Vacuum Ultraviolet Illumination Research Articles

Uniform broad-area deposition and patterning of SiO2 nanofilms by 172 nm photochemical conversion of liquid tetraethoxysilane layers at 300 K

Oxide films of the quality required for the fabrication of electronic and photonic devices are typically deposited at elevated temperatures and thermal equilibrium, thereby adversely impacting thermal budgets. We report the deposition and patterning of silicon dioxide (SiO2) films of high electrical and optical quality on Si(100) or polymer substrates in a N2 atmosphere and at 300 K by the photochemical conversion of thin liquid tetraethoxysilane (TEOS) layers with narrowband vacuum ultraviolet radiation [vacuum ultraviolet (VUV), 172 nm] provided by efficient microplasma lamps. Irradiating liquid TEOS layers, produced by spin-coating the precursor onto a substrate, with a VUV intensity of 13 mW cm−2, yields 40 nm-thick SiO2 films having a dielectric breakdown strength (Eb) of 5 MV cm−1, for which no precedent exists in the deposition of oxide films at 300 K. If room temperature-deposited films are post-annealed at 200 °C, Eb rises to 7.5 MV cm−1, which is <12% below the measured value (8.5 MV cm−1) for 40 nm SiO2 films grown by thermal oxidation. The deposition of 1 µm thick, stoichiometric SiO2 films requires only 20 min of VUV illumination at low optical fluences, and films with thicknesses of ∼35–60 nm exhibit a refractive index of 1.45 (633 nm). X-ray photoelectron spectrometry and Rutherford backscattering analysis indicate that, despite the deposition temperature, hydrocarbon impurity levels are near or below the detection limit. The capability for depositing 960 nm-thick SiO2 films uniformly (to within 0.6%) by liquid → solid photochemical conversion over a 5 cm diameter Si substrate and patterning films onto flexible polymer substrates has also been demonstrated.

Surface modification of polymers by vacuum ultraviolet Illumination containing low wavelength below 160 nm and microfluidic applications of irradiated polycarbonate

Microfluidic devices are applied in various fields including medicine and drug discovery. Recently, plastics are used for the device due to low cost. One of the challenges is the bonding process. A bonding method that preserves the microstructure is required. Recently, room temperature bonding methods using vacuum ultraviolet (VUV) light have been reported, but the applicable materials are limited. Here, we propose a new method using VUV with a wavelength shorter than 160 nm. As a result of the surface analysis, it was confirmed that the substrate surface becomes hydrophilic upon VUV irradiation. As a result of the bonding test, it was confirmed that it was possible to bond polycarbonate (PC) to each other (maximum bonding force: 2.03 MPa), PC to glass (maximum bonding force: 10.72 MPa), and cyclic olefin polymer to glass (maximum bonding force: 3.51 MPa), which have not been reported so far. The results suggest that one of these bonding mechanisms is the addition of hydrophilicity by VUV irradiation. Based on the results, for the first time, a PC-made microfluidic device was successfully developed using the bonding method. This result greatly contributes to the development of the manufacturing process of plastic microfluidic devices.

Novel insight into the effect of relative humidity on the acetone photocatalytic decomposition using Ag−CeO2 under vacuum ultraviolet illumination: Catalytic synergies, performance, and mechanistic interpretation

The present work reports on the impact of relative humidity (RH) on the process of acetone oxidation in the absence and presence of Ag − CeO2 photocatalysts, including 185 nm vacuum UV (VUV) photolysis, ozonation, ozonation + 254 nm UV photolysis, and 254 nm UV photolysis treatment processes. The photocatalysts were evaluated by Raman, SEM, XRD, HRTEM, EPR, EDS, and XPS techniques. The information from characterization tests revealed that modification of ceria structure by the introduction of Ag nanoparticles increases the photocatalyst oxygen storage ability, which promotes the oxygen vacancies formation, resulting in the lattice oxygen activation. The attained data demonstrated the dual role of RH in the process of acetone oxidation under VUV irradiation. Compared to the dry condition, the introduction of RH = 25, 45, and 65% to the VUV photolysis system enhances the conversion of acetone remarkably from 40% to 60, 68, and 72%, respectively. However, the acetone oxidation did not change significantly in the presence of RH = 25–65% over the Ag − CeO2/VUV system. This observation confirms the dual role of RH; while RH boosted the gas phase photooxidation reactions, it poisoned the gas–solid interface, leading to an inhibitory effect on the catalytic oxidation reactions. Moreover, the results showed that the lower level of relative humidity (e.g., RH = 5%) over the Ag − CeO2 under VUV light not only enhances the acetone oxidation to 88% but also, through the participation of the catalytic ozonation, decomposed the formed O3 and increased the selectivity of the reaction towards complete oxidation up to 97%. The acetone oxidation mechanism was studied where the EPR results show that both.OH and.O2– contribute to acetone oxidation. Finally, the Ag − CeO2/VUV reusability was assessed in a way that the in-situ procedure for photocatalyst regeneration was utilized to inhibit the catalyst deactivation.

Degradation mechanism of BPA under VUV irradiation: efficiency contribution and DFT calculations.

Bisphenol A (BPA) is regarded as a hazardous pollutant that exists widely in aquatic environments, posing a severe threat to human health. In this study, a vacuum ultraviolet (VUV) lamp emitting a hybrid of 254nm and 185nm light was used to degrade BPA. Results indicated that photolysis via 254nm wavelength accounted for 24.93% for BPA decay, while indirect oxidation was responsible for 52.27% of decay. Results confirmed that the degradation of BPA under VUV illumination mainly occurred via photo-excited degradation and ·OH electrophilic addition reactions based on average local ionization energy (ALIE) calculation and density functional theory (DFT) calculations. Therefore, only light with a wavelength of 254nm was able to induce the first three excited states of BPA, forming the electron transition type of n → π* from O atom to a single benzene ring and π → π* in the single benzene ring. Indirect oxidation by ·OH occurred as it preferentially attacked the C6 atom in BPA ring A. Moreover, the energy required for photo-excited degradation was about twofold than that of ·OH electrophilic addition reactions.

VUV Pump and Probe of Phase Separation and Oxygen Interstitials in La2NiO4+y Using Spectromicroscopy

While it is known that strongly correlated transition metal oxides described by a multi-band Hubbard model show microscopic multiscale phase separation, little is known about the possibility to manipulate them with vacuum ultraviolet (VUV), 27 eV lighting. We have investigated the photo-induced effects of VUV light illumination of a super-oxygenated La2NiO4+y single crystal by means of scanning photoelectron microscopy. VUV light exposure induces the increase of the density of states (DOS) in the binding energy range around Eb = 1.4 eV below EF. The photo-induced states in this energy region have been predicted due to clustering of oxygen interstitials by band structure calculations for large supercell of La2CuO4.125. We finally show that it is possible to generate and manipulate oxygen rich domains by VUV illumination as it was reported for X-ray illumination of La2CuO4+y. This phenomenology is assigned to oxygen-interstitials ordering and clustering by photo-illumination forming segregated domains in the La2NiO4+y surface.

(Invited) Vacuum Ultraviolet Photochemical Atomic Layer Deposition of Alumina and Titania Films

Conventional atomic layer deposition (ALD) is a thermo-chemical process where co-reagents are sequentially pulsed in cycles onto a heated substrate. As an alternative to substrate heating, various forms of other “energy – enhanced” ALD processes are being investigated. Herein, the photochemical atomic layer deposition of Al2O3 and TiO2thin films at 60°C is reported using a shuttered vacuum ultraviolet (VUV) light source to excite molecular oxygen as a co-reagent with the metal precursors. The growth mechanisms using trimethyl aluminium (TMA) and titanium isopropoxide precursors, are investigated using in-situ quartz crystal microbalance (QCM) and post-deposition ellipsometric measurements. The figure below illustrates the mass gain / loss processes during a series of ALD cycles of TMA with O2/ UV excitation from a VUV lamp with peak emissions at 125 nm (9.9 eV) and 160 nm (7.7 eV). Further, the figure shows the growth per cycle (GPC) derived from the QCM data and also separately from single wavelength (633 nm) ellipsometry measurements made on silicon (100) samples exposed to the same growth conditions. Both data sets indicate that as the VUV illumination time is extended beyond 5 s to 10 s the overall mass gain per cycle saturates and becomes approximately constant, which is consistent with other self limiting ALD processes. The photochemical process is exploited to achieve area-selective film deposition. Selective area deposition was achieved by projecting the VUV light through a metalized magnesium fluoride photolithographic mask. The selectivity of deposition is a function of the UV exposure time and the mask – substrate separation. The photochemical atomic layer deposition of TiO2 thin films from titanium isopropoxide exhibits similar growth characteristics to the Al2O3 photochemical process, however subtle differences in the growth mechanisms are attributed to the different reactions of the excited oxygen species (O( 3P)) with the Al-CH3 and Ti-OCH(CH3)2 bonds. Figure 1

Influence of VUV illumination on breakdown mechanics: pre-ionization, direct photoionization, and discharge initiation

A microdischarge (MD) vacuum ultraviolet (VUV) light source is fired onto a N2–NO (99.92 : 0.08%) target gas. The minor gas constituent, NO, was chosen for its ionization potential (9.23 eV) and photoionization cross-section (1.4 × 10−18 cm2) at the wavelength of interest (121.6 nm, 10.2 eV). The result is a plasma generated entirely by volume photoionization in a N2–NO background. Using a very low electric field amplitude, charge carriers are drifted though the photoplasma at picoampere levels, serving as a non-invasive diagnostic. Using a simple one-dimensional fluid approximation for the low electric field condition, theoretical predictions of photoplasma current were found to be in meaningful agreement with experimental data. The impact of direct photoionization and pre-ionization on nanosecond timescale high voltage breakdown yielded two primary observations: (1) a significant reduction in the formative delay time necessary for spark formation, and (2) almost complete elimination of the statistical delay time. Again utilizing one-dimensional fluid approximations, reasonable agreement between experimental and simulated breakdown voltage was observed. Utilizing the same VUV source to illuminate a HV spark gap biased to about 95% self-breakdown voltage revealed that direct volume photoionization alone was insufficient to trigger breakdown of the high voltage gap. However, permitting electrode illumination, the same source was found to be capable of triggering breakdown in the undervoltaged gap, albeit with a large temporal jitter.

Rubidium metavanadate formation at room temperature under vacuum ultraviolet irradiation from metal-organic compositions

White light emitting phosphor RbVO 3 films have been successfully fabricated by means of a vacuum ultraviolet (VUV) irradiation using an excimer lamp after spin coating of metal-organic solution onto substrates. The metal-organic carboxylates coated on substrates decomposed and reacted under the VUV irradiation. The metal-organic bonds were efficiently cleaved by the VUV illumination not only in air but also in vacuum, however, there was not a strong driving force for the reaction process to the formation of RbVO 3 in the vacuum atmosphere. On the contrary, the reaction and crystallization simultaneously proceeded under photo-chemically produced active oxygen and/or ozone atmospheres due to the VUV illumination in air. The reaction between the photo-activated Rb–O and V–O species could be strongly enhanced by the oxidation atmospheres at the moment of the metal-organic bond cleavage under the VUV irradiation.

Effective energy transfer between Ce 3+and Eu 2+ in CaAl 2Si 2O 8 host under vacuum ultraviolet illumination

We presented the energy transfer from Ce 3+ to Eu 2+ in CaAl 2Si 2O 8 host. The Ce 3+-doped CaAl 2Si 2O 8 phosphor had a strong emission band at 378 nm under the vacuum ultraviolet (VUV) light. This emission spectrum of Ce 3+ well overlapped with the excitation spectrum of Eu 2+ under the UV illumination. As a result, the energy transfer from Ce 3+ to Eu 2+ in CaAl 2Si 2O 8 matrix was observed under VUV excitation, which resulted in a significant enhancement of the emission peak intensity at 446 nm. More details about the luminescent properties were presented.

Processing and properties of transparent sulfated TiO 2 thin films using sol–gel method

For this study, transparent sulfated TiO 2 thin films were prepared from Ti alkoxide. Vacuum ultraviolet illumination in air at 180 °C was demonstrated to be an effective pretreatment for reproducible preparation of crack-free sulfated TiO 2 thin films. The film provides a high photoinduced wettability conversion rate and photocatalytic decomposition efficiency on gaseous n-hexane under proper conditions. This result is attributable to the accelerated decomposition of intermediates by a strong inorganic acid on the surface or to the resultant high quantum yield by suppressing the recombination between electrons and holes on the film surface.

Effects of amide types DCCAs on the properties of Y2O3:Eu phosphor powders with spherical shape and fine size

The effects of amide types drying control chemical additives on the morphologies, BET surface areas, pore sizes, crystal structures, and photoluminescence intensities of the Y 2 Ο 3 :Eu phosphor powders prepared by spray pyrolysis were investigated. The phosphor powders obtained from the spray solutions with N,N-dimethylacetamide (DMA) and N,N-dimethylformamide (DMF) had filled inner structures, low surface areas and high photoluminescence intensities under vacuum ultraviolet illumination. The photoluminescence intensity of the phosphor powders obtained from the spray solution with DMA was 120% of that of the phosphor powders obtained from the spray solution without drying control chemical additive.

Outgassing Measurements Combined with Vacuum Ultraviolet Illumination of the Deposited Materials

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Effect of Boric Acid Flux and Drying Control Chemical Additive on the Characteristics of Y2O3:Eu Phosphor Particles Prepared by Spray Pyrolysis

Y2O3:Eu phosphor particles with a spherical shape, a fine size, and high brightness under vacuum ultraviolet (VUV) illumination were prepared by spray pyrolysis. The polymeric precursors were needed to modify the spray solution to prepare spherical particles. The use of only flux with the polymeric precursors did not produce dense and spherical Y2O3:Eu particles; that is, the produced particles were very porous. The addition of both a drying control chemical additive (DCCA) and boric acid to a spray solution containing polymeric precursors was found to produce Y2O3:Eu phosphor particles with a dense structure while maintaining the spherical morphology. According to X-ray diffraction (XRD) analysis, the DCCA induced an enhancement of crystallinity. The use of boric acid flux improved the photoluminescence (PL) intensity under VUV illumination. In addition, the use of both boric acid flux and a DCCA with polymeric precursors further improved the PL intensity owing to the enhancement of the morphology and the crystallinity.

Effects of Vacuum Ultraviolet Light Illumination on the Residual Stress in Sol–Gel-derived Titanium Dioxide Films

Abstract This study investigated effects of vacuum ultraviolet (VUV) illumination on the residual stress of sol–gel-derived titanium dioxide (TiO2) films. After firing at 500 °C, VUV pre-illumination to TiO2 precursor film decreases residual stress more than 30%. This effect is explained by the great shrinkage that occurs during VUV illumination.

Effects of vacuum ultraviolet light illumination and seeding on crystallization of sol–gel-derived titanium dioxide precursor films using plasma treatment

The current study investigated the effects of vacuum ultraviolet (VUV) illumination and seeding on the crystallization of sol–gel-derived titanium dioxide (TiO2) precursor films on Si(100) and Pyrex glass substrates by plasma treatment. To improve the film microstructure to one that is amenable to crystallization, VUV illumination is found to be effective. However, crystallization behavior suggests that a proper balance between decomposition of organic compounds and densification by dehydration is important for the treatment. The combination of seeding and VUV illumination is effective even for crystallization by plasma treatment on both Si(100) and Pyrex glass substrates.

Thermalization range distributions for photoelectrons injected into dielectric liquids

A photocurrent can be produced by direct illumination of a dielectric liquid with photons having energies in the vacuum ultraviolet (VUV). The kinetic energy initially imparted to an electron by absorption of a photon is subsequently dissipated through random collisions with molecules in the liquid. With the parent cation at the origin, the distance, r, traveled by the electron in coming into thermodynamic equilibrium with the liquid is called the thermalization range. Electron energy thermalization range distribution functions were determined for electrons created by photoionization of cyclohexane, 2,2-dimethylbutane, tetramethylsilane, and two polydimethylsiloxane oils. Except for cyclohexane, the function [r2/2(B3)3]exp(−r/B3), where B3 is an energy-dependent range parameter, gave the best agreement between the calculated and experimental photocurrents at all photon energies considered. Using a Monte Carlo simulation (Goulet type) to represent the slowing down of the electrons after their injection into the liquid, it was found that four to ten collisions along the thermalization path were sufficient to generate this range distribution function. Electrons can also be injected into these liquids by UV illumination of a photocathode. By taking into account electron scattering back to the image charge in the photocathode, it was possible to use the Monte Carlo simulation to demonstrate the consistency between the range distribution function for electron injection by UV exposure of the photocathode and the range distribution function for electron injection by direct VUV illumination of the bulk liquid.

Creation of trapped electrons and holes in alkaline-earth fluoride crystals doped by rare-earth ions

Defects in Ce 3+- and Eu 2+-doped alkaline-earth fluorides, created by vacuum ultraviolet (VUV) photons with energy lower than that of the band gap, were investigated by various methods: thermostimulated luminescence, photostimulated luminescence and optical absorption. The CaF 2:Eu 2+ thermoluminescence curves in the range of 60–330 K due to various types of trapped holes were the same after VUV illumination as after X-ray irradiation. Thermoluminescence curves of Ce 3+-doped alkaline-earth fluorides created by VUV illumination or X-ray irradiation were generally similar. However, V k thermoluminescence peaks were absent in VUV-illuminated CaF 2:Ce 3+ and SrF 2:Ce 3+ crystals. Creation of Ce 2+ characteristic bands was observed in photostimulated luminescence spectra as well as in optical absorption spectra of vacuum ultraviolet-illuminated or X-ray-irradiated Ce 3+-doped crystals. The proposed mechanism of creation of trapped hole and trapped electron defects by vacuum ultraviolet illumination involves charge transfer-type transitions, in which the electron transfers from valence band to an impurity level, lying in the band gap. Comparison of all involved energies of transitions in the crystals investigated shows that the sum of all transition energies is less than that of the band gap by 1–3 eV. This energy difference can be considered as the energy of lattice relaxation around created Ce 2+ or Eu + ions.

Precursor Type Influence on the Morphology and VUV Characteristics of GdPO[sub 4]:Tb Phosphor Particles Prepared by Spray Pyrolysis

The influence of the precursor types of phosphorus such as (NH 4 ) 2 HPO 4 , NH 4 H 2 PO 4 , (NH 4 ) 3 PO 4 , and H 3 PO 4 on the morphology and vacuum ultraviolet (VUV) characteristics of GdPO 4 :Tb phosphor particles prepared by spray pyrolysis was investigated. All prepared GdPO 4 :Tb phosphor particles had spherical shape and dense structure except for the sample prepared with H 3 PO 4 , which had hollow morphology after the 3 h post-treatment anneal at 1050°C. The phosphorus type also affected the photoluminescence (PL) intensity of GdPO 4 :Tb phosphor particles. The highest PL intensity under VUV excitation was obtained with (NH 4 ) 2 HPO 4 used as the precursor. For optimizing the PL intensity of spherical particles, the average particle size of GdPO 4 :Tb phosphor particles prepared using (NH 4 ) 2 HPO 4 was changed from 0.5 to 1.9 μm and the effect on PL intensity was investigated. The optimized GdPO 4 :Tb particles have a good excitation spectrum comparable to that of the commercial Zn 2 SiO 4 :Mn phosphor particles under VUV illumination from 140 to 220 nm.

Polymer photoablation under windowless VUV hydrogen or helium discharge lamp

In this work, the photoablation of PolyPhenyl-Quinoxaline (PPQ) films is investigated by using the vacuum ultraviolet (VUV) incoherent emission (λ<160nm) from a novel windowless hydrogen (or helium) discharge lamp. A tentative explanation of the film etching process is given through the photochemical reactions which take place in the polymer under VUV illumination.

Photoinduced paramagnetic centers in amorphous silicon oxynitride

Room-temperature metastable paramagnetic point defects are created in plasma-enhanced chemical vapor deposited silicon oxynitride films by exposing the films to ultraviolet and vacuum ultraviolet illumination. We compare the response of films with low, intermediate, and high oxygen content. The defects include unpaired electrons on silicons bonded to nitrogen atoms (K centers), unpaired electrons on bridging nitrogens, and what appear to be overcoordinated nitrogen centers.