Broad-Band-Emissive Organic–Inorganic Hybrid Semiconducting

Article pubs.acs.org/IC

Broad-Band-Emissive Organic−Inorganic Hybrid Semiconducting Nanowires Based on an ABX3‑Type Chain Compound Zhenyue Wu,†,‡ Lina Li,† Chengmin Ji,*,† Guoming Lin,† Sasa Wang,†,‡ Yaoguo Shen,†,‡ Zhihua Sun,† Sangen Zhao,† and Junhua Luo*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China ‡ University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100039, P. R. China S Supporting Information *

ABSTRACT: Organic−inorganic hybrid lead halide (e.g., CH3NH3PbX3, where X = CI, Br, and I) nanowires (NWs) with remarkable electric and optical properties have recently garnered increasing attention, owing to their structural flexibility and tunability compared to inorganic semiconducting NWs. While most recently reported NWs are limited to methylammonium/ formamidinium three-dimensional lead halide perovskites, it is urgent to develop new organic−inorganic hybrid semiconducting NWs. Here, broad-band-emissive single-crystal semiconductive NWs based on a new ABX3-type organic−inorganic chain hybrid, (2-methylpiperidine)lead tribromide, are reported. It is believed that this work will enrich the organic−inorganic hybrid semiconducting NWs and may provide potential applications for LED displaying.

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INTRODUCTION Semiconducting nanowires (NWs), since first being reported by Hiruma et al. in the 1990s,1 have been widely investigated in multifarious areas, including solar cells,2 light-emitting diodes (LEDs),3 lasers,4 field-effect transistors,5 and photodetectors.6 Different from quantum dots and nanosheets, one-dimensional (1D) semiconducting NWs are restricted in two dimensions, thereby permitting electrons, holes, or photons to transport freely along the third dimension, which endows NWs with fascinating electric and optical performances.7 For example, a 3.4% power conversion efficiency for the core/shell silicon NW solar cell was reported,8 which drives the development of nanophotoelectrical devices. In addition, a series of semiconducting NWs, such as GaN,9 CdS,10 CdSxSe1−x,11 and GaAs,12 with laser emitting from the ultraviolet to the nearinfrared region have been triumphantly synthesized. However, until now, most researches on semiconducting NWs are mainly concentrated on the pure inorganic semiconducting materials. Recently, organic−inorganic hybrid lead halide perovskite NWs have garnered increasing attention as a novel class of promising semiconducting photoelectric materials because of their tunable structures and outstanding semiconducting © XXXX American Chemical Society

features, including faster carrier separation, long carrier lifetimes, low trap state densities, and high luminescence efficiency.13 For instance, as a result of the faster carrier separation and lateral conductivity of NWs, an organic− inorganic perovskite NW solar cell with a high power conversion efficiency of 14.71% was first reported by Park’s group.14 Horvath and his colleagues fabricated perovskite NW photodetector devices with high responsivity and sensitivity.15 Meanwhile, methylammonium- and formamidinium-based lead halide organic−inorganic hybrid perovskite semiconducting NWs displayed low lasing thresholds, high-quality factors, and high photoluminescence quantum yields.16 What is more, the emission color of the NWs could be tuned in the visible spectrum by chemical replacements. Nevertheless, most recently reported organic−inorganic hybrid lead halide semiconducting NWs are merely limited to formamidinium- and methylammonium-based three-dimensional perovskites.14−17 At the same time, broad-band-emissive organic−inorganic hybrids have attracted great interest because of their possible Received: February 26, 2017

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DOI: 10.1021/acs.inorgchem.7b00521 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry application in white-light displays.18 Thus, exploring new broad-band-emissive organic−inorganic hybrid semiconducting NWs is urgent and still a challenge. In this context, we demonstrated the successful fabrication of organic−inorganic hybrid semiconducting NWs from a newly synthesized ABX3-type lead halide, (2-methylpiperidine)lead tribromide (1). 1 features an infinite double-chain framework constructed through inorganic PbBr6 octahedra. Interestingly, the high-quality single-crystal NWs for 1 with high aspect ratios show broad-band emission. In addition, the semiconducting properties of the prepared NWs of compound 1 were investigated by positive temperature-dependent conductivity and density functional theory (DFT) calculations. Such new kinds of organic−inorganic hybrid broad-band-emissive NWbased 1D infinite double-chain structures may provide potential applications for broad-band white-light displays.19

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Table 1. Crystal Data and Structure Refinement for Compound 1 empirical formula fw cryst syst, space group unit cell dimens a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z, calcd density (g/cm3) F(000) limiting indices reflns collected/unique completeness (%) data/restraints/param final R indices [I > 2σ(I)] R indices (all data)

EXPERIMENTAL SECTION

Chemicals. All chemicals were purchased and used without any further purification: hydrogen bromide (HBr; 40%, Macklin), methanol (99.5%, SCRC), ethanol (99.7%, SCRC), isopropyl alcohol (99.7%, SCRC), lead acetate trihydrate (99.5%, Aladdin), and 2methylpiperidine (99%, Aladdin). Fabrication of 2-Methylpiperidine Bromide. 2-Methylpiperidine bromide was fabricated by adding an equal number of moles of 2-methylpiperidine to a dilute aqueous HBr solution. The precipitate was collected by evaporating the solvent. The product, 2methylpiperidine bromide, was recrystallized with ethanol several times. Finally, the purified product was dried at 50 °C in a vacuum oven for 24 h. Synthesis and Crystal Growth of Compound 1. Pb(Ac)2·3H2O (3.79 g, 0.01 mol) was dissolved in an aqueous HBr solution (40%, 50 mL) in a clean 100 mL beaker. Thereafter, a white precipitate was generated immediately when 2-methylpiperidine (1.00 g, 0.01 mol) was slowly dropped into the solution, and the precipitate dissolved after 30 min of heating. Finally, colorless single crystals were obtained by slowly evaporating the solvent at room temperature after several weeks. For the thermostability of compound 1, thermogravimetry measurements show that compound 1 is stable up to 150 °C (Figure S1). Preparation of NWs for Compound 1. The clean, ozone-treated glass slide was coated with 30 μL of a saturated methanol solution of lead acetate trihydrate at 4500 rpm for 50 s. Then, the slide was annealed at 90 °C for 30 min. After cooling, the slide was transported into 5 mL of an isopropyl alcohol solution, which contains 5 mg of 2methylpiperidine bromide. After 24 h, white floccule was produced on the surface of the glass slide. The white floccule was carefully collected by centrifugation and washed several times with isopropyl alcohol. Powder X-ray Diffraction (XRD) and Single-Crystal Structure Determination. Powder XRD patterns were meaured on a Rigaku DMAX 2500 PC X-ray diffractometer at room temperature in the 2θ range of 5−50° with a step size of 0.02°. Single-crystal XRD patterns were obtained on a SuperNova diffractometer with Mo Kα radiation (λ = 0.71073 Å). Absorption corrections were applied by using a multiscan program. The crystal structure was solved by direct methods and refined by the full matrix method based on F2 using the SHELXLTL software package and SHELXS-97.20 The crystal data and details of data collection and refinement for the compounds were summarized in Table 1. Characterization. The morphology images of the samples were taken by scanning electron microscopy (SEM; JEOL JSM-6700F) and transmission electron microscopy (TEM; Tecnai F20). X-ray photoelectron spectroscopy (XPS) was measured on a Thermo Scientific ESCALAB 250 Xi XPS system. Diffuse-reflectance spectra were recorded on a Lamda 950 using BaSO4 as a standard, and temperaturedependent photoluminescence (PL) spectra were acquired on the FLS980 (Edinburgh) spectrometer. Temperature-dependent current−

a

C6H14Br3NPb 547.10 orthorhombic, Pbcn 13.2134(5) 21.9770(10) 8.4266(3) 90 90 90 2447.01(17) 8, 2.970 1952 −16 ≤ h ≤ 12, −19 ≤ k ≤ 27, −10 ≤ l ≤ 10 6910/2511 99.7 2511/36/151 R1 = 0.0434, wR2 = 0.0904 R1 = 0.0619, wR2 = 0.1006

R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = {∑[w(Fo2 − Fc2)2]/∑w[(Fo)2]2}1/2.

voltage (I−V) curves were measured on a Keithley 4200-SCS instrument.

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RESULTS AND DISCUSSION Colorless single crystals of compound 1 were first prepared from the aqueous HBr solution (see the Experimental Section), and the purity was confirmed by powder XRD (Figure 1b). Structure analyses manifest that compound 1 pertains to the orthorhombic system with a space group of Pbcn, and the asymmetric unit is composed of one inorganic [PbBr3]− (Br1, Br2, Br3, and Br4 with occupancies of 0.5, 1, 0.5, and 1, respectively) anion and one protonated organic 2-methylpiperidine cation. XPS measurements further confirm that the Pb/Br atomic ratio of compound 1 is 1:3.02 (Figure S2), which is consistent with the results of structure analyses. For the inorganic part of compound 1, each Pb atom is coordinated by six Br atoms, including one terminal Br atom, two doublebridged Br atoms, and three triple-bridged Br atoms. It is quite evident that the configuration of the PbBr6 octahedron is slightly distorted, as deduced from the inhomogeneity bond lengths of Pb−Br. For example, Pb1−Br1 = 2.974 Å, Pb1−Br2 = 2.825 Å, Pb1−Br3 = 2.978 Å, Pb1−Br4 = 3.096 Å, Pb1−Br4a = 3.089 Å, and Pb1−Br4b = 3.253 Å (Figure 1a). It is worth noting that the bond length of Pb1−Br4b is remarkably longer than any other Pb−Br bond length. Two transformative PbBr6 octahedra form a dinuclear [Pb2Br6]2− cluster via face-sharing (Figure 1d), and the dinuclear [Pb2Br6]2− clusters are further interlinked together into a 1D single chain by sharing corners. Finally, two such single-chain configurations constitute the 1D infinite transformative double-chain structure (Figures 1c and S3) of compound 1 extending along the c axis by sharing edges (Figure 1e). The geometrical distortions of the 1 lattice closely affect the optical performance of the material by controlling defect formation.21 Remarkably, disordered organic 2-methylpiperidine cations are boned to the 1D infinite double-chain framework through weak N−H···Br hydrogen bonds with lengths of 3.2584 and 3.518 Å, respectively (Figures S4 and S5). Such a 1D infinite double-chain structure is quite similar to the reported organic−inorganic hybrid lead halide compounds B

DOI: 10.1021/acs.inorgchem.7b00521 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) PbBr6 distorted octahedron. The Pb−Br bond lengths are indicated. (b) XRD patterns for powder and NWs of compound 1. (c) 1D infinite double chain composed of PbBr6 octahedra. (d) Face-sharing PbBr6 octahedron. (e) Edge-sharing PbBr6 octahedron.

Figure 2. (a) Temperature-dependent conductivity of compound 1 measured on the powder pallet. (b) Calculated band structure. (c) Density of states and PDOS of compound 1.

such as [Mn(en)3][Pb2I6],22 [H2dppip][Pb2I6]·H2O,23 and [H2dabco][Pb2Br6]·H2O.24 However, NWs based on such a 1D infinite double-chain organic−inorganic hybrid metal halide are rarely reported. To understand much better the semiconducting properties of the prepared NWs of 1, variable-temperature conductivity was performed at different temperatures via a two-point pressedpellet method. As shown in Figure S6, the slope of the I−V curve gradually increases with increasing temperature, which means that the resistivity of the compound progressively decreases. The conductivity of compound 1 varies exponentially upon heating (Figure 2a), which is a typical feature of a semiconductive compound.25 The band structure and partial density of states (PDOS) contribute to the comprehension of

the electronic origin of the semiconducting properties of the prepared NWs. The valence-band maximum (VBM) is located at the G point, while the conduction-band minimum (CBM) lies in the middle of the G and Z points. The calculated band gap is about 3.03 eV (Figure 2b), slightly smaller than the experimental value of 3.26 eV (Figure S7), owing to the restriction of the DFT methods.26 1 is a representative wideband-gap semiconductor, resembling some Pb-based organic− inorganic hybrid materials, such as bis(benzylammonium)lead tetrachloride25a and (R-NH3)2PbCl4.27 Furthermore, the PDOS reveals that Pb s/p and Br s/p states obviously overlap (Figure 2c), which means strong interactions between the Pb and Br atoms. The VBM bands principally originate from the nonbonding states of Br 4p, and the CBM bands mainly arise C

DOI: 10.1021/acs.inorgchem.7b00521 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a and b) SEM and (c) TEM images of NWs based on compound 1. Inset of part c: Selected-area electron diffraction pattern of NWs.

from the unoccupied Pb 6p orbitals. Undoubtedly, the inorganic part of compound 1 mainly contributes to its band gap. High-quality single-crystal NWs based on compound 1 were successfully fabricated by depositing a lead acetate thin film on a glass slide and then immersing it in a (2-methylpiperidine) bromide/isopropyl alcohol solution at room temperature (see the Experimental Section). The powder XRD pattern of the prepared semiconducting NWs is completely consistent with the fitting data (Figure 1b, blue curve), which illustrates the high purity and good crystallinity of the prepared NWs, and the strong diffractions can be assigned to (110), (040), (330), and (260) facets, respectively. Moreover, after 4 weeks of storage in an isopropyl alcohol solution at room temperature, there was no change observed for the powder XRD pattern of the prepared NWs (Figure S8), indicating its excellent stability in solution. The selected-area electron diffraction pattern indicates that the prepared NWs are single crystalline and grow in length along the [001] direction (Figure 3c, inset). Furthermore, elemental mapping images of the single-crystal NWs are determined by SEM and energy-dispersive X-ray spectroscopy, which show that the Br and Pb atoms disperse uniformly in the prepared NWs and the Pb/Br atomic ratio is 1:2.7 (Figure S9). For the morphology of the NWs, a typical SEM image in Figure 3a indicates that the length of the NWs reaches several hundreds of microns. Because of the different interaction lengths of the exciton and phonon in different lengths of NWs,28 photoelectric performances may be modulated in such long organic−inorganic metal halide NWs by micronanofabrication. SEM and TEM images (Figures 3b,c and S10) reveal that the widths of the NWs vary from 100 to 500 nm, exhibiting a smooth surface of the prepared semiconducting NWs. Besides, the prepared NWs of 1 show high aspect ratios, more than 650 by the counting length and width of an individual NW (Figure S11), which is extremely in favor of improving the electrical performance.29 These results clearly demonstrate that high-quality organic−inorganic hybrid metal halide single-crystal semiconducting NWs with smooth surfaces and high aspect ratios have been successfully fabricated at room temperature. At present, great efforts have been devoted to exploring organic−inorganic hybrid semiconducting NWs for luminescence performances, such as lasers,16,17a LEDs,17b etc. In order to uncover the optical properties of the prepared semiconducting NWs of compound 1, typical UV−vis absorption and temperature-dependent PL spectra were measured. As shown in Figure 4a, the absorption edge of the prepared NWs is around 375 nm. In addition, the NWs show broad-band emission with a 220 nm full width at half-maximum and a large red shift at 300 K, and the broad peak has a maximum value at

Figure 4. (a) UV−vis absorption and PL spectra of compound 1. (b) Temperature-dependent PL spectra of compound 1 taken from 77 to 300 K.

630 nm (1.97 eV; Figure 4a). Such a broad-band emission is mainly attributed to the self-trapped excitons resulting from structural deformation.21,30 The self-trapping excitons have been observed in a recently reported range of compounds, including pure inorganic CsPbI3,31 organic−inorganic hybrid (EDBE)[PbX4] (X = Cl and Br),32 (N-MEDA)[PbBr4],33 (C6H13N3)PbBr4,34 (C6H5C2H4NH3)2PbX4 (X = Cl, Br, and I).35 The broad-band emission of the prepared semiconducting NWs becomes narrower as the temperature decreases (Figure 4b) because of strong coupling between the electron and phonon in the deformed inorganic metal halide lattice.30,36 Interestingly, orange emission can be clearly observed for the D

DOI: 10.1021/acs.inorgchem.7b00521 Inorg. Chem. XXXX, XXX, XXX−XXX

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fabricated organic−inorganic hybrid semiconducting NWs based on a newly synthesized double-chain metal halide under laser irradiation of 365 nm (Figure S12).

CONCLUSION In summary, we have successfully obtained an organic− inorganic hybrid ABX3-type semiconducting metal halide compound, 1, featuring a 1D infinite double-chain structure constructed by distorted PbBr6 octahedra. More importantly, high-quality and high-aspect-ratio (>650) single-crystal semiconducting NWs based on compound 1 have been successfully fabricated via a solution method at room temperature and display broad-band emission due to deformation of the inorganic metal halide double-chain lattice. Hence, we believe that this work will broaden the organic−inorganic hybrid semiconducting NW family and open up opportunities for exploring new NWs. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00521. Structures, SEM and TEM images, PL and XPS spectra, TGA−DTA curves, optical image, XRD patterns, and additional information (PDF) Accession Codes

CCDC 1507618 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Junhua Luo: 0000-0002-7673-7979 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21525104, 21622108, 21601188, 91422301, 21373220, 51402296, 21571178, 51502288, and 51502290) and Strategic Priority Research Program of Chinese Academy of Sciences (Grant XDB20000000). Z.S. is thankful for support by the State Key Laboratory of Luminescence and Applications (Grant SKLA-2016-09). Z.S. and S.Z. are thankful for support from the “Chunmiao Projects” of Haixi Institute of Chinese Academy of Sciences (Grants CMZX-2013-002 and CMZX-2014-003). L.L. and S.Z. are thankful for support from the Youth Innovation Promotion of CAS (Grants 2016274 and 2015240) E

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b00521 Inorg. Chem. XXXX, XXX, XXX−XXX