Excitation Energy Dependence of Photoluminescence Quantum Yields in Semiconductor Nanomaterials with Varying Dimensionalities

Abstract

The excitation energy dependence (EED) of the photoluminescence quantum yield (Φ PL ) of semiconductor nanoparticles with varying dimensionalities is reported. Specifically, the EEDs of CdSe quantum dots, CdSe quantum platelets, CdSe quantum belts, and CdTe quantum wires were determined via measurements of individual Φ PL values and photoluminescence efficiency (PL Eff (E)) spectra. There is a general trend of overall decreasing efficiency for radiative recombination with increasing excitation energy. In addition, there are often local minima in the PL Eff (E) spectra that are most often at energies between quantumconfinement transitions. The average PL lifetimes of the samples do not depend on the excitation energy, suggesting that the EED of Φ PL arises from charge carrier trapping that competes efficiently with intraband carrier relaxation to the band edge. The local minima in the PL Eff (E) spectra are attributed to excitation into optically coupled states that results in the loss of carriers in the semiconductor. The EED data suggest that the PL Eff (E) spectra depend on the sample synthesis, preparation, surface passivation, and environment.

Excitation Energy Dependence of Semiconductor Nanocrystal Emission Quantum Yields

“…Today, conventional wisdom suggests that observations of EED QYs stems from NC sensitivities to ensemble-specific parameters such as NC size, size-distribution, choice of surface ligand, and starting emission QYs. This would then rationalize in a self-consistent fashion, which is why some studies report NC EED QYs ,,,− while others do not. ,,, …”

“…Consequently, PLE signals are divided by measured absorptance spectra in Figures (b, e, h) (solid blue lines). What results are signals referred to in the literature as PL efficiencies (η PL ). ,,,,, Resulting CsPbBr 3 NC η PL values are shown in Figures (c, f, i) using open red circles.…”

“…Application of near-unity QY NCs to demonstrating condensed phase laser cooling, however, requires better understanding of how NC QYs are impacted by excitation conditions such as excitation intensity and/or excitation wavelength. The latter is of particular concern given prior studies, suggesting that varying NC excitation energies leads to different emission QYs. − Such excitation energy-dependent (EED) emission QYs were first demonstrated by Hoheisel et al who showed through photoluminescence excitation (PLE) measurements that CdSe NC emission QYs decreased monotonically when ensembles were excited to the blue of their band edges. This was rationalized as exciting particles into a continuum of states, distinct from confined NC states.…”

“…CdSe NCs . We now investigate whether EED QYs exist in CdSe NCs as this has been the prototypical system for demonstrating their existence. − ,, As with CsPbBr 3 , three different sizes were made using an existing literature approach . Details of the synthesis have been provided in the SI.…”

“…This would then rationalize in a self-consistent fashion, which is why some studies report NC EED QYs 24,26,27,31−33 while others do not. 25,28,31,33 Because of their importance for verifiable demonstrations of condensed phase laser cooling, 11 we assess here whether CsPbBr 3 and CdSe NCs exhibit EED QYs and whether observations of EED QYs relate to ensemble parameters that involve the above-mentioned parameters (i.e., NC composition, size, size-distribution, surface passivation, and starting emission QY). Assessments are made using two independent approaches to measuring NC QYs.…”