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. Here we demonstrate that in situ high-temperature annealing of hBN-encapsulated monolayer WS2 on a suspended microheater leads to the emergence of spectrally isolated single-photon emitters at cryogenic temperatures. Annealing at temperatures around 1100 K produces a sharp emission line, X-L, red-shifted by approximately 80 meV from the neutral exciton and exhibiting a linewidth below 200 & micro;eV. Photoluminescence excitation spectroscopy and power-dependent measurements show that X-L originates from annealing-induced defects in the WS2 monolayer, while second-order photon correlation measurements reveal clear antibunching with g((2))(0)<0.5. These results establish high-temperature in situ annealing as a controlled means to access defect-bound excitonic states and single-photon emission in van der Waals materials.

Layered magnetic materials potentially hold the key to future applications based on optical control and manipulation of magnetism. NiPS3, a prototype member of this family, is antiferromagnetic below 155 K and exhibits sharp photoluminescence associated to a transition between a triplet ground state and a singlet excited state. The nature of the luminescent transition is a matter of current debate and so is an eventual fundamental link of this excitation to magnetism. Here we provide answers through experiments and calculations. We fabricate samples with metal and ligand substitutions which alter the Neel transition temperature and measure the effects of these changes on the temperature dependent photoluminescence. We perform crystal field and charge transfer multiplet calculations to explain the origin of the excitation and identify the effects of the magnetic ground state on its properties. These measurements and calculations provide a comprehensive explanation for the observed properties and a template for finding similar materials exhibiting spin-flip luminescence.

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.

The electronic structure of semiconducting 2D materials such as transition metal dichalcogenides (TMDs) is known to be tunable by its environment, from simple external fields applied with electrical contacts up to complex van der Waals heterostructure assemblies. However, conventional alloying from reference binary TMD compounds to composition-controlled ternary alloys also offers unexplored opportunities. In this work, we use nano-angle resolved photoemission spectroscopy (nano-ARPES) and density functional theory (DFT) calculations to study the structural and electronic properties of different alloy compositions of bulk WS2(1-x)Se2x. Our results demonstrate the continuous variation of the band structure and the progressive evolution of the valence band splitting at the K points from 420 to 520 meV in bulk WS2(1-x)Se2x. We also carried out scanning tunneling microscopy (STM) measurements and DFT to understand the possible S or Se substitutions variants in WS2(1-x)Se2x alloys, with different local atomic configurations. Our work opens up perspectives for the fine control of the band dispersion in van der Waals materials and demonstrate how the band structure can be tuned in bulk TMDs. The collected information can serve as a reference for future applications.

The electronic structure of semiconducting 2D materials such as transition metal dichalcogenides (TMDs) is known to be tunable by its environment, from simple external fields applied with electrical contacts up to complex van der Waals heterostructure assemblies. However, conventional alloying from reference binary TMD compounds to composition-controlled ternary alloys also offers unexplored opportunities. In this work, we use nano-angle resolved photoemission spectroscopy (nano-ARPES) and density functional theory (DFT) calculations to study the structural and electronic properties of different alloy compositions of bulk WS2(1-x)Se2x. Our results demonstrate the continuous variation of the band structure and the progressive evolution of the valence band splitting at the K points from 420 to 520 meV in bulk WS2(1-x)Se2x. We also carried out scanning tunneling microscopy (STM) measurements and DFT to understand the possible S or Se substitutions variants in WS2(1-x)Se2x alloys, with different local atomic configurations. Our work opens up perspectives for the fine control of the band dispersion in van der Waals materials and demonstrates how the band structure can be tuned in bulk TMDs. The collected information can serve as a reference for future applications.

Straintronics involves the manipulation and regulation of the electronic characteristics of 2D materials through the use of macro- and nano-scale strain engineering. In this study, an atomic force microscope (AFM) coupled with an optical system is used to perform indentation measurements and tip-enhanced photoluminescence (TEPL), allowing to extract the local optical response of a suspended monolayer membrane of ternary WSSe at various levels of deformation, up to strains of 10%. The photoluminescence signal is modeled considering the deformation, stress distribution, and strain dependence of the WSSe band structure. An additional TEPL signal is observed that exhibits significant variation under strain, with 64 meV per percent of elongation. This peak is linked to the highly strained 2D material lying right underneath the tip. The amplification of the signal and its relation to the excitonic funneling effect are discussed in a more comprehensive model. The diffusion caused by Auger recombination against the radiative excitonic decay will also be compared. TEPL is used to examine and comprehend the local physics of 2D semi-conducting materials subjected to extreme mechanical strain. Chemical vapor deposition-fabricated 2D ternaries possess high strain resistance, comparable to the benchmark MoS2, and a high Young's modulus of 273 GPa. Straintronics involves the manipulation and regulation of the electronic characteristics of 2D materials through the use of macro- and nano-scale strain engineering. In this study, an atomic force microscope (AFM) coupled with an optical system is used to perform indentation measurements and tip-enhanced photoluminescence (TEPL), allowing to extract the local optical response of a suspended monolayer membrane of ternary WSSe at various levels of deformation, up to strains of 10%. image

We have used a polarized spatially resolved microluminescence technique to investigate photocarrier charge and spin transport at 6 K in a GaAs nanowire (NW; n-type doping level approximate to 1017 cm-3). Because of the difference in expansion coefficients of the NW and of its SiO2 substrate, the NW is under strain, as revealed by the splitting between light- and heavy-hole emissions in the luminescence intensity spectrum. Light valence levels lie above the heavy valence ones, which is attributed as being caused by a tensile strain along both the axial and the lateral directions of the NW, equivalent to a compressive strain in the direction of light excitation. The symmetry group of the perturbed nanowire is then lowered to C2v. No spin polarization can be evidenced for the heavy valence levels. The electron spin polarization decays up to a distance of 5 mu m from the excitation spot, because of spin relaxation, and stays constant for larger distances because of the increased value of the drift velocity. Remarkably, the light-hole spin polarization exhibits damped spatial oscillations over as much as 5 mu m. Analysis of the effect of strain on valence states shows that these oscillations are caused by the spin-orbit interaction in the light valence level. It is found that, for the C2v point group, the corresponding Hamiltonian is linear in momentum. This spin-orbit interaction causes coherent oscillations rather than a spin relaxation process since transport essentially has a drift character in the internal electric field. The equivalent effective magnetic field induced by spin-orbit interaction and strain is, taking a light-hole g factor of 1, of the order of 60 mT.

A systematic study of spin‐dependent recombination (SDR) under steady‐state optical pumping conditions as a function of nitrogen content, x , in dilute nitride alloys of the form GaAs 1− x N x is reported. Use of high‐excitation power densities up to 10 7 W cm −2 allows measurement of the full SDR versus power curves, even at relatively high nitrogen contents of x = 0.039. Alloy contents for are determined within by fitting the photoluminescence (PL) spectra using a Roosbroeck–Shockley relation, and values consistent with those obtained by studying the intensity of the GaN‐like LO 2 Raman mode are found. PL intensity increases by a factor known as the SDR ratio when switching from linearly to circularly polarized pump excitation. This factor reaches 5 for x = 0.022 and decreases with increasing x , falling to 1.5 for x = 0.039. Moreover, the excitation power required for maximum SDR increases with increasing x , varying from 0.6 mW for x = 0.022 to 15 mW for x = 0.039. These observations indicate an increase in the density of electronically active defects with increasing nitrogen content, both responsible for the SDR and other, standard Shockley–Read–Hall centers. The result demonstrates the importance of including nonspin‐dependent recombination channels in a complete model of SDR.

HgTe,thanks to its unique spectral tunability in the infrared,is the only material able to cover near-, short-, and mid-wave infrared.Current best devices rely on electrodes made from transparent conductiveoxides and gold, but so far, none of these completely fit for theintended purpose. Gold is not compatible with Si foundries, and transparentconductive oxides are highly lossy in this spectral range, limitingelectrode transparency. Metal-based electrodes appear as good alternativecandidates but require further investigations. While obvious constraintsof work function get raised, chemical stability appears equally important.Here, we screen the use of Au, Al, Ag, and Zn as possible metals andreveal that in the case of Ag, dramatic transformations of Ag andHgTe are observed. Especially, a cation exchange procedure can occurover a solid-state film without intentional heating of the sample.This process has then been studied by combining both structural andelectronic probes. This work points out the importance of the carefulchoice of surrounding electrodes in the case of HgTe since the observedmechanism is likely not limited to Ag. On the other hand, both Auand Al appear stable toward this transformation.

We have investigated the laser-induced valley polarization and coherence of encapsulated MoS 2 monolayer as a function of temperature, power density, and spatial position. Besides a non-monotonic dependence on temperature, recently attributed to a dependence of the valley relaxation time on the momentum scattering rate, we observe a two-fold increase of the valley polarization when increasing the laser excitation power. We attribute this effect to a local heating induced by the energy relaxation of photoexcited excitons and to an increase of the exciton-exciton scattering rate. In contrast, only a moderate enhancement of valley coherence is observed, which exhibits a dramatic drop after further increasing the excitation power. We attribute this behaviour to the detrimental role of exciton-exciton interactions on the pure dephasing rate responsible for the loss of coherence between the valleys. This manifests itself by a strong dip in the spatial profile of the valley coherence at high photoexcited densities.

We have used a polarized microluminescence technique to investigate photocarrier charge and spin transport in n-type depleted GaAs nanowires (≈ 10 cm−3 doping level). At 6K, a longdistance tail appears in the luminescence spatial profile, indicative of charge and spin transport, only limited by the length of the NW. This tail is independent on excitation power and temperature. Using a self-consistent calculation based on the drift-diffusion and Poisson equations as well as on photocarrier statistics (Van Roosbroeck model), it is found that this tail is due to photocarrier drift in an internal electric field nearly two orders of magnitude larger than electric fields predicted by the usual ambipolar model. This large electric field appears because of two effects. Firstly, for transport in the spatial fluctuations of the conduction band minimum and valence band maximum, the electron mobility is activated by the internal electric field. This implies, in a counter intuitive way, that the spatial fluctuations favor long distance transport. Secondly, the range of carrier transport is further increased because of the finite NW length, an effect which plays a key role in one-dimensional systems.

The roadmap of future innovative device developments foresees the reduction of material dimensions down to nanometer scale and the incorporation of novel degrees of freedom. For instance, electrons and holes in 2D semiconductors like MoS2 monolayers exhibit a unique coupling between the spin and the crystal momentum, also referred to as the valley. A crucial requirement for future applications is therefore the possibility to initialise the spin/valley degree of freedom in these materials. Here we investigate the optical initialisation of the valley degree of freedom in charge-tunable MoS2 monolayers encapsulated with hexagonal boron nitride at cryogenic temperatures. We report in photoluminescence a large steady state valley polarization of the different excitonic complexes following circularly-polarized laser excitation. We reveal efficient valley initialisation of positively-charged excitons, which have so far proved to be elusive in non-encapsulated monolayers due to defect and laser-induced large electron doping. We find that negatively-charged excitons present a polarization of 70% which is unusually large for non-resonant excitation. We attribute this large valley polarization to the particular band structure of MoS2. In addition, we demonstrate that circular excitation induces a dynamical polarization of resident electrons and holes––as recently shown in tungsten-based monolayers.

Hyperspectral imaging at cryogenic temperatures is used to investigate exciton and trion propagation in MoSe_2 monolayers encapsulated with hexagonal boron nitride (hBN). Under a tightly focused, continuous-wave laser excitation, the spatial distribution of neutral excitons and charged trions strongly differ at high excitation densities. Remarkably, in this regime the trion distribution develops a halo shape, similar to that previously observed in WS2 monolayers at room temperature and under pulsed excitation. In contrast, the exciton distribution only presents a moderate broadening without the appereance of a halo. Spatially and spectrally resolved luminescence spectra reveal the buildup of a significant temperature gradient at high excitation power, that is attributed to the energy relaxation of photoinduced hot carriers. We show, via a numerical resolution of the transport equations for excitons and trions, that the halo can be interpreted as thermal drift of trions due to a Seebeck term in the particle current. The model shows that the difference between trion and exciton profiles is simply understood in terms of the very different lifetimes of these two quasiparticles.

Monolayers of transition metal dichalcogenides are ideal materials to control both spin and valley degrees of freedom either electrically or optically. Nevertheless, optical excitation mostly generates excitons species with inherently short lifetime and spin/valley relaxation time. Here we demonstrate a very efficient spin/valley optical pumping of resident electrons in n-doped WSe2 and WS2 monolayers. We observe that, using a continuous wave laser and appropriate doping and excitation densities, negative trion doublet lines exhibit circular polarization of opposite sign and the photoluminescence intensity of the triplet trion is more than four times larger with circular excitation than with linear excitation. We interpret our results as a consequence of a large dynamic polarization of resident electrons using circular light.

We investigate with polarized microphotoluminescence the optical pumping of the valley degree of freedom in charge-tunable MoS2 monolayers encapsulated with hexagonal boron nitride at cryogenic temperatures. We report a large steady state valley polarization of the different excitonic complexes following circularly-polarized laser excitation 25 meV above the neutral exciton transition. For the first time in this material we reveal efficient valley pumping of positively-charged trions, which were so far elusive in non-encapsulated monolayers due to defect and laser-induced large electron doping. We find that negatively-charged trions present a polarization of 70 which is unusually large for non-resonant excitation. We attribute this large valley polarization to the particular band structure of MoS2, where an optically dark exciton ground state coexists with a bright conduction band ordering in the single-particle picture, leading to a supression of the valley relaxation for negatively-charged trions. In addition, we demonstrate that circular excitation induces a dynamical polarization of resident electrons and holes, as recently shown in tungsten-based monolayers. This manifest itself as a variation in the intensity of different excitonic complexes under circular and linear excitation.

We have investigated the photocarrier charge transport in n-type metallic GaAs nanowires ( 10^17 cm^-3 doping level), grown by hydride vapor phase epitaxy (HVPE) on Si(111) substrates. Analysis of the luminescence intensity spatial profiles for selected energies in the spectrum allows us to determine the spatial distribution of photoelectrons, minority photoholes and electrons of the Fermi sea as a function of distance from the light excitation spot. This analysis shows that charge can be transported over record distances larger than 25 (micro)m at 6K, in spite of the expected localization of minority holes in the potential fluctuations generated by statistical fluctuations of the donor concentration. It is shown that transport is little affected by the fluctuations because of the build up of large internal electric fields which strongly increase the hole and electron mobilities and therefore enable transport. Comparison of the spatial profiles of the emissions due to hot electrons and to the Fermi sea gives evidence for at least three spatial zones, including a zone of excess intrinsic electrons near the excitation spot and a zone of depletion of these electrons at a distance larger than 2-10 (micro)m depending on excitation power. The internal outward electric field increases the kinetic energy of photoholes in the fluctuations so that after a given distance to the excitation spot, there occurs ballistic transport over the fluctuations.

We have investigated the steady-sate valley polarization and valley coherence of encapsulated MoS2 monolayer as a function of the temperature and the power density with a continuous wave laser excitation. Both valley polarization and coherence exhibit a non-monotonic dependence on sample temperature, attaining a local maximum at T=40 K. This has been recently attributed to a motional narrowing effect: an enhancement of the valley relaxation time occurs when the scattering rate increases. At a fixed temperature of T=6 K, a two-fold increase of the steady-state valley polarization is achieved by increasing the laser excitation power, which we attribute to a local heating induced by the energy relaxation of photoexcited excitons outside the light cone and to an increase in the exciton-exciton scattering rate. In contrast, in the same power range only a moderate enhancement of valley coherence is observed. Further increasing the excitation power leads to a small reduction of valley polarization but a dramatic loss of valley coherence. Supported by spatial imaging of the excitonic luminescence and polarization, we attribute this behaviour to the detrimental role of exciton-exciton interactions on the pure dephasing rate.

Photon helicity-dependent photocurrent is measured at zero magnetic field on a device based on an ensemble of InGaAs/GaAs quantum dots that are embedded into a GaAs-based p-i-n diode. Our main goal is to take advantage of the long electron spin relaxation time expected in these nano-objects. In these experiments, no external magnetic field is required thanks to the use of an ultrathin magnetic CoFeB/MgO electrode, presenting perpendicular magnetic anisotropy (PMA). We observe a clear asymmetry of the photocurrent measured under respective right and left polarized light that follows the hysteresis of the magnetic layer. The amplitude of this asymmetry at zero magnetic field decreases with increasing temperatures and can be controlled with the bias. Polarization-resolved photoluminescence is detected in parallel while the device is operated as a photodetector. This demonstrates the multifunctional capabilities of the device and gives valuable insights into the spin relaxation of the electrons in the quantum dots.

Pour répondre aux besoins croissants de la microélectronique, il a été proposé de transmettre l’information par le spin de l’électron, cette variable quantique dont l’analogue classique est la rotation sur lui-même. Il serait naturel de penser que la charge de l’électron et son spin se transportent de manière identique, si l’on excepte la tendance de toute orientation de spin à disparaitre par relaxation. Et pourtant, plusieurs études fondamentales montrent que le spin peut se transporter de façon indépendante de l’électron qui le porte, et donc « vivre sa vie » comme une particule autonome. Cet article est consacré à ces résultats fortement contre-intuitifs.

In semiconductors under tightly-focused photocarrier excitation, the lateral variation of carrier concentration induces a lateral variation of photovoltage. In chemically-passivated p-type GaAs films at 300 K, we show experimentally and theoretically that the photovoltage lateral dependence is able to block the photoelectron diffusion, thus reducing the effective charge diffusion constant by a factor of ≈5 with respect to surface-free conditions and producing a self-trapping of photoelectrons. This effect is not present for surface-free and for oxidized samples, for which the photovoltage magnitude is strongly reduced. The photovoltage lateral variation also induces a coupling between charge and spin diffusion. Because of this coupling, the effective spin diffusion constant is significantly larger than the effective charge one.