Silica Gels Doped with Gold Nanoparticles: Preparation, Structure and Optical Properties

A novel, one-pot sol-gel preparation scheme leading to reproducible incorporation of 20-40 nm sized gold nanoparticles (AuNPs) in SiO2 gels is developed based on in situ reduction during gelation using chloroauric acid and ascorbic acid. Variation in the preparation conditions affects the chemical composition, optical properties and size distribution of the AuNPs incorporated in the silica gels. Different organic dopants, i.e., oleic acid, acetic acid or dodecanethiol, are applied to modify the final composite material and to control the rate of reduction and growth of the AuNPs in the gels. The synthesized samples are characterized by UV/Vis/NIR spectroscopy, X-ray diffraction, transmission electron microscopy, thermal conductivity measurements and DTA/TG measurements. The optical properties of the obtained composites are explained using Mie theory. The incorporation of AuNPs leads to an increase in the thermal conductivity of the silica gels. The best process method in this contribution is the use of NaOH as a gelation catalyst and oleic acid as an organic modifier, leading to 20 nm AuNPs dispersed in the silica matrix.

First-Principles Calculation of MoO2 and MoO3 Electronic and Optical Properties Compared with Experimental Data

MoO₃ and MoO₂ systems have attracted particular attention for many widespread applications thanks to their electronic and optical peculiarities; from the crystallographic point of view, MoO₃ adopts a thermodynamically stable orthorhombic phase (α-MoO₃) belonging to the space group Pbmn, while MoO₂ assumes a monoclinic arrangement characterized by space group P2₁/c. In the present paper, we investigated the electronic and optical properties of both MoO₃ and MoO₂ by using Density Functional Theory calculations, in particular, the Meta Generalized Gradient Approximation (MGGA) SCAN functional together with the PseudoDojo pseudopotential, which were used for the first time to obtain a deeper insight into the nature of different Mo-O bonds in these materials. The calculated density of states, the band gap, and the band structure were confirmed and validated by comparison with already available experimental results, while the optical properties were validated by recording optical spectra. Furthermore, the calculated band-gap energy value for the orthorhombic MoO₃ showed the best match to the experimental value reported in the literature. All these findings suggest that the newly proposed theoretical techniques reproduce the experimental evidence of both MoO₂ and MoO₃ systems with high accuracy.

A Simple Method for Estimation of the Scattering Exponent of Nanostructured Glasses

For most of nanostructured glasses (NGs) (phase-separated glasses and glass-ceramics), the light scattering coefficient (turbidity) is described by a power function of the inverse wavelength with an exponent which differs appreciably from the Rayleigh value 4 and is called the scattering exponent. The knowledge of the scattering exponent of a material is important from both fundamental and practical points of view. Previously, we developed three rather complex methods to determine the scattering exponent. Here, we present a novel simple express method for its estimation. In the method, the measured optical density for only one sample is used, the refractive index of the material is not required, and the dispersion of refractive index is assumed to be insignificant. The method is based on the differentiation of the measured optical density with respect to the wavelength. The scattering exponent values obtained by the new method for NGs of different types are in good agreement with those found by the traditional methods. The new method is found to be applicable even to NGs with high dispersion of refractive index. Thus, the new method does not require the data on the refractive index dispersion and can be applied without restrictions.

The Electronic Structures and Energies of the Lowest Excited States of the Ns0, Ns+, Ns- and Ns-H Defects in Diamond

This paper reports the energies and charge and spin distributions of the mono-substituted N defects, N⁰_s, N⁺_s, N^-_s and N_s-H in diamonds from direct Δ-SCF calculations based on Gaussian orbitals within the B3LYP function. These predict that (i) N_s⁰, N_s⁺ and N_s^- all absorb in the region of the strong optical absorption at 270 nm (4.59 eV) reported by Khan et al., with the individual contributions dependent on the experimental conditions; (ii) N_s-H, or some other impurity, is responsible for the weak optical peak at 360 nm (3.44 eV); and that N_s⁺ is the source of the 520 nm (2.38 eV) absorption. All excitations below the absorption edge of the diamond host are predicted to be excitonic, with substantial re-distributions of charge and spin. The present calculations support the suggestion by Jones et al. that N_s⁺ contributes to, and in the absence of N_s⁰ is responsible for, the 4.59 eV optical absorption in N-doped diamonds. The semi-conductivity of the N-doped diamond is predicted to rise from a spin-flip thermal excitation of a CN hybrid orbital of the donor band resulting from multiple in-elastic phonon scattering. Calculations of the self-trapped exciton in the vicinity of N_s⁰ indicate that it is essentially a local defect consisting of an N and four nn C atoms, and that beyond these the host lattice is essential a pristine diamond as predicted by Ferrari et al. from the calculated EPR hyperfine constants.

A comparable study of the optical spectra and EPR parameters for 3d1 - and 3d9 -doped MgNH4 PO4 ·6H2 O

VO²⁺ (3d¹ ) and Cu²⁺ (3d⁹ ) are the two complementary states that usually show opposite distortions when they are doped in crystals. In this work, the optical absorption spectra (OAS), electron paramagnetic resonance (EPR) parameters, and local structure (LS) for VO²⁺ (and Cu²⁺ ) in MgNH₄ PO₄ ·6H₂ O (MPPH) are uniformly investigated on the basis of the high-order perturbation formulas for a 3d¹ (and 3d⁹ ) ion in tetragonally compressed (and elongated) octahedra, respectively. In the calculated formulas, the required crystal-field parameters can be obtained from the superposition model and reasonably linked with the LS distortion for VO²⁺ and Cu²⁺ centers. Based on the calculations, the tetragonal compressed [VO(H₂ O)₅ ]²⁺ cluster (and tetragonal elongated [Cu(H₂ O)₆ ]²⁺ cluster) is found to suffer tetragonal compression ratio of 1.65% and tetragonal elongation ratio of 3.8% along C₄ -axis, respectively, due to the Jahn-Teller (JT) effect. The theoretical EPR parameters based on the above lattice distortions agree well with the experimental data, and the LS of the VO²⁺ and Cu²⁺ centers in MPPH is discussed.

Deciphering excited state properties utilizing algebraic diagrammatic construction schemes of decreasing order

Excited state properties are difficult to trace back to the common molecular orbital picture when the excited state wavefunction is a linear combination of two or more Slater determinants. Here, a theoretical methodology is introduced based on the algebraic diagrammatic construction scheme for the polarization propagator (ADC(n)) that allows to make this connection and to eventually derive structure-function relationships. The usefulness of this approach is demonstrated by an analysis of the transition dipole moments of the low-lying 1B_3u and 2B_3u states of anthracene and (1,4,5,8)-tetraazaanthracene.

Multi-orbital tight binding model for the electronic and optical properties of armchair graphene nanoribbons in the presence of a periodic potential

The influences of an external electric field with uniform or modulated potential on the electronic and optical properties of armchair graphene nanoribbons (GNRs) are explored using the multi-orbital tight-binding Hamiltonian. The interplay between an electric field and interaction between (s,p_x,p_y,p_z) orbitals remarkably enriches the main features of band structures and absorption spectra. The applied electric field can notably alter the energy dispersions ofπandσbands, leading to the deformation of band-edge states, open and close of a band gap, and modification of the Fermi energy. The vertical optical excitations happen among theπbands, while their available channels depend on the Fermi level which is controlled by theσ-edge bands and a finite potential. With the rich and unique properties, GNRs are suitable candidates for applications in the fields of photodetectors, nanoelectronics, and spintronics. The calculated results are expected to be examined by the angle-resolved photoemission spectroscopies and optical spectroscopies.

n-Doping of Quantum Dots by Lithium Ion Intercalation

The optical properties of colloidal quantum dots (QDs) are controllable through introduction of excess electrons or holes into their delocalized bands. Crucial to robust and energy-efficient electronic doping of QDs is suitable charge compensation. Compensation by surface modification and substitutional impurities are however not sufficiently controllable to enable effective doping of QDs. This article describes electrochemical n-type doping of CdSe QDs where injected electrons are compensated by interstitial Li⁺ to form Li_xCdSe, x ≤ 0.3. n-type degenerate doping reversibly decreases absorption into the lowest-energy excitonic state of the QD, activates intraband optical transitions, and shifts the photoluminescence of the QD to higher energy. This work establishes electrochemical interstitial doping as a reversible and highly controllable method for tuning the optical properties of colloidal QDs.

Plasmonic Resonances of Metal Nanoparticles: Atomistic vs. Continuum Approaches

The fully atomistic model, ωFQ, based on textbook concepts (Drude theory, electrostatics, quantum tunneling) and recently developed by some of the present authors in Nanoscale, 11, 6004-6015 is applied to the calculation of the optical properties of complex Na, Ag, and Au nanostructures. In ωFQ, each atom of the nanostructures is endowed with an electric charge that can vary according to the external electric field. The electric conductivity between nearest atoms is modeled by adopting the Drude model, which is reformulated in terms of electric charges. Quantum tunneling effects are considered by letting the dielectric response of the system arise from atom-atom conductivity. ωFQ is challenged to reproduce the optical response of metal nanoparticles of different sizes and shapes, and its performance is compared with continuum Boundary Element Method (BEM) calculations.

Photoexcitation Processes in Oligomethine Cyanine Dyes for Dye-Sensitized Solar Cells-Synthesis and Computational Study

We report density functional theory (DFT) calculations of three newly synthesized oligomethine cyanine-based dyes as potential TiO₂-sensitizers in dye-sensitized solar cells. The three dyes have π-symmetry and the same acceptor side, terminating in the carboxylic anchor, but they differ through the π-bridge and the donor groups. We perform DFT and time-dependent DFT studies and present the electronic structure and optical properties of the dyes alone as well as adsorbed to the TiO₂ nanocluster, to provide some predictions on the photovoltaic performance of the system. We analyze theoretically the factors that can influence the short circuit current and the open circuit voltage of the dye-sensitized solar cells. We examine the matching of the absorption spectra of the dye and dye-nanocluster system with the solar irradiation spectrum. We display the energy level diagrams and discuss the alignment between the excited state of the dyes and the conduction band edge of the oxide as well as between the redox level of the electrolyte and the ground state of the dyes. We determine the electron density of the key molecular orbitals and analyze comparatively the electron transfer from the dye to the semiconducting substrate. To put our findings in the right perspective we compare the results of our calculations with those obtained for a coumarin-based dye used in fabricating and testing actual devices, for which experimental data regarding the photovoltaic performance are available.

Environmental Screening Effects in 2D Materials: Renormalization of the Bandgap, Electronic Structure, and Optical Spectra of Few-Layer Black Phosphorus

Few-layer black phosphorus has recently emerged as a promising 2D semiconductor, notable for its widely tunable bandgap, highly anisotropic properties, and theoretically predicted large exciton binding energies. To avoid degradation, it has become common practice to encapsulate black phosphorus devices. It is generally assumed that this encapsulation does not qualitatively affect their optical properties. Here, we show that the contrary is true. We have performed ab initio GW and GW plus Bethe-Salpeter equation (GW-BSE) calculations to determine the quasiparticle (QP) band structure and optical spectrum of one-layer (1L) through four-layer (4L) black phosphorus, with and without encapsulation between hexagonal boron nitride and sapphire. We show that black phosphorus is exceptionally sensitive to environmental screening. Encapsulation reduces the exciton binding energy in 1L by as much as 70% and completely eliminates the presence of a bound exciton in the 4L structure. The reduction in the exciton binding energies is offset by a similarly large renormalization of the QP bandgap so that the optical gap remains nearly unchanged, but the nature of the excited states and the qualitative features of the absorption spectrum change dramatically.

A New Potent Inhibitor of Glycogen Phosphorylase Reveals the Basicity of the Catalytic Site

The design and synthesis of a glucose-based acridone derivative (GLAC), a potent inhibitor of glycogen phosphorylase (GP) are described. GLAC is the first inhibitor of glycogen phosphorylase, the electronic absorption properties of which are clearly distinguishable from those of the enzyme. This allows probing subtle interactions in the catalytic site. The GLAC absorption spectra, associated with X-ray crystallography and quantum chemistry calculations, reveal that part of the catalytic site of GP behaves as a highly basic environment in which GLAC exists as a bis-anion. This is explained by water-bridged hydrogen-bonding interactions with specific catalytic site residues.

Imaging the Ultrafast Photoelectron Transfer Process in Alizarin-TiO2

In this work, we adopt a quantum mechanical approach based on time-dependent density functional theory (TDDFT) to study the optical and electronic properties of alizarin supported on TiO2 nano-crystallites, as a prototypical dye-sensitized solar cell. To ensure proper alignment of the donor (alizarin) and acceptor (TiO2 nano-crystallite) levels, static optical excitation spectra are simulated using time-dependent density functional theory in response. The ultrafast photoelectron transfer from the dye to the cluster is simulated using an explicitly time-dependent, one-electron TDDFT ansatz. The model considers the δ-pulse excitation of a single active electron localized in the dye to the complete set of energetically accessible, delocalized molecular orbitals of the dye/nano-crystallite complex. A set of quantum mechanical tools derived from the transition electronic flux density is introduced to visualize and analyze the process in real time. The evolution of the created wave packet subject to absorbing boundary conditions at the borders of the cluster reveal that, while the electrons of the aromatic rings of alizarin are heavily involved in an ultrafast charge redistribution between the carbonyl groups of the dye molecule, they do not contribute positively to the electron injection and, overall, they delay the process.

The feasibility of coherent energy transfer in microtubules

It was once purported that biological systems were far too 'warm and wet' to support quantum phenomena mainly owing to thermal effects disrupting quantum coherence. However, recent experimental results and theoretical analyses have shown that thermal energy may assist, rather than disrupt, quantum coherent transport, especially in the 'dry' hydrophobic interiors of biomolecules. Specifically, evidence has been accumulating for the necessary involvement of quantum coherent energy transfer between uniquely arranged chromophores in light harvesting photosynthetic complexes. The 'tubulin' subunit proteins, which comprise microtubules, also possess a distinct architecture of chromophores, namely aromatic amino acids, including tryptophan. The geometry and dipolar properties of these aromatics are similar to those found in photosynthetic units indicating that tubulin may support coherent energy transfer. Tubulin aggregated into microtubule geometric lattices may support such energy transfer, which could be important for biological signalling and communication essential to living processes. Here, we perform a computational investigation of energy transfer between chromophoric amino acids in tubulin via dipole excitations coupled to the surrounding thermal environment. We present the spatial structure and energetic properties of the tryptophan residues in the microtubule constituent protein tubulin. Plausibility arguments for the conditions favouring a quantum mechanism of signal propagation along a microtubule are provided. Overall, we find that coherent energy transfer in tubulin and microtubules is biologically feasible.

Fluorene-based sensitizers with a phenothiazine donor: effect of mode of donor tethering on the performance of dye-sensitized solar cells

Two types of fluorene-based organic dyes featuring T-shape/rod-shape molecular configuration with phenothiazine donor and cyanoacrylic acid acceptor have been synthesized and characterized as sensitizers for dye-sensitized solar cells. Phenothiazine is functionalized at either nitrogen (N10) or carbon (C3) to obtain T-shape and rod-like organic dyes, respectively. The effect of structural alternation on the optical, electrochemical, and the photovoltaic properties is investigated. The crystal structure determination of the dye containing phenyl linker revealed cofacial slip-stack columnar packing of the molecules. The trends in the optical properties of the dyes are interpreted using time-dependent density functional theory (TDDFT) computations. The rod-shaped dyes exhibited longer wavelength absorption and low oxidation potentials when compared to the corresponding T-shaped dyes attributable to the favorable electronic overlap between the phenothiazine unit and the rest of the molecule in the former dyes. However, the T-shaped dyes showed better photovoltaic properties due to the lowest unoccupied molecular orbital (LUMO) energy level favorable for electron injection into the conduction band of TiO2 and appropriate orientation of the phenothiazine unit rendering effective surface blocking to suppress the recombination of electrons between the electrolyte I3(-) and TiO2. The electrochemical impedance spectroscopy investigations provide further support for the variations in the electron injection and transfer kinetics due to the structural modifications.

Quantum mechanical calculations unveil the structure and properties of the absorbing and emitting excited electronic states of guanine quadruplex

Herein, a full quantum mechanical study, in solution, of several models of guanine-quadruplex helices, both parallel and antiparallel, containing up to eight guanine residues, in their electronic excited state is reported. By exploiting TD-DFT calculations and including solvent effects by the polarizable continuum model, we provide the first atomistic description of the processes triggered by the absorption of UV light, reproducing and assigning the experimental optical and electronic circular dichroism spectra. The absorbing excited states are delocalized over multiple bases, whereas emission involves a stacked guanine dimer or a monomer. Several states, with a varying degree of localization and charge-transfer character, rule the photoexcited dynamics, which are deeply affected by the quadruplex topology. The lowest excited-state minimum for parallel quadruplex is an asymmetric excimer involving two stacked guanines, with a small charge transfer character, whereas for the anti-parallel structure, with the same topology of the thrombin binding aptamer, it is a fully symmetric excimer, characterized by a strong decrease of the stacking distance. A monomer-like decay path is the most relevant nonradiative decay pathway. Insights on the effect of the ions (K(+) or Na(+)) on the excited state decay are also provided.

Aligned Linear Arrays of Crystalline Nanoparticles

Fabrication of one-dimensional arrays of crystalline nanoparticles with tunable particle size and spacing (down to 20 nm) is demonstrated. The individual nanocrystals are pentagonal prisms, and the arrays are up to 11 μm in length, with some arrays containing >50 nanocrystals. Precise particle morphology and interparticle spacing can be maintained down the array. The far-field scattering spectra of the arrays show the near-fields of the nanocrystals are coupled. The method is fast and produces precise, well-defined, coupled plasmonic arrays with optical properties that match well to theory.

Density Functional Theory (DFT) Study of Coumarin-based Dyes Adsorbed on TiO₂ Nanoclusters-Applications to Dye-Sensitized Solar Cells

Coumarin-based dyes have been successfully used in dye-sensitized solar cells, leading to photovoltaic conversion efficiencies of up to about 8%. Given the need to better understand the behavior of the dye adsorbed on the TiO₂ nanoparticle, we report results of density functional theory (DFT) and time-dependent DFT (TD-DFT) studies of several coumarin-based dyes, as well as complex systems consisting of the dye bound to a TiO₂ cluster. We provide the electronic structure and simulated UV-Vis spectra of the dyes alone and adsorbed to the cluster and discuss the matching with the solar spectrum. We display the energy level diagrams and the electron density of the key molecular orbitals and analyze the electron transfer from the dye to the oxide. Finally, we compare our theoretical results with the experimental data available and discuss the key issues that influence the device performance.

Triple-Decker Motif for Red-Shifted Fluorescent Protein Mutants

Among fluorescent proteins (FPs) used as genetically encoded fluorescent tags, the red-emitting FPs are of particular importance as suitable markers for deep tissue imaging. Using electronic structure calculations, we predict a new structural motif for achieving red-shifted absorption and emission in FPs from the GFP family. By introducing four point mutations, we arrive to the structure with the conventional anionic GFP chromophore sandwiched between two tyrosine residues. Contrary to the existing red FPs in which the red shift is due to extended conjugation of the chromophore, in the triple-decker motif, the chromophore is unmodified and the red shift is due to π-stacking interactions. The absorption/emission energies of the triple-decker FP are 2.25/2.16 eV, respectively, which amounts to shifts of ∼40 (absorption) and ∼25 nm (emission) relative to the parent species, the I form of wtGFP. Using a different structural motif based on a smaller chromophore may help to improve optical output of red FPs by reducing losses due to radiationless relaxation and photobleaching.

Origin of Optical Excitations in Fluorine-Doped Titania from Response Function Theory: Relevance to Photocatalysis

We investigate the effect of fluorine doping on the optical spectra of stoichiometric and reduced TiO2 anatase, brookite, and rutile using density functional methods. The present approach is able to reproduce the main features of experiments and high-level quasiparticle calculations for undoped titania but at a much lower computational cost, thus allowing the study of doped titania, which requires large supercells. Whereas the simulated spectra of F-substituted brookite and rutile do not show any significant new feature, a relatively intense new band near the visible region is predicted for F-substituted anatase. This allows one to suggest assigning the spectral features near the visible region, observed on multiphase F-doped titania samples, to the presence of anatase. The physical origin of the new absorption band in F-doped anatase is unambiguously attributed to the presence of Ti(3+) centers.