We have investigated the temperature dependence of photoluminescence ͑PL͒ properties of a number of self-organized InAs/GaAs heterostructures with InAs layer thickness ranging from 0.5 to 3 ML. The temperature dependence of InAs exciton emission and linewidth was found to display a significant difference when the InAs layer thickness is smaller or larger than the critical thickness around 1.7 ML. The fast redshift of PL energy and an anomalous decrease of linewidth with increasing temperature were observed and attributed to the efficient relaxation process of carriers in multilayer samples, resulting from the spread and penetration of the carrier wave functions in coupled InAs quantum dots. The measured thermal activation energies of different samples demonstrated that the InAs wetting layer may act as a barrier for the thermionic emission of carriers in high-quality InAs multilayers, while in InAs monolayers and submonolayers the carriers are required to overcome the GaAs barrier to escape thermally from the localized states. ͓S0163-1829͑96͒06440-5͔Spontaneous formation of three-dimensional ͑3D͒ islands during Stranski-Krastanov-like growth of highly strained InAs layers on GaAs substrates by molecular-beam epitaxy ͑MBE͒ has been proposed as a promising way for fabricating high-quality InAs quantum dots ͑QD's͒ in GaAs.1-6 When the thickness of an InAs layer is beyond a critical thickness of around 1.7 ML, the structure is usually composed of an InAs wetting layer and conelike InAs islands deposited on it. The density, size distribution, uniformity, and coverage of such InAs islands were found to be growth condition dependent, and have been investigated by TEM and atomic force microscopy ͑AFM͒ in recent years. [7][8][9] The optical studies 10-13 revealed its excellent radiative recombination, which usually gives a broadband with a reported full width at half maximum ͑FWHM͒ in the range of 50-130 meV. Recent work by Lubyshev et al. 13 presented the unusual temperature dependence of exciton energy in InAs multilayer structures. An anomalous decrease of the FWHM was detected and explained in terms of the tunneling process between InAs dots.In this paper, we studied the exciton relaxation and thermal activation in InAs multilayer structures via the analysis of cw photoluminescence ͑PL͒ data under different temperatures. It is found that the temperature dependence of the exciton energy and linewidth is significantly different from that obtained in InAs monolayers and submonolayers. The unusual temperature behavior in InAs multilayers is associated with the relaxation effect of carriers, resulting from the spread and penetration of the wave functions of carriers in coupled InAs QDs. In the study of the thermal activation process, we found that potential barriers for InAs excitons to escape thermally from the localized states are different for different structures. For high-quality InAs multilayers, the barrier could be the wetting layer, while in the case of monolayers or submonolayers the carriers are required to ove...

Palladium-catalyzed asymmetric allylic alkylation (AAA) has become one of the most powerful tools for constructing chiral centers via carbon-carbon bond forming reactions. 1 However, regioselectivity is still not fully resolved when monosubstituted allyl substrates are used, 1-4 and the carbon nucleophiles are restricted mainly to "soft", stabilized carbanions. 1 In the past several years, great progress has been made in Pd-catalyzed AAA reactions in the control of both regio-and enantioselectivities involving monosubstituted allyl substrates 2-4 and nonstabilized ketone enolates. 3a,5-7 Procedures have also been developed for installment of two chiral centers through combination of allyl substrates and nucleophiles, such as in Pd-and Mo-catalyzed reactions of azalactone with cyclohexyl acetate, PhCHdCHCH(OAc) 2 , 8a and ArCHdCHCH 2 X, 8b in Ir-catalyzed reactions of glycine derivatives with monoallyl substrates, 9 or Pd-catalyzed reactions of cyclohexanones with 1,3disubstituted allyl acetate, 5b,g all showing good to excellent regio-, diastereo-, and enantioselectivities. High regio-and diastereoselectivities were also achieved in the Rh-catalyzed reaction of acyclic R-alkoxy ketones with branched allyl carbonates. 10 However, chiral allyl carbonates must be used in order to obtain enantioenriched products. To the best of our knowledge, a method to generate two chiral centers by transition-metal-catalyzed asymmetric reactions of simple ketone enolates and monosubstituted allyl substrates is still lacking. Recently, we have developed a series of chiral ferrocene ligands, which have been utilized successfully in Pdcatalyzed regio-and enantioselective AAA. 3 Subsequent extension to include acyclic ketone enolates as the nucleophiles was achieved by us and others. 5h,6b,7 In this paper, we describe a Pd-catalyzed AAA of monosubstituted allyl substrates with simple acyclic ketone enolates, while establishing two chiral centers with high regio-, diastereo-, and enantioselectivity.

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