Graphene Boron Nitride Research Articles

Effects of band gap on the magic-angle of twisted bilayer graphene

Band flattening has been observed in various materials with twisted bilayer structures, such as graphene, MoS2, and hexagonal boron nitride (hBN). However, the unique phenomenon of magic-angle has only been reported in the twisted bilayer graphene (tBG) and not in the twisted bilayer semiconductors or insulators. We aim to investigate the impact of gap opening and interlayer coupling strength on the magic-angle in the tBG. Our results based on the continuum model Hamiltonian with mass term indicate that the presence of a band gap hinders the occurrence of the magic-angle, but strengthening the interlayer coupling tends to restore it. By introducing layer asymmetry, such as interlayer bias or mass difference between layers, the flat bands become more dispersive. Furthermore, we have explored the influence of the Moiré’s potential due to the hBN substrate by calculating the quasi-band-structure of the hetero-structure tBG/hBN. Our findings indicate that the conclusions drawn from using the mass term remain valid despite the presence of the Moiré’s potential due to the hBN substrate.

New methods to increase PVR in gate controllable armchair graphene-boron nitride RTDs

New methods to increase PVR in gate controllable armchair graphene-boron nitride RTDs

Computational Assessment of the Biocompatibility of Two-Dimensional g-C3N3 Toward Lipid Membranes.

One of the most recent additions to the family of two-dimensional (2D) materials, graphitic C3N3 (g-C3N3), has been considered a viable contender for biomedical applications, although its potential toxicity remains elusive. We perform all-atom molecular dynamics simulations to decipher the interactions between model lipid membranes and g-C3N3 as a first step toward exploring the cytotoxicity induced at the nanoscale. We show that g-C3N3 can easily insert into the cellular membranes following a multistage mechanism consisting of simultaneous desolvation of the 2D material along with enrichment of nanomaterial-lipid interactions. Free energy calculations indicate that g-C3N3 is more stable in a membrane-bound state compared to an aqueous solution; however, the insertion of the material does not disturb the structural integrity of lipid membranes. After being inserted into a membrane, g-C3N3 is unlikely to be released into the cellular environment and is incapable of extracting lipid molecules from the membrane. The nature of interaction between the 2D material and membranes is found to be independent of the nanomaterial size. Also, the performance of g-C3N3 toward biomolecular delivery is shown to be significantly improved compared to the state-of-the-art 2D materials graphene and hexagonal boron nitride (h-BN). It is revealed that, the affinity of g-C3N3 toward lipid membranes is weaker compared to the nanotoxic graphene and h-BN, while being marginally higher than h2D-C2N, which in turn, increases the biocompatibility of the material, thereby brightening its future as a noncytotoxic material for forthcoming biomedical applications.

Ferroelectric polarization of graphene/h-BN bilayer of different stacking orders

In recent years, two-dimensional van der Waals materials including graphene and hexagonal boron nitride (h-BN) have attracted extensive interest from researchers of many fields. In this paper, the polarization properties of graphene/h-BN van der Waals heterostructures with different stacking orders are explored by first-principle calculations. Notably, the direction of graphene/h-BN heterostructure polarization can be flipped (from AB stacking to BA stacking) by changing its stacking method. According to differential charge density analysis, the interlayer charge redistribution is the main reason driving the polarization flip. Also, we find that the polarization increases with compression of the layers and tensile strain of the lattice. In addition, we investigated the effect of the twisting angle on the polarization. The results show that polarization flip occurs during interlayer rotation and, interestingly, the magnitude and direction of polarization remain constant over a range of twist angles. The results of this study provide insight into the polarization properties of graphene/h-BN heterostructures, which is important for their potential applications in electronic devices.

Unveiling Liquid-Phase Exfoliation of Graphite and Boron Nitride Using Fluorescent Dyes Through Combined Experiments and Simulations.

Liquid-phase exfoliation (LPE) in aqueous solutions provides a simple, scalable, and green approach to produce 2D materials. By combining atomistic simulations with exfoliation experiments, the interaction between a surfactant and a 2D layer at the molecular scale can be better understood. In this work, two different dyes, corresponding to rhodamine B base (Rbb) and to a phenylboronic acid BODIPY (PBA-BODIPY) derivative, are employed as dispersants to exfoliate graphene and hexagonal boron nitride (hBN) through sonication-assisted LPE. The exfoliated 2D sheets, mostly as few-layers, exhibit good quality and high loading of dyes. Using molecular dynamics (MD) simulations, the binding free energies are calculated and the arrangement of both dyes on the layers are predicted. It has been found that the dyes show a higher affinity toward hBN than graphene, which is consistent with the higher yields of exfoliated hBN. Furthermore, it is demonstrated that the adsorption behavior of Rbb molecules on graphene and hBN is quite different compared to PBA-BODIPY.

Understanding macroscopic thermal conduction in composites reinforced with 2D nanosheets

Two-dimensional (2D) nanosheets, such as graphene and hexagonal boron nitride, are considered as the most promising fillers for enhancing thermal conductivity of polymers and phase-change materials. Nevertheless, the effect of various 2D nanosheets on the effective thermal conductivity of composites is not fully understood, and the corresponding prediction model is still lacking, since numerous influence factors and complex thermal transfer networks are involved. This paper aims to study the macroscopically effective thermal conductivity of the nanosheets-reinforced composites in a systematical way, and develop a robust machine learning based prediction model. To this end, a series of representative volume elements are reconstructed based on the SEM observations of experimental samples, and high-throughput simulations are performed via the updated lattice Boltzmann scheme proposed in our recent work. The effects of shape, size, orientation, intrinsic thermal conductivity, interface thermal resistance, surface coating, and hybrid filling of the 2D nanosheets are clarified. This work could provide a deep insight into the effective thermal conductivity of the nanosheets-reinforced composites, and may offer important guidelines for the custom-design of polymer and phase-change composites with targeted thermal performances.

Visible-frequency plasmonic enhancement at the edge of graphene/h-BN heterostructures on silicon substrate

Heterostructures of graphene and hexagonal boron nitride (G/h-BN) have been widely studied for controlling and utilizing graphene electronic properties. Here we characterize specific optical and electronic properties of G/h-BN heterostructures made of a high-quality single layer chemical vapor deposition (CVD) graphene laid over h-BN flakes, with focus on plasmonic effects. We compare the G/h-BN properties on Si and SiO2 substrates by micro-Raman spectroscopy mapping, Kelvin probe force microscopy, optical and atomic force microscopy. We observe highly enhanced Raman intensity (up to 280 %) from Si as well as graphene along the G/h-BN edge. It is attributed to localized concentration of electrons in graphene and suitable perpendicular orientation of plasmonic vibrations at the edge. The plasmonic Raman enhancement occurs under a visible light excitation (532 nm) and the effect can be tuned by the h-BN flake thickness (10–150 nm). The enhancement is specific to G/h-BN/Si structures, on G/h-BN/SiO2 structures the Raman signal is suppressed while I2D/IG ratio is increased. Vice versa, change of surface potential under visible light illumination (photovoltage) is on G/h-BN/Si negligible (within 10 mV) compared to the G/h-BN/SiO2 structures. These results open new prospects for broad utilization of localized visible plasmonic effects in graphene.

Enhanced thermoelectric performance of a wide-bandgap twisted heterostructure of graphene and boron nitride.

The manipulation of the relative twist angle between consecutive layers in two-dimensional (2D) materials dramatically modulates their electronic characteristics. Twisted bilayer graphene (tblg) and twisted boron nitride (tBN) exhibit Moiré patterns that have the potential to revolutionize various fields, from electronics to quantum materials. Here, the electronic and thermoelectric properties of 21.8° tblg and 21.8° tBN and a 21.8° twisted graphene/boron nitride (Gr/BN) heterostructure were investigated using density functional theory and Boltzmann transport theory. The twisted Gr/BN heterostructure possesses a wide band gap of 1.95 eV, which overcomes the limitations of the absence of a band gap of graphene and boron nitride's extremely wide band gap. A significant increase in thermoelectric power factor was obtained for the heterostructure compared to its parent materials, 21.8° tblg and 21.8° tBN. It has a thermal conductivity of 5.88 W m-1 K-1 at 300 K, which is much lower than those of 21.8° tblg and 21.8° tBN. It is observed that graphene plays an important role in electron transport or power factor enhancement, whereas BN helps in reducing the thermal conductivity in twisted Gr/BN systems. A strong role of boundary scattering in thermal transport compared to electrical transport was observed. A high figure of merit (ZT) of 1.28 for the twisted Gr/BN heterostructure at a ribbon width of L = 10 nm and T = 900 K was obtained. This suggests its suitability as an effective material for thermoelectric applications.

Possible formation of H2 hydrates in different nanotubes and surfaces using molecular dynamics simulation.

In this work, we simulated water molecules confined in carbon, boron nitride (BN), and silicon carbide (SiC) nanotubes with similar sizes. We also simulated water molecules confined between parallel graphene, BN, and SiC surfaces in two cases: (a) a similar geometric surface density of water of 0.177/Å2, in which the number of gas molecules was 18% of the total water molecules, and (b) a similar density profile of water of 0.04-0.05 dalton per Å3. To examine H2 hydrate formation, we added guest H2 molecules to the confined water molecules in the nanotube and surface systems. We analyzed the formed shapes, adsorption energies, radial distribution functions (RDFs), and self-diffusion coefficients of the confined molecules in gas hydrate formation. Our results showed that a more ordered heptagonal ice nanotube was formed in the BN nanotube than that in the other systems. After the addition of H2 molecules in the different nanotubes, some of the H2 molecules occupied the wall of the ice nanotube and some of them positioned in the hollow space. Although gas hydrates were created in all surface systems, ordered gas hydrate shapes were formed only in the graphene system. The adsorption energy for guest H2 molecules between the different surfaces was negative, which means that the formation of H2 hydrates between these surfaces is a spontaneous process (unlike that in the nanotube systems). According to RDF results, the BN nanotube and graphene surfaces are proper systems to form more ordered H2 hydrate structures. The confined water molecules have much higher diffusion coefficients in the BN nanotube and graphene surfaces than in the other systems. The F 4 parameter also substantiated hydrate formation in the different nanostructures. In a new configuration of BN and SiC systems with density profiles similar to that of the graphene system, the H2 hydrate was not formed completely as in the case of the graphene system. H2 hydrates formed in the new BN and SiC surfaces were less than those formed in the primary structures (with a geometrical density similar to that of the graphene system) and the graphene system.

Growing sp2 materials on transition metals: calculated atomic adsorption energies of hydrogen, boron, carbon, nitrogen, and oxygen atoms, C2 and BN dimers, C6 and (BN)3 hexamers, graphene and h-BN with and without atomic vacancies.

The growth of graphene and hexagonal boron nitride on hot transition metal surfaces involves the adsorption of precursor molecules, and their dissociation and assembly into two-dimensional honeycomb lattices. In a recent account it was found that h-BN may be distilled on a rhodium metal surface, which yields higher quality h-BN [Cun et al., ACS Nano, 2020, 15, 1351]. In this context, we calculated in a systematic approach the adsorption energies and sites of hydrogen, boron, carbon, nitrogen, and oxygen atoms and from the site dependence the activation energy for diffusion. Existing computed values of the solvation energy into the bulk were compared to the present ones with our calculation scheme and found to be in good agreement. For the distinction of different systems we introduce the concepts of epiphilicity and epiphobicity. The adsorption energies and stabilities of the C2 and BN dimers, the C6 and (BN)3 ring-hexamers and the graphene and h-BN monolayers allow the prediction of the performance of different substrates in chemical vapor deposition (CVD) processes for the growth of graphene and h-BN. Finally, vacancy creation energies were calculated as a criterion for the stability of graphene and h-BN on metallic substrates.

Atomistic insights into the nucleation and growth of hexagonal boron nitride and graphene heterostructures.

Graphene and hexagonal boron nitride (hBN) are two-dimensional (2D) materials with a similar atomic structure but drastically different although complementary electronic properties. The large-scale synthesis of h-BN/graphene heterostructures with high crystallographic quality is required to fully benefit of the graphene electronic properties. In this study, we examine numerically the interaction of graphene precursors on hBN and of hBN precursors on graphene to gain deep insight of the CVD and MBE growth mechanism of graphene/hBN heterostructures. Density functional theory (DFT) calculations reveal the adsorption and diffusion behaviors for B, N, and C atoms on these surfaces. In particular, the adsorption energy is found to be similar to the diffusion barriers, except for the nearly free diffusion of B atoms on both graphene and hBN. We have also investigated the transition from individual atoms to graphene or h-BN seeds by considering the stability of linear chains as well as branched and ring seeds. Furthermore, for larger clusters, the triangular h-BN domains are found to be equally thermodynamically stable on graphene regardless of their orientation. These findings provide preliminary hints for the ability of graphene to grow on hBN layers and hBN layer on graphene.

Unexpected reduction in thermal conductivity observed in graphene/h-BN heterostructures.

Heterostructures find wide-ranging applications in fields such as thermal management, thermoelectric energy conversion, and nanoelectronics. This study provides new insights into the thermal conductivity of parallel heterointerfaces by investigating a longitudinal heterostructure composed of graphene and hexagonal boron nitride (h-BN) using molecular dynamics simulations. Interestingly, it is observed that this unique heterostructure possesses a lower thermal conductivity compared to pure h-BN. The analysis reveals that phonon scattering is enhanced by stress at the interface of the heterostructure and the mass distribution through it. The heterostructure model introduced in this study presents new insights for controlling phonon transportation in nanoscale structures.

Phase-field crystal modeling of graphene/hexagonal boron nitride interfaces.

Two-dimensional (2D) materials such as graphene and hexagonal boron nitride (h-BN) are an essential class of materials with enhanced structural and electronic properties compared to their bulk counterparts. The phase-field crystal (PFC) model can reach diffusive time scales to study nucleation, growth of crystallites, and relaxation of strain-driven 2D monolayers that are much larger in comparison to molecular dynamics (MD) and quantum mechanical density functional theory (QMDFT) methods while retaining atomic resolution. The model also naturally incorporates an atomic length scale and elastic and plastic deformations. We simulate the morphological transition of the crystal growth of various equilibrium crystal shapes. In this work, we generalize the one-mode PFC model to study the graphene/h-BN heterostructure interface by using conserved dynamics to describe the dynamics of the model. The model was used to find the equilibrium shape of the crystal of the h-BN crystal embedded in a graphene monolayer.

Synergistic Effect of Graphene and Boron Nitride on the Electrical Conductivity and Mechanical Properties of Hybrid Polymer Composites

This work examines the combined influence of graphene and hexagonal boron nitride (h-BN) on the electrical conductivity, mechanical characteristics, and thermal stability of polyvinylidene fluoride (PVDF)-based hybrid polymer composites. Graphene and h-BN were exfoliated and amalgamated in different ratios (1:1, 2:1, and 3:1), thereafter integrated into the PVDF matrix at nanofiller concentrations of 0.5 wt%, 1.0 wt%, 2.0 wt%, and 5.0 wt%. Mechanical tests demonstrated substantial improvements in tensile strength and Young’s modulus, especially for composites with a 2:1 graphene-to-h-BN ratio. Electrical conductivity significantly enhanced with increasing nanofiller content, reaching a peak of 5.0 × 10⁻² S/m at a 5.0 wt% nanofiller concentration with a 3:1 graphene-to-h-BN ratio. Thermal stability has also improved with degradation temperatures increased by up to 70°C more as compared to PVDF. The findings demonstrate that the ideal nanofiller structure (2:Adding one graphene ratio at 5.0 wt% strikes a balance of these properties enabling the composites to be applied in electronics, sensors, and advanced structures

Covalent functionalization of germanene employing computational simulations.

Computational simulations through density functional theory in conjunction with M06-L and HSE functional have been carried out to investigate the chemical reactivity of the germanene monolayer. It is exceptionally reactive, with an average reaction energy of -60.4 kcal mol-1 for the nineteen functional groups considered: H, F, Cl, Br, O, S, Se, Ge, OH, SH, CH3, CF3, NH, NH2, C6H5, C6H4, CCl2, CBr2, and the azomethine ylide. The results indicate that oxygen is the most reactive reagent (-96.9 kcal mol-1), followed by fluorine (-83.1 kcal mol-1). Germanene presents a rich organic chemistry, and functionalization with azomethine ylides, benzynes, and carbenes can be easily accomplished as indicated by the reaction energies computed, which lie between -45 and -65 kcal mol-1. Furthermore, germanene is significantly more reactive than graphene and hexagonal boron nitride monolayers since the reaction energy for germanene is more than 40 kcal mol-1 lower. Although, in general, germanene is slightly more reactive than black and blue phosphorene and less prone to oxidation, but its oxidation when exposed to air occurs spontaneously. The addition of functional groups works cooperatively. The reaction energies become lower as the number of functional groups increases, thus favouring the agglomeration of functional groups attached unless the steric effect alters this pattern. Finally, we analyzed the electronic properties of functionalized germanene. It is possible to fine-tune the band gap of germanene from 0.1 to 2 eV using different functional groups and coverages. For O-50% and S-50% functionalized germanene, we found that carrier recombination is the most difficult due to the considerable differences between the effective masses of holes and electrons, which is promising for optical applications.

Enhancing Li+ recovery in brine mining: integrating next-gen emotional AI and explainable ML to predict adsorption energy in crown ether-based hierarchical nanomaterials.

Artificial intelligence (AI) is being employed in brine mining to enhance the extraction of lithium, vital for the manufacturing of lithium-ion batteries, through improved recovery efficiencies and the reduction of energy consumption. An innovative approach was proposed combining Emotional Neural Networks (ENN) and Random Forest (RF) algorithms to elucidate the adsorption energy (AE) (kcal mol-1) of Li+ ions by utilizing crown ether (CE)-incorporated honeycomb 2D nanomaterials. The screening and feature engineering analysis of honeycomb-patterned 2D materials and individual CE were conducted through Density Functional Theory (DFT) and Gaussian 16 simulations. The selected honeycomb-patterned 2D materials encompass graphene, silicene, and hexagonal boron nitride, while the specific CEs evaluated are 15-crown-5 and 18-crown-6. The crown-passivated 2D surfaces held a significant adsorption site through van der Waals forces for efficient recovery of Li+ ions. ENN predicted the targeted adsorption sites with high precision and minimal deviation. The eTAI (XAI) based Shapley Additive exPlanations (SHAP) was also explored for insight into the feature importance of CE embedded 2D nanomaterials for the recovery of Li+ ions. The extreme gradient boosting algorithm (XGBoost) model demonstrated a RT-2-MAPE = 0.4618% and ENN-2-MAPE = 0.4839% for the feature engineering analysis. This research would be an insight into the AI-driven nanotechnology that presents a viable and sustainable approach for the extraction of natural resources through the application of brine mining.

Design of three-dimensional B-C-N structures via tailoring GNRs and BNNRs

A series of porous 3D Cx(BN)y crystal structures with full sp2 hybridization are obtained by polymerizing edge-functionalized graphene nanoribbons (GNRs) and boron nitride nanoribbons(BNNRs). Herein, the crystal structure and properties of Cx(BN)y are systematically and comprehensively investigated by first-principles calculations, deriving that it is feasible to achieve performance tuning by controlling the nanoribbon width. Thermodynamic stability, these structures have energy advantages with respect to the previously proposed B-C-N structures, and the thermodynamic stability of the structures deteriorates with the increase of the carbon content. Electrical properties, the bandgap value is tunable between 0.1 and 1.1 eV, and the bandgap value increases with the width of BNNRs at the same width of GNRs. Mechanical properties, these structures are mechanically stable, and exhibit unexpected superstretching properties, with tensile lengths up to 2.4 times the original length, bulk modulus tunable between 137-167GPa, shear modulus tunable between 81-115GPa, and the incompressibility of the structure decreases as the nanoribbon width increases.

Phonon Thermal Transport at Interfaces of a Graphene/Vertically Aligned Carbon Nanotubes/Hexagonal Boron Nitride Sandwiched Heterostructure

Molecular dynamics simulation is used to calculate the interfacial thermal resistance of a graphene/carbon nanotubes/hexagonal boron nitride (Gr/CNTs/hBN) sandwiched heterostructure, in which vertically aligned carbon nanotube (VACNT) arrays are covalently bonded to graphene and hexagonal boron nitride layers. We find that the interfacial thermal resistance (ITR) of the Gr/VACNT/hBN sandwiched heterostructure is one to two orders of magnitude smaller than the ITR of a Gr/hBN van der Waals heterostructure with the same plane size. It is observed that covalent bonding effectively enhances the phonon coupling between Gr and hBN layers, resulting in an increase in the overlap factor of phonon density of states between Gr and hBN, thus reducing the ITR of Gr and hBN. In addition, the chirality, size (diameter and length), and packing density of sandwich-layer VACNTs have an important influence on the ITR of the heterostructure. Under the same CNT diameter and length, the ITR of the sandwiched heterostructure with armchair-shaped VACNTs is higher than that of the sandwiched heterostructure with zigzag-shaped VACNTs due to the different chemical bonding of chiral CNTs with Gr and hBN. When the armchair-shaped CNT diameter increases or the length decreases, the ITR of the sandwiched heterostructure tends to decrease. Moreover, the increase in the VACNT packing density also leads to a continuous decrease in the ITR of the sandwiched heterostructure, attributed to the extremely high intrinsic thermal conductivity of CNTs and the increase of out-of-plane heat transfer channels. This work may be helpful for understanding the mechanism for ITR in multilayer vertical heterostructures, and provides theoretical guidance for a new strategy to regulate the interlayer thermal resistance of heterostructures by optimizing the design of sandwich layer thermal interface materials.

Mechanically robust and thermally enhanced sand‐polyacrylamide‐2D nanofiller composite hydrogels for water shutoff applications

AbstractHerein, we report a facile preparation method for mechanically robust and thermally enhanced sand‐polyacrylamide (PAM)‐2D‐nanofillers composite hydrogels and their application in water shutoff. To prepare the sample, 4 wt% of aqueous PAM solution is mixed with organic cross‐linkers of hydroquinone (HQ) and hexamethylenetetramine (HMT) in a 1:1 weight ratio and aqueous potassium chloride (KCl) solution and with a specific amount of 2D‐nanofillers such as commercial graphene (CG) nanosheets or boron nitride nanoparticles (BN NPs). A specific amount of the above solution is added to sand, well mixed, and subsequently cured at 150°C for 8 h. The prepared composite hydrogels were characterized by Fourier‐transform infrared spectroscopy (FT‐IR) for chemical composition and X‐ray diffraction (XRD) for successful hydrogel coating onto the sand particles. Thermal stabilities of the samples have been evaluated by differential scanning calorimetry (DSC). Mechanical properties (storage modulus [G′]; loss modulus [G″]; gel strength (G′/G″); and damping factor [G″/G′]) of the samples were determined using dynamic mechanical analyses. The thermal stability of the samples has reached as high as 193.4°C, while the gel strength is found to be a maximum of 16.2. The water swelling ratio for the composite hydrogel has reached a maximum of 1100% within 1 h.

Nonlinearity induced negative Poisson’s ratio of two-dimensional nanomaterials

Materials exhibiting a negative Poisson’s ratio have garnered considerable attention due to the improved toughness, shear resistance, and vibration absorption properties commonly found in auxetic materials. In this work, the nonlinear effect on the Poisson’s ratio was derived theoretically and verified by first-principle calculations and molecular dynamics simulations of two-dimensional nanomaterials including graphene and hexagonal boron nitride. The analytic formula explicitly shows that the Poisson’s ratio depends on the applied strain and can be negative for large applied strains, owing to the nonlinear interaction. Both first-principle calculations and molecular dynamics simulations show that the nonlinear effect is highly anisotropic for graphene, where the nonlinearity-induced negative Poisson’s ratio is much stronger for the strain applied along the armchair direction. These findings provide valuable insights into the behavior of materials with negative Poisson’s ratios and emphasize the importance of considering nonlinear effects in the study of the Poisson’s ratio of two-dimensional materials.