Emission Quenching in PbSe Quantum Dot Arrays by Short-Term Air Exposure

Clear evidence for two emitting states in PbSe nanocrystals (NCs) has been observed. The flow of population between these two states as temperature increases is interrupted by the presence of nonradiative trap states correlated with the exposure of the NC film to air. Quenching of the higher-energy emission begins after only seconds of exposure, with the effect saturating after several days. Unlike short-term oxygen-related effects in solution, the emission quenching appears to be irreversible, signaling a distinction between surface reactivity in NCs in films and that in solution. The origin of the two emissive centers and the impact of trapping on other NC film properties (e.g., electron/hole mobilities) remain important issues to be resolved

Close Packing of Nitroxide Radicals in Stable Organic Radical Polymeric Materials

The relationship between the polymer network and electronic transport properties for stable radical polymeric materials has come under investigation owing to their potential application in electronic devices. For the radical polymer poly­(2,2,6,6-tetramethylpiperidine-4-yl-1-oxyl methacrylate), it is unclear whether the radical packing is optimal for charge transport partially because the relationship between radical packing and molecular structure is not well-understood. Using the paramagnetic nitroxide radical as a probe of the polymer and synthetic techniques to control the radical concentration on the methyl methacrylate backbone, we investigate the dependence of radical concentration on molecular structure. The electron paramagnetic resonance data indicate that radicals in the PTMA assume a closest approach distance to each other when more than 60% of the backbone is populated with radical pendant groups. Below 60% coverage, the polymer rearranges to accommodate larger radical–radical spacing. These findings are consistent with theoretical calculations and help explain some experimentally determined electron-transport properties

Control of PbSe Quantum Dot Surface Chemistry and Photophysics Using an Alkylselenide Ligand

We have synthesized alkylselenide reagents to replace the native oleate ligand on PbSe quantum dots (QDs) in order to investigate the effect of surface modification on their stoichiometry, photophysics, and air stability. The alkylselenide reagent removes all of the oleate on the QD surface and results in Se addition; however, complete Se enrichment does not occur, achieving a 53% decrease in the amount of excess Pb for 2 nm diameter QDs and a 23% decrease for 10 nm QDs. Our analysis suggests that the Se ligand preferentially binds to the {111} faces, which are more prevalent in smaller QDs. We find that attachment of the alkylselenide ligand to the QD surface enhances oxidative resistance, likely resulting from a more stable bond between surface Pb atoms and the alkylselenide ligand compared to Pb-oleate. However, binding of the alkylselenide ligand produces a separate nonradiative relaxation route that partially quenches PL, suggesting the formation of a dark hole-trap

Synthesis and Spectroscopy of Silver-Doped PbSe Quantum Dots

Electronic impurity doping of bulk semiconductors is an essential component of semiconductor science and technology. Yet there are only a handful of studies demonstrating control of electronic impurities in semiconductor nanocrystals. Here, we studied electronic impurity doping of colloidal PbSe quantum dots (QDs) using a postsynthetic cation exchange reaction in which Pb is exchanged for Ag. We found that varying the concentration of dopants exposed to the as-synthesized PbSe QDs controls the extent of exchange. The electronic impurity doped QDs exhibit the fundamental spectroscopic signatures associated with injecting a free charge carrier into a QD under equilibrium conditions, including a bleach of the first exciton transition and the appearance of a quantum-confined, low-energy intraband absorption feature. Photoelectron spectroscopy confirms that Ag acts as a p-type dopant for PbSe QDs and infrared spectroscopy is consistent with <i>k</i>·<i>p</i> calculations of the size-dependent intraband transition energy. We find that to bleach the first exciton transition by an average of 1 carrier per QD requires that approximately 10% of the Pb be replaced by Ag. We hypothesize that the majority of incorporated Ag remains at the QD surface and does not interact with the core electronic states of the QD. Instead, the excess Ag at the surface promotes the incorporation of <1% Ag into the QD core where it causes p-type doping behavior