High-temperature activation of single-photon emitters in monolayer WS2

Research on the quantum nature of matter and light has flourished in recent years, as the field has grown for theorists and experimentalists alike. New research directions, such as the study of novel phases in quantum materials, have laid the ground for new foundational theories and future quantum technologies. In particular, novel topological materials and superconducting hybrid systems are promising for the development of novel superconductors and quantum computers. This Focus Issue aims to highlight some of the latest achievements in the field of quantum matter. It includes 3 Reviews, 1 Perspective, and 3 Research Articles on a range of topics, including single-photon emitters for quantum technologies, topological quantum materials, and quantum sensing and metrology: Rare-Earth Doped Thin Films for Optical Quantum Technologies (Philippe Goldner et al., 10.1002/qute.202500026): A Review article on rare earth-doped thin films—materials that are highly promising for use in on-chip photonic circuits thanks to their excellent coherence properties. The integration of rare-earth thin films into photonic platforms could potentially enable photonic quantum technologies, such as optical quantum memories, which are relevant for communications and processing. Cuprate twistronics for quantum hardware (Nicola Poccia et al., 10.1002/qute.202500203): A topical Review on the emerging field of cuprate twistronics, which involves stacking and twisting ultrathin layers of cuprate superconductors. This technique creates Moiré patterns and new electronic states, uncovering new opportunities in both applied (e.g., quantum hardware) and fundamental physics. Quantum spin Hall effects in van der Waals materials (Qiong Ma et al., 10.1002/qute.202500327): A Review of the Quantum Spin Hall (QSH) effect in two-dimensional van der Waals (vdW) materials. The QSH effect is promising for the development of low-power electronics and topological quantum computing. The article explores the main developments in the field, emerging research directions, as well as experimental challenges. Beyond kagome: p-bands in kagome metals (Alexander Tsirlin et al., 10.1002/qute.202500336): A Perspective arguing that p-bands (electron bands from non-transition metals) play a critical, often overlooked, role in the electronic instabilities, such as charge-density waves and superconductivity, observed in kagome lattice metals. High-purity single-photon emission in the telecom O-band from droplet-epitaxy InAs quantum dots integrated into a GaAs/AlGaAs planar microcavity on vicinal GaAs(111) (Battulga Munkhbat et al., 10.1002/qute.202500159): A Research article on developing a reliable source of single photons in the telecom O-band (a critical wavelength for fibre communication) using InAs quantum dots grown via droplet epitaxy. Single-photon emission from such sources could potentially be useful for building global quantum communication networks. Can Janus monolayer transition-metal dihalides be bright excitonic insulators? (Yuanchang Li et al., 10.1002/qute.202500345): This Research article explores Janus monolayers (asymmetric 2D materials) as potential candidates for bright excitonic insulators (EIs). EIs are a state of matter where electrons and holes spontaneously form excitons that condense, and bright EIs could provide a unique platform for coherent quantum optical devices. Highly sensitive magnetometer with continuous excitation Ramsey protocol by utilizing long dephasing time of millimetre-scale spin ensembles in (111) diamond (Mutsuko Hatano et al., 10.1002/qute.202500056): This Research article presents a method to significantly enhance the sensitivity of magnetometers by utilizing the long dephasing time of nitrogen-vacancy (NV) centre spin ensembles in a specific orientation of diamond. NV-diamond magnetometers are key for high-precision quantum sensing. Through this Focus Issue, we aim to highlight the major advances in quantum matter research and to offer perspectives on key directions for future research. We hope that readers will find the content of this collection both inspiring and useful.

New two‐dimensional (2D) van der Waals materials are emerging, and their optical parameters urgently need to be acquired experimentally. Most of the reported results come from traditional metrologies used in the past for bulk materials. This unavoidably causes controversy regarding the correctness and accuracy of results, as well as the physical model. Surface/interface plasmon polariton waves are sub‐diffraction‐limited and very sensitive to surroundings. Here, 2D van der Waals materials with metal embodied are presented. Surface/interface exciton‐plasmon polaritons come into being with excitons in 2D transition‐metal dichalcogenides (TMDCs), which are physically taken as the boundary conditions with complex optical conductivities at the interfaces. Complex optical conductivities of 2D TMDCs are measured by means of interference behavior of surface/interface polariton waves. The size requirement of 2D van der Waals materials becomes relaxed, and complex optical conductivities can be measured for single‐crystalline WS2 and MoS2 monolayers with small sizes. Furthermore, a remarkable capability is manifested by yielding complex optical conductivity of van der Waals heterostructures. The proposed metrology can apply to other newly developed materials with modified underlying physical models, as well as various types of 2D van der Waals materials.

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.

High-quality van der Waals heterostructures assembled from hBN-encapsulated monolayer transition metal dichalcogenides enable observations of subtle optical and spin-valley properties whose identification was beyond the reach of structures exfoliated directly on standard SiO2/Si substrates. Here, we describe different van der Waals heterostructures based on uncapped single-layer MoS2 stacked onto hBN layers of different thicknesses and hBN-encapsulated monolayers. Depending on the doping level, they reveal the fine structure of excitonic complexes, i.e. neutral and charged excitons. In the emission spectra of a particular MoS2/hBN heterostructure without an hBN cap we resolve two trion peaks, T1 and T2, energetically split by about 10 meV, resembling the pair of singlet and triplet trion peaks (T S and T T ) in tungsten-based materials. The existence of these trion features suggests that monolayer MoS2 has a dark excitonic ground state, despite having a ‘bright’ single-particle arrangement of spin-polarized conduction bands. In addition, we show that the effective excitonic g-factor significantly depends on the electron concentration and reaches the lowest value of −2.47 for hBN-encapsulated structures, which reveals a nearly neutral doping regime. In the uncapped MoS2 structures, the excitonic g-factor varies from −1.15 to −1.39 depending on the thickness of the bottom hBN layer and decreases as a function of rising temperature.

One of the major challenges in the van der Waals (vdW) integration of two-dimensional (2D) materials is achieving high-yield and high-throughput assembly of predefined sequences of monolayers into heterostructure arrays. Mechanical exfoliation has recently been studied as a promising technique to transfer monolayers from a multilayer source synthesized by other techniques, allowing the deposition of a wide variety of 2D materials without exposing the target substrate to harsh synthesis conditions. Although a variety of processes have been developed to exfoliate the 2D materials mechanically from the source and place them deterministically onto a target substrate, they can typically transfer only either a wafer-scale blanket or one small flake at a time with uncontrolled size and shape. Here, we present a method to assemble arrays of lithographically defined monolayer WS2 and MoS2 features from multilayer sources and directly transfer them in a deterministic manner onto target substrates. This exfoliate–align–release process—without the need of an intermediate carrier substrate—is enabled by combining a patterned, gold-mediated exfoliation technique with a new optically transparent, heat-releasable adhesive. WS2/MoS2 vdW heterostructure arrays produced by this method show the expected interlayer exciton between the monolayers. Light-emitting devices using WS2 monolayers were also demonstrated, proving the functionality of the fabricated materials. Our work demonstrates a significant step toward developing mechanical exfoliation as a scalable dry transfer technique for the manufacturing of functional, atomically thin materials.

Point defects in crystalline solids behave as optically addressable individual quantum systems when present in sufficiently low concentrations. In two-dimensional (2D) semiconductors, such quantum defects hold potential as versatile single photon sources. Here, we report the synthesis and optical properties of Nb-doped monolayer WS2 in the dilute limit where the average spacing between individual dopants exceeds the optical diffraction limit, allowing the emission spectrum to be studied at the single-dopant level. We show that these individual dopants exhibit common features of quantum emitters, including narrow emission lines (with linewidths <1 meV), strong spatial confinement, and photon antibunching. These emitters consistently occur within a narrow spectral range across multiple samples, distinct from common quantum emitters in van der Waals (vdW) materials that show large ensemble broadening. Analysis of the Zeeman splitting reveals that they can be attributed to bound exciton complexes comprising dark excitons and negatively charged Nb.

Moiré heterostructures produced by twisted heterojunction of transition-metal dichalcogenides are recognized as novel platforms for unique and tunable means of controlling the optical phenomena including photoluminescence (PL). Despite some interesting results on the PL peak shifts by the heterojunction at twist angles (θ) far from 0 or 60°, all of them are redshifts. Here, we first report blue shift of energy and strong enhancement of intensity in the PL by twisted heterojunction of MoS2 and WS2 monolayers (MLs) in a particular range of θ. The PL peak energy of the heterobilayer steeply increases (about 120 meV) as θ gets closer to 15 or 52° from 3 or 57°, respectively and reaches a plateau at around 2.01 eV in the θ range from 15 to 52°, higher than that of the separate MoS2 or WS2 ML. The PL intensity shows a similar θ-dependent behavior with its magnitude in the plateau being ∼4 or 80 times larger than that of the WS2 or MoS2 ML, respectively. These novel light-emission behaviors are well explained with reference to theoretical predictions on the avoided crossing between the intralayer and interlayer excitons. Our findings highlight extendable tuning and remarkable enhancement of light emission from two-dimensional semiconductors by a simple approach of twisted heterojunction in a proper θ range, very useful for their optoelectronic device applications.

Semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising materials for an array of applications, ranging from conventional field-effect transistors, photodetectors, and light-emitting diodes to their more recent use in quantum photonic technologies. Chemical functionalization of 2D TMDs with organic ligands and adlayers provides an additional means for customizing their electronic and optical properties. While many pathways have been reported for the chemical functionalization of 2D TMDs, their frequent reliance on solution-based methods results in limited control over adlayer thickness and coverage, thus hindering utility in high-performance applications. Here we describe the vapor-phase functionalization of a 2D TMD with carbene ligands, specifically tungsten disulfide (WS2) with N-heterocyclic carbenes (NHCs), resulting in molecularly smooth, thin, and uniform adlayers. Reacting NHCs with monolayer WS2 reduces the broad photoluminescence background observed at cryogenic temperatures by 58%, which facilitates the detection of single-photon emitters from strained monolayer WS2, as indicated by second order correlation values (g(2)) as low as 0.17 ± 0.07. Chemical characterization coupled with density functional theory calculations suggests that the NHC adlayer has a dual defect-passivation and doping effect on monolayer WS2 that results in enhanced single-photon emission. Overall, this study establishes vapor-phase carbene functionalization as a homogeneous surface modification scheme for tailoring the quantum emission properties of semiconducting 2D TMDs.

Two-dimensional (2D) semiconductors possess strongly bound excitons, opening novel opportunities for engineering light–matter interaction at the nanoscale. However, their in-plane confinement leads to large non-radiative exciton–exciton annihilation (EEA) processes, setting a fundamental limit for their photonic applications. In this work, we demonstrate suppression of EEA via enhancement of light–matter interaction in hybrid 2D semiconductor–dielectric nanophotonic platforms, by coupling excitons in WS2 monolayers with optical Mie resonances in dielectric nanoantennas. The hybrid system reaches an intermediate light–matter coupling regime, with photoluminescence enhancement factors up to 102. Probing the exciton ultrafast dynamics reveal suppressed EEA for coupled excitons, even under high exciton densities >1012 cm−2. We extract EEA coefficients in the order of 10−3, compared to 10−2 for uncoupled monolayers, as well as a Purcell factor of 4.5. Our results highlight engineering the photonic environment as a route to achieve higher quantum efficiencies, for low-power hybrid devices, and larger exciton densities, towards strongly correlated excitonic phases in 2D semiconductors.

Realization of wafer-scale single-crystal films of transition metal dichalcogenides (TMDs) such as WS2 requires epitaxial growth and coalescence of oriented domains to form a continuous monolayer. The domains must be oriented in the same crystallographic direction on the substrate to inhibit the formation of inversion domain boundaries (IDBs), which are a common feature of layered chalcogenides. Here we demonstrate fully coalesced unidirectional WS2 monolayers on 2 in. diameter c-plane sapphire by metalorganic chemical vapor deposition using a multistep growth process to achieve epitaxial WS2 monolayers with low in-plane rotational twist (0.09°). Transmission electron microscopy analysis reveals that the WS2 monolayers are largely free of IDBs but instead have translational boundaries that arise when WS2 domains with slightly offset lattices merge together. By regulating the monolayer growth rate, the density of translational boundaries and bilayer coverage were significantly reduced. The unidirectional orientation of domains is attributed to the presence of steps on the sapphire surface coupled with growth conditions that promote surface diffusion, lateral domain growth, and coalescence while preserving the aligned domain structure. The transferred WS2 monolayers show neutral and charged exciton emission at 80 K with negligible defect-related luminescence. Back-gated WS2 field effect transistors exhibited an ION/OFF of ∼107 and mobility of 16 cm2/(V s). The results demonstrate the potential of achieving wafer-scale TMD monolayers free of inversion domains with properties approaching those of exfoliated flakes.

We have identified excited exciton states in monolayers of MoS2 and WS2 supported on fused silica by means of photoluminescence excitation spectroscopy. In monolayer WS2, the positions of the excited A exciton states imply an exciton binding energy of 0.32 eV. In monolayer MoS2, excited exciton transitions are observed at energies of 2.24 and 2.34 eV. Assigning these states to the B exciton Rydberg series yields an exciton binding energy of 0.44 eV.

Guided modes in nanometer thick anisotropic van der Waals materials are experimentally investigated and their refractive indices in visible wavelengths are extracted. Our method involves near-field scanning optical microscopy of waveguide (transverse electric) and surface plasmon polariton (transverse magnetic) modes in h-BN/SiO2/Si and Ag/h-BN stacks, respectively. We determine the dispersion of these modes and use this relationship to extract anisotropic refractive indices of h-BN flakes. In the wavelength interval 550–700 nm, the in-plane and out-of-plane refractive indices are in the range 1.98–2.12 and 1.45–2.12, respectively. Our approach of using near-field scanning optical microscopy allows for the direct study of the interaction between light and two-dimensional van der Waals materials and heterostructures.

Single-photon emitters in gallium nitride (GaN) are gaining interest as attractive quantum systems due to the well-established techniques for growth and nanofabrication of the host material, as well as its remarkable chemical stability and optoelectronic properties. We investigate the optical properties at room temperature and cryogenic temperature and demonstrate resonant excitation of quantum emitters in gallium nitride (GaN). The emitters are stable under non-resonant excitation and exhibit nearly Fourier transform limited lines of ~ 250 MHz under coherent excitation, the narrowest reported to date for GaN. Furthermore, we investigate the nature of such single photon emitters in GaN with a systematic analysis of various samples produced under different growth conditions. We explore the effect that intrinsic structural defects (dislocations and stacking faults), doping and crystal orientation in GaN have on the formation of quantum emitters. We investigate the relationship between the position of the emitters-determined via spectroscopy and photoluminescence measurements-and the location of threading dislocations-characterised both via atomic force microscopy and cathodoluminescence. We find that quantum emitters do not correlate with stacking faults or dislocations; instead, they are more likely to originate from point defects or impurities whose density is modulated by the local extended defect density.

The recently discovered negatively charged tin-vacancy centre in diamond is a promising candidate for applications in quantum information processing (QIP). We here present a detailed spectroscopic study encompassing single photon emission and polarisation properties, the temperature dependence of emission spectra as well as a detailed analysis of the phonon sideband and Debye–Waller factor. Using photoluminescence excitation spectroscopy we probe an energetically higher lying excited state and prove fully lifetime limited linewidths of single emitters at cryogenic temperatures. For these emitters we also investigate the stability of the charge state under resonant excitation. These results provide a detailed insight into the spectroscopic properties of the SnV− centre and lay the foundation for further studies regarding its suitability in QIP.

Quantum technology has grown out of quantum information theory and now provides a valuable tool that researchers from numerous fields can add to their toolbox of research methods. To date, various systems have been exploited to promote the application of quantum information processing. The systems that can be used for quantum technology include superconducting circuits, ultracold atoms, trapped ions, semiconductor quantum dots, and solid-state spins and emitters. In this review, we will discuss the state-of-the-art of material platforms for spin-based quantum technology, with a focus on the progress in solid-state spins and emitters in several leading host materials, including diamond, silicon carbide, boron nitride, silicon, two-dimensional semiconductors, and other materials. We will highlight how first-principles calculations can serve as an exceptionally robust tool for finding novel defect qubits and single-photon emitters in solids, through detailed predictions of electronic, magnetic, and optical properties.