Lattice deformation is a powerful way to engineer the properties of 2D materials, making their precise measurement an important challenge for both fundamental science and technological applications. Here, we demonstrate that boron-vacancy () color centers in hexagonal boron nitride (hBN) enable quantitative strain sensing with sub-micrometer spatial resolution. Using this approach, we precisely quantify the strain-induced shift of the Raman mode in a multilayer hBN flake under uniaxial stress, establishing centers as a new tool for strain metrology in van der Waals heterostructures. Beyond strain sensing, our work also highlights the unique multimodal sensing functionalities offered by centers, which will be valuable for future studies of strain-engineered 2D materials.

Van der Waals heterostructures made from atomically thin transition metal dichalcogenides (TMD) and graphene have emerged as a building block for optoelectronic devices. Such systems are also uniquely poised to investigate interfacial coupling as well as photoinduced charge and energy transfer in the 2D limit. Recent works have revealed efficient photoluminescence quenching and picosecond transfer in TMD/graphene heterostructures. However, key questions regarding the transfer mechanisms remain. Here, employing time-resolved photoluminescence spectroscopy with 1 ps resolution in MoSe_2 monolayer directly coupled to a few-layer “staircase-like” graphene flake, we consistently observe an exciton transfer time of ≈ 2.5 ps at cryogenic temperature that is marginally affected by the number of graphene layers. Remarkably, exciton transfer vanishes in samples consisting in an MoSe_2 monolayer separated from graphene by a thin dielectric spacer of hexagonal boron nitride, as soon as the spacer thickness reaches 1 nm. These results suggest that charge tunnelling processes govern exciton dynamics. Other mechanisms mediated the dipolar interactions (Förster-type energy transfer) have no measurable impact on bright excitons (with near-zero center of mass momentum) but may accelerate the relaxation of finite momentum “hot” excitons, leading to larger photoluminescence quenching than anticipated based on the measurements of the photoluminescence decay rates. Our work provides important insights into charge and energy transfer in van der Waals materials with direct implications for energy harvesting and funneling.

Inspirées par le succès du graphène, les recherches sur les cristaux atomiques bidimensionnels (2D) se sont développées extrêmement rapidement au cours de la dernière décennie. La famille des matériaux 2D présente une variété exceptionnelle de propriétés électroniques liées à leur faible dimensionnalité, des isolants aux métaux, semi-métaux, isolants topologiques, semi-conducteurs et supraconducteurs. Des couches atomiquement minces avec des propriétés différentes peuvent être assemblées en empilements verticaux communément appelés hétérostructures de van der Waals, permettant la conception de matériaux artificiels avec des propriétés physiques originales. Nous montrons dans cet article que de telles hétérostructures permettent de contrôler les propriétés optiques d’une monocouche atomique d’un semi-conducteur de manière spectaculaire.

GaN-based spin light-emitting diodes (spin-LEDs) are attractive for realizing room-temperature spin-controlled light emission in efficient, color-tunable devices. In this work, we demonstrate spin injection in a GaN spin-LED fabricated on a Si substrate using wafer-bonding technology. An inverted n-i-p LED structure with n-type GaN on top facilitates the injection of spin-polarized electrons from a ferromagnetic Au/Co/MgO spin-injector layer grown by molecular beam epitaxy. At room temperature, an electroluminescence circular polarization of approximately 8% is measured under an applied 1 T out-of-plane magnetic field in a surface-emitting geometry. After considering contributions from the Zeeman effect and magnetic circular dichroism, about 6% circular polarization is attributed to the pure spin injection into GaN. This work is focused on the detailed structural and interface characterization of the spin-LED and allows to understand its spin and optoelectronic properties. These results highlight the potential of GaN-based spin-optoelectronic devices compatible with Si platforms.

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

Despite decades of research, demonstration of all-optical detection and control of free electron spins in silicon remains elusive. Here, we directly probe the electron spin properties in bulk silicon by measuring the polarization of luminescence following circularly polarized light excitation. The all-optical experiments performed for both direct and indirect gap excitation allow not only an experimental determination of the optical selection rules in silicon for the different phonon-assisted transitions but they also lead to the measurement of the spin relaxation of electrons in conditions that are not accessible using transport techniques. We also measure the spin properties of free excitons in bulk silicon, a very little explored field.

Monolayers of group-VI transition-metal dichalcogenides (TMDs) are two-dimensional semiconductors that exhibit exceptionally strong light-matter coupling yet typically suffer from low emission quantum yields. In this letter, we investigate the heavily n-doped regime of a WSe_2 monolayer and show that multi-particle excitonic complexes produce photoluminescence signals up to two orders of magnitude stronger than in the neutral state. Time-resolved photoluminescence and differential reflectivity measurements reveal that the quantum yield rises with carrier density and exceeds 50

Lamellar crystalline materials, whose layers are bond by van der Waals forces, can be stacked to form ultrathin artificial heterostructures, and in particular vertical magnetic junctions when some of the stacked materials are (ferro)magnetic. Here, using the room temperature van der Waals ferromagnet 1$T$-CrTe$_2$, we report a method for patterning lateral magnetic junctions. Exploiting the heat-induced phase transformation of the material into Cr$_x$Te$_y$ compounds ($x/y>1/2$), we use local laser heating to imprint patterns at the micron-scale. Optimizing laser heat dissipation, we further demonstrate the crucial role of the substrate to control the phase transformation. If plain, unstructured poorly heat-conducting substrates allow for direct writing of magnetic patterns, structured $h$-BN layers can serve as heat stencils to draw potentially thinner patterns. Besides, $h$-BN encapsulation turns out to be heat-protective (in addition from protecting against oxidation as it is generally used for), allowing the demonstration of room temperature ferromagnetism in $<$7~nm-thick 1$T$-CrTe$_2$.

Single photon emitters (SPEs) are building blocks of quantum technologies. Defect engineering of two-dimensional (2D) materials is ideal to fabricate SPEs, wherein spatially deterministic and quality-preserving fabrication methods are critical for integration into quantum devices and cavities. Existing methods use combination of strain and electron irradiation, or ion irradiation, which make fabrication complex, and limited by surrounding lattice damage. Here, only ultra-low energy electron beam (e-beam) irradiation (5 keV) is utilized to create dilute defect density in hBN-encapsulated monolayer MoS2, with ultra-high spatial resolution (<50 nm, extendable to 10 nm). Cryogenic photoluminescence spectra exhibit sharp defect peaks, following power-law for finite density of single defects, and characteristic Zeeman splitting for MoS2 defect complexes. The sharp peaks have low spectral jitter (<200 mu eV), and are tunable with gate-voltage and e-beam energy. Use of low-momentum electron irradiation, ease of processing, and high spatial resolution, will disrupt deterministic creation of high-quality SPEs.

Lamellar crystalline materials, whose layers are bound by van der Waals forces, can be stacked to form ultrathin artificial heterostructures and, in particular, vertical magnetic junctions when some of the stacked materials are (ferro)magnetic. Here, using the room temperature van der Waals ferromagnet 1T-CrTe2, we report a method for patterning lateral magnetic junctions. Exploiting the heat-induced phase transformation of the material into CrxTey compounds (x/y>1/2), we use local laser heating to imprint patterns at the micron-scale. Optimizing laser heat dissipation, we further demonstrate the crucial role of the substrate to control the phase transformation. If plain, unstructured poorly heat-conducting substrates allow for direct writing of magnetic patterns, structured h-BN layers can serve as heat stencils to draw potentially thinner patterns. Besides, h-BN encapsulation turns out to be heat-protective (in addition to protecting against oxidation, for which it is generally used), allowing the demonstration of room temperature ferromagnetism in <7 nm-thick 1T-CrTe2.

Magnetic two-dimensional (2D) crystals were isolated about a decade ago, triggering a tremendous research activity worldwide. This colloquium raises a stiff question: what is really new about them? At first sight, they seem to be purer implementations of 2D spin models than traditional systems such as ultra-thin films. Yet, they partly realized their promises so far, and whether they give fresh perspectives on long-standing predictions in statistical physics is still an open question. Undoubtedly, they are uniquely amenable to electric-field effect, susceptible to mechanical deformation, and sensitive to moirés, for example. They represent interesting platforms for exploring, challenging, or simply revisiting a wide range of phenomena in condensed matter magnetism. This colloquium intends to offer a critical, yet not necessarily skeptical, overview of the field, clarifying what we believe could be unique with 2D magnets, related quasi-2D van der Waals magnets, and their heterostructures.

Tuning the density of resident electrons or holes in semiconductors provides crucial insight into the composition of excitonic complexes that are observed as absorption or photoluminescence resonances in optical studies. Moreover, we can change the way these resonances shift and broaden in energy by controlling the quantum numbers of the resident carriers with magnetic fields and doping levels, and by selecting the quantum numbers of the photoexcited or recombining electron-hole (e-h) pair through optical polarization. We discuss the roles of distinguishability and optimality of excitonic complexes, showing them to be key ingredients that determine the energy shifts and broadening of optical resonances in charge-tunable semiconductors. A distinguishable e-h pair means that the electron and hole undergoing photoexcitation or recombination have quantum numbers that are not shared by any of the resident carriers. An optimal excitonic complex refers to a complex whose particles come with all available quantum numbers of the resident carriers. All optical resonances may be classified as either distinct or indistinct depending on the distinguishability of the e-h pair, and the underlying excitonic complex can be classified as either optimal or suboptimal. The universality of these classifications, inherited from the fundamental Pauli exclusion principle, allows us to understand how optical resonances shift in energy and whether they should broaden as doping is increased. This understanding is supported by conclusive evidence that the decay of optical resonances cannot be simply attributed to enhanced screening when resident carriers are added to a semiconductor. Finally, applying the classification scheme in either monolayer or moire heterobilayer systems, we relate the energy shift and amplitude of the neutral exciton resonance to the compressibility of the resident carrier gas.

One of the main interests of 2D materials is their ability to be assembled with many degrees of freedom for tuning and manipulating excitonic properties. There is a need to understand how the structure of the interfaces between atomic layers influences exciton properties. Here we use cathodoluminescence and time-resolved cathodoluminescence experiments to study how excitons interact with the interface between two twisted hexagonal boron nitride (hBN) crystals with various angles. An efficient capture of free excitons by the interface is demonstrated, which leads to a population of long-lived and interface-localized (2D) excitons. Temperature dependent experiments indicate that for high twist angles, these excitons localized at the interface further undergo a selftrapping. It consists in a distortion of the lattice around the exciton on which the exciton traps itself. Our results suggest that this exciton-interface interaction causes the broad 4-eV optical emission of highly twisted hBN-hBN structures. Exciton self-trapping is finally discussed as a common feature of sp2 hybridized boron nitride polytypes and nanostructures due to the ionic nature of the B-N bond and the small size of their excitons.

Boron vacancies (V_B^-) in hexagonal boron nitride (hBN) have emerged as a promising platform for two-dimensional quantum sensors capable of operating at atomic-scale proximity. However, the mechanisms responsible for photoluminescence quenching in thin hBN sensing layers when placed in contact with absorptive materials remain largely unexplored. In this Letter, we investigate non-radiative Förster resonance energy transfer (FRET) between V_B^- centers and either monolayer graphene or 2D semiconductors. Strikingly, we find that the FRET rate is negligible for hBN sensing layers thicker than 3 nm, highlighting the potential of V_B^- centers for integration into ultra-thin quantum sensors within van der Waals heterostructures. Furthermore, we experimentally extract the intrinsic radiative decay rate of V_B^- defects.

The optical emission spectra of semiconducting transition metal dichalcogenide monolayers highlight fascinating recombination processes of charged excitons (trions). When charge-tunable WSe2 monolayers are moderately doped with electrons, a strong luminescence peak emerges just below the well-understood spectral lines associated with the recombination of negatively charged bright and dark trions. Despite previous investigations, its origin remains elusive. Here, we demonstrate that this luminescence peak is the result of electron-electron assisted recombination that brightens the dark trion emission. Supporting evidence for this second-order recombination process comes from identifying the brightened emission of positively charged dark trions when the monolayer is electrostatically doped with holes. Remarkably, the discovered hole-hole assisted luminescence peak emerges in the near-infrared, about 500 meV below the well-studied spectral region of excitons and trions. In magnetophotoluminescence experiments, we find that the g-factor of this new transition (g = +4) has an opposite sign compared to the well-known g-factor of neutral or charged excitons. This allows us to propose a mechanism of brightening of the positively charged dark trion involving the Gamma valley of the valence band.

Studies of excitonic transport in transition metal dichalcogenide monolayers have attracted increasing interest in recent years in order to develop nano-optoelectronic devices made with 2D materials. These studies began with low to moderate optical excitation regimes, and more recently have focused on high injection regimes where nonlinear effects appear. This article is focused on the transport of biexcitons by spatially and temporally resolved photoluminescence spectroscopy at high excitation flux. The study is carried out on a high-quality WSe$_2$ monolayer encapsulated in hexagonal boron nitride. The results show that a Seebeck current affects transport in connection with the presence of hot biexcitons. In particular, we observe the formation of spatial rings, also called halos, which have been observed in other excitonic gases. These results tend to generalize the importance of high-energy populations in excitonic transport in TMD, even for complex and heavy excitonic particles.

Boron vacancies in hexagonal boron nitride (hBN) are among the most extensively studied optically active spin defects in van der Waals crystals, due to their promising potential to develop two-dimensional (2D) quantum sensors. In this letter, we demonstrate the tunability of the charge state of boron vacancies in ultrathin hBN layers, revealing a transition from the optically active singly negatively charged state to the optically inactive doubly negatively charged state when sandwiched between graphene electrodes. Notably, there is a photoluminescence quenching of a few percent upon the application of a bias voltage between the electrodes. Our findings emphasize the critical importance of considering the charge state of optically active defects in 2D materials, while also showing that the negatively charged boron vacancy remains robust against external perpendicular electric fields. This stability makes it a promising candidate for integration into various van der Waals heterostructures.

Among a broad diversity of color centers hosted in layered van der Waals materials, the negatively charged boron vacancy (VB-) center in hexagonal boron nitride (hBN) is garnering considerable attention for the development of quantum sensing units on a two-dimensional platform. In this work, we investigate how the optical response of an ensemble of VB- centers evolves with the hBN thickness in a range of a few to hundreds of nanometers. We show that the photoluminescence intensity features a nontrivial evolution with thickness, which is quantitatively reproduced by numerical calculations taking into account thickness-dependent variations of the absorption, radiative lifetime, and radiation pattern of VB- centers. Besides providing an important resource to optimize the performances of quantum sensing units based on VB- centers in hBN, the thickness-dependent nanophotonic effects discussed in this work generally apply to any type of color center embedded in a van der Waals material.

When two BN layers are stacked in parallel in an AB or BA arrangement, a spontaneous out-of-plane electric polarization arises due to charge transfer in the out-of-plane B-N bonds. The ferroelectric switching from AB to BA (or BA to AB) can be achieved with a relatively small out-of-plane electric field through the in-plane sliding of one atomic layer over the other. However, the optical detection of such ferroelectric switching in hBN has not yet been demonstrated. In this study, we utilize an adjacent WSe2 monolayer to detect the ferroelectric switching in BN. This dynamic coupling between a two-dimensional (2D) ferroelectric and a 2D semiconductor allows for the fundamental investigation of the ferroelectric material using a nondestructive, local optical probe, offering promising applications for compact and nonvolatile memory devices.

The negatively charged boron vacancy (V-B ) center in hexagonal boron nitride (hBN) is currently garnering considerable attention for the design of two-dimensional (2D) quantum sensing units. Such developments require a precise understanding of the spin-dependent optical response of V-B centers, which still remains poorly documented despite its key role for sensing applications. Here we investigate the spin-dependent photodynamics of V-B centers in hBN by a series of time-resolved photoluminescence (PL) measurements. We first introduce a robust all-optical method to infer the spin-dependent lifetime of the excited states and the electron spin polarization of V-B centers under optical pumping. Using these results, we then analyze PL time traces recorded at different optical excitation powers with a seven-level model of the V-B center and we extract all the rates involved in the spin-dependent optical cycles, both under ambient conditions and at liquid helium temperature. These findings are finally used to study the impact of a vector magnetic field on the optical response. More precisely, we analyze PL quenching effects resulting from electron spin mixing induced by the magnetic field component perpendicular to the V-B quantization axis. All experimental results are well reproduced by the seven-level model, illustrating its robustness to describe the spin-dependent photodynamics of V-B centers. This work provides important insights into the properties of V-B centers in hBN, which are valuable for future developments of 2D quantum sensing units.