Conformal SiO2 coating of sub-100 nm diameter channels of polycarbonate etched ion-track channels by atomic layer deposition

Controllable Shrinking Fabrication of Solid-State Nanopores

Nanopores have attracted widespread attention in DNA sequencing and protein or biomarker detection, owning to the single-molecule-scale detection accuracy. Despite the most use of naturally biological nanopores before, solid-state nanopores are widely developed with strong robustness, controllable sizes and geometries, a wide range of materials available, as well as flexible manufacturing. Therefore, various techniques typically based on focused ion beam or electron beam have been explored to drill nanopores directly on free-standing nanofilms. To further reduce and sculpt the pore size and shape for nano or sub-nano space-time sensing precision, various controllable shrinking technologies have been employed. Correspondingly, high-energy-beam-induced contraction with direct visual feedback represents the most widely used. The ability to change the pore diameter was attributed to surface tension induced original material migration into the nanopore center or new material deposition on the nanopore surface. This paper reviews typical solid-state nanopore shrinkage technologies, based on the careful summary of their principles and characteristics in particularly size and morphology changes. Furthermore, the advantages and disadvantages of different methods have also been compared completely. Finally, this review concludes with an optimistic outlook on the future of solid-state nanopores.

Ion-Track Template Synthesis and Characterization of ZnSeO3 Nanocrystals

ZnSeO3 nanocrystals with an orthorhombic structure were synthesized by electrochemical and chemical deposition into SiO2/Si ion-track template formed by 200 MeV Xe ion irradiation with the fluence of 107 ions/cm2. The lattice parameters determined by the X-ray diffraction and calculated by the CRYSTAL computer program package are very close to each other. It was found that ZnSeO3 has a direct band gap of 3.8 eV at the Γ-point. The photoluminescence excited by photons at 300 nm has a low intensity, arising mainly due to zinc and oxygen vacancies. Photoluminescence excited by photons with a wavelength of 300 nm has a very low intensity, presumably due to electronic transitions of zinc and oxygen vacancies.

Conical Nanotubes Synthesized by Atomic Layer Deposition of Al2O3, TiO2, and SiO2 in Etched Ion-Track Nanochannels

Etched ion-track polycarbonate membranes with conical nanochannels of aspect ratios of ~3000 are coated with Al₂O₃, TiO₂, and SiO₂ thin films of thicknesses between 10 and 20 nm by atomic layer deposition (ALD). By combining ion-track technology and ALD, the fabrication of two kinds of functional structures with customized surfaces is presented: (i) arrays of free-standing conical nanotubes with controlled geometry and wall thickness, interesting for, e.g., drug delivery and surface wettability regulation, and (ii) single nanochannel membranes with inorganic surfaces and adjustable isoelectric points for nanofluidic applications.

Molecular Design of Solid-State Nanopores: Fundamental Concepts and Applications

Solid-state nanopores are fascinating objects that enable the development of specific and efficient chemical and biological sensors, as well as the investigation of the physicochemical principles ruling the behavior of biological channels. The great variety of biological nanopores that nature provides regulates not only the most critical processes in the human body, including neuronal communication and sensory perception, but also the most important bioenergetic process on earth: photosynthesis. This makes them an exhaustless source of inspiration toward the development of more efficient, selective, and sophisticated nanopore-based nanofluidic devices. The key point responsible for the vibrant and exciting advance of solid nanopore research in the last decade has been the simultaneous combination of advanced fabrication nanotechnologies to tailor the size, geometry, and application of novel and creative approaches to confer the nanopore surface specific functionalities and responsiveness. Here, the state of the art is described in the following critical areas: i) theory, ii) nanofabrication techniques, iii) (bio)chemical functionalization, iv) construction of nanofluidic actuators, v) nanopore (bio)sensors, and vi) commercial aspects. The plethora of potential applications once envisioned for solid-state nanochannels is progressively and quickly materializing into new technologies that hold promise to revolutionize the everyday life.

Oriented crystallization of PEG induced by confinement in cylindrical nanopores: structural and thermal properties

Nanoporous ion track-etched polycarbonate is ideally suited for the study of confined polymers via small angle X-ray scattering (SAXS) due to the strictly parallel orientation of the pores as well as their uncorrelated lateral distribution. Nanopores with radii ranging from 17 to 213 nm are prepared and coated with SiO2via atomic layer deposition in order to obtain a well-defined and homogeneous surface. A low molecular weight polyethylene glycol (PEG) homopolymer with a semicrystalline lamellar bulk structure is introduced into the nanopores via melt infiltration. At high temperatures SAXS measurements confirm a uniform filling of the pores with amorphous polymer. Upon cooling below the melting point of PEG, a concentrical structure of semicrystalline lamellae is revealed for large pore radii. We introduce models which successfully describe the combined scattering from nanopores and semicrystalline or amorphous PEG inside. DSC measurements of the confined polymer show a decrease of melting temperature and heat of fusion per gram polymer upon reduction of the pore radius and hint at a change in the lamellar configuration.

Low-Temperature Plasma-Enhanced Atomic Layer Deposition of SiO2 Using Carbon Dioxide

In this work, we report the successful growth of high-quality SiO₂ films by low-temperature plasma-enhanced atomic layer deposition using an oxidant which is compatible with moisture/oxygen sensitive materials. The SiO₂ films were grown at 90 °C using CO₂ and Bis(tertiary-butylamino)silane as process precursors. Growth, chemical composition, density, optical properties, and residual stress of SiO₂ films were investigated. SiO₂ films having a saturated growth-per-cycle of ~ 1.15 Å/cycle showed a density of ~ 2.1 g/cm³, a refractive index of ~ 1.46 at a wavelength of 632 nm, and a low tensile residual stress of ~ 30 MPa. Furthermore, the films showed low impurity levels with bulk concentrations of ~ 2.4 and ~ 0.17 at. % for hydrogen and nitrogen, respectively, whereas the carbon content was found to be below the measurement limit of time-of-flight elastic recoil detection analysis. These results demonstrate that CO₂ is a promising oxidizing precursor for moisture/oxygen sensitive materials related plasma-enhanced atomic layer deposition processes.

Monitoring the Process of Nanocavity Formation on a Monomolecular Level

Controlling the synthesis of nanostructured surfaces is essential to tailor the properties of functional materials such as catalysts. We report on the synthesis of nanocavities of 1–2 nm dimension on planar Si-wafers by sacrificial nanotemplating and atomic layer deposition (ALD). It is shown that the process of nanocavity formation can be directly monitored on a monomolecular level through imaging with an atomic force microscope (AFM). In particular, by employing the AFM peak force tapping mode the simultaneous mapping of surface topography and tip-surface adhesion forces is accessible, which is useful for the assignment of topographical features and determining the orientation of the template molecules on the wafer surface. Detailed analysis based on the three-dimensional AFM topography allows for a quantification of the template and nanocavity surface coverage. The results are of importance for a detailed understanding of the processes underlying template-based nanocavity formation on oxide surfaces.

Nanoscale Structuring in Confined Geometries using Atomic Layer Deposition: Conformal Coating and Nanocavity Formation

Nanoscale structuring in confined geometries using atomic layer deposition (ALD) is demonstrated for surfaces of nanochannels in track-etched polymer membranes and in mesoporous silica (SBA-15). Suitable process conditions for conformal ALD coating of polymer membranes and SBA-15 with inorganic oxides (SiO2, TiO2, Al2O3) were developed. On the basis of the oxide-coated layers, nanochannels were further structured by a molecular-templated ALD approach, where calixarene macromolecules are covalently attached to the surface and then embedded into an Al2O3 layer. The removal of calixarene by ozone treatment results in 1–2 nm wide surface nanocavities. Surfaces exposed to different process steps are analyzed by small angle X-ray scattering (SAXS) as well as by X-ray photoelectron and infrared spectroscopy. The proposed nanostructuring process increases the overall surface area, allows controlling the hydrophilicity of the channel surface, and is of interest for studying water and ion transport in confinement.

Properties of Hydrogen-Bonded Liquids at Interfaces

Effects of interfaces on hydrogen-bonded liquids play major roles in nature and technology. Despite their importance, a fundamental understanding of these effects is still lacking. In large parts, this shortcoming is due to the high complexity of these systems, leading to an interference of various interactions and effects. Therefore, it is advisable to take gradual approaches, which start from well designed and defined model systems and systematically increase the level of intricacy towards more complex mimetics. Moreover, it is necessary to combine insights from a multitude of methods, in particular, to link novel preparation strategies and comprehensive experimental characterization with inventive computational and theoretical modeling. Such concerted approach was taken by a group of preparative, experimentally, and theoretically working scientists in the framework of Research Unit FOR 1583 funded by the Deutsche Forschungsgemeinschaft (German Research Foundation). This special issue summarizes the outcome of this collaborative research. In this introductory article, we give an overview of the covered topics and the main results of the whole consortium. The following contributions are review articles or original works of individual research projects.

Surface Enhanced DNP Assisted Solid-State NMR of Functionalized SiO2 Coated Polycarbonate Membranes

Surface enhanced solid-state NMR by dynamic nuclear polarization (DNP SENS) enables the characterization of the inner-pore surface functionalization of porous etched ion-track membranes exhibiting low specific surface areas compared to typical SBA- or MCM-type mesoporous silica materials. The membranes were conformally coated with a 5 nm thin SiO2 layer by atomic layer deposition. This layer was subsequently modified by aminopropyl silane linkers that allow further functionalization via the terminal amine group. The results evidence that in principle DNP SENS is a capable tool to analyze more complex porous systems, e.g. bioinspired functional etched ion-track membranes down to the molecular level. These results are relevant also for single nanopore systems, for which a direct analysis of the channel surface functionalization is not feasible by classical characterization methods. The applicability of DNP SENS to complex porous systems requires the optimization of the sample preparation and measurement parameters.

Water/PEG Mixtures: Phase Behavior, Dynamics and Soft Confinement

Polyethylene glycol is water soluble and forms an eutectic system with water. The eutectic temperature is −19 °C for M=1500 g mol−1 and increases with molecular weight. The dielectric relaxation spectrum of the mixtures exhibits a strong loss maximum in ϵ″ (ω) similar to pure water. Relaxation time increases with the addition of PEG. Activation energies exhibit a maximum of 0.35 eV at molar fraction χp ≈0.2. This compares well with results on ethanol water mixtures. Adding PEG molecules to nanoscopic water droplets of inverse microemulsions has only small impact on the bending modulus κ of a non-ionic microemulsion. In AOT based microemulsions an increase or decrease of κ is found in dependence on the size of the droplets. This is in accordance with the variation of the dynamic percolation transition in the same systems.

Ab-Initio Molecular Dynamics Simulations and Calculations of Spectroscopic Parameters in Hydrogen-Bonding Liquids in Confinement (Project 8)

We investigate the effect of several nanoscale confinements on structural and dynamical properties of liquid water and binary aqueous mixtures. By means of molecular dynamics simulations based on density functional theory and atomistic force fields. Our main focus is on the dependence on the structure and the hydrogen-bonding-network of the liquids near the confinement interface at atomistic resolution. As a complementary aspect, spatially resolved profiles of the proton NMR chemical shift values are used to quantify the local strength of the hydrogen-bond-network.

Multimaterial Nanoporous Membranes Shaped through High Aspect-Ratio Sacrificial Silicon Nanostructures

We present an innovative fabrication method for solid-state nanoporous membranes based on the casting of sacrificial silicon nanostructures. The process allows the individual definition of geometry and placement of each nanopore through e-beam lithography and is compatible with a wide range of materials without the need to adapt the process to the materials used. We demonstrate the fabrication of membranes integrating high aspect-ratio nanopores with critical dimensions as small as 30 nm, 1.2 μm in length, with round or elongated shapes, and made of silicon dioxide or amorphous carbon. The capability to engineer nanoporous membranes made of a variety of materials and with tailored designs will lead to new applications in the field of electrochemical sensing, flow modulation, or the chemical functionalization of nanopores.

Investigating Polymer-Metal Interfaces by Grazing Incidence Small-Angle X-Ray Scattering from Gradients to Real-Time Studies

Tailoring the polymer-metal interface is crucial for advanced material design. Vacuum deposition methods for metal layer coating are widely used in industry and research. They allow for installing a variety of nanostructures, often making use of the selective interaction of the metal atoms with the underlying polymer thin film. The polymer thin film may eventually be nanostructured, too, in order to create a hierarchy in length scales. Grazing incidence X-ray scattering is an advanced method to characterize and investigate polymer-metal interfaces. Being non-destructive and yielding statistically relevant results, it allows for deducing the detailed polymer-metal interaction. We review the use of grazing incidence X-ray scattering to elucidate the polymer-metal interface, making use of the modern synchrotron radiation facilities, allowing for very local studies via in situ (so-called "stop-sputter") experiments as well as studies observing the nanostructured metal nanoparticle layer growth in real time.

Fabrication of multilayered nanofluidic membranes through silicon templates

We present a new fabrication method for solid-state nanoporous membranes based on sacrificial template structures made of silicon. The process consists of creating membranes by evaporating thin-films on sacrificial templates which, after their selective removal, opens the nanopores and releases the free-standing membranes. This way it is possible to define the geometry of the pore by design and to build the membrane by stacking thin-films of various materials through evaporation. Such a membrane with controlled porosity, pore geometry, thickness and nano-channel composition provides new opportunities for selective chemical functionalization, gating, electrical sensing or electrical stimulation inside the nanopore.