Letter pubs.acs.org/JPCL
Charge Trapping versus Exciton Delocalization in CdSe Quantum Dots Jamie J. Grenland, Cassandra J. A. Maddux, David F. Kelley, and Anne Myers Kelley* Chemistry and Chemical Biology, University of California, Merced, 5200 North Lake Road, Merced, California 95343, United States S Supporting Information *
ABSTRACT: The spectroscopic and photophysical similarities and differences between charge trapping by surface ligands on CdSe quantum dots and charge delocalization into the shell in excited CdSe core/shell nanocrystals are discussed. Optical absorption and resonance Raman spectroscopies are used to study small CdSe quantum dots coated with organic ligands that accept electrons (methyl viologen) or holes (phenothiazine, 4-methylbenzenethiol), as well as with semiconductor shells that delocalize electrons (CdS) or holes (CdTe). The organic ligands have only a small effect on the optical absorption spectrum and contribute negligibly to the resonance Raman spectra, indicating little participation of ligand orbitals in the initial excitation. The semiconductor shells more strongly red-shift the absorption spectrum by delocalizing the electron and/or hole into the shell, and vibrations of the shell appear in the resonance Raman spectrum, showing that the shell is involved in the vertical excitation. The qualitative differences between ligand and semiconductor shells are discussed in terms of the energetics and coupling strengths.
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absorption by delocalizing the hole wave function,20,21 but it is now understood that most of the effect arises from deposition of a CdS shell by decomposition of the unstable ligand and reaction with residual cadmium.22−24 For nearly spherical quantum dots (QDs), the envelope functions of the electron and hole wave functions are approximately described as the eigenstates of a particle in a sphere (1S, 2S, 1P, etc.). The lowest-energy excitation, 1Se1S3/2, creates both electron and hole in 1S functions with the hole having an angular momentum of 3/2. The second feature observed in well-resolved absorption spectra is the 1Se2S3/2, which involves the same electron state but a higherenergy hole state. A ligand or semiconductor shell that delocalizes electrons into the shell should lower the energy of the 1Se state, red-shifting the lowest energy exciton, but should not affect the hole states and therefore should not change the splitting between the 1Se1S3/2 and 1Se2S3/2 transitions. In contrast, delocalization of the hole wave function should redshift the lowest energy exciton by a smaller amount (because the quantum confinement energy of the hole is much smaller than that of the electron) and should also reduce the 1Se1S3/2− 1Se2S3/2 splitting. Resonance Raman (RR) spectra also provide a specific probe of the spatial extent of the initially created exciton because the resonantly enhanced vibrations are those that occupy the same region of space as the exciton.25,26 RR spectroscopy has been widely applied to QDs carrying the usual types of ligands, but such spectra do not show features attributable to ligand vibrations or even to the semiconductor-ligand bond stretch27−31 except in a few special cases.32 By contrast, RR
emiconductor nanocrystals have their lowest-energy optical transitions at higher energy than the corresponding bulk material. This “quantum confinement” results from the photogenerated electron and/or hole being confined to a smaller volume than in a bulk crystal. The spectrum can be shifted to lower energies by making the nanocrystal larger or by adding a shell of a second semiconductor, allowing the electron or hole to occupy a larger volume. For example, in a CdSe/CdS core/shell quantum dot, the lowest-energy excitation leaves the hole largely confined to the core while the electron is delocalized over both core and shell, red-shifting the absorption compared to a bare core.1−3 Semiconductor nanocrystals are usually synthesized with ligands attached to the surface atoms to passivate charge trapping surface defects as well as aid solubility and protect against aggregation and undesirable chemical reactions.4 Ligands with appropriate orbital energies can also accept or donate electrons (accept holes) to the photoexcited semiconductor, trapping either the positive or negative charge on the ligand and quenching the emission. Such ligands for CdSe are phenothiazine5−8 and alkylthiols9−11 (electron donors/hole acceptors), and methyl viologen (electron acceptor).12−16 These ligands typically have little effect on the optical absorption in the region of the lowest-energy exciton, indicating that, upon vertical excitation, the electron and hole wave functions are not significantly delocalized into the ligand. The transition from an electron or hole that is delocalized throughout the nanocrystal to a localized charge on a single ligand occurs following photoexcitation and may involve significant geometric relaxation. Aromatic thiols represent an intermediate case in which hole trapping on the surface is accompanied by a modest red-shift of the lowest excitonic transition, indicating some delocalization of the hole into the ligands.17−19 Aromatic dithiocarbamates were initially thought to be organic ligands that strongly red-shift the lowest excitonic © XXXX American Chemical Society
Received: August 24, 2017 Accepted: October 3, 2017 Published: October 3, 2017 5113
DOI: 10.1021/acs.jpclett.7b02242 J. Phys. Chem. Lett. 2017, 8, 5113−5118
Letter
The Journal of Physical Chemistry Letters
CdTe shell broadens the lowest excitonic absorption and redshifts it to 534 nm, with the second transition no longer evident in these broader spectra. Addition of a thin CdS shell red-shifts the two lowest absorptions to 551.7 and about 510 nm, respectively. Only the CdSe cores and the CdSe/CdS core/ shells exhibited significant photoluminescence; all three ligands and the CdTe shells quenched the emission to
