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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, Departament d’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 3.3 eV A Γ9 B Γ7 C * Growth techniques Optical Properties rf. magnetron sputtering Pulsed laser deposition 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, (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., Nanoletters 7, 1323 (2008) Theoretically Single – wire PL spectra Size-dependent exciton-polariton coupling ΔωLTBulk ≈ 2 to 12 meV ΔωLTNWs ≈ 60 to 164 meV !!! B. Gil and A. V. Kavokin, Appl. Phys. Lett., Vol. 81, 748 (2002) L. K. Van Vugt. et. Al, Phys. Rev. Lett 97, (2006)

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 SEM images – Three samples Vapour-liquid-solid
d = 70 nm d = 110 nm d = 170 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
FX BX Room temperature - Free exciton 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 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 BULK (λ << d) NW (d < λlight)
The DX are spatially localized states !!! 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. The different lifetimes probe the influence of the NWs size on the e.m. field inside the NWs.

11 Lifetime vs. NWs diameter
J. S. Reparaz, M. Wagner, A. Hofmann, et. al., unpublished (2010) 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

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 Cavity modes Coupling to the external e.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 Thank you !!  Come down you messy cat !!!

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