Excitation Energy Dependence of the Photoluminescence Quantum Yield of Core/Shell CdSe/CdS Quantum Dots and Correlation with Circular Dichroism Irina V. Martynenko, †,‡ Anvar S. Baimuratov, † Victoria A. Osipova, † Vera A. Kuznetsova, § Finn Purcell-Milton, § Ivan D. Rukhlenko, †,∥ Anatoly V. Fedorov, † Yurii K. Gun’ko, †,§ Ute Resch-Genger,* ,‡ and Alexander V. Baranov † † ITMO University, 49 Kronverksky Pr., St. Petersburg 197101, Russia ‡ Federal Institute for Materials Research and Testing (BAM), Division Biophotonics, Richard-Willstaetter-Strasse 11, 12489 Berlin, Germany § School of Chemistry and CRANN, Trinity College Dublin, Dublin 2, Ireland ∥ Monash University, Clayton Campus, Clayton, Victoria 3800, Australia * S Supporting Information ABSTRACT: Quantum dot (QD) based nanomaterials are very promising materials for the fabrication of optoelectronic devices like solar cells, light emitting diodes (LEDs), and photodetectors as well as as reporters for chemo- and biosensing and bioimaging. Many of these applications involve the monitoring of changes in photoluminescence intensity and energy transfer processes which can strongly depend on excitation wavelength or energy. In this work, we analyzed the excitation energy dependence (EED) of the photoluminescence quantum yields (PL QYs) and decay kinetics and the circular dichroism (CD) spectra of CdSe/CdS core/shell QDs with different thicknesses of the surface passivation shell. Our results demonstrate a strong correlation between the spectral position of local maxima observed in the EED of PL QY and the zero-crossing points of the CD profiles. Theoretical analysis of the energy band structure of the QDs with effective mass approximation suggests that these structures could correspond to exciton energy levels. This underlines the potential of CD spectroscopy for the study of electronic energy structure of chiroptically active nanocrystals which reveal quantum confinement effects. ■ INTRODUCTION A quantum dot (QD) is a semiconductor nanoparticle with a size in the quantum confinement region that shows unique size- tunable optical properties including a large absorption cross section and narrow photoluminescence (PL) with a high quantum yield (PL QY). Therefore, these nanostructures are very promising materials for optoelectronic devices 1 such as solar cells, LEDs, photodetectors, and even qubits in future quantum computers. 2 QDs represent also interesting optical reporters for chemical sensing, 3 biosensing, and bioimaging. 4 For most of these applications, PL intensities are measured to monitor changes in the local QD environment. 5 The interpretation of changes in PL intensity or the comparison of measurements performed at different excitation wavelengths or energies can be hampered by a possible excitation energy dependence (EED) of PL QYs of QDs, particularly for high energy excitation. 6 This can principally affect applications such as the monitoring energy transfer processes using QDs as donors or acceptors 7−10 or the use of QDs as optical reporters, studies of the blinking and charging dynamics of QDs as well as measurements of PL QYs of QD samples. Although there is some evidence for an EED of the PL QY of QDs, the occurrence and origin of this effect is still debated. Moreover, it can be affected by the size of PL QY value of the respective QD sample. Some groups did not observe an EED of PL QY of spherical QDs like core-only CdTe with PL QY between about 0.25 and 0.75 11 and CdSe cores for excitation wavelengths relatively close to the band gap 12 or claimed the absence of an EED for differently sized CdSe QDs despite hints for it. 13 Moreover, only recently, a unity PL QY was reported for CdSe/CdS nanorods excited at 3.057 eV (405 nm), where most of the absorption occurred directly into the shell, thereby demonstrating complete shell-to-core energy transfer. 14 Other research groups, however, noticed an EED of PL QY for CdSe, CdSe/CdS, CdSe/ZnS, CdSe/ZnS/CdSe, CdTe, and InP QDs, particularly a decrease in PL QY for excitation energies above the effective band gap. 6,15−21 Explanations for this observation range from an overestimation of the absorption of the samples at higher energies to surface mediated nonradiative deactivation of the excitons formed upon light absorption. Possible nonradiative decay pathways are the coupling of the charge carriers to the organic ligands on the QD surface as suggested Received: October 27, 2017 Revised: December 20, 2017 Published: December 21, 2017 Article pubs.acs.org/cm Cite This: Chem. Mater. 2018, 30, 465-471 © 2017 American Chemical Society 465 DOI: 10.1021/acs.chemmater.7b04478 Chem. Mater. 2018, 30, 465−471 Downloaded via ITMO UNIV on July 30, 2019 at 11:59:19 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.