Structural selection and phase engineering in NaNbO₃ thin films and free-standing membranes. Epitaxial strain, Ca–Mn co-doping and film/membrane architecture for structural and functional tuning toward photocatalysis and multifunctional applications

This dissertation examines structural selection and phase engineering in sodium niobate, NaNbO₃ (NNO), thin films and free-standing membranes fabricated by pulsed laser deposition (PLD). The roles of epitaxial strain, thickness, Ca–Mn co-doping, and sacrificial-layer design were examined by combining X-ray diffraction (XRD), reciprocal-space mapping (RSM), Raman spectroscopy, atomic force microscopy (AFM), and electrical measurements. In pure NNO, structural evolution depends on deposition temperature, thickness, and substrate-induced strain. On NdGaO₃ (NGO) substrate at 600 °C, the films start in the strained ferroelectric Q phase and then pass to the relaxed antiferroelectric P phase as thickness increases. At temperatures higher than 700 °C, the evolution changes: the ferroelectric Q phase remains stable over a much wider thickness interval, up to about 45 nm on NGO and up to about 125 nm on SrTiO₃ (STO), and only beyond this interval does the relaxed antiferroelectric P phase appear. On NGO (101), suitable growth conditions make it possible to obtain fully single-phase layers, either pure Q or pure P, without detectable mixed contribution. These results show that the structural phase of NNO can be selected through substrate choice and growth conditions. For the investigated Ca,Mn co-doped composition, Ca₀.₂₅Na₀.₇₅Nb₀.₈₇₅Mn₀.₁₂₅O₃, the films preserve single-oriented epitaxial growth within the explored thickness and temperature intervals. In contrast with pure NNO, the symmetric XRD θ–2θ scans remain those of one NNO-based perovskite phase over the investigated range. The optimal growth conditions identified for this composition are pO₂ = 1 mbar and T = 700 °C, with NGO and LSAT as the most suitable substrates. The transition from supported films to free-standing membranes is governed by the sacrificial layer. Replacing Sr₃Al₂O₆ (SAO) with SrCa₂Al₂O₆ (SC₂AO) shifts the sacrificial-layer lattice parameter toward better structural correspondence with NNO and improves the membrane state after release, with larger continuous areas and lower scratch and folding density. For Ca,Mn co-doped films, lattice engineering alone is not sufficient: the sacrificial-layer composition must iii also be adjusted, since interfacial chemistry during growth and release affects membrane integrity. On the tuned sacrificial layers, both pure and Ca,Mn co-doped NNO preserve the same thicknessdependent strained-to-relaxed evolution observed on the reference substrates. In the pure case, this remains coupled to the Q/P structural coexistence, whereas in the Ca,Mn co-doped case no thickness-driven structural-phase change is resolved in symmetric XRD θ–2θ scans. The sacrificial layer itself also changes during film overgrowth and oxygen exposure, so reproducible membrane fabrication depends on both its initial state and its evolution during growth. Overall, the dissertation shows how phase selection, strain state, and sacrificial-layer design act together in pure and Ca–Mn co-doped sodium niobate thin films and free-standing membranes.

Questa tesi esamina la selezione strutturale e il phase engineering in film sottili e membrane autoportanti di niobato di sodio, NaNbO₃ (NNO), ottenuti mediante pulsed laser deposition (PLD). Il lavoro considera il ruolo di strain epitassiale, spessore, co-drogaggio Ca–Mn e progettazione del layer sacrificale, combinando diffrazione a raggi X, reciprocal-space mapping, spettroscopia Raman, microscopia a forza atomica e misure elettriche. Nel sistema del niobato di sodio puro, l’evoluzione strutturale dipende congiuntamente da temperatura di deposizione, spessore e strain imposto dal substrato. Su NdGaO₃ a 600 °C il film cresce inizialmente nella fase ferroelettrica Q strained e passa poi alla fase antiferroelettrica P relaxed all’aumentare dello spessore. A temperature superiori a 700 °C, la fase ferroelettrica Q resta stabile fino a circa 45 nm su NdGaO₃ e fino a circa 125 nm su SrTiO₃; solo oltre compare la fase antiferroelettrica P relaxed. Per il composto co-drogato Ca₀.₂₅Na₀.₇₅Nb₀.₈₇₅Mn₀.₁₂₅O₃, i film mantengono crescita epitassiale single-oriented e, nell’intervallo esplorato, restano strutturalmente single-phase nelle scansioni XRD θ–2θ. Vista la particolare sensibilità dei film di niobato di sodio alle strutture sottostanti, la fabbricazione delle membrane dipende in modo decisivo dal layer sacrificale. Il passaggio da SAO a SC₂AO migliora la corrispondenza strutturale con NNO e la qualità della membrana dopo il rilascio. Nel complesso, la tesi mostra come selezione di fase, strain e layer sacrificale agiscano insieme nei film e nelle membrane di NNO puro e co-drogato.