Phase Transitions in Spin-Crossover Thin Films Probed by Graphene Transport Measurements.

Future multifunctional hybrid devices might combine switchable molecules and 2D material-based devices. Spin-crossover compounds are of particular interest in this context since they exhibit bistability and memory effects at room temperature while responding to numerous external stimuli. Atomically thin 2D materials such as graphene attract a lot of attention for their fascinating electrical, optical, and mechanical properties, but also for their reliability for room-temperature operations. Here, we demonstrate that thermally induced spin-state switching of spin-crossover nanoparticle thin films can be monitored through the electrical transport properties of graphene lying underneath the films. Model calculations indicate that the charge carrier scattering mechanism in graphene is sensitive to the spin-state dependence of the relative dielectric constants of the spin-crossover nanoparticles. This graphene sensor approach can be applied to a wide class of (molecular) systems with tunable electronic polarizabilities.

1/f noise in graphene nanopores.

Graphene nanopores are receiving great attention due to their atomically thin membranes and intrinsic electrical properties that appear greatly beneficial for biosensing and DNA sequencing. Here, we present an extensive study of the low-frequency 1/f noise in the ionic current through graphene nanopores and compare it to noise levels in silicon nitride pore currents. We find that the 1/f noise magnitude is very high for graphene nanopores: typically two orders of magnitude higher than for silicon nitride pores. This is a drawback as it significantly lowers the signal-to-noise ratio in DNA translocation experiments. We evaluate possible explanations for these exceptionally high noise levels in graphene pores. From examining the noise for pores of different diameters and at various salt concentrations, we find that in contrast to silicon nitride pores, the 1/f noise in graphene pores does not follow Hooge's relation. In addition, from studying the dependence on the buffer pH, we show that the increased noise cannot be explained by charge fluctuations of chemical groups on the pore rim. Finally, we compare single and bilayer graphene to few-layer and multi-layer graphene and boron nitride (h-BN), and we find that the noise reduces with layer thickness for both materials, which suggests that mechanical fluctuations may be the underlying cause of the high 1/f noise levels in monolayer graphene nanopore devices.

Spin ½ Delafossite honeycomb compound Cu5SbO6.

Cu(5)SbO(6) is found to have a monoclinic, Delafossite-derived structure consisting of alternating layers of O-Cu(I)-O sticks and magnetic layers of Jahn-Teller distorted Cu(II)O(6) octahedra in an edge sharing honeycomb arrangement with Sb(V)O(6) octahedra. This yields the structural formula Cu(I)(3)Cu(II)(2)Sb(V)O(6). Variants with ordered and disordered layer stacking are observed, depending on the synthesis conditions. The spin ½ Cu(2+) ions form dimers in the honeycomb layer. The magnetic susceptibility measured between 5 and 300 K is characteristic of the presence of a singlet-triplet spin gap of 189 K. High resolution synchrotron X-ray diffraction studies indicate that changes in the intra- or interdimer distances between 300 and 20 K, such as might indicate an increase in strength of the Peierls-like distortion through the spin gap temperature, if present, are very small. A comparison to the NaFeO(2)-type Cu(2+) honeycomb compounds Na(3)Cu(2)SbO(6) and Na(2)Cu(2)TeO(6) is presented.

Origin of predominance of cementite among iron carbides in steel at elevated temperature.

A long-standing challenge in physics is to understand why cementite is the predominant carbide in steel. Here we show that the prevalent formation of cementite can be explained only by considering its stability at elevated temperature. A systematic highly accurate quantum mechanical study was conducted on the stability of binary iron carbides. The calculations show that all the iron carbides are unstable relative to the elemental solids, α-Fe and graphite. Apart from a cubic Fe23C6 phase, the energetically most favorable carbides exhibit hexagonal close-packed Fe sublattices. Finite-temperature analysis showed that contributions from lattice vibration and anomalous Curie-Weis magnetic ordering, rather than from the conventional lattice mismatch with the matrix, are the origin of the predominance of cementite during steel fabrication processes.

Superconductivity in CuxBi2Se3 and its implications for pairing in the undoped topological insulator.

Bi2Se3 is one of a handful of known topological insulators. Here we show that copper intercalation in the van der Waals gaps between the Bi2Se3 layers, yielding an electron concentration of approximately 2x10{20} cm{-3}, results in superconductivity at 3.8 K in CuxBi2Se3 for 0.12

Cluster-glass behavior of a highly oxygen deficient perovskite, BaBi(0.28)Co(0.72)O(2.2).

A highly oxygen deficient perovskite, BaBi(0.28)Co(0.72)O(2.2), was synthesized by solid state reaction. The crystal structure was determined by means of neutron and x-ray powder diffraction. The material exhibits semiconducting behavior with an energy gap of 1.8 eV. The electron diffraction study does not reveal long range Bi:Co ordering; instead it shows the existence of short range ordering in this phase. The AC and DC magnetic susceptibility studies reveal cluster-glass behavior, which has its origin in the interacting ferromagnetic clusters present.

Downsizing of robust Fe-triazole@SiO2 spin-crossover nanoparticles with ultrathin shells.

A chemical protocol to design robust hybrid [Fe(Htrz)2(trz)](BF4)@SiO2 nanoparticles (NPs) with sizes as small as 28 nm and ultrathin silica shells below 3 nm has been developed. These NPs present a characteristic abrupt spin transition with a subsequent decrease in the width of the thermal hysteresis upon reducing the NP size.

Atomic-scale electron microscopy at ambient pressure.

We demonstrate a novel nanoreactor for performing atomic-resolution environmental transmission electron microscopy (ETEM) of nanostructured materials during exposure to gases at ambient pressures and elevated temperatures. The nanoreactor is a microelectromechanical system (MEMS) and is functionalized with a micrometer-sized gas-flow channel, electron-transparent windows and a heating device. It fits into the tip of a dedicated sample holder that can be used in a normal CM microscope of Philips/FEI Company. The nanoreactor performance was demonstrated by ETEM imaging of a Cu/ZnO catalyst for methanol synthesis during exposure to hydrogen. Specifically, the nanoreactor facilitated the direct observation of Cu nanocrystal growth and mobility on a sub-second time scale during heating to 500 degrees C and exposure to 1.2 bar of H(2). For the same gas reaction environment, ETEM images show atomic lattice fringes in the Cu nanocrystals with spacing of 0.18 nm, attesting the spatial resolution limit of the system. The nanoreactor concept opens up new possibilities for in situ studies of nanomaterials and the ways they interact with their ambient working environment in diverse areas, such as heterogeneous catalysis, electrochemistry, nanofabrication, materials science and biology.

Frustrated ferroelectricity in niobate pyrochlores.

The crystal structures of the A(2)B(2)O(7-x) niobium based pyrochlores Y(2)(Nb(0.86)Y(0.14))(2)O(6.91), CaYNb(2)O(7), and Y(2)NbTiO(7) are reported, determined by means of powder neutron diffraction. These compounds represent the first observation of B-site displacements in the pyrochlore structure: the B-site ions are found to be displaced from the ideal pyrochlore positions, creating electric dipoles. The orientations of these dipoles are fully analogous to orientations of the magnetic moments in Ising spin based magnetically frustrated pyrochlores. Diffuse scattering in electron diffraction patterns shows that the displacements are only short range ordered, indicative of geometric frustration of the collective dielectric state of the materials. Comparison to the crystal structure of the Nb(5+) (d(0)) pyrochlore La(2)ScNbO(7) supports the prediction that charge singlets, driven by the tendency of Nb to form metal-metal bonds, are present in these pyrochlores. The observed lack of long range order to these singlets suggests that Nb(4+) based pyrochlores represent the dielectric analogy to the geometric frustration of magnetic moments observed in rare earth pyrochlores.

Angular and local spectroscopic analysis to probe the vertical alignment of N-doped well-separated carbon nanotubes.

Vertically aligned well-separated N-doped multiwalled carbon nanotubes (CNTs) were grown on a silicon substrate by plasma enhanced chemical vapor deposition (PECVD). Angular near-edge X-ray absorption fine structure (NEXAFS) was used to investigate the vertical alignment of as-grown CNTs. In addition, both individual tubes and tube bundles were characterized by high-resolution electron energy loss spectroscopy (HREELS). Simultaneous analysis of both spectroscopic techniques provides information on chemical environment, orbital orientation between carbon and heteroatoms, and local curvature effects. We demonstrate the utility of NEXAFS as an in situ probe of CNTs.

Enhancement of resolution in core-loss and low-loss spectroscopy in a monochromated microscope.

The significant enhancement of the energy resolution in the new generation of commercially available monochromated transmission electron microscopes presents new challenges in term of selecting the correct experimental conditions and understanding the various effects that can potentially influence the quality of the EELS data. In this respect we investigated the effect of point spread function of the detector and spectrum-diffraction mixing on the energy resolution and the intensity of the zero loss peak tails. Alternative approaches to improve the energy resolution by mathematical methods have been tested. By using a simple and commonly available test case (Si L(2,3) edges) we assessed the efficiency of the deconvolution algorithms to improve the resolution. The results show that the deconvolution is not always successful in improving the resolution of the core loss EELS data and the results may not always be reliable. Contrary to this, the application of the Richardson-Lucy deconvolution algorithm on some bandgap measurements data appears to be very effective. The procedure proved successful in removing the contribution of the zero-loss peak tails and allows an easier access to spectroscopic information starting at energy losses as low as of 0.5 eV with monochromated spectra and 1 eV with the non-monochromated spectra.

Image deconvolution in spherical aberration-corrected high-resolution transmission electron microscopy.

The method of image deconvolution developed previously for FEG high-resolution transmission electron microscope (HRTEM) without a spherical aberration (C(s)) corrector was for the first time applied to FEG HRTEM with a C(s)-corrector. The principle and the procedure of image deconvolution are briefly described. Four qualified [1 1 0] images of Si were selected from a through-focus series to perform image deconvolution. The projected potential is successfully derived from all the images, and the obtained "dumbbell" structure maps of Si [1 1 0] are in good agreement with the calculated potential map. The criterion of selecting qualified images for performing image deconvolution is indicated. The possibility of applying image deconvolution to defect study and to ab initio crystal structure determination is discussed.

Cerenkov losses: a limit for bandgap determination and Kramers-Kronig analysis.

Measuring low energy losses in semiconductors and insulators with high spatial resolution becomes attractive with the increasing availability of modern transmission electron microscopes (TEMs) equipped with monochromators, C(s) correctors and energy filters. In this paper, we demonstrate that Cerenkov losses pose a limit for the interpretation of low energy loss spectra (EELS) in terms of interband transistions and bandgap determination for many materials. If the velocity of a charged particle in a medium exceeds the velocity of light, photons are emitted leading to a corresponding energy loss of a few electronvolt. Since these losses are strong for energies below the onset of interband transitions, they change the apparent loss function of semiconductors and insulators, with the risk of erroneous interpretation of spectra. We measured low energy losses of Si and GaAs with a monochromated TEM demonstrating the effect of sample thickness on Cerenkov losses. Angle resolved EELS and energy filtered diffraction patterns (taken without a monochromator) show the extremely narrow angular distribution of Cerenkov losses. The latter experiment provides a method that allows to decide whether Cerenkov radiation masks the very low loss signal in EELS.

Translocation of double-strand DNA through a silicon oxide nanopore.

We report double-strand DNA translocation experiments using silicon oxide nanopores with a diameter of about 10 nm . By monitoring the conductance of a voltage-biased pore, we detect molecules with a length ranging from 6557 to 48 500 base pairs. We find that the molecules can pass the pore both in a straight linear fashion and in a folded state. Experiments on circular DNA further support this picture. We sort the molecular events according to their folding state and estimate the folding position. As a proof-of-principle experiment, we show that a nanopore can be used to distinguish the lengths of DNA fragments present in a mixture. These experiments pave the way for quantitative analytical techniques with solid-state nanopores.

Exit wave reconstructions using through focus series of HREM images.

The through focus exit wave reconstruction technique uses a series of high resolution electron microscopy images to reconstruct the complex electron wavefunction at the exit plane of the specimen. The feasibility of the through focus exit wave reconstruction method and its most important limitations are discussed. It is shown that-provided all aberrations of the microscope are well corrected for-a large improvement in the interpretability of the information can be obtained.

Determination of absolute configurations of crystal structures using electron diffraction patterns by means of least-squares refinement.

A simple method is reported to determine the absolute configuration of the crystal structure from electron diffraction patterns taken from very small areas. The method is based on the differences in the Friedel reflections, which are in general much larger than for X-rays due to the dynamical behaviour of the electron scattering. We express the absolute configuration with a parameter similar to the one Flack (Acta Cryst. A 39 (1983) 876) introduced in X-ray crystallography. This parameter is added to a refinement procedure that uses a multi-slice calculation to calculate diffraction patterns. The validity and strength of the method are shown with simulated and experimental data sets of GaN in the [0 1 0]-zone and a more complex compound, Ce5Cu19P12 in the [0 0 1] zone.

The effect of inelastic scattering on crystal structure refinement from electron diffraction patterns recorded under almost parallel illumination.

Recently a number of crystal structures were determined using electron diffraction data with an almost parallel electron beam. In many cases no energy filtering was applied. On the other hand, the contrast in convergent beam electron diffraction patterns is greatly improved by energy filtering of the electrons. To investigate whether energy filtering will improve the accuracy of the structure analysis from diffraction data recorded under an almost parallel beam condition, we recorded diffraction patterns of the [100] zone of YBa(2)Cu(3)O(7) using unfiltered electrons, zero-loss electrons and plasmon-loss electrons, respectively. Subsequently, the structure is refined based on these different electron diffraction datasets, using the program MSLS (Acta Crystallogr. A 54 (1998) 91). The results show that the obtained atomic positions are not significantly different for the chosen filter conditions. Even with amorphous carbon deposited on the specimen, which will cause a significant increase (>40 times) of energy-loss electrons, the structure refinement led to the same atomic positions.

Investigation of hexagonal and cubic GaN by high-resolution electron energy-loss spectroscopy and density functional theory.

High-resolution electron energy-loss spectroscopy in a transmission electron microscope is a very powerful method for the study of electronic structure of materials. The fine structure of Ga L(2,3) and N ionization edges in c-GaN and h-GaN was studied using a TEM equipped with a monochromator and high-resolution energy spectrometer. The experimental results were compared with the results of calculation based on the density functional theory using the Wien2k code and show that the best fit is achieved when the core hole effect is taken into account. The effect of the core hole value and the supercell size on the energy-loss near-edge structure have been investigated. A different behaviour was found for c-GaN and h-GaN: better agreement is obtained for a 0.5 core hole for h-GaN and for a full core hole for c-GaN. The anisotropic behaviour of the experimental spectra and calculated spectra for h-GaN have been studied and the "magic" angle was determined.

Determination of the orientation in small areas of the exit wave.

Local differences in thickness and misorientation in a through-focus reconstructed exit wave hamper direct interpretation. These local thicknesses and misorientations can be refined on an atomic scale using the intensities of the Fourier components with the refinement procedure in which a least-squares refinement is combined with a multi-slice calculation. The method was applied to the superconductor La3Ni2B2N3. The crystal tilt and specimen thickness can be determined with an R-value of 8-25%, with a better R-value for thinner areas. Significant differences in the refined tilts, thicknesses and R-values are observed when reconstructed exit waves that are corrected for mechanical vibration are compared with reconstructed exit waves, which are uncorrected.

Atomic imaging in aberration-corrected high-resolution transmission electron microscopy.

In recent years, the successful implementation of a spherical-aberration corrector in a Philips CM 200 FEG ST microscope achieved by Haider et al. has attracted a great deal of attention. However, thus far extensive applications of this novel high-resolution transmission electron microscope (HRTEM) to materials research have been hampered by the problems concerning optimum imaging conditions and image interpretation. In this paper, we present our points of view concerning atomic imaging in an aberration-corrected HRTEM. Since atomic resolution images can also be obtained with other techniques such as through-focus exit-wave function reconstruction (TF-EWR), we have to emphasis that the strength of the aberration-corrected HRTEM particularly lies on its ability to resolve the atomic structure in real time. However, for this purpose it is mandatory that the image contrast be related in a one-to-one function with the projected structure of the object. We analyzed the atomic imaging conditions in much detail and we come to the following conclusion: this novel facility is no doubt a powerful and advanced HRTEM instrument in achieving atomic images with its highest resolution (information limit). We furthermore demonstrate that the combination of the new microscope and TF-EWR will yield optimal results.