Narrow Results

Modest fill factors (~0.2) and efficiencies for sensitized photocurrent generation are observed with porphyrins adsorbed to saturation on a nanocrystalline SnO₂ thin film employed as the working electrode in a photoelectrochemical cell. No dye aggregation is observed at the metal oxide/adsorbate interface, and no advantage in the photosensitization efficiency is seen with two porphyrins that exhibit a stable liquid crystalline phase over another porphyrin that does not.

A novel photocurrent effect induced by Terahertz (THz) radiation in two-dimensional (2D) electron systems is reported. It was found that when the system is tuned to electron cyclotron resonance conditions, THz radiation induces a visible signal related to currents flowing in the plane of 2D electron systems without any external bias. This effect is employed as a technique for the study of the integer Quantum Hall samples in the absence of external bias. Our data indicate that at the integer ν the photocurrents are circulating currents in the dissipationless quantum Hall regime.

Scanning photocurrent microscopy (SPCM) is a powerful experimental tool used to investigate spatially resolved optoelectronic properties of semiconductors and their nanostructures. Raster-scanned laser excitation generates a position-dependent photocurrent map from which carrier diffusion length, electric field distribution, doping concentration and more can be explored. In this review, we will briefly discuss the history of the technique, the theory behind locally injected carrier transport in semiconductors, the SPCM experimental setup, and recent applications of SPCM in semiconductor nanostructures. Particularly, we have shown that the minority carrier diffusion length can also be obtained by SPCM in two-dimensional semiconductors and that the local excitation can result in an internal electric field because of the difference in electron and hole mobilities.

In this work, we consider the results of studying the spectral distribution, the temperature dependence of photoconductivity, and the dependence of the spectral distribution of photoconductivity on the applied electric field of doped with rare earth elements GaTe single crystals in the temperature range of 30–300 K. As the temperature decreases to 30 K, the impurity peaks disappear, and the intense maximum shifts to the short wavelength region. The increase in impurity photoconductivity with increasing temperature is due to the thermooptical filling of acceptor levels with electrons and their further transition to the conduction band under the action of illumination. Such a temperature dependence of the photoconductivity is explained by the presence of acceptor levels in the band gap.

This study reports on the deposition of highly transparent conducting n-type zinc oxide (ZnO) thin films on FTO substrates, via an optimized doping process. Our work is focused on doping zinc oxide with vanadium (V) using spray pyrolysis technique and ensure the synthesis of nanoparticles-shaped ZnO, with an improved optical, microstructural and electrical properties for solar cells applications, as optical window material. Undoped and V-doped ZnO thin films, with careful optimized amounts (2, 4, 6 and 8 at.%), were grown at maintained 550^∘C pre-heated substrate during the deposition process, which enables us to obtain nano-sized ZnO particles. We proved that 4 at.% is the optimum V content that enhances the crystallinity of the grown thin film noticeably. With an average transmittance of 80%, the deposited thin films revealed high transparency in the visible domain with a slight decrease in optical transmission which might result from additional scattering. UV-Visible analysis showed that increasing V amounts, a resulting decrease in the energy bandgap ($E_g$) is obtained from 3.26eV to 3.17eV for 4 at.% of V content. Moreover, deep level defects in zinc oxide can be reduced with vanadium doping and consequently strengthen the UV emission. The UV emission peak intensity rises with increasing V-doping amount then decreases slightly at 8% of V content. The electrical properties measurements showed a decrease in resistivity from 2.8 10${}^{-2}$$\mathrm{\Omega }$⋅cm to 0.9 10${}^{-2}$$\mathrm{\Omega }$⋅cm when doping with 4 at.% of V. The crucial effect of the V-doping of ZnO was also demonstrated via the enhancement of carrier mobility that attains 38.5cm²/V⋅s at the optimum vanadium content. The photocurrent analysis revealed much higher visible light absorption in the V-doped zinc oxide thin films than that of undoped film. The photocatalytic activity enhancements are attributed to the lower recombination rate of the photogenerated electron-hole pairs, the narrowed bandgap, yielding a higher photocatalytic performance.

The photogalvanic effect (PGE) can produce a photocurrent independent of a p–n junction or bias voltage, but the resulting photocurrent is usually very small. An appropriate lateral heterojunction can effectively enhance PGE. Here, we studied the PGE of a monolayer (ML) Janus MoSSe-CrSSe lateral heterojunction through ab initio quantum transport simulation. Compared with that of a homogeneous material, PGE is enhanced due to the reduced symmetry of the lateral heterojunction, and the maximum increase ratio in the photocurrent is more than two orders of magnitude greater than that of the homogeneous ML MoSSe and CrSSe photodetectors. A peak maximum photocurrent of 13.85 a${}_0^2$/photon and high polarization sensitivity with a maximum extinction ratio of 308 are obtained for the ML MoSSe-CrSSe lateral heterojunction. These favorable properties indicate that the MoSSe-CrSSe lateral heterojunction is a promising candidate for photodetectors.

Low efficiency of Terahertz (THz) radiation and radiation power have limited the development of THz Science and Technology. Thus, with the aims of achieving greater photoconductive current and improving radiation characteristics of THz photoconductive antenna (PCA), this study utilized Extraordinary Optical Transmission (EOT) of light passing through various subwavelength metal structures to control and restrain the light wave in subwavelength scale. Furthermore, by grooving the grating electrode structure, the influence of metal grating’s EOT on the transmission field of PCA were investigated and analyzed. Simulation results show that the effect of local electric field enhancement is significant. When the incident power is 0.1W, the peak value of the local electric field reaches $7.04\times 10^7$V/m. In addition, comparing to the grating electrode with no groove structure in which the field intensity was less than $4.28\times 10^6$V/m, the local electric field increased by 16.4 times, respectively. Correspondingly, the photocurrent intensity of the improved photoconductive plasmonic structure is increased by 72.3 times. In conclusion, the improved plasma photoconductive structure was shown to obviously enhance the transmission field strength of semiconductor materials and the current of the PCA, and accordingly, to improve the THz radiation capability.

In this article, titanium oxide nanotube arrays (TiO₂–NTAs) were fabricated by anodic oxidation in an ethylene glycol (EG) electrolyte solution containing 0.25 wt.% NH₄F. By varying anodized time and annealed temperature, the obtained nanotube arrays behaved different photocatalytic (PC) activities and photocurrent properties. These samples were characterized by scanning electronic microscope (SEM), X-ray powder diffraction (XRD). It was indicated in SEM images that TiO₂ nanotube manifests highly ordered structure which, however, has been completely destroyed when the temperature comes to 800°C. XRD manifested that TiO₂ nanotubes with various kinds of length all possessed anatase crystallite when annealed at 500°C; meanwhile, with certain length, TiO₂–NTAs annealed at series calcination temperature range of 300–600°C also presented anatase crystallite, which is gradually enhanced with the increment of temperature. At 700°C, mixed structure was observed which was made up of proportions of overwhelming anatase and toothful rutile. Methyl blue (MB) degradation and photocurrent measurement testified that TiO₂–NTAs under 4 h oxidation and 3 h of 600°C calcination manifested the highest activity and photocurrent density.

The defects induced by a spike rapid thermal annealing (RTA) process in crystalline silicon (c-Si) solar cells were investigated by the photoluminescence (PL) technique and the transmission electron microscopy (TEM), respectively. Dislocation defects were found to form in the near-surface junction region of the monocrystalline Si solar cell after a spike RTA process was performed at 1100${}^{\circ }$C. Photo J–V characteristics were measured on the Si solar cell before and after the spike RTA treatments to reveal the effects of defects on the Si cell performances. In addition, the Silvaco device simulation program was used to study the effects of defects density on the cell performances by fitting the experimental data of RTA-treated cells. The results demonstrate that there was an obvious degradation in the Si solar cell performances when the defect density after the spike RTA treatment was above $1\times 10^{13}$cm${}^{-3}$.

Undoped and magnesium-doped zinc oxide thin films were prepared by the sol–gel method. Results from X-ray diffraction indicated that the films exhibited a hexagonal wurtzite structure and were highly oriented along the $c$-axis. The intensity of the (002) diffraction peak increased with increasing the Mg doping concentration. Also, Mg doping inhibited the growth of crystallite size which decreased from 46nm to 38nm with doping concentration. Morphological studies by atomic force microscopy (AFM) indicated the uniform thin film growth and the decreasing of grain size and surface roughness with Mg doping. Optical analysis showed that the average transmittance of all films was above 90% in the visible range and Mg doping has significantly enhanced the bandgap energy of ZnO. Two Raman modes assigned to $E_{2L}$ and $E_{2H}$ for the ZnO wurtzite structure were observed for all films. UV emission peak and three defect emission peaks in the visible region were observed by photoluminescence measurements at room temperature. The intensity ratio of UV emission to the visible emission increased with the Mg concentration. Photocurrent measurements revealed that all films presented the photoresponses with $n$-type semiconducting behavior and their generated photocurrents were reduced by Mg doping. The prepared thin films of high quality with improved properties by Mg doping could be proposed to workers in the field of optoelectronic devices for using them as a strong candidate.

SrTiO₃ and SrTiO₃:Nd films of 110nm and 210nm thickness were fabricated using the sol–gel technology on silicon. Their current–voltage characteristics were investigated with and without illumination. The film structures are photosensitive and exhibit the hysteresis on the forward and reverse bias with loop broadening at the reverse part.

Molecular systems capable of photoinduced vectorial electron transfer (ET) were assembled using the Langmuir-Blodgett technique. The active part of the systems consisted of phytochlorin-fullerene donor-acceptor layers and polythiophene secondary donor layers. Such structures can promote multi-step inter-layer ET in a direction determined by the film architecture. Transient photovoltage and photocurrent were studied and ET rates and efficiencies were determined. The primary intramolecular charge separated state was formed upon photoexcitation in less than 1 ns and it recombined in approximately 50 ns. Addition of the polythiophene layer at the side of the phytochlorin moieties increased the distance of charge transfer and the lifetime of the charge separated state by promoting spontaneous interlayer electron transfer from the polymer to the phytochlorin cation.

Supramolecular design principles for a porphyrin-sensitized, wet-type solar cell are described. To construct efficient organic photocurrent-generating systems, the following two important targets exist: (i) kinetic control of photoinduced electron-transfer processes by spatial, three-dimensional alignment of photo-functional molecules (sensitizers, electron donors, acceptors, and mediators) and (ii) highly dense deposition of composites of the photo-functional molecules on an electrode. These objectives can be achieved by tailoring a photoactive multilayer using supramolecular interactions, such as molecular adsorption, inclusion, coordination, and recognition. Using these interactions, it is expected that the reduction of the costs of synthesis and the combinational fabrication of a simplified-molecular device will become possible. In addition, recent approaches toward the construction of supramolecular porphyrin-sensitized photovoltaic cells are introduced.

Tetrabenzoporphyrin films on indium-tin-oxide electrodes were prepared by continuous spin-coating of indium-tin-oxide electrodes with a soluble precursor, tetrabicyclo[2.2.2]octadiene-fused porphyrin, and subsequent thermal conversion of the precursor to tetrabenzoporphyrin by annealing the modified electrodes. When the tetrabenzoporhyrin-modified indium-tin-oxide working electrode was irradiated in a three-electrode system, using Pt as a counter electrode and Ag/Ag⁺ as a reference electrode in the presence of hexyl viologen as an electron acceptor, a cathodic photocurrent was observed. A double layer structure consisting of tetrabenzoporphyrin and [6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM) films and a triple layer structure consisting of tetrabenzoporphyrin; a mixture of tetrabenzoporphyrin and PCBM; and PCBM films were also prepared on indium-tin-oxide electrodes by repeated spin-coating. The incident photon to photocurrent efficiency values of up to 6.8% were obtained for the triple layer structure, in which the mixed layer contained tetrabenzoporphyrin and PCBM molecules in a 7:3 ratio. Action spectra of the triple layer structure showed that visible light from 380 to 700 nm sensitized the system for photocurrent generation.

The photoelectroectrochemical studies of water soluble octacarboxylated oxotitanium (OTiOCPc), zinc (ZnOCPC), hydroxyaluminium ((OH)AlOCPc), dihydroxysilicon ((OH)₂SiOCPc), hydroxygallium (OHGaOCPc) and low symmetry zinc monocarboxy (ZnMCPc) phthalocyanines were performed. The dyes were adsorbed to nanoporous ZnO electrodeposited in the presence of eosin Y as structure directing agent (SDA) on FTO substrates by refluxing or soaking the films in a solution containing the dye of interest such that a full surface coverage was achieved. High external (IPCE) and internal (APCE) quantum efficiencies of up to 50.6% and 96.7% were achieved for the OTiOCPc complex. There was a lower overall cell efficiency for cells sensitized with phthalocyanines containing hydroxyl as axial ligand ZnO/(OH)₂SiOCPc, ZnO/(OH)GaOCPc and (OH)AlOCPc because of strong aggregation on the surface of the electrodes. To further suppress dye aggregation, the zinc complex of a new monocarboxylated phthalocyanine sensitizer with bulky naphtho side groups (ZnMCPc) was employed. Among the studied sensitizers, ZnMCPc gave the highest overall cell efficiency of phthalocyanine electrodeposited on ZnO of η = 0.48%.

Zinc-porphyrin(ZnP)–viologen(V²⁺) linked compound containing six methylene group (ZnP(6)V)–silver nanoparticle (AgNP) composite films was fabricated by combining electrostatic layer-by-layer adsorption and the Langmuir–Blodgett method. The incident photo to photocurrent efficiency (IPCE) values of the ZnP(6)V–AgNP composite films are higher than those of the ZnP(6)V films and much higher than those of ZnP derivative films without V²⁺ moiety as a reference. The large increase in the IPCE values of the ZnP(6)V–AgNP composite films likely comes from a combination of localized surface plasmon resonance (LSPR) from AgNPs and photoinduced intramolecular electron-transfer upon linking to a V²⁺ moiety. The photocurrents of the ZnP(6)V–AgNP composite films and the ZnP(6)V films increase upon application of a magnetic field. Magnetic field effects (MFEs) were clearly observed for both ZnP(6)V–AgNP composite films and the ZnP(6)V films. Photocurrents increase with magnetic field under low magnetic fields (B ≤ 150–300 mT) and are constant under high magnetic fields (B > 150–300 mT). MFEs can be explained by a radical pair mechanism. The magnitude of the MFEs in the ZnP(6)V–AgNP composite films is higher than that in the ZnP(6)V films. A remarkable increase in photocurrent for the ZnP(6)V–AgNP composite films was observed because of LSPR from the AgNPs in the presence of a magnetic field when compared with the ZnP(6)V films in the absence of a magnetic field.

Ag nanoparticles were deposited by polyol process onto the as-synthesized anatase TiO₂ cubes with single crystals by hydrothermal method. The materials were characterized by SEM, XRD, TEM, XPS and UV-vis spectroscopy and their photoconversion efficiencies were also evaluated. The photocurrent measurements revealed that the modification of the anatase TiO₂ cubes with Ag nanoparticles improved the photoelectrochemical properties of electrodes.

We report on the sensitive detection of glucose using silicon nanowire array field-effect-transistor (SiNW-FET) upon illumination. The uniformly distributed and size-controlled SiNWs were fabricated by "top-down" approach. The fabricated SiNW-FET device was evaluated for detection of glucose in the range of 100–900 mg/dL. The SiNW-FET shows enhanced sensitivity of 0.988 ± 0.030 nA (mg/dl)^-1 upon illumination at 480 nm light as compared to without illumination as 0.486 ± 0.014 nA (mg/dL)^-1. The presented SiNW-FET device is fast, stable and sensitive to light as well as to bio analyte, and hence can be utilized as sensitive biological sensing platform.

Top-down silicon nanowire (SiNW) fabrication mechanisms for connecting electrodes are widely utilized because they provide good control of the diameter to length ratio. The representative mechanism for the synthesis of SiNWs, a top-down approach, has limitations on the control of their diameter following lithography technologies, requires a long manufacturing process and is not cost-effective. In this study, we have implemented the bottom-up growth of horizontal SiNWs(H-SiNWs) on Si/SiO₂ substrates directly by plasma enhanced chemical vapor deposition (PECVD) under about 400°C. The HAuCl4 solution as a catalyst and SiH₄ gas as a precursor are used for the synthesis of H-SiNWs. After optimization of synthesis conditions, we evaluated the photoelectric properties of the H-SiNWs under illumination with different light intensities. Further, we demonstrated the feasibility of H-SiNW devices for the detection of biotinylated DNA nanostructures and streptavidin interaction.

WO₃/Cu composite film electrodes were synthesized by hydrothermal combined electrodeposition method. Characterization of samples was conducted by SEM, XRD and XPS, which showed WO₃/Cu composite films had been synthesized. The diffusion coefficient, reversibility, response time, coloration efficiency and transmission rate of the samples were obtained by electrochemical and spectral measurements. The photocurrent and photoelectric catalysis degradation efficiency of the samples were obtained by photocurrent and photoelectric catalysis measurements. The WO₃/Cu composite films improve electrochromic performance, photocurrent and photoelectric catalytic activity compared with pure WO₃ nanoblocks, and the WO₃/Cu composite film obtained by depositing Cu nanoparticles at 50 s shows the highest electrochromic performance, photocurrent and photoelectric catalytic activity. Meanwhile, the direct photocatalytic and electric catalytic activity of the composite film are also discussed. The combined effects of the lower band gap and the Schottky junction lead to significant enhancement in the electrochromic and photoelectrochemical properties of the WO₃/Cu composite film.