Ultraviolet Ozone Treatment Research Articles

Mechanical and chemical analysis of plasma and ultraviolet–ozone surface treatments for thermal bonding of polymeric microfluidic devices

Here we have demonstrated that radio frequency plasma and ultraviolet-ozone (UVO) surface modifications are effective treatments for enabling the thermal bonding of polymeric microfluidic chips at temperatures below the T(g) (glass transition temperature) of the polymer. The effects of UVO and plasma treatments on the surface properties of a cyclic polyolefin and polystyrene were examined with X-ray photoelectron spectroscopy (XPS), contact angle measurements, atomic force microscopy (AFM) surface roughness measurements and surface adhesion measurements with AFM force-distance data. Three-point bending tests using a dynamic mechanical analyzer (DMA) were used to characterize the bond strength of thermally sealed polymer parts and the cross-sections of the bonded microchannels were evaluated with scanning electron microscopy (SEM). The experimental results demonstrated that plasma and UVO surface treatments cause changes in the chemical and physical characteristics of the polymer surfaces, resulting in a decrease in T(g) at the surface, and thus allowing the microfluidic chips to be effectively bonded at temperatures lower than the T(g) of the bulk polymer without losing the intended channel geometry.

Electro-oxidation of formic acid catalyzed by FePt nanoparticles

The electrocatalytic oxidation of formic acid at a gold electrode functionalized with FePt nanoparticles was studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in a mixed solution of 0.1 M HCOOH and 0.1 M HClO4. The FePt bimetallic nanoparticles, with a mean diameter of 3 nm, were prepared by a chemical reduction method. The Au/FePt nanostructured electrode was prepared firstly by the deposition of FePt nanoparticles onto a clean Au electrode surface, followed by ultraviolet ozone treatment to remove the organic coating. In CV measurements, two well-defined anodic peaks were observed at +0.20 and +0.51 V (vs. a Ag/AgCl quasi-reference). The anodic peak at +0.20 V was attributed to the oxidation of HCOOH to CO2 on surface unblocked by CO, whereas the peak at +0.51 V was ascribed to the oxidation of surface-adsorbed CO (an intermediate product of HCOOH oxidation) and further oxidation of bulk HCOOH. From the onset potential and current density of the electro-oxidation of HCOOH, FePt nanoparticles exhibit excellent electrocatalytic activities as compared to Pt and other metal alloys. EIS measurements were carried out to further examine the reaction kinetics involved in the HCOOH electro-oxidation. The EIS responses were found to be strongly dependent on electrode potentials. At potentials more positive than -0.25 V (vs. Ag/AgCl), pseudo-inductive behavior was typically observed. At potentials between +0.3 and +0.5 V, the impedance response was found to reverse from the first quadrant to the second quadrant; such negative Faradaic impedance was indicative of the presence of an inductive component due to the oxidation of surface-adsorbed CO. The impedance responses returned to normal behavior at more positive potentials (+0.6 to +0.9 V). The mechanistic variation was attributed to the formation of different intermediates (CO or oxygen containing species) on the electrode surface in different potential regions. Two equivalent circuits were proposed to model these impedance behaviors.

Rapid formation of soft hydrophilic silicone elastomer surfaces

We report on the rapid formation of hydrophilic silicone elastomer surfaces by ultraviolet/ozone (UVO) irradiation of poly(vinylmethyl siloxane) (PVMS) network films. Our results reveal that the PVMS network surfaces render hydrophilic upon only a short UVO exposure time (seconds to a few minutes). We also provide evidence that the brief UVO irradiation treatment does not cause dramatic changes in the surface modulus of the PVMS network. We compare the rate of formation of hydrophilic silicone elastomer surfaces made of PVMS to those of model poly(dimethyl siloxane) (PDMS) and commercial-grade PDMS (Sylgard-184). We find that relative to PVMS, 20 times longer UVO treatment times are needed to oxidize the PDMS network surfaces in order to achieve a comparable density of surface-bound hydrophilic moieties. The longer UVO treatment times for PDMS are in turn responsible for the dramatic increase in surface modulus of UVO treated PDMS, relative to PVMS. We also study the formation of self-assembled monolayers (SAMs) made of semifluorinated organosilane precursors on the PVMS-UVO and PDMS-UVO network surfaces. By tuning the UVO treatment times and by utilizing mono- and tri-functional organosilanes we find that while mono-functionalized organosilanes attach directly to the substrate, SAMs of tri-functionalized organosilanes form in-plane networks on the underlying UVO-modified silicone elastomer surface, even with only short UVO exposure times.

Surface treatment of ferromagnetic nanoparticle-assembly by UV irradiation with ozone

Fe–Pt nanoparticles of 3.5 nm in average particle diameter are obtained with the simultaneous chemical reduction of platinum acetylacetonate and the thermal decomposition of iron pentacarbonyl. Because the as-synthesized particles are superparamagnetic at room temperature, they must be transformed to L1 0 ordered phase by annealing at temperatures above 773 K to apply to high-density magnetic recording media. However, the annealing at such high temperatures induces the coalescence and inter-particle sintering. In the present paper, to solve the problem caused by the annealing, we use ultraviolet-ozone treatment to modify particle surface. Even after annealing at 773 K, the crystalline size is kept at 4.4 nm. Furthermore, coercivity is higher than that without the ultraviolet-ozone treatment.

Work-function changes of treated indium-tin-oxide films for organic light-emitting diodes investigated using scanning surface-potential microscopy

Local surface work-function (WF) measurement on indium-tin-oxide (ITO) films prepared by different cleaning methods for use as anode materials in organic light-emitting diodes were studied using scanning surface-potential microscopy. The ITO WF changes with standard wet-cleaning treatment correspond directly to asperities on the film surface. The maximum difference value (MDV) in the local WF reached 0.41eV. However, after wet-cleaning ITO with the ultraviolet ozone or O-plasma treatment, the WF distribution is more uniform than the original distribution. Owing to the sufficient ITO surface oxidization, the mean local WF value increases effectively to more than 5.00eV and the MDV is less than 0.05eV. Furthermore, the changes in roughness and conductivity on the ITO surface at different treatment times are also discussed. The proper exposure time for the O-plasma treatment can thereby be determined.

Improved performance of OLEDs with ITO surface treatments

In this work, we performed systematic study of the influence of oxidative (ultraviolet (UV) ozone) and acid treatments and their combinations on the ITO parameters (work function, carrier concentration and mobility, surface roughness and morphology) and the performance of organic light emitting diodes (OLEDs) fabricated on ITO treated with different treatments. ITO substrates were characterized by Hall measurements, Seebeck coefficient measurements, and surface probe microscopy. After characterization of ITO, we deposited 100 nm ( N, N′-di(naphthalene-1-yl)- N, N′-diphenyl-benzidine (NPB) film on the surface of ITO substrates treated with different treatments and characterized it with surface probe microscopy and Seebeck coefficient measurements. Then, OLEDs with structure ITO/NPB/rubrene/tris (8-hydroxyquinoline) aluminum (Alq 3)/LiF/Al were fabricated. The device performance was related to the ITO surface morphology and the ITO work function, while there was no obvious relationship to the ITO carrier concentration and mobility. From the treatments investigated here, it is not possible to distinguish whether relatively flat surface with isolated high spikes is sufficient on its own for good performance, or high work function value together with such surface morphology is required. The maximum luminous efficiency of 2.0 lm/W was obtained for ITO/NPB/rubrene/Alq 3/LiF/Al OLED with HCl followed by UV ozone treatment compared to 1.7 lm/W with UV ozone treatment only.

Polymer dielectric treatment using ultraviolet-ozone: A photoelectron spectroscopy and ellipsometric porosimetry study

Ultraviolet-ozone (UV-O3) treatment of polyarylene, a semiconductor dielectric polymer, has been characterized using contact angle measurements, x-ray photoelectron spectroscopy, spectroscopic ellipsometry (SE), and ellipsometric porosimetry. Significant modification of the film surface composition is already observed after a short exposure time (30s). Longer treatment time leads to further increase in oxygen concentration and formation of an oxidized surface layer and densification of the film as evidenced by solvent adsorption data. Results obtained from SE indicate few changes in optical parameters, thickness, and refractive index (RI) of the film, up to 5min of UV-O3 treatment. Increasing the exposure time further leads to a substantial change in film thickness and an increase in RI. The same trend was observed for porous polyarylene but to a stronger extent. A dense layer at the porous polymer surface is only formed after 20min of treatment. Under these conditions, the optical properties of the material are strongly modified, with a substantial thickness shrinkage and increase in RI.

Effect of ultraviolet–ozone treatment of indium–tin–oxide on electrical properties of organic light emitting diodes

We report the change of surface electronic structure of indium–tin–oxide (ITO) as a function of ultraviolet (UV)–ozone treatment time. The voltage of organic light emitting diodes at a current density of 100 mA/cm2 was reduced as the surface treatment time using UV–ozone was lengthened. X-ray photoelectron spectroscopy results showed that the relative concentration of carbon atoms decreased, but oxygen concentration increased relatively with UV–ozone treatment. This led to the increase in the ITO work function via the reduction of operation voltage.

Surface Modification of Sylgard-184 Poly(dimethyl siloxane) Networks by Ultraviolet and Ultraviolet/Ozone Treatment

We report on the surface modification of Sylgard-184 poly(dimethyl siloxane) (PDMS) networks by ultraviolet (UV) radiation and ultraviolet/ozone (UVO) treatment. The effects of the UV light wavelength and ambient conditions on the surface properties of Sylgard-184 are probed using a battery of experimental probes, including static contact angle measurements, Fourier transform infrared spectroscopy, near-edge X-ray absorption fine structure, and X-ray reflectivity. Our results reveal that when exposed to UV, the PDMS macromolecules in the surface region of Sylgard-184 undergo chain scission, involving both the main chain backbone and the side groups. The radicals formed during this process recombine and form a network whose wetting properties are similar to those of a UV-modified model PDMS. In contrast to the UV radiation, the UVO treatment causes very significant changes in the surface and near-surface structure of Sylgard-184. Specifically, the molecular oxygen and ozone created during the UVO process interact with the UV-modified specimen. As a result of these interactions, the surface of the sample contains a large number of hydrophilic (mainly –OH) groups. In addition, the material density within the first ≈5 nm reaches about 50% of that of pure silica. A major conclusion that can be drawn from the results and analysis described in this work is that the presence of the silica fillers in Sylgard-184 does not alter the surface properties of the UVO- and UV-modified Sylgard-184.

Nanoscale surface electrical properties of indium–tin–oxide films for organic light emitting diodes investigated by conducting atomic force microscopy

Nanoscale surface electrical properties of indium–tin–oxide films prepared by different cleaning methods for use as anode materials in organic light emitting diodes are studied by conducting atomic force microscopy. It is found that most of the surface area possesses a nonconducting feature, and an ultraviolet-ozone treatment produces the most nonconductive sample. The conducting regions, which distribute randomly and range from 6 to 50 nm in size, are attributed to the existence of Sn-rich oxide by a comparison with reported scanning electron microscopy images. After scanning the tip with a bias of −8 V on the nonconducting regions, oxide decomposition occurs on as-received and wet-cleaning processed samples, whereas no structure change appears on the ozone treated sample. The results indicate that the generation of stable oxide after ozone treatment is one of the origins for improved device performance.

Frictional Force Microscopy of Oxidized Polystyrene Surfaces Measured Using Chemically Modified Probe Tips

Chemical force microscopy (CFM) using probe tips that have been chemically functionalized to give hydroxylated (polar) or methylated (apolar) surfaces has been used to investigate frictional properties of polystyrene (PS) films that have been oxidatively modified to varying degrees using an ultraviolet-ozone (UVO) treatment. Oxygen chemisorption levels and functional group chemistry of the PS films have been followed using X-ray photoelectron spectroscopy (XPS), and the resulting data have been correlated with lateral (frictional) force measurements for each surface type. XPS analysis showed that UVO treatments introduced polar species at the film surfaces via the formation of C−O, CO, and O−CO functional groups. CFM was performed on the native and treated films using tips that were either hydroxyl- or methyl-terminated. The treated films exhibit substantially higher friction than untreated films when imaging with the hydroxyl-terminated tips. Friction is reduced when nonpolar methyl-functionalized tips a...

Characterization of treated indium–tin–oxide surfaces used in electroluminescent devices

The influence of oxidative and reductive treatments of indium–tin–oxide (ITO) on the performance of electroluminescent devices is presented. The improvement in device performance is correlated with the surface chemical composition and work function. The work function is shown to be largely determined by the surface oxygen concentration. Oxygen-glow discharge or ultraviolet–ozone treatments increase the surface oxygen concentration and work function in a strongly correlated manner. High temperature, vacuum annealing reduces both the surface oxygen and work function. With oxidation the occupied, density of states (DOS) at the Fermi level is also greatly reduced. This process is reversible by vacuum annealing and it appears that the oxygen concentration, work function, and DOS can be cycled by repeated oxygen treatments and annealing. These observations are interpreted in terms of the well-known, bulk properties of ITO.

Surface preparation and characterization of indium tin oxide substrates for organic electroluminescent devices

The cleanliness of indium tin oxide (ITO) sub- strates used in organic light-emitting diodes (OLEDs) is in- vestigated by contact angle measurement and by X-ray pho- toemission spectroscopy (XPS). It was found that ultraviolet (UV) ozone treatment is quite effective in removing organic contamination on the ITO surface. The degree of surface con- tamination was checked by changes in contact angles and by XPS. Strong correlation can be established between these two techniques. OLEDs fabricated from UV-irradiated ITO sub- strates exhibit low turn-on voltage and superior brightness. There are now widespread interests in using organic or poly- meric fluorescent thin films for fabricating electroluminescent (EL) devices (1, 2). The basic structure of an organic EL de- vice consists of one or more layers of organic fluorescent materials sandwiched between an anode and a metal cathode. In most cases, a thin film of indium tin oxide (ITO), with a thickness in the order of 0:1 mm, is used as the anode ma- terial. The ITO layer can be prepared by standard sputtering techniques onto a glass plate or a plastic substrate. Since ITO is a transparent conductor, it also functions as the viewing side for the EL device. Although there are now numerous re- ports on EL devices fabricated by many different materials, few reports have been devoted to the preparation of ITO for EL devices. Since organic EL devices are thin film devices, a small amount of contamination on the surface of the an- ode can severely alter the work function, or the interfacial barrier height between the organic layer and the anode. Un- less the anode is thoroughly cleaned, the electrical and optical characteristics can be highly unstable and unpredictable. It is, therefore, of utmost importance that the ITO substrate is carefully cleaned before deposition of the organic layers. A variety of methods (3-8) have been developed in the preparation of ITO surfaces for organic light-emitting diodes (OLEDs). A summary of these methods is shown in Table 1.

Ultrathin Aluminosilicate Films from Langmuir−Blodgett Multilayers

Multilayer films incorporating both silicon and aluminum have been prepared by Langmuir−Blodgett deposition and subsequently decomposed by exposure to ultraviolet ozone (UVO) to yield films with a composition consistent with the formation of an aluminosilicate. The morphology of the deposited films was studied by atomic force microscopy both before and after UVO treatment, and their chemical structures and compositions were investigated by FTIR and XPS. Two film systems were investigated, namely, cyclic polysiloxanes with carboxylic acid side groups and octadecyltrimethoxysilane (OTMS). Aluminum was introduced into the films from a 10-5 M aluminum nitrate subphase at pH ∼ 4 in the first case and at pH = 9.5 in the second. It is shown that uptake of aluminum and control of aluminosilicate formation is likely to be easier with the OTMS system. It is also shown that inclusion of an ether linkage in the hydrophobic tails of the polysiloxanes leads to a much more rapid removal of carbon from the films during U...

Quantum confined luminescence in Si/SiO2 superlattices.

Superlattices of $\mathrm{Si}/{\mathrm{SiO}}_{2}$ have been grown at room temperature with atomic layer precision using state of the art molecular beam epitaxy and ultraviolet ozone treatment. Photoluminescence was observed at wavelengths across the visible range for Si layer thicknesses $1<d<3\phantom{\rule{0ex}{0ex}}\mathrm{nm}$. The fitted peak emission energy $E(\mathrm{eV}){\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.60+0.72d}^{\ensuremath{-}2}$ is in accordance with effective mass theory for quantum confinement by the wide-gap ${\mathrm{SiO}}_{2}$ barriers and also with the bulk amorphous Si band gap. Measurements of the conduction and valence band shifts by x-ray techniques correlate with $E(d)$, confirming the role of quantum confinement and indicating a direct band-to-band recombination mechanism.

XPS of oxidized wool fibre surfaces

AbstractOxygen plasma and ultra violet (UV) ozone treatments have been used to oxidize the surfaces of batches of natural wool fibres and woven cloths. The changes in surface composition and chemistry induced by these treatments have been followed using X‐ray photoelectron spectroscopy (XPS).Oxidation of surface proteinaceous di‐sulphide sulphur to sulphonic acid groups (SO3H) containing S(6+) is shown to occur for both treatments. However, oxidation at levels of approximately 90% is apparent for the UV ozone treatment which is significantly higher than levels achieved using oxygen plasmas (30%). Both treatments also lead to oxidation of surface carbon species. The data presented indicate that the UV ozone treatment used is capable of producing surface sulphur and carbon chemistry of the type usually obtained industrially by wet chemical methods which have the disadvantage of producing chlorinated effluent.

UV ozone modification of wool fibre surfaces

An ultraviolet (UV) ozone treatment has been used to oxidise the surfaces of batches of natural wool fibres. The changes in surface composition and chemistry induced by this treatment have been followed using X-ray photoelectron spectroscopy (XPS). Oxidation of surface di-sulphide sulphur to sulphonic acid groups (- SO 3 H) containing S 6+ is observed at levels of approximately 90% conversion. This is significantly higher than levels previously achieved using oxygen plasmas. The treatment also appears to cause reaction of the proteinaceous carbon, leading to an increase in carbon-oxygen, particularly carbonyl, functionality. The data presented indicate that the treatment used is capable of producing surface sulphur and carbon chemistry of the type usually obtained by wet chlorination.

Influence of Surface Pre-Cleaning on Electrical Properties of Rapid Thermal Oxide and Rapid Thermal Chemical Vapor Deposition Oxide

ABSTRACTVarious surface pre-cleaning processes for rapid thermal in-situ polysilicon/oxide/silicon stacked gate formation have been evaluated. MOS capacitors have been fabricated to assess the effects of surface pre-cleaning on the quality of both Rapid Thermal Oxide (RTO) and Rapid Thermal Chemical Vapor Deposition (RTCVD) oxide. Measurement results have shown that, 1) High temperature (≥ 900 °C) rapid thermal cleaning in Ar, H2 or high vacuum (10−8 Torr) ambients can lead to MOS gates with high leakage current if RTO is used to form the gate oxide, 2) The standard Huang clean and ultra-violet ozone (UV/O3) treatments can improve the film quality for both deposited and thermally grown oxide, and 3) Compared with RTO, the breakdown field of the RTCVD oxide is less dependent on the surface pre-cleaning treatment. These results indicate that silicon wafer surface cleaning techniques typically used for silicon epitaxial processes are not necessarily applicable to oxide film formation in RTP reactors.

A study of the use of ultraviolet–ozone cleaning for reduction of the defect density on molecular beam epitaxy grown GaAs wafers

A problem with molecular beam epitaxy growth of semiconductors is the presence of defects. Since most of them originate at the substrate/epitaxial interface, special attention must be given to the treatment of the surface before growth. It must be clean on an atomic scale. This study has found that a combination of ultraviolet–ozone treatment and chemical etching produces the lowest defect density in the grown layer.