Alkali triel chalcogenide nanocrystals: a molecular reactivity approach to ternary phase selectivity

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, and NaTr₃Ch₅, where Tr = Ga and In and Ch = S, Se, and 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.⁻¹, we find that LiTrCh₂ nanocrystals easily adopt tetragonal (chalcopyrite) and orthorhombic polymorphs separated by only 2.7–6.2 meV f.u.⁻¹ 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 thermodynamically more stable tetragonal I$\bar{4}2$d phase, whereas mildly reactive diphenyl diselenide favors the kinetic, metastable orthorhombic Pna2₁ phase. Density functional theory calculations confirm the relative energies 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.. Authors: Pavel, Md Riad Sarkar [Iowa State Univ., Ames, IA (United States)]; Santhiran, Anuluxan [Iowa State Univ., Ames, IA (United States); Ames Laboratory (AMES), Ames, IA (United States)] (ORCID:000000025067157X); Dalberg, Seth [Iowa State Univ., Ames, IA (United States)]; Rossini, Aaron J. [Iowa State Univ., Ames, IA (United States); Ames Laboratory (AMES), Ames, IA (United States)] (ORCID:0000000216799203); Vela, Javier [Iowa State Univ., Ames, IA (United States); Ames Laboratory (AMES), Ames, IA (United States)] (ORCID:0000000151246893). DOE Contract: AC02-07CH11358