Synthesis of nanoparticles in the gas phase for electronic, optical and magnetic applications—a review
Introduction
Nanoparticles have attracted the attention of an increasing number of researchers from several disciplines in the last 10 years. 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. The term nanoparticle is generally used now in the materials science community to indicate particles with diameters smaller than 100 nm (El-Shall and Edelstein, 1996). However, in most of the discussed applications in this review, the particle sizes are less than 50 nm, in accordance with the definition by the editors of this special issue. The term nanoparticles is used here interchangeably to refer to particles in aerosols and particles in other situations, respectively. A closely related but not identical concept, “cluster”, indicates smaller nanoparticles with less than 104 molecules or atoms, corresponding to a diameter of only a few nanometers.
Nanomaterials or nanostructured materials have a characteristic length scale of less than 100 nm, and therefore also include uni-axially stacked multilayers and coatings. A further subset can be distinguished in these nanomaterials, i.e. the nanophase materials which have a three-dimensional structure with a domain size smaller than 100 nm. Nanophase materials are usually produced by compaction of a powder of nanoparticles. They are characterized by a large number of grain boundary interfaces in which the local atomic arrangements are different from those of the crystal lattice (Weissmüller, 1996).
The small size of nanoparticles, which is responsible for the different properties (electronic, optical, electrical, magnetic, chemical and mechanical) of nanoparticles and nanostructured materials with respect to the bulk material, makes them suitable for new applications. Having a size between the molecular and bulk solid-state structures, nanoparticles have hybrid properties which are incompletely understood today, creating a challenge for theoreticians as well. Some examples of these properties are lower melting temperature (Goldstein et al., 1992), increased solid–solid phase transition pressure (Tolbert and Alivisatos, 1995), lower effective Debye temperature (Fujita et al., 1976), decreased ferroelectric phase transition temperature (Ishikawa et al., 1988), higher self-diffusion coefficient (Horvath et al., 1987), changed thermophysical properties (Qin et al., 1996) and catalytic activity (Sarkas et al., 1993).
In this review, besides nanoparticles we also discuss “nanocomposites” which consist of nanoparticles dispersed in an continuous matrix, creating a compositional heterogeneity of the final structure. The nanocomposites usually involve a ceramic or polymeric matrix and are not restricted only to thin films. These materials show interesting properties such as alloying of normally immiscible materials (Linderoth and Moerup, 1990) and higher critical superconductor transition temperature (Goswami et al., 1993).
Since size-dependent properties are related to the aforementioned structure of nanostructured materials, these materials are considered also with respect to their potential technological applications. This has led before the 1990s to applications such as supported nanoscale catalysts and pigments, based mainly on the large surface area to volume ratio in these systems. After 1980, a renewed interest took place in nanomaterials research. Brus (1983)suggested quantum confinement effects to occur specifically in semiconductor nanoparticles. Birringer et al. (1984)developed a method for synthesizing amounts of weakly agglomerated nanoparticles for producing nanophase materials with a large volume fraction of grain boundaries. Improved mechanical properties of nanophase ceramics were observed in these materials, such as increased hardness. Finally, one of many other important findings was the giant magnetoresistance in nanocomposites discovered by Carey et al. (1992).
Since the special issue is devoted to the aerosol community this review paper is limited to nonvacuum systems, i.e. atmospheric and low-pressure systems, by choosing an arbitrary lower limit at 1 mbar. This is roughly in accordance with the tradition of this journal, which has included in the past papers on low-pressure impactors but excluded results of high-vacuum cluster research.
One important field of research deals with aerosol-assisted processes used for film synthesis in which liquid droplets are used only as source and transport vehicles to the substrate. These processes are called aerosol-assisted chemical vapor deposition (AACVD) (Xu et al., 1994), aerosol metal-organic CVD (A-MOCVD) (Fröhlich et al., 1995), the pyrosol process (Blandenet et al., 1981) and aerosol CVD (Tourtin et al., 1995). Common to these processes is the evaporation of micron-sized droplets in proximity to the substrate to produce epitaxial films (Jergel et al., 1992). However, in this review we will be dealing mainly with the formation of films where nanoparticles retain their discreteness.
Developing methods for the synthesis of nanoparticles was also taking place in other than gas-phase processes such as the research of nanoparticles in colloidal systems where stabilization is used to prevent coagulation (Peled, 1997). However, gas-phase processing systems may prove better in some cases because of their following inherent advantages:
- 1.
(a) Gas-phase processes are generally purer than liquid-based processes since even the most ultra-pure water contains traces of minerals, detrimental for electronic grade semiconductors. These impurities seem to be avoidable today only in vacuum and gas-phase systems.
- 2.
(b) Aerosol processes have the potential to create complex chemical structures which are useful in producing multicomponent materials, such as high-temperature superconductors (Kodas et al., 1988).
- 3.
(c) The process and product control is usually very good in aerosol processes. Particle size, crystallinity, degree of agglomeration, porosity, chemical homogeneity, stoichiometry, all these properties can be controlled with relative ease by either adjusting the process parameters or adding an extra processing step, e.g. sintering or size fractionation.
- 4.
(d) Being a nonvacuum technique, aerosol synthesis provides a cheap alternative to expensive vacuum synthesis techniques in thin or thick film synthesis (Wang et al., 1990). Furthermore, the much higher deposition rate as compared to vacuum techniques may enable mass production.
- 5.
(e) An aerosol droplet resembles a very small reactor in which chemical segregation is minimized, as any phases formed cannot leave the particle (Kodas et al., 1989).
- 6.
(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.
Section snippets
Electronic, optical and magnetic applications of nanoparticles
In this section, the use of nanoparticles for electronic, optical and magnetic applications is discussed. A broad spectrum of materials including insulators, semiconductors, superconductors, metals and alloys, optical, and artificially structured materials can be used for these purposes. Most applications use films composed of nanoparticles, supported by a substrate material. In this section the main phenomena making specific applications possible by nanoparticles are reviewed, while in Section
Synthesis methods of nanoparticles in the gas phase
Most synthesis methods of nanoparticles in the gas phase are based on homogeneous nucleation in the gas phase and subsequent condensation and coagulation. The ablation of a solid source with a pulsed laser can also yield nanoparticles, but the formation mechanism does not involve a homogeneous nucleation step. Instead, clusters or even larger particles are ejected from the surface and may undergo coagulation. A micron-sized aerosol droplet may also yield nanoparticles by evaporating a
Synthesis of nanoparticles in the gas phase for electronic, optical and magnetic applications
In the preceding sections the applications of nanoparticles and the synthesis methods in the gas phase were described. In the final section of this review we bring most of the relevant works done using gas-phase methods to obtain nanoparticles or nanoparticle-based films and powders for electronic, optical and magnetic applications.
Conclusions
As shown at length in this review, there is a great deal of interest today in the special properties of nanoparticles and their potential applications. In this work, we gave also an overview of the potential applications using nanoparticles for electronic, optical and magnetic applications stemming from different scientific disciplines. It is anticipated then to see many new applications as they will be developed in the years to come. As one can gather from this review, a large number of
Acknowledgements
The authors would like to thank Mrs Doris Ponten for her assistance in the literature research. This work was supported by the Deutsche Forschungs Gemeinschaft in the framework of the special research program SFB209.
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