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 spectraFX 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 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 !!!