| 1. |
- Clark, Pip C.J., et al.
(author)
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The passivating effect of cadmium in PbS/CdS colloidal quantum dots probed by nm-scale depth profiling
- 2017
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In: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3364 .- 2040-3372. ; 9:18, s. 6056-6067
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Journal article (peer-reviewed)abstract
- Achieving control of the surface chemistry of colloidal quantum dots (CQDs) is essential to fully exploit their properties in solar cells, but direct measurement of the chemistry and electronic structure in the outermost atomic layers is challenging. Here we probe the surface oxidation and passivation of cation-exchanged PbS/CdS core/shell CQDs with sub nm-scale precision using synchrotron-radiation-excited depth-profiling photoemission. We investigate the surface composition of the topmost 1-2.5 nm of the CQDs as a function of depth, for CQDs of varying CdS shell thickness, and examine how the surface changes after prolonged air exposure. We demonstrate that the Cd is localized at the surface of the CQDs. The surface-localized products of oxidation are identified, and the extent of oxidation quantified. We show that oxidised sulfur species are progressively eliminated as Cd replaces Pb at the surface. A sub-monolayer surface 'decoration' of Cd is found to be effective in passivating the CQDs. We show that the measured energy-level alignments at PbS/CdS colloidal quantum dot surfaces differ from those expected on the basis of bulk band offsets, and are strongly affected by the oxidation products. We develop a model for the passivating action of Cd. The optimum shell thickness (of around 0.1 nm, previously found to give maximised power conversion efficiency in PbS/CdS solar cells) is found to correspond to a trade-off between the rate of oxidation and the introduction of a surface barrier to charge transport.
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| 2. |
- Fairclough, Simon M., et al.
(author)
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Growth and Characterization of Strained and Alloyed Type-II ZnTe/ZnSe Core-Shell Nanocrystals
- 2012
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In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 116:51, s. 26898-26907
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Journal article (peer-reviewed)abstract
- We investigate the growth and the physical and optical properties of type-II heterostructured ZnTe/ZnSe colloidal nanocrystals, focusing on the role of the 7% lattice mismatch between the two materials in determining growth homogeneity and band structure. We find that the lattice mismatch between the two materials places limitations on the range of structures that can be grown, and for those in which coherent growth is achieved we present clear evidence that the low bulk modulus ZnTe cores are compressed by the higher modulus ZnSe shells, accentuating the red-shift of the excitonic state with increasing shell thickness. By employing a variety of characterization tools we build a clear picture of the core-shell architecture. We show how strain is manifested in structures with sharp core-shell interfaces and how intentional alloying of the interface can influence the growth and exciton energies. We show that a (2,6)-band effective mass model is able to distinguish between the as-grown "sharp" and "alloyed" interfaces, indicating that the alloyed structures incorporate reduced strain.
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| 3. |
- Hübner, Sariel, et al.
(author)
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Islands and streams : clusters and gene flow in wild barley populations from the Levant.
- 2012
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In: Molecular Ecology. - 0962-1083 .- 1365-294X. ; 21:5, s. 1115-29
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Journal article (peer-reviewed)abstract
- The domestication of plants frequently results in a high level of genetic differentiation between domesticated plants and their wild progenitors. This process is counteracted by gene flow between wild and domesticated plants because they are usually able to inter-mate and to exchange genes. We investigated the extent of gene flow between wild barley Hordeum spontaneum and cultivated barley Hordeum vulgare, and its effect on population structure in wild barley by analysing a collection of 896 wild barley accessions (Barley1K) from Israel and all available Israeli H. vulgare accessions from the Israeli gene bank. We compared the performance of simple sequence repeats (SSR) and single nucleotide polymorphisms (SNP) marker data genotyped over a core collection in estimating population parameters. Estimates of gene flow rates with SSR markers indicated a high level of introgression from cultivated barley into wild barley. After removing accessions from the wild barley sample that were recently admixed with cultivated barley, the inference of population structure improved significantly. Both SSR and SNP markers showed that the genetic population structure of wild barley in Israel corresponds to the three major ecogeographic regions: the coast, the Mediterranean north and the deserts in the Jordan valley and the South. Gene flow rates were estimated to be higher from north to south than in the opposite direction. As has been observed in other crop species, there is a significant exchange of alleles between the wild species and domesticated varieties that needs to be accounted for in the population genetic analysis of domestication.
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| 4. |
- Syres, Karen L., et al.
(author)
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Adsorbate-Induced Modification of Surface Electronic Structure: Pyrocatechol Adsorption on the Anatase TiO2 (101) and Rutile TiO2 (110) Surfaces
- 2012
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In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 116:44, s. 23515-23525
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Journal article (peer-reviewed)abstract
- Photoemission and near-edge X-ray absorption fine structure (NEXAFS) techniques have been used to study the adsorption of pyrocatechol on anatase TiO2 (101) and rutile TiO2 (110) single crystals. Photoemission results suggest the pyrocatechol molecule adsorbs on both surfaces predominantly in a bidentate geometry. Using the searchlight effect, the carbon K-edge NEXAFS spectra recorded for pyrocatechol on anatase TiO2 (101) and rutile TiO2 (110) show the phenyl rings in the pyrocatechol molecule to be oriented at 27 +/- 6 degrees and 2.3 +/- 8 degrees, respectively, from the surface normal. Experimental data and computational models of the pyrocatechol-anatase TiO2 interface indicate the appearance of new occupied and unoccupied states on adsorption due to hybridization between the electronic states of the pyrocatechol molecule and the surface. The atomic character of the new orbitals created facilitates direct photoinjection from pyrocatechol into anatase TiO2 and induces a strong final state effect in the carbon K-edge NEXAFS spectrum.
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