Hexagonal Boron Nitride Crystals Research Articles

Supercontinuum second harmonic generation spectroscopy of atomically thin semiconductors.

Two-dimensional semiconductors have recently emerged as promising materials for novel optoelectronic devices. In particular, they exhibit favorable nonlinear optical properties. Potential applications include broadband and ultrafast light sources, optical signal processing, and generation of nonclassical light states. The prototypical nonlinear process second harmonic generation (SHG) is a powerful tool to gain insight into nanoscale materials because of its dependence on crystal symmetry. Material resonances also play an important role in the nonlinear response. Notably, excitonic resonances critically determine the magnitude and spectral dependence of the nonlinear susceptibility. We perform ultrabroadband SHG spectroscopy of atomically thin semiconductors by using few-cycle femtosecond infrared laser pulses. The spectrum of the second harmonic depends on the investigated material, MoS2 or WS2, and also on the spectral and temporal shape of the fundamental laser pulses used for excitation. Here, we present a method to remove the influence of the laser by normalization with the flat SHG response of thin hexagonal boron nitride crystals. Moreover, we exploit the distinct angle dependence of the second harmonic signal to suppress two-photon photoluminescence from the semiconductor monolayers. Our experimental technique provides the calibrated frequency-dependent nonlinear susceptibility χ(2)(ω) of atomically thin materials. It allows for the identification of the prominent A and B exciton resonances, as well as excited exciton states.

Phonon Polariton-assisted Infrared Nanoimaging of Local Strain in Hexagonal Boron Nitride.

Strain plays an important role in condensed matter physics and materials science because it can strongly modify the mechanical, electrical, and optical properties of a material and even induce a structural phase transition. Strain effects are especially interesting in atomically thin two-dimensional (2D) materials, where unusually large strain can be achieved without breaking them. Measuring the strain distribution in 2D materials at the nanometer scale is therefore greatly important but is extremely challenging experimentally. Here, we use near-field infrared nanoscopy to demonstrate phonon polariton-assisted mapping and quantitative analysis of strain in atomically thin polar crystals of hexagonal boron nitride (hBN) at the nanoscale. A local strain as low as 0.01% can be detected using this method with ∼20 nm spatial resolution. Such ultrasensitive nanoscale strain imaging and analysis technique opens up opportunities for exploring unique local strain structures and strain-related physics in 2D materials. In addition, experimental evidence for local strain-induced phonon polariton reflection is also provided, which offers a new approach to manipulate light at deep subwavelength scales for nanophotonic devices.

Optical contrast signatures of hexagonal boron nitride on a device platform

Atomic layers of hexagonal boron nitride (h-BN) crystal, primarily serving as atomically-smooth dielectric layers in two-dimensional (2D) electronics and structural materials in 2D nanoelectromechanical systems (NEMS), have recently emerged as a promising platform for nanoscale optics and photonics. Ultrawide bandgap (~5.9 eV) of h-BN promises ultraviolet (UV) and deep ultraviolet (DUV) responsivity; it can host defect states related to single photon generation. Meanwhile, it also gives rise to the visibility challenge of these atomically-thin crystals, because of the optical transparency in the visible range. In this work, we conduct a comprehensive study, synergizing numerical modeling and experimental efforts, to reveal the optical contrast signatures of h-BN on various substrates. We demonstrate that the visualization and thickness identification based on optical contrast are applicable to mechanically suspended h-BN crystals, without interference enhancement from the commonly used oxidized silicon (SiO2 on Si) wafer. The understanding and protocols developed here offer a non-contact, rapid thickness estimation method for h-BN thin films on suspended device platforms, where conventional contact methods such as scanning probes are incapable or cumbersome. The results and methods presented here also serve as valuable guidelines for engineering the h-BN platform towards 2D nanophotonic and optoelectronic applications.

Plasmons in realistic graphene/hexagonal boron nitride moiré patterns

Van der Waals heterostructures employing graphene and hexagonal boron nitride (hBN) crystals have emerged as a promising platform for plasmonics thanks to the tunability of their collective modes with carrier density and record values for plasmonics figures of merit. In this Article we investigate theoretically the role of moir\'e-pattern superlattices in nearly aligned graphene on hBN by using continuum-model Hamiltonians derived from ab initio calculations. We calculate the system's energy loss function for a variety of chemical potential values that are accessible in gated devices. Our calculations reveal that the electron-hole asymmetry of the moir\'e bands leads to a remarkable asymmetry of the plasmon dispersion between positive and negative chemical potentials, showcasing the intricate band structure and rich absorption spectrum across the secondary Dirac point gap for the hole bands.

Dipole modelling for a robust description of subdiffractional polariton waves.

The nanophotonics of van der Waals (vdW) materials relies critically on the electromagnetic properties of polaritons defined on sub-diffraction length scales. Here, we use a full electromagnetic Hertzian dipole antenna (HDA) model to describe the hyperbolic phonon polaritons (HP2s) in vdW crystals of hexagonal boron nitride (hBN) on a gold surface. The HP2 waves are investigated by broadband synchrotron infrared nanospectroscopy (SINS) which covers the type I and type II hyperbolic bands simultaneously. Basically, polariton waves, observed by SINS, are assigned to the resultant electric field from the summation over the irradiated electric fields of dipoles distributed along the crystal edge and at the tip location and a non-propagating field. The values of polariton momenta and damping extracted from the HDA model present excellent agreement with theoretical predictions. Our analysis shows that the confinement factor of type I HP2s exceeds that of the type II ones by up to a factor of 3. We extract anti-parallel group velocities (vg) for type I (vg,typeI = -0.005c, c is the light velocity in a vacuum) in relation to type II (vg,typeII = 0.05c) polaritonic pulses, with lifetimes of ∼0.6 ps and ∼0.3 ps, respectively. Furthermore, by incorporating consolidated optical-near field theory into the HDA model, we simulate real-space images of polaritonic standing waves for hBN crystals of different shapes. This approach reproduces the experiments with a minimal computational cost. Thus, it is demonstrated that the HDA modelling self-consistently explains the measured complex-valued polariton near-field, while being a general approach applicable to other polariton types, like plasmon- and exciton-polaritons, active in the wide range of vdW 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.

Infrared Fingerprints of Natural 2D Talc and Plasmon–Phonon Coupling in Graphene–Talc Heterostructures

Two-dimensional (2D) materials occupy noteworthy place in nanophotonics providing for subwavelength light confinement and optical phenomena dissimilar to those of their bulk counterparts. In the mid-infrared, graphene-based heterostructures and van der Waals crystals of hexagonal boron nitride (hBN) overwhelmingly concentrate the attention by exhibiting real-space nano-optics from plasmons, phonon-polaritons and hybrid plasmon phonon-polaritons quasiparticles. Here we present the mid-infrared nanophotonics of talc, a natural atomically flat layered material, and graphene-talc (G-talc) heterostructures using broadband synchrotron infrared nano-spectroscopy. We achieve wavelength tuning of the talc resonances, assigned to in- and out-of-plane vibrations by changing the thickness of the crystals, which serves as its infrared fingerprints. Moreover, we encounter coupling of the graphene plasmons polaritons with surface optical phonons of talc. As in the case of the G-hBN heterostructures, this coupling configures hybrid surface plasmon phonon-polariton modes causing 30 % increase in intensity for the out-of-plane mode, blue-shift for the in-plane mode and we have succeeded in altering the amplitude of such hybridization by varying the gate voltage. Therefore, our results promote talc and G-talc heterostructures as appealing materials for nanophotonics, like hBN and G-hBN, with potential applications for controllably manipulating infrared electromagnetic radiation at the subdiffraction scale.

Edge controlled growth of hexagonal boron nitride crystals on copper foil by atmospheric pressure chemical vapor deposition

We report on a precursor supply technique controlled h-BN crystal growth over the catalytic activity of Cu by APCVD.

Isotope engineering of van der Waals interactions in hexagonal boron nitride.

Hexagonal boron nitride is a model lamellar compound where weak, non-local van der Waals interactions ensure the vertical stacking of two-dimensional honeycomb lattices made of strongly bound boron and nitrogen atoms. We study the isotope engineering of lamellar compounds by synthesizing hexagonal boron nitride crystals with nearly pure boron isotopes (10B and 11B) compared to those with the natural distribution of boron (20 at% 10B and 80 at% 11B). On the one hand, as with standard semiconductors, both the phonon energy and electronic bandgap varied with the boron isotope mass, the latter due to the quantum effect of zero-point renormalization. On the other hand, temperature-dependent experiments focusing on the shear and breathing motions of adjacent layers revealed the specificity of isotope engineering in a layered material, with a modification of the van der Waals interactions upon isotope purification. The electron density distribution is more diffuse between adjacent layers in 10BN than in 11BN crystals. Our results open perspectives in understanding and controlling vanderWaals bonding in layered materials.

Large-Scale Growth of High-Quality Hexagonal Boron Nitride Crystals at Atmospheric Pressure from an Fe–Cr Flux

Single crystals of hexagonal boron nitride (hBN) have recently been envisioned for electronic, optoelectronic, and nanophotonic applications. In this study, the production of large-scale, high-quality hBN single crystals via precipitation from a new solvent composed of Fe and Cr was demonstrated to be viable at atmospheric pressure. The clear and colorless crystals have a maximum domain size of around 2 mm and a thickness of around 200 μm. The Raman spectra and photoluminescence emission spectra demonstrate that the crystals produced with this solvent are pure hBN phase, and with low defect and residual impurity concentrations. The use of an Fe–Cr mixture provides a lower cost alternative to the more common Ni–Cr solvent for growing large hBN of comparable quality.

Hexagonal boron nitride nanomechanical resonators with spatially visualized motion

Atomic layers of hexagonal boron nitride (h-BN) crystal are excellent candidates for structural materials as enabling ultrathin, two-dimensional (2D) nanoelectromechanical systems (NEMS) due to the outstanding mechanical properties and very wide bandgap (5.9 eV) of h-BN. In this work, we report the experimental demonstration of h-BN 2D nanomechanical resonators vibrating at high and very high frequencies (from ~5 to ~70 MHz), and investigations of the elastic properties of h-BN by measuring the multimode resonant behavior of these devices. First, we demonstrate a dry-transferred doubly clamped h-BN membrane with ~6.7 nm thickness, the thinnest h-BN resonator known to date. In addition, we fabricate circular drumhead h-BN resonators with thicknesses ranging from ~9 to 292 nm, from which we measure up to eight resonance modes in the range of ~18 to 35 MHz. Combining measurements and modeling of the rich multimode resonances, we resolve h-BN’s elastic behavior, including the transition from membrane to disk regime, with built-in tension ranging from 0.02 to 2 N m−1. The Young’s modulus of h-BN is determined to be EY≈392 GPa from the measured resonances. The ultrasensitive measurements further reveal subtle structural characteristics and mechanical properties of the suspended h-BN diaphragms, including anisotropic built-in tension and bulging, thus suggesting guidelines on how these effects can be exploited for engineering multimode resonant functions in 2D NEMS transducers.

Assessing Hexagonal Boron Nitride Crystal Quality by Defect Sensitive Etching

Journal Article Assessing Hexagonal Boron Nitride Crystal Quality by Defect Sensitive Etching Get access T Hoffman, T Hoffman Dept of Chemical Engineering, Kansas State University, Manhattan, Kansas, USA Search for other works by this author on: Oxford Academic Google Scholar Y Zhang, Y Zhang Dept of Chemical Engineering, Kansas State University, Manhattan, Kansas, USA Search for other works by this author on: Oxford Academic Google Scholar S Liu, S Liu Dept of Chemical Engineering, Kansas State University, Manhattan, Kansas, USA Search for other works by this author on: Oxford Academic Google Scholar N Khan, N Khan School of Science and Technology, Georgia Gwinnett College, Lawrenceville, Georgia, USA Search for other works by this author on: Oxford Academic Google Scholar ME Twigg, ME Twigg United States Naval Research Laboratory, Washington, DC, USA Search for other works by this author on: Oxford Academic Google Scholar ND Bassim, ND Bassim Dept of Materials Science and Engineering, JHE 357, McMaster University, Hamilton, Ontario, CANADA Search for other works by this author on: Oxford Academic Google Scholar JH Edgar JH Edgar Dept of Chemical Engineering, Kansas State University, Manhattan, Kansas, USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 23, Issue S1, 1 July 2017, Pages 1518–1519, https://doi.org/10.1017/S143192761700825X Published: 04 August 2017

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.

Experimentally determined edge orientation of triangular crystals of hexagonal boron nitride

This work demonstrates a method to directly determine edge orientations of three‐fold symmetric two‐dimensional crystals in real space, without resolving the edge structure. In an annular dark‐field scanning transmission electron microscope, the orientation of an edge is derived from the overall edge direction recorded at a low magnification and the crystal orientation revealed by atomic resolution, element‐contrast imaging at a high magnification. By applying this method, the edge orientation of equilateral triangle‐shaped hexagonal boron nitride (h‐BN) crystals grown by chemical vapor deposition on Cu is experimentally determined for the first time. The experimentally determined nitrogen‐terminated zigzag edges validate the kinetic theory of h‐BN crystal growth, which is a useful case study of the growth of 2D crystals lacking inversion symmetry.

The synthesis of hexagonal boron nitride via chemical vapor deposition

Hexagonal boron nitride, a two dimensional wide bandgap insulator material which has a similar structure with graphene, has many excellent physical properties. For instance, hexagonal boron nitride has super mechanical properties, chemical and thermal stabilities, resulting in its wide potential applications in the areas of non-linear optics, ultraviolet laser and protection layers. In addition, due to its atomically smooth surface that is free of trapped charge and dangling bonds, hexagonal boron nitride is a super dielectric material for graphene electrical devices. Moreover, hexagonal boron nitride can be fabricated into planar or vertical hetero-structures with other two dimensional materials, which show unique properties and potential applications in novel electronic devices. How to synthesize large scale and high quality hexagonal boron nitride is currently key scientific issue. Up to now, many strategies, including mechanical exfoliation, deposition by ion beam sputtering, co-segregation, chemical vapor deposition and so on, have been developed for growing this material. Among these methods, chemical vapor deposition provides a versatile platform for modulating the growth and etching process of this material and producing a large scale material with high quality. This review summarizes a series of work regarding the synthesis of hexagonal boron nitride by chemical vapor deposition method, in which most recent work is also presented, and discusses the choices of reaction precursors and substrates in details. The use of solid borane ammonia complex leads to better controls over the properties of hexagonal boron nitride such as layer numbers and uniformity compared with the use of mixed B and N compounds as precursors. On the other hand, the types, surface structures and properties of the supporting substrates are critical for the growth of hexagonal boron nitride. For example, Cu substrate has advantages for growing single layered hexagonal boron nitride crystal or film, while the use of Ni or Fe substrates usually leads to multi-layered materials that are applicable to different applications. This difference largely results from two different growth mechanisms in two typical systems, i.e., surface diffusion and co-segregation mechanism. Moreover, owing to the nature of complex systems of chemical vapor deposition, kinetic controls over the growth and etching of hexagonal boron nitride can result in a more precise controls over size, shape and edge structures of this material, and the related progresses are presented. Finally, the key challenge and future directions are also presented. From the basic scientific research and large-scale application points of view, there are still plenty room for further studies of controllable synthesis of hexagonal boron nitride. How to optimize the growth process, further understand the growth mechanism, and realize the large sized layer number controlled growth of single crystal hexagonal boron nitride are important issues required to be solved in future. Further explorations of a better substrate such as an alloy possibly leads to combined advantages of components in an alloy, overcoming the difficulties associated with uniform production of high quality hexagonal boron nitride crystal or film. In addition, searching better reaction precursors also deserves attention, and working on this may be rewarding.

Synthesis of multilayered hexagonal boron nitride microcrystals as a potential hydrogen storage element

Top-down approach has been used to synthesize pure, highly crystalline, multilayered micron size crystals of hexagonal boron nitride (BNMCs) at the top of Silicon substrate at 800°C by using bulk boron nitride powder as a precursor. The synthesized crystals have different interlayers spacing from left to right (0.33nm, 0.37nm and 0.35nm) and at the center (~0.24nm). The former spacing corresponds to d(002) spacing whereas the later corresponds to d(010) spacing in h-BN. The sharpness of the peaks in XRD, Raman and FTIR spectrums correspond to highly crystalline nature of BNMCs whereas the locations of the peaks verify the h-BN nature of BNMCs. The B-N bonded BNMCs with larger surface area can be an excellent choice as a hydrogen storage element.

Graphene-Hexagonal Boron Nitride Heterostructure as a Tunable Phonon–Plasmon Coupling System

The layered van der Waals (vdW) heterostructure, assembled from monolayer graphene, hexagonal boron nitride (h-BN) and other atomic crystals in various combinations, is emerging as a new paradigm with which to attain desired electronic and optical properties. In this paper, we study theoretically the mid-infrared optical properties of the vdW heterostructure based on the graphene–h-BN system. The light–matter interaction of this heterostructure system is described by the hyperbolic phonon–plasmon polaritons which originate from the coupling modes of surface plasmon polaritons (SPPs) in graphene with hyperbolic phonon polaritons (HPPs) in h-BN. By numerical simulation, we find that the coupling modes are governed by the Fermi level of monolayer graphene, the thickness of the h-BN slab and the mode excitation sequence of SPPs and HPPs. Moreover, the response of the coupling modes of the graphene–h-BN heterostructure on a noble metal layer is also proposed in this paper.

Centimeter-sized epitaxial h-BN films

We report the growth and transfer of centimeter-sized, epitaxial hexagonal boron nitride (h-BN) few-layer films using Ni(111) single-crystal substrates. The h-BN films were heteroepitaxially grown on 10 × 10 mm2 or 20 × 20 mm2 Ni(111) substrates using atmospheric pressure chemical vapor deposition with a single ammonia-borane precursor. The grown films were transferred to arbitrary substrates via an electrochemical delamination technique, and the remaining Ni(111) substrates were repeatedly re-used. A careful analysis of the growth parameters revealed that the crystallinity and area coverage of the h-BN films were mostly sensitive to the sublimation temperature of the ammonia-borane source. Moreover, various physical characterizations confirmed that the grown films exhibited the typical characteristics of hexagonal boron nitride layers over the entire area. Furthermore, the heteroepitaxial relationship between h-BN and Ni(111) and the overall crystallinity of the film were thoroughly investigated using a synchrotron radiation X-ray diffraction analysis including θ–2θ scans, grazing incident diffraction, and reciprocal space mapping. The crystallinity at the microscopic scale was further investigated using transmission electron microscopy (TEM)-based techniques, including selective area electron diffraction pattern mapping, electron back-scattered diffraction, and high-resolution TEM. Transparent and flexible ceramic films for electronic devices can now be made at centimetre scales by a low-cost technique. Hexagonal boron nitride (h-BN) crystals are attractive platforms for building next-generation circuits because their flat, graphite-like surfaces resist mechanical and chemical damage while offering excellent signal isolation. Gyu-Chul Yi from Seoul National University and colleagues have developed a way to improve large-scale manufacturing of these films using a reusable nickel template. Nickel has a surface structure that helps h-BN films grow with high crystallinity following chemical vapor deposition, but detaching the resulting ceramic layer has always proved troublesome. The Korean team solved this problem through an electrochemical delamination process that seamlessly separates h-BN films with dimensions up to 20 × 20 square millimetres using a stream of hydrogen bubbles. Centimeter-sized, epitaxial hexagonal boron nitride (h-BN) few-layer films were heteroepitaxially grown on Ni(111) single-crystal substrates using atmospheric pressure chemical vapor deposition with ammonia-borane single precursor. The grown films were transferred to arbitrary substrates via an electrochemical delamination technique, and the remaining Ni(111) substrates were repeatedly re-used. Over repeated growth and transfer after the initial annealing, no significant degradation of Ni(111) substrates was observed and the crystallinity of h-BN layers was reproduced reliably. The grown h-BN films showed typical physical characteristics of h-BN, with a high uniformity over a wide area. The large-area synthesis and transfer of atomically thin uniform epitaxial h-BN layers can be applied in various fields where high quality two-dimensional insulating layers are required.

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.

Scaling approach to tight-binding transport in realistic graphene devices: The case of transverse magnetic focusing

Ultra-clean graphene sheets encapsulated between hexagonal boron nitride crystals host two-dimensional electron systems in which low-temperature transport is solely limited by the sample size. We revisit the theoretical problem of carrying out microscopic calculations of non-local ballistic transport in such micron-scale devices. By employing the Landauer-Buttiker scattering theory, we propose a novel scaling approach to tight-binding non-local transport in realistic graphene devices. We test our numerical method against experimental data on transverse magnetic focusing (TMF), a textbook example of non-local ballistic transport in the presence of a transverse magnetic field. This comparison enables a clear physical interpretation of all the observed features of the TMF signal, including its oscillating sign.