hBN Crystals Research Articles

Launching Phonon Polaritons by Natural Boron Nitride Wrinkles with Modifiable Dispersion by Dielectric Environments.

Interference-free hyperbolic phonon polaritons (HPPs) excited by natural wrinkles in a hexagonal boron nitride (hBN) microcrystal are reported both experimentally and theoretically. Although their geometries are off-resonant with the excitation wavelength, the wrinkles compensate for the large momentum mismatch between photon and phonon polariton, and launch the HPPs without interference. The spatial feature of wrinkles is about 200 nm, which is an order of magnitude smaller than resonant metal antennas at the same excitation wavelength. Compared with phonon polaritons launched by an atomic force microscopy tip, the phonon polaritons launched by wrinkles are interference-free, independent of the launcher geometry, and exhibit a smaller damping rate (γ ≈ 0.028). On the same hBN microcrystal, in situ nanoinfrared imaging of HPPs launched by different mechanisms is performed. In addition, the dispersion of HPPs is modified by changing the dielectric environments of hBN crystals. The wavelength of HPPs is compressed twofold when the substrate is changed from SiO2 to gold. The findings provide insights into the intrinsic properties of hBN-HPPs and demonstrate a new way to launch and control polaritons in van der Waals materials.

High mobility dry-transferred CVD bilayer graphene

We report on the fabrication and characterization of high-quality chemical vapor-deposited (CVD) bilayer graphene (BLG). In particular, we demonstrate that CVD-grown BLG can be detached mechanically from the copper foil by a hexagonal boron nitride (hBN) crystal after oxidation of the copper-to-BLG interface. Confocal Raman spectroscopy reveals an AB-stacking order of the BLG crystals and a high structural quality. From transport measurements on fully encapsulated hBN/BLG/hBN Hall bar devices, we extract charge carrier mobilities up to 180 000 cm2/(Vs) at 2 K and up to 40 000 cm2/(Vs) at 300 K, outperforming state-of-the-art CVD bilayer graphene devices. Moreover, we show an on-off ratio of more than 10 000 and a band gap opening with values of up to 15 meV for a displacement field of 0.2 V/nm in such CVD grown BLG.

Low frequency Raman spectroscopy of few-atomic-layer thick hBN crystals

Hexagonal boron nitride (hBN) has recently gained a strong interest as a strategic component in engineering van der Waals heterostructures built with 2D crystals such as graphene. This work reports micro-Raman measurements on hBN flakes made of a few atomic layers, prepared by mechanical exfoliation. The temperature dependence of the Raman scattering in hBN is investigated first such as to define appropriate measurements conditions suitable for thin layers avoiding undesirable heating induced effects. We further focus on the low frequency Raman mode corresponding to the rigid shearing oscillation between adjacent layers, found to be equal to 52.5 cm−1 in bulk hBN. For hBN sheets with thicknesses below typically 4 nm, the frequency of this mode presents discrete values, which are found to decrease down to 46.0(5) cm−1 for a three-layer hBN, in good agreement with the linear-chain model. This makes Raman spectroscopy a relevant tool to quantitatively determine in a non destructive way the number of layers in ultra thin hBN sheets, below 8 L, prior to their integration in van der Waals heterostructures.

The Effect of Adjacent Materials on the Propagation of Phonon Polaritons in Hexagonal Boron Nitride

In order to apply the ability of hexagonal boron nitride (hBN) to confine energy in the form of hyperbolic phonon polariton (HPhP) modes in photonic-electronic devices, approaches to finely control and leverage the sensitivity of these propagating waves must be investigated. Here, we show that by surrounding hBN with materials of lower/higher dielectric responses, such as air and silicon, lower/higher surface momenta of HPhPs can be achieved. Furthermore, an alternative method for preparing thin hBN crystals with minimum contamination is presented, which provides opportunities to study the sensitivity of the damping mechanism of HPhPs on adsorbed materials. Infrared scanning near-field optical microscopy (IR-SNOM) results suggest that the reflections at the upper and lower hBN interfaces are primary causes of the damping of HPhPs, and that the damping coefficients of propagating waves are highly sensitive to adjacent layers, suggesting opportunities for sensor applications.

Hexagonal Boron Nitride Self-Launches Hyperbolic Phonon Polaritons.

Hexagonal boron nitride (hBN) is a 2D material that supports traveling waves composed of material vibrations and light, and is attractive for nanoscale optical devices that function in the infrared. However, the only current method of launching these traveling waves requires the use of a metal nanostructure. Here, we show that the polaritonic waves can be launched into the 2D structure by folds within hBN, alone, taking advantage of the intrinsic material properties. Our findings suggest that structural continuity between the fold and hBN crystal is crucial for creating self-launched waves with a constant phase front. This approach offers a single material system to excite the polaritonic modes, and the approach is applicable to a broad range of 2D crystals and thus could be useful in future characterization.

Morphology-Controlled Synthesis of Hexagonal Boron Nitride Crystals by Chemical Vapor Deposition

Synthesis of hexagonal boron nitride (hBN) crystals with a controlled morphology is a major challenge due to various kinetic and thermodynamic factors at the edge. Supplying BN building blocks by heating precursor ammonia borane at a temperature of 90 °C produced triangular crystals, whereas, at higher heating (130 °C), hBN crystals with a hexagonal morphology were observed. The shape of crystals could also be modulated from hexagonal to triangular under a continuous reduced supply of BN building blocks. We attributed these phenomena to a different growth mechanism dependent on the concentration of BN radicals in the growth region. With a low supply of BN building blocks, crystal growth is the edge attachment limited, producing triangles, whereas, under higher concentration of BN building blocks, crystal growth is limited by diffusion producing hexagons. The presence of alternating B and N terminated vertices in hexagonal hBN crystals offers the possibility of hBN/graphene in-plane heterostructure synthesis with B–C and N–C bonding at the interface, which is not possible with N terminated triangular crystals. Supplying the controlled amount of BN building blocks to synthesize triangular and hexagonal crystals and modulation from hexagons to triangles is a major achievement (in the present work) due to single parameter control over the experiment.

Efficient single photon emission from a high-purity hexagonal boron nitride crystal

Among a variety of layered materials used as building blocks in van der Waals heterostructures, hexagonal boron nitride (hBN) appears as an ideal platform for hosting optically-active defects owing to its large bandgap ($\sim 6$ eV). Here we study the optical response of a high-purity hBN crystal under green laser illumination. By means of photon correlation measurements, we identify individual defects emitting a highly photostable fluorescence under ambient conditions. A detailed analysis of the photophysical properties reveals a high quantum efficiency of the radiative transition, leading to a single photon source with very high brightness. These results illustrate how the wide range of applications offered by hBN could be further extended to photonic-based quantum information science and metrology.

Tunable perfect absorption at infrared frequencies by a graphene-hBN hyper crystal

In this article, we have theoretically demonstrated that the perfect absorption at infrared frequencies can be achieved and controlled by using a graphene-hexagonal Boron Nitride (hBN) hyper crystal. hBN, the latest natural hyperbolic material, can be regarded as an excellent substrate to form a hyper crystal with graphene. Although the perfect absorption by a half-space of hBN crystal can be achieved due to its high optical anisotropy, but the perfect absorption can only appear at certain fixed wavenumber and incidence angle. By introducing a graphene-hBN hyper crystal, we can get perfect absorption at different wavenumbers and incidence angles by varying the Fermi energy level of graphene sheets via electrostatic biasing. We show that the perfect absorption can be realized at different Fermi energies for TM waves.

Critical coupling using the hexagonal boron nitride crystals in the mid-infrared range

We theoretically demonstrate the perfect absorption phenomena in the hexagonal boron nitride (hBN) crystals in the mid-infrared wavelength ranges by means of critical coupling with a one-dimensional photonic crystal spaced by the air. Different from the polymer absorbing layer composed by a metal-dielectric composite film, the hyperbolic dispersion characteristics of hBN can meet the condition of critical coupling and achieve the total absorption in the mid-infrared wavelength ranges. However, the critical coupling phenomenon can only appear in the hBN crystals with the type II dispersion. Moreover, we discuss the influence of the thickness of hBN, the incident angle, and the thickness and permittivity of the space dielectric on the total absorption. Ultimately, the conditions for absorption enhancement and the optimization methods of perfect absorption are proposed, and the design rules for a totally absorbing system under the different conditions are achieved.

Synthesis of Hexagonal Boron Nitride Monolayer: Control of Nucleation and Crystal Morphology

Monolayer hexagonal boron nitride (hBN) attracts significant attention due to the potential to be used as a complementary two-dimensional dielectric in fabrication of functional 2D heterostructures. Here we investigate the growth stages of the hBN single crystals and show that hBN crystals change their shape from triangular to truncated triangular and further to hexagonal depending on copper substrate distance from the precursor. We suggest that the observed hBN crystal shape variation is affected by the ratio of boron to nitrogen active species concentrations on the copper surface inside the CVD reactor. Strong temperature dependence reveals the activation energies for the hBN nucleation process of ∼5 eV and crystal growth of ∼3.5 eV. We also show that the resulting h-BN film morphology is strongly affected by the heating method of borazane precursor and the buffer gas. Elucidation of these details facilitated synthesis of high quality large area monolayer hexagonal boron nitride by atmospheric pressure ...

Amplitude- and Phase-Resolved Nanospectral Imaging of Phonon Polaritons in Hexagonal Boron Nitride

Phonon polaritons are quasiparticles resulting from strong coupling of photons with optical phonons. Excitation and control of these quasiparticles in 2D materials offer the opportunity to confine and transport light at the nanoscale. Here, we image the phonon polariton (PhP) spectral response in thin hexagonal boron nitride (hBN) crystals as a representative 2D material using amplitude- and phase-resolved near-field interferometry with broadband mid-IR synchrotron radiation. The large spectral bandwidth enables the simultaneous measurement of both out-of-plane (780 cm-1) and in-plane (1370 cm-1) hBN phonon modes. In contrast to the strong and dispersive in-plane mode, the out-of-plane mode PhP response is weak. Measurements of the PhP wavelength reveal a proportional dependence on sample thickness for thin hBN flakes, which can be understood by a general model describing two-dimensional polariton excitation in ultrathin materials.

High pressure synthesis of BN and BCN crystals and their functionalization

Hexagonal BN (hBN) and cubic BN (cBN) are known as the representative crystal structures of BN. The former is chemically and thermally stable, and has been widely used as an electrical insulator and heat-resistant materials. The latter, which is a high-density phase, is an ultra-hard material second only to diamond. Recently, some progresses in the synthesis of high purity BN crystals were achieved by using Ba-BN as a solvent material at high pressure crystal growth [1]. Band-edge natures (cBN Eg=6.2eV and hBN Eg=6eV) were characterized by their optical properties. The key issue to obtain high purity crystals is to reduce oxygen and carbon contamination in the growth circumstances. It should be emphasized that hBN exhibits attractive potential for deep ultraviolet (DUV) light emitter [2 ] and also superior properties as substrate of graphene devices [3]. In this study, the effect of carbon impurity in BN is investigated. Three types of experimental approaches were carried out; (1) synthesis of high purity hBN single crystals and its characterization with respect to residual carbon, (2) high temperature solid state diffusion of carbon into hBN and its characterization, and (3) high temperature annealing of turbostratic B-C-N(t-BCN) compound under high pressure. t-BCN flakes obtained by chemical vapour deposition process was annealed near 3000C and 2GPa so as to become well crystallized.At annealing near 3000C at 2GPa with graphite, carbon incorporation of 1E21/cm3 in hBN was achieved with exhibiting totally different Cathode Luminescence spectra feature with high purity hBN crystals. Since major carbon contribution may affect the crystal structure of the 2-D layers stacking in hBN system, phase stability of BCN ternary phase will be introduced by the experimental results of high temperature annealing. Furthermore, effect of carbon impurity upon the synthesis of wurtzite BN from highly crystalline hBN via martensitic phase transformation will also be introduced.

Crystal growth of hexagonal boron nitride (hBN) from Mg–B–N solvent system under high pressure

Transparent and colorless hexagonal boron nitride (hBN) single crystals were grown from the Mg–B–N system using high-pressure–high-temperature cubic anvil technique. By varying the synthesis conditions we could determine the sequence of phase transformations occurring in the Mg–B–N system, construct the pressure–temperature (P–T) phase diagram and discuss the possible growth mechanism. The largest plate-like-shaped hBN crystals with sizes up to 2.5mm in length and up to 10µm in thickness were grown at 30kbar and 1900–2100°C. The hBN crystals exhibited strong, narrow diffraction peaks typical of well-ordered stacking crystal planes, with the c-axis perpendicular to the crystal face. A characteristic Raman peak observed at 1367cm−1 with a full width at half maximum of 8cm−1 corresponds to the E2g vibration mode and indicates the high purity and order of hBN crystals grown by this method. From the practical point of view this work can stimulate further explorations of the Mg–B–N solvent system to obtain isotopically-enriched h10BN crystals, which can act as a key element in solid-state neutron detector devices.

Characterization of bulk hexagonal boron nitride single crystals grown by the metal flux technique

The optical and physical properties of hexagonal boron nitride single crystals grown from a molten metal solution are reported. The hBN crystals were grown by precipitation from a nickel–chromium flux with a boron nitride source, by slowly cooling from 1500°C at 2–4°C/h under a nitrogen flow at atmospheric pressure. The hBN crystals formed on the surface of the flux with an apparent crystal size up to 1–2mm in diameter. Individual grains were as large as 100–200µm across. Typically, the flakes removed from the metal were 6–20µm thick. Optical absorption measurements suggest a bandgap of 5.8eV by neglecting the binding energy of excitons in hBN. The highest energy photoluminescence peak was at 5.75eV at room temperature. The hBN crystals typically had a pit density of 5×106cm−2 after etching in a molten eutectic mixture of potassium hydroxide and sodium hydroxide. The quality of these crystals suggests they are suitable as substrates for two dimensional materials such as graphene and gallium nitride based devices.

Growth of hBN Using Metallic Boron: Isotopically Enriched h10BN and h11BN

ABSTRACTHexagonal boron nitride (hBN) crystals enriched in 10B and 11B isotopes were synthesized using a high temperature (1500° C) Ni-Cr-B reactive-precipitation growth under a N2 atmosphere. Two growth mechanisms were observed: conventional defect-facilitated bulk growth which produced crystals with a platelet-like habit with width and thickness of 20-30 μm and 5 μm, respectively, and vapor-liquid-solid interface growth of hBN whiskers with lengths and diameters as large as 70 μm and 5 μm, respectively. Similar growth mechanisms were seen for samples enriched in either isotope. Isotopic analysis via secondary-ion mass spectrometry showed boron concentrations of 84.4 at% and 93.0 at% for the majority isotopes in the 10B-rich and 11B-rich samples, respectively. Raman spectroscopy showed an increase in peak Raman shift for the 10B-rich sample, having two barely resolved peaks at 1393.5 and 1388.8 cm-1, and a decrease for the 11B-rich sample, having peak at 1359.5 cm-1 (FWHM of 9.4 cm-1), compared to that of natural hBN, with its peak at 1365.8 cm-1 (FWHM of 10.3 cm-1). Raman shift showed a linear trend with increasing 10B concentration allowing for a calibration curve to be developed to estimate 10B enrichment in hBN using non-destructive methods.

Transferring Few-Layer Graphene Sheets on Hexagonal Boron Nitride Substrates for Fabrication of Graphene Devices

We have developed a dry transfer method that allows graphene to be transferred from polymer- thyl-methacrylate (PMMA)/Si (silicon) substrates on commercially available hexagonal boron ni- tride (hBN) crystals. With this method we are able to fabricate graphene devices with little wrin- kles and bubbles in graphene sheets, but that do not degrade the electronic quality more than the SiO2 substrate does. For hBN to perform the function described above substrate cleanliness is critical to get high quality graphene devices. Using hBN as a substrate, graphene exhibits enhanced mobility, reduced carrier inhomogeneity, and reduced intrinsic doping compared to graphene on SiO2 substrate.

Optimization of Ni–Cr flux growth for hexagonal boron nitride single crystals

Hexagonal boron nitride (hBN) single crystals were grown using a Ni–Cr flux growth method. The crystallization cooling rate, soak temperature and soak time were controlled to determine their effect on crystal size and quality. A cooling rate of 2°C/h produced the best quality hBN crystals. The maximum crystal width increased with soak temperature from 1mm at 1450°C to 5mm at 1700°C. The crystal thickness decreased with soak temperature from 500µm at 1500°C to 40µm at 1700°C. A soak time of 24 to 48h produced the maximum crystal thickness. X-ray diffraction and Raman spectroscopy confirmed that the crystals were highly ordered and of high purity.

Deep Ultraviolet Light-Emitting Hexagonal Boron Nitride Synthesized at Atmospheric Pressure

Materials emitting light in the deep ultraviolet region around 200 nanometers are essential in a wide-range of applications, such as information storage technology, environmental protection, and medical treatment. Hexagonal boron nitride (hBN), which was recently found to be a promising deep ultraviolet light emitter, has traditionally been synthesized under high pressure and at high temperature. We successfully synthesized high-purity hBN crystals at atmospheric pressure by using a nickel-molybdenum solvent. The obtained hBN crystals emitted intense 215-nanometer luminescence at room temperature. This study demonstrates an easier way to grow high-quality hBN crystals, through their liquid-phase deposition on a substrate at atmospheric pressure.

Synthesis of high-purity boron nitride single crystals under high pressure by using Ba–BN solvent

High-purity cubic boron nitride (cBN) and hexagonal boron nitride (hBN) single crystals were synthesised at 4.5 GPa and 1500 °C using barium boron nitride as a solvent. Secondary ion mass spectrometry was used to analyse impurities in the crystals. Fine cBN and hBN crystals, whose oxygen and carbon concentrations were less than 10 18 atoms/cm 3, were obtained, and their band-edge optical properties were measured by cathodoluminescence spectroscopy. High-purity hBN single crystals exhibited intense ultraviolet emission, demonstrating their promise for use as deep ultraviolet-light emitters.

Synthesis of Cubic and Hexagonal Boron Nitrides by Using Ni Solvent under High Pressure

The reaction boundary between boron nitride (BN) and nickel (Ni) metal was studied at high pressures (HP) of 4.5–6 GPa in the high temperature (HT) 1300–1900 °C region. It can be seen that Ni dissolves BN followed by the precipitation of BN crystals under HP and HT. Full conversion from hexagonal BN (hBN) to cubic BN (cBN) took place above 6 GPa and 1700 °C while cBN started to crystallize from 5.5 GPa and 1500 °C. Near the reaction boundary in the cBN formed region, hBN also spontaneously recrystallized. Although crystallized cBN exhibited irregularity in shape, the recrystallized hBN exhibited crystalline facets and intrinsic optical characteristics. It was found that high quality hBN crystals can be synthesized in the Ni–BN system under HP and HT.