Radiative and Nonradiative Lifetime Engineering of Quantum Dots in Multiple Solvents by Surface Atom Stoichiometry and Ligands

Abstract

CdTe quantum dots have unique characteristics that are promising for applications in photoluminescence, photovoltaics or optoelectronics. However, wide variations of the reported quantum yields exist and the influence of ligand-surface interactions that are expected to control the excited state relaxation processes remains unknown. It is important to thoroughly understand the fundamental principles underlying these relaxation processes to tailor the QDs properties to their application. Here, we systematically investigate the roles of the surface atoms, ligand functional groups and solvent on the radiative and non-radiative relaxation rates. Combining a systematic synthetic approach with X-ray photoelectron, quantitative FT-IR and time-resolved visible spectroscopies, we find that CdTe QDs can be engineered with average radiative lifetimes ranging from nanoseconds up to microseconds. The non-radiative lifetimes are anticorrelated to the radiative lifetimes, although they show much less variation. The density, nature and orientation of the ligand functional groups and the dielectric constant of the solvent play major roles in determining charge carrier trapping and excitonic relaxation pathways. These results are used to propose a coupled dependence between hole-trapping on Te atoms and strong ligand coupling, primarily via Cd atoms, that can be used to engineer both the radiative and non-radiative lifetimes.

Photophysical Properties of CdSe/CdS core/shell quantum dots with tunable surface composition

“…The nonradiative rate increase originates from the increase in the number mid-gap hole states. Similar studies have found comparable patterns in CdTe nanoparticles with increasing telluride concentration on surfaces 19 , where hole traps and less passivated telluride surfaces contribute to increased nonradiative rates. Further, selenium rich surfaces on CdSe cores also produce insufficiently passivated particle surfaces 54 and comparable changes in fluorescent lifetimes 21 …”

“…Due to their large surface to volume ratio, the surface of QDs has been historically recognized as integral in influencing the optical properties of colloidal QDs 6,[18][19][20][21][22][23][24][25][26] . For example, early synthetic breakthroughs that produced highly-fluorescent QDs were enabled, in part, by the discovery that phosphine ligands provided effective passivation of dangling bonds at the nanoparticle surface 27 .…”

Are Bidentate Ligands Really Better than Monodentate Ligands For Nanoparticles?

“…22 Briefly, ODA-QDs were precipitated from toluene by the addition of acetone (VWR), centrifuged at 1900g (4000rpm on a Clinical 50 centrifuge, VWR) and the supernatant discarded. DHLA or MPA was dissolved in methanol, and the solution adjusted to pH 10 by the addition of tetramethylammonium hydroxide pentahydrate (Alfa Aesar).…”

“…The surface atom ratio may indeed result in different affinities to ligand functional groups, as previously shown for core-only CdTe. 22 Quantifying this ratio for core-shell QDs is more difficult due to the lack of techniques that probe surface atoms without interference from internal atoms, but will lead to a more thorough understanding of non-specific binding mechanisms, and will be the focus of future studies.…”

Drastic difference between hole and electron injection through the gradient shell of CdxSeyZn1−xS1−y quantum dots

“…5,22,27 However, such ligand exchange can induce numerous surface states acting as hole traps. 22,28 Indeed, a drastic decrease in the PL QY of the core QDs was observed after ligand exchange due to the quenching by the surface hole traps. 16 However, the PL QY decreased significantly less if the shell is coated well around the core (see the inset in Fig.…”