Simon Johnsen

Nanostructures Boost the Thermoelectric Performance of PbS

In situ nanostructuring in bulk thermoelectric materials through thermo-dynamic phase segregation has established itself as an effective paradigm for optimizing the performance of thermoelectric materials. In bulk PbTe small compositional variations create coherent and semicoherent nanometer sized precipitates embedded in a PbTe matrix, where they can impede phonon propagation at little or no expense to the electronic properties. In this paper the nanostructuring paradigm is for the first time extended to a bulk PbS based system, which despite obvious advantages of price and abundancy, so far has been largely disregarded in thermoelectric research due to inferior room temperature thermoelectric properties relative to the pristine fellow chalcogenides, PbSe and PbTe. Herein we report on the synthesis, microstructural morphology and thermoelectric properties of two phase (PbS)(1-x)(PbTe)(x)x = 0-0.16 samples. We have found that the addition of only a few percent PbTe to PbS results in a highly nanostructured material, where PbTe precipitates are coherently and semicoherently embedded in a PbS matrix. The present (PbS)(1-x)(PbTe)(x) nanostructured samples show substantial decreases in lattice thermal conductivity relative to pristine PbS, while the electronic properties are left largely unaltered. This in turn leads to a marked increase in the thermoelectric figure of merit. This study underlines the efficiency of the nanostructuring approach and strongly supports its generality and applicability to other material systems. We demonstrate that these PbS-based materials, which are made primarily from abundant Pb and S, outperform optimally n-type doped pristine PbTe above 770 K.

Thallium chalcohalides for X-ray and γ-ray detection.

We report that the chalcohalide compound Tl(6)SeI(4) is a promising material for efficient X-ray and γ-ray detection. This material has a higher figure of merit than the current state-of-the-art material for room-temperature operation, Cd(0.9)Zn(0.1)Te (CZT). We have synthesized high-quality single-crystalline wafers of Tl(6)SeI(4) with detector-grade resistivities and good carrier transport of both electrons and holes. We demonstrate that pulse height spectra recorded using Co-57 radiation show an energy resolution matching that of a commercial CZT detector material.

Photoluminescent properties of semiconducting Tl6I4Se

The photoluminescence (PL) from the wide bandgap semiconductor Tl6I4Se, a promising candidate material for gamma ray detection, was studied at low temperature. The Tl6I4Se single crystal was grown by the Bridgman method. For undoped material, we observed a single broad peak at ~1.61 eV with a full width at half maximum of 112 meV at 20 K, which is attributed to donor–acceptor pair (DAP) recombination involving shallow donors and deep acceptors. From the thermal quenching of the integrated PL peak intensity, thermal activation energy of the donor level of 52 meV was obtained. At high excitation intensities a blue-shift of peak emission energy in DAP recombination was observed. From the PL measurements, the ionization energies of the donor and acceptor levels were estimated at 52 and 290 meV, respectively.

Tl-based wide gap semiconductor materials for x-ray and gamma ray detection

The optical and electronic properties of Tl-chalcogenide, wide gap semiconductors, TlGaSe2, Tl6I4Se, and Tl2Au4S3 for x-ray and γ ray detection were characterized. The semiconductor crystals are grown by the modified Bridgman method. The optical absorption and band gap energy of the materials were determined from UV-Vis-near IR transmission and reflection spectra. The mobility-lifetime products were measured. For Tl6I4Se the values were comparable to those of CdZnTe. We measured room temperature detector response to x-ray and γ ray radiations. Under 57Co radiation, Tl6I4Se has a well-resolved spectral response and peak FWHM comparable to those of Cd0.9Zn0.1Te.