Surface Triboelectrification of MXenes with Fluorine Groups for Flexible Energy Harvesting and Sensing

Triboelectric nanogenerator (TENG) has attracted attention for flexible electronics in the past decade. MXene is gradually applied in TENGs for performance enhancement, but the surface triboelectrification mechanism still needs to be further investigated experimentally. Herein, an experimental approach of regulating MXene fluorine groups by alkalization as a contrast to validate the mechanism is proposed. The MXene film and alkalized MXene film are studied to exploit the surface state. Compared with alkalized MXene-based TENGs, MXene-based TENG obtains enhancement in open-circuit voltage, short-circuit current, and output power by 4.1 times, 4.6 times, and even 136.6 times, respectively. The excellent performance enables flexible sensing and energy harvesting. Notably, this work reveals MXene surface triboelectrification mechanism and might motivate a new approach for TENG performance enhancement.     

M‐shaped Grating by Nanoimprinting: A Replicable,Large‐Area,Highly Active Plasmonic Surface‐Enhanced Raman Scattering Substrate with Nanogaps

Plasmonic nanostructures separated by nanogaps enable strong electromagnetic‐field confinement on the nanoscale for enhancing light‐matter interactions, which are in great demand in many applications such as surface‐enhanced Raman scattering (SERS). A simple M‐shaped nanograting with narrow V‐shaped grooves is proposed. Both theoretical and experimental studies reveal that the electromagnetic field on the surface of the M grating can be pronouncedly enhanced over that of a grating without such grooves, due to field localization in the nanogaps formed by the narrow V grooves. A technique based on room‐temperature nanoimprinting lithography and anisotropic reactive‐ion etching is developed to fabricate this device, which is cost‐effective, reliable, and suitable for fabricating large‐area nanostructures. As a demonstration of the potential application of this device, the M grating is used as a SERS substrate for probing Rhodamine 6G molecules. Experimentally, an average SERS enhancement factor as high as 5×10⁸ has been achieved, which verifies the greatly enhanced light–matter interaction on the surface of the M grating over that of traditional SERS surfaces.     

Nanostructured surfaces and assemblies as SERS media

Metallic nanostructures attract much interest as an efficient media for surface-enhanced Raman scattering (SERS). Significant progress has been made on the synthesis of metal nanoparticles with various shapes, composition, and controlled plasmonic properties, all critical for an efficient SERS response. For practical applications, efficient strategies of assembling metal nanoparticles into organized nanostructures are paramount for the fabrication of reproducible, stable, and highly active SERS substrates. Recent progress in the synthesis of novel plasmonic nanoparticles, fabrication of highly ordered one-, two-, and three-dimensional SERS substrates, and some applications of corresponding SERS effects are discussed.     

Monolayer Graphene Coupled to a Flexible Plasmonic Nanograting for Ultrasensitive Strain Monitoring

Plasmonically coupled graphene structures have shown great promise for sensing applications. Their complex and cumbersome fabrication, however, has prohibited their widespread application and limited their use to rigid, planar surfaces. Here, a plasmonic sensor based on gold nanowire arrays on an elastomer with an added graphene monolayer is introduced. The stretchable plasmonic nanostructures not only significantly enhance the Raman signal from graphene, but can also be used by themselves as a sensor platform for 2D strain sensing. These nanowire arrays on an elastomer are fabricated by template‐stripping based nanotransfer printing, which enables a simple and fast production of stable nanogratings. The ultrasmooth surfaces of such transferred structures facilitate reliable large‐area transfers of graphene monolayers. The resulting coupled graphene‐nanograting construct exhibits ultrahigh sensitivity to applied strain, which can be detected by shifts in the plasmonic‐enhanced Raman spectrum. Furthermore, this sensor enables the detection of adsorbed molecules on nonplanar surfaces through graphene‐assisted surface enhanced Raman spectroscopy (SERS). The simple fabrication of the plasmonic nanowire array platform and the graphene‐coupled devices have the potential to trigger widespread SERS applications and open up new opportunities for high‐sensitivity strain sensing applications.     

Space‐Confined Growth of Individual Wide Bandgap Single Crystal CsPbCl3 Microplatelet for Near‐Ultraviolet Photodetection

Perovskite photodetectors (PDs) with tunable detection wavelength have attracted extensive attention due to the potential application in the field of imaging, machine vision, and artificial intelligence. Most of the perovskite PDs focus on I‐ or Br‐based materials due to their easy preparation techniques. However, their main photodetection capacity is situated in the visible region because of their narrower bandgap. Cl‐based wide bandgap perovskites, such as CsPbCl₃, are scarcely reported because of the bad film quality of the spin‐coated Cl‐based perovskite, due to the poor solubility of the precursor. Therefore, ultraviolet detection using high‐quality full inorganic perovskite films, especially with high thermal stability of materials and devices, is still a big challenge. In this work, high‐quality single crystal CsPbCl₃ microplatelets (MPs) synthesized by a simple space‐confined growth method at low temperature for near‐ultraviolet (NUV) PDs are reported. The single CsPbCl₃ MP PDs demonstrate a decent response to NUV light with a high on/off ratio of 5.6 × 10³ and a responsivity of 0.45 A W^?1 at 5 V. In addition, the dark current is as low as pA level, leading to detectivity up to 10¹¹ Jones. Moreover, PDs possess good stability and repeatability.     

Hybridizing Plasmonic Materials with 2D‐Transition Metal Dichalcogenides toward Functional Applications

Recently, 2D transition metal dichalcogenides (TMDs) have become intriguing materials in the versatile field of photonics and optoelectronics because of their strong light–matter interaction that stems from the atomic layer thickness, broadband optical response, controllable optoelectronic properties, and high nonlinearity, as well as compatibility. Nevertheless, the low optical cross‐section of 2D‐TMDs inhibits the light–matter interaction, resulting in lower quantum yield. Therefore, hybridizing the 2D‐TMDs with plasmonic nanomaterials has become one of the promising strategies to boost the optical absorption of thin 2D‐TMDs. The appeal of plasmonics is based on their capability to localize and enhance the electromagnetic field and increase the optical path length of light by scattering and injecting hot electrons to TMDs. In this regard, recent achievements with respect to hybridization of the plasmonic effect in 2D‐TMDs systems and its augmented optical and optoelectronic properties are reviewed. The phenomenon of plasmon‐enhanced interaction in 2D‐TMDs is briefly described and state‐of‐the‐art hybrid device applications are comprehensively discussed. Finally, an outlook on future applications of these hybrid devices is provided.     

Forward/Backward Switching of Plasmonic Wave Propagation Using Sign‐Reversal Coupling

Backward waves with wave‐front propagation opposite in direction to that of energy flow have attracted considerable interest in the context of photonic metamaterials. However, switching between forward and backward waves in the same frequency range has remained a challenge. Here, on a platform of coupled designer surface plasmon resonators in the microwave regime, multiband forward/backward switching of plasmonic wave propagation is demonstrated. This approach makes use of sign‐reversal coupling that occurs when switching the coupling configuration between tightly confined photonic modes. Direct experimental measurements of plasmon dispersion curves confirm the forward/backward propagation of plasmonic waves in the same frequency range. This study provides a solution to forward/backward switching of subwavelength plasmonic wave propagation, and may find potential applications in photonic integrated systems.     

Controlled Assembly of Highly Raman‐Enhancing Silver Nanocap Arrays Templated by Porous Anodic Alumina Membranes

A convenient nanoscale technique is reported for the fabrication of highly ordered hemispherical silver nanocap arrays templated by porous anodic alumina (PAA) membranes as robust and cost‐efficient surface‐enhanced Raman scattering (SERS) substrates. This geometry produces a high Raman signal due to its periodic hexagonal arrangements and control of the gap between the nanostructures in the sub‐10‐nm regime. The surface structure can be tuned further to optimize the enhancement factor according to optional PAA fabrication and silver deposition parameters. Finite‐difference time‐domain calculations indicate that the structure may possess excellent SERS characteristics due to the high density and abundance of hot spots.     

Highly-ordered, 3D petal-like array for surface-enhanced Raman scattering

Despite the great potential of the application of surface-enhanced Raman scattering (SERS), the difficulty in fabricating suitable SERS substrates is still a problem. Based on the self-assembly of silica nanoparticles, a simple method is here proposed to fabricate a highly-ordered, 3D, petal-like arrayed structure (3D PLAS) that serves as a promising SERS substrate for both its high reproducibility and enormous SERS enhancement. Such a novel structure is easily achieved by anisotropically etching a self-assembly bilayer of silica nanoparticles, followed by metal deposition. The SERS performance of the 3D PLAS and its relationship with the main parameters, including the etching time, the diameter of silica nanoparticles, and the deposited metal film, are characterized using 632.8 nm incident light. With Rhodamine 6G as a probe molecule, the spatially averaged SERS enhancement factor is on the order of 5 × 10(7) and the local enhancement factor is much higher, both of which can be improved further by optimizing the parameters.     

Ultrathin Molybdenum Dioxide Nanosheets as Uniform and Reusable Surface‐Enhanced Raman Spectroscopy Substrates with High Sensitivity

Metal oxides have advantages over the traditional noble metals to be used as substrate materials for surface‐enhanced Raman spectroscopy (SERS) with low cost, versatility, and biocompatibility, but their enhancement factors are generally quite low with a poor limit of detection. Here, ultrathin molybdenum dioxide (MoO₂) nanosheets synthesized by chemical vapor deposition demonstrated in large area are used as SERS substrates with superior signal uniformity in the whole area with a limit of detectable concentration down to 4 × 10^?8m and enhancement factor up to 2.1 × 10⁵, exceeding that of 2D materials and comparable to that of noble metal films. More practically important, the planar MoO₂ substrate is more robust than noble metals and shows excellent reusability and uniformity, which is usually prohibited for nanostructured or nanoparticle‐based metal oxide substrates. The enhancement is mainly attributed to the surface plasmon resonance effect as evidenced by the first principle calculations and UV–vis absorption spectroscopy characterization, which can be further increased by decreasing the thickness of the MoO₂ nanosheets. The overall superior performance makes the MoO₂ nanosheets an ideal substrate for practical SERS applications.     

Plasmonic coupling interference (PCI) nanoprobes for nucleic acid detection

A label-free approach using plasmonic coupling interference (PCI) nanoprobes for nucleic acid detection using surface-enhanced Raman scattering (SERS) is described. To induce a strong plasmonic coupling effect, a nanonetwork of silver nanoparticles with the Raman label located between adjacent nanoparticles is assembled by Raman-labeled DNA-locked nucleic acid (LNA) duplexes. The PCI method then utilizes specific nucleic acid sequences of interest as competitor elements for the Raman-labeled DNA strands to interfere the formation of nanonetworks in a competitive binding process. As a result, the plasmonic coupling effect induced through the formation of the nanonetworks is significantly diminished, resulting in a reduced SERS signal. The potential of the PCI technique for biomedical applications is illustrated by detecting single-nucleotide polymorphism (SNP) and microRNA sequences involved in breast cancers. The results of this study could lead to the development of nucleic acid diagnostic tools for biomedical diagnostics and biosensing applications using SERS detection.