Family of Chiral Ferroelectric Compounds with Widely Tunable Band Gaps

Abstract

Intense research activities have been exploring the possibility of harnessing solar energy via photovoltaic and photocatalytic applications of ferroelectric materials using the built-in electric field for an efficient separation of photoexcited charge carriers. However, one of the most important bottlenecks in these efforts is to find ferroelectricity in suitably low-band-gap materials for harvesting a sizable part of the solar spectrum, with most of the known ferroelectric materials having band gaps larger than 2.5 eV. In the present work, we show that the known chiral and polar compound, (R-/S-MBA) 2 CuCl 4 , with MBA = α-methyl benzylamine, is also ferroelectric and its ligand-to-metal charge transfer (LMCT) band gap (∼2.53 eV) can be systematically decreased via substitution of Cl − with Br − forming the solid solution (R-/S-MBA) 2 CuCl 4−x Br x . These compounds retain their chiral ferroelectric state until x = 2 and reach a significantly low band gap of ∼2.09 eV for (R-/S-MBA) 2 CuCl 2 Br 2 , which is the smallest band gap reported so far among layered hybrid ferroelectric materials. We elucidate the origin of the band gap reduction and other changes in the electronic structure with the help of state-of-the-art electronic structure calculations. Chiral ferroelectrics constitute an interesting class of materials, with ferroelectricity being able to discriminate between electron and hole charge transports, while chirality may have the ability to discriminate between up-and downspin transports.

Giant Anisotropic Thermal Expansion Phase Transition of Silver Iodide Anionic Organic–Inorganic Hybrid

“…As shown in the solid-state ultraviolet–visible spectra (Figure S5), both 1 and 2 exhibit a sharp absorption in the range from 200 to 350 nm with the absorption edge at 340 and 307 nm, respectively. The band gaps of 1 and 2 are calculated to be 3.66 and 4.14 eV respectively, which are higher than some inorganic semiconductors such as LiNbO 3 and BaTiO 3 . Under ultraviolet light excitation, 1 and 2 have no obvious visible luminescence emission at room temperature (Figure a).…”

Chirality-Tunable Nonlinear Hall Effect

“…Just as the enantiomers of a chiral molecule are related by a mirror reflection, enantiomorphic pairs of a chiral crystal are related by screw rotations. Chiral materials often exhibit distinct optical, electronic, or mechanical characteristics, making chirality an important consideration in the design and study of materials with applications ranging from pharmaceuticals to advanced materials. − In recent years, a number of intriguing aspects of chiral materials have been discovered, including interplay with topology, − phonons, − and ferroelectricity. − …”