We present a one-pot
synthesis of nanocrystal heterostructures containing metal selenide
cores shelled with tungsten-based metal selenides. This synthesis
is enabled by the use of oleic acid as a W-coordinating ligand, which
inhibits W reactivity and allows for the formation of core nanocrystals
prior to W-based selenide secondary growth. As a case study, we show
that high amounts of oleic acid lead to slower, edge-preferred growth
of WSe2 on Ni3Se4, whereas lower
amounts of oleic acid lead to uninhibited growth of WSe2, fully covering the Ni3Se4 nanocrystals. The
Ni3Se4 nanocrystal cores can readily be removed
to access hollow WSe2 nanocrystals. The manipulation of
W-precursor reactivity can be extended to form Co3Se4/WSe2, Cu2–x
Se/Cu2WSe4, and Cu2WSe4/WSe2 core/shell heterostructures. These results demonstrate
the exploitation of ligand coordination to enable easy, solution-phase
syntheses of exotic or complex nanostructures.
Coincident photon histogram measurements of fluorescence antibunching via confocal microscopy correlated with atomic force microscopy were carried out on (i) individual CdSe/ZnS core/shell quantum dots (QDs), (ii) several well separated QDs, and (iii) clusters of QDs. Individual QDs and well separated QDs showed the expected degree of antibunching for a single emitter and several independent emitters, respectively. The degree of antibunching in small, compact clusters was more characteristic of a single emitter than multiple emitters. The antibunching in clusters provides strong evidence of nonradiative energy transfer between QDs in a cluster. A minimal phenomenological model of energy transfer gives reasonable quantitative agreement with the experimental results.
We report a low-temperature colloidal synthesis of WSe2 nanocrystals from tungsten hexacarbonyl and diphenyl diselenide in trioctylphosphine oxide (TOPO). We identify TOPO-substituted intermediates, W(CO)5TOPO and cis-W(CO)4(TOPO)2 by infrared spectroscopy. To confirm these assignments, we synthesize aryl analogues of phosphine-oxide-substituted intermediates, W(CO)5TPPO (synthesized previously, TPPO = triphenylphosphine oxide) and cis-W(CO)4(TPPO)2 and fac-W(CO)3(TPPO)3 (new structures reported herein). Ligation of the tungsten carbonyl by either the alkyl or aryl phosphine oxides results in facile labilization of the remaining CO, enabling low-temperature decomposition to nucleate WSe2 nanocrystals. The reactivity in phosphine oxides is contrasted with syntheses containing phosphine ligands, where substitution results in decreased CO labilization and higher temperatures are required to induce nanocrystal nucleation.
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