Field and photo-field electron emission from self-assembled Ge–Si nanostructures with quantum dots

Monolayer and multilayer Ge nanocluster structures were prepared on Si(1 0 0) using molecular beam epitaxy. The cluster size was 10 nm and cluster density was 10¹⁰ cm⁻². A stable field electron emission was obtained from these structures, showing current peaks in the current–voltage characteristics, which may be attributed to the resonant electron tunneling via the energy levels of the nanocluster potential well. For cluster multilayers, the current–voltage curves also showed current peaks with a complex shape. The cluster multilayer structures had a considerable temperature sensitivity, as well as photosensitivity, in the wavelength range from 0.4 to 10 μm.     

Theoretical and experimental study of the biamperometry for irreversible redox couple in flow system

The biamperometry for the direct determination of irreversible redox analytes in flow system has been proposed based on coupling two independent and irreversible couples to form the biamperometric detection scheme. In this work, the method is studied both theoretically and experimentally. Equations describing the current–voltage characteristics and the current–concentration relationship are presented. The influence of the applied potential difference (ΔE) and the half-wave potential difference (ΔE_1/2) between two irreversible couples on the method are discussed. It shows that small ΔE_1/2 is favorable to construct the biamperometric detection system and to achieve high sensitivity and selectivity. Increasing ΔE leads to an increase in sensitivity. This is, however, accompanied by a decrease in selectivity and signal-to-noise ratio. To construct the biamperometric scheme for the irreversible systems with large ΔE_1/2, two approaches, adjusting acidity of supporting electroyte or adding new irreversible couple, are proposed by taking uric acid/platinum oxide and phenol/permanganate systems as examples. Uric acid and phenol are, respectively, detected in a flow injection system with a biamperometric detector.     

Native surface structure of natural soil particles determined by combining atomic force microscopy and X-ray photoelectron spectroscopy

A procedure was developed for handling natural soil particles and probing their native surface structure by atomic force microscopy (AFM) under water. This procedure was used to investigate the nanometer scale organisation of organic matter at the surface of sand particles taken from three soil horizons. The latter were selected for the contrasted properties of their organic matter, namely Podzol E and Bh horizons and a Cambisol A–B horizon. The presence of an adsorbed layer was visualised at the surface of Podzol Bh and Cambisol particles in the form of aggregated structures that interacted with the AFM probe. Surface analysis by X-ray photoelectron spectrometry (XPS) confirmed the carbonaceous nature of this adsorbed layer. Displacement of organic matter by the scanning probe was directly evidenced for Podzol Bh sand particles. Such displacement was not observed for Cambisol particles. A dramatic effect of drying on the concentration, nanometer scale distribution and properties of the adsorbed organic matter was clearly demonstrated by combining AFM imaging and XPS analysis. The procedure developed here gives access to direct, nanoscale information of the surface structure of sand particles and offers promising prospects for the characterisation of other environmentally-relevant particles in native conditions.     

Study of the influence of substituents on spectroscopic and photoelectric properties of zinc phthalocyanines

In this paper we describe conversion of light energy into electric energy in a photoelectrochemical cell containing zinc phthalocyanine (ZnPc) dyes. For all dyes investigated in liquid polyvinyl alcohol with dimethyl sulfoxide solution and located in the photoelectrochemical cell the following measurements have been done: absorption, fluorescence, photoacoustic spectra, photovoltaic spectra, kinetics of photocurrent and current–voltage characteristics. It has been shown that all dyes located in the photoelectrochemical cell are able to convert light into electric energy but with different effectiveness. The influence of substituted different peripheral groups to ZnPc core and the correlation between the molecular structure and effectiveness of solar to electric energy conversion were observed and described. The unique behavior of ZnPc substituted with fluorines was indicated.     

Model of intradiffusional kinetics of adsorption on active carbons

A model is proposed for the intradiffusional kinetics of adsorption, taking into account the different mobilities of the molecules adsorbed in micro-, supermicro-, and mesopores and the bonding of such pores in an active carbon particle. Within the framework of the model it has been shown that the effective diffusion coefficient of the adsorbed molecules depends on the structural and sorption characteristics of the material and the concentration conditions of the process being carried out. The model has been used to make an analysis of experimental data for adsorption of organic materials dissolved in water by active carbon having nonuniform pores.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 1, pp. 79–83, January–February, 1986.     

Fundamental studies on the intermediate layer of a bipolar membrane: Part III. Effect of starburst dendrimer PAMAM on water dissociation at the interface of a bipolar membrane

Starburst dendrimer polyamidoamine (PAMAM) with ellipsoidal or spheroidal shape is structure-regular and has much more amino groups than conventional polymers. This paper investigates the possibility of these amino groups on water dissociation in a bipolar membrane interface. To do this, a bipolar membrane is prepared by casting the solution of sulfonated poly(phenylene oxide) (SPPO) in dimethyl formamide (DMF) on a commercial anion exchange membrane that is immersed in PAMAM aqueous solution in advance. The existence of PAMAM adsorbed on the membrane is proved by X-ray photoelectron spectroscopy (XPS), and the adsorption amount is evaluated by weighting method. The junction thickness of the prepared bipolar membrane is determined by electrochemical impedance spectroscopy (EIS), and the performance is evaluated by current–voltage curves. The experiments show that both the generation and concentration of PAMAM would strongly affect the characteristics of the bipolar membranes. There exists a transitional concentration for various generations PAMAMs to catalyze effectively the water dissociation, and above or below the transitional concentration the performance of bipolar membranes is decreasing. The higher the generation, the lower the concentration. Moreover, at a fixed solution concentration, there is not the simple relation of monotone decreasing or increasing between the performance of bipolar membranes and the generations of PAMAMs. All these can be explained according to the characteristics of PAMAMs combined with available water dissociation theory.     

Biomolecular photodiode consisting of cytochrome c-adsorbed hetero-Langmuir–Blodgett films

Photoinduced electron transfer in biomolecular photodiode consisting of protein-adsorbed hetero-Langmuir–Blodgett (LB) films was investigated. Four kinds of functional molecules, cytochrome c, viologen, flavin, and ferrocene, were used as a second electron acceptor, a first electron acceptor, an electron sensitizer, and an electron donor, respectively. The hetero-LB film was fabricated by subsequently depositing ferrocene, flavin, and viologen onto the pretreated ITO glass. Cytochrome c-adsorbed hetero-LB films were prepared by soaking the hetero-LB films into the phosphate buffer solution containing cytochrome c. To verify the optimal adsorption conditions of cytochrome c molecules onto the viologen LB layers, the UV-absorption spectrum and atomic force microscopy observations of LB films were performed. Finally, the metal/insulator/metal structured molecular device was constructed by depositing aluminum onto the surface of the cytochrome c-adsorbed hetero-LB films. For photoelectric response properties, the current–voltage characteristic and photoswitching effect of the proposed molecular photodiode were investigated. To verify the charge shift, transient photocurrent of the molecular photodiode was measured.     

Galvanomagnetic size studies of metallic surface processes

This review gives an account of basic ideas concerning scattering of current carriers at metallic surfaces. It deals with the physical picture and theory of surface-sensitive galvanomagnetic size effects, i.e., transverse magnetic resistance, static skin effect, Sondheimer oscillations, and conduction electron focusing. These processes are employed to study the peculiarities of current carrier surface scattering with regard to the electron–hole transfers. Diffraction of conduction electrons at adsorbed submonolayer lattices is considered. An outlook is given of the application of these phenomena to the study of adsorption and ordering of adsorbed submonolayer films of various symmetries and chemical content, involving, in particular, precursor states. Influence of the diffusion of adsorbed atoms into the substrate on the kinetic size effects is considered in detail.     

Gas sorption properties of microporous metal organic frameworks

A low-temperature gas sorption study has been carried out on four three-dimensional microporous metal organic framework (MMOF) structures and two two-dimensional layered structures. The pore characteristics are analyzed based on the argon adsorption-desorption isotherms at 87 K. The results from hydrogen sorption experiments conducted at 77 and 87 K show that all MMOFs have a relatively high hydrogen uptake, with adsorbed hydrogen densities falling in the range of liquid hydrogen. Isosteric heats of hydrogen adsorption data calculated based on the Clausius-Clapeyron equation are consistent with these observations, indicating strong sorbent-sorbate interactions.     

Length dependence of coherent electron transportation in metal–alkanedithiol–metal and metal–alkanemonothiol–metal junctions

We have applied the elastic-scattering Green’s function theory to study the coherent electron transportation processes in both metal–alkanedithiol–metal (gold–[S(CH₂)_nS]–gold, n = 8–14) and metal–alkanemonothiol–metal (gold–[H(CH₂)_nS]–gold, n = 8–14) at the hybrid density functional theory level. It is shown that the current decreases exponentially with the molecular length. At the low temperature limit the electron decay rate, β, for alkanedithiol junction is found to be around 0.30/CH₂ at 1.0 V bias, much smaller than the calculated value of 0.60/CH₂ for alkanemonothiol junction. The decay rate for alkanedithiol junction at the room temperature is neither sensitive to the activation of the Au–S stretching vibrational mode nor to the external bias. The calculated current–voltage characteristics and decay rates for both junctions are in excellent agreement with the corresponding experimental results.     

Probing organic field effect transistors in situ during operation using SFG

In this communication, we report results obtained using surface-sensitive IR+Visible Sum Frequency Generation (SFG) nonlinear optical spectroscopy on interfaces of organic field effect transistors during operation. We observe remarkable correlations between trends in the surface vibrational spectra and electrical properties of the transistor, with changes in gate voltage (VG). These results suggest that field effects on electronic conduction in thin film organic semiconductor devices are correlated to interfacial nonlinear optical characteristics and point to the possibility of using SFG spectroscopy to monitor electronic properties of OFETs.     

Voltage-Height Spectroscopy in the <Emphasis Type="Italic">Ex Situ</Emphasis> Configuration of a Scanning Tunneling Microscope

Basic regularities of electrochemical processes in the gap of an ex situ scanning tunneling microscope in conditions of condensation of air moisture at the sample surface are considered on a qualitative level. A layer of condensed moisture is viewed as an electrolyte in a two-electrode cell. The depolarizers present in this layer may experience electrochemical conversions on the tip and in an area of the sample surface near the tip. As a result, the recorded “tunneling” current includes electrochemical constituents. Depending on the electrochemical processes in the gap, various dependences of the tip-sample distance on the current and applied voltage can be expected. For preliminary diagnostics of processes in the gap it is suggested to use voltage-height spectra, whose shape and characteristic heights are sensitive to the nature and location of redox active species. Experimental data for various films on conducting supports (quasi-two-dimensional adsorbed layers of hemin and peroxidase, electrodeposited nonstoichiometric tungsten oxides, doped tin dioxide, solid electrolyte with ionic conduction) are presented as an examples.__________Translated from Elektrokhimiya, Vol. 41, No. 5, 2005, pp. 583–595.Original Russian Text Copyright © 2005 by Yusipovich, Vassiliev.     

Nonlinear electrical dipolar switching at single molecular scale

At the single molecular scale (less than 2 Å × 2 Å × 9.8 Å), the nonlinear electrical dipolar switching behavior from crystalline two-monolayer polyvinylidene fluoride films was measured using a scanning tunneling microscope (STM). The atomic structure of the polymer chain was clearly imaged by the STM. The nonlinear switching behavior at the single molecular scale appears as the hysteresis in the tunneling current–voltage relationship with switching onset voltage of 0.19 V/monomer. The nonlinear dipolar switching behavior at the single molecular scale has many potential applications including single molecular scale switching devices and re-writable non-volatile memories.     

New possibility for an organic semiconductor: a smectic liquid crystalline semiconductor having a long conjugated core and two long alkyl chains

We report a new possibility for liquid crystalline organic semiconductors. These materials exhibit smectic liquid crystalline phases, in which the molecules assume a smectic molecular order by self-assembly. Because of the strong dispersion force among long alkyl chains, on cooling, smectic molecular order was retained at room temperature. A charge transport ability was also retained. The conductivity of a device having smectic liquid crystalline order is about 5×10⁷ that of a device with no smectic order. The current - voltage characteristic of the device has a very sharp increase at low threshold voltage (5 V). A high carrier mobility of 1.8×10^-2 was observed in the smectic phase of one of the compounds studied (e).     

Synthesis and characterization of P2O5–SiO2–X (X = phosphotungstic acid) glasses as electrolyte for low temperature H2/O2 fuel cell application

A new class of proton conducting glass membranes for hydrogen fuel cell applications are being developed using phosphotungstic acid. These glasses are being design to yield high proton conductivities could be potential substitutes for electrolytes in H₂/O₂ fuel cell. P₂O₅–SiO₂–PWA glasses have been non-crystalline phases confirmed by structural studies. The glass materials showed good mechanical and thermal stability, and also found a maximum proton conductivity of 9.1 × 10⁻² S/cm at 90 °C and 30% RH. The average pore size less than 5 nm was determined by Barrett–Joyner–Halenda (BJH) desorption method. The electrochemical activity was investigated by polarization curves and current–voltage profiles. A maximum power density value of 10.2 mW/cm² was obtained using 0.15 mg/cm² of Pt/C loaded on electrode and 5P₂O₅–87SiO₂–8PWA glasses at 30 °C and 30% humidity.     

Characterization of solid-state dye-sensitized solar cells utilizing high absorption coefficient metal-free organic dyes

Solid-state dye-sensitized solar cells were fabricated using the organic hole-transporting medium (HTM) 2,2'7,7'-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene (spiro-MeOTAD), and three organic indoline-based sensitizer dyes with high molar extinction coefficients. The cells were characterized by several techniques, including spectral response measurements, photovoltage decay transients, intensity modulated photovoltage spectroscopy (IMVS), and charge extraction. The differences in apparent electron lifetime observed for cells fabricated using the three dyes are attributed in part to changes in the surface dipole potential at the TiO2/spiro-MeOTAD interface, which shift the TiO2 conduction band energy relative to the Fermi level of the HTM. These energy shifts influence both the open circuit voltage (as a result of changes in free electron density) and the short circuit current (as a consequence of changes in the overlap between the dye LUMO level and the conduction band). A self-consistent approach was used to derive the positions of the conduction band relative to the spiro-MeOTAD redox Fermi level for cells fabricated using the three dyes. The analysis also provided estimates of the free electron lifetime in spiro-MeOTAD cells. In order to evaluate the possible contribution of the adsorbed dyes to the observed changes in surface dipole potential, their dipole moments were estimated using ab initio density functional theory (DFT) calculations. Comparison of the calculated dipole contributions with the experimentally measured shifts in conduction band energy revealed that other factors such as proton adsorption may be predominant in determining the surface dipole potential.     

Electrokinetic study of adsorption of ionic surfactants on titania from organic solvents

The electrokinetic potential of titania was studied as a function of concentration of SDS, DOSS and CTMABr in a series of solvents. In water and 50–50 water–methanol mixture, which are the most polar studied solvents, the organic ion is adsorbed on titania and the small inorganic ion remains in solution. In hexane the adsorption behavior is reversed, that is, the organic ion remains in solution and the small inorganic ion is adsorbed on titania. The borderline between these two types of behavior corresponds to solvent dielectric constant of about 25. In solvents, which have a dielectric constant in this range (methanol and 1-propanol) the adsorption preferences vary from one surfactant to another. The affinities of the organic ion and of the small inorganic ion to the surface are often similar, and then none of the ionic components is preferentially adsorbed, and the electrokinetic potential is not affected. In such cases, ionic surfactants are not suitable as agents for regulation of zeta potential.     

In situ AFM study of sorbed humic acid colloids at different pH

Humic acid colloids adsorbed on the basal plane of cleaved muscovite are investigated under in situ conditions by non-contact mode atomic force microscopy (AFM) in liquid (also called fluid tapping-mode AFM). Structures are found to be of nanometer scale, consisting of flat particles (8–13 nm in diameter), aggregates of particles (20–100 nm), chain-like assemblies, networks and torus-like structures. In contrast to former investigations colloids are investigated in aquatic solution and structures are not influenced by sample preparation. Nanostructure, surface coverage and particle sizes are found to depend on solution pH. Humic colloids can be distinguished from surface roughness and background noise by image processing. Furthermore, an approach to quantify the surface coverage is discussed. Therefore, non-contact mode AFM in liquid is shown to be a powerful method to study the interaction of colloids at solid–liquid interfaces.     

Dye-sensitized solar cells made from BaTiO3-coated TiO2 nanoporous electrodes

We reported on the preparation of a thin BaTiO₃-coated layer (2.27 nm) on the surface of TiO₂ and its further application in the dye-sensitized solar cells (DSCs). The as-prepared BaTiO₃–TiO₂ films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscope (TEM). The performances of the DSCs with and without BaTiO₃ coating were analyzed by cyclic voltammograms (CVs), electrochemical impedance spectroscopy (EIS), and current–voltage measurements. It was found that the BaTiO₃–TiO₂ films with about 12 μm thickness increased the dye adsorption, resulting in increased J_sc. In the meantime, the BaTiO₃ modification on the TiO₂ surface is beneficial to the formation of an energy barrier against the electron transfer from TiO₂ to I₃⁻, providing the increase of V_oc due to the increased electron density in the TiO₂ that is caused by the increased electron lifetime.     

Anomalous chemical shifts of organic molecules adsorbed on spherical carbon adsorbents

The adsorption of simple organic molecules on synthetic carbon adsorbents of the SCA brand, thermovacuum treated at 2273 K, was studied by the PMR method. It was found that adsorbed molecules appear in the PMR spectra in the form of one or several signals, shifted toward strong magnetic fields in comparison with the proton signal of molecules in the condensed phase. The strongest shielding effect on the part of the surface of the material is experienced by adsorbent molecules in the SCA micropores. It was shown that the chemical shift does not depend on the nature of the adsorbed molecules and is apparently determined by the site of coordination in the interplanar gap of the graphitized portions of the carbon material. It is suggested that the anomalous values of the chemical shift of the protons of the adsorbed molecules are due to the influence of ring currents of the graphitized surface.Translated from Teoreticheskaya Éksperimental'naya Khimiya, Vol. 28, No. 1, pp. 80–85, January–February, 1992.