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

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

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

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

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

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

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

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

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

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

Growth mechanism Size comparison: Au particle: ~15 – 20 nm Nanosheet base width: ~120 - 500 nm  Nanostructures unlikely to grow by VLS mechanism alone  Combination of VLS and vapor-solid (VS) growth mechanisms nanosheet Au

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

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

Taper angle of nanosheet 9° 45° [0001] [0001] Increased inclination angle  larger (0001) facet area exposed  increased sidewall growth rate at (0001) facet  larger taper angle (wider sheet)

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

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

The work is supported by the NANOMAT bilateral program under Project No. 182092/S10 and the National Science Foundation under Grant No. DMR-0606054