Three different types of water-soluble Cd 1Àx Zn x S quantum dots (QDs) having nearly identical sizes and compositions have been synthesized via simple and low-cost methods to understand the effect of internal structures on the optical properties of QDs. Compared with the other two types, CdS@ZnS core-shell Cd 1Àx Zn x S QDs and alloy Cd 1Àx Zn x S QDs, composition-gradient CdS@ZnS core-shell Cd 1Àx Zn x S (G-Cd 1Àx Zn x S) QDs, which have been prepared by exchanging the Cd 2+ ions of CdS QDs partially with Zn 2+ ions conserving the shapes and sizes, have shown the longest lifetime and the highest quantum yield of photoluminescence due to the smallest nonradiative and the largest radiative decay constants of photogenerated charge carriers. The composition-gradient ZnS shells, which passivate the CdS cores optimally alleviating the lattice strain caused by the lattice mismatches between the CdS cores and the ZnS shells, have been considered to be the main reason for the enhanced optical properties of G-Cd 1Àx Zn x S QDs. Among our prepared G-Cd 1Àx Zn x S QDs, the quantum yield and the lifetime of photoluminescence are the highest (22%) and the longest (290 ns), respectively, due to the smallest nonradiative decay constant when about half of the Cd 2+ ions in CdS QDs are replaced by Zn 2+ ions with composition gradients from their surfaces, suggesting that internal structures play an important role in the relaxation dynamics of photoexcited charge carriers.

Various types of 2% Cu-incorporated CdS (Cu:CdS) quantum dots (QDs) with very similar sizes have been prepared via a water soluble colloidal method. The locations of Cu impurities in CdS host nanocrystals have been controlled by adopting three different synthetic ways of doping, exchange, and adsorption to understand the impurity location-dependent relaxation dynamics of charge carriers. The oxidation state of incorporated Cu impurities has been found to be +1 and the band-gap energy of Cu:CdS QDs decreases as Cu2S forms at the surfaces of CdS QDs. Broad and red-shifted emission with a large Stokes shift has been observed for Cu:CdS QDs as newly produced Cu-related defects become luminescent centers. The energetically favored hole trapping of thiol molecules, as well as the local environment, inhibits the radiative recombination processes of Cu:CdS QDs, thus resulting in low photoluminescence. Upon excitation, an electron is promoted to the conduction band, leaving a hole on the valence band. The hole is transferred to the Cu+ d-state, changing Cu+ into Cu2+, which then participates in radiative recombination with an electron. Electrons in the conduction band are ensnared into shallow-trap sites within 52 ns. The electrons can be further captured on the time scale of 260 ns into deep-trap sites, where electrons recombine with holes in 820 ns. Our in-depth analysis of carrier relaxation has shown that the possibilities of both nonradiative recombination and energy transfer to Cu impurities become high when Cu ions are located at the surface of CdS QDs.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-017-1832-3) contains supplementary material, which is available to authorized users.

Water plays a crucial role in many principal biological phenomena such as enzymatic catalysis and proton pumping through a membrane protein channel.1-4 Moreover, in biological systems, water is usually contained in a small pocket of a membrane, 5-8 and such confined water, which is generally called a water nanopool, [8][9][10] shows peculiar properties differing considerably from the properties of bulk water. 8-16The confinement effect and the enclosing interfacial surfaces of waterpools are the main factors to determine the properties, such as polarity, viscosity, and H-bonding ability, of water nanopools.5-13 For example, the dielectric constant of a water nanopool has been reported to be much lower than that of bulk water (ε = 78.5)17 but similar to that of an alcohol (ε = 30-40).18,19 On the other hand, biological processes often take place based on proton relay along a hydrogen (H)-bonded chain, [1][2][3][4][20][21][22][23][24] and the dynamics of biological proton relay is determined by the size, the structure, and the motion of a water cluster which is the prime agent in most of biological systems. 3,[13][14][15][24][25][26] Thus, for better understanding of cellular dynamics, it is necessary to investigate the properties of a biologically relevant water nanopool as a biomimetic system of water confined in a cell membrane. In this regard, water nanopools confined in reverse micelles, which are formed by surfactant molecules having polar headgroups pointing inward and dispersed in hydrocarbon solvents, can be good model systems of biological water. 6,18,19,27 It is a unique feature of reverse micelles that they can make nonpolar media to solubilize a large amount of water by encapsulating water molecules in their inner polar cores; 28 reverse micelles are surrounded by a layer of surfactant molecules such as Aerosol-OT (sodium 1,4-bis-2-ethylhexylsulfosuccinate, AOT) and immersed in a nonpolar solvent, so that nanometer-sized droplets of a polar solvent such as water are formed inside. The polar headgroups of surfactant molecules point inward toward a polar solvent pool, and the alkyl chains of the surfactant molecules point outward toward a nonpolar solvent. About 20 surfactant molecules of AOT form a reverse micelle having a diameter of 3.0 nm above the critical concentration of 1 mM in a hydrocarbon solvent such as n-heptane.29 By adding water to the AOT solution, microemulsions of nanometer-sized water droplets surrounded by AOT molecules are formed, and the size of water nanopools confined in AOT reverse micelles increases as the concentration of water increases. In n-heptane, the diameters in nanometers of the waterpools have been reported to be about 0.3w 0 , in which w 0 is the molar ratio of water to AOT. 30The catalytic properties of water nanopools depend strongly on the hydration extent of reverse micelles and on the solvation structure of reactants relevant to the polarity of waterpools confined in reverse micelles. [5][6][7][8]17,18 The peculiar structure of waterpools contained in reverse mi...