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Förster resonant energy transfer (FRET)-mediated exciton diffusion through artificial nanoscale building block assemblies could be used as an optoelectronic design element to transport energy. However, so far, nanocrystal (NC) systems supported only diffusion lengths of 30 nm, which are too small to be useful in devices. Here, we demonstrate a FRET-mediated exciton diffusion length of 200 nm with 0.5 cm2/s diffusivity through an ordered, two-dimensional assembly of cesium lead bromide perovskite nanocrystals (CsPbBr3 PNCs). Exciton diffusion was directly measured via steady-state and time-resolved photoluminescence (PL) microscopy, with physical modeling providing deeper insight into the transport process. This exceptionally efficient exciton transport is facilitated by PNCs' high PL quantum yield, large absorption cross section, and high polarizability, together with minimal energetic and geometric disorder of the assembly. This FRET-mediated exciton diffusion length matches perovskites' optical absorption depth, thus enabling the design of device architectures with improved performances and providing insight into the high conversion efficiencies of PNC-based optoelectronic devices.

Monolayer transition metal dichalcogenides (TMDCs) are promising materials for next generation optoelectronic devices. The exciton diffusion length is a critical parameter that reflects the quality of exciton transport in monolayer TMDCs and limits the performance of many excitonic devices. Although diffusion lengths of a few hundred nanometers have been reported in the literature for asexfoliated monolayers, these measurements are convoluted by neutral and charged excitons (trions) that coexist at room. temperature due to natural background doping. Untangling the diffusion of neutral excitons and trionsusingis paramount to understand the fundamental limits and potential of new optoelectronic device architectures made possible TMDCs. In this work, lw.e measure the diffusion lengths of neutral excitons and trions in monolayer MoS2 by tuning the background carrier concentration using a gate voltage and utilizing both steady state and transient. spectroscopy. We observe diffusion lengths of 1.5 mu m and 300 nm for neutral excitons and trions, respectively, at an optical power density of 0.6 W cm(-2).

Halide perovskite nanocrystals hold promise for printable optoelectronic and photonic applications. Doping enhances their functionalities and is being investigated for substituting lead with environmentally friendlier elements. The most investigated dopant is Mn 2+ that acts as a color center sensitized by the host excitons. The sensitization mechanism is far from understood and no comprehensive picture of the energy-transfer process has been proposed. Similarly, the role of shallow states, particularly abundant in defect tolerant materials, is still unknown. Here, we address this problem via spectroscopic studies at controlled excitation density and temperature on Mn:CsPbCl 3 nanocrystals. Our results indicate a two-step process involving exciton localization in a shallow metastable state that mediates the thermally assisted sensitization of the Mn 2+ emission, which is completely quenched for T < 200 K. At T ≤ 60 K, however, such emission surprisingly reappears, suggesting direct energy transfer from band-edge states. Electron spin resonance supports this picture, revealing the signatures of conformational rearrangements below 70 K, possibly removing the potential barrier for sensitization. Our results demystify anomalous behaviors of the exciton-to-Mn 2+ energy-transfer mechanism and highlight the role of shallow defects in the photophysics of doped perovskite nanostructures

Durante la mia tesi di dottorato mi sono occupata dello studio spettroscopico di nanocristalli colloidali di semiconduttore e in particolare della correlazione tra le loro superfici e la fotofisica, che ho studiato per mezzo di spettroelettrochimica (SEC) e spettroscopia ottica in atmosfera controllata. Più precisamente, ho studiato e modellizzato il comportamento di diversi tipi di nanocristalli sottoponendoli a variazioni controllate delle condizioni ossidanti/riducenti dell’ambiente che li circonda, con l’obiettivo di implementarne l’uso in sensori ottici di ossigeno. L’elevato rapporto superficie-volume tipico dei nanocristalli fa sì che la loro fotoluminescenza sia fortemente influenzata da un’ampia distribuzione di stati introdotti da difetti superficiali. Un portatore catturato da una trappola in superficie, infatti, non è più disponibile per la ricombinazione radiativa, e abbassa la resa quantica del nanocristallo. Tramite SEC, si può applicare una differenza di potenziale elettrochimico (EC) a un sottile film di nanocristalli depositati su un substrato trasparente e conduttivo. La fotolumiscenza del campione viene eccitata e raccolta sia in continua sia temporalmente risolta tramite appositi rivelatori. L’applicazione di una differenza di potenziale EC negativo corrisponde ad aumentare il livello di Fermi del nanocristallo, riempiendone gradualmente i difetti superficiali e attivando l’intrappolamento di lacune. La fotoluminescenza che ne risulta dipende dall’effetto dominante tra lo spegnimento della stessa dovuto all’intrappolamento di lacune e l’aumento derivante dalla soppressione dell’intrappolamento di elettroni. Per ciascun materiale ho effettuato misure di SEC ed esperimenti in atmosfera controllata, per arrivare alla realizzazione di diversi tipi di sensori ottici di ossigeno (Pressure Sensitive Paints, PSPs), usati per monitorare il flusso di ossigeno nei pressi di superfici complesse o miniaturizzate. Tipicamente consistono in un mezzo poros, The main research theme of my PhD has been the spectroscopic investigation of colloidal semiconductor nanocrystals (NCs), with a focus on the correlation between their surfaces and their photophysics, and was conducted by means of spectroelectrochemistry (SEC) and optical spectroscopy under controlled atmosphere. Specifically, I aimed to understand and model the NCs behavior in a changing oxidative/reducting environment, with the ultimate goal to implement their use as active material in optical oxygen pressure sensors. The high surface-to-volume ratio typical of NCs causes their photoluminescence (PL) efficiency to be strongly affected by a broad distribution of surface defect states. If captured by a surface trap, a photogenerated electron (or hole) becomes unavailable for the radiative recombination, thus lowering the overall PL efficiency of the NCs. By means of SEC, an electrochemical (EC) potential can be applied to a thin film of NCs deposited onto a transparent and conductive substrate, whose PL is excited and collected via dedicated instruments for either continuous or time-resolved measurements. The application of a negative EC potential corresponds to raising the Fermi level of the NCs, thus gradually filling the surface defects and activating their hole-trapping capability. The PL intensity is thus determined by the competition between the quenching effect of hole withdrawal and the brightening effect of suppressed electron trapping. For each material system I performed side-by-side SEC measurements and spectroscopic experiments under controlled atmosphere, and eventually demonstrated different types of optical oxygen pressure sensors, also called pressure-sensitive paints (PSPs), i.e, all-optical probes for monitoring oxygen flows in the vicinity of complex or miniaturized surfaces. They typically consist in a porous binder embedding an oxygen sensitive chromophore, whose PL intensity changes accordingly to the oxygen partial pressure. By employing cesi

Electronic doping of colloidal semiconductor nanostructures holds promise for future device concepts in optoelectronic and spin-based technologies. Ag+ is an emerging electronic dopant in iii-v and ii-vi nanostructures, introducing intragap electronic states optically coupled to the host conduction band. With its full 4d shell Ag+ is nonmagnetic, and the dopant-related luminescence is ascribed to decay of the conduction-band electron following transfer of the photoexcited hole to Ag+. This optical activation process and the associated modification of the electronic configuration of Ag+ remain unclear. Here, we trace a comprehensive picture of the excitonic process in Ag-doped CdSe nanocrystals and demonstrate that, in contrast to expectations, capture of the photohole leads to conversion of Ag+ to paramagnetic Ag2+. The process of exciton recombination is thus inextricably tied to photoinduced magnetism. Accordingly, we observe strong optically activated magnetism and diluted magnetic semiconductor behaviour, demonstrating that optically switchable magnetic nanomaterials can be obtained by exploiting excitonic processes involving nonmagnetic impurities.

Ternary CuInS2 nanocrystals (CIS NCs) are attracting attention as nontoxic alternatives to heavy metal-based chalcogenides for many technologically relevant applications. The photophysical processes underlying their emission mechanism are, however, still under debate. Here we address this problem by applying, for the first time, spectro-electrochemical methods to core-only CIS and core/shell CIS/ZnS NCs. The application of an electrochemical potential enables us to reversibly tune the NC Fermi energy and thereby control the occupancy of intragap defects involved in exciton decay. The results indicate that, in analogy to copper-doped II-VI NCs, emission occurs via radiative capture of a conduction-band electron by a hole localized on an intragap state likely associated with a Cu-related defect. We observe the increase in the emission efficiency under reductive electrochemical potential, which corresponds to raising the Fermi level, leading to progressive filling of intragap states with electrons. This indicates that the factor limiting the emission efficiency in these NCs is nonradiative electron trapping, while hole trapping is of lesser importance. This observation also suggests that the centers for radiative recombination are Cu2+ defects (preexisting and/or accumulated as a result of photoconversion of Cu1+ ions) as these species contain a pre-existing hole without the need for capturing a valence-band hole generated by photoexcitation. Temperature-controlled photoluminescence experiments indicate that the intrinsic limit on the emission efficiency is imposed by multiphonon nonradiative recombination of a band-edge electron and a localized hole. This process affects both shelled and unshelled CIS NCs to a similar degree, and it can be suppressed by cooling samples to below 100 K. Finally, using experimentally measured decay rates, we formulate a model that describes the electrochemical modulation of the PL efficiency in terms of the availability of intragap elect

Lead halide perovskite nanocrystals (NCs) are emerging as optically active materials for solution-processed optoelectronic devices. Despite the technological relevance of tracing rational guidelines for optimizing their performances and stability beyond their intrinsic resilience to structural imperfections, no in-depth study of the role of selective carrier trapping and environmental conditions on their exciton dynamics has been reported to date. Here we conduct spectro-electrochemical (SEC) experiments, side-by-side to oxygen sensing measurements on CsPbBr3 NCs for the first time. We show that the application of EC potentials controls the emission intensity by altering the occupancy of defect states without degrading the NCs. Reductive potentials lead to strong (60%) emission quenching by trapping of photogenerated holes, whereas the concomitant suppression of electron trapping is nearly inconsequential to the emission efficiency. Consistently, oxidizing conditions result in minor (5%) brightening due to suppressed hole trapping, confirming that electron traps play a minor role in nonradiative decay. This behavior is rationalized through a model that links the occupancy of trap sites with the position of the NC Fermi level controlled by the EC potential. Photoluminescence measurements in controlled atmosphere reveal strong quenching by collisional interactions with O2, which is in contrast to the photobrightening effect observed in films and single crystals. This indicates that O2 acts as a scavenger of photoexcited electrons without mediation by structural defects and, together with the asymmetrical SEC response, suggests that electron-rich defects are likely less abundant in nanostructured perovskites than in the bulk, leading to an emission response dominated by direct interaction with the environment.

Ratiometric pressure sensitive paints (r-PSPs) are all-optical probes for monitoring oxygen flows in the vicinity of complex or miniaturized surfaces. They typically consist of a porous binder embedding mixtures of a reference and a sensor chromophore exhibiting oxygen-insensitive and oxygen-responsive luminescence, respectively. Here, we demonstrate the first example of an r-PSP based on a single two-color emitter that removes limitations of r-PSPs based on chromophore mixtures such as different temperature dependencies of the two chromophores, cross-readout between the reference and sensor signals and phase segregation. In our approach, we utilize a novel "double-sensor" r-PSP that features two spectrally separated emission bands with opposite responses to the O2 pressure, which boosts the sensitivity with respect to traditional reference-sensor pairs. Specifically, we use two-color-emitting dot-in-bulk CdSe/CdS core/shell nanocrystals, exhibiting red and green emission bands from their core and shell states, whose intensities are respectively enhanced and quenched in response to the increased oxygen partial pressure that effectively tunes the position of the nanocrystal's Fermi energy. This leads to a strong and reversible ratiometric response at the single particle level and an over 100% enhancement in the pressure sensitivity. Our proof-of-concept r-PSPs further exhibit suppressed cross-readout thanks to zero spectral overlap between the core and shell luminescence bands and a temperature-independent ratiometric response between 0 and 70 °C

Colloidal quantum dots (QDs) are emerging as true candidates for light-emitting diodes with ultrasaturated colors. Here, we combine CdSe/CdS dot-in-rod heterostructures and polar/polyelectrolytic conjugated polymers to demonstrate the first example of fully solution-based quantum dot light-emitting diodes (QD-LEDs) incorporating all-organic injection/transport layers with high brightness, very limited roll-off and external quantum efficiency as high as 6.1%, which is 20 times higher than the record QD-LEDs with all-solution-processed organic interlayers and exceeds by over 200% QD-LEDs embedding vacuum-deposited organic molecules.

Colloidal quantum wells combine the advantages of size-tunable electronic properties with vast reactive surfaces that could allow one to realize highly emissive luminescent-sensing varnishes capable of detecting chemical agents through their reversible emission response, with great potential impact on life sciences, environmental monitoring, defence and aerospace engineering. Here we combine spectroelectrochemical measurements and spectroscopic studies in a controlled atmosphere to demonstrate the 'reversed oxygen-sensing' capability of CdSe colloidal quantum wells, that is, the exposure to oxygen reversibly increases their luminescence efficiency. Spectroelectrochemical experiments allow us to directly relate the sensing response to the occupancy of surface states. Magneto-optical measurements demonstrate that, under vacuum, heterostructured CdSe/CdS colloidal quantum wells stabilize in their negative trion state. The high starting emission efficiency provides a possible means to enhance the oxygen sensitivity by partially de-passivating the particle surfaces, thereby enhancing the density of unsaturated sites with a minimal cost in term of luminescence losses.

Colloidal "dot-in-bulk" nanocrystals (DiB NCs) consist of a quantum confined core embedded into a bulklike shell of a larger energy gap. The first reported example of this class of nanostructures are CdSe/CdS DiB NCs that are capable of producing tunable two-color emission under both weak continuous-wave optical excitation and electrical charge injection. This property is a consequence of a Coulomb blockade mechanism, which slows down dramatically intraband relaxation of shell-localized holes when the core is already occupied by a hole. Here, we demonstrate electrochemical control of dual emission from DiB NCs. Spectro-electrochemical (SEC) experiments are used to tune and probe the photoluminescence (PL) intensity and branching between the core and the shell emission channels as a function of applied electrochemical potential (VEC). To interpret the SEC data we develop a model that describes the changes in the intensities of the shell and core PL bands by relating them to the occupancies of electron and hole traps. Specifically, application of negative electrochemical potentials under which the Fermi level is shifted upward in energy leads to passivation of electron traps at the surface of the CdS shell thereby increasing the total PL quantum yield by favoring the shell emission. Simultaneously, the emission color changes from red (VEC = 0) through yellow to green (VEC = -1). Time-resolved PL measurements indicate that as the Fermi level approaches the NC conduction band-edge electrons are injected into the NC quantized states, which leads to typical signatures of negative trions observed under optical excitation. Application of positive potentials leads to activation of electron traps, which quenches both core and shell PL and leads to the reduction of the overall PL quantum efficiency. A high sensitivity of emission intensity (especially pronounced for the shell band) and the apparent emission color of DiB NCs to local electrochemical environment can enable inter

Luminescent solar concentrators are cost-effective complements to semiconductor photovoltaics that can boost the output of solar cells and allow for the integration of photovoltaic-active architectural elements into buildings (for example, photovoltaic windows). Colloidal quantum dots are attractive for use in luminescent solar concentrators, but their small Stokes shift results in reabsorption losses that hinder the realization of large-area devices. Here, we use Stokes-shift-engineeredCdSe/CdS quantum dots with giant shells (giant quantum dots) to realize luminescent solar concentrators without reabsorption losses for device dimensions up to tens of centimetres. Monte-Carlo simulations show a 100-fold increase in efficiency using giant quantum dots compared with core-only nanocrystals. We demonstrate the feasibility of this approach by using high-optical-quality quantum dot-polymethylmethacrylate nanocomposites fabricated using a modified industrial method that preserves the light-emitting properties of giant quantum dots upon incorporation into the polymer. Study of these luminescent solar concentrators yields optical efficiencies >10% and an effective concentration factor of 4.4. These results demonstrate the significant promise of Stokes-shift-engineered quantum dots for large-area luminescent solar concentrators. © 2014 Macmillan Publishers Limited. All rights reserved.