Growth and characterization of films of perovskite based organic-inorganic hybrid materials for  electronics
Creators: name: Chiarella, Fabio
Publisher: name: Università degli Studi di Napoli Federico II
Year: 2006

description: [ITALIANO] In questo lavoro di tesi l’attenzione è stata rivolta allo studio di composti ibridi self-assembled per l’elettronica. Il primo passo è stato la sintesi (dove è stato possibile) e la classificazione strutturale delle polveri policristalline dei composti in esame ovvero alogenuri di metalli (Cu, Sn) intercalati con ammine semplici (formula chimica (CnH2n+1NH3)2CuCl4 e CH3NH3SnX3 con X=Cl, Br, I). In particolare i composti CH3NH3SnX3 risultano avere una struttura cubica mentre quelli (CnH2n+1NH3)2CuCl4 mostrano una struttura ortorombica, un discorso particolare va fatto per il composto (CH3NH3)2CuCl4 la cui struttura è molto discussa, probabilmente monoclina anziché ortorombica.
I film sono stati realizzati presso il centro IMEM-CNR di Parma durante la mia permanenza, attraverso una tecnica detta SSTA (Ablazione Termica da Singola Sorgente) in un reattore da alto vuoto partendo dalle polveri come composti precursori. Per i composti dello stagno l’evaporazione è stata effettuata dai sali componenti del composto che volevamo realizzare, anche in questo modo sono stati ottenuti buoni risultati, a testimonianza della buona elasticità della tecnica di deposizione SSTA.
La buona qualità dei film è stata valutata attraverso la caratterizzazione morfologico-strutturale e delle proprietà ottiche. La morfologia superficiale dei film è stata analizzata con tecniche microscopiche (microscopio ottico, AFM) che ha mostrato una struttura a grani piccoli dei film mentre con la tecnica di diffrazione da raggi X (radiazione CuK-alfa del rame) è stato realizzato un ampio studio sistematico della struttura reticolare dei composti. L’ottimizzazione del processo di deposizione ci ha portato ad ottenere film monofasici cristallinamente ben strutturati, dall’analisi diffrattometrica infatti si osserva come i composti non presentino fasi spurie e risultino ben orientati asse c.
Sui film ottimizzati è stato effettuato uno studio delle proprietà ottiche di assorbimento e fotoemissione al variare della temperatura. Dalle misure di assorbimento è stato possibile osservare le energie delle bande elettroniche in particolare del claster perovskitico CuCl42- con notevole sensibilità a testimonianza delle buone proprietà di ordine strutturale dei film rispetto ai composti bulk. Le misure di fotoluminescenza mostrano invece la presenza  di un picco di emissione a circa 380 nm di natura eccitonica. In particolare il composto (C2H5NH3)2CuCl4 per l’elevata stabilità, bassa igroscopicità buona conducibilità ed intensa fotoluminescenza, risulta un candidato ideale per eventuali studi di elettroluminescenza e realizzazioni di dispositivi LED nell’UV.
Infine sui film ottimizzati si è proceduto alla caratterizzazione della risposta di trasporto elettrico in continua in funzione della realizzazione di un dispositivo preliminare FET (Field Effect Transistor). Le categorie di film studiati risultano isolanti o semiconduttori, tale caratterizzazione è stata effettuata attraverso misura della resistività in funzione della temperatura. Un dispositivo FET è stato realizzato e misurato utilizzando come elemento attivo (canale) un film del composto semiconduttivo CH3NH3SnBr3.
Infine uno studio sulla sensitività all’umidità dei composti proposti è stato effettuato in collaborazione con l’istituto ENEA di portici. I materiali considerati infatti, sono sali igroscopici ed hanno mostrato interessanti caratteristiche per la realizzazione di eventuali sensori di umidità. / [ENGLISH] In this work of thesis study, synthesis, and characterization of two perovskite based hybrid series of compounds was been faced.
The search for new materials plays a central role in the framework of new technologies and in sector of consumer electronics. In the last ten years, in fact, an extraordinary growth has been registered (organic electronics) and the increasing business of connection between communication and information technologies and the foreseen possibility of reaching the physical limit of semiconductor technology, have required the search of new ideas to develop cheaper materials and technologies that could generate new applications and form factors to drive the growing needs for pervasive computing and enhanced connectivity.
The research in organic-inorganic hybrids in the last years has focused on using the range of interactions found within organic and inorganic worlds in order to create, project and engineering new materials with the desired, innovative or increased functionalities relative to that achievable with organic and inorganic materials alone. 
The attention in this work is focused on the perovskite based hybrid compounds, proposed by D. B. Mitzi of IBM research center, that result a remarkable example for potentially useful physical properties including enhanced exciton binding energies, nonlinear optical properties, electroluminescence, organic-like mobility, magnetic properties, conductivity and other properties. 
The advantage of the hybrid compounds is mostly related to the greater thermal stability related to full long-range structural ordering in respect to ordinary organic compounds, without losing flexibility and simple processability that have a principal role in low-cost electronics. Simple processability, in Hybrid organic-inorganic compounds, as the perovskite based hybrids, is in many cases overdrew by self-assembling properties. 
In the proceeding of this thesis work, the attention will be focused on two series of hybrid compounds: (CnH2n+1NH3)2CuCl4 (where n =1,…,4) (called 2D layered structures) and CH3NH3SnX3 (where X =Cl and Br) (called 3D structures) where alkylammonium chains are the organic components while the constituent unit of the inorganic structure is the octahedral cluster MX64-. The bonding between organic and inorganic components is realized by an ordinary ammine group ( NH3+) that realizes an hydrogen/ionic bond with the halogen atoms. 
The choice of these two class of compound is related to the interesting electrical, magnetic and optical properties of relative isostructural compounds reported in literature. The possibility to synthesize and study optical properties of transition metal halide compounds is explored, in view of exploiting such a potentiality and to find new materials for UV-LED applications and conductive compounds with organic-like or enhanced mobility. In the framework of the research of enhanced mobilities materials, in fact, hybrid compounds have drive great interest. As for analogues inorganic oxide-based perovskite layered family, metal-semiconductor transition in function of the perovskite layer thickness have been reported in literature in the tin (II) 100 and 110 oriented perovskite families. This first example of conducting layered organic-inorganic halides enable an examination of the electrical transport properties as a function of the dimensionality of the metal halide framework 
In the development of this thesis it is proposed an introduction on the world of new materials for electronics and a brief overview of hybrid compounds with particular attention on perovskite based ones. Many of the pre-established aims have been reached, in the film deposition, characterizations, and the comprehension of some correlation between the structures of hybrid compounds and their properties. 
Synthesis problems and the possible techniques in the realization of polycrystalline powders and films of the two series of selected materials are discussed. In fact, many potential applications involving the organic-inorganic hybrids depend on the availability of simple and reliable thin film deposition techniques. In this work a single source evaporative technique (Single Source Thermal Ablation) has been utilized for film deposition. The obtained film were primarily investigated and optimized in respect to the crystallographic quality. The technique used for the investigation of the structural properties of the films realized for SSTA is X-rays diffraction. CH3NH3SnCl3 is the more complex  compound to realize in thin film form. Its surface was observed by AFM (Atomic Force Microscope) and SEM (Scanning electron Microscope) analysis. The high flexibility of Single Source Thermal Ablation technique in hybrid films deposition has been demonstrated. The films obtained in the optimized deposition conditions on different substrates (crystalline, amorphous and flexible), results by the different analysis to have good structural, morphological and optical quality.
it is analyzed the change in optical properties with the change of one component in the 2D [(CnH2n+1NH3)2CuCl4] and 3D [CH3NH3SnX3] series structure to understand the possible correlations between structure and optical properties. Besides,  the 2D layered hybrid structures have attracted the scientists’ interest because these systems act as the artificial multi-quantum well structures. In principle in this type of structure it is possible to obtain excitonic states and so photoluminescence and electroluminescence, by quantum and dielectric confinement effects. Besides, the intrinsic two-dimensional layered structure of some hybrids, added to self-assembling properties, can lead to the possibility to design and synthesize an emitter material with the capability to control electron/hole drain and separation. 
Analysis of the absorption data was carried out to test the optical quality of SSTA deposited films as well as to determine the predominant optical transition and electronic configuration, while the presence of excitonic states transition have been investigated by photoluminescence measurements performed on the films. All the analyzed 2D layered copper structures realize a photoluminescence emission in the UV. In particular (C2H5NH3)2CuCl4 compound thanks to its high stability, low hygroscopicity, and good conduction properties, results a good candidate to UV-LED applications. 
Transport properties of perovskite based hybrid materials have been discussed. Resistivity and its temperature behaviour of deposited films of the two series are measured by standard technique while the CH3NH3SnBr3 film mobility was carried out by field effect analysis. A field effect device on SrTiO3 dielectric substrate has been realized and characterized varying gate voltages and temperature. Finally a study on the sensitivity to humidity of the much hygroscopic compounds are reported. In the structure-properties correlations, in the 2D compounds that optical properties don’t depend by changing of alkylammonium molecule and that room temperature resistivity decrease with increase of perovskite sheets coupling have been observed, while in 3D tin(II) structures, energy gap and resistivity increase when cell parameter decrease. By the field effect analysis it has been observed that in the proposed hybrid compounds conduction is due to hole carrier. Preliminary measures on sensitivity to humidity have shown interesting sensitivity properties of this hybrid in particular in 3D ones.