Narrow Results

An indium tin oxide (ITO) electrode was coated by a poly(2-vinylpyridine) (P-2VP) film incorporating tetraphenylporphyrin zinc (ZnTPP) as a redox center. A cyclic voltammogram (CV) exhibited a couple of redox waves at 0.7 V (vsAg/AgCl). The potential-step chronoamperospectrometry (PSCAS) measurement was carried out on the modified electrode immersed in a 0.1 MLiClO₄ aqueous solution at pH 3.5 under argon atmosphere. Assuming a random distribution of the redox species in the macromolecular structure, the charge transfer distance between the adjacent ZnTPP complexes was obtained as 1.08 nm. The charge hopping distance between the redox centers (1.01 nm) and the bounded motion distance (0.07 nm after 2h) were obtained according to their different response time in the electrochemical measurement. Considering the contributions from both the charge hopping and bounded motion, the second order rate constant of electron hopping was 4.1 × 10⁴ M⁻¹s⁻¹.

The spectra of porphyrins meso-substituted by 4-dimethylaminophenyl groups and protonated at the central pyrrole nitrogens show strong new bands in the visible (450–600 nm) and far-red (700–780 nm). On complete protonation the observed spectra are essentially normal except for enhanced intensity in the farthest red band at ∼ 650 nm. The new bands observed with partial protonation have been qualitatively interpreted as hyperporphyrin spectra due to transitions from 'the filled π orbitals on the amino nitrogen atoms… to the porphyrin LUMOs' [Ojadi ECA, Linschitz M, Gouterman M, Walter RI, Lindsey JS, Wagner RW,]. We now report spectra calculated using the CAChe computer program package. The best structures are determined by energy minimization using a semi-empiral AM1 algorithm; a semi-empirical INDO/CI calculation is then used to obtained the excited state transition energies and intensities. The calculated spectra, although generally blue-shifted compared to experiment, reproduce the hyperporphyrin spectra in those cases where they are observed. In these cases, in contrast to species with normal porphyrin spectra, the HOMO or HOMO-1 is not an orbital on the porphyrin ring but is rather a π-orbital on the unprotonated amino nitrogen that extends over the associated phenyl ring. The calculations suggest that the hyperporphyrin transitions are of Ph(π) → Por(π*) origin, where Ph(π) is an orbital on the phenylamine substitutent and Por(π*) is a LUMO on the porphyrin. It is further suggested that in the fully protonated species the intensity enhancement of the farthest red band in an otherwise normal porphyrin spectrum may relate to phenyl character in the HOMO or HOMO-1.

We performed ab-initio, local density functional calculations of the electronic structure, charge transfer, and optical properties of MgB₂, using the LCAO formalism. The Fermi level of MgB₂ cuts through relatively narrow electron bands which have a dominant contribution from B(2p) states. There is a substantial charge transfer from magnesium to boron atoms. We found the ionic formula for this material to be . A clearly metallic distribution of the electronic charge density in the plane of boron atoms is interwoven with a visibly covalent one in the direction perpendicular to this plane. The calculated optical conductivities from the direct inter-band transitions exhibit a strong anisotropy between σ_xx(ω) or σ_yy(ω) and σ_zz(ω). Due to our application of the BZW procedure, major peaks in the density of states above the Fermi level are at markedly higher energies (1–1.5 eV) than the results of previously reported ones. A similar pattern is followed by our findings for optical conductivities.

We report effects of heat treatments on physical properties and finding optimal heating condition to add value to Thai blue sapphires. The color of sapphire arises from the presence of trace 3d-transition ions in its crystal lattice. For blue sapphire, the color is due to a charge transfer mechanism between Fe²⁺ and Ti⁴⁺ ions. However, iron may adopt both Fe³⁺ and Fe²⁺ due to oxygen vacancies. Fe³⁺ and Fe²⁺ yield sapphire yellow and green colors, respectively. Therefore, we have to convert as many as possible of Fe³⁺ to Fe²⁺ by heating the blue sapphire in N₂ atmosphere for 12 h. Experimental results reveal that the ratio of lattice parameter c/a increases with the heating temperature and reaches maximum at 1700°C, which can be caused by displacement of Fe³⁺ ions or more Fe³⁺ ions being converted to Fe²⁺. ESR signals show that the number of Fe³⁺ ions decreases roughly linearly with the heating temperature. The intense sky blue color was achieved after the 1500°C heat treatment, having the ratio ~0.78. The optimal heat treatment should therefore be at 1500°C in flowing N₂ atmosphere for Thai blue sapphires which yield intense sky blue color and good crystal clarity. The blue sapphires exhibited good clarity but light sky blue due to the increase in lightness after the treatment at 1700°C. A monoclinic distortion of the corundum structure has been found to start at the 1600°C treatment by ESR spectrometer. This is also clearly evident from low angle shifts of XRD peaks after heating at 1700°C. We can therefore conclude that the color change of Thai blue sapphires arises from the conversion of Fe³⁺ to Fe²⁺ and thus the change in crystal field. The monoclinic distortion of the crystal structure may also play an important role in coloring the sapphires after the heat treatment at 1600–1700°C.

The effect of aperiodicity on the charge transfer process through DNA molecules is investigated using a tight-binding model. Single-stranded aperiodic Fibonacci polyGC and polyAT sequences along with aperiodic Rudin–Shapiro poly(GCAT) sequences are used in the study. Based on the tight-binding model, molecular orbital calculations of the DNA chains are performed and ionization potentials compared, as this might be relevant to understanding the charge transfer process. Charges migrate through the sequences in a multistep hopping process. Results for current conduction through aperiodic sequences are compared with those for the corresponding periodic sequences. We find that dinucleotide aperiodic Fibonacci sequences decrease the current while tetranucleotide aperiodic Rudin–Shapiro sequences increase the current when compared with the corresponding periodic sequences. The conductance in all cases decays exponentially as the sequence length increases.

The present study is geared towards investigating methods to increase the tendency of fullerene structures to aggregate with biological systems. To accomplish this task, the encapsulation of metals inside a fullerene structure was performed. The calculations performed demonstrate that the Ca@C₆₀ structure leads to stronger interactions with amino acids at the DFT-BLYP/DND level of theory. Correlations of the dissociation energies, HOMO/LUMO band gaps and hardness are discussed.

Europium chalcogenides receive greater interest because of their interesting properties such as valence transition, semiconductor to metallic transition and structural phase transition etc. In this paper, the charge transfer in Europium Sulphide (EuS) is analyzed by experimental and theoretical X-ray diffraction (XRD) data. The direction and amount of charge transfer are inferred by plotting and comparing the structure factors of the components. The charge transfer parameter thus obtained is 0.29 electron from Eu to S which confirms the intermediate valence fluctuation. Further, the charge transfer in EuS, EuSe and EuTe, is found to be decreasing with the increase of lattice constant and energy gap which correlates the valency and lattice size.

First-principles calculations based on Hartree–Fock (HF) and density functional theory (DFT) levels of approximation have been carried out in order to study the stability of graphene clusters as a function of number of carbon atoms (N). The variation of calculated binding energy per carbon atom with corresponding number of carbon atoms (N) in graphene cluster almost saturates after the cluster size consisting of 96 carbon atoms, with binding energy per carbon atom 8.24 eV/atom. Adsorption of halogen atoms, (F, Cl and Br), on hydrogen passivated graphene (H-graphene) was also studied systematically through first-principles DFT calculations by taking five different H-graphene clusters. The calculations showed that the most stable adsorption site for halogen adatoms on H-graphene being T site with binding energy 2.41 eV (F), 1.48 eV (Cl) and 1.19 eV (Br) on the H-graphene cluster consisting 96 carbon atoms. Moreover, on increasing the size of H-graphene cluster, the binding energy of halogen adatoms found to be increasing. The distances of adatom from the nearest carbon atom of H-graphene sheet were 1.47 Å (F), 2.71 Å (Cl) and 3.01 Å (Br), however, the adatom heights from the H-graphene basal plane were 2.22 Å (F), 2.90 Å (Cl), and 3.19 Å (Br). The bonding of halogen adatoms on H-graphene were through the charge transfer; 0.30 |e| (F), 0.37 |e| (Cl) and 0.19 |e| (Br), from H-graphene to adatoms and includes the negligible local distortion in the underlying planner H-graphene. Charge redistribution upon adsorption induces significant dipole moments 2.16 D (F), 4.81 D (Cl) and 3.08 D (Br) on H-graphene. The calculated HOMO–LUMO energy gap of adatom-H-graphene and H-graphene does not differ significantly up to the cluster size N = 30, however, beyond N = 30 adsorption of halogen adatoms significantly opens the HOMO–LUMO energy gap on H-graphene and the opening of HOMO–LUMO energy gap also saturates from N = 96. Correlation of computed HOMO–LUMO energy gap and corresponding binding energy of adatom-H-graphene systems have been also studied.

In this work, the transport properties of carbon atom chains sandwiched between Au (100) nanoelectrodes with different anchored structures have been studied using a combination of first principles and nonequilibrium Green’s function. The results show that the transport properties of the carbon chain are not only related to the chain length and the number of C atoms (odd/even), but also related to the anchoring structure of nanoelectrodes. For the same chain length and bias voltage, the current of the H–H model is about three times that of the T–T model due to more charge transfer. Meanwhile, it was also found that the C atom in contact with the electrode has a large amount of charge transfer, which provides an effective way for the electronic doping of semiconductor devices. These findings can provide theoretical support for the design of molecular devices based on the C-chain.

Electron spectra of DNA model compounds, adenosine-thymidine and guanosine-cytidine nucleoside base pairs, as well as the relevant homogeneous stacked base pair steps in A-DNA and B-DNA conformations, were investigated using ZINDO semiempirical quantum-chemical method. This work confirms that, in DNA with intact Watson–Crick hydrogen bonding and base stacking, the highest occupied molecular orbitals (HOMO) are residing on purine base residues, whereas the lowest unoccupied molecular orbitals (LUMO) — on pyrimidine base residues. In general, the present results are satisfactorily comparable with the available experimental data. The role of charge transfer excitations in the polymer DNA 260 nm spectral band is discussed.

A detailed study of charge transport properties of synthetic and genomic DNA sequences is reported. Genomic sequences of the Chromosome 22, λ-bacteriophage, and D1s80 genes of Human and Pygmy chimpanzee are considered in this work, and compared with both periodic and quasiperiodic (Fibonacci) sequences of nucleotides. Charge transfer efficiency is compared for all these different sequences, and large variations in charge transfer efficiency, stemming from sequence-dependent effects, are reported. In addition, basic characteristics of tunneling currents, including contact effects, are described. Finally, the thermoelectric power of nucleobases connected in between metallic contacts at different temperatures is presented.

Charge transfer in Ytterbium Selenide (YbSe) is analyzed by using experimental and theoretical XRD data. The direction and amount of charge transfer are inferred by plotting and comparing the structure factors of the components. The charge transfer parameters thus obtained are 0.36ē from Yb to Se.

In doped organic electroluminescent devices (OLEDs), the changes of the fluorescence spectra occurred due to the energy and charge transfer from guest to host. A quantitative relation between the energy and charge transfer for doping OLEDs was studied by means of a Hamiltonian model. It was found that there is a corresponding relation between the EL intensity of doped PVK and the amount of the transferred charge and the change of the energy derived from charge transfer with dopant concentration, and the more the amount of the transferred charge and the decreasing of the energy from charge transfer, the higher the EL intensity in doped OLEDs. We can deduce that the efficient energy transfer results from the transferred charge between PVK and perylene. Based on the transport mechanism of carriers in conjugated polymer, an expression of conductivity was presented. Calculated results indicated that there is also a corresponding relation between the EL intensity of doped PVK and conductivities with dopant concentration. The larger the conductivities, the higher the EL intensity in doped OLEDs.

The interaction between C₆₀ and quantum dots (QDs) has attracted much attention recently due to its potential applications in solar cells, for example, as a promising renewable energy source. In this paper, monolayer self-assembled CdSe/ZnS core/shell QDs films were prepared and drop-coated with different amounts of C₆₀. A significant photoluminescence quenching and an emission lifetime reduction were observed in the QDs-C₆₀ composite films, and the quenching efficiency was increased with the increasing of the C₆₀ content. Together with the reduced emission lifetime, the results here provide direct evidence for the charge transfer process in the QDs-C₆₀ system.

Graphenes with varying number of layers can be synthesized by different strategies. Thus, single-layer graphene is obtained by the reduction of single layer graphene oxide, CVD and other methods besides micromechanical cleavage. Few-layer graphenes are prepared by the conversion of nanodiamond, arc-discharge of graphite and other means. We briefly present the various methods of synthesis and the nature of graphenes obtained. We then discuss the various properties of graphenes. The remarkable property of graphene of quenching fluorescence of aromatic molecules is shown to be associated with photo-induced electron transfer, on the basis of fluorescence decay and time-resolved transient absorption spectroscopic measurements. The interaction of electron donor and acceptor molecules with few-layer graphene samples has been discussed. Decoration of metal nano-particles on graphene sheets and the resulting changes in electronic structure are examined. Few-layer graphenes exhibit ferromagnetic features along with antiferromagnetic properties, independent of the method of preparation. Graphene-like MoS₂ and WS₂ have been prepared by chemical methods, and the materials are characterized by electron microscopy, atomic force microscopy (AFM) and other methods. Boron nitride analogues of graphene have been obtained by a simple chemical procedure starting with boric acid and urea and have been characterized by various techniques.

Single charge transfer process in collision of energetic protons with molecular hydrogens is theoretically studied using a first-order two-effective-center Born approximation. The correct boundary conditions are incorporated in the formalism and the Hartree–Fock molecular wave function for molecular targets and the residual ions are used to calculate the transition amplitude. The interference patterns in the capture differential cross-sections (DCSs) for a given fixed orientation of the molecule, due to the scattering from the two-atomic centers in the molecular targets, are examined. The dependence of the DCSs upon the angle between the molecular axis and the direction of the incident velocity is theoretically investigated. Both average differential and integral cross-sections are calculated. The obtained results are compared with the available experimental data.

The morphology, stability and electronic properties of Pd and Au monolayers supported by MoS₂ substrate have been investigated by using the first principles calculations based on periodic density functional theory (DFT). The results demonstrated that the most stable site is top of Mo top of S ($T_S$) for the adsorption of Pd and Au monolayers on MoS₂. We found that Pd/MoS₂ with lower adsorption energy of $-$0.54eV is energetically more stable than Au/MoS₂. Due to the emergence of impurities state in the bandgap of MoS₂, Pd/MoS₂ and Au/MoS₂ display metallic character upon coating of metal monolayers. We demonstrated that the Pd monolayer strongly hybrids with underlying Mo and S around the Fermi level, which is achieved by the intermediate Pd–S–Mo hybridized chains. On the contrary, intensity of impurities states around the fermi level is much weaker for Au/MoS₂, which can be explained by weak hybridizations between sp state of Au and 4$d$ state of nearest neighboring Mo. The calculated results demonstrated that work functions are markedly modulated to 4.99 and 6.23eV after coating of Pd and Au monolayer, respectively, which can be qualitatively explained by the fact that Pd (as accepter) received charge from the MoS₂ host, while Au donated charge to the host. These findings promise potential applications in the fields of nanoelectronics in future, such as it's helpful to choose suitable electrode materials for MoS₂-based nanodevices.

We have studied the adsorption of SO₂ molecule on the pristine and nitrogen-doped Au decorated TiO₂ nanoparticles using density functional theory (DFT) calculations, taking into attention van der Waals (vdW) interactions. Theoretically, it was found that SO₂ molecule adsorbs on the Au decorated TiO₂ in a bridge geometry. The adsorption of SO₂ molecule on both the Au and TiO₂ sides were considered. On the TiO₂ side, the fivefold coordinated titanium sites were found to be the most stable binding sites, providing double contacting point between the nanoparticle and SO₂. The results suggest that the adsorption of SO₂ on the N-doped Au decorated TiO₂ is more energetically favorable than the adsorption on the pristine one. Thus, nitrogen doping is dominant during the adsorption of SO₂. The changes in the electronic structure of the system and the net charge transfer upon adsorption of SO₂ molecule were studied in detail. Also, HOMO–LUMO isosurfaces of the adsorption systems were presented for discussion of results. By considering large overlaps in the projected density of states (PDOSs) of the interacting atoms, we found that the molecule is bound most strongly to the Au and fivefold coordinated titanium positions. The inclusion of vdW interactions is important for obtaining final equilibrium adsorption configurations of SO₂ on Au decorated TiO₂ nanoparticles. Our results thus suggest that N-doped Au decorated TiO₂ would be an efficient sensor for SO₂ gas detection in the environment.

A dibenzothiophene-based two-photon photopolymerization (TPP) initiator named E,E-1,4-bis[3-vinyl-dibenzothiophene]-2,5-dimethoxybenzene (VBPMOB) with intense two-photon fluorescence (TPF) was investigated by UV–visible, fluorescence, pump–probe and time-resolved fluorescence spectroscopies. The maximum two-photon absorption (TPA) cross-section of VBPMOB in DMF was determined to be 274GM. In addition, the TPA cross-section at 800nm was 230GM, which is the most commonly used wavelength for TPP. The ultrafast spectra result indicated that after excitation, the VBPMOB molecule will first undergo a fast internal conversion process to form an intramolecular charge transfer (ICT) state with a certain width followed by relaxation of the ICT state with fluorescence emission.

Presented in this paper is the result of a study on the utilization of Al-doped ZnO (AZO) and Ti-doped ZnO (TiZO) as a blocking layer (BL) in Dye-Sensitized Solar Cells (DSSCs). The DSSCs structure was FTO/BL/TiO₂/dye/electrolyte/Pt on FTO. The crystallinity and crystallite size of those BLs were shown to be affected by the dopant type and its concentration. From the photovoltaic characteristics of those DSSCs, the TiZO layer showed better contribution in enhancing the open voltage as a BL in comparison to the AZO. The effect of these layers on the solar cell characteristics in the fabricated DSSCs was found to be not merely due to the difference in their Ohmic resistance. There is also the contribution of back recombination kinetics from the photoexcited electrons via the dopant states in the AZO and TiZO layers, which are different among the layers due to the difference in dopant states distributions below their conduction bands.