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We report and explain circular dichroism in semiconductor quantum dots. CdS nanocrystals capped with penicillamine enantiomers were prepared and found to be both highly luminescent and optically active. No new features in circular dichroism were observed as the nanocrystal grew larger. Density functional calculations reveal that penicillamine strongly distorts surface Cd, transmitting an enantiomeric structure to the surface layers and associated electronic states. The quantum dot core is found to remain undistorted and achiral.

Materials based on Cu2O are potential p-type transparent semiconducting oxides. Developing an understanding of the mechanism leading to p-type behaviour is important. An accepted origin is the formation of Cu vacancies. However, the way in which this mechanism leads to p-type properties needs to be investigated. This paper presents a first principles analysis of the origin of p-type semiconducting behaviour in Cu2O with 1.5 and 3% Cu vacancy concentrations. Plane wave density functional theory (DFT) with the Perdew-Burke Ernzerhof (PBE) exchange-correlation functional is applied. In order to investigate the applicability of DFT, we firstly show that CuO, with 50% Cu vacancies cannot be described with DFT and in order to obtain a consistent description of CuO, the DFT + U approach is applied. The resulting electronic structure is consistent with experiment, with a spin moment of 0.64 mu(B) and an indirect band gap of 1.48 eV for U = 7 eV. However, for a 3% Cu vacancy concentration in Cu2O, the DFT and DFT + U descriptions of Cu vacancies are similar, indicating that DFT is suitable for a small concentration of Cu vacancies; the formation energy of a Cu vacancy is no larger than 1.7 eV. Formation of Cu vacancies produces delocalised hole states with hole effective masses consistent with the semiconducting nature of Cu2O. These results demonstrate that the p-type semiconducting properties observed for Cu2O are explained by a small concentration of Cu vacancies.

We examine the effect of growth temperature in the 150−300 °C range on the structural and morphological properties of Al2O3 films deposited using atomic layer deposition, contrasting the effect of H2O and O3 as oxygen sources. Trimethylaluminum (TMA) is the metal source. A mechanism for the O3 reaction is investigated using ab initio calculations and provides an explanation for the observed temperature dependence. The simulations show that hydroxyl groups are produced at the surface by the oxidation of adsorbed methyl groups by O3. This is confirmed by the measured rates; both H2O and O3 processes show molar growth rates per cycle that decrease with increasing reactor temperature, consistent with a decrease in hydroxyl coverage. At no temperature does the O3 process deposit more Al2O3 per cycle than the H2O process. Morphological characterization shows that O3 as the oxygen source yields lower-quality films than H2O; the films are less dense and rougher, especially at low growth temperatures. The existence of voids correlates with the low film electronic density. This may indicate the low mobility of surface hydroxyl at low temperatures, an effect that is washed out by repeated exchange with the vapor phase in the H2O case.

We studied the reduction of CuO(111) surface using density functional theory (DFT) with the generalized gradient approximation corrected for on-site Coulomb interactions (GGA + U) and screened hybrid DFT (HSE06 functional). The surface reduction process by oxygen vacancy formation and H2 adsorption on the CuO(111) surface is investigated as two different reduction mechanisms. It is found that both GGA + U and HSE06 predict the same trend in the relative stability of oxygen vacancies. We found that loss of the subsurface oxygen is initially thermodynamically favourable. As the oxygen vacancy concentration increases, mixture of subsurface and surface vacancies is energetically preferred over full reduction of the surface or subsurface monolayer. The reduction of Cu2+ to Cu+ is found to be more favourable than that of Cu+ to Cu0 in the most stable vacancy structures at all concentrations. Consistent with the oxygen vacancy calculations, H2 adsorption occurs initially on under-coordinated surface oxygen. Water molecules are formed upon the adsorption of H2 and this gives a mechanism for H2 reduction of CuO to Cu. Ab initio atomistic thermodynamics shows that reducing CuO to metallic Cu at the surface is more energetically difficult than in the bulk so that the surface oxide protects the bulk from reduction. Using H2 as the reducing agent, it is found that the CuO surface is reduced to Cu2O at approximately 360 K and that complete reduction from Cu2O to metallic Cu occurs at 780 K

Density functional calculations have been performed to study the structure and energetics of mainly neutral with n up to 153. Clusters up to n \ 15 have been investigated systematically, in part by simulated Al n annealing techniques. No pronounced islands of stability are found in this range although and are of Al 7Al 13 special importance. The treatment of selected larger clusters with di †erent packings and cluster shapes leads to the following conclusions. Icosahedral packing is favoured only for n \ 13 ; around n \ 55, decahedral packing is most stable, whereas fcc is deÐnitely preferred, energetically, for n [ 80. Among clusters of comparable size and packing those with dominantly (111) surfaces are found to be most stable. These trends are rationalized by considering the average number of next neighbours and the internal strain arising from a mismatch of bond distances. Extrapolation of the computed cluster binding energies yields a cohesive energy of bulk Al of 3.3È3.4 eV, in close agreement with experiment (3.36 eV).

Point defects in metal oxides such as TiO 2 are key to their applications in numerous technologies. The investigation of thermally induced nonstoichiometry in TiO 2 is complicated by the difficulties in preparing and determining a desired degree of nonstoichiometry. We study controlled self-doping of TiO 2 by adsorption of 1/8 and 1/16 monolayer Ti at the ͑110͒ surface using a combination of experimental and computational approaches to unravel the details of the adsorption process and the oxidation state of Ti. Upon adsorption of Ti, x-ray and ultraviolet photoemission spectroscopy ͑XPS and UPS͒ show formation of reduced Ti. Comparison of pure density functional theory ͑DFT͒ with experiment shows that pure DFT provides an inconsistent description of the electronic structure. To surmount this difficulty, we apply DFT corrected for on-site Coulomb interaction ͑DFT+ U͒ to describe reduced Ti ions. The optimal value of U is 3 eV, determined from comparison of the computed Ti 3d electronic density of states with the UPS data. DFT+ U and UPS show the appearance of a Ti 3d adsorbate-induced state at 1.3 eV above the valence band and 1.0 eV below the conduction band. The computations show that the adsorbed Ti atom is oxidized to Ti 2+ and a fivefold coordinated surface Ti atom is reduced to Ti 3+ , while the remaining electron is distributed among other surface Ti atoms. The UPS data are best fitted with reduced Ti 2+ and Ti 3+ ions. These results demonstrate that the complexity of doped metal oxides is best understood with a combination of experiment and appropriate computations.

Starting from the theoretical prediction of the ␥-Al 2 O 3 structure using density-functional theory in the generalized gradient approximation, we have studied the (1 1 1), (0 0 1), (1 1 0), and (1 5 0) surfaces. The surface energies and their corresponding structures are computed and compared with predictions for (0 0 0 1) ␣-Al 2 O 3 and available experimental results for ␥-alumina surfaces. (1 1 1) and (0 0 1) surfaces are predicted to be equally stable, but to show quite different structure and reactivity. Whereas a low coverage of highly reactive trigonal Al occurs on (1 1 1), (0 0 1) exhibits a more dense plane of both five-coordinate and tetrahedral Al. Microfaceting of a (1 1 0) surface into (1 1 1)-like planes is also observed. The implications for the structure of ultrathin dielectric films and for the surfaces of disordered transition aluminas are discussed.