Photoluminescence studies

Turning to non-metallic catalysts, photoluminescence studies of alkaline-earth oxides in dre near-ultra-violet region show excitation of electrons corresponding to duee types of surface sites for the oxide ions which dominate the surface sUmcture. These sites can be described as having different cation co-ordination, which is normally six in the bulk, depending on the surface location. Ions on a flat surface have a co-ordination number of 5 (denoted 5c), those on the edges 4 (4c), and dre kiirk sites have co-ordination number 3 (3c). The latter can be expected to have higher chemical reactivity than 4c and 5c sites, as was postulated for dre evaporation mechanism. 

C.L. Gettinger, A.J. Heeger, J.H. Drake, and D.J. Pine, A photoluminescence study of poly (phenylene vinylene) derivatives the effect of intrinsic persistence length, J. Chem. Phys., 101 1673-1678, 1994. 

In 1958, the first SiC conference was held in Boston, Massachusetts. However, after this, the interest in SiC rapidly declined and the 1960s and 1970s are characterized by a low interest in SiC. Research was still ongoing, mainly in the former Soviet Union. In the United States, the work done by Westinghouse and the University of Pittsburgh is primarily notable. Indeed, the photoluminescence studies made by Choyke, Patrick, and Hamilton are still very relevant and often cited [21]. 

Time-resolved measurements of photogenerated (very intense illumination, up to 0.56 GW/cm ) electron/hole recombination on CD (selenosulphate/NTA bath) CdSe of different crystal sizes has shown that the trapping of electrons, probably in surface states, occurs in ca. 0.5 ps, and a combination of (intensity-dependent) Auger recombination and shallow-trapped recombination occurs in a time frame of ca. 50 ps. A much slower (not measured) decay due to deeply trapped charges also occurred [102]. A different time-resolved photoluminescence study on similar films attributed emission to recombination from localized states [103]. In particular, the large difference in luminescence efficiency and lifetime between samples annealed in air and in vacuum evidenced the surface nature of these states. 

A photoluminescence study of CdSe deposited from a selenourea/ammonia solution onto glass at 80°C and relatively low pH (7-8) was made [31]. An emission peak centered at 1.445 eV (860 nm) was observed with a tail to the low energy side. Such an emission must be due to deep traps, since the shift from the bandgap emission is ca. 0.4 eV, a value close to, or somewhat less than, that for films deposited from selenosulphate solution (see earlier). Annealing in air shifted the emission to higher energies. 

An important aspect of semiconductor films in general with regard to electronic properties is the effect of intrabandgap states, and particularly surface states, on these properties. Surface states are electronic states in the forbidden gap that exist because the perfect periodicity of the semiconductor crystal, on which band theory is based, is broken at the surface. Change of chemistry due to bonding of various adsorbates at the surface is often an important factor in this respect. For CD semiconductor films, which are usually nanocrystalline, the surface-to-volume ratio may be very high (several tens of percent of all the atoms may be situated at the surface for 5 mn crystals), and the effects of such surface states are expected to be particularly high. Some aspects of surface states probed by photoluminescence studies are discussed in the previous section. 

A3.5 Time-resolved photoluminescence studies of GaN A3.6 Persistent photoconductivity in GaN A3.7 Electrical transport in wurtzite and zincblende GaN A3.8 Characterisation of III-V nitrides by capacitance transient spectroscopy... 

Figure 2.28 An ORTEP view of the crystal structure of Eu(L )3(TPTZ) with partial atomic labeling. Thermal ellipsoids are drawn at the 50% probability level [25]. (Reprinted from Polyhedron, 26, C.R. De Silvaa, J.R. Maeyera, A. Dawsona and Z. Zheng, Adducts of lanthanide [l-diketonates with 2,4,6-tri(2-pyridyl)-l,3,5-triazine synthesis, structural characterization, and photoluminescence studies, 1229-1238, 2007, with permission from Elsevier.)...

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