Liquid-crystalline ordering helps block copolymer self-assembly

Interaction between liquid-crystalline elastic deformation and microphase separation in liquid-crystalline block copolymers enables them to supramolecularly assemble into ordered nanostructures with high regularity. With the help of liquid-crystalline alignment, parallel and perpendicular patterning of nanostructures is fabricated with excellent reproducibility and mass production, which provides nanotemplates and nanofabrication processes for preparing varieties of nanomaterials. Furthermore, nanoscale microphase separation improves the optical performance of block-copolymer fi lms by eliminating the scattering of visible light, leading to advanced applications in optical devices and actuators. Recent progress in liquid-crystalline block copolymers, including their phase diagram, structure-property relationship, nanostructure control and nanotemplate applications, is reviewed.     

Integration of coplanar (Ba,Sr)TiO3 microwave phase shifters onto Si wafers using TiO2 buffer layers.

In this paper, a Ba0.6Sr0.4TiO3 (BST) tunable phase shifter with TiO2 films as microwave buffer layer between BST and silicon (Si) substrates is presented. The TiO2 buffer layer is grown by atomic layer deposition (ALD) onto Si substrate followed by pulsed laser deposition (PLD) of BST thin films onto the TiO2 buffer layer. The phase shifter fabricated on BST films grown on TiO2/Si substrate shows a good figure of merit (FOM) of 75.4 degrees/dB by exhibiting improved tunablity while retaining an appropriate dielectric Q as compared to 55.1 degrees/dB of BST/MgO structure. The TiO2 buffer layer grown by ALD enables successful integration of BST-based microwave tunable devices with high resistive Si wafer.     

Integration of coplanar (Ba,Sr)TiO/sub 3/ microwave phase shifters onto Si wafers TiO/sub 2/ buffer layers

In this paper, a Ba/sub 0.6/Sr/sub 0.4/TiO/sub 3/ (BST) tunable phase shifter with TiO/sub 2/ films as microwave buffer layer between BST and silicon (Si) substrates is presented. The TiO/sub 2/ buffer layer is grown by atomic layer deposition (ALD) onto Si substrate followed by pulsed laser deposition (PLD) of BST thin films onto the TiO/sub 2/ buffer layer. The phase shifter fabricated on BST films grown on TiO/sub 2//Si substrate shows a good figure of merit (FOM) of 75.4/spl deg//dB by exhibiting improved tunability while retaining an appropriate dielectric Q as compared to 55.1/spl deg//dB of BST/MgO structure. The TiO/sub 2/ buffer layer grown by ALD enables successful integration of BST-based microwave tunable devices with high resistive Si wafer.     

Nanostructuring of Nickel Hydroxide via a Template Solution Approach for Efficient Electrochemical Devices

Nanostructuring is a key approach in enhancing the performance of electrochemical devices. In this work, nanostructuring is achieved by the electrodeposition of nickel hydroxide nanowire arrays, with both open‐ended and close‐ended structures, through anodized aluminium oxide (AAO) templates that are directly fabricated on indium tin oxide/glass substrates. The open‐ended and close‐ended nanostructures are compared together with identically fabricated thin films to show the effects of nanostructuring. Open‐ended nanowire arrays demonstrated the best electrochemical activity with superior transmittance modulation and faster activation, while the thin film showed the worst performance. In comparing with the close‐ended structures, enhanced performance is observed for the open‐ended structures despite the use of less material for the latter. This demonstrates that in designing nanostructures or porous materials, it is important for the porosity to have both interconnectivity and exposure to the electrolyte in electrochemical reactions.     

Vertically aligned one-dimensional AlN nanostructures on conductive substrates: Synthesis and field emission

Herein we report the synthesis of vertically aligned AlN nanostructures on conductive substrates through the chemical reaction between AlCl₃ and NH₃ in the temperature range of 650-850 °C. The morphologies of the AlN nanostructures could be controllably modulated from cone-like to rod-like geometries by increasing the reaction temperature. The formation mechanism of the AlN nanostructures on the nitrified Ti substrates has been discussed based on the analysis of the intermediate products. The field emission (FE) property of AlN nanocones grown on the nitrified Ti substrate is better than that for AlN nanocones on Si substrate. The improvement of FE property can be attributed to the lower resistance between AlN nanocones and the nitrified Ti substrate because the conductive titanium nitride film can directly contact with AlN emitters while a high-resistive silica layer would easily form between Si substrate and AlN nanocones. These results indicate that the deposition of nanoscale filed emitters on conductive substrates is an effective way to improve the FE behavior, and may find potential applications in FE devices.     

TEM-based metrology for HfO2 layers and nanotubes formed in anodic aluminum oxide nanopore structures

Nanotubes are fabricated by atomic layer deposition (ALD) into nanopore arrays created by anodic aluminum oxide (AAO). A transmission electron microscopy (TEM) methodology is developed and applied to quantify the ALD conformality in the nanopores (thickness as a function of depth), and the results are compared to existing models for ALD conformality. ALD HfO2 nanotubes formed in AAO templates are released by dissolution of the Al2O3, transferred to a grid, and imaged by TEM. An algorithm is devised to automate the quantification of nanotube wall thickness as a function of position along the central axis of the nanotube, by using a cylindrical model for the nanotube. Diffusion-limited depletion occurs in the lower portion of the nanotubes and is characterized by a linear slope of decreasing thickness. Experimentally recorded slopes match well with two simple models of ALD within nanopores presented in the literature. The TEM analysis technique provides a method for the rapid analysis of such nanostructures in general, and is also a means to efficiently quantify ALD profiles in nanostructures for a variety of nanodevice applications.     

Fabrication of a one-dimensional array of nanopores horizontally aligned on a Si substrate

A one-dimensional array of nanopores horizontally aligned on a silicon substrate was successfully fabricated by anodic aluminum oxidation (AAO) using a modified two-step procedure. SEM pictures show clear nanostructures of well-aligned one-dimensional nanopore arrays without cracks at the interfaces of the sandwiched structures. The processes are compatible with the planar silicon integrated circuit processing technology, promising for applications in nanoelectronics. The formation mechanism of a single nanopore array on Si substrates was also discussed.     

In situ growth of single-crystal TiO2 nanorod arrays on Ti substrate: Controllable synthesis and photoelectrochemical water splitting

Despite one-dimensional (1D) semiconductor nanostructure arrays attracting increasing attention due to their many advantages,highly ordered TiO2 nanorod arrays (TiO2 NR) are rarely grown in situ on Ti substrates.Herein,a feasible method to fabricate TiO2 NRs on Ti substrates by using a through-mask anodization process is reported.Self-ordered anodic aluminum oxide (AAO) overlaid on Ti substrate was used as a nanotemplate to induce the growth of TiO2 NRs.The NR length and diameter could be controlled by adjusting anodization parameters such as electrochemical anodization voltage,anodization time and temperature,and electrolyte composition.Furthermore,according to the proposed NR formation mechanism,the anodized Ti ions migrate and deposit in the AAO nanochannels to form Ti(OH)4 or amorphous TiO2 NRs under electric field,owing to the confinement effect of the template.Photoelectrochemical tests indicated that,after hydrogenation,the TiO2 NRs presented higher photocurrent density under simulated sunlight and visible light illuminations,suggesting their potential use in photoelectrochemical water splitting,photocatalysis,solar cells,and sensors.     

Thermal stability of atomic layer deposited Ru layer on Si and TaN/Si for barrier application of Cu interconnection

The thermal stability of thin Ru single layer and Ru/TaN bilayers grown on bare Si by plasma enhanced atomic layer deposition (PEALD) have been studied with Cu/Ru, Cu/Ru/TaN structures as a function of annealing temperature. To investigate the characteristics as a copper diffusion barrier, a 50 nm thick Cu film was sputtered on Ru and Ru/TaN layers and each samples subjected to thermal annealing under N2 ambient with varied temperature 300, 400, and 500 degrees C, respectively. It was found that the single 5 nm thick ALD Ru layer acted as an effective Cu diffusion barrier up to 400 degrees C. On the other hand ALD Ru (5 nm)/TaN (3.2 nm) showed the improved diffusion barrier characteristics even though the annealing temperature increased up to 500 degrees C. Based on the experimental results, the failure mechanism of diffusion barrier would be related to the crystallization of amorphous Ru thin film as temperature raised which implies the crystallized Ru grain boundary served as the diffusion path of Cu atoms. The combination of ALD Ru incorporated with TaN layer would be a promising barrier structure in Cu metallization.     

Structure of post-annealed ferroelectric PbZrxTi1-xO3 and SrBi2Ta2O9 thin films

In order to fabricate good quality ferroelectric thin films, PbZr_xTi_(1-x)O₃ (PZT) and SrBi₂Ta₂O₉ (SBT) films were fabricated on SiO₂/Si(100) substrates and on Pt/Ti/ SiO₂/Si(100) substrates by pulsed laser excimer deposition (PLD). X-ray diffraction, Rutherford backscattering analysis, and atomic force microscopy were used to characterize the structural properties of the samples, which were post-annealed at different temperatures. The results showed that the PZT and SBT films fabricated on Pt/Ti/SiO₂/Si(100) substrates and annealed at 700 °C exhibited optimum properties.     

Fabrication and optical property of vertically-aligned ZnO/Si double nanostructures

We fabricated the vertically-aligned zinc oxide (ZnO)/silicon (Si) double nanostructures by simple processes using the metal-assisted chemical etching and a subsequent hydrothermal synthesis, and their optical property was investigated. For efficient antireflection characteristics, Si nanostructures were optimized by changing the size of the dewetted silver (Ag) at different etching times. The thermally dewetted Ag nanoparticles or semi-island films as metal catalysts were controlled by the Ag film thickness and dewetting temperature. To form the ZnO/Si double nanostructures, ZnO nanorods were synthesized on the chemically etched Si nanostructures using a thin sputtered ZnO seed layer. The grown ZnO nanorod arrays (NRAs) exhibited good crystallinity and further reduced the surface reflection due to their antireflective property. The ZnO/Si double nanostructures showed the increased peak intensity of X-ray diffraction as well as the significantly reduced solar weight reflectance of 6.05% compared to 11.71% in the ZnO NRAs on the flat Si substrate. Also, the enhanced antireflection property of ZnO/Si double nanostructures was theoretically analyzed by performing the rigorous coupled wave analysis simulation.     

Fabrication of flexible and vertical silicon nanowire electronics

Vertical silicon nanowire (SiNW) array devices directly connected on both sides to metallic contacts were fabricated on various non-Si-based substrates (e.g., glass, plastics, and metal foils) in order to fully exploit the nanomaterial properties for final applications. The devices were realized with uniform length Ag-assisted electroless etched SiNW arrays that were detached from their fabrication substrate, typically Si wafers, reattached to arbitrary substrates, and formed with metallic contacts on both sides of the NW array. Electrical characterization of the SiNW array devices exhibits good current-voltage characteristics consistent with the SiNW morphology.     

Optical and field-emission properties of ZnO nanostructures deposited using high-pressure pulsed laser deposition

ZnO nanostructures were deposited on GaN (0001), Al2O3 (0001), and Si (100) substrates using a high-pressure pulsed laser deposition (PLD) method. Vertically aligned hexagonal-pyramidal ZnO nanorods were obtained on the Al2O3 and Si substrates whereas interlinked ZnO nanowalls were obtained on the GaN substrates. A growth mechanism has been proposed for the formation of ZnO nanowalls based on different growth rates of ZnO polar and nonpolar planes. Both ZnO nanorods and nanowalls exhibit a strong E2H vibration mode in the micro-Raman spectra. The corresponding fluorescence spectra of ZnO nanorods and nanowalls showed near band emission at 3.28 eV. The ZnO nanorods grown on the Si substrates exhibited better crystalline and optical properties compared with the ZnO structures grown on the GaN and Al2O3 substrates. The high aspect ratio, good vertical alignment, and better crystallinity of the ZnO nanorods with tapered tips exhibited promising field emission performance with a low turn-on field of 2 V/μm, a high current density of 7.7 mA/cm2, and a large field enhancement factor.     

Precise control of highly ordered arrays of nested semiconductor/metal nanotubes

Lithographically defined microporous templates in conjunction with the atomic layer deposition (ALD) technique enable remarkable control of complex novel nested nanotube structures. So far three-dimensional control of physical process parameters has not been fully realized with high precision resolution, and requires optimization in order to achieve a wider range of potential applications. Furthermore, the combination of composite insulating oxide layers alternating with semiconducting layers and metals can provide various types of novel applications and eventually provide unique and advanced levels of multifunctional nanoscale devices. Semiconducting TiO₂ nanotubes have potential applications in photovoltaic devices. The combination of nanostructured semiconducting materials with nested metal nanotubes has the potential to produce novel multifunctional vertically-ordered three-dimensional nanodevices. Platinum growth by ALD has been explored, covering the initial stages of the thin film nucleation process and the synthesis of high aspect ratio nanotube structures. The penetration depth of the Pt into porous templates having various pore sizes and aspect ratios has been investigated. Several multi-walled nested TiO₂-Pt nanotubes in series have been successfully fabricated using microporous Si templates. These innovative nested nanostructures have the potential to produce novel multifunctional vertically-ordered three-dimensional nanodevices in photovoltaic and sensing technologies.      

Site-controlled fabrication of dimension-tunable Si nanowire arrays on patterned (001)Si substrates

In this study, we fabricated well-ordered arrays of site-controlled, vertically-aligned Si nanowires on the desired areas of pre-patterned (001)Si substrates by employing the nanosphere lithographic technique in combination with the Au-assisted selective etching process. The results of transmission electron microscopy and selected-area electron diffraction analysis show that the Si nanowires that fabricated on the patterned (001)Si substrates have a single-crystalline nature and form along the [001] direction. The length of the Si nanowires was found to increase linearly with the Au-assisted etching time. Scanning electron microscopy images clearly revealed that by adjusting the sizes of the nanosphere template and the etching temperature and time, the diameter and length of the patterned Si nanowires could be effectively tuned and accurately controlled. Furthermore, the diameters of the Si nanowires produced at various temperatures and time were found to be relatively uniform over the entire length. The combined approach presented here provides the capability to fabricate a variety of size-, length-tunable 1D Si-based nanostructures on various patterned Si-based substrates.     

Low cost fabrication of the large-area anti-reflection films from polymer by nanoimprint/hot-embossing technology

In this paper, we present a low cost approach to produce large-area polymer sheets with sub-wavelength nanostructures. The fabricated polymer films would have great potentials to attach to optical or solar-cell-related consumer products when anti-reflection/anti-glaring is mandatory. We employed a special electron cyclotron resonance (ECR) plasma process to fabricate the SWSs with large area directly on silicon substrates. Homogeneously distributed nanotips on the full 4 inch silicon substrate were fabricated by using gas mixtures of silane, methane, argon and hydrogen. An Ni-Co metal mold with a hardness of 550 HV was produced through the replication of the Si mold by electroplating. Afterwards, the molding process was applied to manufacture the nanostructures in PMMA plates in large volume. The nanostructures in PMMA plates with aspect ratios of 4 and diameters of 150?nm were fabricated. The fabricated PMMA sheets could generate the gradient of the refractive indices, absorb the light and greatly reduce the reflectivity. Compared with the PMMA without SWSs, the reflectivity of PMMA with SWSs decreased dramatically from 4.25% to 0.5% at the wavelength of light from 400 to 800?nm.     

Influences of Ti film thickness on electrochemical properties of Si/Ti/Cu film electrodes

Si and Si/Ti films were fabricated on a Cu current collector (substrate) using the DC sputtering system. The Ti film as a buffer layer was inserted between the Si film and the Cu current collector. Their structural and electrochemical properties were investigated with various Ti film thicknesses of 20-90 nm. The Si and Ti films deposited on a polycrystalline Cu substrate were amorphous. The Si/Ti/Cu film electrode exhibited better electrochemical properties than the Si/Cu electrode in terms of capacity, charge-discharge efficiency, and cycleability. In the Si/Ti/Cu electrode, the film electrode with a 55 nm Ti film thickness showed the best electrochemical properties: 367 microA h/cm2 initial capacity, 91% efficiency, and 50% capacity retention after 100 cycles. These good electrochemical properties are attributed to the enhanced adhesion between the Si and Ti films. Additionally, the modified surface morphology of Si film with a cluster structure could withstand the lateral volume change during the charge-discharge process.     

Fabrication of superhydrophobic surfaces by a Pt nanowire array on Ti/Si substrates

Superhydrophobic surfaces were prepared on Ti/Si substrates via the fabrication of a platinum (Pt) nanowire array. The Pt nanowire array was obtained by dc electrodeposition of Pt into the pores of an anodic aluminium oxide (AAO) template on the substrate followed by the removal of the template. Transmission electron microscopy (TEM) examination demonstrated that all the nanowires have uniform diameter of about 30?nm. Field emission scanning electron microscopy (FE-SEM) showed that the structures at both the micrometre scale and nanometre scale bestowed the prerequisite roughness on the surfaces. The chemical surface modification made the Pt nanowire array superhydrophobic. The surface modified Pt nanowire array exhibited superhydrophobicity even in corrosive solutions over a wide pH range, such as acidic or basic solutions. The results demonstrated that the Pt nanowire array will have good potential applications in the preparation of superhydrophobic surfaces.     

Atomic layer deposition of lead sulfide quantum dots on nanowire surfaces

Quantum dots provide unique advantages in the design of novel optoelectronic devices owing to the ability to tune their properties as a function of size. Here we demonstrate a new technique for fabrication of quantum dots during the nucleation stage of atomic layer deposition (ALD) of PbS. Islands with sub-10 nm diameters were observed during the initial ALD cycles by transmission electron microscopy, and in situ observations of the coalescence and sublimation behavior of these islands show the possibility of further modifying the size and density of dots by annealing. The ALD process can be used to cover high-aspect-ratio nanostructures, as demonstrated by the uniform coating of a Si nanowire array with a single layer of PbS quantum dots. Photoluminescence measurements on the quantum dot/nanowire composites show a blue shift when the number of ALD cycles is decreased, suggesting a route to fabricate unique three-dimensional nanostructured devices such as solar cells.     

Fabrication of highly ordered anodic aluminium oxide templates on silicon substrates

The controlled fabrication of highly ordered anodic aluminium oxide (AAO) templates of unprecedented pore uniformity directly on Si, enabled by new advances on two fronts - direct and timed anodisation of a high-purity Al film of unprecedented thickness (50 mum) on Si, and anodising a thin but pre-textured Al film on Si, has been reported. To deposit high-quality and ultra-thick Al on a non-compliant substrate, a prerequisite for obtaining highly ordered pore arrays on Si by self-organisation while retaining a good adhesion, a specially designed process of e-beam evaporation followed by in situ annealing has been deployed. To obtain an AAO template with the same high degree of ordering and uniformity but from a thin Al film, which is not achievable by the self-organisation alone, pre-patterning of the thin Al surface by reactive ion etching using a freestanding AAO mask that was formed in a separate process was performed. The resultant AAO/Si template provides a good platform for integrated growth of nanotube, nanowire or nanodot arrays on Si. Template-assisted growth of carbon nanotubes (CNTs) directly on Si was demonstrated via a chemical vapour deposition method. By controllably removing the AAO barrier layer at the bottom of the pores and partially etching back the AAO top surface, new CNT/Si structures were obtained with potential applications in field emitters, sensors, oscillators and photodetectors.