Charge trapping is one of several factors that limit the performance of organic electronic materials, yet even in pentacene, a prototypical small‐molecule semiconductor, the precise chemical nature of charge trapping remains poorly understood. Here the effects of three chemical trap‐precursor candidates are examined by layering thin‐film pentacene transistors with different pentacene defect species. The resulting charge trapping is studied in each device via scanning‐probe electric force microscopy coupled with variable‐wavelength sample illumination. Firstly, it is found that layering with pentacen‐6(13H)‐one (PHO) readily produces uniform charge trapping everywhere in the transistor channel, as expected for an active blanket‐deposited trap‐precursor. However, layering with 6,13‐dihydropentacene (DHP) produces fewer, more‐isolated traps, closely resembling the surface potential distribution in pristine pentacene thin films. Secondly, the rates of trap‐clearing versus illuminating wavelength (trap‐clearing spectra) are measured, revealing enhanced trap‐clearing rates at wavelengths assigned to the absorption of either pentacene or the charged trap species. The trap‐clearing spectrum for the PHO‐layered sample closely resembles the spectrum obtained from pentacene aged in a working transistor, while the trap‐clearing spectrum for the DHP‐layered sample resembles the spectrum observed in pristine pentacene. We conclude that PHO competently creates traps in pentacene that match the expected trap‐clearing spectrum for degraded pentacene, while DHP does not, and that the chemical trap species in aged pentacene is very likely PHO+.
Copper (Cu) and iron (Fe) are essential micronutrients that are toxic when accumulating in excess in cells. Thus, their uptake by roots is tightly regulated. While plants sense and respond to local Cu availability, the systemic regulation of Cu uptake has not been documented in contrast to local and systemic control of Fe uptake. Fe abundance in the phloem has been suggested to act systemically, regulating the expression of Fe uptake genes in roots. Consistently, shoot-to-root Fe signaling is disrupted in Arabidopsis thaliana mutants lacking the phloem companion cell-localized Fe transporter, OLIGOPEPTIDE TRANSPORTER 3 (AtOPT3). We report that AtOPT3 also transports Cu in heterologous systems and contributes to its delivery from sources to sinks in planta. The opt3 mutant contained less Cu in the phloem, was sensitive to Cu deficiency and mounted a transcriptional Cu deficiency response in roots and young leaves. Feeding the opt3 mutant and Cu- or Fe-deficient wild-type seedlings with Cu or Fe via the phloem in leaves downregulated the expression of both Cu and Fe-deficiency marker genes in roots. These data suggest the existence of shoot-to-root Cu signaling, highlight the complexity of Cu/Fe interactions, and the role of AtOPT3 in fine-tuning root transcriptional responses to the plant Cu and Fe needs.
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