Graphene Lattice Research Articles

The effective regulation of heterogeneous N-heterocyclic carbenes: structures, electronic properties and transition metal adsorption.

Heterogeneous N-heterocyclic carbene materials have attracted increasing interest in the fields of materials science and catalysis due to their unique properties and potential applications. However, current heterogeneous systems primarily focus on a single class of carbene. In this work, we simultaneously introduce two classes of typical five-membered carbenes into a graphene lattice, forming a series of novel two-dimensional heterogeneous N-heterocyclic carbene nanomaterials (2D-NCMs) composed of multiple carbenes. First-principles calculations demonstrate the thermodynamic stability of the designed 2D-NCMs, as well as their diverse electronic properties ranging from metallic to semiconducting. The incorporation of carbenes in the 2D-NCMs enables them to adsorb both acidic BCl3 and basic CO molecules, thus exhibiting unique amphoteric properties. Furthermore, the 2D-NCMs exhibit remarkable adsorption capacities for ten transition metals, highlighting their promising potential for future catalytic applications. By adjusting the proportions of the two classes of carbenes, we can effectively regulate the electronic properties and adsorption capacities of small molecules and transition metals in the 2D-NCMs. This study presents a novel strategy for designing and regulating the properties of heterogeneous N-heterocyclic carbenes, offering significant implications in the fields of catalysis and materials science.

Growth of 1D ClAlPc molecular chains mediated by graphene moiré patterns.

The on-surface formation of iso-oriented 1D molecular architectures, with high structural perfection, on 2D materials has been a long-sought objective. However, such realization has been troublesome and limited, and it still remains an experimental challenge. Here, the quasi-1D stripe-like moiré pattern, arising at the interface of graphene grown on Rh(110), has been used to guide the formation of 1D molecular wires of π-conjugated, non-planar, chloro-aluminum phthalocyanine (ClAlPc) molecules, brought together by van der Waals interactions. Using scanning tunnelling microscopy (STM) under ultra-high vacuum (UHV) at 40 K, the preferential adsorption orientations of the molecules at low coverages have been investigated. The results shed light on the potential signature of graphene lattice symmetry breaking, induced by the incommensurate quasi-1D moiré pattern of Gr/Rh(110), as the subtle mechanism behind this templated growth of 1D molecular structures. For coverages close to 1 ML, the molecule-molecule interactions favor a closely packed square lattice arrangement. The present work provides new insights to tailor 1D molecular structures on graphene grown on a non-hexagonal metal substrate.

Advances in solar energy harvesting integrated by van der Waals graphene heterojunctions.

Graphene has garnered increasing attention for solar energy harvesting owing to its unique features. However, limitations hinder its widespread adoption in solar energy harvesting, comprising the band gapless in the molecular orbital of graphene lattice, its vulnerability to oxidation in oxidative environments, and specific toxic properties that require careful consideration during development. Beyond current challenges, researchers have explored doping graphene with ionic liquids to raise the lifespan of solar cells (SCs). Additionally, they have paid attention to optimizing graphene/Si Schottky junction or Schottky barrier SCs by enhancing the conductivity and work function of graphene, improving silicon's reflectivity, and addressing passivation issues at the surface/interface of graphene/Si, resulting in significant advancements in their power conversion efficiency. Increasing the functional area of graphene-based SCs and designing efficient grid electrodes are also crucial for enhancing carrier collection efficiency. Flaws and contaminants present at the interface between graphene and silicon pose significant challenges. Despite the progress of graphene/Si-based photovoltaic cells still needs to catch up to the efficiency achieved by commercially available Si p-n junction SCs. The low Schottky barrier height, design-related challenges associated with transfer techniques, and high lateral resistivity of graphene contribute to this performance gap. To maximize the effectiveness and robustness of graphene/Si-based photovoltaic cells, appropriate interlayers have been utilized to tune the interface and modulate graphene's functionality. This mini-review will address ongoing research and development endeavors using van der Waals graphene heterojunctions, aiming to overcome the existing limitations and unlock graphene's full potential in solar energy harvesting and smart storage systems.

Localized and quasi-localized energy levels in the electron spectrum of graphene with isolated boron and nitrogen substitutions

Based on the calculation and analysis of local Green’s functions of impurity atoms of low concentration in a two-dimensional graphene lattice, the conditions for the formation and characteristics of local discrete levels with energies lying outside the band of the quasi-continuous spectrum and quasi-localized states with energies near the Fermi one are determined. Specific calculations were performed for boron and nitrogen impurity atoms, which can actually replace carbon in graphite and graphene nanostructures. For a boron impurity that forms local discrete levels outside the band of the quasi-continuous spectrum, sufficiently simple analytical expressions for the conditions for their formation, energy, intensity at the impurity atom, and damping parameter are obtained. An analysis of the formation of states quasi-localized on nitrogen impurities with energy near the Fermi level in graphene nanostructures was carried out.

The Fluorination of Boron‐Doped Graphene for CFx Cathode with Ultrahigh Energy Density

The enhancement of the fluorination degree of carbon fluorides (CFx) compounds is the most effective method to improve the energy densities of Li/CFx batteries because the specific capacity of CFx is proportional to the molar ratio of F to C atoms (F/C). In this study, B‐doped graphene (BG) is prepared by using boric acid as the doping source and then the prepared BG is utilized as the starting material for the preparation of CFx. The B‐doping enhances the F/C ratio of CFx without hindering the electrochemical activity of the C–F bond. During the fluorination process, B‐containing functional groups are removed from the graphene lattice. This facilitates the formation of a defect‐rich graphene matrix, which not only enhances the F/C ratio due to abundant perfluorinated groups at the defective edges but also serves as the active site for extra Li+ storage. The prepared CFx exhibits the maximum specific capacity of 1204 mAh g−1, which is 39.2% higher than that of CFx obtained directly from graphene oxide (without B‐doping). An unprecedented energy density of 2974 Wh kg−1 is achieved for the as‐prepared CFx samples, which is significantly higher than the theoretically calculated energy density of commercially available fluorinated graphite (2180 Wh kg−1). Therefore, this study demonstrates a great potential of B‐doping to realize the ultrahigh energy density of CFx cathodes for practical applications.

Optical Goos-Hänchen effect in uniaxially strained graphene.

We prove the existence of relatively large Goos-Hänchen (GH) shifts for graphene in the presence of an applied strain in different crystallographic directions for p and s polarized beams. It is shown that GH shifts are smoothly increased by stretching the graphene's lattice. Moreover, we investigate the GH effect for strained graphene as a function of Fermi energy, which can be controlled by external factors such as gate voltage. We show that applied strain along zigzag and armchair orientations gives different results for GH shifts, which could provide a proper tool for the detection of strain in graphene.

On the computational modeling, additive manufacturing, and testing of tube-networks TPMS-based graphene lattices and characterizing their multifunctional properties

Lattices based on triply periodic minimal surfaces (TPMSs), which have been receiving increasing interest due to advances in additive manufacturing, are known now to be outperforming other cellular materials in several properties, enabling wider multifunctional applications. In this work, we focus on fabricating graphene TPMS lattices, viz., Gyroid, Schoen's I-WP (IWP), and Diamond, and investigate their multifunctional properties experimentally and computationally. The three-dimensional (3D) graphene structures were synthesized using a self-assembly hydrothermal-assisted dip-coating technique and the resulting lattices were tested for their mechanical, thermal, and electrical properties and compared to finite element simulation results. The graphene TPMS lattices resulting from the fabrication take the topology of a new class of TPMS architectures that are referred to here as the TPMS tube-networks. IWP demonstrated the highest elastic modulus as well as electrical and thermal conductivities. This study shows that 3D porous tube graphene can be utilized in designing new lightweight structural materials of low density with controllable thermal and electrical properties and mechanical strength with a potential to be employed in multifunctional engineering applications.

Electrical, luminescence, and microwave-assisted reduction behaviour of alkali vs. alkaline earth metal ion-modified graphene oxide membranes

Ions are often introduced into graphene oxide (GO) membranes during their preparation and application, which may lead to changes in their properties. In this study, GO membranes regulated by ions were prepared by incorporating alkali and alkaline earth metals. We found that the introduction of different ions led to opposite changes in the electrical properties of the reduced GO (RGO) membranes. The introduction of alkali metal ions improves their conductivity, whereas the introduction of alkaline earth metal ions results in a decrease in their conductivity. Furthermore, we found that the introduction of ions resulted in significantly different changes in the luminescence spectra, which could be used to detect alkali and alkaline earth metal ions. In addition, the introduction of alkali metal and alkaline earth metal ions resulted in significantly different changes in the luminescence spectra and microwave reduction effect of the RGO membranes. The introduction of alkaline earth metal ions was more helpful for the removal of oxygen-containing groups and the restoration of the graphene lattice structure when exposed to microwave irradiation. These results suggest that the properties of GO membranes can be regulated by regulating the introduction of ions, and these findings have potentially important applications in the preparation and application of GO membranes and ion detection.

Coexistence of Urbach-Tail-Like Localized States and Metallic Conduction Channels in Nitrogen-Doped 3D Curved Graphene.

Nitrogen (N) doping is one of the most effective approaches to tailor the chemical and physical properties of graphene. By the interplay between N dopants and 3D curvature of graphene lattices, N-doped 3D graphene displays superior performance in electrocatalysis and solar-energy harvesting for energy and environmental applications. However, the electrical transport properties and the electronic states, which are the key factors to understand the origins of the N-doping effect in 3D graphene, are still missing. The electronic properties of N-doped 3D graphene are systematically investigated by an electric-double-layer transistor method. It is demonstrated that Urbach-tail-like localized states are located around the neutral point of N-doped 3D graphene with the background metallic transport channels. The dual nature of electronic states, generated by the synergistic effect of N dopants and 3D curvature of graphene, can be the electronic origin of the high electrocatalysis, enhanced molecular adsorption, and light absorption of N-doped 3D graphene.

Robust Room Temperature Ferromagnetism In Cobalt Doped Graphene by Precision Control of Metal Ion Hybridization

AbstractGraphene‐based magnetic materials exhibit novel properties and promising applications in the development of next‐generation spintronic devices. Modern synthesis techniques have paved the way to design precisely the local environments of metal atoms anchored onto a nitrogen‐doped graphene matrix. Herein, it is demonstrated that grafting cobalt (Co) into the graphene lattice induces robust and stable room‐temperature ferromagnetism. These comprehensive experiments and first‐principles calculations unambiguously identify that the mechanism for this unusual ferromagnetism is π‐d orbital hybridization between Co dxz and graphene pz orbitals. Here, it is found that the magnetic interactions of Co–carbon ions are mediated by the spin‐polarized graphene pz orbitals, and room temperature ferromagnetism can be stabilized by electron doping. It is also found that the electronic structure near the Fermi level, which sets the nature of spin polarization of graphene pz bands, strongly depends on the local environment of the Co moiety. This is the crucial, previously missing, ingredient that enables control of the magnetism. Overall, these observations unambiguously reveal that engineering the atomic structure of metal‐embedded graphene lattices through careful d to p orbital interactions opens a new window of opportunities for developing graphene‐based spintronics devices.

Using polycyclic aromatic hydrocarbons for graphene growth on Cu(111) under ultra-high vacuum

Ultra-high vacuum deposition of the polycyclic aromatic hydrocarbons azupyrene and pyrene onto a Cu(111) surface held at a temperature of 1000 K is herein shown to result in the formation of graphene. The presence of graphene was proven using scanning tunneling microscopy, x-ray photoelectron spectroscopy, angle-resolved photoemission spectroscopy, Raman spectroscopy, and low energy electron diffraction. The precursors, azupyrene and pyrene, are comparatively large aromatic molecules in contrast to more commonly employed precursors like methane or ethylene. While the formation of the hexagonal graphene lattice could naively be expected when pyrene is used as a precursor, the situation is more complex for azupyrene. In this case, the non-alternant topology of azupyrene with only 5- and 7-membered rings must be altered to form the observed hexagonal graphene lattice. Such a rearrangement, converting a non-alternant topology into an alternant one, is in line with previous reports describing similar topological alterations, including the isomerization of molecular azupyrene to pyrene. The thermal synthesis route to graphene, presented here, is achievable at comparatively low temperatures and under ultra-high vacuum conditions, which may enable further investigations of the growth process in a strictly controlled and clean environment that is not accessible with traditional precursors.

Differences in water and vapor transport through angstrom-scale pores in atomically thin membranes

The transport of water through nanoscale capillaries/pores plays a prominent role in biology, ionic/molecular separations, water treatment and protective applications. However, the mechanisms of water and vapor transport through nanoscale confinements remain to be fully understood. Angstrom-scale pores (~2.8–6.6 Å) introduced into the atomically thin graphene lattice represent ideal model systems to probe water transport at the molecular-length scale with short pores (aspect ratio ~1–1.9) i.e., pore diameters approach the pore length (~3.4 Å) at the theoretical limit of material thickness. Here, we report on orders of magnitude differences (~80×) between transport of water vapor (~44.2–52.4 g m−2 day−1 Pa−1) and liquid water (0.6–2 g m−2 day−1 Pa−1) through nanopores (~2.8–6.6 Å in diameter) in monolayer graphene and rationalize this difference via a flow resistance model in which liquid water permeation occurs near the continuum regime whereas water vapor transport occurs in the free molecular flow regime. We demonstrate centimeter-scale atomically thin graphene membranes with up to an order of magnitude higher water vapor transport rate (~5.4–6.1 × 104 g m−2 day−1) than most commercially available ultra-breathable protective materials while effectively blocking even sub-nanometer (>0.66 nm) model ions/molecules.

Exploring the impact of doping and co-doping with B and N on the properties of graphene oxide and its photocatalytic generation of hydrogen

The photocatalytic production of hydrogen was studied in graphene oxide materials doped with nitrogen or/and boron by hydrothermal treatments. Characterization of the materials was carried out by XRD, FTIR, XPS, Raman, UV–Vis, and photoluminescence spectroscopies, FESEM and TEM. The study of hydrogen evolution in the water splitting reaction was done using UV light as source of irradiation and methanol as hole scavenger. Boron-doped graphene oxide with the highest bulk electrical resistance exhibited the highest photocatalytic hydrogen generation, due to interstitial positioning of boron in the graphene lattice, which improved the light absorption coefficient, formation of inter-gap states and reduced charge recombination. This phenomenon is hypothesized for the first time as “decentralized reaction clusters”, which spread across the graphene lattice and produce hydrogen independently. Nitrogen-doped graphene oxide showed high electrical conductivity due to a significant removal of oxygen functional groups, and improved carrier density. Partially reduced nitrogen and boron co-doped graphene oxide showed the highest electrical conductivity, due to the presence of more electron-donating nitrogen configurations, such as pyrrolic N and pyridinic N. Nitrogen and boron co-doping of graphene oxide allows to modify the conduction band and valence bands, thus improving the electrical conductivity.

Machine Learning Diffusion Monte Carlo Energies.

We present two machine learning methodologies that are capable of predicting diffusion Monte Carlo (DMC) energies with small data sets (≈60 DMC calculations in total). The first uses voxel deep neural networks (VDNNs) to predict DMC energy densities using Kohn-Sham density functional theory (DFT) electron densities as input. The second uses kernel ridge regression (KRR) to predict atomic contributions to the DMC total energy using atomic environment vectors as input (we used atom-centered symmetry functions, atomic environment vectors from the ANI models, and smooth overlap of atomic positions). We first compare the methodologies on pristine graphene lattices, where we find that the KRR methodology performs best in comparison to gradient boosted decision trees, random forest, Gaussian process regression, and multilayer perceptrons. In addition, KRR outperforms VDNNs by an order of magnitude. Afterward, we study the generalizability of KRR to predict the energy barrier associated with a Stone-Wales defect. Lastly, we move from 2D to 3D materials and use KRR to predict total energies of liquid water. In all cases, we find that the KRR models are more accurate than Kohn-Sham DFT and all mean absolute errors are less than chemical accuracy.

Visible light emission enhancement from a graphene-based metal Fabry-Pérot cavity.

The high saturation current density and ultrafast heating modulation of graphene makes it a competitive candidate for future thermal emission source. However, the low emissivity and easy oxidation under high temperature in air limit graphene application in the spectral range from the visible to near infrared. Here, we report a visible graphene thermal emitter based on the metal Fabry-Pérot (FP) cavity, which can greatly enhance the emissivity of graphene at wavelength around 637 nm and protect graphene from oxidation. We investigate the temperature characteristics of the emitter, and find the temperature of hot electrons in graphene is much higher than that of graphene lattice. Moreover, we also demonstrate the wavelength and intensity of graphene emission could be controlled by tuning the dielectric thickness between two gold layers. These results are helpful in the development of advanced graphene electro-thermal emission controlling application.

High-performance flexible electrothermal Joule heaters from laser reduced F-N Co-doped graphene oxide with extended Sp2 networks

Phonons play an important role in heat generation and propagation across graphene lattice; however, this is hindered by graphene heteroatom doping which generates highly defected structures. Consequently, heteroatom doping has not been primarily explored in electrothermal materials despite presenting a cost-effective and metal-free method of improving electronic activity in flexible 2D materials. Herein, we present a facile and scalable technique for fabrication of fluorine-nitrogen co-doped laser reduced graphene oxide (FN-G) Joule heaters by ultrasonic wetchemical doping and maskless laser writing method. Films containing 6.14 at. % F and 3.22 at. % N, low sheet resistance of 24.49 O/sq. and uniquely high degree of atomic ordering indicated by an ID/IG of 0.16 were successfully fabricated. The heaters showed highly competitive transient electrothermal behaviour with a saturation temperature of 365 °C for only 9 V applied voltage, high heating rate of 385.33 °C/s, and average response time of 0.68 s. An excellent temperature distribution, low power requirement, and a net-zero degradation in the electrical and electrothermal properties even after 500 bending cycles was demonstrated. These high-performance features render the FN-G heater suitable for applications in wearable electronics powered by low voltage portable electrochemical cells.

Gate-Tunable Resonance State and Screening Effects for Proton-Like Atomic Charge in Graphene.

The ability to create a robust and well-defined artificial atomic charge in graphene and understand its carrier-dependent electronic properties represents an important goal toward the development of graphene-based quantum devices. Herein, we devise a new pathway toward the atomically precise embodiment of point charges into a graphene lattice by posterior (N) ion implantation into a back-gated graphene device. The N dopant behaves as an in-plane proton-like charge manifested by formation of the characteristic resonance state in the conduction band. Scanning tunneling spectroscopy measurements at varied charge carrier densities reveal a giant energetic renormalization of the resonance state up to 220 meV with respect to the Dirac point, accompanied by the observation of gate-tunable long-range screening effects close to individual N dopants. Joint density functional theory and tight-binding calculations with modified perturbation potential corroborate experimental findings and highlight the short-range character of N-induced perturbation.

Carbonized Bacterial Cellulose Decorated with ZIF-8 Derived Iron Single-Atom Catalyst Nanocages As a Cathode Host for Lithium-Sulfur Batteries

Li-S chemistry is regarded as promising next-generation battery technology. However, its complex redox process and long-chain lithium polysulfides shuttling are hindering the development of practical Li-S battery technology. Various electrocatalysts are being investigated to address the problem by expediting reaction kinetics. Here we report Fe single metal atom catalyst (Fe-SAC) used in the sulfur cathode of Li-S batteries.Fe-SAC is synthesized using a modified metal-organic framework (MOF)-based approach, in which the low vapor pressure Fe element substitutes Zn during pyrolysis of ZIF-8 to attain Fe-N4 moieties by being coordinated with four substitutional nitrogen atoms in the graphene lattice. A freestanding 3D structure with Fe-SAC-functionalized carbon nanocages linked by a carbon nanofiber network is applied as the sulfur cathode scaffold. Here the nanocages are derived from Fe-doped ZIF with Fe-N4 SAC moieties embedded on the inside and outside surfaces. The carbon nanofiber network is derived from the carbonization of bacterial cellulose.We report several methods for assessing Fe-SAC catalytic function on sulfide redox reactions for the prepared electrode in cells with lithium polysulfides electrolytes. Electrochemical studies compare the performance of the nanostructured electrodes with or without Fe-SAC to confirm the catalytic effects in improving the Li-S battery performance. Accordingly, the prepared composite cathode delivers a high initial capacity of 1338 mAh/g at 0.1C and excellent rate capability at 2C with a reversible capacity of 840 mAh/g.

Thermal Effect on Quantum Correlations of Two Interacting Qubits in Graphene Lattices

Thermal Effect on Quantum Correlations of Two Interacting Qubits in Graphene Lattices

Periodic Structural Defects in Graphene Sheets Engineered via Electron Irradiation.

Artificially-induced defects in the lattice of graphene are a powerful tool for engineering the properties of the crystal, especially if organized in highly-ordered structures such as periodic arrays. A method to deterministically induce defects in graphene is to irradiate the crystal with low-energy (<20 keV) electrons delivered by a scanning electron microscope. However, the nanometric precision granted by the focused beam can be hindered by the pattern irradiation itself due to the small lateral separation among the elements, which can prevent the generation of sharp features. An accurate analysis of the achievable resolution is thus essential for practical applications. To this end, we investigated patterns generated by low-energy electron irradiation combining atomic force microscopy and micro-Raman spectroscopy measurements. We proved that it is possible to create well-defined periodic patterns with precision of a few tens of nanometers. We found that the defected lines are influenced by electrons back-scattered by the substrate, which limit the achievable resolution. We provided a model that takes into account such substrate effects. The findings of our study allow the design and easily accessible fabrication of graphene devices featuring complex defect engineering, with a remarkable impact on technologies exploiting the increased surface reactivity.