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Title: Experimental Observation of Quantum Confinement in the Conduction Band of CdSe Quantum Dots

Abstract

X-ray absorption spectroscopy has been used to characterize the evolution in the conduction band (CB) density of states of CdSe quantum dots (QDs) as a function of particle size. We have unambiguously witnessed the CdSe QD CB minimum (CBM) shift to higher energy with decreasing particle size, consistent with quantum confinement effects, and have directly compared our results with recent theoretical calculations. At the smallest particle size, evidence for a pinning of the CBM is presented. Our observations can be explained by considering a size-dependent change in the angular-momentum-resolved states at the CBM.

Authors:
; ; ; ;  [1]; ;  [2]
  1. Lawrence Livermore National Laboratory, Livermore, California 94550 (United States)
  2. Naval Research Laboratory, Washington D.C. 20375 (United States)
Publication Date:
OSTI Identifier:
20951216
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review Letters; Journal Volume: 98; Journal Issue: 14; Other Information: DOI: 10.1103/PhysRevLett.98.146803; (c) 2007 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ABSORPTION SPECTROSCOPY; ANGULAR MOMENTUM; CADMIUM SELENIDES; CONFINEMENT; PARTICLE SIZE; QUANTUM DOTS; X-RAY SPECTROSCOPY

Citation Formats

Lee, Jonathan R. I., Meulenberg, Robert W., Klepeis, John E., Terminello, Louis J., Buuren, Tony van, Hanif, Khalid M., and Mattoussi, Hedi. Experimental Observation of Quantum Confinement in the Conduction Band of CdSe Quantum Dots. United States: N. p., 2007. Web. doi:10.1103/PHYSREVLETT.98.146803.
Lee, Jonathan R. I., Meulenberg, Robert W., Klepeis, John E., Terminello, Louis J., Buuren, Tony van, Hanif, Khalid M., & Mattoussi, Hedi. Experimental Observation of Quantum Confinement in the Conduction Band of CdSe Quantum Dots. United States. doi:10.1103/PHYSREVLETT.98.146803.
Lee, Jonathan R. I., Meulenberg, Robert W., Klepeis, John E., Terminello, Louis J., Buuren, Tony van, Hanif, Khalid M., and Mattoussi, Hedi. Fri . "Experimental Observation of Quantum Confinement in the Conduction Band of CdSe Quantum Dots". United States. doi:10.1103/PHYSREVLETT.98.146803.
@article{osti_20951216,
title = {Experimental Observation of Quantum Confinement in the Conduction Band of CdSe Quantum Dots},
author = {Lee, Jonathan R. I. and Meulenberg, Robert W. and Klepeis, John E. and Terminello, Louis J. and Buuren, Tony van and Hanif, Khalid M. and Mattoussi, Hedi},
abstractNote = {X-ray absorption spectroscopy has been used to characterize the evolution in the conduction band (CB) density of states of CdSe quantum dots (QDs) as a function of particle size. We have unambiguously witnessed the CdSe QD CB minimum (CBM) shift to higher energy with decreasing particle size, consistent with quantum confinement effects, and have directly compared our results with recent theoretical calculations. At the smallest particle size, evidence for a pinning of the CBM is presented. Our observations can be explained by considering a size-dependent change in the angular-momentum-resolved states at the CBM.},
doi = {10.1103/PHYSREVLETT.98.146803},
journal = {Physical Review Letters},
number = 14,
volume = 98,
place = {United States},
year = {Fri Apr 06 00:00:00 EDT 2007},
month = {Fri Apr 06 00:00:00 EDT 2007}
}
  • Recent theoretical descriptions as to the magnitude of effect that quantum confinement has on he conduction band (CB) of CdSe quantum dots (QD) have been conflicting. In this manuscript, we experimentally identify quantum confinement effects in the CB of CdSe QDs for the first time. Using X-ray absorption spectroscopy, we have unambiguously witnessed the CB minimum shift to higher energy with decreasing particle size and have been able to compare these results to recent theories. Our experiments have been able to identify which theories correctly describe the CB states in CdSe QDs. In particular, our experiments suggest that multiple theoriesmore » describe the shifts in the CB of CdSe QDs and are not mutually exclusive.« less
  • The energies of the lowest excited state of hexagonal and zinc-blende CdSe spherical quantum dots are calculated using empirical pseudopotentials of the bulk semiconductor. The lowest excited state energies computed for hexagonal clusters are in reasonable agreement with recent experiments. We predict that zinc-blende and hexagonal dots have nearly the same lowest exciton energy shifts down to very small dot sizes. We have found that small changes in the pseudopotentials can turn the lowest energy transitions of small particles into indirect gaps. 29 refs., 6 figs.
  • Coordination of phenyldithiocarbamate (PTC) ligands to solution-phase colloidal CdSe quantum dots (QDs) decreases the optical band gap, E g, of the QDs by up to 220 meV. These values of ΔE g are the largest shifts achieved by chemical modification of the surfaces of solution-phase CdSe QDs and are—by more than an order of magnitude in energy—the largest bathochromic shifts achieved for QDs in either the solution or solid phases. Measured values of ΔE g upon coordination to PTC correspond to an apparent increase in the excitonic radius of 0.26 ± 0.03 nm; this excitonic delocalization is independent of themore » size of the QD for radii, R = 1.1-1.9 nm. Density functional theory calculations indicate that the highest occupied molecular orbital of PTC is near resonant with that of the QD, and that the two have correct symmetry to exchange electron density (PTC is a π-donor, and the photoexcited QD is a π-acceptor). We therefore propose that the relaxation of exciton confinement occurs through delocalization of the photoexcited hole of the QD into the ligand shell.« less