Cathodoluminescence Spectroscopy of Complex Dendritic Au Architectures for Application in Plasmon‐Mediated Photocatalysis and as SERS Substrates

Complex 3D metallic nanostructures with large surface areas and broadband absorption are attractive candidates for efficient photocatalysis and spectroscopy. Here, hierarchical Au dendrites are self-assembled over centimeter-scales by electrodeposition and the plasmon modes are locally mapped using cathodoluminescence spectroscopy. A correlation between the spatial and spectral distribution of the plasmonic “hot-spots” and the morphology of these structures are demonstrated. Electrodynamic simulations show that the spectra of the plasmon modes are determined by the local geometry of sharp features. Their performance as both surface-enhanced Raman scattering (SERS) substrates and as photocatalysts for the N-demethylation reaction of methylene blue is investigated. High hot-spot densities result in larger SERS enhancement, while the sample with the lowest hot-spot density has a reaction yield 136% larger than the sample with the highest density. These findings indicate that maximizing the hot-spot density is not sufficient to optimize plasmonic substrates for all applications. The spectral and spatial distribution of the plasmon resonances will modify the hot electron generation efficiency and need to be considered for plasmon-enhanced photocatalysis. This work extends the understanding of light–matter interactions in complex 3D structures and provides direction for the rational design of plasmonic architectures for different applications.The authors acknowledge the financial support by the Australian Reasearch Council (ARC). F.J.B. gratefully acknowledges the support of the ARC DECRA Fellowship (DE180100383) and META-ACTIVE—International Research Training Group (IRTG 2675)

Multiscale Disordered Photonics Metamaterials: Novel Complex Media for Sensing Applications

The twentieth century was a golden period for the global scientific advancement: revolutionary ideas and progressive thought were setting the fundamental for our modern world. The drastic advance of mathematics has led physics and chemistry to develop relativistic and quantum theories to understand the unexplored macro- and nano- world, respectively. Biology was doing giant leaps in the comprehension of life with the deduction of the double helical structure of DNA and the discovery of the basic constituent of proteins. Meanwhile, with the help of the extraordinary engineering and technological advancement, medicine was developing an increasing understanding of a wide spectrum of diseases. Those fundamental steps were the outcome of an evolution of the concept of scientific method: the human intellect moved from the standard iterative observation & testing process to a deeper phenomenological understanding of the complicatedness of Nature by falsifiable theories and paradigm shifts. This consequence was dictated by an increased variability of the considered problems which led to the advent of the probabilistic and statistic mathematics theories to deal, predict and understand organized complexity. Indeed, the Natural word is plenty of -at first sight- irregular and disordered structures which follow self-similar patterns, for instance, snowflakes, lightning bolts, heartbeat, and pulmonary vessels. These systems all fall in the category of random fractals: complex objects that show a statistical recursive pattern at different magnifications. Enabled by nanotechnology, the human being is trying to harness the richness of this inherently disordered word by developing new strategies for the understanding, replication and taming of complexity for different applications. Efficient proof of concepts for the use of these disordered structures are always more popular and can be found in different research fields, ranging from energy harvesting, bio-imaging, and advanced opto-electronic materials. On the other side, in this frenetic modern age, the human being is demanding always more measurement to track, monitor and communicating information essential to our daily life. In this big scenario, technological advances demand more and more precise measurement of existence or absence of harmful substances, with low concentrations, in various environments. Sensing technology has already impacted many aspect of our daily lives, through many application, including but not limited to gas alarms, medical diagnostics, healthcare, safety, defense and security, automotive and environmental monitoring. Particularly, gas and biomolecule sensing is becoming considered as the key technology to avoid or reduce growing global challenges including global warming, pollution, safety, non-invasive diagnostic and security. High sensitivity, fast response, and good selectivity are the target requirements for a high throughput sensor in today's technology. Among all the nanotechnology-enabled gas sensors, the optical ones are most promising and attracting options thanks to their high sensitivity, long lifetime, high resistivity to electromagnetic noise and the possibility to work at room temperature. This thesis work aims to study, understand and exploit the properties and advantages of the light-matter interaction in complex systems for sensing applications, with particular focus on the interaction between metallic and dielectric materials. The candidate has hence focused on the optical properties of disordered metallo-dielectric networks, exploiting the localized surface plasmons polaritons and their intrinsic sensitivity to the local environment to develop a scalable and efficient platform for sensing applications

Investigation of the mechanisms of plasmon-mediated photocatalysis: synergistic contribution of near-field and charge transfer effects

Plasmonic photocatalysis is an attractive way to drive and enhance chemical reactions. The relative importance of thermal and non-thermal effects in driving the reaction is still under debate in the literature, and the lack of a complete theoretical framework, discrepancies in nomenclature and contrasting experimental results continue to hinder the understanding of the underlying reaction mechanism. Particularly for small (<50 nm) nanoparticles (NPs), this has been exacerbated by limited in situ investigation of the relationships between morphology and performance. Here, we study the N-demethylation reaction of methylene blue (MB) adsorbed on disordered Au nanoparticle arrays with average size <50 nm as a model plasmonic photocatalytic system, by means of surface-enhanced Raman spectroscopy (SERS). The highest reaction yield is found under laser excitation at 633 nm, which overlaps both the plasmon and molecular resonance, however traces of products are found also off-resonance under illumination at 785 nm. Critically, we find that the reaction rate decreases as particle radius increases, showing a reduction of ca. 80% when the NP radius is increased from 5 nm to 32 nm. By employing multiscale modelling, we systematically report a mechanistic analysis of photothermal effects, near-field enhancements and hot-electron transfer in this system and address their relationship with NP size and reaction yield. This work demonstrates that both near-fields and hot-electrons synergistically cooperate in enhancing the N-demethylation reaction of MB and indicates that photothermal effects do not play a dominant role in this reaction. The results of this investigation contribute to the mechanistic understanding of plasmon-mediated reactions and provide insights for improved design of metal–molecule interfaces for efficient and selective photocatalysis

Self-assembly of Au Nano-islands with Tuneable Organized Disorder for Highly Sensitive SERS

Aggregates of disordered metallic nano-clusters exhibiting long-range organized fractal properties are amongst the most efficient scattering enhancers, and they are promising as high performance surface-enhanced Raman scattering (SERS) substrates. However, the low reproducibility of the disordered structures hinders the engineering and optimization of well-defined scalable architectures for SERS. Here, a thermophoretically driven Au aerosol deposition process is used for the self-assembly of thin films consisting of plasmonic nano-islands (NIs) with a controllable and highly reproducible degree of disorder. The intrinsic Brownian motion of the aerosol deposition process results in long-range periodicity with self-similar properties and stochastically distributed hot-spots, providing a facile means for the reliable fabrication of crystalline Au substrates with uniform disorder over large-surfaces. These morphological features result in the generation of a high density of hot-spots, benefitting their application as SERS substrates. NI substrates with an optimal uniform disorder demonstrate a SERS enhancement factor (EF) of 107–108 with nanomolar concentrations of Rhodamin-6G. These findings provide new insights into the investigation of light scattering with disordered structures, paving the way toward low-cost scalable self-assembly optoelectronic materials with applications ranging from ultrasensitive spectroscopy to random lasing and photonic devices

Hybrid plasmonic-semiconducting fractal metamaterials for superior sensing of volatile compounds

Localized surface plasmon resonance (LSPR) is a subwavelength optical phenomenon that has found widespread use in bio-and chemical-sensing applications thanks to the possibility to efficiently transduce refractive index changes into wavelength shifts. However, is it very hard to transpose the successes demonstrated in liquid and physiological environment toward the detection of gasous molecules. In fact, the latter typically adsorb in an unspecific manner and induce very minute refractive index changes tipicaly below the sensor sensitivity. Here, we show first insights on the aerosol large-scale self-Assembly of metasurfaces made of monocrystalline Au nanoislands with uniform disorder over large scale. Notably, these architectures show tuneable disorder levels and demonstrate high-quality LSPR, enabling the fabrication of highly performing optical gas sensors detecting down to 10-5 variations in refractive index. Next, we use our aerosol synthesis method to integrate tailored fractals of dielectric TiO2 nanoparticles onto resonant plasmonic metasurfaces. We show how this integration strongly enhances the interaction between the plasmonic field and volatile organic molecules and provides a means for their selective detection. Interesting, the improved performance is the result of a synergetic behavior between the dielectric fractals and the plasmonic metasurface: in fact, upon this integration, the enhancement of plasmonic field is drastically extended, all the way up to a maximum thickness of 1.8 μm. Optimal dielectric-plasmonic structures allow measurements of changes in the refractive index of the gas mixture down to &lt;8x10-6at room temperature and selective identification of three exemplary volatile organic compounds (VOCs). These findings provide a basis for the development of a novel family of hybrid dielectric-plasmonic materials with application extending from light harvesting and photo-catalysts to contactless sensors for non-invasive medical diagnostics

High‐Temperature One‐Step Synthesis of Efficient Nanostructured Bismuth Vanadate Photoanodes for Water Oxidation

Authors acknowledge the financial supports of the Australian Research Council (ARC) DP150101939, ARC DE160100569, Westpac 2016 Research Fellowship, the ActewAGL Endowment Fund, and the Research School of Engineering of the ANU. Authors also acknowledge the Centre for Advanced Microscopy (CAM) with funding through the Australian Microscopy and Microanalysis Research Facility (AMMRF), and the Australian National Fabrication Facility (ANFF), ACT Node. A.N.S. acknowledges funding by the ARC through the Centre of Excellence for Electromaterials Science (CE140100012)

Self-assembly of noble metal-free graphene-copper plasmonic metasurfaces

The strong light confinement and near field enhancement by metallic scatters enabled the development of a large family of plasmonic-based technologies, including broadly used gold metasurfaces. Despite progress, the engineering of non-precious metal plasmonic devices remains challenging, due to the limited chemical stability of most nanostructured metals. Here, we report the preparation of earth-abundant plasmonic metasurfaces by the engineering of copper-graphene nano-resonators, and their use as localized surface plasmon resonance (LSPR) sensors. We achieve the large-scale self-assembly of Cu nanocrystals, featuring a protective graphene film, by one-step reduction of CuO nanoparticle networks in a hydrocarbon-containing atmosphere. Microscopic and spectroscopic investigations reveal that coalescence and reduction of the CuO nanoparticles during graphene growth result in the formation of graphene-encapsulated metallic Cu nano-islands (NIs). These Cu-graphene metasurfaces can detect down to 1% concentrations of toluene gas at room temperature, displaying a reproducible and rapid LSPR shift of 0.2 nm. Finite-difference time-domain (FDTD) simulation and structural characterization reveal that the graphene layer significantly improves the Cu crystals’ long-term stability, leading to a prolonged LSPR performance over periods of three months. These insights provide promising directions for the development of earth-abundant plasmonic materials with applications ranging from biosensing to photo-catalysis and other optoelectronic devices.</p

The effect of β-sheet breaker peptides on metal associated Amyloid-β peptide aggregation process

Far-UV Circular Dichroism experiments and Atomic Force Microscopy tomography are employed to assess the impact of β-sheet breaker on the Aβ(1−40) peptide aggregation process in the presence of Cu(II) or Zn(II) transition metals. In this work we focus on two specific 5-amino acids long β-sheet breakers, namely the LPFFD Soto peptide, already known in the literature, and the LPFFN peptide recently designed and studied by our team. We provide evidence that both β-sheet breakers are effective in reducing the Aβ(1−40) aggregation propensity, even in the presence of metal ions

The effect of β-sheet breaker peptides on metal associated Amyloid-β peptide aggregation process

Far-UV Circular Dichroism experiments and Atomic Force Microscopy tomography are employed to assess the impact of beta-sheet breakers on the A beta(1-40) peptide aggregation process in the presence of Cu2+ or Zn2+ transition metals. In this work we focus on two specific 5-amino acids long beta-sheet breakers, namely the LPFFD Soto peptide, already known in the literature, and the LPFFN peptide recently designed and studied by our team. We provide evidence that both 13 -sheet breakers are effective in reducing the A beta(1-40) aggregation propensity, even in the presence of metal ions

Multifunctional nanostructures of Au-Bi2O3 fractals for CO2 reduction and optical sensing

The development of nanomaterials with multifunctional properties presents a viable business case for potential scale-up of nanomaterial fabrication. Hence, the design and engineering of structures as well as tuning of active sites are crucial in generating multifunctional properties in nanomaterials. In this regard, we demonstrate a three-dimensional (3D) fractal structure of Au–Bi2O3 with a fractal dimension (Df) of z 1.80, which is obtained from the small-angle X-ray scattering (SAXS) measurement and through the box counting algorithm. The fractal structures, fabricated via a one-step direct synthesis, gives a homogeneous distribution of catalytically active nanocrystals Au and Bi2O3 on a 3D platform with a large active surface area, resulting in a strong enhancement of its localized electric field. Therefore, when applied as a catalyst for electrochemical CO2 reduction reactions (CO2RR) and optical gas sensing, the material displays an excellent performance. Specifically, the fractal structure exhibits a high selectivity towards the formation of formate, achieving a very high faradaic efficiency of 97% and high massspecific formate current density of 54 mA mg 1 at 1.1 V vs. a reversible hydrogen electrode (RHE). Similarly, this structure displayed a plasmonic shift as high as 5 nm for 4 vol% acetone sensing with a detection limit of 100 ppm towards different volatile organic compounds (VOCs).Authors acknowledge the nancial supports of the Australian Research Council (ARC) DP150101939, ARC DE160100569, ARC Research Hub on Integrated Energy Storage Solutions IH180100020, Westpac 2016 Research Fellowship, and the Research School of Engineering of the ANU. Authors also acknowledge the Centre for Advanced Microscopy (CAM) with funding through the Australian Microscopy and Microanalysis Research Facility (AMMRF). R. D. & R. A. would like to acknowledge funding from the UNSW Digital Grid Futures Institute. T. T.-P. also thanks Dr Felipe Kremer for the assistance with the TEM measurement at CAM and Mr Mahdiar Taheri with the support on the TGA measurement. Part of the research was undertaken at the SAXS/WAXS beamline at the Australian Synchrotron, part of ANSTO, and we thank the beamline scientists for their technical assistance