Single-Photon Emitters Achieve Activation at 1100 K in Monolayer WS₂

The quest for robust, single-photon sources is central to advances in quantum technologies, and recent research focuses on harnessing defects within two-dimensional materials to achieve this goal. Gyeongjun Lee, Antoine Borel, and Takashi Taniguchi, alongside colleagues at various institutions, now demonstrate a method for reliably creating such sources within monolayer tungsten disulphide. The team successfully activates spectrally isolated single-photon emitters by applying high temperatures to the material during its fabrication, a process that induces specific defects. This technique yields emitters exhibiting clear antibunching, a key signature of single-photon behaviour, and represents a significant step towards scalable quantum light sources based on van der Waals materials.

Controlled activation of defect-bound excitonic states in two-dimensional semiconductors provides a route to isolated quantum emitters and a sensitive probe of defect physics. This work demonstrates that in situ high-temperature annealing of hexagonal boron nitride-encapsulated monolayer tungsten disulphide on a suspended microheater leads to the emergence of spectrally isolated single-photon emitters at cryogenic temperatures. Annealing at temperatures around 1100 Kelvin produces a sharp emission line, designated XL, red-shifted by approximately 80 meV from the neutral exciton and exhibiting a linewidth below 200 microelectronvolts. Photoluminescence excitation spectroscopy further characterises these emitters, revealing their unique properties and formation mechanisms.

Stable Single-Photon Emitters in 2D Materials

Scientists are developing methods to create stable, bright, and reproducible single-photon emitters, essential building blocks for future quantum technologies. These emitters rely on carefully engineered defects within two-dimensional materials like molybdenum disulphide and tungsten disulphide. The research focuses on controlling these defects to achieve the desired emission characteristics. A key challenge is creating emitters that maintain their performance over time and emit a strong signal. Researchers are exploring techniques to ensure consistent properties and reproducibility in the creation of these emitters.

Studies demonstrate the potential of sulfur vacancies, missing sulfur atoms within the material, as promising candidates for single-photon emission. Scientists are investigating methods to intentionally create these vacancies in a controlled manner. Helium ion beam techniques are used to create defects in tungsten disulphide, while oxygen annealing is employed to generate color centers in hexagonal boron nitride. Understanding the role of these defects and chalcogen vacancies in molybdenum disulphide is crucial for optimizing emitter performance. Various techniques, including optical spectroscopy, scanning tunneling microscopy, and atomic force microscopy, are used to characterize the materials and identify defects.

Photoluminescence and Raman spectroscopy provide insights into the vibrational and emission properties of the materials. Annealing processes are used to create defects, and exfoliation techniques are employed to obtain the two-dimensional materials. These advancements pave the way for developing materials for quantum computing, quantum cryptography, and highly sensitive quantum sensors. Studying the properties of defects in these materials also provides valuable insights into fundamental physics.

Isolated Single-Photon Emitters Formed in WS2

Scientists have successfully created isolated single-photon emitters in two-dimensional materials using a novel high-temperature annealing process. Heating encapsulated monolayer tungsten diselenide to approximately 1100 Kelvin induces the formation of spectrally isolated single-photon emitters at cryogenic temperatures. Measurements reveal a sharp emission line, designated XL, red-shifted by approximately 80 meV from the neutral exciton, with a remarkably narrow linewidth below 200 microelectronvolts. This narrow linewidth confirms the high optical quality of the samples and the effectiveness of the encapsulation process in minimizing disorder.

Photoluminescence excitation spectroscopy and power-dependent measurements demonstrate that the XL emission originates from defects induced within the tungsten diselenide monolayer during the annealing process. Second-order photon correlation measurements confirm the single-photon nature of the emission, yielding a clear antibunching effect with values less than 0.5. These results establish high-temperature in situ annealing as a controlled method for accessing defect-bound excitonic states and generating single-photon emission in van der Waals materials. Further analysis reveals a recombination time in the nanosecond range at cryogenic temperatures, supporting the efficient radiative decay of excitons localized at the induced defect sites. The team observed sharp excitonic features in the low-temperature photoluminescence spectra, including the neutral exciton, trions, dark-exciton, and biexciton, with linewidths of only a few meV, providing a direct reference for identifying defect-localized emission. Photoluminescence excitation spectroscopy data reveals clear resonances at the A and B excitons and at the 2s excited states, intrinsic to pristine tungsten diselenide, which will be important for characterizing the single-photon emitters.

High-Temperature Annealing Creates Single-Photon Emitters

Researchers have demonstrated a method for creating isolated, single-photon emitters in monolayer tungsten diselenide, a two-dimensional material, through controlled high-temperature annealing. By heating the material to approximately 1100 Kelvin while suspended on a micro-membrane, the team activated specific defects that become the source of these light-emitting states. These emitters exhibit a narrow emission spectrum and demonstrate clear antibunching, a key characteristic confirming their single-photon nature. This achievement establishes high-temperature annealing as a reliable technique for accessing and controlling defect-bound excitonic states within van der Waals materials.

The process differs from methods relying on strain or disorder, instead highlighting the activation of intrinsic defects as the primary mechanism. Importantly, the ability to suspend the material allows for real-time monitoring of its optical properties during the annealing process, a capability difficult to achieve with conventional bulk semiconductors, and extends the applicability of the method to other layered materials. Future work could investigate the impact of doping on these emitters and their interaction with a spin-valley-polarized electron gas, potentially enabling electrical or optical tuning of their properties.