Chiral Ligand-Free, Optically Active Nanoparticles Inherently Composed of Chiral Lattices at the Atomic Scale.

Bulk metals lack chirality. Recently, metals have been sculptured with metastable chirality varying from the micro- to nano-scale. The manipulation of molecular chirality could be novelly performed using metals composed of chiral lattices at atomic scales (i.e., chiral nanoparticles or CNPs) if one could fundamentally understand the interactions between molecules and the chiral metal lattices. The incorporation of chiral ligands has been generally adapted to form metal CNPs. However, post-fabrication removal of chiral ligands usually causes relaxation of the metastable chiral lattices to thermodynamically stable achiral structures, and thus the coexisting chiral ligands will unavoidably disturb or screen the interactions of interest. Herein, a concept of metal CNPs that are free of chiral ligands and consist of atomically chiral lattices is introduced. Without chiral ligands, shear forces applied by substrate rotation along with the translation of incident atoms lead to imposing the metastable chiral lattices onto metals. Metal CNPs show not only the chiroptical effect but the enantiospecific interactions of chiral lattices and molecules. These two unique chiral effects have resulted in the applications of enantiodifferentiation and asymmetric synthesis. Prospectively, the extension in composition space and constituent engineering will apply alloy CNPs to enantiodiscrimination, enantioseperation, bio-imaging, bio-sensing, and asymmetric catalysis.

Chirality Transfer in Galvanic Replacement Reactions.

Demand for the transfer of chirality from a pre-engineered nanoparticle to any other metal is of fundamental importance for developing a wide range of chirality-related applications. Herein, we show that binary alloy chiral nanoparticles (CNPs) with an engineerable composition can be formed from metallic CNPs with intrinsic structural chirality serving as sacrificial templates (STs), via a galvanic replacement reaction (GRR). This GRR-mediated chirality transfer is a general phenomenon and results in the formation of Cu-Ag CNPs with solid morphology and mesoporous CNPs made of Ag-Au, Ag-Pt, and Ag-Pd. Our study imposes a new component, i.e., structural chirality, on the GRR. The insights from our study improve our fundamental understanding of the GRR principle and devise a versatile method to generate mesoporous alloy CNPs for developing prominent chirality-related applications in asymmetric catalysis, enantiodifferentiation, enantioseparation, biodetection, and bioimaging.

Significant Enhancement of Circular Polarization in Light Emission through Controlling Helical Pitches of Semiconductor Nanohelices.

Circularly polarized light emission (CPLE) can be potentially applied to three-dimensional displays, information storage, and biometry. However, these applications are practically limited by a low purity of circular polarization, i.e., the small optical dissymmetry factor gCPLE. Herein, glancing angle deposition (GLAD) is performed to produce inorganic nanohelices (NHs) to generate CPLE with large gCPLE values. CdSe NHs emit red CPLE with gCPLE = 0.15 at a helical pitch (P) ≈ 570 nm, having a 40-fold amplification of gCPLE compared to that at P ≈ 160 nm. Ceria NHs emit ultraviolet-blue CPLE with gCPLE ≈ 0.06 at P ≈ 830 nm, with a 103-fold amplification compared to that at P ≈ 110 nm. Both the photoluminescence and scattering among the close-packed NHs complicatedly account for the large gCPLE values, as revealed by the numerical simulations. The GLAD-based NH-fabrication platform is devised to generate CPLE with engineerable color and large gCPLE = 10-2-10-1, shedding light on the commercialization of CPLE devices.

One-Fold Anisotropy of Silver Chiral Nanoparticles Studied by Second-Harmonic Generation.

Second-harmonic generation (SHG) integrated with diverse nonlinear optical activity characterization has high sensitivity to detect the symmetry of materials at an interface, but the study is in its infancy. Here, we employ SHG with linear dichroism (or SHG-LD) to study the chiroptical origin of silver (Ag) chiral nanoparticles (CNPs) deposited by glancing angle deposition (GLAD). It is found that Ag CNPs show the chiroptical activity ascribed to not only the structural chirality (i.e., atomically chiral lattices) but also one-fold anisotropy at an interface due to the substrate rotation during GLAD. Therefore, the SHG-LD shows great potential to provide valuable complementary information to study the chiroptical properties of chiral metamaterials.

Extension of Compositional Space to the Ternary in Alloy Chiral Nanoparticles through Galvanic Replacement Reactions.

Metal chiral nanoparticles (CNPs), composed of atomically chiral lattices, are an emerging chiral nanomaterial showing unique asymmetric properties. Chirality transmission from the host CNPs mediated with galvanic replacement reactions (GRRs) has been carried out to extend their compositional space from the unary to binary. Further compositional extension to, e.g., the ternary is of fundamental interest and in practical demand. Here, layer-by-layer glancing angle deposition is used to dope galvanically "inert" dopant Au in the host Cu CNPs to generate binary Cu:Au CNPs. The "inert" dopants serve as structural scaffold to assist the chirality transmission from the host to the third metals (M: Pt and Ag) cathodically precipitating in the CNPs, enabling the formation of polycrystalline ternary Cu:Au:M CNPs whose compositions are tailored with engineering the GRR duration. More scaffold Au atoms are favored for the faster chirality transfer, and the Au-assisted chirality transfer follows the first-order kinetics with the reaction rate coefficient of ≈0.3 h-1 at room temperature. This work provides further understanding of the GRR-mediated chirality transfer and paves the way toward enhancing the application functions in enantiodifferentiation, enantioseperation, asymmetric catalysis, bioimaging, and biodetection.