Gas Phase Growth Techniques for Quantum Dots

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15 Νοε 2013 (πριν από 3 χρόνια και 7 μήνες)

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1

Gas Phase Growth Techniques for Quantum Dots

Weiqiang Wang
1,*
, Muzhou Jiang
2

1
Department of Mechanical Engineering, University of Rochester;
2
Department of Electrical and Computer Engineering
, University of
Rochester; *Corresponding author(Email: camp21340
0@hotmail.com)


Abstract
:
An overview of methods for preparing
quantum dots (
nanoparticles
)

in the gas phase is
given, and recent developments and advances for gas phase synthesis techniques are discussed.

Developments in instrumentation for monitoring
gas
-
phase synthesis of nanoparticles, in modeling
these processes, and in producing multi
-
component nanoparticles are also

included. The most
important developments relate to improved control and understanding of nanoparticle aggregation
and

coalescence durin
g synthesis.


1. Introduction

From the end of last century, researchers in many
different

disciplines

trend to pay attention
to nano
-
scale
materials and related applications.
The term “nanoparticle” came into frequent use in the early 1990s together with
t
he related concepts, “nanoscaled” or “nanosized” particle. Until then, the more general terms submicron and
ultrafine particles were used.

From a scientific point of view, n
anoparticles (Fig. 1) are of great scientific interest
as they are effectively a br
idge between bulk materials and atomic or molecular structures. A bulk material should
have constant physical properties regardless of its size, but at the nano
-
scale this is often not the case.
Size
-
dependent properties are observed such as quantum confin
ement in semiconductor particles, surface plasmon
resonance in some metal particles and superparamagnetism in magnetic materials.







Fig.1. TEM image of nanoparticles typical of those

produced in
many vapor
-
phase processes. These particular particles

are silicon
produced by laser pyrolysis of silane.

[5]

Fig.2.
Researchers at Los Alamos National Laboratory have
developed a wireless nanodevice that efficiently produces visible
light, t
hrough energy transfer from nano
-
thin layers of quantum
wells to nanocrystals above the nanolayers.


2

Nanoparticles have been suggested recently for various potential applications in electronics (Fig. 2) where
q
uantum con
fi
nement e
ff
ects may be of advantage. When electrons are confined to a small domain such as a
nanoparticle the system is called a “quantum dot” or zero
-
dimensional structure. Then the electrons are
behaving like “particles
-
in
-
a
-
box” and their res
ulting new energy levels are determined by quantum
“confinement” effects.

As a result, discrete energy levels are needed to describe the electron excitation and
transport in quantum dots.

The corresponding wave functions are spatially localized within the
quantum dot, but
extend over many periods of the crystal lattice.

Being zero
-
dimensional, quantum dots have a sharper density of states than higher
-
dimensional structures.
As a result, they have superior transport and optical properties, and are being res
earched for use in diode lasers,
amplifiers, and biological sensors.

Scientists make efforts to give this invisible matter to boarder applications,
f
or

instance,

fabrication of optical memories

and
organic dyes

in modern biological analysis
.


M
ethods for t
he synthesis of nanoparticles
are

taking place in other than gas
-
phase
growth

technology.
However, gas
-
phase processing systems may
have some advantages over other methods

in some cases because
of their following inherent advantages:

(a) Gas
-
phase processe
s are generally purer than liquid
-
based processes since even the

most ultra
-
pure water
contains traces of minerals
, which

seem to be avoidable today only in vacuum and gas
-
phase systems.

(b) Aerosol processes have the potential to create complex chemical s
tructures which are useful in producing
multicomponent materials, such as high
-
temperature superconductors

[1]
.

(c) The process and product control is usually very good in aerosol processes.

(d) Being a nonvacuum technique, aerosol synthesis provides a ch
eap alternative to expensive vacuum synthesis
techniques in thin or thick
fi
lm synthesis
[2]
. Furthermore, the much higher deposition rate as compared to
vacuum techniques may enable mass production.

(e)

An aerosol droplet resembles a very small reactor in

which chemical segregation is minimized, as any phases
formed cannot leave the particle
[3]
.

(f)

Gas
-
phase processes for particle synthesis are usually continuous processes, while liquid
-
based synthesis
processes or milling processes are often performed i
n a batch form. Batch processes can result in product
characteristics which vary from one batch to another.


2. S
ynthesis method of quantum dots using gas phase growth technology

Most synthesis methods of nanoparticles in the gas phase are based on homogen
eous

nucleation in the gas phase
and subsequent condensation and coagulation. Once nucleation occurs,

remaining supersaturation can be
relieved by condensation

or reaction of the
gas
-
phase molecules on the

resulting particles, and particle growth
will occu
r rather

than further nucleation. Therefore, to prepare small

particles, one wants to create a high degree
of supersaturation,

thereby inducing a high nucleation density, and

then immediately quench the system, either
by removing

the source of supersaturat
ion or slowing the kinetics, so

that the particles do not grow. In most
cases, this happens

rapidly (milliseconds to seconds) in a relatively uncontrolled

fashion, and lends itself to
continuous or quasi
-
continuous

operation. This contrasts with many collo
idal

syntheses of nanoparticles that are
carried out in discrete

batches under well
-
controlled conditions with batch

times of hours to days.

Finally, initiating homogeneous nucleation synthesis of nanoparticles in the gas phase inside aerosol
droplets can
result in many nanosized nuclei in the droplet, which upon drying will yield nanoparticles. These
methods will be described in detai
l

in the following sections.

2
.1.
Homogeneous nucleation
synthesis

The generation of nanoparticles from the
gas

phase requir
es the establishment of supersaturation
.
A means of

3

achieving the supersaturation

required to induce homogeneous nucleation of particles

is chemical reaction.
Chemical precursors are heated

and
/
or mixed to induce gas
-
phase reactions that produce

a state of

supersaturation in the gas phase.

2.1.1 Homogeneous nucleation reactors

Furnace flow reactors

Oven sources are the simplest systems to produce a saturated vapor for substances
having a large vapor pressure at intermediate temperatures up to about 1700
˚C. A crucible containing the source
material is placed in a heated flow of inert carrier gas. This has the disadvantage that the operating temperature
is limited by the choice of crucible material and that impurities from the crucible might be incorporate
d in the
nanoparticles. Nanoparticles are formed by subsequent cooling, such as natural cooling or dilution cooling. For
very small particles a rapid temperature decrease is needed which can be achieved by the free jet

expansion
method described later. Mat
erials with too low vapor pressure for obtaining appreciable particle density have to
be fed in the form of suitable precursors, such as organometallics or metal carbonyls, in the furnace. A recent
developed method of “aerotaxy” utilizing self
-
limited reac
tion between III V semiconductor particles in furnace
reactor is shown in Fig. 3.


Fig.

3
Schematic diagram of the aerosol generation, sizing and reaction process: aerotaxy.

[6]


Plasma reactors

A
nother means of providing the energy needed to

induce reac
tions that lead to supersaturation
and particle

nucleation is to inject the precursors into thermal

plasma. This generally decomposes them fully
into

atoms, which can then react or condense to form

particles when cooled by mixing with cool gas or

expansion

through a nozzle.

(Fig. 4)


Fig.
4

Schematic diagram of a
plasma
reactor

[9]




4

L
aser reactors
An alternate means of heating the particles to induce homogeneous nucleation is absorption of
laser energy. Compared to heating the gases in a furnace, this all
ows highly localized heating and rapid cooling,
since only the gas
(
or a portion of the gas
)
is heated, and its heat capacity is small. Heating is generally done
using an infrared
(
CO
2
)
laser, whose energy is either absorbed by one of the precursors or by
an inert

photosensitizer such as sulfur hexafluoride. The
iron

particles shown in Fig.6 were prepared by laser pyrolysis.

The main advantage

of laser
-
heating in gas
-
fl
ow systems is the absence of heated walls which reduces the

danger of product contaminati
on.





`



Flame reactors

Rather than supplying energy externally to induce

reaction and particle nucleation, one can carry
out the

particle synthesis within a flame, so that the heat needed

is produced in situ by the combustion reactions
(Fig.7)
.

Thi
s is

by far the most commercially successful approach to

nanoparticle synthesis
-
producing millions
of metric

tons per year of carbon black and metal oxides. However,

the coupling of the particle production to the
flame

chemistry makes this a complex proces
s that is rather

difficult to control. It is primarily useful for making

oxides, since the flame environment is quite oxidizing.

Recent advances are expanding flame synthesis to a

wider variety of materials and providing greater control

over particle morph
ology. (Fig. 8)





Fig. 5. Shematic diagram of the experimental apparatus. 1,
TEA CO
2

laser; 2, aperture; 3,

BaF
2

lens; 4, KBr

window; 5,
Pyrex glass irradiation cell; 6, Fe(
CO)
5

[7]

Fig. 6. Transmission Electron micrograph of the iron
ultrafine particles (magnification, 5* 10
5
) [7]

Fig. 7. Schematic of the experimental setup of a flame
reactor. [8]

FIG.

8.

TEM micrographs of TiO
2

particles made in a
nonstabilized, lamina
r flame as a function of the applied positive
field strength, with the needle electrodes kept at 0.1 cm from the
burner face: (a) No electric field, (b) 12 kV/cm, (c) 12.25 kV/cm,
and (d) 12.75 kV/cm. [8]


5

2.1.2
. Sputtering

Sputtering is a method of vaporizing materials from a solid surface by

bombardment with high
-
velocity ions of
an inert gas,
such as

Ar or Kr, causing an ejection of

atoms and clusters.
So this method is also cal
led
inert gas
evaporation (IGE) method
.

This process must be carried out in vacuum systems, below
0.1 Pa
, as a higher
pressure hinders

the transportation of the sputtered material. Instead of ions, electrons from an electron gun

can
be also used.
As early
as in 1982,
Iwama et al.
[12]

operated an electron gun at 10
-
3

Pa

separated by

a
di
ff
erential pumping system from a 1
00

Pa

evaporation chamber in order to evaporate Ti

and Al targets in a N
2

or NH
3

atmosphere, producing TiN and AlN nanoparticles smaller

th
an 10 nm. G
u
nther and Kumpmann
[13]

applied an electron beam

to
bulk oxides in an inert gas

atmosphere with pressures up to 5
00 Pa

in order to
produce 5 nm amorphous Al
2
O
3

and SiO
2

particles

and
crystalline Y
2
O
3

oxide powders.
They also found t
he
primary p
article size is rather insensitive to variations in gas pressure and evaporation rate. Magnetron
sputtering can be
used in a higher pressure level. A schematic picture of m
agnetron sputtering
system is shown
below.
Hahn and Averback
[14]

showed that a magn
etron sputter source can

be operated in the
100 Pa

range,
and
can be used for metals with high heat of vaporization
. T
hey successfully synthesized Al, Mo, Cu
91
Mn
9
,
Al
52
Ti
48

and ZrO
2

nanoparticles with diameters of 7
-
50nm.

Urban et al. [
15
] recently

demonst
rated formation of
nanoparticles of a dozen

different metals using magnetron sputtering of metal

targets. They formed collimated
beams of the nanoparticles

and deposited them

as nanostructured films on

silicon substrates. Sputtering has the
advantage that
it is mainly the target material which is heated and that

the composition of the sputtered material
is the same as that of the target.

The low pressure system provided a
very clean environment for powder
synthesis
, but it also
makes

further processing of t
he nanoparticles in aerosol form

difficult.


Fig.9.
Schematic drawing of the deposition system.

[15]

.

2.1.3
.
Inert gas condensation.

One of the earliest methods used to synthesize nanoparticles,

which is also p
erhaps the most straightforward
method of a
chieving

supersaturation
,
is the evaporation of a material in a cool inert gas, usually He or Ar, at
low
-
pressures

conditions, of the order of 1
00

Pa
. It is usually called ‘‘inert gas evaporation’’. This method is
well suited for

production of metal nanopa
rticles, since many metals

evaporate at reasonable rates at attainable
temperatures.

By including a reactive gas, such as oxygen, in the cold

gas stream, oxides or other compounds of

6

the evaporated

material can be prepared.


Common

vaporization methods are

resistive evaporation,
[16]
laser evaporation and sputtering. A
convective flow of inert gas

passes over the evaporation source and transports the nanoparticles formed above
the

evaporative source via thermophoresis towards a substrate with a liquid N
2

co
oled surface

[17]
.
A basic
experimental system is shown in Fig.10. Later, people developed several modification for this method. One

modification
from
Birringer and Gleiter
[18]
which consists of a scraper and a collection

funnel allows the
production of r
elatively large quantities of nanoparticles, which are

agglomerated but do not form hard
agglomerates and which can be compacted in the

apparatus itself without exposing them to air. Increased
pressure or increased molecular

weight of the inert gas leads t
o an increase in the mean particle size. Another
method replaces the evaporation

boat by a hot
-
wall tubular reactor into which an organometallic precursor in a
carrier

gas is introduced. This process is known as chemical vapor condensation referring to the

chemical
reactions taking place as opposed to the inert gas condensation method
.

[19]

T
he gas deposition method is also
used in industry. In this method,

nanoparticles are formed by evaporation in an inert gas at atmospheric pressure
and

transported by a
special designed transfer pipe to the spray chamber at a pressure of about

30 Pa
. By moving
the nozzle at the end of the transfer pipe, the particles which have

a mean velocity of 300 m/s can be deposited
in required places on the substrate in the spray

ch
amber. Using this technique writing micron
-
sized patterns was
demonstrated
[20]
.



Fig. 10.
Cross
-
section sketch of the inert gas condensation system.

[21]
Inconel pipe (1), the crucible containing the bismuth melt (2), the
furnace (3), the evaporation zo
ne (4), and the cap
-
free diluter (5). An inset schematic shows the diluter conCguration with gas return cap (6)
used to introduce the quenching gas perpendicular to the Bi
-
laden carrier gas jet.


A

systematic modeling study of

this method

is presented by
W
egner et al.

[21]

They

applied
this method
to
preparation of bismuth nanoparticles

(Fig.11.)
,

and

both visualization and computational

fluid dynamics
simulation of the flow fields in their

reactor

were achieved
. They clearly showed that they could control
the


7

particle size distribution by controlling the flow field

and the mixing of the cold gas with the hot gas carrying

the evaporated metal.

Most recent

advances in this method

have been in preparing composite nanoparticles and in

controlling the
morphology

of single
-
component nanoparticles

by controlled sintering after particle formation.

Nanda et al.

[22]

studied the in
-
flight sintering of PbS nanoparticles.

They were able to tune the band
-
gap of these semiconductor

nanoparticles by changing the particle s
ize

and morphology.



Fig. 11.
TEM picture ofspherical bismuth particles collected by thermophoretic


sampling from the gas phase.
[21]


2.1.4
.
Expansion
-
cooling.

Expansion of a condensable gas through a nozzle leads to

cooling of the gas and a subsequent

homogeneous
nucleation and condensation. In order to produce

nanoparticles smaller than 5 nm, supersonic free jets
expanding in a vacuum chamber with

pressures smaller than 10
-
2

Pa

have been used.
[23]

In the work of

Bowles
et al. (1981)
[24],

an inert gas
containing a metal vapor was subjected to multiple

expansions. After a first sonic
expansion, a mixture of molecular clusters was prepared

turbulently with a quench gas and undergoes a second
sonic expansion resulting

in homogeneous nucleation. Then a clus
ter growth region in a subsonic, low
-
pressure,

fast
-
flow reactor produced nanoparticles with mean sizes below 2.5 nm. A
lso a

controlled mean size ranging
from the dimer up to several thousand of the monomer species is possible. Converging nozzles which cre
ate an
adiabatic expansion in a low
-
pressure flow

have also been used to produce nanoparticles (Bayazitoglu et al.,
1996).

[25]

Although the

particles sizes are larger than in a vacuum expansion, particles of the order of 100 nm
were

obtained with a relati
vely high production rate. They
also
studied the effects of nozzle initial pressures and

nozzle half angles on the nucleation and, therefore, on size distribution of the exiting particles.

And
the width of
particle size distribution increased with the

incr
ease of nozzle pressure. A modified method of
producing 4

10
nm

sized nanoparticles by expanding a thermal plasma carrying vapor
-
phase precursors

through a ceramic
-
lined
subsonic nozzle

has also been developed to obtain a narrow size distribution
.
[26]


8

2.1
.5
Laser vaporization

This technique uses a laser which evaporates a sample

target in an inert gas flow reactor.
(Fig.12)
The source
material is locally heated

to a high temperature enabling thus vaporization. The vapor is cooled by collisions
with the

ine
rt gas molecules and the resulting supersaturation induces nanoparticle formation.

Laser
vaporization techniques provide several advantages over other heating

methods such as the production of a
high
-
density vapor of any metal;

the generation of a directio
nal high
-
speed metal vapor from the solid

target,
which can be useful for directional deposition of the particles; the

control of the evaporation from specific areas
of the target; and the

simultaneous or sequential evaporation of several different targets
.

[27]
Nanocomposites
can also be produced, Kato [
29
] used a continuous
-
wave CO
2

laser with a power of 100 W to prepare
nanoparticles between 6 and 100 nm of many complex refractory oxides such as Fe
3
O
4
, CaTiO
3

and Mg
2
SiO
4

from powders, single crystals or

sintered blocks.

A
modified
method which combines laser vaporization of metal
targets with controlled condensation in a diffusion cloud chamber is used to synthesize nanoscale metal oxide
and metal carbide particles (10
-
20 nm)
, and
very porous aggregates

w
ere obtained
.
[30]



Fig.12.
The schematic diagram of the laser vaporization flowtube reactor
. [28]


2.1.6 Spark source

A high
-
current spark between two solid electrodes

can be used to evaporate the electrode material for creating
nanoparticles. At the

ele
ctrodes a plasma is formed.
(Fig.13)
This technique is used for materials with a high
melting point

such as Si or C, which cannot be evaporated in a furnace.

Reactive evaporation is also possible by
adding a suitable

reactant gas.



Fig.13.
A schematic di
agram of the spark source used to generate luminescent nanometer
-
scale clusters.
[31]


9

2.2
.
Laser ablation

Laser ablation is a technique in which a pulsed laser rapidly heats a very thin (<100 nm)

layer of substrate
material

(Fig.14)
, resulting in the format
ion of an energetic plasma above the

substrate. This technique should
be distinguished from laser vaporization, as apart from

atoms and ions also fragments of solid or liquid material
are ablated from the substrate

surface which vary in size from sub
-
nanom
etric to micrometric. Therefore, it
cannot be

considered as a pure homogeneous nucleation process. The pulse duration and energy

determines the
relative amounts of ablated atoms and particles. The nonequilibrium nature

of the short
-
pulse (10

50 ns) laser
h
eating enables the synthesis of nanoparticles of materials

which normally would decompose when vaporized
directly. The material removal rate by laser

ablation decreases with longer target exposure times, therefore the
target is usually rotated.

When used f
or producing films, this technique is called pulsed laser deposition (PLD).




Fig. 14.
Schematic drawing of the laser ablation chamber.

[35]


Examples of nanoparticle preparation using this method include
magnetic oxide

nanoparticles by Shinde
S.R. et al
.
[32]
, titania nanoparticles

by Harano et al.
[33]
, and hydrogenated
-
silicon nanoparticles

by Makimura
et al.
[34]
.

T
he

operating conditions
can be altered

to
select

particle formation
or film formation
. Yamamoto and
Mazumder
[35]

showed that laser ablati
on of NbAl
3

at He pressures of 0.1

Torr

did not produce any
nanoparticles while an operating

pressure of 1
Torr

resulted in the formation of 6 nm nanoparticles with the
same

stoichiometry as the substrate

(Fig.15)
. Typical production rates are in the order

of micrograms per

pulse
with pulse frequencies of about 50 Hz, yielding 10

100 mg powder per hour.


10


Fig. 15.
Production rate as a function of He back
-
filled gas

pressure changing laser pulse energy.

[35]


The theoretical development and

analysis

for
las
er ablation
technique were presented by
Marine et al.
[36]
using
Zeldovich and Raizer theory of condensation
.

Reactive

laser ablation in which a reaction of the ablated
material with the reactor gas occurs is also

used. Johnston et al.

[37]

ablated an Al t
arget in an O
2

atmosphere,
producing Al
2
O
3

nanoparticles.


2.3
. S
pray systems

A simple way to produce nanoparticles is to evaporate micron
-
sized droplets of a dilute

solution. By choosing
the appropriate solute concentration, nanosized particles consisting

of the solid residue can be obtained.
However, a

serious problem here is that all the

impurities present in the liquid will concentrate in the solid
residue. Rather than delivering the nanoparticle precursors into

a hot reactor as a vapor, one can use a n
ebulizer
to

directly inject very small droplets of precursor solution.

This has been called spray pyrolysis, aerosol
decomposition

synthesis,
or
droplet
-
to
-
particle conversion.
A
recent example of

this
is

preparation of TiO
2

nanoparticles by Ahonen et al.

[38] They studied the
size change and crystallization of monodisperse titanium
dioxide particles in an aerosol flow

reactor
, and
observed
i
ncreasing mobility diameters of constant volume
particles
at
above
1000۫
C for
60

nm particles and above
1200۫
C for 12
0 nm particles.

Moreover, they successfully
formed
single
-
crystal particles

at these temperatures. (Fig.16)



11


Fig. 16.
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 .

[38]


Furthermore, it is necessary to start from small droplet sizes which are di
ffi
cult to

obtain in normal spray
systems.

Chen et al.
[39]

showed that an electrospray system operated in the cone
-
jet mode

(Fig.17)
could yield
small droplets with a na
rrow size distribution. To avoid droplet explosion

during evaporation, the highly charged
aerosol is first passed through a radioactive

neutralizer before the evaporation takes place. It is important here to
avoid droplet

explosion since that would deterio
rate the narrowness of the size distribution. Using

a sucrose
solution, particles as small as 4 nm were obtained.


Fig. 17.
Schematic diagram of the eiectrospraying system.

[39]


In another work (Hull et al.,

1997)

[40]

Ag particles with a mean size of 1
0 nm were produced by
electrospraying a dilute

AgNO3 solution in methanol onto a grounded substrate.

Kim and Rye

[41]

developed a
special charge injection technique in order to obtain

very high charge densities.
Their e
lectrospray atomization
produced subm
icrometer precursor droplets which were dispersed in air and carried through an electric furnace
for thermal decomposition for several seconds.

It is stated that the higher the surface charge density of the

electrospray jet is, the smaller is the size of t
he ejected droplets. Spraying a 10 vol% TEOS solution in

ethanol
in a chamber filled with O
2

resulted in 30

100 nm sized
nano

particles.




12

3. Recent developments and prospective advances for gas phase techniques.

The
gas
-
phase or
vapor
-
phase synthesis of n
anocrystals (quantum dots) has several advantages over traditional

liquid
-
phase techniques, including better compatibility with existing operations in the microelectronics

industry.
On the other hand, it makes size control and surface passivation a more ch
allenging task.
To this understanding,
developments for gas phase
synthesis

lies in either expanding its advantages or overcoming its shortcoming.
Here, we present three aspects of advances of gas phase synthesis techniques.


3.1
Advances in instrumentati
on

Because vapor
-
phase nanoparticle synthesis often

takes place on short timescales, in small regions of a

reactor,
and in complex mixtures, improvements in

methods for characterization of reactor conditions and

particle
formation are essential to improved

understanding

and control of particle formation. Thus, a few

examples of
the current state
-
of
-
the art are included

here.

Nakata et al.
[42]
used a combination of laser
-
spectroscopic imaging techniques
and laser ablation
to image
the

plume of Si atoms and
clusters formed during synthesis

of Si nanoparticles. They investigated the
dependence

of the particle formation dynamics on the background

gas, and found that it was substantial.

Cho
and

Choi
[43]

combined localized thermophoretic sampling and in situ lig
ht scattering measurements to
characterize

particle concentration, size, and morphology during

flame synthesis of silica nanoparticles.

Kim

et
al.[44]
synthesized nano
-
sized Al
2
O
3

powders by a thermal MOCVD (Metal Organic Chemical Vapor
Deposition) combine
d with plasma.

Methods combining TEM imaging for in
-
situ investigation have also been
developed. [45]



3.2 Advances in modeling and simulation

Because in situ characterization and control of many

vapor
-
phase nanoparticle syntheses is difficult, modeling

s
tudies can play an important role in the development

and improvement of these processes. Several of the

studies cited above had significant modeling components.

Some additional advances in the modeling of

vapor
-
phase particle synthesis are included here.

A
ristizabal

et al. [46] developed
a two
-
dimensional axisymmetric turbulent model of a particle generator
with radial injection of a quenching gas

to gain a better understanding of the particle forming process.

The
model provides

information on distributions

of flow, temperature and concentration fields and particle
generation within the reactor

as well as mixing cup data as a function of reactor length.

There have

recently been
many important developments in modeling

multidimensional particle size distributi
ons, where both

particle
volume and surface area or some other pair of

particle characteristics are explicitly treated. These

include
methods presented by Muhlenweg et al.
[47]
, Tsantilis et al.
[48]
, Lee et al.
[49].

Continuing

improvements in simulation
methodologies, along with

inevitable advances in computing
power, are beginning

to make possible the coupling of detailed chemical

reaction kinetics, multidimensional
particle size distributions,

and computational dynamics simulations

in two or even three
dimensions to create
models that

quantitatively describe the details of particle formation

processes and that compare reasonably with
experiment.


3.3 Advances in synthesis of multi
-
component nanoparticles

M
ulti
-
component nanoparticles offers many possibi
lities for optoelectronic applications
. For example, AlInGaN
semiconductor quantum dots

in the blue and ultraviolet emission range has special flexibility
s
ince band gap and
lattice constant can be adjusted independently, the following reduction of strain
and defects in the

active layer
will enhance some characteristics of nitride laser diodes, like output power or lifetime.


13

Yin

et al. [50]
prepared InAs mid
-
infrared emissive quantum dots on a graded

InxGa1
-
xAs/InP matrix with
more uniform size and higher d
ot density by low pressure metal organic chemical vapor

deposition
(LP
-
MOCVD) under safer growth conditions.

The emission wavelength of the

QDs reaches >2.1 µm.

Solorzano

et al. [51]
investigate the growth of AlxInyGa1
-
x
-
yN and the formation mechanism
, the
y found

the formation of quantum dots depends on the main

epi
taxy parameters like growth temperature, amount of
deposited material, TMAl flow or even its carrier gas.

They also
achieved a narrow distribution of quantum dots
and a large dot density

by MOVPE.


4. Summary

In this paper, we presented a
n overview of meth
ods for preparing nanoparticles in the gas phase
, and recent
developments and advances for gas phase synthesis techniques are discussed.
As can
be seen,
a large number of
synthesis methods of nanoparticles in the gas phase have

been developed in the last 4
0 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, and

includes
multi
-
component and d
oped materials.

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 nano
particles
.


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