UV Illumination In Air Research Articles

Bifacial Passivation of Organic Hole Transport Interlayer for NiO x -Based p-i-n Perovskite Solar Cells.

Methoxy‐functionalized triphenylamine‐imidazole derivatives that can simultaneously work as hole transport materials (HTMs) and interface‐modifiers are designed for high‐performance and stable perovskite solar cells (PSCs). Satisfying the fundamental electrical and optical properties as HTMs of p‐i‐n planar PSCs, their energy levels can be further tuned by the number of methoxy units for better alignment with those of perovskite, leading to efficient hole extraction. Moreover, when they are introduced between perovskite photoabsorber and low‐temperature solution‐processed NiOx interlayer, widely featured as an inorganic HTM but known to be vulnerable to interfacial defect generation and poor contact formation with perovskite, nitrogen and oxygen atoms in those organic molecules are found to work as Lewis bases that can passivate undercoordinated ion‐induced defects in the perovskite and NiOx layers inducing carrier recombination, and the improved interfaces are also beneficial to enhance the crystallinity of perovskite. The formation of Lewis adducts is directly observed by IR, Raman, and X‐ray photoelectron spectroscopy, and improved charge extraction and reduced recombination kinetics are confirmed by time‐resolved photoluminescence and transient photovoltage experiments. Moreover, UV‐blocking ability of the organic HTMs, the ameliorated interfacial property, and the improved crystallinity of perovskite significantly enhance the stability of PSCs under constant UV illumination in air without encapsulation.

Frabrication and properties of self-powered ultraviolet detectors based on one-demensional ZnO nanomaterials

ZnO micro/nanowires were synthesized by chemical vapor deposition method. The morphology and structure of the products have been characterized by using scanning electron microscopy (SEM), X-ray diffraction (XRD), photoluminescence (PL) and micro-Raman scattering spectrometer, etc. Results show that the surface of the highly uniform ZnO wire is smooth and the as-synthesized ZnO wires show high crystal quality. Three types of UV detector are constructed using a single ZnO nanowire with different contact characteristics, and their corresponding performances are investigated systematically by using Keithley 4200-SCS and other equipments. All of the three different devices exhibit good rectifying characters and significant responsivity to ultraviolet light. The devices show self-driven features at zero bias. Compared with the devices made from Schottky contact and ZnO/PEDOT:PSS film, the present single ZnO nanowire/p-Si film devices with heterojunctions have the best self-powered function, which can be attributed to the stronger built-in electric field as well as the smaller dark current due to the insulating layer on the p-Si film. At zero bias, the fabricated ZnO nanowire/p-Si film device can deliver a dark current of 1.210-3 nA and a high photosensitivity of about 4.5103 under UV illumination. The response of the devices made from ZnO nanowire/p-Si film to UV illumination in air is pretty fast with the rise time of about 0.7 s and the fall time of about 1 s, which could be attributed to the fact that the photo-generated electron-hole pairs in the depletion layer is quickly separated by the built-in electric field, leading to a rapid response speed and a larger photocurrent. Comparison among the three kinds of devices indicates that the devices made from ZnO nanowire/p-Si film are the best candidate for UV detectors.

Electrical Conductivity and Gas-sensing Properties of Mg-doped and Undoped Single-crystalline In2O3 Thin Films: Bulk vs. Surface

This study aims to provide a better fundamental understanding of the gas-sensing mechanism of In2O3-based conductometric gas sensors. In contrast to typically used polycrystalline films, we study single crystalline In2O3 thin films grown by molecular beam epitaxy (MBE) as a model system with reduced complexity. Electrical conductance of these films essentially consists of two parallel contributions: the bulk of the film and the surface electron accumulation layer (SEAL). Both these contributions are varied to understand their effect on the sensor response. Conductance changes induced by UV illumination in air, which forces desorption of oxygen adatoms on the surface, give a measure of the sensor response and show that the sensor effect is only due to the SEAL contribution to overall conductance. Therefore, a strong sensitivity increase can be expected by reducing or eliminating the bulk conductivity in single crystalline films or the intra-grain conductivity in polycrystalline films. Gas-response measurements in ozone atmosphere test this approach for the real application.

Fabrication and fast photoresponse properties of ZnO nanowires photodetectors

One-dimensional ZnO nanostructure is especially attractive because of its unique properties such as high surface-to-volume ratio and a large exciton binding energy, but how to put it into a device is still a challenge. In this article, we show that a novel lateral metal-semiconductor-metal ultraviolet detector composed of ZnO nanowires is fabricated on glass substrate by a single-step hydrothermal approach. The fabricated photodetector demonstrates that the response to UV illumination in air is fast, the rise time is about 4 s, and the fall time is about 5 s, which could be attributed to the fact that the adsorption and the desorption of water molecules in the air onto oxygen vacancy of the nanowire significantly influence the photoresponse.

Thermal annealing effects on the dynamic photoresponse properties of Al-doped ZnO nanowires network

In this study, UV photodetectors based on a network of aluminium-doped zinc oxide (AZO) nanowires were manufactured at a low cost; for this purpose, a fast and simple fabrication process that involved dropping nanowires dispersion solution was employed instead of the conventional e-beam lithography process that is used to manufacture single nanowire–based UV photodetectors. It was demonstrated that nanowire network–based UV photodetectors provide a much faster UV photoresponse than conventional single nanowire–based UV photodetectors. The fast UV photoresponse of the fabricated UV photodetector can be attributed to the fact that the potential barriers formed in the nanowire network junctions effectively block the flow of electrons during the process of photocurrent decay. Furthermore, the UV photoresponse under illumination by a 254 nm UV source was studied as a function of the annealing temperature of the AZO nanowires network at a bias of 5 V. The fabricated UV photodetector showed the fastest response of 2 s to UV illumination in air when the sample was annealed in air for 1 h at 300 °C.

High-performance UV detector made of ultra-long ZnO bridging nanowires

A nanowatt UV photoconductive detector made up of ultra-long(∼100 µm) ZnO bridging nanowires has been fabricated by a single-step chemical vapor deposition(CVD) process. The electrodes, forming comb-shaped thick ZnO layers, and the sensingelements, consisting of ZnO nanowires bridging the electrodes, were fabricatedsimultaneously in a single-step CVD process. The device showed drastic changes(10–105 times) in current under a wide range of UV irradiances(10−8–10−2 W cm−2). Moreover, the detector exhibited fast response (rise and decay times ofthe order of 1 s) to UV illumination in air, but no response to visible light(hν<3.2 eV). Our approach provides a simple and cost-effective way to fabricate high-performance‘visible-blind’ UV detectors.

Fabrication of ZnO Bridging Nanowire Device by a Single-Step Chemical Vapor Deposition Method

ABSTRACTZnO nanowires are directly integrated into a working device by a single-step chemical vapor deposition (CVD) method. Gold catalyst is patterned on a quartz glass substrate using a comb-shaped shadow mask and then ZnO is grown on the patterned substrate by CVD. Thick ZnO layers formed on the gold-patterned areas serve as native electrodes. Ultra-long (˜100 μm) ZnO nanowires grown across the gap between the ZnO electrodes and the nanowires serve as the sensing elements of the device. The device exhibits high sensitivity and fast response to UV illumination in air. Our method can be used to fabricate other metal oxide semiconductor bridging nanowire devices, which have promising applications in photodetection and gas sensing.

Simple fabrication of a ZnO nanowire photodetector with a fast photoresponse time

A zinc oxide (ZnO) nanowire photodetector was fabricated by a simple method of growing ZnO nanowires bridging the gap of two patterned zinc electrodes. The nanowire growth is self-catalytic, involving the direct heating of patterned Zn electrodes at 700°C in an O2∕Ar gas flow of 20SCCM (standard cubic centimeter per minute at STP)/80SCCM, respectively, at atmospheric pressure for 3h. The fabricated photodetector demonstrated fast response of shorter than 0.4ms to UV illumination in air, which could be attributed to the adsorption, desorption, and diffusion of water molecules in the air onto the nanowire significantly influencing the photoresponse.

Keto defect sites in fluorene-based organic field-effect transistors: The origin of rapid degradation on the performance of the device

The effect of keto defects in fluorene units on the performance of organic field-effect transistors (OFETs) was examined based on fluorene end-capped fused bithiophenes (BFTT) and biphenyl end-capped fused bithiophene oligomers (BPTT). The formation of keto defects after various periods of UV illumination in air on BFTT films was confirmed by the increase of the long-wavelength emission at 2.1–2.3eV in the photoluminescent (PL) spectrum and the generation of a Fourier transfer infrared (FTIR) peak at 1721cm−1, corresponding to the carbonyl stretching mode of the fluorenone moiety. For both BPTT films irradiated in air and BFTT in nitrogen, i.e., a keto-free system, no increase in long-wavelength emission in the PL spectrum, was found and the peak corresponding to the carbonyl stretching mode of the fluorenone moiety was absent in the FTIR spectrum. The threshold voltage, i.e., switch-on voltage, of the OFETs was increased and the field-effect mobility and on-state drain current were rapidly decreased after the formation of ketonic defects, since these defects induce the formation of numerous trap sites in the band gap of the semiconducting conjugated oligomer. The density of trap sites (Nt) generated after the formation of keto defects was determined using space-charge-limited current spectroscopy. A Nt of around 2.7×1015cm−3 was found for the BFTT film due to the formation of keto defects after 6h of UV irradiation.

Photo-induced effects in Ge25Ga10S65 glasses studied by XPS and XAS

The present paper focuses on the structural, electronic, and compositional properties of Ge25Ga10S65 glasses before and after UV illumination in air using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) techniques. The XPS Ge 3d spectra reveal the existence of GeO bonds in the surface region of illuminated glass. In the case of this sample, XAS O K-edge spectra showed the formation of an enriched region of oxygen atoms in the glass bulk, indicating a different bonding structure of oxygen at the surface and in the bulk of the glass. Moreover, the structural changes that occur after UV illumination in the glass sample are identified as the formation of a homogeneous germanium oxide surface layer followed by an intermediary Ge25Ga10S65−yOz subsurface region.

Bulk and track etching of PET studied by spectrophotometer

UV–VIS spectra of poly(ethylene terephthalate) (PET) solutions formed by etching PET in NaOH solution were analyzed with respect to the etching time. A linear relationship between absorptions centered at 4.45 and 5.11 eV with weight loss of PET in NaOH solution was established. The relation was applied to study the influence of UV light illumination on bulk etching of PET and to evaluate pore size of etched-through tracks. It is found that bulk etching of PET can be greatly enhanced by UV illumination in air in the wavelength range around 313 nm . A surface area of about 350 nm in thickness shows a 23 times increase in bulk-etching rate after illuminated for 6 h . The phenomenon is attributed to the oxygen-assisted photo-degradation through generating of new photo-unstable species. The enhancement in bulk etching was immediately reduced as the etching proceeds below the surface with an exponential decay constant of about 1.5 μm −1 . Etching of Xe ion irradiated PET films gives extra etching products with similar chemical structure as revealed by spectrophotometer measurements. Quantitative analysis of etching products from latent tracks implies that pores of about 14.6 nm in radius are formed after etching in 0.74 N NaOH at 40°C for 35 min , which is in agreement with the conductometric measurement.

A new polystyrene–TiO 2 nanocomposite film and its photocatalytic degradation

A new kind of photodegradable polystyrene (PS)–TiO 2 nanocomposite film was synthesized. The TiO 2 nanoparticles were first modified by grafting polymer on its surface (G-TiO 2), and then the PS–G-TiO 2 composite films were formed through polymerization. The photocatalytic degradation of the composite films was investigated. The as-prepared films were characterized by scanning electron microscope (SEM), gel permeation chromatogram (GPC), FT-IR and UV-Vis spectroscopy, and the photoinduced weight-loss. The results show that PS–G-TiO 2 nanocomposite films could be efficiently photocatalytically degraded under UV illumination in air. The weight-loss of the PS–G-TiO 2 film reached 29%; the average molecular weight reduced to 1/4 of the original molecular weight under UV-light irradiation for 300 h, implying that some benzene rings had been cleaved. The photocatalytic degradation mechanism of the films is briefly discussed.

Photoreactivity of Alkylsiloxane Self-Assembled Monolayers on Silicon Oxide Surfaces

We have combined contact angle measurements, Fourier transform infrared spectroscopy (FT-IR), and atomic force microscopy (AFM) to investigate the UV photoreactivity of octadecylsiloxane (ODS) SAMs (self-assembled monolayers) in air. Contact angle measurements present evidence that ozone is not the active agent in alkylsiloxane SAM degradation under UV illumination in air. A combination of UV and oxygen is necessary for monolayer degradation to proceed. AFM measurements on submonolayer coverage SAMs provide direct evidence of the photodegradation of ODS SAMs and reveal the role of defects in the degradation process. FT-IR and AFM results suggest that the hydrocarbon chain is the reactive site of the monolayers. A microscopic mechanism of the photoreactivity involving hydrogen abstraction is suggested based on the mechanism of gas-phase oxidation of alkanes. Our results have implications in clarifying the contribution from the reactivity of alkyl chains during the photomodification of SAMs. This clarificat...

Ultraviolet light induced changes in polyimide liquid-crystal alignment films

Ultraviolet light induced changes in polyimide liquid-crystal alignment films were investigated. Infrared, UV-visible, x-ray photoelectron spectroscopy, and electron-spin-resonance measure- ments indicated that bond breaking and subsequent oxidation reactions occur in polyimide films (SE7210, OCG284, DuPont 2555 and 2540) during the broadband UV illumination in air. Mechanical rubbing has no effect on the optical and magnetic properties but it causes the removal of the UV-exposed film. Capacitance–voltage measurements indicate that there is a slight decrease in dielectric constant and creation of net negative charges in the film after UV exposure. Sur- face tension of polyimide films before and after UV illumination and changes in the pretilt angle of the polyimide surface following UV exposure have also been studied. The decrease in pretilt angle following UV illumination is attributed to an increase in surface tension. Our results indicate that a simple UV technique can be used to achieve domain divided liquid-crystal pixel electrode design with improved viewing characteristics.

Photo-induced Transformation, upon UV Illumination in Air, of Hyponitrite SpeciesN2O22- Preadsorbed onTiO2 Surface

Ultraviolet photo-excitation of hydroxylated TiO2 surfaces in the presence of a mixture of N2/O2 gas (air) produces some adsorbed nitrogen oxide species which were previously identified as hyponitrite, N2O22-. The present paper reports selected results relating to the photo-induced transformations, upon UV illumination in air, of N2O22- species preadsorbed on TiO2 surfaces. It provides some information about the mechanism by which dinitrogen is photofixed on TiO2 surface and of its phototransformation.