Øystein Prytz

Nanoscale inclusions in the phonon glass thermoelectric material Zn4Sb3

We have investigated the thermoelectric material Zn4Sb3 using transmission electron microscopy (TEM). Nanoscale inclusions with a diameter of about 10 nm were observed, constituting on the order of 1% by volume of the material. Studies using energy filtered imaging, electron diffraction, and high-angle annular dark-field STEM indicate that the inclusions consist of Zn. These inclusions are expected to scatter the medium and long-wavelength phonons effectively, thus contributing to phonon glass behavior which results in the exceptionally low thermal conductivity for this thermoelectric material.

The influence of exact exchange corrections in van der Waals layered narrow bandgap black phosphorus

We study the electronic structure of black phosphorus by combining state-of-the-art density functional theory, multiple scattering calculations and electron energy loss spectroscopy. The hybrid functionals HSE03 and PBE0 are tested to investigate whether they give an improved description compared to the more traditional PZ-LDA and PBE-GGA functionals. These calculations are compared with investigations of the conduction band using electron energy loss spectroscopy and calculations based on the real-space multiple scattering approach, and previous determinations of the bandgap. The hybrid functional HSE03 gives an improved correspondence with these experiments. Comparisons of the calculated valence band with previous XPS studies yield acceptable agreement for the traditional functionals, while the results from the hybrid functionals are less satisfactory, since the hybrid functionals overestimate the valence band width significantly.

The Lorenz function: Its properties at optimum thermoelectric figure-of-merit

The Lorenz function is investigated for different scattering mechanisms, temperatures, chemical potentials, and charge carrier concentrations. We show that at optimum thermoelectric figure-of-merit, the deviation from the degenerate and non-degenerate limits is significant, where the magnitude of the deviation is determined by the scattering mechanism. Furthermore, the Lorenz functions are parameterized as a function of charge carrier concentration, temperature, and effective mass for different scattering mechanisms.

Electronic structure of thermoelectric Zn–Sb

The electronic structures of the two main compounds of the binary zinc antimonides that are stable at room temperature, Zn(1)Sb(1) and β-Zn(4)Sb(3), were probed with x-ray photoelectron spectroscopy. Additionally, electron energy loss measurements and density functional theory calculations are presented. The compounds are found to share a very similar electronic structure. They both feature only small charge transfers and differ moderately in their screening potentials. These results are in line with recent theoretical works on the Zn-Sb system and are discussed in light of the reported thermoelectric performance of the materials.

Reduction of lattice thermal conductivity from planar faults in the layered Zintl compound SrZnSb2

phonon scattering. 12 In the present study we perform transmission electron microscopy (TEM) investigations of the SrZnSb2 compound. We show that planar defects are present, but that these are not due to twinning. Instead, the defects are out-of-phase boundaries created by rigid shifts in the b–c plane of the material. These defects are discussed in light of the similar SrZnBi2 compound, and we report on calculations of the energetics of the observed defects. Furthermore, the effect of FIG. 1. (Color online) The crystal structure of (a) SrZnSb2 and (b) SrZnBi2. The arrangement of Zn and Sb/Bi atoms is very similar in the two structures, while the sequence of Sr atoms is different. A rigid shift of half the unit cell by the vector [0 1=2 1=2] would give similar arrangement of Sr atoms, while

Nanoscale mapping of optical band gaps using monochromated electron energy loss spectroscopy

Using monochromated electron energy loss spectroscopy in a probe-corrected scanning transmission electron microscope we demonstrate band gap mapping in ZnO/ZnCdO thin films with a spatial resolution below 10 nm and spectral precision of 20 meV.

New filled P-based skutterudites—promising materials for thermoelectricity?

The crystal structure, thermodynamic stability and electronic structure of the 75 filled and unfilled skutterudite compounds MxCo4- yFeyP12, with M being La, Y and Sc, x = {0, 0.125, 0.25, 0.50, 1} and y={0, 1, 2, 3, 4} are predicted from periodic density functional calculations. The rattling amplitude of the filling atom M, which is important for the thermal conductivity of the compounds, is found to depend mostly on the type of filling element and only to a small extent on the filling fraction, or the fraction of Fe in the structure. The calculated ground state stabilities show that none of the Sc-filled skutterudites are thermodynamically stable at 0 K, while the only stable combination of x and y among the studied Y-filled ones is YCoFe3P12. The La-filled skutterudites, on the other hand, are thermodynamically stable in a variety of combinations, with maximum stability when y=3x, that is three Fe atoms per filling atom. Based on the calculated electronic structure we also expect filled skutterudites with y=3x to be most interesting for thermoelectric applications.

Automated approaches for band gap mapping in STEM-EELS

Band gap variations in thin film structures, across grain boundaries, and in embedded nanoparticles are of increasing interest in the materials science community. As many common experimental techniques for measuring band gaps do not have the spatial resolution needed to observe these variations directly, probe-corrected Scanning Transmission Electron Microscope (STEM) with monochromated Electron Energy-Loss Spectroscopy (EELS) is a promising method for studying band gaps of such features. However, extraction of band gaps from EELS data sets usually requires heavy user involvement, and makes the analysis of large data sets challenging. Here we develop and present methods for automated extraction of band gap maps from large STEM-EELS data sets with high spatial resolution while preserving high accuracy and precision.

Band gap maps beyond the delocalization limit: correlation between optical band gaps and plasmon energies at the nanoscale

Recent progresses in nanoscale semiconductor technology have heightened the need for measurements of band gaps with high spatial resolution. Band gap mapping can be performed through a combination of probe-corrected scanning transmission electron microscopy (STEM) and monochromated electron energy-loss spectroscopy (EELS), but are rare owing to the complexity of the experiments and the data analysis. Furthermore, although this method is far superior in terms of spatial resolution to any other techniques, it is still fundamentally resolution-limited due to inelastic delocalization of the EELS signal. In this work we have established a quantitative correlation between optical band gaps and plasmon energies using the Zn1−xCdxO/ZnO system as an example, thereby side-stepping the fundamental resolution limits of band gap measurements, and providing a simple and convenient approach to achieve band gap maps with unprecedented spatial resolution.

The temperature-dependency of the optical band gap of ZnO measured by electron energy-loss spectroscopy in a scanning transmission electron microscope

The optical band gap of ZnO has been measured as a function of temperature using Electron Energy-Loss Spectroscopy (EELS) in a (Scanning) Transmission Electron Microscope ((S)TEM) from approximately 100 K up towards 1000 K. The band gap narrowing shows a close to linear dependency for temperatures above 250 K and is accurately described by Varshni, Bose-Einstein, Passler and Manoogian-Woolley models. Additionally, the measured band gap is compared with both optical absorption measurements and photoluminescence data. STEM-EELS is here shown to be a viable technique to measure optical band gaps at elevated temperatures, with an available temperature range up to 1500 K and the benefit of superior spatial resolution.