The Two-Dimensional AxCdxBi4–xQ6 (A = K, Rb, Cs; Q = S, Se

May 3, 2017 - ... conductivity of 0.33 W·m–1·K–1 at 773 K. The cesium ions between the layers of Cs1.43Cd1.43Bi2.57S6 are mobile and can be topo...
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The Two-Dimensional AxCdxBi4−xQ6 (A = K, Rb, Cs; Q = S, Se): Direct Bandgap Semiconductors and Ion-Exchange Materials Jing Zhao,†,‡ Saiful M. Islam,‡ Oleg Y. Kontsevoi,§ Gangjian Tan,‡ Constantinos C. Stoumpos,‡ Haijie Chen,‡ R. K. Li,† and Mercouri G. Kanatzidis*,‡ †

Beijing Center for Crystal Research and Development, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China ‡ Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States § Department of Physics & Astronomy, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: We report the new layered chalcogenides AxCdxBi4−xQ6 (A = Cs, Rb, K; Q = S and A = Cs; Q = Se). All compounds are isostructural crystallizing in the orthorhombic space group Cmcm, with a = 4.0216(8) Å, b = 6.9537(14) Å, c = 24.203(5) Å for Cs1.43Cd1.43Bi2.57S6 (x = 1.43); a = 3.9968(8) Å, b = 6.9243(14) Å, c = 23.700(5) Å for Rb1.54Cd1.54Bi2.46S6 (x = 1.54); a = 3.9986(8) Å, b = 6.9200(14) Å, c = 23.184(5) Å for K1.83Cd1.83Bi2.17S6 (x = 1.83) and a = 4.1363(8) Å, b = 7.1476(14) Å, c = 25.047(5) Å for Cs1.13Cd1.13Bi2.87Se6 (x = 1.13). These structures are intercalated derivatives of the Bi2Se3 structure by way of replacing some Bi3+ atoms with divalent Cd2+ atoms forming negatively charged Bi2Se3-type quintuple [CdxBi2−xSe3]x− layers. The bandgaps of these compounds are between 1.00 eV for Q = Se and 1.37 eV for Q = S. Electronic band structure calculations at the density functional theory (DFT) level indicate Cs1.13Cd1.13Bi2.87Se6 and Cs1.43Cd1.43Bi2.57S6 to be direct band gap semiconductors. Polycrystalline Cs1.43Cd1.43Bi2.57S6 samples show n-type conduction and an extremely low thermal conductivity of 0.33 W·m−1·K−1 at 773 K. The cesium ions between the layers of Cs1.43Cd1.43Bi2.57S6 are mobile and can be topotactically exchanged with Pb2+, Zn2+, Co2+ and Cd2+ in aqueous solution. The intercalation of metal cations presents a direct “soft chemical” route to create new materials.

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INTRODUCTION

Quaternary A/M/Bi/Q (A = Li, K, Rb, Cs; M = Sn, Pb, Ag, Cd; Q = S, Se Te) systems define a large composition space with many fascinating compounds such as the A2[M5+nSe9+n] (A = Rb, Cs; M = Bi, Ag, Cd),21 the Am[M′1+lSe2+l]2m[M″2l+nSe2+3l+n] (A = alkali and alkali earth element; M′ and M″ = the main group element) series,10,15,24 Cs4[Bi2n+4Te3n+6],25−29 CsMmBi3Te5+m,30,31 and tetradymite homologous series [MTe]n[Bi2Te3]m (M = Ge, Sn, Pb).32−40 Chalcogenide compounds with labile cations in layers present unique opportunities for ion-exchange chemistry, such as (Me 2 NH 2 ) 1.33 (Me 3 NH) 0.66 Sn 3 S 7 ·1.25H 2 O (FJSM-SnS), 41 K2xMnxSn3−xS6 (KMS-1),42 K2xMgxSn3−xS6 (x = 0.5−1) (KMS-2),43 K2xSn4−xS8−x (x = 0.65−1) (KTS-3),44 βCsPbBi3Se6 and RbPbBi3Se6.45 Ion-exchange is an effective “soft” chemical synthetic approach to prepare new and metastable compounds.46 Herein, we report the synthesis, crystal structure and ionexchange properties of the new layered quaternary semiconductors AxCdxBi4−xQ6 (A = Cs, Rb, K; Q = S and A = Cs; Q = Se). The crystal structures were determined by single-crystal

Two-dimensional (2D) metal chalcogenides are being intensely investigated for their interesting chemical, electrical and optical properties.1 The highly anisotropic nature of the layered materials may lead to the tailoring of the band gaps2,3 and can give rise to interesting physical properties such as charge density waves, anisotropic transport properties, spontaneous magnetization, two-dimensional (2D) electron gas physics, superconductivity, etc.4−9 Bismuth chalcogenide compounds have been widely investigated,10−13 and many of them possess layered or tunneled structures.14−17 The stereochemical activity of 6s2 lone pair enables Bi3+ atoms to occupy sites with coordination numbers varying from 3 to 9. The coordination geometries are generally distorted such as capped octahedra,18 capped trigonal prisms,19 or trigonal bipyramids.20 Often, when other atoms are present the Bi3+ atom sites can be occupied with similarly sized atoms, such as Pb, Sn, lanthanides, alkali metals, or alkaline earth metals thereby exhibiting mixed occupancy.1,14,21,22 The Bi-Q octahedra in these structures generally form large blocks which can be conceptually thought to derive from the NaCl-, Bi2Te3-, CdI2-, and Sb2Se3-type structures.19,23 © 2017 American Chemical Society

Received: March 6, 2017 Published: May 3, 2017 6978

DOI: 10.1021/jacs.7b02243 J. Am. Chem. Soc. 2017, 139, 6978−6987

Article

Journal of the American Chemical Society Table 1. Crystal Data and Structural Refinement Statistics for AxCdxBi4−xQ6 at 293(2) Ka empirical formula

Cs1.13Cd1.13Bi2.87Se6

Cs1.43Cd1.43Bi2.57S6

Rb1.54Cd1.54Bi2.46S6

K1.83Cd1.83Bi2.17S6

1078.14 Cmcm 4.0216(8) 6.9537(14) 24.203(5) 676.8(2) 2 5.290 40.144 911 0.042 × 0.023 × 0.002 3.37−24.96° −4 ≤ h ≤ 4, 0 ≤ k ≤ 8, 0 ≤ l ≤ 28 583 356 [Rint = 0.0173] 99.7%

1011.79 Cmcm 3.9968(8) 6.9243(14) 23.700(5) 655.9(2) 2 5.123 41.906 863 0.559 × 0.245 × 0.003 3.44−24.88° −4 ≤ h ≤ 4, −8 ≤ k ≤ 7, −28 ≤ l ≤ 25 1559 326 [Rint = 0.0916] 94.5%

923.38 Cmcm 3.9986(8) 6.9200(14) 23.184(5) 641.5(2) 2 4.780 34.241 798 0.118 × 0.018 × 0.003 3.52−24.99° −4 ≤ h ≤ 4, −8 ≤ k ≤ 8, −27 ≤ l ≤ 27 1932 336 [Rint = 0.0465] 99.1%

data/restraints/parameters goodness-of-fit final R indices [>2σ(I)]

1350.55 Cmcm 4.1363(8) 7.1476(14) 25.047(5) 740.5(3) 2 6.057 53.076 1117 0.113 × 0.081 × 0.006 3.25−24.98° −4 ≤ h ≤ 4, −8 ≤ k ≤ 7, −29 ≤ l ≤ 29 1925 375 [Rint = 0.0993] 98.4% full-matrix least-squares on F2 375/2/27 1.226 R1 = 0.0488, wR1 = 0.1029

326/2/27 1.230 R1 = 0.0556, wR1 = 0.1155

336/2/26 1.221 R1 = 0.0298, wR1 = 0.0732

R indices [all data]

R2 = 0.0569, wR2 = 0.1078

356/2/26 1.218 R1 = 0.0386, wR1 = 0.0878 R2 = 0.0541, wR2 = 0.0912 1.892 and −2.537

R2 = 0.0597, wR2 = 0.1175

R2= 0.0355, wR2 = 0.0760

2.416 and −1.409

1.352 and −1.450

formula weight space group a (Å) b (Å) c (Å) volume (Å3) Z density (calculated) (g/cm3) absorption coefficient (mm−1) F(000) crystal size (mm3) θ range for data collection (deg.) index ranges reflections collected independent reflections completeness to θ = 24.96° refinement method

largest difference peak and hole (e·Å−3) a

1.904 and −1.864

R = Σ||F0| − |Fc||/Σ|F0|, wR = {Σ[w(|F0| − |Fc| ) ]/Σ[w(|F0| )]} 2

2 2

4

1/2

2

and calc w = 1/[σ (Fo ) + (0.0521P) + 3.9364P], where P = (Fo2 + 2Fc2)/3

2

finally cooled to room temperature (RT) in 7 h. The product was ground into powder and used for further reactions. Cs1.43Cd1.43Bi2.57S6. A mixture of 0.075 g Cs2S (0.25 mmol), 0.257 g Bi2S3 (0.5 mmol), 0.056 g Cd (0.5 mmol) and 0.024 g S (0.75 mmol) were loaded in a 9 mm carbon coated fused silica tube. The mixture was heated to 700 °C in 24 h and stayed at 700 °C for 3 days, followed by slow cooling to 450 °C in 96 h and held at 450 °C for 1 h, finally cooled to RT in 4 h. This experiment yielded black plate-like crystals which were large enough (with the longest edge up to 0.1 mm) for single crystal X-ray diffraction measurements. Pure phase of Cs1.43Cd1.43Bi2.57S6 (1.85 g in total) was synthesized by the same starting molar ratio with a slightly different experimental procedure which is as followed. The mixture was loaded into a 9 mm carbon coated fused silica tube. The sealed tube was placed in a 13 mm tube and sealed. The mixture was heated to 300 °C in 7 h and then slowly heated to 450 °C in 12 h in order to minimize the evaporation of S, then heated to 700 °C in 12 h and soaked for 36 h, followed by cooling to 450 °C in 12 h and then held for 1 h at 450 °C, finally cooled down to RT in 2 h. This procedure led to the formation of a negligible amount of yellow crystals of CdS (confirmed by EDS) in the black matrix of Cs1.43Cd1.43Bi2.57S6. The ingot was ground thoroughly and annealed at 630 °C in a 9 mm carbon coated fused silica tube for 12 h. After annealing the yellow crystals disappeared and Cs1.43Cd1.43Bi2.57S6 with a purity of ∼99% was obtained. Cs1.13Cd1.13Bi2.87Se6. 0.082 g Cs2Se (0.24 mmol), 0.044 g Cd (0.39 mmol), 0.176 g Bi (0.84 mmol) and 0.139 g Se (1.76 mmol) were combined together in a 9 mm carbon coated fused silica tube. The mixture was heated to 750 °C in 24 h and soaked for 24 h, followed by slow cooling to RT in 48 h. Black plate-like Cs1.13Cd1.13Bi2.87Se6 crystals with dimensions up to 0.1 mm, which can be used for single crystal X-ray diffraction measurement, were obtained. The yield of Cs1.13Cd1.13Bi2.87Se6 is ∼100% with no detectable other phases was observed by powder X-ray diffraction (PXRD). Rb1.54Cd1.54Bi2.46S6. 0.051 g Rb2S (0.25 mmol), 0.056 g Cd (0.50 mmol), 0.257 g Bi2S3 (0.50 mmol) and 0.024 g S (0.75 mmol) were combined together in a 9 mm carbon coated fused silica tube. The mixture was heated to 350 °C in 12 h and held at 350 °C for 12 h for

X-ray diffraction analysis and found that Bi2Se3-type quintuple [CdxBi4−xQ6]n−x (Q = S and Se) layers are separated by disordered alkali metal ions. The intercalated Cs+ cations of Cs1.43Cd1.43Bi2.57S6 can be exchanged with other cations topotactically (i.e., without structural change of the [CdxBi4−xQ6]n−x layers) and thus pave the way to a “soft chemistry”47−49 route to new materials. Thermal conductivity measurements showed that Cs1.43Cd1.43Bi2.57S6 possesses a surprisingly low value of 0.33 W·m−1·K−1 at 773 K.

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2

EXPERIMENTAL SECTION

Reagents. All chemicals were used as obtained: potassium metal (98%, Sigma-Aldrich); rubidium metal (99.9%, Strem Chemicals, Inc.); cesium metal (99.9%, Strem Chemicals, Inc.); cadmium metal (99.9%, Strem Chemicals, Inc.), bismuth metal (99.9%, Strem Chemicals, Inc.), selenium pellets (99.99%, Sigma-Aldrich), sulfur pellets (99.99%, Sigma-Aldrich). K2S2, Rb2S, Cs2S, Cs2Se were synthesized by reacting stoichiometric amounts of the elements in liquid ammonia as described previously;50 CdCl2 (ACS grade, Mallinckrodt Baker, Inc.), Co(NO3)2·6H2O (ACS grade, Mallinckrodt Baker, Inc.), ZnCl2 (ACS grade, Columbus Chemical Industries, Inc.), Pb(NO3)2 (ACS grade, Columbus Chemical Industries, Inc.) and CsCl (99.99%, Sigma-Aldrich) were used for ion exchange experiments. Synthesis. For the synthesis of AxCdxBi4−xQ6 (A = Cs, Rb, K; Q = S and A = Cs; Q = Se), all manipulations of the starting materials were carried out in a dry, nitrogen-filled glovebox. The starting materials were transferred into fused silica tubes. The tubes were flame-sealed under a vacuum of ∼10−4 mbar and then the samples were heated in a programmable furnace. Bi2S3. A mixture of 4.180 g (20 mmol) of Bi and 0.962 g (30 mmol) of S were transferred into a 9 mm fused silica tube. The tube was heated to 350 °C in 12 h, stayed at 350 °C for 7 h for the totally melt of S, and then heated to 700 °C in 12 h, held at 700 °C for 1 day, 6979

DOI: 10.1021/jacs.7b02243 J. Am. Chem. Soc. 2017, 139, 6978−6987

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Journal of the American Chemical Society the totally melt of S, then heated to 800 °C in 12 h and soaked for 12 h, followed by slow cooling to 300 °C in 96 h and then held for 1 h, finally cooled to RT in 3 h. Crystals suitable for single crystal X-ray diffraction were obtained with a yield of ∼95%. K1.83Cd1.83Bi2.17S6. 0.036 g K2S2 (0.25 mmol), 0.056 g Cd (0.50 mmol), 0.257 g Bi2S3 (0.50 mmol) and 0.008 g S (0.25 mmol) were combined together in a 9 mm carbon coated fused silica tube. The mixture was heated to 350 °C in 12 h and held for 12 h for the totally melt of S, then heated to 800 °C in 12 h and soaked for 12 h, followed by slow cooling to 300 °C in 96 h and held at 300 °C for 1 h, finally cooled to RT in 3 h. Crystals were obtained with a yield of ∼90%. Powder X-ray Diffraction. PXRD measurements were performed by using a Rigaku Miniflex powder X-ray diffractometer with Nifiltered Cu Kα radiation operating at 40 kV and 15 mA. Calculated PXRD spectra were obtained by using the CIFs of refined structures in the Visualizer software package of the program FINDIT. Single Crystal X-ray Diffraction. A STOE IPDS II single crystal diffractometer operating at 50 kV and 40 mA was used to conduct Xray diffraction measurements with Mo Kα radiation (λ = 0.71073 Å). Single crystals of AxCdxBi4−xQ6 (A = Cs, Rb, K; Q = S and A = Cs; Q = Se) were adhered to the tip of a glass fiber with glue. Data collection was performed using X-Area software; integration was carried out in XRED, and numerical absorption correction was performed with XSHAPE; both X-RED and X-SHAPE are programs provided by STOE.51 The crystal structures were solved via direct methods and refined with the SHELXTL program package.52 The crystal data and structure refinement results are shown in Table 1 and the selected bond lengths of Cs1.13Cd1.13Bi2.87Se6 and Cs1.43Cd1.43Bi2.57S6 are shown in Table 2. Atomic coordinates, equivalent isotropic displacement

Scanning Electron Microscopy. A Hitachi S-3400 scanning electron microscope equipped with a PGT energy-dispersive X-ray analyzer was used to conduct energy dispersive spectroscopy (EDS). The EDS spectra were collected by using parameters of 25 kV accelerating voltage, 80 mA probe current, and 60 s acquisition time. The compositions reported here are the results of averaging a large number of independent measurements from a given sample. Thermal Analysis. 60 mg pure Cs1.43Cd1.43Bi2.57S6 powder sample was placed in a fused silica ampule, evacuated to ∼10−4 mbar and flame-sealed. Differential thermal analysis (DTA) measurement was conducted on a Shimadzu DTA-50 thermal analyzer using a temperature rate of ±10 °C/min and a maximum temperature of 800 °C. Melting and crystallization temperatures were recorded at the minimum of endothermic valleys and the maximum of exothermic peaks. The DTA product was examined with PXRD after the experiment. Spark Plasma Sintering (SPS). The obtained Cs1.43Cd1.43Bi2.57S6 ingot was crushed into fine powders and subsequently densified at 898 K for 10 min in a 10 mm diameter graphite die under an axial compressive stress of 30 MPa using spark plasma sintering (SPS) method (SPS-211LX, Fuji Electronic Industrial Co., Ltd.). High dense disk-shaped pellet was obtained. The density of the sample was achieved ∼95% of its theoretical density. Electrical Properties. Highly dense SPS-processed pellet was cut into a bar perpendicular to the sintering pressure direction with dimensions of 3.2 mm × 2.7 mm × 8.9 mm for Cs1.43Cd1.43Bi2.57S6. The sample was spray-coated with boron nitride spray to minimize outgassing, except where needed for electrical contact with the thermocouples, heater and voltage probes. Simultaneous measurements of the Seebeck coefficients and electrical conductivities were conducted on an Ulvac Riko ZEM-3 instrument under a low-pressure helium atmosphere from RT to 773 K. The uncertainty of the Seebeck coefficient and electrical conductivity measurements is 5%.54 Thermal Conductivity. The samples were coated with a thin layer of graphite to minimize errors from the emissivity of the material. The total thermal conductivity was calculated from κtot = DCpd, where the thermal diffusivity coefficient (D) was measured using the laser flash diffusivity method in a Netzsch LFA457 instrument, and the density (d) was determined using the dimensions and mass of the sample. The specific heat capacity (Cp) was estimated by Dulong-Petit law Cp = 3R/ M̅ where R is the gas constant 8.314 J·mol−1·K−1 and M̅ is the average molar mass. The thermal diffusivity data were analyzed using a Cowan model with pulse correction. The uncertainty of the thermal conductivity is estimated to be within 8%, considering all the uncertainties from D, Cp and d.54 Ion-Exchange Experiments. A typical ion-exchange experiment of Cs1.43Cd1.43Bi2.57S6 with CdCl2, Co(NO3)2·6H2O, ZnCl2 and Pb(NO3)2 is as follows. In 0.2 mol/L solution of CdCl2, Co(NO3)2· 6H2O, ZnCl2 and Pb(NO3)2 in water (10 mL), the ground polycrystalline powder of Cs1.43Cd1.43Bi2.57S6 (10.0 mg) were added, respectively. The mixture kept shaking for 24 h at RT. Then the ion exchanged material was isolated by filtration (through filter paper, Whatman no. 1), and washed several times with water and then dried in air. The reverse ion exchange of the Pb ion exchanged sample was conducted in CsCl solution. Ion-exchanged crystals were analyzed with energy dispersive spectroscopy (EDS) analysis. The EDS spectra were collected using 25 kV accelerating voltage, 80 mA probe current and 60 s acquisition time. Cs1.43Cd1.43Bi2.57S6 (110 mg) ground polycrystalline powder was added in 0.2 mol/L solution of CdCl2, Co(NO3)2· 6H2O, ZnCl2 and Pb(NO3)2 in water (20 mL) and kept under magnetic stirring for 24 h at RT. The solution was filtered and the black ion-exchanged sample was isolated and characterized with PXRD and UV−visible spectroscopy. Band Structure Calculations. In order to investigate the electronic structure of Cs1.43Cd1.43Bi2.57S6 and Cs1.13Cd1.13Bi2.87Se6, first-principle calculations were carried out with the density functional theory using the Projector Augmented Wave method55 implemented in Vienna Ab-initio Simulation Package.56,57 The energy cut off for plane wave basis was set to 350 eV and the Monkhorst−Pack k-point grids of 13 × 13 × 2 were used for Brillouin zone (BZ) sampling. The

Table 2. Bond Lengths (Å) for Cs1.43Cd1.43Bi2.57S6 and Cs1.13Cd1.13Bi2.87Se6 at 293(2) K with Estimated Standard Deviations in Parentheses Cs1.43Cd1.43Bi2.57S6

d (Å)

Cs1.13Cd1.13Bi2.87Se6

d (Å)

Bi(1)−S(1)#1 Bi(1)−S(1)#2 Bi(1)−S(1) Bi(1)−S(2) Bi(1)−S(2)#3 Bi(1)−S(2)#1 Cs(1)−S(1)#2 Cs(1)−S(1)#4 Cs(1)−S(1)#5 Cs(1)−S(1)#1 Cs(1)−S(1)#6 Cs(1)−S(1)#7 Cs(2)−S(1)#6 Cs(2)−S(1)#1 Cs(2)−S(1)#4 Cs(2)−S(1)#2 Cs(2)−S(1)#15

2.717(4) 2.718(2) 2.718(2) 2.9372(7) 2.9394(6) 2.9394(6) 3.657(8) 3.657(8) 3.662(5) 3.662(5) 3.662(5) 3.662(5) 3.652(9) 3.652(9) 3.665(5) 3.665(5) 3.665(5)

Bi(1)−Se(2) Bi(1)−Se(2)#1 Bi(1)−Se(2)#2 Bi(1)−Se(1) Bi(1)−Se(1)#3 Bi(1)−Se(1)#4 Cs(1)−Se(2)#9 Cs(1)−Se(2)#10 Cs(1)−Se(2)#3 Cs(1)−Se(2)#11 Cs(1)−Se(2) #8 Cs(1)−Se(2)#4 Cs(2)−Se(2)#9 Cs(2)−Se(2)#2 Cs(2)−Se(2)#13 Cs(2)−Se(2)#1 Cs(2)−Se(2)#14

2.826(3) 2.831(2) 2.831(2) 3.022(1) 3.025(1) 3.025(1) 3.72(1) 3.752(6) 3.752(6) 3.752(6) 3.72 (1) 3.752(6) 3.73(1) 3.747(6) 3.747(6) 3.747(6) 3.747(6)

parameters and anisotropic displacement parameters of AxCdxBi4−xQ6 (A = Cs, Rb, K; Q = S and A = Cs; Q = Se) are given in Table S1−S8. Selected bond lengths of Rb1.53Cd1.54Bi2.46S6 and K1.83Cd1.83Bi2.17S6 are shown in Table S9. Infrared and Ultraviolet−Visible Spectroscopy. A Shimadzu UV-3600 PC double-beam, double-monochromatic spectrophotometer was used to collect diffuse-reflectance spectra. BaSO4 was served as the reference and taken to have 100% reflectance. Samples were ground into a fine powder and placed on top of a bed of compressed BaSO4. The collected reflectance data were converted to absorbance via the Kubelka−Munk equation: α/S = (1−R)2/2R, where R is reflectance, α is the absorption coefficient, and S is the scattering coefficient.53 The fundamental absorption edge was estimated by linearly extrapolating the absorption edge against the energy axis. 6980

DOI: 10.1021/jacs.7b02243 J. Am. Chem. Soc. 2017, 139, 6978−6987

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Journal of the American Chemical Society generalized gradient approximation (GGA), functional of Perdew− Burke−Ernzerhof (PBE)58 was employed and the spin−orbit coupling was included in the calculation. 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.

Crystal Structure of AxCdxBi4−xQ6 (A = Cs, Rb, K; Q = S and A = Cs; Q = Se). The AxCdxBi4−xQ6 compounds crystallize in the orthorhombic space group Cmcm, having similar lattice parameters which are summarized in Table 1. All the compounds are isostructural and their unit cells shrink from Cs to Rb to K and from Se to S. This observation is in agreement with their ionic sizes. For the sake of economy only the representative crystal structure of Cs1.43Cd1.43Bi2.57S6 will be discussed in detail. Cs1.43Cd1.43Bi2.57S6 has a layered structure, which consists of infinite anionic [CdxBi4−xS6]n−x layers separated by disordered Cs+ cations between them, Figure 2a.

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RESULTS AND DISCUSSION Synthesis and Thermal Behavior. Plate-like crystals of Cs1.43Cd1.43Bi2.57S6, Rb1.54Cd1.54Bi2.46S6, K1.83Cd1.83Bi2.17S6 and Cs1.13Cd1.13Bi2.87Se6 with sizes up to 0.1 mm were obtained by high temperature solid state synthesis which was used for single crystal X-ray diffraction. Pure phases of Cs1.13Cd1.13Bi2.87Se6 and Cs1.43Cd1.43Bi2.57S6 were obtained and that was validated by comparing the experimental and calculated PXRD patterns, Figure 1a. All compounds are insoluble in water and organic

Figure 2. Unit cell of Cs1.43Cd1.43Bi2.57S6 viewed along the a-axis and SEM image of the Cs1.43Cd1.43Bi2.57S6 crystal used for X-ray single crystal diffraction.

The layers adopt the hexagonal motif of the Bi2Se3 structure type. Scanning electron microscopy (SEM) confirms the layered nature of the compound revealing a step-like texture on the plate-like crystals which arises from the stacking of the individual layers, Figure 2b. Each layer consists of [CdxBi4−xS6]n−x octahedra, which are formed by mixed Cd/Bi metal sites (64% Bi and 36% Cd) coordinating with six sulfur anions, Figure 3. Atoms S1 and S2 are respectively 3- and 6coordinated to the Cd/Bi atoms with the former acting as terminal ligands at the layer edges and the latter occupying the

Figure 1. (a) Comparison of powder X-ray diffraction spectrum of synthesized and simulated spectrum; A: CsCdBi3Se6 simulated, B: Cs1.13Cd1.13Bi2.87Se6 simulated, C: Cs1.13Cd1.13Bi2.87Se6 synthesized, D: Cs1.43Cd1.43Bi2.57S6 simulated, E: Cs1.43Cd1.43Bi2.57S6 synthesized. (b) The DTA curves of Cs1.43Cd1.43Bi2.57S6.

solvents such as ethanol, acetone, DMF and hydrazine. Energydispersive X-ray spectroscopy (EDS) shows that all the elements are present in the approximate desired stoichiometric ratios as obtained by the crystal structure refinements, Figure S1. The thermal behavior of Cs1.43Cd1.43Bi2.57S6 was investigated by DTA, Figure 1b. There is a single endothermic peak at 780 °C on heating. On cooling, a single exothermic peak occurs at 769 °C corresponding to the crystallization of the compound. These results were repeatable over two DTA cycles. The material obtained after DTA has the same PXRD pattern as the sample before heating, Figure S2. DTA and PXRD experiments revealed that Cs1.43Cd1.43Bi2.57S6 melts congruently.

Figure 3. Graphite-like disordered cesium layer viewed along the c-axis of Cs1.43Cd1.43Bi2.57S6. Coordination environment of Cd/Bi site and the coordination environments of two crystallographically different S atoms. The bond lengths are indicated in angstroms. 6981

DOI: 10.1021/jacs.7b02243 J. Am. Chem. Soc. 2017, 139, 6978−6987

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Journal of the American Chemical Society

sites, it cannot accommodate compositions that are off stoichiometry in the Cd/Bi ratio. This structure can only be adopted if the Cd:Bi ratio is 1:3. When it deviates from 1:3 such as in Cs1.43Cd1.43Bi2.57S6 the formula becomes off stoichiometric with more Cs atoms required to be accommodated. The most favorable structure is then the flat layers of Bi2Se3 type and the ample space afforded between the layers so all Cs atoms can fit. Optical Absorption and Band Structure Calculations. The band gaps of Cs1.43Cd1.43Bi2.57S6, Rb1.54Cd1.54Bi2.46S6, K1.83Cd1.83Bi2.17S6 and Cs1.13Cd1.13Bi2.87Se6 are 1.28 eV, 1.33 eV, 1.37 and 1.00 eV respectively, see Figure 5. In order to

intralayer interstitial sites. The Cd/Bi−S bond distances range between 2.720(4) and 2.9394(6) Å. The average Cd/Bi−S distance is 2.8282 Å, which is very similar to the Bi−S distances reported in KBi6.33S10 and K2Bi8S13.59 The octahedral environment of the Cd2+ ions is rather unusual because Cd2+ generally prefers a tetrahedral coordination environment in chalcogenides. There are two crystallographically independent Cs sites, and both of them are partly occupied with a refined fractional occupancy of 40% and 31%. All Cs atoms have relatively large thermal factors, which is the characteristic for loosely, ionically bound intercalated cations. To get a realistic Cs−Cs bond distances, certain Cs atoms in partially occupied crystallographically sites were omitted in Figure 3. A Cs−Cs bond distance of 4.02 Å was obtained, which is comparable to the Cs−Cs bond distances reported in CsBi4Te6 (4.403 Å)28 and Cs0.65Pb3.8Bi11.2Se21(4.172 Å).60 As depicted in Figure 4a and b, the idealized [CdxBi4−xS6]n−x slab can be viewed as a fragment excised from the hexagonal

Figure 5. Optical absorption spectra of crystalline Cs1.43Cd1.43Bi2.57S6 (A), Rb 1.54 Cd 1.54 Bi 2.46 S 6 (B), K 1.83 Cd 1.83 Bi 2. 17 S 6 (C) and Cs1.13Cd1.13Bi2.87Se6 (D), with band gaps of 1.28, 1.33, 1.37, and 1.00 eV, respectively.

understand the nature of the electronic transitions in AxCdxBi4−xQ6, electronic band structure calculations were performed for Cs1.43Cd1.43Bi2.57S6 and Cs1.13Cd1.13Bi2.87Se6 as representative examples. Both compounds have mixed occupancy of Cd and Bi sites in the [CdxBi4−xQ6]−x layers. In order to perform electronic structure calculations, a structural model was required that approximates mixed occupation of these sites by a periodic array of fully occupied Cd and Bi sites. An unlimited number of such models can be proposed with different arrangements and short- and longrange order within the Cd/Bi sublattice. In the model utilized in this work the disordered [CdxBi4−xQ6]n−x layers were approximated by [Cd2Bi2Q6]n−2 layers ordered in such a way that each of two Cd/Bi sublayers was fully occupied by either Cd or Bi atoms. Additionally, disordered Cs+ cations were approximated by a layer of ordered Cs+ cations satisfying the charge neutrality condition. The formal compositions of the models are Cs2Cd2Bi2S6 and Cs2Cd2Bi2Se6, respectively. The calculated electronic structure of Cs2Cd2Bi2S6 is shown in Figure 6a in the form of the electronic band structure plotted along the lines between the high-symmetry points in the BZ and the electronic density of states (DOS) projected onto the atomic sites, Figure 6b. The valence band maximum (VBM) and the conduction band minimum (CBM) are both located at the Γ point indicating that Cs2Cd2Bi2S6 has a direct band gap. It should be noted that the conduction band (CB) near the CBM shows significant dispersion, whereas valence band (VB) is almost flat between Γ and A points. In addition, VB approaches VBM at several other points. This creates a possibility of multiple optical transitions between band edges, including indirect transitions. From the PDOS picture shown in Figure 6b the VBM is formed almost exclusively by weakly coupled S p states, resulting in mostly flat bands of the VBM. In contrast,

Figure 4. Structure comparison between (a) Bi 2 Se 3 , (b) Cs1.43Cd1.43Bi2.57S6 and (c) CsCdBi3Se6. The green, blue, orange, red, yellow balls are Bi, Bi/Cd, Se, Cs, S, respectively.

Bi2Se3-type structure along the [101] direction (or intercalated Bi2Te3-type) with a thickness of one monolayer. The AxCdxBi4−xQ6 structure derives from the substitution of some of Bi3+ atoms for Cd2+ atoms in the Bi2Se3 structure type. This generates a net with negative charge and requires another cation to balance charge, resulting in a rigid Bi2Se3-type [CdxBi4−xS6]n−x layer accepting monovalent alkali cations between them. The AxCdxBi4−xQ6 compounds have a similar structure with β-CsPbBi3Se6 and RbPbBi3Se6.45 There is also a similarity to the series [(PbSe) 5 ][(Bi 2 Se 3 ) 3 ] 61 and PbmBi2nTe3n+m [that is, (PbTe)m(Bi2Te3)n],17 which possesses alternating two-dimensional infinite slabs of [Pb1−xBixQ]+x and [PbxBi4−xQ6]−x (Q = Se and Te). Cs1.43Cd1.43Bi2.57S6 structure is chemically related to, but it is structurally different from CsCdBi3Se6, which crystallizes in the orthorhombic space group Pnma, Figure 4c.21 Figure 1a shows the differences of the simulated PXRD patterns between Cs1.43Cd1.43Bi2.57S6 and CsCdBi3Se6. CsCdBi3Se6 also has a layered structure with the alkali metal ions residing between the slabs. In CsCdBi3Se6, the layers are corrugated and the Cs cations are ordered as there is only one fully occupied crystallographic position of the cesium cation. In CsCdBi3Se6, three out of four crystallographic metal sites are mixed occupied by Cd and Bi atoms, with the fourth site exclusively occupied by bismuth. In contrast, in Cs1.43Cd1.43Bi2.57S6 the layers are flat and there is only one metal site in which both Cd and Bi occupy. Because the CsCdBi3Se6 structure is stoichiometric and has ordered Cs 6982

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Figure 6. (a) Electronic band structure; (b) projected electronic density of states of Cs2Cd2Bi2S6; (c) electronic band structure; (d) projected electronic density of states of Cs2Cd2Bi2Se6.

CBM is composed of Bi p and S p states that form very strongly hybridized orbitals and are responsible for the dispersive character of the CBM. Importantly, there is no contribution from Cd states near VBM or CBM, even though Cd atoms occupy the same mixed positions in the lattice as Bi atoms; there is also no contribution from Cs states. The interaction between Bi and S orbitals is therefore solely responsible for the formation of VBM and CBM. It can be concluded that the variation in the concentration of Cd and Bi atoms will not qualitatively change the character of the band edges. Therefore, the electronic band structure of the idealized Cs2Cd2Bi2S6 compound should maintain all the important features and be a reasonable representation of the electronic band structure of the actual compound, Cs1.43Cd1.43Bi2.57S6. The electronic structure of Cs2Cd2Bi2Se6, presented in Figure 6c is very similar to that of Cs2Cd2Bi2S6. Cs2Cd2Bi2Se6 also has a direct band gap at Γ point. The VB near VBM is extremely flat and extends toward the A point, while CBM exhibits a significant dispersion. From the PDOS plots in Figure 6d it can be seen that the CBM is composed of very strongly hybridized Bi p and Se p orbitals resulting in a highly dispersive character of the CBM. The weakly coupled Se p states dominate VBM leading to the mostly flat bands of the VBM.

The calculated values of the band gaps for Cs2Cd2Bi2S6 and Cs2Cd2Bi2Se6 are 0.86 and 0.60 eV, respectively, which are underestimated compared to the experimental values of 1.28 and 1.00 eV for Cs1.43Cd1.43Bi2.57S6 and Cs1.13Cd1.13Bi2.87Se6, respectively. The underestimation of bandgaps compared to the experimentally obtained values is a well-known tendency of semilocal exchange-correlation functional like PBE.62,63 Similarly to the case of Cs2Cd2Bi2S6 the electronic band structure of the idealized Cs2Cd2Bi2Se6 is a good representation of the electronic band structure of the actual Cs1.13Cd1.13Bi2.87Se6 compound. Ion-Exchange Chemistry of Cs1.43Cd1.43Bi2.57S6. The structure of Cs1.43Cd1.43Bi2.57S6 features a layered framework with disordered alkali metal cations between the layers; we examined its ability to undergo cation-exchange reactions. To check the feasibility of ion exchange of Cs+ ions in exiting the structure of Cs1.43Cd1.43Bi2.57S6, we immersed a polycrystalline sample in a solution of Co2+, Cd2+, Pb2+ and Zn2+ ions, respectively, for 24 h. EDS of the exchanged materials showed an average ratio of Cd1.94Bi2.59S6 for Cd2+, Co0.93Cd0.83Bi3.02S6 for Co2+, Pb0.90Cd1.43Bi2.57S6 for Pb2+ and Cs0.34Zn0.48Cd1.03Bi2.84S6Cl0.4 for Zn2+. The ion-exchange of Cs+ cations by Co2+, Cd2+ and Pb2+ is complete. The reason for 6983

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Journal of the American Chemical Society the incomplete ion-exchange of Zn2+ may be due to the higher affinity of Co2+, Cd2+ and Pb2+ toward sulfur atoms. The ion exchange process is shown in Figure 7a and the mapping of the

Figure 7. (a) Intercalative mechanism of capture of Cd2+, Co2+, Pb2+ and Zn2+ ions by Cs1.43Cd1.43Bi2.57S6 through exchange of Cs+. (b) SEM images of Pb exchanged products and its elemental distribution maps of Pb, Cd, Bi and S.

elemental distribution in the crystal after ion exchange is shown in Figure 7b for Pb2+ and Figure S3 shows the mapping of the elemental distribution of Co2+, Cd2+ and Zn2+ ion exchanged crystals. Energy-dispersive X-ray spectroscopy of the ion-exchanged samples shows the Bi and S ratio is almost stable that means the ions do not go into the interlayer sites, Figure S4. The process of ion exchange using Pb2+ as an example can be described by the following equation:

Figure 8. (a) Optical absorption spectra of Co2+, Cd2+, Pb2+ and Zn2+ ion exchanged products. (b) PXRD patterns of the ion exchanged products and the pristine Cs1.43Cd1.43Bi2.57S6.

X-ray Bragg peaks of the samples after ion exchange indicate that the quintuple layer structure is retained but the interlayer spacing is changed reflecting the size effect of the intercalating ion. The peaks are also broadened because of the simultaneous effects from a reduced size of the microcrystals (crystallites) and to an increased amount of lattice disorder (microstrain).65 For the Cd2+, Co2+and Pb2+ exchanged sample PXRD analysis showed a shift of the (002) and (006) basal Bragg peaks to higher 2θ values (lower d-spacing). This is in accordance to smaller ionic size of the ions and also related to the stronger covalent interaction between Cd2+, Co2+, Pb2+-S compared to the Cs+-S interactions.66 In the case of Zn2+ exchange the PXRD pattern shifted slightly to lower angles suggesting an interlayer expansion; this suggests that Zn intercalates between the layers in the form of hydrated species, i.e., ZnClxH2O(4−x) and this was further confirmed by the observation of the Cl peaks in the EDS spectrum, Figure S4.67−69 Although the selenide analog Cs1.13Cd1.13Bi2.87Se6, was not investigated we expect it to exhibit similar ion-exchange properties. The Pb2+ ion exchanged sample of 50 mg was immersed into a 1 mol/L CsCl solution and was shaken for up to 48 h for the reverse ion exchange. EDS analysis showed that Pb cannot be back ion exchanged with Cs and this was further confirmed by the PXRD which showed no obvious change after the experiment, Figure S5. This shows the new composition Pb0.90Cd1.43Bi2.57S6 is very stable and the ion exchange that leads to it is irreversible.

Cs1.43Cd1.43Bi 2.57S6 + 0.715Pb(NO3)2 → Pb0.715Cd1.43Bi 2.57S6 + 1.43CsNO3

when the Pb2+ is exchanged the band gap of the pristine Cs1.43Cd1.43Bi2.57S6 undergoes a big red-shift from 1.28 to 0.82 eV, Figure 8a. This shift can be attributed to the fact that the character of the material changes from two-dimensional to 3dimensional because the Pb2+ ions form new bonding interactions of Pb−S thereby directly connecting the slabs in the stacking direction of the [Cd1.43Bi2.57S6]1.43− slabs.64 These additional covalent bonds broaden the bands in the material thereby narrowing the bandgap. This is a general argument and should apply to any incoming metal ion that can form bonding interactions with the chalcogen atoms of the slabs. Thus, the corresponding Cd2+ and Co2+ ion exchanged samples also have red-shifted bandgaps of 1.06 and 1.14 eV. With Zn2+exchange, the band gap is 1.26 eV, which is almost the same as Cs1.43Cd1.43Bi2.57S6. The ion exchange experiments show that four new compounds can be obtained by this mild “soft chemical” route. The PXRD of the exchanged materials suggest topotactic ion exchange with retention of the parent structure, Figure 8b. The 6984

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Figure 9. (a) Seebeck coefficient, (b) electrical conductivity and (c) thermal conductivity as a function of temperature for Cs1.43Cd1.43Bi2.57S6 (Per: the perpendicular direction of SPS pressure, Par: the parallel direction of SPS pressure).

Seebeck, Electrical Conductivity and Thermal Transport Properties. The Seebeck coefficient and electrical conductivity were measured perpendicular to the SPS pressure direction by using the SPSed polycrystalline Cs1.43Cd1.43Bi2.57S6 pellet. The Seebeck coefficient of Cs1.43Cd1.43Bi2.57S6 is −525 μV·K−1 at 325 K, indicating n-type transport behavior, Figure 9a. The Seebeck coefficient exhibits negative temperature dependence, with the lowest absolute value of 320 μV·K−1 at 726 K. The electrical conductivity of Cs1.43Cd1.43Bi2.57S6 is low with 0.17 S·cm−1 at 325 K, which suggests a low level of carrier concentration, Figure 9b. Electrical conductivity increases steadily with increasing temperature and reaches 3.6 S·cm−1 at 773 K. This thermally activated temperature dependence is in accordance with the semiconducting nature of the compound. The thermal diffusivity was measured parallel to the sintering pressure. Figure S6 shows the measured thermal diffusivity as a function of temperature. The thermal conductivity of Cs1.43Cd1.43Bi2.57S6 is lower than 0.43 W·m−1·K−1 at RT and decreases with rising temperature reaching 0.33 W·m−1·K−1 at 773 K, Figure 9c. Due to the strongly anisotropic structure of Cs1.43Cd1.43Bi2.57S6 a preferential orientation of the crystallites might happen when doing SPS sintering. Another sample obtained by the same SPS procedure mentioned above was used for thermal conductivity measurements perpendicular to the sintering pressure. From Figure 9c, the thermal conductivity along the direction parallel to the SPS pressing direction is 0.1 W·m −1 ·K −1 lower than that perpendicular to the SPS sintering direction. To further confirm this low thermal conductivity another batch sample was synthesized and SPSed to cut parallel to the SPS pressure direction to characterize the thermoelectric properties. From Figure 9a and b the Seebeck coefficient is not affected by the measurement direction but the electrical conductivity parallel to the SPS pressure direction is much lower with the highest value of 0.78 S·cm−1 at 770 K. This is consistent with the layered nature of Cs1.43Cd1.43Bi2.57S6. The thermal conductivity obtained in the new batch agreed well with the previous values measured parallel to the SPS pressure direction. The thermal conductivity can be lowered in materials by the random alloying of atoms of differing atomic mass, the rattling of ions in the structure or nanostructuring.70 The point defects associated with the mixed occupation of Bi/Cd in the quintuple layers and the disordered and mobile Cs+ cations in the

interlayer space are responsible for the ultralow thermal conductivity. Cd and Bi have high mass contrast Cd (Z = 48) and Bi (Z = 83). The mobility of the Cs+ cations (as evident in the very large atomic displacement parameters of Cs+ atoms as determined from the single crystal refinement) can be likened to a rattling action which in many other materials has been shown to give very low thermal conductivity values. Examples with similar ultralow values and cations with believed rattling activity include Cs2Hg6S771 and Ba8Au16P30,72 which feature anionic three-dimensional frameworks and cavities filled with large cations of Cs+ and Ba2+.

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CONCLUDING REMARKS Chemical diversity can be generated by starting from the twodimensional Bi2Se3 structure and substituting a pair of Bi3+ atoms with A+ and Cd2+ atoms, respectively. This strategy led to the discovery of four new layered chalcogenides Cs1.43Cd1.43Bi2.57S6, Rb1.54Cd1.54Bi2.46S6, K1.83Cd1.83Bi2.17S6 and Cs 1 . 1 3 Cd 1 . 1 3 Bi 2 . 8 7 Se 6 featuring hexagonal quintuple [CdxBi4−xQ6]n−x (Q = S, Se) slabs similar to the Bi2Se3 archetype structure except that they are separated by disordered Cs+ ions. Cs1.43Cd1.43Bi2.57S6 and Cs1.13Cd1.13Bi2.87Se6 exhibit strong optical absorption with direct bandgaps of 1.28 and 1.00 eV, respectively. Ion exchange experiments show that the Cs+ layer in Cs1.43Cd1.43Bi2.57S6 can be exchanged topotactically by Cd2+, Co2+, Pb2+ and Zn2+ leading to yield novel materials that may not be accessible through direct solid state synthesis. Cs1.43Cd1.43Bi2.57S6 sample shows n-type conductivity. The layered structure yields very low thermal conductivity of 0.43 to 0.33 W·m−1·K−1 over the temperature range of 325 to 773 K. These phases are chemically functional derivatives of the parent Bi2Se3 structure. They present a dramatic widening of the gap from 0.3 eV in Bi2Se3 to 1.0 eV in Cs1.13Cd1.13Bi2.87Se6 and at the same time impart facile ion-exchange functionality of the products. The latter allows further chemical diversity to become possible, leading to new materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b02243. Tables of atomic coordinates and displacement parameters of Cs 1.43 Cd 1.43 Bi 2.57 S 6 , Rb 1.54 Cd 1.54 Bi 2.46 S 6 , K1.83Cd1.83Bi2.17S6 and Cs1.13Cd1.13Bi2.87Se6; Table of 6985

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bond distances of Rb1.54Cd1.54Bi2.46S6 and K1.83Cd1.83Bi2.17S6; SEM image and energy-dispersive Xray spectroscopy spectrum of Cs 1.43Cd 1.43 Bi 2.57S 6 , Rb1.54Cd1.54Bi2.46S6, K1.83Cd1.83Bi2.17S6 and Cs1.13Cd1.13Bi2.87Se6; PXRD before and after DTA of Cs1.43Cd1.43Bi2.57S6; SEM images and Energy-dispersive X-ray spectroscopy spectrum of Cd2+, Co2+ and Zn2+ exch anged products; Thermal di ffusivity of Cs1.43Cd1.43Bi2.57S6 (PDF) X-ray crystallographic data of Cs1.43Cd1.43Bi2.57S6 (CIF) X-ray crystallographic data of Rb1.54Cd1.54Bi2.46S6 (CIF) X-ray crystallographic data of K1.83Cd1.83Bi2.17S6 (CIF) X-ray crystallographic data of Cs1.13Cd1.13Bi2.87Se6 (CIF)

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Constantinos C. Stoumpos: 0000-0001-8396-9578 Mercouri G. Kanatzidis: 0000-0003-2037-4168 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation Grant DMR-1410169 (synthesis). This work was supported by the Department of Energy, Office of Science, Basic Energy Sciences under grant DE-SC0014520 (thermal transport, electronic structure calculations). SMI is supported by MRSEC program (NSF DMR-1121262). OYK is supported by DHS-ARI grant 2014-DN-077-ARI086-01. This work made use of the EPIC facility (NUANCE Center-Northwestern University), which has received support under the State of Illinois, Northwestern University, and the National Science Foundation with grants DMR-1121262 through the MRSEC program at the Materials Research Center, and EEC-0118025/ 003 through The Nanoscale Science and Engineering Center. 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 US Department of Energy under Contract No. DE-AC0205CH11231.

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DOI: 10.1021/jacs.7b02243 J. Am. Chem. Soc. 2017, 139, 6978−6987

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DOI: 10.1021/jacs.7b02243 J. Am. Chem. Soc. 2017, 139, 6978−6987