Exploring Perovskite-Metal Nanoassemblies for Photocatalytic Applications

The development of sustainable, renewable and economic chemical processes lies in the centre of todayâs global energy challenge. Like in plants, artificial photosynthesis offers an auspicious solution in harvesting solar energy and storing it in chemical bonds. A number of potential architectures and material systems have been proposed. Among these, the construction of light harvesting antennas funnelling energy towards a catalytic center is a promising idea that mimics the natural photosynthetic system. Semiconductor nanocrystals (NCs) and plasmonic metal NCs are ideal candidates to develop such a concept. The former possesses a tunability of optical properties which is superior to other light absorbers. The latter are interesting photocatalysts able to steer reaction selectivities in a unique way related to their plasmon decay. Among semiconductor NCs, the recently emerged lead halide perovskite NCs represent ideal FRET-type donors due to their high quantum yields and short photoluminescence lifetimes. Yet, incorporating them into a multicomponent light harvesting assemblies is challenging due to their inherent instability issues in conditions which are normally used to drive water splitting or CO2 reduction. Hence, this thesis focuses on exploring the viability of a photocatalytic assembly including perovskite and metal NCs, starting from enhancing the environmental stability of perovskite NC films, then moving towards investigating the optical and structural changes resulting from their interfacing with plasmonic metal NCs, and finally demonstrating an exemplary assembly platform to study energy transfer between perovskite and metal NCs that ultimately reveals improved photocatalytic efficiencies compared to the single components. Firstly, the fabrication of CsPbX3 NC aluminium oxide (AlOx) nanocomposites by a low temperature atomic layer deposition (ALD) process is proposed as a novel protection scheme. The nucleation and growth of AlOx on the NC surface was investigated by a miscellanea of techniques, highlighting the importance of the interaction between the ALD precursors and the NC surface to uniformly coat the film. These nanocomposites show enhanced stability under exposure in air, irradiation, heat, and upon immersion in water for 1 hour. A deeper understanding of the perovskiteâmetal chemistry is crucial to elucidate the instability problems at the assembly and device level. In the second part, we study the reactions occurring between CsPbX3 (X = Br, BrI, I) perovskite and plasmonic metals (M = Ag, Cu, Au) NCs. We demonstrate a fast optical and structural degradation of perovskites, particularly of iodine containing analogs, driven by the formation of metal halides. While the encapsulation of perovskite NCs in inorganic matrices has been shown to be effective in enhancing their stability, the feasibility of extracting electronic energy from these composite systems still needs to be studied. In this final part, we explore the capacity of CsPbBr3 NC/AlOx nanocomposite films to drive chemical reactions by coupling them to plasmonic Ag NCs. AlOx is used both as a stabilizing layer and as a spacer to study distance-dependent energy transfer, which reveals a migration of energy from the perovskite toward the AgNCs. We then utilize this pooled energy for a plasmon-mediated dye desorption where we demonstrate enhancement effects of spectral and spatial absorption on the reaction outcome due to the coupling to perovskites NCs

Tunable Metal Oxide Shell as a Spacer to Study Energy Transfer in Semiconductor Nanocrystals

Colloidal semiconductor nanocrystals (NCs) are promising components in various optoelectronic and photocatalytic devices; however, the mechanism of energy transport in these materials remains to be further understood. Here, we investigate the distance dependence of the electronic interactions between CsPbBr3 nanocubes and CdSe nanoplateles using an alumina (AlOx) shell as a spacer. CsPbBr3AlOx corepshell NCs are synthesized via colloidal atomic layer deposition (c-ALD), which allows us to fine-tune the oxide thickness and thus the distance d between the two NCs. This versatile material platform shows that the electronic interactions between the CsPbBr3 NCs and the CdSe nanoplatelets can be tuned from electron to energy transfer by increasing the shell thickness, whereas previous studies on the same system had been limited to the former. Considering the applicability of the c-ALD to different NCs, we suggest that metal oxide shell spacers synthesized by this approach can generally be used to study energy-transfer mechanisms at the nanoscale

Exploring Energy Transfer in a Metal/Perovskite Nanocrystal Antenna to Drive Photocatalysis

The use of all-inorganic perovskite nanocrystals (PeNCs) in photocatalytic systems has been limited because of their instability in polar solvents. Encapsulation of PeNCs in inorganic or polymeric matrices has been shown to be effective in overcoming such instability issues, yet studies on charge and energy extraction from these composite systems are still rare. Herein, we explore the capacity of CsPbBr3 PeNC/AlOx composite films to drive chemical reactions by coupling them to plasmonic AgNCs. AlOx is used both as a stabilizing layer and as a spacer to study distance dependent excitation energy transfer, which reveals a migration of energy from the PeNCs toward the AgNCs. We then utilize this pooled energy for a plasmon-mediated methylene blue desorption where we demonstrate enhancement effects of spectral and spatial absorption on the reaction outcome due to the coupling to PeNCs

Universal Oxide Shell Growth Enables in Situ Structural Studies of Perovskite Nanocrystals during the Anion Exchange Reaction

The ability to tune thin oxide coatings by wet-chemistry is desirable for many applications, yet it remains a key synthetic challenge. In this work, we introduce a general colloidal atomic layer deposition (c-ALD) synthesis to grow an alumina shell with tunable thickness around nanocrystalline cores of various compositions spanning from ionic semiconductors (i.e., CsPbX3, with X = Br, I, Cl) to metal oxides and metals (i.e., CeO2 and Ag). The distinctive characteristics of each core (i.e., emission, facile surface functionalization, stability) allowed us to optimize and to elucidate the chemistry of the c-ALD process. Compared to gas-phase ALD, this newly developed synthesis has the advantage of preserving the colloidal stability of the nanocrystalline core while controlling the shell thickness from 1 to 6 nm. As one example of the opportunities offered by the growth of a thin oxide shell, we study the anion exchange reaction in the CsPbX3 perovskites nanocrystals by in situ X-ray diffraction, which had been impeded so far by the instability of this class of materials and by the fast exchange kinetics

Optimizing the Atomic Layer Deposition of Alumina on Perovskite Nanocrystal Films by Using O2 as a Molecular Probe

Encapsulation methods have been shown to be effective in imparting improved stability to metal-halide perovskite nanocrystals. Here, we optimize the atomic layer deposition of amorphous alumina as a protective layer for CsPbBr3 quantum dot thin films. We use oxygen as a molecular diffusion probe to assess the uniformity of the deposited alumina layer

Synthesis and Size-Dependent Optical Properties of Intermediate Band Gap Cu3VS4 Nanocrystals

Intermediate band gap semiconductors are an underex-plored class of materials with unique optical properties of interest for photovoltaic and optoelectronic applications. Herein, we synthesize highly crystalline cubic Cu3VS4 nanocrystals with tunable edge length of 9, 12, and 18 nm. Because size control is achieved for the first time for this semiconductor, particular emphasis is laid on the structural/compositional analysis, the formation mechanism, and the size dependent optical properties. The corresponding UV-vis spectra reveal three absorption peaks in the visible range, resulting from the intermediate band gap electronic structure of Cu3VS4, which blue shift with decreasing size. Density functional theory reveals these size dependent optoelectronic properties to result mostly from weak quantum confinement. The reported results pave the way toward further fundamental investigations of the physicochemical properties of intermediate band gap semiconductors in the nanoscale regime for solar energy harvesting

Persistent Enhancement of Exciton Diffusivity in CsPbBr3 Nanocrystal Solids

In semiconductors, exciton or charge carrier diffusivity is typically described as an inherent material property. Here, we show that the transport of excitons (i.e., bound electron-hole pairs) in CsPbBr3 perovskite nanocrystals (NCs) depends markedly on how recently those NCs were occupied by a previous exciton. Using fluence- and repetition-rate-dependent transient photoluminescence microscopy, we visualize the effect of excitation frequency on exciton transport in CsPbBr3 NC solids. Surprisingly, we observe a striking dependence of the apparent exciton diffusivity on excitation laser power that does not arise from nonlinear exciton-exciton interactions nor from thermal heating of the sample. We interpret our observations with a model in which excitons cause NCs to undergo a transition to a metastable configuration that admits faster exciton transport by roughly an order of magnitude. This metastable configuration persists for ~microseconds at room temperature, and does not depend on the identity of surface ligands or presence of an oxide shell, suggesting that it is an intrinsic response of the perovskite lattice to electronic excitation. The exciton diffusivity observed here (>0.15 cm2/s) is considerably higher than that observed in other NC systems on similar timescales, revealing unusually strong excitonic coupling in a NC material. The finding of a persistent enhancement in excitonic coupling between NCs may help explain other extraordinary photophysical behaviors observed in CsPbBr3 NC arrays, such as superfluorescence. Additionally, faster exciton diffusivity under higher photoexcitation intensity is likely to provide practical insights for optoelectronic device engineering

Color-tunable and high quantum-yield luminescence from a biomolecule-inspired single species emitter of white light

Single-species light emitters with high photoluminescence quantum yields (PLQYs) and broad- spectrum color tunability are sought-after for applications ranging from bio-imaging to artificial lighting. We explore a new strategy to design such emitters, inspired by bioluminescent fireflies and click-beetles. These organisms use a single molecular substrate, D-Luciferin (LH2), to emit light ranging in color from green to red. By combining LH2 with metals, we synthesize new bio-analogous, color-tunable, luminescent metal complexes. The copper complex forms an organic molecule of intrinsic microporosity (OMIM), which crystallizes into a stable structure with intermolecular voids. By changing the composition of guest molecules in the voids, we can tune the emitted color. The optimum composition gives nearly perfect white light, with the highest PLQY reported for a single-species white- light emitter. Similarities between our OMIM and the luciferase active site provide a new approach to investigating the heavily-debated mechanisms underlying in-vivo bioluminescence color variations. Moreover, as a proof of principle, we show that these materials can be used in a new type of light- emitting device (LED). The current generation of LEDs requires at least two active layers to achieve color tunability. The tunability is intrinsic in our materials, and therefore may lead to simpler device fabrication