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2009 SPIE Photonics WestJanuary 24-29, 2009, San Jose, California
Synthesis of ZnO Nanoparticles Using a Low Temperature Vapor Phase
Transport Process
Curtis TaylorUniversity of Florida, Dept. of Mechanical and Aerospace Engineering,
Gainesville, Florida
Tarek TradUniversity of Texas-Brownsville, Dept. of Chemistry, Brownsville, Texas
Kurt Eyink, David Look, and David TomichUnited States Air Force Research Laboratory, Wright-Patterson AFB, Dayton, Ohio
SPIE Photonics West 2009Quantum Dots, Particles, and Nanoclusters
2009 SPIE Photonics West, Jan. 24-29, 2009 Slide
Outline
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2009 SPIE Photonics West, Jan. 24-29, 2009 Slide
ZnO Nanostructures for Novel Optoelectronic Devices
Unique electronic properties of ZnO:• Direct wide band gap = 3.37 eV• Large exciton binding energy ~ 60 meV
Applications:• high efficiency field emitters• piezoelectric transducers• transparent thin film transistors• light emitting diodes (LEDs)• hybrid organic solar cells
Hybrid polymer-nanowire solar cell
thin film ZnO transistor
for transparent flex circuitry
D. Wang et al. Nano Letters 7(4), 1003-1009 (2007)
nanowire photodetector
high efficiency ZnO nanowire field emitters
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2009 SPIE Photonics West, Jan. 24-29, 2009 Slide
Chemical and Physical Synthesis of ZnO Nanostructures
• Problem: ZnO nanostructures synthesized by wet chemical or physical methods at high temperatures (> 800 C)
• wet chemical methods difficult to integrate with existing silicon fabrication and processing
• physical methods generally provide higher crystalline quality material than chemical
• physical methods not amenable to flexible electronics or substrates
• Need for low-temperature physical synthesis techniques
chemical: spray pyrolysis synthesis
Huang M. et alMadler, L et al.
Chemical• sol-gel• polymer stabilization• reversed micelles• alkoxide-assisted
Physical• Vapor Liquid Solid (VLS)• Vapor Solid (VS)
• CVD• MOCVD• etc.
nanoparticles
physical: VLS synthesis
nanorods
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2009 SPIE Photonics West, Jan. 24-29, 2009 Slide
Low Temperature Physical Synthesis of ZnO Quantum Dots
• Vapor Phase Transport (VPT) synthesis
• Zinc acetate is used as a precursor
• ZnO quantum dots are grown at ~ 500 °C
Lu et al. Applied Physics Letters, 88, 063110, 2006
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2009 SPIE Photonics West, Jan. 24-29, 2009 Slide
NEW APPROACH--Low Temperature Carbothermal Vapor Phase Synthesis of ZnO Nanoparticles
• Inner tube allows for LT VPT synthesis by carbothermal decomposition of ZnO powder
• Facile route to synthesis of high quality ZnO nanoparticles
• Substrate temperatures as low as 225 °C
• amenable for polymer and other flexible substrates
• Tunable structural and electronic properties
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2009 SPIE Photonics West, Jan. 24-29, 2009 Slide
Experimental Details
ZnO + C Zn + CO2Zn + O2 ZnO
Substrate: Si(100) with 3 nm thin film of Au deposited by thermal evaporation
Precursor: 1:1 equimolar mixture of high purity ZnO/C
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2009 SPIE Photonics West, Jan. 24-29, 2009 Slide
ZnO Mesoparticles (>100 nm) Formation
0.5 cm
Reaction Conditions• substrate Ts = 270 °C • precursor Tp = 950 °C
Time:• 1 hour reaction time
60° tilted image
• faceted morphology
• particle density gradient across substrate
• multilayer formation at substrate edge
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2009 SPIE Photonics West, Jan. 24-29, 2009 Slide
Ts ~ 270 °C Tp = 950 °C Reaction time = 1 hourAr gas flow rate = 139 sccmAverage particle diameter = 248 nm
Particle stacking is observed towards substrate edge
Mesoparticles
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2009 SPIE Photonics West, Jan. 24-29, 2009 Slide
• No particle stacking observed
• Uniform dispersion of nanoparticles
• Narrow size distribution
Nanoparticles
Ts ~ 270 °C Tp = 950 °C Reaction time = 10 minutesAr gas flow rate = 139 sccmAverage particle diameter = 80 nm
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2009 SPIE Photonics West, Jan. 24-29, 2009 Slide
Reaction Conditions and the Effect on Average Particle Diameter
0
100
200
300
400
0 75 150 225 300
Part
icle
Dia
mete
r (n
m)
Gas Flow (sccm)
Time Gas Flow Rate
Temperature:(a) Tp = 950 °C, Ts = 275 °C for 1 hour
dav = 277 nm(a) Tp = 900 °C, Ts = 225 °C for 1 hour
dav = 113 nm
(a) (b)
55.00
116.25
177.50
238.75
300.00
0 15 30 45 60
Part
icle
Dia
mete
r (n
m)
Time (minutes)
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2009 SPIE Photonics West, Jan. 24-29, 2009 Slide
Composition and Crystalline Quality of Nanoparticles
XRD peaks indexed to ZnO structures
EDX indicates no impurities present in ZnO formation
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EDX
EDX
XRD
2009 SPIE Photonics West, Jan. 24-29, 2009 Slide
Optical Quality - LT PhotoluminescenceStrong Luminescence and Optical Size-Effect Observed
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• Peak at 3.366 eV --attributed to surface-related exciton [D. Stichtenoth et. al, 2007]
•Low Temperature PL ~ 4 K
• Results corroborate 3.366 eV feature as surface related since it dominates with decrease in NP size -- increase in surface area
surface exciton feature
2009 SPIE Photonics West, Jan. 24-29, 2009 Slide
Conclusions and Future Directions
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• Novel method for the physical synthesis of ZnO nanoparticles at substrate temperatures as low as 225 °C
• Tuning particle properties (size, photoluminescence, structural quality, and density) was realized by changing reaction conditions such as temperature, time, and carrier gas flow
• ZnO nanoparticles show strong luminescence and optical size effects
• Future directions:
• Optimize reaction conditions to reach quantum confinement
• Obtain high resolution structural characterization
• Embed particles in polymer to fabricate novel hybrid (polymer/metal oxide) photovoltaic devices potentially on flexible substrates