DUV Excitonic Emission from Monolayer h-BN Studied by Cathodoluminescence Performed in a Scanning Tunneling Microscope

Hexagonal boron nitride (h-BN) is a van der Waals solid with a wide electronic band gap. Like other 2D materials, it has different optical and electronic properties when in monolayer form with respect to its bulk counterpart. In 2019, photoluminescence and optical reflectance spectroscopy measurements on h-BN showed the indirect to direct band gap transition from few layers to monolayer thickness.[1] In particular, for a sample composed of h-BN monolayers grown on HOPG by high-temperature molecular beam epitaxy, the excitonic emission was observed at ~6.1 eV.[1,2] Raman scattering prevented the exact determination of the excitonic emission energy. Supporting this result, recent cathodoluminescence (CL) measurements carried out on h-BN/HOPG reported the presence of a weak emission peak attributed to phonon-assisted recombination of direct excitons.[3] As well as allowing easy access to high excitation energy, CL avoids the presence of Raman signals close to the luminescence which greatly simplifies data interpretation. Despite the high surface coverage of monolayer hBN grown on HOPG,[2] these samples have regions both with thicker layers and disordered growth. Therefore, it is mandatory to have high resolution images of the region probed by CL in order to avoid missing the excitonic emission which is specific of the monolayer. Here, we studied the deep ultraviolet (DUV) emissions of h-BN monolayers grown on HOPG using low energy CL performed in a scanning tunneling microscope (STM). In this setup, it is possible to excite regions as small as tens of nm with free electrons of 100 to 300 eV which greatly enhances the interaction at the surface. Also, this system uses an off axis parabolic mirror with high light collection efficiency which is fully compatible with low temperature operation.[4] In this way, it was possible to locally excite the sample in previously characterized regions and observe the excitonic emission from a monolayer of h-BN. From the analysis of local spectra, it was possible to precisely determine the h-BN monolayer excitonic emission energy.

Acknowledgements:
This work was supported by the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) projects 2021/06893-3, 2023/04168-5 and by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior projects 88887.517233/2020-00 and 88887.716201/2022-00.

References:
[1] Elias, C. et al. Nat. Commun., 10, 2639 (2019).
[2] Cheng, T. S. et al. J. Vac. Sci. Technol. B 36, 02D103 (2018).
[3] Shima, K. et al. Sci. Rep. 14, 169 (2024).
[4] Román, R. J. P. et al. Rev. Sci. Instrum. 1 April 2022; 93 (4): 043704.