The chiral nano-world: chiroptically active quantum nanostructures

Optically Active CdSe-Dot/CdS-Rod Nanocrystals with Induced Chirality and Circularly Polarized Luminescence

Ligand-induced chirality in semiconductor nanocrystals (NCs) has attracted attention because of the tunable optical properties of the NCs. Induced circular dichroism (CD) has been observed in CdX (X = S, Se, Te) NCs and their hybrids, but circularly polarized luminescence (CPL) in these fluorescent nanomaterials has been seldom reported. Herein, we describe the successful preparation of l- and d-cysteine-capped CdSe-dot/CdS-rods (DRs) with tunable CD and CPL behaviors and a maximum anisotropic factor ( g_lum) of 4.66 × 10^-4. The observed CD and CPL activities are sensitive to the relative absorption ratio of the CdS shell to the CdSe core, suggesting that the anisotropic g-factors in both CD and CPL increase to some extent for a smaller shell-to-core absorption ratio. In addition, the molar ratio of chiral cysteine to the DRs is investigated. Instead of enhancing the chiral interactions between the chiral molecules and DRs, an excess of cysteine molecules in aqueous solution inhibits both the CD and CPL activities. Such chiral and emissive NCs provide an ideal platform for the rational design of semiconductor nanomaterials with chiroptical properties.

Optically Active Semiconductor Nanosprings for Tunable Chiral Nanophotonics

The search for the optimal geometry of optically active semiconductor nanostructures is making steady progress and has far-reaching benefits. Yet the helical springlike shape, which is very likely to provide a highly dissymmetric optical response, remains somewhat understudied theoretically. Here we comprehensively analyze the optical activity of semiconductor nanosprings using a fully quantum-mechanical model of their electronic subsystem and taking into account the anisotropy of their interaction with light. We show that the circular dichroism of semiconductor nanosprings can exceed that of ordinary semiconductor nanocrystals by a factor of 100 and be comparable to the circular dichroism of metallic nanosprings. It is also demonstrated that nanosprings can feature a total dissymmetry of optical response for certain ratios between their length and coil height. The magnitude and sign of the circular dichroism signal can be controlled by stretching or compressing the nanosprings, which makes them a promising material base for optomechanical sensors, polarization controllers, and other types of optically active nanophotonic devices.

Optical Activity of Semiconductor Gammadions beyond Planar Chirality

We present rigorous analysis of optical activity of chiral semiconductor gammadions whose chirality in three dimensions is caused by the nonuniformity of thickness in the transverse plane. It is shown that such gammadions not only distinguish between the two circular polarizations upon scattering and reflection of light, like all two-dimensional semiconductor nanostructures with planar chirality do, but also exhibit circular dichroism and circularly polarized luminescence. Chiral semiconductor gammadions whose charge carriers are mostly confined to the arms are found to feature both high dissymmetry of optical response and a constant-sign circular dichroism signal over a wide frequency range. It is also shown that the strength of the gammadion's chiroptical response is determined solely by two geometric factors: the variation range of the gammadion's thickness and the arms' curvature. Our seminal theoretical study is intended to lay the foundation for future applications of semiconductor gammadions in chiral nanophotonics and nanotechnology.

Imprinting Chirality onto the Electronic States of Colloidal Perovskite Nanoplatelets

The direct synthesis of chiroptical organic-inorganic methylammonium lead bromide perovskite nanoplatelets that are passivated by R- or S-phenylethylammonium ligands is reported. The circular dichroism spectra can be divided into two components: (1) a region associated with a charge transfer transition between the ligand and the nanoplatelet, 300-350 nm, and (2) a region corresponding to the excitonic absorption maximum of the perovskite, 400-450 nm. The temperature- and concentration-dependent circular dichroism spectra indicate that the chiro-optical response arises from chiral imprinting by the ligand on the electronic states of the quantum-confined perovskite rather than chiral ligand-induced stereoselective aggregation.

Induction of Chirality in Two-Dimensional Nanomaterials: Chiral 2D MoS2 Nanostructures

Two-dimensional (2D) nanomaterials have been intensively investigated due to their interesting properties and range of potential applications. Although most research has focused on graphene, atomic layered transition metal dichalcogenides (TMDs) and particularly MoS₂ have gathered much deserved attention recently. Here, we report the induction of chirality into 2D chiral nanomaterials by carrying out liquid exfoliation of MoS₂ in the presence of chiral ligands (cysteine and penicillamine) in water. This processing resulted in exfoliated chiral 2D MoS₂ nanosheets showing strong circular dichroism signals, which were far past the onset of the original chiral ligand signals. Using theoretical modeling, we demonstrated that the chiral nature of MoS₂ nanosheets is related to the presence of chiral ligands causing preferential folding of the MoS₂ sheets. There was an excellent match between the theoretically calculated and experimental spectra. We believe that, due to their high aspect ratio planar morphology, chiral 2D nanomaterials could offer great opportunities for the development of chiroptical sensors, materials, and devices for valleytronics and other potential applications. In addition, chirality plays a key role in many chemical and biological systems, with chiral molecules and materials critical for the further development of biopharmaceuticals and fine chemicals, and this research therefore should have a strong impact on relevant areas of science and technology such as nanobiotechnology, nanomedicine, and nanotoxicology.

Enantioselective cellular localisation of europium(iii) coordination complexes

The selective mitochondrial localisation of the Λ enantiomer of three different emissive europium(iii) complexes in NIH 3T3 and MCF7 cells contrasts with the behaviour of the Δ enantiomer, for which a predominant lysosomal localisation was observed by confocal microscopy. In each case, cell uptake occurs via macropinocytosis.

Circular Dichroism of CdSe Nanocrystals Bound by Chiral Carboxylic Acids

Chiral semiconductor nanocrystals, or quantum dots (QDs), are promising materials for applications in biological sensing, photonics, and spin-polarized devices. Many of these applications rely on large dissymmetry, or g-factors, the difference in absorbance between left- and right-handed circularly polarized light compared to the unpolarized absorbance. The majority of chiral QDs, specifically CdSe, reported to date have used thiolated amino acid ligands to introduce chirality onto the nanoparticles, but these systems have ultimately reported small g-factors of ∼2 × 10^-4. In an effort to realize chiral CdSe QDs with higher g-factors and to expand the set of designer chiral semiconductor nanocrystals, we have employed chiral carboxylic acids as a distinct class of ligands for chiral CdSe nanoparticles. Through this family of chiral carboxylic acid ligands, we performed a direct comparison between carboxylate-bound and thiolate-bound chiral CdSe QDs. Spectral analysis revealed that the resulting circular dichroism shifts originate from the splitting of the exciton by the ligand-nanocrystal interaction. Subsequent examination of a series of chiral carboxylic acid ligands revealed a 30-fold range in g-factor through relatively small changes in the structure of the ligand. Finally, we showed that increasing the number of stereocenters on the ligand can further enhance the dissymmetry factors. This versatile and tunable combination of nanocrystals and ligands will inform future designs of chiral nanomaterials and their applications.

An organometallic route to chiroptically active ZnO nanocrystals

The unique optical properties of zinc oxide nanocrystals (ZnO NCs) are strongly dependent on both the properties and the composition of the inorganic core-organic ligand interface. Developing a novel organometallic self-supporting approach, we report on the synthesis and characterization of ZnO nanocrystals coated by chiral monoanionic aminoalcoholate ligands. The resulting ZnO NCs are both chiroptically active and possess size dependent optical properties. The size and in consequence the emission color of the ZnO NCs could be simply adjusted by the characteristic of the aminoalcohol used.

The effect of surface ligands on the optical activity of mercury sulfide nanoparticles

Mercury sulfide (HgS) nanoparticles (NPs) were prepared in the presence of water-soluble thiols as capping ligands in aqueous solutions. Chiral thiol ligands successfully afforded the formation of the chiral cinnabar phase (α-HgS), leading to optically active NPs, while two achiral thiols preserved β-HgS NPs with an achiral crystalline system. The profiles of UV-vis absorption and circular dichroism (CD) spectra of chiral NPs were dependent on the chemical structures of the chiral ligands. Cysteine-based derivatives gave HgS NPs demonstrating almost mirror image CD profiles even though they possess identical stereochemistry. The water soluble chiral ligands on the NPs were replaced with an achiral ligand, 1-dodecanethiol, by the spontaneous phase transfer method. The ligand-exchanged NPs with the achiral thiol preserved the optical activity with a feature of the CD profile similar to that of the original NPs in water, demonstrating the chiral memory effect in the NP-core. The dissymmetry factor in optical absorption decreased by almost half, which could be attributed to the amorphous phase formed by the chemical etching with an excess amount of dodecanethiol. The optical activity showed a higher thermal stability compared to that of NPs before the ligand-exchange.

Mid-infrared Plasmonic Circular Dichroism Generated by Graphene Nanodisk Assemblies

It is very interesting to bring plasmonic circular dichroism spectroscopy to the mid-infrared spectral interval, and there are two reasons for this. This spectral interval is very important for thermal bioimaging, and simultaneously, this spectral range includes vibrational lines of many chiral biomolecules. Here we demonstrate that graphene plasmons indeed offer such opportunity. In particular, we show that chiral graphene assemblies consisting of a few graphene nanodisks can generate strong circular dichroism (CD) in the mid-infrared interval. The CD signal is generated due to the plasmon-plasmon coupling between adjacent nanodisks in the specially designed chiral graphene assemblies. Because of the large dimension mismatch between the thickness of a graphene layer and the incoming light's wavelength, three-dimensional configurations with a total height of a few hundred nanometers are necessary to obtain a strong CD signal in the mid-infrared range. The mid-infrared CD strength is mainly governed by the total dimensions (total height and helix scaffold radius) of the graphene nanodisk assembly and by the plasmon-plasmon interaction strength between its constitutive nanodisks. Both positive and negative CD bands can be observed in the graphene assembly array. The frequency interval of the plasmonic CD spectra overlaps with the vibrational modes of some important biomolecules, such as DNA and many different peptides, giving rise to the possibility of enhancing the vibrational optical activity of these molecular species by attaching them to the graphene assemblies. Simultaneously the spectral range of chiral mid-infrared plasmons in our structures appears near the typical wavelength of the human-body thermal radiation, and therefore, our chiral metastructures can be potentially utilized as optical components in thermal imaging devices.

Chiral nanoparticles in singular light fields

The studying of how twisted light interacts with chiral matter on the nanoscale is paramount for tackling the challenging task of optomechanical separation of nanoparticle enantiomers, whose solution can revolutionize the entire pharmaceutical industry. Here we calculate optical forces and torques exerted on chiral nanoparticles by Laguerre-Gaussian beams carrying a topological charge. We show that regardless of the beam polarization, the nanoparticles are exposed to both chiral and achiral forces with nonzero reactive and dissipative components. Longitudinally polarized beams are found to produce chirality densities that can be 109 times higher than those of transversely polarized beams and that are comparable to the chirality densities of beams polarized circularly. Our results and analytical expressions prove useful in designing new strategies for mechanical separation of chiral nanoobjects with the help of highly focussed beams.

A new approach towards spintronics-spintronics with no magnets

We review a recently discovered phenomenon, the chiral induced spin selectivity (CISS) effect, that can enable a new technology for the injection of spin polarized current without the need for a permanent magnetic layer. The effect occurs in chiral molecules and systems without parity symmetry, i.e. systems that do not have inversion symmetry. The theoretical foundations for the effect are presented first and then followed by several examples of spin-valves that are based on chiral systems. The CISS-based spin valves introduce the possibility to inject spin current without the use of a permanent magnet and to achieve relatively large magnetoresistance at room temperature.

Cooperative expression of atomic chirality in inorganic nanostructures

Cooperative chirality phenomena extensively exist in biomolecular and organic systems via intra- and inter-molecular interactions, but study of inorganic materials has been lacking. Here we report, experimentally and theoretically, cooperative chirality in colloidal cinnabar mercury sulfide nanocrystals that originates from chirality interplay between the crystallographic lattice and geometric morphology at different length scales. A two-step synthetic scheme is developed to allow control of critical parameters of these two types of handedness, resulting in different chiral interplays expressed as observables through materials engineering. Furthermore, we adopt an electromagnetic model with the finite element method to elucidate cooperative chirality in inorganic systems, showing excellent agreement with experimental results. Our study enables an emerging class of nanostructures with tailored cooperative chirality that is vital for fundamental understanding of nanoscale chirality as well as technology applications based on new chiroptical building blocks.

Chiral carbon dots and their effect on the optical properties of photosensitizers

In this work, we have successfully prepared intrinsically chiral carbon dots from chiral precursors. We have also demonstrated that the chirality of these carbon dots can affect the optical properties of photosensitizer molecules like azobenzene.

Circularly polarized luminescence in chiral silver nanoclusters

Sets of mirror image circular dichroism (CD) and circularly polarized luminescence (CPL) spectra are for the first time demonstrated using enantiomeric dihydrolipoic acid (DHLA)-capped silver nanoclusters.

Mixing of quantum states: A new route to creating optical activity

The ability to induce optical activity in nanoparticles and dynamically control its strength is of great practical importance due to potential applications in various areas, including biochemistry, toxicology, and pharmaceutical science. Here we propose a new method of creating optical activity in originally achiral quantum nanostructures based on the mixing of their energy states of different parities. The mixing can be achieved by selective excitation of specific states or via perturbing all the states in a controllable fashion. We analyze the general features of the so produced optical activity and elucidate the conditions required to realize the total dissymmetry of optical response. The proposed approach is applicable to a broad variety of real systems that can be used to advance chiroptical devices and methods.

Completely Chiral Optical Force for Enantioseparation

Fast and reliable separation of enantiomers of chiral nanoparticles requires elimination of all the forces that are independent of the nanoparticle handedness and creation of a sufficiently strong force that either pushes different enantiomers in opposite directions or delays the diffusion of one of them with respect to the other. Here we show how to construct such a completely chiral optical force using two counterpropagating circularly polarized plane waves of opposite helicities. We then explore capabilities of the related enantioseparation method by analytically solving the problem of the force-induced diffusion of chiral nanoparticles in a confined region, and reveal that it results in exponential spatial dependencies of the quantities measuring the purity of chiral substances. The proposed concept of a completely chiral optical force can potentially advance enantioseparation and enantiopurification techniques for all kinds of chiral nanoparticles that strongly interact with light.

Circular Dichroism of Electric-Field-Oriented CdSe/CdS Quantum Dots-in-Rods

Here we report anisotropy of intrinsic chiroptical response in CdSe/CdS quantum dot-in-rod systems. These nanostructures being oriented in an external electric field demonstrate dependence of circular dichroism signal on the orientation of the nanocrystals. The type of circular polarization in these nanostructures correlates with preferential direction of linear polarization, and the degree of circular polarization is the maximal for the first circular dichroism band corresponding to the absorption band edge. We also support our experimental data with a theoretical model. Using this model, we show a direct connection between theoretically derived morphological parameters of twisting in nanocrystals lattices and calculated from experimental data parameters of circular dichroism anisotropy.

Shape-induced optical activity of chiral nanocrystals

We present a general approach to analyzing the optical activity of semiconductor nanocrystals of chiral shapes. By using a coordinate transformation that turns a chiral nanocrystal into a nanocuboid, we calculate the rotatory strengths, dissymmetry factors, and peak values of the circular dichroism (CD) signal upon intraband transitions inside the nanocrystal. It is shown that the atomic roughness of the nanocrystal surface can result in rotatory strengths as high as 10^-36  erg×cm³ and in peak CD signals of about 0.1  cm^-1 for typical nanocrystal densities of 10¹⁶  cm^-3. The developed approach may prove useful for other nanocrystal shapes whereas the derived expressions apply directly for the modeling and interpretation of experimental CD spectra of quantum dots, nanorods, and nanoplatelets.