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.

Stretchable, Transparent, and Ultra-Broadband Terahertz Shielding Thin Films Based on Wrinkled MXene Architectures.

With the increasing demand for terahertz (THz) technology in security inspection, medical imaging, and flexible electronics, there is a significant need for stretchable and transparent THz electromagnetic interference (EMI) shielding materials. Existing EMI shielding materials, like opaque metals and carbon-based films, face challenges in achieving both high transparency and high shielding efficiency (SE). Here, a wrinkled structure strategy was proposed to construct ultra-thin, stretchable, and transparent terahertz shielding MXene films, which possesses both isotropous wrinkles (height about 50 nm) and periodic wrinkles (height about 500 nm). Compared to flat film, the wrinkled MXene film (8 nm) demonstrates a remarkable 36.5% increase in SE within the THz band. The wrinkled MXene film exhibits an EMI SE of 21.1 dB at the thickness of 100 nm, and an average EMI SE/t of 700 dB µm-1 over the 0.1-10 THz. Theoretical calculations suggest that the wrinkled structure enhances the film's conductivity and surface plasmon resonances, resulting in an improved THz wave absorption. Additionally, the wrinkled structure enhances the MXene films' stretchability and stability. After bending and stretching (at 30% strain) cycles, the average THz transmittance of the wrinkled film is only 0.5% and 2.4%, respectively. The outstanding performances of the wrinkled MXene film make it a promising THz electromagnetic shielding materials for future smart windows and wearable electronics.

Plasma Treatment for Achieving Oxygen Substitution in Layered MoS2 and the Room-Temperature Mid-Infrared (10 µm) Photoresponse.

Highly sensitive photodetectors in the mid-infrared (MIR, 3-15 µm) are highly desired in a growing number of applications. However, only a handful of narrow-band-gap semiconductors are suitable for this purpose, most of which require cryogenic cooling to increase the signal-to-noise ratio. The realization of high-performance MIR photodetectors operating at room temperature remains a challenge. Herein, we report on plasma-treated few-layer MoS2 for room-temperature MIR (10 µm) photodetection. Oxygen plasma treatment, which is a mature microfabrication process, is employed. The ion kinetic energy of oxygen plasma is adjusted to 70-130 eV. A photoresponsivity of 0.042 mA/W and a detectivity of 1.57 × 107 Jones are obtained under MIR light (10 µm) illumination with an average power density of 114.6 mW/cm2. The photoresponse is attributed to the introduction of electronic states in the band gap of MoS2 through oxygen substitution. A graphene/plasma-treated MoS2/graphene device is further demonstrated to shorten the active channel while maintaining the illumination area. The photoresponsivity and detectivity are largely boosted to 1.8 A/W and 2.64 × 109 Jones, respectively. The excellent detective performance of the graphene/plasma-treated MoS2/graphene device is further demonstrated in single-detector MIR (10 µm) scanning imaging. This work offers a facile approach to constructing integrated MoS2-based MIR photodetectors.

Néel-type optical target skyrmions inherited from evanescent electromagnetic fields with rotational symmetry.

Optical skyrmions have recently attracted growing interest due to their potential applications in deep-subwavelength imaging and nanometrology. While optical skyrmions have been successfully demonstrated using different field vectors, the study of their generation and control, as well as their general correlation with electromagnetic (EM) fields, is still in its infancy. Here, we theoretically propose that evanescent transverse-magnetic-polarized (TM-polarized) EM fields with rotational symmetry are actually Néel-type optical target skyrmions of the electric field vectors. Such optical target skyrmions are independent of the operation frequency and medium. Our proposal was verified by numerical simulations and real-space nano-imaging experiments performed on a graphene monolayer, where the target skyrmions could be as small as ∼100 nm in diameter. The results can therefore not only further our understanding of the formation mechanisms of EM topological textures, but also provide guidelines for the facile construction of EM skyrmions that may impact future information technologies.

Motor imagery electroencephalogram classification algorithm based on joint features in the spatial and frequency domains and instance transfer.

Introduction: Motor imagery electroencephalography (MI-EEG) has significant application value in the field of rehabilitation, and is a research hotspot in the brain-computer interface (BCI) field. Due to the small training sample size of MI-EEG of a single subject and the large individual differences among different subjects, existing classification models have low accuracy and poor generalization ability in MI classification tasks. Methods: To solve this problem, this paper proposes a electroencephalography (EEG) joint feature classification algorithm based on instance transfer and ensemble learning. Firstly, the source domain and target domain data are preprocessed, and then common space mode (CSP) and power spectral density (PSD) are used to extract spatial and frequency domain features respectively, which are combined into EEG joint features. Finally, an ensemble learning algorithm based on kernel mean matching (KMM) and transfer learning adaptive boosting (TrAdaBoost) is used to classify MI-EEG. Results: To validate the effectiveness of the algorithm, this paper compared and analyzed different algorithms on the BCI Competition IV Dataset 2a, and further verified the stability and effectiveness of the algorithm on the BCI Competition IV Dataset 2b. The experimental results show that the algorithm has an average accuracy of 91.5% and 83.7% on Dataset 2a and Dataset 2b, respectively, which is significantly better than other algorithms. Discussion: The statement explains that the algorithm fully exploits EEG signals and enriches EEG features, improves the recognition of the MI signals, and provides a new approach to solving the above problem.

A Centimeter-Scale Type-II Weyl Semimetal for Flexible and Fast Ultra-Broadband Photodetection from Ultraviolet to Sub-Millimeter Wave Regime.

Flexible photodetectors with ultra-broadband sensitivities, fast response, and high responsivity are crucial for wearable applications. Recently, van der Waals (vdW) Weyl semimetals have gained much attention due to their unique electronic band structure, making them an ideal material platform for developing broadband photodetectors from ultraviolet (UV) to the terahertz (THz) regime. However, large-area synthesis of vdW semimetals on a flexible substrate is still a challenge, limiting their application in flexible devices. In this study, centimeter-scale type-II vdW Weyl semimetal, Td -MoTe2 films, are grown on a flexible mica substrate by molecular beam epitaxy. A self-powered and flexible photodetector without an antenna demonstrated an outstanding ability to detect electromagnetic radiation from UV to sub-millimeter (SMM) wave at room temperature, with a fast response time of ≈20 µs, a responsivity of 0.53 mA W-1 (at 2.52 THz), and a noise-equivalent power (NEP) of 2.65 nW Hz-0.5 (at 2.52 THz). The flexible photodetectors are also used to image shielded items with high resolution at 2.52 THz. These results can pave the way for developing flexible and wearable optoelectronic devices using direct-grown large-area vdW semimetals.

A multimode photodetector with polarization-dependent near-infrared responsivity using the tunable split-dual gates control.

As the limited carrier densities in atomic thin materials can be well controlled by electrostatic gates, p-n junctions based on two-dimensional materials in the coplanar split-gate configuration can work as photodetectors or light-emitting diodes. These coplanar gates can be fabricated in a simple one-step lithography process and are frequently used in hybrid integration with on-chip optical structures. However, the polarization-dependent responsivity of such a configuration is less explored in the near-infrared band, and a clear understanding is still missing. Here we fabricate near-infrared tunable multiple modes twisted bilayer graphene photodetector enabled by the coplanar split-gate control and confirm that the photothermoelectric effect governs the photovoltage mechanism of the p-n junction mode. Our study also elucidates that the discrepancy of the responsivities under different linear polarizations is owing to the different cavity modes and provides a valuable example for designing chip-integrated optoelectronic devices.

A planar plasmonic nano-gap and its array for enhancing light-matter interactions at the nanoscale.

Gap surface plasmon (GSP) modes, the localized electromagnetic modes existing between two metal structures separated by a nano-gap, are able to support subwavelength confinement and enhancement of a light field upon resonance excitation. Such features can greatly facilitate various light-matter interactions at the nanoscale. Here, we demonstrate a planar nano-gap architecture existing between a pair of tip-shaped gold pads. The nano-gap gives rise to plasmon resonances with strong light confinement close to the tip surfaces in the visible to near-infrared spectral region. Accordingly, we showed that the plasmonic gold nano-gap can exhibit strong intrinsic second-harmonic generation (SHG) and significantly enhance the Raman scattering signal from small molecules. Furthermore, by arranging the nano-gap into arrays, a stronger SHG signal can be obtained. In addition, the surface enhanced Raman scattering (SERS) activity is also improved by two orders of magnitude compared to that of a single nano-gap. Overall, the findings in our study have demonstrated the potential applications of a plasmonic nano-gap and its arrays for signal generation and sensitive chemical sensing at the nanoscale.

Controlling and Focusing In-Plane Hyperbolic Phonon Polaritons in α-MoO3 with a Curved Plasmonic Antenna.

Hyperbolic phonon polaritons (HPhPs) sustained in polar van der Waals (vdW) crystals exhibit extraordinary confinement of long-wave electromagnetic fields to the deep subwavelength scale. In stark contrast to uniaxial vdW hyperbolic materials, recently emerged biaxial hyperbolic materials, such as α-MoO3 and α-V2 O5 , offer new degrees of freedom for controlling light in two-dimensions due to their distinctive in-plane hyperbolic dispersions. However, the control and focusing of these in-plane HPhPs remain elusive. Here, a versatile technique is proposed for launching, controlling, and focusing in-plane HPhPs in α-MoO3 with geometrically designed curved gold plasmonic antennas. It is found that the subwavelength manipulation and focusing behaviors are strongly dependent on the curvature of the antenna extremity. This strategy operates effectively in a broadband spectral region. These findings not only provide fundamental insights into the manipulation of light by biaxial hyperbolic crystals at the nanoscale but also open up new opportunities for planar nanophotonic applications.

Highly Sensitive and Ultra-Broadband VO2(B) Photodetector Dominated by Bolometric Effect.

In this study, Wadsley B phase vanadium oxide (VO2(B)) with broad-band photoabsorption ability, a large temperature coefficient of resistance (TCR), and low noise was developed for uncooled broad-band detection. By using a freestanding structure and reducing the size of active area, the VO2(B) photodetector shows stable and excellent performances in the visible to the terahertz region (405 nm to 0.88 mm), with a peak TCR of -4.77% K-1 at 40 °C, a peak specific detectivity of 6.02 × 109 Jones, and a photoresponse time of 83 ms. A terahertz imaging ability with 30 × 30 pixels was demonstrated. Scanning photocurrent imaging and real-time temperature-photocurrent measurements confirm that a photothermal-type bolometric effect is the dominating mechanism. The study shows the potential of VO2(B) in applications as a new type of uncooled broad-band photodetection material and the potential to further raise the performance of broad-band photodetectors by structural design.

Optimization Strategies for High Photoluminescence Quantum Yield of Monolayer Chemical Vapor Deposition Transition Metal Dichalcogenides.

Chemical vapor deposition (CVD) is a promising method to obtain monolayer transition metal dichalcogenides (TMDCs) with high quality and enough size to meet the requirements of practical photoelectric devices. However, the as-grown monolayers often exhibit a lower PL performance due to the stress between the as-grown TMDCs flakes and the substrate. Therefore, finding a facile method to effectively promote the photoluminescence quantum yield (PL QY) of CVD monolayer TMDCs with a clean surface is highly desirable for practical applications. In this work, based on the CVD monolayers MoS2 and MoSe2, the effect of various stress relaxation methods on the TMDCs PL enhancement is systemically studied. By comparing the different kinds of volatile solution treatment processes, as well as the traditional transfer process, it can be found that the volatile solution with a moderate volatilization rate such as ethanol or IPA is a preferred option to improve the PL performance of the CVD monolayer TMDCs, which also surpasses the traditional transfer method by avoiding wrinkles, defects, and contamination to the samples. PL QY of ethanol-treated CVD samples could increase by 6 times on average. Significantly, PL QY of CVD MoSe2 treated by ethanol can reach ∼16%, which is at the forefront of the previous reports of 2D MoSe2. Our study demonstrated an optimized method to enhance the PL QY of CVD monolayer TMDCs, which would facilitate TMDCs optoelectronics.

Mid-Infrared Photodetection of Type-II Dirac Semimetal 1T-PtTe2 Grown by Molecular Beam Epitaxy.

Mid-infrared (MIR) photodetection is of significance in civil and military applications because it shows superiority in absorbing the vibration of various molecules and covering atmospheric transmission windows. Recently, the PtTe2, a typical type-II Dirac semimetal, has come under the spotlight due to its unique photodetection sensibility in the MIR region and robust stability in the atmosphere. Here, the high-quality and large-scale 1T-PtTe2 thin films with air stability were grown by molecular beam epitaxy. Broadband photoresponse of the photodetectors of PtTe2 from 420 nm to 10.7 µm shows high responsivity and detectivity of 0.2 mA W-1 and 2.6 × 107 Jones at 10.7 µm and 1.6 mA W-1 and 2.2 × 108 Jones at 4.7 µm under the atmosphere, respectively. Moreover, the photodetectors exhibit high sensitivity in visible and near-infrared regions (8.2 mA W-1 at 650 nm and 15.6 mA W-1 at 960 nm). The power- and polarization-dependent photoresponse measurements reveal the linear relationship of power photoresponse and obvious anisotropic photoresponse (the ratio of anisotropy ellipse is 8.3 at 10.7 µm), respectively. These results suggest that the PtTe2 could be expected to be an advanced photodetection material for polarization angle-sensitive detection, infrared imaging, and photodetection from the visible to MIR range.

Probing the in-Plane Near-Field Enhancement Limit in a Plasmonic Particle-on-Film Nanocavity with Surface-Enhanced Raman Spectroscopy of Graphene.

When the geometric features of plasmonic nanostructures approach the subnanometric regime, nonlocal screening and charge spill-out of metallic electrons will strongly modify the optical responses of the structures. While quantum tunneling resulting from charge spill-out has been widely discussed in the literature, the near-field enhancement saturation caused by the nonlocal screening effect still lacks a direct experimental verification. In this work, we use surface-enhanced Raman spectroscopy (SERS) of graphene to probe the in-plane near-field enhancement limit in gold nanosphere-on-film nanocavities where different layers of graphene are sandwiched between a gold nanosphere and a gold film. Together with advanced transmission electron microscopy cross-sectional imaging and nonlocal hydrodynamic theoretical calculations, the cavity gap width correlated SERS and dark-field scattering measurements reveal that the intrinsic nonlocal dielectric response of gold limits the near-field enhancement factors and mitigates the plasmon resonance red-shift with decreasing the gap width to less than one nanometer. Our results not only verify previous theoretical predictions in both the near-field and far-field regime but also demonstrate the feasibility of controlling the near- and far-field optical response in such versatile plasmonic particle-graphene-on-film nanocavities, which can find great potential in applications of graphene-based photonic devices in the visible and near-infrared region.

A Plasmon-Mediated Electron Emission Process.

Light-driven electron emission plays an important role in modern optoelectronic devices. However, such a process usually requires a light field with either a high intensity or a high frequency, which is not favorable for its implementations and difficult for its integrations. To solve these issues, we propose to combine plasmonic nanostructures with nanoelectron emitters of low work function. In such a heterostructure, hot electrons generated by plasmon resonances upon light excitation can be directly injected into the adjacent emitter, which can subsequently be emitted into the vacuum. Electron emission of high efficiency can be obtained with light fields of moderate intensities and visible wavelengths, which is a plasmon-mediated electron emission (PMEE) process. We have demonstrated our proposed design using a gold-on-graphene (Au-on-Gr) nanostructure, which can have electron emission with light intensity down to 73 mW·cm-2. It should be noted that the field electron emission is not involved in such a PMEE process. This proposal is of interest for applications including cold-cathode electron sources, advanced photocathodes, and micro- and nanoelectronic devices relying on free electrons.

1T' Transition Metal Telluride Atomic Layers for Plasmon-Free SERS at Femtomolar Levels.

Plasmon-free surface enhanced Raman scattering (SERS) based on the chemical mechanism (CM) is drawing great attention due to its capability for controllable molecular detection. However, in comparison to the conventional noble-metal-based SERS technique driven by plasmonic electromagnetic mechanism (EM), the low sensitivity in the CM-based SERS is the dominant barrier toward its practical applications. Herein, we demonstrate the 1T' transition metal telluride atomic layers (WTe2 and MoTe2) as ultrasensitive platforms for CM-based SERS. The SERS sensitivities of analyte dyes on 1T'-W(Mo)Te2 reach EM-comparable ones and become even greater when it is integrated with a Bragg reflector. In addition, the dye fluorescence signals are efficiently quenched, making the SERS spectra more distinguishable. As a proof of concept, the SERS signals of analyte Rhodamine 6G (R6G) are detectable even with an ultralow concentration of 40 (400) fM on pristine 1T'-W(Mo)Te2, and the corresponding Raman enhancement factor (EF) reaches 1.8 × 109 (1.6 × 108). The limit concentration of detection and the EF of R6G can be further enhanced into 4 (40) fM and 4.4 × 1010 (6.2 × 109), respectively, when 1T'-W(Mo)Te2 is integrated on the Bragg reflector. The strong interaction between the analyte and 1T'-W(Mo)Te2 and the abundant density of states near the Fermi level of the semimetal 1T'-W(Mo)Te2 in combination gives rise to the promising SERS effects by promoting the charge transfer resonance in the analyte-telluride complex.

Highly Confined and Tunable Hyperbolic Phonon Polaritons in Van Der Waals Semiconducting Transition Metal Oxides.

2D van der Waals (vdW) layered polar crystals sustaining phonon polaritons (PhPs) have opened up new avenues for fundamental research and optoelectronic applications in the mid-infrared to terahertz ranges. To date, 2D vdW crystals with PhPs are only experimentally demonstrated in hexagonal boron nitride (hBN) slabs. For optoelectronic and active photonic applications, semiconductors with tunable charges, finite conductivity, and moderate bandgaps are preferred. Here, PhPs are demonstrated with low loss and ultrahigh electromagnetic field confinements in semiconducting vdW α-MoO3 . The α-MoO3 supports strong hyperbolic PhPs in the mid-infrared range, with a damping rate as low as 0.08. The electromagnetic confinements can reach ≈λ0 /120, which can be tailored by altering the thicknesses of the α-MoO3 2D flakes. Furthermore, spatial control over the PhPs is achieved with a metal-ion-intercalation strategy. The results demonstrate α-MoO3 as a new platform for studying hyperbolic PhPs with tunability, which enable switchable mid-infrared nanophotonic devices.