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The photovoltaic devices offer promising eco‐friendly solution for self‐powered flexible electronics. However, their fabrication on flexible substrate is not easy due to mismatches between the requirements of conventional microfabrication and the thermal, and mechanical features of the substrates. Herein, direct roll printed nanoscale photoactive electronic layers are presented, which are further processed to develop ≈315 µm2 sized miniaturized photovoltaic microcells. Using a set of 32 microcells, connected in parallel configuration, indoor light harvesting is shown at a maximum power density of ≈10 µW cm−2 under white LED illumination. Further, the dual functionality of developed microcells i.e., energy harvesting as well as wideband photodetection is demonstrated. As self‐powered photo sensors the developed photovoltaic microcells exhibit distinctive photo responses under white LED‐UV (365 nm)‐ NIR (850 nm) light illumination, with exceptionally high‐speed response (rise time τRise = 205 µs and fall time τFall = 2000 µs), and a peak responsivity of 2.48 A W−1 to UV light at zero bias voltage. The presented results show the potential usage of printed multifunctional photovoltaic microcells in a wide variety of applications such as self‐powered wearable and flexible electronic systems for health monitoring and indoor robotics. [ABSTRACT FROM AUTHOR]

Heterogeneous integration on flexible substrates has been explored in recent years to meet the high‐performance and flexible form factors requirements of several emerging applications. Reliable interconnects, in both 2D and 3D layouts, are critical for the effective use of these systems. But it is challenging to employ conventional bonding and interconnect technology due to considerable mismatches in mechanical and thermal properties. For example, the ultra‐thin chips (UTCs) are too fragile to withstand the static or oscillating forces applied during conventional bonding. In this regard, printing of metal tracks on flexible substrates is a promising alternative to access the contact pads on integrated circuits (ICs), as the interconnects on diverse substrates can be realized at room temperature processing using as much materials as needed. Further, it is easier to attain step coverage. Considering the above attractive features, and the challenges associated with conventional bonding techniques, this article focuses on the development of high‐resolution printing interconnects in both 2D and 3D layouts and explores various printing routes available for high density integration. The article also discusses major conventional interposers/interconnects forming methods and their limitations for flexible hybrid electronics along with few examples of printed interconnects to highlight the opportunities for future. [ABSTRACT FROM AUTHOR]

Flexible hybrid electronics (FHE) is an emerging area that combines printed electronics and ultra‐thin chip (UTC) technology to deliver high performance needed in applications such as wearables, robotics, and internet‐of‐things etc. The integration of UTCs on flexible substrates and the access to devices on them requires high resolution interconnects, which is a challenging task as thermal and mechanical mismatches do not allow conventional bonding methods to work. To address this challenge, the resource‐efficient, area‐efficient, and low‐cost printing routes for obtaining vertical interconnection accesses (VIAs) are demonstrated here. It is demonstrated how high‐resolution printers (electrohydrodynamic and extrusion‐based direct‐ink writing printers) can be used for patterning of high‐resolution, freeform, vertical conductive structures. To access the transistors on UTCs, the VIAs, obtained using conventional photolithography and plasma etching steps, are filled with conductive silver nanoparticle‐based ink/paste using high‐resolution printers. Comprehensive studies are performed to compare and benchmark in terms of: i) the printing speed and throughput of the printers, ii) the electrical performance of vertically connected transistors in UTCs, and iii) the electrical performance stability of FHE system (interconnects and UTCs) under mechanical bending conditions. This in‐depth study shows the potential use of printing technologies for development of high‐density 3D integrated FHE systems. [ABSTRACT FROM AUTHOR]

Wearable electronic devices‐laden systems such as electronic‐skin (e‐Skin) have been explored in recent years to enable advances in applications such as Internet of Things, healthcare, and robotics. The power requirement of multitudes of devices in the e‐Skin is a major hurdle for its wider uptake. Herein, a solar cells‐based energy generating e‐Skin is presented and how the energy outputs of solar cells can be innovatively processed for multimodal sensing is demonstrated. By reading the variations and energy output patterns of the e‐Skin, present on a robotic arm, multiple parameters can be sensed including object motion, color detection, and ambient temperature. With the accurate tracking of shadow sensing, for an object moving in horizontal and vertical directions with respect to the solar skin, information can be obtained such as the velocity and acceleration of moving object. In this regard, the presented e‐Skin can also be seen to have vision capability. The presented multifunctional energy‐generating e‐Skin shows an energy surplus of >1 mW (effective module area of 20 cm2) under white light illumination of 4,450 lux, which is sufficient for continuous powering of portable low‐powered devices. Finally, we demonstrate the e‐Skin application for energy‐autonomous hand gestures recognition in robotics. [ABSTRACT FROM AUTHOR]

Ferroelectric nanomaterials often suffer from severe polarization loss compared to their bulk due to a size-induced alteration in their crystalline structure, making them inefficient for piezoelectric applications. Discovering nanomaterials with efficient piezoelectric properties is therefore a challenging task. We report here a direct observation of a single-phase ferroelectric structure with stripe domains stabilized by size-induced thermal residual stress in NaNbO3 nanowires (NWs) and demonstrate their excellent efficiency for lead-free piezoelectric nanocomposites. Polymer composites containing NaNbO3 NWs exhibit piezoelectric coefficients and figure-of-merit values comparable to those of KNbO3 NWs and approximately 9 and 100 times higher, respectively, than those of the reference devices using competing BaTiO3 NWs. The remarkable performance of NaNbO3 NWs compared to BaTiO3 NWs contradicts the ranking of bulk properties, claiming that NaNbO3 ceramics are significantly less active than BaTiO3. However, this counterintuitive behavior can be well understood if we consider structure modifications of these materials at the nanoscale, with a size-induced antiferroelectric-to-ferroelectric transition in NaNbO3 NWs and ferroelectric-to-paraelectric transition in BaTiO3 NWs. These findings are further supported by second harmonic generation characterizations, revealing substantially stronger second harmonic intensities for NaNbO3 and KNbO3 NWs compared to BaTiO3 NWs. Our work confirms the critical role of structural properties in the macroscopic piezoelectric performance of nanomaterials beyond the ranking of the bulk properties. With their scalable synthesis and high aspect ratio, ferroelectric NaNbO3 NWs hold great promise for the large-scale production of efficient, lead-free piezoelectric nanocomposites. [ABSTRACT FROM AUTHOR]

Multimaterial 3D printing is an attractive route for low‐cost fabrication of electronic systems having different types of embedded devices. Herein, tunable thermoplastic polyurethane (TPU)‐based conductive composite filaments are presented for development of either strain sensors or different circuit elements. The filaments are developed with two filler materials, namely, silver and multiwalled carbon nanotubes (MWCNT). The influences of filler aspect ratio (AR), concentration, functionalization, and morphology on the composites' mechanical, thermal, and electrical properties are studied. Printed tracks of the 10 wt% high‐AR MWCNT/TPU filament exhibit a maximum electrical conductivity of 0.92 S cm−1 and withstand powers >1 W and currents >100 mA. The filament shows negligible change in impedance over the frequency range 1 kHz–1 MHz and a change in the resistance of <5% with 90° bending. Conversely, printed tracks using filaments with 3 wt% low‐AR MWCNT exhibit a change in resistance of ≈30% with 90° bending, allowing a clear distinction between various bending angles, and thus could be used for embedded strain/bend sensors. These results suggest that, with the correct optimization, multimaterial additive manufacturing can be utilized with tunable conductive filaments to fabricate complex 3D electronic systems by constructing reliable circuit tracks, bendable interconnects, and sensors. [ABSTRACT FROM AUTHOR]

Flexible hybrid electronics (FHE) offers potential for fast computation and communication needed in applications such as human–machine interfaces, electronic skin, etc. FHE typically comprises devices that can vary from nano‐ to chip scale, and their integration using a common process is often challenging. Herein, a printed electronics route is presented to integrate the ultrathin chips (chip‐scale) and nanowires (NWs)‐based electronic layers (nanoscale) on the same substrate. The fabrication process is categorized into three stages: i) direct transfer printing of ultrathin chips (UTCs), ii) contact printing of nanoscale structures, and iii) metal printing using the direct ink write (DIW) method to define electrodes/interconnects. The UTC printing process is carefully optimized by studying the performance of transistors present on them. Electrical data collected from 14 transistors located on 3 different chips show negligible variation in performance after they are transfer printed—thus confirming the efficacy of the printing technique. The superior grade quality of ZnO‐NWs‐based electronic layers printed on the same substrate is also demonstrated by constructing UV photodetectors using DIW printing. The photodetectors show high responsivity (≈2 × 107 A W−1) and specific detectivity (≈5 × 1015 Jones) at a low UV intensity of 0.5 µW cm−2. [ABSTRACT FROM AUTHOR]

Electronic systems possessing skin‐like morphology and functionalities (electronic skins [e‐skins]) have attracted considerable attention in recent years to provide sensory or haptic feedback in growing areas such as robotics, prosthetics, and interactive systems. However, the main focus thus far has been on the distributed pressure or force sensors. Herein a thermoreceptive e‐skin with biological systems like functionality is presented. The soft, distributed, and highly sensitive miniaturized (≈700 µm2) artificial thermoreceptors (ATRs) in the e‐skin are developed using an innovative fabrication route that involves dielectrophoretic assembly of oriented vanadium pentoxide nanowires at defined locations and high‐resolution electrohydrodynamic printing. Inspired from the skin morphology, the ATRs are embedded in a thermally insulating soft nanosilica/epoxy polymeric layer and yet they exhibit excellent thermal sensitivity (−1.1 ± 0.3% °C−1), fast response (≈1s), exceptional stability (negligible hysteresis for >5 h operation), and mechanical durability (up to 10 000 bending and twisting loading cycles). Finally, the developed e‐skin is integrated on the fingertip of a robotic hand and a biological system like reflex is demonstrated in response to temperature stimuli via localized learning at the hardware level. [ABSTRACT FROM AUTHOR]

Flexible electronics with high-performance devices are crucial for transformative advances in several emerging and traditional applications. To address this need, herein we present p -type silicon (Si) nanoribbons (NR)-based high-performance field-effect transistors (FETs) developed using an innovative direct roll transfer stamping (DRTS) process. First, ultrathin Si NRs (∼70 nm) are obtained from silicon on insulator wafers using the conventional top-down method, and then the DRTS method is employed to directly place the NRs onto flexible substrates at RT. The NRFETs are then developed following the RT fabrication process which includes deposition of high-quality SiN x dielectric. The fabricated p -channel transistors demonstrate high linear mobility ∼100 ± 10 cm2 V−1 s−1, current on/off ratio >104, and low gate leakage (<1 nA). Further, the transistors showed robust device performance under mechanical bending and at a wide temperature range (15 °C–90 °C), showing excellent potential for futuristic high-performance flexible electronic devices/circuits. [ABSTRACT FROM AUTHOR]

This paper describes a facile strategy of micromolding‐in‐capillary process to fabricate stretchable strain sensors wherein, the sensing material is wrapped within silicone rubber (Dragon Skin [DS]) to form a sandwich‐like structure. Two different 1D sensing materials are exploited to fabricate and study strain sensing performance of such device structure, namely multi‐walled carbon nanotubes (MWCNTs) and silver nanowires (AgNWs). The fabricated strain sensors using MWCNT exhibits wide sensing range (2–180%), and moderately high sensing performance with outstanding durability (over 6000 cycles). It is found that MWCNT presents a strong strain‐dependent character in the 45–120% elongation regime. On the other hand, very high gauge factor of >106 is achieved using AgNWs at 30% strain with good stability (over 100 cycles). Sensing mechanisms for both 1D conductive sensing materials are discussed. They can be applied for human motion monitoring such as finger, knee, and wrist bending movements to enable human physiological parameters to be registered and analyzed continuously. They are also employed in multichannel and interactive electronic system to be used as a control mechanism for teleoperation for robotic end‐effectors. The developed sensors have potential applications in health diagnosis and human–machine interaction. [ABSTRACT FROM AUTHOR]

Energy Autonomous Wearable Sensors (EAWS) have attracted a large interest due to their potential to provide reliable measurements and continuous bioelectric signals, which help to reduce health risk factors early on, ongoing assessment for disease prevention, and maintaining optimum, lifelong health quality. This review paper presents recent developments and state-of-the-art research related to three critical elements that enable an EAWS. The first element is wearable sensors, which monitor human body physiological signals and activities. Emphasis is given on explaining different types of transduction mechanisms presented, and emerging materials and fabrication techniques. The second element is the flexible and wearable energy storage device to drive low-power electronics and the software needed for automatic detection of unstable physiological parameters. The third is the flexible and stretchable energy harvesting module to recharge batteries for continuous operation of wearable sensors. We conclude by discussing some of the technical challenges in realizing energy-autonomous wearable sensing technologies and possible solutions for overcoming them. [ABSTRACT FROM AUTHOR]

In the present work, we report the high performance of zinc oxide (ZnO) nanosheet (NS) based source-gated transistors (SGTs) with asymmetric Schottky source and ohmic drain contacts: low saturation drain-source voltages (~ 2 V) in the output scans (even at high gate voltages), high current on/off ratio (> 10 7 ) and low off-currents (0.1 pA). For a deeper understanding of the device mechanism and charge transport at metal–semiconductor contact interface, temperature dependent current–voltage studies have been performed. They revealed that the device operation can be ascribed to 3 main processes: i) the reverse biased Schottky source contact, which essentially controls charge carrier injection in to the NS channel, ii) effective manipulation of the source barrier by the gate field and iii) modulation of the depletion region beneath the source contact. These results are likely to improve the future generations of the ZnO based SGTs which offer several advantages for thin-film transistor design, including low power dissipation, small signal amplification, and as active load for the electronic circuits. [ABSTRACT FROM AUTHOR]

A vertically integrated nanogenerator (VING) refers to the new generation of mechanical energy harvesting devices: arrays of piezoelectric and semiconducting nanowires are grown perpendicularly to a substrate and appropriate electrodes enable charge circulation through an external electrical load. In this Letter, an equivalent circuit of VING is proposed. This model takes into account the effect of the polymer matrix that surrounds the nanowire lattice on the electromechanical behaviour of the generator. The active part of the material is then seen as a 1–3 piezocomposite. Simulations were made to study the VING performance in a context of body movement energy harvesting. In particular, it is shown that the strain amplitude and the strain rate of the piezocomposite have an impact on nanogenerator output voltage and output current, respectively. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) Part of Focus Issue on “Semiconductor Nanowires” (Eds.: Chennupati Jagadish, Lutz Geelhaar, Silvija Gradecak) A vertically integrated nanogenerator (VING) is an energy harvesting device made up of arrays of piezoelectric and semiconducting nanowires grown orthogonally to a substrate and entangled in a polymer matrix that convert mechanical energy into electricity. An equivalent circuit model of VING is established. It can be a valuable tool for the identification of the parameters that most influence its performance and for the optimisation of its design. [ABSTRACT FROM AUTHOR]

Printing technologies are changing the face of electronics with features such as resource-efficiency, low-cost, and novel form factors. While significant advances have been made in terms of organic electronics, the high-performance and stable transistors by printing, and their large-scale integration leading to fast integrated circuits remains a major challenge. This is because of the difficulties to print high-mobility semiconducting materials and the lack of high-resolution printing techniques. Herein, we present silicon based printed n- and p-channel transistors to demonstrate the possibility of developing high-performance complementary metal-oxide-semiconductor (CMOS) computing architecture. The direct roll transfer printing is used here for deterministic assembly of high-mobility single crystal silicon nanoribbons arrays on a flexible polyimide substrate. This is followed by high-resolution electrohydrodynamic printing to define source/drain/gate electrodes and to encapsulate, thus leading to printed devices. The printed transistors show effective peak mobilities of 15 cm 2 /(V s) (n-channel) and 5 cm 2 /(V s) (p-channel) at low 1 V drain bias. Furthermore, the effect of electrical, mechanical, and thermal stress on the performance and stability of the encapsulated transistors is investigated. The transistors showed stable transfer characteristics even after: (i) continuous 4000 transfer cycles, (ii) excruciating 10000 bending cycles at different bending radii (40, 25, and 15 mm), and (iii) between 15 and 60 °C temperatures.

The production of large quantities of single crystalline semiconducting ZnO nanowires (NWs) at low cost can offer practical solutions to realizing several novel electronic/optoelectronic and sensor applications on an industrial scale. The present work demonstrates high-density single crystalline NWs synthesized by a multiple cycle hydrothermal process at ∼100 °C. The high carrier concentration in such ZnO NWs is greatly suppressed by a simple low cost thermal annealing step in ambient air at ∼450 °C. Single ZnO NW FETs incorporating these modified NWs are characterized, revealing strong metal work function-dependent charge transport, unobtainable with as-grown hydrothermal ZnO NWs. Single ZnO NW FETs with Al as source and drain (s/d) contacts show excellent performance metrics, including low off-state currents (fA range), high on/off ratio (10(5)-10(7)), steep subthreshold slope (<600 mV/dec) and excellent field-effect carrier mobility (5-11 cm(2)/V-s). Modified ZnO NWs with platinum s/d contacts demonstrate excellent Schottky transport characteristics, markedly different from a reference ZnO NW device with Al contacts. This included abrupt reverse bias current-voltage saturation characteristics and positive temperature coefficient (∼0.18 eV to 0.13 eV). This work is envisaged to benefit many areas of hydrothermal ZnO NW research, such as NW FETs, piezoelectric energy recovery, piezotronics and Schottky diodes.