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.
In this work, Cd-free, oleophilic (Cu)ZnInS(core)ÀZnS(shell) core/shell quantum dots (QDs) with varying Cu content in the core and shell thickness were synthesized by heating-up method (core) followed by hot-injection route (shell). Effect of Cu ion doping (in core) and shell growth on the structural and optical properties of (Cu)ZnInSÀZnS core/shell QDs is reported. Incorporation of Cu ions causes the crystalline phase transformation of zinc blende ZnInS ternary alloy to wurtzite CuÀZnÀInÀS quaternary alloy. The (Cu)ZnInS alloyed core QDs are encapsulated with a wider bandgap ZnS shell with gradient interface. The photoluminescence of (Cu)ZnInS core QDs exhibits wide tunable multicolor emissions from green (535 nm) to deep red (683 nm) with large Stokes shift. After the growth of ZnS layer, the PL peak position and absorption edge undergo a hypsochromic shift with enhanced photoluminescence quantum yield up to 47.31%, which is due to the partial cation exchange of Cu + and In 3 + with Zn 2 + , yielding a graded core/shell structure with smooth potential interface. The radiative excited state lifetime is lengthened from 22.91 ns to 291.11 ns. The mercaptoundecanoic acid (MUA) functionalized QDs have been successfully used for in vitro fluorescence cellular imaging of HEK-293 and HeLa cells. These QDs are cell permeable and suitable for visible light induced multicolor fluorescence-based cellular imaging.[a] J.
Development of nontoxic quantum dots (QDs) exhibiting superior photoluminescence properties is a major challenge in the commercialization of QDs-based solid-state lighting or display device applications. This work reports the synthesis of heavy-metal-free Ag:Zn−In−S/ZnS (Ag:ZIS/ZS) core/shell QDs via the heating-up method followed by a nonaqueous hot-injection route. Formation of the QDs, their growth, and structural qualities were studied by high-resolution transmission electron microscopy and XRD analysis. The optical properties of these QDs were investigated through absorption, photoluminescence, and time-resolved fluorescence decay spectroscopic measurements. Surface passivation of the core Ag:ZIS QDs was done by coating ZnS layers on them. The successful doping of Ag + into the ZnInS host material changed the emission characteristics; the influence of Ag loading and the shell thickness on the optical features were studied. The shell thickness had a strong impact on the photophysics of the nanocrystal with enhanced radiative lifetime. By regulating the Ag concentration, the Ag:ZIS core QDs showed a wide ranging tunable emission from 392 to 547 nm with a prominent blue shift with the introduction of ZnS shell layers. The blue shift in the band gap energy and emission wavelength has been reported to be due to the cation-exchange process of Ag:ZIS/ZS core/shell QDs. The radiative excited state lifetime is prolonged with subsequent injection of shells with the highest reported value being 504.72 ns. These nontoxic core/shell QDs, endowed with notable spectral emission within the visible spectrum, can be used as a down-converting material by tailoring the dopant concentration and shell precursor injection. Furthermore, a nanocomposite film of PMMA−QDs was synthesized by the drop-casting method, and its overall luminous efficacy complements that of parental core/shell QDs without quenching. The transmittance plot delineates that the optical transparency of the film can be engineered over the entire visible spectrum by varying the dopant (Ag) concentration.
Dot-in-dot core/shell Mn:ZnS/ZnS QDs as a good fluorescent agent for bioimaging and a turn-off fluorescent probe for detection of heavy metal ions.
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