Synthesis and Characterization of Inorganic Perovskites for Nanophotonics

In questa tesi di dottorato vengono affrontati diversi aspetti riguardanti la fisica delle perovskiti inorganiche alogenate. In particolare mi focalizzerò su: • Proprietà intrinseche del materiale come il disordine strutturale e dinamico, approfondito da uno studio della fotoluminescenza in un grande insieme di campioni CsPbBr3 che si differenziano per morfologia e protocollo di sintesi. Siccome il disordine pregiudica la forma spettrale della linea della fotoluminescenza e gioca un ruolo importante nelle proprietà di trasporto del supporto al bordo della banda, l’indagine presentata in questa tesi è di interesse per l’ampio insieme delle applicazioni optoelettroniche delle perovskiti alogenate. I risultati ottenuti durante l’attività di dottorato contribuiscono a chiarire le principali caratteristiche del disordine in questa classe di semiconduttori. • L’interazione tra i microcristalli di CsPbBr3 e il substrato/metasuperficie su cui sono depositati. Infatti, l’integrazione di perovskiti alogenate su metasuperfici e strutture fotoniche è una ricerca in fase iniziale e un’indagine approfondita è necessaria prima dell’implementazione di dispositivi fotonici innovativi. In particolare presenterò una serie di risultati sperimentali riguardanti effetti non lineari (comportamento superlineare e sublineare) nell’emissione radiativa di film CsPbBr3 depositati da spin-coating su un particolare tipo di metasuperficie realizzata da microrisonatori core-shell (T-Rex) assemblati in pattern esagonali su un substrato di silicio. I risultati sperimentali evidenziano la presenza di Amplified Spontaneous Emission (ASE) che, a seconda della struttura del campione, si presenta e non si presenta con un aumento superlineare dell’emissione. In particolare si distinguono due processi: l’amplificazione della luce da emettitori disposti in modo casuale e l’amplificazione da una distribuzione abbastanza ordinata di nanocristalli intorno a un risonatore. • CsPbBr3 ha caratteristiche ottimali come adsorbitore e catalizzatore nei pro- cessi di assorbimento degli inquinanti dalle acque reflue industriali. Nella parte finale del mio lavoro mostrerò i risultati preliminari riguardanti il rapido assorbimento del Blu di Metilene (MB), una sostanza tipicamente utilizzata nell’industria tessile, disciolta in soluzione acquosa quando le nanopolveri di CsPbBr3 (NPs) sono incapsulate all’interno di sferette di chitosano (CH-BB). Un aumento significativo della percentuale di adsorbimento di MB si ottiene in presenza delle nanopolveri di perovskite rispetto all’uso delle sferette di chitosano vuote.In this PhD thesis several aspects of the physics of halide inorganic perovskites are addressed. In particular I will focus on: • Intrinsic properties of the material such as the structural and dynamical disorder by a deep study of the photoluminescence in a large set of CsPbBr3 samples differing for morphology and synthesis protocol. Being the disorder affecting the spectral line shape of the photoluminescence, and playing a major role in the carrier transport properties at the band edge, the investigation here presented is of interest for the large set of optoelectronic applications of halide perovskites. The results obtained during the PhD activity contribute to clarify the main characteristics of disorder in this class of semiconductors. • The interaction between CsPbBr3 microcrystals and the substrate/metasurface where they are deposited. In fact, the integration of halide perovskites in metasurfaces and photonic structures is a research at the early stage and deep investigation is required before the implementation of innovative photonic devices. In particular I will present a set of experimental results concerning non-linear effects (superlinear and sublinear behavior) in the radiative emission of CsPbBr3 films deposited by spin-coating on a particular type of metasurface realized by core-shell microresonators (T-Rex) assembled in an esagonal pattern on a silicon substrate. The experimental results evidence the presence of Amplified Spontaneous Emission (ASE) which, depending on the sample structure, does or does not show up with a superlinear increase of the emission. In particular we distinguish between two processes: light amplification by randomly arranged emitters and amplification by a quite ordered distribution of nanocrystals around a resonator. • CsPbBr3 has optimal characteristics as adsorber and catalyst in the processes of absorption of pollutants from industrial waste water. In the final part of my work I will show preliminary results concerning the fast adsorption of Methylene Blue (MB), a substance typically used in the textile industry, dissolved in aqueous solution when CsPbBr3 nanopowders (NPs) are encapsulated inside chitosan bubbles (CH-BB). A significant increase of the adsorption percentage of MB is obtained in presence of the perovskite nanopowders respect to the use of empty chitosan bubbles

Large-Area Nanocrystalline Caesium Lead Chloride Thin Films: A Focus on the Exciton Recombination Dynamics

Caesium lead halide perovskites were recently demonstrated to be a relevant class of semiconductors for photonics and optoelectronics. Unlike CsPbBr3 and CsPbI3, the realization of high-quality thin films of CsPbCl3, particularly interesting for highly efficient white LEDs when coupled to converting phosphors, is still a very demanding task. In this work we report the first successful deposition of nanocrystalline CsPbCl3 thin films (70–150 nm) by radio frequency magnetron sputtering on large-area substrates. We present a detailed investigation of the optical properties by high resolution photoluminescence (PL) spectroscopy, resolved in time and space in the range 10–300 K, providing quantitative information concerning carriers and excitons recombination dynamics. The PL is characterized by a limited inhomogeneous broadening (~15 meV at 10 K) and its origin is discussed from detailed analysis with investigations at the micro-scale. The samples, obtained without any post-growth treatment, show a homogeneous PL emission in spectrum and intensity on large sample areas (several cm2). Temperature dependent and time-resolved PL spectra elucidate the role of carrier trapping in determining the PL quenching up to room temperature. Our results open the route for the realization of large-area inorganic halide perovskite films for photonic and optoelectronic devices

Amplified Spontaneous Emission in CsPbBr_3 Decorated Metasurfaces

Halide perovskites are extremely interesting semiconductors for innovative optoelectronics devices and sensors. In particular, being direct band-gap semiconductors, they are efficient emitters of both incoherent and coherent light [1]. Therefore the possibility of use as active medium in lasers, light amplification systems (resonators and waveguides) and other photonic devices is driving a large part of research in these years [2]. Among inorganic halide perovskites, CsPbBr3 is of interest because of its efficient green emission ( ≈2.3 eV) which makes it a valid substitute of indium-gallium nitrides to solve the green gap problem. In this contribution we present experimental results concerning the light emission in a set of CsPbBr3 films deposited, by spin-coating, on a silicon substrate and on metasurfaces realized by micro/nanospheres having a core of SiO2 and a shell of TiO2 (T-Rex ) on a silicon substrate [3]. Samples were excited after continuous wave laser excitation at 405 nm and with 300 ps long pulses at 266 nm. We investigate both the linear regime, typical of photoluminescence, and the non-linear regime bringing clear evidence of the presence of Amplified Stimulated Emission (ASE) up to 200 K in a back-scattering geometry without the need of a waveguide configuration ( see Figure_PL_and_ASE_CsPbBr3). Depending on the sample structure and increasing the excitation density a transition from the superlinear regime to a linear/sublinear regime is revealed. The significant differences found between samples, differing in their morphology, allow to distinguish between anamplification due to randomly arranged emitters and an amplification due to quite ordered nanocrystals. In particular an enhancement of the photoluminescence and ASE is found when nanocrystals are arranged on the T-Rex indicating a major role played by the metasurfaces in the light extraction. [1] X. Li et al.: “All inorganic halide perovskites nanosystem: Synthesis, structural features, optical properties and optoelectronic applications”, Small 13, 1603996 (2017). [2] see “Halide Perovskites for Photonics”, edited by A. Vinattieri and G. Giorgi (AIP Publishing, New York, 2021). [3] I. Alessandri, “Enhancing raman scattering without plasmons: Unprecedented sensitivity achieved by TiO2 shell-based resonators”, Journal of the American Chemical Society 135, 5541–5544 (2013

Analysis of the Urbach tail in caesium lead halide perovskites

The role of structural and dynamical disorder in semiconductors is a topic of fundamental relevance because of its contribution to the spectral line shape of the photoluminescence, and it plays a major role in ruling the carrier transport properties at the band edge. In this regard, a class of semiconductors, i.e., halide perovskites, deeply investigated in the last decade, shows a peculiar degree of disorder, which has only been recently under investigation. The interest to study disorder in halide perovskites is related to the large set of innovative applications of this class of materials, spanning from energy harvesting to high brilliance incoherent and coherent light emitters. In this perspective, we show that quantitative information on the disorder in halide perovskites can be extracted by deep analysis of the photoluminescence in different experimental conditions. Our study, conducted on a large set of samples of a metal halide perovskite, CsPbBr3, prepared with various synthesis/deposition methods, clarifies the relative weight of the static and dynamic contributions. A comparison with theoretical predictions is provided, gaining insights into the exciton/carrier–phonon interaction in metal halide perovskites

A new route for caesium lead halide perovskite deposition

Inorganic metal halide perovskites are relevant semiconductors for optoelectronic devices. The successful deposition of thin films of CsPbBr3 and CsPbCl3 has recently been obtained by Radio-Frequency magnetron sputtering. In this work we compare the morphological, structural and optical characteristics of the two materials obtained with this deposition technique. A detailed photoluminescence (PL) spectroscopy study of the as-grown samples was conducted at the macro and micro scale in a wide temperature range (10-300 K) to fully characterize the PL on sample areas of square centimeters, to assess the origin of the inhomogeneous broadening and to quantify the PL quantum yield quenching. Our results prove that this technique allows for the realization of high quality nanometric films with controlled thickness of relevance for optoelectronic applications

A new route for high quality nanometric films of inorganic halide perovskites

Successful deposition of CsPbX3 (X = Br, Cl) thin films (50-500 nm) on several kind of substrates has been realized by Radio-Frequency Magnetron Sputtering. The technique allows for high homogeneity of the samples on large areas (several cm2) not only in terms of morphology but also referring to the optical properties. In particular, high resolution (in space, spectrum and time) photoluminescence studies in a wide temperature range (10-300 K) reveal that the low inhomogeneous broadening comes from a submicron size disorder while no significant contribution arises from a micrometric or even larger disorder. Given the relevance of inorganic halide perovskites for innovative optoelectronic devices, our results prove the scalability of this technique. Moreover, the successful deposition on several different substrates open the route for an easy integration in multi-layered structures

A new route for high quality nanometric films of inorganic halide perovskites

Successful deposition of CsPbX3 (X = Br, Cl) thin films (50-500 nm) on several kind of substrates has been realized by Radio-Frequency Magnetron Sputtering. The technique allows for high homogeneity of the samples on large areas (several cm2) not only in terms of morphology but also referring to the optical properties. In particular, high resolution (in space, spectrum and time) photoluminescence studies in a wide temperature range (10-300 K) reveal that the low inhomogeneous broadening comes from a submicron size disorder while no significant contribution arises from a micrometric or even larger disorder. Given the relevance of inorganic halide perovskites for innovative optoelectronic devices, our results prove the scalability of this technique. Moreover, the successful deposition on several different substrates open the route for an easy integration in multi-layered structures