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Size-dependent recombination dynamics in ZnO nanowires

Size-dependent recombination dynamics in ZnO nanowires. J. S. Reparaz 1 , M. R. Wagner 1 , A. Hoffmann 1 , F. Güell 2 , A. Cornet 3 , and J . R. Morante 2,3. 1 Institut für Festkörperphysik, Technische Universität Berlin, Germany.

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Size-dependent recombination dynamics in ZnO nanowires

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  1. Size-dependent recombination dynamics in ZnO nanowires J. S. Reparaz1, M. R. Wagner1, A. Hoffmann1, F. Güell2, A. Cornet3, and J. R. Morante2,3 1Institut für Festkörperphysik, Technische Universität Berlin, Germany. 2MIND & M-2E, IN2UB, Departamentd’Electrònica, Universitat de Barcelona, Spain. 3Institut de la Recerca de l’Energia de Catalunya (IREC), Barcelona, Spain.

  2. Outline I) Motivation II) Growth procedure III) Optical investigation on NWs with different diameters IV) Single-wire spectroscopy V) Conclusions

  3. Briefly on some ZnO basic properties Band structure  direct bandgap Wurtzite structure  4 at. / cell CB Γ7 Γ7 A 3.3 eV Γ9 B Γ7 C * Growth techniques Optical Properties Pulsed laser deposition rf. magnetron sputtering Free excitons (FE) Bound excitons (BE) Donor acceptor pairs (DAP) Two electron satelites Phonon replicas Molecular beam epitaxy Chemical vapour deposition * M. R. Wagner et. al. , PRB 80, 205203 (2009)

  4. I) Motivation • Lowest dimensional system suitable for conductivity measurements • Non-toxic and highly bio-compatible • The electronic states in the NWs core are sensitive to the surface states C. Lao et. al., Nanoletters7, 1323 (2008) Theoretically Single – wire PL spectra Size-dependent exciton-polariton coupling ΔωLTBulk ≈ 2 to 12 meV ΔωLTNWs ≈ 60 to 164 meV !!! L. K. Van Vugt. et. Al, Phys. Rev. Lett97, 147401 (2006) B. Gil and A. V. Kavokin, Appl. Phys. Lett., Vol. 81, 748 (2002)

  5. “The active media is the cavity itself” Can we learn something on size-dependentpolariton fields in the NWs??

  6. II) ZnO NWs growth SEM images – Three samples Vapour-liquid-solid d = 170 nm d = 70 nm d = 110 nm SiO2/Si substrate Au deposition Au drops formation ( ≈ 900 ºC) HRTEM images ZnO atmosphere NWs nucleation NWs growth

  7. III) Results & Experimental Setup Beam Splitter Ti:Sa LBO CCD Spectrom. 70 nm Pulsed: 2 ps 355 nm Piezo XYZ MCP 63x He Anti- vibrations system Sample Cryostat High spatial resolution 1) 50 x objective  500 nm 2) Piezo-XYZ stage  50 nm 3) Horizontally aligned NWs Pump

  8. Photoluminescence spectra e- FX BX Room temperature - Free exciton hν Acceptors VB Low temperatures - Free exciton - Bound excitons - Surface excitons - Free to bound - DAP - Two electron satelites

  9. Photoluminescence spectra DX=3.365 eV observed in allthesamples We use this DX tostudy theNWsdiameter Influenceonthee.m.filedinsidetheNWs J. S. Reparaz, M. Wagner, A. Hofmann, et. al., Appl. Phys. Lett., 96, 053105 (2010)

  10. The DX are spatially localized states !!! Time resolved spectra BULK (λ << d) Plane wave E=E0exp(-ikr-wt) NW (d < λlight) i) The NWs shape influences the polaritons field ii) Emission from excitons in different spatial positions in the NWs influence the recombination times. Thedifferentlifetimesprobetheinfluence of theNWssizeonthee.m.field insidetheNWs.

  11. Lifetime vs. NWs diameter J. S. Reparaz, M. Wagner, A. Hofmann, et. al., unpublished (2010) Thelifetime of the DX excitonsincreasesapproximatellylinearlywith NWsdiameter. Thisresultsfromtheinfluence of theNWssizeonthee.m.fieldspatialdistribution

  12. On the precursor influence…

  13. V) Single-wire spectroscopy PL spectra PL mapscan

  14. V) Single-wire spectroscopy PL spectra PL mapscan Sub-wavelength polariton guiding

  15. V) Single-wire spectroscopy Time resolved spectra PL mapscan Cavitymodes Couplingtothe externale.m.field • The lifetime depends on the position • on the NWs. • We observe the presence of a ZnO WL.

  16. VI) Conclusions • The DX recombination times have shown to be an useful tool for proving the size influence on the e.m. field inside ZnO NWs. • The lifetime of the neutral donor bound excitons depends • on the NWs size  size-dependent polariton field. We find an approximately linear relation for the investigated sizes. • Single-wire spectroscopy has revealed that the recombination dynamics depend on the position on the NWs, decreasing closer to the tip • The presence of a ZnO WL was observed by studying single NWs.

  17. Thankyou !!  Come down youmessycat !!!

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