STM investigation of substitute effect on oligothiophene adlayer at Au(111) substrate

Molecular adlayers of a series of oligothiophenes with carboxylic groups and alkyl substitutes, DTDA, TTDA, QTDA, and PTDA, are investigated by STM at Au(111) surface. The effect of molecular structure including alkyl chain and thiophene backbone on the adlayer structure is revealed. DTDA and TTDA with two and three thiophene rings, self-assembled into highly ordered long-range two dimensional structures via hydrogen bondings, while QTDA with four thiophene rings formed short-range ordered structure with hexagonal symmetry. PTDA with five thiophene rings showed disordered pattern due to its strong molecule-substrate interaction. The results are compared with those on HOPG surface, showing that the molecule-substrate interaction plays an important role in the adlayer formation of these oligothiophenes.     

Losing the expression of molecular chirality in self-assembled physisorbed monolayers

STM imaging on graphite of the S-enantiomer of a chiral diacetylene isophthalic acid derivative reveals that molecular chirality is not expressed in the monolayer due to a specific molecular conformation preventing the stereogenic center to transfer its chiral information.     

Solvent Mediated Nanoscale Quasi-Periodic Chirality Reversal in Self-Assembled Molecular Networks Featuring Mirror Twin Boundaries

Grain boundaries in polycrystals have a prominent impact on the properties of a material, therefore stimulating the research on grain boundary engineering. Structure determination of grain boundaries of molecule-based polycrystals with submolecular resolution remains elusive. Reducing the complexity to monolayers has the potential to simplify grain boundary engineering and may offer real-space imaging with submolecular resolution using scanning tunneling microscopy (STM). Herein, the authors report the observation of quasi-periodic nanoscale chirality switching in self-assembled molecular networks, in combination with twinning, as revealed by STM at the liquid/solid interface. The width of the chiral domain structure peaks at 12–19 nm. Adjacent domains having opposite chirality are connected continuously through interdigitated alkoxy chains forming a 1D defect-free domain border, reflecting a mirror twin boundary. Solvent co-adsorption and the inherent conformational adaptability of the alkoxy chains turn out to be crucial factors in shaping grain boundaries. Moreover, the epitaxial interaction with the substrate plays a role in the nanoscale chirality reversal as well.     

Direct evidence of arsenic(III)-carbonate complexes obtained using electrochemical scanning tunneling microscopy

Electrochemical scanning tunneling microscopy (ECSTM), ion chromatography (IC), and electrospray ionization-mass spectrometry/mass spectrometry were applied to investigate the interactions between arsenite [As(III)] and carbonate and arsenate [As(V)] and carbonate. The chemical species in the single and binary component solutions of As(III), As(V), and carbonate were attached to a Au(111) surface and then imaged in a 0.1 M NaClO4 solution at the molecular level by ECSTM. The molecules formed highly ordered adlayers on the Au(111) surface. High-resolution STM images revealed the orientation and packing arrangement of the molecular adlayers. Matching the STM images with the molecular models constructed using the Hyperchem software package indicated that As(III) formed two types of complexes with carbonate, including As(OH)2CO3- and As(OH)3(HCO3-)2. No complexes were formed between As(V) and carbonate. IC chromatograms of the solutions revealed the emergence of the new peak only in the aged As(III)-carbonate solution. MS spectra showed the presence of a new peak at m/z 187 in the aged As(III)-carbonate solution. The results obtained with the three independent methods confirmed the formation of As(OH)2CO3-. The results also indicated that As(OH)3 could be associated with HCO3- through a hydrogen bond. The knowledge of the formation of the As(III) and carbonate complexes will improve the understanding of As(III) mobility in the environment and removal of As(III) in water treatment systems.     

手性材料手性参数的圆波导测量方法

本文提出了用普通微波圆波导测量系统测量了手性材料复手性参数的方法。该方法还能同时测量手性材料复介电常数和复磁导率。为了得到手性参数,首先需要测量短路和开路时的复反射系数,及电磁波通过手性材料后的偏转角和轴比。测量结果与自由空间法的测量结果一致。     

Chiral guanosine 5′-monophosphate-capped gold nanoflowers: Controllable synthesis, characterization, surface-enhanced Raman scattering activity, cellular imaging and photothermal therapy

Plasmonics and chirality in metal nanomaterials are intriguing and inspiring phenomena. Nanoscale chirality of metal nanomaterials has emerged as a hot topic in the past several years. Generally, most plasmon-induced circular dichroism (CD) responses of nanomaterials (> 10 nm) have been artificially created by modifying pre-made achiral nanomaterials with chiral agents, because the in situ generation of plasmon-induced CD responses of nanomaterials with larger size (> 10 nm) is not easy. Herein, we report a simple one-pot green synthesis of chiral gold nanoflowers (GNFs) with abundant petal-shaped tips in the chiral reduction environment arising from the presence of chiral guanosine 5??-monophosphate (5??-GMP) and the chiral reducing agent L-ascorbic acid (L-AA). Different reducing agents can impact the shape and chirality of the products. In addition, the size and chirality of the GNFs can be controlled by adjusting the reaction time. The as-synthesized GNFs have good biocompatibility and can be used for surface-enhanced Raman scattering (SERS) enhancement, cellular dark-field imaging and photothermal therapy.      

Enantiomeric interactions between nucleic acid bases and amino acids on solid surfaces

Molecular interaction between nucleic acid bases and amino acids is a fundamental process in biology. The adsorption of the molecules on surfaces provides the opportunity to study such interactions in great detail by exploiting the high-resolution imaging capabilities of scanning tunnelling microscopy (STM). The chemisorption of prochiral molecules, such as adenine, on a metal surface causes the adsorbed species to become chiral. Subsequent interactions with inherently chiral molecules may then lead to the formation of diastereoisomers, if the enantiomeric interaction process is sufficiently strong. In the case of adenine adsorption on Cu[110], the chiral adsorbates form homochiral chains. Here, we show that the adenine chain direction is fully correlated with the chirality, and that the alpha-amino acid, phenylglycine, shows a strong chiral preference in its interaction with these chains. STM images clearly demonstrate that S-phenylglycine (R-phenylglycine) binds only to chains rotated 19.5 degrees (anti-) clockwise from the [001] direction. Closer examination reveals that the enantiomeric interaction involves double rows of phenylglycine molecules and the adenine chains. This is the first observation at the molecular level of diastereoisomeric interaction, and demonstrates that STM is a powerful method for studying the details of these interactions.     

Chirality dependence of carbon single-walled nanotube material properties: axial Young's modulus

The potential use of individual carbon nanotubes as nano devices warrants detailed investigation of their mechanical behavior based on structural and geometrical configurations. The objective of this paper is to unravel the structural and chirality dependence of the axial Young's modulus of a carbon single-walled nanotube by analytical and numerical approaches. In this work, we employ the general homogenization composite shell model developed based on the asymptotic homogenization technique for analytical modeling of single-walled nanotubes. We derive the working formulae for the effective elastic properties of carbon single-walled nanotube of any chirality and predict the structural and chiral dependence of the effective axial Young's modulus of the nanotube. Also, a finite element analysis on the chirality dependence of the axial Young's modulus of the carbon nanotube is reported. The outcomes of our analyses are compared with available experimental and simulation results.     

Formation of chiral branched nanowires by the Eshelby Twist

Manipulating the morphology of inorganic nanostructures, such as their chirality and branching structure, has been actively pursued as a means of controlling their electrical, optical and mechanical properties. Notable examples of chiral inorganic nanostructures include carbon nanotubes, gold multishell nanowires, mesoporous nanowires and helical nanowires. Branched nanostructures have also been studied and been shown to have interesting properties for energy harvesting and nanoelectronics. Combining both chiral and branching motifs into nanostructures might provide new materials properties. Here we show a chiral branched PbSe nanowire structure, which is formed by a vapour-liquid-solid branching from a central nanowire with an axial screw dislocation. The chirality is caused by the elastic strain of the axial screw dislocation, which produces a corresponding Eshelby Twist in the nanowires. In addition to opening up new opportunities for tailoring the properties of nanomaterials, these chiral branched nanowires also provide a direct visualization of the Eshelby Twist.     

Chiral pinwheel clusters lacking local point chirality

The supramolecular pinwheel cluster is a unique chiral structure with evident handedness. Previous studies reveal that the chiral pinwheels are composed of chiral or achiral molecules with polar groups, which result in strong intermolecular interactions such as hydrogen-bonding or dipole interactions. Herein, it is shown that the simple linear aromatic molecule, pentacene, can be self-assembled into large chiral pinwheel clusters on the semimetal Bi(111) surface, due to enhanced intermolecular interactions. The pentacene pinwheels reveal two levels of organizational chirality: the chiral hexamers resulting from asymmetric shifting along the long molecular axis, and chiral arrangement of six hexamers with a rotor motif. Furthermore, a new relation between the local point chirality and organizational chirality is identified from the pinwheels: the former is not essential for the latter in 2D pinwheel clusters of the pentacene molecule.     

Chiral nihility effects on energy flow in chiral materials

The propagation of electromagnetic plane waves in an isotropic chiral medium is characterized, and a special interest is shown in chiral nihility and the effects of chirality on energy transmission. In particular, the wave impedance is matched to that of free space. Moreover, the refractive index n is also matched in impedance to that of free space when an appropriate value of the chirality is chosen. A "chiral nihility" medium is explored in which both the permittivity and the permeability tend to zero. Some specific case studies of chiral nihility are presented, and Brewster angles are found to cover an extremely wide range. The E-field distributions in these different cases where the chiral slab is placed in free space are analyzed by using the appropriate constitutive relations. It is shown from numerical calculations that one can obtain some critical characteristics of the effects of chirality on energy transmission and reflection, such as transparency and power tunneling.     

Observation of multiple vibrational modes in ultrahigh vacuum tip-enhanced Raman spectroscopy combined with molecular-resolution scanning tunneling microscopy

Multiple vibrational modes have been observed for copper phthalocyanine (CuPc) adlayers on Ag(111) using ultrahigh vacuum (UHV) tip-enhanced Raman spectroscopy (TERS). Several important new experimental features are introduced in this work that significantly advance the state-of-the-art in UHV-TERS. These include (1) concurrent sub-nm molecular resolution STM imaging using Ag tips with laser illumination of the tip-sample junction, (2) laser focusing and Raman collection optics that are external to the UHV-STM that has two cryoshrouds for future low temperature experiments, and (3) all sample preparation steps are carried out in UHV to minimize contamination and maximize spatial resolution. Using this apparatus we have been able to demonstrate a TERS enhancement factor of 7.1 × 10(5). Further, density-functional theory calculations have been carried out that allow quantitative identification of eight different vibrational modes in the TER spectra. The combination of molecular-resolution UHV-STM imaging with the detailed chemical information content of UHV-TERS allows the interactions between large polyatomic molecular adsorbates and specific binding sites on solid surfaces to be probed with unprecedented spatial and spectroscopic resolution.     

Chiral expressions at metal surfaces

The manifestation of chirality at surfaces has attracted much attention in recent years. In this review, some of main features of chiral endowments by complex organic molecules at defined metal surfaces are reviewed. Detailed surface spectroscopic data have enabled a hierarchy of chiral expressions to be delineated, from point group chirality expressed by local chiral motifs, to space group chirality in which these motifs act as building blocks which self-assemble into organised chiral structures, to deeper propagation of chirality into the metal leading to chiral reconstructions. Chiral endowments by both chiral and achiral molecules is discussed alongside the implications for progressing chirality from the local to the global level.     

Site-Selective Chiral Growth of Anisotropic Au Triangular Nanoplates for Tuning the Optical Chirality

Site-selective chiral growth of anisotropic nanoparticles is of great importance to realize the plasmonic nanostructures with delicate geometry and desired optical chirality; however, it remains largely unexplored. This work demonstrates a controlled site-selective chiral growth system based on the seed-mediated growth of anisotropic Au triangular nanoplates. The site-selective chiral growth involves two distinct underlying pathways, faceted growth and island growth, which are interswitchable upon maneuvering the interplay of chiral molecules, surfactants, and reducing agents. The pathway switch governs the geometric and chirality evolution of Au triangular nanoplates, giving rise to tailorable circular dichroism spectra. The ability to tune the optical chirality in a controlled manner by manipulating the site-selective chiral growth pathway opens up a promising strategy for exploiting chiral metamaterials with increasing architectural complexity in chiroptical applications.     

Cathodic electropaint insulated tips for electrochemical scanning tunneling microscopy

A method of preparing tungsten tips insulated for in situ scanning tunneling microscopy (STM) work is presented. Tips were electrocoated at low applied voltages using an organic solution of the cathodic electropaint rather than the more frequently utilized electropaint emulsions. The insulated tips were then characterized using cyclic voltammetry and electrochemical STM (EC-STM). They displayed low Faradaic leakage currents under electrochemical conditions and provided for high-resolution STM imaging in electrolyte solution. Flat terraces on a monocrystalline Au surface and adlayers of 5,10,15,20-tetraphenyl-21H,23H-porphine cobalt(II) were imaged under potential control in dilute HClO4 using the fabricated tips, and atomic or molecular resolution images were obtained in both cases. The developed procedure allows for the fast, reproducible generation of large numbers of electrically insulated tungsten tips suitable for EC-STM imaging experiments.     

Empirical procedure that uses molecular structure to predict enantioselectivity of chiral stationary phases.

A total of 121 racemic compounds were separated in the normal-phase mode on a (S)-(1-naphthylethyl)carbamoylated beta-cyclodextrin (S-NEC-beta-CD) bonded phase and 74 on the R equivalent (R-NEC) chiral stationary phase (CSP). All compounds are of the type that have four substituents on a stereogenic center, rather than an "axis of chirality". It is shown that the binary solvent pair used as the mobile phase has a significant influence on chiral recognition. However, the proportions of the components of a specific pair have little effect. From the results, the individual contributions to chiral recognition by these CSPs were estimated for 81 different substituents of the stereogenic center. Varying the arrangement of these 81 substituents could produce over 1.6 million compounds. Hydrogen was chosen as the reference substituent and was assigned a 0 cal/mol free energy. The chiral recognition increased when sp2-hybridized carbons were connected to the stereogenic center. Conversely, sp3-hybridized carbons decreased the enantioselectivity. Amido groups increased the chiral recognition, especially when associated with pi-acid (3,5-dinitrobenzoyl) or pi-basic (naphthyl) groups. This approach does not allow one to know which enantiomer elutes first. However, the "substituent energy" list for chiral compounds can be used to obtain an estimated value for the enantioselectivity of a compound by adding the energy contributions of the four substituents connected to the stereogenic center. In this way one can predict a priori whether or not a compound will separate on a CSP and estimate its separation factor (alpha). Theoretically, this approach can be used for most CSPs, provided a sufficient data base is generated on them.     

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.     

Technique for the Evaluation of Chirality, Permittivity, and Permeability of a Reciprocal Chiral Slab Through the Utilization of the Time-Harmonic Green Functions

In this paper, a technique is developed to compute values of the chirality, permittivity, and permeability of a dielectric reciprocal chiral slab from the knowledge of limited laboratory measurement information. The chirality parameter is a function of frequency and depth of the slab. The developed technique is based on the use of coupled differential equations for the Green functions associated with electromagnetic propagation through the slab. The technique makes use of the $N$th degree interpolation polynomials to approximate the Green functions and the chiral parameter among other functions using Legendre–Gauss–Lobatto collocation points. Numerical results from a laboratory measurement simulation are given to demonstrate the validity of the proposed technique.      

Using optical vortex to control the chirality of twisted metal nanostructures

We discovered for the first time that light can twist metal to control the chirality of metal nanostructures (hereafter, chiral metal nanoneedles). The helicity of optical vortices is transferred to the constituent elements of the irradiated material (mostly melted material), resulting in the formation of chiral metal nanoneedles. The chirality of these nanoneedles could be controlled by just changing the sign of the helicity of the optical vortex. The tip curvature of these chiral nanoneedles was measured to be <40 nm, which is less than 1/25th of the laser wavelength (1064 nm). Such chiral metal nanoneedles will enable us to selectively distinguish the chirality and optical activity of molecules and chemical composites on a nanoscale and they will provide chiral selectivity for nanoscale imaging systems (e.g., atomic force microscopes), chemical reactions on plasmonic nanostructures, and planar metamaterials.     

Formation of chiral surface with enantiomeric tartaric acid on gemini-structured self-assembled monolayers

Chiral surfaces were prepared by L, D, and Meso-tartaric acids (TAs) adsorbed on gemini-structured self-assembled monolayers (SAMs) composed of ethylenebis [(12-mercaptododecyl) dimethyl ammonium bromide] (HS-gQA-SH). The formation and structure of the chiral surfaces were characterized by surface plasmon resonance spectroscopy (SPR) and Fourier transform infrared-reflection adsorption spectroscopy (FTIR-RAS). The thickness of enantiomeric TA layers on the HS-gQA-SH SAM was estimated to be c.a. 5-6 angstroms regardless of their chirality, in good agreement with the height of TA molecules anchoring on the surface with two COOH groups. All the TAs on the HS-gQA-SH SAM exhibit the same ionization state independent of their chirality in their vibration bands of carboxylic groups. We attempted a second-layer adsorption of the enantiomeric TAs on L-TA monomolecular layer (L-TA SAM) precomposed on the HS-gQA-SH. A strong affinity between first and second TA layers resulted in the film growth when their chirality is identical (i.e., L-TA on L-TA SAM). We found the structure of second L-TA layer was completely different from that of the first layer, where a crystalline-like L-TA phase was found as a predominant component. Our results imply a preferential crystalline growth of chiral molecules on the same chiral surface, which may lead to a work for optical resolution into two enantiomers at a solid-liquid interface.