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