TlSbS2, a Semiconductor for Hard Radiation Detection - ACS

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TlSbS2, a Semiconductor for Hard Radiation Detection Wenwen Lin, Haijie Chen, Jiangang He, Constantinos C. Stoumpos, Zhifu Liu, Sanjib Das, Joon-Il Kim, Kyle M McCall, Bruce W. Wessels, and Mercouri G. Kanatzidis ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00891 • Publication Date (Web): 29 Sep 2017 Downloaded from http://pubs.acs.org on October 1, 2017

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TlSbS2: a Semiconductor for Hard Radiation Detection Wenwen Lin,† Haijie Chen,† Jiangang He, ‡ Constantinos C. Stoumpos,† Zhifu Liu, ‡ Sanjib Das,‡ Joon-IL Kim,‡ Kyle M. McCall,†,‡ Bruce W. Wessels,‡ and Mercouri G. Kanatzidis†,* †

Department of Chemistry, and ‡Department of Materials Science and Engineering, Northwestern

University, Evanston, Illinois 60208, United States

ABSTRACT: We report the quasi-2D semiconductor compound TlSbS2 as a new hard radiation detection material. This compound crystallizes in the triclinic P-1 space group, with a direct bandgap of 1.67 eV and high chemical stability. Thanks to its congruent melting at 484 oC, one centimeter-size single crystals were grown from stoichiometric melts by the Bridgman method. The device exhibits a high resistivity of > 1010 Ω·cm, and responds to 22.4 keV Ag X-rays and 5.5 MeV a-particles from 241Am at room temperature. Power dependent photoluminescence spectra at 17 K reveal that the near-band emission bands peaked at 1.61 eV and 1.53 eV can be ascribed to donor-acceptor pair recombination. The mobility-lifetime product for electrons along the perpendicular direction with respect to the (0k0) cleavage planes was estimated as 2.4 10-6 cm2·V-

1

, based on spectral response against a-particles. Drift mobility measurements based on a time of flight

technique using a-particle response reveals an electron mobility of 13.2 ± 2.6 cm2·V-1·s-1. Electronic band structure calculations based on the density functional theory indicate that the lowest effective mass, and thus the best charge transport, is along the (0k0) planes.

KEYWORDS: hard radiation detection; crystal growth; semiconductor detector; photon detection

Hard radiation detectors operating at room temperature are highly sought after for applications in nuclear medicine, nonproliferation of nuclear materials and outer space exploration. A potential hard radiation detection material must simultaneously satisfy a series of requirements on physical properties including sufficiently wide bandgap (> 1.5 eV) to suppress thermal ionization of carriers at room temperature, high mass density and average atomic number to guarantee high photon stopping power, high chemical stability and high carrier mobility-lifetime product µτ to allow for high carrier collection and spectroscopic performance.1-3 The µτ is regarded as the figure of merit for assessing detection performance. Due to these strict requirements, very few semiconductors have been identified as hard radiation detection materials. Even the most commercialized detection material, the pseudo-binary Cd0.9Zn0.1Te (CZT),4 still struggles with the intrinsic drawbacks associated with its defects, including Te precipitates, macro-scale defects and poor uniformity.5-6 TlBr is another material which has shown spectroscopic detection performance with high mobility-lifetime (µτ) products (electron: ~10-3 cm2·V-1).7-10 However, TlBr also suffers from intrinsic polarization-induced instability and poor hardness that is unfavorable to material processing.1,

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Other intensively investigated 1

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semiconductor detection materials such as HgI2, PbI2 and BiI3 are highly dense, highly resistive and easy to obtain large-size crystals, but are plagued by excessively high anisotropy and mechanical deformation due to poor hardness that inhibits device fabrication.12-14 Therefore, it is important to explore alternative low-cost semiconductor detection materials without the drawbacks that have limited the above materials. To date, the leading semiconductor detection materials mainly consist of binary or pseudo-binary compounds. However, several Tl-based ternary chalcogenides and halides including TlSn2I5, Tl6SeI4, Tl6SI4, TlGaSe2, TlInSe2, Tl4HgI6, Tl3AsSe3 and TlPbI3, have been shown to be photoresponsive to hard radiation.15-26 Compared to the leading binary semiconductor compounds, these Tl based ternary compounds have superior photon stopping power owing to the high atomic number (Z = 81) of Tl and higher densities (> 7 g·cm-3). In addition, the lower melting points (< 600 oC) of these compounds allow for easier material purification/crystal growth protocols and a low concentration of thermally activated defects, which act as trapping and recombination centers to reduce performance. Therefore, the exploration of Tl-based ternary compounds is a promising approach to finding new detection materials with high performance. In this work, we present a first assessment of the Tl based ternary chalcogenide compound TlSbS2 for hard radiation detection. This compound is also known as the mineral of weissbergite from the Carlin-type gold deposit,27

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signifying high chemical stability. Importantly, an earlier study on this compound revealed that

TlSbS2 has strong photoconductivity under visible light,29-30 suggesting excellent intrinsic charge transport for photoelectric conversion. This feature indicates its excellent potential as a hard radiation detector material. However, there is no reports on its application in the radiation detection field to date. TlSbS2 features a high average atomic number (Z) of 41 and a high density of 6.101 g·cm-3 with a comparable photon stopping power with that of CZT with a Z of 49 and a density of 5.78 g·cm-3, as shown in Figure 1.31 Because of congruent melting at 484 oC centimeter-size TlSbS2 single crystals could be grown from the melt using a modified Bridgman method. A detector made of a TlSbS2 plate with an orientation of cleaved (0k0) facet detects 22.4 keV Ag X-rays and 5.5 MeV a-particles from 241Am radiation source. Based on spectral response for 5.5 MeV a-particles, the mobility-lifetime product for electrons along the perpendicular direction to the cleavage planes was estimated. Electronic band structure calculations suggest that the facile electron transport is along the (0k0) cleavage planes. TlSbS2, the newest addition to the family of Tl based ternary semiconductor compounds for hard radiation detection, is a highly stable material which promises high performance upon further optimization of material purification and crystal growth.

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Figure 1. Attenuation lengths for TlSbS2 and CZT as a function of photon energy.

■ RESULTS AND DISCUSSION Crystal Structure. In order to get precise and accurate information of the as-synthesized TlSbS2 crystal, we carried out X-ray diffraction and structure refinement. As listed in Table 1, consistent with the previous result,32 TlSbS2 is well refined with the triclinic P-1 space group with a = 6.1259(12) Å, α = 101.46(3)°; b = 6.3161(13) Å, β = 98.22(3)°; c = 11.793(2) Å, γ = 104.10(3)°. The formula of TlSbS2 can be written as (Tl+)(Sb3+)(S2-)2. As shown in Figure 2a, TlSbS2 displays a quasi-layered structure with Tl-Sb-S slabs stacking along the b axis. The nearest bonding between the layers is the Tl2-S bond of 3.461(8) Å, indicating a weak but still bonding interaction between the layers. The detailed Tl and Sb coordination environments are given in Figure 2b. Both Tl and Sb are surrounded with four S atoms, with bond lengths ranging from 3.038(1) Å to 3.200(1) Å and 2.396(1) Å to 2.950(9) Å, respectively. Figure 2c shows a magnified single TlSbS2 layer, viewed along the b axis. It has distorted Tl-Sb-S cubes with bridging Sb-S bonds connecting them, forming a 2D slab extending along the ac plane. The slab stacks in the third direction via longer Sb-S and Tl-S bonds.


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Figure 2. (a) Crystal structure of TlSbS2 viewed along the a axis, indicating a layered structure in the b direction. The pink, orange, and green circles are Tl, Sb, and S atoms, respectively. The nearest bonding between the layers is the Tl2-S bond of 3.461(8) Å. (b) 4-coordinated Tl and Sb atoms with bond lengths labeled. (c) Magnified single TlSbS2 layer viewed along the b axis. It is composed of distorted Tl-Sb-S cubes that are connected via Sb-S bonding.

Crystal Growth and Characterization. Polycrystalline TlSbS2 was synthesized from the direct combination reaction of elementary precursors in stoichiometric amount using in a rocking furnace at 600 ºC. Subsequently, the as-synthesized raw material was subject to crystal growth via a vertical two-zone Bridgman method to yield large-size single crystalline boule. Figure 3a shows the pristine cleaved chunks and thin plates from boule with a diameter of 10 mm under ambient light. The cleaved plates appear black and have mirrorlike surfaces. The as-grown crystal was composed of pure TlSbS2 phase as evidenced by the X-ray diffraction pattern of powdered specimen from the boule, as shown in Figure 3b. The X-ray diffraction pattern on the cleaved crystal facet shows one diffraction peak which matches well with the (020) peak of the simulated pattern at 2θ = 30.033º, suggesting that the cleaved facet consists of (0k0) planes that align perfectly parallel to the crystallographic c-axis. TlSbS2 congruently melts at 484 ºC, which agrees well with the reported value, Figure 3c.28 Compared to the high melting point (1092 ºC) of the leading detection material CZT,33 the much lower melting point of TlSbS2 is not only beneficial for suppressing the formation of thermal activated defects but also for facile material purification and crystal growth. Most importantly, TlSbS2 is free of phase transitions between the ambient temperature and melting point, which is favorable for obtaining single crystals. Figure 3d shows the UV-vis diffuse reflection spectrum. The bandgap was determined to be 1.67 eV, which 4 ACS Paragon Plus Environment

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agrees well with the reported value 1.68 eV.34 This bandgap is narrow enough to generate more photo-

induced electron-hole pairs due to a relatively low pair creation energy,35-37 and yet large enough to suppress the thermal generation and recombination of carriers at room temperature. 38

Figure 3. (a) Image of pristine chunks and cleaved plates from a TlSbS2 boule. (b) PXRD patterns of powdered specimen from TlSbS2 boule, XRD pattern of cleaved plate and simulated XRD pattern according to the solved crystal structure. (c) Differential thermal analysis (DTA) curves of TlSbS2. (d) UV-vis-Near IR optical absorption spectrum of TlSbS2. Optical Properties. Figure 4a shows a PL spectrum from a typical TlSbS2 single crystal with an orientation of (0k0) using a laser power of 15 mW collected at 17 K. The sample exhibits a broad emission with a full width at half maximum (FWHM) of 172 meV, which is the overlap of two Gaussian peaks located at 1.53 and 1.61 eV. As the room-temperature bandgap is 1.67 eV based on optical absorption spectrum (Figure 3d), both PL peaks are attributed to defects. To investigate the nature of these defects, the dependence of PL emission on laser excitation intensity was measured from 3 mW to 18 mW at 17 K. As shown in Figure 4b, the PL intensity was found to increase with increasing laser power. The PL intensity is generally proportional to Lk, where L is the laser power and k is the exponent,39 where typical values of k are either in the 0 < k < 1 or 1 < k < 2 range. A k value in the range 0 < k < 1 suggests donor-acceptor pair (DAP) recombination or free-to-bound transitions, while 1 < k < 2 corresponds to exciton-like transitions. Figure 4c shows the plots of log (PL intensity) vs. log 5 ACS Paragon Plus Environment

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(laser power) for the two observed peaks, with corresponding k values of (0.91 ± 0.12) and (0.83 ±0.06), respectively. The k values for both peaks are of < 1 (within error margin), indicating that both transitions are due to DAP or free-to-bound recombination. However, slight blue shift of the both peaks with laser power (Figure 4d) confirms that both can be attributed to DAP recombination. As the PL spectra reveal the presence of defects at at 1.53 and 1.61 eV, it should also be noted that the crystal growth needs to be further refined,

Spectrum_15 mW Gaussian peak 1 Gaussian peak 2 Cumulative fit

1.2 0.8 1.61 eV

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which is currently underway.

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Laser power (mW)

Figure 4. (a) PL spectrum from a typical TlSbS2 single crystal measured at 17 K using a 405-nm laser power of 15 mW; red and green dotted lines indicate two Gaussian peaks used to fit the spectrum. (b) Excitation intensity-dependent PL spectra from the corresponding sample, measured at 17 K. (c) Plots of log(PL intensity) vs. log(laser power) for the observed peaks. (d) Plots of peak energy vs. laser power for the two observed peaks.

Charge Transport Properties and Detector Performance. The leakage currents of the planar detector made from a TlSbS2 single crystalline (0k0) cleaved plate was measured at room temperature. The detector was made from a 0.2 mm thick TlSbS2 cleaved plate with carbon paint electrodes, as shown in Figure 5a. The bias voltage was applied along the direction perpendicular to the (0k0) cleaved facet. Figure 5b shows the current-voltage (I-V) characteristics of the detector in the dark. The I-V curve of detector is very linear in the 6 ACS Paragon Plus Environment

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bias range from -100 V to +100 V, demonstrating the absence of space charge effects which would be detrimental for performance. The resistivity estimated from the I-V curve is around 2 × 1010 Ω·cm, makes possible very low background noise for hard radiation detection. This high resistivity on the order of 1010 Ω·cm can be easily obtained, regardless of growth conditions. In contrast, it is not straightforward to obtain a high resistivity above the order of 109 Ω·cm for CZT crystals without delicate controlling on growth conditions and composition of starting materials. Figure 5c shows the sensitive photoresponse of the detector upon 22.4 keV X-ray irradiation by switching the Ag Kα X-ray source on and off at a detector bias of 100 V. The ratio of photocurrent to dark current is ~150 : 1, indicating the detector is highly responsive to X-rays.

Figure 5. (a) The TlSbS2 detector made from a 0.2 mm thick cleaved plate with lateral dimensions of 5 mm × 4 mm. (b) I-V characteristic of the TlSbS2 detector in the dark (0.2 mm thickness). (c) Photocurrent response to 22.4 keV Ag X-rays at a bias of 100 V by switching the X-ray source on and off.

Spectroscopic measurements using alpha particle emission from a carried out under ambient conditions. The distance between the

241

241

Am radiation source (1 µCi) were

Am radiation source and the detector was set

to ~3 mm to minimize the absorption of α-particles emission in the air. Figure 6a shows the spectral responses of the detector under different bias voltages with alpha emission source.


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Figure 6. (a) The spectral response of 0.2 mm thick TlSbS2 detector upon α-particles. (b) Mobility-lifetime product for electrons estimated from the Hecht equation. (c) A typical transient pulse from one radiation event recorded by the preamplifier with a rise time of ~0.28 µs. (d) Histogram of electron rise time distribution at 100 V bias.

The most important figure of merit to evaluate the performance of semiconductor detector materials for their radiation detection efficiency is the mobility-lifetime product, µτ, for both electron carriers and hole carriers. For the mobility-lifetime product measurement, we used a setup similar to that described by A. Many,40 and further developed by Ruzin et al.

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to estimate the carrier mobility-lifetime product from the voltage

dependence of gamma ray spectrum. The mobility-lifetime product for electrons is derived from the analysis of charge collection efficiency (CCE) under each bias voltage applied to the sample, specifically. The Hecht equation for a single charge carrier relates the charge collection efficiency (CCE) to the sample bias voltage V, the bias voltage applied to the sample as

() =

=

1

−

(1),

where CCE(V) is the charge collection efficiency under the applied V, Q is the measured photopeak/shoulder channel number at bias V, L (0.02 cm) is the thickness of detector, and Q0 refers to the theoretical saturated 8 ACS Paragon Plus Environment

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channel number of the photopeak/shoulder. The mobility-lifetime product for electrons (µeτe) and Q0 can be derived from the experimental data of CCE(V) and Q. Since there is no resolved photopeak in the spectra, the maximum channel positions instead of peak channel numbers were used to fit the single-carrier Hecht equation. Figure 6b shows the datasets obtained, and from the fit of the experimental results the mobilitylifetime product for electron carriers was estimated to be 2.6 × 10-6 cm2·V-1. Furthermore, we were able to estimate the mobility of the carriers from the rise time under given bias voltage. Figure 6c shows the temporal behavior of one waveform signal after preamplifier with a bias voltage of 100 V. This typical transient electron pulse has a rise time is on the order of 0.28 µs. Figure 6d shows the histogram of the rise times for external electric field at E = 5000 V·cm-1. The averaged rise time is 0.31 ± 0.06 µs. The relationship between the carrier drift velocity Vdf and is given by

=

where

=

(2),

is the distance the carriers moved before reaching the electrode, tdf is the carrier drift time, is the

carrier mobility, and E is the applied external electric field. In the alpha emission case with electron carrier collection, we assume that d ≈ L = 0.2 mm, that is, the electrons had to travel through the length of the detector. This is justified because attenuation length of α-particles is several microns in dense materials such as TlSbS2, negligible compared to the detector thickness. The estimated mobility is 13.2 ± 2.6 cm2/V/s for this sample.

Electronic Structure. The PBEsol

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relaxed lattice parameters are a = 6.039 Å, b = 6.288 Å, c = 11.653

Å, α = 102.88°, β = 96.83°, and γ = 105.26°, which agree with the experimental values very well. The band structure and density of state (DOS) of TlSbS2 with the fully relaxed crystal structure are shown in Figure 7a. TlSbS2 is a direct band gap semiconductor with the valence band maximum and conduction band minimum located at the X point of the Brillouin Zone, consistent with the experimental measurement. Although the calculated band gap (0.92 eV) is smaller than the experimental value due to the limitation of the semilocal density functional, it is well accepted that semilocal density functionals can still describe the main band characters, e.g., band dispersion and band composition, rather accurately. Along with S 3p orbitals, the top of the valence band has many states from Tl 6s and Sb 5s orbitals due to the stereochemically active lone-pair electrons of Tl+ (5d106s2) and Sb3+ (4d105s2) cations. This explains the observation that the top of the valence band shows a relatively large energy dispersion. The conduction band is composed of Sb 5p and Tl 6p orbitals. The calculated electron effective masses (me) are 0.44 m0 (X-Γ, a axis), 0.60 m0 (X-V, b axis), and 1.07 m0 (XU, c axis). The hole effective masses (mh) are 0.64 m0 (X-Γ, a axis), 4.26 m0 (X-V, b axis), 1.12 m0 (X-U, c axis), indicating a large anisotropy which suggests that this compound has a strong two-dimensional character at least regarding hole transport. The small electron effective mass points to a potential a high carrier 9 ACS Paragon Plus Environment

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mobility. Therefore, the calculations suggest that the best charge transport is along the (0k0) cleavage planes. Figure 7b shows the calculated ration of electrical conductivity (σ) over relaxation time (τ), σ/τ, of TlSbS2 (within the relaxation time approximation). It is clear that electrical conductivity along a-axis (xx direction) is much higher than b-axis (yy direction) in a given electron life time for electron and hole. Therefore, the calculations suggest that the best charge transport is along the direction which is parallel to the (0k0) cleavage planes.

Figure 7. (a) The band structure and density of state (DOS) of TlSbS2 based on the DFT calculations. (b) Electric conductivity over relaxation time (σ/τ) as a functional of the electron chemical potential.

■ CONCLUSION The mineral weissbergite TlSbS2 was identified as a promising candidate for hard radiation detection at room temperature. This quasi two-dimensional compound features high photon stopping power and a high resistivity above 1010 Ω·cm. It is congruently melting for facile crystal growth, and one centimeter-sized single crystals of TlSbS2 were readily grown from the melt via Bridgman method. A room temperature radiation detector was made from a single crystalline TlSbS2 cleaved plate, and it exhibited reasonable photoresponse for hard Ag Xrays and α-particles from 241Am. The spectral photoresponse for 5.5 MeV α-particles gives a mobility-lifetime

product for electrons of 2.4 10-6 cm2·V-1 along the direction perpendicular to the (0k0) cleavage planes. Drift mobility measurements based on electron rise time of the pulse induced by α-particles reveal an electron drift mobility of 13.2 ± 2.6 cm2·V-1·s-1. Power dependent photoluminescence spectra at room temperature suggest that the near-band emission bands can be ascribed to donor-acceptor pair recombination. DFT calculations of the electronic structure reveal strong anisotropy of carrier effective masses, indicating that the most favorable charge transport is along the (0k0) cleavage plane. It is interesting to note here that the electron mobility found in the TlSbS2 samples is comparable to that in TlBr.

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In TlBr strong correlation was found between the 10

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material processing and purification conditions, and detector performance. The optimization of purification processes has shown to raise its mobility-lifetime from ~ 10-6 cm2·V-1 to > 10-3 cm2·V-1.

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Therefore, future

work will focus on purification of raw materials prior to crystal growth to further improve its detection performance and studying the anisotropy of detection performance.

■ METHODS Synthesis and Crystal Growth. The precursors for synthesizing TlSbS2 raw material include Tl rod (99.999% purity, Alfa Aesar), Sb shots (99.999% purity, Alfa Aesar) and S pieces (99.999% purity, Alfa Aesar). TlSbS2 raw material was synthesized by the direct chemical combination of Tl, Sb and S with the stoichiometric ratio in a fused silica ampoule sealed at a vacuum pressure of 310-4 mbar. The synthesis was performed at 600 ºC for 24 h in a rocking furnace, followed by slow cooling to room temperature. The resultant raw material was loaded into a conical-tip quartz ampoule with a thickness of 1.5 mm and an inner diameter of 10 mm, which was sealed at a vacuum pressure of 2 10-4 mbar. The crystal growth was carried out in a two-zone Bridgman furnace.

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Before crystal growth, the ampoule was held in the hot zone of

Bridgman furnace for 24 h to ensure complete melting of raw material. Subsequently, the ampoule was translated downward from the hot zone (620 ºC) to the cold zone (200 ºC) at a translation speed of 1.0 mm·h-1 and a temperature gradient of 27 ºC·cm-1. The crystal was annealed in-situ after the crystal growth process was finished at cold zone for 48 hours to reduce thermal stress.

Single-crystal X-ray Diffraction. Single crystals with well-defined facets were selected for X-ray diffraction at room temperature (293 K) by using X-Area software with STOE IPDS 2 single-crystal diffractometer at 50 kV and 40 mA with Mo Kα radiation (λ = 0.71073 Å).45 The crystal structure was solved via direct methods and refined with the SHELX package.46 Details of the single-crystal structural refinement are listed in Table S1-S4 in the Supporting Information.

Crystal Processing and Characterization. Some cleaved plates were extracted from the as-grown boule using a razor blade. These thin plates are shiny and mirror-like, and can be directly fabricated as detectors without further polishing and surface etching. In order to analyze the phase purity of as-grown single crystals, powder X-ray diffraction (PXRD) pattern of ground specimen from crystals was collected using a Sicalibrated CPS 120 INEL diffractometer operating at 40 kV and 20 mA (Cu Ka radiation λ = 1.5418 Å). Photoluminescence spectra on TlSbS2 samples were collected at 17 K using a 405-nm CW semiconductor laser (OBIS laser from Coherent, Inc.). A long-pass filter with a cut-on wavelength of 650-nm (Thorlabs, Inc) was used to filter the scattered and reflected laser light before the entrance slit to the monochromator. The photoluminescence (PL) spectra were resolved by a 500 SPEX grating monochromator equipped with a Hamamatsu photomultiplier tube (R928 PMT). An optical chopper (frequency of ~710 Hz) and a lock-in amplifier were used to improve the signal-to-noise ratio. The orientation of the cleaved plates was determined 11 ACS Paragon Plus Environment

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using X-ray diffraction measurements. The bandgap was determined by solid-state diffuse reflectance UV-visnear IR spectroscopy using a Shimadzu UV-3600PC double-beam, double-monochromator spectrophotometer. The spectrophotometer operates in the 200-2500 nm region using BaSO4 as the 100% reflectance reference as described elsewhere.

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The thermal stability of TlSbS2 was assessed by differential thermal analysis (DTA)

using a Netzsch STA 449F3 Jupiter thermal analyzer. Powdered crystalline material (~60 mg) was flame-

sealed in a tiny quartz ampoule evacuated to 10−4 mbar. A similarly sealed ampoule of ∼60 mg of Al2O3 was used as reference sample. Sample was heated to 550 °C at 5 °C·min-1 and then cooled at -5 °C·min-1 to 70 °C. Samples were cycled for a second time under the same experimental conditions. Device Fabrication. The mirror-like TlSbS2 cleaved plate with a dimension of 4 × 5 mm2 and a thickness of 0.2 mm was selected for fabricating detector on a glass substrate. The top electrode, with a diameter of 1.5 mm, was made by applying fast-dry carbon paint, while the bottom of cleaved plate was fully covered by carbon paint for the second electrode. The electrodes were bonded to Cu foil straps using 0.1 mm diameter Cu wires.

Charge Transport and X-Ray Photocurrent Measurement. The direct current (DC) current-voltage (I-V) measurements in the dark were performed to assess the leakage current. DC conductivity was measured on a Keithley 6517B electrometer and a Keithley 6105 resistivity adapter. Electromagnetic interference and stray photoconductive responses are eliminated by a metallic enclosure. In order to estimate photoresponse upon X-rays, photocurrent measurements were performed using 22.4 keV Ag X-rays as irradiation source. Ag X-rays were generated from a CPS 120 INEL diffractometer operating at an accelerating voltage of 40 kV and a current of 2 mA.

Hard Radiation Spectroscopy Measurements. Spectral response of the TlSbS2 detector was performed using a homemade system including an eV-550 preamplifier box, a spectroscopy amplifier (ORTEC, Model 572A) and a computer installed multichannel pulse height analyzer (Model ASPEC-927). The final signals were read into the MAESTRO-32 software. Alpha particle spectroscopy measurements were carried out in atmosphere and the distance between 241Am radiation source (activity: ~1 µCi) and detector was set to be ~3 mm. The optimal linear amplifier gain, amplifier shaping time and recording time are 100, 6.0 µs and 300 s, respectively. All measurements were performed under the cathode irradiation configuration. Pulse height spectra without radiation source were collected as background noise under the same experimental conditions.

Calculations on Electronic Structure. In order to shed light on the potential of the detection performance, we studied the electronic structure of TlSbS2 based on density functional theory (DFT) calculations. All the calculations in this study are performed using the PBEsol exchange-correlation 12 ACS Paragon Plus Environment

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functional42 and projector-augmented wave (PAW) pseudo potential48-49 as implemented in the Vienna Ab initio Simulation Package (VASP). 50-51 The plane wave basis set with an energy cutoff of 350 eV and a 12 × 6 × 12 gamma centered Monkhorst-Pack K-mesh were used. The electronic structure calculation including spinorbital coupling (SOC) is conducted on the fully relaxed crystal structure adopted from experiment. The electron and hole effective masses were obtained by fitting a small range of the bottom of the conduction and the top of the valence band using a quadratic function. The electrical conductivity was calculated by solving the electronic Boltzmann transport equation within the constant relaxation time approximation using the BoltzTrap code.52 The electronic structure was resolved on a 20 × 20 × 10 k-mesh, including SOC.

■ ASSOCIATED CONTENT Supporting information is available. One CIF file of the X-ray crystallographic data for TlSbS2 is available free of charge via the Internet at http://pubs.acs.org. Correspondence and requests for materials should be addressed to M. K.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Wenwen Lin: 0000-0002-1627-9558 Haijie Chen: 0000-0003-3567-1763 Jiangang He: 0000-0001-9643-3617 Constantinos C. Stoumpos: 0000-0001-8396-9578 Sanjib Das: 0000-0002-5281-4458 Kyle M. McCall: 0000-0001-8628-3811 Bruce W. Wessels: 0000-0002-8957-7097 Mercouri G. Kanatzidis: 0000-0003-2037-4168 Author Contributions

M. K. conceived and supervised the project. W. L. conceived and conducted experiments on synthesis, crystal growth and characterization, detector fabrication, charge transport, detection performance and mobility estimation. H. C. solved and refined crystal structure. J. H. conducted calculations on electronic band structure. C. S. and K. M. helped with the organization of the manuscript. Z. L. and J. K. conducted experiments on detection performance. S. D conducted optical measurements. B. W conceived and supervised the charge transport measurements in the project. 13 ACS Paragon Plus Environment

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Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work is supported by DHS-ARI grant 2014-DN-077-ARI086-01. This work made use of the EPIC facility of the NUANCE Center and IMSERC at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205).

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17. Kahler, D.; Singh, N. B.; Knuteson, D. J.; Wagner, B.; Berghmans, A.; McLaughlin, S.; King, M.; Schwartz, K.; Suhre, D.; Gotlieb, M. Performance of novel materials for radiation detection: Tl3AsSe3, TlGaSe2, and Tl4HgI6. Nucl. Instrum. Meth. A 2011, 652, 183-185. 18. Nguyen, S. L.; Malliakas, C. D.; Peters, J. A.; Liu, Z. F.; Im, J.; Zhao, L. D.; Sebastian, M.; Jin, H.; Li, H.; Johnsen, S.; Wessels, B. W.; Freeman, A. J.; Kanatzidis, M. G. Photoconductivity in Tl6SI4: A novel semiconductor for hard radiation detection. Chem. Mater. 2013, 25, 2868-2877. 19. Onodera, T.; Hitomi, K.; Tada, T.; Shoji, T.; Mochizuki, K. Characterization of thallium bromide detectors made from material purified by the filter method. IEEE Trans. Nucl. Sci. 2013, 60, 3833-3836. 20. Shi, H.; Lin, W.; Kanatzidis, M. G.; Szeles, C.; Du, M.-H. Impurity-induced deep centers in Tl6SI4. J. Appl. Phys. 2017, 121, 145102. 21. Kostina, S. S.; Peters, J. A.; Lin, W.; Chen, P.; Liu, Z.; Wang, P. L.; Kanatzidis, M. G.; Wessels, B. W. Photoluminescence fatigue and inhomogeneous line broadening in semi-insulating Tl6SeI4 single crystals. Semicond. Sci. Tech. 2016, 31, 065009. 22. Das, S.; Peters, J. A.; Lin, W.; Kostina, S. S.; Chen, P.; Kim, J.-I.; Kanatzidis, M. G.; Wessels, B. W. Charge transport and observation of persistent photoconductivity in Tl6SeI4 single crystals. J. Phys. Chem. Lett. 2017, 8, 1538-1544. 23. Hitomi, K.; Onodera, T.; Shoji, T.; Hiratate, Y. Thallium lead iodide radiation detectors. IEEE Trans. Nucl. Sci. 2003, 50, 1039-1042. 24. Kocsis, M. Proposal for a new room temperature X-ray detector-thallium lead iodide. IEEE Trans. Nucl. Sci. 2000, 47, 1945-1947. 25. Alekseev, I. V. Application of TlInSe2 crystals for the detection of hard radiation. Inorg. Mater. 1992, 28, 1961-1964. 26. Lin, W.; Stoumpos, C. C.; Liu, Z.; Das, S.; Kontsevoi, O. Y.; He, Y.; Malliakas, C. D.; Chen, H.; Wessels, B. W.; Kanatzidis, M. G. TlSn2I5, a robust halide antiperovskite semiconductor for γ-ray detection at room temperature. ACS Photonics 2017, 4, 1805-1813. 27. Sobott, R. J. G. Minerals and calculated low-temperature phase-equilibria in the pseudoternary system Tl2S-As2S3-Sb2S3. Miner. Petrol. 1995, 53, 277-284. 28. Dickson, F. W.; Radtke, A. S. Weissbergite, TlSbS2 - new mineral from Carlin gold deposit, Nevada. Am. Mineral 1978, 63, 720-724. 29. Bohac, P.; Bronnima.E; Gaumann, A. Ternary compound TlSbS2 and its photoelectric properties. Mater. Res. Bull. 1974, 9, 1033-1040. 30. Cermak, K.; Juska, G. Drift mobility of holes in TlSbS2. Phys. Status Solidi. A 1985, 91, 219-223. 31. Seltzer, S. M. Calculation of photon mass energy-transfer and mass energy-absorption coefficients. Radiat. Res. 1993, 136, 147-170. 32. Rey, N.; Jumas, J.; Olivier-Fourcade, J.; Philippot, E. Sur les composés III–V–VI: étude structurale du disulfure d'antimoine et de thallium, TlSbS2. Acta. Cryst. C. 1983, 39, 971-974. 33. Steininger, J.; Strauss, A. J.; Brebrick, R. F. Phase diagram of Zn-Cd-Te ternary system. J. Electrochem. Soc. 1970, 117, 1305-1309. 34. Cermak, K.; Lostak, P. Drift mobility of electrons in TlSbS2. Czech J. Phys. 1986, 36, 709-713. 35. Alig, R. C.; Bloom, S. Electron-hole-pair creation energies in semiconductors. Phys. Rev. Lett. 1975, 35, 1522-1525. 36. Alig, R. C.; Bloom, S.; Struck, C. W. Electron-hole-pair creation energies in semiconductors. B Am. Phys. Soc. 1980, 25, 175-175. 37. Ozawa, L.; Hersh, H. N. Creation energy of electron-hole pairs in luminescent semiconductors. J. Electrochem. Soc. 1976, 123, C258-C258. 38. Tuller, H. L.; Bishop, S. R. Point defects in oxides: tailoring materials through defect engineering. Annu. Rev. Mater. Res. 2011, 41, 369-398. 39. Schmidt, T.; Lischka, K.; Zulehner, W. Excitation-power dependence of the near-band-edge photoluminescence of semiconductors. Phys. Rev. B 1992, 45, 8989-8994. 40. Many, A. High-field effects in photoconducting cadmium sulphide. J. Phys. Chem. Solids 1965, 26, 575-585. 15 ACS Paragon Plus Environment

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41. Ruzin, A.; Nemirovsky, Y. Methodology for evaluation of mobility-lifetime product by spectroscopy measurements in CdZnTe spectrometers. J. Appl. Phys. 1997, 8, 4166-4171. 42. Perdew, J. P.; Ruzsinszky, A.; Csonka, G. I.; Vydrov, O. A.; Scuseria, G. E.; Constantin, L. A.; Zhou, X. L.; Burke, K. Restoring the density-gradient expansion for exchange in solids and surfaces. Phys. Rev. Lett. 2008, 100, 3865-3868. 43. Hitomi, K.; Matsumoto, M.; Muroi, O.; Shoji, T.; Hiratate, Y. Characterization of thallium bromide crystals for radiation detector applications. J. Cryst. Growth 2001, 225, 129-133. 44. Bridgman, P. W. Certain physical properties of single crystals of tungsten, antimony, bismuth, tellurium, cadmium, zinc, and tin. Proc. Am. Acad. Arts Sci. 1925, 60, 305-383. 45. X-RED;, X.-A. X.-S., STOE & Cie GMbH: Darmstadt,Germany 2009. 46. Sheldrick, G. M. A short history of SHELX. Acta Cryst. A 2007, 64, 112-122. 47. Liao, J. H.; Kanatzidis, M. G. Quaternary rubidium copper tin sulfides (Rb2Cu2SnS4, A2Cu2Sn2S6 (A = Na, K, Rb, Cs), A2Cu2Sn2Se6 (A = K, Rb), potassium gold tin sulfides, K2Au2SnS4, and K2Au2Sn2S6. Syntheses, structures, and properties of new solid-state chalcogenides based on tetrahedral [SnS4]4- units. Chem. Mater. 1993, 5, 1561-1569. 48. Blochl, P. E. Projector augmented-wave method. Phys. Rev. B 1994, 50, 17953-17979. 49. Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 1999, 59, 1758-1775. 50. Kresse, G.; Furthmuller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comp. Mater. Sci. 1996, 6, 15-50. 51. Kresse, G.; Hafner, J. Ab-initio molecular-dynamics for open-shell transition-metals. Phys. Rev. B 1993, 48, 13115-13118. 52. Madsen, G. K. H.; Singh, D. J. BoltzTraP. A code for calculating band-structure dependent quantities. Comput. Phys. Commun. 2006, 175, 67-71.


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Table 1. Crystal data and structure refinement for TlSbS2 at 293(2) K. Empirical formula

TlSbS2

Formula weight

390.24

Temperature

293(2) K

Wavelength

0.71073 Å

Crystal system

Triclinic

Space group

Volume

P-1 a = 6.1259(12) Å, α = 101.46(3)° b = 6.3161(13) Å, β = 98.22(3)° c = 11.793(2) Å, γ = 104.10(3)° 424.84(17) Å3

Z

4

Density (calculated)

6.101 g·cm-3

Absorption coefficient

44.990 mm-1

F(000)

656

Crystal size

0.1786 × 0.1384 × 0.0193 mm3

θ range for data collection

3.429 to 24.995°

Index ranges

-7 ≤ h ≤ 7, -7 ≤ k ≤ 7, -13 ≤ l ≤ 13

Reflections collected

2751

Independent reflections

1405 [Rint = 0.0959]

Completeness to θ = 25.242°

91.5%

Refinement method

Full-matrix least-squares on F2

Data / restraints / parameters

1405 / 0 / 73

Goodness-of-fit

1.035

Final R indices [I > 2σ(I)]

Robs = 0.0906, wRobs = 0.2435

R indices [all data]

Rall = 0.1063, wRall = 0.2579

Largest diff. peak and hole

4.885 and -4.875 e·Å-3

Unit cell dimensions

R = Σ||Fo|-|Fc|| / Σ|Fo|, wR = {Σ[w(|Fo|2 - |Fc|2)2] / Σ[w(|Fo|4)]}1/2 and w = 1/[σ2(Fo2) + (0.1763P)2] where P = (Fo2 + 2Fc2)/3


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For Table of Contents Use Only

TlSbS2: a Semiconductor for Hard Radiation Detection Wenwen Lin,† Haijie Chen,† Jiangang He, ‡ Constantinos C. Stoumpos,† Zhifu Liu, ‡ Sanjib Das,‡ Joon-IL Kim,‡ Kyle M. McCall,†,‡ Bruce W. Wessels,‡ and Mercouri G. Kanatzidis†,* †

Department of Chemistry, and ‡Department of Materials Science and Engineering, Northwestern

University, Evanston, Illinois 60208, United States Synopsis: The mineral weissbergite TlSbS2 was identified as a promising candidate for hard radiation detection at room temperature. This quasi two-dimensional compound features high photon stopping power and a high resistivity above 1010 Ωcm. A room temperature radiation detector was made from a single crystalline TlSbS2 cleaved plate, and it exhibited reasonable photoresponse for hard Ag X-rays and α-particles from 241Am.


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