Controlling the Vapor Transport Crystal Growth of Hg3Se2I2 Hard

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Controlling the Vapor Transport Crystal Growth of Hg3Se2I2 Hard Radiation Detector Using Organic Polymer Yihui He, Grant C. B. Alexander, Sanjib Das, Zhifu Liu, Ido Hadar, Kyle M McCall, Wenwen Lin, Yadong Xu, Duck Young Chung, Bruce W. Wessels, and Mercouri G. Kanatzidis Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01646 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 5, 2019

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Crystal Growth & Design

Controlling the Vapor Transport Crystal Growth of Hg3Se2I2 Hard Radiation Detector Using Organic Polymer Yihui He1, Grant C. B. Alexander1, Sanjib Das2, Zhifu Liu2, Ido Hadar1, Kyle M. McCall1,2, Wenwen Lin1, Yadong Xu1, Duck Young Chung4, Bruce W. Wessels2,3 and Mercouri G. Kanatzidis1* 1Department

of Chemistry, 2Department of Materials Science and Engineering, 3Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208, USA 4Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA *Corresponding Author: [email protected]

ABSTRACT The chalcohalide compound Hg3Se2I2 with defect anti-perovskite structure has been demonstrated to be a promising semiconductor for room temperature X- and γ-ray detection. In this work, we use transport agents during the vapor growth of Hg3Se2I2 crystals under gradient temperature profiles to dramatically improve the size and yield of Hg3Se2I2 single crystals. Various growth conditions with combinations of organic polymer (polyethylene) with elemental Hg, Se or I2 are compared. The largest single crystals (with size up to 753.5 mm3) were obtained using both polyethylene and excess I2 as the transport agents. The as-prepared detector devices based on these crystals have excellent photo response for a series of radiation sources, including low flux X-ray source, alpha particles and gamma rays. The X ray induced photocurrent of Hg3Se2I2 detectors is three orders of magnitude higher than the dark current, indicating excellent X-ray photosensitivity. Under 241Am α particle source (5.5 MeV), the best energy resolution obtained is ~8.1%. The Hg3Se2I2 device also shows improved detector performance under 57Co

and 137Cs  ray sources. The improved crystal growth and detector performance using this polymer

additive during the vapor transport process further confirms the great potential for the development of Hg3Se2I2 for radiation detection. Keywords: Defect antiperovskite; Hg chalcohalide; Vapor transport; Radiation detection; Alpha particle spectrum 1

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INTRODUCTION Developing high performance hard radiation detectors based on compound semiconductors is imperative owing to their broad potential applications in the fields of homeland security, industrial and medical imaging as well as in fundamental scientific research.1-3 This demand has driven the development of candidate semiconductor materials for several decades. However, so far, no suitable compound exists which simultaneously offers low cost with all of the concomitant properties required of a semiconductor material, namely high density, high atomic number (Z), suitable bandgap (Eg), high electrical resistivity (𝜌) and high mobility-lifetime (τ) product. To date, only a few conventional compounds (binary chalcogenides and halides) have demonstrated encouraging spectroscopic response, and even fewer of these (mainly Cd1-xZnxTe and TlBr) have been developed to the point of commercial applications due to the difficulties or high cost in obtaining high-quality single crystals reproducibly. 4-9

Very recently, halide perovskites materials, such as CsPbBr3, has demonstrated promising properties,

while still under early optimization.10-11 Based on the concept of lattice hybridization 12, mercury chalcohalides Hg3Q2X2 (Q=S, Se and Te; X=Cl, Br and I) have been proposed as an promising group of semiconductors for X- and γ-ray radiation detection at room temperature.

12-19

Among them, Hg3Se2I2 with high effective atomic number (Zeff =

71.8) has demonstrated the most promising properties. Hg3Se2I2 possesses a so-called “defect antiperovskite structure” and fulfills the necessary properties of a semiconductor detector material, with large bandgap (2.12 eV), high density (7.38 g/cm3), high intrinsic resistivity (>1012 Ωcm), and high electron mobility (~ 100 cm2V-1s-1) which is comparable to that of detector-grade HgI2 detectors. 13, 19 The detectors based on as-grown Hg3Se2I2 single crystals yielded resolved

241Am

α particle spectrum

and 241Am γ-ray spectrum, however, the single crystal size that can be grown is still limited. 13 Growth issues of Hg3Se2I2 (incongruent melting) have greatly impeded growth from the melt and consequently the subsequent development of performance optimization. utilized vapor transport method in term of “auto transport”

21

20

In our previous work, we

to grow high-quality Hg3Se2I2 single

crystals plates with length of ~3-5 mm but with very small thickness of ~0.1 mm. However, high reproducibility and high yield for large single crystals will be required from such a process if larger crystals are to be obtained. 2


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In this work in an effort to obtain more practical crystal sizes of cm scale, we describe a more effective and novel approach to improve both the reproducibility and the yield of large Hg3Se2I2 single crystals by employing vapor transport agents. Instead of the “auto-transport” process, we added an organic polymer in the growth chamber during the vapor transport growth and found that it can lead to significantly larger sizes and higher quality Hg3Se2I2 crystals. The role of the polymer is to create via in-situ decomposition effective small molecules that modulate the vapor transport process suppressing random nucleation centers during vapor transport which results in fewer and large single platelets or polyhedra of Hg3Se2I2. We show that the detector performance of these crystals is improved compared to the previous report, showing excellent photo response against low flux X-rays, α particles and  ray sources. EXPERIMENTAL SECTION Vapor transport growth of Hg3Se2I2. The high purity elements were used to synthesize the starting materials (α-HgI2 and HgSe) through vapor transport method. The synthesis method is similar to that described in previous work 13. Caution: Hg metal and its related compounds are highly toxic and great care should be taken with proper protective equipment in both synthesis and handling. For vapor transport, HgQ and α-HgI2 were used with the stochiometric mole ratio of 2:1. Then the mixed binary compounds were sealed in a fused silica tube with the pressure less than 210-3 mbar, and placed horizontally in a two-zone furnace. The nominal temperature at the source end is usually between 460 oC and 360 oC while the cold end is about 10-100 oC lower. The temperature gradient at the transport region should be in the range of 2-20 K/cm. Since the thermodynamic vapor pressure of each component in the Hg-Se-I system is not available yet, it was difficult to predict the optimization temperature for vapor transport accordingly. Thus, the temperature profile, as well as the high and low temperature zones were investigated systematically and from these were determined the optimized growth conditions. The range of vapor transport parameters used in our experiments are shown in Table 1. The optimized temperature profile used for vapor transport by a two-zone horizontal furnace is indicated in Figure 1. For the optimized temperature profile (Figure 1), the source and cold end temperature is 430 oC

and 300 oC, respectively, and the temperature gradient is 12 K/cm across the transport zone.

Additionally, we added low molecular weight polyethylene (PE), (Mw = ~4000), and excess element 3



(such as Hg or I2) or their combinations as transport agents during the vapor transport. 450

o

Temperature / C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Optimized T profile

400

350

300 0

5

10 Distance / cm

15

Figure 1. Optimized temperature profile using a two-zone furnace from hot zone (distance from 0 cm) to cold zone (distance from ~15 cm) used for vapor transport growth of Hg3Se2I2.

Table 1. Potential ranges of vapor transport parameters used in experiments. Parameters

Range

Thot Tcold Temperature gradient Transport time Transport additives Tube geometry Starting materials

460-360 oC 350-250 oC 2-20 K/cm 120 h Polyethylene, I2, Hg, Se > 10 mm (ID) >2g

Single crystal X-ray diffraction. A single crystal of vapor grown Hg3Se2I2 was cleaved under Paratone N oil to produce samples of a suitable size for X-ray diffraction. Crystals were examined using a microscope equipped with polarized filter to determine relative quality. Once selected the small needle of 0.0650.0430.029 mm3 was mounted on a MiTeGenTM mount using Paratone N oil. Under the cooled 100 K nitrogen flow of the cryostream the oil sufficiently solidifies, preventing crystal movement during collection. Diffraction was conducted at 100 K on a Bruker KAPPA APEX diffractometer with a Mo Kα microsource and QuazarTM optics monochromator. Using the COSMO program provided in the APEX 3 software, data collection strategy with a series of 0.5° scans in ω and φ was determined.

22

The exposure time was 10 s/frame, and the crystal-to-detector distance was 60 4


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mm. SADABS was used for face-indexed absorption, incident beam, and decay corrections. 23 The structure solved by the direct method and refined on F2 using the SHELX14 program suite.24 All atoms were refined anisotropically. Further details are presented in Table S1. Electrical Properties and Detector Performance Measurements. Planar-type Au contacts with thickness of ~50 nm on Hg3Se2I2 samples were prepared by e-beam thermal evaporation using a shadow mask. The electrical resistivity was measured in dark using a Keithley 6517B electrometer. The resistivity is calculated from the I-V characteristics between -10 and +10 V. Photoconductivity was evaluated under both ambient light and Ag X-ray tube (Mini-X X-Ray Tube from Amptek, Inc). The – Valance band maximum energy (VBM), was measured by ambient pressure photoemission spectroscopy (See Supporting Information for the details).25 For radiation detection measurements, the γ-ray sources employed were a non-collimated 1 Ci 241Am source with 5.5 MeV α-ray and 59.5 keV γ-ray, 0.2 mCi 57Co and 5 Ci 137Cs 662 keV γ-ray sources. The measurement method is the same as that described in previous work 13, where a bias varying from 10 to 500 V was applied on the bottom electrode while the incident radiation irradiated the top electrode. The energy resolution (ER) of characteristic peak is evaluated as its full width at half maximum (FWHM)/(Channel number) 100%. The mobility was estimated using alpha pulse height spectroscopy

13, 26-27,

according to following

𝑑2

formula, 𝜇 = 𝑈𝑡𝑟 , where U is the applied bias across the thickness d, and tr is the transit time of the carriers through the device. The distance between the α source from the surface and the detector is about 𝑄

5 mm. 28 The mobility-lifetime product (μτ) of electrons was estimated by the Hecht equation, 29 𝜂 = 𝑄0 𝑈

= 𝜇𝜏𝑑2(1 ― 𝑒

𝑑2

― 𝜇𝜏𝑈

), where η is the charge collection efficiency (CCE), Q is the measured maximum

channel number of photo peak/shoulder and Q0 is the theoretical saturated channel number of the photo peak/shoulder. RESULTS AND DISCUSSION Crystal growth The chalcohalide Hg3Se2I2 adopts a unique anti-perovskite structure where half of the Hg atoms is missing compared to the standard anti-perovskite structure,13 which was first reported by Beck et al 31. Due to the incongruent melting of this compound, direct melt freezing methods such as Bridgman 5



cannot be used to grow high-quality single crystals, so a vapor transport growth method 21, 30 is used. This circumvents the issue of off-stoichiometry, but can still pose challenges if the compound possesses phase transitions between the synthesis temperature and room temperature. For example, the varied polymorphs of HgI2 at different temperatures poses crystal growth problem, but Hg3Se2I2 does not exhibit a phase transition ranging from -160 oC to 300 oC 31, which permits the growth of high-quality single crystals. To examine the cell parameters of Hg3Se2I2 at lower temperature, a crystal structure solution was obtained at 100 K by single-crystal X-ray diffraction (Figure 2). Table 2 indicates that in accordance to the previous literature, no phase change or higher symmetry is observed at 100 K in Hg3Se2I2. The cell parameters decrease in both a and c by 1.2% and 0.45% respectively, and the b parameter increases by 0.17%. Change in bond lengths and bond angles between Hg and Se do not account for this effect, but instead it is a consequence of the reduction in distance between the free iodide atoms and the Hg-Se chains (Table S1-S5).

Figure 2. Crystal structure (Unit cell) of Hg3Se2I2 at 100 K indicating quasi-1D ∞1[Hg3Se2]2+ chains along a axis.

Though the compound does not undergo a phase transition, the yield and crystal size remain extremely sensitive to the growth conditions due to the competing binary phases and possible variations in the composition of the vapor during growth. Since the equilibrium partial pressure of HgI2 and HgSe binaries differs greatly,32-33 any temperature fluctuation shall lead to stoichiometric imbalance and the generation of excess nucleation centers. Therefore, it is necessary to control the temperature at the hot and cold zone precisely. 6


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In addition, the transport agents used play a critical role in the vapor transport process. Without transport agents, the as-grown single crystals generally tend to be small needle-like shapes (Figure S1) along [100] direction. In this case, there were few millimeter-size platelets, however, very thin, usually around 0.1-0.2 mm.13 The incongruent sublimation property of the material primarily accounts for the limited single crystal size. Upon heating above 420 oC, Hg3Se2I2 tends to form the HgI2-rich liquid and HgSe solid through the equilibrium peritectic reaction.

20

To ensure the nucleation of pure Hg3Se2I2

crystals occurs without forming HgI2 at the cold side, the pressure balance among each vapor components, such as Hg, I2, HgI2 and Se during vapor transport should be controlled.

Table 2. Crystal data and structure refinement for Hg3Se2I2 at 296 K 31 and 100 K in this work. Empirical formula

Hg3Se2I2

Formula weight

1013.49

1013.49

Temperature

100.02 K

296 K

Wavelength

0.71073 Å

0.71073 Å

Crystal system

Orthorhombic

Orthorhombic

Space group

Imma

Imma

a = 9.6451(4) Å, α = 90°

a = 9.7660(9) Å, α = 90°

b = 19.4145(8) Å, β = 90°

b = 19.381(3) Å, β = 90°

c = 9.5898(4) Å, γ = 90°

c = 9.6332(9) Å, γ = 90°

Unit cell dimensions Volume

1795.74(13)

Z

8

Density (calculated)

Å3

1823.3(3) Å3 8

7.497

g/cm3

7.384 g/cm3

Figure 3 shows a series of Hg3Se2I2 crystals grown with different transport additives under the same temperature profile shown in Figure 1. It is very interesting that the use of polyethylene (PE), an unconventional additive in vapor transport reactions, and an appropriate temperature profile are both essential in obtaining large single crystals apparently mitigating the issue of incongruent evaporation. The amount of PE in the starting materials has substantial influence on both the crystal size and yield. As shown in Figure 3a-3c, 0.5% in wt.% excess of PE gave rise to the largest monocrystalline crystals with around 1 cm size. The color of the as-grown single crystals appears as transparent bright-red, which corresponds well to the bandgap value of 2.12 eV. By using higher amounts (from 2 grams to 9 grams) of starting materials and larger tubes (from 13 mm to 18 mm), the crystals with size larger than 1 cm also increased in concert (Figure 3e). It is also worth noting that the preferred morphology of the as-grown single crystals changes from needles to plates, which has the additional benefit of facilitating 7



detector device fabrication.

Figure 3. Crystal morphology of Hg3Se2I2 grown under optimized temperature profile shown in Figure 1 but with different transport additives. (a) PE excess in 2% wt.% in 13 mm tube. (b) PE excess in 1% wt.% in 13 mm tube. (c) PE excess in 0.5% wt.% in 13 mm tube. (d) HgI2 excess in 10% mole% in 18 mm tube. (e) PE excess in 0.5% wt.% in 18 mm tube. (f) PE excess in 0.5% wt.%.and Hg excess in 0.5% wt.% in 18 mm tube. (g) PE excess in 0.5% wt.%.and iodine excess in 0.5% wt.% in 18 mm tube. (h) single crystals obtained as indicated in (g). The scale bar indicated corresponds to 10 mm.

To further improve the crystal size, excess elemental I2, or Hg, or binary HgI2 are also introduced into starting materials besides PE. The type of transport agents greatly influences the preferable morphology of as-grown crystals. Growth with both PE and excess HgI2 led to formation of polycrystalline samples and low yield of single crystals (Figure 3d). The vapor transport with PE and Hg (Figure 3f) gave rise to thinner crystals (with thickness of ~0.1 mm), but with elongated shape and the edge of the crystals showed serrated feature. The vapor transport efficiency under PE and I2 (Figure 3g) was the highest among all combinations, such as Hg and PE (Figure 3f), HgI2 and PE (Figure 3d). Furthermore, the thickness of the as-grown crystals is improved dramatically under PE and I2 transport. In contrast to PE-only transport, the Hg3Se2I2 grown under PE and I2 was transported to the cold side completely (Figure S2), indicating excellent transport efficiency. The as-grown single crystal maximum size was ~753.5 mm3. The thickness of the crystals was also significantly increased to over 1 mm for most of the crystals. With further improvement of transport efficiency, even larger single crystals are expected to be grown under 8


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PE and I2 conditions. At high transport temperature (>400 oC), Hg halides (HgI2) possess high equilibrium vapor pressure and could sublime to the cool end. Besides, PE decomposes completely into an inert black carbon-based substance and residual gas species, probably in the form of hydrogen. This decomposition may also prompt the formation of intermediate volatile species during vapor transport and accelerate the transport process, such as possible H2Se, CH3I, CH3HgI, CH3SeCH2, CH3Se or CH3SeSeCH3.34 Here we suppose the efficacy of PE is a result of intermediate phases (probably as H2Se34) that promote the transport of Se atoms, which have significant smaller vapor pressure than Hg halides. The mobilization of HgSe via vapor transport with PE, may create more balanced vapor pressures of the gas species in sufficient concentrations to form Hg3Se2I2 crystals at the sink, further promoting the single crystal growth of phase-pure Hg3Se2I2. Detector studies Gold electrodes were evaporated on the two opposite faces of a plate crystal with no indication of chemical reaction between Au and Hg3Se2I2. The thickness of the crystal was 0.5 mm. The valence band maxima (VBM), of the crystal was measured to be 5.93 ± 0.05 eV and the conduction band minima (CBM), was calculated by subtracting the bandgap from the VBM (CBM = 3.81 ± 0.05 eV) (Figure S3). This means that the Au electrodes with work function of ~ 4.85 eV are suitable electrodes. The devices based on the single Hg3Se2I2 crystals demonstrated excellent photoresponse under light source (Figure S4). The intrinsic electrical resistivity of the crystal (Figure S4a) was over extremely high at ~1012 Ωcm,. The symmetrical non-linear feature of current-voltage (I-V) curve is likely due to the competing Poole-Frenkel emission and phonon-assisted tunneling processes35, commonly observed in high resistivity CdTe and CdZnTe detectors 36. Under the bias of 100 V the leakage current was less than 0.25 nA while under ambient white light (~1 mW/cm2) the photocurrent was over 4 A. The ratio of photo/dark current (>16,000) is over one order larger than our precious results using no transport agent. 13 Another intriguing feature is that the device can respond spontaneously even at very low bias. Photoresponse under 100 mV corresponding to an electrical field of 2 V/cm (Figure S4b), indicated the photocurrent was over 4 orders of magnitude above the dark current. Such On/Off test also confirm the reproducible, fast and steady photoresponse process. Under low flux Ag X-ray source (tube voltage 50 kV and tube current 40 A), the photocurrent was three orders higher than the dark current, indicating the excellent photo-sensitivity under low 9



intensity X-ray source (Figure 4). The large On/Off ratio provides high signal to noise (S/N) ratio during X-ray detection.

Dark Current Ag X ray 50 kv 40 uA

1000 100

Current / nA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 18

10 1 0.1

0.01 1E-3 1E-4

-10

-5

0

Voltage / V

5

10

Figure 4. X ray induced photocurrent of Hg3Se2I2 device under Ag X-ray source (Mini-X X-Ray Tube from Amptek, Inc with a distance of ~15 cm). The current under negative bias was intentionally converted to positive values for better illustration.

Under the

241Am

α particle radiation source, the Hg3Se2I2 detector showed greatly improved

spectroscopic response compared to our previous results.

13

The typical kinetic energy for incident α

particles is 5.5 MeV and all the kinetic energy will be attenuated completely within less than tens of micrometers from the entrance the detector surface. 37 The cathode in Figure 5a and anode in Figure 5b was under irradiation, respectively. Thus, the measured signal was generated mainly through drift of electron and hole, respectively. The 241Am α particle (5.5 MeV) spectra in Figure 5a and 5b indicate the distinct transport properties of electrons and holes. With increasing applied bias, the spectral profiles shifted to higher channel numbers, indicating the improvement of charge collection efficiency (CCE). When the anode was under irradiation, the detector resolved the alpha peaks but at lower channel numbers, indicating poor charge collection efficiency for holes. The τ values for electron and hole calculated are ~1.510-5 cm2/V and ~110-6 cm2/V using the Hecht equation, respectively, which can be largely attributed to difference in the effective mass of electron (0.20m0) and hole (0.86m0)

13.

Meanwhile, the energy resolution improved from 50% at 50 V to 9% at 200 V. The time-dependent spectrum confirmed the stability of the device performance (Figure 5c). The best energy resolution under 241Am α particle source achieved was 8.1% when the device (~0.3 mm thickness) was biased at 450 V (Figure 5d).

10


100

0

(c)

30 V 50 V 70 V 100 V 130 V 150 V 200 V

0

200

0

0

-30 V -50 V -70 V -100 V

400 300 200 100 0

2000 4000 6000 8000 Channel number

(d)

200 V-bkgd 60 s 120 s 180 s 240 s

100

(b) 500

Count /a.u.

Count /a.u.

(a) 200

Count / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Count /a.u.

Page 11 of 18

0

200 400 600 800 1000 Channel number

200

450 V 120 s (48 h later)

100 ER=8.1% 0

2000 4000 6000 Channel number

0

1000 2000 3000 4000 5000 Channel number

Figure 5. 241Am α source (5.5 MeV) spectra resolved by Hg3Se2I2 detector grown using 0.5% wt.% PE and 0.5% wt.% iodine as the transport agents. (a) Cathode under irradiation. (b) Anode under irradiation. (c) Time-dependent spectrum recorded over 240 s under 200 V. (d) The spectrum obtained from the champion device recorded at 450 V for 120 s.

The carrier drift mobility of the crystals was evaluated using the rise time tr of transient waveforms from the preamplifier under

241Am

α particle source.

26-27

As Figure 6a shows, the rise times tr

accordingly deceases with increasing bias while its distribution narrows and shifts towards smaller values. The detector responds fast under radiation, for instance, at 130 V, the median rise time was only ~90 ns. The typical waveforms under various bias voltages were extracted directly from the preamplifier (Figure 6b). The pulse amplitude was enhanced with bias, indicating higher charge collection efficiency. Subsequently, those waveforms were shaped further by the amplifier and then gave rise to the pulse height spectrum. The electron mobility values obtained from three growth batches all indicate mobility around 100 cm2/(V·s) (Figure 6c), which confirms the good reproducibility.

11



Figure 6. Box chart (a) of rise time data with scatter plot under various biases with the bottom and top of the box representing the 25th and 75th centiles. Typical transient waveforms (b) under various biases were obtained directly from the preamplifier. The estimation of mobility for electron of Hg3Se2I2 detector (c) from different growth batches by linear fitting with the uncertainty of 10% (error bars). Note that the median value of rise time was chosen to calculate as indicated in (a).

Under a 122 keV

57Co

gamma ray source, an improved characteristic spectrum was obtained

(Figure 7a) compared to our previous work 13. Spectra were collected for four minutes at a bias of 200 V. The spectrum showed a very weak peak around channel number 100 and a shoulder around channel number 300. Such spectroscopic feature is analogous to α-HgI2 and TlBr in their early stages of development.

29, 38-39

The advantage of Hg3Se2I2 is its high electron mobility. It is generally believed

that the carrier lifetime and mobility is inversely correlated to the concentration of ionized point defects. 12


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Page 13 of 18

Thus, better stoichiometric control and purification will be a practical future approach to achieve the full potential of Hg3Se2I2. The device was table in air as it showed consistent and stable response (Figure S5). Under 241Am 59.5 keV  ray source, the detector can resolve the full energy peak with the energy resolution of ~18% (Figure 7b), greatly improved from previous reported ~50%. 13 Furthermore, under 137Cs

662 keV  ray source, the detector was also able to count this high-energy gamma ray (Figure 7c).

The detector was almost saturated under a bias of 200 V (Figure 7c). These results suggest a great promise of Hg3Se2I2 for high energy gamma ray radiation detection at room temperature.

1500 57

Count / a.u.

Co  ray

(c)

Background with (PE and I2)

137

Cs  ray

without (PE and I2)

1000

300 V 250 V 200 V

500 0

(b)

150 V

0

80


800

500 V background 500 V for 300s

60

Count / a.u.

(a)

Count / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 V 70 V 50 V

241

Am  ray

30 V

40 10 V

20 0

300 V-bkgd

0


0

1000 2000 Channel number

Figure 7.  ray response of Hg3Se2I2 grown by using 0.5% wt.% polyethylene and 0.5% wt.% iodine as the transport agents (a) under 122 keV 57Co  ray source. (b) under 241Am 59.5 keV  ray source and (c) under 137Cs 662 keV  ray source.

CONCLUSION The use of PE during the vapor growth of Hg3Se2I2 results in dramatic improvements in both the size and the yield of high-quality Hg3Se2I2 single crystals. The ultimate morphology (such as platelets or 13



Page 14 of 18

polyhedra) of as-grown single crystals is also influenced by the quantity and combinations of transport agents used. Single crystals with sizes up to 753.5 mm3 were successfully grown using the combination of PE and I2 as the transport agents. The crystals exhibited great performance for photoelectric conversion, showing excellent photo response under ambient light, X-ray, α particle source and  ray sources. The X ray induced photocurrent of the Hg3Se2I2 detectors was higher than the dark current by three orders of magnitude. Under a 241Am α particle source, the best energy resolution obtained was ~8.1%. The τ values for electron and hole calculated were ~1.510-5 cm2/V and ~1106

cm2/V using Hecht equation, respectively. The detector also showed reasonable response under 241Am,

57Co

and

137Cs

 ray sources and resolve the full energy peak of

241Am

59.5 keV  ray peak with the

energy resolution of ~18%. Future efforts should focus on improved purification and better stoichiometric control to further improve the room-temperature detector performance of Hg3Se2I2. ASSOCIATED CONTENT Supporting Information. More detailed information about the single crystal diffraction data and structure refinement of Hg3Se2I2 at 100 K, with corresponding atomic coordinates and equivalent isotropic displacement parameters and bond lengths and bond angles, and crystal morphology comparison of Hg3Se2I2 grown without/with transport agent, and photo and gamma ray response of Hg3Se2I2 grown by using 0.5% wt.% PE and 0.5% wt.% iodine as the transport agents. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by the Department of Homeland Security ARI program with grant 2014-DN077-ARI086-01. 14


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For Table of Contents Use Only

Controlling the Vapor Transport Crystal Growth of Hg3Se2I2 Hard Radiation Detector Using Organic Polymer Yihui He1, Grant C. B. Alexander1, Sanjib Das2, Zhifu Liu2, Ido Hadar1, Kyle M. McCall1,2, Wenwen Lin1, Yadong Xu1, Duck Young Chung4, Bruce W. Wessels2,3 and Mercouri G. Kanatzidis1* 1Department

of Chemistry, 2Department of Materials Science and Engineering, 3Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208, USA 4Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA *Corresponding Author: [email protected]

TOC Graphic

Synopsis A chemical vapor transport method with an unconventional transport agent has been developed to grow Hg3Se2I2 crystals. This approach has demonstrated dramatic improvement on the size and yield of Hg3Se2I2 single crystals. As-grown crystals exhibited great performance for photoelectric conversion, showing excellent photo response under ambient light, X-ray, α particle source and  ray sources.

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Controlling the Vapor Transport Crystal Growth of Hg3Se2I2 Hard

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