Flexible Piezoresistive Sensors from Polydimethylsiloxane Films with Ridge-like Surface Structures.

Developing flexible sensors and actuators is of paramount importance for wearable devices and systems. In this research, we developed a simple and facile technique to construct flexible piezoresistive sensors from polydimethylsiloxane films with ridge-like surface structures and laser-induced porous graphene. Using a replication strategy, we prepared the ridge-like surface structures from sandpapers. The piezoresistive sensors exhibit excellent sensitivity with a response time of less than 50 ms and long-term cyclic stability under mechanical loading. The smallest weight they can sense is ~96 mg. We demonstrated applications of the piezoresistive sensors in the sensing of bio-related activities, including muscle contraction, finger flexion, wrist flexion, elbow bending, knee bending, swallowing, respiration, sounds, and pulses.

Multi-band optical resonance of all-dielectric metasurfaces toward high-performance ultraviolet sensing.

All-dielectric sensors featuring low-loss resonances have been proposed instead of plasmonic-based sensors. However, reported dielectric-based sensors generally work in the visible and near-infrared regions and detect the intensity variation of resonant modes because the electromagnetic energy is mainly confined inside dielectric nanoparticles. It is a challenge to adjust the hotspots from the inside to the surface of the all-dielectric metasurface. In this study, highly uniform Si3N4 all-dielectric metasurfaces have been successfully fabricated as sensing platforms by utilizing nanosphere self-assembly and plasma enhanced chemical vapor deposition techniques. Experimental and simulated results demonstrate that proposed Si3N4 all-dielectric metasurfaces exhibit multiple optical resonant modes in the ultraviolet and visible wavelength and present distinct field-confinement in the gaps of nanoparticles. The hotspots have been successfully adjusted to the surface of Si3N4 nanoparticles. Delightedly, Si3N4 all-dielectric metasurfaces show characteristic wavelength shifts with variation of the refractive index, and the sensitivity can reach 707 nm per RIU for trace detection as sensing substrates. Proposed Si3N4 all-dielectric metasurfaces are promising to act as high-sensitive sensing substrates in the ultraviolet and visible wavelength with the ease of high-throughput fabrication.

Light response and adsorption interaction of black phosphorus quantum dots and single-layer graphene phototransistor.

Black phosphorus quantum dots (BPQDs) are synthesized and combined with graphene sheet. The fabricated BPQDs/graphene devices are capable of detecting visible and near infrared radiation. The adsorption effect of BPQDs in graphene is clarified by the relationship of the photocurrent and the shift of the Dirac point with different substrate. The Dirac point moves toward a neutral point under illumination with both SiO2/Si and Si3N4/Si substrates, indicating an anti-doped feature of photo-excitation. To our knowledge, this provides the first observation of photoresist induced photocurrent in such systems. Without the influence of the photoresist the device can respond to infrared light up to 980 nm wavelength in vacuum in a cryostat, in which the photocurrent is positive and photoconduction effect is believed to dominate the photocurrent. Finally, the adsorption effect is modeled using a first-principle method to give a picture of charge transfer and orbital contribution in the interaction of phosphorus atoms and single-layer graphene.

Hybrid metal-dielectric gratings (HMDGs) as an alternative UV-SERS substrate.

Ultraviolet surface-enhanced Raman scattering (UV-SERS) typically occupies an important position because the electronic absorption bands of many biomolecules are located in the deep-ultraviolet (DUV) or ultraviolet (UV) region. Practical application of UV-SERS still relies on uniform, reproducible, and affordable substrates. The conventional aluminum (Al) plasmonic nanostructures are mostly applied to act as UV-SERS substrates, but their intrinsic ohmic loss hinders their practical application. In this study, wafer-scale hybrid metal-dielectric gratings (HMDGs) consisting of aluminum and silicon (Al-Si) have been successfully fabricated as UV-SERS substrates to reduce ohmic dissipation and elevate the detection performance. Well-defined HMDG substrates exhibit tunable hybrid resonant modes in the UV and the visible regions. The adenine biomolecules deposited on HMDG substrates are used to perform SERS measurement with an excitation wavelength of 325 nm. The HMDG nanostructures can obtain as high as 5 orders of magnitude compared with that of Al film as UV-SERS substrates. The proposed HMDG nanostructures have a great advantage in detecting important biomolecules as UV-SERS substrates.

Broadband long-wave infrared high-absorption of active materials through hybrid plasmonic resonance modes.

Broadband high absorption of long-wavelength infrared light for rough submicron active material films is quite challenging to achieve. Unlike conventional infrared detection units, with over three-layer complex structures, a three-layer metamaterial with mercury cadmium telluride (MCT) film sandwiched between an Au cuboid array and Au mirror is studied through theory and simulations. The results show that propagated/localized surface plasmon resonance simultaneously contribute to broadband absorption under the TM wave of the absorber, while the Fabry-Perot (FP) cavity resonance causes absorption of the TE wave. As surface plasmon resonance concentrates most of the TM wave on the MCT film, 74% of the incident light energy is absorbed by the submicron thickness MCT film within the 8-12 µm waveband, which is approximately 10 times than that of the rough same thickness MCT film. In addition, by replacing the Au mirror with Au grating, the FP cavity along the y-axis direction was destroyed, and the absorber exhibited excellent polarization-sensitive and incident angle-insensitive properties. For the corresponding conceived metamaterial photodetector, as carrier transit time across the gap between Au cuboid is much less than that of other paths, the Au cuboids simultaneously act as microelectrodes to collect photocarriers generated in the gap. Thus the light absorption and photocarrier collection efficiency are hopefully improved simultaneously. Finally, the density of the Au cuboids is increased by adding the same arranged cuboids perpendicular to the original direction on the top surface or by replacing the cuboids with crisscross, which results in broadband polarization-insensitive high absorption by the absorber.

Promoted Mid-Infrared Photodetection of PbSe Film by Iodine Sensitization Based on Chemical Bath Deposition.

In recent years, lead selenide (PbSe) has gained considerable attention for its potential applications in optoelectronic devices. However, there are still some challenges in realizing mid-infrared detection applications with single PbSe film at room temperature. In this paper, we use a chemical bath deposition method to deposit PbSe thin films by varying deposition time. The effects of the deposition time on the structure, morphology, and optical absorption of the deposited PbSe films were investigated by x-ray diffraction, scanning electron microscopy, and infrared spectrometer. In addition, in order to activate the mid-infrared detection capability of PbSe, we explored its application in infrared photodetection by improving its crystalline quality and photoconductivity and reducing tge noise and high dark current of PbSe thin films through subsequent iodine treatment. The iodine sensitization PbSe film showed superior photoelectric properties compared to the untreated sample, which exhibited the maximum of responsiveness, which is 30.27 A/W at 808 nm, and activated its detection ability in the mid-infrared (5000 nm) by introducing PbI2, increasing the barrier height of the crystallite boundary and carrier lifetimes. This facile synthesis strategy and the sensitization treatment process provide a potential experimental scheme for the simple, rapid, low-cost, and efficient fabrication of large-area infrared PbSe devices.

Epitaxial Topological Insulator Bi2Te3 for Fast Visible to Mid-Infrared Heterojunction Photodetector by Graphene As Charge Collection Medium.

Three dimensional topological insulators have a thriving application prospect in broadband photodetectors due to the possessed topological quantum states. Herein, a large area and uniform topological insulator bismuth telluride (Bi2Te3) layer with high crystalline quality is directly epitaxial grown on GaAs(111)B wafer using a molecular beam epitaxy process, ensuring efficient out-of-plane carriers transportation due to reduced interface defects influence. By tiling monolayer graphene (Gr) on the as-prepared Bi2Te3 layer, a Gr/Bi2Te3/GaAs heterojunction array prototype was further fabricated, and our photodetector array exhibited the capability of sensing ultrabroad photodetection wavebands from visible (405 nm) to mid-infrared (4.5 µm) with a high specific detectivity (D*) up to 1012 Jones and a fast response speed at about microseconds at room temperature. The enhanced device performance can be attributed to enhanced light-matter interaction at the high-quality heterointerface of Bi2Te3/GaAs and improved carrier collection efficiency through graphene as a charge collection medium, indicating an application prospect of topological insulator Bi2Te3 for fast-speed broadband photodetection up to a mid-infrared waveband. This work demonstrated the potential of integrated topological quantum materials with a conventional functional substrate to fabricate the next generation of broadband photodetection devices for uncooled focal plane array or infrared communication systems in future.

Photonic nanojets with ultralong working distance and narrowed beam waist by immersed engineered dielectric hemisphere.

Engineered spherical micro-lens can manipulate light at sub-wavelength scale and emerges as a promising candidate to extend the focal length and narrow the focal spot size. Here, we report the generation of photonic nanojets (PNJs) with an ultralong working distance and narrowed beam waist by an immersed engineered hemisphere. Simulations show that a two-layer hemisphere of 4.5 µm radius exhibits a PNJ with the working distance of 9.6 µm, full width at half maximum of 287 nm, and length of 23.37 λ, under illumination of a plane wave with a 365 nm wavelength. A geometrical optics analysis indicated that the formed PNJ behind the immersed two-layer hemisphere results from the convergence of light of the outer-hemisphere fringe area, which refracts into and passes through the outer hemisphere and then directly leaves the outer-hemisphere flat surface. Thus the embedded hemisphere is comparable to an immersed focusing lens with high numerical aperture, which can promise both long working distance and narrowed beam waist. This is further demonstrated with the corresponding embedded-engineered single-layer hemisphere, whose spherical face is partly cut parallel to the hemispherical flat surface. In addition, the hemisphere is compatible with adjacent laser wavelengths. Finally, a spot size smaller than 0.5 λ is demonstrated in the lithography simulation. Due to these hemispheres low cost, they have potential in far-field lithography for pattern arrays with line width less than 0.5 λ.

High responsivity and fast UV-vis-short-wavelength IR photodetector based on Cd3As2/MoS2 heterojunction.

High responsivity, fast response time, ultra-wide detection spectrum are pursuing goals for state-of-art photodetectors. Cd3As2, as a three-dimensional (3D) Dirac semimetal, has a zero bandgap, high light absorption rate in broad spectral region, and higher mobility than graphene at room temperature. However, photoconductive detectors based Cd3As2 suffer low quantum efficiency due to the absence of high built-in field. Here, a Cd3As2 nanoplate/multilayer MoS2 heterojunction photodetector was fabricated which achieved a quite high responsivity of 2.7 × 103 A W-1 at room temperature. The photodetector exhibits a short response time of in broad spectra region from ultraviolet (365 nm) to short-wavelength-infrared (1550 nm) and reached 65 µs at 650 nm. This work provides a great potential solution for high-performance photodetector and broadband imaging by combining 3D Dirac semi-metal materials with semiconductor materials.

Three-Dimensional Topological Insulator Bi2Te3/Organic Thin Film Heterojunction Photodetector with Fast and Wideband Response from 450 to 3500 Nanometers.

In the pursuit of broadband photodetection materials from visible to mid-IR region, the fresh three-dimensional topological insulators (3D TIs) are theoretically predicted to be a promising candidate due to its Dirac-like stable surface state and high absorption rate. In this work, a self-powered inorganic/organic heterojunction photodetector based on n-type 3D TIs Bi2Te3 combined with p-type pentacene thin film was designed and fabricated. Surprisingly, it was found that the Bi2Te3/pentacene heterojunction photodetector exhibited a fast and wideband response from 450 to 3500 nm. The optimized responsivity of photodetector reached 14.89 A/W, along with the fast response time of 1.89 ms and the ultrahigh external quantum efficiency of 2840%. Moreover, at the mid-IR 3500 nm, our devices demonstrated a responsivity of 1.55 AW-1, which was several orders of magnitude higher than that of previous 3D TIs photodetector. These excellent properties indicate that the inorganic/organic heterojunction, that is, the combination of 3D TIs with organic materials, is an exciting structure for high performance photodetectors in the wideband detection region. On account of the fact that the device is constructed on mica substrate, this work also represents a potential scenario for flexible optoelectronic devices.

Optical Properties and Sensing Performance of Au/SiO2 Triangles Arrays on Reflection Au Layer.

In order to enhance the refractive index sensing performance of simple particle arrays, a structure, consisting of Au/SiO2 triangle arrays layers and reflection Au substrate, with increasing size and lengthening tips of triangles, is studied. The triangle arrays are modeled after an experimentally realizable "imprint" of microsphere lithography. Numerical calculation was carried out to study its optical properties and spectral sensitivity. The calculation results show that a large local enhancement of electric field (61 times) and simultaneously high absorption is due to combination of the resonance absorption of Au triangle disks, plasmonic couplings between the Au triangle disks and the Au film, and the high-density packing of triangle disks. The absorption peaks were not detuned when the gap between neighboring tips of the triangles varied from 10 to 50 nm. When the thickness of SiO2 layer increased from 10 to 50 nm, the absorption peak shifted to longer wavelengths and the amplitude rises quickly signaling the dominance of the gap mode resonance between the two Au layers. As the thickness of the top Au layer varies from 10 to 50 nm, the absorption peak is also red shifted and the peak amplitude increases. The full width at half maximum of the peaks for high absorption (> 90%) is about 5 nm. When fixing the gap, the thicknesses of Au/SiO2 triangle layer, and increasing the surrounding refractive index from 1.33 to 1.36, the absorption peaks shifted quickly, with a refractive index sensitivity and figure of merit as high as 660 nm per refractive index unit and 132, respectively. Such arrays can be easily fabricated by using microsphere array as projection masks and find application in refractive index monitoring of liquid and identification of gas and liquid phases.

Influence of sphere-surface distance and exposure dose on resolution of sphere-lens-array lithography.

Sphere-surface distance and exposure dose effects on lithography resolution of microsphere lens array are studied. A transparent gap is introduced between photoresist film and polystyrene microspheres to adjust light distribution of photoresist surface. The pores size on the photoresist film varies when the gap thickness is changed. By optimization gap thickness and exposure dose, sub-100 nm surface patterning array is achieved for sphere of 2.06 µm diameter and optical wavelength λ of 400 nm theoretically and experimentally. Furthermore, when smaller polystyrene sphere (1.25 µm diameter) is chosen, spot width of 35 nm is realized by numerical calculation. This nano-fabrication method is simple, low-cost and high-efficiency, which can provide opportunities to make a variety of nano-devices, such as biosensors and nano-antennas.