Alkali Triel Chalcogenide Nanocrystals: A Molecular Reactivity Approach to Ternary Phase Selectivity

Abstract

Alkali-metal based materials are promising building blocks for energy conversion and storage technologies. Here, we use a molecular reactivity-based, solution-phase approach to selectively synthesize multiple phases and specific polymorphs of lithium- and sodium-containing triel chalcogenide nanocrystals (LiTrCh2, NaTrCh2, NaTr3Ch5; Tr = Ga, In; Ch = S, Se, Te). Analogous to the case of binary II-VI and III-V tetrahedral semiconductors, where the two commonly isolated zinc blende and wurtzite polymorphs are separated by only 1-50 meV/f.u., we find that LiTrCh2 nanocrystals easily adopt tetragonal (chalcopyrite) and orthorhombic polymorphs separated by only 2.7-6.2 meV/f.u.. Because of this small energy difference, soft colloidal synthesis succeeds in accessing either one of these polymorphs depending on the specific dichalcogenide precursor used. Highly reactive diethyl diselenide favors the thermodynamic, more stable tetragonal I4 @#x0305;2d phase, whereas mildly reactive diphenyl diselenide favors the kinetic, metastable orthorhomic Pna21 phase. Density functional theory calculations confirm the relative energy among multiple LiTrCh2 polymorphs, and also model the observed powder X-ray diffraction pattern of a new C2 NaIn3Te5 phase. 7Li, 69Ga, and 77Se solid-state NMR spectra are consistent with phase pure, ternary LiGaSe2 nanocrystals. A majority of the nanocrystal compositions are visible light emitters. This work opens the door to new Li/Na-based ternary triel chalcogenide nanostructures for energy storage and conversion applications.