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Soft surfaces of nanomaterials enable strong phonon interactions

Abstract

Phonons and their interactions with other phonons, electrons or photons drive energy gain, loss and transport in materials. Although the phonon density of states has been measured and calculated in bulk crystalline semiconductors1, phonons remain poorly understood in nanomaterials2,3,4,5, despite the increasing prevalence of bottom-up fabrication of semiconductors from nanomaterials and the integration of nanometre-sized components into devices6,7,8. Here we quantify the phononic properties of bottom-up fabricated semiconductors as a function of crystallite size using inelastic neutron scattering measurements and ab initio molecular dynamics simulations. We show that, unlike in microcrystalline semiconductors, the phonon modes of semiconductors with nanocrystalline domains exhibit both reduced symmetry and low energy owing to mechanical softness at the surface of those domains. These properties become important when phonons couple to electrons in semiconductor devices. Although it was initially believed that the coupling between electrons and phonons is suppressed in nanocrystalline materials owing to the scarcity of electronic states and their large energy separation9, it has since been shown that the electron–phonon coupling is large and allows high energy-dissipation rates exceeding one electronvolt per picosecond (refs 10, 11, 12, 13). Despite detailed investigations into the role of phonons in exciton dynamics, leading to a variety of suggestions as to the origins of these fast transition rates14,15 and including attempts to numerically calculate them12,13,16, fundamental questions surrounding electron–phonon interactions in nanomaterials remain unresolved. By combining the microscopic and thermodynamic theories of phonons1,17,18,19 and our findings on the phononic properties of nanomaterials, we are able to explain and then experimentally confirm the strong electron–phonon coupling and fast multi-phonon transition rates of charge carriers to trap states. This improved understanding of phonon processes permits the rational selection of nanomaterials, their surface treatments, and the design of devices incorporating them.

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Figure 1: Inelastic neutron scattering (INS) measurements.
Figure 2: Ab initio molecular dynamics simulations of nanocrystal (NC) vibrations.
Figure 3: Non-radiative transition rates measured by thermal admittance spectroscopy (TAS).

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Acknowledgements

D.B., N.Y., M.Y., W.M.M.L., O.Y., S.V. and V.W. acknowledge funding from the Swiss National Science Foundation through the NCCR Quantum Science and Technology and an ETH Research Grant. K.V. and M.L. acknowledge funding from SNF TORNAD under project 149454. Neutron scattering experiments were performed at the Swiss spallation neutron source SINQ, Paul Scherrer Institute. Molecular dynamics simulations were supported by a grant from the Swiss National Supercomputing Centre (CSCS; project ID s579). We thank D. Norris for access to SEM and EDX, C. Hierold for access to the confocal Raman spectrometer, and M. Wörle for access to XRD at the Small Molecule Crystallography Center at ETH Zürich.

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Contributions

M.Y. and O.Y. designed and conducted the material synthesis; M.Y., O.Y., D.B., N.Y., W.M.M.L. and S.V. performed the sample preparation; D.B., N.Y. and M.Y. characterized the samples; D.B. and F.J. conducted the neutron scattering experiments and data analysis; N.Y. and D.B. performed the ab initio molecular dynamics simulations; K.V. performed density functional theory calculations; D.B., W.M.M.L. and N.Y. fabricated NC diodes and performed thermal admittance spectroscopy; D.B. performed Raman spectroscopy; D.B. developed the theoretical description of the results; D.B. and V.W. devised the experiments; and all authors contributed to the writing of the manuscript.

Corresponding authors

Correspondence to Deniz Bozyigit or Vanessa Wood.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Methods, a Supplementary Discussion, Supplementary References and Supplementary Figures 1-12. (PDF 13641 kb)

Scaled-Up PbS Nanocrystal Synthesis

The video shows a scaled-up NC synthesis where the sulfur precursor (bis(trimetylsilyl)sulfide diluted in 1-octadecene) is rapidly introduced into a two-litre flask containing lead (II) oleate and oleic acid by applying mild underpressure to the flask and opening the stopcock valve of the funnel. (MP4 4557 kb)

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Bozyigit, D., Yazdani, N., Yarema, M. et al. Soft surfaces of nanomaterials enable strong phonon interactions. Nature 531, 618–622 (2016). https://doi.org/10.1038/nature16977

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