Vacuum Ultraviolet Light Irradiation Research Articles

Vacuum ultraviolet-induced surface modification of polyoxymethylene plates for photo-activation bonding

The surface photoactivation technique using vacuum ultra-violet (VUV) light at 172 nm in wavelength has been applied for adhesive-free bonding of polyoxymethylene (POM) plates. The POM surfaces were photochemically modified via VUV light irradiation in dry air conditions, resulting in the formation of oxygen-containing functionalities. The POM plates were successfully bonded with a practical bonding strength below the melting point. The maximum bonding strength was obtained at an irradiation time of 60 min, while a longer irradiation time resulted in lower bonding strength. Surface morphologies of the photoactivated and fractured POM surfaces were observed by a laser scanning microscope. Due to the VUV irradiation, numerous cracks were formed on the POM surfaces. The fractured surface shows that a cohesive failure occurs at the bonding interface. Atomic force microscopy was used to observe the cross-sectional images of the bonding samples. Around 10 μm width, significant phase contrast between the bonding interface and bulk POM indicates the different mechanical properties, due to VUV irradiation. The modified POM surfaces adhered through thermocompression, and the mechanical interlocking and entanglement of POM molecules can result in successful bonding.

Crystallinity improvement of room-temperature PLD-deposited ZnO thin films on cyclo-olefin polymer substrates subject to surface-pretreatment with vacuum-UV-light irradiation

The effect of the surface-pretreatment of cyclo-olefin polymer (COP) substrates with vacuum-ultraviolet (VUV) light irradiation on the crystal growth of ZnO thin films deposited by room-temperature pulsed laser deposition (PLD) was investigated for development of flexible optical devices. 172-nm-excimer light irradiation with durations of 60 to 600 s in air was found to make the COP surface more hydrophilic and smooth. It was verified from measurements of X-ray diffraction, atomic force microscope and high-resolution transmission electron microscope that the ZnO thin films grown on the VUV-light-pretreated COP substrates exhibited enhanced (0001) orientation growth and increased crystallinity compared with the samples grown on substrates not subject to the pretreatment; the samples on the pretreated substrates also showed a smooth surface and atomically sharp interface between the thin film and the substrate. The electrical resistivity of the ZnO thin film grown on the VUV-light-pretreated COP substrate was estimated to be 2.3 × 10−3 Ωcm, which was 40% lower than that of the film grown on the untreated substrate.

Characterization of vacuum ultraviolet-irradiated surface modification of CoO(111) crystal by low-energy atom scattering spectroscopy

The surface of a CoO(111) single crystal was treated with vacuum ultraviolet (VUV) light at a wavelength of 172 nm without heat treatment. We studied the surface structural analysis of CoO(111) before and after VUV light irradiation in air using low-energy atom scattering spectroscopy. The primary beam was 3 keV-4He0, and backscattered 4He particles from Co atoms were detected using a microchannel plate detector. We compared the experimental spectra to simulation results, and the results demonstrated that the rock-salt CoO(111) surface was transformed to a spinel Co3O4(111) surface after VUV light irradiation. We also identified details about the chemical properties and topmost layers of CoO(111) surfaces before and after VUV light irradiation.

Organic monolayers modified by vacuum ultraviolet irradiation for solution-processed organic thin-film transistors

A trimethylsilyl-monolayer modified by vacuum ultraviolet (VUV) light has been investigated for use in solution-processed organic thin-film transistors (OTFTs). The VUV irradiation changed a hydrophobic trimethylsilyl-monolayer formed from hexamethyldisilazane vapor into a hydrophilic surface suitable for solution processing. The treated surface was examined via water contact angle measurement and X-ray photoelectron spectroscopy. An appropriate irradiation of VUV light enabled the formation of a dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) film on a modified monolayer by spin-coating. Consequently, the C8-BTBT-based OTFT with a monolayer modified for an optimal VUV irradiation time exhibited a field-effect mobility up to 4.76 cm2 V−1 s−1. The partial monolayer modification with VUV can be adapted to a variety of solution-processes and organic semiconductors for prospective printed electronics.

Improvement of device performance of Ph-BTBT-10 field-effect transistors fabricated on a HfO2/alicyclic polyimide double-layered gate insulator

We report on the FET properties of 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) on a HfO2/alicyclic polyimide double-layered gate insulator at an elevated temperature. The patterning of semiconductor and polymeric insulator layers is also discussed using the surface selective deposition of solution onto the wettable region (self-assembled monolayer-coated oxide layer) and unwettable region (bare metal oxide layer) obtained by vacuum ultraviolet light (172 nm) irradiation. A multilayered film of 20–30 nm thick Ph-BTBT-10 and a double-layered gate insulator consisting of 30–40 nm thick alicyclic polyimide on high-k metal oxide films leads to a reduction in the operating voltage and the interfacial trap density at the gate insulator interface. The mobility of the FETs was improved from 0.4 to 2.4 cm2 V−1 s−1 by thermal annealing of Ph-BTBT-10 at 120 °C due to the transition from a monolayer to a bilayer structure and the use of alicyclic polyimide as the polymer gate insulator.

Controlled Growth of Organosilane Micropatterns on Hydrophilic and Hydrophobic Surfaces Templated by Vacuum Ultraviolet Photolithography.

In this report, micropatterns of (3-aminopropyl)trimethoxysilane (APTMS) were developed on hydrophilic and hydrophobic surfaces after patterning using 172 nm vacuum ultraviolet (VUV) photolithography. Self-assembled monolayers (SAMs) formed on Si substrates through UV hydrosilylation of 1-hexadecene (HD) and 10-undecenoic acid (UDA) were used as hydrophilic and hydrophobic surfaces, respectively. For templating the HD- and UDA-SAMs, the VUV light was exposed to HD- and UDA-SAMs from the slits of photomasks in atmospheric and evacuated environments, respectively. Various oxygenated groups were generated at the exposed domains of HD-SAM, while the COOH groups were trimmed from the irradiated domains of UDA-SAM. The APTMS molecules were immobilized on the domains that were terminated by oxygenated groups after chemical vapor deposition (CVD). The thicknesses of the developed APTMS micropatterns increased significantly by raising the CVD temperature and in the presence of ambient air in the CVD Teflon container as well. The increase in thicknesses was ascribed to the formation of APTMS multilayers, which were mediated by H3N+ ions. Also, the developed APTMS micropatterns on the UDA-SAM patterned by VUV light irradiation in a high-vacuum environment (HV-VUV) were thicker than those on the VUV/(O) patterned HD-SAM due to the presence of inactive oxygenated groups at the surface of VUV/(O)-terminated domains of HD-SAM such as COO-C and C-O-C groups. The presence of water or ambient air facilitated the silane coupling between the silyl groups with the oxygenated and amino groups The combination of VUV photolithography and the CVD method with control of the conditions would enable us to control the thicknesses and shapes of the developed APTMS micropatterns. These findings illustrate the applicability of VUV photolithography for templating hydrophobic and hydrophobic surfaces toward the development of organosilane architectures, which can be feasible for several applications.

Surface Patterning of Closed Nanochannel Using VUV Light and Surface Evaluation by Streaming Current.

In nanofluidics, surface control is a critical technology because nanospaces are surface-governed spaces as a consequence of their extremely high surface-to-volume ratio. Various surface patterning methods have been developed, including patterning on an open substrate and patterning using a liquid modifier in microchannels. However, the surface patterning of a closed nanochannel is difficult. In addition, the surface evaluation of closed nanochannels is difficult because of a lack of appropriate experimental tools. In this study, we verified the surface patterning of a closed nanochannel by vacuum ultraviolet (VUV) light and evaluated the surface using streaming-current measurements. First, the C18 modification of closed nanochannels was confirmed by Laplace pressure measurements. In addition, no streaming-current signal was detected for the C18-modified surface, confirming the successful modification of the nanochannel surface with C18 groups. The C18 groups were subsequently decomposed by VUV light, and the nanochannel surface became hydrophilic because of the presence of silanol groups. In streaming-current measurements, the current signals increased in amplitude with increasing VUV light irradiation time, indicating the decomposition of the C18 groups on the closed nanochannel surfaces. Finally, hydrophilic/hydrophobic patterning by VUV light was performed in a nanochannel. Capillary filling experiments confirmed the presence of a hydrophilic/hydrophobic interface. Therefore, VUV patterning in a closed nanochannel was demonstrated, and the surface of a closed nanochannel was successfully evaluated using streaming-current measurements.

Silver plasmonic colour change due to chemical/mechanical reactions

Plasmonic colour from metals has been recently attracting attention because it is less likely to deteriorate due to environmental change. However, metals such as silver easily react chemically with the atmosphere. Although plasmonic colour in various materials and structures has been assessed, the effects of chemical changes on plasmonic properties have not been deeply investigated. In this paper, we have evaluated the changes in optical properties and structure of silver plasmonic colour from the viewpoint of a phase diagram. As a result, we report remarkable colour changes and the formation of nanorings with a diameter of approximately 100 nm by vacuum ultraviolet light irradiation and heat annealing.

Effects of Cu Metal Barrier on Electrical Characteristics of Porous Carbon-Doped Oxide Film

Porous carbon-doped oxide (CDO) film is a promising low-dielectric-constant (low-k) material that is used as an insulator in the back-end-of-line interconnects of integrated circuits for 45 nm or below technological nodes. In Cu/porous low-k integrated interconnects, a barrier is required to prevent the migration of Cu into the low-k insulating dielectric, avoiding electrical instability and degradation of reliability. Cobalt (Co) and ruthenium (Ru), deposited by physical vapor deposition (PVD), were evaluated as Cu barriers in this study. Their effects on the electrical characteristics of the porous CDO film were compared. Experimental results revealed that Co is a better barrier against Cu diffusion, enhances adhesion, and generates electric stress-induced traps at a lower rate. However, damage to the underlying porous CDO film during Co or Ru PVD sputtering deposition was observed. More damage was detected when a Co barrier was used because vacuum-ultraviolet light irradiation was emitted by Ar plasma for a longer time. Therefore, Co can serve as an alternative material for Cu barrier layer. However, the method of deposition of the Co barrier must be carefully chosen to alleviate damage to the contacting low-k insulating dielectric.

(Invited) Photoluminescent ZnO Thin Films By Photochemical Gel Conversion Under VUV Irradiation

Zinc oxide (ZnO), as a wide band gap (3.37 eV) semiconductor, are useful materials for wide range of applications in thin-film transistor (TFT), photovoltaics, photocatalysts, sensors, and so on. Large exciton binding energy (60 meV) at room temperature (RT) makes ZnO a promising luminescent material especially in optoelectronic devices such as UV lasers and light emitting diodes (LED). Among them, ZnO nanocrystals (quantum dots) has attracted increasing attention because their optical properties can be tuned by varying the crystal size. However, its nanoscale crystallites tend to aggregate to undergo Ostwald ripening due to their high surface energy. To stabilize them, surface modification are usually necessary. At the same time, the whole processes for nanocrystal synthesis and film fabrication become complicated.Here, we report a simple novel photochemical gel conversion approach for one-step preparation of ZnO thin films with an average nanocrystals in about 2-3 nm size (inset in Fig. 1a), by employing vacuum ultraviolet (VUV) light irradiation in dry N2. Systematic characterization is performed to understand the surface chemistry and the optical properties of ZnO nanocrystals. Thus obtained ZnO films show unique absorption peak at 294 nm (4.2 eV) (Fig. 1a) and strong tunable photoluminescence in the range of 386-496 nm when expose to UV light, from violet, blue to bluish-green upon increasing time of VUV irradiation (Fig. 1b). Clearly blue-shifted absorption peaks of the VUV treated samples are indicative of quantum-confinement effect, as contrasted to the broad character of the annealed film, which originates from band structure of bulky ZnO. The extension of VUV time only results in increase of absorbance, with no change of the peak wavelength, indicating population increase but no crystal growth by Ostwald ripening during the VUV irradiation. Therefore, the color change of the emitted light should be associated with the change of defect state distribution, rather than the size effect. However, such photoluminescence from defects was fairly strong to be clearly recognized by naked eyes, so that the university logo could be patterned by photomasking VUV to be seen under black light was demonstrated (inset in Fig. 1b). Figure 1

Fluorinated TiO2 coupling with α-MnO2 nanowires supported on different substrates for photocatalytic VOCs abatement under vacuum ultraviolet irradiation

The particular role of fluorine (F) doping in the photocatalytic activity of nanocrystalline titanium dioxide (TiO2) towards toluene degradation was systematically studied. TiO2 nanosheet catalysts doped with different amounts of F− (denoted by F-TiO2) were synthesized by hydrothermal method. For comparison, calcination was performed on the catalyst to remove F-. The photocatalytic oxidation (PCO) of toluene over F-TiO2 was found to be much higher than those of TiO2 and commercial P25 under the irradiation of vacuum ultraviolet (VUV) light, with a high toluene degradation efficiency of about 90 %. Moreover, the toluene degradation efficiency increased to 96 % with the addition of only 1 wt.% α-MnO2 nanowires, and the residual ozone (O3) in the system can be completely eliminated. In addition, four different substrates were explored for catalyst coating to facilitate the operation and minimize catalyst loading, which can provide a promising strategy for practical volatile organic compounds (VOCs) abatement in industrial applications.

Effect of Vacuum-Ultraviolet Light Irradiation on Thermal Fusion Bonding Strength of Carbon Fiber-Reinforced Polyimide Composite with Thermoplastic Polyimide Film

Effect of Vacuum-Ultraviolet Light Irradiation on Thermal Fusion Bonding Strength of Carbon Fiber-Reinforced Polyimide Composite with Thermoplastic Polyimide Film

Room temperature direct patterning of nanocrystalline zinc oxide on flexible polymer substrates through vacuum ultraviolet light irradiation

This paper describes a 172-nm vacuum ultraviolet (VUV) light treatment in ambient air towards the formation of nanocrystalline zinc oxide (ZnO) micropatterns. Thin layers of zinc acetate (Zn(OAc)2) as a precursor are spun on substrates, and subsequently treated by the VUV irradiation. The VUV treatments lead to an organic-free ZnO films with wurtzite crystalline structures, determined by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. The nanocrystalline ZnO patterns with a fine feature of 1 μm are fabricated on cycloolefin polymer (COP) substrates via the precise VUV lithography. This one-step ZnO crystallizing and patterning process is suited for polymer substrates because the potentially damaging procedures involving acidic etchants and thermal treatments are no longer needed. The present method proves to be a simple, effective, controllable patterning approach, which might be recommended for the future fabrication of flexible devices.

Effect of vacuum ultraviolet light irradiation on bond strength of carbon fiber/polyimide composite

Effect of vacuum ultraviolet light irradiation on bond strength of carbon fiber/polyimide composite

Interfacial Engineering in Solution Processing of Silicon-Based Hybrid Multilayer for High Performance Thin Film Encapsulation.

Solution processing of thin film encapsulation (TFE) has been a long anticipated technology to bridge the big idea of flexible organic electronics to become real world values, since only small-sized flexible devices are currently achieved with expensive multilayered TFE by complex vacuum processing. Highly demanding conditions are to carry out the process under inert gas, at a low temperature, and without aggressive chemicals to avoid damages to the organic materials. Here we show for the first time a solution-processed TFE to totally equal the level of conventional glass-cap encapsulation to achieve a "ready-to-be-used" stability of an organic light emitting diode (OLED). A seamless organic/inorganic multilayer in a structure such as polydimethylsiloxane (PDMS)/SiOx/SiNy/SiOxNy with a built-in compositional gradient, as we named "PONT", was achieved by a combination of two Si-based polymer coatings, UV-curable PDMS, perhydropolysilazane (PHPS), and their photochemical conversion under irradiation of vacuum ultraviolet (VUV) light (λ = 172 nm) in an N2-filled glovebox at room temperature. PDMS precursors diluted with decamethylcyclopentasiloxane were directly coated to OLED to form a protective layer. The presence of soft, elastic PDMS and its surface conversion to SiOx to improve wetting resulted in strong adhesion at the interfaces and relaxed strain to avoid cracks in ultrathin and high density SiOxNy to serve as a perfect barrier. A remarkably low water vapor transmission rate <10-4 g/m2/day was confirmed for a single PONT as thin as 280 nm. Standardized OLED devices with PONT TFEs have proven 3,864 and 528 h stability under atmospheric (25 °C, 50% relative humidity (RH)) and accelerated (60 °C, 90%RH) degradation tests, respectively, without formation of nonemissive dark spots in OLEDs. The fast processing of PONT TFE can be applied to roll-to-roll fabrication of various organic devices at low cost and in large areas, since direct solution coating as well as VUV irradiation do not cause any noticeable damages to sensitive organic materials.

Organic temperature sensors based on conductive polymers patterned by a selective-wetting method

Abstract Organic temperature sensors are constructed using the temperature dependence of the electrical conductivity of conductive polymers, in which sensor patterns are defined by inkjet printing on selective-wetting surfaces. A poly(3,4-ethylenedioxythiophene) (PEDOT)-based conductive polymer dissolved in a non-aqueous solution was employed as a robust material against humid conditions. The selective-wetting patterns were fabricated using a liquid-repellent fluoropolymer layer and subsequent irradiation of vacuum ultraviolet (VUV) light through a shadow-mask pattern to obtain lyophilic areas. The PEDOT-based conductive polymer was printed by inkjet onto these selective-wetting surfaces, which resulted in the formation of patterns of resistors consisting of the conductive polymer. The patterned resistors exhibited a temperature coefficient of resistance (TCR) of −0.75%/°C at 23 °C, and small variation of resistance coefficient of 4.1%, which is comparable to the variations of metal resistors as references fabricated by photolithography. Organic temperature sensors consisting of the polymer resistors and metal reference resistors were demonstrated to function at -10–80 °C under ambient conditions.

VUV-Assisted Photochemical Sol-Gel Processing of Metal Oxide Thin Films

Metal oxides such as TiO2, ZnO, SnO2 are useful in many of energy devices because of their high chemical stability, n-type semiconductivity and photoelectrochemical properties. Various methods of solution processing of their thin films are known, such as chemical bath deposition, electrodeposition and sol-gel reactions. Typically, high temperatures need to be applied for crystallization to achieve good properties. The need of high temperature however limits the choice of substrates to heat-resistant materials. Also, crystallization often results in pinholes and cracks to damage the performance of devices. We have developed an alternative sol-gel method to obtain ultra-smooth thin films of various metal oxides at room temperature, by employing photochemical decomposition of coated precursor by vacuum ultraviolet (VUV) light irradiation. The method is compatible with conductive plastic film substrates, so that multiple applications in thin film devices such as transparent conductive layer, light emitter, buffer layer in solar cells and thin-film encapsulation (TFE) as an example shown in Fig. 1 can be envisioned. Figure 1

Low Damage Reductive Patterning of Oxidized Alkyl Self-Assembled Monolayers through Vacuum Ultraviolet Light Irradiation in an Evacuated Environment.

Through 172 nm vacuum ultraviolet light irradiation in a high vacuum condition (HV-VUV), well-defined micropatterns with a varied periodic friction were fabricated at the surface of self-assembled monolayers (SAMs) terminated with oxygenated groups. No apparent height contrast between the HV-VUV-irradiated and -masked areas was observed, which indicated the stability of the C-C skeleton of the assembled molecules. The trimming of oxygenated groups occurred through dissociating the C-O bonds and promoting the occurrence of α- and β-cleavages in the C═O-containing components. Hence, the HV-VUV treatment trimmed the oxygenated groups without degrading the C-C skeleton. The HV-VUV treatment influenced the order of the assembled molecules, and the step-terrace structure was distorted. The decrease in friction at the HV-VUV-irradiated domains was attributed to the dissociation of oxygenated groups. (3-Aminopropyl)trimethoxysilane (APTMS) aggregated at the masked areas of the HV-VUV-patterned SAM, where the oxygenated groups worked as anchors. APTMS aggregations did not exist at the irradiated areas, indicating the trimming of the oxygenated groups at these areas. The direct assembling of APTMS on the Si substrate at the irradiated areas was prevented by the remaining C-C skeleton.

Immobilization of Reduced Graphene Oxide on Hydrogen-Terminated Silicon Substrate as a Transparent Conductive Protector.

Silicon is a promising electrode material for photoelectrochemical and photocatalytic reactions. However, the chemically active surface of silicon will be easily oxidized when exposed to the oxidation environment. We immobilized graphene oxide (GO) onto hydrogen-terminated silicon (H-Si) and reduced it through ultraviolet (UV) and vacuum-ultraviolet (VUV) irradiation. This acted as an ultrathin conductive layer to protect H-Si from oxidation. The elemental evolution of GO was studied by X-ray photoelectron spectroscopy, and it was found that GO was partially reduced soon after the deposition onto H-Si and further reduced after UV or VUV light irradiation. The VUV photoreduction demonstrated ca. 100 times higher efficiency compared to the UV reduction based on the irradiation dose. The saturated oxygen-to-carbon ratio (RO/C) of the reduced graphene oxide (rGO) was 0.21 ± 0.01, which is lower than the photoreduction of GO on SiO2 substrate. This indicated the H-Si played an important role in assisting the photoreduction of GO. No obvious exfoliation of rGO was observed after sonicating the rGO-covered H-Si sample in water, which indicated rGO was immobilized on H-Si. The electrical conductivity of H-Si surface was maintained in the rGO-covered region while the exposed H-Si region became insulating, which was observed by conductive atomic force microscopy. The rGO was verified capable to protect the active H-Si against the oxidation under an ambient environment.

Enhancing the electrical conductivity of vacuum-ultraviolet-reduced graphene oxide by multilayered stacking

Vacuum-ultraviolet light irradiation under a high vacuum is a facile method to reduce graphene oxide (GO) sheets and therefore to enhance their electrical conductivity. The aim of this study was to investigate the local electrical properties of a reduced graphene oxide (rGO) monolayer and bilayer by using conductive atomic force microscopy (CAFM). Both the lateral and vertical CAFM measurements showed a higher current signal on the rGO bilayer than on the rGO monolayer. The enlargement of the tip contact area significantly affected the vertical CAFM measurements and enhanced the current signal in the bilayer regions. However, when performing the lateral CAFM measurements, the enlarged tip contact area had no obvious influence on the current signal. The increase in the current signal can be ascribed to the intrinsic enhancement of the electrical conductivity on the rGO bilayer. These results suggested that the stacked rGO sheets formed the new conductive paths for the carrier transportation in the lateral direction. This process can be further applied in producing microconductive patterns in multilayered GO.