Page 58 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Chapter 3
In Figure 3.3(a), no evidence of products related to ferrocene decomposition is detected. As previously mentioned, this could be the reason that at room temperature ferrocene vapor pressure is too low to affect discharge spectra. In comparison, when temperature reaches 323.15 K, emission spectrum shows several lines corresponding to Fe or carbonaceous species. The carbon related emission lines are mainly assigned to CH radicals (370-435 nm) or C2 swan band transitions (460-570 nm).19 A low intensity band observed at 405 nm can be associated with emission from 4050 group of C3 radicals.20,21 Peak observed at 248.5 nm can be ascribed to Fe radicals, based on preceding research and NIST Atomic Spectra Database.22 However, due to quite limited resolution of employed spectrometer, other Fe peaks were not visible in the spectra. Low intensity lines observed at 489.6 nm and 657.2 nm are corresponding to Hβ and Hα respectively. Table 3.2 presents a summary of the intensive emission lines of hydrogen and carbonaceous species observed in the spectra.23,24 As shown in Figure 3.3(c), the raise of temperature to 339.15 K at constant dissipated discharge power results in an apparent increase of these emission line intensities, indicating more precursor molecules are dissociated in the plasma. Obviously, this is because higher concentration of ferrocene vapors can be reached at increased temperature. When higher power is imposed to the reactor, there is no visible increase of Fe line at 248.5 nm. By contrast, a significant increase of line intensities related to H (489.6 nm, 657.2 nm) and carbonaceous species (432 nm, 467-474 nm, 516.5 nm, 558.6 nm and 563.6 nm) are detected. This behavior of spectral intensities can be explained as follows. It is known that cyclopentadienyl rings are quite stable because of their mesomeric nature.25 As reported in Ref. [26], in CVD process the decomposition of metallocene always takes place first by breaking the metal- cyclopentadienylbonds while further fragmentation requires more energy. Therefore, dissociation rate of cyclopentadienyl rings is expected to be a steep function of dissipated discharge power. Higher power of 2.27 W primary leads to the rise in electron density, and the reactions rates involving electrons also increase, which in turn, resulting in higher cyclopentadienyl rings dissociation degree.
Table 3.2 Summary of the carbon and hydrogen intensive emission lines collected from spectra
Species C2
CH
H Ar
System Swan system
4300 Å
3900 Å Balmer series Ar I
Transition
A 3∏→X 3∏, ground state
A 2∆ →X 2∏, ground state B 2∏−→X 2∏, ground state n→2s,2p
4p→4s
Wavelength
467.9 nm (5,4), 468.5 nm (4,3), 469.8 nm (3,2), 471.5 nm (2,1), 473.7 nm (1,0), 516.5 nm (0,0),
558.6 nm (1,2), 563.6 nm (0,1) 432.0 nm (0,0)
389.3 nm (0,0)
489.6 nm (Hβ), 657.2 nm (Hα) 696.5 nm (1s5-2p2), 706.7 (1s5-2p3), 738.4 nm (1s4-2p3), 750.4 (1s5-2p1), 763.5 nm (1s5-2p6), 772.4 (1s3-2p2), 794.8 nm (1s3-2p4), 826.5 (1s2-2p2), 842.5 nm (1s4-2p8)
Therefore, cyclopentadienyl rings cannot be dissociated easily at lower plasma power (1.05 W). By comparison, higher plasma power (2.27 W) leads to an increase in electron temperatures and electron density. More energetic species are produced in plasma, which 46