Creation and Microscopic Origins of Single-Photon Emitters in Transition Metal Dichalcogenides and Hexagonal Boron Nitride

TL;DR

This review surveys irradiation- and strain-based methods to create single-photon emitters in 2D van der Waals materials, focusing on hBN and TMDCs. It emphasizes the diverse defect classes, the mechanisms of defect creation and activation, and the experimental/theoretical efforts to link atomic structure to optical signatures using PLE, STM/CL, and ab-initio calculations. A central theme is the unsettled atomistic origin of many emitters, particularly in hBN, with V_B^- centers, carbon-based complexes, and sulfur vacancies as leading candidates, while strain and defect-bound states shape TMDC emission. The authors advocate a theory-guided, open-data approach combining high-throughput computations, correlative atomistic characterization, and standardized fabrication to achieve deterministic, scalable quantum emitters in 2D materials and enable practical quantum photonic devices.

Abstract

We highlight recent advances in the controlled creation of single-photon emitters in van der Waals materials and in the understanding of their atomistic origin. We focus on quantum emitters created in monolayer transition-metal dichalcogenide semiconductors, which provide spectrally sharp single-photon emission at cryogenic temperatures, and the ones in insulating hBN, which provide bright and stable single-photon emission up to room temperature. After introducing the different classes of quantum emitters in terms of band-structure properties, we review the defect creation methods based on electron and ion irradiation as well as local strain engineering and plasma treatments. A main focus of the review is put on discussing the microscopic origin of the quantum emitters as revealed by various experimental platforms, including optical and scanning probe methods.

Figures (11)

• Figure 1: Quantum emission from defects.a Color centers derived from deep in-gap states with ground state (GS) and multiple excited states (ES). The color center is directly addressable in absorption or emission. b Shallow donors or acceptors states localize excitons into bound complexes by mutual electron-hole interactions. c Defect-localized excitons can hybridize both to deep in-gap states and to shallow extended band states. d Local strains or moiré potentials trap excitons. Point defects may facilitate radiative recombination and single-photon emission.
• Figure 2: Fundamentals of the irradiation-mediated engineering of 2D materials.a Schematic illustration of energy loss per unit length (stopping power) of the ion, with the main loss mechanisms being dependent on ion energy. b Sketch of the number of defects produced in a bulk system, a free-standing, and a supported 2D material by energetic ions as a function of ion energy. c Knock-on damage production mechanism upon electron irradiation, e.g. in the TEM. d Excitation/ionization damage production mechanism. e Beam-induced chemical etching and adatom-mediated damage production upon electron irradiation.
• Figure 3: Strain engineering of SPEs.a AFM topography and b integrated near-field PL (range 1.5–1.6 eV) image of exfoliated monolayer $\mathrm{WSe_2}$ on top of hBN. The white arrows indicate that PL emission is spatially localized on nanobubbles sites. All scale bars, 500 nm. Reproduced with permission from Darlington2020, Nat. Nanotechnol. 15, 854–860 (2020). Copyright 2020 Springer Nature. c Sketch and d AFM image (inset: SEM image) of monolayer $\mathrm{WSe_2}$ on a a star-shaped nanopillar. e (left) In-plane polarization-resolved PL spectrum and (right) corresponding polar plot (degree of polarization about 99%) taken from the P1 region in d. Reproduced with permission from Paralikis2024, npj 2D Mater. Appl. 8, 59 (2024). Copyright 2024 Authors, licensed under a Creative Commons Attribution Non Commercial License 4.0 (CC BY-NC). f AFM image of a region of a bulk $\mathrm{WS_2}$ flake treated with $\mathrm{H^+}$-irradiation, showing formation of nanodomes. Reproduced with permission from Tedeschi2019, Adv. Mater. 31, 1903795 (2019). Copyright 2019 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim. g Working principle of "quantum calligraphy": an AFM tip applies sufficient load to plastically deform a polymer underneath a 2D material, leaving a strain configuration. h Line profile of the strain configuration in g. i PL spectra with SPEs generated by the local strain in g. Reproduced with permission on from Rosenberger2019 ACS Nano 13 (1), 904-912 (2019). Copyright 2019 American Chemical Society.
• Figure 4: SPEs in hBN as fabricated by plasma treatment.a Typical fabrication steps for creating SPEs in exfoliated hBN (top) based on an $\text{O}_2$-plasma treatment (middle) and subsequent high-temperature annealing in controlled atmosphere (bottom). b The luminescence of $\text{O}_2$-plasma treated hBN typically exhibits several lines, which can be grouped into distinct lineshape categories (here group I and II as in Ref. Fischer2021) and which are red-shifted by $\sim$160 - 200 meV with respect to the zero-phonon line of the SPEs. Reprinted from Fischer2021, Science Advances 7, 7138–7155 (2021). Copyright 2021 Authors, licensed under a Creative Commons Attribution Non Commercial License 4.0 (CC BY-NC) License. c Statistics of the zero-phonon line of emitters in hBN as treated with an $\text{Ar}$-plasma at certain plasma powers (top) and with a $\text{H}_2$-plasma (bottom). Reprinted with permission from Zeng2024, ACS Applied Materials & Interfaces 16, 24899–24907 (2024). Copyright 2024 American Chemical Society. d Normalized second-order autocorrelation of an emitter in hBN fabricated by $\text{O}_2$-plasma treatment with $g^{(2)}(0)$ = 0.0034 (47). Reprinted with permission from Vogl2018, ACS Photonics 5, 2305–2312 (2018). Copyright 2018 American Chemical Society. e Luminescence map of SPEs (red circles) in hBN fabricated by an $\text{Ar}$-plasma with a characteristic random spatial distribution. Scale bar: 10 µm. Reprinted with permission from Xu2018, Nanoscale 10, 7957–7965 (2018). Copyright 2018 Royal Society of Chemistry.
• Figure 5: Creation of SPEs utilizing ion irradiation.a With the focused ion beam, defects can be produced in TMDCs embedded in device structures. Reproduced with permission from Hotger2021, Nano Lett. 21 (2), 1040–1046 (2021). Copyright 2021, American Chemical Society. b A proton beam creates chalcogen vacancies in monolayer semiconductors. Reproduced with permission from Zhang2022a, Adv. Optical Mater. 10, 2201350 (2022). Copyright 2022 Wiley‐VCH GmbH. c Sketch of the highly focused ion beam that knocks out boron atoms in hBN. Reproduced with permission from Liang2023, Adv. Optical Mater. 11, 2201941 (2023). Copyright 2022 Wiley‐VCH GmbH. d ODMR spectra of defects in hBN at vanishing and at finite magnetic fields. Reproduced with permission from Gottscholl2020, Nat. Mater. 19, 540–545 (2020). Copyright 2020, under exclusive license to Springer Nature Limited. e Plasmonic enhancement of photoluminescence of defects in hBN created by a focused He$^{+}$ ion beam. Reproduced from Sasaki2023, Appl. Phys. Lett. 122, 244003 (2023), with the permission of AIP Publishing. f Photoluminescence of an ion-irradiated array in hBN after thermal annealing, with associated PL spectra and second-order correlation functions. Reproduced with permission from Liu2023, Adv. Optical Mater. 12, 2302083 (2024). Copyright 2023 Wiley‐VCH GmbH.