Hexagonal boron nitride (hBN) is a ceramic material that can be prepared in nearly atomically thin layers in a similar fashion to graphene.1 As a 2D material, hBN has been attracting a great deal of research interest for its optical properties and potential applications in photonics.
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hBN is isostructural to graphene and the overall structure consists of alternating layers of boron and nitrogen. In its pure form, hBN acts as an insulator but can be integrated with other materials, such as transition metal dichalcogenides, to make microelectronic devices and other technologies.2
Overall, the chemical inertness and mechanical robustness of hBN have made it a highly versatile material and improved levels of synthetic control in the manufacturing of hBN have opened up a number of possibilities for making hBN-based devices.
Applying 2D Materials to Photonics
Another unusual and exceptional property of hBN is its highly efficient deep-ultra-violet emission.3 hBN has an unusually large indirect band gap of about 6 eV, so it emits at very short wavelengths. Still, the emission efficiency from hBN 2D materials was greater than that of many single crystalline direct bandgap semiconductors.
The ability to tune the optical properties of hBN through processes like the systematic variation of the layer thickness or the number and type of defect sites on the material has also made hBN a highly appealing material for the use of photonic and optoelectronic applications.
As a highly efficient absorber, hBN can be used as a detector and sensor of deep ultraviolet radiation, for which few other materials are suitable.3 Depending on the layer structure, it is also possible to get emission in the visible and near-ultraviolet regions in addition to the deep ultraviolet and the thickness of the material also determines a variety of optical nonlinear properties.
hBN is sometimes described as a van der Waals layered material due to the types of intermolecular forces that hold the different layers together. Following demonstrations of the highly bright emission and absorption in the deep ultraviolet, there was also a great deal of research interest into how hBN can be manufactured into practical photonic devices, including waveguides.4
One recent demonstration of hBN as a waveguide showed that it was possible to create nanomechanical structures that could support radiofrequency acoustic wave propagation over a length of 1.2 mm.4
By exploiting the layered crystal structure, the team could fabricate a device with very high transmission in a unique frequency range; the team suggests the unique piezoelectric properties of hBN could be further exploited to create tunable waveguides. In the future, it is hoped that these types of advancements will mean that hBN devices can be integrated at a scale of on-chip structures.
Infrared Nanophotonics
Another key and surprising property of hBN is that it is a hyperbolic material, which is one of the key properties for its use in infrared nanophotonics.5 A hyperbolic material has a very specific dispersion effect on the light, dispersing in a way that is hyperbolic rather than the more traditional elliptical or parabolic dispersion that is seen in more standard materials.
In hBN materials, there is a very strong coupling between the phonons and any infrared light, with the natural hyperbolic bands of the material being in the 780 – 830 cm-1 range and the 1370 – 1610 cm-1 range. What this means is for infrared light, hBN has some unusual light-matter interaction, including negative refraction, thermal radiation enhancement and strong spontaneous enhancement.6
Some key applications of infrared nanophotonics are as optical elements such as waveguides, sensors, filters or even nanoscale lasers. It has historically been quite challenging to make cost-effective, efficient detectors in the infrared region. The development of nanophotonic devices would offer improved efficiency, smaller device footprints, and lower power draw – ideal for devices that need to be used in portable applications.
One key area of improvement for the future development of hBN in nanophotonic applications is the synthesis methods used to create the materials.
While there are many feasible options for creating nanosheets, including liquid phase and mechanical exfoliation, a very high degree of the number of layers is needed and the ability to create larger heterojunctions while ensuring a consistent material quality.6
Methods like chemical vapor deposition look promising for the degree of control they offer for the material formation and the ability to combine hBN materials with other materials such as graphene, but as well as developments on the device fabrication side, there is still work to be done on improving the robustness of the synthetic approaches to create the materials.
Overall, hBN is a highly customizable material that has some remarkable optical properties that can be easily adapted to a number of different applications. As a greater variety of highly sensitive quantum sensors and instrumentation become more commonplace as it becomes easier to fabricate and integrate these technologies into useable devices, hBN and other materials are likely to see even greater use.
References and Further Reading
Molaei, M. J., Younas, M., & Rezakazemi, M. (2021). A Comprehensive Review on Recent Advances in Two-Dimensional ( 2D ) Hexagonal Boron Nitride. Applied Electronic Materials, 3, 5165–5187. https://doi.org/10.1021/acsaelm.1c00720
Zhang, K., Feng, Y., Wang, F., Yang, Z., & Wang, J. (2017). Two dimensional hexagonal boron nitride (2D-hBN): synthesis, properties and applications. Journal of Materials Chemistry C, 5, 11992–12022. https://doi.org/10.1039/c7tc04300g
Moon, S., Kim, J., Park, J., Im, S., Kim, J., Hwang, I., & Kim, J. K. (2023). Hexagonal Boron Nitride for Next-Generation Photonics and Electronics. Advanced Materials, 35, 2204161. https://doi.org/10.1002/adma.202204161
Wang, Y., Lee, J., Zheng, X., & Xie, Y. (2019). Hexagonal Boron Nitride Phononic Crystal Waveguides. ACS Photonics, 6, 3225–3232. https://doi.org/10.1021/acsphotonics.9b01094
Caldwell, J. D., Aharonovich, I., Cassabois, G., Edgar, J. H., Gil, B., & Basov, D. N. (2019). Photonics with hexagonal boron nitride. Nature Reviews Materials, 4, 552–567. https://doi.org/10.1038/s41578-019-0124-1
Shi, N., Li, L., Gao, P., Jiang, X., Hao, J., Ban, C., & Zhang, R. (2023). Synthesis of Two-Dimensional Hexagonal Boron Nitride and Mid- Infrared Nanophotonics. Applied Electronic Materials, 5, 34–65. https://doi.org/10.1021/acsaelm.2c01083
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