No abstract

Abstract

Metallic chiral nanoparticles (CNPs) with a nominal helical pitch (P) of sub‐10 nm contain inherent chirality and are promisingly applied to diverse prominent enantiomer‐related applications. However, the sub‐wavelength P physically results in weak optical activity (OA) to prohibit the development of these applications. Herein, a facile method to amplify the CNPs' OA by alloying the host CNPs with metals through a three‐step layer‐by‐layer glancing angle deposition (GLAD) method is devised. Promoted by the GLAD‐induced heating effect, the solute metallic atoms diffuse into the host CNPs to create binary alloy CNPs. Chiral alloying not only induces the plasmonic OA of the diffused solute and the created alloys but also amplifies that of the host CNPs, generally occurring for alloying Ag CNPs with diverse metals (including Cu, Au, Al, and Fe) and alloying Cu CNPs with Ag. Furthermore, the chiral alloying leads to an enhancement of refractive index sensitivity of the CNPs. The alloy CNPs with amplified plasmonic OA pave the way for potentially developing important chirality‐related applications in the fields of heterogeneous asymmetric catalysis, enantiodifferentiation, enantioseparation, biosensing, and bioimaging.

The bonding between nanocrystals in solution is one of the most important crystal growth pathways to the bottom-up assembly of nanocrystals into hierarchical structures for the fabrication of nanoscale devices and nanomaterials. The main challenge, however, is to understand and avoid defect formation at the bonding interfaces between nanocrystals. It is believed that any misalignment results in defect formation at bonding interface, and defect-free coherent bonding is only possible when bonding lattice planes are perfectly oriented. Recent studies employing in situ imaging showed that the dynamics of nanocrystal bonding in solution is rich. For the first time the pre-alignment of nanocrystals prior to attachment, post-bonding dipole-induced realignment of nanocrystals, and annealing of line dislocations in nanocrystals were observed [1-3]. However, two important key questions to nanocrystal bonding remain unresolved: 1) what are the necessary conditions for nanocrystal attachment in solution that will lead to defect-free coherent bonding? and 2) what is the likelihood of such coherent bonding? Here, we resolved the mechanisms underpinning these phenomena using liquid-cells developed for time-resolved electron microscopy [4]. Our study is the first to give a detailed description of two distinct bonding pathways which shows how attachment geometry of the system affects the bonding of nanocrystals with rotational and translational degrees of freedom [5]. Figure 1A shows the coherent bonding of two nanocrystals initially separated by ~1 nm, and their (111) lattice planes misaligned by 10° (at t = 6.9 s) prior to contact. As the nanocrystals approach each other, their lattice misalignment decreases to 9° upon contact (not shown here), then visibly realigns into coherent bonding (Fig. 1A: t = 25.5 s and 48.2 s). Here, the bonding was coherent because the common (111) lattice planes reflections of the nanocrystals merged into a single reflection with no visible defects (inset t = 48.2 s). Expectedly, defect-mediated bonding occurs when two nanocrystals meet with their (111) lattice planes initially misaligned by a large angle, for example by about 32 o in Figure 1B. Here the nanocrystals do not realign upon bonding and a visible defect forms at their merging interface (t = 6.9 s, black arrow). Post bonding images and splitted Fourier reflections show that the newly formed nanoparticle is not a single crystal (Fig. 1B: t = 11.2 s). The coherent and defect-mediated bonding sequences captured in Figure 1 are remarkable because upon contact the nanocrystals can reorient themselves to a limited extent, either to achieve coherent bonding when the two nanocrystals are initially slightly misaligned, or rotate to a mutual configuration that allows defects to form.The experimental results in Figure 2A show that there is a critical misalignment angle (βcritical ~ 15°) that separates two pathways for bonding: below this critical angle gold nanocrystals can realign for defectfree coherent bonding, and above which the defect are form...

We suggest a new strategy for creating stimuliresponsive bio-integrated optical nanostructures based on Mieresonant silicon nanoparticles covered by an ensemble of similarity negatively charged polyelectrolytes (heparin and sodium polystyrene sulfonate). The dynamic tuning of the nanostructures optical response is due to light-induced heating of the nanoparticles and swelling of the polyelectrolyte shell. The resulting hydrophilic/hydrophobic transitions significantly change the shell thickness and reversible shift of the scattering spectra for individual nanoparticles up to 60 nm. Our findings bring novel opportunities for the application of smart nanomaterials in nanomedicine and bio-integrated nanophotonics.