Thermal Evaporation of Large-Area SnS2 Thin Films with a UV-to-NIR Photoelectric Response for Flexible Photodetector Applications

A Flexible and Wearable Photodetector Enabling Ultra-Broadband Imaging from Ultraviolet to Millimeter-Wave Regimes

Flexible and miniaturized photodetectors, offering a fast response across the ultraviolet (UV) to millimeter (MM) wave spectrum, are crucial for applications like healthcare monitoring and wearable optoelectronics. Despite their potential, developing such photodetectors faces challenges due to the lack of suitable materials and operational mechanisms. Here, the study proposes a flexible photodetector composed of a monolayer graphene connected by two distinct metal electrodes. Through the photothermoelectric effect, these asymmetric electrodes induce electron flow within the graphene channel upon electromagnetic wave illumination, resulting in a compact device with ultra-broadband and rapid photoresponse. The devices, with footprints ranging from 3 × 20 µm2 to 50 × 20 µm2, operate across a spectrum from 325 nm (UV) to 1.19 mm (MM) wave. They demonstrate a responsivity (RV) of up to 396.4 ± 5.1 mV W-1, a noise-equivalent power (NEP) of 8.6 ± 0.1 nW Hz- 0.5, and a response time as small as 0.8 ± 0.1 ms. This device facilitates direct imaging of shielded objects and material differentiation under simulated human body-wearing conditions. The straightforward device architecture, aligned with its ultra-broadband operational frequency range, is anticipated to hold significant implications for the development of miniaturized, wearable, and portable photodetectors.

Flexible and Stretchable Light-Emitting Diodes and Photodetectors for Human-Centric Optoelectronics

Optoelectronic devices with unconventional form factors, such as flexible and stretchable light-emitting or photoresponsive devices, are core elements for the next-generation human-centric optoelectronics. For instance, these deformable devices can be utilized as closely fitted wearable sensors to acquire precise biosignals that are subsequently uploaded to the cloud for immediate examination and diagnosis, and also can be used for vision systems for human-interactive robotics. Their inception was propelled by breakthroughs in novel optoelectronic material technologies and device blueprinting methodologies, endowing flexibility and mechanical resilience to conventional rigid optoelectronic devices. This paper reviews the advancements in such soft optoelectronic device technologies, honing in on various materials, manufacturing techniques, and device design strategies. We will first highlight the general approaches for flexible and stretchable device fabrication, including the appropriate material selection for the substrate, electrodes, and insulation layers. We will then focus on the materials for flexible and stretchable light-emitting diodes, their device integration strategies, and representative application examples. Next, we will move on to the materials for flexible and stretchable photodetectors, highlighting the state-of-the-art materials and device fabrication methods, followed by their representative application examples. At the end, a brief summary will be given, and the potential challenges for further development of functional devices will be discussed as a conclusion.

The Acoustophotoelectric Effect: Efficient Phonon-Photon-Electron Coupling in Zero-Voltage-Biased 2D SnS2 for Broad-Band Photodetection

Two-dimensional (2D) layered metal dichalcogenides constitute a promising class of materials for photodetector applications due to their excellent optoelectronic properties. The most common photodetectors, which work on the principle of photoconductive or photovoltaic effects, however, require either the application of external voltage biases or built-in electric fields, which makes it challenging to simultaneously achieve high responsivities across broad-band wavelength excitation─especially beyond the material's nominal band gap─while producing low dark currents. In this work, we report the discovery of an intricate phonon-photon-electron coupling─which we term the acoustophotoelectric effect─in SnS2 that facilitates efficient photodetection through the application of 100 MHz order propagating surface acoustic waves (SAWs). This effect not only reduces the band gap of SnS2 but also provides the requisite momentum for indirect band gap transition of the photoexcited charge carriers, to enable broad-band photodetection beyond the visible light range, while maintaining pA-order dark currents─ without the need for any external voltage bias. More specifically, we show in the infrared excitation range that it is possible to achieve up to 8 orders of magnitude improvement in the material's photoresponsivity compared to that previously reported for SnS2-based photodetectors, in addition to exhibiting superior performance compared to most other 2D materials reported to date for photodetection.

Narrowband mid-infrared thermal emitters based on the Fabry-Perot type of bound states in the continuum

The development of narrow-band thermal emitters operating at mid-infrared (MIR) wavelengths is vital in numerous research fields. However, the previously reported results obtained with metallic metamaterials were not successful in achieving narrow bandwidths in the MIR region, which suggests low temporal coherence of the obtained thermal emissions. In this work, we demonstrate a new design strategy to realize this target by employing the bound state in the continuum (BIC) modes of the Fabry-Perot (FP) type. When a disk array of high-index dielectric supporting Mie resonances is separated from a highly reflective substrate by a low refractive index spacer layer with appropriate thickness, the destructive interference between the disk array and its mirror with respect to the substrate leads to the formation of FP-type BIC. Quasi-BIC resonances with ultra-high Q-factor (>10³) are achievable by engineering the thickness of the buffer layer. This strategy is exemplified by an efficient thermal emitter operating at a wavelength of 4.587 µm with the on-resonance emissivity of near-unity and the full-width at half-maximum (FWHM) less than 5 nm even along with consideration of metal substrate dissipation. The new thermal radiation source proposed in this work offers ultra-narrow bandwidth and high temporal coherence along with the economic advantages required for practical applications, compared to those infrared sources made from III-V semiconductors.

Enhanced Optical Response of SnS/SnS2 Layered Heterostructure

The SnS/SnS₂ heterostructure was fabricated by the chemical vapor deposition method. The crystal structure properties of SnS₂ and SnS were characterized by X-ray diffraction (XRD) pattern, Raman spectroscopy, and field emission scanning electron microscopy (FESEM). The frequency dependence photoconductivity explores its carrier kinetic decay process. The SnS/SnS₂ heterostructure shows that the ratio of short time constant decay process reaches 0.729 with a time constant of 4.3 × 10^-4 s. The power-dependent photoresponsivity investigates the mechanism of electron-hole pair recombination. The results indicate that the photoresponsivity of the SnS/SnS₂ heterostructure has been increased to 7.31 × 10^-3 A/W, representing a significant enhancement of approximately 7 times that of the individual films. The results show the optical response speed has been improved by using the SnS/SnS₂ heterostructure. These results indicate an application potential of the layered SnS/SnS₂ heterostructure for photodetection. This research provides valuable insights into the preparation of the heterostructure composed of SnS and SnS₂, and presents an approach for designing high-performance photodetection devices.

Fast near-infrared photodetectors from p-type SnSe nanoribbons

Low-dimensional tin selenide nanoribbons (SnSe NRs) show a wide range of applications in optoelectronics fields such as optical switches, photodetectors, and photovoltaic devices due to the suitable band gap, strong light-matter interaction, and high carrier mobility. However, it is still challenging to grow high-quality SnSe NRs for high-performance photodetectors so far. In this work, we successfully synthesized high-quality p-type SnSe NRs by chemical vapor deposition and then fabricated near-infrared photodetectors. The SnSe NR photodetectors show a high responsivity of 376.71 A W^-1, external quantum efficiency of 5.65 × 10⁴%, and detectivity of 8.66 × 10¹¹Jones. In addition, the devices show a fast response time with rise and fall time of up to 43μs and 57μs, respectively. Furthermore, the spatially resolved scanning photocurrent mapping shows very strong photocurrent at the metal-semiconductor contact regions, as well as fast generation-recombination photocurrent signals. This work demonstrated that p-type SnSe NRs are promising material candidates for broad-spectrum and fast-response optoelectronic devices.

Recent advances in self-powered and flexible UVC photodetectors

Ultraviolet-C (UVC) radiation is employed in various applications, including irreplaceable applications in military and civil fields, such as missile guidance, flame detection, partial discharge detection, disinfection, and wireless communication. Although most modern electronics are based on Si, UVC detection technology remains a unique exception because the short wavelength of UV radiation makes efficient detection with Si difficult. In this review, recent challenges in obtaining ideal UVC photodetectors with various materials and various forms are introduced. An ideal photodetector must satisfy the following requirements: high sensitivity, fast response speed, high on/off photocurrent ratio, good regional selectivity, outstanding reproducibility, and superior thermal and photo stabilities. UVC detection is still in its infancy compared to the detection of UVA as well as other photon spectra, and recent research has focused on different key components, including the configuration, material, and substrate, to acquire battery-free, super-sensitive, ultra-stable, ultra-small, and portable UVC photodetectors. We introduce and discuss the strategies for fabricating self-powered UVC photodetectors on flexible substrates in terms of the structure, material, and direction of incoming radiation. We also explain the physical mechanisms of self-powered devices with various architectures. Finally, we present a brief outlook that discusses the challenges and future strategies for deep-UVC photodetectors.

Highly-Responsive Broadband Photodetector Based on Graphene-PTAA-SnS2 Hybrid

The development of wearable systems stimulate the exploration of flexible broadband photodetectors with high responsivity and stability. In this paper, we propose a facile liquid-exfoliating method to prepare SnS₂ nanosheets with high-quality crystalline structure and optoelectronic properties. A flexible photodetector is fabricated using the SnS₂ nanosheets with graphene-poly[bis(4-phenyl) (2,4,6-trimethylphenyl) amine (PTAA) hybrid structure. The liquid-exfoliated SnS₂ nanosheets enable the photodetection from ultraviolet to near infrared with high responsivity and detectivity. The flexible broadband photodetector demonstrates a maximum responsivity of 1 × 10⁵ A/W, 3.9 × 10⁴ A/W, 8.6 × 10² A/W and 18.4 A/W under 360 nm, 405 nm, 532 nm, and 785 nm illuminations, with specific detectivity up to ~10¹² Jones, ~10¹¹ Jones, ~10⁹ Jones, and ~10⁸ Jones, respectively. Furthermore, the flexible photodetector exhibits nearly invariable performance over 3000 bending cycles, rendering great potentials for wearable applications.

Enhanced Photodetection Performance of Photodetectors Based on Indium-Doped Tin Disulfide Few Layers

Two dimensional (2D) tin disulfide (SnS₂) has attracted growing interest as a promising high performance photodetector with superior performance such as fast response time, high responsivity, and good stability. However, SnS₂-based photodetectors still face great challenges, and the photodetection performance needs to be improved for practical applications. Herein, indium-doped SnS₂ (In-SnS₂) few layers were exfoliated from CVT-grown single crystals, which were synthesized by chemical vapor transport. Photodetectors based on In-SnS₂ few layers were fabricated and detected. Compared with photodetectors based on pristine SnS₂, photodetectors based on In-SnS₂ few layers exhibited better photodetection performances, including higher responsivities, higher external quantum efficiencies, and greater normalized detectivities. The responsivity (R), external quantum efficiency (EQE), and normalized detectivity (D*) were increased by up to 2 orders of magnitude after In doping. Considering responsivity and response time, the photodetector based on 1.4 at. % In-SnS₂ few layers exhibited an optimal photodetection performance with a high R of 153.8 A/W, a high EQE of 4.72 × 10⁴ %, a great D* of 5.81 × 10¹² Jones, and a short response time of 13 ms. Our work provides an efficient path to enhance photodetection performances of photodetectors based on SnS₂ for future high-performance optoelectronic applications.

Room-temperature fabrication of SiC microwire photodetectors on rigid and flexible substrates via femtosecond laser direct writing

Flexible ultraviolet (UV) photodetectors (PDs) have gained increasing demand because of their widespread applications in wearable devices. However, difficulties associated with complicated fabrication technologies significantly limit their scope of application. Herein, via the development of a femtosecond laser direct writing (FsLDW) strategy, silicon carbide (SiC) nanoparticles are found to be assembled in a single microwire within 30 s. The surface of the deposited SiC microwire presents a three-dimensional porous structure, which is conducive to improving the responsivity of the device. The responsivity of a SiC-based microwire PD to UV light at 365 nm is found to be 55.89 A W-1 at a 1 V bias. The as-fabricated SiC microwire PDs on a glass substrate exhibit thermal stability at 350 °C, and the response speed of the PDs becomes notably faster at high temperatures, suggesting their promising applications in harsh conditions. Due to the low-temperature processing characteristics of this process, they can be prepared not only on glass substrates, but also on thermosensitive polymer substrates without an extra transfer process. Moreover, the SiC microwires prepared via FsLDW are directly deposited on the flexible substrate, and the prepared flexible SiC-based PDs can still work stably after being bent 2000 times. This research unveils a feasible way to fabricate a PD with excellent thermal stability and mechanical flexibility.