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Tailoring Chiral Gold Nanorods via Single‐Step Seed‐to‐Au(III) Ratio and Unlocking their Potential in Refractive Index Sensing
Full text linkColloidal chiral plasmonic nanoparticles are garnering growing interest due to their interaction with circularly polarized light, offering advanced optical applications. Their circular dichroism (CD) spectra are significantly narrower and more defined than extinction spectra, making them ideal for refractive index-based sensing. Despite progress in colloidal synthesis, this field remains relatively underexplored. In this work, a one-step, seed-mediated route to chiral Au nanorods is introduced in which the molar ratio Au3⁺/Au⁰ enables continuous control of the CD response (intensity, sign, and position), using L- and D-cysteine as chiral inducers. CD-based refractive index sensitivity (RIS) measurements reveal a figure of merit (FoM = RIS/linewidth) exceeding 1000 RIU−1, outperforming the conventional extinction-based approaches. Thin films of C-AuNRs fabricated via a layer-by-layer assembly retain the bisignate CD response and show RIS values comparable to colloidal samples. These films demonstrate excellent stability, reusability, and resilience in highly absorbing media. All the experimental data are supported by advanced calculations performed using full-wave M3 Maxwell's solver and using electron tomography reconstructions as direct input. Finally, their applicability in RI-based quantitative detection of bovine serum albumin (BSA) is demonstrated, highlighting their potential for biomolecular sensing
Mechanistic Dissymmetry between Crystal Growth and Dissolution drives Ratcheted Chiral Amplification
Full text linkComplete chiral amplification of the solid phase arises when mixtures of self-sorting enantiopure crystals undergo cycles of crystal growth and dissolution under solution-phase racemizing conditions. However, despite extensive studies and widespread use, the mechanism underlying such crystallization-induced deracemization remains insufficiently understood, hindering its optimization and broader application. Here, we experimentally dissect the individual contributions of crystal growth and dissolution and use a mass-balance to expose crystal dynamics. Regardless of the racemization rate, we always find a dissymmetry between the growth and the dissolution of the enantiomer populations. These experiments suggest that a fundamental difference between the mechanisms of crystal growth and dissolution enables a ratchet effect that drives chiral amplification. These insights advance our understanding of chiral crystallization mechanisms and provide guidance for optimizing crystallization-induced deracemizations, particularly by separately optimizing growth and dissolution steps to maximize the chiral amplification and deracemization efficiency
Structure-in-Void Quasi-Bound State in the Continuum Metasurface for Deeply Subwavelength Nanostructure Metrology
Full text linkFano lineshapes associated with quasi bound-state-in-the-continuum resonances, that are supported by dielectric metasurfaces, have the advantageous properties of being extremely sensitive to minute geometrical changes in the meta-atoms. We show an approach to determine deep subwavelength feature sizes, comparable to semiconductor critical dimension metrology, by structurally infilling a void of a dielectric disk-hole metasurface design. Our simulated results show a sensitivity of 40.5 nm resonant wavelength shift for a 1 nm feature width (i.e., critical dimension) change, at an optical line width of 1.8 nm. We present both experimental and simulated results of different void infillings and attribute the spectral change of the resonance to the sensitivity to an effective index in the void of the meta-atom, which arises from the filled volume fraction and the material boundaries orientation relative to the local polarization. Treating our metasurface as an effective index sensor, the sensitivity is 262 nm·RIU–1and the figure of merit is 146 RIU–1, which underlies the pronounced resonant wavelength shift driven by similarly large changes in the effective index caused by extremely tiny critical dimension variations. This approach could impact critical dimension measurements in semiconductor metrology, as it works at the high throughput of optical measurements while performing at the high resolution of scanning electron microscopy
Angle-Resolved Cathodoluminescence Interferometry of Plasmonic and Dielectric Scatterers
Full text linkWe demonstrate angle-resolved cathodoluminescence (CL) interferometry from electron-beam-excited plasmonic and dielectric nanostructures placed above a Au-coated substrate. We use 20–30 keV electrons to coherently excite plasmon-mediated radiation, which interferes with its mirror image, providing a method to determine the particle-substrate spacing. In an aloof excitation geometry, transition radiation emitted from the Au substrate adds to the interferogram and provides a means to probe the electron traveling time. The measured CL interferograms are in excellent agreement with a scattering and interferometry model in which a single electron coherently launches plasmons at two separate locations. Polarization-resolved CL measurements confirm the interferometric scattering model. Electron-excited Si Mie scatterers show interferograms modulated with resonantly enhanced emission. CL interferometry enables accurate measurement of critical distances in nanoscale geometries, in particular along the electron-beam direction, which are not easily accessible in electron microscopy, while offering a platform for studying optical interference in complex geometries
Nanoscale Heat Flow and Thermometry in Laser-Heated Resonant Silicon Mie Nanospheres Probed with Spatially Resolved Cathodoluminescence Spectroscopy
Full text linkMany nanoscale technologies depend critically on precise knowledge and control of local temperature and heat flow, making robust nanothermometry essential for designing, optimizing, and ensuring the reliability of next-generation devices. In this work, we introduce a correlative method that combines laser excitation with scanning electron microscopy-based cathodoluminescence (SEM-CL) to probe photothermal effects in situ with nanoscale spatial resolution. We analyze the spatially resolved CL (30 keV) of resonant Mie modes in single silicon nanoparticles under continuous-wave laser irradiation (λ = 442 nm). The 235–250-nm-diameter crystalline nanospheres, placed on a Si3N4 membrane, show a strong electric quadrupole CL resonance of which the peak wavelength reversibly red-shifts upon laser-induced heating. A temperature of up to 585 ± 12 °C is derived from the spectral shifts for the highest laser power used (9.6 mW, ∼1 × 106 W/cm2 at the substrate). Numerical heat flow simulations show that the measured steady-state temperatures are consistent with a geometry in which heat flow occurs through a contact area of up to 100 nm2, depending on laser power, between the Si nanosphere and the Si3N4 membrane. We postulate that this contact forms by reshaping of the particle–membrane geometry as it heats up in the initial phase of the laser irradiation, leading to an equilibrium geometry that results in the measured steady-state temperature. This work shows that CL of resonant nanostructures in combination with simulations can serve as sensitive probes of temperature and thermal conductivity. Spatially resolved CL nanothermometry in a SEM enables studies of nanoscale thermal properties of a wide range of device geometries such as electronic integrated circuits, surface catalysts, photovoltaic devices, and more
Electron-Energy Dependent Excitation and Directional Far-Field Radiation of Resonant Mie Modes in Single Si Nanospheres
Full text linkHigh-energy electron beams with energies in the 15–30 keV range are used to excite optical Mie modes in crystalline Si nanospheres with radius 80–100 nm. Cathodoluminescence (CL) spectra show emission from resonant electric and magnetic dipole and quadrupole modes, with relative intensities that depend strongly on electron energy and impact parameter. The measured trends are explained by a coupling model in which the electron-energy dependent CL excitation probability–and thus the CL emission–is proportional to the Fourier transform of the modal electric field at a spatial frequency determined by the electron velocity. As a result, the coupling to a specific resonant mode is strongly dependent on the electron energy and the impact parameter of the electron beam. This enables the selective enhancement of CL emission from a resonant mode by phase-matching with the electron velocity. A systematic study of spatial excitation probability for the electric dipole mode as a function of electron energy further confirms the validity of the coupling model. Angle-resolved cathodoluminescence measurements show strong directional emission due to far-field interference of coherently excited Mie modes. By varying the electron energy and impact parameter the intensity and interference of these modes can be controlled and the angular distribution tailored. The insights in the localized deep-subwavelength coherent excitation of resonant Mie modes explored here are important for studies in light-emitting nanostructures, sensors, and photovoltaics, in which the interplay of local modes and far-field directional emission must be controlled
Intensity-Modulated Photoluminescence Spectroscopy for Revealing Ionic Processes in Halide Perovskites
Full text linkMobile ions limit halide perovskite device performance, yet quantifying ionic properties remains challenging. Frequency-domain electrical techniques are restricted to operational devices, and the resulting signals are often dominated by interfacial recombination which obscures ionic contributions. Here, we introduce intensity-modulated photoluminescence spectroscopy (IMPLS) as a fully optical alternative, where the amplitude and phase of the photoluminescence intensity is measured as a function of excitation modulation frequency. IMPLS is demonstrated on a Cs0.07(FA0.83MA0.17)0.93Pb(I0.83Br0.17)3 film. Fitting the data with an optical equivalent circuit model reveals two characteristic lifetimes: τchar = 2.1 ms and 77 s, likely corresponding to defect formation and ionic diffusion, respectively. The diffusion feature is consistent with intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) measurements on corresponding full devices. Importantly, IMPLS enables contact-free characterization of slow processes for all perovskite sample types, including films and devices, significantly expanding the techniques available for understanding mobile ions in these materials
Variational Path Sampling of Rare Dynamical Events
Full text linkThis article reviews the concepts and methods of variational path sampling. These methods allow computational studies of rare events in systems driven arbitrarily far from equilibrium. Based upon a statistical mechanics of trajectory space and leveraging the theory of large deviations, they provide a perspective from which dynamical phenomena can be studied with the same types of ensemble reweighting ideas that have been used for static equilibrium properties. Applications to chemical, material, and biophysical systems are highlighted
Supramolecular Preorganization Rhodium and Iridium Metal Complexes Within M12L24 Self-Assembled Nanospheres for the Confined Synthesis Rh/Ir Alloyed Nanoparticles
Full text linkControlling the size and composition of metal nanoparticles is of considerable interest, as these are essential to their catalytic properties. Recently, our group has developed a preorganization strategy for controlled Ir nanoparticle synthesis inside Pt12L24 nanospheres. In the current contribution, we expand this method to the controlled synthesis of Rh nanoparticles. The encapsulated RhI complexes (Rh-s @ G-sphere) led to reasonable size control (2.8 ± 0.9 nm). Next, we demonstrated the formation of Rh-Ir alloyed nanoparticles with varying Rh/Ir compositions, by preorganization of the respective metal complexes inside Pt12L24 nanospheres based on complementary hydrogen bonds before the reduction step that leads to nanoparticle formation. These heterometallic particles were evaluated in the hydrogenation of cinnamaldehyde (7) as a probe reaction. Besides a high activity in this probe reaction, the Rh particles also catalyzed the conversion of the solvent (CH3CN). The formed basic amine leads to follow-up reactions of the product and compatibility issues with the hosting nanosphere. The solvent hydrogenation was effectively suppressed by using the Rh:Ir alloyed nanoparticles, provided that they contain > 66% Ir. Compared to the monometallic Ir particles (Ir-s @ G-sphere), the Rh:Ir alloyed nanoparticles displayed higher catalytic activity, reaching optimal selectivity and activity at an 8:16–Rh:Ir ratio. The combined catalytic results illustrate that preorganization of the metal complexes in the nanosphere before the reduction with hydrogen effectively facilitates the formation of Rh:Ir alloyed nanoparticles, which allows for tuning a catalyst to create a more active and selective catalyst compared to monometallic or nonencapsulated Rh/Ir particles
Information advantage in sensing revealed by Fano-resonant Fourier scatterometry
Full text linkFano resonances in nanophotonic structures are attractive for sensing due to their ultanarrow resonant linewidths and high local fields. Conventional read out schemes rely on measuring a frequency shift in Fano scattering spectra as function of perturbation. We experimentally demonstrate that angle-resolved analysis of the scattering of a Fano resonant structure is quantitatively more informative than measuring spectral shifts. We theoretically discuss how a perturbation affects fundamental nanophotonic properties of a Fano resonant metasystem, and how these are transduced to an observable far field response. We perform a rigorous experimental study in which we characterize deeply subwavelength perturbations in a Fano resonant dielectric metasurface using a conventional spectral approach, and a Fourier scatterometry based approach, and show that perturbations can lead to marked directional scattering in Fourier space. We finally quantitatively compare these two sensing methods in terms of their inherent Fisher information content, and show that an information advantage is obtained when the signal is resolved in Fourier space