Optical gap

Ogut S, Chelikowsky J R and Louie S G 1997 Quantum confinement and optical gaps in Si nanocrystals Phys. Rev. Lett. 79 1770... 

The absorption and photoluminescence (PL) spectra of a-6T measured at 80 K are shown in Figure 7-24. There is an onset of absorption at 2.1 eV, with several sharp peaks and a maximum at 2.8 eV. A second absorption band is seen at 4.5 eV and is due to direct excitation of the thiophene ring. We take the first peak as the 0-0 transition and calculate an optical gap j,=2.4eV. The PL spec-... 

The optical gap of a-C H films was found to continuously decrease with increasing self-bias [42]. The gap shrinking was found to be strongly correlated to the variation of Raman spectra that is related to the increase of the graphitic clusters present in the a-C H films. Accordingly, the electrical resistivity of C H films was found to strongly decrease with substrate bias [43]. 

This picture was found to be consistent with the comparison of Raman spectra and optical gap of a-C H films deposited by RFPECVD, with increasing self-bias [41], It was found that both, the band intensity ratio /d//g and the peak position (DQ increased upon increasing self-bias potential. At the same time, a decrease on the optical gap was observed. Within the cluster model for the electronic structure of amorphous carbon films, a decrease in the optical gap is expected for the increase of the sp -carbon clusters size. From this, one can admit that in a-C H films, the modifications mentioned earlier in the Raman spectra really correspond to an increase in the graphitic clusters size. 

In reality, several factors were mentioned as being responsible for this behavior, such as variations in bond angle distortion, in the internal stress or in the hydrogen content [40, 76], but all of them are also strongly correlated with the variation of optical gap width in amorphous carbon films. Theoretical work on Raman spectroscopy on DLC materials gave additional support for Dillon s interpretation [77]. 

In fact, an apparent doping effect was also reported by Schwan et al. [39] in a-C(N) H films deposited by the highly ionized plasma beam deposition technique in C2H2-N2 atmospheres. Schwan et al. also observed thermally activated behavior for the conductivity. As reported by Silva et al. [14], they also observed increasing optical gap, and decreasing ESR spin signal, but the Urbach energy was found to increase. 

SCHEME 2.14 Tuning the energy levels of PPV by introducing pendant charge-transporting units. The HOMO defined as the -/P value (determined from the UPS experiments) and the LUMO was deduced by adding the optical gap to the HOMO value. 

The product obtained from the high-pressure reaction of benzene has been identified as amorphous [309]. The amorphous character of the sample prevents the obtainment of the Raman spectra. Other physical-chemical properties of the reaction product are the following refractive index n = 1.75 density p = 1.39 g/cm elastic constant Bq = 80 GPa optical gap 2.5 eV. These values must to be considered only as typical values of the properties because, as described above, the reaction product is reported to change according to the... 

Bandgap measurements for Cu sulphides and selenides are complicated by the fact that these semiconductors are normally degenerate, with high free-carrier absorption in the near-infrared and possible Moss-Burstein shifts (due to saturation of the top of the valence band by holes) in the optical gap. It is quite possible that variations in bandgaps in these materials are due to differences in stoichiometry, phase, and doping rather than to any quantum size effect. Only studies where crystal size can be estimated and are possibly in the quantum size range are given here. 

Ionic fluorides with large optical gaps exhibit high transparency to electromagnetic radiation. MgF2, for instance, is transparent from 10 cm (corresponding to the energy threshold for the electronic transition from the valence band to the conduction band) to 10 cm (maximum frequency of lattice vibrations). The transparency of metal fluorides has led to their use as windows and prisms in optical instruments (see... 

Fig. 19. Predicted dependence of the photoionization spectral dependence on the Franck-Condon factor [dF c—see Eq. (53)]. The parameter values are appropriate for the electron cross section ( ) for in GaP. The level depth is E, = 0.9 eV, the band gap is Et = 2.2 eV, the average optical gap (the Penn gap) is Ep = 5.8 eV, and the temperature is 400°K. [After Jaros (1977, Fig. 5e).]...

The onset of photoconductivity is determined from the intersection point of the x-axis (dark current) and the extrapolated line from the linear part of the conductivity. All spectra show a pronounced onset of photoconductivity at 2.40 eV (Na S), 2.25 eV (K S), 2.15 eV (Na Se), and 2.10 eV (K Se), respectively, which is found to be in remarkable agreement with the optical gap. This proves that the photoconductivity is a bulk property instead of a surface conduction phenomenon. 

Experimental and theoretical optical gaps for cetineites with A = Na+, K+ and... 

As it was expected the optical gap follows the chemical composition in the same way as the lattice constants. Thus, the optical properties of cetineites can be tuned to a certain extent. Figure 5b shows the dependence of the optical band gap on the lattice constants of the cetineite crystals. 

The fluorescence quantum yield of 448 is 0.14, a sixfold increase relative to that of the parent. In comparison, the fluorescence quantum yield of 449 (0.01) is comparable to that of the parent compound. The phosphorescence emission quantum yield of 449 is 0.56 in a frozen matrix as expected as a result of the intramolecular heavy atom effect. In contrast, the phosphorescence is effectively shut off in the anti-isomer where the quantum yield is only 0.04. This observation suggests that the electronic excited state structures and nonradiative decay channels very considerably with constitution of the isomers. The optical gap energy was 3.1 (3.3) eV for 448 (449). 

The optical and PL spectroscopies have been undertaken to understand the structure-property correlations of this important family of triplet-emitting polymers. The red shift in the absorption features upon coordination of the metal groups is consistent with there being an increase in conjugation length over the molecule through the metal center. The trade-olf relationship between the phosphorescence parameters (such as emission wavelength, quantum yield, rates of radiative and nonradiative decay) and the optical gap will be formulated. For systems with third-row transition metal chromophores in which the ISC efficiency is close to 100%,76-78 the phosphorescence radiative (kr)y, and nonradiative (/cm)p decay rates are related to the measured lifetime of triplet emission (tp) and the phosphorescence quantum yield ([Pg.300]

---