Showing papers on "Photoconductivity published in 2010"

Atomically thin MoS2: a new direct-gap semiconductor

Abstract: The electronic properties of ultrathin crystals of molybdenum disulfide consisting of N=1,2,…,6 S-Mo-S monolayers have been investigated by optical spectroscopy Through characterization by absorption, photoluminescence, and photoconductivity spectroscopy, we trace the effect of quantum confinement on the material's electronic structure With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by more than 06 eV This leads to a crossover to a direct-gap material in the limit of the single monolayer Unlike the bulk material, the MoS₂ monolayer emits light strongly The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 10⁴ compared with the bulk material

Light Trapping in Silicon Nanowire Solar Cells

Abstract: Thin-film structures can reduce the cost of solar power by using inexpensive substrates and a lower quantity and quality of semiconductor material. However, the resulting short optical path length and minority carrier diffusion length necessitates either a high absorption coefficient or excellent light trapping. Semiconducting nanowire arrays have already been shown to have low reflective losses compared to planar semiconductors, but their light-trapping properties have not been measured. Using optical transmission and photocurrent measurements on thin silicon films, we demonstrate that ordered arrays of silicon nanowires increase the path length of incident solar radiation by up to a factor of 73. This extraordinary light-trapping path length enhancement factor is above the randomized scattering (Lambertian) limit (2n2 ∼ 25 without a back reflector) and is superior to other light-trapping methods. By changing the silicon film thickness and nanowire length, we show that there is a competition between impr...

Observation of long-range exciton diffusion in highly ordered organic semiconductors

Abstract: Excitons in polycrystalline and disordered films of organic semiconductors have been shown to diffuse over distances of 10-50 nm. Here, using polarization- and wavelength-dependent photoconductivity in the highly ordered organic semiconductor rubrene, we show that the diffusion of triplet excitons in this material occurs over macroscopic distances (2-8 μm), comparable to the light absorption length. Dissociation of these excitons at the surface of the crystal is found to be the main source of photoconductivity in rubrene. In addition, we observe strong photoluminescence quenching and a simultaneous enhancement of photoconductivity when the crystal surface is functionalized with exciton splitters. In combination with time-resolved measurements, these observations strongly suggest that long-lived triplet excitons are indeed generated in molecular crystals by fission of singlets, and these triplets provide a significant contribution to the surface photocurrent generated in organic materials. Our findings indicate that the exciton diffusion bottleneck is not an intrinsic limitation of organic semiconductors.

Single-crystalline CdS nanobelts for excellent field-emitters and ultrahigh quantum-efficiency photodetectors.

Abstract: The single crystalline CdS nanobelts were synthesized by an improved vapor-liquid-solid (VLS) process Field emission measurements show that the nanostructures have a low turn-on field of 37 V&m-1 at a current density of 10 μA cm-1, a low threshold field of 93 V μm cm-1 and a high enhancement factor of 1298 When assembled into nanoscale visible light photodetectors, the CdS nanobelts showed good sensitivity and wavelength selectivity The results imply that the present CdS nanobelts are excellent candidates for applications in high-performance field emitters and photodetectors

Photoinduced Degradation of Polymer and Polymer–Fullerene Active Layers: Experiment and Theory

Abstract: As organic photovoltaic effi ciencies steadily improve, understanding degradation pathways becomes increasingly important. In this paper, the stability under prolonged illumination of a prototypical polymer:fullerene active layer is studied without the complications introduced by additional layers and interfaces in complete devices. Combining contactless photoconductivity with spectroscopy, structural characterization at the molecular and fi lm level, and quantum chemical calculations, the mechanism of photoinduced degradation in bulk heterojunctions of poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) is studied. Bare fi lms are subjected to four conditions for 1000 h with either constant illumination or dark and either ambient or inert atmosphere. All samples are found to be intrinsically stable for 1000 + h under inert conditions, in contrast to complete devices. While PCBM stabilizes P3HT fi lms exposed to air, its fullerene cage is found to undergo a series of oxidations that are responsible for the deterioration of the photoconductivity of the material. Quantum chemical calculations show that PCBM oxides have deeper LUMO levels than pristine PCBM and therefore act as traps for electrons in the PCBM domains.

Ultrahigh‐Performance Solar‐Blind Photodetectors Based on Individual Single‐crystalline In2Ge2O7 Nanobelts

Abstract: [∗] Dr. L. Li , Dr. T. Y. Zhai , Dr. X. S. Fang , Prof. Y. Bando , Prof. D. Golberg International Center for Materials Nanoarchitectonics (MANA) National Institute for Materials Science (NIMS) Namiki 1–1, Tsukuba, Ibaraki, 305–0044 (Japan) E-mail: LI.Liang@nims.go.jp; ZHAI.Tianyou@nims.go.jp; FANG.Xiaosheng@nims.go.jp Prof. P. S. Lee , Dr. C. Y. Yan School of Materials Science and Engineering Nanyang Technological University (NTU) (Singapore) Dr. M. Y. Liao , Prof. Y. Koide Sensor Materials Center, NIMS Namiki 1–1, Tsukuba, Ibaraki, 305–0044 (Japan) E-mail: Meiyong.Liao@nims.go.jp

Ultrathin n-Type Organic Nanoribbons with High Photoconductivity and Application in Optoelectronic Vapor Sensing of Explosives

Abstract: Well-defined ultrathin nanoribbons have been fabricated from an amphiphilic electron donor−acceptor (D−A) supramolecule comprising perylene tetracarboxylic diimide as the backbone scaffold to enforce the one-dimensional intermolecular assembly via strong π-stacking. These nanoribbons demonstrated high photoconductivity upon illumination with white light. The high photoconductivity thus obtained is likely due to the optimal molecular design that enables a good kinetic balance between the two competitive processes, the intramolecular charge recombination (between D and A) and the intermolecular charge transport along the nanoribbon. The photoconduction response has also proven to be prompt and reproducible with the light turning on and off. The photogenerated electrons within the nanoribbon can be efficiently trapped by the adsorbed oxygen molecules or other oxidizing species, leading to depletion of the charge carriers (and thus the electrical conductivity) of the nanoribbon, as typically observed for n-ty...

Ultraviolet ZnO Nanorod Photosensors

Abstract: This study fabricates and characterizes ultraviolet (UV) photosensors with ZnO nanorods (NRs). The NR arrays were selectively grown in the gap between interdigitated (IDT) electrodes of devices using hydrothermal solution processes and a lithography-based technique. Compared with a conventional ZnO photosensor without NRs, the proposed UV NR photosensors have much higher photoresponse in the UV region. Additionally, the photoconductive gain of an NR photosensor increased as UV illumination time increased; it varied at 34.45−5.32 × 102 under illumination by 18.28 mW/cm2 optical power. Consequently, the substantial photoconductive gain can be attributed to high surface-to-volume ratio of ZnO NRs. The high density of hole-trap states on NR surfaces lead to a persistent photoconductivity (PPC) state, promoting the transport of carriers through devices.

Near-IR Photoresponse in New Up-Converting CdSe/NaYF4:Yb,Er Nanoheterostructures

Abstract: We report the synthesis of new nanoheterostructures consisting of lanthanide-doped NaYF4 nanocrystals dendritically decorated with CdSe quantum dots. Strong coupling between NaYF4:Yb,Er and CdSe leads to up-conversion of near-IR photons and subsequent energy transfer from lanthanide nanocrystals to CdSe. This is manifested in characteristic CdSe emission and leads to pronounced photoconductivity of the films upon sub-band-gap irradiation at 980 nm.

Position dependent photodetector from large area reduced graphene oxide thin films

Abstract: We fabricated large area infrared photodetector devices from thin film of chemically reduced graphene oxide (RGO) sheets and studied their photoresponse as a function of laser position. We found that the photocurrent either increases, decreases, or remain almost zero depending upon the position of the laser spot with respect to the electrodes. The position sensitive photoresponse is explained by Schottky barrier modulation at the RGO film-electrode interface. The time response of the photocurrent is dramatically slower than single sheet of graphene possibly due to disorder from the chemical synthesis and interconnecting sheets.

20 μs Photocurrent Response from Lithographically Patterned Nanocrystalline Cadmium Selenide Nanowires

Abstract: Lithographically patterned nanowire electrodeposition (LPNE) provides a method for patterning nanowires composed of nanocrystalline cadmium selenide (nc-CdSe) over wafer-scale areas. We assess the properties of (nc-CdSe) nanowires for detecting light as photoconductors. Structural characterization of these nanowires by X-ray diffraction and transmission electron microscopy reveals they are composed of stoichiometric, single phase, cubic CdSe with a mean grain diameter of 10 nm. For nc-CdSe nanowires with lengths of many millimeters, the width and height dimensions could be varied over the range from 60 to 350 nm (w) and 20 to 80 nm (h). Optical absorption and photoluminescence spectra for nc-CdSe nanowires were both dominated by band-edge transitions. The photoconductivity properties of nc-CdSe nanowire arrays containing approximately 350 nanowires were evaluated by electrically isolating 5 microm nanowire lengths using evaporated gold electrodes. Photocurrents, i(photo), of 10-100 x (i(dark)) were observed with a spectral response characterized by an onset at 1.75 eV. i(photo) response and recovery times were virtually identical and in the range from 20 to 40 micros for 60 x 200 nm nanowires.

Electrical Transport and High‐Performance Photoconductivity in Individual ZrS2 Nanobelts

Abstract: Individual ZrS(2)-nanobelt field-effect transistors were fabricated using a photolithography process. Temperature-dependent electrical transport revealed different electrical conductivity mechanism at different working temperature regions. ZrS(2)-nanobelt photodetectors demonstrated a high-performance visible-light photoconductivity.

Diameter-dependent internal gain in ohmic Ge nanowire photodetectors.

Abstract: We report a diameter-dependent photoconduction gain in intrinsic Ge nanowire (NW) photodetectors. By employing a scanning photocurrent imaging technique, we provide evidence that the photocarrier transport is governed by the hole drift along the Ge NWs, ensuing the higher internal gain up to ∼103 from the thin NWs. It is found that the magnitudes of both gain and photoconductivity are inversely proportional to the NW diameter ranging from 50 to 300 nm. We attribute our observations to the variation in the effective hole carrier density upon varying diameters of Ge NWs, as a result of field effects from the diameter-dependent population of the surface-trapped electrons, along with a model calculation. Our observations represent inherent size effects of internal gain in semiconductor NWs, thereby provide a new insight into nano-optoelectronics.

Electrical transport and high-performance photoconductivity in individual ZrS(2) nanobelts

Abstract: Individual ZrS(2)-nanobelt field-effect transistors were fabricated using a photolithography process. Temperature-dependent electrical transport revealed different electrical conductivity mechanism at different working temperature regions. ZrS(2)-nanobelt photodetectors demonstrated a high-performance visible-light photoconductivity.

Photoluminescence and Photoconductivity of ZnS-Coated ZnO Nanowires

Abstract: ZnO nanowires (NWs) with a ZnS coating are synthesized in order to modify the surface without changing the diameter of the NWs. They have the wurtzite ZnO at the core and a cubic ZnS at the outer layer. The NWs show a sharp ultraviolet and a broad visible emission of the photoluminescence spectra. Surface modification has led to a change in the position of the maxima of the visible emission in ZnO-ZnS NWs. The photocarrier relaxation under steady UV illumination occurs in ZnO NW arrays but is absent in ZnO-ZnS NW arrays. The dark current value for both type of NWs are similar, whereas the photocurrent value is much higher in the surface-modified NWs. Higher photocurrent value indicates a transport of the photogenerated carriers from the ZnS layer to ZnO during UV illumination. The carrier transport mechanism is proposed through a model.

Persistent photoconductivity in Hf–In–Zn–O thin film transistors

Abstract: Passivated Hf–In–Zn–O (HIZO) thin film transistors suffer from a negative threshold voltage shift under visible light stress due to persistent photoconductivity (PPC). Ionization of oxygen vacancy sites is identified as the origin of the PPC following observations of its temperature- and wavelength-dependence. This is further corroborated by the photoluminescence spectrum of the HIZO. We also show that the gate voltage can control the decay of PPC in the dark, giving rise to a memory action.

Coherent Control of Ballistic Photocurrents in Multilayer Epitaxial Graphene Using Quantum Interference

Abstract: We report generation of ballistic electric currents in unbiased epitaxial graphene at 300 K via quantum interference between phase-controlled cross-polarized fundamental and second harmonic 220 fs pulses. The transient currents are detected via the emitted terahertz radiation. Because of graphene's special structure symmetry, the injected current direction can be well controlled by the polarization of the pump beam in epitaxial graphene. This all optical injection of current provides not only a noncontact way of injecting directional current in graphene but also new insight into optical and transport process in epitaxial graphene.

Plasmon-induced electrical conduction in molecular devices.

Abstract: Metal nanoparticles (NPs) respond to electromagnetic waves by creating surface plasmons (SPs), which are localized, collective oscillations of conduction electrons on the NP surface. When interparticle distances are small, SPs generated in neighboring NPs can couple to one another, creating intense fields. The coupled particles can then act as optical antennae capturing and refocusing light between them. Furthermore, a molecule linking such NPs can be affected by these interactions as well. Here, we show that by using an appropriate, highly conjugated multiporphyrin chromophoric wire to couple gold NP arrays, plasmons can be used to control electrical properties. In particular, we demonstrate that the magnitude of the observed photoconductivity of covalently interconnected plasmon-coupled NPs can be tuned independently of the optical characteristics of the molecule—a result that has significant implications for future nanoscale optoelectronic devices.

Phototransistor-based optoelectronic tweezers for dynamic cell manipulation in cell culture media

Abstract: Optoelectronic tweezers (OET), based on light-induced dielectrophoresis, has been shown as a versatile tool for parallel manipulation of micro-particles and cells (P. Y. Chiou, A. T. Ohta and M. C. Wu, Nature, 2005, 436, 370–372).1 However, the conventional OET device cannot operate in cell culture media or other high-conductivity physiological buffers due to the limited photoconductivity of amorphous silicon. In this paper, we report a new phototransistor-based OET (Ph-OET). Consisting of single-crystalline bipolar junction transistors, the Ph-OET has more than 500× higher photoconductivity than amorphous silicon. Efficient cell trapping of live HeLa and Jurkat cells in Phosphate Buffered Saline (PBS) and Dulbecco's Modified Eagle's Medium (DMEM) has been demonstrated using a digital light projector, with a cell transport speed of 33 µm/sec, indicating a force of 14.5 pN. Optical concentration of cells and real-time control of individually addressable cell arrays have also been realized. Precise control of separation between two cells has also been demonstrated. We envision a new platform for single cell studies using Ph-OET.

Photoluminescence and photoconductive characteristics of hydrothermally synthesized ZnO nanoparticles

Abstract: In the present paper, ZnO nanoparticles (NPs) with particle size of 20–50 nm have been synthesized by hydrothermal method. UV-visible absorption spectra of ZnO nanoparticles show absorption edge at 372 nm, which is blue-shifted as compared to bulk ZnO. Photoluminescence (PL) and photoconductive device characteristics, including field response, light intensity response, rise and decay time response, and spectral response have been studied systematically. The photoluminescence spectra of these ZnO nanoparticles exhibited different emission peaks at 396 nm, 416 nm, 445 nm, 481 nm, and 524 nm. The photoconductivity spectra of ZnO nanoparticles are studied in the UV-visible spectral region (366–691 nm). In spectral response curve of ZnO NPs, the wavelength dependence of the photocurrent is very close to the absorption and photoluminescence spectra. The photo generated current, Ipc = (Itotal - Idark) and dark current Idc varies according to the power law with the applied field IpcαVr and with the intensity of illumination IpcαILr, due to the defect related mechanism including both recombination centers and traps. The ZnO NPs is found to have deep trap of 0.96 eV, very close to green band emission. The photo and dark conductivities of ZnO NPs have been measured using thick film of powder without any binder.

Ultraviolet photoconductance of a single hexagonal WO3 nanowire

Abstract: The ultraviolet (UV) photoconductance properties of a single hexagonal WO3 nanowire have been studied systematically. The conductance of WO3 nanowires is very sensitive to ultraviolet B light and a field-effect transistor (FET) nanodevice incorporating a single WO3 nanowire exhibits excellent sensitivity, reversibility, and wavelength selectivity. A high photoconductivity gain suggests that WO3 nanowires can be used as the sensing element for UV photodetectors. Measurements under UV light in vacuum show that the adsorption and desorption of oxygen molecules on the surface of the WO3 nanowire can significantly influence its photoelectrical properties. The WO3 nanowires have potential applications in biological sensors, optoelectronic devices, optical memory, and other areas.

Hydrogen generation by solar water splitting using p-InGaN photoelectrochemical cells

Abstract: Photoelectrochemical effects in p-InxGa1−xN (0≤x≤0.22) alloys have been investigated. Hydrogen generation was observed in p-InGaN semiconducting electrodes under white light illumination with additional bias. It was found that p-InGaN alloys possess much higher conversion efficiencies than p-GaN. Time dependent photocurrent density characteristics showed that the stability of p-InGaN in aqueous HBr is excellent. The photocurrent density was found to increase almost linearly with hole mobility and excitation light intensity.

Steady-state and transient photoconductivity in c-axis GaN nanowires grown by nitrogen-plasma-assisted molecular beam epitaxy

Abstract: Analysis of steady-state and transient photoconductivity measurements at room temperature performed on c-axis oriented GaN nanowires yielded estimates of free carrier concentration, drift mobility, surface band bending, and surface capture coefficient for electrons. Samples grown (unintentionally n-type) by nitrogen-plasma-assisted molecular beam epitaxy primarily from two separate growth runs were examined. The results revealed carrier concentration in the range of (3–6)×1016 cm−3 for one growth run, roughly 5×1014–1×1015 cm−3 for the second, and drift mobility in the range of 500–700 cm2/(V s) for both. Nanowires were dispersed onto insulating substrates and contacted forming single-wire, two-terminal structures with typical electrode gaps of ≈3–5 μm. When biased at 1 V bias and illuminated at 360 nm (3.6 mW/cm2) the thinner (≈100 nm diameter) nanowires with the higher background doping showed an abrupt increase in photocurrent from 5 pA (noise level) to 0.1–1 μA. Under the same conditions, thicker (151...

Intersublevel infrared photodetector with strain-free GaAs quantum dot pairs grown by high-temperature droplet epitaxy.

Abstract: Normal incident photodetection at mid infrared spectral region is achieved using the intersublevel transitions from strain-free GaAs quantum dot pairs in Al0.3Ga0.7As matrix. The GaAs quantum dot pairs are fabricated by high temperature droplet epitaxy, through which zero strain quantum dot pairs are obtained from lattice matched materials. Photoluminescence, photoluminescence excitation optical spectroscopy, and visible-near-infrared photoconductivity measurement are carried out to study the electronic structure of the photodetector. Due to the intersublevel transitions from GaAs quantum dot pairs, a broadband photoresponse spectrum is observed from 3 to 8 μm with a full width at half-maximum of ∼2.0 μm.

Instability in threshold voltage and subthreshold behavior in Hf–In–Zn–O thin film transistors induced by bias-and light-stress

Abstract: Electrical bias and light stressing followed by natural recovery of amorphous hafnium-indium-zinc-oxide (HIZO) thin film transistors with a silicon oxide/nitride dielectric stack reveals defect density changes, charge trapping and persistent photoconductivity (PPC) In the absence of light, the polarity of bias stress controls the magnitude and direction of the threshold voltage shift (ΔVT), while under light stress, VT consistently shifts negatively In all cases, there was no significant change in field-effect mobility Light stress gives rise to a PPC with wavelength-dependent recovery on time scale of days We observe that the PPC becomes more pronounced at shorter wavelengths

Plasmonic nanoparticle enhanced photocurrent in GaN/InGaN/GaN quantum well solar cells

Abstract: We demonstrate enhanced external quantum efficiency and current-voltage characteristics due to scattering by 100 nm silver nanoparticles in a single 2.5 nm thick InGaN quantum well photovoltaic device. Nanoparticle arrays were fabricated on the surface of the device using an anodic alumina template masking process. The Ag nanoparticles increase light scattering, light trapping, and carrier collection in the III-N semiconductor layers leading to enhancement of the external quantum efficiency by up to 54%. Additionally, the short-circuit current in cells with 200 nm p-GaN emitter regions is increased by 6% under AM 1.5 illumination. AFORS-Het simulation software results were used to predict cell performance and optimize emitter layer thickness.

Visible-blind deep-ultraviolet Schottky photodetector with a photocurrent gain based on individual Zn2GeO4 nanowire

Abstract: We report on the visible-blind deep-ultraviolet (DUV) photodetectors with metal-semiconductor-metal (MSM) Schottky contacts based on individual Zn2GeO4 nanowire single-crystals. At an 8 V bias voltage, the device shows an extremely low dark current (<0.1 pA), a responsivity of 38.3 A/W (corresponding gain ∼200), a high DUV-to-visible discrimination ratio up to ∼104, and a relatively fast response time upon 245 nm DUV illumination. By analyzing the light-intensity-dependent photocurrent generation and carrier transport, the photogenerated holes trapped in Schottky barrier and shrinking of depletion region under DUV illumination at the metal/Zn2GeO4 interface are proposed for the carrier injection and the photocurrent gain.

Ultrafast terahertz conductivity dynamics in mesoporous TiO2: Influence of dye sensitization and surface treatment in solid-state dye-sensitized solar cells

Abstract: We have used optical-pump terahertz-probe spectroscopy to explore the photoinduced conductivity dynamics in mesoporous anatase TiO2 films, commonly employed as the electron-transporting electrode in dye-sensitized solar cells. We find an intrinsic mobility value of 0.1 cm2/(V s) and diffusion length of ∼20 nm for electron motion through the TiO2 matrix. The photoconductivity dynamics in TiO2 films, both before and after sensitization with a ruthenium bypyridyl complex termed Z907, were examined in order to study the charge injection, trapping, and recombination time scales. We observe a biphasic charge injection from Z907, with a fast sub-500 fs component, followed by a slower 70−200 ps component. This is followed by photoconductivity decay over the first few nanoseconds, predominantly reflecting charge carrier trapping. In addition, we have utilized terahertz spectroscopy to investigate the influence of treating the titania surface with TiCl4 on early-time charge dynamics. In the solar cells, surface tre...