This document summarizes a new low-temperature vapor phase transport process for synthesizing ZnO nanoparticles. ZnO powder and carbon are used as precursors and decomposed at temperatures as low as 225°C to form ZnO nanoparticles. Reaction conditions like time, temperature, and gas flow rate can be adjusted to control the nanoparticle size, with smaller sizes exhibiting stronger surface-related photoluminescence. This new method allows for the low-temperature physical synthesis of high-quality ZnO nanoparticles suitable for applications in flexible electronics and optoelectronics.

1. Synthesis of ZnO Nanoparticles Using
a Low Temperature Vapor Phase
Transport Process
Curtis Taylor
University of Florida, Dept. of Mechanical and Aerospace Engineering,
Gainesville, Florida
Tarek Trad
University of Texas-Brownsville, Dept. of Chemistry, Brownsville, Texas
Kurt Eyink, David Look, and David Tomich
United States Air Force Research Laboratory, Wright-Patterson AFB, Dayton, Ohio
SPIE Photonics West 2009
Quantum Dots, Particles, and Nanoclusters
2009 SPIE Photonics West
January 24-29, 2009, San Jose, California

2. Outline
2009 SPIE Photonics West, Jan. 24-29, 2009 Slide 2

3. ZnO Nanostructures for Novel Optoelectronic Devices
Unique electronic properties of ZnO: thin film ZnO
• Direct wide band gap = 3.37 eV transistor
• Large exciton binding energy ~ 60 meV for transparent
flex circuitry
Applications:
• high efficiency field emitters
• piezoelectric transducers
• transparent thin film transistors
• light emitting diodes (LEDs) high efficiency
• hybrid organic solar cells ZnO nanowire
field emitters
nanowire
photodetector
Hybrid polymer-nanowire solar cell
D. Wang et al. Nano Letters 7(4), 1003-1009 (2007)
2009 SPIE Photonics West, Jan. 24-29, 2009 Slide 3

4. Chemical and Physical Synthesis of ZnO
Nanostructures
Chemical Physical
• sol-gel • Vapor Liquid Solid (VLS)
• polymer stabilization • Vapor Solid (VS)
nanoparticles nanorods
• reversed micelles • CVD
• alkoxide-assisted • MOCVD
• etc. Madler, L et al. Huang M. et al
chemical: spray physical: VLS
pyrolysis synthesis synthesis
• 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
2009 SPIE Photonics West, Jan. 24-29, 2009 Slide 4

5. Low Temperature Physical Synthesis of ZnO
Quantum Dots
Lu et al. Applied Physics Letters, 88, 063110, 2006
• Vapor Phase Transport (VPT) synthesis
• Zinc acetate is used as a precursor
• ZnO quantum dots are grown at ~ 500 °C
2009 SPIE Photonics West, Jan. 24-29, 2009 Slide 5

6. 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
2009 SPIE Photonics West, Jan. 24-29, 2009 Slide 6

7. Experimental Details
Substrate: Si(100) with 3 nm thin
film of Au deposited by thermal
evaporation
Precursor: 1:1 equimolar mixture
of high purity ZnO/C
ZnO + C Zn + CO
2Zn + O2 ZnO
2009 SPIE Photonics West, Jan. 24-29, 2009 Slide 7

8. ZnO Mesoparticles (>100 nm) Formation
Reaction Conditions
• substrate Ts = 270 °C 60° tilted
image
• precursor Tp = 950 °C
Time:
• 1 hour reaction time
• faceted morphology
• particle density
gradient across
substrate
• multilayer formation
at substrate edge 0.5 cm
2009 SPIE Photonics West, Jan. 24-29, 2009 Slide 8

9. Particle stacking is
observed towards
substrate edge
Mesoparticles
Ts ~ 270 °C Tp = 950 °C
Reaction time = 1 hour
Ar gas flow rate = 139 sccm
Average particle diameter = 248 nm
2009 SPIE Photonics West, Jan. 24-29, 2009 Slide 9

10. • No particle
stacking
observed
• Uniform
dispersion of
nanoparticles
• Narrow size
distribution
Nanoparticles
Ts ~ 270 °C Tp = 950 °C
Reaction time = 10 minutes
Ar gas flow rate = 139 sccm
Average particle diameter = 80 nm
2009 SPIE Photonics West, Jan. 24-29, 2009 Slide 10

11. Reaction Conditions and the Effect on Average
Particle Diameter
Time Gas Flow Rate
300.00 400
Particle Diameter (nm)
Particle Diameter (nm)
238.75 300
177.50 200
116.25 100
55.00 0
0 15 30 45 60 0 75 150 225 300
Time (minutes) Gas Flow (sccm)
(a) (b)
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
2009 SPIE Photonics West, Jan. 24-29, 2009 Slide 11

12. EDX
Composition and Crystalline Quality of
Nanoparticles
EDX XRD
EDX indicates no impurities XRD peaks indexed to ZnO
present in ZnO formation structures
2009 SPIE Photonics West, Jan. 24-29, 2009 Slide 12

13. Optical Quality - LT Photoluminescence
Strong Luminescence and Optical Size-Effect Observed
• Low Temperature PL
~4K
• Peak at 3.366 eV --
attributed to surface- surface exciton
feature
related exciton
[D. Stichtenoth et. al, 2007]
• Results corroborate
3.366 eV feature as
surface related since it
dominates with
decrease in NP size --
increase in surface area
2009 SPIE Photonics West, Jan. 24-29, 2009 Slide 13

14. Conclusions and Future Directions
• 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
2009 SPIE Photonics West, Jan. 24-29, 2009 Slide 14