Two-dimensional materials have fascinating properties distinct from their bulk counterparts. In particular, the class of semiconducting materials known as transition metal dichalcogenides (TMDs) change from being indirect band gap materials to direct band gap materials at the monolayer limit. This, in turn, makes their light-matter interaction unique. When an electron in a direct band gap material absorbs light energy in the form of photons, it is excited to a higher energy state and forms an electron-hole pair called an exciton. When the electron settles back to its lower energy state, it emits photons, and these photons can tell us many things about the properties of the material, including imperfections in the crystal lattice. Point defects are defects of zero dimensionality; one such example in TMD is an atomic vacancy, where the regular lattice is missing an atom where one should be. Vacancy sites provide the conditions for a particular type of emission called single photon emission, or SPE, in which an exciton can emit only one photon per recombination lifetime with a very narrow energy distribution [1-3]. Such emitters are of great interest for work in quantum information processing; however, some challenges exist in detecting and utilizing them. One of the Stern group’s research areas focuses on the isolation, detection, and control of SPE through the means of chemical functionalization. Functionalization in general refers to a treatment that modifies some optical or electronic aspect of a 2D material. In this case, it is the application of specific chemical schemes that interact with SPE sites to control and isolate them (Figure 1).
Figure 1
Thus far the most extensive work has been done with aryl diazonium chemistry and the TMD tungsten diselenide (WSe2). WSe2 is known to have SPE, but those emitters are random on the material and are often crowded within many other kinds of non-SPE defect emitter sites. However, with the treatment of nitrobenzene diazonium tetrofluoroborate (4-NBD), SPE can be preferentially chosen out of the spectrum by creating chemical oligomers that have transitions resonant in energy with the broad WSe2 defect spectrum. The interaction of the oligomer energy states with the defect levels passivates much of the crowded emission spectrum and leaves behind the SPE transitions as the preferred exciton recombination pathway (Figure 2) [4]. Based on these results, the group continues to investigate the possibilities of chemical and physical bonding between TMDs and new chemical schemes.
Figure 2
Further Reading
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- Zhong Lin et al. “Defect engineering of two-dimensional transition metal dichalcogenides.” 2016 2D Mater. 3 0220022.
- Philipp Tonndorf et al. “Single-photon emission from localized excitons in an atomically thin semiconductor,” Optica 2, 347-352 (2015)
- Branny, A., Kumar, S., Proux, R. et al. “Deterministic strain-induced arrays of quantum emitters in a two-dimensional semiconductor.” Nat Commun 8, 15053 (2017)
- Utama, M.I.B., Zeng, H., Sadhukhan, T. et al. “Chemomechanical modification of quantum emission in monolayer WSe2.” Nat Commun 14, 2193 (2023)