Nonpolar Quantum Structures

CaN/AlCaN Quantum Structures Crown by PA Molecular Beam Epitaxy 

GaN/AICaN MQW deposited by PAM BE on 1120) direction (c) defective area of the GaN bulk substrates (a) epitaxial growth in structure grown in nonpolar (1120) polar (0001) direction (b) defect-free direction (courtesy Julita Smalc). 

An advantage of the PAM BE method was that both thicknesses and compositions of the epitaxial layers comprising the MQWs were the same for both polar and nonpolar growth directions. 

Because of the high structural quality of the high-pressure polar substrates, the multilayer structures of polar orientation were also almost defect free. The ones of nonpolar orientation contained areas with structural defects induced by imperfections in the substrate. 

For optical pumping experiments, the GaN-AlGaN-based laser structures have also been grown by PAM BE on the nonpolar GaN quasi- wafers. Fivefold GaN-AlGaN MQWs with quantum well thicknesses of 5 and 2 nm surrounded by 12% AlGaN barriers were prepared. Then the substrates with the laser structures were cleaved along (1010) planes to get lasers of 700-, 500-, and 300-gm cavity lengths. 

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The research on bulk crystallization of GaN by HVPE as well as on the epitaxial growth and studies of nonpolar GaN-based quantum structures was supported by Polish Committee for Scientific Research GRANT Nr 3T08A 033 29 and GRANT NrlP03B05329. 

It is seen that the fluorescence quantum yield and lifetime of G19 gradually decreases with increasing solvent polarity. For example, the insertion of 20% ACN by volume into toluene leads to a decrease of a factor of two. Based on these results we can conclude that G19 is very sensitive to solvent polarity and can be used as an efficient probe to test the polarity of its microenvironment. A reverse trend of the absorption peak at 1 1 mixture of ACN and toluene (50%T in Fig. 22b) corresponds to a change of the sign of due to a transition from a polyene-like structure in nonpolar toluene to a polymethine-like structure in polar ACN. 

Another example of fluorescence intensity modulation in cou-marins is the 3-azido substitution that quenches the fluorescence completely. These compounds are used as starting material for the synthesis of fluorescent triazolocoumarins by click chemistry [31], Interestingly, the fluorescence of some coumarins depends strongly on the solvent. This is the case for 7-alkoxycoumarins that have been used as probes for microenvironments [32], 7-hydroxycoumarin that is pH sensitive, and 7-NR2 substituted coumarins such as coumarin 120 whose quantum yield is reduced in nonpolar solvents due to a change in the 3D structure [33],... 

The fluorescence emission maximum, quantum yield, and lifetime of a fluorophore are very sensitive to its immediate environment. A blue shift in the emission maximum and an increase in the fluorescence quantum yield or lifetime is generally observed when a fluorophore is transferred form a polar solvent to a nonpolar one or when it binds to a hydro-phobic protein site. Furthermore, fluorescence quenching or enhancement may result from interactions of the fluorophore with various structural elements in its vicinity. 

Fluorescence techniques have been used with great success in the study of PEO-fe-PSt micelles [64]. In this study, the effect of polymer concentration on the fluorescence of pyrene present in water at saturation was studied. Three features of the absorption and emission spectra change when micellization occurs. First, the low-energy band of the (S2-So) transition is shifted from 332.5 to 338 nm. Second, the lifetime of the pyrene fluorescence decay increases from 200 to ca. 350 ns, accompanied by a corresponding increase in the fluorescence quantum yield. Third, the vibrational fine structure changes, as the transfer of pyrene from a polar environment to a nonpolar one suppresses the permissibility of the symmetry-forbidden (0,0) band. 

Multinuclear Li and NMR and quantum-chemical investigations by Muller and coworkers showed that it is possible to determine the structure of active species in methacrylate polymerization in the presence of triethylaluminum (AlEts) in nonpolar solvents, such as toluene. Reaction of eibLi and trialkylaluminum with methyl pivalate (mpiv), in equimolar amounts, leads to an equilibrium of monomer 46 and dimer 47 complexes (eibLi mpiV"AlEt3) (n = 1, 2), consistent with kinetic studies. Methyl pivalate was... 

An analogous structure for 2-oxo-l,3-bis(phenylhydrazono)cyclopen-tane (268) was likewise based on spectroscopic evidence.526 Quantum-mechanical calculation (HMO) of the bonding energies of various tautomers (such as 268, 269, and 270) indicates505 that the most stable tautomeric structure is the chelated bis(phenylhydrazone) 269. It seems that interconversion of the tautomers 268,269, and 270 occurs in polar solvents and that the dichela ted structure 271 is preponderant in the solid state or in nonpolar solvents (Scheme 63).526... 

In this chapter, we review a number of DG-based models. Initially, we discuss solvation models, i.e., nonpolar and polar solvation models at equilibrium. To improve the accuracy and make our models robust, quantum mechanics is applied to the solute s electron structure. As an important extension, we also consider... 

The obtained results can be explained as follows. In the pH region where the necklace of pearls structure exists, a fraction of probes are entrapped in compact globules and experience the nonpolar medium. The other probes attached to the stretched parts of the chains are exposed to polar water molecules. The probes are thus effectively distributed in two different microenvironments. The short lifetime corresponds to the water-exposed and the long one to the globule-embedded probes. The pre-exponential factors, when corrected by corresponding quantum yields, give the number fractions of both types of probes in the macroscopic ensemble. In the first approximation, the factors reflect the average numbers of monomer units in stretched and collapsed parts of the chain. 

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