Atomically thin transition metal dichalcogenides (TMDCs) have attracted great interest as a new class of two-dimensional (2D) direct band gap semiconducting materials. The controllable modulation of optical and electrical properties of TMDCs is of fundamental importance to enable a wide range of future optoelectronic devices. Here we demonstrate a modulation of the optoelectronic properties of 2D TMDCs, including MoS2, MoSe2, and WSe2, by interfacing them with two metal-centered phthalocyanine (MPc) molecules: nickel Pc (NiPc) and magnesium Pc (MgPc). We show that the photoluminescence (PL) emission can be selectively and reversibly engineered through energetically favorable electron transfer from photoexcited TMDCs to MPcs. NiPc molecules, whose reduction potential is positioned below the conduction band minima (CBM) of monolayer MoSe2 and WSe2, but is higher than that of MoS2, quench the PL signatures of MoSe2 and WSe2, but not MoS2. Similarly, MgPc quenches only WSe2, as its reduction potential is situated below the CBM of WSe2, but above those of MoS2 and MoSe2. The quenched PL emission can be fully recovered when MPc molecules are removed from the TMDC surfaces, which may be refunctionalized and recycled multiple times. We also find that photocurrents from TMDCs, probed by photoconductive atomic force microscopy, increase over 2-fold only when the PL is quenched by MPcs, further supporting the photoinduced charge transfer mechanism. Our results should benefit design strategies for 2D inorganic-organic optoelectronic devices and systems with tunable properties and improved performances.

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optical transparency, and mechanical flexibility of the sensor are considered as essential requirements for use in future tactile sensing applications. To fulfill these demands, a broad range of materials, fabrication processes, and structural designs of the tactile sensor have been developed, [8] while sensing principles are mainly classified as either resistive [9][10][11] or capacitive types. [12][13][14] Many of resistive sensors use nanomaterial-embedded composites and exploit changes in contact resistance between the nanomaterials in the composite matrix (such as elastomer) under pressure loading, showing improved pressure sensitivity and mechanical flexibility compared to silicon-or metal-based piezoresistive sensors. [15] However, resistive tactile sensors suffer from signal drift due to temperature changes and require high power consumption. In addition, complicated circuit arrangement for multipoint recognition is regarded as a drawback to be addressed. [7,16] Compared to resistive tactile sensing mechanisms, capacitive tactile sensors have advantages in terms of temperature independence, low power consumption, stability against long-term signal drift, and easy multipoint recognition by simple assembly of row and column electrodes. [16,17] In general, the structure of a capacitive sensor consists of two parallel electrodes with a dielectric layer between them. Highly compressible dielectric materials are essential to achieve high sensitivity; a lower Young's modulus of the dielectric leads to greater deformation when pressure is applied to the sensor, resulting in a larger change in capacitance. Accordingly, considerable efforts have been devoted to using elastomers with low Young's modulus as dielectric materials, including polydimethylsiloxane (PDMS), [18] polyurethane, [19] or Ecoflex. [12] However, these low-modulus elastomers also tend to have high viscoelasticity, slowing their response and relaxation times. [20] To overcome this limitation and further improve the sensitivity, a few tactile sensors make use of the strategy of structuring the dielectric layer by fabricating a microstructured surface in an orderly fashion. [7,20,21] The microstructured dielectric layer led to much higher sensitivity and faster response/relaxation time by allowing a larger deformation compared to conventional capacitive sensors with a plain dielectric layer under equal applied pressure. Nevertheless, The development of sensitive, flexible, and transparent tactile sensors is of great interest for next-generation flexible displays and human-machine interfaces. Although a few materials and structural designs have been previously developed for high-performance tactile sensors, achieving flexibility, full transparency, and highly sensitive multipoint recognition without crosstalk remains a significant challenge for such systems. This work demonstrates a capacitive tactile sensor composed of two sets of facing graphene electrodes separated by spacers, which forms an air dielectric between them. The air gap facilitates mor...

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have emerged as attractive direct bandgap semiconducting materials with remarkable properties. Recently, TMDC-based electronic and optoelectronic systems have been demonstrated with various chemical doping and functionalization approaches for modulating their physical properties and enhancing device performances. However, the dependence of intrinsic properties of TMDCs on diverse solvents, which are used necessarily in fabrication processes and chemical doping, remains largely unaddressed. Here we report a charge transfer mechanism in TMDCs by commonly used solvents such as chloroform, toluene, acetone, and 2-propanol, which significantly changes the physical properties of monolayer MoS2 and WSe2. We find that the relative difference in electronegativity between solvents and TMDCs drives the transfer of electrons from or to the TMDCs, which results in photoluminescence (PL) enhancement or quenching depending on the change of carrier density in TMDCs. The analysis of exciton and trion spectral weights in MoS2 as a function of solvent electronegativity provides evidence of charge transfer. Finally, conductive atomic force microscopy (C-AFM) on TMDCs before and after immersion in the solvents further supports the charge transfer mechanism and resulting changes in carrier density. Our results highlight the importance of selection of solvents for solution-processed 2D TMDC devices and systems.

Nanomaterials have drawn great interest in recent years due to their extraordinary properties that make them advantageous in food packaging applications. Specifically, nanoparticles can impart significant barrier properties, as well as mechanical, optical, catalytic, and antimicrobial properties into packaging. Silver nanoparticles (AgNPs) and nanoclay account for the majority of the nano-enabled food packaging on the market, while others, such as nano-zinc oxide (ZnO) and titanium, share less of the current market. In current food packaging, these nanomaterials are primarily used to impart antimicrobial function and to improve barrier properties, thereby extending the shelf life and freshness of packaged food. On the other hand, there is growing concern about the migration of nanomaterials from food contact materials to foodstuffs and its associated potential risks. Indeed, insufficient data about environmental and human safety assessments of migration and exposure of nanomaterials are hindering their market growth. To overcome this barrier, the public believes that legislation from government agencies is critical. This review provides an overview of the characteristics and functions of major nanomaterials that are commonly applied to food packaging, including available and near- future products. Migration research, safety issues, and public concerns are also discussed.

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