Piezophototronic Effect in Nanosensors

Advancements in nanomaterials for nanosensors: a comprehensive review

Nanomaterials (NMs) exhibit unique properties that render them highly suitable for developing sensitive and selective nanosensors across various domains. This review aims to provide a comprehensive overview of nanomaterial-based nanosensors, highlighting their applications and the classification of frequently employed NMs to enhance sensitivity and selectivity. The review introduces various classifications of NMs commonly used in nanosensors, such as carbon-based NMs, metal-based NMs, and others, elucidating their exceptional properties, including high thermal and electrical conductivity, large surface area-to-volume ratio and good biocompatibility. A thorough examination of literature sources was conducted to gather information on NMs-based nanosensors' characteristics, properties, and fabrication methods and their application in diverse sectors such as healthcare, environmental monitoring, industrial processes, and security. Additionally, advanced applications incorporating machine learning techniques were analyzed to enhance the sensor's performance. This review advances the understanding and development of nanosensor technologies by providing insights into fabrication techniques, characterization methods, applications, and future outlook. Key challenges such as robustness, biocompatibility, and scalable manufacturing are also discussed, offering avenues for future research and development in this field.

Frictional behavior of one-dimensional materials: an experimental perspective

The frictional behavior of one-dimensional (1D) materials, including nanotubes, nanowires, and nanofibers, significantly influences the efficient fabrication, functionality, and reliability of innovative devices integrating 1D components. Such devices comprise piezoelectric and triboelectric nanogenerators, biosensing and implantable devices, along with biomimetic adhesives based on 1D arrays. This review compiles and critically assesses recent experimental techniques for exploring the frictional behavior of 1D materials. Specifically, it underscores various measurement methods and technologies employing atomic force microscopy, electron microscopy, and optical microscopy nanomanipulation. The emphasis is on their primary applications and challenges in measuring and characterizing the frictional behavior of 1D materials. Additionally, we discuss key accomplishments over the past two decades in comprehending the frictional behaviors of 1D materials, with a focus on factors such as materials combination, interface roughness, environmental humidity, and non-uniformity. Finally, we offer a brief perspective on ongoing challenges and future directions, encompassing the systematic investigation of the testing environment and conditions, as well as the modification of surface friction through surface alterations.

Color-Shifting Iontronic Skin for On-Site, Nonpixelated Pressure Mapping Visualization

Mimicking the function of human skin is highly desired for electronic skins (e-skins) to perceive the tactile stimuli by both their intensity and spatial location. The common strategy using pixelated pressure sensor arrays and display panels greatly increases the device complexity and compromises the portability of e-skins. Herein, we tackled this challenge by developing a user-interactive iontronic skin that simultaneously achieves electrical pressure sensing and on-site, nonpixelated pressure mapping visualization. By merging the electrochromic and iontronic pressure sensing units into an integrated multilayer device, the interlayer charge transfer is regulated by applied pressure, which induces both color shifting and a capacitance change. The iontronic skin could visualize the trajectory of dynamic forces and reveal both the intensity and spatial information on various human activities. The integration of dual-mode pressure responsivity, together with the scalable fabrication and explicit signal output, makes the iontronic skin highly promising in biosignal monitoring and human-machine interaction.

Lead-Free Perovskite Cs3Bi2I9-Derived Electroactive PVDF Composite-Based Piezoelectric Nanogenerators for Physiological Signal Monitoring and Piezo-Phototronic-Aided Strain Modulated Photodetectors

In recent years, lead-free perovskite materials are exponentially emerging in photovoltaic and optoelectronic applications due to their low toxicity and superior optical properties. On the other hand, the demand for flexible, wearable, and lightweight optoelectronic devices is significantly growing in sensor and actuator technologies. In this scenario, lead-free perovskite-based flexible piezoelectric polymer composites have sparked considerable attention in this field due to their excellent piezo-, pyro-, ferroelectric, and photovoltaic properties. Thus, in this work, a long-term stable lead-free Cs₃Bi₂I₉-PVDF composite is introduced. The in situ growth of the Cs₃Bi₂I₉ perovskite induces 92% yield of the electroactive phase in the PVDF matrix. The possible mechanism behind the electroactive β-phase transformation is presented via interfacial interactions of PVDF moieties with the Cs₃Bi₂I₉ (CBI) perovskite, which also give rise to long-term environmental stability. Next, a piezoelectric nanogenerator (PNG) has been fabricated with the Cs₃Bi₂I₉-PVDF composite for mechanical energy harvesting, biophysiological motion monitoring, and voice recognitions that have potential utility in the health-care sector. Furthermore, a photodetector is developed to realize the piezo-phototronic effect. It exhibits a fast photoswitching behavior with rise and decay times of 141 and 278 ms, respectively. Thus, it is confirmed that the flexible Cs₃Bi₂I₉-PVDF composite has shown tremendous potential to be used as an optical signal-modulated piezo-responsive wearable sensor.

Bimodal Tactile Sensor without Signal Fusion for User-Interactive Applications

Tactile sensors with multimode sensing ability are cornerstones of artificial skin for applications in humanoid robotics and smart prosthetics. However, the intuitive and interference-free reading of multiple tactile signals without involving complex algorithms and calculations remains a challenge. Herein a pressure-temperature bimodal tactile sensor without any interference is demonstrated by combining the fundamentally different sensing mechanisms of optics and electronics, enabling the simultaneous and independent sensing of pressure and temperature with the elimination of signal separation algorithms and calculations. The bimodal sensor comprises a mechanoluminescent hybrid of ZnS-CaZnOS and a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) thermoresistant material, endowing the unambiguous transduction of pressure and temperature into optical and electrical signals, respectively. This device exhibits the highest temperature sensitivity of -0.6% °C^-1 in the range of 21-60 °C and visual sensing of the applied forces at a low limitation of 2 N. The interference-free and light-emitting characteristics of this device permit user-interactive applications in robotics for encrypted communication as well as temperature and pressure monitoring, along with wireless signal transmission. This work provides an unexplored solution to signal interference of multimodal tactile sensors, which can be extended to other multifunctional sensing devices.

Scalable Van der Waals Encapsulation by Inorganic Molecular Crystals

Encapsulation is critical for devices to guarantee their stability and reliability. It becomes an even more essential requirement for devices based on 2D materials with atomic thinness and far inferior stability compared to their bulk counterparts. Here a general van der Waals (vdW) encapsulation method for 2D materials using Sb₂ O₃ layer of inorganic molecular crystal fabricated via thermal evaporation deposition is reported. It is demonstrated that such a scalable encapsulation method not only maintains the intrinsic properties of typical air-susceptible 2D materials due to their vdW interactions but also remarkably improves their environmental stability. Specifically, the encapsulated black phosphorus (BP) exhibits greatly enhanced structural stability of over 80 days and more sustaining-electrical properties of 19 days, while the bare BP undergoes degradation within hours. Moreover, the encapsulation layer can be facilely removed by sublimation in vacuum without damaging the underlying materials. This scalable encapsulation method shows a promising pathway to effectively enhance the environmental stability of 2D materials, which may further boost their practical application in novel (opto)electronic devices.

Electricity Generation and Self-Powered Sensing Enabled by Dynamic Electric Double Layer at Hydrogel-Dielectric Elastomer Interfaces

The electric double layer (EDL) at liquid-solid interfaces is fundamental to many research areas ranging from electrochemistry and microfluidics to colloidal chemistry. Here, we demonstrate the electricity generation by mechanically modulating the EDL at the hydrogel-dielectric polymer interfaces. It is found that contact electrification between the hydrogel and the dielectric polymer could charge the dielectric polymer surface at first; the mechanical deformation of the pyramid-shaped hydrogel results in the periodic variation of the EDL area and capacitance, which then induces an alternative current in the external circuits. This mechano-to-electrical energy conversion mechanism is then utilized to construct soft stretchable self-powered pressure sensors by designing dynamic EDL at hydrogel-dielectric elastomer interfaces. The sensitivity is optimized to 1.40 kPa^-1 in the low-pressure range of 31-300 Pa by increasing the elastomer roughness. Its antifreeze performance is also improved by adding ethylene glycol into the hydrogel. The capability in detecting subtle human activities is further demonstrated. This mechano-electrical energy conversion and the corresponding self-powered sensor can be widely applied in future soft electronics.

Piezo-phototronic effect on photocatalysis, solar cells, photodetectors and light-emitting diodes

The piezo-phototronic effect (a coupling effect of piezoelectric, photoexcitation and semiconducting properties, coined in 2010) has been demonstrated to be an ingenious and robust strategy to manipulate optoelectronic processes by tuning the energy band structure and photoinduced carrier behavior. The piezo-phototronic effect exhibits great potential in improving the quantum yield efficiencies of optoelectronic materials and devices and thus could help increase the energy conversion efficiency, thus alleviating the energy shortage crisis. In this review, the fundamental principles and challenges of representative optoelectronic materials and devices are presented, including photocatalysts (converting solar energy into chemical energy), solar cells (generating electricity directly under light illumination), photodetectors (converting light into electrical signals) and light-emitting diodes (LEDs, converting electric current into emitted light signals). Importantly, the mechanisms of how the piezo-phototronic effect controls the optoelectronic processes and the recent progress and applications in the above-mentioned materials and devices are highlighted and summarized. Only photocatalysts, solar cells, photodetectors, and LEDs that display piezo-phototronic behavior are reviewed. Material and structural design, property characterization, theoretical simulation calculations, and mechanism analysis are then examined as strategies to further enhance the quantum yield efficiency of optoelectronic devices via the piezo-phototronic effect. This comprehensive overview will guide future fundamental and applied studies that capitalize on the piezo-phototronic effect for energy conversion and storage.

Mixed Triboelectric and Flexoelectric Charge Transfer at the Nanoscale

The triboelectric effect is a ubiquitous phenomenon in which the surfaces of two materials are easily charged during the contact-separation process. Despite the widespread consequences and applications, the charging mechanisms are not sufficiently understood. Here, the authors report that, in the presence of a strain gradient, the charge transfer is a result of competition between flexoelectricity and triboelectricity, which could enhance charge transfer during triboelectric measurements when the charge transfers of both effects are in the same direction. When they are in the opposite directions, the direction and amount of charge transfer could be modulated by the competition between flexoelectric and triboelectric effects, which leads to a distinctive phenomenon, that is, the charge transfer is reversed with varying forces. The subsequent results on the electrical power output signals from the triboelectrification support the proposed mechanism. Therefore, the present study emphasizes the key role of the flexoelectric effect through experimental approaches, and suggests that both the amount and direction of charge transfer can be modulated by manipulating the mixed triboelectric and flexoelectric effects. This finding may provide important information on the triboelectric effect and can be further extended to serve as a guideline for material selection during a nanopatterned device design.

A Self-Powered Photodetector Based on MAPbI3 Single-Crystal Film/n-Si Heterojunction with Broadband Response Enhanced by Pyro-Phototronic and Piezo-Phototronic Effects

Pyro-phototronic and piezo-phototronic effect have shown their important roles for high performance heterojunction-based photodetectors (PDs). Here, a coupling effect of pyro-phototronic and piezo-phototronic effect is utilized to fabricate a self-powered and broadband PD based on the MAPbI₃ single-crystal film/n-Si heterojunction. First, by using the pyro-phototronic effect derived from MAPbI₃ , the maximum photoresponsivity of the self-powered PD is 1.5 mA W^-1 for 780 nm illumination, which is enhanced by more than 20 times in consideration of the relative peak-to-peak output current. Light-induced temperature change in MAPbI₃ film will create pyro-charges distributed at heterojunction interface, resulting in a downward bending of the energy band, facilitating the transport of photon-generated electrons and holes, and generating spike-like output currents. Second, piezo-phototronic effect is further introduced by applying vertical pressures onto the PD. With a vertical pressure of 155 kPa, the responsivity can be improved by more than 120% compared to the condition with no pressure. The overall enhancement is due to the piezo-phototronic and pyro-phototronic coupling effects which utilize the polarization charges to modulate the band structure of heterojunction. These results provide a promising approach to develop high-performance self-powered and broadband perovskite-based PDs by coupling pyro-phototronic and piezo-phototronic effect.