Size-Tunable, Bright, and Stable PbS Quantum Dots: A Surface Chemistry Study

Abstract

PbS Qdots are synthesized using PbCl2 and elemental sulfur as precursors. The available size range is significantly expanded using tri-n-octylphosphine (TOP), enabling the synthesis of monodisperse suspensions of Qdots with a mean size varying between 3 and 10 nm. The ligand composition and dynamics are investigated with nuclear magnetic resonance (NMR) spectroscopy. We show that the Qdots are passivated solely by highly dynamic OlAm ligands, even when TOP is employed during synthesis. In this respect, TOP is a compound strongly modifying the Qdot synthesis, without affecting the final Qdot surface chemistry. Next, the OlAm ligands are exchanged for oleic acid (OlAc). NMR data show that the OlAc ligands are tightly bound to the Qdot surface, with a coverage of 3.0±0.4 nm(-2). In addition, we demonstrate that they are bound as oleate ions. Combining this with the inorganic Qdot composition, we observe that charge-neutral Qdots are obtained when taking into account the charge of the stoichiometric PbS Qdot core, the surface excess of Pb ions, the surface-adsorbed Cl ions and the oleate ligands. The Qdot suspensions are stable under atmospheric conditions, showing no changes in the NMR and absorbance spectra for several weeks. Finally, we determine the photoluminescence quantum yield (PL QY) for OlAc-capped PbS Qdots, synthesized either with or without TOP. In both cases, they are highly luminescent, with PL QY values varying between 20 and 90%, depending on the Qdot size.

Lead sulphide nanocrystal photodetector technologies

“…* = 0.1 , m is the free electron mass), giving rise to discrete quantized energy states which may be observed in optical absorption and emission spectra, Fig 1b. The onset of wavefunction confinement at relatively large NC diameters allows bandgap 4 tuning over a large energy range (0.6-1.6 eV), with optical absorption extending into to the UV. 4 Additionally, as a direct bandgap semiconductor PbS NCs have a high molar absorption coefficient (~10 6 M -1 cm -1 at 400 nm). 5…”

The role of surface passivation for efficient and photostable PbS quantum dot solar cells

“…While improvement in PCE of QD solar cells has been thoroughly sought after over the years, the photostability of this technology still remains an open challenge to further increase its technology readiness level, albeit initial promising results on low-performance PbS QD solar cells 11 . The Achilles' heel of this photovoltaic technology is the surface of QDs, which remarkably impacts QDs' energy bands 12 , mid-gap states 13 , carrier transport 14 and stability 15 .…”

A top–down strategy towards monodisperse colloidal lead sulphide quantum dots

“…Because of the striking size-dependence of characteristics, monodisperse QDs, with a size dispersion (or polydispersity) so10% (ref. 8), are of great importance in realizing functional control and manipulation, such as achieving very sharp photoluminescence and a precise tuning of the emission by varying QDs sizes 9 . Moreover, monodisperse colloidal QDs have been regarded as promising building blocks for advanced crystal superlattices and devices with abnormal electrical and optical behaviours, resulting from the quantum mechanical and dipolar interactions between nanocrystal units 10 .…”