Refined Synthesis and Crystal Growth of Pb2P2Se6 for Hard

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Refined Synthesis and Crystal Growth of Pb2P2Se6 for Hard Radiation Detectors Peng L. Wang, Svetlana S Kostina, Fang Meng, Oleg Y. Kontsevoi, Zhifu Liu, Pice Chen, John A. Peters, Micah Hanson, Yihui He, Duck Young Chung, Arthur J. Freeman, Bruce W. Wessels, and Mercouri G. Kanatzidis Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00684 • Publication Date (Web): 26 Jul 2016 Downloaded from http://pubs.acs.org on July 30, 2016

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Crystal Growth & Design

Refined Synthesis and Crystal Growth of Pb2P2Se6 for Hard Radiation Detectors Peng L. Wang,† Svetlana S. Kostina,‡ Fang Meng,£ Oleg Y. Kontsevoi,§ Zhifu Liu,‡ Pice Chen,‡ John A. Peters,‡ Micah Hanson,‡ Yihui He,† Duck Young Chung,£ Arthur J. Freeman,§,^ Bruce W. Wessels,‡ and Mercouri G. Kanatzidis*,† †Department of Chemistry, ‡Department of Materials Science and Engineering, and §Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, United States £ Materials Science Division, Argonne National Laboratory, Lemont, Illinois, 60439, United States

Abstract The refined synthesis and optimized crystal growth of high quality Pb2P2Se6 single crystals are reported. Improved experimental procedures were implemented to reduce the oxygen contamination and improve the stoichiometry of the single crystal samples. The impact of oxygen contamination and the nature of the stoichiometry deviation in the Pb2P2Se6 system were studied by first-principles density functional theory (DFT) electronic structure calculations as well as experimental methods. The DFT calculations indicated that the presence of interstitial oxygen atoms (Oint) leads to the formation of a deep level located near the middle of the gap, as well as a shallow acceptor level near the valence band maximum. In addition, total energy calculations of the heat of formation of Pb2P2Se6 suggests that the region of thermodynamic stability is sufficiently wide. By refining the preparative procedures, high quality Pb2P2Se6 single crystal samples were reproducibly obtained. These Pb2P2Se6 single crystals exhibited excellent optical transparency, electrical resistivity in the range of 1011 Ω·cm, and a significant increase in photoconductivity. Infrared photoluminescence of the Pb2P2Se6 single crystals was observed 1 ACS Paragon Plus Environment


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over the temperature range of 15-75K. Detectors fabricated from boules yielded clear spectroscopic response to both Ag Kα X-ray and

57

Co γ-ray radiation. The electron and hole

mobility-lifetime product (µτ) of the current Pb2P2Se6 detectors were estimated to be 3.1×10-4 and 4.8×10-5 cm2/V respectively.

^Deceased

2 ACS Paragon Plus Environment

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1. Introduction Semiconducting hard radiation detectors have many applications in high-energy physics, biomedical imaging and national security.1-4 However, only a few semiconductors have been successfully developed as radiation detectors, as a set of strict physical property requirements must be simultaneously satisfied to yield X-ray and γ-ray response.5 The general requirements for a detector material are: high purity, heavy element constituents (high Z) to increase the mass density and the interaction between the incident radiation and the material’s electrons, a large band gap (>1.6 eV) to reduce dark current and a high carrier-mobility and lifetime product (i.e. µτ) to improve detector response.4 For a given material, its effectiveness as a radiation detector (µτ) is related to the material purity and crystalline quality. The commercial benchmark X-/γ-ray detector material, Cd1−xZnxTe (CZT), is a solid solution of cadmium telluride and zinc telluride. As a result, CZT suffers from non-uniform zinc distribution and micron-scale compositional variations rooted from its intrinsic chemical properties.6,7 Other well studied detector materials such as TlBr and HgI2 exhibit issues with stability, phase change, and mechanical property that hinder their crystal growth developments and detector properties.8,9 Therefore, it is important to identify and evaluate new materials for potential γ-ray radiation detection applications. Our group has investigated a number of novel semiconductor compounds for hard radiation detectors.10-19 The concepts of “dimensional reduction” and lattice hybridization5,20-22 were used to lead the material discovery efforts, where binary semiconductors with high mass density but small gaps were combined with ones with wide band gaps to form ternary compounds with desirable properties. In addition, the pool of potential detector materials was further expanded by considering heavy element chemical compounds incorporating light p-block



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elements, such as Si and P. An example of such a material is the lead seleno-phosphate, Pb2P2Se6, where phosphorous with a high electronegativity was introduced into the leadselenium system to form a desirable band gap without significantly compromising the high mass density. The Pb2P2Se6 compound adopts a P21/c space group with unit cell dimensions, a = 6.910 Å, b = 7.670 Å, c =11.816 Å and β = 124.35°.23 From a structural point of view, the Pb2P2Se6 phase can be considered a salt of Pb2+ cations with the ethane-like [P2Se6]4- anions and belongs to the large class of chalcophosphates.24-27 The phosphorus atoms stabilize the unique [P2Se6]4cluster which creates a Pb/Se/P network with an indirect band gap of 1.88 eV. On the other hand, the limited interaction between lead and phosphorus allows the lead and selenium atomic states to dominate the electronic structure of Pb2P2Se6 in the vicinity of the Fermi level, thus giving rise to robust chemical and mechanical properties similar to II-VI semiconductors. In addition to its desirable electronic structure, the Pb2P2Se6 compound has an average atomic number (Z) of 39.8 and a high mass density of 6.14 g·cm-3,28 which result in a high attenuation coefficient for high energy radiations. Previously, we have demonstrated the potential of Pb2P2Se6 as a room temperature hard radiation detector.29 However, the Pb2P2Se6 samples exhibited large variations in spectroscopic response to hard radiation. While a few of Pb2P2Se6 samples yielded partially resolved γ-ray spectra, many specimens did not. A major cause of the observed variation in sample quality seems to be the subtle stoichiometric imbalance due to variations in preparative procedures. In order to further develop the Pb2P2Se6 as a hard radiation detector, it is essential to improve the reproducibility of the detector material quality. For instance, a higher phase uniformity within an ingot will allow us to study the distributions of impurities, defects, and


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secondary phases along the growth direction. In addition, better reproducibility across the growth runs would result in identification of the crucial processes affecting the final sample quality. In this article, we report on the refined synthesis, crystal growth and post-growth processes required to produce Pb2P2Se6 with improved electrical and optical properties as well as reproducible responses to both X-ray (Ag Kα source) and γ-ray radiation. The impact of stoichiometry control on the optical and electrical properties of the detector samples is elucidated by a series of comparative measurements. 2. Experimental 2.1. Synthesis and Crystal Growth The Pb2P2Se6 samples were synthesized from lead metal (rod, Alfa Aesar, 99.9999% wt.), selenium (pellets, Stanford Advanced Materials, 99.99999% wt.) and red phosphorous (pieces, Alfa Aesar, 99.999% wt.). The as-purchased lead was etched with 5% HNO3 solution then rinsed with pure ethanol in order to remove the surface oxide. The Pb rod was cut into a section of ~10 g. This practice minimized the contamination of the Pb metal from cutting and handling. Red phosphorus and elemental selenium were weighed with respect to the exact mass of the lead metal. The silica ampoules and plugs (silica rods fitted to ampoule) used for synthesis and crystal growth were first etched with a 2% HF solution in HNO3 in order to remove any surface layer of metallic impurities. The ampoules and plugs were subsequently rinsed with deionized water and 200 proof ethanol. The tubes and the plugs were then baked in a furnace at 1000°C for 20 hours to remove organic contamination. The furnace was cooled to 200°C after the baking process. The baked silica ampoules/plugs were immediately used for sample loading in a moisture controlled, Class 1000 clean room (30% humidity at 25°C). For each synthesis, up to 30 grams



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of stoichiometric starting materials were loaded into tapered fused silica ampoules (13 mm ID, 1 mm wall thickness and 250 mm in length). After loading the starting materials, the silica plugs (12 mm Φ, 100 cm long) were inserted to rest directly on top of the raw materials. The loaded sample ampoules were subjected to a bake-out procedure under vacuum using a setup shown in the Supporting Information. This setup was composed of a DC-powered heating tape and a graphite tube. While the DC-powered heating tape provided the thermal energy, the high thermal conductivity of the graphite tube ensured the temperature uniformity. The starting materials were baked at 85°C under a vacuum of ~10-4 torr for 2 hours before the ampoules were sealed. The loaded samples were sealed under vacuum by flame starting at the top of the silica plug. The schematic of the vacuum sealing configuration is demonstrated in the Supporting Information (Figure S1), along with a sealed sample. For the initial material synthesis, the sealed samples were heated to 850°C over 72 hours and held at the maximum temperature for 48 hours before they were cooled to room temperature over 12 hours. The polycrystalline samples were directly subjected to crystal growth in a modified vertical two-zone Bridgman furnace equipped with a computer controlled linear stage. The temperature of the hot zone was 850°C, while the cold zone was 650°C. The crystal growth was carried out with a lowering rate of 1 mm per hour. Once the samples were completely in the cold zone, they were annealed in situ at 650°C for 24 hrs. Finally, the grown crystals were cooled to room temperature over 24 hours. The temperature gradient in the two-zone Bridgman furnace was estimated to be 8°C/cm. Crack-free crystal ingots were obtained from the crystal growth (Figure 1a). 2.2. Post-growth Processes and Detector Fabrication


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The Pb2P2Se6 single crystals obtained from the Bridgman growth procedure were cut into 2 mm thick wafers perpendicular to the growth direction using a Struers Accutom-50 waferizing saw equipped with a 300 µm wide diamond-impregnated blade. The sample wafers were ground to

Refined Synthesis and Crystal Growth of Pb2P2Se6 for Hard

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