including graphene, [10,[13][14][15][16] carbon nanotubes (CNTs), [5,17] conductive elastomers, [4,18] silicon nanowires and nanostrips, [19] and metal nanowires and nanoparticles. [20] Among them, graphene has been the most studied due to its excellent electrical conductivity, [21] high transmittance, [22] outstanding mechanical properties, [23] and large surface area. [24,25] It is noteworthy that graphene can be safely employed in devices being in direct contact with human skin, enabling applications as tattoo sensors. [26] To evaluate the performance of a pressure sensor, several parameters need to be taken into account such as sensitivity, response time, detection limit, linearity range, cyclability, power consumption, and robustness. The sensitivity of the pressure sensor, defined as the ratio between the change in the electrical signal output and the applied pressure, is probably the most important figure-of-merit of the sensor. Sensors featuring high sensitivities are capable of detecting extremely small changes in the pressure, and can be exploited even to transduce muscle movements [16,27] as well as the subtle vibrations of sound [6,11,28,29] into electrical outputs. Compared to the complicated fabrication methods such as microelectromechanical systems [30,31] and microfluidics techniques, [7] the engineering of the structure of active material represents the simplest and the most straightforward approach for the fabrication of pressure sensors in which a small applied pressure can determine subtle structural changes in the electroactive material. For example, upon applying a pressure, cracks and structural defects can be generated, which results in modification of the percolation pathways for charge transport, and can ultimately result in large variations in the electrical output. [13,32,33] Moreover, the contact resistance at the electrode-active layer interface can be modulated by pressure resulting into an improvement of the sensitivity. [12,18,34,35] By using such a strategy, Suh and coworkers. [35] demonstrated a strain-gauge sensor, which is based on two interlocked arrays of Pt-coated polyurethane acrylate nanofibers supported on thin poly(dimethylsiloxane) layers. Furthermore, a change in capacitance can be induced by pressure, which is the working principle of capacitive pressure sensor. In that case the sensitivity can be improved by microstructurationThe development of pressure sensors is crucial for the implementation of electronic skins and for health monitoring integrated into novel wearable devices. Tremendous effort is devoted toward improving their sensitivity, e.g., by employing microstructured electrodes or active materials through cumbersome processes. Here, a radically new type of piezoresistive pressure sensor based on a millefeuille-like architecture of reduced graphene oxide (rGO) intercalated by covalently tethered molecular pillars holding on-demand mechanical properties are fabricated. By applying a tiny pressure to the multilayer structure, the electron tunnelling ruling th...
2D materials (2DMs), which can be produced by exfoliating bulk crystals of layered materials, display unique optical and electrical properties, making them attractive components for a wide range of technological applications. This review describes the most recent developments in the production of high-quality 2DMs based inks using liquid-phase exfoliation (LPE), combined with the patterning approaches, highlighting convenient and effective methods for generating materials and films with controlled thicknesses down to the atomic scale. Different processing strategies that can be employed to deposit the produced inks as patterns and functional thin-films are introduced, by focusing on those that can be easily translated to the industrial scale such as coating, spraying, and various printing technologies. By providing insight into the multiscale analyses of numerous physical and chemical properties of these functional films and patterns, with a specific focus on their extraordinary electronic characteristics, this review offers the readers crucial information for a profound understanding of the fundamental properties of these patterned surfaces as the millstone toward the generation of novel multifunctional devices. Finally, the challenges and opportunities associated to the 2DMs' integration into working opto-electronic (nano) devices is discussed.
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