The 2D–3D growth mode transition during the initial stages of growth of highly strained InGaAs on GaAs is used to obtain quantum-sized dot structures. Transmission electron micrographs reveal that when the growth of In0.5Ga0.5As is interrupted exactly at the onset of this 2D–3D transition, dislocation-free islands (dots) of the InGaAs result. Size distributions indicate that these dots are ∼300 Å in diameter and remarkably uniform to within 10% of this average size. The areal dot densities can be varied between 109 and 1011 cm−2. The uniformity of the dot sizes is explained by a mechanism based on reduction in adatom attachment probabilities due to strain. We unambiguously demonstrate photoluminescence at ∼1.2 eV from these islands by comparing samples with and without dots. The luminescent intensities of the dots are greater than or equal to those of the underlying reference quantum wells.

letters to nature 944 NATURE | VOL 389 | 30 OCTOBER 1997 pyrochlore, a large degree of spin-polarization is primarily due to the extremely small number of carriers (estimated at 0.0001-0.0005 carriers per formula unit from Hall effect measurements 6 ). Thus even moderate exchange splitting of Tl 6s-derived bands can fully polarize the conducting electrons.What, then, can account for the different temperature dependences of the transport process at the grain boundary between the two systems? In the perovskite manganites, the surface magnetism will in general be quite different from that of the bulk, owing to the lower effective magnetic coupling, the tendency of spin canting (an important feature in the perovskite manganites 21 ), as well as the disorder due to the surface termination of the crystal structure. Also important is the influence of the Jahn-Teller lattice distortions at the surface. One possible consequence of these effects would be a lower surface T C and surface spin polarization with respect to the bulk. The hopping electron is strongly coupled to these surface spins by the large on-site J H , and thus the TMR is susceptible to fast degradation with increasing temperature via the magnetically altered surface spins. In the pyrochlore, however, there is no strong J H coupling the conducting electrons to the Mn spins, and our results imply less sensitivity to the effects of the surface.Another distinction can be visualized by considering the temperature dependence of MR* in absolute units (r 0 MR* shown in the lower panel of Fig. 3. In the perovskite, the absolute magnitude of MR* diminishes rapidly with increasing temperature, much below T C . In contrast, the same data for the pyrochlore is virtually constant for all temperatures up to T C . This difference may indicate that for Tl 2 Mn 2 O 7 , the potential barrier ∆ B is less than the electron energy E ↑ (referring to Fig. 4). In this case the grain boundary transport would not be considered tunnelling, but rather a scattering process across a disordered region acting as a series resistance. This implies that the low-field MR can be made even larger by raising the relative resistance of the grain boundaries with respect to the bulk resistivity, much as a thin-film tunnel junction increases this ratio with respect to bulk polycrystalline samples in perovskite manganites. Ⅺ

An unusual luminescent inorganic oxide, Sr2CeO4, was identified by parallel screening techniques from within a combinatorial library of more than 25,000 members prepared by automated thin-film synthesis. A bulk sample of single-phase Sr2CeO4 was prepared, and its structure, determined from powder x-ray diffraction data, reveals one-dimensional chains of edge-sharing CeO6 octahedra, with two terminal oxygen atoms per cerium center, that are isolated from one another by Sr2+ cations. The emission maximum at 485 nanometers appears blue-white and has a quantum yield of 0.48 +/- 0.02. The excited-state lifetime, electron spin resonance, magnetic susceptibility, and structural data all suggest that luminescence originates from a ligand-to-metal Ce4+ charge transfer.

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We report on GaAs/MCM-41 heterostructures synthesized by deposition of GaAs into the channels of MCM-41 using metalloorganic chemical vapor deposition. MCM-41 consists of an ordered array of silica tubules comprising pores with uniform and controllable diameter in the nanometer range. The GaAs/MCM-41 heterostructures show blue-shifted absorption and broad visible photoluminescence even at room temperature. The photoluminescence maximum depends on the MCM-41 pore diameter supporting formation of size-quantized semiconductor crystallites whose growth is restricted by the diameter of the pores of MCM-41. The shift in the photoluminescence spectra as a function of the excitation wavelength suggests a broad size distribution of the GaAs particles crystallized on the outside of MCM-41 and a relatively narrow size distribution of the GaAs particles inside the channels of MCM-41.