Connecting the Dots: The Kinetics and Thermodynamics of Hot, Cold, and Surface-Trapped Excitons in Semiconductor Nanocrystals

Abstract

The excitonics of semiconductor nanocrystals (NC) depend upon temperature in a complex manner due to the interplay between the kinetics of hot exciton relaxation/trapping and the thermodynamics leading to cold exciton recombination. We apply a semiclassical electron transfer model of surface trapping to temperature-dependent absorption and emission data to elucidate a microscopic picture of the factors which govern the fate of hot and cold excitons. The linear absorption spectra reveal a unique temperature-dependence to the energies of higher excitonic states, while oscillator strength is shown to be temperature invariant. We identify the phonon based origin to the anomalous low temperature peak energy trend in photoluminescence (PL) spectra. PL intensities, PL lifetimes, and absorption spectra are used to demonstrate that variation of quantum yield with temperatures arises from the thermally controlled fraction of NC which emit, rather than from an activated nonradiative pathway common to all NCs. Experimental quantum yield spectra are shown for several NCs and we perform a much-needed analysis of the role of surface PL in quantum yield. Finally, we show that a semiclassical electron transfer model including hot excitonic effects can explain experimental quantum yield spectra and suggests how to probe kinetic trapping processes via simple steady-state spectroscopy.

Temperature-Dependent Exciton and Trap-Related Photoluminescence of CdTe Quantum Dots Embedded in a NaCl Matrix: Implication in Thermometry

“…In principal, the radiative rate is assumed to be temperature independent except for some temperaturedependence variation due to the Boltzmann distribution of electronic states or via non-Condon effects. [ 49 ] Radiative recombination rates are faster than the corresponding nonradiative recombination rates up to T = 260 K and show no substantial difference in a wide temperature range (80-360 K), remaining in the range of (0.09-0.22) × 10 7 s −1 . In contrast, a substantial continuous increase by more than one order of magnitude of the nonradiative recombination rates is observed for all the samples as the temperature is raised from 200 to 360 K. Cross-overs of the temperature dependences of radiative and nonradiative rates for differently sized QDs are in the range of 240-275 K and correspond to the temperature at which PL QY equals 50% for each of the samples.…”

Linking surface chemistry to optical properties of semiconductor nanocrystals

“…26 Similarly, mobile atomic vacancies can be arranged to cause emitting and non-emitting configurations. Mobile traps, localized over several surface atoms, can be caused by additional ligands on an otherwise electrically balanced NC and stop an NC from emitting, which can explain our group's observation on temperature dependent PL of NCs.…”

“…[26][27][28][29] Based upon measurements of the PL spectra of a variety of NC-ligand systems over a broad temperature range, a simple electron transfer model emerged which can uniquely describe all these surface phenomena. [26][27][28][29] Based upon measurements of the PL spectra of a variety of NC-ligand systems over a broad temperature range, a simple electron transfer model emerged which can uniquely describe all these surface phenomena.…”

Bridging Energetics and Dynamics of Exciton Trapping in Core–Shell Quantum Dots

“…28 Microscopically, this is consistent with theoretical predictions relating the excitation energy dependence of PLQY to energy dependent carrier trapping rates. [30][31] In addition to this, the dependence of PLQY on excitation energy and shell thickness provides further information on the nature of these traps. All our measurements have been performed at room temperature and only changing the shell thickness within a core/shell series while maintaining 13 the same CdSe core dimension.…”

“…13 By means of a semi-classical electron transfer model, Kambhampati related the excitation energy dependence (EED) of PLQY with energy-dependent trapping rates in core CdSe QDs. 30 The outcome of that analysis led to the prediction that the PLQY may exhibit a dependence on the excitation energy that markedly increases on lowering the temperature. Kambhampati's analysis was based on data from ultrafast time resolved measurements.…”