Conducting polymer electrochemical switching as an easy means for designing active plasmonic devices

Due to the continuously increasing demand for ultimate miniaturization of electronic and photonic systems, molecular electronics and plasmonic devices are currently booming as alternative technologies because of their very promising potential in writing, reading, storing, and processing information at the nanoscale. Conducting polymers or oligomers have been proposed and used as basic building blocks in molecular and plastic electronics since the end of the 80s. Plasmonics is, on the other hand, an emerging branch of photonics which uses nanostructured materials that support surface plasmons. Among plasmonic devices, active plasmonic devices are still lacking. In this work, we report on new active molecular plasmonic devices in which the electrochemical switching of a nanometric film of conductive polymer between its reduced and oxidized state is used in order to control, switch, and modulate localized surface plasmon (LSP) resonance of gold nanoparticle arrays.     

Recent progress in probing atomic and molecular quantum coherence with scanning tunneling microscopy

Quantum coherent physics and chemistry concern the creation and manipulation of an excited-state manifold that contains the superposition and entanglement of multiple quantum levels. Electromagnetic waves such as light and microwave can be used to generate and probe different quantum coherent phenomena. The recent advances in scanning tunneling microscopy (STM) techniques including ultrafast laser coupled STM and electron spin resonance STM combine electromagnetic excitation with tunneling electron detection, bringing the investigation of quantum coherence down to the atomic and molecular level. Here, we survey the latest STM studies of different quantum coherent phenomena covering molecular vibration, electron transfer, surface plasmon resonance, phonon, spin oscillation, and electronic transition, and discuss the state and promise of characterizing and manipulating quantum coherence at the atomic or molecular scale.     

STM studies of photochemistry and plasmon chemistry on metal surfaces

We review our recent studies of photochemistry and plasmon chemistry of dimethyl disulfide, (CH₃S)₂, molecules adsorbed on metal surfaces using a scanning tunneling microscope (STM). The STM has been used not only for the observation of surface structures at atomic spatial resolution but also for local spectroscopies. The STM combined with optical excitation by light can be employed to investigate chemical reactions of single molecules induced by photons and localized surface plasmons. This technique allows us to gain insights into reaction mechanisms at a single molecule level. The experimental procedures to examine the chemical reactions using the STM are briefly described. The mechanism for the photodissociation reaction of (CH₃S)₂ molecules adsorbed on metal surfaces is discussed based on both the experimental results obtained with the STM and the electronic structures calculated by density functional theory. The dissociation reaction of the (CH₃S)₂ molecule induced by the optically excited plasmon in the STM junction between a Ag tip and metal substrate is also described. The reaction mechanism and pathway of this plasmon-induced chemical reaction are discussed by comparison with those proposed in plasmon chemistry.     

Properties of large organic molecules on metal surfaces

The adsorption of large organic molecules on surfaces has recently been the subject of intensive investigation, both because of the molecules’ intrinsic physical and chemical properties, and for prospective applications in the emerging field of nanotechnology. Certain complex molecules are considered good candidates as basic building blocks for molecular electronics and nanomechanical devices. In general, molecular ordering on a surface is controlled by a delicate balance between intermolecular forces and molecule–substrate interactions. Under certain conditions, these interactions can be controlled to some extent, and sometimes even tuned by the appropriate choice of substrate material and symmetry. Several studies have indicated that, upon molecular adsorption, surfaces do not always behave as static templates, but may rearrange dramatically to accommodate different molecular species. In this context, it has been demonstrated that the scanning tunnelling microscope (STM) is a very powerful tool for exploring the atomic-scale realm of surfaces, and for investigating adsorbate–surface interactions. By means of high-resolution, fast-scanning STM unprecedented new insight was recently achieved into a number of fundamental processes related to the interaction of largish molecules with surfaces such as molecular diffusion, bonding of adsorbates on surfaces, and molecular self-assembly. In addition to the normal imaging mode, the STM tip can also be employed to manipulate single atoms and molecules in a bottom–up fashion, collectively or one at a time. In this way, molecule-induced surface restructuring processes can be revealed directly and nanostructures can be engineered with atomic precision to study surface quantum phenomena of fundamental interest. Here we will present a short review of some recent results, several of which were obtained by our group, in which several features of the complex interaction between large organic molecules and metal surfaces were revealed. The focus is on experiments performed using STM and other complementary surface-sensitive techniques.     

Role of Nanocavity Plasmons in Tunneling Electron Induced Light Emission on and near a Molecule

By using a microscopic quantum model, we study theoretically different roles of nanocavity plasmons in scanning tunneling microscope (STM) induced light emission upon selective initial excitation of molecules or plasmons. The time evolution and spectroscopic properties of the emission from the coupled plasmon-molecule system in each case are studied using time-dependent quantum mater equations. When the STM tip is placed on the molecule to ensure direct carrier injection induced molecular excitation, the major role of the plasmons is to enhance the molecular emission via increasing its radiative decay rate, resulting in sharp molecule-specific emission peaks. On the other hand, when the STM tip is located in close proximity to the edge of the molecule but without direct carrier injection into the molecule, the role of the plasmon-molecule coupling is to cause destructive interferences between the two quantum objects, leading to the occurrence of Fano dips around the energy of the molecular exciton in the plasmonic emission spectra.     

Dithiocarbamate-capped silver nanoparticles

Nanometer-sized silver particles were synthesized by using didecylamine dithiocarbamates as the protecting ligands. With control of the initial ligand-metal feed ratios, the core diameter of the resulting particles was found to vary from about 5 to 2.5 nm, as determined by transmission electron microscopic measurements. The core size dispersity was also found to decrease with increasing feed ratio. In UV-visible spectroscopic studies, the particle surface plasmon resonance peak diminished in intensity as the particle core size decreased, whereas in electrochemical measurements, smaller sized particles gave rise to well-defined quantized charging voltammetric features, in contrast to the featureless responses with the larger particles. Such single electron-transfer behaviors were consistent with those observed in STM measurements involving individual nanoparticles. Overall, this study provides an effective approach to the synthesis of stable nanometer-sized silver nanoparticles with interesting electronic and electrochemical properties.     

局域表面等离子体共振辅助的金纳米棒/聚合物纳米复合材料的双光子聚合

本文研究了金纳米棒的局域表面等离子体共振效应在双光子聚合过程中的作用,即当激发光与金纳米棒表面等离子体共振波长相匹配时,会在金纳米棒表面产生很强的局域电磁场,从而引发双光子聚合。通过采用与金纳米棒表面等离子体共振波长相同的飞秒激光,在低于光刻胶聚合阈值的功率下照射含有金纳米棒的光刻胶,制备聚合物包覆金纳米棒的纳米复合材料。透射电子显微镜结果表明,当飞秒激光功率为0.6 W、光斑直径为1.6 cm、照射时间为0.3 s时,金纳米棒表面成功聚合上厚度为5 nm左右的聚合物。本研究在制备聚合物/金属纳米粒子方面提供了一种简单可行的方法,有望在纳米光子学、纳米传感器等新兴领域得到应用。     

Regulating the Interactions of Adsorbates on Surfaces by Scanning Tunneling Microscopy Manipulation

Scanning tunneling microscopy (STM) manipulation has received wide attention in the surface science community since the pioneering work of Eigler to construct surface nanostructures in an atom by atom fashion. Lots of scientists have been inspired and devoted to study the surface issues with the help of STM manipulations and great achievements have been obtained. In this Minireview, we mainly describe the recent progress in applying STM manipulations to regulate the inter‐adsorbate and adsorbate–substrate interactions on solid surfaces. It was shown that this technique could not only differentiate intermolecular interactions but also construct molecular nanostructures by regulating different kinds of inter‐adsorbate interactions or adsorbate–substrate interactions.     

Stimuli induced structural changes of gold nanoparticle assemblies having sequential alternating amphiphilic peptides at the surface

Gold nanoparticles having sequential alternating amphiphilic peptide chains, Phe-(Leu-Glu)8, on the surface have been prepared. We describe structural control of the amphiphilic peptide coated gold nanoparticle assembly by a conformational transition of the surface peptides. Under the acidic condition, the conformation of the surface amphiphilic peptide was converted to a beta-sheet structure from an aggregated alpha-helix by incubation. Under this condition, the amphiphilic peptide coated gold nanoparticles formed a nanosheet assembly. The plasmon absorption maximum of the gold nanoparticles shifted to a shorter wavelength with the formation of the beta-sheet assembly of the surface peptide. This suggests that the structure of the peptide coated gold nanoparticle assembly could be controlled by the conformational transition of the surface peptide. Furthermore, the core gold nanoparticle could be fixed in the beta-sheet assembly in the state that stood alone. This system may be useful for novel molecular devices that exhibit quantized properties.     

Surface-enhanced resonance Raman scattering and background light emission coupled with plasmon of single Ag nanoaggregates

We investigated the optical properties of isolated single aggregates of Ag nanoparticles (Ag nanoaggregates) on which rhodamine 6G molecules were adsorbed to reveal experimentally a correlation among plasmon resonance Rayleigh scattering, surface-enhanced resonance Raman scattering (SERRS), and its background light emission. From the lack of excitation-laser energy dependence of background emission maxima we concluded that the background emission is luminescence, not Raman scattering. The polarization dependence of both SERRS and background emission was the same as that of the lowest-energy plasmon resonance maxima, which is associated with a longitudinal plasmon. From the common polarization dependence, we identified that the lowest-energy plasmon is coupled with both SERRS and background emission. In addition, we revealed that the lowest-energy plasmon with a higher quality factor (Q factor) yields larger SERRS and background emission intensity. Also, we identified that the Q factor dependence of the SERRS intensity was similar to that of the background emission intensity. This similarity directly supported us to demonstrate an enhancement of both SERRS and background emission by coupling with a common plasmon radiative mode.     

Dimer pinning and the assignment of semiconductor-adsorbate surface structures

It has been observed in scanning tunneling microscopy (STM) that the adsorption of molecules on the (001) surface of a Group IV semiconductor can lead to an asymmetric ordering of the dimers immediately adjacent to the adsorbate. This so-called pinning may occur along the dimer row on only one, or both sides of the adsorbate. Here we present a straightforward methodology for predicting such pinning and illustrate this approach for several different adsorbate structures on the Si(001) surface. This approach extends earlier work by including the effects of coupling across the adsorbate as well as the nearest-neighbor interactions between the chemisorbed dimer and its adjacent dimers. The results are shown to be in excellent agreement with the room temperature experimental STM data. The examples also show how this approach can serve as a powerful tool for discriminating between alternative possible adsorbate structures on a dimerized semiconductor (001) surface, especially in cases of molecular adsorption where the STM measurements provide insufficient details of the underlying atomic structure.     

Organized Monolayers of Biological Macromolecules on Au(111) Surfaces

Single-crystal electrochemistry and scanning tunneling microscopy directly in aqueous electrolyte solution (in situ STM) are established in physical electrochemistry but new in studies of adsorption and interfacial electrochemistry of biological macromolecules. These high-resolution techniques have now been applied comprehensively to proteins and other biomolecules in recent studies, discussed in this report. Focus is on three systems. The first one is a pair of amino acids, cysteine and cystine. These are strongly adsorbed via thiolate and disulfide, respectively, with identical reductive desorption and in situ STM patterns. Long-range lateral order can be imaged to molecular resolution. The amino acids are also reference molecules for the blue single-copper protein Pseudomonas aeruginosa azurin. This protein assembles in two well-defined orientations. One applies on bare Au(111) to which the protein is linked via its surface disulfide group. This orients the copper center away from the electrode surface. The other mode is by hydrophobic interactions with variable-length alkanethiols self-assembled on Au(111). In this mode the copper center is directed towards the surface. Adsorption and long-range electron tunneling in both modes have been characterized in detail using different electrochemical and spectroscopic techniques, as well as STM. Other data show that penta-(A–T) oligonucleotide adsorbed via a covalently bound thiol linker also displays reductive desorption and in situ STM to molecular resolution. The three systems thus appear to open new perspectives for broader use of high-resolution electrochemical techniques in comprehensive investigations of large biological molecules.     

Design of chiral dimesogens containing cholesteryl groups; formation of new molecular organizations and their application to molecular photonics

Photoresponsive liquid crystals and organogels are finding increasing application in information technology and photonics. In this tutorial review, the authors describe how weak intermolecular interactions facilitate molecular organization of cholesterol-containing dimesogens to form such materials. Design considerations and photoresponsive properties of both organogels and glassy liquid crystals are discussed and their applications to molecular photonics highlighted. The review will be of value to readers interested in the development of new materials which respond to the different properties of light.     

Fused Triangulene Dimers: Facile Synthesis by Intramolecular Radical-Radical Coupling and Application for Near-Infrared Lasers

Large graphene-like molecules with four zigzag edges are ideal gain medium materials for organic near-infrared (NIR) lasers. However, synthesizing them becomes increasingly challenging as the molecular size increases. In this study, we introduce a new intramolecular radical-radical coupling approach and successfully synthesize two fused triangulene dimers ( 1 a / 1 b ) efficiently. X-ray crystallographic analysis of 1 a indicates that there is no intermolecular π-π stacking in the solid state. When the more soluble derivative 1 b is dispersed in polystyrene thin films, amplified spontaneous emission in the NIR region is observed. Using 1 b as the active gain material, we fabricate solution-processed distributed feedback lasers that exhibit a narrow emission linewidth at around 790 nm. The laser devices also exhibit low thresholds with high photostability. Our study provides a new synthetic strategy for extended nanographenes, which have diverse applications in electronics and photonics.     

Study of adsorption and decomposition of H2O on Ge(100)

The adsorption and decomposition of water on Ge(100) have been investigated using real-time scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations. The STM results revealed two distinct adsorption features of H2O on Ge(100) corresponding to molecular adsorption and H-OH dissociative adsorption. In the molecular adsorption geometry, H2O molecules are bound to the surface via Ge-O dative bonds between the O atom of H2O and the electrophilic down atom of the Ge dimer. In the dissociative adsorption geometry, the H2O molecule dissociates into H and OH, which bind covalently to a Ge-Ge dimer on Ge(100) in an H-Ge-Ge-OH configuration. The DFT calculations showed that the dissociative adsorption geometry is more stable than the molecular adsorption geometry. This finding is consistent with the STM results, which showed that the dissociative product becomes dominant as the H2O coverage is increased. The simulated STM images agreed very well with the experimental images. In the real-time STM experiments, we also observed a structural transformation of the H2O molecule from the molecular adsorption to the dissociative adsorption geometry.     

Multifunctional polymers as multi-role materials for photonics

Multifunctional polymers can play multi-roles such as laser light source, passive interconnects, optical signal processing and optical data storage in the development of photonics technology. In this paper, two approaches to achieve nanostructure control to introduce multifunctionality at both molecular and bulk levels are presented. A novel concept of multiphasic nanostructured composites is discussed. Various applications of a specific multifunctional property introduced by strong two-photon absorption and efficient fluorescence are presented. The combined action of these two functions produces upconverted emission. Specific applications discussed are upconversion lasing, optical data storage, confocal microscopy, and photodynamic therapy.     

Scanning tunneling microscopy image simulation of the rutile (110) TiO2 surface with hybrid functionals and the localized basis set approach

In this work we present a simplified procedure to use hybrid functionals and localized atomic basis sets to simulate scanning tunneling microscopy (STM) images of stoichiometric, reduced and hydroxylated rutile (110) TiO2 surface. For the two defective systems it is necessary to introduce some exact Hartree-Fock exchange in the exchange functional in order to correctly describe the details of the electronic structure. Results are compared to the standard density functional theory and planewave basis set approach. Both methods have advantages and drawbacks that are analyzed in detail. In particular, for the localized basis set approach, it is necessary to introduce a number of Gaussian function in the vacuum region above the surface in order to correctly describe the exponential decay of the integrated local density of states from the surface. In the planewave periodic approach, a thick vacuum region is required to achieve correct results. Simulated STM images are obtained for both the reduced and hydroxylated surface which nicely compare with experimental findings. A direct comparison of the two defects as displayed in the simulated STM images indicates that the OH groups should appear brighter than oxygen vacancies in perfect agreement with the experimental STM data.     

Fluorescence Emission Mediated by Metal-Dielectric-Metal Fishnet Metasurface: Spatially Selective Excitation and Double Enhancement

Enhancement of fluorescent radiation is of great importance for applications including biological imaging, high-sensitivity detectors, and integrated light sources. Strong electromagnetic fields can be created around metallic nanoparticles or in gap of nanostructures, where the local state density of radiating mode is then dramatically enhanced. While enhanced fluorescent emission has been demonstrated in many metallic nanoparticles and nanoparticle pairs, simultaneous mediation of absorption and emission processes of fluorescent emitters remains challenging in metallic nanostructures. Here, we investigate fluorescent emission mediated by metal-dielectric-metal fishnet metasurface, in which localized surface plasmon (LSP) and magnetic plasmon polaritons (MPPs) modes are coupled with absorption and emission processes, respectively. For absorption process, coupling of the LSP mode enables spatially-selective excitation of the fluorescent emitters by rotating the polarization of the pump laser beam. In addition, the polarization-dependent MPP mode enables manipulation of both polarization and wavelength of the fluorescent emission by introducing a rectangular fishnet structure. All the experimental observations are further corroborated by finite-difference time-domain simulations. The structure reported here has great potentialfor application to color light-emitting devices and nanoscale integrated light sources.     

Scanning tunneling microscopy as a tool to study catalytically relevant model systems

The surface science approach to catalysis, pioneered by 2007 Nobel Laureate in chemistry Gerhard Ertl, has helped revolutionize our understanding of heterogeneous catalysis at the atomic level. In this tutorial review we show how the scanning tunnelling microscope (STM), in combination with this surface science approach, is a very important tool for the study of catalytically relevant model systems. We illustrate how the high spatial and temporal resolution of the STM can be used to obtain quantitative information on elementary processes involved in surface catalyzed reactions. Furthermore, we show that the STM is an outstanding surface science tool to bridge the materials gap and the pressure gap between surface science experiments and real catalysis. Finally, we show that we are approaching an era where the atomic-scale insight gained from fundamental STM surface science studies can be used for the rational design of new catalysts from first principles.     

Color-tunable ultralong organic phosphorescence materials for visual UV-light detection

Ultralong organic phosphorescence(UOP) materials have roused considerable attention in the field of photonics and optoelectronics owing to the feature of long-lived emission lifetimes. However, to develop UOP materials with color-tunability is still a formidable challenge. Here, we report a class of UOP materials containing carbonyl, amino or amide groups, exhibiting colortunable persistent luminescence ranging from blue(458 nm) to yellow-green(508 nm) under different UV wavelength excitation. Taken theoretical and experimental results together, we conclude that the excitation dependent color-tunable UOP emission is ascribed to multiple emission centers from single molecular and aggregated states in crystal. Given color-tunable UOP feature, these materials are used to successfully realize visual UV-light detection. This finding not only provides a strategy to design new organic phosphorescent molecules with colorful emission, but also extends the scope of the applications of purely organic phosphorescent materials.