Influence of Structural and Electronic Properties on the Collective Excitations of Ag/Cu(111)

Herein, we report on the interplay between structural and electronic properties in the dynamical screening processes and the excitation of collective modes in Ag/Cu(111). For the (9 × 9)-Ag/Cu(111) structure, the excitation of the two-dimensional plasmon, observed in Ag/Si(111), is forbidden by both the high corrugation of the Ag layer and the presence of the underlying metal substrate. The increase of the sp density of states at the Fermi energy induces the appearance of a broad peak at 7–8 eV assigned to a free-electron collective excitation.     

Collective Electronic Excitations in Thin Ag Films on Ni(111)

The collective electronic excitations in thin Ag films deposited onto the Ni(111) surface were studied by high-resolution electron energy loss spectroscopy. A broad loss peak at 7.7 eV was assigned to the Ag multipole plasmon, in excellent agreement with calculations based on s-d polarization model. Ag multipole plasmon was excited only at grazing incidence. Furthermore, a strong dependence on the impinging energy exists. Multipole plasmon could be measured only for a very strict range of primary electron beam energies and it was excited by electrons scattered at a reflection plane located just underneath the jellium edge. Such mode was found to be dramatically more sensible to the state of the surface with respect to ordinary surface plasmon. Moreover, we report experimental evidence of interference effects in surface plasmon excitation.     

Energy loss of hydrogen- and helium-ion beams in DNA: calculations based on a realistic energy-loss function of the target

We have calculated the electronic energy loss of proton and α-particle beams in dry DNA using the dielectric formalism. The electronic response of DNA is described by the MELF-GOS model, in which the outer electron excitations of the target are accounted for by a linear combination of Mermin-type energy-loss functions that accurately matches the available experimental data for DNA obtained from optical measurements, whereas the inner-shell electron excitations are modeled by the generalized oscillator strengths of the constituent atoms. Using this procedure we have calculated the stopping power and the energy-loss straggling of DNA for hydrogen- and helium-ion beams at incident energies ranging from 10 keV/nucleon to 10 MeV/nucleon. The mean excitation energy of dry DNA is found to be I = 81.5 eV. Our present results are compared with available calculations for liquid water showing noticeable differences between these important biological materials. We have also evaluated the electron excitation probability of DNA as a function of the transferred energy by the swift projectile as well as the average energy of the target electronic excitations as a function of the projectile energy. Our results show that projectiles with energy ?100 keV/nucleon (i.e., around the stopping-power maximum) are more suitable for producing low-energy secondary electrons in DNA, which could be very effective for the biological damage of malignant cells.     

Excitation energy transfer in phycobiliproteins of the cyanobacterium <Emphasis Type="Italic">Acaryochloris marina</Emphasis> investigated by spectral hole burning

The cyanobacterium Acaryochloris marina developed two types of antenna complexes, which contain chlorophyll-d (Chl d) and phycocyanobilin (PCB) as light-harvesting pigment molecules, respectively. The latter membrane-extrinsic complexes are denoted as phycobiliproteins (PBPs). Spectral hole burning was employed to study excitation energy transfer and electron–phonon coupling in PBPs. The data reveal a rich spectral substructure with a total of four low-energy electronic states whose absorption bands peak at 633, 644, 654, and at about 673 nm. The electronic states at ~633 and 644 nm can be tentatively attributed to phycocyanin (PC) and allophycocyanin (APC), respectively. The remaining low-energy electronic states including the terminal emitter at 673 nm may be associated with different isoforms of PC, APC, or the linker protein. Furthermore, the hole burning data reveal a large number of excited state vibrational frequencies, which are characteristic for the chromophore PCB. In summary, the results are in good agreement with the low-energy level structure of PBPs and electron–phonon coupling parameters reported by Gryliuk et al. (BBA 1837:1490–1499, 2014) based on difference fluorescence line-narrowing experiments.     

Ab initio molecular dynamics simulations of the adsorption of the methylguanidine or methylguanidinium on Ag(111)

A density-functional and Car–Parrinello molecular dynamics methods were employed to study the adsorption of the methylguanidine or methylguanidinium on Ag(111) surface with Vanderbilt pseudopotentials and PBE functional. The geometry, interacting energy, vibrational frequency, Mayer bond order and electrostatic fit charges were calculated. The results show that the methylguanidine interacts with the Ag(111) surface mainly through the interaction between the sp² hybridisation imine nitrogen and its nearest silver atom on top site, assisted with the Ag???H interaction, with the most stabilising interacting energy ?78.83 kJ/mol. The Car–Parrinello molecular dynamics results at 293.15 or 300.00 K indicate that the Ag???N interaction exists stably for more than 6 ps and the Mayer bond order analysis shows that it is the main interaction in adsorption. For the methylguanidinium on Ag(111) surface, the weak interaction between N?H and its neighbour silver atoms, with the energy of ?40.73–?42.68 kJ/mol and the interacting time of 0.2–0.3 ps at 300 K, could not keep it steady on Ag(111). The CP dynamics results show that only the methylguanidine could adsorb on Ag(111) at the room temperature.     

Uphill energy transfer in photosystem I from <Emphasis Type="Italic">Chlamydomonas reinhardtii</Emphasis>. Time-resolved fluorescence measurements at 77 K

Energetic properties of chlorophylls in photosynthetic complexes are strongly modulated by their interaction with the protein matrix and by inter-pigment coupling. This spectral tuning is especially striking in photosystem I (PSI) complexes that contain low-energy chlorophylls emitting above 700 nm. Such low-energy chlorophylls have been observed in cyanobacterial PSI, algal and plant PSI–LHCI complexes, and individual light-harvesting complex I (LHCI) proteins. However, there has been no direct evidence of their presence in algal PSI core complexes lacking LHCI. In order to determine the lowest-energy states of chlorophylls and their dynamics in algal PSI antenna systems, we performed time-resolved fluorescence measurements at 77 K for PSI core and PSI–LHCI complexes isolated from the green alga Chlamydomonas reinhardtii. The pool of low-energy chlorophylls observed in PSI cores is generally smaller and less red-shifted than that observed in PSI–LHCI complexes. Excitation energy equilibration between bulk and low-energy chlorophylls in the PSI–LHCI complexes at 77 K leads to population of excited states that are less red-shifted (by ~?12 nm) than at room temperature. On the other hand, analysis of the detection wavelength dependence of the effective trapping time of bulk excitations in the PSI core at 77 K provided evidence for an energy threshold at ~?675 nm, above which trapping slows down. Based on these observations, we postulate that excitation energy transfer from bulk to low-energy chlorophylls and from bulk to reaction center chlorophylls are thermally activated uphill processes that likely occur via higher excitonic states of energy accepting chlorophylls.     

Mycocrystallization of gold ions by the fungus Cylindrocladium floridanum

The size and morphology determines the thermodynamic, physical and electronic properties of metal nanoparticles. The extracellular synthesis of gold nanoparticles by fungus, Cylindrocladium floridanum, which acts as a source of reducing and stabilizing agent has been described. The synthesized nanoparticles were characterized using techniques such as UV–Vis spectroscopy, X-ray diffraction (XRD), scanning electron microscopy, energy dispersive X-ray analysis (EDAX), and high-resolution transmission electron microscopy (HR-TEM). Based on the evidence of HR-TEM, the synthesized particles were found to be spherical with an average size of 19.05 nm. Powder XRD pattern proved the formation of (111)-oriented face-centered cubic crystals of metallic gold. This microbial approach by fungus for the green synthesis of spherical gold nanoparticles has many advantages such as economic viability, scaling up and environment friendliness.     

Absolute and effective cross-sections for low-energy electron-scattering processes within condensed matter

Within the last two decades, a number of experimental techniques have been developed to measure mean free paths and absolute and effective cross-sections for various processes related to the interaction of low-energy electrons with condensed matter. In all of the experiments, a monochromatic electron beam impinges on a thin multilayer film composed of atoms and/or molecules condensed on a metal or semiconductor substrate held at cryogenic temperatures in an ultra-high-vacuum system. Depending on the apparatus, cross-sections are obtained from low-energy electron transmission (LEET), high-resolution electron energy loss (HREEL), x-ray photoelectron (XPS) spectroscopy, electron-stimulated desorption (ESD) of neutral and ions, or a combination of these techniques. Quasi-elastic and inelastic mean free paths have been extracted from LEET data. This method has also served to generate absolute cross-sections for electron trapping and fragment production from the dissociation of transient molecular anions. In amorphous ice, a complete set of absolute cross-sections for all inelastic losses by 1–20 eV electrons has been obtained from HREEL data. Effective cross-sections for neutral and ionic radical formation were generated by desorption and XPS experiments. These various methods are briefly described in this article, and the corresponding cross-sections in the range 0–20 eV summarized. Received: 10 September 1998 / Accepted: 22 October 1998     

Dispersion and Damping of Gold Surface Plasmon

High-resolution electron energy loss spectroscopy was used to investigate the surface plasmon dispersion in (111)-oriented Au films grown on Cu(111). The measured dispersion of the plasmon mode was positive, as found for Ag. The centroid of the induced charge associated to the plasmon field lies well inside the jellium edge. The damping relation of the Au surface plasmon presented a critical wave vector of 0.11 Å^?1. For higher values of the parallel momentum transfer, the line width of Au surface plasmon considerably increased as a consequence of the opening of a new decay channel via single-particle transitions.     

Plasmon Spectroscopy for Subnanometric Copper Particles: Dielectric Function and Core–Shell Sizing

In the last years, there has been a growing interest in the study of transition metal nanoparticles (Nps) due to their potential applications in several fields of science and technology. In particular, their optical properties are governed by the characteristics of the dielectric function of the metal, its size and environment. This work analyses the separated contribution of free and bound electrons on the optical properties of copper Nps. Usually, the contribution of free electrons to the dielectric function is corrected for particle size through the modification of the damping constant, which is changed as usual introducing a term inversely proportional to the particle’s radius to account for the extra collisions with the boundary when the size approaches the electronic mean free path limit (about 10 nm). For bound electron contribution, the interband transitions from the d-band to the conduction band are considered together with the fact that the electronic density of states in the conduction band must be made size-dependent to account for the larger spacing between electronic energy levels as the particle decreases in size below 2 nm. Taking into account these specific modifications of free and bound electron contributions to the dielectric function, it was possible to fit the bulk complex dielectric function, and consequently, determine optical parameters and band energy values such as the coefficient for bound electron contribution Q _bulk?=?2?×?10²⁴, gap energy E _g?=?1.95 eV, Fermi energy E _F?=?2.15 eV, and damping constant for bound electrons γ _b?=?1.15?×?10¹⁴ Hz. With both size-dependent contributions to the dielectric function, extinction spectra of copper Nps in the subnanometer radius range can be calculated using Mie’s theory and its behaviour with size can be analysed. These studies are applied to fit experimental extinction spectra of very small spherical core–shell Cu–Cu₂O Nps generated by ultrafast laser ablation of a solid target in water. Theoretical calculations for subnanometric core radius are in excellent agreement with experimental results obtained from core–shell colloidal Nps. From the fitting, it is possible determining core radius and shell thickness of the Nps, showing that optical extinction spectroscopy is a good complementary technique to standard high-resolution electron microscopy for sizing spherical nanometric-subnanometric Nps.     

Plasmonic and Energy Studies of Ag Nanoparticles in Silica-Titania Hosts

The present work reports an investigation of surface plasmon resonance (SPR) of silver nanoparticles in SiO₂–TiO₂ hosts. The surface plasmon resonance of silver nanoparticles was observed in the wavelength range 300–400 nm. Numerical calculation of SPR of silver nanoparticles with spherical morphology was done on the basis of discrete dipole approximation (DDA) method. The observed fluorescence spectrum fits well with the theoretically calculated one. The luminescence enhancement is attributed to the strong local electric field which increases the exciting and emitting photons coupled to SPR. An effort has been made to study the surface plasmon mediated excitation energy transfer (EET) between two spherical metal nanoparticles. The van der Waals (vdW) energy between plasmonic silver nanoparticles in the present hosts has been estimated.     

Intraprotein electron transfer in a ruthenium-modified Tyr83-His plastocyanin mutant: evidence for strong electronic coupling

A site-directed mutant of spinach plastocyanin, Pc(Tyr83-His), has been modified by covalent attachment of a photoactive [Ru(bpy)₂(im)]²⁺ complex to the His83 residue. The residue is surface exposed and located about 10–12?Å from the copper ion at the entrance of a proposed natural electron transfer pathway from cytochrome f. Electron transfer within the Ru-Pc complex has been studied with time-resolved optical spectroscopy using two different approaches. In the first, the fully reduced [Cu(I), Ru(II)] protein was photoexcited and subsequently oxidized by an external quencher, forming the [Cu(I), Ru(III)] protein. This was followed by an electron transfer from reduced Cu(I) to Ru(III). In the second method, the initially oxidized Cu(II) ion acted as an internal quencher for excited Ru(II) and the photoinduced reduction of the Cu(II) ion was followed by a thermal recombination with the Ru(III) ion. The reoxidation of the Cu ion, which has an estimated driving force of 0.56?eV, occured with a rate constant k _et?=?(9.5±1.0)×10⁶?s^–1, observed with both methods. The results suggest a strong electronic coupling (H _DA>0.3?cm^–1) along the Ru-His(83)-Cys(84)-Cu pathway.     

Electron Energy Loss and One- and Two-Photon Excited SERS Probing of “Hot” Plasmonic Silver Nanoaggregates

We report electron energy loss spectroscopy (EELS) and one- and two-photon excited surface-enhanced Raman scattering (SERS) and hyper Raman studies on plasmonic silver nanoaggregates. By comparison with computations, EELS imaging reveals an inverse relationship between local field intensity in an optical experiment and electron energy loss intensity at energies corresponding to excitation wavelengths used for optical probing. This inverse relation exists independent on specific nanoaggregate geometries and is mainly controlled by the gap size between the particles forming the aggregate. The ratio between two- and one-photon excited SERS measured at different excitation wavelengths provides information about local fields in the hottest spots and their dependence on the photon energy. Our data verify experimentally the predicted increase of local optical fields in the hot spots with increasing wave lengths. The reported findings show new experimental ways to characterize local fields of plasmonic nanostructures. This is of particular importance for complex structures which are not easily approachable by computations.     

Negative-polarity fast ionization wave in molecular gases: Electric field, electron density, and energy branching

The charge density per unit length, the longitudinal component of the electric field, and the electron density behind the front of a fast ionization wave initiated by a nanosecond negative voltage pulse in air, N₂, and H₂ in the 1-to 24-torr pressure range are reconstructed from the experimental data. It is shown that the electron density behind the wave front depends weakly on the sort of gas used and, at relatively high pressures (8–24 torr), is (2–3)×10¹² cm^?3. The energy deposited in the internal degrees of freedom is analyzed. It is shown that, for all gases used, most of the deposited energy (40–60%) is spent on the excitation of the electron degrees of freedom. The fraction of the energy deposited in the high-energy degrees of freedom (ionization and dissociation) monotonically decreases with increasing the pressure, whereas the fraction of the energy spent on the excitation of the low-energy degrees of freedom (rotational and vibrational) monotonically increases.     

Bimetallic Cobalt‐Based Phosphide Zeolitic Imidazolate Framework: CoPx Phase‐Dependent Electrical Conductivity and Hydrogen Atom Adsorption Energy for Efficient Overall Water Splitting

Cobalt‐based bimetallic phosphide encapsulated in carbonized zeolitic imadazolate frameworks has been successfully synthesized and showed excellent activities toward both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Density functional theory calculation and electrochemical measurements reveal that the electrical conductivity and electrochemical activity are closely associated with the Co₂P/CoP mixed phase behaviors upon Cu metal doping. This relationship is found to be the decisive factor for enhanced electrocatalytic performance. Moreover, the precise control of Cu content in Co‐host lattice effectively alters the Gibbs free energy for H* adsorption, which is favorable for facilitating reaction kinetics. Impressively, an optimized performance has been achieved with mild Cu doping in Cu_0.3Co_2.7P/nitrogen‐doped carbon (NC) which exhibits an ultralow overpotential of 0.19 V at 10 mA cm^–2 and satisfying stability for OER. Cu_0.3Co_2.7P/NC also shows excellent HER activity, affording a current density of 10 mA cm^–2 at a low overpotential of 0.22 V. In addition, a homemade electrolyzer with Cu_0.3Co_2.7P/NC paired electrodes shows 60% larger current density than Pt/RuO₂ couple at 1.74 V, along with negligible catalytic deactivation after 50 h operation. The manipulation of electronic structure by controlled incorporation of second metal sheds light on understanding and synthesizing bimetallic transition metal phosphides for electrolysis‐based energy conversion.     

Efficiency of excitation energy trapping in the green photosynthetic bacterium Chlorobaculum tepidum

During the millions of years of evolution, photosynthetic organisms have adapted to almost all terrestrial and aquatic habitats, although some environments are obviously more suitable for photosynthesis than others. Photosynthetic organisms living in low-light conditions require on the one hand a large light-harvesting apparatus to absorb as many photons as possible. On the other hand, the excitation trapping time scales with the size of the light-harvesting system, and the longer the distance over which the formed excitations have to be transferred, the larger the probability to lose excitations. Therefore a compromise between photon capture efficiency and excitation trapping efficiency needs to be found. Here we report results on the whole cells of the green sulfur bacterium Chlorobaculum tepidum. Its efficiency of excitation energy transfer and charge separation enables the organism to live in environments with very low illumination. Using fluorescence measurements with picosecond resolution, we estimate that despite a rather large size and complex composition of its light-harvesting apparatus, the quantum efficiency of its photochemistry is around ~87% at 20?°C, ~83% at 45?°C, and about ~81% at 77?K when part of the excitation energy is trapped by low-energy bacteriochlorophyll a molecules. The data are evaluated using target analysis, which provides further insight into the functional organization of the low-light adapted photosynthetic apparatus.     

Quadratic Dispersion and Damping Processes of π Plasmon in Monolayer Graphene on Pt(111)

High-resolution electron energy-loss spectroscopy has been used to study the π plasmon in monolayer graphene grown on Pt(111). A quadratic dispersion has been observed, in contrast to the linear dispersion reported for monolayer graphene grown on SiC(0001) and in agreement with recent experiments on graphene/Ni(111). Despite the weak interaction of the monolayer graphene with the Pt(111) surface, our results indicate that the screening by the underlying metal substrate strongly influences both the dispersion relation and the damping processes of the plasmon mode of π electrons.     

Electronic excitations in condensed biological matter

In living matter, electronic excitations may have a collective character which is reviewed here in simple physical terms. In liquids and ordered solids the collective excitations appear as plasmons or excitons. Plasmons are delocalized electronic perturbations of a huge number of oscillating electrons decaying very quickly into localized electronic perturbations, mainly low-energy ionizations. Excitons are very light, moving quantum quasi-particles carrying energy, charge and information in structured biological systems. In deformable soft structures collective excitations appear as solitons behaving as rather massive quasi-particles of combined quantum and classical character. Solitons are relatively stable micro-objects able to transfer energy, charge, mass, and biological information along such biological structures as (chains of) macromolecules, fibres, membranes and surfaces. Some photobiological and radiation biological consequences of collective electronic excitations are suggested.     

Electronic fine structure calculation of metal complexes with three-open-shell <Emphasis Type="Italic">s</Emphasis>, <Emphasis Type="Italic">d,and p</Emphasis> configurations

The ligand field density functional theory (LFDFT) algorithm is extended to treat the electronic structure and properties of systems with three-open-shell electron configurations, exemplified in this work by the calculation of the core and semi-core 1s, 2s, and 3s one-electron excitations in compounds containing transition metal ions. The work presents a model to non-empirically resolve the multiplet energy levels arising from the three-open-shell systems of non-equivalent ns, 3d, and 4p electrons and to calculate the oscillator strengths corresponding to the electric-dipole 3d ^m  → ns ¹3d ^m 4p ¹ transitions, with n = 1, 2, 3 and m = 0, 1, 2, …, 10 involved in the s electron excitation process. Using the concept of ligand field, the Slater-Condon integrals, the spin-orbit coupling constants, and the parameters of the ligand field potential are determined from density functional theory (DFT). Therefore, a theoretical procedure using LFDFT is established illustrating the spectroscopic details at the atomic scale that can be valuable in the analysis and characterization of the electronic spectra obtained from X-ray absorption fine structure or electron energy loss spectroscopies.     

Solvent-Induced Morphological Changes of Polyhedral Silver Nanoparticles in Epoxy Resin

The use of just one material as reducing and capping agent during the synthesis of noble metal nanoparticles is of great interest for potential applications. This paper reports a simple method to prepare polyhedral silver nanoparticles at 80 °C using an epoxy resin (Araldite 506) as both reducing and capping agent. The formation of metal nanoparticles was investigated by Fourier-transform infrared spectroscopy, atomic force microscopy, transmission electronic microscopy, high-resolution transmission electronic microscopy and UV-vis spectroscopy. The proposed mechanism for the reduction of silver ions involves radicals as precursors of ketones and other products originated by thermo-oxidation of the resin. The nucleation of small spherical and polyhedral seeds (～7 nm) of polycrystalline silver and twin planes lead to big polyhedral silver nanoparticles of average size, 68 nm. On the other hand, the polyhedral silver nanoparticles dispersed in toluene changed to prolate-like particles, and their dispersion in dimethyl-sulphoxide and formamide originated elongated polyhedrons and concave nanostructures, respectively. These structural changes lead to unusual solvent-induced optical properties. For instance, the polyhedral nanoparticles dispersed in toluene red-shifted their surface plasmon resonance from 425 to 540 nm, in dimethyl-sulphoxide the spectrum exhibited a peak at 418 nm and a shoulder at 520 nm, and for the silver nanoparticles in formamide a broad band with maximum peak at 420 nm was observed. It is showed that the solvent/resin system works itself as structure-directing agent of silver nanoparticles. These results open the doors to achieve silver nanostructures highly sensitive to the dielectric environment, an ideal condition for applications in colorimetric sensors of molecules of biological or chemical interest.