Tape-Based Photodetector: Transfer Process and Persistent Photoconductivity

Inorganic Semiconductor-Based Flexible UV Photodetector Arrays Achieved by Specific Flip-Chip Bonding

In recent years, flexible UV photodetectors (PDs) with complex environmental adaptability and great wearability have attracted the attention of researchers worldwide. Wide bandgap inorganic semiconductor materials with excellent optoelectronic properties and mechanical stability are key functional materials for UV PD devices. However, the high temperature processing and inherent brittleness limit the further application of high-quality inorganic semiconductors in the field of flexible optoelectronics. In this work, we develop a specific flip-chip bonding fabrication technique that utilizes high-temperature treated inorganic semiconductor materials for high-performance flexible UV detection devices. Leveraging this technique, a 7 × 7 pixel flexible UV photodetector array (UV-FPDA) device based on a vertical architecture Mg-doped ZnO/NiO (Mg:ZnO/NiO) heterojunction transistor is built. The UV-FPDAs exhibit a high responsivity of 75.8 A/W and an outstanding detectivity of 8.5 × 1012 Jones. Besides, the UV-FPDAs also demonstrate excellent bending stability. Furthermore, the photoresponse characteristics of each pixel are trained and learned by an artificial neural network to achieve clear imaging of UV light information. Our results provide a new pathway for the application of inorganic semiconductors in the field of high-performance flexible UV photodetection.

Origin of photoinduced DC current and two-level population dynamics in a single molecule

Studying the photoinduced changes of materials with atomic-scale spatial resolution can provide a fundamental understanding of light-matter interaction. A long-standing impediment has been the detrimental thermal effects on the stability of the tunneling gap from intensity-modulated laser irradiation of the scanning tunneling microscope junction. Photoinduced DC current transduces photons to an electric current and is widely applied in optoelectronics as switches and signal transmission. Our results revealed the origin of the light-induced DC current and related it to the two-level population dynamics and related nonlinearity in the conductance of a single molecule. Here, we compensated for the near-visible laser-induced thermal effects to demonstrate photoinduced DC current spectroscopy and microscopy and to observe the persistent photoconductivity of a two-level pyrrolidine molecule. The methodology can be generally applied to the coupling of light to scan probes to investigate light-matter interactions at the atomic scale.

Transfer Printing of Ordered Plasmonic Nanoparticles at Hard and Soft Interfaces with Increased Fidelity and Biocompatibility Supports a Surface Lattice Resonance

Transfer printing, the relocation of structures assembled on one surface to a different substrate by adjusting adhesive forces at the surface-substrate interface, is widely used to print electronic circuits on biological substrates like human skin and plant leaves. The fidelity of original structures must be preserved to maintain the functionality of transfer-printed circuits. This work developed new biocompatible methods to transfer a nanoscale square lattice of plasmon resonant nanoparticles from a lithographed surface onto leaf and glass substrates. The fidelity of the ordered nanoparticles was preserved across a large area in order to yield, for the first time, an optical surface lattice resonance on glass substrates. To effect the transfer, interfacial adhesion was adjusted by using laser induction of plasmons or unmounted adhesive. Optical and confocal laser scanning microscopy showed that submicron spacing of the square lattice was preserved in ≥90% of transfer-printed areas up to 4 mm2. Up to 90% of ordered nanoparticles were transferred, yielding a surface lattice resonance measured by transmission UV-vis spectroscopy.

Self-assembly, alignment, and patterning of metal nanowires

Armed with the merits of one-dimensional nanostructures (flexibility, high aspect ratio, and anisotropy) and metals (high conductivity, plasmonic properties, and catalytic activity), metal nanowires (MNWs) have stood out as a new class of nanomaterials in the last two decades. They are envisaged to expedite significantly and even revolutionize a broad spectrum of applications related to display, sensing, energy, plasmonics, photonics, and catalysis. Compared with disordered MNWs, well-organized MNWs would not only enhance the intrinsic physical and chemical properties, but also create new functions and sophisticated architectures of optoelectronic devices. This paper presents a comprehensive review of assembly strategies of MNWs, including self-assembly for specific structures, alignment for anisotropic constructions, and patterning for precise configurations. The technical processes, underlying mechanisms, performance indicators, and representative applications of these strategies are described and discussed to inspire further innovation in assembly techniques and guide the fabrication of optoelectrical devices. Finally, a perspective on the critical challenges and future opportunities of MNW assembly is provided.

Emerging Optoelectronic Devices Based on Microscale LEDs and Their Use as Implantable Biomedical Applications

Thin-film microscale light-emitting diodes (LEDs) are efficient light sources and their integrated applications offer robust capabilities and potential strategies in biomedical science. By leveraging innovations in the design of optoelectronic semiconductor structures, advanced fabrication techniques, biocompatible encapsulation, remote control circuits, wireless power supply strategies, etc., these emerging applications provide implantable probes that differ from conventional tethering techniques such as optical fibers. This review introduces the recent advancements of thin-film microscale LEDs for biomedical applications, covering the device lift-off and transfer printing fabrication processes and the representative biomedical applications for light stimulation, therapy, and photometric biosensing. Wireless power delivery systems have been outlined and discussed to facilitate the operation of implantable probes. With such wireless, battery-free, and minimally invasive implantable light-source probes, these biomedical applications offer excellent opportunities and instruments for both biomedical sciences research and clinical diagnosis and therapy.

A New Strategy to Fabricate Nanoporous Gold and Its Application in Photodetector

Nanoporous gold (NPG) plays an important role in high-performance electronic devices, including sensors, electrocatalysis, and energy storage systems. However, the traditional fabricating methods of NPG, dealloying technique or electrochemical reduction technique, usually require complex experimental procedures and sophisticated equipment. In this work, we reported a unique and simple method to prepare the NPG through a low-temperature solution process. More importantly, the structure of the NPG-based electrode can be further controlled by using the post-treatment process, such as thermal treatment and plasma treatment. Additionally, we also demonstrate the application of the resulting NPG electrodes in flexible photodetectors, which performs a higher sensitivity than common planar photodetectors. We believe that our work opens a possibility for the nanoporous metal in future electronics that is flexible, large scale, with facile fabrication, and low cost.

GaOx@GaN Nanowire Arrays on Flexible Graphite Paper with Tunable Persistent Photoconductivity

Flexible optoelectronic synaptic devices that functionally imitate the neural behavior with tunable optoelectronic characteristics are crucial to the development of advanced bioinspired neural networks. In this work, amorphous oxide-decorated GaN nanowire arrays (GaO_x@GaN NWAs) are prepared on flexible graphite paper. A GaO_x@GaN NWA-based flexible device has tunable persistent photoconductivity (PPC) and shows a conversible fast/slow decay process (SDP). Photoconductivity can be modulated by single or double light pulses with different illumination powers and biases. PPC gives rise to the high-performance SDP such as a long decay time of 2.3 × 10⁵ s. The modulation mechanism is proposed and discussed. Our results reveal an innovative and efficient strategy to produce decorated NWAs on a flexible substrate with tunable optoelectronic properties and exhibit potential for flexible neuromorphic system applications.

Optically Programmable Plateau-Rayleigh Instability for High-Resolution and Scalable Morphology Manipulation of Silver Nanowires for Flexible Optoelectronics

The ability to engineer microscale and nanoscale morphology upon metal nanowires (NWs) has been essential to achieve new electronic and photonic functions. Here, this study reports an optically programmable Plateau-Rayleigh instability (PRI) to demonstrate a facile, scalable, and high-resolution morphology engineering of silver NWs (AgNWs) at temperatures <150 °C within 10 min. This has been accomplished by conjugating a photosensitive diphenyliodonium nitrate with AgNWs to modulate surface-atom diffusion. The conjugation is UV-decomposable and able to form a cladding of molten salt-like compounds, so that the PRI of the AgNWs can be optically programmed and triggered at a much lower temperature than the melting point of AgNWs. This PRI self-assembly technique can yield both various novel nanostructures from single NW and large-area microelectrodes from the NW network on various substrates, such as a nanoscale dot-dash chain and the microelectrode down to 5 μm in line width that is the highest resolution ever fabricated for the AgNW-based electrode. Finally, the patterned AgNWs as flexible transparent electrodes were demonstrated for a wearable CdS NW photodetector. This study provides a new paradigm for engineering metal micro-/nanostructures, which holds great potential in fabrication of various sophisticated devices.

A Fully Integrated Wireless Flexible Ammonia Sensor Fabricated by Soft Nano-Lithography

Flexible ammonia (NH₃) sensors based on one-dimensional nanostructures have attracted great attention due to their high flexibility and low power consumption. However, it is still challenging to reliably and cost-effectively fabricate ordered nanostructure-based flexible sensors. Herein, a smartphone-enabled fully integrated system based on a flexible nanowire sensor was developed for real-time NH₃ monitoring. Highly aligned, sub-100 nm nanowires on a flexible substrate fabricated by facile and low-cost soft lithography were used as sensitive elements to produce impedance response. The detection signals were sent to a smartphone and displayed on the screen in real time. This nanowire-based sensor exhibited robust flexibility and mechanical durability. Moreover, the integrated NH₃ sensing system presented enhanced performance with a detection limit of 100 ppb, as well as high selectivity and reproducibility. The power consumption of the flexible nanowire sensor was as low as 3 μW. By using this system, measurements were carried out to obtain reliable information about the spoilage of foods. This smartphone-enabled integrated system based on a flexible nanowire sensor provided a portable and efficient way to detect NH₃ in daily life.