TY - JOUR
T1 - Imaging Excited Orbitals of Quantum Dots
T2 - Experiment and Electronic Structure Theory
AU - Nienhaus, Lea
AU - Goings, Joshua J.
AU - Nguyen, Duc
AU - Wieghold, Sarah
AU - Lyding, Joseph W.
AU - Li, Xiaosong
AU - Gruebele, Martin
N1 - Publisher Copyright:
© 2015 American Chemical Society.
PY - 2015/11/25
Y1 - 2015/11/25
N2 - 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.
AB - 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.
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U2 - 10.1021/jacs.5b09272
DO - 10.1021/jacs.5b09272
M3 - Article
C2 - 26518039
AN - SCOPUS:84948659059
SN - 0002-7863
VL - 137
SP - 14743
EP - 14750
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 46
ER -