Semiconductors extinction coefficient

For a typical monomolecular coverage, T — 10 10 mol/cm2, an electrode roughness factor r = 1000 and an extinction coefficient ads = 107 cm2/mol, the light-harvesting efficiency is, in comparison to the preceding case, very high, intimate contact with the semiconductor surface, hence the conditions for charge injection from S into the semiconductor are almost ideal (q9j—>100 per cent). 

Separation of bulk and surface properties in macroscopic semiconductors is less than straight forward and requires highly sensitive experimental techniques. In contrast, the large surface-to-volume ratios in nanosized semiconductor particles render the examination of surface processes in and/or on these colloids to be experimentally feasible. Advantage has been taken of pulse radiolysis to inject electrons (in aqueous, N20-saturated solutions which contained 2-propanol see Eqs. 22,23, and 25) or holes (in aqueous, N20-saturated solutions which did not contain 2-propanol see Eqs. 22 and 23) into nanosized semiconductor particles [601, 602], Electron injection into CdS particles, for example, decreased the extinction coefficient at 470 nm (the absorption onset) by — 5 x 104 M-1cm-1 (Fig. 98) [576]. Hole injection resulted in the appearance of a transient absorption band in the long-wavelength region and in much less... 

There are a number of advantages of using colloidal, semiconductors in artificial photosynthesis. They are relatively inexpensive. They have broad absorption spectra and high extinction coefficients at appropriate band gap energies. Nevertheless, they can be made optically transparent enough to allow direct flash photolytic investigations of electron transfers. They can be modified by derivatization or sensitizer adsorption. Importantly, electrons produced by band gap excitation can be used directly without relays for catalytic water reduction (Figure IB). 

A sensitizer is of paramount importance to photovoltaic performance. The sensitizer is attached to the surface of a mesoporous wide band-gap semiconductor serving as electron transporter. While the trivial ultraviolet absorption for 375 with a molar extinction coefficient (s) of 50.0 x 103 M [ cm-1 peaks at 372 nm, the s value of its low-energy band at 525 nm (mainly stemming from the intramolecular charge transfer transition) is 44.8 x 103 IVT1 cm-1 (08JA9202). 

Dyes such as erythrosin B [172], eosin [173-177], rose bengal [178,179], rhodamines [180-185], cresyl violet [186-191], thionine [192], chlorophyll a and b [193-198], chlorophyllin [197,199], anthracene-9-carboxylate [200,201], perylene [202,203] 8-hydroxyquinoline [204], porphyrins [205], phthalocyanines [206,207], transition metal cyanides [208,209], Ru(bpy)32+ and its analogs [83,170,210-218], cyanines [169,219-226], squaraines [55,227-230], and phe-nylfluorone [231] which have high extinction coefficients in the visible, are often employed to extend the photoresponse of the semiconductor in photoelectro-chemical systems. Visible light sensitization of platinized Ti02 photocatalyst by surface-coated polymers derivatized with ruthenium tris(bipyridyl) complex has also been attempted [232,233]. Because the singlet excited state of these dyes is short lived it becomes essential to adsorb them on the semiconductor surface with... 

Armelao et al. (2005) fabricated LaCoOs thin films by the combination of chemical vapor deposition (CVD) and sol-gel methods. Two sequences were adopted to prepare the target film (i) sol-gel of Co-O on CVD La-O (ii) CVD of Co-O on sol-gel La-O. Losurdo et al. (2005) further investigated the spectroscopic properties of these films by ellipsometry in the near-IR and UV range. The former film has a larger crystallite size, a lower refractive index, and a higher extinction coefficient. It also presents a semiconductor-to-metal transition at a temperature of 530 K. Contrarily, the latter film has a smaller crystallite size, a higher refractive index, a lower extinction coefficient and a semiconductor behavior. 

At this point it should be noted that both the index of refraction and the extinction coefficient are extremely frequency dependent. Consequently, the properties of absorption, reflection, and phase shift which depend on them will also show a frequency dependence. Value of r] and k for most metals and semiconductors are tabulated in the American Institute of Physics Handbook ... 

Fig. 6. Refractive indices and extinction coefficients (both given at a wavelength of 550 ran) of several metals and semiconductor materials, as found in the literature. Some isovalue-curves of nk product are shown (most optical constants values are extracted from Palik, 1985, and from J. A. Woollam WVASE software, 2009).

A very promising electrically conductive omnidirectional reflector suitable for use in LEDs is shown in Fig. 1.17 [62, 63], The reflector comprises the LED semiconductor material with a refractive index ns, a low-refractive index layer (nu), and a metal with a complex refractive index Nm = nm + i km, where km is the extinction coefficient. 

---