Hexagonal Boron Nitride Crystal Growth from Iron, a Single Component Flux

Abstract

The highest quality hexagonal boron nitride (hBN) crystals are grown from molten solutions. For hBN crystal growth at atmospheric pressure, typically the solvent is a combination of two metals, one with a high boron solubility and the other to promote nitrogen solubility. In this study, we demonstrate that high-quality hBN crystals can be grown at atmospheric pressure using pure iron as a flux. The ability to produce excellent-quality hBN crystals using pure iron as a solvent is unexpected, given its low solubility for nitrogen. The properties of crystals produced with this flux matched the best values ever reported for hBN: a narrow Raman E2g vibration peak (7.6 cm–1) and strong phonon-assisted peaks in the photoluminescence spectra. To further test their quality, the hBN crytals were used as a substrate for WSe2 epitaxy. WSe2 was deposited with a low nucleation density, indicating the low defect density of the hBN. Lastly, the carrier tunneling through our hBN thin layers (3.5 nm) follows the Fowler–Nordheim model, with a barrier height of 3.7 eV, demonstrating hBN’s superior electrical insulating properties. This ability to produce high-quality hBN crystals in such a simple, environmentally friendly and economical process will advance two-dimensional material research by enabling integrated devices.

Defect Engineering of Monoisotopic Hexagonal Boron Nitride Crystals via Neutron Transmutation Doping

“…The PL signals corresponding to phonon-assisted emission in h-BN are observed for energies between 5.7 and 5.9 eV . These phonon replicas in the PL spectra from our hBN crystals are strong evidence of their excellent crystallinity. − The set of emission lines between 5.4 and 5.7 eV (so-called “D-lines”) was attributed to inter- K valley scattering assisted by phonons at the K point of the Brillouin zone where stacking fault defects in the bulk hBN provide the density of states to make this scattering observable . Not surprisingly, the near band-edge emission from the pBN sample was much broader and did not exhibit any sharp structure compared to that found from the hBN single crystal samples.…”

Polytypes of sp2-Bonded Boron Nitride

“…However, its X-ray diffraction spectrum is not as good as those of a high-quality hBN sample. In Figure 9, we gathered typical PL spectra recorded on BN samples, grown with Cr + Ni + C (green), Fe + V (red), and a very high-quality hBN sample grown using Fe only as a metal solvent [137] (blue). The astonishing feature is a new photoluminescence line at 6.035 eV in samples grown from Cr + Ni + C and Fe + V that we attribute to the signature of the direct bandgap of the AB stacking, i.e., bBN, and that is never detected in high-purity BN samples.…”

“…Note that it is worthwhile attributing the 6.035 eV luminescence to the recombination at the bottleneck of the lowest polariton branch of the direct exciton polariton systems; thus, the value of the excitonic gap is higher than this value by an amount dictated by the magnitude of the longitudinal-transverse (LT) splitting between the branches of dispersion of the exciton-polariton. This LT splitting is huge in hBN [119], about 400 meV, and from the band structure calculations referenced above [57,62,[121][122][123][124][125][127][128][129][130][131][132][133]136,137], there is no reason for it to be drastically different in bBN.…”

Isotope effect on the thermal expansion coefficient of atomically thin boron nitride

“…h 10 BN crystals were synthesized and grown by the nickel-chromium solvent method [19,39]. The h 11 BN crystals were synthesized and grown from a pure iron solvent [40]. According to our previous study, the 10 BN and 11 BN had 99.2% and 99.9% isotope purity, respectively [21].…”