Imaging Excited Orbitals of Quantum Dots

Experiment and Electronic Structure Theory

Lea Nienhaus, Joshua J. Goings, Duc Nguyen, Sarah Wieghold, Joseph W. Lyding, Xiaosong Li, Martin Gruebele

Research output: Contribution to journalArticle

Abstract

Electronically excited orbitals play a fundamental role in chemical reactivity and spectroscopy. In nanostructures, orbital shape is diagnostic of defects that control blinking, surface carrier dynamics, and other important optoelectronic properties. We capture nanometer resolution images of electronically excited PbS quantum dots (QDs) by single molecule absorption scanning tunneling microscopy (SMA-STM). Dots with a bandgap of ∼1 eV are deposited on a transparent gold surface and optically excited with red or green light to produce hot carriers. The STM tip-enhanced laser light produces a large excited-state population, and the Stark effect allows transitions to be tuned into resonance by changing the sample voltage. Scanning the QDs under laser excitation, we were able to image electronic excitation to different angular momentum states depending on sample bias. The shapes differ from idealized S- or P-like orbitals due to imperfections of the QDs. Excitation of adjacent QD pairs reveals orbital alignment, evidence for electronic coupling between dots. Electronic structure modeling of a small PbS QD, when scaled for size, reveals Stark tuning and variation in the transition moment of different parity states, supporting the simple one-electron experimental interpretation in the hot carrier limit. The calculations highlight the sensitivity of orbital density to applied field, laser wavelength, and structural fluctuations of the QD.

Original languageEnglish (US)
Pages (from-to)14743-14750
Number of pages8
JournalJournal of the American Chemical Society
Volume137
Issue number46
DOIs
StatePublished - Nov 25 2015

Fingerprint

Quantum Dots
Semiconductor quantum dots
Electronic structure
laser
Imaging techniques
scanning tunnelling microscopy
Lasers
Hot carriers
experiment
Experiments
image resolution
angular momentum
defect
gold
Scanning Tunnelling Microscopy
spectroscopy
wavelength
Methyl Green
Stark effect
Blinking

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

Cite this

Imaging Excited Orbitals of Quantum Dots : Experiment and Electronic Structure Theory. / Nienhaus, Lea; Goings, Joshua J.; Nguyen, Duc; Wieghold, Sarah; Lyding, Joseph W.; Li, Xiaosong; Gruebele, Martin.

In: Journal of the American Chemical Society, Vol. 137, No. 46, 25.11.2015, p. 14743-14750.

Research output: Contribution to journalArticle

Nienhaus, Lea ; Goings, Joshua J. ; Nguyen, Duc ; Wieghold, Sarah ; Lyding, Joseph W. ; Li, Xiaosong ; Gruebele, Martin. / Imaging Excited Orbitals of Quantum Dots : Experiment and Electronic Structure Theory. In: Journal of the American Chemical Society. 2015 ; Vol. 137, No. 46. pp. 14743-14750.
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