214 research outputs found
Disorder control in crystalline GeSb2Te4 and its impact on characteristic length scales
Crystalline GeSb2Te4 (GST) is remarkable material, as it allows to
continuously tune the electrical resistance by orders of magnitude without
involving a phase transition or stoichiometric changes, just by altering the
short-range order. While well-ordered specimen are metallic, increasing amounts
of disorder can eventually lead to an insulating state with vanishing
conductivity in the 0K limit, but a similar number of charge carriers. These
observations make disordered GST one of the most promising candidates for the
realization of a true Anderson insulator. While so far the low-temperature
properties have mostly been studied in films of small grain size, here a
sputter-deposition process is employed that enables preparation of a large
variety of these GST states including metallic and truly insulating ones. By
growing films of GST on mica substrates, biaxially textured samples with huge
grain sizes are obtained. A series of these samples is employed for transport
measurements, as their electron mean free path can be altered by a factor of
20. Yet, the mean free path always remains more than an order of magnitude
smaller than the lateral grain size. This proves unequivocally that grain
boundaries play a negligible role for electron scattering, while intragrain
scattering, presumably by disordered vacancies, dominates. Most importantly,
these findings underline that the Anderson insulating state as well as the
system's evolution towards metallic conductivity are indeed intrinsic
properties of the material
Monatomic phase change memory
Phase change memory has been developed into a mature technology capable of
storing information in a fast and non-volatile way, with potential for
neuromorphic computing applications. However, its future impact in electronics
depends crucially on how the materials at the core of this technology adapt to
the requirements arising from continued scaling towards higher device
densities. A common strategy to finetune the properties of phase change memory
materials, reaching reasonable thermal stability in optical data storage,
relies on mixing precise amounts of different dopants, resulting often in
quaternary or even more complicated compounds. Here we show how the simplest
material imaginable, a single element (in this case, antimony), can become a
valid alternative when confined in extremely small volumes. This compositional
simplification eliminates problems related to unwanted deviations from the
optimized stoichiometry in the switching volume, which become increasingly
pressing when devices are aggressively miniaturized. Removing compositional
optimization issues may allow one to capitalize on nanosize effects in
information storage
Mg Deficiency in Grain Boundaries of n-Type Mg_3Sb_2 Identified by Atom Probe Tomography
Highly resistive grain boundaries significantly reduce the electrical conductivity that compromises the thermoelectric figureâofâmerit zT in nâtype polycrystalline Mg_3Sb_2. In this work, discovered is a Mg deficiency near grain boundaries using atomâprobe tomography. Approximately 5 at% of Mg deficiency is observed uniformly in a 10 nm region along the grain boundary without any evidence of a stable secondary or impurity phase. The offâstoichiometry can prevent nâtype dopants from providing electrons, lowering the local carrier concentration near the grain boundary and thus the local conductivity. This observation explains how nanometer scale compositional variations can dramatically determine thermoelectric zT, and provides concrete strategies to reduce grainâboundary resistance and increase zT in Mg_3Sb_2âbased materials
The importance of surface adsorbates in solution-processed thermoelectric materials: the case of SnSe
Solution synthesis of particles emerges as an alternative to prepare thermoelectric materials with less demanding processing conditions than conventional solid-state synthetic methods. However, solution synthesis generally involves the presence of additional molecules or ions belonging to the precursors or added to enable solubility and/or regulate nucleation and growth. These molecules or ions can end up in the particles as surface adsorbates and interfere in the material properties. This work demonstrates that ionic adsorbates, in particular Na+ ions, are electrostatically adsorbed in SnSe particles synthesized in water and play a crucial role not only in directing the material nano/microstructure but also in determining the transport properties of the consolidated material. In dense pellets prepared by sintering SnSe particles, Na remains within the crystal lattice as dopant, in dislocations, precipitates, and forming grain boundary complexions. These results highlight the importance of considering all the possible unintentional impurities to establish proper structureâproperty relationships and control material properties in solution-processed thermoelectric materials.Peer ReviewedPostprint (author's final draft
Discovering electron transfer driven changes in chemical bonding in lead chalcogenides (PbX, where X = Te, Se, S, O)
Understanding the nature of chemical bonding in solids is crucial to
comprehend the physical and chemical properties of a given compound. To explore
changes in chemical bonding in lead chalcogenides (PbX, where X = Te, Se, S,
O), a combination of property-, bond breaking- and quantum-mechanical bonding
descriptors have been applied. The outcome of our explorations reveals an
electron transfer driven transition from metavalent bonding in PbX (X = Te, Se,
S) to iono-covalent bonding in beta-PbO. Metavalent bonding is characterized by
adjacent atoms being held together by sharing about a single electron and small
electron transfer (ET). The transition from metavalent to iono-covalent bonding
manifests itself in clear changes in these quantum-mechanical descriptors (ES
and ET), as well as in property-based descriptors (i.e. Born effective charge,
dielectric function, effective coordination number (ECON) and mode-specific
Grueneisen parameter, and in bond breaking descriptors (PME). Metavalent
bonding collapses, if significant charge localization occurs at the ion cores
(ET) and/or in the interatomic region (ES). Predominantly changing the degree
of electron transfer opens possibilities to tailor materials properties such as
the chemical bond and electronic polarizability, optical band gap and optical
interband transitions characterized by the imaginary part of the dielectric
function. Hence, the insights gained from this study highlight the
technological relevance of the concept of metavalent bonding and its potential
for materials design
Dynamic doping and Cottrell atmosphere optimize the thermoelectric performance of n-type PbTe
High thermoelectric energy conversion efficiency requires a large
figure-of-merit, zT, over a broad temperature range. To achieve this, we
optimize the carrier concentrations of n-type PbTe from room up to hot-end
temperatures by co-doping Bi and Ag. Bi is an efficient n-type dopant in PbTe,
often leading to excessive carrier concentration at room temperature. As
revealed by density functional theory calculations, the formation of Bi and Ag
defect complexes is exploited to optimize the room temperature carrier
concentration. At elevated temperatures, we demonstrate the dynamic dissolution
of Ag2Te precipitates in PbTe in situ by heating in a scanning transmission
electron microscope. The release of n-type Ag interstitials with increasing
temperature fulfills the requirement of higher carrier concentrations at the
hot end. Moreover, as characterized by atom probe tomography, Ag atoms
aggregate along parallel dislocation arrays to form Cottrell atmospheres. This
results in enhanced phonon scattering and leads to a low lattice thermal
conductivity. As a result of the synergy of dynamic doping and phonon
scattering at decorated dislocations, an average zT of 1.0 is achieved in
n-type Bi/Ag-codoped PbTe between 400 and 825 K. Introducing dopants with
temperature-dependent solubility and strong interaction with dislocation cores
enables simultaneous optimization of the average power factor and thermal
conductivity, providing a new concept to exploit in the field of
thermoelectrics
Mg Deficiency in Grain Boundaries of n-Type Mg_3Sb_2 Identified by Atom Probe Tomography
Highly resistive grain boundaries significantly reduce the electrical conductivity that compromises the thermoelectric figureâofâmerit zT in nâtype polycrystalline Mg_3Sb_2. In this work, discovered is a Mg deficiency near grain boundaries using atomâprobe tomography. Approximately 5 at% of Mg deficiency is observed uniformly in a 10 nm region along the grain boundary without any evidence of a stable secondary or impurity phase. The offâstoichiometry can prevent nâtype dopants from providing electrons, lowering the local carrier concentration near the grain boundary and thus the local conductivity. This observation explains how nanometer scale compositional variations can dramatically determine thermoelectric zT, and provides concrete strategies to reduce grainâboundary resistance and increase zT in Mg_3Sb_2âbased materials
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