Electronic interactions between "pea" and "pod": the case of oligothiophenes encapsulated in carbon nanotubes

One of the most challenging strategies to achieve tunable nanophotonic devices is to build robust nanohybrids with variable emission in the visible spectral range, while keeping the merits of pristine single-walled carbon nanotubes (SWNTs). This goal is realized by filling SWNTs ("pods") with a series of oligothiophene molecules ("peas"). The physical properties of these peapods are depicted by using aberration-corrected high-resolution transmission electron microscopy, Raman spectroscopy, and other optical methods including steady-state and time-resolved measurements. Visible photoluminescence with quantum yields up to 30% is observed for all the hybrids. The underlying electronic structure is investigated by density functional theory calculations for a series of peapods with different molecular lengths and tube diameters, which demonstrate that van der Waals interactions are the bonding mechanism between the encapsulated molecule and the tube.

Host tree age as a selective pressure leading to local adaptation of a population of a polyphagous Lepidoptera in virgin boreal forest

We tested the hypothesis that host tree age may act as a selective factor and lead to local adaptation of the hemlock looper (Lambdina fiscellaria), a geometrid Lepidoptera that has a wide geographical distribution and has evolved in different eco-zones characterized by different levels of floristic composition, age structure and fragmentation level. Considering that hemlock looper outbreaks mainly occurred in old forests, we compared the biological performances of two populations. The first population was collected in the northern virgin boreal forest, which is dominated by mature and overmature coniferous stands that have not suffered from human disturbance. The other population was collected in the southern mixed-wood forest, which is more diversified and has been modified by forest harvesting. Larvae were reared under controlled conditions on foliage from three age classes of balsam fir trees: juvenile, mature and overmature. Although we measured significant variations of biological performances between the two populations, no significant effect of the age of the balsam fir trees could be detected for males from both populations or for females from the southern population. However, northern females were strongly affected by the age of balsam fir trees on which they fed, as their pupal weight was 12% higher and their fecundity increased by 27% on overmature trees compared with juvenile ones. These results indicate that under the same selective pressure, females adapt their strategy to maximize their fitness, and thus they appear as the driving force of evolution through the local adaptation concept. Furthermore, the two populations evolved in distinct habitats and their adaptation reflects selective pressures occurring inside their original environment. This is the first report on local adaptation of an herbivore that is mediated by host tree age. Changes in forest age structure may have a considerable impact on insect local adaptation and presumably on their population dynamics.

Magnetic circular dichroism in EELS: Towards 10nm resolution

We describe a new experimental setup for the detection of magnetic circular dichroism with fast electrons (EMCD). As compared to earlier findings the signal is an order of magnitude higher, while the probed area could be significantly reduced, allowing a spatial resolution of better than 40 nm. A simplified analysis of the experimental results is based on the decomposition of the mixed dynamic form factor S(q-->,q-->('),E) into a real part related to the scalar product and an imaginary part related to the vector product of the scattering vectors q--> and q-->('). Following the recent detection of chiral electronic transitions in the electron microscope the present experiment is a crucial demonstration of the potential of EMCD for nanoscale investigations.

Optimal aperture sizes and positions for EMCD experiments

The signal-to-noise ratio (SNR) in energy-loss magnetic chiral dichroism (EMCD)--the equivalent of X-ray magnetic circular dichroism (XMCD) in the electron microscope--is optimized with respect to the detector shape, size and position. We show that an important increase in SNR over previous experiments can be obtained when taking much larger detector sizes. We determine the ideal shape of the detector but also show that round apertures are a good compromise if placed in their optimal position. We develop the theory for a simple analytical description of the EMCD experiment and then apply it to dynamical multibeam Bloch wave calculations and to an experimental data set. In all cases it is shown that a significant and welcome improvement of the SNR is possible.

Magnetic circular dichroism in electron energy loss spectrometry

The measurement of circular dichroism in the electron microscope is a new, emerging method and, as such, it is subject to constant refinement and improvement. Different ways can be envisaged to record the signal. We present an overview of the key steps in the energy-loss magnetic chiral dichroism (EMCD) experiment as well as a detailed review of the methods used in the intrinsic way where the specimen is used as a beam splitter. Lateral resolution up to 20-30 nm can be achieved, and the use of convergent beam techniques leads to an improved S/N ratio. Dichroic effects are shown for Ni and Co single crystal; as a counterexample, measurements were carried also for a non-magnetic (Ti) sample, where no dichroic effect was found.

Atomoxetine, but not paroxetine, blocks norepinephrine reuptake in depressed patientss

Purpose: Paroxetine is a potent serotonin (5-HT) reuptake inhibitor. However, a purported norepinephrine (NE) reuptake blockade action remains to be established. Atomoxetine is a potent NE reuptake inhibitor with the indication of attention deficit hyperactivity disorder (ADHD). The present study was aimed at confirming a NE reuptake inhibitory action with ascending doses of atomoxetine and possibly with paroxetine in depressed patients. Methods: Patients were randomized to escalating doses of either paroxetine (20 to 50 mg/day), or atomoxetine (25-80 mg/day) in a four to six week period. Inhibition of NE reuptake was assessed using the attenuation of systolic blood pressure (SBP) elevations produced by intravenous injections of tyramine. Tyramine penetrates into peripheral NE terminals via the NE reuptake transporter and releases NE. Then, NE acts on the vascular adrenoceptors, which causes an elevation of SBP. Drugs that block NE reuptake attenuate the pressor effects of tyramine. Two-way ANOVA for repeated measures for doses of tyramine and treatments were used to assess the effects of the different drug regimens on the pressor response to loads of 3–6 mg of tyramine. Sixteen patients with unipolar major depressive disorder were assessed weekly after increasing the dose of paroxetine (9 patients) and atomoxetine (7 patients). Results: Atomoxetine exerted a robust inhibition of the tyramine response, starting at the dose of 25mg/day in a dose-dependent pattern. Neither the low nor the high doses of paroxetine altered the tyramine pressor response. Conclusions: These results provide evidence that atomoxetine started significantly inhibiting NE reuptake at subtherapeutic dose for ADHD, whereas paroxetine leaves the activity of the NE transporter unaltered, even at the highest recommended dose for depression.

Practical aspects of running the WIEN2k code for electron spectroscopy

The Wien2k code is widely used for the calculation of electron energy loss spectra. Low loss spectra can be calculated with the OPTIC package while core loss spectra are calculated with the TELNES program. A new version, TELNES.2, takes into account the effects of relativity for anisotropic materials. In this paper we discuss the effects of different parameters used for the self-consistent calculation of the electron density on the obtained spectra. We give an overview of possibilities for the calculation of complicated systems requiring a super-cell, like defects or disordered systems. We discuss the problem of the core hole and of the calculation of orientation-sensitive spectra and give an overview of results already published.

Geographic biotype and host-associated local adaptation in a polyphagous species, Lambdina fiscellaria (Lepidoptera: Geometridae) feeding on balsam fir on Anticosti Island, Canada

The debate about mechanisms underlying the evolution of host specialization by herbivorous insects remains open. Natural selection may act locally and lead to different patterns of geographic variation in life history traits of polyphagous herbivores. The hypothesis of genetically-based trade-offs in offspring performance on different hosts has been proposed but this has rarely been demonstrated. Under laboratory conditions, the biological performance of two populations of the hemlock looper Lambdina fiscellaria (Guenée), a highly polyphagous lepidopteran, was compared when reared on three different tree host species: balsam fir, eastern hemlock and sugar maple. One population originated from Anticosti Island, Québec, Canada, where the insect has evolved without having access to two of the three tree species tested, the other being from the mainland where all tree species are present. When reared on balsam fir foliage, which was naturally available to each population, larvae from Anticosti Island underwent four instars compared with five for the mainland population, indicating the existence of geographic biotypes in L. fiscellaria. When reared on the foliage of non-naturally available host trees, larvae from Anticosti Island had a higher incidence of supernumerary instars. This is a unique example where local adaptation to environmental conditions of an insect herbivore is expressed through a differential number of larval instars. Moreover, the Anticosti Island population showed a higher growth related index on the host available to both populations indicating that a fitness trade-off was the evolutionary process underlying the local adaptation of this population on balsam fir.

Experimental and theoretical determination of low electron energy loss spectra of Ag and Ru

We show the experimental and calculated q-dependent low energy loss electron energy loss spectrum of Ru and Ag. The spectra were calculated within the time-dependent density-functional theory including local-field effects. For Ag, the momentum transfer was parallel to the (110) direction. For Ru the three main directions (010), (110) and (001) were investigated. The agreement between theory and experiment is very good for Ag and for momentum transfers parallel to the (001) direction of Ru. For momentum transfers parallel to the in-plane directions (110) and (010) the agreement for Ru is not satisfactory, which could be attributed to relativistic effects or to strong localization of the 4d states of Ru.

ELNES at magic angle conditions

If one needs to cancel the effects of the anisotropy of the sample in a EELS experiment in the TEM, a particular couple of values for the collection and convergence angle must be used, called magic angle conditions (MAC). Recent developments in the theory have shown that a full relativistic treatment is mandatory to correctly describe this effect and that the MAC are strongly dependent on the acceleration voltage. We show how the analytical formula can be derived and give the exact analytical solution for the MAC which can then be easily applied to every practical case. We show the consequences of the energy dependence of the magic angle and that the parallelity of the beam will be the limiting factor for high acceleration voltages while for low acceleration voltages the contribution coming from Bragg spots may make it impossible to reach MAC.

Short note on parallel illumination in the TEM

Parallel illumination conditions are required for several experiments in the transmission electron microscope (TEM). The image rotation induced by the helical trajectory of electrons passing through the magnetic field of the TEM lenses inevitably induces an inclination of the beam relative to the optical axis in the object plane--even for an electron which travels parallel to the optical axis in the far field. This angle (shear angle) is vectorially added to the convergence angle; it depends both on the distance to the optical axis and the magnetic field. By using a beam tilt compensation method, the minimum shear angle is found to be of the order of 1 mrad for a field of view of 2 microm in a 200 kV TEM. In practice, "parallel illumination" can only be obtained for fields of view 1 microm.

Detection of magnetic circular dichroism using a transmission electron microscope

A material is said to exhibit dichroism if its photon absorption spectrum depends on the polarization of the incident radiation. In the case of X-ray magnetic circular dichroism (XMCD), the absorption cross-section of a ferromagnet or a paramagnet in a magnetic field changes when the helicity of a circularly polarized photon is reversed relative to the magnetization direction. Although similarities between X-ray absorption and electron energy-loss spectroscopy in a transmission electron microscope (TEM) have long been recognized, it has been assumed that extending such equivalence to circular dichroism would require the electron beam in the TEM to be spin-polarized. Recently, it was argued on theoretical grounds that this assumption is probably wrong. Here we report the direct experimental detection of magnetic circular dichroism in a TEM. We compare our measurements of electron energy-loss magnetic chiral dichroism (EMCD) with XMCD spectra obtained from the same specimen that, together with theoretical calculations, show that chiral atomic transitions in a specimen are accessible with inelastic electron scattering under particular scattering conditions. This finding could have important consequences for the study of magnetism on the nanometre and subnanometre scales, as EMCD offers the potential for such spatial resolution down to the nanometre scale while providing depth information--in contrast to X-ray methods, which are mainly surface-sensitive.

The magic angle: a solved mystery

We resolve the long-standing mysterious discrepancy between the experimental magic angle in EELS--approximately 2theta(E)--and the quantum mechanical prediction of approximately 4theta(E). A relativistic approach surpassing the usually applied kinematic correction yields a magic angle close to the experimental value. The reason is that the relativistic correction of the inelastic scattering cross section in anisotropic systems is significantly higher than in isotropic ones.

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.

A proposal for dichroic experiments in the electron microscope

Building upon the similarities between inelastic electron scattering and X-ray absorption we show that dichroism can be observed in electron energy loss spectrometry (EELS) in the transmission electron microscope (TEM). Natural or magnetic linear dichroism can be studied in electron scattering experiment with definite wave vector transfer in the interaction.The detection of circular dichroism in the TEM relies on interferometric EELS in a particular scattering geometry that allows extraction of the mixed dynamic form factor from energy loss spectra. Similarities between dichroic signals in energy loss near edge structures and X-ray absorption near edge structures are discussed, and a new experimental setup for dichroic measurements in the TEM is proposed.

Improvement of energy loss near edge structure calculation using Wien2k

The density functional theory (DFT) is a recognised method for the calculation of electronic properties of materials. As such it can also be used for the calculation of energy loss near edge structures. Some care has to be taken since the DFT is intended for ground state calculation. The effect of the core hole left by the excited electron is different in an insulator and in a metal and can be observed in both cases. For an insulator (MgO, Si), a supercell calculation is needed while in the case of copper, extremely good agreement with experiment can be obtained with a partial core hole calculation. In the particular case of the WIEN code (APW method) we show that calculation of low lying edges (Si L at 99eV) where the initial state is not strongly localised can only be done within the dipole approximation and with some care. Random alloys (CuNi) have been calculated previously using a supercell; we show that a particular version of the virtual crystal approximation gives promising results.

Anisotropy and collection angle dependence of the oxygen K ELNES in V2O5: a band-structure calculation study

We present a theoretical study of the anisotropy and collection angle dependence of the oxygen K ELNES in V2O5. Ab initio band-structure calculations were performed with WIEN97, a program package based on the full potential linearised augmented plane waves (FP-LAPW) method. An analysis of the site and angular momentum projected DOS allowed the identification of differently coordinated oxygens and the separation of the oxygen K-edge into contributions from terminal (vanadyl) oxygens, bridging oxygens and chain oxygens. The major contribution to the anisotropy of the O K-edge ELNES could be assigned to transitions at the vanadyl oxygen. Theoretical calculations predict that the extent of changes in the ELNES would be large enough for detection in collection angle dependent O K-edge measurements. A variation in the fine structure of the O K-edge with decreasing collection angle was confirmed by experiments.

High resolution EELS using monochromator and high performance spectrometer: comparison of V2O5 ELNES with NEXAFS and band structure calculations

Using single crystal V2O5 as a sample, we tested the performance of the new aberration corrected GATAN spectrometer on a monochromatised 200 kV FEG FEI (S)TEM. The obtained V L and O K ELNES were compared with that obtained in a common GATAN GIF and that in the new spectrometer, without monochromatised beam. The performance of the new instrumentation is impressive: recorded with an energy-resolution of 0.22 eV, the V L(3) edge reveals all the features due to the bulk electronic structure, that are also revealed in near-edge X-ray absorption fine structure (NEXAFS) with a much higher energy-resolution (0.08 eV). All features of the ELNES and NEXAFS are in line with a theoretical spectrum derived from band-structure calculations.