Charge transfer between ZnO crystals and dye layers

The interaction between crystal and adsorbed dye molecules has been studied under well defined conditions by measurements of field effect and spectrally sensitized photoconductivity. The (101̄0) surfaces of n-type ZnO crystals (band gap 3.3 eV) are cleaned in ultrahigh vacuum. A pretreatment with atomic hydrogen produces an accumulation layer. Merocyanine (polymethine) dye molecules are deposited by sublimation in the same vacuum (coverage (1–2000) × 10¹⁴ cm^?2. Optical excitation of the dye causes a sensitized photoconductivity in the ZnO crystal close to the surface. The spectra distribution resembles the absorption spectrum of the dye with a maximum at 2.3 eV. An electric field applied perpendicular to the dye covered surface induces charge carriers in the crystal and changes the surface conductivity (field effect). Additional excitation of the dye by light causes a slow relaxation of the field-induced change of surface conductivity. This relaxation is observed for both signs of the field. Furthermore a memory of the dye covered crystals has been found. It can be programmed by field and light, read out via the surface conductivity and quenched by light. A phenomenological model for relaxation and memory is refined by kinetic equations and by considerations about charge transport within the dye layer. The observations can only be explained by a charge transfer between crystal and dye operating in both directions. From these results the following conclusion is drawn for the mechanism of spectrally sensitized photoconductivity of the present system: An electron transfer between dye molecules and crystal represents the decisive step rather than an energy transfer.     

Interaction of dyes with nanoclusters adsorbed on the surface of AgBr microcrystals

We have performed a comparative luminescence investigation into spectral sensitization with dyes of holographic emulsions that contain AgBr microcrystals and are subjected to chemical sulfur or reducing sensitization. We show that, upon spectral sensitization of silver sulfide (Ag₂S) _n nanoclusters located on AgBr microcrystals, the polylayer adsorption of dyes on the surface of nanoclusters is observed, which is caused by van der Waals forces in the J-aggregated state. For silver oxide (Ag₂O) _m nanoclusters located on AgBr micro-crystals, the adsorption of dyes on their surface occurs only if chlorine atoms of heterocyclic residues of the dye interact with the nanocluster surface, which determines the adsorption of the dye as its chemisorption. In the remaining investigated cases, the polylayer adsorption of dyes during the spectral sensitization occurs not on the surface of nanoclusters but rather on the surface of AgBr microcrystals, initially in the J-aggregated state and then in the molecular- and H-aggregated states.     

感绿光谱增感染料在氯化银微晶表面的电子陷阱效应

光谱增感技术可使卤化银感光材料实现对全波段感光，同时光谱增感技术在现代光信息记录与存储、光电器件、太阳能转换与存储等领域具有重要的应用.应用微波吸收介电谱技术研究了立方体氯化银吸附感绿菁染料后的光电子衰减特性，建立了氯化银光电子衰减动力学模型，根据此模型结合光电子衰减实验结果对光谱增感染料吸附在卤化银表面的电子陷阱效应进行了分析.研究结果表明：当染料以单分子态吸附在卤化银表面时，染料起浅电子陷阱效应；染料以J聚集体吸附在卤化银表面时，染料起到了深电子陷阱效应.浅电子陷阱与深电子陷阱效应的临界浓度为每40g 关键词： 感绿染料 氯化银 光电子衰减 电子陷阱效应     

Experimental studies on the spectral sensitization of photoconductivity on zinc-oxide crystals

The photoconductivity at the prism-surfaces(1$\text{1}$00) of ZnO-crystals was studied before and after adsorption of a merocyanine-dye under well defined experimental conditions in ultrahigh vacuum.The undyed crystal exhibited a pronounced photoconductivity only for fundamental absorption (UV-light, band-band excitation). The dyed crystal showed a very enhanced photoconductivity in the visible surmounting even the values measured for band-band light. The two-component system merocyanine dye/ZnO-surface is thus well suited for studies of spectral sensitization.The influence of different parameters was studied, e.g. pretreatment of the surface with oxygen or hydrogen (variation of surface conductivity) and the effect of temperature between 90 and 298 K. The conductivity of the crystals in the dark may be lowered or increased during dye-deposition. The spectrally sensitized photoconductivity was found negative or positive. No decrease was found for samples with low surface conductivity before dyeing.An activation energy of 34 meV was deduced from the temperature dependence of the initial slope of the sensitized photoconductivity. The sign of the photosignal may change between 90 and 298 K. Pretreatment or treatment of the dyed surface with oxygen or atomic hydrogen did not influence the spectral distribution of photoconductivity in the sensitization region.     

Sensitized anti-Stokes luminescence centers in microcrystals of Zn<Subscript>0.6</Subscript>Cd<Subscript>0.4</Subscript>S solid solutions with adsorbed dye molecules and few-atomic silver clusters

The sensitized anti-Stokes luminescence excited by radiation with wavelengths from 610 to 750 nm and flux densities of 10¹⁴–10¹⁵ quanta/(cm²·s) is detected for microcrystals of Zn _0.6 Cd _0.4 S solid solutions with adsorbed organic malachite green and methylene blue dye molecules. The position of its excitation spectra coincides with that of the absorption spectra of adsorbed dye molecules, which suggests the cooperative mechanism of its occurrence. The possibility of amplification of the anti-Stokes luminescence by means of adsorption of silver atoms and few-atomic silver clusters, in addition to the dye molecules, on the Zn _0.6 Cd _0.4 S surface is investigated. It is assumed that in the latter case, the anti-Stokes luminescence is excited as a result of two-quantum optical transitions with electron or electron excitation energy transfer from the dye molecules adsorbed on the Zn _0.6 Cd _0.4 S surface to silver atoms and few-atomic silver clusters creating deep local states with photoionization energies of 1.8–2.0 eV in the gap. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 21–26, March, 2008.     

A lattice model of nearest-neighbor hopping conduction and its application to neutron-doped Ge: Ga

A model of hopping conduction between nearest neighbors is developed in which the majority and compensating dopant atoms are assumed to form a unified simple cubic lattice in a crystalline matrix. The hopping of carriers occurs when thermally activated “equalization” of majority impurity levels takes place, while the compensating impurities block the corresponding sites. The range of relatively high temperatures is considered in which the interactions giving rise to a Coulomb gap can be neglected and the density of states of the majority impurity band is Gaussian. The concentration dependences of the activation energy for hopping conductivity ? ₃ (nonmonotonic and having a maximum) and the preexponential factor σ₃ are found. The results are compared with experimental data obtained by different authors for neutron-doped Ge: Ga.     

Columinescence of dye molecules in nanostructures of metal ion complexes

The mechanism of columinescence (fluorescence sensitization) of dyes incorporated in nanostructures of metal complexes is studied. It is shown for the first time that the columinescence of dyes is due to the transfer of excitation energy from ligands and metal ions of complexes that form nanostructures. It is proven that the dye columinescence of rhodamine 6G (R6G) molecules incorporated into nanostructures of Al(DBM)₃phen, Al(DBM) _n (OH)_{6 ? 2n} , and Eu(DBM)₃phen (DBM is dibenzoylmethane) nanostructures is completely determined by the singlet excitation energy migration from ligands to R6G molecules. It is shown that, at small concentrations of R6G, the R6G columinescence intensity is lower in nanostructures of metal complexes with a high probability of S-T conversion and that this difference disappears at large concentrations of R6G. In the case of Nile blue (whose S ₁ level lies below the ⁵ D ₀ level of Eu(III)) incorporated in nanostructures of Eu(DBM)₃phen complexes, as well as in nanostructures of Al(DBM)₃phen and Gd(DBM)₃phen complexes with admixture of Eu complexes, we observed the S-S energy transfer from DBM to NB in addition to the delayed sensitized fluorescence of NB previously observed in nanostructures of Eu complexes, which was caused by the energy transfer from the ⁵ D ₀ level of Eu(III) to NB. At dye concentrations below 100 nM, the efficiency of NB sensitization due to the migration of singlet excitation energy from DBM is lower than in the case of the energy transfer from Eu(III) ions, while, at large concentrations of the dye, the S-S energy transfer successfully competes with the sensitization of NB by Eu(III) ions. The use of dye columinescence makes it possible to easily determine dye concentrations of 2–100 nM in solutions with standard spectrofluorimeters.     

Surface conduction mechanisms and the electrical properties of Al2O3 humidity sensor

The electrical properties of Al₂O₃ humidity sensor are governed by the variations in the surface conductivity of a porous anodic Al₂O₃ film with humidity. We propose a two-carrier mechanism to describe charge transport on porous alumina. At low humidities, a phonon-induced electron tunneling is postulated between donor water sites; at high humidities protonic conduction dominates. A physical model of surface conduction is developed using energy-band concepts. Based on this model, theoretical expressions are presented for the surface conductivity. The model predicts a log-normal distribution of surface conductivity amongst the pores which conforms to the observed electrical properties of the sensor. The experimental plot of log σ versus T^-1 shows that the activation energy varies on transition from the liquid-water state to electron tunneling on the hydroxylated surface. The model also prescribes the Al₂O₃ film-growth parameters for low-and high-humidity sensors which will be useful for fabricating devices of desired response characteristics.     

Holographic recording on the basis of forbidden singlet-triplet electronic transitions

The possibility of recording holograms in thick layers of a photopolymeric material on the basis of a forbidden singlet-triplet transition of a sensitizing dye, erythrosine, is theoretically substantiated and experimentally confirmed. A single hologram and 16 superimposed holograms are recorded with a low-power He-Ne laser (632 nm) at high concentrations of the sensitizing dye and high optical densities in the range of its main absorption. The diffraction efficiency of the single hologram is ～50%. The dependence of the increase in the diffraction efficiency of a transmission grating in samples of photopolymeric material on the intensity of incident radiation of a Kr⁺ laser (647 nm) is studied. The observed linear dependence of the maximal rate of increase in the diffraction efficiency in the photopolymer on the incident light intensity is explained by the manifestation of a two-step excitation of the dye (T ₁←S ₀, T ₂←T ₁) in the samples studied.     

The mutual influence of two different dyes on their sensitized fluorescence (cofluorescence) in nanoparticles from complexes

We have studied the fluorescence sensitization and quenching for pairs of different dyes simultaneously incorporated into nanoparticles from complexes M(diketone)₃phen, where M(III) is La(III), Lu(III), or Sc(III); diketone is p-phenylbenzoyltrifluoroacetone (PhBTA) or naphthoyltrifluoroacetone (NTA); and phen is 1,10-phenanthroline. We have shown that, upon formation of nanoparticles in the solution in the presence of two dyes the concentrations of which are either comparable with or lower than the concentration of nanoparticles (<20 nM), the intensities of the sensitized fluorescence of dyes in nanoparticles in binary solutions and in solutions of either of the dyes coincide. We have found that the intensity of sensitized fluorescence of small (<20 nM) concentrations of rhodamine 6G (R6G) or Nile blue (NB) increases by an order of magnitude upon simultaneous introduction into nanoparticles of 1 μM of coumarin 30 (C30), while the intensity of fluorescence of C30 sensitized by complexes decreases by an order of magnitude. The same effect is observed as 1 μM of R6G are introduced into nanoparticles with NB ([NB] ≤ 20 nM). The increase in the fluorescence of dye molecules upon their incorporation from the solution into nanoparticles from complexes is noticeably lower than that expected from the proposed ratio of concentrations of complexes and dyes in nanoparticles. Analysis of the obtained data indicates that the introduction of large concentrations of C30 or R6G dyes into nanoparticles makes it possible to prevent large energy losses due to impurities or upon transition to a triplet state that arises during the migration of the excitation energy over S ₁ levels of complexes. Energy accumulated by these dyes is efficiently transferred to another dye that is present in the solution at lower concentrations and that has a lower-lying S ₁ level, which makes it possible to increase its fluorescence by an order of magnitude upon its incorporation into nanoparticles.     

Modeling the structure and spectral properties of sensitizing black dye for nanocrystalline TiO<Subscript>2</Subscript> solar cells

Spectral and electronic properties of a sensitizing black dye; a 4,4′,4′′-tricarboxy-2,2′:6′,2′-terpyridine)tris(isocyanato) ruthenium(II) complex; for nanocrystalline TiO₂ solar cells have been investigated by modern methods of quantum chemistry. The light absorption mechanism in the lowest excited triplet states of the dye was studied. The efficiency of electron injection into nanocrystalline TiO₂ is shown to depend on both the nature of charge-transfer states and asymmetric deformation of an excited triplet term of the black dye.     

Efficient solar energy conversion using TiO2 nanotubes produced by rapid breakdown anodization – a comparison

We report on dye‐sensitization of different TiO₂ nanotube layers, their photoelectrochemical response and their efficiency for solar energy conversion. The tubes compared in this study were either grown by controlled Ti anodization in fluoride containing electrolytes or by rapid breakdown anodization (RBA) of Ti in fluoride free electrolytes. After converting the different tube layers to anatase and sensitizing with Ru‐dye (N719), clearly layers consisting of RBA‐NTs show a significantly higher photoresponse and conversion efficiencies than tubes formed under self‐ordering conditions. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A quantitative understanding of pressure dependent conductivity of FeSi1−xGex

A new density of states model, referred to as the Gaussian density of states, is proposed for the quantitative understanding of the electrical conductivity behaviour of FeSi Kondo insulating system. The effects of electron correlation and disorder, responsible for the physical properties of this system, are judiciously incorporated in this model. Within the framework of this model, a detailed quantitative analysis of the temperature and pressure dependent electrical conductivity data of FeSi_1−xGe_x (x=0.0, 0.05 and 0.20) reported by Awadhesh Mani et al. [Phys. Rev. B 63 (2001) 115103] has been carried out. From these analyses the complicated pressure dependence of energy gap seen experimentally in these samples could be satisfactorily rationalized.     

Study of density of localized states in Cd4Se96−xSx (x=0, 4, 8, 12) chalcogenide semiconductor

Thin films of Cd₄Se_96−xS_x (x=0, 4, 8, 12) chalcogenide semiconductor were deposited by the thermal evaporation technique on glass substrates. XRD pattern of CdSeS alloys show that the grain size decreases with the concentration of Sulfur (S). The surface morphology changes due to the addition of sulfur content. The effect of sulfur on the DC conductivity has been investigated, which show that the DC conductivity is a thermally activated process. The Mott parameter shows that dominate conduction is in the localized states, also the addition of sulfur in Cd–Se results an increase in electrical conductivity, which may be due to shift of Fermi level. Current–voltage (I–V) measurements at different fixed temperatures show two regions; Ohmic conduction at low bias having a unit slope, and non ohmic conduction at high bias. Observation of the data shows that conduction is dominated by trap limited space charge limited conduction (SCLC), from where the density of state has been calculated using SCLC measurement data. The increase in the density of states with sulfur concentration may be due to the increase in the defect states.     

Enhanced performance of bi-layer Nb2O5 coated TiO2 nanoparticles/nanowires composite photoanode in dye-sensitized solar cells

Dye sensitized solar cells (DSSCs) were fabricated based on coumarin NKX-2700 dye sensitized bi-layer photoanode and quasi-solid state electrolyte sandwiched together with cobalt sulfide coated counter electrode. A novel bi-layer photoanode has been prepared using composite mixtures of 90 wt.% TiO₂ nanoparticles + 10 wt.% TiO₂ nanowires (TNPWs) as active layer and Nb₂O₅ is coated on the active layer, which acts as scattering layer. Hafnium oxide (HfO₂) was applied over the TNPWs/Nb₂O₅ photoanode film, as a blocking layer. TiO₂ nanoparticles (TNPs), TiO₂ nanowires (TNWs) and TNPWs/Nb₂O₅ were characterized by X-ray diffractometer (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The sensitizing organic dye coumarin NKX-2700 displayed maximum absorption wavelength (λ_max) at 525 nm, which could be observed from the UV–vis spectrum. DSSC-1 fabricated with composite bi-layer photoanode revealed enhanced photo-current efficiency (PCE) as compared to other DSSCs and illustrated photovoltaic parameters; short-circuit current J_SC = 18 mA/cm², open circuit voltage (V_OC) = 700 mV, fill factor (FF) = 64% and PCE (η) = 8.06%. The electron transport and charge recombination behaviors of DSSCs were investigated by electrochemical impedance spectra (EIS) and the results illustrated that the DSSC-1 showed the lowest charge transport resistance (R_tr) and the longest electron lifetime (τ_eff). Therefore, in the present investigation, it could be concluded that the novel bi-layer photoanode with blocking layer increased the short circuit current, electron transport and suppressed the recombination of charge carriers at the photoanode/dye/electrolyte interface in DSSC-1.     

Hydrothermal synthesis of TiO2 nanocrystals in different basic pHs and their applications in dye sensitized solar cells

In this research TiO₂ nanocrystals with sizes about 11–70 nm were grown by hydrothermal method. The process was performed in basic autoclaving pH in the range of 8.0–12.0. The synthesized anatase phase TiO₂ nanocrystals were then applied in the phtoanode of the dye sensitized solar cells. It was shown that the final average size of the nanocrystals was larger when the growth was carried out in higher autoclaving pHs. The photoanodes made of TiO₂ nanocrystals prepared in the pHs of 8.0 and 9.0 represented low amounts of dye adsorption and light scattering. The performance of the corresponding dye sensitized solar cells was also not acceptable. Nevertheless, the energy conversion efficiency was better for the state of pH of 9.0. For the photoanodes made of TiO₂ nanocrystals prepared at autoclaving pH of 10.0, the dye adsorption and light scattering were quite higher. The photovoltaic characteristics of the best cell in this state were 15.25 mA/cm², 740 mV, 0.6 and 6.8% for the short-circuit current density, open-circuit voltage, fill factor and efficiency, respectively. The photoanodes composed of TiO₂ nanocrystals prepared in autoclaving pHs of 11.0 and 12.0 demonstrated lower amount of dye adsorption and higher light scattering. This was quite considerable for the state of pH of 12.0. The energy conversion efficiencies were consequently decreased compared to that of the pH of 10.0. The optimum situation was finally discussed based on the nanocrystals size and its influence on the sensitization and light harvesting efficiency.     

Critical behavior of the surface of FeBO3 single crystals

The behavior of the surface and near-surface layers of macroscopic FeBO₃ single crystals is studied over the temperature range from 291 K to Neél temperature (T _N ) using depth-selective conversion-electron Mössbauer spectroscopy. Three different phases or states, namely, an antiferromagnetically ordered phase (similar to the crystal bulk state), a surface phase, and a transition layer between them coexist near the Neél point in a surface layer ～500 nm thick. The critical parameters found for the bulk phase agree well with the theoretical critical index ν_th?0.63 predicted by the 3D Ising model. As the crystal surface is approached, the critical parameter β increases to 0.51(2) but remains smaller than the value of β=0.8 for the surface of a semi-infinite Heisenberg model. Therefore, the effective dimensionality of the system, being equal to 3 in the bulk, decreases at the crystal surface.     

Molecular dynamics study for the thermal conductivity of diatomic liquid

The thermal conductivity of diatomic liquids was analyzed using a nonequilibrium molecular dynamics (NEMD) method. Five liquids, namely, O₂, CO, CS₂, Cl₂ and Br₂, were assumed. The two-center Lennard-Jones (2CLJ) model was used to express the intermolecular potential acting on liquid molecules. First, the equation of state of each liquid was obtained using MD simulation, and the critical temperature, density and pressure of each liquid were determined. Heat conduction of each liquid at various liquid states [metastable (ρ=1.9ρ_cr), saturated (ρ=2.1ρ_cr), and stable (ρ=2.3ρ_cr)] at T=0.7T_cr was simulated and the thermal conductivity was estimated. These values were compared with experimental results and it was confirmed that the simulated results were consistent with the experimental data within 10%. Obtained thermal conductivities at saturated state were reduced by the critical temperature, density and mass of molecules and these values were compared with each other. It was found that the reduced thermal conductivity increased with the increase in the molecular elongation. Detailed analysis of the molecular contribution to the thermal conductivity revealed that the contribution of the heat flux caused by energy transport and by translational energy transfer to the thermal conductivity is independent of the molecular elongation while the contribution of the heat flux caused by rotational energy transfer to the thermal conductivity increases with the increase in the molecular elongation. Moreover, by comparing the reduced thermal conductivity at various states, it was found that the increase of thermal conductivity with the increase in the density, or pressure, was caused by the increase of the contribution of energy transfer due to molecular interaction.     

Photostimulated luminescence flash: from scientific photography to photonics of nanostructured materials

We have analyzed the possibilities of using the phenomenon of photostimulated luminescence flash for optical diagnosing of energy levels of structural and impurity defects of semiconductor crystals and nanostructures. New data on the spectra of deep localized states associated with adsorbed few-atom clusters Zn _n on the surface of ZnS; clusters Cd _n , Cu _n , and Ag _n on the surface of CdS; and clusters Ag _n on the surface of AgBr(I) have been presented, as well as results of investigation of photostimulated assembling processes of few-atom clusters on the surface of crystals using this phenomenon. We are the first to show the potential of the luminescence flash technique for studying the mutual arrangement of the levels of dye molecules and the bands of the crystal on the surface of which they are adsorbed, as well as of the spectra of localized states in colloidal CdS semiconductor quantum dots.     

TiO2表面质子化对染料敏化太阳能电池性能及吸附染料稳定性的影响

采用致密平整TiO₂薄膜作为染料敏化太阳能电池光电极,并研究了HCl处理表面质子化对电池性能的影响. 结果表明,HCl处理后电池的短路电流显著提升,电池的开路电压则有轻微的下降,电池电流提升了31%,而能量转化效率则提升了25%. 这是因为TiO₂的表面质子化增强了吸附染料与TiO₂间的电学耦合,提高了染料中激发电子向TiO₂导带的注入速率. 而电压的下降,一方面是由于质子化会引起TiO₂导带能级