Transcript Fullerenes + SWNT*s - Time resolved PL

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.
Outline
I) Motivation
II) Growth procedure
III) Optical investigation on NWs with different diameters
IV) Single-wire spectroscopy
V) Conclusions
Briefly on some ZnO basic properties
Wurtzite structure  4 at. / cell
Band structure  direct bandgap
3.3 eV
CB
Γ7
A
Γ7
B
Γ9
C
Γ7
*
Growth techniques
Optical Properties
rf. magnetron
sputtering
Molecular beam
epitaxy
Pulsed laser
deposition
Chemical vapour
deposition
i)
ii)
iii)
iv)
v)
Free excitons (FE)
Bound excitons (BE)
Donor acceptor pairs (DAP)
Two electron satelites
Phonon replicas
* M. R. Wagner et. al. , PRB 80, 205203 (2009)
I) Motivation
1000
ZnO nanowires
publications
500
• Lowest dimensional system suitable for
conductivity measurements
0
• 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, 147401 (2006)
“The active media is the cavity itself”
Can we learn something on sizedependent polariton fields in the NWs??
II) ZnO NWs growth
SEM images – Three samples
Vapour-liquid-solid
d = 70 nm
d = 110 nm
SiO2/Si substrate
Au deposition
Au drops formation
( ≈ 900 ºC)
ZnO atmosphere
NWs nucleation
NWs growth
HRTEM images
d = 170 nm
III) Results & Experimental Setup
Beam Splitter
Ti:Sa
LBO
70 nm
Pulsed: 2 ps
CCD
Spectrom.
355 nm
Piezo
XYZ
MCP
63x
He
Antivibrations
system
Sample
Cryostat
High spatial resolution
Pump
1) 50 x objective  500 nm
2) Piezo-XYZ stage  50 nm
3) Horizontally aligned NWs
Photoluminescence spectra
FX
BX
ZnO NWs - 300 K
Room temperature
Free exciton
Intensity (arb. units)
- Free exciton
e-
hν
2.7
3.0
Energy (eV)
Low temperatures
- Free exciton
- Bound excitons
- Surface excitons
- Free to bound
- DAP
- Two electron satelites
3.3
Acceptors
VB
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, 053105 (2010)
Time resolved spectra
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.
Lifetime vs. NWs diameter
420
ZnO NWs
4 K - Au sputt.
long component
360
Time (ps)
300
240
E = 3.357 eV
Short: BE
120
E = 3.361 eV
80
E = 3.365 eV
J.S. Reparaz, et. al., Appl. Phys. Lett. 96, 053105
40
45
60
75
90
105
120
135
NWs diameter (nm)
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
On the precursor influence…
ZnO NWs
Au-colloids
Intensisty (arb. units)
4K
Au-sputtering
d = 130 nm, L = 1 
d = 85 nm, L = 0.8 
d = 65 nm, L = 0.8 
d = 50 nm, L = 0.7 
3.30
3.31
3.32
3.33
3.34
3.35
Energy (eV)
3.36
3.37
3.38
3.39
V) Single-wire spectroscopy
PL spectra
PL mapscan
ZnO single NW
Intensity (arb. units)
300 K
1 (center)
3 (WL)
380
390
2 (tip)
400
Wavelength (nm)
410
V) Single-wire spectroscopy
PL spectra
PL mapscan
ZnO NWs
Intensity (arb. units)
300 K
3 nm = 20 meV
L=5
380
390
400
410
Wavelength (nm)
Sub-wavelength polariton guiding
V) Single-wire spectroscopy
Time resolved spectra
PL mapscan
ZnO single NW
300 K
Intensity (arb. units)
= 382 nm
1) Tcenter = 288 ps
2) Ttip = 211 ps
0
1
2
Time (ns)
• The lifetime depends on the position
on the NWs.
Coupling to the
external e.m. field
• We observe the presence of a ZnO WL.
Cavity modes
3
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.
Thank you !!

Come down
you messy cat !!!