Gas Phase Growth Techniques for Quantum Dots

Weiqiang WangDepartment of Mechanical Engineering, University of Rochester

Muzhou JiangDepartment of Electrical and Computer Engineering, University of Ro

chester

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

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.

Quantum dots

Great potential in applications

Cells labelled with quantum dotsNanocrystal LEDs Brighten

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

•Furnace flow reactors•Plasma reactors•Laser reactors•Flame reactors

Aerosol reactors Inert gas condensationinert gas evaporation (IGE) Laser vaporizationExpansion-coolingSpark source

Other methodsLaser ablationSpray systems

Furnace flow reactorsHeating 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；

+ high cooling rates

— uniformity of the products

Schematic diagram of a plasma reactor

React or condense afterwards;

Laser reactorsUsing 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

+ inexpensive method+ simplest method for producing very high temperatures (< 3000K)+ most commercially successful method

Employing the flame heat to initiate chemical reactions

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 evaporation (IGE) (Sputtering)

• 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;

-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

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

The flow in the condenser using ammonium chloride particles2

bismuth particles2

Modeling

Velocity vectors calculated for the configuration.2

Inert gas evaporation (IGE) (Sputtering)

Schematic drawing of the deposition system.

TEM: bright field dark field

Urban et al. (2002) J Vac Sci Technol B 20:995-999.

Alternative: electron beam.

Synthesis for Al, Mo, Cu91Mn9, Al52Ti48

and ZrO2 Al2O3 and SiO2 nanoparticles.

Laser related techniques• Laser vaporization

• Laser ablation

-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;

-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

The schematic diagram of the laser vaporization reactor.

SEM of weblike agglomeration of Si and Ge nanocrystals.

Shoutian et al (1999) J. Cluster Sci. 10, 533-547

XRD spectrum of Ge nanocrystals. (a) Freshly made particles; (b) after 2 months of storage in air.

Laser ablation

Schematic drawing of the laser ablation chamber. Si cluster size distribution for deposits prepared at different laser fluence.

Marine et al. (2000) Appl Surf Sci. 154-155, 345-352.

Laser ablation

The operating conditions can be altered to select particle formation or film formation.

Production rate as a function of He back-filled gaspressure changing laser pulse energy.

Yamamoto et al. (1996) Nanostruct. Materials 7, 305–312.

Theoretical development

Ablation crater morphology

Crater structures obtained with Nd:YAG laser at 266 nm, 4 mJ, 10 Hz. (A, B) copper and (C, D) silicon.

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.

SEM pictures of zinc particles formed in nozzle.

Bayazitoglu et al (1996) Nanostruct. Materials 7, 789–803.

Modifications

-multiple expansions;-expansion of a thermal plasma;-use a ceramic-lined subsonic nozzle;

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) 1300 ۫C .

Ahonen et al. (2001) J Aerosol Sci. 32, 615-630.

Example of electrospray system

Schematic diagram of the eiectrospraying system

Measured size distributions

Shapes of liquid meniscus

Chen et al (1995) Nanostruct. Materials 6, 309–312.

Recent developments and prospective advances for gas phase techniques

• Advances in instrumentation;• Advances in modeling and simulation;• Advances in synthesis of multi-component nanopa

rticles;

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, 1640–1643.2Cho 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.

Modeling and simulation

Aristizabal et al. (2006) Aerosol Sci. 37, 162–186

A two-dimensional axisymmetric turbulent model of a particle generator with radial injection of a quenching gas.

Modeling and simulationSimulation 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

AFM scans of AlGaInN particles.

Example of semiconductor quantum dots.

Solorzano et al. (2004) J. Cryst. Growth. 272, 186–191.

Diagram of the growth process.

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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