Atomic layers of hybridized boron nitride and graphene domains

Optical analysis of the refractive index and birefringence of hexagonal boron nitride from the visible to near-infrared

Two-dimensional materials such as hexagonal boron nitride (h-BN), graphene, and transition metal dichalcogenides have drawn great attention in various fields of photonics and electronics. Among them, h-BN has recently emerged as a promising material platform to study integrated quantum photonics due to its ultrabright quantum light emission capabilities. However, the fundamental optical properties of h-BN have not yet been investigated in the visible and near-infrared (NIR) spectrum thoroughly. In this Letter, we report the refractive indices of h-BN thin films in the visible to NIR range. To the best of our knowledge, this is the first experimental observation of h-BN birefringence. Accurate parameters of refractive indices enable more precise design of h-BN-based photonic devices in the integrated photonics platforms.

Highly Compressive Boron Nitride Nanotube Aerogels Reinforced with Reduced Graphene Oxide

Boron nitride nanotubes (BNNTs), structural analogues of carbon nanotubes, have attracted significant attention due to their superb thermal conductivity, wide bandgap, excellent hydrogen storage capacity, and thermal and chemical stability. Despite considerable progress in the preparation and surface functionalization of BNNTs, it remains a challenge to assemble one-dimensional BNNTs into three-dimensional (3D) architectures (such as aerogels) for practical applications. Here, we report a highly compressive BNNT aerogel reinforced with reduced graphene oxide (rGO) fabricated using a freeze-drying method. The reinforcement effect of rGO and 3D honeycomb-like framework offer the BNNTs/rGO aerogel with a high compression resilience. The BNNTs/rGO aerogels were then infiltrated with polyethylene glycol to prepare a kind of phase change materials. The prepared phase change material composites show zero leakage even at 100 °C and enhanced thermal conductivity, due to the 3D porous structure of the BNNTs/rGO aerogel. This work provides a simple method for the preparation of 3D BNNTs/rGO aerogels for many potential applications, such as high-performance polymer composites.

Defective graphene domains in boron nitride sheets

Novel two-dimensional materials have emerged as hybrid structures that combine graphene and hexagonal boron nitride (h-BN) domains. During their growth process, structural defects such as vacancies and change of atoms connectivity are unavoidable. In the present study, we use first-principle calculations to investigate the electronic structure of graphene domains endowed with a single carbon atom vacancy or Stone-Wales defects in h-BN sheets. The results show that both kinds of defects yield localized states within the bandgap. Alongside this change in the bandgap configuration, it occurs a splitting of the spin channels in such a way that electrons with up and down spins populate different energy levels above and below the Fermi level, respectively. Such a spin arrangement is associated to lattice magnetization. Stone-Wales defects solely point to the appearance of new intragap levels. These results demonstrated that vacancies could significantly affect the electronic properties of hybrid graphene/h-BN sheets. Graphical Abstract A Boron-Nitride sheet doped with a vacancy endowed Carbon domain.

The geometry of hexagonal boron nitride clusters in the initial stages of chemical vapor deposition growth on a Cu(111) surface

To understand the nucleation process in the growth of hexagonal boron nitride (h-BN) on transition metal substrates by chemical vapor deposition (CVD), the energy of formation and stability of h-BN clusters of different geometries on a pristine Cu(111) surface were systematically investigated using density functional theory calculations. We find that unlike carbon clusters, h-BN clusters on Cu supports can undergo two possible transformations of the minimum-energy structure at a critical size of 13. Different from freestanding h-BN clusters, on a Cu(111) surface, h-BN chains are more stable than h-BN rings and thus dominate the minimum-energy structure for cluster sizes lower than the critical size. Thus, depending on the experimental conditions of CVD, one-dimensional Bn-1Nn (N-rich environment) or BnNn-1 (B-rich) chains are first created, and they transform to two-dimensional sp2 networks or h-BN islands, but for a BnNn chain, the transformation to a two-dimensional sp2 network h-BN island does not occur. In contrast to carbon islands where pentagons are readily formed, odd-membered rings are extremely rare in h-BN islands, where the transformation to the most stable structure occurs through a combination of trapeziums and hexagons at the edges, so as to avoid B-B and N-N bonds. Moreover, on a Cu(111) surface, trapeziums are destabilized when the four edges are connected to other hexagons because of additional curvature energy, thus favoring the nucleation of planar nuclei. A deep insight into h-BN cluster formation on a Cu support is vital to understanding the growth mechanism of h-BN on a transition metal surface in CVD experiments to further improve experimental designs in the CVD growth of h-BN.

Deposition of Multiscale Thickness Graphene Coating by Harnessing Extreme Heat and Rapid Quenching: Toward Commercialization

Deposition of graphene as a coating material over large-scale areas is an intense topic of research because of complexities involved in the existing deposition techniques. Higher defects and compromised properties restricted in realizing the full potential of graphene coating. This work aims to deposit graphene coatings by adopting a traditional technique, that is, plasma spraying, which has inherent merits of extremely high cooling rate (∼10⁶ K/s) and low plasma exposure time (∼0.1-10 μs). Graphene nanoplatelets (GNPs) were spray-dried into spherical agglomerates (∼60 μm dia.) and coatings were deposited over a wide range of surfaces. Continuous monitoring of temperature and velocity of in-flight GNPs was done using a diagnostic sensor. Deposition of GNP coatings was the result of striking of quasi-2D melted GNPs with higher velocity (∼197 m/s) toward the substrate. Postcharacterizations confirmed that GNPs did not collapse even after being exposed to harsh environments in plasma. Instead, high temperatures proved to be beneficial in purifying the commercial GNPs. The coatings were transparent even in the short-wavelength infrared region and remained electrically conductive. A proof-of-concept was established by carrying out preliminary corrosion and antifriction tests. Outstanding reduction of ∼3.5 times in corrosion rate and 3 times in coefficient of friction was observed in GNP-deposited coating. It is envisaged that graphene coating by plasma spraying can bring a revolution in commercial sectors.

Interface and defect engineering of hybrid nanostructures toward an efficient HER catalyst

The hydrogen evolution reaction (HER) plays a key role in hydrogen production for clean energy harvesting. Designing novel efficient and robust electrocatalysts with sufficient active sites and excellent conductivity is one of the key parameters for hydrogen production using water splitting devices. Recently, low-dimensional carbon materials have gained attention as metal-free catalysts for hydrogen production. Such nanostructures need to be engineered to improve their catalytic activity. Here, we designed and synthesized a B and N doped carbon nanostructure (CNS)-hBN heterostructure as an improved HER catalyst. The hBN layers on CNS could provide exposed defects and edges that act as active sites for proton adsorption and reduction. The composition, structure and chemical properties of the B and N doped CNS-hBN heterostructure were tuned to obtain excellent HER activity. Detailed morphological, structural and electrochemical characterization demonstrated that the synergistic effect rising from the interaction between B and N doped CNS and hBN structures contributes to enhance the electrocatalytic performances. To get more insight into the role of defects and doping, we performed density functional theory (DFT) calculations on the CNS-hBN heterostructure.

Green function, quasi-classical Langevin and Kubo-Greenwood methods in quantum thermal transport

With the advances in fabrication of materials with feature sizes at the order of nanometers, it has been possible to alter their thermal transport properties dramatically. Miniaturization of device size increases the power density in general, hence faster electronics require better thermal transport, whereas better thermoelectric applications require the opposite. Such diverse needs bring new challenges for material design. Shrinkage of length scales has also changed the experimental and theoretical methods to study thermal transport. Unsurprisingly, novel approaches have emerged to control phonon flow. Besides, ever increasing computational power is another driving force for developing new computational methods. In this review, we discuss three methods developed for computing vibrational thermal transport properties of nano-structured systems, namely Green function, quasi-classical Langevin, and Kubo-Green methods. The Green function methods are explained using both nonequilibrium expressions and the Landauer-type formula. The partitioning scheme, decimation techniques and surface Green functions are reviewed, and a simple model for reservoir Green functions is shown. The expressions for the Kubo-Greenwood method are derived, and Lanczos tridiagonalization, continued fraction and Chebyshev polynomial expansion methods are discussed. Additionally, the quasi-classical Langevin approach, which is useful for incorporating phonon-phonon and other scatterings is summarized.

Tuning the Electronic and Magnetic Properties of Graphene Flake Embedded in Boron Nitride Nanoribbons with Transverse Electric Fields: First-Principles Calculations

The electronic and magnetic properties of h-BN nanoribbions embedded with graphene nanoflakes (CBNNRs) are systematically studied by ab initio calculations. The CBNNRs with zigzag or armchair edges are all bipolar magnetic semiconductors (BMSs). The band gaps of zigzag CBNNRs (zCBNNRs) change linearly with the transverse electric field (E-field) for the first-order Stark effect, whereas for the armchair CBNNRs (aCBNNRs), the band gaps vary quadratically with the E-field for the second-order Stark effect. For zCBNNRs and aCBNNRs, they could transform from BMS to spin gapless semiconductor (SGS), metal, and half-metal (HM) under different transverse E-fields. The CBNNRs may transform into a semiconductor or HM, under the same E-fields, depending on the position of graphene flakes. The CBNNRs introduce local magnetic moment at carbon atoms, and the magnetic moment is determined by the size of the graphene flakes. These observations open the door to applications of CBNNRs in spintronic devices.

Recent Advances in 2D Lateral Heterostructures

Recent developments in synthesis and nanofabrication technologies offer the tantalizing prospect of realizing various applications from two-dimensional (2D) materials. A revolutionary development is to flexibly construct many different kinds of heterostructures with a diversity of 2D materials. These 2D heterostructures play an important role in semiconductor and condensed matter physics studies and are promising candidates for new device designs in the fields of integrated circuits and quantum sciences. Theoretical and experimental studies have focused on both vertical and lateral 2D heterostructures; the lateral heterostructures are considered to be easier for planner integration and exhibit unique electronic and photoelectronic properties. In this review, we give a summary of the properties of lateral heterostructures with homogeneous junction and heterogeneous junction, where the homogeneous junctions have the same host materials and the heterogeneous junctions are combined with different materials. Afterward, we discuss the applications and experimental synthesis of lateral 2D heterostructures. Moreover, a perspective on lateral 2D heterostructures is given at the end.

Two-dimensional hexagonal boron-carbon-nitrogen atomic layers

Two-dimensional (2D) hexagonal boron-carbon-nitrogen (h-BCN) atomic layers are expected to possess interesting properties complementary to those of graphene and h-BN, enabling a rich variety of electronic structures, properties and applications. Herein, we demonstrate a novel method to synthesize 2D h-BCN atomic layers with a full range of compositions by ion beam sputtering deposition under a mixed Ar/CH4 atmosphere. The h-BCN layers have been thoroughly characterized by various techniques, aiming at the determination of their structure evolution and properties. We find that homogeneous h-BCN layers consisting of graphene and h-BN nanodomains can be obtained at an appropriate C content, whereas too high or too low C contents result in the segregation of large-sized graphene or h-BN islands. Furthermore, the band gap of h-BCN layers slightly decreases with the increasing C content, while their electric properties can be tuned from insulating to highly conducting. This work provides a novel approach for synthesizing 2D h-BCN atomic layers and paves the way for the development of h-BCN-based devices.

Tuning the electronic structure properties of MoS2 monolayers with carbon doping

The structural and electronic properties of MoS₂ sheets doped with carbon line domains are theoretically investigated through density functional theory calculations. It is primarily studied how the system's electronic properties change when different domain levels are considered. These changes are also reflected in the geometry of the system, which acquires new properties when compared to the pristine structure. We predict, both qualitative and quantitatively, how the energy gap changes as a function of domain types. Strikingly, the band structure for the doped system shows semiconducting behavior with an indirect-bandgap, which is narrower than the one for bulk MoS₂. This is an important feature as far as gap tuning engineering is concerned. It has a profound impact on the applicability of these systems in electronic devices, where an indirect bandgap favors the quantum yield efficiency.

Chiral heteronanotubes: arrangement-dominated chiral interface states and conductivities

Structural analogue between pure carbonic nanostructures and their boron nitride counterparts provides possibilities for the fabrication of BCN hetero-nanomaterials, which have attracted widespread interest and been synthesized with stacked-layer, monolayer and tubular morphologies. In this work, the arrangement-dominated chiral interface states and conductivities of BCN heteronanotubes are investigated in detail by first principles calculations. The π-conjugated states can be driven by the high potential barrier of insulating BN domains to form chiral transport states along the interfaces. The emerging antiparallel and parallel chiral interface states play a dominant role for resonant transport and provide possibilities for the formation of chiral currents. Moreover, the unidirectional chiral currents have advantages to induce a magnetic field which can reach over 0.1 T. In contrast to the parallel-arranged chiral heteronanotubes, the antiparallel-arranged chiral heteronanotubes with the same stoichiometry have narrower band-gaps and stronger chiral conductivities. Such arrangement-dominated chiral transport interface states endow CHNTs with potential application in magneto-electronics.

High-Performance WS2 Monolayer Light-Emitting Tunneling Devices Using 2D Materials Grown by Chemical Vapor Deposition

The solid progress in the study of a single two-dimensional (2D) material underpins the development for creating 2D material assemblies with various electronic and optoelectronic properties. We introduce an asymmetric structure by stacking monolayer semiconducting tungsten disulfide, metallic graphene, and insulating boron nitride to fabricate numerous red channel light-emitting devices (LEDs). All the 2D crystals were grown by chemical vapor deposition (CVD), which has great potential for future industrial scale-up. Our LEDs exhibit visibly observable electroluminescence (EL) at both 5.5 V forward and 7.0 V backward biasing, which correlates well with our asymmetric design. The red emission can last for at least several minutes, and the success rate of the working device that can emit detectable EL is up to 80%. In addition, we show that sample degradation is prone to happen when a continuing bias, much higher than the threshold voltage, is applied. Our success of using high-quality CVD-grown 2D materials for red light emitters is expected to provide the basis for flexible and transparent displays.

Potential Application of h-BNC Structures in SERS and SEHRS Spectroscopies: A Theoretical Perspective

In this work, the electronic and optical properties of hybrid boron-nitrogen-carbon structures (h-BNCs) with embedded graphene nanodisks are investigated. Their molecular affinity is explored using pyridine as model system and comparing the results with the corresponding isolated graphene nanodisks. Time-dependent density functional theory (TDDFT) analysis of the electronic excited states was performed in the complexes in order to characterize possible surface and charge transfer resonances in the UV region. Static and dynamic (hyper)polarizabilities were calculated with coupled-perturbed Kohn-Sham theory (CPKS) and the linear and nonlinear optical responses of the complexes were analyzed in detail using laser excitation wavelengths available for (Hyper)Raman experiments and near-to-resonance excitation wavelengths. Enhancement factors around 10³ and 10⁸ were found for the polarizability and first order hyperpolarizability, respectively. The quantum chemical simulations performed in this work point out that nanographenes embedded within hybrid h-BNC structures may serve as good platforms for enhancing the (Hyper)Raman activity of organic molecules immobilized on their surfaces and for being employed as substrates in surface enhanced (Hyper)Raman scattering (SERS and SEHRS). Besides the better selectivity and improved signal-to-noise ratio of pristine graphene with respect to metallic surfaces, the confinement of the optical response in these hybrid h-BNC systems leads to strong localized surface resonances in the UV region. Matching these resonances with laser excitation wavelengths would solve the problem of the small enhancement factors reported in Raman experiments using pristine graphene. This may be achieved by tuning the size/shape of the embedded nanographene structure.

New Family of Two-Dimensional Ternary Photoelectric Materials

Screening unique two-dimensional (2D) materials with high mobility and applicable band gaps is motivated by not only the interest in basic science but also the practical applications for photoelectric materials. In this work, we have systematically studied a new family of 2D ternary quintuple layers (QLs), named ABC (A = Na, K, and Rb; B = Cu, Ag, and Au; C = S, Se, and Te). Our results indicate that the QLs of KCuTe, KAgS, KAgSe, KAuTe, RbCuTe, RbAgSe, and RbAgTe host direct band gaps. Moreover, KCuTe, RbCuTe, and RbAgTe QLs show extremely high mobilities of ∼10⁴ cm² V^-1 s^-1. Interestingly, the linear scaling between exciton binding energy and quasiparticle band gap for ABC QLs exhibits an unexpected deviation with the 1/4 law. In addition, KAgSe, KAgS, RbAgSe, and RbAgTe show outstanding power energy conversion efficiencies of up to 21.5%, suggesting that they are good potential donor materials. Our results provide many potential candidates for applications in photoelectric materials, which may be realized in experiments due to the possible exfoliation from their parent compounds.

A comparative study of mechanisms of the adsorption of CO2 confined within graphene-MoS2 nanosheets: a DFT trend study

The space within the interlayer of 2-dimensional (2D) nanosheets provides new and intriguing confined environments for molecular interactions. However, atomic level understanding of the adsorption mechanism of CO₂ confined within the interlayer of 2D nanosheets is still limited. Herein, we present a comparative study of the adsorption mechanisms of CO₂ confined within graphene-molybdenum disulfide (MoS₂) nanosheets using density functional theory (DFT). A comprehensive analysis of CO₂ adsorption energies (E _AE) at various interlayer spacings of different multilayer structures comprising graphene/graphene (GrapheneB) and MoS₂/MoS₂ (MoS₂B) bilayers as well as graphene/MoS₂ (GMoS₂) and MoS₂/graphene (MoS₂G) hybrids is performed to obtain the most stable adsorption configurations. It was found that 7.5 Å and 8.5 Å interlayer spacings are the most stable conformations for CO₂ adsorption on the bilayer and hybrid structures, respectively. Adsorption energies of the multilayer structures decreased in the following trend: MoS₂B > GrapheneB > MoS₂G > GMoS₂. By incorporating van der Waals (vdW) interactions between the CO₂ molecule and the surfaces, we find that CO₂ binds more strongly on these multilayer structures. Furthermore, there is a slight discrepancy in the binding energies of CO₂ adsorption on the heterostructures (GMoS₂, MoS₂G) due to the modality of the atom arrangement (C-Mo-S-O and Mo-S-O-C) in both structures, indicating that conformational anisotropy determines to a certain degree its CO₂ adsorption energy. Meanwhile, Bader charge analysis shows that the interaction between CO₂ and these surfaces causes charge transfer and redistributions. By contrast, the density of states (DOS) plots show that CO₂ physisorption does not have a substantial effect on the electronic properties of graphene and MoS₂. In summary, the results obtained in this study could serve as useful guidance in the preparation of graphene-MoS₂ nanosheets for the improved adsorption efficiency of CO₂.

Boron-oxygen complex yields n-type surface layer in semiconducting diamond

Diamond is a wide-bandgap semiconductor possessing exceptional physical and chemical properties with the potential to miniaturize high-power electronics. Whereas boron-doped diamond (BDD) is a well-known p-type semiconductor, fabrication of practical diamond-based electronic devices awaits development of an effective n-type dopant with satisfactory electrical properties. Here we report the synthesis of n-type diamond, containing boron (B) and oxygen (O) complex defects. We obtain high carrier concentration (∼0.778 × 10²¹ cm^-3) several orders of magnitude greater than previously obtained with sulfur or phosphorous, accompanied by high electrical conductivity. In high-pressure high-temperature (HPHT) boron-doped diamond single crystal we formed a boron-rich layer ∼1-1.5 μm thick in the {111} surface containing up to 1.4 atomic % B. We show that under certain HPHT conditions the boron dopants combine with oxygen defects to form B-O complexes that can be tuned by controlling the experimental parameters for diamond crystallization, thus giving rise to n-type conduction. First-principles calculations indicate that B₃O and B₄O complexes with low formation energies exhibit shallow donor levels, elucidating the mechanism of the n-type semiconducting behavior.

Isoelectronic analogues of graphene: the BCN monolayers with visible-light absorption and high carrier mobility

By employing particle-swarm optimization (PSO) and first-principles computations, we theoretically predicted five stable phases of graphene-like borocarbonitrides (g-BCN) with the stoichiometric ratio of 1:1:1 and uniformly distributed B, C, N atoms, which are the isoelectronic analogues of graphene. These g-BCN monolayers are effectively stabilized by their relatively high proportion of robust C-C or B-N bonds and strong partial ionic-covalent B-C and C-N bonds within them, leading to pronounced thermal and kinetic stability. The visible-light absorption and high carrier mobility of the investigated g-BCN monolayers indicate their possible applications in high-efficiency photochemical processes and electronic devices. Our computations could provide some guidance for designing the graphene-like materials with earth-abundant elements, as well as some clues for the experimental synthesis and practical applications of ternary BCN nanosheets.

The chemical space of B, N-substituted polycyclic aromatic hydrocarbons: Combinatorial enumeration and high-throughput first-principles modeling

Combinatorial introduction of heteroatoms in the two-dimensional framework of aromatic hydrocarbons opens up possibilities to design compound libraries exhibiting desirable photovoltaic and photochemical properties. Exhaustive enumeration and first-principles characterization of this chemical space provide indispensable insights for rational compound design strategies. Here, for the smallest seventy-seven Kekulean-benzenoid polycyclic systems, we reveal combinatorial substitution of C atom pairs with the isosteric and isoelectronic B, N pairs to result in 7 453 041 547 842 (7.4 tera) unique molecules. We present comprehensive frequency distributions of this chemical space, analyze trends, and discuss a symmetry-controlled selectivity manifestable in synthesis product yield. Furthermore, by performing high-throughput ab initio density functional theory calculations of over thirty-three thousand (33k) representative molecules, we discuss quantitative trends in the structural stability and inter-property relationships across heteroarenes. Our results indicate a significant fraction of the 33k molecules to be electronically active in the 1.5-2.5 eV region, encompassing the most intense region of the solar spectrum, indicating their suitability as potential light-harvesting molecular components in photo-catalyzed solar cells.

Self-Ordered Orientation of Crystalline Hexagonal Boron Nitride Nanodomains Embedded in Boron Carbonitride Films for Band Gap Engineering

Boron carbonitride (BCN) films containing hybridized bonds involving elements B, C, and N over wide compositional ranges enable an abundant variety of new materials, electronic structures, properties, and applications, owing to their semiconducting properties with variable band gaps. However, it still remains challenging to achieve band gap-engineered BCN ternary with a controllable composition and well-established ordered structure. Herein, we report on the synthesis and characterization of hybridized BCN materials, consisting of self-ordered hexagonal BN (h-BN) crystalline nanodomains, with its aligned basal planes preferentially perpendicular to the substrate, depending on the growth conditions. The observation of the two sets of different band absorptions suggests that the h-BN nanodomains are distinguished enough to resume their individual band gap identity from the BCN films, which decreases as the carbon content increases in the BCN matrix, due to the doping and/or boundary effect. Our results reveal that the structural features and band gap of this form of hybrid BCN films are strongly correlated with the kinetic growth factors, making it a great system for further fundamental physical research and for potential in the development of band gap-engineered applications in optoelectronics.

Borocarbonitrides as Metal-Free Catalysts for the Hydrogen Evolution Reaction

Hydrogen generation by water splitting is clearly a predominant and essential strategy to tackle the problems related to renewable energy. In this context, the discovery of proper catalysts for electrochemical and photochemical water splitting assumes great importance. There is also a serious intent to eliminate platinum and other noble metal catalysts. To replace Pt by a non-metallic catalyst with desirable characteristics is a challenge. Borocarbonitrides, (B_x C_y N_z ) which constitutes a new class of 2D material, offer great promise as non-metallic catalysts because of the easy tunability of bandgap, surface area, and other electronic properties with variation in composition. Recently, B_x C_y N_z composites with excellent electrochemical and photochemical hydrogen generation activities have been found, especially noteworthy being the observation that B_x C_y N_z with a carbon-rich composition or its nanocomposites with MoS₂ come close to Pt in electrocatalytic properties, showing equally good photochemical activity.

Doping Nanoscale Graphene Domains Improves Magnetism in Hexagonal Boron Nitride

Carbon doping can induce unique and interesting physical properties in hexagonal boron nitride (h-BN). Typically, isolated carbon atoms are doped into h-BN. Herein, however, the insertion of nanometer-scale graphene quantum dots (GQDs) is demonstrated as whole units into h-BN sheets to form h-CBN. The h-CBN is prepared by using GQDs as seed nucleations for the epitaxial growth of h-BN along the edges of GQDs without the assistance of metal catalysts. The resulting h-CBN sheets possess a uniform distrubution of GQDs in plane and a high porosity macroscopically. The h-CBN tends to form in small triangular sheets which suggests an enhanced crystallinity compared to the h-BN synthesized under the same conditions without GQDs. An enhanced ferromagnetism in the h-CBN emerges due to the spin polarization and charge asymmetry resulting from the high density of CN and CB bonds at the boundary between the GQDs and the h-BN domains. The saturation magnetic moment of h-CBN reaches 0.033 emu g^-1 at 300 K, which is three times that of as-prepared single carbon-doped h-BN.

Boron-Carbon-Nitride Sheet as a Novel Surface for Biological Applications: Insights from Density Functional Theory

Understanding the interaction between nanoscale materials and nucleobases is essential for their use in nanobiotechnology and nanomedicine. Our ab initio calculations indicate that the interaction of nucleobases [adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U)] with boron-carbon-nitride (BCN) is mainly governed by van der Waals interactions. The adsorption energies, ranging from -0.560 to -0.879 eV, decrease in the order of G > A > T > C > U, which can be attributed to π-π interactions and different side groups of the nucleobases. We found that anions (N and O atoms) of nucleobases prefer to stay on top of cation (B) of the substrate. The results also showed that BCN exhibits superior binding strength than graphene and boron-nitride-based materials. We also found that upon adsorption, the fundamental properties of BCN and nucleobases remains unaltered, which suggests that BCN is a promising template for self-assembly of nucleobases.

Tunable, Multifunctional Ceramic Composites via Intercalation of Fused Graphene Boron Nitride Nanosheets

Ternary two-dimensional (2D) materials such as fused graphene-boron nitride (GBN) nanosheets exhibit attractive physical and tunable properties far beyond their parent structures. Although these features impart several multifunctional properties in various matrices, a fundamental understanding on the nature of the interfacial interactions of these ternary 2D materials with host matrices and the role of their individual components has been elusive. Herein, we focus on intercalated GBN/ceramic composites as a model system and perform a series of density functional theory calculations to fill this knowledge gap. Propelled by more polarity and negative Gibbs free energy, our results demonstrate that GBN is more water-soluble than graphene and hexagonal boron nitride (h-BN), making it a preferred choice for slurry preparation and resultant intercalations. Further, a chief attribute of the intercalated GBN/ceramic is the formation of covalent C-O and B-O bonds between the two structures, changing the hybridization of GBN from sp² to sp³. This change, combined with the electron release in the vicinity of the interfacial regions, leads to several nonintuitive mechanical and electrical alterations of the composite such as exhibiting higher young's modulus, strength, and ductility as well as sharp decline in the band gap. As a limiting case, though both tobermorite ceramic and h-BN are wide band gap materials, their intercalated composite becomes a p-type semiconductor, contrary to intuition. These multifunctional features, along with our fundamental electronic descriptions of the origin of property change, provide key guidelines for synthesizing next generation of multifunctional bilayer ceramics with remarkable properties on demand.

Recent progress in synthesis, properties, and applications of hexagonal boron nitride-based heterostructures

Featuring an absence of dangling bonds, large band gap, low dielectric constant, and excellent chemical inertness, atomically thin hexagonal boron nitride (h-BN) is considered an ideal candidate for integration with graphene and other 2D materials. During the past years, great efforts have been devoted to the research of h-BN-based heterostructures, from fundamental study to practical applications. In this review we summarize the recent progress in the synthesis, novel properties, and potential applications of h-BN-based heterostructures, especially the synthesis technique. Firstly, various approaches to the preparation of both in-plane and vertically stacked h-BN-based heterostructures are introduced in detail, including top-down strategies associated with exfoliation transfer processes and bottom-up strategies such as chemical vapor deposition (CVD)-based growth. Secondly, we discuss some novel properties arising in these heterostructures. Several promising applications in electronic and optoelectronic devices are also reviewed. Finally, we discuss the main challenges and possible research directions in this field.

Graphene-hBN non-van der Waals vertical heterostructures for four- electron oxygen reduction reaction

A novel vertical non-van der Waals (non-vdW) heterostructure of graphene and hexagonal boron nitride (G/hBN) is realized and its application in direct four-electron oxygen reduction reaction (ORR) in alkaline medium is established. The G/hBN differs from previously demonstrated vdW heterostructures, where it has a chemical bridging between graphene and hBN allowing a direct charge transfer - resulting in high ORR activity. The ORR efficacy of G/hBN is compared with that of graphene-hBN vdW structure and individual layers of graphene and hBN along with that of benchmark platinum/carbon (Pt/C). The ORR activity of G/hBN is found to be on par with Pt/C in terms of current density but with much higher electrochemical stability and methanol tolerance. The onset potential of the G/hBN is found to be improved from 780 mV at a glassy carbon electrode to 930 mV and 940 mV in gold and platinum electrodes, respectively, indicating its substrate-dependent catalytic activity. This opens possibilities of new benchmark catalysts of metals capped with G/hBN atomic layers, where the underneath metal is protected while keeping the activity similar to that of pristine metal. Density functional theory-based calculations are found to be supporting the observed augmented ORR performance of G/hBN.

New 2D Structural Materials: Carbon-Gallium Nitride (CC-GaN) and Boron-Gallium Nitride (BN-GaN) Heterostructures-Materials Design Through Density Functional Theory

New class of ternary nanohetrostructures have been proposed by mixing 2D gallium nitride (GaN) with graphene and 2D hexagonal boron nitride (BN) with an aim towards desgining innovative 2D materials for applications in electronics and other industries. The structural stability and electronic properties of these nanoheterostructures have been analyzed using first-principles based calculations done in the framework of density functional theory. Different structure patterns have been analyzed to identify the most stable structures. It is found to be more energetically favorable that the carbon atoms occupy the positions of the nitrogen atoms in a clustered pattern in CC-GaN heterostructures, whereas boron doping is preferred in the reverse order, where isolated BN and GaN layered configurations are preferred in BN-GaN heterostructures. These 2D nanoheterostructures are energetically favored materials with direct band gap and have potential application in nanoscale semiconducting and nanoscale optoelectronic devices.

The Influence of Copper Substrates on Irradiation Effects of Graphene: A Molecular Dynamics Study

In this paper, classical molecular dynamics simulations are conducted to study the graphene grown on copper substrates under ion beam irradiation, in which the emphasis is put on the influence copper substrate on a single graphene layer. It can be inferred that the actual transmission and distribution of kinetic energy from incident ion play important roles in irradiation-defects forming process together. The minimum value needed to generate defects in supported graphene is higher than 2.67 keV, which is almost twice the damage threshold as the suspended graphene sheet. This work indicates the presence of copper substrate increases the damage threshold of graphene. Additionally, our results provide an atomistic explanation for the graphene with copper substrate under ion irradiation, which is very important for engineering graphene.

Growth of lateral graphene/h-BN heterostructure on copper foils by chemical vapor deposition

The synthesis of lateral heterostructures assembled by atomically-thin materials with distinct intrinsic properties is important for future heterojunction-embedded two-dimensional (2D) devices. Here we report an etching-assisted chemical vapor deposition method to synthesize large-area continuous lateral graphene/hexagonal boron nitride (Gr/h-BN) heterostructures on carbon-containing copper foils. The h-BN film is first synthesized on the copper foil, followed by hydrogen etching, and then epitaxial graphene domains are grown to form continuous lateral heterostructures. Analyses, including Raman spectroscopy, atomic force microscopy, scanning electron microscopy, x-ray photoelectron spectroscopy, and ultraviolet-visible absorption spectroscopy, are used to characterize the coexistence of both materials and the highly continuous nature of this lateral heterostructure. This facile and scalable synthesizing method enables the potential usage of Gr/h-BN heterostructure in both fundamental studies and related 2D devices.

A simple landscape of metastable state energies for two-dimensional cellular matter

The mechanical behavior of cellular matter in two dimensions can be inferred from geometric information near its energetic ground state. Here it is shown that the much larger set of all metastable state energies is universally described by a systematic expansion in moments of the joint probability distribution of size (area) and topology (number of neighbors). The approach captures bounds to the entire range of metastable state energies and quantitatively identifies any such state. The resulting energy landscape is invariant across different classes of energy functionals, across simulation techniques, and across system polydispersities. The theory also finds a threshold in tissue adhesion beyond which no metastable states are possible. Mechanical properties of cellular matter in biological and technological applications can thus be identified by visual information only.

Triangular graphene nanofragments: open-shell character and doping

In this work we study the intricacies of the electronic structure properties of triangular graphene nanofragments (TGNFs) in their ground and low-lying excited states by means ofab initioquantum chemistry calculations.

Ba4(BS3S)2S4: a new thioborate with unprecedented [BS3-S] and [S4] fundamental building blocks

The first BS-S group, BS₃S, with π-conjugation composed of planar BS₃ and a non-polar covalent S–S bond.

Understanding the enhanced electrical properties of free-standing graphene paper: the synergistic effect of iodide adsorption into graphene

Free-standing graphene (FSG) paper plays a vital role in a wide variety of applications as an electrode material. Specifically, the electrical properties of FSG are the most important factor affecting its use as an electrode material. Herein, the vacuum filtration technique is utilized to fabricate GO paper, which is then reductively treated with HI. Initially, the electrical conductivity is measured for GO papers with different thicknesses by varying the concentration of GO precursor as well as the reduction time. The FSG paper with a thickness of 3 microns exhibits the lowest sheet resistance and further characterization is carried out to reveal the origin of this enhancement of electrical properties. The low resistance is attributed to its crystalline nature, stacking height (L_c), in-plane crystallite size (L_a) and defect density (n_D). Meanwhile, iodide ions intercalated into the graphene layers act as hole-carriers, and their intercalation is favoured over adsorption at the surface.

Synergistic effect of electrical properties for tri-iodide adsorbed free-standing graphene paper upon different exposure time of GO papers in HI reducing agent for 0.5 hour (FSG1), 1 hour (FSG2) and 24 hour (FSG3).

Modifying the electronic and geometrical properties of mono/bi-layer graphite-like BC2N via alkali metal (Li, Na) adsorption and intercalation: computational approach

The boron/carbon/nitrogen-based materials have received much attention in condensed matter physics and material sciences due to their novel optoelectronic properties.

Boron nitride nanochannels encapsulating a water/heavy water layer for energy applications

Water interaction and transport through nanochannels of two-dimensional (2D) nanomaterials hold great promises in several applications including separation, energy harvesting and drug delivery. However, the fundamental underpinning of the electronic phenomena at the interface of such systems is poorly understood. Inspired by recent experiments, herein, we focus on water/heavy water in boron nitride (BN) nanochannels – as a model system – and report a series of ab initio based density functional theory (DFT) calculations on correlating the stability of adsorption and interfacial properties, decoding various synergies in the complex interfacial interactions of water encapsulated in BN nanocapillaries. We provide a comparison of phonon vibrational modes of water and heavy water (D₂O) captured in bilayer BN (BLBN) to compare their mobility and group speed as a key factor for separation mechanisms. This finding, combined with the fundamental insights into the nature of the interfacial properties, provides key hypotheses for the design of nanochannels.

Single layer water (SLW) on BN layer and encapsulated between bilayer BN (BLBN) as nanochannel.

Realizing high thermoelectric performance with comparable p- and n-type figure-of-merits in a graphene/h-BN superlattice monolayer

We demonstrate that the superlattice monolayer consisting of light, earth-abundant, and environmentally friendly elements can be designed as perfect TE modules with comparable p- and n-type energy conversion efficiency.

Anticarcinogenic activity of blue fluorescent hexagonal boron nitride quantum dots: as an effective enhancer for DNA cleavage activity of anticancer drug doxorubicin

Blue fluorescent hexagonal boron nitride quantum dots (h-BNQDs) of ∼10 nm size as an effective enhancer for DNA cleavage activity of anticancer drug doxorubicin (DOX) were synthesized using simple one-step hydrothermal disintegration of exfoliated hexagonal boron nitride at very low temperature ∼ 120 °C. Boron nitride quantum dots (BNQDs) at a concentration of 25 μg/ml enhanced DNA cleavage activity of DOX up to 70% as checked by converting supercoiled fragment into nicked circular PBR322 DNA. The interaction of BNQDs with DOX is proportional to the concentration of BNQDs, with binding constant K b ∼0.07338 μg/ml. In addition, ab initio theoretical results indicate that DOX is absorbed on BNQDs at the N-terminated edge with binding energy -1.075 eV and prevented the normal replication mechanisms in DNA. BNQDs have been shown to kill the breast cancer cell MCF-7 extensively as compared with the normal human keratinocyte cell HaCaT. The cytotoxicity of BNQDs may be correlated with reduced reactive oxygen species level and increased apoptosis in MCF-7 cells, which may be liable to enhance the anticancerous activity of DOX. The results provide a base to develop BNQD-DOX as a more effective anticancer drug.

2D-BCNO with Eu3+: partial energy transfer and direct natural white light for LEDs

2D-BCNO transfers some energy to excite Eu³⁺ ions and together they generate white light.