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International audience; Light management is one of the main challenges to address when designing a sensor from a nanocrystal (NC) array. Indeed, the carrier diffusion length, limited by hopping mechanism, is much shorter than the absorption depth. Several types of resonators (plasmon, Bragg mirror, guided mode, Fabry-Perot cavity) have been proposed to reduce the volume where light is absorbed. All of them are inherently narrow bands, while imaging applications focus on broadband sensing. Here, we propose an infrared sensor in the short and mid-wave infrared (SWIR and MWIR) that combines three different photonic modes to achieve broadband enhancement of the light absorption. Moreover, we show that these three modes can be obtained from a simple structure where the NC film is coupled only to a grating and a top metallic layer. The obtained device achieves a high responsivity of >700 mA.W-1 , a detectivity up to 2x10 10 Jones at 80 K, and a short response time of 11 µs.

International audience; Heterostructured two-dimensional (2D) colloidal nanoplatelets are a class of material that have attracted great interest for optoelectronic applications due to their high photoluminescence yield, atomically tunable thickness and ultralow lasing thresholds. Of particular interest are laterally heterostructured core-crown nanoplatelets with a type-II band alignment, where the in-plane spatial separation of carriers leads to indirect (or charge transfer) excitons with long lifetimes and bright, highly Stokes shifted emission. Despite this, little is known about the nature of the lowest energy exciton states responsible for emission in these materials. Here using polarization-controlled, steady-state and time-resolved photoluminescence measurements, at temperatures down to 1.6 K and magnetic fields up to 30 T, we study the exciton fine structure and spin dynamics of archetypal type-II CdSe/CdTe core-crown nanoplatelets. Complemented by theoretical modelling and zero-field quantum beat measurements we find the bright-exciton fine structure consists of two linearly polarized states with a fine structure splitting  50 μeV and an indirect exciton Landé g-factor of 0.7. In addition, we show the exciton spin lifetime to be in the microsecond range with an unusual B-3 magnetic field dependence. The discovery of linearly polarized exciton states and emission highlights the potential for use of such materials in display and imaging applications without polarization filters. Furthermore, the small exciton fine structure splitting and a long spin lifetime are fundamental advantages when envisaging CdSe/CdTe nanoplatelets as elementary bricks for the next generation of quantum devices, particularly given their ease of fabrication.

International audience; We investigate the potential use of colloidal nanoplates of Sb 2 Te 3 by conducting transport on single particle with in mind their potential use as 3D topological insulator material. We develop a synthetic procedure for the growth of plates with large lateral extension and probe their infrared optical and transport properties. These two properties are used as probe for the determination of the bulk carrier density and agree on a value in the 2–3 × 10 19 cm −3 range. Such value is compatible with the metallic side of the Mott criterion which is also confirmed by the weak thermal dependence of the conductance. By investigating the transport at the single particle level we demonstrate that the hole mobility in this system is around 40 cm 2 V −1 s −1. For the bulk material mixing n-type Bi 2 Te 3 with the p-type Sb 2 Te 3 has been a successful way to control the carrier density. Here we apply this approach to the case of colloidally obtained nanoplates by growing a core-shell heterostructure of Sb 2 Te 3 /Bi 2 Te 3 and demonstrates a reduction of the carrier density by a factor 2.5. Bismuth and antimony chalcogenides (tetradymite group with formula such as (Sb;Bi) 2 (Se;Te) 3) have attracted great interest in the past for their thermoelectric properties 1–5. The heavy mass of these materials leads to a large spin orbit coupling which results in an inverted band structure. Over this past decade, it is the original electronic structure of bismuth and antimony chalcogenides which has driven most of interest in these compounds. Indeed they appear as model 3D topological insulator 6–9 , with conducting surface-states and an insulating core, as long as the material can be obtained under an intrinsic form. Sb 2 Te 3 is a 0.3 eV band gap semiconductor. This material has common antisite defects 10–12 where Sb atoms replace Te atoms, which tends to result in a p-type doping 13. Controlling the bulk carrier density in topological insulator compounds is a key challenge since the Fermi level of the material needs to be close to its Dirac point for electronic transport to be dominated by topologically protected surface states. Moreover, the conductance of the material is the sum of the surface and bulk contribution. Because of the narrow band gap nature of these topolog-ical insulator materials and their deviation from stoichiometry, the bulk is generally not as insulating as desired. By reducing the bulk carrier density and the associated conductance, the weight of the surface contribution in transport is expected to increase and make the surface observation more likely to occur. The Mott criterion can be used to estimate whether the material will behave as a metal or as an insulator. Metallic behavior is expected to occur if >. a n 0 25 0 1/3 14 , where n is the carrier density and = ε ε π a h m e 0 r 2 0 2 the Bohr radius with h the Planck constant, ε 0 the vacuum permitivity, ε r the material dielectric constant, m the effective mass and e the proton charge. Due to large dielectric constant (ε > 50) of Sb 2 Te 3 , the Bohr radius is large which

Nanometer-sized gaps between plasmonically coupled adjacent metal nanoparticles enclose extremely localized optical fields, which are strongly enhanced. This enables the dynamic investigation of nanoscopic amounts of material in the gap using optical interrogation. Here we use impinging light to directly tune the optical resonances inside the plasmonic nanocavity formed between single gold nanoparticles and a gold surface, filled with only yoctograms of semiconductor. The gold faces are separated by either monolayers of molybdenum disulfide (MoS2) or two-unit-cell thick cadmium selenide (CdSe) nanoplatelets. This extreme confinement produces modes with 100-fold compressed wavelength, which are exquisitely sensitive to morphology. Infrared scattering spectroscopy reveals how such nanoparticle-on-mirror modes directly trace atomic-scale changes in real time. Instabilities observed in the facets are crucial for applications such as heat-assisted magnetic recording that demand long-lifetime nanoscale plasmonic structures, but the spectral sensitivity also allows directly tracking photochemical reactions in these 2-dimensional solids.

CdSe/CdTe core-crown type-II nanoplatelet heterostructures are two-dimensional semiconductors that have attracted interest for use in light-emitting technologies due to their ease of fabrication, outstanding emission yields, and tunable properties. Despite this, the exciton dynamics of these complex materials, and in particular how they are influenced by phonons, is not yet well understood. Here, we use a combination of femtosecond vibrational spectroscopy, temperature-resolved photoluminescence (PL), and temperature-dependent structural measurements to investigate CdSe/CdTe nanoplatelets with a thickness of four monolayers. We show that charge-transfer (CT) excitons across the CdSe/CdTe interface are formed on two distinct time scales: initially from an ultrafast (∼70 fs) electron transfer and then on longer time scales (∼5 ps) from the diffusion of domain excitons to the interface. We find that the CT excitons are influenced by an interfacial phonon mode at ∼120 cm-1, which localizes them to the interface. Using low-temperature PL spectroscopy we reveal that this same phonon mode is the dominant mechanism in broadening the CT PL. On cooling to 4 K, the total PL quantum yield reaches close to unity, with an ∼85% contribution from CT emission and the remainder from an emissive sub-band-gap state. At room temperature, incomplete diffusion of domain excitons to the interface and scattering between CT excitons and phonons limit the PL quantum yield to ∼50%. Our results provide a detailed picture of the nature of exciton-phonon interactions at the interfaces of 2D heterostructures and explain both the broad shape of the CT PL spectrum and the origin of PL quantum yield losses. Furthermore, they suggest that to maximize the PL quantum yield both improved engineering of the interfacial crystal structure and diffusion of domain excitons to the interface, e.g., by altering the relative core/crown size, are required.