Figure 1. QDs coated with tri-n-octyl phosphine oxide (tri) and mercaptoacetic acid (mer) under (a) ambient and (b) ultraviolet illumination. The upper layer is water; the lower layer is CCL4.
A range of alloyed QDs were examined via absorption and photoluminescence spectroscopy (Figure 2). Comparative literature values for bulk alloys are included (3). The data reveal resolved electronic transitions, plus fluorescence emission at the band-edge. Note the unexpected depression in band-gap for all nanoparticle sizes at about 60% Te. The generally successful Vegard's law (4) for thin-film and bulk alloys is linear,
Ealloy = xEA + (1 - x)EB
where x = mole fraction, and EA, EB and Ealloy are the band-gaps for pure materials A, B and alloy of A and B respectively. Vegard's law, however, is only a first approximation, and others (5) have found this "optical bowing" in bulk CdSeTe, so this effect is not solely caused by quantum confinement.
Figure 2. Composition versus absorpotion and emission energies for CdSe1-xTex nanoparticles. (a) Absorption and photoluminescence of CDSe0.34Te0.66 QDs; (b) absorption-energy onset related to Te content; (c) emission peak-wavelength versus Te content.
Zunger et al. (6,7) suggest the observed effects arise because
of: (a) the various ionic sizes in the alloy; (b) the various electronegativities