Alkaline-Earth Chalcogenide Nanocrystals: Solution-Phase Synthesis, Surface Chemistry, and Stability

Abstract

Increasing demand for effective energy conversion materials and devices has renewed interest in semiconductors comprised of earth-abundant and biocompatible elements. Alkaline-earth sulfides doped with rare earth ions are versatile optical materials. However, relatively little is known about controlling the dimensionality, surface chemistry, and inherent optical properties of the undoped versions of alkaline-earth mono- and polychalcogenides. We describe the colloidal synthesis of alkaline-earth chalcogenide nanocrystals through the reaction of metal carboxylates with carbon disulfide or selenourea. Systematic exploration of the synthetic phase space allows us to tune particle sizes over a wide range using a mixture of commercially available carboxylate precursors. Solid-state NMR spectroscopy confirms the phase purity of the selenide compositions. Surface characterization reveals that bridging carboxylates and amines preferentially terminate the surface of the nanocrystals. While these materials are colloidally stable in the mother solution, the selenides are susceptible to oxidation over time, eventually degrading to selenium metal through polyselenide intermediates. As part of these investigations, we have developed the colloidal syntheses of barium di- and triselenides, two among few reported nanocrystalline alkaline-earth polychalcogenides. Electronic structure calculations reveal that both materials are indirect band gap semiconductors. The colloidal chemistry presented here may enable the synthesis of more complex, multinary chalcogenide materials containing alkaline-earth elements.

Solution-Phase Synthesis and Photoluminescence of Quaternary Chalcohalide Semiconductors

“…The remaining supercells display much narrower band gaps, which underestimate the experimental value by 50–75%. Considering both the degree of band gap underestimation often observed using TB-LMTO − and the relative total energies, these findings suggest that the Cmcm Pb 2 SbS 2 I 3 polymorph most likely adopts the staggered coloring pattern. Because different Pb/Sb coloring patterns appear to affect the calculated band gap, it may be possible to achieve band gap tunability by controlling the extent of crystallographic disorder in Pb 2 SbS 2 I 3 .…”

Colloidal Synthesis of Multinary Alkali-Metal Chalcogenides Containing Bi and Sb: An Emerging Class of I–V–VI2 Nanocrystals with Tunable Composition and Interesting Properties

“…36 The surface-bound oleylamine exhibits its characteristic peaks at ∼1150 cm −1 (C−N stretching), 1650 cm −1 (N−H bending), and a broad stretching band around ∼3250 cm −1 coming from NH 2 stretching. 42,43 Additionally, the C−S stretching frequencies between 1050 and 650 cm −1 could arise from a surface-bound alkane thiol−selenium complex formed upon the reaction of thiol−amine solution with Se, as per equation 2 mentioned in Figure S16. 44,45 Further, from the XPS analysis of NaBiSe 2 , NaBi 0.5 Sb 0.5 Se 2 , and NaSbSe 2 NCs, the elemental composition and surface chemistry of the NCs are corroborated (Figure 6).…”

Redox Reactions at Colloidal Semiconductor Nanocrystal Surfaces

“…In recent years, dynamic nuclear polarization (DNP) enhanced MAS NMR spectroscopy has been used to overcome the sensitivity issues of MAS NMR methods. , Transfer of spin polarization from added stable organic radicals to the nuclear spin enhances the measurement up to 100-fold, resulting in shorter measurement times by 2–4 orders of magnitude and experiment times of tens of minutes. , Importantly, DNP enhanced NMR increases the intensity of resonances corresponding to nuclei at the NC surface. Studies by Kovalenko, Rossini, and others of CdSe NCs have been able to reveal the coordination environment of core and surface Cd 2+ and Se 2– centers. ,, These methods have also been applied toward other materials such as lead halide perovskite films and alkaline earth chalcogenide NCs …”

“…161,163,164 These methods have also been applied toward other materials such as lead halide perovskite films 165 and alkaline earth chalcogenide NCs. 166 DNP surface enhanced MAS NMR methods will likely soon provide opportunities to characterize NC surface redox chemistry. While these methods are not fast enough to be used yet for in situ measurement of chemical reactions, (i.e., seconds or faster), it would still be possible to induce surface changes via chemical, photochemical, or electrochemical treatment prior to NMR measurement.…”