Cu2I2Se6: A Metal-Inorganic-Framework Wide-bandgap

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Cu2I2Se6: A Metal-Inorganic-Framework Wide-bandgap Semiconductor for Photon Detection at Room Temperature Wenwen Lin, Constantinos C. Stoumpos, Oleg Y. Kontsevoi, Zhifu Liu, Yihui He, Sanjib Das, Yadong Xu, Kyle M McCall, Bruce W. Wessels, and Mercouri G. Kanatzidis J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12549 • Publication Date (Web): 14 Jan 2018 Downloaded from http://pubs.acs.org on January 14, 2018

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Journal of the American Chemical Society

Cu2I2Se6:

A

Metal-Inorganic-Framework

Wide-bandgap

Semiconductor for Photon Detection at Room Temperature

Wenwen Lin,† Constantinos C. Stoumpos,† Oleg Y. Kontsevoi, ‡,# Zhifu Liu,§ Yihui He, † Sanjib Das, § Yadong Xu, † Kyle M. McCall, †,§ Bruce W. Wessels§ and Mercouri G. Kanatzidis†,* †

Department of Chemistry, ‡Department of Physics and Astronomy, #Northwestern-Argonne

Institute of Science and Engineering, and §Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States

ABSTRACT: Cu2I2Se6 is a new wide-bandgap semiconductor with high stability and great potential towards hard radiation and photon detection. Cu2I2Se6 crystallizes in the rhombohedral ത m space group with a density of d = 5.287 g·cm-3 and a wide bandgap Eg of 1.95 eV. R3 First-principles electronic band structure calculations at the density functional theory level indicate an indirect bandgap and a low electron effective mass me* of 0.32. The congruently melting compound was grown in centimeter-size Cu2I2Se6 single crystals using a vertical Bridgman method. A high electric resistivity of ~1012 Ω·cm is readily achieved and detectors made of Cu2I2Se6 single crystals demonstrate high photo sensitivity to Ag Kα X-rays (22.4 keV), and show spectroscopic performance with energy resolutions under

241

Am α-particles (5.5 MeV)

radiation. The electron mobility is measured by a time of flight technique to be ~46 cm2·V-1·s-1. This value is comparable to that of one of the leading γ-ray detector materials TlBr, and is a factor of 30 higher than mobility values obtained for amorphous Se for X-rays detection.

■ INTRODUCTION Hard radiation (X-rays, α-particles, and γ-rays) detectors operating at room temperature are highly sought after for applications in nuclear medicine, nonproliferation of nuclear materials and outer space exploration. Compared with the traditional scintillator detectors which require bulky photomultipliers, semiconductor detectors promise higher resolution and can be made much more compact owing to the direct conversion of incident photons into electric signals. An ideal semiconductor with high detection performance for hard radiation should meet a series of strict 1

ACS Paragon Plus Environment

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physical properties including high photon stopping power (d > 5 g·cm-3), high resistivity (ρ > 108 Ω·cm), reasonably wide bandgap to suppress formation of carriers at room temperature (1.5 eV < Eg < 2.5 eV), high carrier mobility-lifetime product, low-concentration of carrier trapping centers and high chemical stability.1 Therefore, only a few compounds have been identified as hard radiation semiconductor detection materials. To date, the leading materials with spectroscopic performance are Cd0.9Zn0.1Te (CZT), TlBr, HgI2 and PbI2.2-9 However, some serious issues such as intrinsic defects (CZT),10-11 polarization effects (TlBr),1,

12

and low mechanical processability

(PbI2 and HgI2) are plaguing these compounds.7-9 The recently developed organic-inorganic halide perovskite semiconductor CH3NH3PbX3 (X = Cl, Br or I) has attracted great attention due to its remarkable carrier mobility, low densities of traps and long carrier lifetimes.13-16 The detectors made of CH3NH3PbX3 have shown photoresponse for X-rays and γ-rays.17-18 Nevertheless, the ionic conductivity which is intrinsic to the materials leads to space-charge effects, which in turn greatly reduces the charge collection efficiency. Moreover, these materials are subject to poor chemical stability,19-20 low photon stopping power due to low densities (3.80 - 4.15 g·cm-3) and undesirable phase transitions.21 Thus, we are motivated to explore and study alternative semiconductor compounds which could have high-performance. In this work, we present Cu2I2Se6 as a new semiconductor compound which exhibits very promising properties for hard radiation detection. Cu2I2Se6 is a three-dimensional (3D) metal inorganic framework consisting of [Cu2I2] nodes and Se6 linkers.22 The compound derives directly from elemental selenium, an “ancient” semiconductor with high photoconductivity contrast ratio under energetic photons.23 One of the main technological problems, however, in employing Se in X-ray detection devices is the presence of different allotropes with the semi-insulating, photoconducting, amorphous Se (Eg ~ 2.2 eV) crystallizing into the semiconducting black crystalline Se (Eg ~ 1.7 eV) upon prolonged irradiation. 24-25 The instability of amorphous Se arises from the inhomogeneity of its components which comprise of random distributions of Se6, Se7, Se8 molecular rings and finite size [Se]n chains.26 Cu2I2Se6 on the other hand is a highly crystalline compound with defined crystallographic positions for the Se6 rings which are held together by direct coordination to the Cu atoms. The Cu-Se bonding “locks” the crystal structure so that prolonged irradiation does not cause a phase change to a different allotrope. Cu2I2Se6 has a bandgap of ~ 2.0 eV,27 which is suitable for 2


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suppressing room temperature electron ionization and obtaining high resistivity. In addition, this compound has a high chemical stability and a dense crystal packing (d = 5.287 g·cm-3) which guarantees a high photon stopping power (Supporting Information, Figure S1).28 Therefore, Cu2I2Se6 fulfills many of the prerequisites to obtain a functional hard radiation detection material. In this work, we demonstrate that Cu2I2Se6 is very promising for hard radiation detection at room temperature. We report optimized synthesis, crystal growth, crystal structure, charge transport properties, detection performance, and electronic structure calculations for Cu2I2Se6. The compound has a wide bandgap of Eg = 1.95 eV and melts congruently at a relatively low temperature (T = 397 oC), which allows for simpler material purification and crystal growth. The compound is free of phase transitions between its melting point and ambient temperature and is highly air-stable. Centimeter-sized Cu2I2Se6 crystals were grown from the stoichiometric melt by the typical vertical Bridgman method,29 yielding large single crystalline boules which were subsequently processed to fabricate detectors. Owing to its wide bandgap, its resistivity reaches the order of ρ = 1012 Ω·cm, which is ideal for fabrication of detectors with a low dark current. The planar-type detector made of a Cu2I2Se6 single crystal exhibits photo sensitivity to hard 22.4 keV Ag X-rays, and shows spectroscopic performance for 5.5 MeV

241

Am α-particles. Drift mobility

measurements using α-particles reveal an electron mobility µe of 46 ± 9 cm2·V-1·s-1 which is comparable to the leading detector material TlBr.12

■ EXPERIMENTAL SECTION Synthesis and Crystal Growth. The synthesis of Cu2I2Se6 polycrystalline raw material was performed by the direct combination of elements (2.50 g Cu foil, purity of 99.99%; 5.07 g I2 lumps, purity of 99.999%; 9.32 g Se shots, purity of 99.999%; all from Alfa Aesar) in an evacuated silica ampoule at 500 °C for 24 h in a rocking furnace, followed by slow cooling to room temperature in 12 h. The heating temperature for synthesis was 500 ºC, well above the melting point (397 ºC) of Cu2I2Se6 to ensure its complete melting. The temperature of the furnace was increased slowly to avoid any possibility of explosion due to high vapor pressure of the I2 and Se precursors. After synthesis, the polycrystalline raw material was put into a conical-bottom quartz ampoule with an inner diameter of 10 mm, which was sealed at a vacuum pressure of 1 × 3



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10-4 mbar. A single crystalline boule of Cu2I2Se6 was grown from the stoichiometric melt by the vertical Bridgman method equipped with a translation platform. At the beginning of the growth process, the ampoule was held in the hot zone (420 ºC) of a two-zone Bridgman furnace for 12 h for a complete melting of polycrystalline raw material. The ampoule was subsequently translated from the hot zone to cold zone at a speed of 1.0 mm·h-1. In order to generate a temperature gradient of 19 ºC·cm-1, the temperature of cold zone was set to 150 ºC. After crystal growth, the ingot was annealed in-situ at 150 ºC for 48 h in the Bridgman furnace without translation. Finally, the ingot was cooled down to room temperature in 24 h to avoid cracks due to thermal stress. Crystal Processing and Characterization. The resulting boule was cut along the direction perpendicular to the growth direction by using a Struers Accutom-50 saw with a 300 µm wide diamond-impregnated blade. One wafer was extracted from the middle section of ingot. Subsequently, the wafer was polished with silicon carbide sand paper and alumina slurries with a particle size of 0.05-1 µm. After fine polishing, no further surface etching and passivation were performed on the polished surface. In order to analyze the phase purity of the as-grown crystal, powder X-ray diffraction (PXRD) pattern of ground crystals was collected using a Si-calibrated Rigaku Miniflex 600 diffractometer operating at 40 kV and 15 mA (Cu Ka radiation λ=1.5406 Å). The XRD powder pattern was refined using the Jana2006 software suite.30 Single Crystal X-ray Diffraction. Single-crystal X-ray diffraction was performed at 298(2) K

with

a

STOE

image

plate

diffraction

system

(IPDS)

II

diffractometer

using

graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Data reduction and numerical absorption corrections were done using STOE X-Area software. The structure was solved by direct methods and refined by full-matrix least-squares on F2 (all data) using the Jana2006 software suite.30 Thermal displacement parameters were refined anisotropically for all atomic positions. Optical

Properties

Measurements.

Solid-state

diffuse

reflectance

UV-vis-NIR

spectroscopy at 295 K was performed with a Shimadzu UV-3600PC double-beam, double-monochromator spectrophotometer operating in the 300-2500 nm region, using BaSO4 as a 100% reflecting reference. Thermal Analysis. To assess the thermal stability of Cu2I2Se6, differential thermal analysis 4


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(DTA) was performed using a Netzsch STA 449F3 Jupiter thermal analyzer. Ground crystalline material (∼50 mg) was flame sealed in a silica ampoule evacuated to 10−4 mbar. As a reference, a similarly sealed ampoule of ∼30 mg of Al2O3 was used. The sample was heated to 650 ºC at 5 ºC·min-1 and then cooled at -5 ºC·min-1 to 20 ºC. Sample was cycled for a second time at 5 ºC·min-1 to 650 ºC and then cooled at −5 ºC·min-1 to 70 ºC. Band Structure Calculations. To obtain the electronic structure of Cu2I2Se6, first-principles calculations were carried out within the density functional theory formalism using the Projector Augmented Wave method implemented in the Vienna Ab-initio Simulation Package.31-33 The energy cut off for plane wave basis was set to 350 eV and the Monkhorst-Pack k-point grid of 5 × 5 × 5 was used for Brillouin zone (BZ) sampling.34 For exchange-correlation function, the generalized gradient approximation (GGA) was employed within Perdew-Burke-Ernzerhof (PBE) formalism.35 To obtain the ground states for each compound, the crystal structures, the lattice parameters and the positions of atoms in the cells were relaxed until the atomic forces on each atom are less than 0.01 eV·Å-1. The hole and electron effective mass components were obtained as the inverse of the eigenvalues of the tensor of second derivatives of the band dispersions calculated numerically using the finite difference method for the valence band maximum and conduction band minimum bands, respectively. Device Fabrication and X-ray Photocurrent Measurements. The sample was mounted on a 1-square inch glass substrate. The contacts were fabricated by applying colloidal fast-dry carbon paint on the parallel surfaces of the wafer. The diameter of the electrode on top of sample is around 2 mm. Cu wires were attached to the contacts made by carbon paint, and then attached to Cu foil attached to the glass substrate. The thickness of device is around 1.0 mm, and the diameter of the wafer is 10 mm. The DC I-V measurements in dark were performed using a Keithley 6517B electrometer and a Keithley 6105 resistivity adapter. Electromagnetic interference and photoconductive responses are eliminated by a metallic enclosure. Photocurrent measurements were performed using 22.4 keV Ag Kα X-ray as irradiation source. Ag X-ray was generated from a CPS 120 INEL diffractometer operating at an accelerating voltage of 40 kV and a tube current of 2 mA. Hard Radiation Spectroscopy Measurements. An un-collimated 241Am α-particle source 5



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was used to characterize the detector radiation response upon 5.5 MeV α-particles. The activity of the α-source was around 1.0 µCi. The measurements were carried out also in the atmosphere with a source-detector distance of ~2 mm. The fabricated device was connected to an eV-550 preamplifier box. Various bias voltages from 100 to 300 V were applied. For the single carrier measurement, the cathode on the top of wafer was under irradiation to ensure that the measured signals were induced by electron drift through the whole thickness of material. The signals were transferred to an ORTEC amplifier (Model 572A) with linear amplifier gain of 50, amplifier shaping time of 2.0 µs and collection time of 300 s before it is evaluated by a dual 16 K input multichannel analyzer (Model ASPEC-927) and read into the MAESTRO-32 software. For carrier mobility measurements, the complete transient waveforms from the preamplifier with a maximum time resolution of 4 ns were recorded by using a custom interface based on National Instruments software. Every transient waveform was analyzed to determine the rise time corresponding to the transit time between 10% and 90% of the amplitude of the transient pulses.

■ RESULTS AND DISCUSSION Synthesis, Crystal Growth and Characterization. Polycrystalline Cu2I2Se6 raw material was synthesized by a direct combination of Cu foils, Se shots, and I2 lumps with the appropriate stoichiometry at 500 oC for 24 h in a rocking furnace. The raw material was subsequently used to grow a single crystalline boule via the vertical two-zone Bridgman method. Figure 1a shows a pristine crystal of Cu2I2Se6 with a diameter of 1 cm under ambient light. The entire boule appears black in color and has good coherence. No visible cracks were observed, suggesting that the crystal can endure the large temperature gradient of 19 K·cm-1 which was employed during Bridgman growth. Since this compound has a 3D inorganic lattice, the as-grown ingot cannot be cleaved. The ingot is a pure phase as judged by powder X-ray diffraction pattern on a ground ingot specimen, as shown in Figure 1b. Differential thermal analysis (Figure 1c) reveals that this compound melts congruently at 397 oC, which agrees with the reported value 394 oC.36 Importantly, the compound has no phase transitions between melting point and ambient temperature, which is beneficial in obtaining high-quality single crystals. In addition, the low melting point is not only favorable for material purification and crystal growth, but also for 6


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suppressing the formation of thermally activated defects.37 Figure 1d shows the optical absorption spectrum and a bandgap of 1.95 eV, corresponding well to the dark red color of powder. Cu2I2Se6 is well-suited for hard radiation detection, as its bandgap is narrow enough to create more photoinduced electron-hole pairs due to a lower pair creation energy,38-40 and yet large enough to suppress the formation of thermally activated charge carriers.37 Crystal Structure. Cu2I2Se6 crystallizes in the rhombohedral R3തm space group, with a = 14.0392(9) Å, c = 14.1531(10) Å, V = 2415.83(10) Å3, Z = 9, and has a calculated density of d = 5.287 g·cm-3. The crystal structure of Cu2I2Se6 consists of a 3D inorganic coordination framework comprised of coordinatively unsaturated [Cu2I2] dimers linked together through molecular Se6 rings (Figures 2a, 2b). The [Cu2I2] molecules are formed through symmetric bridging of the two metals with two iodide ions leaving two coordination sites available on each Cu ion. These two sites are occupied by two Se atoms from two adjacent Se6 rings to complete a tetrahedral coordination geometry around the CuI ions, forming a dimer type reminiscent of the Al2Cl6 molecular structure (Figure 2c). The [Cu2I2] dimers are co-planar in the c-direction but they rotate about the screw 3-fold axis which runs along the iodide ions. The molecular Se6 rings adopt a regular chair configuration and exhibit two different coordination motifs. The ring comprising of Se1 atoms, which sits on an inversion center about the proper 3-fold symmetry axis, coordinates to six Cu ions, while the ring comprising of three Se2 and three Se3 atoms, also sitting on the proper 3-fold axis, binds to three Cu atoms bonding exclusively through Se2 in a cis, cis, cis coordination mode with Se3 remaining unbound (Figure 2d). All Se6 rings stack along the crystallographic c-axis filling the “channels” formed between the [Cu2I2] dimers. This configuration motif leads to a cage that is a convex 14-face polyhedron (triangular orthobicupola, Johnson polyhedron #27) 3

with a volume of ~ 110 Å (Figure 2e). All bonding parameters (see tables in SI) are in good agreement with the expected bond length and angles. The Cu-I bond length is 2.635(1) Å, slightly elongated respect to γ-CuI (ZnS-type, 2.620 Å),41 while the Cu-Se distance varies slightly depending on whether the Cu binds to the fully coordinated Se6 ring (Cu-Se1 = 2.463(1) Å) or the half-coordinated Se6 ring (Cu-Se2 = 2.450(1) Å) with both distances being smaller than the Cu-Se distances in Cu2-xSe (Na2O-type, 2.501 Å).42 Likewise, the bonding parameters in the Se6 vary according to the number of Cu ions attached to the 7



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ring; the ring bearing six Cu ions has a Se-Se distance of Se1-Se1 = 2.381(1) Å and a Se1-Se1-Se1 bond angle of 98.62(3)º whereas the ring that bears three Cu ions has a Se-Se distance of Se1-Se1 = 2.360(1) Å and bond angles of Se2-Se3-Se2 = 99.69(3)º and Se3-Se2-Se3 = 102.20(3)º. These parameters indicate that the Se6 rings in Cu2I2Se6 are slightly puckered with respect to the Se6 rings in the rhombohedral Se6 allotrope which have Se-Se distances of 2.356(9) and Se-Se-Se angles of 101.1(3)º, likely as a result of the coordination of the rings to the Cu ions.43 Remarkably, Cu2I2Se6 represents a chemically stable compound made from a structural blending of two unstable binary compounds; CuI is well-known for its tendency to lose Cu and to release I2, while molecular Se is metastable with respect to its various allotropes. Electronic Structure. The density-functional-theory (DFT) calculated electronic structure of Cu2I2Se6 is shown in Figure 3a. The calculated band structure shows that Cu2I2Se6 has an indirect band gap with the conduction band minimum (CBM) located at the F point and the valence band maximum (VBM) at the L point. The valence bands near the VBM, consisting of Cu 3d, Se 4p and I 5p orbitals, show almost no dispersion, whereas the CBM consists mainly of Se p orbitals and has significant dispersion that translates into low electron effective masses. The calculated principal electron effective masses are me,xx= 0.32 m0, me,yy= 0.33 m0, and me,zz= 0.90 m0 along the three crystallographic directions. Note that the me,xx and me,yy components are significantly lower than the me,zz component of the electron effective mass tensor. According to these characteristics a higher µτ value for electrons is expected from detectors made of Cu2I2Se6 if the electric field is applied along the ab plane of the crystal. From the projected electronic density of states (PDOS) calculations (Figure 3c), it can be seen that the CBM is dominated by Se p states with some contribution from Cu d-Se p hybridized orbitals. We thus conclude that the high dispersion of the CBM and low electron effective masses are mainly due to strong Se ppσ* orbital interaction in the Se6/Cu2I2 sheets along the ab plane in the structure (see Figure 2b). In contrast to electrons, the calculated hole effective masses are significantly higher and are also very anisotropic: mh,xx = 2.1 m0, mh,yy = 4.0 m0, me,zz= 14.3 m0, therefore hole mobility is expected to be low. The calculated PDOS (Figure 3c) shows that the VBM mainly consists of contributions from Cu d, Se p and I p orbitals, with the Cu d contribution being dominant. It can be therefore inferred that VBM is formed by a sequence of localized Cu d-Se p and Cu d-I p interactions taking 8


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place between the Se6/Cu2I2 and I planes along the c direction in the structure. Such weak and localized interactions are responsible for the nearly flat character of the VBM bands and high hole effective masses. Charge Transport Properties and Photoconductivity Measurements. The current leakage of a detector made from Cu2I2Se6 single crystalline wafer was measured. The typical detector was made from a 1.0 mm thick Cu2I2Se6 wafer with carbon paint electrodes of 2 mm in diameter. Figure 4a show the I-V characteristics of the detector in the dark. The I-V curve of detector is almost linear in the bias range from -100 V to +100 V, suggesting high field stability and absence of space charge effect.44 The resistivity derived from the I-V curve by a linear fitting is ~1.5 × 1012 Ω·cm, which reveals a very low background noise for hard radiation detection. It is important to point out that thus high resistivity can be easily obtained without delicate control on growth conditions, extensive purification of starting materials or intentional doping. In contrast, it is not straightforward to obtain a high resistivity 1010 Ω·cm for CZT crystals without doping and careful control on the melt composition.45 Figure 4b shows the spontaneous and sensitive photoresponse of the detector against a Ag Kα (22.4 keV) X-ray source at an applied bias of 100 V by switching the Ag X-ray source on and off. The ratio of photocurrent to dark current was ~750:1, indicating the detector is highly photosensitive to X-rays. Detection Performance. We also tested the detection performance of the material against 5.5 MeV α-particles from an un-collimated

241

Am radiation source with an activity of 1 µCi. Figure

5a shows the α-particle spectral response as a function of applied voltage under electron-collection configuration (cathode irradiation). The detector clearly shows a photopeak resolving the characteristic energy peak of α-particles (Figure 5a), as indicated by the significant count rates of the detected photons. The α-particle induced signal can be easily distinguished from the background noise. Importantly, the photopeak shifts to higher energy channels with increasing applied bias voltage, which is a decisive criterion to verify that the signal arises from the α-particles source and not from artifacts induced by the high voltage. The spectral energy resolution is currently limited by the insufficiently high carrier collection efficiency due to electron trapping and pair recombination centers induced by defect states. Hence, purification on raw materials is crucial in future studies in order to further improve detection performance by 9



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eliminating recombination and trapping centers. The electron mobility (µe) of the detector, an important figure of merit for detection material, was estimated by measuring the electron drift time (tdrift) for electrons from an interaction close to the cathode as the electron cloud drifts through the entire thickness of the detector.46-48 The µe can be estimated by the following equation ߤୣ =

௏೏ೝ೔೑೟ ா

=

஽మ

(1)

௎௧೏ೝ೔೑೟

where, Vdrift, D, E and U are the electron drift velocity, detector thickness, applied electric field and bias voltage, respectively. The electron drift time tdrift is measured by recording the electron rise time from output pulse collected by the preamplifier. One hundred measurements of electron rise time were performed and averaged in order to enhance the reliability of the data. Figure 5b shows the distribution of electron rise time at various voltage biases. With increasing bias, the average electron rise time decreases because of increasing electron drift velocity. Figure 5c illustrates a typical electron output pulse with an electron rise time of 1.18 µs using α-particles from the

241

Am source. Since the attenuation length (< 20 µm) of the α-particles is much smaller

than the detector thickness,49 the electron drifting time approximately equals the electron rising time. As shown in Figure 5d, based on Equation 1, the electron mobility of Cu2I2Se6 detector was estimated to be 46 ± 9 cm2·V-1·s-1 by linearly fitting the electron drift velocity as a function of electric field. This value is comparable to that of one of the leading detector materials TlBr (10 50 cm2·V-1·s-1) and is a factor of 30 higher than mobility values obtained for amorphous Se. 12, 23

■ CONCLUSIONS In conclusion, centimeter-sized single crystals of Cu2I2Se6 can be grown readily using the Bridgman method from the stoichiometric melt, owing to its congruent melt at a relatively low temperature. The compound has a metal inorganic framework 3D structure resembling the structure of rhombohedral Se. Band structure calculations suggest an indirect bandgap with that favorable electron transport along the crystallographic ab-plane which is superior to that of amorphous Se. Detectors made of Cu2I2Se6 wafers show a clear response to Ag Κα X-rays (22.4 keV) and the material demonstrates spectroscopic performance under 241Am α-particles (5.5 MeV) at room temperature. Its wide bandgap of 1.95 eV, high resistivity of 1012 Ω·cm, and good 10


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electron mobility of 46 ± 9 cm2·V-1·s-1 without rigorous purification make Cu2I2Se6 a promising material for hard radiation detection. Band structure calculations suggest an indirect bandgap with that favorable electron transport along the crystallographic ab-plane which is superior to that of amorphous Se.

■ ASSOACIATED CONTENTS Supporting Information Supporting information is available. The CIF file of the single-crystal refinement along with the corresponding crystallographic tables are provided as supporting information. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Wenwen Lin: 0000-0002-1627-9558 Constantinos C. Stoumpos: 0000-0001-8396-9578 Oleg Y. Kontsevoi: 0000-0001-6075-630X Zhifu Liu: 0000-0001-9087-1114 Sanjib Das: 0000-0002-5281-4458 Yadong Xu: 0000-0002-1017-9337 Kyle M. McCall: 0000-0001-8628-3811 Bruce W. Wessels: 0000-0002-8957-7097 Mercouri G. Kanatzidis: 0000-0003-2037-4168 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 11



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(NSF NNCI-1542205). Computing resources were provided by the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

■ REFERENCES

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Page 14 of 17

Figure 1. Crystal growth and characterization of Cu2I2Se6. (a) Pristine crystal grown from a stoichiometric melt using a modified vertical two-zone Bridgman method. (b) Power X-ray diffraction pattern of a pulverized ingot specimen and simulated pattern from the refined crystal structure. (c) Differential thermal analysis (DTA) scan of Cu2I2Se6, which is a congruently melting compound. (d) Optical absorption spectrum and bandgap of Cu2I2Se6.

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Figure 2. The crystal structure of Cu2I2Se6. View of the unit cell along (a) the c-axis and (b) the ab-plane. (c) Side view of the [Cu2I2(Se6)4] dimer highlighting the bonding interactions between the (Cu2I2) and the Se6 linkers. (d) The two distinct coordination modes of the Se6 linkers featuring a µ6 mode for the Se1 ring and a µ3 mode for Se2/Se3 ring which bind exclusively through Se2. (e) The triangular orthobicupolar cavity in the inorganic framework formed between two consecutive Se6 rings Se1 and Se2/Se3 rings.

Figure 3. Electronic band structure of Cu2I2Se6. (a) Electronic band structure. (b) View of first Brillouin zone in reciprocal space showing the principle directions. (c) Projected electronic 15



density of states. The Fermi level (EF) is set to zero energy.

100 V 300 V background 300 600 900

Channel

Amplitude (a.u.)

200 V

300 V (c) 200 V (d)

100 V

0 1 2 3 4 5 Rise time (µs)

Rise time 1.18 µs

0

60 120 180 240

200 Rise time (µs) cm·s-1)

(b)

(×103

0

300 V

Drift velocity

(a)

Counts (a.u.)

Figure 4. Charge transport properties and detection performance of the Cu2I2Se6 detector made from 1.0 mm thick wafer. (a) I-V characteristic in the dark. The inset is the fabricated planar-type detector. (b) Photocurrent response of the corresponding Cu2I2Se6 detector to 22.4 keV Ag X-rays measured by switching the X-ray source ON and OFF at an applied bias of 100 V (ON-OFF ratio 750:1).

Counts (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

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150

Experimental data Fitting line

100 50 1000

2000

3000

Electrical field (V·cm-1)

Figure 5. Detection performance and electron mobility estimation of Cu2I2Se6 detector. (a) 241Am α-particle (5.5 MeV) spectral response under various biases. (b) The histogram of electron rise time distribution for detector under various biases. (c) A typical electron transient pulse from one radiation event collected by preamplifier at a bias of 100 V for estimating electron rise time. (d) The linear fitting of electron drift mobility according to Equation (1).

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Table of Contents

17


Cu2I2Se6: A Metal-Inorganic-Framework Wide-bandgap

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