The need for high‐quality large‐scale monolayers of layered materials pushes the development of scalable gold‐mediated exfoliations. Gold proves to be a suitable adhesive for exfoliation of several 2D materials. However, the extension to other noble metals remains underwhelming as gold outperforms all previously studied metals by a large margin. This is attributed to compromised stability against oxidation and surface contamination of less noble metals, leading to nonideal interfaces for exfoliation. The closest competitor to gold is silver, where gold still leads by a factor 100 regarding exfoliated layer size. In this work, a silver‐mediated exfoliation process performing on par with gold is presented. The combination of freshly cleaved silver surfaces with a low‐temperature annealing is found to be crucial. The exfoliation yield shows a dependence with annealing temperature, leading to loss in exfoliation performance for higher temperature. Raman studies indicate inhomogeneous strain for the MoS2/Ag interface at these temperatures, which hints at the competing factors of thermal activation versus oxidation of silver. Finally, a transfer process is implemented to promote silver to a fully functional exfoliation substrate. Ultimately, heating up exfoliations tips the strict balance between interfacial interactions and surface contaminations toward robust high monolayer yield exfoliation as demonstrated for silver.
We have synthesized and characterized a thioindigo photoswitch featuring phenolic residues forming an intramolecular hydrogen bond exclusively in the cis isomer. In acetone–water mixtures, wavelength‐dependent photostationary states reached their maximum and minimum values of up to 95 % and down to 11 % cis isomers at circa λ=580 and 480 nm, respectively. Within a few hours, room‐temperature thermal isomerization is negligible in neutral and mildly acidic solutions. Protic media decrease trans→cis but increase cis→trans quantum yields. In CD3OD solution, in which light of any wavelength absorbed triggers the latter process almost exclusively, a 19F NMR titration is used to calculate pKa values of 7.2 (trans) and 5.0 (cis; both rescaled to water) corresponding to a change in dissociation constant of 2.2 orders of magnitude. Future derivatives featuring solubility and reversible photochromism in water may provide pH modulation of similar amplitude in a range adjustable by the choice of phenolic substituents.
Most electrical sensor and biosensor elements require reliable transducing elements to convert small potential changes into easy to read out current signals. Offering inherent signal magnification and being operable in many relevant environments field‐effect transistors (FETs) are the element of choice in many cases. In particular, using electrolyte gating, numerous sensors and biosensors have been realized in aqueous environments. Over the past years, electrolyte‐gated FETs have been fabricated using a variety of semiconducting materials, including graphene, ZnO, as well as conjugated molecules and polymers. Above all, using conducting polymers top‐performing devices have been achieved. Herein, an approach to use a transition metal dichalcogenide (TMDC)‐based monolayer device as a transducing element is presented. Using MoS2 monolayers, it is shown that such electrolyte‐gated devices may be regarded as very promising transducing elements for sensor and biosensor applications, enabled by their high sensitivity for environmental changes and the possibility of using the naturally occurring sulfur vacancies as grafting points of biorecognition layers. Furthermore, the behavior of such a device under prolonged operation in a dilute biologically relevant electrolyte such as phosphate buffered saline solution (PBS) is reported.
As a direct‐bandgap transition semiconductor with high carrier mobility, monolayer (ML) transition metal dichalcogenides (TMDCs) have attracted significant attention as a promising class of material for photodetection. It is reported that these layers exhibit a persistent photoconductance (PPC) effect, which is assigned to long‐lasting hole capture by deep traps. Therefore, TMDCs‐based photodetectors show a high photoresponse but also a slow response. Herein, intensity‐modulated photocurrent spectroscopy (IMPS) with steady‐state background illumination is performed to investigate the photoresponse dynamics in a hybrid photodetector based on ML MoS2 covered with an ultrathin layer of phthalocyanine (H2Pc) molecules. The results demonstrate that adding the H2Pc layer speeds up the photoresponse of the neat ML‐MoS2 photodetector by almost two orders of magnitude without deteriorating its responsivity. The origin of these improvements is revealed by applying the Hornbeck–Haynes model to the photocarrier dynamics in the IMPS experiment. It is shown that the improved response speed of the hybrid device arises mostly from a faster detrapping of holes in the presence of the H2Pc layer, while the trap densities remain rather unchanged. Meanwhile, the additional absorption of photons in the H2Pc layer contributes to photocarrier generation, resulting in an enlarged responsivity of the hybrid device.
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