Near-infrared optical characteristics of chalcogenide-bound Nd3+ molecules and clusters

摘要

The optical properties of the nanoscale neodymium ceramic cluster (THF)(8)Nd8O2Se2(SePh)(16) (Nd-8) and molecular (DME)(2)Nd(SC6F5)(3) (Nd1) were studied by optical absorption, photoluminescence, and time-resolved spectroscopy. Both complexes exhibited emission characteristic of solid-state materials with bands centered at 927, 1078, 1360, and 1843 nm for Nd8 and 897, 1071, 1347, and 1824 nm for Nd1. The observed red-shift in the absorption and emission bands of Nd8 is attributed to the increased covalency and nephelauxetic effect. Using the calculated radiative decay time, the quantum efficiency of the F-4(3/2) -> I-4(1/2) transition is calculated to be 16% in Nd8 and 9% in Nd1 with corresponding stimulated emission cross sections of 3.04 x 10(-20) cm(2) in Nd8 and 1.61 x 10(-20) cm(2) in Nd1 that are comparable to those of solid-state inorganic systems. This efficiency is the highest reported value for "molecular" neodymium compounds. This finding, along with the novel 1.8 mu m emission, is attributed to the absence of direct Nd3+ coordination with fluorescence quenching vibrational groups such as hydrocarbon or hydroxide groups. The direct coordination of S, Se, and F accounts for the improved fluorescence spectral properties, because these heavy anions facilitate a low phonon energy host environment for neodymium. Monte Carlo simulation permitted analysis of energy transfer processes to show the primary source of fluorescence quenching. Cross relaxation is responsible for the quenching of the F-4(3/2) -> 4I(15/2) emission whereas excitation migration quenches the 4 F3/2 -> 4 I-9/2 emission. These processes are mediated by a dipole-dipole interaction for Nd8 and a quadrupole-quadrupole interaction for Nd1.