Keywords: molybdenum disulfides, growth mechanism, chemical vapor deposition, transmission electron microscopy Understanding the microscopic mechanism of chemical vapor deposition (CVD) growth of two-dimensional molybdenum disulfide (2D MoS2) is a fundamental issue towards the function-oriented controlled growth. In this work, we report results on revealing the growth kinetics of 2D MoS2 via capturing the nucleation seed, evolution morphology, edge structure and terminations at the atomic scale during CVD growth using the transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) studies. The direct growth of few-and mono-layer MoS2 onto graphene based TEM grids allow us to perform the subsequent TEM characterization without any solution-based transfer. Two forms of seeding centers are observed during 2 characterizations: (i) Mo-oxysulfide (MoOxS2-y) nanoparticles either in multi-shelled fullerene-like structures or in compact nanocrystals for the growth of fewer-layer MoS2;(ii) Mo-S atomic clusters in case of monolayer MoS2. In particular, for the monolayer case, at the early stage growth, the morphology appears in irregular polygon shape comprised with two primary edge terminations: S-Mo Klein edge and Mo zigzag edge, approximately in equal numbers, while as the growth proceeds, the morphology further evolves into near-triangle shape in which Mo zigzag edge predominates. Results from density-functional theory calculations are also consistent with the inferred growth kinetics, and thus supportive to the growth mechanism we proposed. In general, the growth mechanisms found here should also be applicable in other 2D materials, such as MoSe2, WS2 and WSe2 etc.
Phosphine-free, highly
luminescent one-dimensional Mn2+ ion-doped ZnSe(core)/ZnS(shell)
nanorods (NRs) were synthesized
by heating-up method (core) followed by hot injection route (shell).
Effect of Mn2+ doping and shell thickness on structural
and optical properties is reported. The NRs were formed with wurtzite-structured
Mn:ZnSe core (diameter 2.52 nm) encapsulated epitaxially by a wurtzite-structured
ZnS shell (thickness of 3.51 nm) with 3.1% lattice mismatch that alters
the band alignment of the overall core–shell structure. A redshift
was observed in optical absorption and photoluminescence (PL) emission
due to an overall size increase with increasing shell thickness. Because
of the reduction of defects/traps by surface passivation, the maximum
photoluminescence quantum yield (QY) was obtained to be 49.35%. The
exciton radiative lifetime for the core–shell NRs (1.678 ms)
was more prolonged than that of the core (0.573 ms). A clear dependence
of QY and lifetime was established on the Mn2+ content
and ZnS shell thickness. The core–shell structure with thickest
shell showed good photostability. Water solubility was achieved by
ligand exchange with bifunctional 11-mercaptoundecanoic acid without
modifying the optical and microstructural properties. These core–shell
NRs were successfully tested for bioimaging of human HEK293 and HeLa
cells with good permeability to cells. Toxicity was observed to be
3% for the 100 μg/mL dose.
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