Photon-Counting Detector (PCD) capable of resolving the energies of single X-ray photons is critical in medical imaging (e.g., Computed Tomography). A high count rate and negligible polarization is essential for a PCD. Besides, there has been a critical need to develop high-Z sensor for synchrotron X-ray facility. The very high X-ray fluxes (e.g., 1e6 – 1e12 photons/s/mm2) involved in both applications makes it very challenging for detector operation. Here, we demonstrate that our perovskite CsPbBr3 single crystal detectors have good performance for these applications.
We conduct Angle Resolved Photoemission Spectroscopy (ARPES) investigation on 2H-TaS2, a prototypical incommensurate Charge Density Wave (CDW) material. A comparative study of the low-energy electronic structures of 2H-TaS2 and two other related compounds, 2H-TaSe2 and 2H-NbSe2, identifies several generic features of their CDW orders. Firstly, Fermi surface (FS) nesting alone doesn’t seem to give rise to the CDW instability in these compounds. Secondly, partial gapping of the underlying FS surface in the CDW state is common to each of these materials. Finally, the CDW energy gap, unlike the energy gap in a superconductor, is not symmetric with respect to the chemical potential.
Thallium based chalcogenide and halide semiconductors such as Tl4HgI6, TlGaSe2, Tl6SeI4 and Tl6SI4 are promising materials for room-temperature hard radiation detection. They feature appropriate band gaps, high mass densities and facile growth technology. However, these materials are being plagued by the Tl oxides impurity from Tl precursor or Tl containing binary precursors, which leads to problems including tube breakage, parasitic nucleation and detector performance deterioration. In this work, we present a facile way to chemically reduce Tl oxidations, and then eliminate oxygen impurity by adding high-purity graphite powder during synthesis and crystal growth. We also further investigated the reactivity between Tl oxides and graphite. The detector performance of Tl6SeI4 crystal was dramatically improved after lowering/removing the oxygen impurities. This result not only indicates the significance of removing oxygen impurity for improving detector performance. Our results suggest that the chemical reduction method we developed by adding carbon powder during synthesis is highly effective in substantially reducing oxygen impurities from Tl containing materials.
CsPbBr3 has direct band gap (orange color, 2.25 eV), high density (4.85 g/cm3), attenuation coefficient comparable to CZT, and high resistivity ~10^9 ohm∙cm. These fundamental physical properties of CsPbBr3well meet the requirements for gamma-ray detector materials. CsPbBr3 exhibits the carrier mobility-lifetime product in the order of 10^-4 cm2/V promising enough to be further developed for practical applications. The major challenge in the process to further enhance the detection performance is the carrier traps present at a deep level of the energy gap which should be minimized. We report the synthesis, purification, crystal growth and physical characterization of the CsPbBr3 crystals obtained by new processes we developed for highly pure materials with reduced carrier traps. The starting binary materials were prepared by reaction of Cs2CO3/HBr and Pb(ac)2/HBr in aqueous solution. Purification of materials was performed by sublimation, bromination with HBr gas, and filtration of molten materials. Large single crystals were grown by the vertical Bridgman and EelectroDynamic Gradient method and cut to the dimensions appropriate for assessment of the material for gamma-ray detector applications. All characterization including optical characteristics, charge transport properties, photoconductivity, and gamma-ray spectroscopy from the new single crystals of CsPbBr3 will be presented. In addition, the charge carrier traps profile has been studied for this compound by Deep-Level Transient Spectroscopy (DLTS), Thermally Stimulated Luminescence (TSL), and Photoluminescence (PL) and will be presented.
Christos Malliakas, Arief Wibowo, Zhifu Liu, John Peters, Maria Sebastian, Hosub Jin, Duck-Young Chung, Arthur Freeman, Bruce Wessels, Mercouri Kanatzidis
We demonstrate that mercury and antimony compounds with chalcogens (Q = S, Se, Te) and halogens (X = I, Cl, Br)
can be a promising family for radiation detection materials. Chalcogen p-orbitals are usually located near the Fermi level
and they are responsible for relative high mobilities but at the same time band gap decreases (from S to Te) due to their
extended interactions. Halogens on the other hand have their bands well below the Fermi level and salts between
transition metals and halogen are usually insulators. Incorporation of halogen atoms in a mercury or antimony
chalcogenide framework can give rise to intermediate properties between the two end members (HgQ and HgX2), i.e.
structures composed of heavy elements (Z < 40), wide band gap (1.6 - 2.5 eV), and high carrier mobilities. As a proof of
concept, we will present two new chalcohalide families, Hg3Q2X2 and SbQX. Crystal growth of the Hg3Te2Br2 phase
(7.8 g/cm3 and 2.5 eV) by a vapor transport method gave mm-sized single crystals with electrical resistivity values more
in the GΩ.cm range. Preliminary data for mobility-lifetime products for both electron and hole carriers were around 10-5
cm2/V. SbSeI (5.8 g/cm3 and 1.7 eV) sample grown by relatively fast Bridgman technique showed an MΩ.cm range (2.8
x 106 Ω.cm) resistivity with a similar order of magnitude (10-4 cm2/V) of mobility-lifetime products for both electron and
hole carriers.
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