Excimer laser projection micromachined free-standing polymer films

Projection etching techniques using 308 nm and 248 nm excimer laser illumination have been investigated as a means of micromachining thin free-standing polymer films. Consideration is given to the choice of operating parameters (wavelength, fluence) to minimise damage to the machined structures. High quality grid patterns have been produced on free-standing mylar films with a thickness down to 1.5 μm.     

Influence of the substrate hydrophilicity on the grid assisted deposition of tris-(8-hydroxyquinolinato) aluminum(III) thin films

The influence of the substrate wetting properties on the grid assisted deposition of tris-(8-hydroxyquinolinato)aluminum(III) (Alq₃) onto Si/SiO_x surfaces was investigated. Different degrees of hydrophilicity/hydrophobicity on the Si/SiO_x were obtained by changing the surface chemical functionalities with wet treatments. We observed that the deposited Alq₃ films can be spatially controlled and assembled either into continuous grid-like stripes or ordered dots depending upon the wetting properties of the substrate.     

Pulsed laser deposition of NiTi shape memory effect thin films

2 O₃(100) substrates. We also produced free-standing NiTi films by deposition on KBr substrates and subsequent substrate removal by immersion in water. The presence of the solid-solid phase transformation responsible for the shape memory effect has been demonstrated through temperature-dependent X-ray diffraction and four-probe resistance versus temperature measurements. On cooling the deposited film, the austenite-martensite transformation was measured at around 195 K; on heating the film the reverse transformation was around 250 K. Evidence of the shape-memory effect for free-standing films was obtained in a bending deformation-shape recovery experiment. Received: 31 July 1996/Accepted: 6 January 1997     

Electrochemical fabrication of a novel conducting polythiophene film junction

A compact conductive polythiophene (PT) film junction was prepared by potential controlled electrochemical doping after electropolymerization of thiophene. The polythiophene film was cation-doped on one side, while its other side was anion-doped, which resulted in a polythiophene p-n junction film diode. The free-standing polythiophene film junction diode was flexible and was 1.5 times stronger than aluminum metal. After treatment by a strong electric field, the polythiophene p-n junction exhibits a novel electric property like an intelligent electric switch.     

图形硅衬底GaN基发光二极管薄膜去除衬底及AlN缓冲层后单个图形内微区发光及 应力变化的研究

研究了图形硅衬底上外延生长的氮化镓(GaN)基发光二极管(LED)薄膜、去除硅衬底后的无损自由状态LED薄膜以及去除氮化铝(AlN)缓冲层后的自由状态LED薄膜单个图形内的微区光致发光(PL)性能, 用荧光显微镜与扫描电镜观测了去除AlN缓冲层前后LED薄膜断面弯曲状况的变化. 研究结果表明: 1)去除硅衬底后, 自由支撑的LED薄膜朝衬底方向呈柱面弯曲状态, 且相邻图形的柱面弯曲方向不一致, 当进一步去除AlN缓冲层后薄膜会由弯曲变为平整; 2)LED薄膜在去除硅衬底前后同一图形内不同位置的PL谱具有显著差异, 而当去除AlN缓冲层后不同位置的PL谱会基本趋于一致; LED薄膜每一位置的PL 谱在去除硅衬底后均出现明显红移, 进一步去除AlN缓冲层后PL谱出现程度不一的微小蓝移; 3)自由支撑的LED薄膜去除AlN缓冲层后, PL光强随激光激发密度变化的线性关系增强, 光衰减得到改善.     

Electronic properties and interfacial coupling in Pb islands on single-crystalline graphene

Introducing metal thin films on two-dimensional (2D) material may present a system to possess exotic properties due to reduced dimensionality and interfacial effects. We deposit Pb islands on single-crystalline graphene on a Ge(110) substrate and studied the nano- and atomic-scale structures and low-energy electronic excitations with scanning tunneling microscopy/spectroscopy (STM/STS). Robust quantum well states (QWSs) are observed in Pb(111) islands and their oscillation with film thickness reveals the isolation of free electrons in Pb from the graphene substrate. The spectroscopic characteristics of QWSs are consistent with the band structure of a free-standing Pb(111) film. The weak interface coupling is further evidenced by the absence of superconductivity in graphene in close proximity to the superconducting Pb islands. Accordingly, the Pb(111) islands on graphene/Ge(110) are free-standing in nature, showing very weak electronic coupling to the substrate.     

Electron beam nanofabrication of ferromagnetic nanostructures in TEM

Electron beam (e-beam) fabrication of nanostructures by transmission electron microscopy (TEM) is rapidly developing into a top-down nanofabrication method for the sub-5 nm fabrication of structures that cannot usually be realised using resist based lithographic techniques or by the focused ion beam patterning methods. We describe the usage of a variety of e-beam shapes, including point and elliptical line focus, as well as a comparison of LaB₆ and field-emission guns (FEGs), to achieve versatile sculpting of nanodot arrays, nanobridges and nanotips. We operate our patterning on free-standing nickel (Ni) thin film laterally connected to a silicon (Si) substrate as well as to free-standing Ni nanotips, where we achieve a novel three-dimensional (3D) nano-sculpting methodology.     

Parallel laser microfabrication of terahertz metamaterials and its polarization-dependent transmission property

Large-area split ring resonator (SRR) array was fabricated by femtosecond laser micro-lens array lithography at a fast speed. Transmission spectra of the SRR arrays at different incident terahertz wave polarization states were characterized by terahertz time domain spectroscopy. A polarization-dependent transmission property of the SRR array was observed. Polarization-dependent loss (PDL) spectrum was characterized to investigate the polarization properties of the terahertz metamaterials. The PDL characterization can eliminate the substrate effect and provide a flexible platform to study the characteristics of free-standing terahertz metamaterials.     

Transfer bonding technology for batch fabrication of SMA microactuators

Currently, the broad market introduction of shape memory alloy (SMA) microactuators and sensors is hampered by technological barriers, since batch fabrication methods common to electronics industry are not available. The present study intends to overcome these barriers by introducing a wafer scale transfer process that allows the selective transfer of heat-treated and micromachined shape memory alloy (SMA) film or foil microactuators to randomly selected receiving sites on a target substrate. The technology relies on a temporary adhesive bonding layer between SMA film/foil and an auxiliary substrate, which can be removed by laser ablation. The transfer technology was tested for microactuators of a cold-rolled NiTi foil of 20 μm thickness, which were heat-treated in free-standing condition, then micromachined on an auxiliary substrate of glass, and finally selectively transferred to different target substrates of a polymer. For demonstration, the new technology was used for batch-fabrication of SMA-actuated polymer microvalves.     

Characterization of free-standing GaN substrate grown through hydride vapor phase epitaxy with a TiN interlayer

Large-scale GaN free-standing substrate was obtained by hydride vapor phase epitaxy directly on sapphire with porous network interlayer. The bottom surface N-face and top surface Ga-face showed great difference in anti-etching and optical properties. The variation of optical and structure characteristics were also microscopically identified using spatially resolved cathodoluminescence and micro-Raman spectroscopy in cross-section of the GaN substrate. Three different regions were separated according to luminescent intensity along the film growth orientation. Some tapered inversion domains with high free carrier concentration of 5 × 10¹⁹ cm⁻³ protruded up to the surface forming the hexagonal pits. The dark region of upper layer showed good crystalline quality with narrow donor bound exciton peak and low free carrier concentration. Unlike the exponential dependence of the strain distribution, the free-standing GaN substrate revealed a gradual increase of the strain mainly within the near N-polar side region with a thickness of about 50 μm, then almost kept constant to the top surface.     

The fabrication technique and electrical properties of a free-standing GaN nanowire

We fabricated a free-standing structure of a GaN nanowire by selectively etching Si₃N₄, previously grown on a SiO₂ substrate, for application to three-dimensional integrated circuits such as nanorelays and actuators. In the nanowire-deposition process we adopted electrophoresis and reactive ion etching techniques to achieve a well-aligned and free-standing nanowire. The electrical transport measurements were performed from room temperature down to liquid-nitrogen temperature. The current–voltage (I–V) characteristics showed a rectifying behavior in the whole temperature range. We analyze this property as a Schottky barrier formation between the nanowire and electrodes. PACS  61.46.+w; 73.22.-f; 73.40.Ei; 81.07.Bc; 81.16.Rf     

Inelastic light scattering and X-ray diffraction from thick free-standing porous silicon films

The results of Raman scattering and X-ray diffraction studies of thick, free-standing, porous Si layers with thickness up to 500 μm are presented. The Raman scattering spectra have a distinctive difference from previous data for porous Si films on Si substrate and for thin, free-standing, porous Si layers. The experimental data can be explained by a modified phonon confinement model that accounts for a comprehensive strained Si nanocrystal. The comprehensive strain is a tensile one, and the value of stress can be up to 3 GPa. This interpretation is supported by data of X-ray diffraction measurements.     

On the charge transfer in the “single-sheet graphene-intercalated metal layer-SiC substrate” system

The proposed scheme for the consideration of charge transfer in the three-layer Gr/Me/SiC system (where Gr is a single-sheet graphene, Me is an intercalated metal layer, and SiC is a substrate) contains three stages. At the first stage, a metal monolayer adsorbed on silicon carbide is considered and the charge of adatoms in this monolayer is calculated. At the second stage, the shift of the Dirac point of free-standing single-layer graphene in an electrostatic field induced by charged adatoms of the monolayer is estimated. At the third stage, a weak interaction between Me/SiC and free-standing graphene is included, which allows electrons to tunnel but does not significantly distort the density of states of free-standing graphene. Estimations are performed for n- and p-type 6H-SiC(0001) substrates and Cu, Ag, and Au layers. The charge state of the graphene sheet and the shift of the Dirac point with respect to the Fermi level of the system are calculated. A comparison with the available experimental and theoretical results shows that the proposed scheme works quite satisfactorily.     

First-principles study of graphyne on SiO2

Employing first-principles density-functional calculations, we study the rectangularly symmetric 6,6,12-graphyne deposited on an SiO₂ surface. We show that, on a substrate without H-passivation, the carbon layer (distorted from the planar structure) is covalently bonded to the substrate and, on a substrate with H-passivation, the graphyne (still keeping the planar structure) exhibits a weak van der Waals (vdW) bonding to the underlying structure. But the characteristic spectrum of the free-standing graphyne does not appear in both cases. The results suggest that, even the graphyne in planar geometry, a small planar irregular deformation induces the gap opening at the Dirac points.     

毫米级栅型电场分布FEP薄膜驻极体的制备及其电荷存储性能研究

采用电晕注极和热注极技术, 在厚度为25 μm的氟化乙丙烯共聚物(FEP)表面制备了宽度为2 mm和3 mm的具有栅型电场分布的驻极体, 研究了注极温度和电极宽度对其电荷存储性能的影响. 样品注极后经150天的存储, 栅型电场分布变得清晰而有规律, 覆盖铝电极区电位已衰减至接近零, 未覆盖铝电极区仍保持高电位; 对电极宽度为2 mm和3 mm的样品, 覆盖铝电极区与未覆盖铝电极区的表面电位差分别为110 V和130 V(电场强度差分别为44 kV/cm和52 kV/cm). 表面电位跟踪测试结果表明: 电晕注极样品初始表面电位高于热注极样品; 在相同的注极方法下, 注极温度越高初始表面电位越高, 电极宽度越小初始表面电位越低. 依据电晕注极和热注极原理对实验结果的分析表明, FEP和金属铝在电荷存储性能上的差异是FEP表面蒸镀铝电极后能获得栅型电场分布的原因所在.     

Enhanced energy storage behaviors in free-standing antiferroelectric Pb(Zr_(0.95)Ti_(0.05))O_3 thin membranes

Free-standing antiferroelectric Pb(Zr_(0.95)Ti_(0.05)O_3(PZT(95/5)) thin film is fabricated on 200-nm-thick Pt foil by using pulsed laser deposition.X-ray diffraction patterns indicate that free-standing PZT(95/5) film possesses an α-axis preferred orientation.The critical electric field for the 300-nm-thick free-standing PZT(95/5) film transiting from antiferroelectric to ferroelectric phases is increased to 770 kV/cm,but its saturation polarization remains almost unchanged as compared with that of the substrate-clamped PZT(95/5) film.The energy storage density and energy efficiency of the substrate-clamped PZT(95/5) film are 6.49 J/cm~3 and 54.5%,respectively.In contrast,after removing the substrate,the energy storage density and energy efficiency of the free-standing PZT(95/5) film are enhanced up to 17.45 J/cm~3 and 67.9%,respectively.     

Graphene oxide templated alumina nanosheet for the removal of As(V)

The current paper demonstrates a simple preparative method of alumina nanosheet through controlled hydrolysis and condensation on the surface of graphene oxide (GO). Taking into account the moisture sensitivity of the alumina precursor, its hydrolysis was carried out after introduction of interaction between the functional groups of GO and the alumina precursor in a non-aqueous solvent. On pyrolysis of the aluminum oxide-GO composite, a free-standing alumina nanosheet was obtained, which was further confirmed by XRD, TEM, STEM-EDX, FE-SEM, TGA, and N₂ physisorption. Due to a favorable nanosheet structure, the alumina nanosheets showed a better performance in the removal of As (V) when compared to alumina prepared without GO template, even though the latter had a larger surface area than the alumina nanosheet.     

Aluminum nitride substrates for ultraviolet light-emitting diode structures

Currently, wafers of aluminum nitride cut from bulk aluminum nitride crystals (AlN) grown by sublimation are considered promising substrates for obtaining light-emitting diode structures based on nitrides of the third group. In this study, the structural characteristics and electrical properties of AlN, as a prospective substrate material for light-emitting diode heterostructures based on AlGaN/GaN, were investigated. The substrate working surface ((0001) plane, Al-polar) was specifically prepared for epitaxial growth using chemical-mechanical polishing. The surface roughness (“epi-ready”), as estimated by atomic force microscopy, did not exceed 0.3 nm.     

Fabrication and Characterization of Large Free-Standing Polarizer Grids for Millimeter Waves

Construction of large aperture free-standing metal wire grids is demonstrated for the lower end of the millimeter wave spectral region. For the constructed two grids the co-polarized and cross-polarized components of transmitted power were measured at 45° oblique incidence. The measurements were performed as a function of wire orientation angle and in more detail at selected angles. The results are in good agreement with the theoretical results presented in the literature. In order to save time and costs the construction apparatus was simplified from those reported previously by other authors. It was shown that for this frequency range the grid characteristics are not degraded when such an apparatus is applied. One of the constructed grids will be used in a calibration system of the Helsinki University of Technology (HUT) polarimetric radiometer.     

Wide band measurement of power transmission coefficients and polarizing efficiency of free standing wire grids in the far infrared

In this paper we describe wide band measurements of power transmission coefficients and the degree of polarization of a set of free-standing wire grids in the far infrared. The measurements have been performed in two different spectral regions, with the grid parameter d/g ranging between 0.4 and 0.003. The results are compared with theoretical predictions and the accuracy of the experimental method is fully discussed.