Unidirectional Electron Flow Research Articles

Surface plasmon induced direct detection of long wavelength photons

Millimeter and terahertz wave photodetectors have long been of great interest due to a wide range of applications, but they still face challenges in detection performance. Here, we propose a new strategy for the direct detection of millimeter and terahertz wave photons based on localized surface-plasmon-polariton (SPP)-induced non-equilibrium electrons in antenna-assisted subwavelength ohmic metal–semiconductor–metal (OMSM) structures. The subwavelength OMSM structure is used to convert the absorbed photons into localized SPPs, which then induce non-equilibrium electrons in the structure, while the antenna increases the number of photons coupled into the OMSM structure. When the structure is biased and illuminated, the unidirectional flow of the SPP-induced non-equilibrium electrons forms a photocurrent. The energy of the detected photons is determined by the structure rather than the band gap of the semiconductor. The detection scheme is confirmed by simulation and experimental results from the devices, made of gold and InSb, and a room temperature noise equivalent power (NEP) of 1.5 × 10−13 W Hz−1/2 is achieved.

Triboelectric-Potential-Regulated Charge Transport Through p-n Junctions for Area-Scalable Conversion of Mechanical Energy.

Regulation of charge-transport direction is realized through the coupling of triboelectrification, electrostatic induction, and semiconducting properties for area-scalable conversion of mechanical energy. The output current from each unit triboelectric generator can always constructively add up due to the unidirectional flow of electrons. This work proposes a practical and general route to area-scalable applications of the triboelectric generator and other energy-harvesting techniques.

Determining complete electron flow in the cofactor photoreduction of oxidized photolyase

The flavin cofactor in photoenzyme photolyase and photoreceptor cryptochrome may exist in an oxidized state and should be converted into reduced state(s) for biological functions. Such redox changes can be efficiently achieved by photoinduced electron transfer (ET) through a series of aromatic residues in the enzyme. Here, we report our complete characterization of photoreduction dynamics of photolyase with femtosecond resolution. With various site-directed mutations, we identified all possible electron donors in the enzyme and determined their ET timescales. The excited cofactor behaves as an electron sink to draw electron flow from a series of encircling aromatic molecules in three distinct layers from the active site in the center to the protein surface. The dominant electron flow follows the conserved tryptophan triad in a hopping pathway across the layers with multiple tunneling steps. These ET dynamics occur ultrafast in less than 150 ps and are strongly coupled with local protein and solvent relaxations. The reverse electron flow from the flavin is slow and in the nanosecond range to ensure high reduction efficiency. With 12 experimentally determined elementary ET steps and 6 ET reaction pairs, the enzyme exhibits a distinct reduction-potential gradient along the same aromatic residues with favorable reorganization energies to drive a highly unidirectional electron flow toward the active-site center from the protein surface.

Nano-size effects on CuO/TiO2 catalysts for highly efficient H2 production under solar light irradiation

Solar light induced interfacial charge transfer of electrons from TiO2 to CuO in a water-glycerol mixture produced 99,823 μmol h(-1) g(-1)catalyst of hydrogen gas. The dispersed CuO/TiO2 photocatalyst in solution exhibited uni-directional electron flow and capture at the Schottky barrier facilitating charge separation and electron transfer resulting in enhanced H2 production performance.

Relating the Photodynamics of Squaraine-Based Dye-Sensitized Solar Cells to the Molecular Structure of the Sensitizers and to the Presence of Additives

Dye-sensitized solar cells (DSSCs) fabricated with TiO2 nanoparticle thin films and sensitized with four types of indole-based squaraines, SQs (symmetric or asymmetric and varying the length and nature of the alkyl side chain substituents), have been prepared. We have studied the influence of the presence of different additives in the electrolyte solutions (tert-butyl pyridine and/or Li+ cations) on the electron transfer dynamics by means of femtosecond transient absorption spectroscopy and flash photolysis. We obtained the rate constants for the electron injection, kei = 2, 3, 8, and 14 × 1010 s–1, for complete solar cells with an iodide-based electrolyte. The asymmetric SQ showed the largest kei value, 14 × 1010 s–1, in line with a unidirectional flow of electrons from the lowest unoccupied molecular orbital (LUMO) orbital of the SQ to the sub-bandgap states of the TiO2, which leads to a more efficient electron injection than that in the symmetric SQs. Addition of tert-butyl pyridine to the electrolyte solution (I–/I3– in acetonitrile) causes a 5–10-fold deceleration of the electron injection (for example, τobs = 2–11 ps in SQ 41). When including the Li+ cation together with the tert-butyl pyridine, the injection is still slower than in cells without any additive (τobs = 2 vs 7 ps in SQ 41), which reflects a stronger influence of the tert-butyl pyridine in the electron injection process. The effective lifetimes for the charge regeneration reaction, τobs, range from 2 to 25 μs for the complete cells with an iodide-based electrolyte. The fastest regeneration occurs in the SQs with the CF3– groups anchored to the side chains and, especially in SQ 26, with two CF3– groups. This result suggests that the inductive effect of the CF3– groups in the structure of SQ 26 and SQ 41 leads to a higher positive charge density in the π-conjugated system, which promotes a higher local concentration of iodide near the oxidized dye and therefore faster regeneration kinetics. Moreover, addition of Li+ cations to the electrolyte accelerates the regeneration reaction, which is ascribed to its interaction with the backbone of the SQ, favoring the approach of the I– species. Using the transient absorption results, we calculated the electron injection efficiency, φei, and compared it with the short-circuit current density, Jsc, of the complete cells. Thus, in the complete cells sensitized with SQ 41 and SQ 4, φei are the highest ones and present comparable values, 0.93 and 0.90, respectively. On the contrary, cells sensitized with SQ 26 and SQ 2 present lower values, 0.47 and 0.75, respectively. A similar tendency is observed for the values of Jsc. On the basis of this good correlation (φei vs Jsc), we can suggest that the electron injection reaction is partially responsible for the photon losses and derive the reasons why this occurs.

Near‐IR Absorbing Solar Cell Sensitized With Bacterial Photosynthetic Membranes

Current interest in natural photosynthesis as a blueprint for solar energy conversion has led to the development of a biohybrid photovoltaic cell in which bacterial photosynthetic membrane vesicles (chromatophores) have been adsorbed to a gold electrode surface in conjunction with biological electrolytes (quinone [Q] and cytochrome c; Magis et al. [2010] Biochim. Biophys. Acta 1798, 637-645). Since light-driven current generation was dependent on an open circuit potential, we have tested whether this external potential could be replaced in an appropriately designed dye-sensitized solar cell (DSSC). Herein, we show that a DSSC system in which the organic light-harvesting dye is replaced by robust chromatophores from Rhodospirillum rubrum, together with Q and cytochrome c as electrolytes, provides band energies between consecutive interfaces that facilitate a unidirectional flow of electrons. Solar I-V testing revealed a relatively high I(sc) (short-circuit current) of 25 μA cm(-2) and the cell was capable of generating a current utilizing abundant near-IR photons (maximum at ca 880 nm) with greater than eight-fold higher energy conversion efficiency than white light. These studies represent a powerful demonstration of the photoexcitation properties of a biological system in a closed solid-state device and its successful implementation in a functioning solar cell.

Dye-sensitized solar cell with a titanium-oxide-modified carbon nanotube transparent electrode

Transparent and conductive carbon-based materials are promising for window electrodes in solid-state optoelectronic devices. However, the catalytic activity to redox reaction limits their application as a working electrode in a liquid-type dye-sensitized solar cell (DSSC). In this letter, we propose and demonstrate a transparent carbon nanotubes (CNTs) film as the working electrode in a DSSC containing iodide/triiodide redox couples. This implementation is realized by inhibiting the charge-transfer kinetics at CNT/redox solution interface with an aid of thin titanium oxide film that facilitates the unidirectional flow of electrons in the cell without sacrificing the electrical and optical properties of CNT.

Gold Nanoparticle-Functionalized Carbon Nanotubes for Light-Induced Electron Transfer Process

The modified electronic properties at the heterojunctions of Au nanoparticle-decorated single-walled carbon nanotubes (SWNTs) have been utilized for photoinduced electron transfer by anchoring a photoactive molecule, namely ruthenium trisbipyridine (Ru(bpy)32+). A unidirectional electron flow was observed from the excited state of Ru(bpy)32+ to carbon nanotubes when the chromophores were linked through Au nanoparticles (SWNT−Au−Ru2+). In contrast, photoinduced electron transfer was not observed from the excited state of Ru(bpy)32+ neither to SWNT nor Au nanoparticles when these components were linked directly. The charge equilibration occurring at the SWNT−Au heterojunctions, due to the differences in electrochemical potentials, result in the formation of a localized depletion layer at the bundled carbon nanotube walls, which may act as acceptor sites of electrons from *Ru(bpy)32+. The charge separation in SWNT−Au−Ru2+ nanohybrids was sustained for several nanoseconds before undergoing recombination, maki...

Molecular engineering of panchromatic unsymmetrical squaraines for dye-sensitized solar cell applications

Three unsymmetrical squaraine dyes JK-64, JK-65, and JK-64Hx, containing a bulky spirobifluorene or hexyloxyphenyl unit are designed and synthesized. These sensitizers, when anchored onto a TiO2 surface, exhibit decreased aggregation as well as enhanced unidirectional flow of electrons. Under standard global AM 1.5 solar conditions, an optimized JK-64Hx sensitized cell gave a short-circuit photocurrent density (Jsc) of 12.82 mA cm−2, an open-circuit voltage (Voc) of 0.54 V and a fill factor (ff) of 0.75, corresponding to an overall conversion efficiency (η) of 5.20%.

Insights into the role of nucleotide-dependent conformational change in nitrogenase catalysis: Structural characterization of the nitrogenase Fe protein Leu127 deletion variant with bound MgATP

In the present work, determination of the structure of the nitrogenase Leu 127 deletion variant Fe protein with MgATP bound is presented, along with density functional theory calculations, to provide insights into the roles of MgATP in the nitrogenase reaction mechanism. Comparison of the MgATP-bound structure of this Fe protein to the nucleotide-free form indicates that the binding of MgATP does not alter the overall structure of the variant significantly with only small differences in the conformation of amino acids in direct contact with the two bound MgATP molecules being seen. The earlier observation of splitting of the [4Fe–4S] cluster into two [2Fe–2S] clusters was observed to be unaltered upon binding MgATP. Density functional theory was used to probe the assignment of ligands to the two [2Fe–2S] rhombs. The Mg 2+ environment in the MgATP-bound structure of the Leu127 deletion Fe protein is similar to that observed for the Fe protein in the nitrogenase Fe protein: MoFe protein complex stabilized by MgADP and tetrafluoroaluminate suggesting that large scale conformational change implicated for the Fe protein may not be mediated by changes in the Mg 2+ coordination. The results presented here indicated that MgATP may enhance the stability of an open conformation and prohibit intersubunit interactions, which have been implicated in promoting nucleotide hydrolysis. This could be critical to the tight control of MgATP hydrolysis observed within the nitrogenase complex and may be important for maintaining unidirectional electron flow toward substrate reduction.

Photosystem II proteins PsbL and PsbJ regulate electron flow to the plastoquinone pool.

The psbEFLJ operon of tobacco plastids encodes four bitopic low molecular mass transmembrane components of photosystem II. Here, we report the effect of inactivation of psbL on the directional forward electron flow of photosystem II as compared to that of the wild type and the psbJ deletion mutant, which is impaired in PSII electron flow to plastoquinone [Regel et al. (2001) J. Biol. Chem. 276, 41473-41478]. Exposure of Delta psbL plants to a saturating light pulse gives rise to the maximal fluorescence emission, Fm(L), which is followed within 4-6 s by a broader hitherto not observed second fluorescence peak in darkness, Fm(D). Conditions either facilitating oxidation or avoiding reduction of the plastoquinone pool do not affect the Fm(L) level of Delta psbL plants but prevent the appearance of Fm(D). The level of Fm(D) is proportional to the intensity and duration of the light pulse allowing reduction of the plastoquinone pool in dark-adapted leaves prior to the activation of PSI and oxidation of plastoquinol. Lowering the temperature decreases the Fm(D) level in the Delta psbL mutant, whereas it increases considerably the lifetime of Q(A)*- in the Delta psbJ mutant. The thermoluminescence signal generated by Q(A)*-/S(2) charge recombination is not affected; on the other hand, charge recombination of Q(B)*-/S(2,3) could not be detected in Delta psbL plants. PSII is highly sensitive to photoinhibition in Delta psbL. We conclude that PsbL prevents reduction of PSII by back electron flow from plastoquinol protecting PSII from photoinactivation, whereas PsbJ regulates forward electron flow from Q(A)*- to the plastoquinone pool. Therefore, both proteins contribute substantially to ensure unidirectional forward electron flow from PSII to the plastoquinone pool.

Unidirectional electron flow in a nanometer-scale semiconductor channel: A self-switching device

By tailoring the boundary of a narrow semiconductor channel to break its symmetry, we have realized a type of nanometer-scale nonlinear device, which we refer to as self-switching device (SSD). An applied voltage V not only changes the potential profile along the channel direction, but also either widens or narrows the effective channel depending on the sign of V. This results in a diode-like characteristic but without the use of any doping junction or barrier structure. The turn-on voltage can also be widely tuned from virtually zero to more than 10 V, by simply adjusting the channel width. The planar and two-terminal structure of the SSD also allows SSD-based circuits to be realized by only one step of lithography.

Rectified photocurrent of molecular photodiode consisting of cytochrome c/GFP hetero thin films

Photoinduced electron transfer in the molecular electronic device consisting of protein-adsorbed hetero Langmuir–Blodgett (LB) film was investigated. Three kinds of functional molecules, cytochrome c, viologen, and green fluorescent protein (GFP) were used as an electron acceptor, a mediator, and a sensitizer, respectively. The hetero-LB film was fabricated by subsequently depositing cytochrome c and viologen onto the pretreated ITO or quartz glass. GFP adsorbed hetero-LB films were prepared by soaking the hetero-LB films into the buffer solution containing GFP. The MIM (metal/insulator/metal) structured molecular device was constructed by depositing aluminum onto the surface of the GFP-adsorbed hetero LB films. Due to the excitation by irradiation with a 460 nm monochromic light source, the photoinduced unidirectional flow of electrons in the MIM device could be achieved and was detected as photocurrents. The photoswitching function was achieved and the rectifying characteristic was observed in the molecular device. Based on the measurement of transient photocurrent of molecular device, the unidirectional flow of electrons was verified.

Asymmetric Electron Transfer in Reaction Centers of Purple Bacteria Strongly Depends on Different Electron Matrix Elements in the Active and Inactive Branches

We have re-examined the contribution of electronic matrix elements (V1) between the primary electron donor and the accessory bacteriochlorophylls in the active (A) and inactive (B) branches of bacterial reaction centers (RC) to the unidirectional light-induced electron transfer (ET) (a preliminary report was recently given by Kolbasov and Scherz in Photosynthesis: Mechanisms and Effects; Garab, G., Ed.; Kluwer Academic Publishers: Dordrecht, 1998; Vol. II; pp 719−722). Our calculations showed that V1B2 is probably smaller by 3 orders of magnitude than V1A2 in Rb. sphaeroides and by at least 1 order of magnitude in Rps. viridis. These phenomena reflect the quantum interference and mutual cancellation of the resonance integrals corresponding to different ET pathways between atoms of P and the accessory bacteriochlorophyll in the inactive branch. The calculated values of V and the corresponding ET rate constants for mutated RC further support this conclusion. Zhang and Friesner (Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 13603−13605), using more elaborate calculations, showed that V1A/V1B for Rps. viridis can reach a value of 14 for same reason, indicating that differences in the overlap matrix elements are key factors in the unidirectional electron flow in both organisms.

Photoinduced electron transfer in a MIM device composed of ferrocene-flavin-viologen-TCNQ molecular heterojunctions

A molecular photodiode was fabricated with the hetero-type Langmuir–Blodgett (LB) film consisting of two electron acceptors (A 1, A 2), a sensitizer (S) and an electron donor (D). N-Docosylquinolinium TCNQ, N-Allyl- N′-[3-propylamido-N′′,N′′-di( n-octadecyl)]-4,4′-bipyridinium dibromide, 7,8-dimethyl-10-dodecyl isoalloxazine and 1-1′ferrocene- N-dioctadecyl carboxamide were used as A 2, A 1, S and D units, respectively. One-way electron transfer from D to S was observed with cyclic voltammetry for the modified electrode containing complex S/D LB layers. Also, one-way electron transfer in S/A 1, S/A 2 and S/A 1/A 2 was observed based on the fluorescence quenching measurement in the complex LB layers. By aligning hetero-type LB films of A 2/A 1/S/D four units on ITO glass with an aluminum thin film, a molecular photodiode with the metal/insulator/metal (MIM) structure was constructed. Due to excitation by irradiation with a 460 nm monochromatic light source, the photoinduced unidirectional flow of electrons in the MIM device could be achieved and was detected as photocurrents. The direction of energy flow was in accordance with the energy level profile across the LB films. The photoswitching function was achieved and the rectifying characteristic was observed in the molecular device.

Molecular Photodiode Consisted of Flavin/TCNQ Hetero-LB Films

A molecular photodiode with Metal/Insulator/Metal (MIM) structure was fabricated using the hetero-Langmuir-Blodgett (LB) films consisted of an electron acceptor (A) and a sensitizer (S). N-Docosylquinolium-TCNQ and 7,8-dimethyl-10-dodecyl isoalloxazine were used as A and S units, respectively. Due to excitation by irradiation with a 460 nm monochromatic light source, the photo induced unidirectional electron flow in the MIM device could be achieved and was detected as photocurrent. The photos witching function was achieved and the rectifying characteristics was observed in the proposed molecular device.

Molecular Photodiode Consisting of Flavin-Viologen-Tcnq LB Multilayers

A moleculear photodiode was fabricated with the hetero-Langmuir-Blodgett(LB) film consisting of an electron acceptor(A), an electron relay(R) and a sensitizer(S). TCNQ, viologen and flavin derivatives were used as A, R and S units, respectively. By aligning hetero-LB film of A/R/S units on ITO glass with an aluminum thin film, a molecular photodiode with the structure of Metal/Insulator/Metal(MIM) was constructed. Due to excitation by irradiation with a 460 nm monochromatic light source, the photo-induced unidirectional flow of electrons in the MIM device could be achieved and was detected as photocurrents. The direction of energy flow was in accordance with the energy level profile across the LB films. The photoswitching function was achieved and the rectifying characteristic was observed in the molecular device.

Photoinduced electron transfer in a MIM device composed of TCNQ-pyrene-ferrocene LB films

A molecular photodiode was fabricated with the hetero-Langmuir-Blodgett (LB) film consisting of an electron acceptor (A), a sensitizer (S) and an electron donor (D). N-Docosylquinolinium TCNQ, 1O-(1-pyrene) decanoic acid and 1-1′- ferrocene-N- dioctadecylcarboxamide were used as A, S and D units, respectively. One-way electron transfer from D to S was observed with cyclic voltammetry for the modified electrode containing complex S/D LB layers. Also, one-way electron transfer from S to A was observed based on the fluorescence quenching in the complex S/A LB layers. By aligning a hetero-LB film of A/S/D three units on ITO glass with an aluminum thin film, a molecular photodiode with the structure of metal/insulator/metal (MIM) was constructed. Due to excitation by irradiation with a 340 nm monochromatic light source, the photo-induced unidirectional flow of electrons in the MIM device could be achieved and was detected as photocurrents. The direction of energy flow was in accordance with the energy level profile across the LB films. The photo-switching function was achieved and the rectifying characteristic was observed in the molecular device.

Target emission probe for suprathermal electron detection

The design and preliminary results of a target emission probe (TEP), applied to the continuous detection of suprathermal electrons in the edge of fusion plasmas, is presented in this paper. This diagnostic, developed for the RFX reversed field pinch experiment, is based on the measurement of the x-ray emission from a material target, exposed to the edge plasma, due to the bremsstrahlung of the electrons. The diagnostic has been successfully tested in RFX and a clear indication of the presence of a mostly unidirectional fast electron flow in the RFX plasma edge has been found.

A short-time pulsed probe plasma analyser

An automatic short-time pulsed probe plasma analyser is described which has been developed for the investigation of concentric non-equilibrium plasmas with unidirectional electron and ion flow in an ion source with high emission density. Besides plasma potential and ion saturation current the electron energy distribution can be measured up to values of 300 eV and simultaneously integrated to obtain the electron density. These four parameters determined serially in a total of less than 2s at 46 different locations in the plasma are stored in a recirculating shift register memory and displayed on an oscilloscope for further analysis. Because of the short measuring time (compared to the depletion time of the higher energy electrons) distortion-free measurements of widespread electron energy distributions or high resolution measurements of the same are possible. The device has also been applied to the determination of transformation energies of plasma chemical reactions.