Letter | Published:

Soft surfaces of nanomaterials enable strong phonon interactions

Nature volume 531, pages 618622 (31 March 2016) | Download Citation

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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References

  1. 1.

    Quantum Processes in Semiconductors 235–281 (Oxford Univ. Press, 1999)

  2. 2.

    et al. Quantum size dependence of femtosecond electronic dephasing and vibrational dynamics in CdSe nanocrystals. Phys. Rev. B 49, 14435–14447 (1994)

  3. 3.

    & Raman-scattering study of exciton-phonon coupling in PbS nanocrystals. Phys. Rev. B 55, 9860–9865 (1997)

  4. 4.

    et al. Time-resolved intraband relaxation of strongly confined electrons and holes in colloidal PbSe nanocrystals. Phys. Rev. B 72, 195312 (2005)

  5. 5.

    On the kinetics and thermodynamics of excitons at the surface of semiconductor nanocrystals: are there surface excitons? Chem. Phys. 446, 92 (2015)

  6. 6.

    , , & Emergence of colloidal quantum-dot light-emitting technologies. Nature Photon. 7, 13–23 (2012)

  7. 7.

    & The architecture of colloidal quantum dot solar cells: materials to devices. Chem. Rev. 114, 863–882 (2014)

  8. 8.

    , , & Prospects of colloidal nanocrystals for electronic and optoelectronic applications. Chem. Rev. 110, 389–458 (2010)

  9. 9.

    & Phonon scattering and energy relaxation in two-, one-, and zero-dimensional electron gases. Phys. Rev. B 42, 8947–8951 (1990)

  10. 10.

    et al. Breaking the phonon bottleneck in semiconductor nanocrystals via multiphonon emission induced by intrinsic nonadiabatic interactions. Phys. Rev. Lett. 95, 196401 (2005)

  11. 11.

    , , , & Breaking the phonon bottleneck for holes in semiconductor quantum dots. Phys. Rev. Lett. 98, 177403 (2007)

  12. 12.

    , & Breaking the phonon bottleneck in PbSe and CdSe quantum dots: time-domain density functional theory of charge carrier relaxation. ACS Nano 3, 93–99 (2009)

  13. 13.

    & Carrier relaxation in colloidal nanocrystals: bridging large electronic energy gaps by low-energy vibrations. Phys. Rev. B 91, 085305 (2015)

  14. 14.

    , & Photoluminescence of mid-infrared HgTe colloidal quantum dots. J. Phys. Chem. C 118, 2749–2753 (2014)

  15. 15.

    , , & A microscopic picture of surface charge trapping in semiconductor nanocrystals. J. Chem. Phys. 138, 204705 (2013)

  16. 16.

    , & Non-radiative electron–hole recombination in silicon clusters: ab initio non-adiabatic molecular dynamics. J. Phys. Chem. C 118, 20702–20709 (2014)

  17. 17.

    Electron transfer reactions in chemistry theory and experiment. J. Electroanal. Chem. 438, 251–259 (1997)

  18. 18.

    , , & Complex nature of gold-related deep levels in silicon. Phys. Rev. B 22, 3917–3934 (1980)

  19. 19.

    , & Multi-excitation entropy: its role in thermodynamics and kinetics. Rep. Prog. Phys. 69, 1145–1194 (2006)

  20. 20.

    et al. Bright infrared quantum-dot light-emitting diodes through inter-dot spacing control. Nature Nanotechnol. 7, 369–373 (2012)

  21. 21.

    et al. Ultrasensitive solution-cast quantum dot photodetectors. Nature 442, 180–183 (2006)

  22. 22.

    et al. Hybrid passivated colloidal quantum dot solids. Nature Nanotechnol. 7, 577–582 (2012)

  23. 23.

    et al. High performance thermoelectrics from earth-abundant materials: enhanced figure of merit in PbS by second phase nanostructures. J. Am. Chem. Soc. 133, 20476–20487 (2011)

  24. 24.

    et al. Entropically stabilized local dipole formation in lead chalcogenides. Science 330, 1660–1663 (2010)

  25. 25.

    , & Low-frequency Raman scattering from CdS microcrystals embedded in a germanium dioxide glass matrix. Phys. Rev. B 47, 1237–1243 (1993)

  26. 26.

    , & Vibrations of quantum dots and light scattering properties: atomistic versus continuous models. Phys. Rev. B 76, 205425 (2007)

  27. 27.

    & Confinement effects on the vibrational properties of III–V and II–VI nanoclusters. Phys. Rev. B 85, 041306 (2012)

  28. 28.

    , , & Quantification of deep traps in nanocrystal solids, their electronic properties, and their influence on device behavior. Nano Lett. 13, 5284–5288 (2013)

  29. 29.

    et al. Meyer–Neldel behavior of deep level parameters in heterojunctions to Cu(In,Ga)(S,Se)2. Appl. Phys. Lett. 69, 2888–2890 (1996)

  30. 30.

    , , & Theoretical study of electron–phonon relaxation in PbSe and CdSe quantum dots: evidence for phonon memory. J. Phys. Chem. C 115, 21641–21651 (2011)

  31. 31.

    , & Light-driven and phonon-assisted dynamics in organic and semiconductor nanostructures. Chem. Rev. 115, 5929–5978 (2015)

  32. 32.

    & Slow electron cooling in colloidal quantum dots. Science 322, 929–932 (2008)

  33. 33.

    et al. Highly effective surface passivation of PbSe quantum dots through reaction with molecular chlorine. J. Am. Chem. Soc. 134, 20160–20168 (2012)

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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.

Author information

Affiliations

  1. Laboratory for Nanoelectronics, Department of Information Technology and Electrical Engineering, ETH Zurich, CH-8092 Zurich, Switzerland

    • Deniz Bozyigit
    • , Nuri Yazdani
    • , Maksym Yarema
    • , Olesya Yarema
    • , Weyde Matteo Mario Lin
    • , Sebastian Volk
    •  & Vanessa Wood
  2. Nano TCAD Group, Department of Information Technology and Electrical Engineering, ETH Zurich, CH-8092 Zurich, Switzerland

    • Kantawong Vuttivorakulchai
    •  & Mathieu Luisier
  3. Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland

    • Fanni Juranyi

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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.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to Deniz Bozyigit or Vanessa Wood.

Supplementary information

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  1. 1.

    Supplementary Information

    This file contains Supplementary Methods, a Supplementary Discussion, Supplementary References and Supplementary Figures 1-12.

Videos

  1. 1.

    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.

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DOI

https://doi.org/10.1038/nature16977

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