Physical and chemical modifications of PET surface using a laser-plasma EUV source

Extreme ultraviolet (EUV) radiation is the electromagnetic radiation ranging from vacuum ultraviolet to soft X-rays. A single EUV photon carries enough energy to ionize any atom or molecule. The penetration depth of the radiation in any material is very short, ranging from tens to hundreds nanometers. Intense EUV pulses can remove material from the surface or modify its morphology or/and chemical structure. In this work, the radiation from a laser-plasma EUV source based on a double-stream gas-puff target was used for surface modification of polyethylene terephthalate (PET). The PET samples were irradiated with the EUV pulses emitted from krypton plasma and focused with a gold-plated ellipsoidal collector. The spectrum of the focused radiation covered the wavelength range from 9 to 70 nm. The PET samples were irradiated for 1 s–2 min at a 10-Hz repetition rate. Surface morphology of polymer samples after irradiation was investigated using a scanning electron microscope. Changes in chemical surface structure of the irradiated samples were investigated using an X-ray photoelectron spectroscopy. Different kinds of surface microstructures were obtained depending on the EUV fluence in a single pulse and the total EUV fluence. XPS measurements also revealed a modification of the chemical structure.     

Ablation and surface modifications of PMMA using a laser-plasma EUV source

Surface modification and micro-structuring of polymers for different application is mainly performed using ultraviolet (UV) radiation from excimer lamps or excimer lasers. In this case, however, the radiation penetration depth may exceed 100 μm, thereby degrading the polymer deep inside. On the other hand, extreme ultraviolet (EUV) radiation is absorbed in a layer approximately 100 nm thick only. In this work, the radiation from a laser-plasma EUV source based on a double-stream gas-puff target is focused with a gold-coated ellipsoidal collector for surface modification of polymethylmethacrylate (PMMA). The spectrum of the focused radiation consists of a narrow feature with maximum at 10 nm and a long-wavelength tail up to 70 nm. The PMMA samples are mounted in the focal plane of the EUV collector or at some distance downstream this plane and irradiated for 10–60 s with 10-Hz repetition rate. The irradiated polymer samples were investigated using a scanning electron microscope. When the EUV fluence exceeds 10 mJ/cm², smooth ablation of PMMA was obtained. For lower fluences but close to this value, strong surface modifications appeared.     

EUV micropatterning for biocompatibility control of PET

We have investigated the influence of oriented microstructures at modified polyethylene terephthalate (PET) on the adhesion and alignment of Chinese hamster ovary (CHO) cells. For surface modification, the PET foils were exposed to the radiation of a laser-plasma extreme ultraviolet (EUV) source based on a double-stream gas-puff target. The emission of the plasma was focused onto the samples by means of a gold-plated ellipsoidal collector. The spectrum of the focused radiation covered the wavelength range from 9 to 70 nm. The PET samples were irradiated with the EUV pulses at a repetition rate of 10 Hz in a high vacuum. For control experiments, PET samples were also irradiated in air with the light of a 193 nm ArF-excimer laser. Different kinds of surface microstructures were obtained depending on the EUV or laser fluence and pulse number, including oriented wall- and ripple-type structures with lateral structure periods of a few μm. The surface morphology of polymer samples after the irradiation was investigated using a scanning electron microscope (SEM). Changes in chemical surface structure of the irradiated samples were investigated using X-ray photoelectron spectroscopy (XPS). We demonstrated that the cells show good adhesion and align along oriented wall- and ripple-type microstructures on PET surfaces produced by the EUV irradiation.     

PMMA and FEP surface modifications induced with EUV pulses in two selected wavelength ranges

Surface modification of PMMA and FEP polymers using extreme ultraviolet (EUV) in two spectral ranges was investigated. A laser-plasma EUV source based on a double stream gas puff target equipped with an Au coated ellipsoidal collector was used for the experiment. The spectrum of the focused radiation from Kr plasma consisted of a narrow feature with a maximum at 10 nm and a long-wavelength tail up to 70 nm. Al and Zr filters were employed for the selection of radiation from these two spectral regions. The radiation fluences in the two cases were comparable. Polymer samples were mounted in the focal plane of the EUV collector and irradiated for 1–2 min with a 10 Hz repetition rate. Weak ablation accompanied by creation of micro- and nanostructures of different kinds was obtained in both cases. Significant differences in the surface structures after irradiation of PMMA and FEP in these two spectral regions were revealed.     

Detection of surface changes of materials caused by intense irradiation with laser-plasma EUV source utilizing scattered or luminescent radiation excited with the EUV pulses

Extreme ultraviolet (EUV) radiation is absorbed in a thin surface layer of any material. Irradiation of material samples with intense EUV pulses may cause different surface changes. Some of them, especially connected with material desorption, can be clearly visible using an optical or electron microscope. Other changes concerning crystal structure or chemical composition may not be visible under the microscope. They can however be detected using the EUV radiation itself. In this paper a new method of measurement of surface changes by irradiation with a laser-plasma EUV source is presented. The radiation was collected and focused on a material surface using a specially designed multifoil collector. Radiation scattered or excited in the material was detected with the use of a Wolter-type mirror coupled to a back-illuminated CCD camera. Depending on material samples, images with different intensity distributions were registered. For some samples, the intensity distributions of the images obtained before and after irradiation were slightly different. The intensity differences in such cases allowed us to obtain differential images. The appearance of such images was assumed to be evidence of surface changes. PACS 42.62.-b; 61.80.-x; 52.25.Os     

The influence of polychromic light on the surface of MDI based polyurethane elastomer

A polyurethane elastomer was synthesized starting from 4,4′ diphenylmethane diisocyanate and poly(ethyleneadipate)diol. Butylene glycol was used as chain extender. Surface properties after photo-degradation of the elastomer under the action of the radiation with λ > 300 nm was monitored by FT-IR spectroscopy and contact angle measurements. The quality of polymer surface was observed under optical microscope. The formation of photo-Fries rearrangement and Norrish II reaction products during irradiation was associated with the gloss loss (from 100% for non-irradiated sample to 27% after 200 h irradiation time) and modification of wettability. There were also found significant modifications with irradiation time of both the glass transition temperature (T_g decreases from 64 °C for non-irradiated sample to 53 °C after 200 h irradiation) and the swelling coefficient (an increase from 1.2% up to 2.5% is observed after 200 h irradiation).     

Simultaneous treatment of polymer surface by EUV radiation and ionized nitrogen

In this paper chemical modification of a poly(vinylidene fluoride) surface by extreme ultraviolet (EUV) irradiation in a presence of ionized nitrogen was demonstrated for the first time. Nitrogen gas, injected into an interaction region, was ionized and excited by the EUV radiation from a laser-plasma source. The ionization degree and excited states of nitrogen were investigated using EUV spectrometry and the corresponding spectra are presented. Chemical modification of polymer after combined EUV and ionized nitrogen treatment was investigated using X-ray photoelectron spectroscopy. A?significant contribution of nitrogen atoms in near-surface layer of the polymer after the treatment was demonstrated.     

Atomic force microscopy, Raman spectroscopy and nonlinear absorption properties of femtosecond laser irradiated CR-39

Investigations have been performed to explore ultrashort laser irradiation effects on the surface topography as well as structural and nonlinear absorption properties of a polymer CR-39. For this purpose, a CR-39 target was exposed in air to 25 fs, 800 nm Ti:sapphire laser radiation at fluences ranging from 0.25 J cm⁻² to 3.6 J cm⁻². The surface features, structural changes and nonlinear absorption were explored by AFM, Raman Spectroscopy and a Z-scan technique, respectively. Several topographical structures like bumps, explosions and nano cavities have been observed on the irradiated surface. Raman spectroscopy reveals changes in the fundamental structure of the polymer after the irradiation. Nonlinear absorption data contained by the Z-scan technique predict the dominance of three-photon absorption in case of pristine CR-39. Furthermore, nonlinear absorption (three or two photon) increases with increasing laser fluences and is well correlated with surface and structural changes revealed by AFM and Raman spectroscopy.     

Efficient micromachining of poly(vinylidene fluoride) using a laser-plasma EUV source

In this paper an efficient micromachining of poly(vinylidene fluoride) (PVDF) by direct photo-etching with a laser-plasma EUV (extreme ultraviolet) source was demonstrated for the first time. Mass spectroscopy was employed to investigate the ablation products and revealed emission of numerous molecular species of C-containing fragments of the polymer chain. Chemical surface changes after irradiation were investigated using X-ray photoelectron spectroscopy (XPS). The XPS spectra obtained for PVDF samples, irradiated with low and high EUV fluence, indicate significant differences between chemical structures in near-surface layers. It was shown that irradiation with low fluence results in defluorination and thus carbon enrichment of the polymer in near-surface layer. In contrary, irradiation with high fluence leads to intense material ablation and hardly modifies the chemical structure of the remaining material.     

Gold coatings on polyethyleneterephthalate nano-patterned by F2 laser irradiation

In this work we present periodic surface structures generated by linearly polarized F₂ laser light (157 nm) on polyethyleneterephthalate (PET). Atomic force microscopy was used to study the topological changes induced by the laser irradiation. The laser irradiation induces the formation of periodic ripple structures with a width of ca 130 nm and a height of about 15 nm in the fluence range 3.80-4.70 mJ/cm² and the roughness of the polymer surface increases due to the presence of these periodic structures. Subsequently, the laser modified PET foils were coated with a 50 nm thick gold layer by sputtering. After Au deposition on the PET foils with ripple structure, the roughness of surface decreases in comparison to PET with ripples without Au coating. For 50 nm thick Au layers, the ripple structure is not directly transferred to the gold coating, but it has an obvious effect on the grain size of the coating. With considerably thinner Au layers, the ripple structures are smoothened but preserved.     

EUV emission from solids illuminated with a laser-plasma EUV source

In this paper a possibility of performing fluorescence experiments in extreme ultraviolet (EUV) using a laboratory EUV source was demonstrated. Usually such measurements are performed using third-generation synchrotrons because of extremely low probability of fluorescence in this wavelength range. In this work a 10 Hz laser-plasma EUV source based on a gas puff target was used. Fluorescent radiation from Al and Si was registered. Despite relatively low spectral resolution, some differences in Si spectra coming from different kind of samples were revealed.     

Fabrication of 15 nm curvature radius polymer tip probe on an optical fiber via two-photon polymerization and O2-plasma ashing

Polymer tips with 15 nm curvature radius apexes that are capable of scanning the surface of a sample have been fabricated on the surface of an optical fiber using two-photon photopolymerization (TPP) and O₂-plasma ashing with a SCR500 resin. First, the parent conical polymer tip with a 125 nm curvature radius apex was fabricated via TPP using a continuous scanning method and the accumulation of circular layers. Next, the tip was sharpened using an O₂-plasma ashing process with high reproducibility. As a result, the apex radius of curvature had a maximum reduction to 15 nm. In order to evaluate the performance of the 15 nm curvature radius polymer tip, a 30 nm thick gold layer with holes of 250 nm radius and a single layer of polystyrene beads with a 350 nm radius were imaged using a tuning-fork-based atomic force microscope. The topographic images obtained by the 15 nm polymer tip were improved in width and depth compared with those obtained by the 125 nm polymer tip due to the reduction of the imaging artifacts. This method can also be commonly used to reduce the radius of curvature of the polymer tip in order to achieve more accurate imaging.     

Design and performance of two-channel EUV multilayer mirrors with enhanced spectral selectivity

In this paper, we present a study on two-channel multilayer mirrors which can operate at two wavelengths in Extreme Ultraviolet (EUV) spectral range. We propose a new method to design two-channel EUV multilayer mirrors with enhanced spectral selectivity. The mirror structure is a stack of two periodic multilayers separated by a buffer layer. We have defined the main parameters which allow adjustment of the distance between different order Bragg’s peak and of wavelength positions of reflectivity minima. Two mirrors have been designed and deposited for solar EUV telescope applications by using this method. The first mirror reflects Fe IX–X line (17.1 nm) and Fe XVI (33.5 nm) lines with attenuation of the He II line (30.4 nm). The second mirror reflects Fe IX–X and He II lines with attenuation of Fe XV (28.4 nm) and Fe XVI lines. Measurements with synchrotron radiation source confirm that, in both cases, for these mirrors, we are able to adjust reflectivity maxima (Bragg peak position) and minima. Such multilayers offer new possibilities for compact design of multi-wavelength EUV telescopes and/or for high spectral selectivity.     

Ultrashort laser ablation of PMMA and intraocular lenses

The use of intraocular lenses (IOLs) is the most promising method to restore vision after cataract surgery. Several new materials, techniques, and patterns have been studied for forming and etching IOLs to improve their optical properties and reduce diffractive aberrations. This study is aimed at investigating the use of ultrashort laser pulses to ablate the surface of PMMA and intraocular lenses, and thus provide an alternative to conventional techniques. Ablation experiments were conducted using various polymer substrates (PMMA samples, hydrophobic acrylic IOL, yellow azo dye doped IOL, and hydrophilic acrylic IOL consist of 25% H₂O). The irradiation was performed using 100 fs pulses of 800 nm radiation from a regeneratively amplified Ti:sapphire laser system. We investigated the ablation efficiency and the phenomenology of the ablated patterns by probing the ablation depth using a profilometer. The surface modification was examined using a high resolution optical microscope (IOLs) or atomic force microscope—AFM (PMMA samples). It was found that different polymers exhibited different ablation characteristics, a result that we attribute to the differing optical properties of the materials. In particular, it was observed that the topography of the ablation tracks created on the hydrophilic intraocular lenses was smoother in comparison to those created on the PMMA and hydrophobic lens. The yellow doped hydrophobic intraocular lenses show higher ablation efficiency than undoped hydrophobic acrylic lenses.     

Correction of a coherent image during KrF excimer laser ablation using a mask projection

Using masks for laser ablation has proven useful in the fabrication of prototypes for the manufacturing of micro-fluidic devices. In this work, an excimer laser was used to engrave microscopic channels on the surface of polyethylene terephthalate (PET), which showed a high absorption ratio for an excimer laser beam with a wavelength of 248 nm. When 50 μm wide rectangular microscopic channels were made using a 500×500 μm square mask and a magnification ratio of 1/10, ditch-shaped defects were found at both corners. The calculation of the laser beam intensity showed that a coherent image in the PET specimen caused the defects. An analysis based on the Fourier diffraction theory enabled the prediction of a coherent shape at the image plane, as well as a diffracted beam between the mask and the image plane. The analysis also showed that the diameter of the aperture was a predominant factor toward the elimination of ditch-shaped defects in the rectangular microscopic channels on the PET produced by an excimer laser ablation.     

Surface disorder in c-Si induced by swift heavy ions

The disorders induced in crystalline silicon (c-Si) through the process of electronic energy loss in the swift heavy ion irradiation were investigated. A number of silicon <1 0 0> samples were irradiated with 65 MeV oxygen ions at different fluences, 1×10¹³ to 1.5×10¹⁴ ions/cm², and characterized by the Raman spectroscopy, the optical reflectivity, the X-ray reflectivity, the atomic force microscopy (AFM) and the X-ray diffraction (XRD) techniques. The intensity, redshift, phonon coherence length and asymmetric broadening associated with the Raman peaks reveal that stressed and disordered lattice zones are produced in the surface region of the irradiated silicon. The average crystallite size, obtained by analyzing Raman spectrum with the phonon confinement model, was very large in the virgin silicon but decreased to<100 nm dimension in the ion irradiated silicon. The results of the X-ray reflectivity, AFM and optical reflectivity of 200–700 nm radiation indicate that the roughness of the silicon surface has enhanced substantially after ion irradiation. The diffusion of oxygen in silicon surface during ion irradiation is evident from the oscillation in the X-ray reflectivity spectrum and the sharp decrease in the reflectivity of 200–400 nm radiation. The rise in temperature, estimated from the heat spike model, was high enough to melt the local silicon surface. The results of XRD indicate that lattice defects have been induced and a new plane <2 1 1> has been formed in the silicon <1 0 0>after ion irradiation. The results of the present study show that the energy deposited in crystalline silicon through the process of electronic energy loss ～0.944 keV/nm per ion is sufficient to induce disorders of appreciable magnitude in the silicon surface even at a fluence of ～10¹³ ions/cm².     

Periodic surface nanostructures on polycrystalline ZnO induced by femtosecond laser pulses

Periodic surface nanostructures induced by femtosecond laser pulses on polycrystalline ZnO are presented. By translating the sample line-by-line under appropriate irradiation conditions, grating-like nanostructures with an average period of 160 nm are fabricated. The dependence of surface morphologies on the processing parameters, such as laser fluence, pulse number and laser polarization, are studied by scanning electronic microscope (SEM). In addition, photoluminescence (PL) analysis at room-temperature indicates that the PL intensity of the irradiated area increases significantly compared with the un-irradiated area. Using femtosecond laser pulses irradiation to fabricate periodic surface nanostructures on polycrystalline ZnO is efficient, simple and low cost, which shows great potential applications in ZnO-based optoelectronic devices.     

Two ultrasonic applications for the synthesis of nanostructured copper oxide (II)

In this paper, we present two aspects of the ultrasonic for the synthesis of CuO (II) nanostructures. In the first ultrasound application, we made a copper tip for an ultrasonic probe transducer and used it for electrolysis and ultrasound irradiation processes. This method is named direct sonoelectrochemistry and compares with conventional electrochemistry. CuO (II) nanostructures are obtained after sintering for both direct sonoelectrochemistry method and conventional electrochemistry method. In the second application of ultrasound, the copper nanostructures were generated by the ultrasound ablation method, and then, the heating process was performed for oxidation. The formation of the copper and CuO (II) nanostructures is confirmed by the powder X-ray diffraction (XRD), the field emission electron microscopy (FESEM), and transmission electron microscopy (TEM). The results show that the direct sonoelectrochemistry method generates CuO (II) nanostructures 4.2 times more than conventional electrochemistry. The crystallite size in the electrochemistry methods and direct sonoelectrochemistry is 28.44 nm and 26.60 nm, respectively. The direct sonoelectrochemistry way is a very flexible method and parameters in electrochemical, ultrasound, and the relationship between them can play an important role in the process of synthesis of nanostructures. The crystallite size in the ultrasound ablation method is 21.13 nm and 25.23 nm for the copper and CuO (II) nanostructures. The most important advantages of this method are green, fast, and high purity of the produced nanostructures.     

Electron beam-induced structural transformations of MoO3 and MoO3?x crystalline nanostructures

Electron beam-induced damage and structural changes in MoO₃ and MoO_3−x single crystalline nanostructures were revealed by in situ transmission electron microscopy (TEM) examination (at 200 kV) after few minutes of concentrating the electron beam onto small areas (diameters between 25 and 200 nm) of the samples. The damage was evaluated recording TEM images, while the structural changes were revealed acquiring selected area electron diffraction patterns and high resolution transmission electron microscopy (HRTEM) images after different irradiation times. The as-received nanostructures of orthorhombic MoO₃ were transformed to a Magnéli’s phase of the oxide (γ-Mo₄O₁₁) after ~10 min of electron beam irradiation. The oxygen loss from the oxide promoted structural changes. HRTEM observations showed that, in the first stage of the reduction, oxygen vacancies generated by the electron beam are accommodated by forming crystallographic shear planes. At a later stage of the reduction process, a polycrystalline structure was developed with highly oxygen-deficient grains. The structural changes can be attributed to the local heating of the irradiated zone combined with radiolysis.     

Self-assembled gold nanochains hybrid based on insulin fibrils

We reported a facile method for preparing self-assembly gold nanochains by using insulin fibrils as biotemplate in aqueous environment. The gold nanochains hybrid nanostructures, which are insulin fibrils coated by gold nanoparticles, can be fabricated by simply reducing the salt precursors using DMAB. By increasing the molar ratio between salt precursors and insulin, denser hybrid nanochains can be obtained, meanwhile the mean diameter of gold nanoparticles is changing from 8 to 10 nm and then to 12 nm. The fabricated gold nanochains hybrid had helix structure, which was confirmed by circular dichroism spectra. The hybrid nanostructures were also investigated by transmission electron microscope, atomic force microscope, Fourier transform infrared spectra, and UV–Visible spectroscopy. As the wire-like structure become denser, the suspensions show color-changing, corresponding to the surface plasmon resonance red shift, which is attributed to the increasing mean size of nanoparticles. Based on the characterizations, a hypothetic mechanism was suggested to describe the formation processing of hybrid gold nanochains.