N-doped Semiconductor Research Articles

Dark vertical conductance of cavity-embedded semiconductor heterostructures

We present a linear-response nonlocal theory of the electronic conductance along the vertical (growth) direction of a semiconductor heterostructure embedded in a single-mode electromagnetic resonator in the absence of illumination. Our method readily applies to the general class of n-doped semiconductors with parabolic dispersion. The conductance depends on the ground-state properties and virtual collective polaritonic excitations that have been determined via a bosonic treatment in the dipole gauge. We show that, depending on the system parameters, the cavity vacuum effects can enhance or reduce significantly the dark vertical conductance with respect to the bare heterostructure.

Role of Surface Reduction in the Formation of Traps in n-Doped II-VI Semiconductor Nanocrystals: How to Charge without Reducing the Surface.

The efficiency of nanocrystal (NC)-based devices is often limited by the presence of surface states that lead to localized energy levels in the bandgap. Yet, a complete understanding of the nature of these traps remains challenging. Although theoretical modeling has greatly improved our comprehension of the NC surface, several experimental studies suggest the existence of metal-based traps that have not yet been found with theoretical methods. Since there are indications that these metal-based traps form in the presence of excess electrons, the present work uses density functional theory (DFT) calculations to study the effects of charging II–VI semiconductor NCs with either full or imperfect surface passivation. It is found that charge injection can lead to trap-formation via two pathways: metal atom ejection from perfectly passivated NCs or metal–metal dimer-formation in imperfectly passivated NCs. Fully passivated CdTe NCs are observed to be stable up to a charge of two electrons. Further reduction leads to charge localization on a surface Cd atom and the formation of in-gap states. The effects of suboptimal passivation are probed by charging NCs where an X-type ligand is removed from the (100) plane. In this case, injection of even one electron leads to Cd-dimerization and trap-formation. Addition of an L-type amine ligand prevents this dimer-formation and is suggested to also prevent trapping of photoexcited electrons in charge neutral NCs. The results presented in this work are generalized to NCs of different sizes and other II–VI semiconductors. This has clear implications for n-doping II–VI semiconductor NCs without introducing surface traps due to metal ion reduction. The possible effect of these metal ion localized traps on the photoluminescence efficiency of neutral NCs is also discussed.

Observation of crossover from intraband to interband nonlinear terahertz optics.

In this Letter, we investigate the nonlinear effects of extremely intense few-cycle terahertz (THz) pulses (generated from the organic crystal 4-NN, NN-dimethylamino-4'4'-N'N'-methyl-stilbazolium 2, 4, 6 trimethylbenzenesulfonate, with peak electrical fields of a few MV/cm) on the carrier dynamics in n-doped semiconductor thin film In0.53Ga0.47As. By performing open-aperture Z-scan measurements and recording the transmitted THz energy through semiconductor sample, we observed a strong THz absorption bleaching effect at high fields, followed by an absorption enhancement at even higher fields. We attribute our observations to a crossover from pure intraband carrier dynamics to an interplay between intraband carrier heating and interband carrier generations.

Electron Injection on Metal/n-Doped Semiconducting Polymer

Efficient n-type doping was not established since the dopants dose not dissolve in same organic solutions of polymer semiconductors. Evaporative Spray Deposition using Ultra-dilute Solution (ESDUS) method enabled it recently. We have already reported efficient n-type doping of poly(2-methoxy-5-(2’-methyl-hexyloxy)-p- phenylenevinylene) (MEH-PPV: LUMO: 3.1 eV, HOMO: 5.2 eV). The high doping efficiency as much as 15% was realized. In organic devices composed of non-doped organic semiconductors, electron injection barrier is basically equal to the gap between LUMO of the organic semiconductor and work function of the el ectrode. It is still open question how the barrier is formed in the n-doped polymer/electrode. The metal/semiconductor junction of n-doped polymer has not been studied yet. In this research, we have fabricated electron only devices (EOD) of an n-doped polymer semiconductor using electrodes of Al and Ca. The electron injection from Al or Ca was examined.MEH-PPV (Mw = 40,000-70,000 Aldrich) and Cesium carbonate were used as a host polymer and an n-type dopant, respectively. The asymmetric EOD using Al for bottom and top electrodes and and a top Ca electrode having a lower work function (3.0 eV) were fabricated. The current-voltage (J-V) characteristics were measured in vacuum without breaking vacuum after the top electrode deposition. J-V characteristics of asymmetric EOD (Fig. 1), Al(50nm)/MEH-PPV:Cs2CO3 (110nm)/Ca(50nm) are fabricated. The current density was drastically increased as the doping concentration increased. The asymmetric EODs, current density in the forward bias was about 1 order of magnitude larger than that in reverse bias at low doping concentration at 0, 0.2, 2.0 wt%.However, the current density was almost overlapped in forward and reverse bias at the doping concentration of 10wt%. The disappearance of the rectification property suggests that the quantum tunneling takes place due to the very thin depletion layer at Al/MEH-PPV junction at the high doping concentration. In order to evaluate depletion layer width on Al/MEH-PPV junction, C-F measurement was carried out. As a result of the calculation, the depletion layer width of the 0.2 wt% doped device was 16.3 nm, the 2.0 wt% doped device was 9.3 nm, and the 10.0 wt% doped device was 3.3 nm. These results suggest that the depletion layer narrows as the doping concentration increases and the rectification property disappears in the 10 wt% doped device by quantum tunneling. The depletion layer width when quantum tunneling was observed in the p-type organic device, Au/ZnPc:0.3%F4-TCNQ/Au, was 5.2nm. Compared with this depletion layer width of 5.2 nm at metal/p-type organic semiconductor junction, it can be said that the value of 3.3 nm calculated this time is sufficiently narrow to observe quantum tunneling.

Impact of surface states and bulk doping level on hybrid inorganic/organic semiconductor interface energy levels

In applications, surface states and bulk doping concentration are important parameters of inorganic semiconductors, as they determine the bulk properties and substantially influence the properties of interfaces in devices, foremost the electron energy level alignment. In this work, we provide a qualitative model to describe the influence of surface state density and bulk donor concentration on the work function increase upon deposition of strong organic molecular acceptors onto the surface of n-doped inorganic semiconductors. This work function increase due to electron transfer to the molecular layer has two contributions: the formation of an interface dipole and a change of the near-surface space charge region inside the inorganic semiconductor, referred to as surface band bending. By using different surface preparation methods, we show how the surface state density limits the surface band bending change and enhances the interface dipole, both measured independently by photoelectron spectroscopy. In addition, we show that bulk donor concentration variation of the inorganic semiconductor has minor influence on the ratio of the two contributions to the work function change, at least for low to moderate donor concentrations up to 1019 cm−3.

Layer and doping tunable ferromagnetic order in two-dimensional CrS2 layers

Interlayer coupling is of vital importance for manipulating physical properties, e.g. electronic bandgap, in two-dimensional materials. However, tuning magnetic proper-ties in these materials is yet to be addressed. Here, we found a striped antiferromag-netic (sAFM) to ferromagnetic (FM) transition undergoing from monolayer to bilayer and thicker CrS2. This transition is attributed to charge sharing of interlayer S atoms and its resulting charge transfer from Cr d eg to t2g orbitals. The transferred charge reduces a portion of Cr4+ to Cr3+, which enhances the double-exchange mechanism favoring FM rather than sAFM ordering. Doping of electrons shares the same effect with stacking induced charge sharing. Therefore, charge doping could either manip-ulate the magnetic ordering between sAFM and FM or tune CrS2 between p- and n-doped magnetic semiconductors. We also proposed several prototype devices ena-bling to use external electric field for manipulating magnetic orderings of CrS2 layers. These results manifest the role of interlayer coupling in modifying magnetic proper-ties of layered materials

Band-to-Band Tunneling-Dominated Thermo-Enhanced Field Electron Emission from p-Si/ZnO Nanoemitters.

Thermo-enhancement is an effective way to achieve high performance field electron emitters, and enables the individually tuning on the emission current by temperature and the electron energy by voltage. The field emission current from metal or n-doped semiconductor emitter at a relatively lower temperature (i.e., < 1000 K) is less temperature sensitive due to the weak dependence of free electron density on temperature, while that from p-doped semiconductor emitter is restricted by its limited free electron density. Here, we developed full array of uniform individual p-Si/ZnO nanoemitters and demonstrated the strong thermo-enhanced field emission. The mechanism of forming uniform nanoemitters with well Si/ZnO mechanical joint in the nanotemplates was elucidated. No current saturation was observed in the thermo-enhanced field emission measurements. The emission current density showed about ten-time enhancement (from 1.31 to 12.11 mA/cm2 at 60.6 MV/m) by increasing the temperature from 323 to 623 K. The distinctive performance did not agree with the interband excitation mechanism but well-fit to the band-to-band tunneling model. The strong thermo-enhancement was proposed to be benefit from the increase of band-to-band tunneling probability at the surface portion of the p-Si/ZnO nanojunction. This work provides promising cathode for portable X-ray tubes/panel, ionization vacuum gauges and low energy electron beam lithography, in where electron-dose control at a fixed energy is needed.

Charge Transfer across the n-Gallium Phosphide(100) Photoanode/Electrolyte Interface during Photoelectrochemical Water Splitting

A detailed understanding of charge transfer mechanisms at the electrode/electrolyte interface is important for the development of electrodes for efficient photoelectrochemical (PEC) processes. We have studied the charge transfer processes across the n-GaP(100) photoanode/electrolyte interface. In a 0.02 M HCl electrolyte, high photoanodic currents from the n-GaP(100) photoanodes related to photolytic water splitting were measured at low anodic potentials but these photocurrents diminished at cathodic potentials and high anodic potentials. Electrochemical impedance spectroscopy (EIS) was carried out for n-GaP(100) photoanodes at different potentials to analyze the relevant charge transfer processes. Our EIS results suggest that the adsorption of hydroxide on metal-like surface Ga and their subsequent oxidative transformations – formation of these surface species is most favorable at low anodic potentials – is the driving force for high photoanodic currents at low anodic potentials. III-V semiconductors are prone to corrosion during PEC water splitting processes with corrosion-related decrease of efficiency. Our n-GaP(100) photoanodes were surface-conditioned via oxidizing at 0.8 V vs RHE and subsequently hydrogenated to passivate the defects in the oxide film. After this preparation, water splitting was observed at potentials between 0 and 0.3 V. The Nyquist plots derived from our EIS measurements for the n-GaP(100) photoanode consist of three semicircles indicating three distinct charge transfer mechanisms. The plots were analyzed with two different equivalent electrical circuits for the electrode/electrolyte system. These equivalent electrical circuits allowed to evaluate the relevance of different possible charge transfer pathways from the n-GaP(100) photoanode to the electrolyte. For each equivalent electrical circuit, the potential dependence of the resistances and capacitances including constant phase elements were determined from the fit and then compared with the potential variation of the current in the cyclic voltammogram. In a theoretical study Bertoluzzi et al. discussed the PEC implications of charge transfer from an n-doped semiconductor to the electrolyte that takes place via two different pathways1: by a direct charge transfer from the valence band of the semiconductor to the electrolyte, and by an indirect charge transfer from the valence band to the electrolyte via surface states (or defect states at the surface). The latter gives rise to a pronounced maximum of the interface- or defect-related capacitance as a function of potential. Our EIS results suggest that at low anodic potentials, the effective charge transfer process occurs from the valence band of the n-GaP(100) photoanode to defect states present in a thin Ga2O3-like layer at the surface, and subsequently, from these defect states to the electrolyte. This indirect charge transfer pathway is derived from a pronounced maximum of the capacitance as a function of applied potential. Our experimental results also suggest that subsequent charge transfer from these defect states leads to the formation of a molecular hydroxide layer the surface of metal-like Ga regions at the n-GaP(100) photoanode. Metal-like Ga at the surface of the photoanode was identified by XPS. A tafel slop of 63 mV/decade measured at the onset potential of the photoanodic current strongly points to the presence of adsorbed metal hydroxide (GaOH) species and subsequent oxidation products GaO and GaOOH as surface intermediates for the oxygen evolution which are finally catalyzed to molecular oxygen2. Our electrochemical impedance spectroscopy results also suggest that at low anodic potentials, overpotentials for the formation of hydroxide absorbates and their subsequent oxidation are lowest whereas at higher anodic potentials these hydroxide absorbates are stable and do not oxidize further to produce oxygen. At cathodic potentials, on the other hand, these surface hydroxides are reduced. As a consequence, large PEC currents associated with oxygen formation are possible only at low anodic potentials. By analyzing and understanding the charge transfer processes and the effect of different electrochemical parameters on these processes, we were able to optimize the PEC conditions which finally led to photolytic water splitting without applied voltage or addition of catalysts. Bertoluzzi, L; Lopez-Varo, P; Tejada, J. A. J; Bisquert, J., J. Mater. Chem. A., 2016, 4, 2873.Antoine, O; Butel, Y; Durand, R., J. Electroanal. Chem. 2001, 499, 85.

Localized Surface Plasmon Coupling between Mid-IR-Resonant ITO Nanocrystals.

Sn-doped indium oxide (ITO) nanocrystals (NC) provide tunable localized surface plasmon resonance in the mid-infrared. To evaluate the applicability of these n-doped plasmonic semiconductors in field-enhanced spectroscopies, it is necessary to assess how the low, free-electron density affects the E-field localization and plasmon coupling in NC films when compared to metal nanoparticles (NP). In this article, we investigate plasmon coupling between approximate 6 nm diameter ITO NC on the collective resonance and quantify the effect of the electromagnetic field enhancement on the absorbance signal of surface-attached ligands in NC films and monolayers with different ratios of doped and undoped indium oxide NC.

Flexible and high-efficiency Sb2S3/solid carrier solar cell at low light intensity

Producing green and efficient energy sources is a major challenge. As a consequence, the use of photovoltaic devices for conversion of light into electricity is growing worldwide. A lot of effort had been invested to create high-efficient solar cells, but their durability, stability, flexibility and efficiency at low light intensities are still unexplored. Here, we built a flexible solar cell made of p-doped, amorphized a-undoped and n-doped Sb2S3 solid carrier loaded with electrolyte. Indium tin oxide glass was the working electrode, and aluminium was the counter electrode. Every (p–a–n) flexible Sb2S3/solid carrier layers were obtained using a cheap casting/solvent evaporation technique, from a blend consisted of chitosan, polyethylene glycol and electrolyte containing 0.5 M potassium iodide and 0.05 M iodine, and corresponding synthesized amorphized a-undoped and p and n-doped Sb2S3 semiconductor. Results show that flexible Sb2S3 solar cell possesses good stability and efficiency of about 10% at 5% sun. Overall, our findings demonstrate for the first time that flexible solar cell can be made and used for low light intensity applications.

First-principles study of optical properties of germanium doped with phosphorus and bismuth

Using first-principles calculations based on density functional theory, we investigate the electronic structures and optical properties of germanium doped by phosphorus and bismuth with different concentrations. By analyzing the electronic structures and optical properties of the doped systems, we can theoretically analyze and predict the optical and electrical practical applications of N-doped germanium semiconductors. By analyzing and comparing the densities of electronic states before and after doped, we can draw some conclusions. The conclusions show that the Fermi level moves in the direction of conduction band after being doped. Although germanium is an indirect band gap luminescent material, the doped systems all become direct band gap luminescence. Doping more or less affects various optical properties in different energy ranges. In a low energy range, the dielectric function and refractive index of the doped systems are affected. When the doping concentration is 2.083%, the dielectric function and refractive index of the doped system both have a special change. And the absorption of the doped system is changed in the high energy. As the energy increases after the absorption peak, the absorption of the doped system drops faster. The reflectance of the doped system is affected in all the energy ranges. The reflectance of the doped system increases in medium energy. And the reflectance of the doped system is reduced in low energy and high energy range. However, when the doping concentration is 2.083% and the energy is less than 1.7 eV, the reflectance of the doped system is higher than that of the undoped system. The conductivity of the doped system forms two peaks, adding a peak in low energy. The additional peaks in the systems where the doping concentrations are 1.563% and 2.083% are obvious. The peak of the loss function increases after being doped. However, as the doping concentration increases, the increment of the loss function decreases. As the doping concentration increases, the peak is formed at a higher energy. The conclusions are of significance for guiding the optical applications of N-type doped germanium. According to the conclusions, we can adjust the doping concentration and energy range in the optical applications of N-doped germanium.

Phase-sensitive Kerr nonlinearity in a three-level n-doped semiconductor quantum well (SQW)

A three-level n-doped semiconductor quantum well (SQW) based on phase-sensitive Kerr nonlinearity is proposed. The effect of phase difference between applied fields on linear and nonlinear dispersion and absorption is then discussed. It is found that by proper selection of the relative phase of applied fields, the linear and nonlinear absorption converts to linear and nonlinear gain, and simultaneously the Kerr nonlinearity enhances dramatically. Also the impacts of coupling fields intensity on linear and nonlinear absorption and dispersion are discussed.

Polymorphic immune molecules in gametes and gametal fluids

In this work, a ternary photonic crystal having an n-doped semiconductor as a defect layer is considered. Tunable properties of the transmission spectra are investigated for TE waves. The three layers of the proposed photonic crystal are Si, Bi4Ge3O12 and SiO2 and the defect layer is n-GaAs. The transmission properties and defect mode of the structure may be controlled by using the doping rate, applied magnetic field and width of the n-GaAs layer. The defect layer width is found the most efficient factor among these three factors. The analysis is performed in the near-infrared spectral region. The proposed structure can be used to construct filters with narrowband transmission peaks.

High-efficient physical adsorption and detection of formaldehyde using Sc- and Ti-decorated graphdiyne

In sensitive analysis, the ultimate limit is to achieve reliable detection on trace amount of molecules. In this work, Sc- and Ti-decorated graphdiyne were proposed as promising materials for high-efficient molecular detection. Using density functional theory calculations, we investigated the electronic response of single Sc- and Ti-atom-decorated graphdiyne to HCHO (as a typical air pollutant). Thermodynamic analysis predicted that Sc or Ti adatom could be stabilized on the corner sites of single-layer graphdiyne sheet, with migration barriers high enough to prevent Sc or Ti adatom aggregation. The adsorption of HCHO on Sc- or Ti-decorated graphdiyne was found stronger than on pristine graphene or graphdiyne, which provides a prerequisite for molecular sensing. The electronegativity of HCHO leads to strong electronic attraction from Sc or Ti adatom to HCHO, resulting in a remarkable decrease of carrier density in graphdiyne. On Ti-decorated graphdiyne, the electronic attraction of HCHO appears to be stronger than on Sc-decorated graphdiyne and changes the system from metal to n-doped semiconductor. Quantum transport calculations show a decrease of current caused by the adsorbed HCHO. The results systematically exhibit the electronic response of Sc- or Ti-decorated graphdiyne to HCHO and suggest them as promising materials for molecule detection.

Doping Versatile n-Type Organic Semiconductors via Room Temperature Solution-Processable Anionic Dopants.

In this study, we describe a facile solution-processing method to effectively dope versatile n-type organic semiconductors, including fullerene, n-type small molecules, and graphene by commercially available ammonium and phosphonium salts via in situ anion-induced electron transfer. In addition to the Lewis basicity of anions, we unveiled that the ionic binding strength between the cation and anion of the salts is also crucial in modulating the electron transfer strength of the dopants to affect the resulting doping efficiency. Furthermore, combined with the rational design of n-type molecules, an n-doped organic semiconductor is demonstrated to be thermally and environmentally stable. This finding provides a simple and generally applicable method to make highly efficient n-doped conductors which complements the well-established p-doped organics such as PEDOT:PSS for organic electronic applications.

Control of Spin Helix Symmetry in Semiconductor Quantum Wells by Crystal Orientation.

We investigate the possibility of spin-preserving symmetries due to the interplay of Rashba and Dresselhaus spin-orbit coupling in n-doped zinc-blende semiconductor quantum wells of general crystal orientation. It is shown that a conserved spin operator can be realized if and only if at least two growth direction Miller indices agree in modulus. The according spin-orbit field has in general both in-plane and out-of-plane components and is always perpendicular to the shift vector of the corresponding persistent spin helix. We also analyze higher-order effects arising from the Dresselhaus term, and the impact of our results on weak (anti)localization corrections.

Synthesis and characterization of nanocrystalline N-doped semiconductor metal oxide and its visible photocatalytic activity in the degradation of an organic dye

N-doped Zinc Oxide (N-ZnO) was synthesized by a sol-gel cum combustion method and it was characterized by various techniques such as FT-IR, XRD, UV-DRS, FESEM, HRTEM, AFM, BET isotherm and Raman spectroscopy. The prepared material was found to possess a band gap energy of 3.1eV. Surface characterization techniques showed the material to possess a high surface area and surface roughness which would play a role in enhancing its photocatalytic efficiency. The photocatalytic activity of the prepared catalyst was evaluated by the degradation of a model organic dye – Eosin Yellow under visible light irradiation. Preliminary experiments showed the catalyst to be efficient across all pH values. A catalyst dosage of 0.7g/L gave better results and high degradation efficiency was observed for up to 5mg/L dye concentration. Kinetic studies were carried out for different initial dye concentrations from 5mg/L to 50mg/L. Results showed the photodegradation reaction to follow pseudo first order kinetics. The progress of the reaction was monitored by UV–vis absorption studies and reduction in Chemical Oxygen Demand of the solution. The presence of electrolytes did not diminish the photodegradation efficiency of the catalyst. Reusability studies showed the material to retain its original activity even after the 3rd cycle of use.

Electrical and optical conductivities of hole gas in p-doped bulk III–V semiconductors

We study electrical and optical conductivities of hole gas in p-doped bulk III-V semiconductors described by the Luttinger Hamiltonian. We provide exact analytical expressions of the Drude conductivity, inverse relaxation time for various impurity potentials, Drude weight, and optical conductivity in terms of the Luttinger parameters γ1 and γ2. The back scattering is completely suppressed as a result of the helicity conservation of the heavy and light hole states. The energy dependence of the relaxation time for the hole states is different from the Brooks-Herring formula for electron gas in n-doped semiconductors. We find that the inverse relaxation time of heavy holes is much less than that of the light holes for Coulomb-type and Gaussian-type impurity potentials and vice-versa for a short-range impurity potential. The Drude conductivity increases non-linearly with the increase in the hole density. The exponent of the density dependence of the conductivity is obtained in the Thomas-Fermi limit. The Drude weight varies linearly with the density even in the presence of the spin-orbit coupling. The finite-frequency optical conductivity goes as ω, and its amplitude strongly depends on the Luttinger parameters. The Luttinger parameters can be extracted from the optical conductivity measurement.

(Invited) CO2 Photoreduction Using H2o: 4.6% Solar-to-Chemical Conversion Efficiency By Metal-Complex Catalysts Coupled with Semiconductors

Recycling of CO2 is regarded as a technology necessary for next generation beyond solar hydrogen production by water splitting reaction, because organic chemicals are considered to be useful if they were synthesized from CO2 and H2O. In this presentation, the concept of photocatalytic system composed of metal-complex catalysts and semiconductors will be presented. We have proposed the novel concept of selective CO2 photoreduction using a hybrid photocatalyst comprising of p-type N-doped Ta2O5 semiconductor[1] photosensitizer linked with a Ru-complex electrocatalyst ([Ru(dcbpy)2(CO)2]2+), in which conduction band electrons in photoexcited N-Ta2O5 transferred to the Ru-complex within tens of picoseconds. [2-4] Then we have proven that a photocahode of zinc-doped indium phosphide (InP) coated with a ruthenium complex polymer ([Ru{4,4’-di(1H-pyrrolyl-3-propylcarbonate)2,2’-bipyridine}(CO)2] n , RuCP) electrocatalyst can reduce CO2 to formate with high selectively (>70%) under visible light irradiation in water.[5] By wiring the InP/[RuCP] photocathode for CO2 reduction with a TiO2 or SrTiO3-x photoanode for H2O oxidation in two compartment cell separated with proton exchange membrane, solar formate production by the Z-scheme reaction utilizing only sunlight, CO2 and H2O was successfully realized at solar-to-chemical conversion efficiencies (SCE) of 0.04-0.14 % without an external electrical bias application.[6,7] The semiconductor/metal-complex system has recently been integrated to a monolithic tablet-shaped device of [RuCP]/carbon/SiGe-jn/IrOx. By applying functions of selective reduction /oxidation reactions to both catalytic sites, the device operated in a single-compartment reactor in phosphate buffer at pH =6.4 (Fig.1), and it generated formate with a very high SCE of 4.6 %.[8] Recent progress in metal complex photocatalysts [9], semiconductors composed of earth-abundant elements [10,11], and challenge for powdered system for solar CO2 reduction [12] will also be presented briefly. [references] [1] T. Morikawa, S. Saeki, T. M. Suzuki, T. Kajino, T. Motohiro, Appl. Phys. Lett. 2010, 96 142111. [2] S. Sato, T. Morikawa, S. Saeki, T. Kajino, T. Motohiro, Angew. Chem. Int. Ed. 2010, 49,5101-5105. [3] T. M. Suzuki, H. Tanaka, T. Morikawa, M. Iwaki, S. Sato, S. Saeki, M. Inoue, T. Kajino, T. Motohiro, Chem. Commun. 2011, 47, 8673-8675. [4] K. Yamanaka, S. Sato, M. Iwaki, T. Kajino, T. Morikawa, J. Phys. Chem. C 2011, 115, 18348-18353. [5] T. Arai, S. Sato, K. Uemura, T. Morikawa, T. Kajino, T. Motohiro, Chem. Commun. 2010, 46,6944-6946. [6] S. Sato, T. Arai, T. Morikawa, K. Uemura, T. M. Suzuki, H. Tanaka, T. Kajino, J. Am. Chem. Soc., 2011, 133,15240-15243. [7] T. Arai, S. Sato, T. Kajino, T. Morikawa, Energy Environ. Sci., 2013, 6,1274-1282. [8] T. Arai, S. Sato, T. Morikawa, Energy Environ. Sci., 201 5 , 8 ,1998-2002. [9] S. Sato, T. Arai, T. Morikawa, K. Uemura, T. M. Suzuki, H. Tanaka, T. Kajino, J. Am. Chem. Soc., 2011, 133,15240-15243. [10] T. Morikawa, T. Arai, T. Motohiro, A ppl. P hys L ett., 98, 242108 _2011. [11] K. Sekizawa, T. Nonaka, T. Arai, T. Morikawa, ACS Appl. Mater. Interfaces 2014, 6, 10969−10973. [12] T. M. Suzuki, A. Iwase, H. Tanaka, S. Sato, A. Kudo, T. Morikawa, J. Mater. Chem. A, 2015, 3, 13283–13290. Figure 1

(Keynote) Artificial Photosynthesis: Synthesis of Organic Substances from CO2, H2o and Sunlight Using Semiconductor Photoelectrodes Coupled with Metal-Complex Catalysts

Recycling of CO2 is regarded as a technology necessary for next generation beyond solar hydrogen production by water splitting reaction, because organic chemicals are considered to be useful if they were synthesized from CO2 and H2O. In this presentation, the concept of photocatalytic system composed of metal-complex catalysts and semiconductors will be presented. We have proposed the novel concept of selective CO2 photoreduction using a hybrid photocatalyst comprising of p-type N-doped Ta2O5 semiconductor[1] photosensitizer linked with a Ru-complex electrocatalyst ([Ru(dcbpy)2(CO)2]2+), in which conduction band electrons in photoexcited N-Ta2O5 transferred to the Ru-complex within tens of picoseconds. [2-4] Then we have proven that a photocahode of zinc-doped indium phosphide (InP) coated with a ruthenium complex polymer ([Ru{4,4’-di(1H-pyrrolyl-3-propylcarbonate)2,2’-bipyridine}(CO)2] n , RuCP) electrocatalyst can reduce CO2 to formate with high selectively (>70%) under visible light irradiation in water.[5] By wiring the InP/[RuCP] photocathode for CO2 reduction with a TiO2 or SrTiO3-x photoanode for H2O oxidation in two compartment cell separated with proton exchange membrane, solar formate production by the Z-scheme reaction utilizing only sunlight, CO2 and H2O was successfully realized at solar-to-chemical conversion efficiencies (SCE) of 0.04-0.14 % without an external electrical bias application.[6,7] The semiconductor/metal-complex system has recently been integrated to a monolithic tablet-shaped device of [RuCP]/carbon/SiGe-jn/IrOx. By applying functions of selective reduction /oxidation reactions to both catalytic sites, the device operated in a single-compartment reactor in phosphate buffer at pH =6.4 (Fig.1), and it generated formate with a very high SCE of 4.6 %.[8] Recent progress in metal complex photocatalysts [9], semiconductors composed of earth-abundant elements [10,11], and challenge for powdered system for solar CO2reduction [12] will also be presented briefly. [references] [1] T. Morikawa, S. Saeki, T. M. Suzuki, T. Kajino, T. Motohiro, Appl. Phys. Lett. 2010, 96 142111. [2] S. Sato, T. Morikawa, S. Saeki, T. Kajino, T. Motohiro, Angew. Chem. Int. Ed. 2010, 49,5101-5105. [3] T. M. Suzuki, H. Tanaka, T. Morikawa, M. Iwaki, S. Sato, S. Saeki, M. Inoue, T. Kajino, T. Motohiro, Chem. Commun. 2011, 47, 8673-8675. [4] K. Yamanaka, S. Sato, M. Iwaki, T. Kajino, T. Morikawa, J. Phys. Chem. C 2011, 115, 18348-18353. [5] T. Arai, S. Sato, K. Uemura, T. Morikawa, T. Kajino, T. Motohiro, Chem. Commun. 2010, 46,6944-6946. [6] S. Sato, T. Arai, T. Morikawa, K. Uemura, T. M. Suzuki, H. Tanaka, T. Kajino, J. Am. Chem. Soc., 2011, 133,15240-15243. [7] T. Arai, S. Sato, T. Kajino, T. Morikawa, Energy Environ. Sci., 2013, 6,1274-1282. [8] T. Arai, S. Sato, T. Morikawa, Energy Environ. Sci., 201 5 , 8 ,1998-2002. [9] S. Sato, T. Arai, T. Morikawa, K. Uemura, T. M. Suzuki, H. Tanaka, T. Kajino, J. Am. Chem. Soc., 2011, 133,15240-15243. [10] T. Morikawa, T. Arai, T. Motohiro, A ppl. P hys L ett., 98, 242108 _2011. [11] K. Sekizawa, T. Nonaka, T. Arai, T. Morikawa, ACS Appl. Mater. Interfaces 2014, 6, 10969−10973. [12] T. M. Suzuki, A. Iwase, H. Tanaka, S. Sato, A. Kudo, T. Morikawa, J. Mater. Chem. A, 2015, 3, 13283–13290. Figure 1