High efficiency thermoelectric conversion is achieved by using materials with a maximum figure of merit Z=S2σ/k, where S is the Seebeck coefficient, σ and k, the electrical and thermal conductivities, respectively. High quality homogeneous thermoelectric materials, based on PbTe crystals, usually display an elevated value of Z over a narrow temperature range. A maximal value of figure of merit Z, as a function of electron density, is attained only for one specific location of the Fermi level, EF, with respect to the conduction band edge, EC. In order to maintain this optimal Z value, namely, maintain a constant location of the Fermi level, the electron density, which is determined by the dopant concentration, must increase with increasing temperature. We present a method for the generation of a dopant (indium) concentration profile in n-type PbTe crystals that gives rise to a constant location of the Fermi level, and hence, to an optimal value of Z over a wide temperature range. The resulting functionally graded material, based on PbTe〈In〉, displays a practically constant value of the Seebeck coefficient, over the 50–600 °C temperature range.

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Quantum dots (QDs) have applications in optoelectronic devices, quantum information processing and energy harvesting. Although the droplet epitaxy fabrication method allows for a wide range of material combinations to be used, little is known about the growth mechanisms involved. Here we apply direct X-ray methods to derive sub-ångström resolution maps of QDs crystallized from indium droplets exposed to antimony, as well as their interface with a GaAs (100) substrate. We find that the QDs form coherently and extend a few unit cells below the substrate surface. This facilitates a droplet-substrate exchange of atoms, resulting in core-shell structures that contain a surprisingly small amount of In. The work provides the first atomic-scale mapping of the interface between epitaxial QDs and a substrate, and establishes the usefulness of X-ray phasing techniques for this and similar systems.

A key factor in improving quantum dots electrical properties and dots-based devices is the ability to control the crucial parameters of composition, doping, size, and strain distribution of the dots. We show that nanometer-scale work function measurements using ultrahigh vacuum Kelvin probe force microscopy is capable of measuring the strain and composition variations within and around individual QDs. This is accomplished by analyzing the detailed surface potential profiles in and around InSb/GaAs dots.

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