Focus on Hexagonal Boron Nitride

The era of two-dimensional (2D) materials, in its current form, truly began at the time that graphene was first isolated just over 15 years ago. Shortly thereafter, the use of 2D hexagonal boron nitride had expanded in popularity, with use of the thin isolator permeating a significant number of fields in condensed matter and beyond. Due to the impractical nature of cataloguing every use or research pursuit, this review will cover ground in the following three subtopics relevant to this versatile material: growth, electrical measurements, and applications in optics and photonics. Through understanding how the material has been utilized, one may anticipate some of the exciting directions made possible by the research conducted up through the turn of this decade.

Hexagonal boron nitride (hBN) is emerging as essential for 2D material-based technologies. New optoelectronic applications require high crystal quality hBN with low defect density and contamination. While vapor-phase processes like chemical vapor deposition can produce large-area hBN thin films, self-standing hexagonal boron nitride crystals provide exfoliated nanosheets of very high quality. The synthesis of these selfstanding hBN crystals prior to the exfoliation process is the focus of this review. Theses syntheses are described with emphasis on crystal size, purity, and defects. These considerations are based on various recently developed physical studies, including optical characterizations such as luminescence measurements highlighting the crystallinity and structural defects of hBN crystals.

Single-layer hexagonal boron nitride is produced on 2 inch Pt(111)/sapphire wafers. The growth with borazine vapor deposition at process temperatures between 1000 and 1300 K is in situ investigated by photoelectron yield measurements. The growth kinetics is slower at higher temperatures and follows a tanh² law which better fits for higher temperatures. The crystal-quality of hexagonal boron nitride (h-BN)/Pt(111) is inferred from scanning low energy electron diffraction (x-y LEED). The data indicate a strong dependence of the epitaxy on the growth temperature. The dominant structure is an aligned coincidence lattice with 10 h-BN on 9 Pt(1 × 1) unit cells and follows the substrate twinning at the millimeter scale.

The chemical vapour sensing behaviour of pristine and variously modified hexagonal boron nitride (hBN) nanostructures was investigated towards the polar protic analyte in the form of ammonia. Morphological characterization with TEM revealed formation of well-define shaped and crystal sized hBN flakes (2.9 ± 0. 7 µm to 3.3 ± 0.3 µm) by using a low temperature and atmospheric pressure modified polymer derived ceramics (PDCs) route. Room temperature chemical sensing studies showed that the hBN-based devices were sensitive to ammonia, at sensitivity values of 2.8 × 10⁻² ppm⁻¹ for the pristine hBN flakes, and 2.0 × 10⁻² ppm⁻¹, 2.4 × 10⁻² ppm⁻¹, 2.1 × 10⁻² ppm⁻¹ for the 2.5, 5 and 10 wt.% BaF₂ modified hBN flakes, respectively. On the contrary, improvement in structure for the 5 wt.% BaF₂ modified hBN flakes had detrimental influence on the detection performance of ammonia, as evidenced by the poor LoD value of 49.7 ppm, in comparison to 1.1, 2.4 and 1.7 ppm for the pristine, 2.5, and 10 wt.% BaF₂ modified hBN flakes, respectively. The improved sensing performance was attributed to the presence of nitrogen vacancies generated during the modification process, as well as the presence of impurities. Indeed, the values measured were higher than those reported for other 2D nanomaterial based sensors. This study demonstrates the critical role played by structural properties on the surface chemistry in the ammonia sensing properties of hBN flakes. Generally, the study highlighted the potential application of hBN nanostructured materials for detection of ammonia vapours at room temperature.