Yellowish White-Light Emission Involving Resonant Energy Transfer in

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Yellowish White-Light Emission Involving Resonant Energy Transfer in a New One-Dimensional Hybrid Material: (CH N)PbCl 9

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Hamdi Barkaoui, Haitham Abid, Aymen Yangui, Smail Triki, Kamel Boukheddaden, and Younes Abid J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06850 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 3, 2018

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The Journal of Physical Chemistry

Yellowish White-Light Emission Involving Resonant Energy Transfer In a New OneDimensional Hybrid Material: (C9H10N2)PbCl4 H. Barkaoui,a H. Abid,a A. Yangui,b S. Triki,c K. Boukheddaden,d and Y. Abida,* a

Laboratoire de Physique Appliquée, Faculté des Sciences de Sfax, Route de Soukra km 3.5 BP 1171, 3018 Sfax, Tunisia b

Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 1538505, Japan

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Laboratoire de Chimie, Electrochimie Moléculaires et Chimie Analytique, CNRS, Université de Bretagne Occidentale, BP 809, 29285 Brest, France

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Groupe d’Etudes de la Matière Condensée (GEMaC), CNRS, Université de Versailles SaintQuentin-en-Yvelines, 45 Avenue des Etats-Unis 78035 Versailles cedex, France

AUTHOR INFORMATION *Corresponding Author: [email protected]

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ABSTRACT. The present work deals with a new one-dimensional (1D) organic-inorganic hybrid material namely (C9H10N2)PbCl4 (abbreviated as AQPbCl4). Its crystal structure is built up from infinite 1D chain of edge-sharing PbCl6 octahedra surrounded by 3-aminoquinoline (abbreviated as AQ) organic molecules. Contrary to the most organic-inorganic hybrid materials, where the organic moieties act as barriers and the inorganic parts play the role of quantum wells, both inorganic and organic parts in AQPbCl4 are optically active, giving rise to optical properties involving the competition and the interaction of two organic and inorganic emitting entities. Under UV excitation, this hybrid compound shows a strong yellowish white-light emission that can be seen even with the naked eye and at room temperature. Photoluminescence spectrum is composed from a strong and broad yellow band at 538 nm associated with π-π* transition localized within AQ organic molecule and a less intense band in the UV region at 340nm associated with inorganic Wannier exciton confined in the PbCl4 inorganic wires. These attributions were made possible thanks to comparisons with homologous materials and it was supported by theoretical band structure calculations. In addition, both theoretical and experimental results suggest that the emission involves resonant energy transfer mechanism in which the inorganic PbCl4 wires act as a donor and the organic molecules act as an acceptor. Moreover, the temperature dependence study of the photoluminescence led to an estimation of the binding energies of interacting excitons and showed that the energy transfer mechanism is characterized by a remarkable enhancement of the emission band intensity.

INTRODUCTION


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Metal halide based organic-inorganic hybrid (OIH) materials with the general formula RxMyXz (R: protonated amine, M: metal and X: halogen) are quite appealing owing to their structural diversity, interesting magnetic, optical, and electrical properties, as well as their ability to process materials using low temperature techniques.1–3 From a structural point of view, the architecture of OIH materials is characterized by MX6 inorganic octahedral, which share corners, edges or faces and naturally form zero (0D), one (1D), two (2D) or three (3D) dimensional semiconductors networks, surrounded by organic cations through X–H–N hydrogen bonds.4–8 In particular, for 1D systems, it is possible to have combinations of various types of sharing within a single chain (corner, edge and face-sharing octahedra).9 Owing to their unique structural and chemical characteristics, 1D OIH materials form a very stable excitons with a large exciton binding energy of several hundred meV as a result of the strong quantum confinement as well as the large dielectric mismatch between the organic and the inorganic moieties.10 These OIH materials have drawn great attention due to their unique electronic and optical properties which make them highly promising as regards to thin-film field-effect transistors (TFT)11,12 and lightemitting diodes (LED).13 Since 2009, 3D OIH perovskites have represented an exciting new class low-cost solar absorber materials which have revolutionized the photovoltaic landscape by the exceptional growth of their power conversion efficiency from 3.8% to more than 22% in less than 10 years.14–23 Recently, broadband white-light (WL) emission was observed, for the first time, by E.D. Dohner et al.24,25 and then by A. Yangui et al.26,27 in - and -oriented 2D OIH materials based on lead bromide. The chromaticity of the WL emission was partially tuned with the choice of the halogen, and a stable photoluminescence quantum efficiency (PLQE) of 9% has been measured, even after three months of continuous irradiation.25 Since these pioneering


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works, several research groups started exploring this new phenomenon deeply,28–34 which further confirms the great interest of these low-cost single-component WL emitters in solid state lighting applications. More recently, WL emission was evidenced in 1D OIH material based on lead bromide, namely C4N2H14PbBr4, in which the edge sharing octahedral lead bromide chains [PbBr42-] are surrounded by the organic cations C4N2H142+ to form the bulk assembly of coreshell quantum wires. This 1D structure enabled strong quantum confinement with the formation of self-trapped excited states and gave efficient bluish WL emission with PLQE of 20% and 12% for bulk single crystals and microscale crystals.35 Furthermore, WL emission was also detected in a 1D lead chloride based material,36 thus highlighting the great potential of OIH materials as white-light emitters. Besides, this wide-class of materials are known by the combination of enhanced optical nonlinearities from inorganic parts and the high oscillator strength from organic parts37 which may allow a strong Frenkel-Wannier excitons coupling.38 Specific studies have been performed on the resonant energy transfer (RET) within the hybrid materials, from inorganic molecules towards organic molecules39 and vice-versa.40 The energy transfer rate can be influenced by many factors such as temperature,41 pressure,42 anisotropy43 or even exciton dimensionality.44 Resonant energy transfer can lead, in some cases, to the conversion of Mott-Wannier45 excitons into Frenkel46 excitons.47 However, only few studies have focused on energy or charge transfer inside highly confined 1D hybrid systems,48,49 and to the best of our knowledge, RET from PbCl wires has never been reported. Here, we report an experimental and theoretical investigation of structural and optical properties of a new 1D OIH material namely (C9H10N2)PbCl4 (abbreviated as AQPbCl4). Single crystals were synthesized by slow solvent evaporation method at room temperature. Crystal


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structure of AQPbCl4 has been determined by x-ray diffraction (XRD) and the optical properties have been investigated using optical absorption (OA) and luminescence spectroscopies. Experimental results were confirmed by theoretical electronic band structure calculations. EXPERIMENTAL METHODS Crystals synthesis and structure description. Single crystals of AQPbCl4 were synthesized by slow solvent evaporation at room temperature.50,51 First, the 3-aminiquimoline ammonium salt (N-C9H6-NH2).2HCl (abbreviated as AQ(HCl)2) was obtained using the reaction between 3aminiquimoline N-C9H6-NH2 with an aqueous solution of HCl (37%). Then, 1 mmol of the resulting AQ(HCl)2 and 1 mmol of PbCl2 with excess aqueous HCl solution (37%) were well stirred and kept in the dark at ambient temperature.52 Three days later, yellow platelets of AQPbCl4 were formed. Good quality single crystals of approximate dimensions of 0.40 x 0.35 x 0.01 mm3 were selected for the XRD investigations. Data collection was carried out, at 170 K, on a supernova 4-circle micro-source diffractometer (Oxford Diffraction) equipped with a two-dimensional ATLAS detector, and using graphite monochromatized MoKα radiation (λ= 0.71073 Å). The crystal structure was solved by the direct method and refined by the full matrix least-square technique using the SHELX-97 crystallographic software package.53 Details of crystal structure data are summarized in Table 1. AQPbCl4 crystallizes as an OIH material in the monoclinic space group P 21/n with a primitive unit cell of dimensions: a = 7.3244(2) Å, b = 11.8952(2) Å, c = 16.1425(4) Å and four formula units (Z = 4). The tridimensional view projection of the AQPbCl4 crystal structure is shown in Figure 1. It consists of a superposition of two sub-lattices: 1D [PbCl4]n2n- chains extended along


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the b-axis and separated by [C9H10N2]2+ organic cations. Each lead atom is bound to six chlorine atoms forming edge-sharing PbCl6 octahedra. Connections between organic and inorganic moieties are provided by charge-assisted weak hydrogen bonds between the -NH3+ terminals and Cl- ions and connections between the organic cations are provided by Van-Der-Walls interactions. The examination of the distance and bonds revealed an ionic character of the lead chloride bonds and a slight distortion of PbCl6 octahedra. Values of the Pb-Cl bond lengths vary between 2.703 Å and 3.088 Å, which is closer to the sum of the ionic radii of lead and chlorine atoms (ri = 1.19 + 1.81 = 3 Å) than to that of their covalent radii (rc = 1.47 + 0.99 = 2.46 Å), indicating that these bonds have a strong ionic character.54 In addition, the Pb-Cl-Pb angles range from 86.31° to 93.24 and those of Cl-Pb-Cl are in the range of 86.31°-93.24° and 170.8°-172.89° for the adjacent and opposite chlorine, respectively, showing the presence of a slight distortion in PbCl6 octahedra. Optical measurements. All optical measurements were performed on single crystals of AQPbCl4 and AQ(HCl)2. Photoluminescence (PL) measurements were performed using a JOBIN YVON HR 320 spectrophotometer under 266 nm and 375 nm excitation wavelength, and with 50 mW excitation power. For temperature dependence measurements, we have used a Janis closed cycle cryostat. Photoluminescence excitation (PLE) was recorded at room temperature on a Fluoromax-4 spectrofluorimeter equipped with a xenon lamp as an excitation source. Room temperature OA measurements were recorded, on the same samples, using a conventional UVvis spectrophotometer (HITACHI, U-3300). Electronic band structure calculations. We used the WIEN2K computer package to perform our calculations.55 The Kohn-Sham equations were solved using all-electron Full-Potential Linearized Augmented Plane Wave (FP-LAPW) method.56,57 The exchange and correlation


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effects were treated by the Becke-Johnson functional.58 In order to achieve a satisfactory degree of convergence of energy eigenvalues, the wave functions in the interstitial regions were expanded in plane wave up to R × K equal to 4 (where R is the minimum radius of the muffin-tin-spheres and K gives the magnitude of the largest k vector in the plane wave expansion). We have chosen the R for Pb, Cl, N, C and H to be 2.5, 2.6, 1.16, 1.25 and 0.54 atomic units (a.u), respectively. Inside the atomic muffin-tin-spheres, the valence wave functions were expanded up to L = 10. The dependence of the total energy on the number of k-points in the irreducible wedge of the first Brillouin zone (BZ) has been explored within the linearized tetrahedron scheme.59 The integrals over the BZ were performed up to 300 k-points and the size of the mesh has been set to 10 ∗ 10 ∗ 3 points. Self-consistency was achieved since the total energy difference between successive iterations was less than 10-5 Ryd per formula unit.


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Table 1. Crystallographic data for AQPbCl4 Empirical formula

(C9H10N2) PbCl4

Formula weight/gmol-1

513.21

Temperature (K)

170

Wavelength (Å)

0.71073

Crystal size (mm3)

0.40 x 0.35 x 0.01

Crystal system

P 21/n

Space group

Monoclinic

a /Å

7.3244(2)

b /Å

11.8952(2

c /Å

16.1425(4)

Volume /Å3

1406.32(6)

Z

4

Densitycalc / gcm3

2.424

Θ range /°

3.43-26.37

Limiting indices h, k, l

-9

Yellowish White-Light Emission Involving Resonant Energy Transfer in

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