Page 40 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Chapter 2
2.2 Characterization Methodologies and Instrumentation Techniques
In the past few decades, the development of new techniques led to a considerable progress in both the synthesis and characterization of nanoparticles. Intensive studies were carried out to study and control the processing parameters for assembling and tailoring their properties to meet particular application, with the ultimate goal to understand how the composition, size, shape, and ordered assembly of nanoparticles affect the bulk properties of materials. With the development of the nanotechnology and instrumentation techniques, currently various techniques have been developed to characterize the microplasma-assisted precursor dissociation process as well as the generated nanoparticles. As a consequence, a better understanding of the complex kinetics and the underlying mechanisms of the process have been achieved in recent years. In this section the in-situ characterization of the plasma- assisted dissociation process will be firstly reviewed, followed by the introduction of the complementary analytical methods for ex-situ characterizing the generated nanoparticles.
2.2.1 In-situ Characterization of the Plasma-assisted Precursors Dissociation Process
The simplest and most convenient way to characterize the microplasma-assisted precursor dissociation process is via the visual observation of the operating plasmas. Due to the presence of electronically excited radicals in the discharges, they will give off photons and emit light when relaxing back to ground state. Meanwhile, since different species have their specific excited states and emit lights of different colors (wavelengths), the visual appearance of plasma can provide some direct and useful information about the radical states existing in the plasma. In the Chapter 3, Figure 3.2 will show the visual appearance of argon discharges with/without ferrocene vapor to illustrate the change of plasma color.
Optical emission spectroscopy (OES) is a non-intrusive technique in plasma diagnostics. It allows an identification of excited states and gives valuable information of reactive species (e.g., excited ions, molecules, etc.) that exist in plasma. In general, by recording the emitted spectrum and correlating the spectral features with emission peaks of the precursor originated radicals, one can not only examine the chemical components in the plasma, but also can have the detailed radiative transition information of the precursor fragments. Based on the intermediate radicals, it is also possible to get an insight into the complex precursor dissociation process and even the underlying mechanisms. Therefore, it has been widely applied for characterizing the plasma-assisted nanofabrication processes.1–4 Moreover, since the rotational temperature of the atmospheric pressure plasma can be regarded as an approximation of the gas temperature, by fitting the experimental spectra profile of certain bands with the simulated ones (by i.e. SPECAIR model), the plasma gas temperature can be estimated.5–8 Among them the second positive system (SPS) of emission bands of nitrogen or the swan bands of C2 molecules has long been utilized to extract information from plasma spectra.9–12 In present research, OES is used during the synthesis of each type of nanomaterials and under various experimental conditions to characterize microdischarge. Detailed information can be referred to the Chapter 3 - the Cahpter 7 (Chapter 3- gas phase synthesis of oxides; Chapter 4- gas phase synthesis of nitrides; Chapter 5- gas phase synthesis of metal nanoparticles; Chapter 6 and Chapter 7- liquid phase synthesis of rare earth oxides).
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