Large-area silica nanotubes with controllable geometry on silicon substrates

The synthesis of a highly uniform, large-scale nanoarrays consisting of silica nanotubes above embedded nanohole arrays in silicon substrates is demonstrated. In situ anodized aluminium oxide (AAO) thin film masks on Si substrates were employed, and the nanotubes were fabricated by Ar ion milling through the masks. The geometries of the nanoarrays, including pore diameter, interpore distance and the length of both nanopores and nanotubes could be controlled by the process parameters, which included that the outer pore diameter of silica tube was tuned from ∼80 nm to ∼135 nm while the inner tube diameter from ∼40 nm to ∼65 nm, the interpore distance of the nanotube arrays was from 100 nm to 180 nm and the length of silica tube changed from ∼90 nm to ∼250 nm. The presented nanostructure fabrication method has strong potential for application in intensity and frequency adjustable high luminescence efficiency optoelectronic devices.     

纳米压印多孔硅模板的研究

纳米压印模板通常采用极紫外光刻、聚焦离子束光刻和电子束光刻等传统光刻技术制备,成本较高.寻找一种简单、低成本的纳米压印模板制备方法以提升纳米压印光刻技术的应用成为研究的重点与难点.本文以多孔氧化铝为母模板,采用纳米压印光刻技术对纳米多孔硅模板的制备进行了研究.在硅基表面成功制备出纳米多孔阵列结构,孔间距为350—560 nm,孔径在170—480 nm,孔深为200 nm.在激发波长为514 nm时,拉曼光谱的测试结果表明,相对于单面抛光的硅片,纳米多孔结构的硅模板拉曼光强有了约12倍左右的提升,对提升硅基光电器件的应用具有重要的意义.最后,利用多孔硅模板作为纳米压印母模板,通过热压印技术,成功制备出了聚合物纳米柱软模板.     

Self-cleaning characteristics on a thin-film surface with nanotube arrays of anodic titanium oxide

Self-cleaning of a surface of nanotube arrays of anodic titanium oxide (ATO) is demonstrated. The ATO was prepared in fluoride ion containing sulfate electrolytes with a structure of 0.4 μm length, 100 nm pores diameter, 120 nm interpore distance, 25 nm pore wall thickness, a 8×10⁹ pores cm⁻² pore density, and 68.2% porosity. Prepared as thin films either directly from a Ti foil or on a glass substrate, these arrays have the property that water drops spread quickly over the surface of the films without irradiation. In contrast, a flat anatase TiO₂ film requires irradiation with UV light for several minutes before the contact angle decreases to zero. The observed self-cleaning behavior of the ATO thin films is due to the capillary effect of the nanochannel structure and the superhydrophilic property of the anatase TiO₂ surface inside the tube.     

SERS imaging of silver coated nanostructured Al and Al2O3 substrates. The effect of nanostructure

The last few years have witnessed rapid development of highly ordered and reproducible surface‐enhanced Raman scattering (SERS) nanostructured substrates for their potential medical and analytical application such as biosensing and bioimaging. In this work, 5‐nm silver films deposited on nanostructured Al and Al₂O₃ templates are investigated as substrates for SERS. The chosen templates show different honeycomb nanostructures with two sets of dimension, i.e. pore diameter of ca. 25 and 50 nm and interpore distance of ca. 56 and 100 nm. The SERS imaging results reveal that the signal of the probe molecule (4‐thiazolidinone‐2‐thione) is distributed inhomogeneously on the substrate surface, and this fact is correlated with the morphology of nanostructures determined by atomic force microscopy. The variation of SERS intensity among the substrates is strongly correlated with the shape and size of potential SERS‐active sites, e.g. nanocups and nanopores. The strongest SERS response is found for the Ag/Al₂O₃ template anodized in sulfuric acid, which represents the nanopore array with the smallest dimensions (e.g. pore diameter, interpore distance etc.). Furthermore, depending on size and nanostructure shape, changes in the adsorption mechanism of the probe molecule are observed. Copyright © 2014 John Wiley & Sons, Ltd.     

Investigation of self-assembled fractal porous-silica over a wide range of length scales using a combined small-angle scattering method

The unique structure of a set of self-assembled porous silica materials was characterized through a combined small-angle scattering (CSAS) method using small- and ultra-small angle neutron scattering as well as small-angle X-ray scattering. The porous silica specimens investigated were prepared by a sol-gel method under the presence of alkylketene dimer (AKD) template particles and through calcination, which leads to the development of porous silica having a mass-fractal structure over length scales from ~ 100 nm to ~ 10 μm. Furthermore, the specimens posses a hierarchical structure, which consist of a fractal porous structure, and also contain primary silica particles less than 10 nm in size, which form a continuous silica matrix. To characterize these complex structures, observation over a broad range of length scales is indispensable. We propose a CSAS technique that serves this purpose well.     

Mesoporous TiO2-B nanowires synthesized from tetrabutyl titanate

Ultralong mesoporous TiO₂-B nanowires were synthesized via a hybrid hydrothermal-ion exchanging-thermal treatment using tetrabutyl titanate (TBOT) as a raw material. The phase transformations and porous structures of TiO₂-B nanowires were characterized and studied by X-ray diffraction (XRD), transmission electron microscopy (TEM) and N₂ adsorption-desorption measurement. Mesoporous TiO₂-B nanowires showed a length of several micrometers and diameter of about 25 nm. The porous structures of obtained TiO₂-B nanowires were demonstrated by BJH pore distribution measurement. The wirelike morphologies and porous structures of monodisperse nanowires calcined at 600 °C showed little change, which indicated that such nanowires possessed high thermal stability. The formation mechanism of TiO₂-B nanowires with mesoporous structures were also discussed based on our experimental results.     

Characterization of multi-scale porous structure of fly ash/phosphate geopolymer hollow sphere structures: From submillimeter to nano-scale

In the present work, the porous structure of fly ash/phosphate geopolymer hollow sphere structures (FPGHSS), prepared by pre-bonding and curing technology, has been characterized by multi-resolution methods from sub-millimeter to nano-scale. Micro-CT and confocal microscopy could provide the macroscopic distribution of porous structure on sub-millimeter scale, and hollow fly ashes with sphere shape and several sub-millimeter open cells with irregular shape were identified. SEM is more suitable to illustrate the distribution of micro-sized open and closed cells, and it was found that the open cells of FPGHSS were mainly formed in the interstitial porosity between fly ashes. Mercury porosimeter measurement showed that the micro-sized open cell of FPGHSS demonstrated a normal/bimodal distribution, and the peaks of pore size distribution were mainly around 100 and 10 μm. TEM observation revealed that the phosphate geopolymer was mainly composed of the porous area with nano-pores and dense areas, which were amorphous Al–O–P phase and α-Al₂O₃ respectively. The pore size of nano-pores demonstrated a quasi-normal distribution from about 10 to 100 nm. Therefore, detailed information of the porous structure of FPGHSS could be revealed using multiple methods.     

Pore size assessment based on wall collision broadening of spectral lines of confined gas: experiments on strongly scattering nanoporous ceramics with fine-tuned pore sizes

Wall collision broadening of absorption lines of gases confined in porous media is a recently opened domain of high-resolution spectroscopy. Here, we present an experimental investigation of its application for pore size assessment. We report on the manufacturing of nanoporous zirconia ceramics with well-defined pore sizes fine-tuned from 50 to 150 nm. The resulting pore structure is characterized using mercury intrusion porosimetry, and the optical properties of these strongly scattering materials are measured using femtosecond photon time-of-flight spectroscopy (transport mean free paths found to be tuned from 2.3 to 1.2 μm as the pore size increase). Wall collision line broadening is studied by performing near-infrared (760 nm) high-resolution diode laser spectroscopy of confined oxygen molecules. A simple method for quantitative estimation of the pore size is outlined and shown to produce results in agreement with mercury intrusion porosimetry. At the same time, the need for improved understanding of wall collision broadening is emphasized.     

Is there an optimal topographical surface in nano-scale affecting protein adsorption and cell behaviors? Part II

Although nano-structured surfaces exhibit superior biological activities to the smooth or micro-structured surfaces, whether there is an optimal topographical surface in nano-scale affecting protein adsorption and cell behaviors is still controversial. In this study, porous aluminum oxide membranes with different pore sizes ranging from 25 to 120 nm were prepared by the anodic oxidation technique. The surface morphology, topography and wettability were analyzed by scanning electron microscope, atomic force microscope and water contact angle measurement, respectively. The results indicated that the synergistic action of the nano-topography structure and hydrophilic/hydrophobic properties resulted in a highest protein adsorption on the aluminum oxide membrane with 80 nm pore size. Additionally, the morphological, metabolic and cell counting methods showed that cells had different sensitivity to porous aluminum oxide membranes with different surface features. Furthermore, this sensitivity was cell type dependent. The optimal pore size of aluminum oxide membranes for cell growth was 80 nm for PC12 cells and 50 nm for NIH 3T3 cells.     

Preparation of Polycaprolactone Tissue Engineering Scaffolds by Improved Solvent Casting/Particulate Leaching Method

The classic solvent casting/particulate leaching method to fabricate PCL scaffolds was improved by using a centrifugal technology, a direct bonding process in preparing salt matrices and a technology of vacuum treatment under heating in the desolvation process. Series operations of preshaping, centrifuging, casting and desolvating were employed during the scaffold's manufacture. The scaffold's properties were characterized including micro‐structures, pore dimensions, porosity and hydrophilicity. The results show that centrifugal technology can improve the pore uniformity of scaffolds. In the bonding process, well‐interconnected porous structures were formed if water content was between 2～7%. The distribution of pore dimensions was from 10 to 80 μm, and the porosities were about 89%. Generally, the porosities formed by vacuum treatment at high temperature are greater than those formed by vacuum treatment at ambient temperature in the desolvation process. The fabricated porous PCL scaffolds with good elasticity and desired thickness could be a good choice for application in soft tissue engineering.     

Analysis of nanopore arrangement and structural features of anodic alumina layers formed by two-step anodizing in oxalic acid using the dedicated executable software

Anodic porous alumina layers were fabricated by a two-step self-organized anodization in 0.3 M oxalic acid under various anodizing potentials ranging from 30 to 60 V at two different temperatures (10 and 17 ^°C). The effect of anodizing conditions on structural features and pore arrangement of AAO was investigated in detail by using the dedicated executable publication combined with ImageJ software. With increasing anodizing potential, a linear increase of the average pore diameter, interpore distance, wall thickness and barrier layer thickness, as well as a decrease of the pore density, were observed. In addition, the higher pore diameter and porosity values were obtained for samples anodized at the elevated temperature, independently of the anodizing potential. A degree of pore order was investigated on the basis of Delaunay triangulations (defect maps) and calculation of pair distribution or angle distribution functions (PDF or ADF), respectively. All methods confirmed that in order to obtain nanoporous alumina with the best, hexagonal pore arrangement, the potential of 40 V should be applied during anodization. It was confirmed that the dedicated executable publication can be used to a fast and complex analysis of nanopore arrangement and structural features of nanoporous oxide layers.     

A comparison of stochastic and effective medium approaches to the backscattered signal from a porous layer in a solid matrix

This paper reports a study of the backscattering behavior of a solid layer containing randomly spaced spherical cavities in the long wavelength limit. The motivation for the work arises from a need to model the responses of porous composite materials in ultrasonic NDE procedures. A comparison is made between models based on a summation over discrete scatterers, which show interesting emergent properties, and an integral formulation based on an ensemble average, and with a simple slab effective medium approximation. The similarities and differences between these three models are demonstrated. A simple quantitative criterion is established which sets the maximum frequency at which ensemble average or equivalent homogeneous medium models can represent echo signal generation in a porous layer for given interpore spacing, or equivalently, given pore size and concentration.     

Orthopositronium annihilation and emission in mesostructured thin silica and silicalite-1 films

Mesoporous silica films and MFI-type pure silica zeolite films were investigated using slow positrons. Detection of the 3γ annihilation fraction was used as a quick test to estimate the emission of orthopositronium (o-Ps) into vacuum. Positronium time-of-flight (TOF) spectroscopy, combined with Monte-Carlo simulation of the detection system was used to determine the energy of o-Ps emitted from the films. Evidence for an efficient o-Ps emission was found in both the mesoporous and silicalite-1. A 3γ fraction in the range of 31-36 % was found in the films with the highest o-Ps yield in each type of porous material, indicating that 40-50 % of the implanted positrons form positronium in the pore systems with very different pore sizes. Time-of-flight measurements showed that the energy of the orthopositronium emitted into vacuum is below 100 meV in the film with 2-3 nm pores at 3 keV positron energy, indicating an efficient slowing down but no complete thermalization in the porous films of 300-400 nm thickness.     

Effect of precursor preoxidation on the structure of phenolic resin-based activated carbon spheres

Activated carbon spheres with 3D hierarchical porous structure were prepared from phenol-formaldehyde resins with oxidation treatment in air and physical activation in an inert atmosphere. Based on the results of thermogravimetric analysis, infrared spectrometer (IR), scanning electron microscopy (SEM) and nitrogen adsorption/desorption, the effect of preoxidation on the morphology and structure of activated carbon spheres was investigated. The results show that decomposition and crosslinking reactions occur during the preoxidation and the structural changes of precursor generated by the preoxidation lead to differences in the pore structure of activated carbon spheres. The carbon spheres exhibit the unique 3D hierarchical porous structure, high specific surface area of 1897 m²/g and high pore volume of 2.22 cm³/g.     

Phase transitions of fluids confined in porous silicon: A differential calorimetry investigation

The phase transitions of non-polar organic fluids and of water, confined in the pores of porous silicon samples, were investigated by Differential Scanning Calorimetry (DSC). Two types of PS samples (p^- and p⁺ type) with different pore size and morphology were used (with spherical pores with a radius of about 1.5 nm and cylindrical shape with a radius of about 4 nm respectively). The DSC results clearly show that the smaller the pores are, the larger is the decrease in the transition temperature. Moreover, a larger hysteresis between melting and freezing is observed for p⁺ type than for p^- type samples. A critical review of the thermodynamical properties of small particles and confined fluids is presented and used to interpret and discuss our DSC results. The effects of the chemical dissolution as well as the influence of anodization time are presented, showing that thick p⁺ type porous silicon layers are non-homogeneous. The DSC technique which was used for the first time to investigate fluids confined in porous silicon, enables us to deduce original information, such as the pore size distribution, the decrease in the freezing temperature of confined water, and the thickness of non-freezing liquid layer at the pore wall surface. Received: 11 May 1998 / Revised and Accepted: 29 July 1998     

Mechanism of primary self-organization in porous silicon with a regular structure

A mechanism for self-organization of a regular system of pores in porous silicon is proposed. According to this mechanism, the self-organization obeys the general kinetic laws for a system of charge carriers. The mean interpore spacing in porous silicon prepared from p-Si and the anodizing current density required for synthesizing porous silicon through anodic etching are evaluated in terms of the proposed mechanism. The results obtained are in good agreement with the available experimental data. The dependence of the mean interpore spacing on the carrier concentration in the initial silicon is predicted to be similar to the function L(n) ～ n^?1/2. The validity of the proposed mechanism is confirmed by computer simulation.     

Random lasing in porous scattering medium

The spectrum behavior evolution and the threshold of random lasing depending on the way of photon walk randomization in an active random medium were investigated. The following three ways of photon walk randomization were implemented: multiple light scattering by corundum and silica particles embedded into a solid polymer solution of dye (astrafloxin), multiple light reflection at sub-millimeter extensive air pores (mean diameter 200 μm) produced in the medium, and the combined action of both these effects. The most effective lasing is observed in the case of an active medium with air pores and scattering particles in the interpore space. Such a combined porous scattering medium acts as a network of dielectric waveguides transmitting effectively the random light. This spatial structure of the random active medium significantly increases the photon path in the medium, thereby promoting photon multiplication due to stimulated emission. In this combined medium the random lasing reveals the narrowest spectrum, the lowest threshold, and the highest density of spectral energy in the spectrum maximum.