| Property | Value |
| Material | hBN - Hexagonal boron nitride |
| Bulk Band Gap | Indirect 5.95 eV |
| Monolayer Band Gap | Direct 6.1 eV |
| Crystal Structure | Hexagonal |
| Crystal Group | P6₃/mmc |
Grade A - 20 crystals between 0.2 - 2mm
Grade B - 100mg 0.1-0.5mm crystals
WHAT IS HEXAGONAL BORON NITRIDE?
Hexagonal boron nitride (hBN) is a material with an analogous structure to graphite, consisting of alternating atoms of boron and nitrogen, arranged in a hexagonal structure. Bulk hBN is an insulator with a band gap of 5.95 eV. Due to its strong emission in the deep ultraviolet, this 2D material is a promising candidate for use in ultraviolet optoelectronics. It also has very high chemical and thermal stability, superior mechanical strength, thermal conductivity, and high optical transparency. Therefore, it can be used under extreme conditions. Furthermore, in the 2D limit its wide-band gap transitions from indirect (bulk) to direct (monolayer). This wide band gap makes this material the thinnest electrically insulating material and can be used in several electronics and spintronics devices. hBN monolayers have applications in photonic devices, piezoelectric actuators and sensors, ultraviolet lasers, biomedicine, fuel cells, nanofillers, electronic packaging, dielectric tunneling, and photoelectric devices.
ELECTRONIC PROPERTIES OF HBN
In bulk, hBN is a semiconductor with an indirect band gap of about 5,95 eV and hexagonal crystal structure with P6₃/mmc crystal structure symmetry. When exfoliated to a single crystal, the band structure evolves and becomes direct, with a size of 6,1 eV.
RAMAN SPECTRUM OF HBN
The Raman spectrum of hBN typically shows two main peaks, the E2g and A1g modes. The E2g mode is a zone-center phonon mode corresponding to an in-plane vibration of the boron and nitrogen atoms. It appears at around 1366 cm-1 and is often called the "first-order peak". The A1g mode, on the other hand, corresponds to an out-of-plane vibration of the boron atoms and appears at around 1582 cm-1. It is sometimes referred to as the "second-order peak". In addition to these two main peaks, other weaker peaks may also be observed in the Raman spectrum of hBN, such as the B1g mode at around 1000 cm-1 and the E1g mode at around 690 cm-1. These additional peaks can provide information on the crystal symmetry, crystal quality, and defects in the hBN sample. The Raman spectrum of hBN is useful for determining the number of layers of hBN, with the peak positions shifting and the intensities changing with the number of layers. Additionally, the Raman spectrum can also provide information on the strain and doping of hBN samples, as well as its thermal and mechanical properties. Overall, the Raman spectrum of hBN is a valuable tool for characterizing and identifying hBN and understanding its fundamental properties.
Phonon Dispersion of hBN
The phonon dispersion of hBN is shown above. As published in “Resonant Raman scattering in cubic and hexagonal boron nitride”, Reich et al, 2005.
References
Direct band-gap crossover in epitaxial monolayer boron nitride. Elias et al, Nature Communications, 2019. High-quality monolayer boron nitride has been successfully grown on graphite substrates using a scalable approach of high-temperature molecular beam epitaxy, revealing the presence of a direct gap of energy 6.1 eV in the single atomic layers and confirming a crossover to direct gap in the monolayer limit.
Hexagonal boron nitride is an indirect bandgap semiconductor. Cassabois et al, Nature Phonics, 2016. This article resolves the controversial issue of the bandgap nature and value of hexagonal boron nitride by providing evidence for an indirect bandgap at 5.955 eV, as well as demonstrating the existence of phonon-assisted optical transitions and measuring an exciton binding energy of about 130 meV by two-photon spectroscopy.