Metasurfaces and Colloidal Suspensions Composed of 3D Chiral Si Nanoresonators

High‐refractive‐index silicon nanoresonators are promising low‐loss alternatives to plasmonic particles in CMOS‐compatible nanophotonics applications. However, complex 3D particle morphologies are challenging to realize in practice, thus limiting the range of achievable optical functionalities. Using 3D film structuring and a novel gradient mask transfer technique, the first intrinsically chiral dielectric metasurface is fabricated in the form of a monolayer of twisted silicon nanocrescents that can be easily detached and dissolved into colloidal suspension. The metasurfaces exhibit selective handedness and a circular dichroism as large as 160° µm^?1 due to pronounced differences in induced current loops for left‐handed and right‐handed polarization. The detailed morphology of the detached particles is analyzed using high‐resolution transmission electron microscopy. Furthermore, it is shown that the particles can be manipulated in solution using optical tweezers. The fabrication and detachment method can be extended to different nanoparticle geometries and paves the way for a wide range of novel nanophotonic experiments and applications of high‐index dielectrics.     

平面手征超常介质研究进展

平面手征超常介质(Planar chiral metamaterials, PCM)是近年迅速发展起来的能获得大旋光性进而控制光偏振的新颖介质.综述了PCM的研究进展,包括金属结构、全介质结构以及单层和双层结构等几种不同情况,这些PCM都能产生非常强的旋光性,在光子学中具有潜在的应用前景,并且有可能成为手征负折射光学材料中有前途的候选者.     

Resolution Limit in Plasmonic Lithography for Practical Applications beyond 2x‐nm Half Pitch

A theoretical model is introduced to evaluate the ultimate resolution of plasmonic lithography using a ridge aperture. The calculated and experimental results of the line array pattern depth are compared for various half pitches. The theoretical analysis predicts that the resolution of plasmonic lithography strongly depends on the ridge gap, achieving values under 1x nm with a ridge gap smaller than 10 nm. A micrometer‐scale circular contact probe is fabricated for high speed patterning with high positioning accuracy, which can be extended to a high‐density probe array. Using the circular contact probe, high‐density line array patterns are recorded with a half pitch up to 22 nm and good agreement is obtained between the theoretical model and experiment. To record the high density line array patterns, the line edge roughness (LER) is reduced to ≈17 nm from 29 nm using a well‐controlled developing process with a smaller molecular weight KOH‐based developer at a temperature below 10°C.     

Resistless Nanoimprinting in Metal for Plasmonic Nanostructures

Metal nanostructures are the main building blocks of metamaterials and plasmonics which show many extraordinary properties not existing in nature. A simple and widely applicable method that can directly pattern metals with silicon molds without the need of resists, using pressures of <4 MPa and temperatures of 25–150 °C is reported. Three‐dimensional structures with smooth and vertical sidewalls, down to sub‐10 nm resolution, are generated in silver and gold films in a single patterning step. Using this nanopatterning scheme, large‐scale vivid images through extraordinary optical transmission and strong surface‐enhanced Raman scattering substrates are realized. Resistless nanoimprinting in metal (RNIM) is a new class of metal patterning that allows plasmonic nanostructures to be fabricated quickly, repeatedly, and at a low‐cost.     

Probing the electromagnetic field distribution within a metallic nanodisk

A Co nanolayer is used as a local probe to evaluate the vertical inhomogeneous distribution of the electromagnetic (EM) field within a resonant metallic nanodisk. Taking advantage of the direct relation between the magneto-optical activity and the electromagnetic field intensity in the Co layer, it is shown that the nonuniform EM distribution within the nanodisk under plasmon resonant conditions has maximum values close to the upper and lower flat faces, and a minimum value in the middle.     

Reconfigurable Plasmonic Chirality: Fundamentals and Applications

Molecular chirality is a geometric property that is of great importance in chemistry, biology, and medicine. Recently, plasmonic nanostructures that exhibit distinct chiroptical responses have attracted tremendous interest, given their ability to emulate the properties of chiral molecules with tailored and pronounced optical characteristics. However, the optical chirality of such human-made structures is in general static and cannot be manipulated postfabrication. Herein, different concepts to reconfigure the chiroptical responses of plasmonic nano- and micro-objects are outlined. Depending on the utilized strategies and stimuli, the chiroptical signature, the 3D structural conformation, or both can be reconfigured. Optical devices based on plasmonic nanostructures with reconfigurable chirality possess great potential in practical applications, ranging from polarization conversion elements to enantioselective analysis, chiral sensing, and catalysis.     

Fabricating a Homogeneously Alloyed AuAg Shell on Au Nanorods to Achieve Strong,Stable, and Tunable Surface Plasmon Resonances

Colloidal metal nanocrystals with strong, stable, and tunable localized surface plasmon resonances (SPRs) can be useful in a corrosive environment for many applications including field‐enhanced spectroscopies, plasmon‐mediated catalysis, etc. Here, a new synthetic strategy is reported that enables the epitaxial growth of a homogeneously alloyed AuAg shell on Au nanorod seeds, circumventing the phase segregation of Au and Ag encountered in conventional synthesis. The resulting core–shell structured bimetallic nanorods (AuNR@AuAg) have well‐mixed Au and Ag atoms in their shell without discernible domains. This degree of mixing allows AuNR@AuAg to combine the high stability of Au with the superior plasmonic activity of Ag, thus outperforming seemingly similar nanostructures with monometallic shells (e.g., Ag‐coated Au NRs (AuNR@Ag) and Au‐coated Au NRs (AuNR@Au)). AuNR@AuAg is comparable to AuNR@Ag in plasmonic activity, but that it is markedly more stable toward oxidative treatment. Specifically, AuNR@AuAg and AuNR@Ag exhibit similarly strong signals in surface‐enhanced Raman spectroscopy that are some 30‐fold higher than that of AuNR@Au. When incubated with a H₂O₂ solution (0.5 m ), the plasmonic activity of AuNR@Ag immediately and severely decayed, whereas AuNR@AuAg retained its activity intact. Moreover, the longitudinal SPR frequency of AuNR@AuAg can be tuned throughout the red wavelengths (≈620–690 nm) by controlling the thickness of the AuAg alloy shell. The synthetic strategy is versatile to fabricate AuAg alloyed shells on different shaped Au, with prospects for new possibilities in the synthesis and application of plasmonic nanocrystals.     

Chiral Surface and Geometry of Metal Nanocrystals

Chirality is a basic property of nature and has great importance in photonics, biochemistry, medicine, and catalysis. This importance has led to the emergence of the chiral inorganic nanostructure field in the last two decades, providing opportunities to control the chirality of light and biochemical reactions. While the facile production of 3D nanostructures has remained a major challenge, recent advances in nanocrystal synthesis have provided a new pathway for efficient control of chirality at the nanoscale by transferring molecular chirality to the geometry of nanocrystals. Interestingly, this discovery stems from a purely crystallographic outcome: chirality can be generated on high-Miller-index surfaces, even for highly symmetric metal crystals. This is the starting point herein, with an overview of the scientific history and a summary of the crystallographic definition. With the advance of nanomaterial synthesis technology, high-Miller-index planes can be selectively exposed on metallic nanoparticles. The enantioselective interaction of chiral molecules and high-Miller-index facets can break the mirror symmetry of the metal nanocrystals. Herein, the fundamental principle of chirality evolution is emphasized and it is shown how chiral surfaces can be directly correlated with chiral morphologies, thus serving as a guide for researchers in chiral catalysts, chiral plasmonics, chiral metamaterials, and photonic devices.     

Efficient and Stable CsPbI3 Solar Cells via Regulating Lattice Distortion with Surface Organic Terminal Groups

All‐inorganic cesium lead iodide perovskites (CsPbI₃) are promising wide‐bandgap materials for use in the perovskite/silicon tandem solar cells, but they easily undergo a phase transition from a cubic black phase to an orthorhombic yellow phase under ambient conditions. It is shown that this phase transition is triggered by moisture that causes distortion of the corner‐sharing octahedral framework ([PbI₆]^4?). Here, a novel strategy to suppress the octahedral tilting of [PbI₆]^4? units in cubic CsPbI₃ by systematically controlling the steric hindrance of surface organic terminal groups is provided. This steric hindrance effectively prevents the lattice distortion and thus increases the energy barrier for phase transition. This mechanism is verified by X‐ray diffraction measurements and density functional theory calculations. Meanwhile, the formation of an organic capping layer can also passivate the surface electronic trap states of perovskite absorber. These modifications contribute to a stable power conversion efficiency (PCE) of 13.2% for the inverted planar perovskite solar cells (PSCs), which is the highest efficiency achieved by the inverted‐structure inorganic PSCs. More importantly, the optimized devices retained 85% of their initial PCE after aging under ambient conditions for 30 days.     

Chirality Transfer from Sub-Nanometer Biochemical Molecules to Sub-Micrometer Plasmonic Metastructures: Physiochemical Mechanisms,Biosensing, and Bioimaging Opportunities

Determining the structural chirality of biomolecules is of vital importance in bioscience and biomedicine. Conventional methods for characterizing molecular chirality, e.g., circular dichroism (CD) spectroscopy, require high-concentration specimens due to the weak electronic CD signals of biomolecules such as amino acids. Artificially designed chiral plasmonic metastructures exhibit strong intrinsic chirality. However, the significant size mismatch between metastructures and biomolecules makes the former unsuitable for chirality-recognition-based molecular discrimination. Fortunately, constructing metallic architectures through molecular self-assembly allows chirality transfer from sub-nanometer biomolecules to sub-micrometer, intrinsically achiral plasmonic metastructures by means of either near-field interaction or chirality inheritance, resulting in hybrid systems with CD signals orders of magnitude larger than that of pristine biomolecules. This exotic property provides a new means to determine molecular chirality at extremely low concentrations (ideally at the single-molecule level). Herein, three strategies of chirality transfer from sub-nanometer biomolecules to sub-micrometer metallic metastructures are analyzed. The physiochemical mechanisms responsible for chirality transfer are elaborated and new fascinating opportunities for employing plasmonic metastructures in chirality-based biosensing and bioimaging are outlined.     

Chirality and Chiroptical Effects in Plasmonic Nanostructures: Fundamentals,Recent Progress,and Outlook

Strong chiroptical effects recently reported result from the interaction of light with chiral plasmonic nanostructures. Such nanostructures can be used to enhance the chiroptical response of chiral molecules and could also significantly increase the enantiomeric excess of direct asymmetric synthesis and catalysis. Moreover, in optical metamaterials, chirality leads to negative refractive index and all the promising applications thereof. In this Progress Report, we highlight four different strategies which have been used to achieve giant chiroptical effects in chiral nanostructures. These strategies consecutively highlight the importance of chirality in the nanostructures (for linear and nonlinear chiroptical effects), in the experimental setup and in the light itself. Because, in the future, manipulating chirality will play an important role, we present two examples of chiral switches. Whereas in the first one, switching the chirality of incoming light causes a reversal of the handedness in the nanostructures, in the second one, switching the handedness of the nanostructures causes a reversal in the chirality of outgoing light.     

Spiral Patterning of Au Nanoparticles on Au Nanorod Surface to Form Chiral AuNR@AuNP Helical Superstructures Templated by DNA Origami

Plasmonic motifs with precise surface recognition sites are crucial for assembling defined nanostructures with novel functionalities and properties. In this work, a unique and effective strategy is successfully developed to pattern DNA recognition sites in a helical arrangement around a gold nanorod (AuNR), and a new set of heterogeneous AuNR@AuNP plasmonic helices is fabricated by attaching complementary‐DNA‐modified gold nanoparticles (AuNPs) to the predesigned sites on the AuNR surface. AuNR is first assembled to one side of a bifacial rectangular DNA origami, where eight groups of capture strands are selectively patterned on the other side. The subsequently added link strands make the rectangular DNA origami roll up around the AuNR into a tubular shape, therefore giving birth to a chiral patterning of DNA recognition sites on the surface of AuNR. Following the hybridization with the AuNPs capped with the complementary strands to the capture strands on the DNA origami, left‐handed and right‐handed AuNR@AuNP helical superstructures are precisely formed by tuning the pattern of the recognition sites on the AuNR surface. Our strategy of nanoparticle surface patterning innovatively realizes hierarchical self‐assembly of plasmonic superstructures with tunable chiroptical responses, and will certainly broaden the horizon of bottom‐up construction of other functional nanoarchitectures with growing complexity.