High-quality quasi-bound state in the continuum enabled single-nanoparticle virus detection.

Bound states in the continuum (BICs) have emerged as a powerful platform for boosting light-matter interactions because they provide an alternative way of realizing optical resonances with ultrahigh quality(Q-) factors, accompanied by extreme field confinement. In this work, we realized an optical biosensor by introducing a quasi-BIC (qBIC) supported by an elaborated all-dielectric dimer grating. Thanks to the excellent field confinement within the air gap of grating enabled by such a high-Q qBIC, the figure of merit (FOM) of a biosensor is up to 18,908.7 RIU-1. Furthermore, we demonstrated that such a high-Q grating can help push the limit of optical biosensing to the single-particle level. Our results may find exciting applications in extreme biochemical sensing like COVID-19 with ultralow concentration.

Efficient monodisperse upconversion composite prepared using high-density local field and its dual-mode temperature sensing.

Enhanced upconversion via plasmonics has considerable potential in biosensors and solar cells; however, conventional plasmonic configurations such as core-shell assemblies or nanoarray platforms still suffer from the compromise between the enhancement factor and monodispersity, which has failed to meet the requirement of the materials for the in vivo all-solution-prepared solar cells and biosensors. We herein report a monodisperse metal-dielectric-metal (MDM) type upconverted hybrid material with high efficiency. The lanthanide-doped upconversion nanoparticles (UCNPs) were sandwiched by two gold nanodisk mirrors, and the highly localized excitation field around the UCNPs together with the efficient coupling enhanced the upconversion. The upconversion intensity can then be effectively regulated and improved by three to four orders of magnitude. As per the measurement of the temperature-dependent fluorescence intensity and spectra shift, a dual-mode nanothermometer based on our proposed hybrid materials was demonstrated. This MDM-type upconverted hybrid material demonstrated the merits of high efficiency and monodispersity, which demonstrated promise in in vivo biosensors and solar cell fabrication techniques such as spin-coating and roll-to-roll.

Ultrastrong Infrared Emission at 1460 nm from Lanthanide-Doped Nanoparticles with Interfacial Sensitization.

Fluorescent imaging in the second near-infrared window (NIR-II; 1000-1700 nm) has recently received tremendous attention due to its excellent tissue probing depths and high resolution. Under NIR pumping, lanthanide-doped nanoparticles can emit infrared light covering a wide range of 800-3000 nm which has good potential for NIR-II imaging and detection. However, the low efficiency hinders their application. Here, we report intense infrared emission at 1460 nm from lanthanide-doped core/shell nanoparticles with efficient interfacial sensitization. The emitter Tm3+ ion and the sensitizer Yb3+ and Nd3+ ions are spatially separated in core and shells so that the efficient interfacial energy transfer is established between Tm3+ and Yb3+/Nd3+ ions, while thermal vibration spread of high concentration of Yb3+ ions and cross-relaxation among Tm3+, Yb3+, and Nd3+ ions are suppressed. As a result, the ultrastrong NIR-II emission at 1460 nm is achieved, which is more than 100-times that in classic core/shell nanoparticles doped with Tm3+ (NaYF4:20%Yb,0.5%Tm@ NaYF4).

Tailoring linear and nonlinear plasmons of metal/MoS2/metal nanostructures.

We investigated the linear and nonlinear response of the localized surface plasmons (LSPs) and surface plasmon polaritons (SPPs) in metal and MoS2 nanostructures. The results show that the response of LSPs and SPPs has an important influence on the energy exchange. SPPs with unique non-radiative characteristics can be used as energy recovery tanks to reuse the radiated energy of LSPs and promote the production of hot carriers. The energy exchange through plasmon modes can promote the transfer of hot electrons in the Au grating, the MoS2 layer, and the metal film. The fundamental field induces the increase of the second harmonic wave by introducing the second-order nonlinear source. In addition, the evolution of the lifetime of linear and nonlinear plasmonic modes is also investigated to study the underlying mechanism of the micro process in the plasmonic-photonic interaction. The plasmonic energy exchanging configuration overcomes the challenge by utilizing hot carriers. It is instructive in terms of improving the linear and nonlinear performance of plasmonic opto-electronic devices.

Pushing Optical Virus Detection to a Single Particle through a High-Q Quasi-bound State in the Continuum in an All-dielectric Metasurface.

Bound states in the continuum (BICs) have emerged as a powerful platform for boosting light-matter interactions because they provide an alternative way of realizing optical resonances with ultrahigh quality factors, accompanied by extreme field confinement. In this work, we realized an optical biosensor by harnessing a quasi-BIC (qBIC) supported by an all-dielectric metasurface with broken symmetry, whose unit cell is composed of a silicon cuboid with two asymmetric air holes. Thanks to the excellent field confinement within the air gap of a metasurface enabled by such a high-Q qBIC, the figure of merit (FOM) of the biosensor is up to 2136.35 RIU-1. Futhermore, we demonstrated that such a high-Q metasurface can push the detection limit to a few virus particles. Our results may find exciting applications in extreme biochemical sensing like COVID-19 with ultralow concentrations.

Interfacial contact barrier and charge carrier transport of MoS2/metal(001) heterostructures.

The rapid rise of two-dimensional (2D) materials has aroused increasing interest in the fields of microelectronics and optoelectronics; various types of 2D van der Waals heterostructures (vdWHs), especially those based on MoS2, have been widely investigated in theory and experiment. However, the interfacial properties of MoS2 and the uncommon crystal surface of traditional three-dimensional (3D) metals are yet to be explored. In this paper, we studied heterostructures composed of MoS2 and metal(001) slabs, based on the first-principles calculations, and we uncovered that MoS2/Au(001) and MoS2/Ag(001) vdWHs reveal Schottky contacts, and MoS2/Cu(001) belongs to Ohmic contact and possesses ultrahigh electron tunneling probability at the equilibrium distance. Thus, the MoS2/Cu(001) heterostructure exhibits the best contact performance. Further investigations demonstrate that external longitudinal strain can modulate interfacial contact to engineer the Schottky-Ohmic contact transition and regulate interfacial charge transport. We believe that it is a general strategy to exploit longitudinal strain to improve interfacial contact performance to design and fabricate a multifunctional MoS2-based electronic device.

First-principles study on photoelectric properties of all-inorganic two-dimensional double perovskite Cs3AgBiBr7.

Two-dimensional (2D) all-inorganic double perovskite materials have attracted great interest owing to their unique photoelectric characteristics, such as high quantum efficiency and relative stability. However, few studies have been conducted on the 2D all-inorganic double perovskite Cs3AgBiBr7, and its photoelectric properties are unclear. In this study, we present a detailed investigation of the band structure, optical absorption spectrum, carrier mobility and exciton binding energy of the double perovskite Cs3AgBiBr7 based on the first-principles. The results show that this system has an indirect band gap and low carrier mobility, high exciton binding energy (2041.38 meV) and significant light absorption in the UV region. We also find that the material may be a potential exciton insulation candidate owing to the exciton binding energy beyond the band gap. Our calculated results also show that low dimensional perovskite Cs3AgBiBr7 is more suitable for luminescence than a photovoltaic device. We hope our theoretical results will inspire and promote the experimental exploration of 2D all-inorganic double perovskite materials for photoelectric applications.

An optical strategy for detecting hypochlorite in vitro and cells with high selectivity and stability based on a lanthanide-doped upconversion probe.

The excessive use of sodium hypochlorite disinfectant for preventing COVID-19 can be harmful to the water environment and humans. More importantly, owing to hypochlorite being a biomarker of immune responses in living organisms, its abnormal production can damage nucleic acids and protein molecules, eventually causing many diseases (even cancer). Exploring a reliable, rapid, and non-invasive method to monitor the hypochlorite level in vitro and in cells can be significant. Herein, we report a novel ratiometric fluorescence sensing strategy based on Astrazon Brilliant Red 4G dye-sensitized NaGdF4:Yb3+, Er3+@NaYF4 core-shell upconversion nanoparticles (UCNPs@ABR 4G). Based on the combination mechanism of the fluorescent resonant energy transfer effect (FRET) and redox, a linear model of fluorescence intensity ratio and hypochlorite concentration was constructed for a fast response and high selectivity monitoring of hypochlorite in vitro and in vivo. The detection limit was calculated to be 0.39 µM. In addition, this sensing strategy possessed good stability and circularity, making it valuable both for the quantitative detection of hypochlorite in water and for the visualization of intracellular hypochlorite. The proposed optical probe is promising for the efficient and stable non-invasive detection of hypochlorite.

Polarization-sensitive switchable display through critical coupling between graphene and a quasi-BIC.

Enhanced light-matter interaction of a local field is of prime importance in optics as it can improve the performance of nanophotonic devices. Such enhancement can be achieved by utilizing the optical bound states in the continuum (BICs). In this study, a dielectric metasurface is proposed that could enhance the light-matter interactions in graphene. A symmetry-protected BIC was observed in such a metasurface, which could transform into a quasi-BIC with a high quality (Q-) factor when the in-plane symmetry is broken. As the graphene monolayer was introduced into the system, its absorption was enhanced by the quasi-BIC resonance. By optimizing the graphene Fermi energy and the asymmetry parameter of the metasurface to satisfy the critical-coupling condition, a tunable absorber could be achieved. The absorbing intensity could be efficiently modulated by varying the polarization direction of the incident light, the maximum difference of which was up to 95.4%. Also, further investigation showed that such a feature indicates potential application in digital switches and image displays, which could be switched by incident polarization only, and therefore without dependence on an additional structural change.

Optical Fermi level-tuned plasmonic coupling in a grating-assisted graphene nanoribbon system.

A novel graphene-based grating-coupled metamaterial structure is proposed, and the optical response of this structure can be obviously controlled by the Fermi level, which is theoretically regulated by the electric field of an applied voltage. The upper graphene monolayer can be intensely excited with the aid of periodic grating and thus it can be considered a bright mode. Meanwhile, the lower graphene monolayer cannot be directly excited, but it could be indirectly activated by the help of bright mode. The plasmonic polaritons resulting from the light-graphene interaction resonance can lead to a destructive interference effect, leading to a plasmonic induced transparency. This structure has a simple construction and retains the integrity of graphene. In the meantime, it can achieve a good tuning effect by adjusting the voltage regulation of microstructure array and it can obtain an outstanding reflection efficiency. Thus, this graphene-based metamaterial structure with these properties is very suitable for the plasmonic optical reflector. In contacting with the characteristics of material, the group delay of this device can reach to 0.3ps, which can well match the slow light performance. Therefore, the device is expected to make some contribution in optical reflection and slow light devices.

The InSe/SiH type-II van der Waals heterostructure as a promising water splitting photocatalyst: a first-principles study.

Photocatalytic water splitting for hydrogen production has attracted increasing research attention in recent years, and great efforts have been made in order to find the ideal photocatalyst. In this work, we proposed a two-dimensional material-based van der Waals (vdW) heterostructure constructed by vertically stacked indium selenide (InSe) and silicane (SiH) and studied the feasibility of using it as a possible photocatalyst for water splitting by using first-principles methods. The results show that the InSe/SiH is a direct band gap semiconductor with appropriate gap value and band edge position for photocatalysts in water splitting. Importantly, this heterostructure presents type-II band alignment at the equilibrium configuration, which supports the effective separation of photoexcited electrons and holes. A built-in electric field set up within the interface of the heterostructure will further hinder the electron-hole recombination and thus improve the photocatalytic efficiency. In addition, compared with separated InSe and SiH monolayers, the heterostructure exhibits enhanced light absorption capabilities in ultraviolet and visible light regions. These findings indicate that the InSe/SiH vdW heterostructure is a promising candidate for photocatalysts for solar water splitting.

A promising blue phosphorene/C2N van der Waals type-II heterojunction as a solar photocatalyst: a first-principles study.

An appropriate band structure and effective carrier separation are very important for the performance of a solar photocatalyst. In this paper, based on first-principles calculations, it was predicted that blue phosphorene (BlueP) and a C2N monolayer can form a promising metal-free type-II heterojunction. The electronic structure of the BlueP/C2N heterojunction facilitated the overall water splitting reactions well. The projected band structure showed that the conduction band edge was contributed by C2N and the valence band edge was dominated by BlueP. Under the combination of the driving force of the band offset and the built-in electric field between the two layers, the photo-generated electrons and holes were transferred spontaneously to the conduction band of C2N and the valence band of BlueP, respectively. An effective carrier separation in the heterostructure was thus achieved. More notably, the obtained light absorption of the BlueP/C2N junction showed an obvious red-shift, which greatly extended the area of light adsorption to the visible light region. We further proposed that strain could also be used to modulate the band gap and the band edge positions of the heterojunction. Our results not only provide a theoretical design, but also reveal the fundamental separation mechanism of the photo-generated carriers in the BlueP/C2N heterojunction.

Mechanistic Analysis of Embedded Copper Oxide in Organic Thin-Film Transistors with Controllable Threshold Voltage.

The modulation of threshold voltage (V TH) of organic thin-film transistors (OTFTs) was investigated by embedding a thin CuO layer between the two semiconductor layers. The results showed that the V TH of OTFTs with a CuO layer can be effectively tuned by controlling the positive gate-to-source voltage (V GS0) under stress of gate-to-source voltages. The V TH shifts from -3.67 to -0.82 V when the positive V GS0 varies from 30 to 50 V. This can be explained by the mechanism of trapping electrons at the interface between the CuO charge-separation layer and the active layer. To confirm the role of the CuO layer acting as the charge-separation source, two organic thin-film diodes, indium-tin oxide(ITO)/tris (8-quinolinolato) aluminum(III) (Alq3)/pentacene/Al (inverted-stack diode) and ITO/Alq3/CuO/pentacene/Al (inverted-stack diode with a CuO layer), were fabricated and their diode current characteristics were measured. For the second device, a large current flow was observed at positive bias on the ITO electrodes, which is ascribed to the internal bipolar charge separation within the added CuO zone.

[Influence of nitrogen doping on electron structure and optical properties of amorphous carbon thin films].

Nitrogen doped amorphous carbon (a-C : N) thin films were prepared by DC magnetron sputtering. The films were investigated by AES, UV-Vis and ellipsometer. A parameter 'D' defined as the distance between the maximum of positive going excursion and the minimum of negative going excursion was calculated in the derivative AES spectra. The values of 'D' were used to calculate the percentage of sp2 hybrid bonds. The optical transmission and the optical band gap of the films were characterized by an UV-Vis spectrophotometer. The results showed that the optical band gap decreased and then increased with the increase of N2 gas source. The transmission and refractive index changed in reverse order. It was demonstrated that the thin film with low percentage of nitrogen was beneficial to the formation of sp3 hybrid bonds and caused the optical band gap of the thin film to increase. As a result, the thin film should be prepared under low percentage of nitrogen pressure to ensure that it possesses fine optical properties.