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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]

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]

Nano/microstructures of compound semiconductors such as gallium arsenide (GaAs) demonstrate enormous potential for advanced photonic technologies as they provide realistic means for miniaturization of optoelectronic devices that feature better performance and low power consumption. However, intimately integrating them onto flexible substrates is challenging and restricts their use in the next generation of applications such as wearables and soft robotics. Herein, printed arrays of well‐defined and laterally aligned semi‐insulating (undoped) and doped GaAs microstructures are presented to develop high‐performance flexible broadband photodetectors. The direct roll transfer printed GaAs microstructures‐based photodetectors exhibit excellent performance under ultraviolet and near‐infrared illumination, including ultrafast response (2.5 ms) and recovery (8 ms) times, high responsivity (>104 AW–1), detectivity (>1014 Jones), external quantum efficiency (>106), and photoconductive gain (>104) at low operating voltage of 1 V. The achieved performance is among the best reported for broadband photodetectors but with an added benefit of the developed devices having a flexible form factor. Further, the photodetectors show stable performance under mechanical bending (500 cycles) and twisting loading. The developed materials and manufacturing route can enable high‐speed communications and computation via high‐performance flexible electronics and optoelectronics and transform numerous emerging applications such as wearable systems and internet of things. [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]

High‐performance flexible electronics developed with resource efficient printing route will transform the way future electronics is manufactured and used to advance applications such as healthcare, Internet of Things, wearables, consumer electronics, etc. Herein, an innovative approach is presented that involves, for the first time, the in‐tandem use of contact and transfer printing methods to realize high‐quality electronic layers at selected locations on rigid (Si/SiO2), flexible (polyimide), and biodegradable (magnesium (Mg) foils). Superior grade quality of printed electronic layers is demonstrated by realizing transistors and printed UV photodetectors (PDs) employing high‐resolution electrohydrodynamic printing. The all‐printed PDs show extremely high performance for UV detection, with extraordinary high responsivity (>107 A W−1) and specific detectivity (≈1017 Jones) values at low UV intensity of 0.1 µW cm−2. Finally, the fabricated PDs on Mg foil are dissolved in deionized water at room temperature. Thus, in‐tandem contact and transfer printing has potential for ecofriendly development of transient electronics. Further, the approach allows printing of wide range of nanomaterials and heterostructures or complex superlattice structures, which can open exciting new possibilities for high‐performance electronics. [ABSTRACT FROM AUTHOR]

Innovative methods to fabricate and integrate biodegradable high‐grade electronics on green substrates are needed for the next generation of robust high‐performance transient electronics. This is also needed to alleviate the growing problem of electronic waste (e‐waste). Herein, the authors present the n‐channel silicon (Si) nanoribbons‐based high‐performance transistors developed on biodegradable metal (magnesium) foils using the direct transfer printing method. The developed transistors present high effective mobility of >600 cm2 V−1 s−1, high on/off current ratio (Ion/off) of >104, negligible hysteresis, transconductance of 0.19 mS, and an on‐current of 1.6 mA at a bias of 2 V. Further, the transistors show stable device performance under temperature stress (5–50 °C), gate‐bias stress, continuous long‐term transfer scans for 24 h (>3000 cycles), and aging test (up to 100 days) demonstrating the excellent potential for futuristic high‐performance robust transient devices and circuits. Finally, the effect of transience on the electrical functioning of devices on Mg foils (at pH 8) and degradation of Mg foils at different pH values is studied by hydrolysis. The outcome from these experiments demonstrates the potential of direct transfer printing for high‐performance transient electronics and also as the new avenue toward zero e‐waste. [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]

"Heterogeneous Integration" is a promising approach for high‐performance hybrid flexible electronics that combine printed electronics and silicon technology. Despite significant progresses made by integrating rigid silicon chips on flexible substrates, the integration of flexible ultra‐thin chips (UTCs) on flexible foils remains a challenge as they are too fragile for conventional bonding methods. Reliable interconnects (low‐resistivity and mechanical robustness) and bonding of UTCs are critical to the realization of hybrid flexible systems. Herein, using a non‐contact printing approach, an easy and cost‐effective method for accessing UTCs on flexible foils is demonstrated. The high‐viscosity conductive paste, extruded from a high‐resolution printer (1–10 µm line width), is used here to connect the metal oxide semiconductor field effect transistors (MOSFETs) on UTCs with the extended pads on flexible printed circuit boards (PCBs). The electrical characterization of MOSFETs, before and after printing the interconnects, reveals an acceptable level of variation in device mobility (change from 780 to 630 cm2 V−1s−1). This is due to the drop in effective drain bias voltage as a marginally small electrical resistance (≈30 Ω) is added by the printed interconnects. The bonded UTCs show robust device performance under bending conditions, indicating high reliability of both the chip thinning and bonding methods. [ABSTRACT FROM AUTHOR]

Wearable UV photodetectors (PDs) have attracted interest recently for detection of excess exposure of the skin to the UV radiation. Despite numerous advances made in this direction, many challenges remain, particularly in terms of device reliability under extreme mechanical deformations simultaneously and self‐powering, etc. Herein, a self‐powered stretchable PD developed with kirigami‐inspired honeycomb‐patterned zinc oxide (ZnO) nanowires (NWs) and coupled with a triboelectric nanogenerator (TENG) is presented. After studying in detail the influence of ZnO NWs dispersion medium and metal‐ZnO NWs contacts, a novel fabrication approach employing the structural engineering on NWs‐elastomer composite is used to achieve high stretchability. The fabricated ZnO NWs‐based UV PDs, embedded inside kirigami‐inspired honeycomb‐patterned elastomeric substrate, exhibit unprecedented stretchability (up to 125%) and high‐performance with photo/dark current ratio of ≈5 × 105, responsivity of ≈54 A W−1, and a fast recovery time of 100 ms. Further, the stretchable PD is coupled with flexible TENGs to demonstrate a self‐powered system for potential application in real‐time UV radiation monitoring using advanced wearable healthcare technology. [ABSTRACT FROM AUTHOR]

Transfer printing of high mobility inorganic nanostructures, using an elastomeric transfer stamp, is a potential route for high-performance printed electronics. Using this method to transfer nanostructures with high yield, uniformity and excellent registration over large area remain a challenge. Herein, we present the 'direct roll transfer' as a single-step process, i.e., without using any elastomeric stamp, to print nanoribbons (NRs) on different substrates with excellent registration (retaining spacing, orientation, etc.) and transfer yield (∼95%). The silicon NR based field-effect transistors printed using direct roll transfer consistently show high performance i.e., high on-state current (Ion) >1 mA, high mobility (μeff) >600 cm2/Vs, high on/off ratio (Ion/off) of around 106, and low hysteresis (<0.4 V). The developed versatile and transformative method can also print nanostructures based on other materials such as GaAs and thus could pave the way for direct printing of high-performance electronics on large-area flexible substrates. [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]

Nanowire source-gated field-effect transistors (NW SGT) are demonstrated using hydrothermally grown ZnO NWs. Device quality ZnO NWs with moderate n-type doping are achieved by thermal annealing in ambient air at ∼550 °C. A single ZnO NW device with Au source-drain contacts (s/d) is found to operate under source-gating mode, with characteristics markedly different from a reference device with ohmic contacts. The NW SGT shows exceptionally early drain current-voltage saturation ( IDSAT- VDSAT) below 1 V. The change in saturation with the gate voltage ( VG) is over 80 times lower than a reference device with ohmic contacts. This device behavior is attributed to the source-gate overlap, enabling gate field penetration inside the depleted source. Current modulation is obtained by a combination of gate-induced image force barrier lowering and the high internal electric fields at source pinch-off. Effective Schottky barrier heights are extracted from activation energy measurements, revealing systematic barrier lowering with increasing VG. These features of the device lead us to conclude that the single NW field-effect transistor (FET) with Schottky contacts operated under SGT mode. [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]

The Printed Electronics (PE) is expected to revolutionise the way electronics will be manufactured in the future. Building on the achievements of the traditional printing industry, and the recent advances in flexible electronics and digital technologies, PE may even substitute the conventional silicon-based electronics if the performance of printed devices and circuits can be at par with silicon-based devices. In this regard, the inorganic semiconducting materials-based approaches have opened new avenues as printed nano (e.g. nanowires (NWs), nanoribbons (NRs) etc.), micro (e.g. microwires (MWs)) and chip (e.g. ultra-thin chips (UTCs)) scale structures from these materials have been shown to have performances at par with silicon-based electronics. This paper reviews the developments related to inorganic semiconducting materials based high-performance large area PE, particularly using the two routes i.e. Contact Printing (CP) and Transfer Printing (TP). The detailed survey of these technologies for large area PE onto various unconventional substrates (e.g. plastic, paper etc.) is presented along with some examples of electronic devices and circuit developed with printed NWs, NRs and UTCs. Finally, we discuss the opportunities offered by PE, and the technical challenges and viable solutions for the integration of inorganic functional materials into large areas, 3D layouts for high throughput, and industrial-scale manufacturing using printing technologies. [ABSTRACT FROM AUTHOR]