We report on the observation of chirality induced spin selectivity for electrons transmitted through monolayers of oligopeptides, both for energies above the vacuum level as well as for bound ...electrons and for electrons conducted through a single molecule. The dependence of the spin selectivity on the molecular length is measured in an electrochemical cell for bound electrons and in a photoemission spectrometer for photoelectrons. The length dependence and the absolute spin polarization are similar for both energy regimes. Single molecule conductance studies provide an effective charge transport barrier between the two spin channels and it is found to be on the order of 0.5 eV.
In electron-transfer processes, spin effects normally are seen either in magnetic materials or in systems containing heavy atoms that facilitate spin-orbit coupling. We report spin-selective ...transmission of electrons through self-assembled monolayers of double-stranded DNA on gold. By directly measuring the spin of the transmitted electrons with a Mott polarimeter, we found spin polarizations exceeding 60% at room temperature. The spin-polarized photoelectrons were observed even when the photoelectrons were generated with unpolarized light. The observed spin selectivity at room temperature was extremely high as compared with other known spin filters. The spin filtration efficiency depended on the length of the DNA in the monolayer and its organization.
We report on a bottom-up approach of the selective and precise growth of subnanometer wide straight and chevron-type armchair nanoribbons (GNRs) on a stepped Au(788) surface using different specific ...molecular precursors. This process creates spatially well-aligned GNRs, as characterized by STM. High-resolution direct and inverse photoemission spectroscopy of occupied and unoccupied states allows the determination of the energetic position and momentum dispersion of electronic states revealing the existence of band gaps of several electron volts for straight 7-armchair, 13-armchair, and chevron-type GNRs in the electronic structure.
Abstract
We present temperature programmed desorption (TPD) measurements of CO, CH4, O2 and CO2 from the forsterite(010) surface in the sub-monolayer and multilayer coverage regimes. In the case of ...CO, CH4 and O2, multilayer growth begins prior to saturation of the monolayer peak, resulting in two clearly distinguishable desorption peaks. On the other hand, a single peak for CO2 is observed which shifts from high temperature at low coverage to low temperature at high coverages, sharpening upon multilayer formation. The leading edges are aligned for all the molecules in the multilayer coverage regime indicating zero order desorption. We have extracted multilayer desorption energies for these molecules using an Arrhenius analysis. For sub-monolayer coverages, we observe an extended desorption tail to higher temperature. Inversion analysis has been used to extract the coverage dependent desorption energies in the sub-monolayer coverage regime, from which we obtain the desorption energy distribution. We found that owing to the presence of multiple adsorption energy sites on the crystalline surface the typical desorption energies of these small molecules are significantly larger than obtained in previous measurements for several other substrates. Therefore molecules bound to crystalline silicate surfaces may remain locked in the solid state for a longer period of time before desorption into the gas phase.
This work demonstrates the chiral-induced spin selectivity effect for inorganic copper oxide films and exploits it to enhance the chemical selectivity in electrocatalytic water splitting. Chiral CuO ...films are electrodeposited on a polycrystalline Au substrate, and their spin filtering effect on electrons is demonstrated using Mott polarimetry analysis of photoelectrons. CuO is known to act as an electrocatalyst for the oxygen evolution reaction; however, it also generates side products such as H2O2. We show that chiral CuO is selective for O2; H2O2 generation is strongly suppressed on chiral CuO but is present with achiral CuO. The selectivity is rationalized in terms of the electron spin-filtering properties of the chiral CuO and the spin constraints for the generation of triplet oxygen. These findings represent an important step toward the development of all-inorganic chiral materials for electron spin filtering and the creation of efficient, spin-selective (photo)electrocatalysts for water splitting.
The thermal desorption of ammonia (NH3) from single crystal forsterite (010) has been investigated using temperature-programmed desorption. The effect of defects on the desorption process has been ...probed by the use of a rough cut forsterite surface prepared from the cleaved forsterite sample. Several approaches have been used to extract the desorption energy and pre-exponential factor describing the desorption kinetics. In the sub-monolayer coverage regime, the NH3 desorption shows a broad distribution of desorption energies, indicating the presence of different adsorption sites, which results in an apparent coverage-dependent desorption energy. This distribution is sensitive to the surface roughness with the cut forsterite surface displaying a significantly broader distribution of desorption energies compared to the cleaved forsterite surface. The cut forsterite surface exhibits sites with desorption energies up to 62.5 kJ mol−1 in comparison to a desorption energy of up to 58.0 kJ mol−1 for the cleaved surface. Multilayer desorption is independent of the nature of the forsterite surface used, with a desorption energy of (25.8 ± 0.9) kJ mol−1. On astrophysically relevant heating time-scales, the presence of a coverage-dependent desorption energy distribution results in a lengthening of the NH3 desorption time-scale by 5.9 × 104 yr compared to that expected for a single desorption energy. In addition, the presence of a larger number of high-energy adsorption sites on the rougher cut forsterite surface leads to a further lengthening of ca. 7000 yr.
Plant hydraulic characteristics were studied in diploid, tetraploid and hexaploid cytotypes of Atriplex canescens (Chenopodiaceae) to investigate the potential physiological basis underlying the ...intraspecific habitat differentiation among plants of different ploidy levels.
Populations of A. canescens from different habitats of the Chihuahuan Desert (New Mexico, USA) were analyzed using flow cytometry to determine ploidy levels. Traits related to xylem water transport efficiency and safety against drought-induced hydraulic failure were measured in both stems and leaves.
At the stem level, cytotypes of higher ploidy showed consistently lower leaf-specific hydraulic conductivity but greater resistance to drought-induced loss of hydraulic conductivity. At the leaf level, comparisons in hydraulics between cytotypes did not show a consistent pattern, but exhibited high plasticity to proximal environmental conditions related to soil water availability.
The results suggest that a trade-off between stem hydraulic efficiency and safety across ploidy levels underlies niche differentiation among different cytotypes of A. canescens. Polyploidization may have been facilitated by environmental heterogeneity related to water availability, and variation in water-related physiology found in the present study suggests an important functional basis for the niche differentiation and coexistence of A. canescens cytotypes in desert environments.
Premise of the study: Atripliceae (Chenopodiaceae), including Atriplex (300 spp.) as the largest genus of the family, are an ecologically important group of steppes and semideserts worldwide. ...Relationships in Atripliceae are poorly understood due to obscure and potentially convergent morphological characters. Methods: Using sequence variation of two chloroplast markers (rbcL gene, atpB-rbcL spacer) and one nrDNA marker (ITS) analyzed with BEAST, we investigated the systematics and biogeography of Atripliceae. We surveyed flower morphology and fruit anatomy to study the evolution of flowers and fruits in the tribe. Key results: Female flowers with persistent foliar cover (the diagnostic character of traditional Atripliceae) evolved three times in Chenopodioideae, in Atripliceae s.s., Axyrideae, and Spinacia. Atripliceae s.s. started to diversify during the Early Miocene in Eurasia, separating into the Archiatriplex and the Atriplex clades. The former consists of eight species-poor, disjunct, and morphologically heterogeneous genera and is likely a relictual lineage. The Atriplex clade comprises the majority of species and evolved one C₄ lineage 14.1-10.5 Ma, which diversified rapidly worldwide. The C₄ Atriplex entered North America during the Middle/Late Miocene and spread to South America subsequently. Australia was colonized by two C₄ lineages both arriving during the Late Miocene. One of them diversified rapidly, giving rise to most Australian Atriplex species. Conclusions: Atripliceae s.s. comprise Archiatriplex, Atriplex, Exomis, Extriplex, Grayia, Halimione, Holmbergia, Manochlamys, Proatriplex, and Stutzia. Microgynoecium is included based on morphology but only weak molecular support. Axyris, Krascheninnikovia, and Ceratocarpus (here described as Axyrideae) and Spinacia are excluded from Atripliceae.
Plant–pollinator interactions are essential for maintaining both pollinator and plant communities in native and agricultural environments. Animal-instigated pollination can be complex. Plants are ...usually visited by a number of different animal species, which in turn may visit flowers of several plant species. Therefore, the identification of the pollen carried by flower visitors is an essential first step in pollination biology. The skill and time required to identify pollen based on structure and morphology has been a major stumbling block in this field. Advances in the genetic analysis of DNA, using DNA barcoding, extracted directly from pollen offers an innovative alternative to traditional methods of pollen identification. This technique, which is reviewed in detail, can be used on pollen loads sampled from bees in the field and from specimens in historic collections. Here the importance of pollination, the role-players involved, their management and the evolution of their interactions, behaviour and morphology are reviewed – with a special focus on South African bees. Significance: • Pollen metabarcoding will enable the identification of pollen for a multitude of uses, including agriculture, conservation and forensics. • Plant–pollinator interaction documentation through pollen identification gives a more certain record of a visitor being a pollinator rather than a flower visitor that could be a nectar gatherer.
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