RESUMO
A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.
RESUMO
Porous materials display enhanced scattering mechanisms that greatly influence their transport properties. Metal-assisted chemical etching (MACE) enables fabrication of porous silicon nanowires starting from a doped Si wafer by using a metal template that catalyzes the etching process. Here, we report on the low thermal conductivity (κ) of individual porous Si nanowires (NWs) prepared from MACE, with values as low as 0.87 W·m-1·K-1 for 90 nm diameter wires with 35-40% porosity. Despite the strong suppression of long mean free path phonons in porous materials, we find a linear correlation of κ with the NW diameter. We ascribe this dependence to the anisotropic porous structure that arises during chemical etching and modifies the phonon percolation pathway in the center and outer regions of the nanowire. The inner microstructure of the NWs is visualized by means of electron tomography. In addition, we have used molecular dynamics simulations to provide guidance for how a porosity gradient influences phonon transport along the axis of the NW. Our findings are important towards the rational design of porous materials with tailored thermal and electronic properties for improved thermoelectric devices.
RESUMO
This paper presents a novel 3D method to correct for absorption in energy dispersive X-ray (EDX) microanalysis of heterogeneous samples of unknown structure and composition. By using STEM-based tomography coupled with EDX, an initial 3D reconstruction is used to extract the location of generated X-rays as well as the X-ray path through the sample to the surface. The absorption correction needed to retrieve the generated X-ray intensity is then calculated voxel-by-voxel estimating the different compositions encountered by the X-ray. The method is applied to a core/shell nanowire containing carbon and oxygen, two elements generating highly absorbed low energy X-rays. Absorption is shown to cause major reconstruction artefacts, in the form of an incomplete recovery of the oxide and an erroneous presence of carbon in the shell. By applying the correction method, these artefacts are greatly reduced. The accuracy of the method is assessed using reference X-ray lines with low absorption.
RESUMO
Electron tomography allows the 3D quantitative characterization of nanostructures, provided a monotonic relationship is fulfilled between the projected signal and the atomic number and thickness of the specimen. This requirement is not satisfied if the micrographs are affected by (i) diffraction contrast, (ii) detector saturation or (iii) contrast inversion due to absorption (high-angle scattering) at high thickness. Artefacts related to the non-monotonic tomography acquisition are examined using computer simulations and experimental tilt series of tungsten tips and CeO(2) nanoparticles. Conditions are derived under which in spite of the non-linear artefacts the information is sufficient for reconstructing the 3D morphology of convex objects by geometric tomography.
RESUMO
Piezoelectric nanoactuation, which is rapidly becoming established as state-of-the-art positioning control in transmission electron microscopy (TEM), is extended here to include a rotational degree of freedom. A piezoelectric goniometer with both translational and rotary drive action has been designed with high level of miniaturization to fit into a standard TEM specimen holder shaft without compromising any of the performance of the default TEM goniometer and without any modifications to the TEM. Enhanced functionality of such a goniometer-in-goniometer is outlined and experimental results for electron tomography of nanostructures over a full tilt range of views, without any missing angles, are demonstrated.
