Molten-Salt-Assisted Annealing for Making Colloidal ZnGa2 O4 :Cr Nanocrystals with High Persistent Luminescence.

Persistent luminescent nanocrystals (PLNCs) in the sub-10 nm domain are considered to be the most fascinating inventions in lighting technology owing to their excellent performance in anti-counterfeiting, luminous paints, bioimaging, security applications, etc. Further improvement of persistent luminescence (PersL) intensity and lifetime is needed to achieve the desired success of PLNCs while keeping the uniform sub-10 nm size. In this work, the concept of molten salt confinement to thermally anneal as-synthesized ZnGa2 O4 :Cr3+ (ZGOC) colloidal NCs (CNCs) in a molten salt medium at 650 °C is introduced. This method led to significantly monodispersed and few agglomerated NCs with a much improved photoluminescence (PL) and PersL intensity without much growth in the size of the pristine CNCs. Other strategies such as i) thermal annealing, ii) overcoating, and iii) the core-shell strategy have also been tried to improve PL and PersL but did not improve them simultaneously. Moreover, directly annealing the CNCs in air without the assistance of molten salt could significantly improve both PL and PersL but led to particle heterogeneity and aggregation, which are highly unsuitable for in vivo imaging. We believe this work provides a novel strategy to design PLNCs with high PL intensity and long PersL duration without losing their nanostructural characteristics, water dispersibility and biocompatibility.

Bright persistent green emitting water-dispersible Zn2GeO4:Mn nanorods.

Luminescent materials with bright and persistent emission have been attracting significant attention from the scientific community owing to their tremendous potential in the area of bioimaging, solid state lighting, and security applications. Moreover, when they are in a nanodomain and are water dispersible, their potential for in vivo and in vitro imaging is further increased. Keeping this in mind, we have synthesized water dispersible nanorods (NRs) of Zn2GeO4 (ZGO) and Mn2+-doped Zn2GeO4 (ZGOM) using the hydrothermal method. For the ZGOM NRs, we have also optimized the dopant Mn2+ concentration, the pH value of the reaction medium, and the precursor Zn/Ge molar ratio. The ZGO NRs emit a bright bluish white (cool-white) emission under UV irradiation which is ascribed to the various kinds of defects present in NRs. On the other hand, the ZGOM NRs display a highly bright green luminescence under UV irradiation with a quantum yield (QY) of 52%. The best emission output was observed with a dopant concentration of 2.0 mol% Mn2+, pH of 10.5, and Zn : Ge = 1 : 2. This fact suggests that zinc deficiency and excess of germanium lead to enhancement in emission intensity. More importantly, the undoped ZGO NRs could not display any kind of persistent luminescence, whereas the ZGOM NRs show excellent green afterglow properties (even visible with the naked eye) upon irradiation with 254 nm UV light. Such water dispersible nanocrystals with high QY and afterglow properties offer great potential for developing cost-effective and environmentally benign phosphors for multifunctional applications in society.

Persistent luminescent sub-10 nm Cr doped ZnGa2O4 nanoparticles by a biphasic synthesis route.

This communication highlights a simple and facile biphasic synthesis of sub-10 nm Cr doped ZnGa2O4 nanoparticles (NPs) for the first time. These smallest Cr:ZnGa2O4 NPs demonstrate stable persistent luminescence emission more than 40 min after excitation. This synthesis strategy not only enables the controlled synthesis of these mixed metal oxide NPs unprecedentedly with smallest size to date but also allows them to be solution processable, which is advantageous for relevant applications with feasible and economic device fabrication.

Tuning the emission colors of semiconductor nanocrystals beyond their bandgap tunability: all in the dope.

Adopting the concept of one dopant for one color, all the prominent emitting colors in the visible windows are obtained by doping selective dopants (Ag, Cu, Ni, and Cu) in an appropriate host (alloy of Cdx Zn1-x S) with fixed size/composition and bandgap. Analyzing the origin of these emissions the relative position of respective dopant states are correlated.

Multifunctional Doped Semiconductor Nanocrystals.

Multifunctional nanomaterials with combined magnetic and optical properties remain one of the most demanded materials in upcoming research. To obtain these materials, we report here several doped semiconductor nanocrystals that simultaneously show tunable emission in a visible and NIR spectral window, above-room-temperature ferromagnetism, and improved conductivity. These nanocrystals are designed by inserting Ni(II) as a dopant in various semiconducting hosts with binary, alloyed, and ternary composition, and the induced multifunctional properties are observed to be stable and reproducible. These semiconducting materials combined with fluorescence and magnetic properties would be useful for a wide range of applications spanning from life science to modern developing device technology.

Doping Cu in semiconductor nanocrystals: some old and some new physical insights.

Cu-doped inorganic semiconductors with concomitant optical properties have garnered enormous research interest in the last two decades. However, uncertainties over the origin of Cu emission, its oxidation state, resemblance with trap state emission, position of Cu d-state, emission spectral width, and moreover understanding of the doping mechanism restricted the wide development of the synthetic methodology for high-quality Cu-doped nanocrystals. It has been shown recently that the emission from Cu-doped semiconductor nanocrystals can span over a wide spectral window and could be a potential color tunable dispersed nanocrystal emitter. Herein, we report the size and composition of variable Cu-doped ZnS/Zn(1−x)Cd(x)S zinc-blende (ZB) surface alloyed nanocrystals with intense, stable, and tunable emission covering the blue to red end of the visible spectrum. Further, the Cu dopant emission is distinguished from trap state emission, and the composition variable spectral broadening has been justified on the account of a different environment around the Cu ions in the host lattice. Whereas some findings are in agreement with past reports, several new physical insights presented here would help the community for an in-depth mechanistic study on Cu doping. Moreover, these doped nanocrystal emitters can be a promising candidate for application ranging from optoelectronics to bio-labeling.

Prevention of photooxidation in blue-green emitting Cu doped ZnSe nanocrystals.

This communication highlights unstable blue-green emitting Cu doped ZnSe nanocrystals stabilized by diluting the surface Se with a calculated amount of S.