Many potential applications of monolayer transition metal dichalcogenides (TMDs) require both high photoluminescence (PL) yield and high electrical mobilities. However, the PL yield of as prepared TMD monolayers is low and believed to be limited by defect sites and uncontrolled doping. This has led to a large effort to develop chemical passivation methods to improve PL and mobilities. The most successful of these treatments is based on the nonoxidizing organic “superacid” bis(trifluoromethane)sulfonimide (TFSI) which has been shown to yield bright monolayers of molybdenum disulfide (MoS2) and tungsten disulfide (WS2) but with trap-limited PL dynamics and no significant improvements in field effect mobilities. Here, using steady-state and time-resolved PL microscopy we demonstrate that treatment of WS2 monolayers with oleic acid (OA) can greatly enhance the PL yield, resulting in bright neutral exciton emission comparable to TFSI treated monolayers. At high excitation densities, the OA treatment allows for bright trion emission, which has not been demonstrated with previous chemical treatments. We show that unlike the TFSI treatment, the OA yields PL dynamics that are largely trap free. In addition, field effect transistors show an increase in mobilities with the OA treatment. These results suggest that OA serves to passivate defect sites in the WS2 monolayers in a manner akin to the passivation of colloidal quantum dots with OA ligands. Our results open up a new pathway to passivate and tune defects in monolayer TMDs using simple “wet” chemistry techniques, allowing for trap-free electronic properties and bright neutral exciton and trion emission.

Recently, Ruddlesden-Popper 2D perovskites (RPPs) solar cells and Light-Emitting Diodes (LED) have shown promising eciencies and improved stability in comparison to 3D halide perovskites. Here, the exciton recombination dynamics is investigated at room temperature in pure phase RPPs crystals (C 6 H 5 C 2 H 4 NH 3) 2 (CH 3 NH 3) n1 Pb n I 3n+1 (n=1, 2, 3 and 4) by time-resolved photoluminescence in a large range of power excitations. As the number of perovskite layers increases, we detect the presence of an increasing fraction of out-of-equilibrium free carriers just after photoexcitation, on a picosecond timescale, while the dynamics is characterized by the recombination of excitons with long lifetime spanning on several tens of nanoseconds. At low excitation power, the PL decays are non-exponential due to defect-assisted recombination. At high uence, defects are lled and many body interactions become important. Similarly to other 2D systems, Exciton-Exciton Annihilation (EEA) is then the dominant recombination path in a high density regime below the Mott transition.

Understanding the surface properties of organic-inorganic lead-based perovskites is of high importance to improve the device's performance. Here, we have investigated the differences between surface and bulk optical properties of CHNHPbBr single crystals. Depth-resolved cathodoluminescence was used to probe the near-surface region on a depth of a few microns. In addition, we have studied the transmitted luminescence through thicknesses between 50 and 600 μm. In both experiments, the expected spectral shift due to the reabsorption effect has been precisely calculated. We demonstrate that reabsorption explains the important variations reported for the emission energy of single crystals. Single crystals are partially transparent to their own luminescence, and radiative transport is the dominant mechanism for propagation of the excitation in thick crystals. The transmitted luminescence dynamics are characterized by a long rise time and a lengthening of their decay due to photon recycling and light trapping.

Structural defects vary the optoelectronic properties of monolayer transition metal dichalcogenides, leading to concerted efforts to control defect type and density via materials growth or postgrowth passivation. Here, we explore a simple chemical treatment that allows on–off switching of low-lying, defect-localized exciton states, leading to tunable emission properties. Using steady-state and ultrafast optical spectroscopy, supported by ab initio calculations, we show that passivation of sulfur vacancy defects, which act as exciton traps in monolayer MoS2 and WS2, allows for controllable and improved mobilities and an increase in photoluminescence up to 275-fold, more than twice the value achieved by other chemical treatments. Our findings suggest a route for simple and rational defect engineering strategies for tunable and switchable electronic and excitonic properties through passivation.

The variation of the optical absorption of carbon nanotubes with their geometry has been a long standing question at the heart of both metrological and applicative issues, in particular because optical spectroscopy is one of the primary tools for the assessment of the chiral species abundance of samples. Here, we tackle the chirality dependence of the optical absorption with an original method involving ultra-efficient energy transfer in porphyrin/nanotube compounds that allows uniform photo-excitation of all chiral species. We measure the absolute absorption cross-section of a wide range of semiconducting nanotubes at their S22 transition and show that it varies by up to a factor of 2.2 with the chiral angle, with type I nanotubes showing a larger absorption. In contrast, the luminescence quantum yield remains almost constant.The versatility of the physical properties of Single-Wall carbon Nanotubes (SWNTs) with respect to their geometry (the so-called (n, m) chiral species) is very attractive for applications [1][2][3][4], but on the other hand, the uncontrolled mixtures of species produced by regular synthesis methods blur out their specific properties. Post-growth sorting methods now allow to enrich samples in some specific species [5], but they also miss a tool for the quantitative assessment of their outcome. Optical techniques such as absorption, photoluminescence (PL) or resonant Raman spectroscopies are the primary tools to this end. However, these techniques can neither give a quantitative estimate of the species concentration nor their relative abundance to-date because they miss the knowledge of the (n, m)-dependence of the optical cross-section at the nanotubes' resonances (S 11 and S 22 ). Although several studies pointed that the optical properties of carbon nanotubes depend on their chiral angle, they all actually dealt with a combination of physical parameters (such as absorption cross-section, Raman scattering cross-section or PL quantum efficiency). As a result, the literature gives quite contradictory or inconclusive results, some of them pointing to a larger abundance of near armchair nanotubes (interpreted as energetically favored in the growth process) whereas other studies concluded for a larger optical cross-section for large chiral angles [6][7][8][9][10].Here, we propose an original method for assessing the chirality dependence of the absorption cross-section of semiconducting carbon nanotubes, by means of noncovalent functionalization with tetraphenyl porphyrin (TPP) molecules (Inset of Figure 1). This functionalization gives rise to an extremely efficient energy transfer [11] that allows to excite uniformly the whole set of carbon nanotubes regardless of their chirality. By comparison with the PL signal obtained in the regular excitation scheme (on the intrinsic S 22 transition of the SWNTs) of the same sample, we can single out the contribution of the absorption cross-section in the chiral dependence of the PL intensity. We show that the main variation of this absorption cross-sect...

Halide perovskites are emerging as valid alternatives to conventional photovoltaic active materials owing to their low cost and high device performances. This material family also shows exceptional tunability of properties by varying chemical components, crystal structure, and dimensionality, providing a unique set of building blocks for new structures. Here, highly stable self‐assembled lead–tin perovskite heterostructures formed between low‐bandgap 3D and higher‐bandgap 2D components are demonstrated. A combination of surface‐sensitive X‐ray diffraction, spatially resolved photoluminescence, and electron microscopy measurements is used to reveal that microstructural heterojunctions form between high‐bandgap 2D surface crystallites and lower‐bandgap 3D domains. Furthermore, in situ X‐ray diffraction measurements are used during film formation to show that an ammonium thiocyanate additive delays formation of the 3D component and thus provides a tunable lever to substantially increase the fraction of 2D surface crystallites. These novel heterostructures will find use in bottom cells for stable tandem photovoltaics with a surface 2D layer passivating the 3D material, or in energy‐transfer devices requiring controlled energy flow from localized surface crystallites to the bulk.

Halide perovskites have shown great potential for light emission and photovoltaic applications due to their remarkable electronic properties and compatibility with cost-effective fabrication techniques. Although the device performances are promising, they are still limited by microscale heterogeneities in their photophysical properties. In particular, the relation between local heterogeneities and the diffusion of charge carriers at the surface and in the bulk, crucial for efficient collection of charges in a light harvesting device, is not well understood.Here, a photoluminescence tomography technique is developed in a confocal microscope using one-and two-photon excitation to distinguish between local surface and bulk diffusion of charge carriers in methylammonium lead bromide single crystals. The local temporal diffusion is probed at various excitation depths to build statistics of local electronic diffusion coefficients.The measured values range between 0.3 to 2 cm 2 .s -1 depending on the local trap density and the morphological environmenta distribution that would be missed from analogous macroscopic or surface-measurements. Tomographic images of carrier diffusion were reconstructed to reveal buried crystal defects that act as barriers to carrier transport. This work reveals a new framework to understand and homogenise diffusion pathways, which are extremely sensitive to local properties and buried defects.Over the past ten years, halide perovskites have emerged as strong candidates for various light-harvesting and light-emission applications [1][2][3] . The performances of perovskitebased photovoltaics (PV) and light-emitting diodes (LEDs) are now competing with mature, commercial technologies [4] . This rapid development has been made possible by the design of new halide perovskite compositions [5][6][7] which generally share properties of remarkably long carrier diffusion lengths (0.1-1 μm) [8,9] even when simple cost-effective fabrication techniques are employed. However, for halide perovskites to reach their full potential, one has to understand the microscopic heterogeneities that still limit their performances [10,11] . For instance, local defects, both at the surface and inside the bulk, trap charge carriers thus limiting their ability to diffuse through the material. It is therefore critical to investigate the diffusion mechanisms at the local scale to identify these trap sites and elucidate ways to mitigate their influence on carrier diffusion and recombination.Methylammonium lead bromide (MAPbBr3, MA=CH3NH3 + ) single crystals have remarkable photophysical properties as highlighted in recent reports on amplified spontaneous emission [12] and lasing phenomena [13,14] , two-photon absorption [15,16] , extreme sensitivity to environment [17] , excitonic properties [18,19] , and long carrier diffusion lengths [20] . Additionally, their optical properties are well-documented including their refractive index [21,22] and exciton binding energy [23] , and photon reabsorption has been quantified [22,24,25...

Graphene nanoribbons synthesized by the bottom-up approach with optical energy gaps in the visible are investigated by means of optical spectroscopy. The optical absorption and fluorescence spectra of two graphene nanoribbons with different structures are reported as well as the life-time of the excited states. The possibility of the formation of excimer states in stacks of individual graphene nanoribbons is discussed in order to interpret the broad and highly Stokes-shifted luminescence lines observed on both structures. Finally, combined atomic force microscopy and confocal fluorescence measurements have been performed on small aggregates, showing the ability of graphene nanoribbons to emit light in the solid state. These observations open interesting perspectives for the use of graphene nanoribbons as near-infrared emitters.