Ultrafast Response Time Research Articles

Out-of-plane polarization induces a picosecond photoresponse in rhombohedral stacked bilayer WSe2.

Constructing van der Waals materials with spontaneous out-of-plane polarization through interlayer engineering expands the family of two-dimensional ferroelectrics and provides an excellent platform for enhancing the photoelectric conversion efficiency. Here, we reveal the effect of spontaneous polarization on ultrafast carrier dynamics in rhombohedral stacked bilayer WSe2. Using precise stacking techniques, a 3R WSe2-based vertical heterojunction was successfully constructed and confirmed by polarization-resolved second harmonic generation measurements. Through output characteristics and the scanning photocurrent map under zero bias, we reveal a non-zero short-circuit current in the graphene/3R WSe2/graphene heterojunction region, demonstrating the bulk photovoltaic effect. Furthermore, the out-of-plane polarization enables the 3R WSe2 heterojunction region to achieve an ultrafast intrinsic photoresponse time of approximately 3 ps. The ultrafast response time remains consistent across varying detection powers, demonstrating environmental stability and highlighting the potential in optoelectronic applications. Our study presents an effective strategy for enhancing the response time of photodetectors.

High-Performance Gate-Voltage-Tunable Photodiodes Based on Nb2Pd3Se8/WSe2 Mixed-Dimensional Heterojunctions.

The mixed-dimensional (MD) van der Waals (vdWs) heterojunction for photodetectors has garnered significant attention owing to its exceptional compatibility and superior quality. Low-dimensional material heterojunctions exhibit unique photoelectric properties attributed to their nanoscale thickness and vdWs contact surfaces. In this work, a novel MD vdWs heterojunction composed of one-dimensional (1D) Nb2Pd3Se8 nanowires and two-dimensional (2D) WSe2 nanosheets is proposed. The heterojunction's energy band engineering is accomplished by manipulating the Fermi level of the bipolar 2D material via gate voltage, resulting in a rectification characteristic that can be adjusted with gate voltage. Under 685 nm laser irradiation, it demonstrates exceptional self-powered photodetection performance, attaining a photoresponsivity of 1.45 A W-1, an ultrahigh detectivity of 6.8 × 1012 Jones, and an ultrafast response time of 37/64 μs at zero bias. In addition, a broadband photodetector from 255 to 1064 nm is realized. These results demonstrate the great potential of Nb2Pd3Se8/WSe2 MD heterostructures for advanced electronic and optoelectronic devices.

A multifunctional and tough lotus root starch-based bio-photonic hydrogel for stretchable fabric pattern color change and water rewriting.

Photonic crystal hydrogels (PCHs) are innovative materials that translate imperceptible deformations and humidity changes into visible colors, broadening the applications of photonics in bioengineering and smart materials. To overcome poor mechanical properties of traditional PCHs limited by weak intermolecular forces, we designed a PCH with a dual-network framework comprising N-isopropylacrylamide-co-acrylamide (NIPAM-co-AM) and biomass lotus root starch (LR). Since LR is rich in hydroxyl groups, it can undergo molecular linkage entanglement with the NIPAM-co-AM hydrogel matrix, forming hydrogen bonds that significantly enhance the mechanical properties of the PCH. We have prepared PCH films capable of conveying multidimensional information about the extent and distribution of transient deformation by encapsulating magnetic photonic crystal microspheres (MPCMs) within a hydrogel matrix. The PCH films were found to have ultrafast response time (< 10 s), full-color tunable range, high spatial resolution, excellent mechanical toughness (tensile up to 0.17 MPa) and sensitivity (2.52 nm %-1), and were able to sense relative humidity (RH) from 11 % to 98 %. Equipped with a dynamically reconfigurable lattice, this dual-responsive (tensile/humidity) PCH not only facilitates the creation of water-rewritable photonic films but also holds promise for integration into structural color elastic fabrics, thereby presenting novel prospects for smart textile applications.

Self-powered visualized tactile-acoustic sensor for accurate artificial perception with high brightness and record-low detection limit.

The growth of the Internet of Things has focused attention on visualized sensors as a key technology. However, it remains challenging to achieve high sensing accuracy and self-power ability. Here, we propose a self-powered visualized tactile-acoustic sensor (SVTAS) based on an elaborated triboelectrification-induced electroluminescence (TIEL) unit. To date, it features a high brightness of 0.5 mW cm-2 (32 cd m-2) and a record-low detection limit of 0.5 kPa in horizontal-sliding mode. Meanwhile, the SVTAS is applicable to convert acoustic waves into TIEL signals in contact-separation mode, showing the highest response to the 44.07 Hz sound, a high signal-to-noise ratio of 8.7 dB-1, and an ultrafast response time of 0.8 ms. Furthermore, advanced artificial visualized perception systems are constructed with excellent performance in recognizing motion trajectories and human speech with different words/sentences. This work paves the way for the highly efficient and sustainable development of new-generation self-powered visualized perception systems, contributing a solution to wireless communication free from electromagnetic interference.

High-Energy Density Pure Polyvinylidene Difluoride with the Magnetic Field Modulation of Free-Volume Pore Size and Other Microstructures.

PVDF polymer dielectrics, renowned for their ultra-high-power density, ultra-fast response times, remarkable toughness, and lightweight properties, constitute the essential material foundation for the development of dielectric capacitors. Nevertheless, the low-energy density of these dielectrics presents a challenge to the advancement of dielectric capacitors. In this paper, in the process of preparing monolayer pure PVDF dielectric films by the solution casting method, a fixed-direction magnetic field and a rotating magnetic field were introduced in the horizontal direction, respectively, and this investigation explores the impact of magnetic field modulation on the polymer films' free-volume pore size, grain size, phase structure, dielectric properties, and energy storage capabilities by altering the duration and orientation of the magnetic field's influence. This study also discusses how microscopic alterations, particularly in the free-volume pore size, affect the macroscopic dielectric properties. Polymer films treated with a magnetic field of constant orientation for 3 min were obtained with the smallest free-volume hole size of 2.91 Å, the highest γ-phase contents of 54.8%, the smallest grain size of 68 Å, the largest electrical displacement of 10.64 and a very high discharge energy density of 12.68 J/cm3 (a 200% enhancement over pure PVDF).

Next-Generation Self-Powered Photodetectors using 2D Bismuth Oxide Selenide Crystals.

The concept of self-powered photodetectors has attracted significant attention due to their versatile applications in areas such as intelligent systems and hazardous substance detection. Among these, p-n junction and Schottky junction photodetectors are the most widely studied types; however, their fabrication processes are often complex and costly. To overcome these challenges, we focused on the emerging self-powered, ultrasensitive photodetector platform based on photoelectrochemical (PEC) principles. This platform leverages the unique properties of the emerging material bismuth oxide selenide (Bi2O2Se), which features a wide bandgap (∼2 eV) and a high absorption coefficient. We utilized chemical exfoliation to obtain thin layers of Bi2O2Se, enabling highly efficient photodetection. The device characterization demonstrated impressive performance metrics, including a responsivity of 97.1 μA W-1 and a specific detectivity of 2 × 108 cm Hz 1/2 W-1. The PEC photodetector also exhibits broad-spectrum sensitivity, from blue to infrared wavelengths, and features an ultrafast response time of ∼82 ms and a recovery time of ∼86 ms, highlighting its practical potential. Moreover, these self-powered photodetectors show excellent stability in electrochemical environments, positioning them promising candidates for integration into future high-efficiency devices.

A hot exciton organic glassy scintillator for high-resolution X-ray imaging.

Hot exciton organic scintillators offer promising prospects due to their efficient generation of bright triplet excitons and ultrafast response time, having potential applications in security detection and medical diagnostics. However, fabricating large-area, highly transparent scintillator screens still remains challenging, impeding the realization of high-resolution X-ray imaging. Herein, we firstly demonstrate a novel highly-transparent hot exciton organic glassy scintillator (>87% transmittance @ 450-800 nm), produced using a low-temperature melt-quenching method with 2',5'-difluoro-N4,N4,N4'',N4''-tetraphenyl-[1,1':4',1''-terphenyl]-4,4''-diamine (DTPA2F) powder. Remarkably, compared to crystalline DTPA2F, which has a photoluminescence quantum yield of 67.8% and a relative light yield of 46 400 ± 406 photons MeV-1, the DTPA2F glass retains 49.8% and 28 341 ± 246 photons MeV-1, respectively. This results in a low detection limit of about 53.7 nGy s-1 and an ultrafast decay time of 1.66 ns for DTPA2F glass. Besides, it exhibits excellent environmental stability with no recrystallization or degradation after over 100 days of exposure to ambient conditions. Furthermore, the scintillator screen demonstrates exceptional spatial resolution of 38.5 lp mm-1 for X-ray imaging. It provides a simple molecular design strategy and a screen fabrication method for developing large-area, highly-transparent, efficient and ultrafast organic glassy scintillators.

Observation of Body‐Centered‐Cubic Cu Nanocluster with Partial Cu(0) Character

The structure of nanoparticles is a key determinant of their material properties. The synthesis of non‐face‐centered‐cubic (FCC) Cu nanoclusters (NCs) with Cu(0) character is still challenging due to their instability. Herein, a Cu(0)‐containing Cu NC is first reported to contain a metallic framework of 23 Cu atoms with a body‐centered‐cubic Cu@Cu8 core, stabilized by 6 triphenylphosphine (TPP) and 18 4‐methylbenzenethiolate ligands, as determined by single‐crystal X‐ray crystallography. The observed structure stands in stark contrast to the FCC and other structures reported for Cu NCs. Moreover, this newly formed NC could react with Au+ ion with controlled concentration, rapidly yielding another identifiable and bright red‐emitting AuCu14 cluster upon irradiation. Based on this structural conversion, a new “turn‐on” type of photoluminescent probe can be developed for the detection of Au+ ion with an ultrafast response time (30 s) at a low detection limit (0.13 ppm). This work provides a new way of understanding the growth of metal clusters and exploring their applications in luminescent sensors.

Study of argon plasma modified Pd/AlGaN/GaN high electron mobility transistor for superior hydrogen gas detection

The surface and microstructure of the sensitive material play an important role in the gas adsorption and detection. However, few report of the surface modification affection on the work function-based gas sensor is carried out. Herein, facile Ar+ modification is proposed and utilized to modify the surface of the Pd sensitive layer, which acts as the gate of a AlGaN/GaN high electron mobility transistor (HEMT) based hydrogen (H2) gas sensor. Results show that the Pd surface is significantly changed to smoother microstructure by 5 s Ar+ modification. Compared with the device without Ar+ modification, the response values increase by ∼ twice at all operating temperatures. Interestingly, the response time is pronouncedly shortened from 10 to 2.3 s at 500 ppm, suggesting the significant effect of Ar+ modification in the enhancement of gas detection. At the optimized operating temperature of 120 °C, a high response of 739% and an ultrafast response time of 1.4 s are observed for the 500 ppm H2 gas. In addition, this sensor exhibits a low limit of detection ∼200 ppb, along with excellent repeatability, consistency, and selectivity. Our study indicates that the facile Ar+ modification of the gate surface is a highly effective technology in the enhancement of gas detection and may provide a novel approach for the design and development of all the work function type gas sensors.

Femtosecond Third-Order Nonlinear Electronic Responses of 2D Metallic NbSe2

This manuscript reports on the third-order nonlinear optical responses of two-dimensional metallic NbSe2 suspended in acetonitrile (ACN). The standard Z-scan technique was employed with 190 fs optical pulses at 790 nm, a repetition rate of 750 Hz, and an intensity ranging from 30 to 300 GW/cm2. A self-focusing nonlinear refractive index (NLR), n2=+(1.8±0.1)×10−15 cm2/W, and a nonlinear absorption (NLA) coefficient, α2=+(3.5±0.2)×10−2 cm/GW, were measured, with the NLA arising from a two-photon process. Aiming to further understand the material’s electronic nonlinearities, we also employed the Optical Kerr Gate (OKG) to evaluate the material’s time response and measure the NLR coefficient in an optical intensity range different from the one used in the Z-scan. For optical pulses of 170 fs at 800 nm and a repetition rate of 76 MHz, the modulus of the NLR coefficient was measured to be n2=4.2±0.5×10−14 cm2/W for intensities up to 650 MW/cm2, with the material’s time response limited by the pulse duration. The ultrafast time response and electronic optical nonlinearities are explained based on the material’s 2D structure.

Patterned 3D-graphene for self-powered broadband photodetector

The remarkable ultra-wideband energy capabilities of zero-bandgap two-dimensional (2D) graphene have made it an outstanding material for photon absorption and charge carrier generation across a broad spectrum. However, atomically thick 2D-graphene only exhibits a light absorption efficiency of 2.3%, posing a challenge for graphene-based photodetectors to harness photon energy effectively. This study utilized plasma-enhanced chemical vapor deposition technology and a mask to create in situ patterned 3D-graphene/Si-based Schottky heterojunctions. The porous structure and natural nano-resonant cavities of the 3D-graphene enhanced the probability of light absorption, while the curved and sharp edges and corners of the patterned 3D-graphene concentrated the local electromagnetic field and induced localized surface plasmon resonance. Both theoretical and experimental analyses confirmed the improved light absorption mechanisms and charge transfer of photogenerated carriers under the built-in electric field. The photodetector based on the patterned 3D-graphene/Si Schottky structure exhibits excellent broadband response characteristics spanning from 380 to 1550 nm, with responsivity and specific detectivity of 68.47 A/W and 1.43 × 1012 Jones at a wavelength of 1550 nm. Furthermore, the photodetector demonstrated ultrafast microsecond-level response times (116/120 μs rise/fall times) along with exceptional stability and reliability. The potential applications of as-fabricated photodetector include logic devices such as “AND” and “OR” gates and information encryption. This research holds valuable scientific significance for the design of future optoelectronic devices and provides crucial insights and technical support for developing high-performance Si-based graphene detectors.

Active friction-regulated inertia impact piezoelectric actuator

In inertia impact piezoelectric actuators, the phenomenon of high-frequency drive-induced back-stepping poses a significant limitation to their overall performance. The ultra-fast response time of the piezoelectric stack enables the resolution of this issue. This paper introduces an inertia impact piezoelectric actuator operating under a novel Dual-Stack Motion Mode (DCMM), diverging from the traditional operation in the Single-Stack Motion Mode (STMM) that involves a solitary piezoelectric stack (PES) for active friction control. A comprehensive description of the actuator's structure and its operational principles under DCMM is provided. By constructing and experimentally evaluating the actuator using a controlled variable approach, a comparative analysis of performance between DCMM and STMM across various scenarios including different inertial mass blocks, driving voltages, frequencies, and load conditions was conducted. The experimental results indicate that DCMM significantly enhances the actuator's output performance, achieving a maximum speed of 1142.79 μm/s and a stable single-step displacement of 0.5 μm. The actuator features a simple yet effective structure and driving mechanism, allowing for multiple driving modes through the assembly of different inertial masses, thereby providing a substantial competitive advantage in output performance. The feasibility of using DCMM to improve actuator performance is corroborated by both theoretical and experimental studies. The ultra-fast response of the piezoelectric stacks expands the operational bandwidth of the actuator, achieving a seamless integration of speed and precision.

Perovskite quantum dots/WSe2 mixed-dimensional van der Waals heterostructure for photoelectric enhancement and polarization sensitivity

The performance of some heterostructures based devices is limited by strong interlayer coupling and limited electronic structure modulation. Meanwhile, the detection of polarized light holds great significance in future photoelectric fields such as cryptography. In this work, zero-dimensional/two-dimensional (0D/2D) mixed-dimensional van der Waals heterostructure (MvdWH) polarization-sensitive photodetector composed of solution-synthesized CH3NH3PbI3(MAPbI3) perovskite quantum dots (QDs) and chemical vapor deposition (CVD)-grown WSe2 was designed, forming a type II band structure. The MvdWH effectively enhance the carrier transmission rate, and improve the efficiency of charge separation, realizing an ultra-high responsiveness of 217.8 A/W and an ultrasensitive photodetectivity of 2.56 × 1011 Jones under 633 nm illumination. The device shows ultra-fast response times of 4.5/4.9 μs, which is due to the introduction of 0D QDs will shorten the electron transport path and reduces trap-mediated non-radiative recombination. Further, the in-plane anisotropy of MAPbI3 QDs was identified by angle-resolved absorption spectroscopy and angle-resolved polarization Raman spectroscopy, respectively. The MAPbI3/WSe2 device exhibits polarization-sensitive optoelectronic detection with an anisotropy ratio of 1.71, and powerful polarization-sensitive imaging ability, implying that MAPbI3 QDs endows the device with polarization detection and imaging capabilities. These results indicate that the construction of 0D/2D MvdWH opens a new avenue for ultrasensitive and polarization-sensitive photodetection, which depicts opportunities for designing multifunctional, high-performance imaging optoelectronic devices.

MXene/MWCNTs-based capacitive pressure sensors combine high sensitivity and wide detection range for human health and motion monitoring

Wearable pressure sensors with exceptional performance are playing a crucial role in various fields such as electronic skin, human motion monitoring, medical diagnosis, and human-computer interaction. However, achieving both high sensitivity and a wide sensing range with simple fabrication and low cost has proven to be a significant challenge for sensors. In this study, a simple capacitive pressure sensor is proposed that multi-walled carbon nanotubes (MWCNTs) are introduced into MXene (Ti3C2TX) as a ‘bridge’. Then the electrode which has excellent conductivity is obtained by filtration of MXene solution and MXene/MWCNTs solution at intervals. Additionally, green and degradable cellulose paper is used as the dielectric layer. The fabricated pressure sensor exhibits a high sensitivity of 4.7 kPa−1 for low pressure from 0 to 1 kPa, a wide detection range of 0–700 kPa, ultra-fast response and recovery times of 46 ms and 62 ms, respectively, and an extremely low detection limit of 0.32 Pa. The sensor remains stable even after 4500 cycles and can accurately monitor the movement of the human joints. Furthermore, it can track human physiological signals, which is beneficial for medical diagnosis and disease prevention, and has significant potential for application in the wearable technology field.

Ultrafast response and highly sensitivity of flexible acoustic sensor based on the controllable hierarchical Nano/Micro-Structure

Flexible and highly sensitive acoustic sensors have become more and more essential for successful military detection. It puts a big barrier for the large-scale production and application. In this work, we have a low-cost and facile approach constructing a hierarchical nano/micro-structure based on the nanosheets of MXene (Ti3C2Tx) and the gold micro-wrinkles on polydimethylsiloxane (PDMS) surface. The period micro-wrinkles can be easily controlled by the thickness of gold layer and the pre-strain on PDMS through the uniaxial stretching method. The formed hierarchical structure can endow the high sensitivity (15 kPa−1 when applied pressure less than 50 Pa), an ultra-fast response time of 15 ms, a minimum detection of 2 Pa, which can achieve the precise acoustic detection of low frequency. Moreover, the flexible sensor also can detect the large pressure about 1000 Pa, and show stable repeatability over 1500 cycles at 400 Pa.

Highly-sensitive ultrathin flexible pressure sensor based on quasi-2D conductive network for human physiological signal monitoring

Ultrathin flexible pressure sensors are highly desirable for human physiological signal monitoring due to their portability and ability to conform to the body. However, existing ultrathin pressure sensors face challenges in balancing the sensitivity and their range. A spatial-dimensionally constrained strategy was proposed to construct an ultrathin active layer. In detail, multi-walled carbon nanotubes (MWCNTs) with an average length of ∼1.3 μm were intentionally spin-coated into a thin thermoplastic polyurethane (TPU) film with a thickness of 1.5 μm. By doing so, a quasi-2D MWCNTs conductive network is constructed, which enabled the sensor to have ultrahigh sensitivity of 27.04 kPa−1, a wide liner detection range of 0.2–24 kPa, ultrafast response time of 40 ms and a recovery time of 10 ms, and high durability in 1500 cycles under 0.2 kPa with loading/unloading frequency of 2 Hz. Thanks to this superior performance, the ultrathin pressure sensor can be used to monitor human movement and physiological signals, including finger touch, finger bending, wrist bending, and deep breathing. Moreover, the sensor can be attached to the side wall of the tip of the renal ureteroscope, allowing for the monitoring of renal pelvic pressure. Therefore, this spatial-dimensionally constrained strategy will open up a new way to fabricate highly sensitive ultrathin pressure sensors and have great application prospects in health monitoring.

Ultrafast response self-powered UV photodetectors based on ZnO/SiO2/Si heterojunction diodes

This paper presents ultrafast response, self-powered UV photodetectors (PDs) based on ZnO/SiO2/Si heterojunction diodes, utilizing zinc oxide (ZnO) nanorods as the photosensitive layer. This configuration results in photodetectors exhibiting unprecedentedly rapid response times, with rise and decay times reduced to 440 ns and 320 ns, respectively. Our study, focusing on ZnO nanorod PDs, not only demonstrates ultrafast response times but also highlights the role of hydrothermal synthesis temperature in tuning the devices’ performance. These findings represent a significant leap forward in the development of high-performance, self-powered UV PDs. The effects of different hydrothermal temperature on the morphology, crystal structure, and optoelectrical properties were systematically investigated using various characterization techniques, including field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD) analysis, photoluminescence analysis, and current-voltage (I-V) measurements.

Ionic liquid-reinforced transparent, stretchable, conductive organic ionic gel with ultra-high sensory capability and ultra-robust impact-resistance

Traditional conductive hydrogels face challenges in environmental stability and mechanical limitations, constraining their applications in wearable devices. Here, we present a novel organic ionic gel by the construction of PVA/EG double polymer network and ionic liquid, named POIG. It showcases outstanding tensile strength (3.01 MPa) and extensibility (820 %). POIG excels in both low and high-speed impacts, dissipating over 90 % of energy in low-speed impacts and outperforming commercial foam five times in high-speed impacts (1.4 kJ/m). The addition of an ionic liquid in POIG creates a stable, uniform, and highly ordered internal conductive structure, leading to ultra-low strain responsiveness (0.2 %), ultra-fast response times (≈10 ms), high linearity (R2 = 0.999), and excellent sensing performance. High stability is also confirmed through 4000 cycles of both tensile and compression testing. Additionally, we developed a real-time sign language recognition system using deep learning, accurately identifying ten gestures with 98.5 % accuracy. This research offers a durable and innovative organic material for future wearable technology and smart devices.

β-enhanced humidity robust high performance triboelectric nanogenerator for energy harvesting and sensors

Limited output current and harsh environmental conditions are big challenges to harvesting electrical energy from triboelectric nanogenerators (TENG) in sensing applications. One contributing factor to this issue is matching the density of state between two layers of TENG for maximum charge transfer. To ensure a stable output performance, a superhydrophobic interface between the TENG's two layers has been achieved by applying a novel fluorescence enhanced hexamethyldisiloxane (HMDSO) dielectric barrier discharge (DBD) plasma. The plasma coating not only improve the hydrophobicity but also the interface contacts between the two layers of TENG and therefore a maximum charge transfer and ultra-high current density are obtained. The plasma treatment increases the surface roughness and introduces functional groups such as amino (-NH2), hydroxyl (-OH), and siloxane (-Si-O-Si) on the PMMA film, which increases the electron affinity difference between the two layers, increases humidity resistance. The modified TENG generates a maximum stable output voltage of 1400 V and a maximum current of 125 µA with a power density of 55 W/m2, which is significantly higher than those reported in previous studies. Therefore, the TENG has been used as a self-powered droplet detector with an ultra-fast response time of 1.8 ms and has the potential to be used as a self-powered humidity sensor.

Preparation and Application of Nanostructured ZnO in Radiation Detection.

In order to adapt to the rapid development of high-speed imaging technology in recent years, it is very important to develop scintillators with an ultrafast time response. Because of its radiation-induced ultrafast decay time, ZnO has become an important material for radiation detection and dosimetry. According to different detection sources and application scenarios, ZnO is used in various radiation detectors in different structures, including nanoarrays and nanocomposites. In this paper, the synthesis methods and research status of various nanostructured ZnO-based materials and their applications in the detection of high-energy rays (X-rays, γ-rays) and high-energy particles (α, β and neutron) are reviewed. The performance discussion mainly includes spatial resolution, decay time and detection efficiency.