UV-ozone Treatment Research Articles

Selective surface modification of ethylene-vinyl acetate and ethylene polymer blend by UV–ozone treatment

Ethylene-vinyl acetate (EVA) copolymers intended for sport sole manufacturing may contain noticeable amounts of polyethylene (LDPE) for improving abrasion resistance and decrease cost; however, this blend (EVA–PE) had low polarity and showed poor adhesion. In this study an effective environmentally friendly and fast surface treatment based on UV–ozone has been used to increase the wettability, polarity and roughness of EVA–PE material. Both the length of the UV–ozone treatment and the distance between the material surface and the UV-radiation source were tested. The UV–ozone treated EVA–PE material was characterized by ATR-IR spectroscopy using Ge prism, water contact angle measurements, X-Ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Adhesion properties were obtained from T peel tests of as-received and UV–ozone treated EVA–PE/polyurethane adhesive/leather joints.The more extended length of treatment and the shorter UV source–substrate distance increased the wettability of the EVA–PE material. Oxidation of the EVA–PE surface was produced by UV–ozone treatment creating new carbonyl groups mainly, and the amounts of hydroxyl and carboxylic groups were increased. The UV–ozone treatment produced ablation and etching of the EVA–PE material surface, mainly in the vinyl acetate, creating a particular roughness consisting on ruffles with deep crevices; this topography was also produced by heating produced during UV–ozone treatment. For low length of UV treatment or high UV source–material distance, the modifications of the EVA–PE material were mainly produced in the ethylene causing the selective removal of vinyl acetate, whereas more aggressive conditions produced strong oxidation in the EVA–PE material. Finally, adhesive strength was noticeably increased in the UV–ozone treated EVA–PE/polyurethane adhesive joints, and a cohesive failure in the leather was obtained.

Enhanced charge transport and photovoltaic performance of PBDTTT-C-T/PC70BM solar cells via UV–ozone treatment

In this work, the electron transport layer of PBDTTT-C-T/PC70BM polymer solar cells were subjected to UV-ozone treatment, leading to improved cell performances from 6.46% to 8.34%. The solar cell efficiency reached a maximum of 8.34% after an optimal 5 minute UV-ozone treatment, and then decreased if treated for a longer time. To the best of our knowledge, the mechanism behind the effects of UV-ozone treatment on the improvement of charge transport and cell performance is not fully understood. We have developed a fundamental understanding of the UV-ozone treatment mechanism, which explains both the enhancements in charge transport and photovoltaic performance at an optimal treatment time, and also the phenomenon whereby further treatment time leads to a drop in cell efficiency. Transient photocurrent measurements indicated that the cell charge transport times were 1370 ns, 770 ns, 832 ns, 867 ns, and 1150 ns for the 0 min, 5 min, 10 min, 15 min, and 20 min UV-ozone treatment times, respectively. Therefore the 5 min UV-ozone treatment time led to the shortest transport time and the most efficient charge transport in the cells. The 5 min UV-ozone treated sample exhibited the highest peak intensity (E2) in the Raman spectra of the treated films, at about 437 cm(-1), indicating that it possessed the best wurtzite phase crystallinity of the ZnO films. Further increasing the UV-ozone treatment time from 5 to 20 min induced the formation of p-type defects (e.g. interstitial oxygen atoms), pushing the ZnO Fermi-level further away from the vacuum level, and decreasing the wurtzite crystallinity.

Curvature-controlled wrinkle morphologies

Wrinkling of spherical surfaces represents a promising pathway for the formation of complex hierarchical structures and patterns. Using a swelling-based approach, we demonstrate the relationship between applied compressive stress, radius of curvature, and wrinkle pattern morphology. Surface features with spherical curvature are fabricated on poly(dimethylsiloxane) elastomers and a stiff surface oxide layer is generated using UV–ozone treatment. Swelling of this surface layer with ethanol vapor results in the formation of periodic wrinkles on the spherical caps. The radius of curvature is found to influence the critical wrinkling stress of the system, allowing cap size to independently dictate wrinkle morphology from non-wrinkled caps at small radii, through hexagonal dimple and long ridge structures as the cap radius increases. A similar transition is observed at constant radius by varying the UV–ozone treatment time, which affects the oxide film thickness and composition. The influence of curvature on the critical wrinkling stress is confirmed by modulating the atmospheric concentration of ethanol, and measuring the applied stress for the lowest concentration for which wrinkling occurs.

Fabrication of Electroosmotic Flow Pump on Polymethylemethacrylate Substrate Using Hot Embossing

Improved fabrication processes of a micro electroosmotic flow pump using hot embossing are described. The microchannels in the micropump were fabricated by hot embossing on a polymethylmethacrylate (PMMA) substrate. A silicon micromachined mold was pressed into the PMMA substrate at a temperature of 145 °C to form microchannel patterns on the substrate. The depth and width of the microchannels were 50 μm and 100 μm, respectively. Aluminum electrodes were deposited using thermal vacuum deposition. A UV ozone treatment was performed to improve adhesion between the PMMA substrate and a PMMA capping layer. This UV ozone treatment enhanced adhesion and resulted in the reduction of the adhesion temperature as low as 70 °C, and nearly no deformation of the microchannels was observed. As a result, the electroosmotic flow pump exhibited the flow rate of 0.5 μl/min when a voltage of 50 V was given between the electrodes separated 8 mm each other.

A UV-ozone treated amorphous barium–strontium titanate dielectric thin film for low driving voltage flexible organic transistors

Reducing the operating power and making them suitable for portable and wearable electronic applications are critical steps for the further development of organic field effect transistors (OFETs). One of the possible approaches to achieve this is by using a high dielectric constant (high-k) dielectric layer. Here, we propose a new kind of high-k amorphous Ba0.7Sr0.3TiO3 (BST) material derived by a layer-by-layer sol–gel method and solidified by UV-ozone treatments under atmospheric air and pressure. The dielectric constant of the amorphous BST thin film is around 11. Without employing the self-assembled monolayers (SAMs), the pentacene OFET on a polyethylene naphthalate (PEN) substrate shows saturation under −2.5 V, the saturation mobility of 0.252 cm2 V−1 s−1 with negligible hysteresis effects. The chemical composition of the dielectric film and the morphology structure are studied by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) respectively. We also performed compressive and tensile bending tests on the OFETs and the dielectric layer. Due to the amorphous nature of the BST thin film, it shows a constant C–f relationship in both compressive and tensile bending up to a bending radius of 3.3 mm. It demonstrates high application potential of the current BST thin film in flexible electronics and circuits. To confirm the application potentials, we develop an organic inverter based on the transistor and a resistor, and a gain as high as 20 can be achieved.

Effect of interface on surface morphology and proton conduction of polymer electrolyte thin films

To understand the relationship between surface morphology and proton conduction of polymer electrolyte thin films, perfluorinated ionomer Nafion® thin films were prepared on different substrates such as glassy carbon (GC), hydrophilic-GC (H-GC), and platinum (Pt) as models for the ionomer film within a catalyst layer. Atomic force microscopy coupled with an electrochemical (e-AFM) technique revealed that proton conduction decreased with film thickness; an abrupt decrease in proton conductance was observed when the film thickness was less than ca. 10 nm on GC substrates in addition to a significant change in surface morphology. Furthermore, thin films prepared on H-GC substrates with UV-ozone treatment exhibited higher proton conduction than those on untreated GC substrates. However, Pt substrates exhibited proton conduction comparable to that of GCs for films thicker than 20 nm; a decrease in proton conduction was observed at ∼5 nm thick film but was still much higher than for carbon substrates. These results indicate that the number of active proton-conductive pathways and/or the connectivity of the proton path network changed with film thickness. The surface morphology of thinner films was significantly affected by the film/substrate interface and was fundamentally different from that of the bulk thick membrane.

Enhancing the performance of P3HT:ICBA based polymer solar cells using LiF as electron collecting buffer layer and UV–ozone treated MoO3 as hole collecting buffer layer

Efficient bulk-heterojunction (BHJ) polymer solar cells (PSCs) based on poly (3-hexylthiophene) (P3HT): indene-C60 bisadduct (ICBA) were fabricated with lithium fluoride (LiF) as electron collecting buffer layer and UV–ozone (UVO) treated molybdenum trioxide (MoO3) as hole collecting buffer layer. LiF is observed to be critical for the device performance, significantly enhancing the fill factor (FF) and the efficiency compared to the device without LiF layer. MoO3 layer is also critical to the device performance, efficiently extracting holes to prevent the exciton quenching and reducing the interfacial resistance because of alignment of energy band. After UVO treatment on MoO3 film, its surface morphology, transmittance and film quality can be improved, therefore, it is beneficial to the performance of the PSCs. The optimal thickness of MoO3 and UVO treatment time on MoO3 was 12nm and 15min, respectively. Based on these optimal parameters, a power conversion efficiency (PCE) of 6.43% was obtained under AM1.5G 100mW/cm2 illumination.

Extension of stability in organic photovoltaic cells using UV/ozone-treated graphene sheets

Organic photovoltaic (OPV) cells with a longer lifetime than that of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)-based OPV cells were fabricated using UV/ozone-treated graphene sheets as hole extraction layers (HELs). The sheet resistance increased from 1.1kΩ/sq to infinity and the transmittance at 550nm decreased from 95.2% to 93.8% after UV–ozone treatment for 9min due to the formation of carbon–oxygen functionalization layers. The intensity of the C–C, C–O, and C=O bonds in X-ray photoemission spectroscopy spectra increased with increasing UV–ozone treatment time, indicating the bonding formation between carbon and oxygen. Ultraviolet photoemission spectroscopy data showed that the work function of graphene increased from 4.3eV to 4.85eV after UV/ozone treatment for 9min. However, the OPV cell with 5min-treated graphene HEL displayed the best efficiency of 3.0% because the graphene sheet was damaged by UV–ozone exposure after 7min. The efficiency of the PEDOT:PSS-based device rapidly decreased to 0% after 14h exposure in humid conditions while the UV/ozone-treated device continued to operate for 26h. These results suggested that UV/ozone-treated graphene under optimized condition is a good HEL candidate in OPV cells as a replacement for the conventional PEDOT:PSS layer.

Solar Cells: Inverted Polymer Solar Cells with Reduced Interface Recombination (Adv. Energy Mater. 11/2012)

Due to intrinsic defects, zinc oxide nanoparticles may induce interfacial recombination when used as the electron extraction layer in an inverted polymer solar cell. On page 1333, Franky So, John R. Reynolds, and co-workers demonstrate that UV-ozone treatment is an effective way to passivate defects in ZnO NPs and reduce interface recombination in a polymer solar cell. As a result, the power conversion efficiency of inverted PDTGTPD: PC71BM cells is improved to 8.1%.

Pattern transfer of nanostructures onto glass using nanoimprint films

We report a method to fabricate periodic nanostructures of silica beads onto a glass substrate by using a nanoimprint film having a hole pattern that acts as a template for an orderly arrangement of the beads within the pattern. Before using the film, its surface was modified from hydrophobic to hydrophilic by UV ozone treatment and surfactant sodium dodecyl sulfate. It was found that the periodic nanostructures of the beads were successfully transferred from the nanoimprint film onto the surface of the glass substrate by heat treatment. Our findings indicate that this pattern-transfer method is a promising candidate for the inexpensive fabrication of large-area periodic nanostructures onto glass.

UV ozone treatment for improving contact resistance on graphene

Optimized UV ozone cleaning of graphene layers on SiO2/Si substrates is shown to improve contact resistance of e-beam evaporated Ti/Au contacts by three orders of magnitude (3 × 10−6 Ω-cm2) compared to untreated surfaces (4 × 10−3 Ω-cm2). Subsequent annealing at 300 °C lowers the minimum value achieved to 7 × 10−7 Ω-cm2. Ozone exposure beyond an optimum time (6 min in these experiments) led to a sharp increase in sheet resistance of the graphene, producing degraded contact resistance. The UV ozone treatment is a simple and effective method for producing high quality contacts to graphene.

Ultraviolet-ozone-treated PEDOT:PSS as anode buffer layer for organic solar cells

Ultraviolet-ozone-treated poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)was used as the anode buffer layer in copper phthalocyanine (CuPc)/fullerene-based solar cells. The power conversion efficiency of the cells with appropriated UV-ozone treatment was found to increase about 20% compared to the reference cell. The improved performance is attributed to the increased work function of the PEDOT:PSS layer, which improves the contact condition between PEDOT:PSS and CuPc, hence increasing the extraction efficiency of the photogenerated holes and decreasing the recombination probability of holes and electrons in the active organic layers.

Polymer Photovoltaic Cells Based on Ultraviolet-Ozone-Treated Vanadium-Doped Indium Oxide Anodes

Polymer photovoltaic (PV) cells based on UV-ozone(UVO)-treated indium vanadium oxide (IVO) anodes are developed. The performance of cells with UVO-treated IVO film anodes without interfacial layers was significantly improved compared with those containing untreated IVO anodes. The origin of the enhancement is investigated by atomic force microscopy and photoelectron spectroscopy measurements. The results demonstrate that UVO treatment can smooth the IVO surface, and increase the work function of IVO films due to the removal of carbon contamination and a dipole resulting from a surface rich in negatively charged oxygen. UVO-treated IVO films are potential electrode materials for polymer PV cells.

Enhanced performance of organic electroluminescent diodes by UV–ozone treatment of molybdenum trioxide

The operating voltage is reduced and luminous efficiency is improved in the organic light-emitting diodes (OLEDs) with UV–ozone treatment of molybdenum trioxide (MoO3) anode buffer layer. The interface of indium tin oxide (ITO)/MoO3 was investigated by X-ray photoelectron spectroscopy. It was found that the binding energies of Mo 3d, O 1s, In 3d and C 1s were shifted to higher after UV–ozone treated MoO3. The enhanced properties are impervious to the UV–ozone treatment of ITO. Therefore, the improved performance can be attributed that the binding energies of Mo 3d, O 1s, In 3d and C 1s are changed to higher and also that the sheet resistance of ITO/MoO3 films is decreased after UV–ozone treated MoO3.

Inverted Polymer Solar Cells with Reduced Interface Recombination

Interface recombination induced by the defect states in zinc-oxide-nanoparticle-based electron extraction layer is reported as a significant loss-mechanism of photocurrent collection. By choosing appropriate UV–ozone treatment conditions on the zinc oxide layer, inverted polymer solar cells show reduced interface recombination and thus improved power conversion efficiencies of up to 8.1%.

Synthesis of Co3O4 nanosheets via electrodeposition followed by ozone treatment and their application to high-performance supercapacitors

A thin film of Co3O4 nanosheets is electrodeposited on a flexible Ti substrate by a one-step potentiostatic method, followed by an UV–ozone treatment for 30 min. The films before and after the UV–ozone treatment are characterized with X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The film is composed of Co(OH)2 before UV–ozone treatment, and of Co3O4 after the treatment. The morphologies of both films are examined by scanning electron microscopy (SEM) and transmission electron microscope (TEM). The obtained films are composed of nanosheets, and there is no change in their sheet-like morphology before and after the UV–ozone treatment. When applied for a supercapacitor, the Co3O4 modified Ti electrode (Co3O4/Ti) shows a far higher capacitance than that of the Co(OH)2 modified Ti electrode. The electrodeposition time and NaOH concentration in the electrolyte are optimized. A remarkably high specific capacitance of 1033.3 F g−1 is obtained for the Co3O4 thin film at a charge–discharge current density of 2.5 A g−1. The long-term stability data shows that there is still 77% of specific capacitance remaining after 3000 repeated charge–discharge cycles. The high specific capacitance and long-term stability suggest the potential use of Co3O4/Ti for making a flexible supercapacitor.

Factors controlling the resistance of Ohmic contacts to n-InGaAs

The authors report specific contact resistance (ρc) values for Mo-, Ti-, TiW-, Pd-, and Pt-based Ohmic contacts to n+-In0.86Ga0.14As that are deposited with either collimated sputter or electron-beam deposition. Palladium-based contacts with 5 nm of electron-beam evaporated Pd have a specific contact resistance of 7.6 ± 0.5 × 10−9 Ω cm2, while identical collimated sputter deposited contacts have a specific contact resistance of 4.1 ± 0.4 × 10−9 Ω cm2. Contacts with a very thin evaporated Pd layer (2 nm) or a sputtered Pd/Ti 1/1 nm × 4 multilayer have stable ρc values of 5.4 ± 0.5 × 10−9 and 5.0 ± 0.5 × 10−9 Ω cm2, respectively, after 2 h at 270 °C. The ρc of sputter deposited TiW-based contacts is an order of magnitude lower than for identical evaporated contacts (1.6 ± 0.3 × 10−8 vs 2.6 ± 0.3 × 10−7 Ω cm2) and is stable during annealing, while sputter deposition of Pt-based contacts yields rc values that are half an order of magnitude lower than similar evaporated contacts (4.5 ± 1.0 × 10−9 vs 1.7 ± 0.2 × 10−8 Ω cm2). However, the ρc values of the sputtered Pt contacts approach those of evaporated contacts after annealing. Surface preparations consisting of UV-ozone treatments followed by immersion in buffered oxide etch or ammonium hydroxide yield ρc values that are half an order of magnitude lower than values yielded by similar preparations in which HCl is used for oxide removal. The authors also discuss how to avoid over- and underestimation artifacts that may be encountered in measurements of very low specific contact resistances of Ohmic contacts to semiconductors that have low sheet resistances.

Comparison of Biocompatibility and Adsorption Properties of Different Plastics for Advanced Microfluidic Cell and Tissue Culture Models

Microfluidic technology is providing new routes toward advanced cell and tissue culture models to better understand human biology and disease. Many advanced devices have been made from poly(dimethylsiloxane) (PDMS) to enable experiments, for example, to study drug metabolism by use of precision-cut liver slices, that are not possible with conventional systems. However, PDMS, a silicone rubber material, is very hydrophobic and tends to exhibit significant adsorption and absorption of hydrophobic drugs and their metabolites. Although glass could be used as an alternative, thermoplastics are better from a cost and fabrication perspective. Thermoplastic polymers (plastics) allow easy surface treatment and are generally transparent and biocompatible. This study focuses on the fabrication of biocompatible microfluidic devices with low adsorption properties from the thermoplastics poly(methyl methacrylate) (PMMA), polystyrene (PS), polycarbonate (PC), and cyclic olefin copolymer (COC) as alternatives for PDMS devices. Thermoplastic surfaces were oxidized using UV-generated ozone or oxygen plasma to reduce adsorption of hydrophobic compounds. Surface hydrophilicity was assessed over 4 weeks by measuring the contact angle of water on the surface. The adsorption of 7-ethoxycoumarin, testosterone, and their metabolites was also determined after UV-ozone treatment. Biocompatibility was assessed by culturing human hepatoma (HepG2) cells on treated surfaces. Comparison of the adsorption properties and biocompatibility of devices in different plastics revealed that only UV-ozone-treated PC and COC devices satisfied both criteria. This paper lays an important foundation that will help researchers make informed decisions with respect to the materials they select for microfluidic cell-based culture experiments.

An improved SPM-based contact tester for the study of microcontacts

An improved scanning probe microscope based contact tester has been designed, constructed and used for cyclic testing of various metal contacts. The tester is designed for contact material evaluation, especially for metal-contact MEMS switch applications, and is capable of simultaneous measurements of contact force and resistance. The tester uses a specially designed silicon force sensor with an integrated contact bump and a mating silicon pillar in order to simulate switch operation. The sensor and the pillar are coated with contact materials, allowing a wide range of contact metals and metal pairs to be evaluated. The testing takes place within a custom-built test chamber in which both plasma and UV-ozone treatments are available for contact cleaning and surface modification. It was found that a dissimilar contact pair of Au–Ru with O2 plasma cleaning can provide low contact adhesion and low resistance as compared with Au, Ru and Ir contact pairs. Layered structures with a thin layer of Ru on top of Au were also modeled and tested. Ru layers between 50 and 100 nm were effective in reducing adhesion and contact resistance provided the contact force is greater than 300 µN.

High intensity UV radiation ozone treatment of nanocrystalline TiO2 layers for high efficiency of dye-sensitized solar cells

We have improved the conversion efficiency of dye-sensitized solar cells (DSSCs) by using high intensity UV-ozone treatment on nanocrystalline TiO2 layers after sintering. The UV light sources used had powers of 1650W and 500W. UV-ozone treatment of nanocrystalline TiO2 layers was performed for 10min with a fixed distance between TiO2 layers and UV light source of about 3cm. Experimental results indicate that photocurrent and power conversion efficiency of cells increased when using high power 1650W and 500W UV-ozone treatment on TiO2 layers. The short circuit current density (Jsc) increased to 25.29±0.23mA/cm2 and 22.22±0.47mA/cm2, the open circuit voltage (Voc) to 0.73±0.02V and 0.74±0.02V, the fill factor (FF) to 0.66±0.01 and 0.71±0.03 and the power conversion efficiency to 11.95±0.45% and 11.12±0.28% respectively. The DSSCs were characterized for absorption dye, chemical structure on nanocrystalline TiO2 layer and electrochemical properties by UV–visible spectroscopy, XPS, XRD, contact angle measurement and impedance spectroscopy respectively.