Undoped absorption spectra

Fig. 9.31 Absorption spectrum of PAc doped with AsF5 (1) undoped, and to an electrical conductivity of (2) 2 x 103, (3) 5 x 103 and (4) 5x 104 Reprinted from Tanaka and Tanaka (1986), by courtesy of Marcel Dekker, Inc.

In course of subsequent work Bubeck, Tieke, and Wegner discovered that the action spectrum for photopolymerization of undoped diacetylene multilayers extends into the visible provided some polymer formed in course of previous UV-irradiation is present. Since obviously excitation of the polymer can sensitize the reaction this effect has been termed self-sensitization. Checking the absorption spectrum of the polymer produced via self-sensitization assured that the final product is identical with the product obtained under UV excitation of the monomer. Later work by Braunschweig and Bassler demonstrated, that the effect is not confined to multilayer systems but is also present in partially polymerized single crystalline TS-6, albeit with lower efficiency. Interestingly, the action spectrum of self-sensitization follows the action spectrum for excitation of an electron from the valence band of the polymer backbone to the conduction band rather than the excitonic absorption spectrum of the polymer which is the dominant spectral feature in the visible (see Fig. 21). The quantum yield is independent of the electric field, whereas in a onedimensional system the yield of free carriers, determined by thermal dissociation of optically produced, weakly bound geminate electron-hole pairs, is an linear function of an applied electric field 29.30,32,129) Apparently, the sensitizing action does not... 

The first report of the absorption spectrum of acceptors in semiconductors is probably the paper by Burstein et al. [26], showing the LHeT transmission spectra of two p-type silicon samples. In one of these spectra, broad electronic lines, attributed to boron, could be seen at 0.034, 0.040 and 0.043 eV while in the other, from a nominally undoped sample, lines near 0.055 and 0.06 eV were observed, now known to be due to the aluminium acceptor. Since then, many acceptors have been identified in silicon and other semiconductor crystals and with the same technological incentive as for donors, their optical spectroscopy has been widely used to characterize them, evaluate their concentrations, provide physical insight into the VB structures of the crystals and more recently evaluate the effect of impurity isotope broadening in quasi-monoisotopic crystals. 

EL spectra of undoped NPB devices (in solid line) and perylene-doped NPB devices (in dashed line). The dotted line is the absorption spectrum of perylene. (From Tse, S.C. et al., Appl. Pkys. Lett, 90,213502,2007. With permission.)... 

Fig. 4 (a) Typical absorption spectrum showing the lS3/2-lSe peak and schematic of the splitting in presence of magnetic field. The thin black line at the bottom shows a typical schematic of the MCD spectra, (b) Typical MCD spectra for Mn doped and undoped NCs. The inset shows the schematic for the field dependence of the intensity of the MCD spectra for doped and undoped spectra. 

The two electrons transferred from TDAE to PEDOT-PSS are expected to undope the conjugated polymer chains. Since TDAE diffuses into PEDOT-PSS, long exposures to the electron donor induce changes in the optical properties of the polymer film. Optical absorption experiments on 200 nm thick PEDOT-PSS films coated onto a transparent polyethylene terephthalate (PET) substrate. The pol5mier film was exposed to the TDAE vapor in an inert nitrogen atmosphere and shows the difference in absorption spectrum between a film exposed to TDAE and the pristine PEDOT-PSS layer (Figs. 3.10 and 3.11). The modification of the optical properties and the sheet resistance of the pol5mier layer were recorded versus exposure time. The two absorption features at 550 nm and... 

Fig. 12.5 Effect of hydrogen bonding on the in situ electronic absorption spectrum of undoped PDHT films on ITO. Solid line Spectrum recorded at 0.0 V vs. SCE after redox cycling in 0.1 M LiC104-MeCN. Dotted line Same film immersed in hexafluoroisopropanol. Dashed line Same film at 0.0 V vs. SCE after redox cycling in 0.1 M LiC104-MeCN. (From Ref. 95.)...

Infrared spectrum of pyridine adsorbed on undoped alumina-supported vanadium oxide catalyst, after evacuation at 150 °C, shows an absorption band at 1450 cm 1, characteristic for pyridine retained on Lewis acid sites, which has been related to V-free alumina [4,10, 11]. The intensities of the bands at 1450 cm l (related to Lewis acid sites) and 1545 cm I (related to Bronsted acid sites) have been used to determine the numer of Lewis and Bronsted acid sites on the surface of catalysts. The results are outlined in Table 1. 

In 677-SiC, B replaces a Si atom and its ionization energies in the three non-equivalent sites measured by admittance spectroscopy are 0.27, 0.31, and 0.38 eV [56], In undoped and boron-doped p-type 6H-SiC samples, a photoionization spectrum with a temperature-dependent threshold between 0.5 and 0.7eV, and a maximum at 1.75 eV has been reported [83]. The difference between the threshold energy and the electrically-measured ionization energy of B (0.3-0.4eV) is attributed to lattice relaxation. This photoionization spectrum is correlated with the observation near LHeT of three narrow absorption lines at 2.824, 2.863, and 2.890 eV tentatively attributed to excitons bound to neutral B at the three possible sites in 6H-SiC. 

Donor-acceptor absorption can also be observed in semiconductors, but this process is weak because of the small overlap of the wavefunctions (like an n -> n transition). Donor-acceptor absorption is best monitored through the emission, that is by excitation spectra. In the normal situation, the donor-acceptor absorption can be observed but the valence band-to-donor and the acceptor-to-conduction band transitions can also be seen, as they also contribute to the luminescence. All three of these transitions are weak but of similar strengths [6]. In undoped AgCl and AgBr, only a very weak excitation spectrum is seen, which consists of a relatively sharp line near the band edge. In Cd2 + doped AgBr both the sharper line, whose onset is about... 

---