We study the electronic properties of GaAs-AlGaAs superlattices with intentional correlated disorder by means of photoluminescence and vertical dc resistance. The results are compared to those obtained in ordered and uncorrelated disordered superlattices. We report the first experimental evidence that spatial correlations inhibit localization of states in disordered low-dimensional systems, as our previous theoretical calculations suggested, in contrast to the earlier belief that all eigenstates are localized. [S0031-9007(99) PACS numbers: 73.20.Dx, 73.20.Jc, In recent years, a number of tight-binding [1][2][3] and continuous [4] models of disordered one-dimensional (1D) systems have predicted the existence of sets of extended states, in contrast to the earlier belief that all the eigenstates are localized in 1D disordered systems. These systems are characterized by the key ingredient that structural disorder is short-range correlated. Because of the lack of experimental confirmations, there are still some controversies as to the relevance of these results and their implications on physical properties. In this context, some authors have proposed finding physically realizable systems that allow for a clear cut validation of the above-mentioned purely theoretical prediction [5][6][7][8]. Given that semiconductor superlattices (SL's) have been already used successfully to observe electron localization due to disorder [9][10][11][12][13][14], these authors have suggested SL's as ideal candidates for controllable experiments on localization or delocalization and related electronic properties [5][6][7][8].To the best of our knowledge, up to now there is no experimental verification of this theoretical prediction owing to the difficulty in building nanoscale materials with intentional and short-range correlated disorder. However, the confirmation of this phenomenon is important both from the fundamental point of view and for the possibility to develop new devices based on these peculiar properties. In this work we present an experimental verification of this phenomenon in semiconductor nanoscale materials, taking advantage of the molecular beam epitaxy growth technique, which allows the fabrication of semiconductor nanostructures with monolayer controlled perfection.We grew several GaAs-Al 0.35 Ga 0.65 As SL's and we studied their electronic properties by photoluminescence (PL) at low temperature and dc vertical transport in the dark. Indeed PL has been proven to be a good technique to study the electronic properties of disordered SL's [9-11], giving transition energies comparable with theoretical calculations of the electronic levels. The electronic states were calculated using a Kronig-Penney model that has been shown to hold in this range of well and barrier thicknesses, with precise results [15]. This allows the analysis of the experimental transition energies for PL and the ascertainment of the localization and delocalization properties of the SL's. The details of the calculations and a schematic view of the cond...

Self-consistent periodic slab calculations based on gradient-corrected density-functional theory ͑DFT-GGA͒ were conducted to probe the potential-energy diagram for the hydrogenation of propanal and acetone on Pt͑111͒. Calculations for molecularly adsorbed species indicate that acetone and propanal are both weakly bound to the surface through oxygen ͑i.e., energy changes of adsorption near Ϫ20 kJ/mol͒. The activation energy barriers are calculated to be ϳ60 and 40 kJ/mol for the addition of an adsorbed hydrogen atom to adsorbed acetone and propanal, leading to adsorbed isopropoxy and n-propoxy species, respectively. The subsequent hydrogenation steps to form adsorbed alcohol species have activation barriers near 15 kJ/mol. These results would suggest that the rate of propanal hydrogenation over Pt should be faster compared to acetone hydrogenation, in contrast to the behavior observed experimentally ͓G.M.R. van Druten and V. Ponec, Applied Catalysis A: General 191, 153 ͑2000͔͒. The origin for the experimentally observed slower rate of propanal hydrogenation over Pt appears to be related to the formation of strongly adsorbed spectator species formed by removal of the ␣-H atom from adsorbed propanal. The calculated energy change for cleavage of this C-H bond, leading to adsorbed propionyl and adsorbed hydrogen atom, is exothermic by 76 kJ/mol.

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Reaction kinetics studies were conducted for the conversions of ethanol and acetic acid over silica-supported Pt and Pt/Sn catalysts at temperatures from 500 to 600 K. Addition of Sn to Pt catalysts inhibits the decomposition of ethanol to CO, CH 4 , and C 2 H 6 , such that PtSn-based catalysts are active for dehydrogenation of ethanol to acetaldehyde. Furthermore, PtSn-based catalysts are selective for the conversion of acetic acid to ethanol, acetaldehyde, and ethyl acetate, whereas Pt catalysts lead mainly to decomposition products such as CH 4 and CO. These results are interpreted using density functional theory (DFT) calculations for various adsorbed species and transition states on Pt(111) and Pt 3 Sn(111) surfaces. The Pt 3 Sn alloy slab was selected for DFT studies because results from in situ 119 Sn Mössbauer spectroscopy and CO adsorption microcalorimetry of silica-supported Pt/Sn catalysts indicate that Pt-Sn alloy is the major phase present. Accordingly, results from DFT calculations show that transition-state energies for C-O and C-C bond cleavage in ethanolderived species increase by 25-60 kJ/mol on Pt 3 Sn(111) compared to Pt(111), whereas energies of transition states for dehydrogenation reactions increase by only 5-10 kJ/mol. Results from DFT calculations show that transition-state energies for CH 3 CO-OH bond cleavage increase by only 12 kJ/mol on Pt 3 Sn(111) compared to Pt(111). The suppression of C-C bond cleavage in ethanol and acetic acid upon addition of Sn to Pt is also confirmed by microcalorimetric and infrared spectroscopic measurements at 300 K of the interactions of ethanol and acetic acid with Pt and PtSn on a silica support that had been silylated to remove silanol groups.

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