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Ternary I-III-VI2 nanocrystals (NCs), such as CuInS2, are receiving attention as heavy-metals-free materials for solar cells, luminescent solar concentrators (LSCs), LEDs, and bio-imaging. The origin of the optical properties of CuInS2 NCs are however not fully understood. A recent theoretical model suggests that their characteristic Stokes-shifted and long-lived luminescence arises from the structure of the valence band (VB) and predicts distinctive optical behaviours in defect-free NCs: the quadratic dependence of the radiative decay rate and the Stokes shift on the NC radius. If confirmed, this would have crucial implications for LSCs as the solar spectral coverage ensured by low-bandgap NCs would be accompanied by increased re-absorption losses. Here, by studying stoichiometric CuInS2 NCs, it is revealed for the first time the spectroscopic signatures predicted for the free band-edge exciton, thus supporting the VB-structure model. At very low temperatures, the NCs also show dark-state emission likely originating from enhanced electron-hole spin interaction. The impact of the observed optical behaviours on LSCs is evaluated by Monte Carlo ray-tracing simulations. Based on the emerging device design guidelines, optical-grade large-area (30×30 cm2) LSCs with optical power efficiency (OPE) as high as 6.8% are fabricated, corresponding to the highest value reported to date for large-area devices.

We report here the synthesis of undoped and Cu-doped Cs2ZnCl4 nanocrystals (NCs) in which we could tune the concentration of Cu from 0.7 to 7.5%. Cs2ZnCl4 has a wide band gap (4.8 eV), and its crystal structure is composed of isolated ZnCl4 tetrahedra surrounded by Cs+ cations. According to our electron paramagnetic resonance analysis, in 0.7 and 2.1% Cu-doped NCs the Cu ions were present in the +1 oxidation state only, while in NCs at higher Cu concentrations we could detect Cu(II) ions (isovalently substituting the Zn(II) ions). The undoped Cs2ZnCl4 NCs were non emissive, while the Cu-doped samples had a bright intragap photoluminescence (PL) at similar to 2.6 eV mediated by band-edge absorption. Interestingly, the PL quantum yield was maximum (similar to 55%) for the samples with a low Cu concentration ([Cu] <= 2.1%), and it systematically decreased when further increasing the concentration of Cu, reaching 15% for the NCs with the highest doping level ([Cu] = 7.5%). The same (similar to 2.55 eV) emission band was detected under X-ray excitation. Our density functional theory calculations indicated that the PL emission could be ascribed only to Cu(I) ions: these ions promote the formation of trapped excitons, through which an efficient emission takes place. Overall, these Cu-doped Cs2ZnCl4 NCs, with their high photo- and radio-luminescence emission in the blue spectral region that is free from reabsorption, are particularly suitable for applications in ionizing radiation detection.

We report on the synthesis of silver (Ag)-doped CdSe nanoplatelets (NPLs) via postsynthesis cation exchange, using silver acetate as the Ag precursor. High-resolution transmission electron microscopy and X-ray diffraction confirmed that the NPLs maintain their morphology and crystal structure after doping when executing the exchange under reduced temperature in an ice bath. Spectroelectrochemistry and transient absorption spectroscopy revealed that Ag + acts as an acceptor dopant. Ag doping results in an emission that is tunable from 609 to 880 nm, with a Stokes shift up to 1 eV and a photoluminescence quantum efficiency exceeding 50%. This is achieved by varying the Ag dopant concentration, which determines the hole energy level, and by controlling the electron energy level via quantum confinement in CdSe NPLs with varying core thickness or in CdSe/CdS core/shell NPLs. As highly fluorescent materials with a strongly suppressed emission reabsorption because of the large Stokes shift, Ag-doped colloidal two-dimensional NPLs offer new opportunities for the development of colloidal nanocrystal-based optoelectronic and photonic devices such as light-emitting diodes or luminescent solar concentrators

Colloidal CdSe nanocrystals (NCs) overcoated with an ultrathick CdS shell, also known as dot-in-bulk (DiB) structures, can support two types of excitons, one of which is core-localized and the other, shell-localized. In the case of weak "sub-single-exciton" pumping, emission alternates between the core-and shell-related channels, which leads to two-color light. This property makes these structures uniquely suited for a variety of photonic applications as well as ideal model systems for realizing complex excitonic quasi-particles that do not occur in conventional core/shell NCs. Here, we show that the DiB design can enable an unusual regime in which the same long-lived resident electron can endow trionlike characteristics to either of the two excitons of the DiB NC (core-or shell-based). These two spectrally distinct trion states are apparent in the measured photoluminescence (PL) and spin dynamics of core and shell excitons conducted over a wide range of temperatures and applied magnetic fields. Low-Temperature PL measurements indicate that core-and shell-based trions are characterized by a nearly ideal (â 100%) emission quantum yield, suggesting the strong suppression of Auger recombination for both types of excitations. Polarization-resolved PL experiments in magnetic fields of up to 60 T reveal that the core-and the shell-localized trions exhibit remarkably similar spin dynamics, which in both cases are controlled by spin-flip processes involving a heavy hole.

We report the composition-dependent optical properties of Bi-doped Cs2Ag1-xNaxInCl6 nanocrystals (NCs) having a double perovskite crystal structure. Their photoluminescence (PL) was characterized by a large Stokes shift, and the PL quantum yield increased with the amount of Na up to ∼22% for the Cs2Ag0.4Na0.6InCl6 stoichiometry. The presence of Bi3+ dopants was crucial to achieve high PL quantum yields (PLQYs) as nondoped NC systems were not emissive. Density functional theory calculations revealed that the substitution of Ag+ with Na+ leads to localization of AgCl6 energy levels above the valence band maximum, whereas doping with Bi3+ creates BiCl6 states below the conduction band minimum. As such, the PL emission stems from trapped emission between states localized in the BiCl6 and AgCl6 octahedra, respectively. Our findings indicated that both the partial replacement of Ag+ with Na+ ions and doping with Bi3+ cations are essential in order to optimize the PL emission of these systems.

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

Fluoromagnetic systems are recognized as an emerging class of materials with great potential in the biomedical field. Here, it is shown how to fabricate fluoromagnetic nanotubes that can serve as multimodal probes for the imaging and targeting of brain cancer. An ionic self-assembly strategy is used to functionalize the surface of synthetic chrysotile nanotubes with pH-sensitive fluorescent chromophores and ferromagnetic nanoparticles. The acquired magnetic properties permit their use as contrast agent for magnetic resonance imaging, and enable the tracking of tumor cell migration and infiltration responsible for metastatic growth and disease recurrence. Their organic component, changing its fluorescence attitude as a function of local pH, targets the cancer distinctive acidity, and allows localizing and monitoring the tumor occurrence and progression by mapping the acidic spatial distribution within biopsy tissues. The fluoromagnetic properties of nanotubes are preserved from the in vitro to the in vivo condition and they show the ability to migrate across the blood brain barrier, thus spontaneously reaching the brain tumor after injection. The simplicity of the synthesis route of these geomimetic nanomaterials combined with their demonstrated affinity with the in vivo condition strongly highlights their potential for developing effective functional materials for multimodal theranostics of brain cancer

In questa tesi ho riportato i risultati ottenuti durante il mio dottorato sullo studio di nuovi materiali per dispositivi di conversione otpo-elettronica, la cui preparazione e processabilità li rende ottimi candidati per il loro scaling-up industriale. Questa tesi è divisa in due argomenti: 1) lo studio di emettori per conentratori solari luminescenti (LSC); 2) lo studio di materiali funzionali per celle solari a eterogiunzione (BHJ-SC) per l’applicazione come layer attivo e come interlayer. Nella prima parte, ho studiato l’applicazione di quantum dots basati sul silicio (Si-QDs) come emettitori in LSC. L’utilizzo di questi nanocristalli è in grado di superare le limitazioni in termini di scaling-up industriale che hanno caratterizzato i QDs fino ad oggi impiegati in LSC. La non tossicità, il basso costo di produzione e la grande disponibilità di materia prima rendono i Si-QDs ottimi candidati per una produzione di LSC su larga scala. L’opportuno confinamento quantico modifica fortemente le proprietà ottiche di questo materiale, conferendogli un elevato Stokes Shift e un’alta efficienza emissiva. Il LSC ottenuto dall’inserimento dei Si.QDs in una matrice copolimerica presenta un’elevata qualità ottica e proprietà vicine ad un LSC ideale, in cui le perdite per auto-assorbimento e scattering, i due più importanti fattori che ne limitano l’efficienza, sono fortemente ridotti. Ho dimostrato infine che le stesse caratteristiche si possono ottenere su LSC con matrice flessibile, preservandone l’efficienza e le proprietà ottiche. Nella seconda parte ho riportato uno studio comparativo sulle performance di BHJ-SC basate su PBDTTPD, un polimero che presenta un ottimo compromesso tra l efficienza di foto-conversione nei dispositivi e complessità molecolare, che lo rendono un promettente candidato per lo scaling-up industriale. Lo studio ha confrontato le efficienze di BHJ-SC basate su due tipologie di PBDTTPD: una preparata via Stille, una sintesi che richiede l’u, In my thesis, I report the results obtained during my three years PhD regarding the study of novel materials for light-energy conversion devices, whose preparation and processing are in agreement with a near-future large scale production. This work is divided into two topics: 1) The study of novel emitters for luminescent solar concentrators (LSCs); 2) The study of novel functional materials for bulk hetero junction solar cells (BHJ-SC) regarding the photoactive layers and interface engineering layers. In the first part, I studied the application of silicon Quantum Dots (Si-QDs) as novel chromophores in LSCs. The use of these semiconductor nanostructures overcomes the limitations in terms of industrial scaling which is characterizing the QDs so far used in LSC. Non-toxicity, low cost of raw material and ultra-abundancy make Si-QDs promising chromophores for large scale production of LSCs. Upon quantum confinement the optical properties of Si-QDs are significantly modified, resulting in a high Stokes Shift quantum efficiency. The device assembled by embedding the Si-QDs in a co-polymeric matrix exhibits a high optical quality and results in an almost ideal LSC in which the losses related to luminescence self-absorption and scattering, the two most important efficiency-limiting factors, are strongly reduced. Similar characteristics were demonstrated to be achieved also in flexible LSCs which still preserve the efficiency and the optical properties of standard flat LSC. In the second part I performed a comparative study about the photovoltaic performances in BHJ-SC of PBDTTPB, a polymer showing a good trade-off between PV performances and structural complexity, which makes it a promising candidate for a future industrial scale production. The comparison of devices PV characteristics was performed between two PBDTTPDs with different synthetic routes: one prepared via Stille reaction, a synthesis involving the use of high toxicity organo-tin compounds and requiring the p

I nanocristalli colloidali a semiconduttore (NC) sono materiali processabili da soluzione che, dalla loro scoperta 30 anni fa, hanno attirato l’attenzione in campo scientifico e tecnologico per le loro proprietà ottiche ed elettriche. Infatti, i NC hanno un ampio range di potenziali applicazioni, che vanno dalle sorgenti luminose, alle celle solari, al bioimaging fino all’informazione quantistica. Ciò è dovuto alla profonda conoscenza e controllo delle loro proprietà elettroniche che si è raggiunto. Infatti, queste ultime si possono modificare controllando la dimensione, la composizione ma anche formando eterostrutture o introducendo impurezze, cioè drogando i NC. A causa dell’ampia varietà di NC che si possono sintetizzare, molti dubbi sui processi fotofisici sottostanti le proprietà ottiche macroscopiche rimangono ancora irrisolti. Dunque, mi sono focalizzato sullo studio di tre sotto-classi di NC: 1) a interfaccia ingegnerizzata; 2) drogati e 3) drogati elettronicamente. Dopo un breve ‘stato dell’arte’ della scienza dei NC colloidali (Capitolo 1), nel secondo Capitolo riporto una studio dettagliato dell’interazione fra i portatori di carica eccitati e l’interfaccia ingegnerizzata dei Dot-in-Bulk core/shell NC, che sono caratterizzati da emissione di fotoluminescenza (PL) sia dagli stati di core che da quelli di shell. Tramite misure di PL ultraveloce, dimostro che la caratteristica struttura all’interfaccia è la motivazione ultima da cui scaturisce la capacità di avere una doppia emissione radiativa, aggiungendo un ulteriore parametro nella chimica dei NC con il quale è possibile modificare le loro proprietà ottiche. Nel Capitolo 3, propongo una nuova strategia di sintesi che permetta di avere NC contenenti tutti un esatto numero di atomi droganti, evitando la distribuzione Poissoniana tipica dei contemporanei metodi di drogaggio. A questo scopo, uso cluster metallici monodispersi come semi di nucleazione per la sintesi dei NC e tramite analisi elemen, Semiconductor colloidal nanocrystals (NCs) are solution-processable materials that have focused scientific and technological attention thanks to their tunable optical and electrical properties. Colloidal NCs have indeed wide applicative perspectives that span from light-emitting diodes, to lasers, from solar cells to luminescent solar concentrators, from bioimaging to quantum information. Such a large range of potential NCs technologies is warranted by the unique knowledge and control that has been achieved over the years about their electronic properties. Specifically, the optical and electric properties of these nanomaterials have been tuned by either controlling their size, composition and shape, or producing multicomponent heterostructures and introducing few atoms of a different chemical element, i.e. doping the NCs. Because of the vast gamut of possibilities that colloidal NCs offer, many questions on the elusive charge carrier dynamics underlying the macroscopic observations are still unanswered. In this picture, my work points toward three different sub-classes of NCs: i) interface engineered NCs; ii) doped NCs and iii) ‘electronic’ doped NCs. After a brief review about the ‘state of the art’ of the colloidal NC science (Chap. 1), in Chap. 2 I show a detailed investigation on the interaction between the photoexcited charge carriers and the engineered interface of Dot-in-Bulk core/shell NC, which are featured by radiative recombination from both the core and shell states. I demonstrate that their uncommon dual emission is due to the peculiar interface structure between the compositional domains and that a fine tuning of the optical properties can be also achieved by modifying the interfacial potential profile. In Chap. 3, I propose a novel synthetic approach to overcome the intrinsic Poisson distribution characteristic of the up-to-date NC doping strategies that are based on stochastic distribution of impurity ions in the NC ensemble. To this aim, I use monod

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.

Halide perovskite nanocrystals (NCs) are promising solution-processed emitters for low-cost optoelectronics and photonics. Doping adds a degree of freedom for their design and enables us to fully decouple their absorption and emission functions. This is paramount for luminescent solar concentrators (LSCs) that enable fabrication of electrode-less solar windows for building-integrated photovoltaic applications. Here, we demonstrate the suitability of manganese-doped CsPbCl3 NCs as reabsorption-free emitters for large-area LSCs. Light propagation measurements and Monte Carlo simulations indicate that the dopant emission is unaffected by reabsorption. Nanocomposite LSCs were fabricated via mass copolymerization of acrylate monomers, ensuring thermal and mechanical stability and optimal compatibility of the NCs, with fully preserved emission efficiency. As a result, perovskite LSCs behave closely to ideal devices, in which all portions of the illuminated area contribute equally to the total optical power. These results demonstrate the potential of doped perovskite NCs for LSCs, as well as for other photonic technologies relying on low-attenuation long-range optical wave guiding.

Metal nanoclusters featuring tunable luminescence and high biocompatibility are receiving attention as fluorescent markers for cellular imaging. The recently discovered ability of gold clusters to scavenge cytotoxic reactive oxygen species (ROS) from the intracellular environment extends their applicability to biomedical theranostics and provides a novel platform for realizing multifunctional luminescent probes with engineered anti-cytotoxic activity for applications in bio-diagnostics and conceivably cellular therapy. This goal could be achieved by using clusters of strongly reactive metals such as silver, provided that strategies are found to enhance their luminescence while simultaneously enabling direct interaction between the metal atoms and the chemical surroundings. In this work, we demonstrate a synergic approach for realizing multifunctional metal clusters combining enhanced luminescence with strong and lasting ROS scavenging activity, based on the fabrication and in situ protection of Ag nanoclusters with a supramolecular mantle of thiolated-Au atoms (Ag/Au-t). Confocal imaging and viability measurements highlight the biocompatibility of Ag/Au-t and their suitability as fluorescent bio-markers. ROS concentration tests reveal the remarkable scavenging activity of Ag-based clusters. Proliferation tests of cells in artificially stressed culture conditions point out their prolonged anti-cytotoxic effect with respect to gold systems, ensuring positive cell proliferation rates even for long incubation time.

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.

Building-integrated photovoltaics is gaining consensus as a renewable energy technology for producing electricity at the point of use. Luminescent solar concentrators (LSCs) could extend architectural integration to the urban environment by realizing electrode-less photovoltaic windows. Crucial for large-Area LSCs is the suppression of reabsorption losses, which requires emitters with negligible overlap between their absorption and emission spectra. Here, we demonstrate the use of indirect-bandgap semiconductor nanostructures such as highly emissive silicon quantum dots. Silicon is non-Toxic, low-cost and ultra-earth-Abundant, which avoids the limitations to the industrial scaling of quantum dots composed of low-Abundance elements. Suppressed reabsorption and scattering losses lead to nearly ideal LSCs with an optical efficiency of i • = 2.85%, matching state-of-The-Art semi-Transparent LSCs. Monte Carlo simulations indicate that optimized silicon quantum dot LSCs have a clear path to i • > 5% for 1â €..m 2 devices. We are finally able to realize flexible LSCs with performances comparable to those of flat concentrators, which opens the way to a new design freedom for building-integrated photovoltaics elements.