Two-dimensional Hexagonal Boron Nitride Research Articles

A novel detection approach for serotonin by graphene quantum dots/two-dimensional (2D) hexagonal boron nitride nanosheets with molecularly imprinted polymer

Serotonin (SER) is the important neurotransmitter and its amount in human body can affect the human life. The low level results in anxiety and carcinoid syndrome. Its high level causes crucial toxicity. Molecularly imprinted approach was used in designing a new electrochemical sensor for serotonin (SER) detection in urine samples. The sensor was prepared based on graphene quantum dots (GQDs) incorporated two-dimensional (2D) hexagonal boron nitride (2D-hBN) nanosheets. All nanomaterials’ formation and properties were highlighted with scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) method, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). SER imprinted voltammetric sensor was improved in presence of 80.0 mM phenol containing 20.0 mM SER by CV. 1.0 × 10−12–1.0 × 10−8 M and 2.0 × 10−13 M were founded as the linearity range and the detection limit (LOD). SER imprinted glassy carbon electrode (GCE) was used for urine sample analysis in presence of the other competitor agents such as dopamine (DOP), tryptophan (TRY) and norepinephrine (NOR). The sensor was also investigated for stability and selectivity.

Strained hexagonal boron nitride: Phonon shift and Grüneisen parameter

Owing to its large electric band gap, two-dimensional (2D) hexagonal boron nitride (hBN) is a very promising candidate for making atomically thin flexible devices. Further, its high mechanical strength makes it suitable as a reinforcing material in composites. It is thus of great interest to study the strain-induced phonon shift of mechanically exfoliated hBN of various thicknesses under tension. It is found that splitting of the ${E}_{2g}$ phonon to two subpeaks due to the lifting of degeneracy under uniaxial loading occurs at low strain level with average shift rates of $\ensuremath{-}8.4\phantom{\rule{0.28em}{0ex}}(1.0)\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}/%$ and $\ensuremath{-}25.2\phantom{\rule{0.28em}{0ex}}(1.5)\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}/%$ for the ${E}_{2g}^{+}$ and ${E}_{2g}^{\ensuremath{-}}$, respectively. These large redshifts of the ${E}_{2g}$ modes with strain are close to the values obtained from graphene and confirm the high stiffness of 2D hBN. Furthermore, the measured ${E}_{2g}$ mode Gr\uneisen parameter is about 1.88 (2), in agreement with the values obtained by classical molecular dynamics (MD) and ab initio MD simulations. The shift rates are similar for hBN with thickness of two to four layers, which is in contrast with graphene, indicating the effective load transfer between the hBN layers. The present results provide useful insights for the use of hBN in electronic devices, 2D heterostructures, and also as a reinforcing agent in composite materials.

Controlled growth and Atomic-scale Characterization of Two-dimensional hexagonal boron nitride crystals

Two-dimensional (2D) hexagonal boron nitride (h-BN) is a fascinating material for variety of applications such as graphene-based devices, transparent/bendable electronics, and deep ultraviolet emitters. However, its technological applications are contingent upon tunable and scalable growth. Here, we demonstrate reproducible and tunable growth of high-quality h-BN crystals on Cu via chemical vapor deposition (CVD) through a precise control of the BN precursor’s flow during growth. We present synthesis of both epitaxially-grown triangular flakes and large-area continuous films with tunable thickness ranging from monolayer to 11-layer thick. Using a combination of electron microscopy imaging, spectroscopy, and diffraction analysis, we thoroughly study morphology, thickness, chemistry, grain size, and atomic structure of the grown h-BN crystals. This study could pave the way for developing controlled and reproducible growth of high-quality h-BN crystals with tunable thickness and morphology.

Role of Carbon Interstitials in Transition Metal Substrates on Controllable Synthesis of High-Quality Large-Area Two-Dimensional Hexagonal Boron Nitride Layers

Reliable and controllable synthesis of two-dimensional (2D) hexagonal boron nitride (h-BN) layers is highly desirable for their applications as 2D dielectric and wide bandgap semiconductors. In this work, we demonstrate that the dissolution of carbon into cobalt (Co) and nickel (Ni) substrates can facilitate the growth of h-BN and attain large-area 2D homogeneity. The morphology of the h-BN film can be controlled from 2D layer-plus-3D islands to homogeneous 2D few-layers by tuning the carbon interstitial concentration in the Co substrate through a carburization process prior to the h-BN growth step. Comprehensive characterizations were performed to evaluate structural, electrical, optical, and dielectric properties of these samples. Single-crystal h-BN flakes with an edge length of ∼600 μm were demonstrated on carburized Ni. An average breakdown electric field of 9 MV/cm was achieved for an as-grown continuous 3-layer h-BN on carburized Co. Density functional theory calculations reveal that the interstitial carbon atoms can increase the adsorption energy of B and N atoms on the Co(111) surface and decrease the diffusion activation energy and, in turn, promote the nucleation and growth of 2D h-BN.

Fabrication and Deterministic Transfer of High-Quality Quantum Emitters in Hexagonal Boron Nitride

Color centers in solid state crystals have become a frequently used system for single photon generation, advancing the development of integrated photonic devices for quantum optics and quantum communication applications. In particular, defects hosted by two-dimensional (2D) hexagonal boron nitride (hBN) are a promising candidate for next-generation single photon sources, due to its chemical and thermal robustness and high brightness at room temperature. The 2D crystal lattice of hBN allows for a high extraction efficiency and easy integration into photonic circuits. Here we develop plasma etching techniques with subsequent high temperature annealing to reliably create defects. We show how different fabrication parameters influence the defect formation probability and the emitter brightness. A full optical characterization reveals the higher quality of the created quantum emitters, represented by a narrow spectrum, short excited state lifetime and high single photon purity. We also investigated the photostability on short and very long timescales. We utilize a wet chemically-assisted transfer process to reliably transfer the single photon sources onto arbitrary substrates, demonstrating the feasibility for the integration into scalable photonic quantum information processing networks.

Single-Photon Emitters in Boron Nitride Nanococoons.

Quantum emitters in two-dimensional hexagonal boron nitride (hBN) are attractive for a variety of quantum and photonic technologies because they combine ultra-bright, room-temperature single-photon emission with an atomically thin crystal. However, the emitter's prominence is hindered by large, strain-induced wavelength shifts. We report the discovery of a visible-wavelength, single-photon emitter (SPE) in a zero-dimensional boron nitride allotrope (the boron nitride nanococoon, BNNC) that retains the excellent optical characteristics of few-layer hBN while possessing an emission line variation that is lower by a factor of 5 than the hBN emitter. We determined the emission source to be the nanometer-size BNNC through the cross-correlation of optical confocal microscopy with high-resolution scanning and transmission electron microscopy. Altogether, this discovery enlivens color centers in BN materials and, because of the BN nanococoon's size, opens new and exciting opportunities in nanophotonics, quantum information, biological imaging, and nanoscale sensing.

Ni foam assisted synthesis of high quality hexagonal boron nitride with large domain size and controllable thickness

The scalable synthesis of two-dimensional (2D) hexagonal boron nitride (h-BN) is of great interest for its numerous applications in novel electronic devices. Highly-crystalline h-BN films, with single-crystal sizes up to hundreds of microns, are demonstrated via a novel Ni foam assisted technique reported here for the first time. The nucleation density of h-BN domains can be significantly reduced due to the high boron solubility, as well as the large specific surface area of the Ni foam. The crystalline structure of the h-BN domains is found to be well aligned with, and therefore strongly dependent upon, the underlying Pt lattice orientation. Growth-time dependent experiments confirm the presence of a surface mediated self-limiting growth mechanism for monolayer h-BN on the Pt substrate. However, utilizing remote catalysis from the Ni foam, bilayer h-BN films can be synthesized breaking the self-limiting effect. This work provides further understanding of the mechanisms involved in the growth of h-BN and proposes a facile synthesis technique that may be applied to further applications in which control over the crystal alignment, and the numbers of layers is crucial.

Tuning the electronic and optical properties of hexagonal boron-nitride nanosheet by inserting graphene quantum dots

It is difficult to integrate two-dimensional (2D) graphene and hexagonal boron-nitride (h-BN) in optoelectronic nanodevices, due to the semi-metal and insulator characteristic of graphene and h-BN, respectively. Using the state-of-the-art first-principles calculations based on many-body perturbation theory, we investigate the electronic and optical properties of h-BN nanosheet embedded with graphene dots. We find that C atom impurities doped in h-BN nanosheet tend to phase-separate into graphene quantum dots (QD), and BNC hybrid structure, i.e. a graphene dot within a h-BN background, can be formed. The band gaps of BNC hybrid structures have an inverse relationship with the size of graphene dot. The calculated optical band gaps for BNC structures vary from 4.71 eV to 3.77 eV, which are much smaller than that of h-BN nanosheet. Furthermore, the valence band maximum is located in C atoms bonded to B atoms and conduction band minimum is located in C atoms bonded to N atoms, which means the electron and hole wave functions are closely distributed around the graphene dot. The bound excitons, localized around the graphene dot, determine the optical spectra of the BNC hybrid structures, in which the exciton binding energies decrease with increase in the size of graphene dots. Our results provide an important theoretical basis for the design and development of BNC-based optoelectronic nanodevices.

Hexagonal Boron Nitride: The Thinnest Insulating Barrier to Microbial Corrosion.

We report the use of a single layer of two-dimensional hexagonal boron nitride (SL-hBN) as the thinnest insulating barrier to microbial corrosion induced by the sulfate-reducing bacteria Desulfovibrio alaskensis G20. We used electrochemical methods to assess the corrosion resistance of SL-hBN on copper against the effects of both the planktonic and sessile forms of the sulfate-reducing bacteria. Cyclic voltammetry results show that SL-hBN-Cu is effective in suppressing corrosion effects of the planktonic cells at potentials as high as 0.2 V ( vs Ag/AgCl). The peak anodic current for the SL-hBN coatings is ∼36 times lower than that of bare Cu. Linear polarization resistance tests confirm that the SL-hBN coatings serve as a barrier against corrosive effects of the G20 biofilm when compared to bare Cu. The SL-hBN serves as an impermeable barrier to aggressive metabolites and offers ∼91% corrosion inhibition efficiency, which is comparable to much thicker commercial coatings such as polyaniline. In addition to impermeability, the insulating nature of SL-hBN suppresses galvanic effects and improves its ability to combat microbial corrosion.

Novel dielectric BN/epoxy nanocomposites with enhanced heat dissipation performance for electronic packaging

Two-dimensional hexagonal boron nitride (h-BN) has excellent and useful mechanical and thermal properties, and can be used as a novel filler to enhance the thermal conductivity of polymer composites. We prepared exfoliated h-BN nanoplatelets (BNNP) with hydroxyl functional groups using a hydroxide-assisted ball milling process, and demonstrated the enhanced thermal conductivity of its epoxy nanocomposites. The prepared BNNP are highly soluble and retain their in-plane structure. The dielectric constant and dielectric loss of the nanocomposites increase with the addition of BNNP owing to interfacial polarization by the large surface area of BNNP and the ionic and electronic relaxation polarizations by the surface functional groups on the BNNP. Notably, the thermal conductivity of the epoxy nanocomposites with 10% mass fraction of BNNP was 0.57 W/m·K, which is 2.85-times higher than that of neat epoxy. Finally, compared with neat epoxy, BNNP nanocomposite exhibits excellent heat dissipation capability in flat chip packaging. This enhanced performance is ascribed to the high quality and dispersion of the BNNP and their strong interfacial bonding with the epoxy matrix, which is produced by the hydroxyl functional groups on the BNNP. The overall results suggest that BNNP nanocomposites have strong potential for application as electronic packaging materials.

Core-Shell Nanoparticles/Two-Dimensional (2D) Hexagonal Boron Nitride Nanosheets with Molecularly Imprinted Polymer for Electrochemical Sensing of Cypermethrin

Cypermethrin ([RS]-alpha-cyano-3-phenoxybenzyl [1RS, 3RS; 1RS, 3SR]-3-[2,2-dichlorovinyl]-2,2-dimethylcyclopropanecarboxylate) is a crucial pyrethroid and it is generally used as poison against crop pests, domestic insects and ectoparasites in farmed fish. A new molecular imprinted sensor approach based on core-shell type nanoparticles (Fe@AuNPs) incorporated two-dimensional (2D) hexagonal boron nitride (2D-hBN) nanosheets was presented for cypermethrin (CYP) detection in wastewater samples. All nanomaterials’ formation and properties were highlighted with scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) method, energy dispersive X-ray analysis (EDX), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). CYP imprinted voltammetric sensor was improved in presence of 80.0 mM phenol containing 20.0 mM CYP by CV. 1.0 × 10−13 - 1.0 × 10−8 M and 3.0 × 10−14 M were founded as the linearity range and the detection limit (LOD). Finally, CYP imprinted glassy carbon electrode (GCE) was used for wastewater sample analysis in presence of the other competitor agents such as deltamethrin (DEL), tetrametrin (TET) and permethrin (PER). In addition, the prepared sensor was investigated in terms of stability and repeatability.

Gold Nanoparticles/Two-Dimensional (2D) Hexagonal Boron Nitride Nanosheets Including Diethylstilbestrol Imprinted Polymer: Electrochemical Detection in Urine Samples and Validation

A new diethylstilbestrol (DES) imprinted sensor approach based on gold nanoparticles (AuNPs) incorporated two-dimensional (2D) hexagonal boron nitride (2D-hBN) nanosheets was presented for DES detection. The methods such as scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) method, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used for all nanomaterials’ characterizations. DES imprinted sensor was prepared in presence of 100.0 mM pyrrole containing 25.0 mM DES by CV. The developed method was validated and 5.0 × 10−12−1.0 × 10−8 M and 1.0 × 10−13 M were founded as the linearity range and the detection limit (LOD). DES imprinted glassy carbon electrode (GCE) was applied in presence of the natural estrogens such as estrone (E1), estradiol (E2) and estriol (E3) and the other agents such as hydroquinone (HYD) and catechol (CAT).

Recent progress in the tailored growth of two-dimensional hexagonal boron nitride via chemical vapour deposition.

Recent years have witnessed many advances in two-dimensional (2D) hexagonal boron nitride (h-BN) materials in both fundamental research and practical applications. This has ultimately been inspired by the unique electrical and optical properties, as well as the excellent thermal and chemical stability of h-BN. However, controllable and scalable preparation of 2D h-BN materials has been challenging. Very recently, the chemical vapour deposition (CVD) technique has shown great promise for achieving high-quality h-BN samples with excellent layer-number selectivity and large-area uniformity, considerably contributing to the latest advancements of 2D material research. In this tutorial review, we provide a systematic summary of the state-of-the-art in the tailored production of 2D h-BN on various substrates by virtue of CVD routes.

Effect of 10B isotope and vacancy defects on the phonon modes of two-dimensional hexagonal boron nitride

We report the details of the effects of the 10B isotope and those of B and N vacancies combined with the isotope on the phonon modes of two-dimensional hexagonal boron nitride (h-BN). The phonon density of states and localization problems are solved using the forced vibrational method, which is suitable for an intricate and disordered system. We observe an upward shift of Raman-active E2g-mode optical phonons (32 cm−1) for a 100% 10B isotope, which matches well with the experiment and simple harmonic oscillator model. However, a downward shift of E2g-mode phonons is observed for B or N vacancies and the combination of the isotope and vacancy-type disordered BN. Strong localized eigenmodes are found for all types of defects, and a typical localization length is on the order of ∼7 nm for naturally occurring BN samples. These results are very important for understanding the heat dissipation and electron transport properties of BN-based nanoelectronics.

Chemical Vapor Deposition Growth of Large Single-Crystal Mono-, Bi-, Tri-Layer Hexagonal Boron Nitride and Their Interlayer Stacking.

Two-dimensional hexagonal boron nitride (h-BN) is a wide bandgap material which has promising mechanical and optical properties. Here we report the realization of an initial nucleation density of h-BN <1 per mm2 using low-pressure chemical vapor deposition (CVD) on polycrystalline copper. This enabled wafer-scale CVD growth of single-crystal monolayer h-BN with a lateral size up to ∼300 μm, bilayer h-BN with a lateral size up to ∼60 μm, and trilayer h-BN with a lateral size up to ∼35 μm. Based on the large single-crystal monolayer h-BN domain, the sizes of the as-grown bi- and trilayer h-BN grains are 2 orders of magnitude larger than typical h-BN multilayer domains. In addition, we achieved coalesced h-BN films with an average grain size ∼100 μm. Various flake morphologies and their interlayer stacking configurations of bi- and trilayer h-BN domains were studied. Raman signatures of mono- and multilayer h-BN were investigated side by side in the same film. It was found that the Raman peak intensity can be used as a marker for the number of layers.

Oxygen-assisted synthesis of hexagonal boron nitride films for graphene transistors

We grow high-quality two-dimensional hexagonal boron nitride (h-BN) films on copper pockets by chemical vapor deposition. A piece of sapphire embedded in the pocket serves as an oxygen supply during the growth process. To obtain clean h-BN films, source powders are placed in a U-shaped quartz tube and heated up in a water bath without the carrier-gas flow. These films are characterized by using SEM, Raman, XPS, and selected area electron diffraction analyses. As dielectric substrates, h-BN films significantly enhance the charge-carrier mobility of graphene transistors. This facile and robust method can be a scalable approach to synthesize large-area high-quality h-BN films for related electronic applications.

Robust 2D Room-Temperature Dilute Ferrimagnetism Enhancement in Freestanding Ammoniated Atom-Thin [0001] h-BN Nanoplates.

The author reports an anthracene vapor-assisted transport growth (∼4.8% in yield) of freestanding atomically thin pristine two-dimensional (2D) hexagonal boron nitride (h-BN) nanoplates directly from bulk powders. A room-temperature dilute magnetism was first observed in the pyrolytic 2D [0001] h-BN nanoplates, which is attributed to the missing N atom numbers (NN) or existence of a nitrogen-vacancy (Nv) with volume fraction ∼1.46%. Upon the postannealing in ammonia, the unsupported ammoniated 2D h-BN nanoplates showed an enhanced robust ferrimagnetism with the effective magnetic moment as high as 0.024 μB/per N, and Néel temperature at 174.9 K. At 295 K, a symmetric electron paramagnetic resonance peak signal was experimentally measured at g ∼ 2.1267, revealing the presence of an unpaired electron trapped at a B atom site in B-rich h-BN. As a promising 2D dilute magnetic semiconductor candidate, our finding favors the ammoniated atom-thin h-BN nanoplate for realization of spintronic nanodevices operating at room temperature.

In-situ deformation mechanism and orientation effects in sintered 2D boron nitride nanosheets

Two-dimensional hexagonal Boron nitride nanosheets (BNNS) are consolidated as a monolithic pellet by spark plasma sintering at 1650°C with a pressure of 50MPa without any sintering aid. X-ray diffraction study confirmed the h-BN structure after sintering of BNNS along with B2O3 and B4C as trace impurities. The preferred orientation of the basal plane (0002) perpendicular to the direction of pressure is observed on the top surface of the pellet after consolidation. High load (several hundred N) in-situ indentation studies inside SEM are carried out in the cross-section and on the top surface of the sintered BNNS pellet to understand the orientation effect on the deformation behavior. Total energy dissipation during indentation along the top surface was 50% greater than the cross-section. Predominant deformation mechanisms observed on the top surface indentation are compression of the layered sheets, delamination and BNNS pile-up. For the cross-section direction, the key deformation mechanisms are cracking and fracturing with an insignificant delamination. The dominant deformation mechanisms in sintered BNNS pellet are directly related to the preferred orientation of the crystallographic planes which was validated with TEM and crystallographic texture studies. Along the top surface, there is no active slip system. However, twinning was observed. In the cross-section, active pyramidal slip system resulted in a plastic deformation. Additionally, the weak van der Waals forces between the layers caused crack propagation along the cross-section.

Aging-resistant nanofluids containing covalent functionalized boron nitride nanosheets

Developing a thermally stable nanofluid that can maintain good thermo-conductive and flow performance at moderate or elevated temperatures for prolonged periods of time is a great challenge in heat transfer applications. Here, the thermal conductivity and rheological properties as well as their thermal stability characteristics of a nanofluid containing two-dimensional (2D) hexagonal boron nitride nanosheets (h-BNNSs) in ethylene glycol (EG) are presented, in comparison with those for a graphene oxide (GO) nanofluid as a counterpart. In place of a surfactant, hydroxyl functional groups covalently bound to the BNNS surface provided excellent compatibility and stable dispersion of the particles within EG at temperatures up to 90 °C. Owing to the percolation effect of the 2D sheets, the thermal conductivity of the EG base fluid was significantly enhanced by 80% at 5 vol% of BNNS, superior to that of the GO fluid. Moreover, the BNNS fluids exhibited excellent long-term stability at 90 °C for 5 d without loss of their high thermal conductivity, low viscosity and electrical insulating property, whereas the GO fluids underwent thermal degradation with irreversible particle aggregation and increasing viscosity due to the selective chemical reduction of the surface functional groups (i.e., C–O groups) of the GO.

Dimeric configurations of atomic hydrogen adsorbed on a monolayer hexagonal boron nitride

Atomic hydrogen adsorbed on the two-dimensional monolayer hexagonal boron nitride is systematically discussed based on dispersion-corrected density function theory (DFT-D). Main emphasis has been placed on aggregation states of two hydrogen atoms, including equal or ectopic adsorption with single side, and double-sided adsorption. The hydrogen atoms are chemisorbed on the top of boron sites, while they are physisorbed on the top of nitrogen or honeycomb sites. Furthermore, two adsorbed hydrogen atoms are most likely to keep close to form meta-TB dimer with single side. Besides, a possible stabilizing mechanism related to the adsorbed performance is investigated.