1. ZnO Nanostructures Grown by Pulsed Laser Deposition
C. Ladam, P.E. Vullum, J. C. Walmsley, Øystein Dahl and R. Fagerberg
SINTEF Materials and Chemistry, Norway
C. Weigand, D. Skåre, J.K. Grepstad and H. Weman
Department of Electronics and Telecommunications, NTNU, Norway
M. Bergren, T.E. Furtak, and R.T. Collins
Department of Physics, Colorado School of Mines, USA

2. Outline Motivation: hybrid polymer/inorganic solar cells
Deposition technique: pulsed laser deposition with Au catalyse
Growth mechanism: VLS and VS growth
Results and discussion
Conclusion

3. Polymer-based solar cells
Advantages:
Very low material and processing costs
Flexible substrates
Status today:
Low efficiency of ~5%
Unstable over a long period of time
Electrode
acceptor -
active
polymer +
Electrode
Electrode
active
polymer
acceptor
active polymer
►Bulk heterojunction:
 Increased interface area
 directed pathway for charge transfer
 higher device efficiencies
Our architecture: the acceptor is ZnO

4. Pulsed laser deposition
Deposition of oxides with very high crystalline quality
Influence of the deposition parameters on the growth mechanisms

5. Vapor-Liquid-Solid growth mechanism
Deposition of 1-2nm Au film prior of ZnO deposition
Option: better control of the growth

6. ZnO nanorods at 800°C and low O2
Ts = 800 °C, 60 min., 5% O2 – 95% Ar
Predominantly vertical nanowires
Diameter: ~60 nm
Length: ~1 µm
Slightly tapered
Au particles at tips
 Vapor-Liquid-Solid (VLS) mechanism
1 μm
1 μm

7. ZnO nanosheets at 600°C and low O2
Ts = 600 °C, 15 min., 5% O2 – 95% Ar
nanosheet
1 μm
width
depth
Planar and tapered, 2-D nanostructure  nanosheet
rough side surfaces

8. Results – ZnO nanosheets at 600°C
TEM analysis
Stacking faults
Growth axis inclined to ZnO c-axis by ~27°
Au particle at tip  VLS growth
Ts = 600 °C, 15 min., 5% O2 – 95% Ar

9. Results – ZnO nanowires at 600°C
TEM analysis
Growth along ZnO c-axis
Stacking-fault free
Smooth side surfaces
Au particle at tip  VLS growth
Ts = 600 °C, 15 min., 5% O2 – 95% Ar

10. ZnO nanosheets at 800°C and high O2
Predominantly nanosheets
Length: ~2-3 µm
Au particles at tips
 Vapor-Liquid-Solid (VLS) mechanism
2 µm
Ts = 800 °C, 60 min, 100% O2

11. Growth mechanism Size comparison:
Au particle: ~15 – 20 nm
Nanosheet base width: ~ nm
 Nanostructures unlikely to grow by VLS mechanism alone
 Combination of VLS and vapor-solid (VS) growth mechanisms
nanosheet Au

12. ZnO nanowire growth mechanism
[0001] VLS growth
Radial VS growth
same surface energy for all sidewall facets:  same slow growth rates  isotropic sidewall growth
ZnO Au
ZnO
ZnO Au Au

13. Nanosheet growth mechanism
Starts as VLS growth inclined to ZnO [0001]-direction (fastest growth)
(0001) facets exposed and has a lower surface energy than other sidewall facets
 Fast VS growth on exposed (0001) facets
 Slow VS growth on other facets
 anisotropic sidewall growth
nanosheet
VLS growth
[0001] Au Au

14. Taper angle of nanosheet9°
45°
[0001]
[0001]
Increased inclination angle
 larger (0001) facet area exposed
 increased sidewall growth rate at (0001) facet
 larger taper angle (wider sheet)

15. Combination of VLS and VS mechanisms
Increase in temperature:
 increase in ZnO diffusion length
 increase in ZnO re-evaporation rate
Increase in oxygen:
 oxidation process (increase in deposition rate)
Oxygen content
temperature

16. Conclusion
ZnO nanosheets and nanowires were grown by pulsed laser deposition using Au catalyst on sapphire.
At Ts = 600 °C
ZnO nanosheets had stacking faults and grew inclined to the ZnO hexagonal c-axis.
stacking-fault free ZnO nanowires grew along the [0001]-direction
At Ts = 800 °C
Growth mode shifted from ZnO nanowires to stacking-fault free ZnO nanosheets with changing gas composition from 5% O2 to pure oxygen
A combined VLS and VS growth mechanism was proposed for ZnO nanosheet formation

17. The work is supported by the NANOMAT bilateral program under Project No /S10 and the National Science Foundation under Grant No. DMR