Size and temperature dependence of the photoluminescence properties of NIR emitting ternary alloyed mercury cadmium telluride quantum dots

Exciton-phonon coupling and nonradiative relaxation processes have been investigated in near-infrared (NIR) emitting ternary alloyed mercury cadmium telluride (CdHgTe) quantum dots. Organically capped CdHgTe nanocrystals of sizes varying from 2.5-4.2 nm have been synthesized where emission is in the NIR region of 650-855 nm. Temperature-dependent (15-300 K) photoluminescence (PL) and the decay dynamics of PL at 300 K have been studied to understand the photophysical properties. The PL decay kinetics shows the transition from triexponential to biexponential on increasing the size of the quantom dots (QDs), informing the change in the distribution of the emitting states. The energy gap is found to be following the Varshni relation with a temperature coefficient of 2.1-2.8 x 10(-4) eV K-1. The strength of the electron-phonon coupling, which is reflected in the Huang and Rhys factor S, is found in the range of 1.17-1.68 for QDs with a size of 2.5-4.2 nm. The integrated PL intensity is nearly constant until 50 K, and slowly decreases up to 140 K, beyond which it decreases at a faster rate. The mechanism for PL quenching with temperature is attributed to the presence of nonradiative relaxation channels, where the excited carriers are thermally stimulated to the surface defect/trap states. At temperatures of different region (<140 K and 140-300 K), traps of low (13-25 meV) and high (65-140 meV) activation energies seem to be controlling the quenching of the PL emission. The broadening of emission linewidth is found to due to exciton-acoustic phonon scattering and exciton-longitudinal optical (LO) phonon coupling. The exciton-acoustic phonon scattering coefficient is found to be enhanced up to 55 MU eV K-1 due to a stronger confinement effect. These findings give insight into understanding the photophysical properties of CdHgTe QDs and pave the way for their possible applications in the fields of NIR photodetectors and other optoelectronic devices.

Stabilization of a Tetrahedral (Mn5+O4) Chromophore in Ternary Barium Oxides as a Strategy toward Development of New Turquoise/Green-Colored Pigments

An experimental investigation of the stabilization of the turquoise-colored chrornophore (Mn5+O4) in various oxide hosts, viz., A(3)(VO4)(2) (A = Ba, Sr, Ca), YVO4, and Ba2MO4 (M = Ti, Si), has been carried out. The results reveal that substitution of Mn5+O4 occurs in Ba-3(VO4)(2) forming the entire solid solution series Ba-3(V1-x MnxO4)(2) (0 < x <= 1.0), while with the corresponding strontium derivative, only up to about 10% of Mn5+O4 substitution is possible. Ca-3(VO4)(2) and YVO4 do not stabilize Mn5+O4 at all. With Ba2MO4 (M = Ti, Si), we could prepare only partially substituted materials, Ba2M1-xMn5+O4+x/2 for x up to 0.15, that are turquoise-colored. We rationalize the results that a large stabilization of the O 2p-valence band states occurs in the presence of the electropositive barium that renders the Mn5+ oxidation state accessible in oxoanion compounds containing PO43-, VO43-, etc. By way of proof-of-concept, we synthesized new turquoise-colored Mn5+O4 materials, Ba-5(BO3)(MnO4)(2)Cl and Ba-5(BO3)(PO4)(MnO4)Cl, based on the apatite-Ba-5(PO4)(3)Cl-structure.