Electron transport and redox reactions in carbon-based molecular electronic junctions

A unique molecular junction design is described, consisting of a molecular mono- or multilayer oriented between a conducting carbon substrate and a metallic top contact. The sp2 hybridized graphitic carbon substrate (pyrolyzed photoresist film, PPF) is flat on the scale of the molecular dimensions, and the molecular layer is bonded to the substrate via diazonium ion reduction to yield a strong, conjugated C-C bond. Molecular junctions were completed by electron-beam deposition of copper, titanium oxide, or aluminium oxide followed by a final conducting layer of gold. Vibrational spectroscopy and XPS of completed junctions showed minimal damage to the molecular layer by metal deposition, although some electron transfer to the molecular layer resulted in partial reduction in some cases. Device yield was high (>80%), and the standard deviations of junction electronic properties such as low voltage resistance were typically in the range of 10-20%. The resistance of PPF/molecule/Cu/Au junctions exhibited a strong dependence on the structure and thickness of the molecular layer, ranging from 0.13 ohms cm2 for a nitrobiphenyl monolayer, to 4.46 ohms cm2 for a biphenyl monolayer, and 160 ohms cm2 for a 4.3 nm thick nitrobiphenyl multilayer. Junctions containing titanium or aluminium oxide had dramatically lower conductance than their PPF/molecule/Cu counterparts, with aluminium oxide junctions exhibiting essentially insulating behavior. However, in situ Raman spectroscopy of PPF/nitroazobenzene/AlO(x)/Au junctions with partially transparent metal contacts revealed that redox reactions occurred under bias, with nitroazobenzene (NAB) reduction occurring when the PPF was biased negative relative to the Au. Similar redox reactions were observed in PPF/NAB/TiO(x)/Au molecular junctions, but they were accompanied by major effects on electronic behavior, such as rectification and persistent conductance switching. Such switching was evident following polarization of PPF/molecule/TiO2/Au junctions by positive or negative potential pulses, and the resulting conductance changes persisted for several minutes at room temperature. The "memory" effect implied by these observations is attributed to a combination of the molecular layer and the TiO2 properties, namely metastable "trapping" of electrons in the TiO2 when the Au is negatively biased.     

In situ Raman spectroscopy of bias-induced structural changes in nitroazobenzene molecular electronic junctions

Carbon/nitroazobenzene (NAB)/titanium/gold molecular electronic junctions with active thicknesses of 7-8 nm were constructed having partially transparent Ti/Au top contacts, which permitted in situ monitoring of molecular structure with Raman spectroscopy for applied biases between +3 and -3 V. Deposition of the Ti/Au top contacts resulted in spectral changes similar to those accompanying NAB reduction in a conventional spectroelectrochemical experiment. Upon application of +3 V (carbon relative to Ti), the spectrum indicated reoxidation of the NAB reduction product, and this redox cycle could be repeated at least three times. When a voltage excursion to -2 or -3 V occurred, the spectra indicated irreversible loss of the nitro group, and a dramatic but reversible decrease in Raman intensity over the entire shift range examined. Negative applied voltage causes formation of reduced NAB and a high oxidation state of titanium, while positive voltage forms oxidized NAB and injects electrons into the titanium oxide layer. The spectral changes were correlated with current/voltage curves in order to probe the mechanism of rectification and conductance switching reported previously. Overall, the combination of spectroscopic and voltammetric results implies a conduction mechanism involving both NAB and titanium oxide, possibly mediated by the injection of carriers into the semiconducting titanium oxide, or by reduction of an insulating titanium oxide to a more conductive form.     

Amperometric biosensor based on glucose dehydrogenase and plasma-polymerized thin films.

A novel design is described for an amperometric biosensor based on NAD(P)-dependent glucose dehydrogenase (GDH) combined with a plasma-polymerized thin film (PPF). The GDH is sandwiched between several nanometer thick acetonitrile PPFs on a sputtered gold electrode (PPF/GDH/PPF/Au). The lower PPF layer plays the role as an interface between enzyme and electrode because it is extremely thin, adheres well to the substrate (electrode), has a flat surface and a highly-crosslinked network structure, and is hydrophilic in nature. The upper PPF layer (overcoating) was directly deposited on immobilized GDH. The optimized amperometric biosensor characteristics covered 2.5-26 mM glucose concentration at +0.6 V of applied potential; the least-squares slope was 320 nA mM(-1) cm(-2) and the correlation coefficient was 0.990. Unlike conventional wet-chemical processes that are incompatible with mass production techniques, this dry-chemistry procedure has great potential for enabling high-throughput production of bioelectronic devices.     

In situ raman spectroelectrochemistry of electron transfer between glassy carbon and a chemisorbed nitroazobenzene monolayer

In situ Raman spectroscopy was used to monitor 4-nitroazobenzene (NAB) in an electrochemical cell, both as a free molecule and as a chemisorbed monolayer on a glassy carbon (GC) electrode surface. Reduction of free NAB exhibited two well-defined voltammetric couples in acetonitrile, and the accompanying spectral changes supported a mechanism involving two successive 1-e(-) transfers. Raman spectra of NAB chemisorbed to GC via diazonium ion reduction were obtained in acetonitrile with a high-sensitivity, line-focused CCD spectrometer. The chemisorbed NAB spectra were quite different from the free NAB spectra, and were sufficiently strong to monitor as a function of applied potential. In the potential range of +400 to -800 mV vs Ag/Ag(+), the intensity of the Raman bands associated with the phenyl-NO(2) moiety varied, implying an electronic interaction between the pi system of the graphitic substrate and the chemisorbed NAB molecules. Negative of -800 mV, a 1-e(-) voltammetric reduction peak was observed, which was reversible on the positive voltage scan. This peak was accompanied by significant spectral changes, particularly the loss of the N=N and NO(2) stretches. The spectra are consistent with formation of a quinoid structure containing a C=C double bond between the NAB and the graphitic surface. The electron transfer and spectral changes occurred over a wider potential range than expected for a conventional Nernstian equilibrium, but did not appear to be broadened by slow electron-transfer kinetics. The results imply a significant perturbation of electron transfer between the GC and the monolayer, caused by strong electronic coupling between the graphitic pi system and the NAB orbitals. Rather than a discrete electron transfer to a free molecule, the electron transfer to chemisorbed NAB is more gradual, and is presumably driven by the electric field at the electrode/solution interface.     

All-carbon molecular tunnel junctions

This Article explores the idea of using nonmetallic contacts for molecular electronics. Metal-free, all-carbon molecular electronic junctions were fabricated by orienting a layer of organic molecules between two carbon conductors with high yield (>90%) and good reproducibility (rsd of current density at 0.5 V <30%). These all-carbon devices exhibit current density-voltage (J-V) behavior similar to those with metallic Cu top contacts. However, the all-carbon devices display enhanced stability to bias extremes and greatly improved thermal stability. Completed carbon/nitroazobenzene(NAB)/carbon junctions can sustain temperatures up to 300 °C in vacuum for 30 min and can be scanned at ±1 V for at least 1.2 × 10(9) cycles in air at 100 °C without a significant change in J-V characteristics. Furthermore, these all-carbon devices can withstand much higher voltages and current densities than can Cu-containing junctions, which fail upon oxidation and/or electromigration of the copper. The advantages of carbon contacts stem mainly from the strong covalent bonding in the disordered carbon materials, which resists electromigration or penetration into the molecular layer, and provides enhanced stability. These results highlight the significance of nonmetallic contacts for molecular electronics and the potential for integration of all-carbon molecular junctions with conventional microelectronics.     

Mechanism of rectification in tunneling junctions based on molecules with asymmetric potential drops

This paper proposes a mechanism for the rectification of current by self-assembled monolayers (SAMs) of alkanethiolates with Fc head groups (SC(11)Fc) in SAM-based tunneling junctions with ultra-flat Ag bottom electrodes and liquid metal (Ga(2)O(3)/EGaIn) top electrodes. A systematic physical-organic study based on statistically large numbers of data (N = 300-1000) reached the conclusion that only one energetically accessible molecular orbital (the HOMO of the Fc) is necessary to obtain large rectification ratios R ≈ 1.0 × 10(2) (R = |J(-V)|/|J(V)| at ±1 V). Values of R are log-normally distributed, with a log-standard deviation of 3.0. The HOMO level has to be positioned spatially asymmetrically inside the junctions (in these experiments, in contact with the Ga(2)O(3)/EGaIn top electrode, and separated from the Ag electrode by the SC(11) moiety) and energetically below the Fermi levels of both electrodes to achieve rectification. The HOMO follows the potential of the Fermi level of the Ga(2)O(3)/EGaIn electrode; it overlaps energetically with both Fermi levels of the electrodes only in one direction of bias.     

Strong effects of molecular structure on electron transport in carbon/molecule/copper electronic junctions

Carbon/molecule/copper molecular electronic junctions were fabricated by metal deposition of copper onto films of various thicknesses of fluorene (FL), biphenyl (BP), and nitrobiphenyl (NBP) covalently bonded to flat, graphitic carbon. A "crossed-wire" junction configuration provided high device yield and good junction reproducibility. Current/voltage characteristics were investigated for 69 junctions with various molecular structures and thicknesses and at several temperatures. The current/voltage curves for all cases studied were nearly symmetric, scan rate independent, repeatable at least thousands of cycles and exhibited negligible hysteresis. Junction conductance was strongly dependent on the dihedral angle between phenyl rings and on the nature of the molecule/copper "contact". Junctions made with NBP showed a decrease in conductivity of a factor of 1300 when the molecular layer thickness increased from 1.6 to 4.5 nm. The slope of ln(i) vs layer thickness for both BP and NBP was weakly dependent on applied voltage and ranged from 0.16 to 0.24 A(-1). These attenuation factors are similar to those observed for similar molecular layers on modified electrodes used to study electrochemical kinetics. All junctions studied showed weak temperature dependence in the range of approximately 325 to 214 K, implying activation barriers in the range of 0.06 to 0.15 eV. The carbon/molecule/copper junction structure provides a robust, reproducible platform for investigations of the dependence of electron transport in molecular junctions on both molecular structure and temperature. Furthermore, the results indicate that junction conductance is a strong function of molecular structure, rather than some artifact resulting from junction fabrication.     

Current rectification in a Langmuir-Schaefer monolayer of fullerene-bis-[4-diphenylamino-4' '-(n-ethyl-n-2' ''-ethyl)amino-1,4-diphenyl-1,3-butadiene] malonate between Au electrodes

Langmuir-Schaefer (LS) monolayer films of fullerene-bis-[4-diphenylamino-4' '-(N-ethyl-N-2' '-ethyl)amino-1,4-diphenyl-1,3-butadiene] malonate, 1, sandwiched between two Au electrodes, exhibit pronounced current asymmetries (rectification) between positive and negative bias at room temperature, with no decay of the rectification after several cycles. The device shows symmetrical through-space tunneling for a bias up to +/-3 V, and asymmetrical, unimolecular, "U" type rectifier behavior in the voltage range from +/-3.0 to +/-5.4 V, with rectification ratios up to 16.5. The rectification is ascribed to the asymmetric placement of the relevant molecular orbitals, with respect to the metallic electrodes.     

Electrodeless determination of the trap density, decay kinetics, and charge separation efficiency of dye-sensitized nanocrystalline TiO(2)

We have studied photoinduced charge separation in a bare, 3.4 microm thick layer of nanocrystalline ("nc") anatase TiO(2) and an nc-TiO(2) layer coated with free-base 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin (H(2)TPPC) using the electrodeless flash-photolysis time-resolved microwave-conductivity technique (FP-TRMC). Photoconductivity transients, resulting from the formation of mobile, conduction band electrons in the semiconductor have been measured on excitation with 3 ns pulses of UV (300 nm) and visible (410-700 nm) light. The product of the yield of formation of mobile charge carriers, phi, and the sum of their mobilities, Sigmamicro, has been determined from the maximum conductivity for light intensities varying from approximately 10(12) to approximately 10(16) photons/cm(2)/pulse. For the bare nc-TiO(2) layer at 300 nm and the coated layer at all wavelengths, phiSigmamicro initially increased with increasing intensity, reached a maximum, and eventually decreased at high intensities. The initial increase is attributed to the gradual filling of (surface) electron trapping sites. This effect was absent when the samples were continuously illuminated with background irradiation at 300 nm with an intensity of 6 x 10(13) photons/cm(2)/s (40 microW/cm(2)), thereby presaturating the trapping sites prior to the laser pulse. The trap-free mobility of electrons within these 9 nm nanoparticles is estimated to be 0.034 cm(2)/Vs at 9 GHz. The eventual decrease in phiSigmamicro at intensities corresponding to an electron occupancy of more than one electron per particle is unaffected by background illumination, and is attributed to a decrease in micro due to electron-electron interactions within the semiconductor particles. The photoconductivity action spectrum of the coated nc-TiO(2) layer closely followed the photon attenuation spectrum in the visible of the porphyrin, with a charge separation efficiency per absorbed photon of 18% at the Soret band maximum. The after-pulse decay of the photoconductivity showed a power law behavior over a time scale of nanoseconds to several hundreds of microseconds, which is attributed to multiple trapping and detrapping events at chemical or physical defects within the semiconductor matrix.     

Layer-by-layer assembly of aqueous dispersible, highly conductive poly(aniline-co-o-anisidine)/poly(sodium 4-styrenesulfonate)/MWNTs core-shell nanocomposites

Poly(aniline-co- o-anisidine) (P(An-co- o-As)) ionomers and poly(sodium 4-styrenesulfonate) (PSS) were layer-by-layer (LbL) assembled on carboxylic acid-functionalized multiwalled carbon nanotubes (MWNTs). The multilayered polyelectrolyte greatly enhanced the dispersibility and stability of MWNTs in aqueous solutions. More importantly, the nanocomposites showed 3 orders of magnitude of conductivity increase, 4.2 S/cm, compared to that of neat ionomers, 0.004 S/cm. The deposition procedure was monitored with zeta (zeta) potential changes. Fourier transform infrared (FT-IR), ultraviolet-visible (UV-vis), and Raman spectra confirmed charge transfer from the quinoid units of the P(An-co- o-As) to MWNTs, which effectively delocalize the electrons. Further, we explored the pH response of the assembled P(An-co- o-As)/PSS/MWNTs multilayer nanocomposites. The sharp transition of the conductivity in the pH range of 2 to 6 makes the nanocomposites promising candidates for chemical-biological sensing.     

氧化石墨烯掺杂PEDOT:PSS作为空穴注入层对有机发光二极管发光性能的影响

研究了氧化石墨烯(GO)掺杂聚(3,4-亚乙二氧基噻吩):聚(苯乙烯磺酸) (PEDOT:PSS)作为空穴注入层对有机发光二极管发光性能的影响. 在PEDOT:PSS水溶液中掺入GO, 经过湿法旋涂和退火成膜后, 不仅提高了空穴注入层的空穴注入能力和导电率, 透光率也得到了相应的提高, 从而使得有机发光二极管(OLED)器件的发光性能得到了提升. 通过优化GO掺杂量发现, 当GO掺杂量为0.8%(质量分数)时, 空穴注入层的透光率达到最大值(96.8%), 此时获得的OLED器件性能最佳, 其最大发光亮度和最大发光效率分别达到17939 cd·m^-2和3.74 cd·A^-1. 与PEDOT:PSS 作为空穴注入层的器件相比, 掺杂GO后器件的最大发光亮度和最大发光效率分别提高了46.6%和67.6%.     

高效白色磷光有机电致发光器件

采用真空热蒸镀方法以4,4'-bis (carbazol-9-yl) biphenyl (CBP)为主体材料、以bis[2-(4-tert-butylphenyl) benzothiazolato-N,^C2] iridium (acetylacetonate) [(t-bt)₂Ir(acac)]磷光染料为掺杂剂构成黄色发光层, 制备了高效白光的有机电致发光器件(OLEDs). OLEDs的器件结构为indiumtin oxide (ITO)/N,N’-bis-(1-naphthyl)-N,N’-biphenyl-1,1’-biphenyl-4,4’-diamine (NPB)/CBP: (t-bt)₂Ir (acac)/NPB/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/8-hydroxy quinoline aluminum(Alq₃)/Mg:Ag, 从ITO阳极开始的第一层NPB为空穴传输层, 第二层超薄的NPB为蓝色发光层, BCP为空穴阻挡层和激子阻挡层, Alq₃为电子传输层. 结果表明, 器件电压在3 V启亮, 在16.5 V时, 器件的最高亮度达到15460 cd·m^-2; 在4 V时, 器件达到最大流明效率为7.5 lm·W^-1, 器件启亮后所发出的白光光谱在低电压时随电压变化有稍微的移动, 但是都在白光范围内变化. 在电压达到8 V后Commission Internationale I’Eclairage(国际照明委员会) (CIE)色坐标为(0.33, 0.32), 并且光谱及色坐标稳定, 不随电压变化而改变, 与最佳的白光坐标(0.33, 0.33)几乎重合. 同时, 从机理上解释了光谱移动和效率衰减的原因, 并探讨了载流子陷阱和能量传递的关系.     

Au-MgB2-Au纳米结点的负微分电阻效应

运用密度泛函理论结合非平衡格林函数的方法对MgB2直线原子链与两半无限Au（100）电极构成纳米结点的电子输运特性进行了第一性原理计算．在模拟Au-MgB2-Au纳米结点的拉伸过程中,计算了结点在不同距离下的结合能与电导．结果发现结点的Au-B键长为1．90A,B-Mg键长为2．22A时,结合能最大,结构最稳定,此时结点平衡电导为0．51G0（G0=2e^0／h）．通过计算投影态密度发现电子通过结点时主要是通过B、Mg原子的px和py电子轨道形成的π键进行传输的．在-1．5～1．5V的电压范围内,结点的电流-电压近似为线性关系,表现出类似金属的导电性质,而当正负电压高于15V时,电流对称性逐渐减小,即存在负微分电阻效应,从不同电压下透射谱的变化对负微分电阻现象进行了分析与讨论．     

Dynamic Random Access Memory Effect and Memory Device Derived from a Functional Polyimide Containing Electron Donor‐Acceptor Pairs in the Main Chain

A functional polyimide, hexafluoroisopropyl bis(phthalic dianhydride)/3,6‐diaminocarbazole (6FDA/DAC), in which DAC serves as electron donor and 6FDA as electron acceptor, has been synthesized in our present work. Electrical characterization results on the sandwiched polyimide memory device (ITO/Thin polyimide Layer/Au) indicate that the polyimide possesses electrical bistability and the device exhibits two accessible conductivity states, which can be reversibly switched from the low‐conductivity (OFF) state to the high‐conductivity (ON) state with an ON/OFF current ratio of about 10⁴. Different from the widely reported write‐once‐read‐many‐times (WORM) effects, the device with the 6FDA/DAC polyimide as the active layer shows dynamic random access memory (DRAM) behavior. The ON state of the device was lost immediately after removal of the applied voltage, while by applying a constant bias (e.g., 3 V) the ON state can be electrically sustained. The roles of donor and acceptor components in the polyimide main chain were elucidated through molecular simulation.

     

Electronic characteristics of fluorene/TiO2 molecular heterojunctions

The electronic properties of molecular junctions of the general type carbon/molecule/TiO2Au were examined as examples of "molecular heterojunctions" consisting of a molecular monolayer and a semiconducting oxide. Junctions containing fluorene bonded to pyrolyzed photoresist film (PPF) were compared to those containing Al2O3 instead of fluorene, and those with only the TiO2 layer. The responses to voltage sweep and pulse stimulation were strongly dependent on junction composition and temperature. A transient current response lasting a few milliseconds results from injection and trapping of electrons in the TiO2 layer, and occurred in all three junction types studied. Conduction in PPFTiO2Au junctions is consistent with space charge limited conduction at low voltage, then a sharp increase in current once the space charge fills all the traps. With fluorene present, there is a slower, persistent change in junction conductance which may be removed by a reverse polarity pulse. This "memory" effect is attributed to a redox process in the TiO2 which generates TiIII and/or TiII, which have much higher conductance than TiO2 due to the presence of conduction band electrons. The redox process amounts to "dynamic doping" of the TiO2 layer by the imposed electric field. The memory effect arises from a combination of the properties of the molecular and oxide layers, and is a special property of the molecular heterojunction configuration.     

Investigation of electrical properties of PANI/chalcogenide junctions

Pure Polyaniline (EB) and Polyaniline doped with different protonic acids (ESs) were chemically synthesized using ammonium peroxydisulphate (APS) as an oxidant. Junctions have been prepared by evaporating chalcogenide materials (ZnSe, CdSe) on conducting polyaniline (EB & ESs) pellets using a vacuum evaporation technique. I–V characteristics of junctions have been studied at room temperature using the Keithley electrometer 6517A. I–V measurements show the rectification effect. A junction of ES[PO₄ ^3?] may be preferred over the other junctions due to its low ideality factor and maximum rectification ratio.      

A molecular half-wave rectifier

This paper describes the performance of junctions based on self-assembled monolayers (SAMs) as the functional element of a half-wave rectifier (a simple circuit that converts, or rectifies, an alternating current (AC) signal to a direct current (DC) signal). Junctions with SAMs of 11-(ferrocenyl)-1-undecanethiol or 11-(biferrocenyl)-1-undecanethiol on ultraflat, template-stripped Ag (Ag(TS)) bottom electrodes, and contacted by top electrodes of eutectic indium-gallium (EGaIn), rectified AC signals, while similar junctions based on SAMs of 1-undecanethiol-SAMs lacking the ferrocenyl terminal group-did not. SAMs in these AC circuits (operating at 50 Hz) remain stable over a larger window of applied bias than in DC circuits. AC measurements, therefore, can investigate charge transport in SAM-based junctions at magnitudes of bias inaccessible to DC measurements. For junctions with SAMs of alkanethiols, combining the results from AC and DC measurements identifies two regimes of bias with different mechanisms of charge transport: (i) low bias (|V| < 1.3 V), at which direct tunneling dominates, and (ii) high bias (|V| > 1.3 V), at which Fowler-Nordheim (FN) tunneling dominates. For junctions with SAMs terminated by Fc moieties, the transition to FN tunneling occurs at |V| ≈ 2.0 V. Furthermore, at sufficient forward bias (V > 0.5 V), hopping makes a significant contribution to charge transport and occurs in series with direct tunneling (V ? 2.0 V) until FN tunneling activates (V ? 2.0 V). Thus, for Fc-terminated SAMs at forward bias, three regimes are apparent: (i) direct tunneling (V = 0-0.5 V), (ii) hopping plus direct tunneling (V ≈ 0.5-2.0 V), and (iii) FN tunneling (V ? 2.0 V). Since hopping does not occur at reverse bias, only two regimes are present over the measured range of reverse bias. This difference in the mechanisms of charge transport at forward and reverse bias for junctions with Fc moieties resulted in large rectification ratios (R > 100) and enabled half-wave rectification.     

V2O5-anchored carbon nanotubes for enhanced electrochemical energy storage

Functionalized multiwalled carbon nanotubes (CNTs) are coated with a 4-5 nm thin layer of V(2)O(5) by controlled hydrolysis of vanadium alkoxide. The resulting V(2)O(5)/CNT composite has been investigated for electrochemical activity with lithium ion, and the capacity value shows both faradaic and capacitive (nonfaradaic) contributions. At high rate (1 C), the capacitive behavior dominates the intercalation as 2/3 of the overall capacity value out of 2700 C/g is capacitive, while the remaining is due to Li-ion intercalation. These numbers are in agreement with the Trasatti plots and are corroborated by X-ray photoelectron spectroscopy (XPS) studies on the V(2)O(5)/CNTs electrode, which show 85% of vanadium in the +4 oxidation state after the discharge at 1 C rate. The cumulative high-capacity value is attributed to the unique property of the nano V(2)O(5)/CNTs composite, which provides a short diffusion path for Li(+)-ions and an easy access to vanadium redox centers besides the high conductivity of CNTs. The composite architecture exhibits both high power density and high energy density, stressing the benefits of using carbon substrates to design high performance supercapacitor electrodes.     

Effect of iodine addition on solid-state electrolyte LiI/3-hydroxypropionitrile (1:4) for dye-sensitized solar cells

It was observed that the ionic conductivity of the solid-state electrolyte LiI/3-hydroxypropionitrile (HPN) = 1:4 (molar ratio) decreased dramatically with increasing iodine (I(2)) concentration, which differs from the conduction behavior of the Grotthuss transport mechanism observed in liquid or gel electrolytes. The short-circuit photocurrent density (J(sc)) of the dye-sensitized solar cell (DSSC) based on this electrolyte system increases with increasing I(2) concentration until LiI/I(2) is 1:0.05 (molar ratio). Beyond this limitation, the J(sc) decreases. At low I(2) concentrations (I(2)/LiI < or = 0.05), the J(sc) is mainly affected by the diffusion of I(3)(-). An increase of the I(2) concentration leads to the enhancement of the diffusion of I(3)(-) and an increase of the J(sc). At high I(2) concentrations (I(2)/LiI > 0.05), the factors, including the increased light absorption by the I(3)(-), the increased recombination of electrons at the photoanode with I(3)(-), and the reduced ionic conductivity of the electrolyte, lead to a decrease of J(sc). At the same time, the open-circuit voltage (V(oc)) of the DSSC decreases monotonically with the ratio of I(2)/LiI due to increased dark current in the DSSC. The increased absorption of visible light by the electrolyte, the enhanced dark current, and the reduced ionic conductivity of the electrolyte contribute to the performance variation of the corresponding solid-state DSSC with increasing I(2) concentration.     

Redox-driven conductance switching via filament formation and dissolution in carbon/molecule/TiO2/Ag molecular electronic junctions

Carbon/molecule/TiO2/Au molecular electronic junctions show robust conductance switching, in which a metastable high conductance state may be induced by a voltage pulse which results in redox reactions in the molecular and TiO2 layers. When Ag is substituted for Au as the "top contact", dramatically different current/voltage curves and switching behavior result. When the carbon substrate is biased negative, an apparent breakdown occurs, leading to a high conductance state which is stable for at least several hours. Upon scanning to positive bias, the conductance returns to a low state, and the cycle may be repeated hundreds of times. Similar effects are observed when Cu is substituted for Au and for three different molecular layers as well as "control" junctions of the type carbon/TiO2/Ag/Au. The polarity of the "switching" is reversed when the Ag layer is between the carbon and molecular layers, and the conductance change is suppressed at low temperature. Pulse experiments show very erratic transitions between high and low conductivity states, particularly near the switching threshold. The results are consistent with a switching mechanism based on Ag or Cu oxidation, transport of their ions through the TiO2, and reduction at the carbon to form a metal filament.