Synthesis, Crystal and Electronic Structures, and Optical Properties of (CH₃NH₃)₂CdX₄ (X = Cl, Br, I)

Abstract

We report the synthesis, crystal and electronic structures, as well as optical properties of the hybrid organic-inorganic compounds MACdX (MA = CHNH; X = Cl, Br, I). MACdI is a new compound, whereas, for MACdCl and MACdBr, structural investigations have already been conducted but electronic structures and optical properties are reported here for the first time. Single crystals were grown through slow evaporation of MACdX solutions with optimized conditions yielding mm-sized colorless (X = Cl, Br) and pale yellow (X = I) crystals. Single crystal and variable temperature powder X-ray diffraction measurements suggest that MACdCl forms a 2D layered perovskite structure and has two structural transitions at 283 and 173 K. In contrast, MACdBr and MACdI adopt 0D KSO-derived crystal structures based on isolated CdX tetrahedra and show no phase transitions down to 20 K. The contrasting crystal structures and chemical compositions in the MACdX family impact their air stabilities, investigated for the first time in this work; MACdCl is air-stable, whereas MACdBr and MACdI partially decompose when left in air. Optical absorption measurements suggest that MACdX have large optical band gaps above 3.9 eV. Room temperature photoluminescence spectra of MACdX yield broad peaks in the 375-955 nm range with full width at half-maximum values up to 208 nm. These PL peaks are tentatively assigned to self-trapped excitons in MACdX following the crystal and electronic structure considerations. The bands around the Fermi level have small dispersions, which is indicative of high charge localization with significant exciton binding energies in MACdX. On the basis of our combined experimental and computational results, MACdX and related compounds may be of interest for white-light-emitting phosphors and scintillator applications.

Broadband Emission in Hybrid Organic–Inorganic Halides of Group 12 Metals

“…These peaks were assigned to the excitonic absorption. 37 Similar to the earlier reports on Cd- and Pb-based hybrid halide perovskites, 12,19−21,38−40 all compounds studied in this work exhibit broadband emission spectra with full width at half maximum (FWHM) values ranging from 162 nm for (R)CdI 3 (DMSO) to 188 nm for (R)ZnBr 3 (DMSO). Importantly, bright broadband white light emission is observed for (R)ZnBr 3 (DMSO).…”

“…A direct comparison with PLQY values for other hybrid halides of group 12 metals cannot be made because of the apparent lack of in-depth investigations of their light emission properties. However, our recent study on (CH 3 NH 3 ) 2 CdX 4 (X = Cl, Br, or I) 19 reports that these compounds do not luminesce at room temperature, which seems to confirm the validity of the strategy of using bulky luminescent organic cations to create 0D structures with molecular organic and inorganic ions to prepare room-temperature luminescent hybrid halides, as employed in this work.…”

“…These values compare well with those reported for other hybrid organic–inorganic halides of Zn and Cd, including [4,4′-H 2 bipy][ZnBr 4 ] ( d Zn–Br = 2.39–2.43 Å), 29 (CH 3 NH 3 ) 2 CdBr 4 ( d Cd–Br = 2.57–2.59 Å), and (CH 3 NH 3 ) 2 CdI 4 ( d Cd–I = 2.72–2.75 Å). 19 In all compounds, the tetrahedral coordination around the metal cations are distorted, with bond angles ranging from 98.97(19)° to 118.37(3)° in (R)CdI 3 (DMSO) (Table S2). It should be noted that the presence of DMSO in the coordination sphere of Cd 2+ undoubtedly contributes to the observed distortion in (R)CdI 3 (DMSO); however, tetrahedral distortions are also observed for cadmium halides with no coordinating solvent molecules including (R) 2 CdBr 4 ·DMSO, which features CdBr 4 2– units, and in literature, for (CH 3 NH 3 ) 2 CdBr 4 and (CH 3 NH 3 ) 2 CdI 4 .…”

“…It should be noted that the presence of DMSO in the coordination sphere of Cd 2+ undoubtedly contributes to the observed distortion in (R)CdI 3 (DMSO); however, tetrahedral distortions are also observed for cadmium halides with no coordinating solvent molecules including (R) 2 CdBr 4 ·DMSO, which features CdBr 4 2– units, and in literature, for (CH 3 NH 3 ) 2 CdBr 4 and (CH 3 NH 3 ) 2 CdI 4 . 19…”

“…1,17,18 Conversely, low-dimensional hybrid halides demonstrate much stronger charge localization and exciton binding energies in excess of 500 meV, values that are comparable to those of tightly bound Frenkel excitons in organic materials. 1,19 Such high exciton binding energies can accordingly result in room-temperature stable excitons and intense excitonic emission. Therefore, preparation and characterization of low-dimensional organic–inorganic hybrid halides for light emission applications have been the focus of several studies published in recent years.…”