Reaction controlled growth with formic acid for high-quality Cs 3 Cu 2 I 5 single crystals

The recent emergence of low-dimensional copper-based halide perovskite Cs 3 Cu 2 I 5 , for use in optoelectronics and radiation detection, has evoked considerable interest. Presently, Cs 3 Cu 2 I 5 single crystals grown using solution methods barely have clearly defined crystal planes and the crystal quality remains inferior. In this work, we have achieved high-quality Cs 3 Cu 2 I 5 single crystals featuring prominent crystal habit planes through reducing the growth rate by controlling the concentration of I − ions via the addition of formic acid. The slow reaction between I 3 − and formic acid at a lower growth temperature can effectively control the concentration of I − ions in the solution, thereby reducing the growth rate and extending the metastable growth range simultaneously. This optimized process can yield high-quality single crystals, with a reduced full width at half maxima (FWHM) of 0.047° for the X-ray rocking curve on the (002) natural crystal habit plane, a substantially improved energy resolution of 7.1% for a photon energy of 511 keV, and an increased absolute light yield to 39 000 photons per MeV. Our study demonstrates that the reaction-controlled growth may be an effective way to improve the crystal quality of the copper-based halide perovskite.