Optical Reflection Spectrum

The charge transport and optical properties of the [Si(Pc)0]-(tos)y)n materials as y=0 -+ 0.67 are reminiscent of the [Si(Pc)0]-(BF4)y)n system, but with some noteworthy differences. Again there is an insulator-to-metal transition in the thermoelectric power near y 0.15-0.20. Beyond this doping stoichiometry, the tosylates also show a continuous evolution through a metallic phase with decreasing band-filling. However, the transition seems somewhat smoother than in the BF4 system for y)>0.40, possibly a consequence of a more disordered tosylate crystal structure. Both [Si(Pc)0]-(tos)y)n optical reflectance spectra and four-probe conductivities are also consistent with a transition to a metal at y 0.15-0.20. Repeated electrochemical cycling leads to considerably more decomposition than in the tetrafluoroborate system. 

Fig. 9.2 Optical reflectance spectra of the sNPS layer depend on the nanocrystallite dimensions 1-5 nm, 2-15 nm, 5-30 nm...

The optical reflectance spectra were dependent on the nanocrystallite structure and dimensions, porosity and the layer thickness (Fig. 9.2). The maximal photosensitivity in the visible wavelength range of the spectra (30-35 mA/Lm) was typical of the sNPS layers with the nanocrystallite dimensions of 15 nm, and it decreased with increasing size of the nanocrystallites. The maximal sensitivity to the ultraviolet irradiation was obtained for sNPS layers with nanocrystalhte dimensions of 20-25 nm. sNPS layers obtained by electrochemical etching as well as by chemical etching showed the photoluminescence typical of this material a broad peak in the visible spectrum with the intensity sufficient for observation of the photoluminescence with a naked eye. sNPS samples obtained by chemical or electrochemical etching had intensive emission with the maximum at A, 640 nm and 700 nm. 

The photocorrosion of the zeolite encapsulated CdS at room temperature in air was followed by recording optical reflectance spectra after various times of illumination with a Xe-high-pressure lamp (150 mW cm-2). Two samples with the same degree of ion-exchange (a = 20%) either dehydrated or not dehydrated prior to sulfidation were used in the photocorrosion experiments. 

The crystal field optical transitions of Cr + in an octahedral site are shown in Fig. 26. The splitting of the upper energy levels by the weak trigonal distortion ( 350 cm ) (67,168-170) of the octahedron cannot be resolved in powder samples of chromia-alumina. The optical reflectance spectra shown in Fig. 27 show a gradual shift of the absorption maxima near 17,000 and 23,000 cm (15,39,171) due to the lattice expansion that results when chromia is added to an alumina lattice. The optical spectra conform to the Tanabe and Sugano (168,172-174) theoretical calculations. The Racah parameter varied with the chromium content (15,171), Variable temperature (169,175) and variable pressure (176,177) optical spectra have been obtained for ruby samples. 

The same chromia-alumina samples which had been examined earlier by X-ray diffraction, gas adsorption, NMR, and ESR were also studied optically 15,179). The spectra obtained with the samples which had been heated at 1400° were discussed in the previous paragraph, so here the results from the optical reflection spectra of the coprecipitated samples calcined at 500, 760 and 900° will be reviewed. 

Fig. 27. Optical reflectance spectra of a-phase chromia-alumiiia (15).

Fio. 28. Optical reflection spectra of reduced 500° chromia-alumina (15). 

Fig. 29. Optical reflection spectra of oxidized and reduced chromia-alumina plus a spectrum of chromic acid (CrOa) on alumina (0.038 mole % Cr) IS).

The optical reflectance spectra were recorded for samples of oxidized coprecipitated chromia-alumina calcined at 500°, 750°, and 900°C. The 0 + concentration measmed optically was found to be proportional to the specific chromia area. This proportionality was observed despite the pronounced decrease in area that resulted from raising the calcination temperature from 500 to 900°. It indicates that only the surface chromium ions take part in the oxidation-reduction process that results from exposure to reactive gases at elevated temperatures. The ESR, NMR, and other measurements described above detected the chromium ions that were relocated within the alumina lattice by the higher calcination temperatures. 

Table 9.4 Materials parameters obtained from the optical reflection spectra of the (Fa)2PFg crystal (28). cop is the unshielded plasma frequency, r the mean scattering time, Coo the background dielectric constant, oq the optical conductivity for oi— O, m pf the effective mass of the charge carriers, vp the Fermi velocity, and Ep the Fermi energy.

With the value of the Fermi velocity Vp from Table 9.3, we obtain from Dy an independent value of the scattering time ry = Dy/v = 1.3 lO s. It is to be sure somewhat different from the values determined from the optical reflection spectra and from the conductivity (Table 9.4), but the differences are negUgible considering the different counterions and the very diverse methods and approximations in the data analyses. 

Figure 9-35. Optical reflectance spectra of the Cei.,TiiP207 sunscreens having various Ce/Ti ratios. (Reprinted with permission from ref 54. Copyright 2003 American Chemical Society)...

Studies of the optical reflection spectra of a POA-TS single crystal fibre subjected to uniaxial stress parallel to the direction hcwed that peak A In Fig. 1 shifted to hl er energy (28). The energy of the peak Increased linearly with strain by 37 meV/% up to the maximum 4% strain at which the fibre broke. 

Fig. 123. Optical reflection spectra of YbS at 6, 47 and ll6kbar. The vertical dashed lines indicate the energy below which interference noise is observed. The spectra are not corrected for losses at the diamond face. (After Syassen et al. 1985.)...

Fig. 14. (top) Optical reflectivity spectra, (bottom) Optical conductivity spectra of Lai.33Sro.67Fe03 as a function of temperature. (Top inset - charge-ordering structure of Lai.33Sro.67Fe03) [232]... 

Figure 13-16 shows optical reflection spectra obtained for three 2D photonic crystals consisting of PLZT ceramic pillars with different values of r and , sample A... 

Major features of the optical reflectance spectra [18] are the characteristic PDA absorption band strongly polarized along the chain direction and a near infrared charge transfer band involving the polymer chain and the electron deficient pyridine rings. A similar charge transfer band has recently been reported for a different PDA [19]. 

---