Transcript Gas Phase Growth Techniques for Quantum Dots

Gas Phase Growth Techniques
for Quantum Dots
Weiqiang Wang
Department of Mechanical Engineering, University of Rochester
Muzhou Jiang
Department of Electrical and Computer Engineering, University of
Rochester
April 1st, 2007
Outline
Introduction;
Gas phase synthesis methods;
• Homogeneous nucleation methods;
• Other methods;
Prospective advances for gas phase techniques;
Summary;
What is quantum dots?
 size-dependent
 discrete energy spectrum
 quantum confinement
 theoretically high quantum yield
 single-electron transistor
 coulomb blockade effect
Quantum dots
Diameter: 2-10 nm
No. of atoms in diameter: 10 - 50
No. of atoms in quantum dot vol. : 102 - 105
various sized cadmium selenide (CdSe) quantum dots.
Great potential in applications
Nanocrystal LEDs Brighten
Cells labelled with quantum dots
Gas Phase Growth Technology
— methods for preparing nanoparticles in the vapor phase
Advantages
+
+
+
+
+
highest purity relative to liquid or solid state process
Cheap alternative to vacuum synthesis
Continuous process
Good performance in producing multicomponent materials
Good in process and product control
Disadvantages
—
Aggregation
Gas phase synthesis methods
Homogeneous nucleation methods
Aerosol reactors
•Furnace flow reactors
•Plasma reactors
•Laser reactors
•Flame reactors
Other methods
Laser ablation
Spray systems
Inert gas condensation
inert gas evaporation (IGE)
Laser vaporization
Expansion-cooling
Spark source
Furnace flow reactors
Heating particles with Oven sources on surfaces
+ simplest heating systems
limited operating temperature
— impurities
—
Shematic diagram of the Furnace flow reactors
Schematic diagram of the aerosol generation, sizing and reaction process.
Deppert et al. (1996) J. Crystal Growth. 169, 13-19
Plasma reactors
Injecting thermal plasma into the sample particles；
Decompose them fully into ions and atoms；
React or condense afterwards;
+ high cooling rates
—
uniformity of the products
Schematic diagram of a plasma reactor
Laser reactors
Using laser energy to heat sample particles
+ highly localized heating and rapid cooling
Shematic diagram of the Laser reactors
TEM of the iron ultrafine particles
Majima et al. (1994) Jpn. J. Appl. Phys. 33, 4759-4763.
Flame reactors
Employing the flame heat to initiate chemical reactions
+ inexpensive method
+ simplest method for producing very high temperatures (< 3000K)
+ most commercially successful method
Schematic of flame reactor.
TEM’s of iron oxide/silica nanocomposites
Zachariah et al. (1995) Nanostruct. Materials 5, 383-392.
Inert gas related techniques
• Inert gas condensation
-early (1960s), straightforward;
-evaporation of a material in a cool inert gas (He or Ar);
-low pressures conditions ~100 Pa;
-suited for production of metal nanoparticles;
-a reactive gas could be included;
-different vaporization methods;
• Inert gas evaporation (IGE) (Sputtering)
-a method of vaporizing materials by bombardment with high
velocity ions of an inert gas (Ar or Kr );
-in vacuum systems, below 0.1 Pa;
-the composition of the sputtered material is the same as that of the target;
-a very clean environment; but further processing difficult;
Inert gas condensation
Al particles1
bismuth particles2
1. Granqvist, et al. (1976) J. Appl. Phys. 47, 2200–2219.
2. Wegner et al. (2002) Chem Eng Sci. 57, 1753-1762.
Cross-section sketch of the inert gas condensation system
Modeling
The flow in the condenser using ammonium chloride particles2
Velocity vectors calculated for the configuration.2
Inert gas evaporation (IGE) (Sputtering)
Alternative: electron beam.
Schematic drawing of the deposition system.
Synthesis for Al, Mo, Cu91Mn9, Al52Ti48
and ZrO2 Al2O3 and SiO2 nanoparticles.
TEM: bright field
dark field
Urban et al. (2002) J Vac Sci Technol B 20:995-999.
Laser related techniques
• Laser vaporization
-uses a laser to evaporate a sample target;
-vapor is cooled by collisions with the inert gas;
-suits for many kinds of materials;
-directional high-speed deposition of the particles;
-the control of the evaporation from specific areas of the target;
-the simultaneous or sequential evaporation of several different targets;
• Laser ablation
-a pulsed laser heats a very thin (<100 nm) layer of substrate material ;
-resulting in the formation of atoms and ions also fragments of solid;
-the pulse duration and energy determines the amounts of ablated particles;
-target is usually rotated;
-when used for producing films, this technique is called pulsed laser
deposition (PLD);
Laser vaporization
SEM of weblike agglomeration of Si and Ge nanocrystals.
The schematic diagram of the laser vaporization reactor.
XRD spectrum of Ge nanocrystals.
(a) Freshly made particles; (b) after 2 months of storage in air.
Shoutian et al (1999) J. Cluster Sci. 10, 533-547
Laser ablation
Schematic drawing of the laser ablation chamber.
Marine et al. (2000) Appl Surf Sci. 154-155, 345-352.
Si cluster size distribution for deposits prepared
at different laser fluence.
Theoretical development
Laser ablation
Ablation crater morphology
The operating conditions can be altered to
select particle formation or film formation.
Crater structures obtained with Nd:YAG laser at
266 nm, 4 mJ, 10 Hz. (A, B) copper and (C, D) silicon.
Production rate as a function of He back-filled gas
pressure changing laser pulse energy.
Yamamoto et al. (1996) Nanostruct. Materials 7, 305–312.
Beam profile and irradiance in adjacent zones
of the crater.
Davide et al. (2006) Spectro Acta Part B: Atomic Spectroscopy. 61, 421-432.
Expansion-cooling
• Expansion of a condensable gas through a nozzle leads to cooling of
the gas and a subsequent homogeneous nucleation and condensation.
Modifications
-multiple expansions;
-expansion of a thermal plasma;
-use a ceramic-lined subsonic nozzle;
SEM pictures of zinc particles formed in nozzle.
Bayazitoglu et al (1996) Nanostruct. Materials 7, 789–803.
Spark source
A high-current spark between two solid electrodes be used to
evaporate the electrode material for creating nanoparticles.
A schematic diagram of the spark source
A low resolution electron micrograph of Si clusters,
and a high resolution electron micrograph of a section
of one chain.
Saunders, W. A. et al (1993) Appl. Phys. lett. 63, 1549–1551.
Spray systems
• A simple way to produce nanoparticles is to
evaporate micron-sized droplets of a dilute
solution.
• To use a nebulizer to directly inject very small
droplets of precursor solution. (spray pyrolysis,
aerosol decomposition synthesis)
• Electrospray system. (small droplet from charged
aerosol.)
Example of aerosol decomposition
Schematic presentation of evolution of particle size, microstructure,
and TEM images of approximately 100 nm TiO2 particles at reactor
temperatures of (a) 800, (b) 1100, and (c) 1300۫C .
Ahonen et al. (2001) J Aerosol Sci. 32, 615-630.
Example of electrospray system
Schematic diagram of the eiectrospraying system
Measured size distributions
Chen et al (1995) Nanostruct. Materials 6, 309–312.
Shapes of liquid meniscus
Recent developments and prospective
advances for gas phase techniques
• Advances in instrumentation;
• Advances in modeling and simulation;
• Advances in synthesis of multi-component
nanoparticles;
Instrumentation
-- Combination of laser-spectroscopic imaging techniques and
laser ablation to image the plume of Si atoms and clusters;1
-- To combine localized thermophoretic sampling and in situ
light scattering measurements to characterize particle size
and morphology;2
-- Synthesis of nano-sized Al2O3 powders by a thermal
MOCVD (Metal Organic Chemical Vapor Deposition)
combined with plasma;3
-- TEM imaging for in-situ investigation;4
1Nakata
et al. (2002) J Appl Phys. 91, 1640–1643.
J, Choi M. (2000) J Aerosol Sci. 31, 1077–1095.
3Kim H, et al. (2006) Key. Eng. Mater. 321-323, 1683-1686.
4Janzen et. al (2002) J Aerosol Sci. 33, 833–841.
2Cho
Modeling and simulation
A two-dimensional axisymmetric turbulent model
of a particle generator with radial injection of
a quenching gas.
Aristizabal et al. (2006) Aerosol Sci. 37, 162–186
Modeling and simulation
Simulation for a electrospary system
CFD results of the reactor tube temperature and flow fields at wall temperatures of (a) 500 and (b) 1500˚C.
Trajectories of three massless particles are shown by solid lines. Colour indicates temperature and black
squares indicate 1s intervals
Ahonen et al. (2001) J Aerosol Sci. 32, 615-630.
Synthesis of multi component
nanoparticles
Example of semiconductor quantum dots.
Diagram of the growth process.
AFM scans of AlGaInN particles.
Solorzano et al. (2004) J. Cryst. Growth. 272, 186–191.
Summary
A large number of synthesis methods of nanoparticles in the gas
phase have been developed in the last 40 years.
New approaches for improving control of particle size,
morphology, and polydispersity are appearing regularly, the
variety of materials that can be prepared as nanoparticles in the
vapor phase is rapidly growing.
Due to its high controllability, and the potential for high purity,
large quantity production, gas phase synthesis of nanoparticles
can be expected to be continue at a rapid pace, and to result in
more examples of gas phase synthesized nanoparticles.
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