Imperial College London

ProfessorStefanMaier

Faculty of Natural SciencesDepartment of Physics

Lee-Lucas Chair in Experimental Physics
 
 
 
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Contact

 

+44 (0)20 7594 6063s.maier Website CV

 
 
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Location

 

Huxley 903Huxley BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

669 results found

Zhang W, Dagar R, Rosenberger P, Sousa-Castillo A, Neuhaus M, Li W, Khan SA, Alnaser AS, Cortes E, Maier SA, Costa-Vera C, Kling MF, Bergues Bet al., 2022, All-optical nanoscopic spatial control of molecular reaction yields on nanoparticles, Optica, Vol: 9, Pages: 551-551

Journal article

Stefancu A, Nan L, Zhu L, Chis V, Bald I, Liu M, Leopold N, Maier SA, Cortes Eet al., 2022, Controlling Plasmonic Chemistry Pathways through Specific Ion Effects, ADVANCED OPTICAL MATERIALS, ISSN: 2195-1071

Journal article

Tilmann B, Pandeya AK, Grinblat G, Menezes LDS, Li Y, Shekhar C, Felser C, Parkin SSP, BedoyaPinto A, Maier SAet al., 2022, Ultrafast sub‐100 fs all‐optical modulation and efficient third‐harmonic generation in Weyl semimetal niobium phosphide thin films, Advanced Materials, Vol: 34, Pages: 1-7, ISSN: 0935-9648

Since their experimental discovery in 2015, Weyl semimetals generated a large amount of attention due their intriguing physical properties that arise from their linear electron dispersion relation and topological surface states. In particular in the field of nonlinear (NL) optics and light harvesting, Weyl semimetals have shown outstanding performances and achieved record NL conversion coefficients. In this context, we perform first steps towards Weyl semimetal nanophotonics by thoroughly characterizing the linear and NL optical behavior of epitaxially grown niobium phosphide (NbP) thin films, covering the visible to near-infrared regime of the electromagnetic spectrum. Despite the measured high linear absorption, third-harmonic generation studies demonstrate high conversion efficiencies up to 10-4%, that can be attributed to the topological electron states at the surface of the material. Furthermore, nondegenerate pump-probe measurements with sub-10 fs pulses reveal a maximum modulation depth of about 1%, completely decaying within 100 fs and therefore suggesting the possibility of developing devices based on NbP with all-optical switching bandwidths of up to 10 THz. Altogether, our work reveals promising NL optical properties of Weyl semimetal thin films that are outperforming bulk crystals of the same material, laying the grounds for nanoscale applications, enabled by top-down nanostructuring, such as light-harvesting, on-chip frequency conversion and all-optical processing.

Journal article

Luo S, Mancini A, Wang F, Liu J, Maier SA, de Mello JCet al., 2022, High-Throughput Fabrication of Triangular Nanogap Arrays for Surface-Enhanced Raman Spectroscopy., ACS Nano

Squeezing light into nanometer-sized metallic nanogaps can generate extremely high near-field intensities, resulting in dramatically enhanced absorption, emission, and Raman scattering of target molecules embedded within the gaps. However, the scarcity of low-cost, high-throughput, and reproducible nanogap fabrication methods offering precise control over the gap size is a continuing obstacle to practical applications. Using a combination of molecular self-assembly, colloidal nanosphere lithography, and physical peeling, we report here a high-throughput method for fabricating large-area arrays of triangular nanogaps that allow the gap width to be tuned from ∼10 to ∼3 nm. The nanogap arrays function as high-performance substrates for surface-enhanced Raman spectroscopy (SERS), with measured enhancement factors as high as 108 relative to a thin gold film. Using the nanogap arrays, methylene blue dye molecules can be detected at concentrations as low as 1 pM, while adenine biomolecules can be detected down to 100 pM. We further show that it is possible to achieve sensitive SERS detection on binary-metal nanogap arrays containing gold and platinum, potentially extending SERS detection to the investigation of reactive species at platinum-based catalytic and electrochemical surfaces.

Journal article

Possmayer T, Tilmann B, Maia LJQ, Maier SA, Menezes LDSet al., 2022, Second to fifth harmonic generation in individual beta-barium borate, OPTICS LETTERS, Vol: 47, Pages: 1826-1829, ISSN: 0146-9592

Journal article

Cortes E, Grzeschik R, Maier SA, Schluecker Set al., 2022, Experimental characterization techniques for plasmon-assisted chemistry, NATURE REVIEWS CHEMISTRY, Vol: 6, Pages: 259-274

Journal article

Zhang C, Luo Y, Maier SA, Li Xet al., 2022, Recent Progress and Future Opportunities for Hot Carrier Photodetectors: From Ultraviolet to Infrared Bands, LASER & PHOTONICS REVIEWS, ISSN: 1863-8880

Journal article

Yang B, Liu K, Li H, Liu C, Fu J, Li H, Huang JE, Ou P, Alkayyali T, Cai C, Duan Y, Liu H, An P, Zhang N, Li W, Qiu X, Jia C, Hu J, Chai L, Lin Z, Gao Y, Miyauchi M, Cortés E, Maier SA, Liu Met al., 2022, Accelerating CO2 electroreduction to multicarbon products via synergistic electric-thermal field on copper nanoneedles., Journal of the American Chemical Society, Vol: 144, Pages: 3039-3049, ISSN: 0002-7863

Electrochemical CO2 reduction is a promising way to mitigate CO2 emissions and close the anthropogenic carbon cycle. Among products from CO2RR, multicarbon chemicals, such as ethylene and ethanol with high energy density, are more valuable. However, the selectivity and reaction rate of C2 production are unsatisfactory due to the sluggish thermodynamics and kinetics of C-C coupling. The electric field and thermal field have been studied and utilized to promote catalytic reactions, as they can regulate the thermodynamic and kinetic barriers of reactions. Either raising the potential or heating the electrolyte can enhance C-C coupling, but these come at the cost of increasing side reactions, such as the hydrogen evolution reaction. Here, we present a generic strategy to enhance the local electric field and temperature simultaneously and dramatically improve the electric-thermal synergy desired in electrocatalysis. A conformal coating of ∼5 nm of polytetrafluoroethylene significantly improves the catalytic ability of copper nanoneedles (∼7-fold electric field and ∼40 K temperature enhancement at the tips compared with bare copper nanoneedles experimentally), resulting in an improved C2 Faradaic efficiency of over 86% at a partial current density of more than 250 mA cm-2 and a record-high C2 turnover frequency of 11.5 ± 0.3 s-1 Cu site-1. Combined with its low cost and scalability, the electric-thermal strategy for a state-of-the-art catalyst not only offers new insight into improving activity and selectivity of value-added C2 products as we demonstrated but also inspires advances in efficiency and/or selectivity of other valuable electro-/photocatalysis such as hydrogen evolution, nitrogen reduction, and hydrogen peroxide electrosynthesis.

Journal article

Wang J, Maier SA, Tittl A, 2022, Trends in Nanophotonics-Enabled Optofluidic Biosensors, ADVANCED OPTICAL MATERIALS, Vol: 10, ISSN: 2195-1071

Journal article

Ren H, Maier SA, 2022, Nanophotonic Materials for Twisted-Light Manipulation, ADVANCED MATERIALS, ISSN: 0935-9648

Journal article

Aigner A, Dawes JM, Maier SA, Ren Het al., 2022, Nanophotonics shines light on hyperbolic metamaterials (vol 11, 9, 2022), LIGHT-SCIENCE & APPLICATIONS, Vol: 11, ISSN: 2047-7538

Journal article

Liu C, Maier SA, Liu C, Maier SAet al., 2022, High-Quality Optical Hotspots with Topology-Protected Robustness, ACS PHOTONICS, Vol: 9, Pages: 241-248, ISSN: 2330-4022

Journal article

Aigner A, Dawes JM, Maier SA, Ren Het al., 2022, Nanophotonics shines light on hyperbolic metamaterials, LIGHT-SCIENCE & APPLICATIONS, Vol: 11, ISSN: 2047-7538

Journal article

Altug H, Oh S-H, Maier SA, Homola Jet al., 2022, Advances and applications of nanophotonic biosensors, NATURE NANOTECHNOLOGY, Vol: 17, Pages: 5-16, ISSN: 1748-3387

Journal article

Büchner R, Weber T, Kühner L, Maier SA, Tittl Aet al., 2021, Tip coupling and array effects of gold nanoatennas in near-field microscopy, ACS Photonics, Vol: 8, Pages: 3486-3494, ISSN: 2330-4022

Scattering-type scanning near-field optical microscopy (s-SNOM) is one of the predominant techniques for the nanoscale characterization of optical properties. The optical response of nanoantennas in s-SNOM is highly sensitive to their environment, including influences of the probing tip or neighboring resonators. Dielectric tips are commonly employed to minimize tip-related perturbations, although they provide a comparatively weak scattering signal. Here we show that when using metallic tips, it is possible to select between distinct weak and strong tip–antenna coupling regimes by careful tailoring of the illumination conditions and resonator orientation. This enables the use of highly scattering metallic instead of dielectric tips for mapping plasmonic modes with comparatively higher signal strengths. This is a particular advantage for the retrieval of near-field spectra, which simultaneously require high near-field signals and unperturbed field patterns. We leverage our approach to analyze the collective effects of nanoantenna arrays, phenomena that are well understood in the optical far-field but have not been extensively studied in the near-field. Probing the dependence of the optical response on the array field size, we identify three regimes: the single rod regime, the intermediate regime, and the array-like regime. We show that these array effects give rise to characteristic spectral features originating from a complex interplay of radiative coupling and plasmon hybridization. These results provide evidence that long-range interactions of antennas also influence the local optical response that is probed in s-SNOM and demonstrate how collective resonances emerge from single building blocks, providing guidelines for optimized array designs for near- and far-field applications.

Journal article

Moretti GQ, Cortes E, Maier SA, Bragas A, Grinblat Get al., 2021, Engineering gallium phosphide nanostructures for efficient nonlinear photonics and enhanced spectroscopies, Nanophotonics, Vol: 10, Pages: 4261-4271, ISSN: 2192-8606

Optical resonances arising from quasi-bound states in the continuum (QBICs) have been recently identified in nanostructured dielectrics, showing ultrahigh quality factors accompanied by very large electromagnetic field enhancements. In this work, we design a periodic array of gallium phosphide (GaP) elliptical cylinders supporting, concurrently, three spectrally separated QBIC resonances with in-plane magnetic dipole, out-of-plane magnetic dipole, and electric quadrupole characters. We numerically explore this system for second-harmonic generation and degenerate four-wave mixing, demonstrating giant per unit cell conversion efficiencies of up to ∼ 2 W−1 and ∼ 60 W−2, respectively, when considering realistic introduced asymmetries in the metasurface, compatible with current fabrication limitations. We find that this configuration outperforms by up to more than four orders of magnitude the response of low-Q Mie or anapole resonances in individual GaP nanoantennas with engineered nonlinear mode-matching conditions. Benefiting from the straight-oriented electric field of one of the examined high-Q resonances, we further propose a novel nanocavity design for enhanced spectroscopies by slotting the meta-atoms of the periodic array. We discover that the optical cavity sustains high-intensity fields homogeneously distributed inside the slot, delivering its best performance when the elliptical cylinders are cut from end to end forming a gap, which represents a convenient model for experimental investigations. When placing an electric point dipole inside the added aperture, we find that the metasurface offers ultrahigh radiative enhancements, exceeding the previously reported slotted dielectric nanodisk at the anapole excitation by more than two orders of magnitude.

Journal article

Kuehne J, Wang J, Weber T, Kuehner L, Maier SA, Tittl Aet al., 2021, Fabrication robustness in BIC metasurfaces, Nanophotonics, Vol: 10, Pages: 4305-4312, ISSN: 2192-8606

All-dielectric metasurfaces supporting photonic bound states in the continuum (BICs) are an exciting toolkit for achieving resonances with ultranarrow linewidths. However, the transition from theory to experimental realization can significantly reduce the optical performance of BIC-based nanophotonic systems, severely limiting their application potential. Here, we introduce a combined numerical/experimental methodology for predicting how unavoidable tolerances in nanofabrication such as random geometrical variations affect the performance of different BIC metasurface designs. We compare several established all-dielectric BIC unit cell geometries with broken in-plane inversion symmetry including tilted ellipses, asymmetric double rods, and split rings. Significantly, even for low fabrication-induced geometrical changes, both the BIC resonance amplitude and its quality factor (Q-factor) are significantly reduced. We find that the all-dielectric ellipses maintain the highest Q-factors throughout the geometrical variation range, whereas the rod and split ring geometries fall off more quickly. The same behavior is confirmed experimentally, where geometrical variation values are derived from automated processing of sets of scanning electron microscopy (SEM) images. Our methodology provides crucial insights into the performance degradation of BIC metasurfaces when moving from simulations to fabricated samples and will enable the development of robust, high-Q, and easy to manufacture nanophotonic platforms for applications ranging from biomolecular sensing to higher harmonic generation.

Journal article

Glass D, Quesada-Cabrera R, Bardey S, Promdet P, Sapienza R, Keller V, Maier SA, Caps V, Parkin IP, Cortes Eet al., 2021, Probing the role of atomic defects in photocatalytic systems through photoinduced enhanced raman scattering, ACS Energy Letters, Vol: 6, Pages: 4273-4281, ISSN: 2380-8195

Even in ultralow quantities, oxygen vacancies (VO) drastically impact keyproperties of metal oxide semiconductors, such as charge transport, surface adsorption,and reactivity, playing central roles in functional materials performance. Currentmethods used to investigate VO often rely on specialized instrumentation under far fromideal reaction conditions. Hence, the influence of VO generated in situ during catalyticprocesses has yet to be probed. In this work, we assess in situ extrinsic surface VOformation and lifetime under photocatalytic conditions which we compare tophotocatalytic performance. We show for the first time that lifetimes of in situ generatedatomic VO play more significant roles in catalysis than their concentration, with strongcorrelations between longer-lived VO and higher photocatalytic activity. Our resultsindicate that enhanced photocatalytic efficiency correlates with goldilocks VOconcentrations, where VO densities must be just right to encourage carrier transportwhile avoiding charge carrier trapping.

Journal article

Luo S, Hoff BH, Maier SA, de Mello JCet al., 2021, Scalable Fabrication of Metallic Nanogaps at the Sub-10 nm Level, ADVANCED SCIENCE, Vol: 8

Journal article

Huettenhofer L, Golibrzuch M, Bienek O, Wendisch FJ, Lin R, Becherer M, Sharp ID, Maier SA, Cortes Eet al., 2021, Metasurface photoelectrodes for enhanced solar fuel generation, Advanced Energy Materials, Vol: 11, ISSN: 1614-6832

Tailoring optical properties in photocatalysts by nanostructuring them can help increase solar light harvesting efficiencies in a wide range of materials. Whereas plasmon resonances are widely employed in metallic catalysts for this purpose, latest advances of nonradiative, dielectric nanophotonics also enable light confinement and enhanced visible light absorption in semiconductors. Here, a design procedure for large-scale nanofabrication of semiconductor photoelectrodes using imprint lithography is developed. Anapole excitations and metasurface lattice resonances are combined to enhance the absorption of the model material, amorphous gallium phosphide (a-GaP), over the visible spectrum. It is shown that cost-effective, high sample throughput is achieved while retaining the precise signature of the engineered photonic states. Photoelectrochemical measurements under hydrogen evolution reaction conditions and sunlight illumination reveal the contributions of the respective resonances and demonstrate an overall photocurrent enhancement of 5.7, compared to a planar film. These results are supported by optical and numerical analysis of single nanodisks and of the upscaled metasurface.https://onlinelibrary.wiley.com/doi/10.1002/aenm.202102877

Journal article

Poblet M, Berte R, Boggiano HD, Li Y, Cortes E, Grinblat G, Maier SA, Bragas Aet al., 2021, Acoustic coupling between plasmonic nanoantennas: detection and directionality of surface acoustic waves, ACS Photonics, Vol: 8, Pages: 2846-2852, ISSN: 2330-4022

Hypersound waves can be efficient mediators between optical signals at the nanoscale. Having phase velocities several orders of magnitude lower than the speed of light, they propagate with much shorter wavelengths and can be controlled, directed, and even focused in a very small region of space. This work shows how two optical nanoantennas can be coupled through an acoustic wave that propagates with a certain directionality. An “emitter” antenna is first optically excited to generate acoustic coherent phonons that launch surface acoustic waves through the underlying substrate. These waves travel until they are mechanically detected by a “receiver” nanoantenna whose oscillation produces a detectable optical signal. Generation and detection are studied in detail, and new designs are proposed to improve the directionality of the hypersonic surface acoustic wave.

Journal article

Sortino L, Zotev PG, Phillips CL, Brash AJ, Cambiasso J, Marensi E, Fox AM, Maier SA, Sapienza R, Tartakovskii AIet al., 2021, Bright single photon emitters with enhanced quantum efficiency in a two-dimensional semiconductor coupled with dielectric nano-antennas, Nature Communications, Vol: 12, ISSN: 2041-1723

Single photon emitters in atomically-thin semiconductors can be deterministically positioned using strain induced by underlying nano-structures. Here, we couple monolayer WSe2 to high-refractive-index gallium phosphide dielectric nano-antennas providing both optical enhancement and monolayer deformation. For single photon emitters formed on such nano-antennas, we find very low (femto-Joule) saturation pulse energies and up to 104 times brighter photoluminescence than in WSe2 placed on low-refractive-index SiO2 pillars. We show that the key to these observations is the increase on average by a factor of 5 of the quantum efficiency of the emitters coupled to the nano-antennas. This further allows us to gain new insights into their photoluminescence dynamics, revealing the roles of the dark exciton reservoir and Auger processes. We also find that the coherence time of such emitters is limited by intrinsic dephasing processes. Our work establishes dielectric nano-antennas as a platform for high-efficiency quantum light generation in monolayer semiconductors.

Journal article

Dagdeviren OE, Glass D, Sapienza R, Cortes E, Maier SA, Parkin IP, Grutter P, Quesada-Cabrera Ret al., 2021, The effect of photoinduced surface oxygen vacancies on the charge carrier dynamics in TiO2 films, Nano Letters: a journal dedicated to nanoscience and nanotechnology, Vol: 21, Pages: 8348-8354, ISSN: 1530-6984

Metal-oxide semiconductors (MOS) are widely utilized for catalytic and photocatalytic applications in which the dynamics of charged carriers (e.g., electrons, holes) play important roles. Under operation conditions, photoinduced surface oxygen vacancies (PI-SOV) can greatly impact the dynamics of charge carriers. However, current knowledge regarding the effect of PI-SOV on the dynamics of hole migration in MOS films, such as titanium dioxide, is solely based upon volume-averaged measurements and/or vacuum conditions. This limits the basic understanding of hole-vacancy interactions, as they are not capable of revealing time-resolved variations during operation. Here, we measured the effect of PI-SOV on the dynamics of hole migration using time-resolved atomic force microscopy. Our findings demonstrate that the time constant associated with hole migration is strongly affected by PI-SOV, in a reversible manner. These results will nucleate an insightful understanding of the physics of hole dynamics and thus enable emerging technologies, facilitated by engineering hole-vacancy interactions.

Journal article

Noor A, Damodaran AR, Lee IH, Maier SA, Oh SH, D'Orazio A, Ciraci Cet al., 2021, Plasmonic Nanopatch Antennas as a Doubly Resonant Platform for Mode-Matched Second-Harmonic Generation, Pages: 305-307

Plasmonic systems are advantageous for nonlinear optics as their ability to strongly confine and enhance the incident light can be exploited to boost the nonlinear interactions. In this contribution, we expand on the possibility of exploiting dielectric-loaded plasmonic film-coupled nanopatch antennas for an optimal second-harmonic generation.

Conference paper

Mao P, Liu C, Li X, Liu M, Chen Q, Han M, Maier SA, Sargent EH, Zhang Set al., 2021, Single-step-fabricated disordered metasurfaces for enhanced light extraction from LEDs., Light: Science and Applications, Vol: 10, ISSN: 2047-7538

While total internal reflection (TIR) lays the foundation for many important applications, foremost fibre optics that revolutionised information technologies, it is undesirable in some other applications such as light-emitting diodes (LEDs), which are a backbone for energy-efficient light sources. In the case of LEDs, TIR prevents photons from escaping the constituent high-index materials. Advances in material science have led to good efficiencies in generating photons from electron-hole pairs, making light extraction the bottleneck of the overall efficiency of LEDs. In recent years, the extraction efficiency has been improved, using nanostructures at the semiconductor/air interface that outcouple trapped photons to the outside continuum. However, the design of geometrical features for light extraction with sizes comparable to or smaller than the optical wavelength always requires sophisticated and time-consuming fabrication, which causes a gap between lab demonstration and industrial-level applications. Inspired by lightning bugs, we propose and realise a disordered metasurface for light extraction throughout the visible spectrum, achieved with single-step fabrication. By applying such a cost-effective light extraction layer, we improve the external quantum efficiency by a factor of 1.65 for commercialised GaN LEDs, demonstrating a substantial potential for global energy-saving and sustainability.

Journal article

Wang J, Kuehne J, Karamanos T, Rockstuhl C, Maier SA, Tittl Aet al., 2021, All-dielectric crescent metasurface sensor driven by bound states in the continuum, Advanced Functional Materials, Vol: 31, ISSN: 1616-301X

Metasurfaces based on quasi-bound states in the continuum (quasi-BICs) constitute an emerging toolkit in nanophotonic sensing as they sustain high quality factor resonances and substantial near-field enhancements. It is demonstrated that silicon metasurfaces composed of crescent shaped meta-atoms provide tailored light-matter interaction controlled by the crescent geometry. Significantly, this metasurface not only exhibits a fundamental quasi-BIC resonance, but also supports a higher-order resonance with tunable electromagnetic field enhancement and advantageous properties for sensing. The higher-order resonance shows twice the sensitivity of the fundamental one for bulk refractive index sensing. It is further demonstrated that both the fundamental and higher-order resonances can be exploited for sensing ultrathin layers of biomolecules in air and buffer solutions. Specifically, when measuring in buffer solution, the figure of merit of the sensor, defined as the change in the spectral position of the resonance normalized to its full width at half maximum, is a factor of 2.5 larger for the higher-order resonance when compared to the fundamental one. Due to its high sensitivity and potential for straightforward microfluidic integration, the silicon crescent metasurface is ideally suited for real-time and in situ biosensing, enabling compact sensing devices for a wide range of diagnostic applications.

Journal article

Ren H, Wang X, Li C, He C, Wang Y, Pan A, Maier SAet al., 2021, Orbital-angular-momentum-controlled hybrid nanowire circuit, Nano Letters: a journal dedicated to nanoscience and nanotechnology, Vol: 21, Pages: 6220-6227, ISSN: 1530-6984

Plasmonic nanostructures can enable compact multiplexing of the orbital angular momentum (OAM) of light; however, strong dissipation of the highly localized OAM-distinct plasmonic fields in the near-field region hinders on-chip OAM transmission and processing. Superior transmission efficiency is offered by semiconductor nanowires sustaining highly confined optical modes, but only the polarization degree of freedom has been utilized for their selective excitation. Here we demonstrate that incident OAM beams can selectively excite single-crystalline cadmium sulfide (CdS) nanowires through coupling OAM-distinct plasmonic fields into nanowire waveguides for long-distance transportation. This allows us to build an OAM-controlled hybrid nanowire circuit for optical logic operations including AND and OR gates. In addition, this circuit enables the on-chip photoluminescence readout of OAM-encrypted information. Our results open exciting new avenues not only for nanowire photonics to develop OAM-controlled optical switches, logic gates, and modulators but also for OAM photonics to build ultracompact photonic circuits for information processing.

Journal article

Li C-H, Maier SA, Ren H-R, 2021, Optical vortices in nanophotonics, CHINESE OPTICS, Vol: 14, Pages: 792-811, ISSN: 2095-1531

Journal article

Davidson-Marquis F, Gargiulo J, Gomez-Lopez E, Jang B, Kroh T, Muller C, Ziegler M, Maier SA, Kubler H, Schmidt MA, Benson Oet al., 2021, Coherent interaction of atoms with a beam of light confined in a light cage, Light: Science and Applications, Vol: 10, ISSN: 2047-7538

Journal article

Wang X, Liu C, Gao C, Yao K, Masouleh SSM, Berté R, Ren H, Menezes LDS, Cortés E, Bicket IC, Wang H, Li N, Zhang Z, Li M, Xie W, Yu Y, Fang Y, Zhang S, Xu H, Vomiero A, Liu Y, Botton GA, Maier SA, Liang Het al., 2021, Self-constructed multiple plasmonic hotspots on an individual fractal to amplify broadband hot electron generation., ACS Nano, Vol: 15, Pages: 10553-10564, ISSN: 1936-0851

Plasmonic nanoparticles are ideal candidates for hot-electron-assisted applications, but their narrow resonance region and limited hotspot number hindered the energy utilization of broadband solar energy. Inspired by tree branches, we designed and chemically synthesized silver fractals, which enable self-constructed hotspots and multiple plasmonic resonances, extending the broadband generation of hot electrons for better matching with the solar radiation spectrum. We directly revealed the plasmonic origin, the spatial distribution, and the decay dynamics of hot electrons on the single-particle level by using ab initio simulation, dark-field spectroscopy, pump-probe measurements, and electron energy loss spectroscopy. Our results show that fractals with acute tips and narrow gaps can support broadband resonances (400-1100 nm) and a large number of randomly distributed hotspots, which can provide unpolarized enhanced near field and promote hot electron generation. As a proof-of-concept, hot-electron-triggered dimerization of p-nitropthiophenol and hydrogen production are investigated under various irradiations, and the promoted hot electron generation on fractals was confirmed with significantly improved efficiency.

Journal article

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