Gate-tuneable single-photon emitters in WSe2 monolayer created via AFM nanoindentation on rigid SiO2/Si substrates

Single-photon emitters (SPEs) hosted by two-dimensional (2D) semiconducting materials are envisioned for next-generation quantum applications. However, SPE creation in 2D semiconductors on rigid substrates like SiO2/Si via nanoindentation is a technological gap, critical for interfacing SPEs with photonic circuits and cavities. Here, we report a protocol for deterministically creating SPEs in monolayer WSe2 on SiO2/Si substrates using a sharp diamond AFM (atomic force microscope) tip. A displacement-controlled indentation process is developed, allowing indent depths > 150 nm necessary for creating SPEs. Sharp defect peaks (~200 μeV) are observed in cryogenic (4K) photoluminescence (PL) spectrum at nanoindented sites and are stable upto ~ 120K. 76% of sites exhibit sharp defect-bound peaks confirmed by power-dependent, temperature-dependent, and time-resolved PL (TRPL). AFM and PL mapping link these peaks to indent periphery. The peaks show sub-linewidth spectral jitter, no blinking, and single-photon nature in second-order autocorrelation measurements. SPEs can be switched on/off, and background emissions suppressed using electrical gating. Gate-voltage dependent TRPL indicate that SPE dynamics can be tuned, depending on nature of SPE, pointing the way to higher-purity SPEs. Our work is directly applicable to other 2D materials and photonic circuit/cavity compatible rigid substrates and is a significant step for scalable SPE technologies.

💡 Research Summary

In this work the authors demonstrate a deterministic method for creating gate‑tuneable single‑photon emitters (SPEs) in monolayer WSe₂ directly on SiO₂/Si substrates using a sharp diamond atomic force microscope (AFM) tip. The key technical advance is the implementation of a displacement‑controlled indentation protocol that precisely drives the tip into the substrate to depths exceeding 150 nm, a threshold identified as necessary for SPE formation. Calibration of the Z‑piezo on bare SiO₂/Si enables repeatable target depths (≈210–220 nm) with a standard deviation of only 3 nm across an array of 41 indents, demonstrating excellent reproducibility.

AFM topography reveals that each indent removes the WSe₂ material at the centre while producing a pronounced bulge (~78 nm high) on the lower side of the indent perimeter. This bulge originates from the ~11° tilt of the AFM cantilever and the large vertical deflection required for deep indentation, which causes asymmetric pile‑up of SiO₂ material on the front face of the tip. The bulge compresses torn edges of the monolayer, generating localized folds or wrinkles that act as strain‑induced defect sites. By contrast, indents made with a conventional instrumented nano‑indenter (vertical shaft) lack this bulge and do not produce any SPE signatures, underscoring the importance of the AFM‑specific geometry.

Cryogenic (4 K) photoluminescence (PL) from the indented regions exhibits sharp, spectrally isolated peaks (Xd) in the 1.5–1.7 eV range. The zero‑phonon line (ZPL) linewidths are as narrow as 0.2 meV, with accompanying phonon sidebands at lower energy. These peaks are absent in pristine (non‑indented) areas, confirming their origin in the nano‑indentation process. Temperature‑dependent PL shows that the peaks persist up to ~120 K, indicating robust thermal stability. Power‑dependent measurements reveal sub‑linear saturation behavior typical of bound excitons, and time‑resolved PL (TRPL) uncovers a biexponential decay with fast (tens of ns) and slow (hundreds of ns) components, reflecting radiative recombination and trap‑mediated processes.

Spectral jitter experiments demonstrate sub‑linewidth stability (fluctuations <10 µeV) and no blinking over extended observation times. Second‑order autocorrelation measurements yield g²(0) < 0.15, confirming true single‑photon emission. Mapping of PL intensity across the array shows that ~76 % of the indents generate localized emission, providing a high yield for deterministic SPE creation.

The authors further integrate a back‑gate electrode on the SiO₂/Si platform. By applying gate voltages, they can switch the SPEs on or off and suppress the broad background L‑peak (≈1.7 eV) that originates from delocalized exciton recombination. Gate‑dependent TRPL reveals that the decay dynamics can be tuned, suggesting that carrier doping modifies the recombination pathways of the defect states. This electrical control offers a practical route to address individual SPEs within larger photonic circuits.

Overall, the study establishes a scalable, substrate‑compatible technique for fabricating high‑quality, electrically controllable SPEs in 2D semiconductors. The method is readily extendable to other transition‑metal dichalcogenides (e.g., MoSe₂, WS₂) and to integration with silicon photonic waveguides, resonators, or cavities, paving the way toward large‑scale quantum photonic architectures.