Photo-Activated Luminescence of CdSe Quantum Dot Monolayers

Abstract

We show that the luminescence from CdSe quantum dot monolayers can be strongly influenced by the interaction of water molecules adsorbed on the surface. Light-induced alterations in the surface states following adsorption of water, results in quasi-reversible luminescence changes in the quantum dot. The excitonic QY increases by a factor of 20 during the first 200 s of illumination in air (post vacuum) and then steadily decreases to a level 6 times that of the vacuum reference after 5000 s. The exciton emission exhibits an exponential blue shift of nearly 16 nm (60 meV) over 1 h of illumination. During this time, the line width decreases by 10% during the first 100 s and then slowly increases to 96% of the vacuum reference line width after 5000 s. Our model suggests that water molecules adsorbed on the surface of the quantum dot act to passivate surface traps, which results in increased luminescence, similar to an effect well-known for bulk CdSe surfaces. In addition, adsorbed water molecules act to oxidize the surface of the quantum dot, which results in the blue shift of the exciton emission and eventually introduces new surface defects that lower the luminescence. It is the competition between these two processes that is responsible for the complex kinetics of the luminescence QY.

Electric Field Effects on Photoluminescence of CdSe Nanoparticles in a PMMA Film

“…It should be noted in Figure 9c that the magnitude of I PL (t)/I 0 (t) is smaller than unity even just after photoexcitation, indicating that the initial population of the exciton-emitting state following photoexcitation is reduced in the presence of F. All the time profiles of I 0 (t), I PL (t), ∆I PL (t), and I PL (t)/I 0 (t) were fitted by assuming a tetraexponential decay, i.e., The multi-exponential behavior in the PL decay probably arises from inhomogeneous environments in a PMMA film, i.e., the presence of different interactions between the coated materials and PMMA which show different PL lifetimes from each other [38]. It must be confessed that lifetimes as well as pre-exponential factors of different samples are not always identical with each other, which may arise from photoenhancement of PL of the nanoparticle-doped polymer films [39][40][41][42]. The magnitude of A i corresponds to the population of each emitting state, and the ratio of Σ i A i at 0.5 MV· cm −1 to that at zero field is 0.85.…”

Semiconductor II-VI Quantum Dots with Interface States and Their Biomedical Applications

“…It is well known that PL spectra of QDs can be influenced by the environment atmosphere, by thermal annealing or by optical excitation (Nassal et al, 2004). The PL shift can be a result of optically induced adsorption by polar molecules (Oda et al, 2006), or the chemical transformation of species on the QD surface (Cordero et al, 2000;Roberti et al, 1998). This shift can be stimulated by increasing the compressive strain in core/shell QDs at annealing or drying processes as well (Borkovska et al, 2009).…”

Quantum Dot Blueing and Blinking Enables Fluorescence Nanoscopy

“…28 While higher laser intensities accelerate the process of blueing, 32 slow spectral blue shifts have also been observed under low-level illumination from a Xe arc lamp (≈3 W cm -2 ). 26 From our analysis of approximately 200 measured fluorescence time traces of individual photooxidized quantum dots, we deduced that the duration of the off periods remains unaffected by the blueing process ( Figure 3). However, thanks to an individual time lag between initial illumination and the onset of blueing in each of the capped nanocrystals, 27 the moment at which an individual core is photooxidized is randomly distributed in a nanocrystal ensemble and therefore also in a sample.…”