Plasmonic nanostructures exhibit intriguing optical properties due to spectrally selective plasmon resonance and thus have broad applications, including biochemical sensing and photoelectric detections. However, excited plasmons are often strongly influenced by the substrates supporting the metallic nanostructures, which not only weakens the intrinsic plasmon coupling effect, but also results in a great reduction of optical near‐field enhancement. Here, a plasmonic nanostructure combining collapsible Au‐nanofingers with selective‐etching that enables Au to be suspended is demonstrated, thus avoiding the undesirable influence of the substrates on the local near‐field distribution and forming symmetric electromagnetic‐field enhancements at both the top and bottom surfaces. The polymer support of the Au‐nanofingers is selectively etched by oxygen plasma, while the Au‐cap retains its original size. After an ultrathin dielectric coating is applied on the Au‐nanofingers, suspended Au‐caps with extremely small dielectric gaps are formed via the collapse of neighboring Au‐nanofingers by exposing them to ethanol. These nanostructures can provide a surface‐enhanced Raman scattering (SERS) enhancement of up to ≈109, which is nearly twice that in the nonsuspended system. As a highly active SERS substrate, the label‐free detection of low‐concentration harmful plastic phthalates in a child's urine without any pretreatment is successfully demonstrated, which suggests that this method is suitable for medical prediagnosis.

Aluminum (Al) can actively support plasmonic response in the ultraviolet (UV) range compared to noble metals (e.g., Au, Ag) and thus has broad applications including UV sensing, displays, and photovoltaics. High-quality Al films with no oxidation are essential and critical in these applications. However, Al is very prone to fast oxidation in air, which critically depends on the fabrication process. Here, we report that by leveraging the in situ sputter etching and sputter deposition of a 1 nm tetrahedral amorphous carbon (ta-C) film on the Al nanostructures, Al plasmonic activity can be improved. The prior sputter etching process greatly reduces the oxidized layer of the Al films, and the subsequent sputter deposition of ta-C keeps Al oxidation-free. The ta-C film outperforms the naturally passivated Al2O3 layer on the Al film because the ta-C film has a denser structure, higher permittivity, and better biocompatibility. Therefore, it can effectively improve the plasmonic response of Al and be beneficial to molecule sensing, which is proved in our experiments and is also verified in simulations. Our results can enable the various applications based on plasmon resonance in the UV range.

The transition metal dichalcogenides (TMDs) have drawn great research attention, motivated by the derived remarkable optoelectronic properties and the potentials for high-efficient excitonic devices. The plasmonic nanocavity, integrating deep-sub wavelength confinement of optical mode with dramatic localized field enhancement, provides a practical platform to manipulate light–matter interaction. In order to obtain strong exciton–plasmon coupling effects, it’s crucial to match the vibration direction of exciton to the available strong localized in-plane electric field. Herein, we demonstrate the coupling effect of in-plane exciton in monolayer tungsten diselenide (WSe2) to deterministic gap-plasmon field which is produced by nanometrically gapped collapsed nanofingers. The gap-plasmon field which is completely parallel to the in-plane excitons in WSe2 will drive a strong exciton–plasmon coupling at room temperature. More interestingly, it is experimentally observed that the luminescence of exciton–polariton cannot be influenced by the temperature in the range from 77 K to 300 K due to the presence of nanofingers. According to the theoretical analysis results, we attribute this finding to the dielectric screening effect arising from the extremely strong localized electric field of plasmonic nanofingers. This work proposes a feasible way to harness and manipulate the exciton of low-dimensional semiconductor, which might be potential for quantum optoelectronics.

Surface-enhanced Raman spectroscopy (SERS) is known as a powerful technique to provide label-free analyses for chemical sensing. While the sensitivity of SERS is heavily dependent on the prepared SERS functional substrates, finding appropriate substrates is critical to achieve sufficient signal enhancement. Meanwhile, the capture of targets to such effective enhancing region becomes challenging especially for low concentration. Here, we report a coupled Au/D-NAs/Au gap plasmon platform that provides remarkable SERS enhancement and sensitivity for mercury ion sensing. Through nanoimprint lithography, large area of Au nanofingers can be fabricated with excellent uniformity and flexibility. These Au nanofingers can be further modified by DNA aptamers to construct Au/DNAs/Au gap plasmon nanostructures with well-defined gap sizes. Due to the subnanometer gap size, strong interaction between collapsed Au nanofingers can be induced to create extremely enhanced near-field with nanoscale mode volume. When Hg 2+ ions exist around the platform, they can specially bind to thymine (T) bases of DNA and form T-Hg 2+ -T pairs between adjacent single strand DNAs. As a result, DNAs that initially lie on the Au surface are forced to stand in rigid duplex-structures. This morphology change can be directly indicated by the ratio between adenine (A) and guanine (G) SERS signals to reveal the Hg 2+ concentration. Given the strong field enhancement as well as the close distance between DNA and hotspot, ultralow Hg 2+ concentration down to 10 À9 M is successfully detected. This work demonstrates a SERS platform with high selectivity and sensitivity, which exhibits significant potential for molecule sensing applications.