives. In the current work, we carried out molecular dynamics simulations to explore the interactions between two droplets (TD) separated by the monolayer graphene under applied electric field. The results indicate that in the absence of the electric field the transmission of molecular interactions can drive migration of two droplets towards each other, implying partial wetting transparency of the graphene. Compared to the cases of single droplet (SD), the critical electric field to induce large deformation is lower for the TD cases. However, the droplet–droplet interactions under strong electric fields suppress the reduction of the number of water molecules in the graphene/droplet interface and the elongation of droplets. To understand the enhancement of interactions between two droplets owing to the electric field, we analyze microscopic structures of interfacial water (dipole orientation distribution and hydrogen bonding). The dipole–dipole interaction through the graphene mainly contributes to the droplet–droplet interactions. It was found that significant deflection of water dipoles for the TD cases occurs as the electric field enhances, but the deflection for the SD cases is smaller and even the droplet is detached from the graphene surface. Our findings provide fundamental insights on the influence of the electric field on molecular interactions between two droplets through the monolayer graphene and clues for developing potential graphene-based nanofluidic applications controlled by external electric field.

Alternately Intercalated Heterostructures of Carbon and MoS2 Monolayers as Saturable Absorber for Ultrashort Femtosecond Mode‐Locked Lasers

Abstract2D van der Waals (VDWs) heterostructures have attracted much attention due to their exotic electronic and optical properties, which provide a new platform for material science and device fabrication. In this work, by coupling atomic interface engineering, an alternately intercalated superstructure of carbon and MoS2 monolayers (m‐C/MoS2) is designed as a novel saturable absorber (SA) for mode‐locked ultrafast laser generation. The novel intercalated structures exhibit excellent properties including wide bandgap, fast inter‐ and intra‐band carrier relaxation time, and superior saturable absorption characteristic, etc. Consequently, a continuous wave mode‐locked Yb:CALGO bulk laser with pulse width as short as 92 fs is achieved by using the prepared m‐C/MoS2 SA mirror, which is much shorter than those obtained with other reported 2D materials. This study provides a valuable strategy for the development of ultrashort pulse femtosecond lasers. The novel VDWs heterostructures also open new avenues toward advanced photonic devices.

Roughening for Strengthening and Toughening in Monolayer Carbon Based Composites.

Three-dimensional (3D) aggregation of graphene is dramatically weak and brittle due primarily to the prevailing interlayer van der Waals interaction. In this report, motivated by the recent success in synthesis of monolayer amorphous carbon (MAC) sheets, we demonstrate that outstanding strength and large plastic-like strain can be achieved in layered 3D MAC composites. Both surface roughening and the ultracompliant nature of MACs count for the high strength and gradual failure in 3D MAC. Such properties are not seen when intact graphene or multiple stacked MACs are used as building blocks for 3D composites. This work demonstrates a counterintuitive mechanism that surface roughening due to initial defects and low rigidity may help to realize superb mechanical properties in 3D aggregation of monolayer carbon.

Hierarchical MoS2/m-C@a-C@Ti3C2 nanohybrids as superior electrodes for enhanced sodium storage and hydrogen evolution reaction

• Alternated intercalation of MoS 2 and carbon monolayers was achieved on Ti 3 C 2 . • Novel structures and strong interfacial coupling insure charge and ion transfer. • The nanohybrids exhibit excellent electrochemical performance for SIBs and HER. Development of sodium-ion batteries (SIBs) and hydrogen evolution reaction (HER) technologies to achieve electrochemical energy storage and conversion has attracted intensive interest. MoS 2 is bifunctionally active towards both SIBs and HER. However, poor electrical conductivity, limited active sites and sluggish ion diffusion kinetics generally give rise to rapid capacity fading, poor cycling stability, and low electrocatalytic activity when used as electrode materials. Herein we designed hierarchical MoS 2 /m-C@a-C@Ti 3 C 2 nanohybrids as electrode materials for SIBs and HER. The nanohybrids are fabricated by vertically anchoring the few-atomic-layered MoS 2 nanosheets, with alternated intercalation of carbon monolayers, on the both sides of carbon-stabilized Ti 3 C 2 substrate. They afford 3D interconnected conductive networks, expanded interlayer distance and strong interfacial coupling, enabling fast charge transfer, abundant active sites, high structural stability and ion diffusion kinetics. Benefited from the synergistic effects, the nanohybrids exhibit ultra-long cycling life of 2000 cycles with high capacities and slow capacity loss per cycle at 2 and 5 A g −1 for Na + storage. They also present high HER activity in both alkaline and acidic solutions (overpotential: 110 and 93 mV ( vs. RHE) at 10 mA cm −2 ). These superior properties demonstrate the great promise of this designed strategy for energy storage and conversion.

G-SixCy as an anode material for potassium-ion batteries insight from first principles

Designing new electrode materials with higher capacity is the key to improve energy density of potassium ion batteries. It is well known that graphene has a low capacity but with high stability, while silicene just opposite. Inspired by this, first-principles calculations were used to evaluate whether siligraphene, the combination of monolayer silicon and monolayer carbon is suitable for Potassium-ion batteries (KIBs) anode material. Our results show that siligraphene not only own high stability of graphene but also own high capacity of silicene. And the theoretic capacity of g-SiC5 and g-Si2C4 are 304.1 mA h/g and 128.6 mA h/g respectively for KIB. The average voltage is about 0.4 V, 0.3 V for g-SiC5 and g-Si2C4. Meanwhile, based on the NEB calculations the low energy barrier of potassium ions diffused on the 2D-siligraphene, with the diffusion barrier is 132 meV for Si2C4 and 23 meV for SiC5, show that this kind of anode materials might process a high migration rate; moreover, from the bandstructure calculations we might deduced that after absorbing potassium ion, siligraphene show excellent electrical conductivity. The molecular dynamics (MD) calculations were also show good stability of siligraphene absorbed potassium ion. Thus, siligraphene is promising anode material candidate for KIBs.

Nanosleeves: Morphology transitions of infilled carbon nanotubes

Morphology instability of substrate-supported carbon atomic layers can be harnessed to modulate physical properties and functions, which has drawn interesting attention. Curvature would be a critical factor affecting surface morphology and its stability characteristics. Infilled carbon nanotubes, that is to say carbon monolayers with curved geometry and infilled substrates, namely nanosleeves, widely exist in the literature and have many potential applications. Here, we reveal an unprecedented rich post-buckling phenomenon of nanosleeves, which involves multiple successive bifurcations: smooth-wrinkle-ridge-sagging transitions. The nanosleeve initially buckles into periodic axisymmetric wrinkles at the threshold and then evolves into a localized mode with one single ridge growing upon further axial compression. Finally, the amplitude of the ridge reaches a limit and the symmetry is broken with the ridge sagging into a recumbent fold that can be axisymmetric or non-axisymmetric. Such rich morphology transition is first revealed by molecular dynamics simulation, and then theoretically predicted by a finite strain core–shell continuum model. An analytical solution of the critical threshold of primary smooth-wrinkle transition is further provided, which explicitly indicates the crucial role played by curvature and van der Waals interaction between the carbon monolayers and infilled metals. Such multiple-bifurcation scenario and curvature-determined mechanism are found to be inherently universal.

Copper−iron dimer for selective C–C coupling in electrochemical CO2 reduction

The electrochemical CO2 reduction to fuels and chemicals using renewable electricity provides a promising strategy for resolving energy environmental crisis and achieving carbon neutral. Progress has been made on catalyst design for CO2 conversion to high valued products. However, the efficient production of multi−carbon compounds is very challenging due to low selectivity and high overpotential. Understanding of catalytic mechanism at the atom level is the key to developing high−performance CO2 reduction catalysts. Herein, employing comprehensive density functional theory computations, we systematically investigated the structures, reaction intermediates, CO2 reduction mechanisms, and the selectivity of state−of−art catalysts—heteronuclear CuFe dimers anchored on nitrogenated carbon monolayers as the CO2 reduction electrocatalysts. The results show that the strong binding between cooperative CuFe dimer and nitrogenated carbon matrix not only prevents the metal from clustering but also dictates favorable electronic structures, that explains their superior CO2 reduction activity, high C2H5OH selectivity and the mechanism of hydrogen evolution inhibition.

First-principles study of two-dimensional puckered and buckled honeycomb-like carbon sulfur systems

The stability and geometrical, mechanical, and electronic properties of monolayer carbon sulfur (CS) systems with honeycomb-like structure are studied by using first-principles calculations based on density functional theory. The results demonstrate that the honeycomb-like CS systems with two types of structure (buckled and puckered) are quite stable. It is found that such puckered and buckled CS nanosheets possess indirect gaps of 1.952 and 2.325 eV, respectively. Interestingly, both puckered and buckled CS monolayer materials exhibit negative out-of-plane Poisson’s ratio, most likely originating from their puckered and buckled nature. Moreover, their bandgap can be effectively modulated by altering bond and bond lengths under uni- and biaxial strains. Meanwhile, an indirect-to-direct gap transition occurs in both monolayers, which is a useful feature for addressing the electron transition difficulties observed for intrinsic indirect nanosheets, thereby enabling their use in solar cells and light-emitting diodes. In particular, the Dirac-like cones of the puckered CS monolayer are identified in the band structure on the application of appropriate zigzag tensile strain. The results of the present calculations are very interesting for the selection of monolayer CS materials for use in electronic devices and provide a promising method to engineer their electronic characteristics for potential applications in future electronic and optoelectronic devices.

Electron-phonon coupling origin of the graphene π* -band kink via isotope effect

The ${\ensuremath{\pi}}^{*}$-band renormalization of Li-doped quasifreestanding graphene has been investigated by means of isotope ($^{13}\mathrm{C}$) substitution and angle-resolved photoemission spectroscopy. The well documented sudden slope change (known as ``kink'') located at 169 meV from the Fermi level in the graphene made of $^{12}\mathrm{C}$ atoms shifts to 162 meV once the carbon monolayer is composed by $^{13}\mathrm{C}$ isotope. Such an energy shift is in excellent agreement with the expected softening of the phonon energy distribution due to the isotope substitution and provides, therefore, an indisputable experimental proof of the electron-phonon coupling origin of this well known many-body feature in the electronic structure of graphene.

A reactive molecular dynamics study on the mechanical properties of a recently synthesized amorphous carbon monolayer converted into a nanotube/nanoscroll.

Recently, laser-assisted chemical vapor deposition has been used to synthesize a free-standing, continuous, and stable monolayer amorphous carbon (MAC). MAC is a pure carbon structure composed of randomly distributed five, six, seven, and eight atom rings, which is different from that of disordered graphene. More recently, amorphous MAC-based nanotubes (a-CNT) and nanoscrolls (a-CNS) were proposed. In this work, we have investigated (through fully atomistic reactive molecular dynamics simulations) the mechanical properties and sublimation points of pristine and a-CNT and a-CNS. The results showed that a-CNT and a-CNS have distinct elastic properties and fracture patterns compared to those of their pristine analogs. Both a-CNT and a-CNS presented a non-elastic regime before their total rupture, whereas the CNT and CNS underwent a direct conversion to fractured forms after a critical strain threshold. The critical strain values for the fracture of the a-CNT and a-CNS are about 30% and 25%, respectively, and they are lower than those of the corresponding CNT and CNS cases. Although less resilient to tension, the amorphous tubular structures have similar thermal stability in relation to the pristine cases with sublimation points of 5500 K, 6300 K, 5100 K, and 5900 K for a-CNT, CNT, a-CNS, and CNS, respectively. An interesting result is that the nanostructure behavior is substantially different depending on whether it is a nanotube or a nanoscroll, thus indicating that the topology plays an important role in defining its elastic properties.

Newly discovered graphyne allotrope with rare and robust Dirac node loop.

Two-dimensional (2D) carbon allotropes with topologically nontrivial states are drawing considerable attention owing to their unique physical properties and great potential applications in the next generation of micro-nano devices. In contrast to the numerous Dirac points predicted in 2D carbon allotropes, systems featuring Dirac nodal lines (loops) are still quite rare. Here, by means of first-principles calculation, we report our newly discovered carbon monolayer 123-E8Y24-1 with robust Dirac nodal line states, which possesses a tetragonal lattice with P4/mmm symmetry and contains 8 sp2 carbon atoms (graphene: E8) and 24 sp carbon atoms (grapheyne: Y24) in the crystalline cell. This 2D material is as energetically stable as the recently experimentally synthesized β-graphdiyne, and it is further predicted to be dynamically, mechanically, and also thermodynamically stable. Owing to its intrinsic geometric characteristics, 123-E8Y24-1 also exhibits obvious Young's modulus anisotropy, with a sizable ratio between the maximum and minimum value of up to 5.8. Remarkably, 123-E8Y24-1 presents a semimetal nature and possesses Dirac nodal line states in the electronic band structure, and such behavior could be kept well under external strain between -10.0% and 8.0%. The electronic properties of 123-E8Y24-1 can be carefully confirmed by constructing a tight-binding (TB) model. The findings presented in this paper reveal a novel 2D Dirac nodal loop carbon sheet, providing a new candidate for carbon-based high-speed electronic devices.

Thermal transport in amorphous graphene with varying structural quality

The synthesis of wafer-scale two-dimensional amorphous carbon monolayers has been recently demonstrated. This material presents useful properties when integrated as coating of metals, semiconductors or magnetic materials, such as enabling efficient atomic layer deposition and hence fostering the development of ultracompact technologies. Here we propose a characterization of how the structural degree of amorphousness of such carbon membranes could be controlled by the crystal growth temperature. We also identify how energy is dissipated in this material by a systematic analysis of emerging vibrational modes whose localization increases with the loss of spatial symmetries, resulting in a tunable thermal conductivity varying by more than two orders of magnitude. Our simulations provide some recipe to design most suitable ‘amorphous graphene’ based on the target applications such as ultrathin heat spreaders, energy harvesters or insulating thermal barriers.

2D Octagon-Structure Carbon and Its Polarization Resolved Raman Spectra.

We predict a new phase of two-dimensional carbon with density functional theory (DFT). It was found to be semimetal with two Dirac points. The vibrational properties and the polarization resolved Raman spectra of the carbon monolayer are predicted. There are five Raman active modes: 574 cm−1 (Eg), 1112 cm−1 (B1g), 1186 cm−1 (B2g), 1605 cm−1 (B2g) and 1734 cm−1 (A1g). We consider the incident light wave vector to be perpendicular and parallel to the plane of the carbon monolayer. By calculating Raman tensor of each Raman active mode, we obtained polarization angle dependent Raman intensities. Our results will help materials scientists to identify the existence and orientation of octagon-structure carbon monolayer when they are growing it.