Controlling contact resistance in organic field‐effect transistors (OFETs) is one of the major hurdles to achieve transistor scaling and dimensional reduction. In particular in the context of ...ambipolar and/or light‐emitting OFETs it is a difficult challenge to obtain efficient injection of both electrons and holes from one injecting electrode such as gold since organic semiconductors have intrinsically large band gaps resulting in significant injection barrier heights for at least one type of carrier. Here, systematic control of electron and hole contact resistance in poly(9,9‐di‐n‐octylfluorene‐alt‐benzothiadiazole) ambipolar OFETs using thiol‐based self‐assembled monolayers (SAMs) is demonstrated. In contrast to common believe, it is found that for a certain SAM the injection of both electrons and holes can be improved. This simultaneous enhancement of electron and hole injection cannot be explained by SAM‐induced work‐function modifications because the surface dipole induced by the SAM on the metal surface lowers the injection barrier only for one type of carrier, but increases it for the other. These investigations reveal that other key factors also affect contact resistance, including i) interfacial tunneling through the SAM, ii) SAM‐induced modifications of interface morphology, and iii) the interface electronic structure. Of particular importance for top‐gate OFET geometry is iv) the active polymer layer thickness that dominates the electrode/polymer contact resistance. Therefore, a consistent explanation of how SAM electrode modification is able to improve both electron and hole injection in ambipolar OFETs requires considering all mentioned factors.
SAM modification in ambipolar transistors is able to improve both electron and hole injection. The results reported here show that gold electrodes modified by 1DT exhibit improved contact resistance for both electrons and holes compared to O2 plasma‐treated gold electrodes. The origin of this unexpected behavior is investigated in terms of SAM‐induced change of work‐function, film thickness, interfacial tunneling resistance and interfacial electronic and molecular structure.
We reveal the rather complex interplay of contact-induced re-orientation and interfacial electronic structure - in the presence of Fermi-level pinning - at prototypical molecular heterojunctions ...comprising copper phthalocyanine (H16CuPc) and its perfluorinated analogue (F16CuPc), by employing ultraviolet photoelectron and X-ray absorption spectroscopy. For both layer sequences, we find that Fermi-level (EF) pinning of the first layer on the conductive polymer substrate modifies the work function encountered by the second layer such that it also becomes EF-pinned, however, at the interface towards the first molecular layer. This results in a charge transfer accompanied by a sheet charge density at the organic/organic interface. While molecules in the bulk of the films exhibit upright orientation, contact formation at the heterojunction results in an interfacial bilayer with lying and co-facial orientation. This interfacial layer is not EF-pinned, but provides for an additional density of states at the interface that is not present in the bulk. With reliable knowledge of the organic heterojunction's electronic structure we can explain the poor performance of these in photovoltaic cells as well as their valuable function as charge generation layer in electronic devices.
Coal has been extracted via surface and sub-surface mining for decades throughout the Appalachian Mountains. New interest in ridge-top mining has raised concerns about possible waterway impacts. We ...examined effects of forestry, mining, and road construction-based disturbance on physico-chemistry and macroinvertebrate communities in east-central Tennessee headwater streams. Although 11 of 30 sites failed Tennessee’s biocriteria scoring system, invertebrate richness was moderately high and we did not find significant differences in any water chemistry or habitat parameters between sites with passing and failing scores. However, conductivity and dissolved solid concentrations appeared elevated in the majority of study streams. Principal components (PCs) analysis indicated that six PCs accounted for ~77 % of among-site habitat variability. One PC associated with dissolved oxygen and specific conductance explained the second highest proportion of among-site variability after catchment area. Specific conductance was not correlated with catchment area but was strongly correlated with mining activity. Composition and success of multivariate models using habitat PCs to predict macroinvertebrate metrics was highly variable. PC scores associated with water chemistry and substrate composition were most frequently included in significant models. These results suggest that impacts of historical and current coal mining remain a source of water quality and macroinvertebrate community impairment in this region, but effects are subtle. Our results suggest that surface mining may have chronic and system-wide effects on habitat conditions and invertebrate communities in Cumberland Plateau streams.
The vertical dimension constitutes an important niche axis along which mosquitoes may adjust their distribution. Here, we evaluated whether the vertical distribution of container-inhabiting Aedes ...mosquitoes differs along a gradient of anthropogenic land-use intensity within an urban landscape. Using a pulley system, we hung oviposition cups at three heights (ground level, 4.5, and 9 m) and in three habitats: forest, park, and a built environment. We hypothesized that mosquito abundance and diversity would be highest in the least disturbed forest habitat, decrease in the park, and be lowest at the UNC-Greensboro campus. We also expected Aedes albopictus (Skuse) and Ae. triseriatus (Say) to mainly oviposit at ground level and Ae. hendersoni (Cockerell) at canopy height. Aedes albopictus was the most common species (68.8%) collected in all three habitat types and was the only species found in the built environment. In that habitat, Ae. albopictus exhibited a bimodal distribution with the lowest activity at the intermediate height (4.5 m). Aedes triseriatus (28.9%) did not differ in egg abundance between the forest and park habitats but did exhibit diverse vertical habitat use while avoiding the canopy in the park habitat. Aedes hendersoni (2.3%) was the most sylvatic species and oviposited only at ground level. Our results indicate that the vertical distribution of mosquitoes is affected by the type of habitat in which they occur, and that this variation could be driven via local-scale modification of microclimatic factors.
The diatom
Didymosphenia geminata
is known to alter benthic habitat and macroinvertebrate diversity and community structure. Associations between macroinvertebrate communities and
D. geminata
biomass ...in riffle and run mesohabitats were investigated in the South Fork Holston River in Tennessee and Virginia, USA. We found that low current velocity, low turbidity, and high dissolved oxygen (DO) were strong predictors of
D. geminata
mat presence.
Didymosphenia geminata
ash-free dry mass was significantly higher in run mesohabitats with low current velocity (CV) than in riffle mesohabitats with higher CV. Macroinvertebrate alpha diversity (Shannon Diversity
H’
) was only marginally significantly different between riffle and runs, while beta diversity (community composition) was highly significantly different between these mesohabitats. NMDS analyses found that
D. geminata
was a relatively unimportant predictor of changes in community structure relative to specific conductance, CV, DO, and turbidity. However, effects of
D. geminata
on macroinvertebrates appear to be very taxon specific with effects on individual taxa potentially masked by tailwater effects on general macroinvertebrate diversity in global analyses. We observed that taxon-specific effects include, but are not limited to, (1) reduction of bryophyte microhabitat utilized by dominant ephemeropterans, trichopterans, amphipods, coleopterans, and some chironomid genera in run mesohabitats from competition with
D. geminata
for substrate attachment space; and (2) differences in utilization of
D. geminata
mat biomass as a food resource and microhabitat for chironomids. Our insights into taxon-specific effects of
D. geminata
on macroinvertebrates open up multiple avenues for experimentation in which to validate our observational findings.
Zircon extracted from drilled oceanic rocks is increasingly used to answer geologic questions related to igneous and sedimentary sequences. Recent zircon studies using samples obtained from marine ...drill cores revealed that drilling muds used in the coring process may contaminate the samples. The JOIDES Resolution Science Operator of the International Ocean Discovery Program has been using two types of clays, sepiolite and attapulgite, which both have salt water viscosifier properties able to create a gel‐like slurry that carries drill cuttings out of the holes several hundred meters deep. The dominantly used drilling mud is sepiolite originating from southwestern Nevada, USA. This sepiolite contains abundant zircon crystals with U‐Pb ages ranging from 1.89 to 2889 Ma and continental trace element, δ18O, and εHf isotopic compositions. A dominant population of 11–16 Ma zircons in sepiolite drilling mud makes identification of contamination in drilled Neogene successions particularly challenging. Interpretation of zircon analyses related to ocean drilling should be cautious of zircon ages in violation of independently constrained age models and that have age populations overlapping those in the sepiolite. Because individual geochronologic and geochemical characteristics lack absolute discriminatory power, it is recommended to comprehensively analyze all dated zircon crystals from cores exposed to drill mud for trace element, δ18O, and εHf isotopic compositions. Zircon analyzed in situ (i.e., in petrographic sections) are assumed to be trustworthy.
Key Points:
Abundant zircon in sepiolite drilling mud used in scientific drilling can contaminate rock samples
Contaminating zircon grains may yield diverse and spurious ages and compositions
Current models for molecular electrical doping of organic semiconductors are found to be at odds with other well-established concepts in that field, like polaron formation. Addressing these ...inconsistencies for prototypical systems, we present experimental and theoretical evidence for intermolecular hybridization of organic semiconductor and dopant frontier molecular orbitals. Common doping-related observations are attributed to this phenomenon, and controlling the degree of hybridization emerges as a strategy for overcoming the present limitations in the yield of doping-induced charge carriers.
C60 adsorbed on a monolayer of hexaazatriphenylene-hexanitrile (HATCN) on Ag(111) is investigated by ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling microscopy. UPS and ...quantum-mechanical modeling show that HATCN chemisorbed on Ag(111) displays metallic character. This metallic molecular layer decouples C60 electronically from the Ag substrate and simultaneously acts both as template for the stable adsorption of isolated C60 molecules at room temperature and as “soft” metallic contact for subsequently deposited molecules.
In this paper, we report UV and X-ray photoelectron spectroscopy studies on layered planar and mixed bulk heterojunctions of Cu-phtalocyanine (CuPc) and C60, a prototypical material pair for organic ...photovoltaic cells (OPVCs). The respective heterojunctions were formed on poly(ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) substrates in order to achieve morphologies comparable to those in actual OPVCs. As a result of a CuPc-to-substrate electron transfer, the work function of pristine PEDOT:PSS is reduced from 5.4 to 4.4 eV. The deposition of C60 onto CuPc, however, leads to a work function increase of 0.2 eV. The codeposition of C60 and CuPc to form mixed bulk heterojunctions resulted in an effective anode work function of 4.6 eV. The energy offset between the highest occupied levels of CuPc and C60 was determined as 1.3 eV for both the layered planar and mixed bulk heterojunction. With reported values of the charge transport gap of C60, we estimate the upper limit of the open circuit voltage to be 1.0 eV for both types of heterojunctions. Our results demonstrate that the energy level offsets are independent of particular interface morphology in C60/CuPc heterojunctions grown on PEDOT:PSS, and that differences in device efficiency are due to other effects.
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