Understanding and tailoring defects in monolayer transition metal dichalcogenides for single photon emission

Abstract

Full text Embargo up to Oct 15, 2026 Point defects in monolayer (ML) transition metal dichalcogenides (TMDs) can behave as single photon emitters (SPEs). SPEs enable foundational quantum phenomena like entanglement and interference and lie at the heart of several cutting-edge technologies, including quantum computing, communication, and sensing. Spatially deterministic and quality-preserving fabrication techniques are critical for integrating SPEs into scalable quantum systems and cavities. This research focuses on a profound understanding of defects, electron-matter interactions, and strain, leading to the creation of SPEs using electron irradiation and strain. Electron irradiation of ML MoS2 (a prototypical TMD) can form defects by knock-on damage mechanism above threshold voltage (80 kV) and via additional energy channels till 20 kV. However, the ultralow acceleration voltage range (2 - 5 kV) is still unexplored for defect formation. This research found clear optical signatures (Raman spectroscopy, photoluminescence) of defect formation at 3 and 5 kV. Simulated Raman spectra suggested sulfur vacancies as possible defects. Further, we studied the role of hydrocarbon contamination in the defect formation process. This research finds a new defect creation regime at ultralow acceleration voltages, and paves the way for understanding electron-matter interactions and defect formation mechanisms. We then modulate the electron dose and develop protocols to create an ultra-dilute density of defects in hBN-encapsulated ML MoS2. We observed spectrally sharp and stable SPE peaks at cryogenic temperature (4 K). The SPE peaks are gate voltage tuneable, originate from sulfur vacancy complexes, and preserve spin nature (via magneto-optics measurements). In a significant advance, SPE writing with ultra-high spatial resolution (~50 nm) was demonstrated, potentially enabling studies of closely spaced arrays of defects and integrating SPEs to cavities. Additionally, we observed a series of spectrally separated SPE peaks, which could be related to SPE-phonon interactions. We then explored strain for modulating the band structure and creating new functionalities in ML TMDs. In existing studies, ML WSe2 is strained using nanoindentation on flexible polymer substrates. However, polymer substrates are not CMOS compatible and can degrade SPE properties. We develop direct deterministic SPE writing on ML WSe2 on SiO2/Si substrate via nanoindentation. Further, we demonstrate the ease of gated device fabrication and find that external gate voltage can turn SPEs on or off.