Strong enhancement of phonon scattering through nanoscale grains in lead sulfide thermoelectrics

We present nanocrystalline PbS, which was prepared using a solvothermal method followed by spark plasma sintering, as a promising thermoelectric material. The effects of grains with different length scales on phonon scattering of PbS samples, and therefore on the thermal conductivity of these samples, were studied using transmission electron microscopy and theoretical calculations. We found that a high density of nanoscale grain boundaries dramatically lowered the thermal conductivity by effectively scattering long-wavelength phonons. The thermal conductivity at room temperature was reduced from 2.5 W m1K 1 for ingot-PbS (grain size 4200 lm) to 2.3 W m1K 1 for micro-PbS (grain size 40.4 lm); remarkably, thermal conductivity was reduced to 0.85 W m1 K 1 for nano-PbS (grain size B30 nm). Considering the full phonon spectrum of the material, a theoretical model based on a combination of first-principles calculations and semiempirical phonon scattering rates was proposed to explain this effective enhancement. The results show that the high density of nanoscale grains could cause effective phonon scattering of almost 61%. These findings shed light on developing high-performance thermoelectrics via nanograins at the intermediate temperature range.This contribution was supported primarily by the startup of the South University of Science and Technology of China, supported by the Shenzhen government, and the national 1000 plan for young scientists. This work was also partially supported by a grant-in-aid of ‘985 Project’ from Xi’an Jiaotong University, the National Natural Science Foundation of China (Grant No. 21201138 and 11204228), the National Basic Research Program of China (2012CB619402 and 2014CB644003) and the Fundamental Research Funds for the Central UniversitiesS

Phase detection limits in off-axis electron holography from pixelated detectors: gain variations, geometric distortion and failure of reference-hologram correction

International audienc

Diffracted phase and amplitude measurements by energy-filtered convergent-beam holography (CHEF)

International audienceInterference between transmitted and diffracted disks in convergent-beam electron diffraction (CBED) patterns using the CBED+EBI method proposed by Herring et al. is explored using different optical configurations on a spherical aberration corrected transmission electron microscope equipped with a biprism and imaging energy filter: the SACTEM-Toulouse. We will relate the amplitude and phase of these interference patterns, which we call convergent-beam holography (CHEF), to microscope transfer theory and the complex amplitudes of the diffracted beams. Experimental CHEF patterns recorded in the absence of aberration correction will be compared with simulations to validate the theory concerning the effect of microscope aberrations and current instabilities. Then, using aberration correction, we propose a scheme for eliminating the effect of the microscope, so that the diffracted amplitudes and phase due to dynamical scattering within the specimen can be studied. Experimental results are compared with simulations performed using the full dynamical theory. The potential for studying diffracted amplitudes and phases using CHEF analysis is discussed

Mapping stress and strain in nanostructures by high-resolution transmission electron microscopy

International audienceWe present the current state-of-the-art of geometric phase analysis (GPA), a technique for measuring stress and strain at the nanoscale by high-resolution transmission electron microscopy (HRTEM). The method will be illustrated with an experimental study of SiGe strained layers using the SACTEM-Toulouse, an aberration-corrected transmission electron microscope. This latest generation machine improves signal-to-noise allowing deformations to be measured to an accuracy of 0.1% at nanometre scale resolution. The relation between strain and deformation will be discussed in the light of thin film relaxation and chemical interdiffusion

Nanoscale measurement of stress and strain by quantitative high-resolution electron microscopy

International audienc

Stress and strain around grain-boundary dislocations measured by high-resolution electron microscopy

International audienc

Reconstruction of a high angle tilt (110)/(001) boundary in Si using O-lattice theory

International audienceHigh angle close to 90° tilt Si boundary created by direct wafer bonding (DWB) using SmartCut® technology is studied in this work. Experimental identification of defects and morphologies at the interface is realized using conventional transmission electron microscopy (TEM) and geometric phase analysis (GPA) of high-resolution TEM images. Atom reconstruction of the interface along the direction is carried out within the frame of the O-lattice theory. We demonstrate that to preserve covalent bonding across the interface it should consist of facets intersected by a maximum of six planes with three 90° Shockley dislocations per facet. For a long enough interface the formation of Frank dislocations is predicted with a period equal 6 times that of Shockley dislocations. Long range undulations of the interface are shown to be related directly to a deviation from exact 90° tilt of the layer with respect to the substrate

Differential phase-contrast dark-field electron holography for strain mapping

cited By 6International audienceStrain mapping is an active area of research in transmission electron microscopy. Here we introduce a dark-field electron holographic technique that shares several aspects in common with both off-axis and in-line holography. Two incident and convergent plane waves are produced in front of the specimen thanks to an electrostatic biprism in the condenser system of a transmission electron microscope. The interference of electron beams diffracted by the illuminated crystal is then recorded in a defocused plane. The differential phase recovered from the hologram is directly proportional to the strain in the sample. The strain can be quantified if the separation of the images due to the defocus is precisely determined. The present technique has the advantage that the derivative of the phase is measured directly which allows us to avoid numerical differentiation. The distribution of the noise in the reconstructed strain maps is isotropic and more homogeneous. This technique was used to investigate different samples: a Si/SiGe superlattice, transistors with SiGe source/drain and epitaxial PZT thin films. © 2015 Elsevier B.V

Poster - Sim2Learn: Simulation of an Electron Microscope to facilitate Mental to Conceptual Model convergence

Scientific instruments such as electron microscopes allow rapid development in many fields of science but are difficult to fully exploit. Even for experienced users, their internal state (shape of the electron beam), represented in conceptual models, remains fairly inaccessible. This prevents the operator from quickly understanding the complete causal link between the observed phenomenon and the resulting image. This paper investigates how a simulation and visualization application of such an instrument, may help users bridge the gap between their mental and the conceptual models of the instrument by displaying an approximation of its internal state in real time. This is supported by a study with 10 microscopists who had to perform a series of plausible tasks on the interface and answer questions. The study shows that users have a better understanding of the instrument’s inner state while using the user interface. These results will help software designers develop the next generation of scientific instrument tools

Electron microscopy by specimen design: application to strain measurements

Abstract A bewildering number of techniques have been developed for transmission electron microscopy (TEM), involving the use of ever more complex combinations of lens configurations, apertures and detector geometries. In parallel, the developments in the field of ion beam instruments have modernized sample preparation and enabled the preparation of various types of materials. However, the desired final specimen geometry is always almost the same: a thin foil of uniform thickness. Here we will show that judicious design of specimen geometry can make all the difference and that experiments can be carried out on the most basic electron microscope and in the usual imaging modes. We propose two sample preparation methods that allow the formation of controlled moiré patterns for general monocrystalline structures in cross-section and at specific sites. We developed moiré image treatment algorithms using an absolute correction of projection lens distortions of a TEM that allows strain measurements and mapping with a nanometer resolution and 10−4 precision. Imaging and diffraction techniques in other fields may in turn benefit from this technique in perspective