Solid-state nanopores are considered a promising tool for the study of biological polymers such as DNA and RNA, due largely to their flexibility in size, potential in device integration and robustness. Here, we show that the precise shape of small nanopores (∼5 nm diameter in 20 nm SiN membranes) can be controlled by using transmission electron microscope (TEM) beams of different sizes. However, when some of these small nanopores are immersed in an aqueous solution, their resistance is observed to decrease over time. By comparing nanopores of different shapes using (scanning) TEM both before and after immersion in aqueous solution, we demonstrate that the stability of small nanopores is related to their three-dimensional geometry, which depends on the TEM beam size employed during pore fabrication. Optimal stability is obtained using a TEM beam size of approximately the same size as the intended nanopore diameter. In addition, we show that thermal oxidation can serve as a means to independently control nanopore size following TEM fabrication. These observations provide key guidelines for the fabrication of stable solid-state nanopores on the scale of nucleic acids and small proteins.

From conductance and noise studies, we infer that nanometer-sized gaseous bubbles (nanobubbles) are the dominant noise source in solid-state nanopores. We study the ionic conductance through solid-state nanopores as they are moved through the focus of an infrared laser beam. The resulting conductance profiles show strong variations in both the magnitude of the conductance and in the low-frequency noise when a single nanopore is measured multiple times. Differences up to 5 orders of magnitude are found in the current power spectral density. In addition, we measure an unexpected double-peak ionic conductance profile. A simple model of a cylindrical nanopore that contains a nanobubble explains the measured profile and accounts for the observed variations in the magnitude of the conductance.

We report on the fabrication and characterization of gold nanoelectrodes with carefully controlled nanometer dimensions in a matrix of insulating silicon nitride. A focused electron beam was employed to drill nanopores in a thin silicon nitride membrane. The size and shape of the nanopores were studied with high-resolution transmission electron microscopy and electron-energy-loss two-dimensional maps. The pores were subsequently filled with gold, yielding conically shaped nanoelectrodes. The nanoelectrodes were examined by atomic and electrostatic force microscopy. Their applicability in electrochemistry was demonstrated by steady-state cyclic voltammetry. Pores with a radii down to 0.4 nm and electrodes with radii down to 2 nm are demonstrated.

The enhanced energy resolution in electron energy loss spectroscopy measurements achieved in transmission electron microscopes equipped with monochromators provides new opportunities for the study of the properties of materials and their defects with an unprecedented spatial resolution. The current systems offering an energy resolution comparable to the one of soft x-ray absorption spectroscopy (about 0.1eV) make it possible to increase the sensitivity to changes in the near-edge structure while maintaining the capability to study small objects with a probe size better than 2nm [1]. With this improved performance, it is therefore possible to study changes in the electron energy loss spectra due to modifications of the structure or the bonding of the sample with minimal contributions due to the poor energy resolution of the instrument. This capability is a necessity for the study of transition metal oxides, and in particular compounds with perovskite-related structures, where subtle changes in the atomic arrangement or distortions in the bond angles can lead to dramatic changes in the physical properties. These structural and bonding effects are studied here by probing the variations of the O-K and Ti-L 23 edges in a series of transition metal oxides.Experiments were carried out on a FEI Tecnai-200FEG equipped with a monochromator[1] and a high-resolution electron energy loss spectrometer [2]. The first applications on a broad range of materials (including oxides, nitrides and fullerenes) and details of the performance of the system have been described elsewhere [3][4][5] and show that an energy resolution of about 0.1eV can be achieved yielding additional information on the near-edge structure spectra.The O-K edge in TiO 2 -brookite (figure 1a) shows two well-resolved peaks attributed to the O2p hybridization with the Ti3d states that are split into t 2g -e g components due to the octahedral crystal field. Experiments with and without the monochromator[3,4] supported by calculations make it possible to deduce that the width of these peaks is intrinsic to the electronic structure of this material and is due to dispersion of the hybridized states and the strong distortions of the octahedra. In the perovskite structures such as CaTiO 3 and SrTiO 3 ( figure 1b and ref. [6]), however, the t 2g peak (at 531eV) is significantly narrower and the e g peak intensity (at 533-534 eV) is significantly reduced due to the lower distortion of the octahedral environment around the cations. This sensitivity to bonding changes can be exploited when studying the variations of the local electronic structure due to substitutions of cations in perovskites. These effects can be probed in the Ba(Ti,Ru)O 3 compound where some of the Ti atoms are substituted by Ru. The main features of the O-K edge of the perovskite spectra are maintained in Ba(Ti,Ru)O 3 (figure 2a) since there are general similarities with the spectra of CaTiO 3 (figure 1b) and BaTiO 3 . Differences exist, however, in the intensity of the t 2g and e g peaks suggesting th...

High-resolution electron energy loss spectroscopy (EELS) provides better sensitivity to fine structures related to bonding in complex materials [1,2]. Using monochromators and improved spectrometers it is possible to achieve 0.1eV resolution with symmetric zero-loss peak profiles leading to improved detection of both core-loss and low-loss features [3] in EELS spectra. This capability makes it possible to study systematically changes in bonding in materials that present interesting physical properties. Oxides with the perovskite structure provide an interesting playing field to study many fundamental changes in structure and physical properties when systematic substitutions of atoms in the crystals are made in a controlled fashion. Substitutions of Ti atoms in BaTiO 3 with other transition metals provide ingenious ways to change the stability of the basic structure and induce phase transformations. For example, Mn substitutions for Ti in BaTiO 3 stabilize the hexagonal phase at room temperature and it has been proposed that this is due to increased metal-metal bonding between adjacent face-sharing octahedra [4]. EELS measurements in Ba(Ti,Mn)O 3 , however, have shown that, with respect to the tetragonal structure, there are only subtle changes in the oxygen K edge fine structures of the hexagonal compound due to overlaps of the Mn 3d electrons and O 2p electrons while the Ti environment is not altered significantly [5]. Ru substitutions for Ti also lead to the stabilization of the hexagonal phase but there are no reports on the effect of the changes in the near-edge structures related to the structural changes. For this particular case, it is of great interest, to study the closely related hexagonal perovskite BaRuO 3 . Probing electronic structure changes in these two materials with respect to BaTiO 3 makes it possible to gain insight on both the electronic structure of these closely related compounds and the origin of the structural changes. We have therefore used EELS with high energy resolution to probe the near edge structures in BaTiO 3 , Ba(Ti,Ru)O 3 and BaRuO 3 .Experiments were carried out on a FEI Tecnai-200FEG equipped with a monochromator and a high-resolution electron energy loss spectrometer as described previously [1]. While the overall structure of the O-K edge is similar for BaTiO 3 and the compounds with Ru substitutions, there are obvious differences in the details. The first peak at the edge threshold corresponds to the Ti 3d-t 2g -O 2p hybrid bands (or 4d bands in BaRuO 3 ) on the three systems. The peak is increasingly narrower in Ba(Ti,Ru)O 3 and BaRuO 3 as compared to BaTiO 3 . At about 2.5eV above the threshold, however, the very weak peak corresponding to the Ti 3d-e g -O 2p in BaTiO 3 and Ba(Ti,Ru)O 3 significantly differs from the BaRuO 3 . This suggests that in Ba(TiRu)O 3 , the Ru 4d states strongly overlap with Ti 3d electrons and the role of Ru appears to be minor from the electronic point of view. This is in marked contrast with the near-edge structure in the hexagonal BaRuO 3 wh...

Teflon AF is a family of amorphous copolymers containing fluoroethylene and dioxole groups. Its splendid properties such as low surface energy, high optical transmission, chemical resistance and low autofluorescence, have made it a desirable surface for the fast generation of molecular phospholipid films, which are being evaluated for biosensing and single molecule spectroscopy. The possibility of confinement of chemical species to a surface-adhered 2-dimesional film, while keeping them mobile within the structure, circumvents many problems of volume-based flow systems (Czolkos et al. 2011). Patterning the Teflon AF by common photolithography is limited to a few specialized processes with micrometer resolution, and it is still difficult to get nano-structured Teflon AF surfaces. It has been shown that a thin film of Teflon AF can be directly patterned by electron beam lithography without the need of further chemical development (Karre et al., 2009), where degradation of the fluorinated dioxole