Excited State Relaxation Mechanisms in InP colloidal Quantum Dots.
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Excited State Relaxation Mechanisms in InP colloidal Quantum Dots. Garry Rumbles, Don Selmarten, Randy E. Ellingson, Jeff Blackburn, Pingrong Yu, Barton B. Smith, Olga I. Micic and Arthur J. Nozik. National Renewable Energy Laboratory Center for Basic Sciences Golden, CO 80401 ABSTRACT We report photoluminescence, linear absorption and femto-second transient bleaching spectra for a colloidal solution of indium phosphide (InP) quantum dots at ambient temperatures. The photoluminescence quantum yield is shown to depend not only upon the size of the quantum dots, with larger dots exhibiting higher quantum yields, but also upon the excitation wavelength. At short wavelengths, photoluminescence excitation spectra deviate markedly from the absorption spectra indicating that an efficient non-radiative deactivation pathway becomes prominent at these higher photon energies. We interpret this observation in terms of an inefficient relaxation mechanism between the second excited state and the lowest energy excited state from which the emission emanates. The results are consistent with the existence of a phonon bottleneck. INTRODUCTION The use of photoluminescence excitation spectroscopy (PLE) for measuring indirectly the absorption spectrum of species that cannot be measured directly is a valuable and reliable tool. For the approach to be valid, however, the intensity of the PL must be directly proportional to the amount of light absorbed and the assumptions made must be recognised in order to avoid errors. For molecular systems in the condensed phase, either solid or solution, it is assumed that relaxation of the initially excited state to the emitting state occurs with an efficiency of 100%. Both internal conversion and vibrational relaxation combine to relax the initially excited state fast and efficiently to achieve this figure and this is often referred to as Kasha’s rule [1]. For systems that contain more than one emitting chromophore, the differing photoluminescence quantum yields (PLQY) must be taken into consideration, as PLE is biased towards the more emissive species. It must also be recognised that PLE spectra are a measure of %Absorption (%A), or 1 − Transmission (1 − T) and this only approximates to the absorption spectrum for weakly absorbing samples, such as dilute solutions and thin films. To correlate the electronic absorption spectra of colloidal quantum dots with the PLE spectra requires careful experimentation. The QD sample is a distribution of dot sizes, each with its own PLQY and emission position. In addition, the relaxation mechanism of the initially excited state to the emitting state may not be as efficient as for molecular systems, as the relaxation of the excited electrons may be hampered by the low density of both electron and phonon states close to the conduction band minimum (CBM). This process, referred to as the phonon bottleneck [2], provides an opportunity to create hot electrons that may be useful in photovoltaic device applications. The phonon bottleneck also provides an opportunity for a
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