Hexagonal Boron Nitride Sheets Research Articles

Flexible substrate based on sandwich-structure hydrocarbon resin/aligned boron nitride composites with high thermal conductivity and low dielectric loss

Flexible circuit board materials with low dielectric constant (εr), low dielectric loss (tanδ) and high thermal conductivity (TC) were highly desired for high-frequency and high-speed electronic equipment. Herein, hexagonal boron nitride (h-BN) sheets were filled into highly elastic styrene ethylene butylene styrene (SEBS) matrix and oriented along in-plane direction regularly by hot-pressing method. SEBS/h-BN films exhibited high TC and outstanding flexibility except relatively high tanδ and water absorption. The sandwich structure was designed utilizing pure cyclic olefin copolymer (COC) films as outer surface layers and SEBS/h-BN film as interlayer. The introduction of COC films reduced the tanδ and water absorption of SEBS/h-BN film without reducing original TC obviously. Hybrid SEBS/COC/BN films owning the same components with sandwich-structure films were also fabricated to verify the advantages of designed structure. Sandwich-structure films always kept good flexibility and exhibited low εr (2.7–3.6), low tanδ (<1 × 10−3, 10 GHz), high TC (3.8–17.6 W m−1 K−1) and low water absorption (<0.1%). These films were also fabricated into flexible copper clad laminates and patch antenna operated in sub-6G band. The antenna exhibited a return loss of −32.8 dB and a gain of 7.04 dBi. The flexible substrates show great potential and satisfy large-scale production demand in high TC flexible circuit board materials field.

Interlayer spacing control of boron nitride sheets with hydrated cations

Two-dimensional (2D) materials are widely used in various fields, among which the application prospect in filtering and purifying water resources is broad. An important factor in sieving water molecules and other substances is the interlayer spacing of 2D material membranes. The interlayer spacing of graphene oxide and graphene membranes can be controlled by hydrated cations because of hydrated cation–π interaction. Similarly, our simulations show that hydrated cations can also control the interlayer spacing between hexagonal boron nitride (h-BN) sheets, which are smaller than that of graphene oxide sheets and graphene sheets. The interlayer distances are 8.70, 8.62, 7.68, 8.21, 8.93, 8.11 and 8.18 Å for hydrated Li+, Na+, K+, Mg2+, Ca2+, Cu2+ and Zn2+, respectively. Both hydration energy and the radius of the hydrated cation are deciding factors for monovalent hydrated cations and divalent hydrated cations in affecting the h-BN layer spacing. Further calculations indicate that unlike graphene and graphene oxide dominated by the hydrated cation–π interaction, the interactions between h-BN sheets and hydrate cations are mainly hydrogen bonds. However, there is an obvious hydrated cation–π interaction between hydrated K+ and the h-BN sheets. Our results are helpful for future ion sieving applications using the h-BN membranes.

Chemical-free exfoliation of hexagonal boron nitride via cavitation-bubble plasma in water

Hexagonal boron nitride (h-BN) sheets have numerous applications, but current low-cost, environmentally friendly techniques for h-BN exfoliation yield undesirably small sheet sizes. We have developed a novel chemical-free method to prepare large h-BN nanosheets (BNNSs) by the exfoliation of h-BN using plasma with cavitation bubbles in water, named here as cavitation-bubble plasma. Unlike ordinary liquid exfoliation methods, which reduce the sheet size of BNNSs compared with that of the starting h-BN, the cavitation-bubble plasma process exfoliated h-BN without reducing the sheet size. Furthermore, the plasma dramatically improved the exfoliation yield.

Theoretical study on S/Fe-doped hexagonal boron nitride (h-BN) sheet: Electronic and optical properties

Limited by the wide band gap of h-BN, the application of h-BN in photocatalysis is extremely challenging. In this paper, a strategy of co-doping monolayer h-BN with sulphur and iron was proposed to enhance the stability and catalytic activity of photocatalysts. Firstly, based on density functional theory (DFT), several models of monolayer h-BN nanosheets with iron decoration was created and optimized. Secondly, the structural, electronic, and optical properties were investigated in this paper, and the calculations proves that the introduction of impurity energy levels mainly composed of Fe 3d orbitals is the key to solving the wide bandgap of boron nitride. The HOMO-LUMO analysis shows S atoms participate in the redistribution of electrons, which inhibits the recombination of electron-hole pairs. This work verified the higher photocatalytic ability and wider light absorption range of monolayer h-BN co-doped with sulfur and iron. This paper has pointed a promising path for the modification of h-BN in the direction of photocatalysis.

Synergistic enhancement of thermal conductivity in thermal interface materials by fabricating3D‐BN‐ZnOscaffolds

AbstractThree‐dimensional boron nitride/zinc oxide (3D‐BN‐ZnO) scaffolds were prepared by the ice‐templating method, in which ZnO particles were in situ formed by sintering. Then, polydimethylsiloxane (PDMS)/3D‐BN‐ZnO composites were fabricated via vacuum infiltration of PDMS prepolymer into the scaffolds and curing. Compared with the composite prepared by simple blending, the thermal conductivity of the composite prepared based on the ice‐templating method is much higher due to the excellent orientation of hexagonal boron nitride (hBN) sheets. Based on the same content of hBN sheets, the further introduction of 2.7 wt% ZnO particles formed in situ by sintering could promote the thermal conductivity of the composite to the value of 1.45 W m−1 K−1, which is 70.6% higher than that of the PDMS/3D‐BN composite. In addition, compared with the PDMS/3D‐BN composite containing more hBN sheets, the PDMS/3D‐BN‐ZnO composite with 2.7 wt% ZnO particles possesses fewer fillers but higher thermal conductivity. It suggests that ZnO particles could synergistically improve the thermal conductivity. As revealed from the morphology of the composites, many ZnO particles are located between hBN sheets and thus construct thermally conductive paths. This study provides a new strategy for the design of thermal interface materials with high thermal conductivity.

A novel three-dimensional boron phosphide network for thermal management of epoxy composites

Recently, construction of effective three-dimensional (3D) heat transfer networks inside polymer has emerged as a promising design strategy to improve the isotropic thermal conductivity for electronic packaging materials. Hexagonal boron nitride (BN) sheets are popular carriers for constructing 3D networks. But removing overheat capability of BN was greatly sacrificed due to size limitation when constructing 3D network. Herein, a novel 3D heat-transferring network, interconnected boron phosphide (BP) grains in-situ growing on Ni foam, was designed to alternate 3D BN network. An isotropic [email protected] network with high-quality and integrity was successfully fabricated by a simple high temperature treatment. The synthesized [email protected] was incorporated into epoxy resin (ER) by infiltration to fabricate composites. ER/[email protected] composite, with strong interfacial adhesion between epoxy and [email protected], achieved a high thermal conductivity of 2.01 W/(mK), which corresponded to 908.53% and 402.00% enhancement compared to pure epoxy and ER/Ni composite. The coefficient of thermal expansion (CTE) of the composite reached as low as 26.95 × 10−6/°C, much smaller than epoxy of 60.69 × 10−6/°C and ER/Ni composite of 59.42 × 10−6/°C. The designed ER/[email protected] composite has distinguished heat removal and CTE with semiconductors from traditional polymer composites containing 3D BN network. This strategy is promising to promote the development of electrical packaging materials with high isotropic thermal conductivity.

Electric Field Control of Magnetism of a Mn Dimer Supported on a Carbon-Doped h-BN Surface

Using density functional theory we show that the interaction between two Mn atoms can be tuned from anti-ferromagnetic (AFM) to ferromagnetic (FM) state by creating charge disproportion between the two on a 2D surface. The non-metallic planar heterostructures, the 2D surface, in our work is designed by doping carbon hexagon rings in a hexagonal boron nitride (h-BN) sheet. In addition, we show that an external electric field can be used to control the charge disproportion and hence the magnetism. In fact, our calculations demonstrate that the magnetic states of the dimer can be switched from AFM to FM or vice versa in an external electric field. The origin of this magnetic switching is explained using the charge transfer from (or to) the Mn dimer to (or from) the 2D material. The switching between anti-ferromagnetic to ferromagnetic states can be useful for future spintronic applications.

Self-assembly and photoinduced fabrication of conductive nanographene wires on boron nitride

Manufacturing molecule-based functional elements directly at device interfaces is a frontier in bottom-up materials engineering. A longstanding challenge in the field is the covalent stabilization of pre-assembled molecular architectures to afford nanodevice components. Here, we employ the controlled supramolecular self-assembly of anthracene derivatives on a hexagonal boron nitride sheet, to generate nanographene wires through photo-crosslinking and thermal annealing. Specifically, we demonstrate µm-long nanowires with an average width of 200 nm, electrical conductivities of 106 S m−1 and breakdown current densities of 1011 A m−2. Joint experiments and simulations reveal that hierarchical self-assembly promotes their formation and functional properties. Our approach demonstrates the feasibility of combined bottom-up supramolecular templating and top-down manufacturing protocols for graphene nanomaterials and interconnects, towards integrated carbon nanodevices.

Exploiting the optical sensing of fluorophore-tagged DNA nucleobases on hexagonal BN and Al-doped BN sheets: a computational study.

Hexagonal boron nitride (h-BN) sheets possess high fluorescence quenching ability and high affinity towards DNA/RNA, and they can be used as a sensing platform for rapid detection. We report the absorption and emission properties of DNA nucleobases such as adenine (A), cytosine (C), guanine (G), and thymine (T) tagged with benzoxazole on h-BN and aluminium-doped h-BN (Al_hBN) sheets. The binding affinity of studied nucleobases on h-BN sheets at the M062X/6-31G* level of theory showed the following adsorption trend: G ≥ T ≥ A > C, which is in good agreement with the previous results. The calculated stability trend of nucleobases on the Al_hBN sheet follows as C > G > A > T at the same level of theory. The physically adsorbed behavior of nucleobases to h-BN sheets was confirmed by the non-covalent interactions (NCIs) and the total density of states (TDOS) plots. The NCI results indicated that van der Waals interactions contribute significantly to the adsorption of nucleobases on h-BN sheets. Atoms in molecules (AIM) calculations revealed the electrostatic interactions between nucleobases and the Al_hBN sheet. The quenching phenomenon of nucleobase-tagged fluorophores on h-BN and Al_hBN sheets was investigated by TD-DFT calculations using the same level of theory. The thymine-tagged fluorophore upon adsorption to the pristine h-BN sheet was found to be blue-shifted (∼43 nm); however, the guanine-tagged fluorophore with Al_hBN showed a remarkable difference from other nucleobase-tagged fluorophores in the absorption and emission spectrum. Guanine-tagged fluorophores showed a smaller blue shift (∼7 nm) in the absorption spectrum; however, it showed a larger red shift (∼55 nm) than the other nucleobase-tagged fluorophores on Al_hBN sheets and can be useful in recognizing a sequence-specific phenomenon as a fluorescent biosensor of DNA and RNA to ascertain the presence of such nucleobases.

Theoretical Study on S/Fe-Doped Hexagonal Boron Nitride (H-Bn) Sheet: Electronic and Optical Properties

Theoretical Study on S/Fe-Doped Hexagonal Boron Nitride (H-Bn) Sheet: Electronic and Optical Properties

First Principles Study of Structural, Electronic and Magnetic Properties of Defected (Monovacant) Hexagonal Boron Nitride Sheet

The structural, electronic, and magnetic properties of vacancy structures with triangular shape are related to the defect in single hexagonal boron nitride (h-BN) sheet. It is investigated by using density functional theory calculations. The first-principles calculations based GGA functionals have been implemented to study the structural, electronic and magnetic properties of pure and defected hexagonal boron nitride (h-BN) monolayer sheet using Quantum ESPRESSO (QE) package, 6.5 version. The calculated values of formation energy reveal the structural stability of the defected systems. The formation energies for B and N vacant system are found to be 16.45 eV and 12.87 eV respectively. This predicts that the N vacant system is more preferable with lower formation energy. The defect in h-BN seems to be changing its band gap and magnetic properties of h-BN system. The 6.25 % B-vacancy results h-BN to be half metallic ferromagnetic with total magnetization of 2.74µB/cell. Further, 6.25 % N-vacancy causes it to be magnetic semiconductor with total magnetization 1.00µB /cell.

Heat transport across graphene/hexagonal-BN tilted grain boundaries from phase-field crystal model and molecular dynamics simulations

We study the interfacial thermal conductance of grain boundaries (GBs) between monolayer graphene and hexagonal boron nitride (h-BN) sheets using a combined atomistic approach. First, realistic samples containing graphene/h-BN GBs with different tilt angles are generated using the phase-field crystal model developed recently [P. Hirvonen et al., Phys. Rev. B 100, 165412 (2019)] that captures slow diffusive relaxation inaccessible to molecular dynamics (MD) simulations. Then, large-scale MD simulations using the efficient GPUMD package are performed to assess heat transport and rectification properties across the GBs. We find that lattice mismatch between the graphene and h-BN sheets plays a less important role in determining the interfacial thermal conductance as compared to the tilt angle. In addition, we find no significant thermal rectification effects for these GBs.

Fabrication of encapsulated graphene-based heterostructure using molybdenum as edge-contacts

Graphene is an intriguing platform to study exotic quantum transport phenomena due to its intrinsically high mobility and remarkable electronic properties. To achieve high-performance device, graphene is usually encapsulated between thin sheets of hexagonal boron nitride (hBN) to protect graphene layer from extrinsic impurities. Cr/Au is typically employed to make contacts with the edges of the heterostructure. In this research, Mo is used as an alternative electrode for graphene without adhesion layer to simplify the fabrication process. hBN-graphene-hBN heterostructures were fabricated by a pick-up technique and etched in O2/CHF3 gases to expose graphene edges. Mo contacts were deposited onto the substrates by sputtering. We achieved ohmic contacts between graphene and Mo. The contact resistance reaches the maximum of around 1,300 Ω·μm at charge neutrality point and decreases to 975 Ω·μm at the density of 4×1012 cm−2. We observed that the contact resistance increases over time likely due to the oxidation of Mo but remained ohmic after 2 months. The intrinsic transport characteristics of graphene can still be obtained by using four-probe measurement. Here, we realized a high-quality twisted bilayer graphene device with a room-temperature mobility of 27,000 cm2/V·s indicating that Mo can be used as edge-contacts to probe the transport properties of graphene.

Valley polarized conductance quantization in bilayer graphene narrow quantum point contact

In this study, we fabricated quantum point contacts narrower than 100 nm by using an electrostatic potential to open the finite bandgap by applying a perpendicular electric field to bilayer graphene encapsulated between hexagonal boron nitride sheets. The conductance across the quantum point contact was quantized at a high perpendicular-displacement field as high as 1 V/nm at low temperature, and the quantization unit was 2e2/h instead of mixed spin and valley degeneracy of 4e2/h. This lifted degeneracy state in the quantum point contact indicates the presence of valley polarized state coming from potential profile or effective displacement field in one-dimensional channel.

Dipole orientation variation of hydration shell around alkali metal cation on hexagonal boron nitride sheet

Orientation arrangement of hydration shells for hydrated cation plays an important role in solid–liquid interfaces. However, there is still poor understanding about the structure of the hydration shells formed at the solid–liquid interface. In this paper, we study the structures and interaction of typical hydrated alkali metal cation (Li+, Na+ and K+) on the flat polar material surface of the hexagonal boron nitride (h-BN) sheet by increasing the number of water molecules from n = 1 to n = 9 through the artificial bee colony (ABC) algorithm and density functional theory calculations. The hydration shell around alkali metal cation could be divided into two groups, one is Li+-(H2O)n and Na+-(H2O)n and the other is K+-(H2O)n. The orientation of hydration shell for K+-(H2O)n complexes exhibits stronger anisotropy behaviour than both Li+-(H2O)n and Na+-(H2O)n complexes on the h-BN sheet. It is found that the stronger orientation anisotropy of K+-(H2O)n originates from the weak cation-hydration energy and its weak influence upon the π-K+ interaction. Our findings provide valuable insights into the microscopic details of the structures and adsorption of hydrated cation on the h-BN sheet.

The Effect of Magnetic Field on Optical Conductivity of Hexagonal Boron-Nitride Sheet

The main purpose of this work is to investigate the effect of external magnetic field on the optical absorption of single-layer hexagonal boron nitride. The optical absorption of the system can be found by calculating the imaginary part of the dielectric function. The optical absorption is proportional to from the real part of the dynamical electrical conductivity function. Based on linear response theory, dynamical conductivity function can be calculated in terms of the current-current correlation function. In this study, the tight-binding model Hamiltonian has been applied to study the electronic band spectrum of the system. Also, the energy spectrum of electrons is calculated in the presence of a magnetic field along perpendicular to the plane. The Green’s function approach has been explored to obtain current-current correlation function and optical conductivity. The results show applying magnetic field has significant effects on dependence of optical absorption of boron nitride system on photon frequency and temperature.

Oxidative Strong Metal-Support Interactions between Metals and Inert Boron Nitride.

The strong metal-support interaction (SMSI) is one of the most important concepts in heterogeneous catalysis, which has been widely investigated between metals and active oxides triggered by reductive atmospheres. Here, we report the oxidative strong metal-support interaction (O-SMSI) effect between Pt nanoparticles (NPs) and inert hexagonal boron nitride (h-BN) sheets, in which Pt NPs are encapsulated by oxidized boron (BOx) overlayers derived from the h-BN support under oxidative conditions. De-encapsulation of Pt NPs has been achieved by washing in water, and the residual ultrathin BOx overlayers work synergistically with surface Pt sites for enhancing CO oxidation reaction. The O-SMSI effect is also present in other h-BN-supported metal catalysts such as Au, Rh, Ru, and Ir within different oxidative atmospheres including O2 and CO2, which is determined by metal-boron interaction and O affinity of metals.

Surface modified boron nitride towards enhanced thermal and mechanical performance of thermoplastic polyurethane composite

Owing to the unpredictable advancement in electronic devices, the demand for highly thermally conductive and electrically insulating material is increasing day by day. Besides, surface treatment of chemically inert hexagonal boron nitride (h-BN) sheets found challenging and a distressing contention towards its acceptable thermal transportation. Herein, h-BN sheets are functionalized with divergent amines (its three isomers) to obtain their astonishing properties like high disperse ability and development of interfacial conduction in polymer composite. This comparative study elucidates the significance of the covalent interaction of amines (-NH2) with h-BN lattice as well as strong hydrogen bonding with thermoplastic polyurethane (TPU) matrix. It is demonstrated that both ortho-phenylenediamine o-PDA-BN and para-phenylenediamine p-PDA-BN filled TPU composites exhibit higher through-plane thermal conductivity (┴TC) of 2.06 and 1.96 W m−1 K−1 respectively by substantially establishing a higher thermal transportation path at high filler loading of 45 wt %. Whereas, meta-phenylenediamine m-PDA-BN composites reveal somewhat low ┴TC of 1.27 W m−1 K−1 at the same filler loading. Interestingly, all fabricated composites consisting of functionalized BN (f-BN) and TPU show unprecedented thermal and mechanical properties at such a high filler loading which demonstrates their great potential for packaging application in microelectronics.

Strong catalyst support interactions in defect-rich γ-Mo2N nanoparticles loaded 2D-h-BN hybrid for highly selective nitrogen reduction reaction

Electrochemical ammonia synthesis by N2 fixation has proven to be a promising alternative to the energy-consuming, befouling Haber-Bosch process. Considering the low faradaic efficiency and sluggish kinetics of Nitrogen Reduction Reaction (NRR), it is significant to design a robust and selective catalyst. Herein, we demonstrate a single step in-situ nitridation method to grow cubic molybdenum nitride (γ-Mo2N) nanoparticles on a 2D hexagonal boron nitride (h-BN) sheets as a potential, cost-effective electrocatalyst for NRR, in which the selectivity for N2 was regulated by interfacially engineering the Mo2N-BN bridge. The maneuverability of h-BN sheets enabled the provocation of N-vacancies governed by the particle size, where the fine-tuning of their significance emanated the highest faradaic efficiency of 61.5 %. Moreover, such non-noble metal-based hybrids delivered a stable performance for 20 h. Therefore, our approach of designing the electronic structure of a catalyst by controlling the defects could be an effective practice for selective NRR.

Realizing a Not-Strong-Not-Weak Polarization Electric Field in Single-Atom Catalysts Sandwiched by Boron Nitride and Graphene Sheets for Efficient Nitrogen Fixation.

Developing efficient single-atom catalysts (SACs) for nitrogen fixation is of great importance while remaining a great challenge. The lack of an effective strategy to control the polarization electric field of SACs limits their activity and selectivity. Here, using first-principles calculations, we report that a single transition metal (TM) atom sandwiched between hexagonal boron nitride (h-BN) and graphene sheets (namely, BN/TM/G) acts as an efficient SAC for the electrochemical nitrogen reduction reaction (NRR). These sandwich structures realize stable and tunable interfacial polarization fields that enable the TM atom to donate electrons to a neighboring B atom as the active site. As a result, the partially occupied pz orbital of a B atom can form B-to-N π-back bonding with the antibonding state of N2, thus weakening the N≡N bond. The not-strong-not-weak electric field on the h-BN surface further promotes N2 adsorption and activation. The NRR catalytic activity of the BN/TM/G system is highly correlated with the degree of positively polarized charges on the TM atom. In particular, BN/Ti/G and BN/V/G are identified as promising NRR catalysts with high stability, offering excellent energy efficiency and suppression of the competing hydrogen evolution reaction.