Maria Sebastian

Distributed Feedback Lasing from a Composite Poly(phenylene vinylene)-Nanoparticle One-Dimensional Photonic Crystal

Nanoparticle one-dimensional photonic crystals exhibit intense, broadband reflectivity coupled with a unique mesoporosity. The latter property allows for infiltration of the one-dimensional photonic crystal with functional materials, such as emitting polymers, which in turn can lead to the fabrication of composites whereby the emitters emission can be modulated by the photon density of states of the photonic crystal. We exploit this interaction in order to produce efficient distributed feedback lasing from a composite poly(phenylene vinylene)-infiltrated nanoparticle one-dimensional photonic crystal.

Photoconductivity in the Chalcohalide Semiconductor, SbSeI: a New Candidate for Hard Radiation Detection

We investigated an antimony chalcohalide compound, SbSeI, as a potential semiconductor material for X-ray and γ-ray detection. SbSeI has a wide band gap of 1.70 eV with a density of 5.80 g/cm(3), and it crystallizes in the orthorhombic Pnma space group with a one-dimensional chain structure comprised of infinite zigzag chains of dimers [Sb2Se4I8]n running along the crystallographic b axis. In this study, we investigate conditions for vertical Bridgman crystal growth using combinations of the peak temperature and temperature gradients as well as translation rate set in a three-zone furnace. SbSeI samples grown at 495 °C peak temperature and 19 °C/cm temperature gradient with 2.5 mm/h translation rate produced a single phase of columnar needlelike crystals aligned along the translational direction of the growth. The ingot sample exhibited an n-type semiconductor with resistivity of ∼10(8) Ω·cm. Photoconductivity measurements on these specimens allowed us to determine mobility-lifetime (μτ) products for electron and hole carriers that were found to be of similar order of magnitude (∼10(-4) cm(2)/V). Further, the SbSeI ingot with well-aligned, one-dimensional columnar needlelike crystals shows an appreciable response of Ag Kα X-ray.

Heavy metal ternary halides for room-temperature x-ray and gamma-ray detection

We report our recent progress on wide bandgap ternary halide compounds CsPbBr3 and CsPbCl3 for room temperature x-ray and gamma-ray detectors. Their bandgaps are measured to be 2.24 eV and 2.86 eV, respectively. The measured mobility-lifetime products of CsPbBr3 are 1.7×10-3, 1.3×10-3 cm2/V, for electron and hole carriers, respectively, comparable to those of CdTe. We measured the room temperature spectral response of CsPbBr3 sample to Ag x-ray radiation. It has a well-resolved spectral response to the 22.4 keV Kα radiation peak and detector efficiency comparable to that of CdZnTe detector at 295 K.

Photo-induced current transient spectroscopy of single crystal Tl6I4Se

The compound Tl6I4Se is a promising wide band gap semiconductor for hard radiation detection at room temperature. To further improve its detection efficiency, native defects have been investigated using photo-induced current transient spectroscopy (PICTS). We observe two shallow acceptor levels with mean activation energies of 76, 175 meV, and two shallow donor defects 62, and 96 meV, respectively. No deeper donor levels are observed. The levels are attributed to native point defects. Defect capture cross sections in the range 10−21 to 10−18 cm2 were measured. The small capture cross sections are attributed to the effective screening of the defects due to a large static dielectric constant.

Mercury and antimony chalcohalide semiconductors as new candidates for radiation detection applications at room temperature

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.

Characterization of thallium-based ternary semiconductor compounds for radiation detection

We report on the characterization of optical and electronic properties of a series of thallium chalcogenide, wide gap ternary semiconductor compounds including Tl6I4Se, Tl6I4S, Tl3SbS3, Tl2SnS3, and Tl7Bi3I16 for ionized radiation detection at the x-ray and γ ray regimes. The semiconductor crystals were grown by the modified Bridgman method. The optical absorption, band gap, mobility-lifetime products for electron and hole carriers were measured at room temperature. For Tl6I4S and Tl6I4Se the mobility-lifetime products are comparable to those of CdZnTe. We measured room temperature detector response to x-ray and γ ray sources. For 137Cs radiation, Tl6I4Se has a well-resolved spectral response comparable to that of CdZnTe.