Gas Phase Growth Techniques for Quantum Dots
Weiqiang Wang1,*, Muzhou Jiang2
1
Department of Mechanical Engineering, University of Rochester; 2Department of Electrical and Computer Engineering, University of
Rochester; *Corresponding author(Email: camp213400@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 during 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
the related concepts, “nanoscaled” or “nanosized” particle. Until then, the more general terms submicron and
ultrafine particles were used. From a scientific point of view, nanoparticles (Fig. 1) are of great scientific interest
as they are effectively a bridge 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 confinement 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, through energy transfer from nano-thin layers of quantum
wells to nanocrystals above the nanolayers.
1
Nanoparticles have been suggested recently for various potential applications in electronics (Fig. 2) where
quantum confinement effects 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 resulting 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 researched for use in diode lasers,
amplifiers, and biological sensors. Scientists make efforts to give this invisible matter to boarder applications,
for instance, fabrication of optical memories and organic dyes in modern biological analysis.
Methods for the 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 processes 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 structures 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 cheap alternative to expensive vacuum synthesis
techniques in thin or thick film 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 in a batch form. Batch processes can result in product
characteristics which vary from one batch to another.
2. Synthesis method of quantum dots using gas phase growth technology
Most synthesis methods of nanoparticles in the gas phase are based on homogeneous 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 occur 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 supersaturation 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 colloidal 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 detail in the following sections.
2.1. Homogeneous nucleation synthesis
The generation of nanoparticles from the gas phase requires the establishment of supersaturation. A means of
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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 incorporated 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. Materials 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 reaction 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 Another means of providing the energy needed to induce reactions 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]
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Laser 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 allows 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 (CO2) 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-flow systems is the absence of heated walls which reduces the
danger of product contamination.
Fig. 5. Shematic diagram of the experimental apparatus. 1,
TEA CO2 laser; 2, aperture; 3, BaF2 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* 105) [7]
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). This 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 process 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 morphology. (Fig. 8)
Fig. 7. Schematic of the experimental setup of a flame
reactor. [8]
FIG. 8. TEM micrographs of TiO2 particles made in a
nonstabilized, laminar 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]
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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 called 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
differential pumping system from a 100 Pa evaporation chamber in order to evaporate Ti and Al targets in a N2
or NH3 atmosphere, producing TiN and AlN nanoparticles smaller than 10 nm. Gunther and Kumpmann [13]
applied an electron beam to bulk oxides in an inert gas atmosphere with pressures up to 500 Pa in order to
produce 5 nm amorphous Al2O3 and SiO2 particles and crystalline Y2O3 oxide powders. They also found the
primary particle 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 magnetron sputtering system is shown
below. Hahn and Averback [14] showed that a magnetron sputter source can be operated in the 100 Pa range,
and can be used for metals with high heat of vaporization. They successfully synthesized Al, Mo, Cu91Mn9,
Al52Ti48 and ZrO2 nanoparticles with diameters of 7-50nm. Urban et al. [15] recently demonstrated 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 the 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 perhaps the most straightforward
method of achieving supersaturation, is the evaporation of a material in a cool inert gas, usually He or Ar, at
low-pressures conditions, of the order of 100 Pa. It is usually called ‘‘inert gas evaporation’’. This method is
well suited for production of metal nanoparticles, 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
5
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 cooled 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 relatively 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 to 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] The 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 chamber. 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 zone (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 Wegner 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
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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 size 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 cluster growth region in a subsonic, low-pressure,
fast-flow reactor produced nanoparticles with mean sizes below 2.5 nm. Also a controlled mean size ranging
from the dimer up to several thousand of the monomer species is possible. Converging nozzles which create 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 relatively 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 increase 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]
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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 inert 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 directional 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 CO2 laser with a power of 100 W to prepare
nanoparticles between 6 and 100 nm of many complex refractory oxides such as Fe 3O4, CaTiO3 and Mg2SiO4
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 were 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 electrodes 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 diagram of the spark source used to generate luminescent nanometer-scale clusters.[31]
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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 formation 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-nanometric 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
heating 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 for 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].
The operating conditions can be altered to select particle formation or film formation. Yamamoto and
Mazumder [35] showed that laser ablation of NbAl3 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.
9
Fig. 15. Production rate as a function of He back-filled gas
pressure changing laser pulse energy. [35]
The theoretical development and analysis for laser 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 target in an O2 atmosphere,
producing Al2O3 nanoparticles.
2.3. Spray 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 nebulizer
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 TiO2
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 increasing mobility diameters of constant volume
particles at above 1000۫C for 60 nm particles and above 1200۫C for 120 nm particles. Moreover, they successfully
formed single-crystal particles at these temperatures. (Fig.16)
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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 difficult 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 narrow 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 deteriorate 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 10 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 electrospray atomization
produced submicrometer 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 the ejected droplets. Spraying a 10 vol% TEOS solution in ethanol
in a chamber filled with O2 resulted in 30–100 nm sized nano particles.
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3. Recent developments and prospective advances for gas phase techniques.
The gas-phase or vapor-phase synthesis of nanocrystals (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 challenging 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 instrumentation
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 light scattering measurements to
characterize particle concentration, size, and morphology during flame synthesis of silica nanoparticles. Kim et
al.[44] synthesized nano-sized Al2O3 powders by a thermal MOCVD (Metal Organic Chemical Vapor
Deposition) combined 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
studies 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.
Aristizabal 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 distributions, 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
Multi-component nanoparticles offers many possibilities for optoelectronic applications. For example, AlInGaN
semiconductor quantum dots in the blue and ultraviolet emission range has special flexibility since 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.
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Yin et al. [50] prepared InAs mid-infrared emissive quantum dots on a graded InxGa1-xAs/InP matrix with
more uniform size and higher dot 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, they found
the formation of quantum dots depends on the main epitaxy 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 an overview of methods 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 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, and includes
multi-component and doped 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 nanoparticles.
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