Reversible and Irreversible Effects of Oxygen on the Optical Properties of CdSe Quantum Wires

Abstract

We investigated the effects of oxygen on the optical properties of wet-chemically synthesized colloidal CdSe quantum wires using confocal spectroscopy. By comparing the photoluminescence characteristics of CdSe quantum wires in oxygen and in inert gas atmospheres, we observed both irreversible photoluminescence enhancement and quenching processes in oxygen as well as reversible changes in the emission spectra and Raman spectra when changing the gas atmosphere during continuous excitation. We attribute the irreversible photoluminescence enhancement to reduction of Auger recombination by formation of oxides on the surface, which eventually leads to photoluminescence quenching. The reversible atmosphere-dependent changes in the emission intensity and wavelengths, accompanied by changes in the Raman spectra, are attributed to electron scavenging by oxygen. This was demonstrated as the presence of excess negative charge carriers led to radiative emission of higher excited states and reduced Raman frequencies when illuminating in inert gas atmospheres. Thereby, this work provides insight into the possible photoinduced effects of oxygen on the optical properties of semiconducting nanoparticles and thus contributes to a more detailed understanding of photobrightening mechanisms.

Tracking Cation Exchange in Individual Nanowires via Transistor Characterization

“…The transport measurements were performed under ambient conditions in the dark to exclude the influence of photoinduced charge carriers. While there are reports that CdSe nanowires can degrade under illumination while exposed to oxygen and that CdS nanowires are sensitive to oxygen adsorption and humidity, we did not observe any influence or degradation during our measurement series. With ongoing cation exchange to Ag 2 Se, we observed an increase in the conductivity, as illustrated in Figure a.…”

Quantum Dot Fluorescent Imaging: Using Atomic Structure Correlation Studies to Improve Photophysical Properties

“…Strikingly, nearly all QD655s photobleach within minutes, and >80% of symm-shelled QDs remain luminescent over the entire excitation duration ( Figure 5 c), emphasizing the importance of symmetrical shell coverage over the core. While the exact role that oxygen has in QD PL performance remains unclear, there are several QD-related factors that may influence how oxygen behaves: (i) the structural quality, 61 − 68 (ii) the medium, in air 64 , 67 or pure oxygen, 62 , 63 , 66 and (iii) the phase, in solution 69 , 70 or thin film. 61 , 64 , 66 In the context of this study, it is likely that oxygen in aCSF photocorrodes QD655s at exposed core facets and degrades coordinating ligands, leading to irreversible photobleaching, as evidenced by plain CdSe core QDs undergoing irreversible photocorrosion in oxygen, 71 while “giant-shell” CdSe/CdS QD experience suppressed blinking and persistent photostability.…”

Redox Reactions at Colloidal Semiconductor Nanocrystal Surfaces

“…Photoinduced charge trapping has also been proposed to induce surface rearrangement or ligand desorption, causing a change in PLQY . Often these PLQY changes are described as “irreversible”, , but some photobrightened or photodarkened samples have been demonstrated to undergo slow recovery processes (hours to days). , …”

“…96 Often these PLQY changes are described as "irreversible", 92,97 but some photobrightened or photodarkened samples have been demonstrated to undergo slow recovery processes (hours to days). 87,98 Lastly, fluorescence intermittency, or "blinking", has also been associated with NC surface defects and photoionization. In CdSe NCs, blinking statistics have been shown to be sensitive to ligand passivation 99 and to surface charging.…”