High Aspect Ratio Pores Research Articles

Fabrication of model ultrafiltration membranes with uniform, high aspect ratio pores

In this manuscript, we report the facile fabrication of large-area model membranes with highly uniform and high aspect ratio pores with diameters <20 nm. These membranes are useful for fundamental investigations of separation by size exclusion in the ultrafiltration regime, where species to be separated from solution have dimensions of 1–100 nm. Such investigations require membranes with narrow pores and high aspect ratios such that the Hagen–Poiseuille equation is followed, enabling well-known models such as the hindered transport model to be evaluated and other affecting factors to be ignored. We demonstrate that the sub-20 nm pores in the membrane are of sufficiently high aspect ratio such that water flux through the membrane is consistent with the Hagen–Poiseuille equation. The fabrication relies on self-assembling block copolymers to form uniform, densely packed patterns with sub-20 nm resolution, sequential infiltration synthesis to convert the block copolymer in situ into a mask with adequate contrast to etch pores with an aspect ratio >5, and low-resolution photolithography to transfer the pattern over a large area into a silicon nitride membrane. Model membranes with narrow pore-size distribution fabricated in this way provide the means to investigate parameters that impact size-selective ultrafiltration separations such as the relationships between solute or particle size and pore size, their distributions, and rejection profiles, and, therefore, test the validity or limits of separation models.

Electroless Plating of Nanoporous Substrates for Thin Film-Solid Oxide Fuel Cells

Thin-film solid oxide fuel cells (TF-SOFCs) are an attractive renewable energy source due to their high efficiency, increased power output and ability to be efficiently stacked for power generation at a small or large-scale. [N. Q. Minh, J Am Ceram Soc. 1993; 76(3):563-588.] Nanoporous Al2O3, also known as anodized aluminum oxide (AAO) is an appropriate substrate for TF-SOFCs due to its high temperature stability, chemically inert nature, and porosity that allows gas flow to the anode. The aligned, nano-sized pores allow for both conformal sputtering on the surface and efficient fuel delivery to the anode, making it particularly suited for thin film deposition. Electroless nickel plating is used to achieve a fully conductive, nanoporous substrate which catalyzes the hydrogen oxidation reaction and improves current collection efficiency in TF-SOFCs. A minimum plating temperature of 60°C is needed to grow nanoscale Ni deposits into a conductive film without compromising the porosity of the surface. Effective stirring methods are used to deliver the plating solution throughout the high-aspect ratio pores of the AAO substrate and create thru-plane conductivity. To achieve conformal plating, the number of pre-treatment steps was optimized to prevent overplating and create a homogenous Ni deposit. Finally, with the creation of a conformally coated, conductive porous substrate, we can improve the efficiency and power output of TF-SOFC performance with commercially relevant active areas (larger than 1 cm2).

Exfoliating kaolin to ultrathin nanosheets with high aspect ratio and pore volume: A comparative study of three kaolin clays in China

A revised intercalation-assisted liquid exfoliation method is proposed to exfoliate three typical kaolin clays (Beihai, Longyan and Suzhou) in China. By combining intercalation, ball milling, ultrasound and freeze-drying technologies, the ultrathin kaolin nanosheets with large lamellar size (1–2 μm), low thickness (∼10 nm), high aspect ratio (∼130) and large pore volume (∼0.3 cm3/g) were obtained. A large number of kaolinite monolayer structures were observed in AFM images, indicating high exfoliation efficiency. The morphology, crystal structure and microstructures of kaolin samples during the intercalation and exfoliation processes are studied in detail by high resolution XRD, SEM, BET and AFM analysis. The mechanism for effective intercalation, exfoliation and stabilization of kaolin clays is clarified. The ultrathin kaolinite nanosheets slightly curved after exfoliation process were found to form some porous structures between the kaolin nanolayers, providing plenty of interlayer spacing for functional loading or interfacial binding. These ultrathin kaolin nanosheets might have the advantages to improve the adhesion ability, loading capacity, and mechanical properties of kaolin-based composite materials.

Rock Physical Modeling and Seismic Dispersion Attribute Inversion for the Characterization of a Tight Gas Sandstone Reservoir

Gas identification using seismic data is challenging for tight gas reservoirs with low porosity and permeability due to the complicated poroelastic behaviors of tight sandstone. In this study, the Chapman theory was used to simulate the dispersion and attenuation caused by the squirt flow of fluids in the complex pore spaces, which are assumed to consist of high aspect-ratio pores (stiff pores) and low aspect-ratio microcracks (soft pores). The rock physics modeling revealed that as the gas saturation varies, P-wave velocity dispersion and attenuation occurs at seismic frequencies, and it tends to move to high frequencies as the gas saturation increases. The velocity dispersion of the tight gas sandstone causes a frequency-dependent contrast in the P-wave impedance between the tight sandstone and the overlying mudstone, which consequently leads to frequency-dependent incidence reflection coefficients across the interface. In the synthetic seismic AVO modeling conducted by integrating the rock physics model and the propagator matrix method, the variations in the amplitudes and phases of the PP reflections can be observed for various gas saturations. The tests of the frequency-dependent AVO inversion of these synthetic data revealed that the magnitude of the inverted P-wave dispersion attribute can be used to indicate gas saturation in tight sandstone reservoirs. The applications of the frequency-dependent AVO inversion to the field pre-stacked seismic data revealed that the obtained P-wave dispersion attribute is positively correlated with the gas production from the pay zone at the well locations. Thus, the methods of the rock physics modeling and the frequency-dependent AVO inversion conducted in this study have good potential for the evaluation of the gas saturation in tight gas sandstone reservoirs.

Ultra-high aspect ratio pores milled in diamond via laser, ion and electron beam mediated processes

Microfabrication techniques are critical for the rapid prototyping and development of applications for cutting edge materials. Recently diamond has gained considerable interest for quantum photonic, biosensing, inertial confinement fusion and magnetometer applications. In this article, ultra-high aspect ratio milling of diamond micropores by photon, ion and electron based methods is reported. A multiphoton absorption laser ablation approach is revealed to rapidly produce sub-10 μm diameter micropores in diamond with an aspect ratio of 14:1 and a tapered profile at the surface interface. Chemically active, oxygen focused ion beam milling produces high-aspect ratio pores in diamond with an aspect ratio of 65:1 and minimal tapering over the length of the pore, overcoming the physical interaction volume limitations imposed in conventional gallium based focused ion beam milling and laser ablation methods. Oxygen-mediated electron beam induced etching is revealed to negate the limitations imposed by physical sputtering mechanisms utilized in focused ion beam milling via the direct initiation of chemical reactions at the receding surface, producing aspect ratios on the order of 200:1. Numerical simulations reveal the physical basis for the superior aspect-ratio pore milling of the oxygen focused ion beam milling and electron beam induced etching methods. Our results demonstrate direct-write methods for the fabrication of ultra-high aspect micropores in diamond and provide insight into the underlying mechanisms of these physical processes. The three methods demonstrated here can be interchanged for applications based on the desired characteristic aspect ratio and process throughput.

Anisotropic pore fabrics in faulted porous sandstones

The fabric of pores in sedimentary rocks around fault zones can be subject to significant modification. Knowledge of how pore fabrics vary during and after faulting is important for understanding how rocks transmit fluids around fault zones, and can help to predict mechanical instability due to changes in pore fluid pressure. Datasets detailing the geometry of pore fabrics in faulted porous rocks are lacking. This paper describes pore fabrics quantified from two outcrops of normally faulted sandstone. The porosity and the size, shape and geometry of pores were quantified from core plugs and thin sections. Results were mapped within a framework of the faults to better illustrate how these datasets may be used to improve understanding of fluid flow around fault zones. Results from a mature, quartz-rich arenite show a change in pore fabric from pores oriented horizontally and parallel to laminations to pores oriented at a low angle to σ1. Pore fabrics quantified from a clay-rich, quartz sub-arkose changed from moderate aspect ratio pores with no preferred orientation, to high aspect ratio pores oriented dominantly sub-parallel to the fault surface. Permeabilities measured on corresponding core plugs showed anisotropy of permeability with maximum permeability oriented down fault dip around both faults.

Effect of reactor pressure on the conformal coating inside porous substrates by atomic layer deposition

Atomic layer deposition (ALD) is renowned for its step coverage in porous substrates. Several emerging applications require a combination of this high step coverage with high throughput ALD, like spatial ALD. Often, high throughput ALD is performed at atmospheric pressure, and therefore, the effect of reactor pressure on the saturation dose is investigated. ALD inside porous substrates is governed by three key parameters: the reaction probability, the pore aspect ratio, and the precursor diffusion coefficient, of which the latter one contains the reactor pressure dependency. The effect of these parameters on the saturation dose is validated using Monte Carlo modeling, where the reactor pressure dependency is included through the mean free path. A reaction-limited and a diffusion-limited regime can be identified, and it is shown that for many realistic experimental conditions, even at low reactor pressures, the saturation dose is in the diffusion-limited regime. An expression for the pressure dependent saturation dose in the diffusion-limited regime is derived. For small pore diameters, the saturation dose is pressure independent, but for larger pores, higher saturation doses are required for atmospheric reactor pressures than for low reactor pressures. However, as high reactor pressures enable much higher precursor partial pressures than low reactor pressures, the resulting saturation times can be much shorter at atmospheric pressure than low pressure. Often, high surface area porous substrates will lead to supply limited conditions, and increased saturation times have to be taken into account. These results show that the atmospheric pressure ALD can be used for high throughput ALD inside porous substrates, as long as high precursor partial pressures and molar flows can be applied. This is experimentally demonstrated by a near 100% step coverage obtained by atmospheric spatial ALD of alumina in high aspect ratio pores.

Controlled SOFC Anode Porosity Using Aligned Sacrificial Fibers

Solid oxide fuel cell anode supports serve a multipurpose role. This layer must be both the mechanical backbone of the cell and an active electrode. While high porosity is desired to maximize reaction sites, it negatively impacts strength. Therefore, efforts should be made to design anode microstructure for high strength while maintaining electrochemical performance. Currently, solid oxide fuel cell anodes are fabricated such that there is little control over the final microstructure. A new technique for creating porous anode support layers in solid oxide fuel cells was developed. Using sacrificial, high aspect ratio pore formers, anode support layers with non-random porosity networks were created utilizing dielectrophoresis. Variation in applied voltage and frequency during dielectrophoretic treatment allowed control over the resulting structure. Microstructure analysis and electrochemical testing of novel anode support layers was performed and compared to results of standard SOFC designs.

(Invited) Porous Silicon for Energy Storage at Microscale: Supercapacitors

The main driving force for continuing investigations on porous silicon (PS) is the expanding area for its potential application. Drug delivery, bio/chemical sensing, energy storage – those are just few samples of a wide range of applications of PS, mainly based on the extremely large surface area of the material.In this presentation we will discuss the application of porous silicon for energy storage at microscale, focusing on the electrochemical double layer capacitors (EDLC). EDLC, or so called supercapacitors, can store several orders of magnitude more energy than standard capacitors, and are slowly approaching the values typical for batteries. At the same time, supercapacitors offer much larger power densities than batteries do, also they can be charged very fast and can perform up to hundreds of thousands charge/discharge cycles. IoT and development of wearable and implantable devices would greatly benefit from this type of miniaturized energy storage element integrated on the same chip together with other active/passive elements and even with the energy harvester. Active carbon is still the most exploited material for supercapacitor electrodes, with carbon nanotubes and, especially, graphene with its large specific surface area, showing very promising results, at least at the laboratory scale. However, fabrication of carbon based electrodes usually includes a high temperature step which complicates their integration into the standard silicon microfabrication process. Porous silicon can be the solution here, as its processing is relatively simple and fully compatible with silicon microfabrication.Here we will review some recent and pioneering achievements in porous silicon based supercapacitor performance. The main obstacle for PS electrode application is chemical activity of the surface and low conductivity of electrode material. Several doping and passivation techniques will be discussed, such as metal sputtering, carbonization (graphene coating) and atomic layer deposition (ALD), all of which reasonably improve the performance (fig. 1). Applicability and efficiency of those techniques depend in large part on the pore size and thickness of the layer, which is also the main factor determining the capacitance of the device. A possible way towards the integration will be also shown by in-chip porous silicon supercapacitor design allowing increase the capacitance without expanding the foot print area of the device on a chip but just exploiting the bulk of silicon chip. A conformal coating with a precise thickness control is an important step in many fabrication processes, including also the supercapacitor. Currently, atomic layer deposition is the most suitable technique to fulfil the requirements. On the other hand, evaluation of the thickness and conformality of the coating inside the high aspect ratio pores is not so trivial task. A high resolution scanning electron microscopy (SEM) is not easy to perform in the case of small pores with such thin deposited layers. An interface contrast development for SEM can be used here, as will be discussed in this presentation. The technique is based on the excellent resistance of Al2O3 against SF6 plasma used for deep reactive ion etching of silicon. During the plasma etch step, the porous silicon “frame” is removed allowing to observe the released ALD layer or multilayer stack which was deposited inside the pores. Acknowledgement: This work has been partially funded by the Academy of Finland through the Finnish Center of Excellence in Atomic Layer Deposition. Figure 1

Catalyst Self-Assembly for Scalable Patterning of Sub 10 nm Ultrahigh Aspect Ratio Nanopores in Silicon.

Nanoporous silicon (NPSi) has received significant attention for its potential to contribute to a large number of applications, but has not yet been extensively implemented because of the inability of current state-of-the-art nanofabrication techniques to achieve sufficiently small pore size, high aspect ratio, and process scalability. In this work we describe the fabrication of NPSi via a modified metal-assisted chemical etching (MACE) process in which silica-shell gold nanoparticle (SiO2-AuNP) monolayers self-assemble from solution onto a silicon substrate. Exposure to the MACE etchant solution results in the rapid consumption of the SiO2 spacer shell, leaving well-spaced arrays of bare AuNPs on the substrate surface. Particles then begin to catalyze the etching of nanopore arrays without interruption, resulting in the formation of highly anisotropic individual pores. The excellent directionality of pore formation is thought to be promoted by the homogeneous interparticle spacing of the gold core nanocatalysts, which allow for even hole injection and subsequent etching along preferred crystallographic orientations. Electron microscopy and image analysis confirm the ability of the developed technique to produce micrometer-scale arrays of sub 10 nm nanopores with narrow size distributions and aspect ratios of over 100:1. By introducing a scalable process for obtaining high aspect ratio pores in a novel size regime, this work opens the door to implementation of NPSi in numerous devices and applications.

High-resolution x-ray imaging using a structured scintillator.

In this study, the authors introduce a new generation of finely structured scintillators with a very high spatial resolution (a few micrometers) compared to conventional scintillators, yet maintaining a thick absorbing layer for improved detectivity. Their concept is based on a 2D array of high aspect ratio pores which are fabricated by ICP etching, with spacings (pitches) of a few micrometers, on silicon and oxidation of the pore walls. The pores were subsequently filled by melting of powdered CsI(Tl), as the scintillating agent. In order to couple the secondary emitted photons of the back of the scintillator array to a CCD device, having a larger pixel size than the pore pitch, an open optical microscope with adjustable magnification was designed and implemented. By imaging a sharp edge, the authors were able to calculate the modulation transfer function (MTF) of this finely structured scintillator. The x-ray images of individually resolved pores suggest that they have been almost uniformly filled, and the MTF measurements show the feasibility of a few microns spatial resolution imaging, as set by the scintillator pore size. Compared to existing techniques utilizing CsI needles as a structured scintillator, their results imply an almost sevenfold improvement in resolution. Finally, high resolution images, taken by their detector, are presented. The presented work successfully shows the functionality of their detector concept for high resolution imaging and further fabrication developments are most likely to result in higher quantum efficiencies.

Coated Porous Si for High Performance On-Chip Supercapacitors

High performance porous Si based supercapacitor electrodes are demonstrated. High power density and stability is provided by ultra-thin TiN coating of the porous Si matrix. The TiN layer is deposited by atomic layer deposition (ALD), which provides sufficient conformality to reach the bottom of the high aspect ratio pores. Our porous Si supercapacitor devices exhibit almost ideal double layer capacitor characteristic with electrode volumetric capacitance of 7.3 F/cm3. Several orders of magnitude increase in power and energy density is obtained comparing to uncoated porous silicon electrodes. Good stability of devices is confirmed performing several thousands of charge/discharge cycles.

Fabrication of porous anodic aluminum oxide by hybrid pulse anodization at relatively high potential

Anodic aluminum oxide (AAO) containing high-aspect ratio pore channels has been widely used as a template for fabricating nanowires or other 1D nanostructures. For AAO prepared in oxalic acid, the anodizing potential is set as 40 V in order to balance the oxidation and dissolution rates. In practice, a higher potential is beneficial in increasing the pore size. However, a higher potential increases the local electrolyte temperature and therefore damages the pore structure. In this article, this problem is resolved by means of a high-potential hybrid pulse anodization (HPA) technique, in which a period of small negative potential is applied to suppress the Joule heating effect during the AAO preparation process. The scanning electron microscope results showed that HPA with an anodizing potential of 60 V resulted in an intact pore structure on the AAO surface. By contrast, the AAO formed using conventional direct current anodization with the same anodizing potential contained many small irregular pores around each original pore. The present results suggest that HPA has significant potential for the fabrication of high-quality nanowires with various diameters for such applications as magnetic recording, super capacitors or field emission devices.

Ultrathin SiO2 Films Grown by Atomic Layer Deposition Using Tris(dimethylamino)silane (TDMAS) and Ozone

Atomic layer deposition of SiO2 using tris(dimethylamino)silane and ozone were studied comprehensively. It was found that SiO2 grew linearly at different temperatures while the growth rate increased at higher temperatures. For thickness <3.5 nm, the direct tunneling current of the ALD SiO2 is comparable to that of thermally grown SiO2 when considering its equivalent oxide thickness (EOT). ALD could grow high-quality conformal SiO2 films on non-silicon substrates and on high aspect-ratio pores and nanostructures. The ALD SiO2 can be used as high-quality gate insulators for thin-film MOS transistors, and insulators for sensor structures and nanostructures on non-silicon substrates.

On the variation in the electrical properties and ac conductivity of through-thickness nano-porous anodic alumina with temperature

The electrical response of self-organized through-thickness anodic alumina with hexagonal arrangement of cylindrical pores has been studied as a function of temperature. Mechanically stable thick porous anodic alumina was prepared, by through-thickness anodic oxidation of aluminum sheet in sulfuric acid, with extremely high aspect ratio pores exhibiting fairly uniform diameter and interpore distance. It was observed that the electrical properties of through-thickness anodic alumina are very sensitive to minute changes in temperature and the role of surface conductivity in governing its electrical response cannot be overlooked. At high frequencies, intrinsic dielectric response of anodic alumina was dominant. The frequency-dependent conductivity behavior at low and intermediate frequencies was explained on the basis of correlated barrier hopping (CBH) and quantum mechanical tunneling (QMT) models, respectively. Experimental data was modeled using an equivalent circuit consisting of Debye circuit, for bulk alumina, parallel to surface conduction path. The surface conduction was primarily based on two circuits in series, each with a parallel arrangement of a resistor and a constant phase element. This suggested heterogeneity in alumina pore surface, possibly related with islands of physisorbed water separated by the regions of chemisorbed water. Temperature dependence of some circuit elements has been analyzed to express different charge migration phenomena occurring in nano-porous anodic alumina.

Tracing the Migration History of Metal Catalysts in Metal-Assisted Chemically Etched Silicon

Three-dimensional (3D) visualization of complex embedded nanopore networks in silicon requires expensive machinery and tedious sample preparation procedures such as electron tomography, also known as 3D transmission electron microscopy. In this article, we report a new, fast, powerful, and low-cost three-dimensional imaging technique with sub-5 nm resolution. This new imaging method is applied to metal-assisted chemically etched monocrystalline Si to demonstrate its capabilities. The AFEI (atomic layer deposition-fill-etch-imaging) technique consists of three simple process steps that are available in most material research settings. First the porous substrate is conformally coated with an atomic layer deposition (ALD) metal oxide layer. ALD is able to penetrate deep into complex, high aspect ratio pores, as it is a sequential gas-phase deposition process. Next, the cross-section of the ALD-filled porous Si substrate is etched with high-density fluorine-based plasma processing, which yields very high selectivity toward Si (e.g., >400:1 for Si:ZnO). This step removes the bulk Si and exposes the metal oxide structures grown inside the pores. In the last step, the sample cross-section is examined using a standard scanning electron microscope at various angles, which allows precise imaging of hidden features and reconstruction of a 3D model of the embedded pore network.

Conformal Coating by High Pressure Chemical Deposition for Patterned Microwires of II–VI Semiconductors

AbstractDeposition techniques that can uniformly and conformally coat deep trenches and very high aspect ratio pores with uniform thickness films are valuable in the synthesis of complex three‐dimensionally structured materials. Here it is shown that high pressure chemical vapor deposition can be used to deposit conformal films of II–VI semiconductors such as ZnSe, ZnS, and ZnO into high aspect ratio pores. Microstructured optical fibers serve as tailored templates for the patterning of II–VI semiconductor microwire arrays of these materials with precision and flexibility. In this way, centimeters‐long microwires with exterior surfaces that conform well to the nearly atomically smooth silica templates can be fabricated by conformal coating. This process allows for II–VI semiconductors, which cannot be processed into optical fibers with conventional techniques, to be fabricated into step index and microstructured optical fibers.

Micromachined Microsieves With High Aspect Ratio Features

Micromachined microsieves fabricated entirely by dry etch processes with pore diameters ranging from 3 to 5 μm are reported. Two microsieve architectures are explored: 2.1 and 5.3 μm thin dielectric layer membranes supported by high aspect ratio rib features (10:1), and 50 μm thick silicon membranes from silicon-on-insulator wafers with no supporting ribs and high aspect ratio pore features (16:1). The flow throughput of each design is evaluated experimentally and theoretically, and the expected relative robustness is assessed and compared to typical microsieve structures reported in the literature. The experimental and theoretical work suggests that both structures have the potential for higher robustness than the typical micromachined microsieve architectures, with reduced but still reasonable flow throughputs in the range of 105 to 106 L/m2-hr-bar, depending on the microsieve porosity. Both microsieve architectures are shown to block monodisperse polymer beads 3 μm in diameter.

Influence of cell aspect ratio on architecture and compressive strength of titanium foams

Titanium foams have been of interest in dental and orthopedic implants over the past few decades on account of their excellent mechanical properties, chemical stability, and biocompatibility. A powerful tool, X-ray computed microtomography was used to measure quantitatively the effect of pore morphology on foam architecture. Mechanical properties of titanium foams with varying pore structure were investigated. Aspect ratio of the pores was quantitatively demonstrated to affect strength, degree of anisotropy and strain-rate sensitivity of the produced titanium foams. Needle-like pored foams showed 30–55% lower strength when compared to the foams having lower aspect ratio pores. Lower aspect ratio pored foams were 3–11%, higher aspect ratio pored foams were 17–34% weaker in the direction parallel to the compaction direction when compared to the perpendicular one. High aspect ratio pores also resulted in more pronounced strain-rate sensitivity.

Micro- and Nanoparticle Translocation through a Solid-State Membrane Pore Thinner than their Diameters

Micron-sized particles have been detected by the resistive-pulse or Coulter counting technique since the 1950's (Coulter, US Pat. 2656508) with 90 nm particle detection reported in 1970 (DeBlois and Bean, Rev. Sci. Inst. 41, 909). Due to the challenges of fabricating submicron pores in those times, the pores used were all several microns in length. Nanofabrication has advanced significantly since then, and in this poster we examine the interesting case of particles translocating through a membrane pore thinner than those particles.With micron-long, high aspect ratio pores, a spherical particle will be entirely contained within the pore for the majority of the time it translocates. Thus, all but the beginning and end of the observed signal is due to the entire particle. In addition, pore resistance dominates over access resistance. In contrast, with low-aspect ratio pores thinner than the particle, the signal reflects the interactions of the particle with both the access resistance volumes and the pore volume. The signal is a complex combination of these interactions, that changes as the particle moves through the pore. Another advantage of shorter pores is that identical particles passing through longer pores give smaller modulations in current, because the proportional change in resistance due to the particle is smaller.We use a focused ion beam to drill nanopores tens to hundreds of nanometers in diameter in silicon nitride membranes, with thicknesses in the same range. Polystyrene microspheres are driven through these pores by an applied electric field. As they pass through, they modulate the flux of ions through the pore, and thus the current flowing between the electrodes. We present experimental and simulated studies of particle translocation, aimed at determining the optimum system for analyzing virus particles using this method.