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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.

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Presentation on theme: "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."— Presentation transcript:

1 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. 2 MIND & M-2E, IN 2 UB, Departament d’Electrònica, Universitat de Barcelona, Spain. 3 Institut 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 Wurtzite structure  4 at. / cell Band structure  direct bandgap 3.3 eV Growth techniques rf. magnetron sputtering Molecular beam epitaxy Pulsed laser deposition Chemical vapour deposition Optical Properties i)Free excitons (FE) ii)Bound excitons (BE) iii)Donor acceptor pairs (DAP) iv)Two electron satelites v)Phonon replicas Γ7Γ7 Γ7Γ7 Γ7Γ7 Γ9Γ9 CB A B C * M. R. Wagner et. al., PRB 80, (2009) *

4 I) Motivation L. K. Van Vugt. et. Al, Phys. Rev. Lett 97, (2006) Single – wire PL spectra B. Gil and A. V. Kavokin, Appl. Phys. Lett., Vol. 81, 748 (2002) Theoretically Size-dependent exciton-polariton coupling Δω LT Bulk ≈ 2 to 12 meV Δω LT NWs ≈ 60 to 164 meV !!! 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., Nanoletters 7, 1323 (2008)

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

6 II) ZnO NWs growth Vapour-liquid-solid SEM images – Three samples HRTEM images d = 70 nmd = 110 nm d = 170 nm SiO 2 /Si substrate Au deposition Au drops formation ( ≈ 900 ºC) ZnO atmosphere NWs nucleation NWs growth

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

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

9 Photoluminescence spectra DX=3.365 eV observed in all the samples We use this DX to study the NWs diameter Influence on the e.m. filed inside the NWs J. S. Reparaz, M. Wagner, A. Hofmann, et. al., Appl. Phys. Lett., 96, (2010)

10 Time resolved spectra i) The NWs shape influences the polaritons field ii) Emission from excitons in different spatial positions in the NWs influence the recombination times. The different lifetimes probe the influence of the NWs size on the e.m. field inside the NWs. The DX are spatially localized states !!! BULK (λ << d) NW (d < λ light ) Plane wave E=E 0 exp(-ikr-wt)

11 Lifetime vs. NWs diameter The lifetime of the DX excitons increases approximatelly linearly with NWs diameter. This results from the influence of the NWs size on the e.m. field spatial distribution J. S. Reparaz, M. Wagner, A. Hofmann, et. al., unpublished (2010)

12 On the precursor influence…

13 V) Single-wire spectroscopy PL mapscan PL spectra

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

15 V) Single-wire spectroscopy PL mapscan Time resolved spectra The lifetime depends on the position on the NWs. We observe the presence of a ZnO WL. Cavity modes Coupling to the external e.m. field

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 Thank you !! Thank you !! Come down you messy cat !!!


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