Exciton-driven change of phonon modes causes strong temperature dependent bandgap shift in nanoclusters

Franziska Muckel, Severin Lorenz, Jiwoong Yang, Taufik Adi Nugraha, Emilio Scalise, Taeghwan Hyeon (), Stefan Wippermann () and Gerd Bacher ()
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Franziska Muckel: Universität Duisburg-Essen
Severin Lorenz: Universität Duisburg-Essen
Jiwoong Yang: Institute for Basic Science (IBS)
Taufik Adi Nugraha: Max-Planck-Institut für Eisenforschung
Emilio Scalise: Max-Planck-Institut für Eisenforschung
Taeghwan Hyeon: Institute for Basic Science (IBS)
Stefan Wippermann: Max-Planck-Institut für Eisenforschung
Gerd Bacher: Universität Duisburg-Essen

Nature Communications, 2020, vol. 11, issue 1, 1-7

Abstract: Abstract The fundamental bandgap Eg of a semiconductor—often determined by means of optical spectroscopy—represents its characteristic fingerprint and changes distinctively with temperature. Here, we demonstrate that in magic sized II-VI clusters containing only 26 atoms, a pronounced weakening of the bonds occurs upon optical excitation, which results in a strong exciton-driven shift of the phonon spectrum. As a consequence, a drastic increase of dEg/dT (up to a factor of 2) with respect to bulk material or nanocrystals of typical size is found. We are able to describe our experimental data with excellent quantitative agreement from first principles deriving the bandgap shift with temperature as the vibrational entropy contribution to the free energy difference between the ground and optically excited states. Our work demonstrates how in small nanoparticles, photons as the probe medium affect the bandgap—a fundamental semiconductor property.

Date: 2020
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DOI: 10.1038/s41467-020-17563-0

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