Chirality Distribution Research Articles

Effect of interfacial Dzyaloshinskii-Moriya interaction on polarized neutrons reflection

The antisymmetric Dzyaloshinskii-Moriya interaction (DMI) in noncentrosymmetric systems leads to various nonuniform chiral magnetic textures. Polarized neutron scattering is a powerful method for investigation such chiral distributions. The most used technique is a small-angle neutrons scattering (SANS). Multilayered magnetic films with the interface induced DMI (iDMI) can’t be investigated by SANS because of their small volume. The appropriate technique is a polarized neutron reflectometry. Within the framework of the continuum theory of micromagnetics, we explore the impact of the iDMI on the polarized neutrons reflection from multilayers with random magnetic anisotropy. It is shown that the iDMI gives rise to a polarization-dependent asymmetric term in the reflection.

Helicoidal particles and swimmers in a flow at low Reynolds number

In this paper, we consider the dynamics of a helicoidal object, which can be either a passive particle or an active swimmer, with an arbitrary shape in a linear background flow at low Reynolds number, and derive a generalized version of the Jeffery equations for the angular dynamics of the object, including a new constant from the chirality of the object as well as the Bretherton constant. The new constant appears from the inhomogeneous chirality distribution along the axis of the helicoidal symmetry, whereas the overall chirality of the object contributes to the drift velocity. Further investigations are made for an object in a simple shear flow, and it is found that the chirality effects generate non-closed trajectories of the director vector which will be stably directed parallel or anti-parallel to the background vorticity vector depending on the sign of the chirality. A bacterial swimmer is considered as an example of a helicoidal object, and we calculate the values of the constants in the generalized Jeffery equations for a typical morphology of Escherichia coli. Our results provide useful expressions for the studies of microparticles and biological fluids, and emphasize the significance of the symmetry of an object on its motion at low Reynolds number.

Photoinduced Chirality Switching of Metal-Inorganic Plasmonic Nanostructures.

Chiral plasmonic nanodevices whose handedness can be switched reversibly between right and left by external stimulation have attracted much attention. However, they require delicate DNA nanostructures and/or continuous external stimulation. In this study, those issues are addressed by using metal-inorganic nanostructures and photoinduced reversible redox reactions at the nanostructures, namely, site-selective oxidation due to plasmon-induced charge separation under circularly polarized visible light (CPL) and reduction by UV-induced TiO2 photocatalysis. We irradiate gold nanorods (AuNRs) supported on TiO2 with right- or left-CPL to generate electric fields with chiral distribution around each AuNR and to deposit PbO2 at the sites where the electric fields are localized, for fixing the chirality to the AuNR. The nanostructures thus prepared exhibit circular dichroism (CD) based on longitudinal and transverse plasmon modes of the AuNRs. Their chirality given by right-CPL (or left-CPL) is locked until PbO2 is rereduced under UV light. After unlocking by UV, the chirality can be switched by left-CPL (or right-CPL) irradiation, resulting in reversed CD signals and locking the switch again. The handedness of the chiral plasmonic nanodevice can be switched reversibly and repeatedly.

Highly enantioselective photo-polymerization enhanced by chiral nanoparticles and in situ photopatterning of chirality

Chiral noble metal nanoparticles has recently gained great interest due to their potential applications including ultrasensitive chiral recognition and asymmetric synthesis. We anticipate that they could be utilized to induce asymmetric photo-polymerization reactions with high enantioselectivity and reactivity. Here, we report such a system. By employing silver nanoparticles modified with cysteine as the chiral inducer, polydiacetylene (PDA) with high chiral asymmetry was obtained from achiral diacetylene monomers triggered with unpolarized UV light. Furthermore, the helical sense of chirality can be controlled by varying the wavelength of UV irradiation. This enables a feasible and economical method to fabricate programmable 2D patterns of chiral PDA with tailored chirality distributions, such as smooth gradients in chirality and micropatterns with tailorable circularly polarized luminescence. Our finding not only opens a pathway for producing programmable chiroptical micropatterns, but also is highly valuable for deeper understanding of symmetry breaking in enantioselective photochemical reactions.

Improved Laser‐Based Photoluminescence on Single‐Walled Carbon Nanotubes

Photoluminescence (PL) is a common tool to characterize various properties of single‐walled carbon nanotube (SWCNT) chirality distribution and the level of tube individualization in SWCNT samples. Most PL studies use conventional lamp light sources whose spectral distribution is filtered with a monochromator, but this results in a still impure spectrum with a low spectral intensity. Tunable dye lasers offer a tunable light source which cover the desired excitation wavelength range with a high spectral intensity, but their operation is often cumbersome. Herein, the design and properties of an improved dye‐laser system which is based on a Q‐switch pump laser are presented. The high peak power of the pump provides an essentially threshold‐free lasing of the dye laser, which substantially improves the operability. It allows operation with laser dyes, such as Rhodamin 110 and Pyridin 1, which are otherwise on the border of operation of the laser. The system allows to cover the 540–730 nm wavelength range with four dyes. In addition, the dye laser output pulses closely follow the properties of the pump, which directly provides a time‐resolved and tunable laser source. The performance of the system by measuring the PL map of a HiPco SWCNTs sample is demonstrated.

Chiral asymmetry detected in a 2D array of permalloy square nanomagnets using circularly polarized x-ray resonant magnetic scattering

The sensitivity of circularly polarized x-ray resonant magnetic scattering (CXRMS) to chiral asymmetry has been demonstrated. The study was performed on a 2D array of Permalloy (Py) square nanomagnets of 700 nm lateral size arranged in a chess pattern, in a square lattice of 1000 nm lattice parameter. Previous x-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM) images on this sample showed the formation of vortices at remanence and a preference in their chiral state. The magnetic hysteresis loops of the array along the diagonal axis of the squares indicate a non-negligible and anisotropic interaction between vortices. The intensity of the magnetic scattering using circularly polarized light along one of the diagonal axes of the square magnets becomes asymmetric in intensity in the direction transversal to the incident plane at fields where the vortex states are formed. The asymmetry sign is inverted when the direction of the applied magnetic field is inverted. The result is the expected in the presence of an unbalanced chiral distribution. The effect is observed by CXRMS due to the interference between the charge scattering and the magnetic scattering.

VQS (Vapor-Quasiliquid-Solid, Vapor-Quasisolid-Solid) Mechanism for Realizing Narrow Distributions of Chirality and Diameters of Single-Walled Carbon Nanotubes (SWCNTs).

Diameter-selective and chiral-selective single-walled carbon nanotubes (SWCNTs), all on a single wafer, have vast technological promise. An investigation has been carried out to study the role of the physicochemical state of the nanoparticle surface, and of the growth parameters and growth mechanism, on the relatively-low-temperature control of SWCNT diameter and chirality. For this, amorphicity, porosity, coarsening, high energy states, etc. resulting from controlled surface disturbance, disorder and/or restructuring of nanoparticle have been considered. Possible relationship between growth rate, chiral angle, diameter, and monodispersity of SWCNTs has been explored. The influence of the nanoparticle's peripheral hill on the diameter selectivity has been examined. Surface segregation and formation of shell have been described. The impact of charge redistribution, creation of contour of charges, and dipole moment in shell in creating chiral-selective SWCNTs has been discussed. The diffusion of the carbon species through the nanopores of the shell via Knudsen dynamics has been elucidated. The role of growth parameters in the growth process has been studied. The fundamental understanding of chiral selectivity and diameter selectivity of SWCNTs has been tried. Possible strategies for realizing narrow diameter and chirality distributions have been presented. The optimized chemical state of nanoparticle surface and the degree of ease and spontaneity, with which SWCNTs are grown on this surface, have been found to govern the chiral-selectivity, diameter-selectivity and high growth rates of SWCNTs. Good agreements of the calculated results with available experiments attest to the validity of the present study.

Improvement in growth yield of single-walled carbon nanotubes with narrow chirality distribution by pulse plasma CVD

A pulse plasma chemical vapor deposition (CVD) technique was developed for improving the growth yield of single-walled carbon nanotubes (SWNTs) with a narrow chirality distribution. The growth yield of the SWNTs could be improved by repetitive short duration pulse plasma CVD, while maintaining the initial narrow chirality distribution. Detailed growth dynamics is discussed based on a systematic investigation by changing the pulse parameters. The growth of SWNTs with a narrow chirality distribution could be controlled by the difference in the nucleation time required using catalysts comprising relatively small or large particles as the key factor. The nucleation can be controlled by adjusting the pulse on/off time ratio and the total processing time.

A robust CoxMg1-xO catalyst for predominantly growing (6, 5) single-walled carbon nanotubes

Chirality-controlled growth of single-walled carbon nanotubes (SWCNTs) by chemical vapor deposition (CVD) is one of the most challenging tasks in carbon nanotube synthesis field. During CVD growth, the catalyst plays crucial roles in governing SWCNT nucleation thermodynamics as well as growth kinetics. However, the performances of catalyst are generally sensitive to the metal loading amount in the catalyst and the reaction conditions, like the partial pressure of carbon source and the reaction time. In this work, we have systematically investigated a robust CoxMg1-xO solid solution, which can predominantly yield (6, 5) SWCNTs in a wide range of Co concentration, with a diversity of CO concentrations or a broad-ranging reaction time. Besides, the effect of reaction temperature on SWCNT chirality distribution is demonstrated, the mechanism of which is clarified with the assistance of environmental transmission electron microscopy. Finally, the chirality distribution of SWCNTs grown using CH4 as the carbon source is presented. The effects of carbon sources are discussed in view of SWCNT growth mode.

Roles of sulfur in floating-catalyst CVD growth of single-walled carbon nanotubes for transparent conductive film applications

Sulfur is an effective promoter for growing single-walled carbon nanotubes (SWCNTs) and tuning their structural properties. In particular, sulfur has been utilized in the floating-catalyst chemical vapor deposition (FC-CVD) process to fabricate SWCNT-based transparent conductive films (TCFs). However, in-situ catalyst nucleation process in conventional FC-CVD, hinders to correlate the substantial role of sulfur in tuning SWCNTs synthesis and enhancing the performance of SWCNT-based TCFs. Herein, we have for the first time, systematically studied the roles of sulfur on yield, morphology, and structure of SWCNTs grown by FC-CVD, using ex-situ Fe and Co catalyst particles. We found that SWCNT yield is largely dependent on the amount of sulfur introduced into the FC-CVD reactor and on catalyst composition. More importantly, the addition of an optimized amount of sulfur resulted in a three-fold enhancement of the opto-electronic performance of SWCNT-TCFs, by increasing diameter and bundle length along with improving the quality of SWCNTs. Surprisingly, electron diffraction analysis revealed that SWCNTs grown from both Fe and Co display wide chirality distributions spanning from zig-zag to armchair edges, indicating that the sulfur promoter has little influence on the chirality modulating of SWCNTs.

Growth of single-walled carbon nanotubes by alcohol chemical vapor deposition with water vapor addition: Narrowing the diameter and chiral angle distributions

We report the structure-controlled growth of single-walled carbon nanotubes (SWCNTs) based on alcohol chemical vapor deposition with the addition of water vapor. The introduction of water vapor during the growth and pretreatment by water vapor was both systematically examined for the growth of SWCNTs. Raman spectroscopy analysis revealed that the addition of water vapor in both cases induced a decrease in the tube diameters, which indicates the importance of catalyst conditioning for the changes in chirality distribution. The selective growth of SWCNTs having small diameters and large chiral angles was achieved by water vapor pretreatment.

Two-dimensional liquid crystal polarization grating via linearly polarized light modified multi-beam polarization interferometry.

Holographic lithography is widely used as an effective approach for two-dimensional (2D) photonic crystal fabrication. However, for the fabrication of 2D polarization structures based on photoaligned liquid crystals (LCs), holographic lithography method is limited. The fabrication requires full coverage of light intensity, 2D chiral distribution and continuously varying polarization direction, which could be hardly guaranteed by multi-beam interference of circularly polarized light (CPL). Herein, we introduce a linearly polarized light (LPL) into a three-CPL interference configuration to improve the interference field and fulfill the critical requirement. The introduced LPL intensity is chosen to be 1/5 of the CPL to guarantee both full coverage of light intensity and well photoalignment defined LC directors. Moreover, the introduction of the weak LPL into multiple CPL interference is shown to give little disturbance to the desired diffraction properties.

Molecular characterization of macroscopic aerogels of single-walled carbon nanotubes

Single-walled carbon nanotubes (SWCNT) can be assembled into various macroscopic architectures, most notably continuous fibers and films, produced currently on a kilometer-per-day scale by floating catalyst chemical vapor deposition and spinning from an aerogel of CNTs. An attractive challenge is to produce continuous fibers with controlled molecular structure with respect to the diameter, chiral angle and ultimately (n,m) indices of the constituent SWCNT “molecules”. This work presents an extensive Raman spectroscopy and high-resolution transmission electron microscopy study of SWCNT aerogels produced by the direct spinning method. By retaining the open structure of the SWCNT aerogel, we reveal the presence of both semiconducting and metallic SWCNTs and determine a full distribution of families of SWCNT grouped by optical transitions. The resulting distribution matches the chiral angle distribution obtained by electron microscopy and electron diffraction. The effect of SWCNT bundling on the Raman spectra, such as the G− line shape due to plasmons activated in the far-infrared and semiconductor quenching, are also discussed. By avoiding full aggregation of the aerogel and applying the methodology introduced, rapid screening of molecular features can be achieved in large samples, making this protocol a useful analysis tool for engineered SWCNT fibers and related systems.

Systematic investigation of the catalyst composition effects on single-walled carbon nanotubes synthesis in floating-catalyst CVD

Enormous technological promise of single-walled carbon nanotubes (SWCNTs) can only be harnessed with the premise of controllable synthesis of SWCNTs, where catalyst composition thermodynamically plays a vital role. Herein, we have systematically investigated the effects of catalyst composition on SWCNTs synthesis, using a novel floating-catalyst chemical vapor deposition (FCCVD) system, consisting of catalyst synthesis via spark generator, FCCVD reactor and a real-time monitor of catalysts and SWCNTs. We synthesized SWCNTs from both monometallic (Fe, Co, Ni), bimetallic (Co-Ni, Co-Fe) catalyst particles and kinetically optimized yield and performance of SWCNTs films. We found that the highest yield of SWCNTs from Fe is 15 times as that of Ni SWCNTs and the Co-Ni SWCNTs film possesses best opto-electronic performance. Interestingly, the mean diameter of SWCNTs was found related to the catalyst particle size distributions, but not composition. Moreover, detailed atomic structures determination revealed that SWCNTs from Fe and Co have a wide chirality distribution spanning from zig-zag to armchair edges. However, Co-Ni SWCNTs have comparatively narrower chirality distribution having 71% SWCNTs in the chiral angle of 15–30°. Our results indicate that catalyst composition can efficiently tune yield and characteristics of SWCNTs, but it does not dramatically shift chirality of FCCVD SWCNTs.

Imaging of Plasmonic Chiral Radiative Local Density of States with Cathodoluminescence Nanoscopy.

Chiral light-matter interactions as an emerging aspect of quantum optics enable exceptional physical phenomena and advanced applications in nanophotonics through the nanoscale exploitation of photon-emitter interactions. The chiral radiative properties of quantum emitters strongly depend on the photonic environment, which can be drastically altered by plasmonic nanostructures with a high local density of states (LDOS). Hence, precise knowledge of the chiral photonic environment is essential for manipulating the chirality of light-matter interactions, which requires high resolution chiral characterization techniques. In this work, chiral radiative LDOS distributions of single plasmonic nanostructures that directly govern the chiral radiative spontaneous decay of quantum emitters are imaged at the nanoscale by using cathodoluminescence nanoscopy, enabling precise and highly efficient control of chiral photon emission for chiroptical technologies. Radiative LDOS hot-spots with the chirality larger than 93% are obtained by properly designing chiral plasmonic modes of Au nanoantennas. After fabricating monolayered WSe2 nanodisks (NDs) at chiral radiative LDOS hot-spots and forming ND/Au hybrid nanostructures, the chiral radiative properties of WSe2 NDs are significantly modified, leading to chiral photoluminescence. Our experimental concept and method provide an effective way to characterize and manipulate chiral light-matter interactions at the nanoscale, facilitating future applications in chiral quantum nanophotonics such as single-photon sources and light emission devices.

Chiral-selective etching effects on carbon nanotube growth at edge carbon atoms.

Chemical vapor deposition (CVD) utilizing metal cluster nanoparticle catalysts is commonly used to synthesize carbon nanotubes (CNT), with oxygen-containing species such as water or alcohol included in the feedstock for enhanced yield. However, the etching effect of these additives on the growth mechanism has rarely been investigated, despite evidence suggesting that etching potentially affects the chirality distribution of product CNTs. We used quantum chemical methods to study how water-based etchant radicals (OH and H) may enhance the chiral selectivity during CVD growth using CNT cap models. Chemical reactivities of the caps with the etchant radicals were evaluated using density functional theory (DFT). It was found that the reactivities on the cap edges correlate with the chirality of the caps. These results suggest that proper selection of etchant species can provide opportunities for selective chirality control of the product CNTs. © 2018 Wiley Periodicals, Inc.

Sign inversion in the terahertz photoconductivity of single-walled carbon nanotube films

In recent years, there have been conflicting reports regarding the ultrafast photoconductive response of films of single walled carbon nanotubes (CNTs), which apparently exhibit photoconductivities that can differ even in sign. Here, we observe explicitly that the THz photoconductivity of CNT films is a highly variable quantity which correlates with the length of the CNTs, while the chirality distribution has little influence. Moreover, by comparing the photo-induced change in THz conductivity with heat-induced changes, we show that both occur primarily due to heat-generated modification of the Drude electron relaxation rate, resulting in a broadening of the plasmonic resonance present in finite-length metallic and doped semiconducting CNTs. This clarifies the nature of the photo-response of CNT films and demonstrates the need to carefully consider the geometry of the CNTs, specifically the length, when considering them for application in optoelectronic devices.

Chiral Ramachandran Plots II: General Trends and Protein Chirality Spectra.

The degree of chirality of protein backbone residues is used to enrich the Ramachandran plot (RP) and create three-dimensional chiral RPs with much more structural information. Detailed comparative analysis of the four classical RPs (general, glycine, proline, and pre-proline) is provided, including statistical analysis of quantitative chirality distributions in the maps and in the secondary structures. Our results show that points with outlier chirality levels represent special transitional points in the folded protein such as α-helix kinks, twists of β-strands, and transition points between secondary structures. A protein chirality spectrum in which the degree of chirality of each residue is plotted against the sequence number explores these special points. More than 65000 residues extracted from 200 high-quality proteins are used for this study, which shows that quantitative chirality is a general and useful structural parameter for protein conformational studies.

Entropy-driven stability of chiral single-walled carbon nanotubes.

Single-walled carbon nanotubes are hollow cylinders that can grow centimeters long via carbon incorporation at the interface with a catalyst. They display semiconducting or metallic characteristics, depending on their helicity, which is determined during their growth. To support the quest for a selective synthesis, we develop a thermodynamic model that relates the tube-catalyst interfacial energies, temperature, and the resulting tube chirality. We show that nanotubes can grow chiral because of the configurational entropy of their nanometer-sized edge, thus explaining experimentally observed temperature evolutions of chiral distributions. Taking the chemical nature of the catalyst into account through interfacial energies, we derive structural maps and phase diagrams that will guide a rational choice of a catalyst and growth parameters toward a better selectivity.

Designing Catalysts for Chirality-Selective Synthesis of Single-Walled Carbon Nanotubes: Past Success and Future Opportunity.

A major obstacle for the applications of single-walled carbon nanotubes (SWNTs) in electronic devices is their structural diversity, ending in SWNTs with diverse electrical properties. Catalytic chemical vapor deposition has shown great promise in directly synthesizing high-quality SWNTs with a high selectivity to specific chirality (n, m). During the growth process, the tube-catalyst interface plays crucial roles in regulating the SWNT nucleation thermodynamics and growth kinetics, ultimately governing the SWNT chirality distribution. Starting with the introduction of SWNT growth modes, this review seeks to extend the knowledge about chirality-selective synthesis by clarifying the energetically favored SWNT cap nucleation and the threshold step for SWNT growth, which describes how the tube-catalyst interface affects both the nucleus energy and the new carbon atom incorporation. Such understandings are subsequently applied to interpret the (n, m) specific growth achieved on a variety of templates, such as SWNT segments or predefined molecular seeds, transition metal (Fe, Co and Ni)-containing catalysts at low reaction temperatures, W-based alloy catalysts, and metal carbides at relatively high reaction temperatures. The up to date achievements on chirality-controlled synthesis of SWNTs is summarized and the remaining major challenges existing in the SWNT synthesis field are discussed.