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Lead-Free Cs4CuSb2Cl12 Layered Double Nanocrystals Tong Cai, Wenwu Shi, Sooyeon Hwang, Kanishka Kobbekaduwa, Yasutaka Nagaoka, Hanjun Yang, Katie Hills-Kimball, Hua Zhu, Junyu Wang, Zhiguo Wang, Yuzi Liu, Dong Su, Jianbo Gao, and Ou Chen*

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ABSTRACT: Concerns about the toxicity of -based perov- skites have aroused great interest for the development of alternative lead-free perovskite-type materials. Recently, theoretical calculations predict that Pb2+ cations can be substituted by a combination of Cu2+ and Sb3+ cations to form a vacancy-ordered layered double perovskite structure with superior optoelectronic properties. However, accessibilities to this class of perovskite-type materials remain inadequate, hindering their practical implementa- tions in various applications. Here, we report the first colloidal synthesis of Cs4CuSb2Cl12 perovskite-type nanocrystals (NCs). The resulting NCs exhibit a layered double perovskite structure with ordered vacancies and a direct of 1.79 eV. A − − composition structure property relationship has been established by investigating a series of Cs4CuxAg2−2xSb2Cl12 perovskite-type NCs (0 ≤ x ≤ 1). The composition induced structure transformation, and thus, the electronic band gap evolution has been explored by experimental observations and further confirmed by theoretical calculations. Taking advantage of both the unique electronic structure and processability, we demonstrate that the Cs4CuSb2Cl12 NCs can be solution-processed as high-speed with ultrafast photoresponse and narrow bandwidth. We anticipate that our study will prompt future research to design and fabricate novel and high-performance lead-free perovskite-type NCs for a range of applications.

■ INTRODUCTION Scheme 1. Schematics of Metal Halide with a Different Crystal Structures Lead halide perovskite nanocrystals (NCs) have attracted profound attention in recent years due to their excellent Downloaded via BROWN UNIV on July 8, 2020 at 19:21:09 (UTC). properties and high potentials for a variety of optoelectronic − applications, such as solar cells,1 5 -emitting diodes,6,7 − ,8,9 etc.10 21 However, one of the major drawbacks that hinder them from further utilization is the presence of lead, which raises concerns over toxicity issues and environmental See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. − pollution.21 23 Therefore, the search for nontoxic alternatives that preserve the superior optoelectronic properties of lead- based perovskite NCs has become a progressively more − important research topic.23 27 One of the most promising approaches is the substitution of lead by non- or less-toxic metal elements. For example, isovalent substitution by divalent 2+ 2+ a cations such as Sn and Ge has been first explored (Scheme (A) AM(II)X3 3D cubic perovskite, (B) A3M(III)2X9 2D layered − 1A).28 30 However, easy oxidation to Sn4+ or Ge4+ in air perovskite, (C) A2M(I)M(III)X6 3D cubic double perovskite, and to the poor crystal structural stability, impeding their uses in (D) A4M(II)M(III)2X12 layered double perovskite. Top: unit cells of different perovskite crystal structures. Bottom: the corresponding future applications.31 Subsequently, heterovalent replacement 3+ models viewed from the [010] zone axis. by M(III) was adopted to obtain Cs3M(III)2X9 (M(III): Bi , Sb3+, etc.) with reduced dimensionality including zero- dimensional (0D) or 2D layered perovskite structures (Scheme Received: May 8, 2020 1B). However, these materials exhibited nonideal optoelec- Published: June 8, 2020 tronic properties including low carrier mobilities and indirect band gaps, which are mainly caused by the isolated networks of − metal-halide octahedral units.32 37 Another alternative strategy

© 2020 American Chemical Society https://dx.doi.org/10.1021/jacs.0c04919 11927 J. Am. Chem. Soc. 2020, 142, 11927−11936 Journal of the American Chemical Society pubs.acs.org/JACS Article

Figure 1. (A) Absorption spectrum of Cs4CuSb2Cl12 layered double perovskite NCs in hexane. Insets: Tauc plot (left) and a photograph of the NC fi hexane solution (right). (B) XRD pattern (gray line) of the Cs4CuSb2Cl12 NCs, tted curve (red line), and constituent peaks (blue line). Black bars indicate the standard peak positions of bulk Cs4CuSb2Cl12 perovskite (ICSD: 243918). (C) EPR spectrum of the Cs4CuSb2Cl12 NCs. (D, E) HR- fi 2+ 3+ XPS spectra of the Cs4CuSb2Cl12 NCs con rming the existence of Cu and Sb ions. The spectra were calibrated using the C 1s peak. (F) Low- fi magni cation TEM image and size distribution histogram (inset). (G, H) HR-TEM images of Cs4CuSb2Cl12 NCs (left panels), the corresponding fast Fourier transform (FFT) patterns (middle panels), and computer-simulated electron diffraction (ED) patterns (right panels) along the [372] and [hk0] ([160] was used for simulation) directions of the layered double perovskite crystal structure. is the substitution of two Pb2+ cations by paired monovalent 1.6 eV and superior optoelectronic properties because of the and trivalent cations to obtain the “elpasolite” double reduced effective photogenerated carrier masses.54 However, perovskite structure with a formula of Cs2M(I)M(III)X6 current synthetic strategies have been limited to either the (M(I), Ag+,Cu+,Au+,Na+,K+, etc.; M(III), Sb3+,Bi3+,In3+, production of polycrystalline bulk and powder materials or a − Tl3+, etc.) (Scheme 1C).38 51 Although double perovskites top-down fabrication process. To date, solution-based colloidal preserve the pristine 3D cubic perovskite structure with synthesis of lead-free layered double perovskite NCs has not structural stability, the wide (larger than 3 eV) and/or indirect yet been reported. Consequently, the intrinsic advantages of band gap characteristics set an intrinsic obstacle for their light colloidal NCs including solution processability, compositional absorbing capability, significantly constraining their widespread and crystal structure control, and optical property tunability practices in applications. have yet to be realized. Continuous efforts in searching other possible lead-free perovskite-type structures with enlarged compositional space ■ RESULTS AND DISCUSSION recently landed on the discovery of layered double perovskites fi 2+ Herein, we report, for the rst time, a colloidal synthesis of with a chemical formula of Cs4M(II)M(III)2X12 (M(II), Cu , 2+ 2+ 3+ 3+ 52−60 Cs4CuSb2Cl12 layered double perovskite NCs using a hot- Mn ,Cd , etc.; M(III), Sb ,Bi , etc.). The layered injection method with fully decoupled cation and anion double perovskite structure consists of one layer of [M(II)- 4− 3− precursors. Through detailed structural and optical character- X6] octahedra sandwiched by two layers of [M(III)X6] izations, we unambiguously show that the resulting NCs octahedra in between two adjacent vacancy layers (Scheme possess a vacancy-ordered layered double perovskite structure 1D). Cs4CuSb2Cl12 layered double perovskite powders (space and a direct band gap of ∼1.79 eV. To demonstrate the fi group: C2/m) with a direct band gap of 1.02 eV were rst composition−structure−property relationship, we have synthe- reported by Solis-Ibarra et al.52 The low-toxicity, high ≤ ≤ sized a series of Cs4CuxAg2−2xSb2Cl12 NCs (0 x 1) with abundancy of both copper and antimony elements, direct controlled cation stoichiometry ratio (i.e., Cu2+ to Ag+). A band gap nature, and superior structural stability render this crystal structural transformation from cubic double perovskite type of material extremely promising for a wide range of structure (x = 0, i.e., Cs2AgSbCl6 NCs) to monoclinic layered optoelectronic applications. Later, the Nag group demon- double perovskite structure (x = 1, i.e., Cs4CuSb2Cl12 NCs) strated a solid-state mechanochemical synthesis of has been observed. The associated electronic band structural Cs4CuSb2Cl12 polycrystalline powder with a long-range transition from indirect to direct band gap has been magnetic ordering observed at a wide temperature range discovered. Density functional theory (DFT) calculations for (i.e., 2−400 K).53 More recently, Kuang et al. fabricated band structure, charge distribution, and bond characteristics Cs4CuSb2Cl12 NCs using a top-down ultrasonic exfoliation strongly support our experimental observations. Finally, taking technique.54 The resulting NCs exhibited a direct band gap of advantage of colloidal stability, solution processability, as well

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Figure 2. (A, D, G, J) HAADF-STEM images of individual Cs4CuSb2Cl12 perovskite NCs along the indicated directions. (B, E, H, K) Pseudocolor images of the corresponding STEM images. Insets are computer-simulated 3D atomic models viewed from the same indicated directions (green, Cs; gray, Cl; orange, Cu; blue, Sb). (C, F, I, L) FFT patterns (top) of the STEM images and the corresponding computer-simulated ED pattern (bottom) of the Cs4CuSb2Cl12 perovskite atomic models. as superior charge transport characteristics, we demonstrate Cs4CuSb2Cl12 layered double perovskite crystal phase with 4− that the Cs4CuSb2Cl12 NCs can serve as a material candidate distorted [CuCl6] octahedra (Figure 1B and Figure S1). The for fabrications of high-speed photodetectors with ultrafast lattice parameters were determined to be a = 13.04 Å, b = 7.30 photoresponse and narrow bandwidth. Our results shed light Å, c = 12.97 Å, β = 111.73° (Figure 1B and Table S1). Room onto synthetic chemistry and photophysics of layered double temperature X-band electron paramagnetic resonance (EPR) perovskite materials, which will promote future opportunities measurements showed a broad peak centered at 3310 G (g in the design and fabrication of colloidal lead-free perovskite factor: 2.079) (Figure 1C). The observed EPR signal further NCs for various optoelectronic applications. confirmed the presence of Cu2+ (electron configuration: 9 fi The lead-free Cs4CuSb2Cl12 layered double perovskite NCs [Ar]3d ) with an unpaired electron. The lack of hyper ne were synthesized using a modified hot-injection method.47,61,62 splitting pattern can be attributed to the exchange interactions In a typical synthesis, metal carboxylate precursors (i.e., cesium between Cu2+ centers within close proximity (Figure S1,Cu− acetate, copper(II) acetate, antimony acetate) were first Cu distance of 7.30 Å in the [010] direction and 7.47 Å in the dissolved in a mixture of oleic acid, oleylamine, and 1- [110] direction) in the crystal structure (Scheme 1, Figure S1). ° octadecene. The resulting solution was heated to 200 CinN2 High-resolution (HR) X-ray photoelectron atmosphere, at which point the chlorotrimethylsilane (TMS- (XPS) spectra further supported the composition and valence Cl) was swiftly injected to trigger the NC and states of Cu2+ and Sb3+ ion centers in the NCs (Figure 1D,E subsequent growth. Upon reaction completion, the mixture and Figure S2). Scanning electron microscopy energy- was rapidly cooled down using an ice−water bath. The as- dispersive X-ray spectroscopy (SEM-EDS) analysis specified fi synthesized Cs4CuSb2Cl12 NCs were puri ed and then a statistical average of the elemental compositions suggesting a dispersed in hexane. The resulting NC solution showed a ratio of Cs:Cu:Sb:Cl as 3.84:1:2.17:13.43 (Figure S3), which dark black color, indicating its strong absorption in the visible matches well with the stoichiometric ratio of Cs4CuSb2Cl12 spectral region (Figure 1A, inset). Absorption measurements NCs with a Cl-terminated surface (i.e., Cl richness). Stabilities showed that the Cs4CuSb2Cl12 NCs possess a broad of the NCs were further investigated. It is shown that the NCs absorption peak centered at 523 nm (half width at half- can preserve the optical properties after being stored under maximum, HWHM: 110 nm) with the absorption onset ambient conditions for at least 2 months (Figure S4) and around 760 nm (Figure 1A). A direct band gap energy of 1.79 maintain the layered double perovskite crystal structure when eV was determined by Tauc plot analysis (Figure 1A, inset).63 heated up to 125 °C(Figure S5), indicating their good stability The larger band gap compared with the bulk materials (i.e., 1 under both ambient and thermal conditions. eV) can be mainly attributed to the nanoscale confinement Transmission electron microscopy (TEM) measurements together with the effects of surface atom arrangements and/or revealed that the Cs CuSb Cl NCs possessed a spherelike − 4 2 12 surface coating .52 54 The powder X-ray diffraction shape with an average diameter of 12.5 ± 1.9 nm (Figure 1F). (XRD) pattern of the sample matched well with the bulk HR-TEM images displayed clear lattice fringes (left panels in fi Cs4CuSb2Cl12 materials (ICSD: 243918), con rming a Figure 1G,H), indicating high crystallinity of the Cs4CuSb2Cl12 monoclinic crystal structure (vacancy-ordered layered double NCs. The measured lattice d-spacings of 4.2, 4.0, and 3.8 Å can perovskite, space group: C2/m). The broadened diffraction be, respectively, assigned to the (203), (112), and (311) peaks were consistent with finite crystalline sizes (Figure crystal planes in the projection viewed along the [372] zone 1B).52 Two sets of characteristic dual peaks located at 33.4° axis (Figure 1G, left). In another HR-TEM image, a set of and 34.8°, and 41.6° and 42.5°, can be assigned to the (404) lattice fringes with a large d-spacing of 12.1 Å can be clearly and (222), and (424) and (511), crystal planes of the visualized (Figure 1H, left), representing a unique crystalline

11929 https://dx.doi.org/10.1021/jacs.0c04919 J. Am. Chem. Soc. 2020, 142, 11927−11936 Journal of the American Chemical Society pubs.acs.org/JACS Article

Figure 3. (A) Scheme of the atomic models for the crystal phase transformation from Cs2AgSbCl6 double perovskite to Cs4CuSb2Cl12 layered double perovskite. (B) XRD patterns of Cs4CuxAg2−2xSb2Cl12 perovskite NCs (left panel, from bottom to top: x = 0.00, 0.10, 0.25, 0.30, 0.40, 0.50, ff 0.75, 0.85, 1.00) and the zoomed-in XRD patterns for (004) di raction peak area of the starting Cs2AgSbCl6 double perovskite NCs (right panel). (C) Absorption spectral evolution of the Cs4CuxAg2−2xSb2Cl12 perovskite NCs (left panel, the spectra are normalized at 350 nm) and four Tauc plots of the selected absorption spectra indicated by gray arrows (right panels). feature of the (001) lattice planes (stacking of the trilayer of and simulated models were also found for two other common octahedra between two adjacent vacancy layers, Scheme 1D). viewing directions, i.e., [101] and [151] directions (Figure Since one set of lattice planes prevents unambiguous 2G−L). Taken together, these results explicitly identified the determination of the viewing direction, we assigned the layered double perovskite structure of the synthesized projection of the image as the [hk0]. The crystal plane and Cs4CuSb2Cl12 NCs, in excellent agreement with the above viewing direction assignments were confirmed by the FFT optical and structural results. patterns of the HR-TEM image (middle panels in Figure It is known that Sb-based halide perovskites can possess 1G,H), which match to the computer-simulated electron fertile polymorphs and undergo a series of compositional diffraction (ED) patterns as shown in the right panels of Figure tuning-induced structural changes from trigonal layered 1G,H. perovskite Cs3Sb2Cl9 to cubic double perovskite Cs2AgSbCl6 fi 47,48,52 High-angle annular dark- eld scanning TEM (HAADF- to monoclinic layered double perovskite Cs4CuSb2Cl12. STEM) measurements were carried out to further confirm the Importantly, this compositional tuning-induced crystal struc- layered double perovskite crystal structure of the Cs4CuSb2Cl12 ture change also leads to an evolution of the electronic NCs with an atomic level of precision (Figure 2). Crystallo- structure with band gap engineering, i.e., from an indirect band graphic analyses have been applied to the obtained HAADF- gap for Cs2AgSbCl6 to a direct band gap for Cs4CuSb2Cl12 STEM images viewed along four different zone axes (Figure (Figure 3A). To construct this unique composition− 2A,D,G,J). For better virtualization and comparison, pseudo- structure−property relationship, we synthesized a series of ≤ ≤ color images and atomic models are also shown in Figure Cs4CuxAg2−2xSb2Cl12 (0 x 1) NCs by tuning the 2B,E,H,K. Figure 2A shows clear periodic lattices with a stoichiometry ratio of Cu2+ and Ag+ precursors (see the SI for layered atomic arrangement. The measured lattice d-spacings details) and correlated the crystal structure evolution to the of 12.1 and 6.4 Å can be, respectively, assigned to the (001) materials’ band gaps. HR-XPS spectra confirmed the + 2+ and (201) crystal planes of the layered double perovskite, coexistence of Ag and Cu in the Cs4CuxAg2−2xSb2Cl12 suggesting that the particle was viewed in the projection along NCs (0 < x < 1), where the 3d signals (3d5/2 and 3d3/2)of + 2+ the [010] zone axis (Figure 2A,B). The cross-fringes in this Ag and 2p signals (2p3/2 and 2p1/2)ofCu can both be projection exhibit an angle of 97.4°, which is in good observed (Figure S6), as compared to the absence of Cu2+ ° + agreement with the theoretically calculated value of 98.1 signals for Cs2AgSbCl6 NCs (x = 0) and Ag signals for (Figure 2A,B). The crystal plane and viewing direction Cs4CuSb2Cl12 NCs (x =1)(Figure S6). TEM images show fi ≤ ≤ assignments were further con rmed by the corresponding that the Cs4CuxAg2−2xSb2Cl12 (0 x 1) NCs undergo a FFT pattern and the computer-simulated ED as shown in change from cubic shape for those with a double perovskite Figure 2C. A different layered atomic lattice was observed structure in the cubic phase (x < 0.5) to a spherelike shape for when a particle was viewed along the [302] direction (Figure the NCs with a layered double perovskite structure in the 2D,E). The measured d-spacings for the (010) and (203) monoclinic phase (x ≥ 0.5) with similar diameters (Figure S7). planes (i.e., 7.3 and 4.2 Å, respectively) and their orthogonal XRD patterns show that the cubic double perovskite crystal lattice relationship matched well with the corresponding structure is maintained when increasing x from 0 to 0.4 (Figure simulated atomic model and ED pattern (Figure 2D−F). 3B). Meanwhile, all the XRD peaks monotonically shifted to Similar levels of agreement between HAADF-STEM images higher diffraction angles, demonstrating a lattice contraction

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Figure 4. Calculated band structures (left panels) and the corresponding DOS (right panels) of the Cs4CuxAg2−2xSb2Cl12 perovskites with x =0 (A), x = 0.25 (B), x = 0.50 (C), x = 1.00 (D). Different dot colors represent the Bloch spectral density (spectral weight). (E) Calculated energy ff Δ Δ − di erences ( ES)ofCs4CuxAg2−2xSb2Cl12 perovskites between the cubic (Ec) and monoclinic (Em) crystal phases ( ES = Ec Em). (F) ff Corresponding calculated band gap energies (EBG)ofCs4CuxAg2−2xSb2Cl12 perovskites with di erent Cu contents (i.e., x = 0, 0.25, 0.50, 0.75, 1.00). with increased Cu2+ amount (lattice parameter a: from 10.52 to the existence of surface trap states as well as strong − to 10.48 Å, Figures S8−S12 and Tables S2−S6), consistent phonon coupling-induced fast carrier trapping processes.27,47 with the substitution of Ag+ (ionic radius: 115 pm) with To gain in-depth understandings of the compositional smaller Cu2+ (ionic radius: 73 pm) cations. Absorption spectra tuning-induced crystal phase and electronic band structure ≤ ≤ of these Cs4CuxAg2−2xSb2Cl12 (0 x 0.4) NCs remained evolutions, DFT calculations based on the generalized gradient nearly unchanged with an indirect band gap of ∼3.05 eV approximation formulated by Perdew−Burke−Ernzerhof were (Figure 3C and Figure S13). With increasing the Cu2+ content performed (Figure 4,seetheSI for calculation de- − (x ≥ 0.5), the (400) diffraction peak (around 33.6°) of the tails).52,57,64 66 We first determined the stable crystal structure ff Δ cubic double perovskite phase became broadened and by calculating the energy di erence ( E S )of gradually split into two distinct peaks (Figure 3B) that can Cs4CuxAg2−2xSb2Cl12 between the cubic (Ec) and monoclinic Δ − be assigned to the (404) and (222) of the monoclinic phase, (Em) phases ( ES = Ec Em) with considerations of the indicating a crystal structure transition from cubic double preferred magnetic ordering (Figure S18). Figure 4E and perovskite to monoclinic layered double perovskite (Figures Figure S19 show that cubic double perovskite phase with a 1B and 3B and Figures S14−S17 and Tables S1 and S7−S10). ferromagnetic ordering is thermodynamically favored when x = This structural transition was further validated by the 0 and 0.25, while the preference shifts to monoclinic layered absorption spectral change. A broad absorption feature started double perovskite with an antiferromagnetic ordering when x to emerge at a long wavelength region (400−700 nm) when x reaches 0.5 and beyond (i.e., x = 0.50, 0.75, and 1). These reached 0.5 and became more and more prominent with calculation results are in accordance with our experimental further increasing Cu2+ content (x > 0.5, Figure 3C). At the observations (Figures 3B and 4E, Figures S17 and S19). Upon same time, the newly emerged absorption peak blue-shifted completing the crystal structure relaxation calculation, the fi from 589 nm (x = 0.5) to 523 nm (x = 1), revealing a slight electronic structures of the Cs4CuxAg2−2xSb2Cl12 NCs with ve increase of the direct band gap energy from 1.67 to 1.79 eV different compositions (i.e., x = 0, 0.25, 0.5, 0.75, 1) were quantified by Tauc plots (Figure 3C and Figure S13). The further calculated, and the results are shown in Figure 4A−D fi appearance of this low-energy absorption feature indicates an and Figure S20. Speci cally, the Cs2AgSbCl6 double perovskite electronic band structural transition from a wide indirect band (x = 0) possesses an indirect band gap of 2.03 eV, where the gap to a narrow direct band gap.52 The direct correlation conduction band minimum (CBM) and valence band between crystal phase and electronic band gap structure maximum (VBM) are located at the L and X symmetry uncovered an effective means to control the optical properties points, respectively (Figure 4A). The (DOS) of the materials through tuning the composition and, thus, the calculation results clearly reveal that the CBM mainly consists crystal structure of the NCs. No detectable of the Cl 3p and Sb 5p orbitals, whereas the VBM is dominated was observed for all the Cs4CuxAg2−2xSb2Cl12 NCs, likely due by the Cl 3p orbital with a small contribution from the Ag 4d

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− ff − Figure 5. (A D) Charge density di erence and (E H) electron localization function (ELF) of Cs4CuxAg2−2xSb2Cl12 perovskites with x = 0 (A, E), x = 0.25 (B, F), x = 0.50 (C, G), and x = 1.00 (D, H). In parts A−D, the yellow and cyan surfaces correspond to charge gain and charge loss, respectively. In parts E−H, top panels display the associated ELF 2D mapping, and the colors in the ELF 2D mapping represent the strength of electron density. The bottom panels show the 1D intensity profiles along the white dash lines in the corresponding top panels, illustrating the ff 4− 5− changes of electron density between di erent ions for clear visualization of gradual distortions of [CuCl6] and [AgCl6] octahedral units.

+ 2+ + orbital (Figure 4A). Compared to Cs2AgSbCl6 double formed between the cations (i.e., Ag ,Cu ,Cs) and anions perovskite, a similar band structure and DOS are observed (i.e., Cl−)(Figure 5E,F). When the Cu2+ concentration is 4− for Cs4Cu0.25Ag1.5Sb2Cl12 perovskite (x = 0.25) with a slightly increased to x = 0.5, the [CuCl6] octahedra become decreased band gap energy of 1.98 eV (Figure 4B). This band distorted with an elongation along the z direction, resulting gap narrowing can be attributed to the enhanced antibonding in the weaker Cu−Cl bonds compared to those in the x−y 4− coupling caused by the introduction of Cu 3d orbitals to the plane (Figure 5C,G). This [CuCl6] octahedral distortion can − ff VBM of the Cs4Cu0.25Ag1.5Sb2Cl12 perovskite (Figure 4B). be ascribed to the Jahn Teller e ect due to the presence of With further increasing the Cu content to x ≥ 0.5, the CBM the Cu2+ ion center (electron configuration: [Ar]3d9) with an shifts to a much lower energy, thus resulting in drastically unpaired electron located in energy-degenerated molecular narrowed band gaps (Figure 4C,D and Figure S20). When orbitals. Meanwhile, the interconnection nature between the fi 4− 5− arriving at the nal Cs4CuSb2Cl12 layered double perovskite neighboring [CuCl6] and [AgCl6] octahedral units leads to 5− (i.e., x = 1), while the VBM is still mainly from the Cl 3p the consequent distortion of [AgCl6] octahedra (Figure fi orbital, the Cu 3d orbital becomes the dominating component 5C,G). In the nal Cs4CuSb2Cl12 perovskites (i.e., x = 1.0), the of the CBM with some contributions from the Cl 3p orbital bond length of Cu−Cl elongates to 2.98 Å in the axial z and a little portion of the Sb 5p orbital. A band gap energy of direction and shortens to 2.32 Å in the equatorial x and y 1.24 eV was obtained with both the CBM and VBM located at directions (Figure 5D,H and Table S11), matching well with the same point in between E and C symmetry points, the values determined by fitting the XRD pattern (2.81 and suggesting a complete transition to direct band gap (Figure 2.30 Å, Figure 1B and Figure S21). The same octahedral 4D). Figure 4F shows a summary of the calculated band gap distortion induced a net charge loss across the neighboring − − energies (EBG). A sudden decrease in band gap energy Cu Cl bonds in the x y plane, which cannot be obviously indicates a transition from indirect to direct band gap (Figure observed in the z direction (cyan surfaces, Figure 5D). 4F), correlating with the thermodynamically favored crystal Consistently, the corresponding ELF 2D mapping shows a low phase evolution (Figure 4E), consistent with the experimental charge density around the Cl atoms in the z direction, results (Figure 3, Figure S13). Together, both calculation and indicating a weaker Cu−Cl bond with an elongated bond experimental efforts unveiled a tunability of electronic length (2.98 Å, Figure 5H). interband transition nature by a composition-induced crystal Band structure, charge distribution, and ELF calculations structure control for Cs4CuxAg2−2xSb2Cl12 perovskite-type provide insights that Cs4CuSb2Cl12 layered double perovskite NCs. NCs possess a narrow direct band gap and superior charge To investigate the charge distribution and bond character- transport characteristics. Previous studies also show that istics of Cs4CuxAg2−2xSb2Cl12 perovskites during the trans- Cs4CuSb2Cl12 perovskites are p-type with formation process, the charge density difference and electron small electron effective mass and excellent photoelectrochem- localization function (ELF) were calculated, and the results are ical response,52,54 indicating its potential to be integrated in shown in Figure 5. With low Cu2+ concentration (x < 0.5), all photodetectors. To explore this opportunity, we fabricated a Ag or Cu atoms donate equal charges to the Cl atoms with thin film-based high-speed through drop casting 5− 4− good preservations of [AgCl6] or [CuCl6] octahedra, a high concentration (20 mg/mL) of Cs4CuSb2Cl12 NC resulting in no obvious charge redistribution (Figure 5A,B). hexane solution on a quartz substrate with a coplanar gold The associated ELF 2D mappings show that ionic bonds are transmission line structure (Figure 6A). A 100 fs pulse

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about this highly promising lead-free perovskite-type material. Furthermore, we demonstrate that the Cs4CuSb2Cl12 layered double perovskite NCs can be solution-processed into high- speed photodetectors with ultrafast photoresponse and narrow bandwidth. We expect that this study will stimulate future opportunities in the design and fabrication of novel lead-free perovskite-type NCs for a spectrum of optoelectronic applications. ■ ASSOCIATED CONTENT *sı Supporting Information The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.0c04919. Additional experimental details, DFT calculations de- tails, XRD analysis, absorption, EPR, XPS spectra, band structures, and magnetic ordering (PDF)

■ AUTHOR INFORMATION Figure 6. (A) Schematic high-speed photodetector device based on Corresponding Author Cs4CuSb2Cl12 NCs. (B) Ultrafast photocurrent peak density and Ou Chen − Department of Chemistry, Brown University, corresponding dark current density as a function of bias voltage Providence, Rhode Island 02912, United States; orcid.org/ measured from the same device. Typical ultrafast photocurrent dependences on various applied bias voltages (C) and laser density 0000-0003-0551-090X; Email: [email protected] (D). The insets are the photocurrent peak dependence with bias Authors voltage (inset in C) and laser density (inset in D). In the insets, m − represents the linear fitting coefficient. Note that the oscillations are a Tong Cai Department of Chemistry, Brown University, result of impedance mismatch, which slightly varies with devices and Providence, Rhode Island 02912, United States; orcid.org/ do not correspond to an internal physical process. 0000-0002-4468-6767 Wenwu Shi − Department of Chemistry, Brown University, (400 nm) was illuminated onto the device, and the ultrafast Providence, Rhode Island 02912, United States; University of photocurrent was collected by a sampling oscilloscope (see Electronic Science and Technology of China, Chengdu 610054, details in the SI). The carrier dynamics characterizations were P. R. China − carried out by the ultrafast photocurrent spectroscopy with a Sooyeon Hwang Center for Functional , − sub-20 ps time resolution.67 69 As shown in Figure 6B, the Brookhaven National Laboratory, Upton, New York 11973, dark current density is in the range of 10−6−10−4 mA/cm2, United States; orcid.org/0000-0001-5606-6728 − which is consistent with the reported values for native organic Kanishka Kobbekaduwa Department of Physics and − ligands capped quantum dot NCs.70 73 The low dark current Astronomy, Ultrafast Photophysics of Quantum Devices density was due to low carrier density as well as the low carrier Laboratory, Clemson University, Clemson, South Carolina mobility. However, the measured ultrafast photocurrent peak 29634, United States fi Yasutaka Nagaoka − Department of Chemistry, Brown for the Cs4CuSb2Cl12 NC thin lm is more than four orders of magnitude higher than that of the dark current (Figure 6B), University, Providence, Rhode Island 02912, United States ∼ Hanjun Yang − Department of Chemistry, Brown University, and 30 times higher than that of the Cs2AgSbCl6 NC-based photodetector under the same condition (Figure S22). These Providence, Rhode Island 02912, United States − results show an excellent performance of the carrier mobility Katie Hills-Kimball Department of Chemistry, Brown for the Cs CuSb Cl NC-based photodetector under illumi- University, Providence, Rhode Island 02912, United States 4 2 12 − nation. The linear dependences of the peak photocurrent Hua Zhu Department of Chemistry, Brown University, density on the applied laser intensity and electrical field Providence, Rhode Island 02912, United States; orcid.org/ (Figure 6C,D), together with a quick rise followed by a fast 0000-0003-2733-7837 − decay (lifetime of ∼150 ps), demonstrate a good performance Junyu Wang Department of Chemistry, Brown University, of the Cs CuSb Cl NC-based high-speed photodetector with Providence, Rhode Island 02912, United States 4 2 12 − a narrow bandwidth (∼10 GHz). Zhiguo Wang University of Electronic Science and Technology of China, Chengdu 610054, P. R. China; orcid.org/0000- ■ CONCLUSION 0002-5652-5362 Yuzi Liu − Center for Nanoscale Materials, Argonne National In conclusion, we report a colloidal synthesis of Cs4CuSb2Cl12 layered double perovskite NCs with a direct band gap of 1.79 Laboratory, Argonne, Illinois 60439, United States; orcid.org/0000-0002-8733-1683 eV. Both experimental results and DFT calculations show that − thecrystalstructure(fromcubicdoubleperovskiteto Dong Su Center for Functional Nanomaterials, Brookhaven monoclinic layered double perovskite) and the corresponding National Laboratory, Upton, New York 11973, United States; orcid.org/0000-0002-1921-6683 electronic band alignment (from indirect to direct band gap) − can be modulated by means of gradually tuning the materials’ Jianbo Gao Department of Physics and Astronomy, Ultrafast ≤ ≤ Photophysics of Quantum Devices Laboratory, Clemson composition (Cs4CuxAg2−2xSb2Cl12,0 x 1). The established composition−structure−property relationship is University, Clemson, South Carolina 29634, United States of great importance for further advancing our understandings Complete contact information is available at:

11933 https://dx.doi.org/10.1021/jacs.0c04919 J. Am. Chem. Soc. 2020, 142, 11927−11936 Journal of the American Chemical Society pubs.acs.org/JACS Article https://pubs.acs.org/10.1021/jacs.0c04919 (11)Chen,Q.;Wu,J.;Ou,X.;Huang,B.;Almutlaq,J.; Zhumekenov, A. A.; Guan, X.; Han, S.; Liang, L.; Yi, Z.; Li, J.; Xie, Funding X.; Wang, Y.; Li, Y.; Fan, D.; Teh, D. B. L.; All, A. H.; Mohammed, O. O.C. acknowledges support from Brown University startup F.; Bakr, O. M.; Wu, T.; Bettinelli, M.; Yang, H.; Huang, W.; Liu, X. All-Inorganic Perovskite Nanocrystal . Nature 2018, 561 funds. K.H.-K. is supported by the U.S. Department of (7721), 88−93. Education GAANN research fellowship (P200A150037). (12) Huang, J.; Lai, M.; Lin, J.; Yang, P. Rich Chemistry in Inorganic Notes Halide Perovskite Nanostructures. Adv. Mater. 2018, 30 (48), The authors declare no competing financial interest. 1802856. (13) Luo, B.; Li, F.; Xu, K.; Guo, Y.; Liu, Y.; Xia, Z.; Zhang, J. Z. 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