Influence of Spectral Diffusion on the Line Shapes of Single CdSe Nanocrystallite Quantum Dots

Abstract

We study the emission line shapes of single CdSe nanocrystallite quantum dots. Single dot line shapes are found to result from rapid spectral shifting of the emission spectrum rather than the intrinsic physics of the quantum dot. A strong dependence of single dot line widths on excitation intensity, wavelength, temperature, and integration time is found and is correlated with the number of times that the quantum dot is excited during the acquisition of a single spectrum. The observed results are consistent with thermally assisted spectral diffusion, activated by the release of excess excitation energy.Recent advances in the detection of single molecules have been responsible for a level of understanding in condensed phase systems that was not previously possible. Of particular interest are incoherent dynamic effects, which can be completely hidden when averaged over an ensemble. In recent years, single chromophore spectroscopy has allowed the direct observation of single enzyme reactions, 1 single energy transfer events, 2 and single molecule rotational dynamics. 3 In addition, new and unexpected physical phenomena have been observed, which appear to be common to many single chromophore systems, such as fluorescence blinking 4-6 and spectral shifting (spectral diffusion). [7][8][9][10][11][12] One field that has greatly benefited from single chromophore spectroscopy is the study of semiconductor quantum dots (QDs). QDs are of great interest due to their unique size dependent optical properties, 13 which can easily be tuned during fabrication. 14 Unfortunately, the characteristics that make QDs interesting also make them inherently difficult to study. Inhomogeneities in size and shape within ensemble samples result in spectral broadening that is many orders of magnitude larger than single QD spectra. 9,15 Though the mechanisms are thought to be quite different, the study of single QDs has revealed phenomena common to other single chromophore systems such as blinking 4 and spectral diffusion. [9][10][11][12] It has also revealed new characteristics specific to QDs such as ultranarrow transition line widths, 9,15 giant Overhauser shifts, 16 multicarrier effects, 17,18 and fluctuating local electric fields. 12 One area of particular interest which can, in principle, be addressed on the single QD level, is the nature of the homogeneous line shape. While theory predicts that QDs should have atomic-like spectral transitions due to long excited-state lifetimes and weak coupling to acoustic phonons, 19 previous ensemble experiments have suggested that line widths in both excitation [20][21][22] and emission 23-25 are quite broad (for example, "homogeneous" line widths extracted from fluorescence line narrowing experiments in CdSe nanocrystallite QDs are reported to be ∼5 meV 26 ). It was expected that single QD spectroscopy would uncover the true "homogeneous" line width. However, while single dot line widths are typically found to be significantly narrower than what is seen in ensemble experiments, many single dot exp...

Transition from silicon nanowires to isolated quantum dots: Optical and structural evolution

“…4. Some of the spectra consist of only the NP line, sometimes homogeneously broadened by effects like spectral diffusion or scattering, 26 while others exhibit some or all of the aforementioned sidebands. Sychugov et al 19 report a blend of different spectra, some containing and some missing, e.g., TO-phonon replicas, and explain the dominance of zero-phonon transitions in silicon nanocrystal spectra by spatial localization of excitons, in addition to the quantum confinement effect.…”

Spontaneous Spectral Diffusion in CdSe Quantum Dots

“…13 This latter SD process is the most commonly studied and the most disruptive process due to the size of the spectral jumps and is the focus of this work. The spectral jumps have been attributed to individual charges moving around the surface, 14,15 although this attribution is merely speculative. For example, scanning electron force microscopy has shown that many, but not all, NCs have or develop surface charges, 16 while SD is a ubiquitous phenomenon observed in all single NC studies.…”

“…18 Such an interpretation is compatible with the SD model originally proposed by Empedocles and Bawendi. 14 Early reports of SD in NCs revealed a thermal activation energy below which the SD process is solely photoactivated. 14,15 This motivates the study of SD at cryogenic temperatures, where competing thermal effects are eliminated, effectively isolating the photophysical properties.…”

“…14 Early reports of SD in NCs revealed a thermal activation energy below which the SD process is solely photoactivated. 14,15 This motivates the study of SD at cryogenic temperatures, where competing thermal effects are eliminated, effectively isolating the photophysical properties. A recent numerical study of the response of surface ligands to photoactivation has revealed that they can "jump" between different lattice sites.…”

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…Temporal variations in the peak emission wavelength are accompanied by, and correlated with, variations in the emission line width [285][286][287][288]. Most of the experiments into spectral diffusion were conducted at cryogenic temperatures [289][290][291][292], but the process occurs at room temperature, too [285][286][287][288]. A direct link between spectral diffusion and blinking was proposed, based on correlations between blinking events (on-off switches) and spectral shifts [290,293].…”