Polymer tips for application of atomic force microscopy

A hydrophobic polymer tip for atomic force microscopy has been fabricated by two-photon adsorbed photopolymerization techniques and has been applied for the high resolution imaging of a hydrophilic metal surface. Using optimized photopolymerization conditions, we have succeeded in fabricating sub-100-nm sized polymer tips. This fabricating resolution of two-photon adsorbed photopolymerization is also confirmed by other supporting experiments. The imaging results show that the capillary-force-induced image distortion can be successfully removed by applying a pure hydrophobic polymer tip with a lateral resolution better than 5 nm, which is difficult to achieve with a commercial tip without any environmental control.     

Spectral degeneracy breaking of the plasmon resonance of single metal nanoparticles by nanoscale near-field photopolymerization

We report on controlled nanoscale photopolymerization triggered by enhanced near fields of silver nanoparticles excited close to their dipolar plasmon resonance. By anisotropic polymerization, symmetry of the refractive index of the surrounding medium was broken: C infinity v symmetry turned to C2v symmetry. This allowed for spectral degeneracy breaking in particles plasmon resonance whose apparent peak became continuously tunable with the incident polarization. From the spectral peak, we deduced the refractive-index ellipsoid fabricated around the particles. In addition to this control of optical properties of metal nanoparticles, this method opens new routes for nanoscale photochemistry and provides a new way of quantification of the magnitude of near fields of localized surface plasmons.     

Fabricating high-aspect-ratio sub-diffraction-limit structures on silicon with two-photon photopolymerization and reactive ion etching

We fabricated sub-micrometer objects with feature sizes about one third of the exposure wavelength using two-photon photopolymerization in an epoxy-based photoresist SU-8 . Owing to the high mechanical strength of this photoresist, an aspect ratio as high as nine was achieved with a 200–300 nm lateral dimension. A simple equation was used to estimate the feature size from the laser parameters such as spot size, exposure time, pulse width, pulse repetition rate, and the material properties including the two-photon absorption coefficient and the exposure threshold dose. Patterns in SU-8 were transferred onto silicon using reactive ion etching, preserving both the feature size and aspect ratio. Vertical sidewalls of the transferred patterns were achieved using the black silicon method. PACS 42.82.Cr; 82.35.Ej; 85.40.Hp     

Structure and optical properties of the silver/polyacrylonitrile nanocomposites

Small-angle X-ray scattering is used to prove the formation of silver nanoparticles with sizes of 5–11 nm in siver/polyacrylonitrile nanocomposites synthesized via photopolymerization of a mixture containing silver, acrylonitrile, and photoinitiator. Optical spectra of nanocomposites obtained under different conditions are studied. The absorption spectra exhibit maxima at wavelengths of 420–450 nm related to the surface plasmon resonance of silver nanoparticles. IR spectra of the nanocomposites prove the formation of polyacrylonitrile in the course of the photopolymerization of monomer. The formation of metal nanoimpurities in polymer matrix leads to an increase in the intensity of photoluminescence and Raman scattering of polyacrylonitrile.     

Real three-dimensional microstructures fabricated by photopolymerization of resins through two-photon absorption

Effective energy windows for two-photon absorption (TPA) photopolymerization of resins were investigated and, with a properly selected laser pulse energy, exquisite three-dimensional (3D) microstructures with submicrometer spatial resolution were achieved. The results show the inherent utility of TPA in the fabrication of real 3D patterns. In particular, we propose and utilize a resin pre-exposure technique by which freely movable components affixed to an axle are built, demonstrating a new application of TPA in laser microfabrication.     

Formation of nano-textured conical microstructures in titanium metal surface by femtosecond laser irradiation

We report for the first time that a regular array of sharp nano-textured conical microstructures are formed on the titanium metal surface by irradiation with ultrafast laser pulses of 130 fs duration, 800 nm wavelength in vacuum (∼1 mbar) or in 100 mbar He. The microstructures are up to 25 μm tall, and taper to about 500 nm diameters at the tip. Irradiation in the presence of SF₆, air or HCl creates a textured surface but does not create sharp conical microstructures. The surfaces of these microstructures exhibit periodic nano-texture of feature size comparable to the wavelength of light consistent with ripple formation. Contrary to pillar formation by femtosecond laser irradiation of silicon where the initial ripples evolve into the pillars and the ripples disappear, the ripples on titanium pillars have a much smaller periodicity than the pillars and remain on the surface of the pillars. The textured surface is pitch black compared to its original silver-grayish color, i.e, it exhibits greatly reduced reflectivity throughout the measured visible spectrum. PACS 52.38.Mf; 79.20.Ds; 81.07.-b; 81.16.-c; 82.53.-k     

Surface plasmon resonance spectroscopy on rotated sub-micrometer polymer gratings generated by UV-laser based two-beam interference

Two-beam interference method was applied to generate gratings having periods of 416 nm and 833 nm by the forth harmonic of a Nd:Yag laser on thin poly-carbonate films spin-coated onto silver layer-covered substrates. The dependence of the modulation depth on the fluence and number of laser pulses was investigated by atomic force microscopy. A secondary pattern appeared on very thin polymer layers thanks to the “p” polarized laser beam illumination induced self-organized processes. The conditions of the emergence of grating-coupling caused additional plasmon resonance peak were determined for the sub-micrometer periodic polymer gratings. Surface plasmon resonance measurements were performed in attenuated total reflection arrangement to determine the effect of the angle between the plasmon propagation direction and the polymer groves on the grating-coupling. The effect of the modulation depth on the grating-coupling caused additional resonance minimum was also analyzed. We found coupling phenomena according to our calculations, the differences between the measured and theoretically predicted resonance curves were explained by the scattering on the complex surface structure.     

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.     

亚微米结构的可见光聚合全息制作

报道用激光全息技术结合可见光光聚合制作周期结构的方法和物理机理;对多束激光相干产生的空间干涉光场进行了讨论;提供了一组激光束的偏振态的最佳组合,使激光全息技术制作理想的亚微米单晶结构更有效和快捷.分析了曝光量与制作效果的关系,此法可实现小球自排无法实现的大幅度占空比调节.与紫外光光聚合相比,可见光光聚合可在监视下调节,操作方便安全. 关键词： 激光全息技术 光学晶格 光子晶体     

Observation of spontaneous pattern with six-fold symmetry in disk-shaped ZnO complex microstructures

Using a simple vapor phase transport technique, we have fabricated unique complex disk-shaped ZnO microstructures comprising a small disk coaxially grown on a large one and observed spatially perfect six-fold symmetric patterns. The observed results can be explained based on the spontaneous nanoindentation (NI) effect under the geometric constraints and the explanation can be extended to fathom the growth mechanism of other highly symmetrical ZnO nanostructures. Our results indicate that NI not only can elucidate the mechanical properties of surfaces and thin films but also is an effective approach to fabricate ordered nanostructures with high precision on the location of the building blocks. PACS 81.16.Rf; 81.07.-b; 81.05.Dz     

Plasmonic resonance enhancement of single gold nanorod in two-photon photopolymerization for fabrication of polymer/metal nanocomposites

We investigated the plasmonic resonance enhanced two-photon photopolymerization (PETPP) using the isolated chemical synthesized gold nanorods for fabrication of polymer/metal nanocomposites. The isolated gold nanorods with the plasmonic resonance band around 750 nm covered by photoresist were irradiated by a femtosecond laser with the wavelength of 780 nm. The PETPP trigged by the plasmonic resonance enhancement of gold nanorods was localized only in the distance smaller than 30 nm from the surface of gold nanorods, which matched the distance of plasmonic resonant enhanced field of the gold nanorod. The shapes of obtained polymer/gold nanocomposites were changed from the “dumbbell” to the “ellipsoid” with the increase of laser irradiating intensity used for PETPP. This study would provide a potential method for fabricating the plasmonic nanomaterials and nanostructures of polymer/metal nanocomposites, which could be expected to be applied in the emerging fields such as nanophotonics, nanobiosensor, nanolithography.     

High speed sub-micrometric microscopy using optical polymer microlens

We report the high speed scanning submicronic microscopy （SSM） using a low cost polymer microlens integrated at the extremity of an optical fiber. These microlenses are fabricated by a free-radical photopolymerization method. Using a polymer microlens with a radius of curvature of 250 nm, a sub-micrometric gold pattern is imaged experimentally by SSM. Different distances between the tip and the sample are used with a high scanning speed of 200 cm/s. In particular, metallic absorption contrasts are described with an optical spatial resolution of 250 nm at the wavelength of 532 nm. Moreover, finite-difference time-domain （FDTD） simulations concerning the focal lengths of microlenses with different geometries and heights support the experimental data.     

Photo-chemical synthesis and deposition of noble metal nanoparticles

Highly dispersed nanoparticles of transition and noble metals are utilized for hydrocarbon reactions and rearrangements important to the chemical industry. The need to obtain 1 to 3 nm particles with narrow size distributions has prompted the development of alternative processing methods. In this paper, a novel, dry method to synthesize nanoparticles from a frozen salt solution is reported. Pd nanoparticles were synthesized by photo-chemical decomposition of palladium acetate (PdAc) within a host matrix of chloroform using an excimer laser operating at 248 nm. Frozen composite targets were ablated at fluences ranging from 0.25 J/cm² to 0.75 J/cm² at a processing pressure of 10 mTorr. The ejected nanoparticles were deposited on continuous carbon coated and lacey carbon transmission electron microscopy (TEM) grids at ambient temperature. Characterization was performed by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDXS). High-resolution TEM analysis showed definitive evidence that the size distributions of the nanoparticles were narrow, exhibiting mean diameters ranging from 2.15 nm to 2.62 nm. PACS 81.15.Fg; 68.37.Lp; 81.07.Wx; 81.07.Bc; 81.10.Dn     

Polymer gratings based on photopolymerization for low-order distributed feedback polymer lasers

Novel polymer distributed feedback(DFB)gratings are fabricated based on photopolymerization to reduce lasing threshold of polymer lasers.A photopolymer formulation sensitive to 355-nm ultraviolet(UV)light is proposed for the fabrication of polymer gratings and it can be used to form polymer films by spin-coating process.A very low surface-relief depth ranging from 12.5 to about 1.0 nm has been demonstrated with a refractive-index modulation of about 0.012.The experimental results indicate that such polymer gratings have promising potentials for the fabrication of low-order DFB organic semiconductor lasers.     

Micromould based laser shock embossing of thin metal sheets for MEMS applications

Laser shock forming is a new material processing technology. Micro-channel with dimension of 260 μm × 59 μm was successfully fabricated on metallic foil surface using laser-generated shock wave. The work piece has a high spatial resolution at the micron-level. A series of experiments was conducted to validate the finite element model. An analysis procedure including dynamic analysis performed by ANSYS/LS-DYNA and static analysis performed by ANSYS is presented in detail to attain the simulation of laser shock embossing to predict the surface deformation. Micromould based laser shock embossing holds promise for achieving precise, well-controlled, low-cost, high efficiency of three-dimensional metallic microstructures. In addition, this technique can fabricate complex 3D microstructures directly by single pulse.     

Size and aerial density distributions of Ge nanocrystals in a SiO2 layer produced by molecular beam epitaxy and rapid thermal processing

We discuss the distribution of size and aerial density of Ge nanocrystals in a metal-oxide-semiconductor (MOS) memory structure fabricated by molecular beam epitaxy combined with rapid thermal processing; the size and aerial density of Ge nanocrystals are controlled by varying the thickness of the deposited Ge layer and the processing time. Variation of tunnel oxide thickness is demonstrated with the extension of the processing time. The effect of processing time and tunnel oxide thickness on the electrical properties of the MOS structures is investigated by high frequency capacitance–voltage measurements. PACS  61.46.+w; 81.07.-b; 81.07.Bc; 81.07.Ta     

Morphology of Si nanowires fabricated by laser ablation using gold catalysts

Si nanowires (NWs) were fabricated successfully by laser ablation using Au as catalyst. Si wafers were used as the collector. The diameters of Si NWs ranged from 20 to 150 nm. Different forms of Si NWs were observed at different local sites inside a furnace: Si NWs with a high aspect ratio of length to diameter, Si NWs with defects and Si NWs with Au-containing nanoparticles being embedded. Especially, a nano-particle embedded Si NW is a new nanostructure that is observed for the first time. PACS 81.07.-b; 81.07.Bc; 81.16.-c; 81.20.-n     

Photopolymerization in water of diacetylenes chemisorbed onto noble metal nanoparticles: a spectroscopic study

Gold and silver nanoparticles of different size, decorated by a diacetylenic monomer having a COOH anchor group, were prepared in aqueous dispersions. Topochemical polymerization of the colloidal suspensions was obtained by UV irradiation and studied by means of spectroscopic techniques (UV–visible absorption, IR and Raman spectroscopies). A variety of polymer phases were found in the case of silver nanohybrids. The main contribution for this investigation was supplied by the Raman spectra, which provide detailed information on the polymer forms and on the preferred conformation of the alkylic side chains. Copyright © 2009 John Wiley & Sons, Ltd.     

Direct micropatterning of polymer materials by ice mold

Micropatterning of functional polymer materials by micromolding in capillaries (MIMIC) with ice mold is reported in this paper. Ice mold was selected due to its thaw or sublimation. Thus, the mold can be easily removed. Furthermore, the polymer solution did not react with, swell, or adhere to the ice mold, so the method is suitable for many kinds of materials (such as P3HT, PMMA Alq₃/PVK, PEDOT: PSS, PS, P2VP, etc.). Freestanding polymer microstructures, binary polymer pattern, and microchannels have been fabricated by the use of ice mold freely.     

Formation of silver particles and silver oxide plume nanocomposites upon pulsed-laser induced liquid-solid interface reaction

Nanodendrites consisting of silver and silver oxide are grown upon a pulsed-laser induced reaction at the interface between the solid target and silver nitrate solution. By using a high-resolution transmission electron microscope (HRTEM) equipped with an energy dispersive X-ray spectrometer (EDS) and selected area diffraction (SAD), the fabricated nanopatterns are identified to be a composite structure that consisting of silver nanoparticles and silver oxide nanoplumes with polycrystalline structure. In detail, these silver nanocrystals are trunks of the nanodendrite, and their size is in the range of 30 to 50 nm. The silver oxide nanoplumes are branches of the nanodendrite, and their width and length are in the ranges of 20 to 50 nm and 30 to 100 nm, respectively. Additionally, we suggested a vapor-liquid mechanism for the formation of the nanopatterns using a pulsed-laser induced liquid-solid interface reaction, in which both silver clusters in vapor and silver ions in solution are simultaneously involved.Received: 6 April 2004, Published online: 5 November 2004PACS: 81.15.Fg Laser deposition - 61.46. + w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 81.07.Bc Nanocrystalline materials