Tunable Plasmon Resonance Research Articles

A Z-scheme photocatalyst for enhanced photocatalytic H2 evolution, constructed by growth of 2D plasmonic MoO3-x nanoplates onto 2D g-C3N4 nanosheets

Light-harvesting capacity and photoexcited charge carrier separation ability are two crucial requirements for high-efficiency semiconductor photocatalysis. Here, we report a plasmonic Z-scheme nanohybrid by hydrothermally in-situ growing two-dimensional (2D) oxygen-deficient molybdenum oxide (MoO3-x) nanoplates onto 2D graphitic carbon nitride (g-C3N4) nanosheets. The resultant 2D/2D MoO3-x/g-C3N4 nanohybrids not only construct a unique Z-scheme heterojunction, which improves the photogenerated charge carrier separation efficiency, but also possess numerous oxygen vacancies on the surface of MoO3-x, which could excite its plasmon resonance for extending spectrum adsorption. Importantly, the plasmon resonance can be readily designed by tailoring the oxygen vacancy concentration via an annealing in air. Benefiting from the synergetic effect of interfacial Z-scheme heterojunction and the tunable plasmon resonance of MoO3-x, the as-obtained nanohybrids achieve a remarkably improved photocatalytic H2 evolution efficiency. The optimal Z-scheme heterostructure presents 2.6 and 1.7 times higher of H2 evolution rate as compared to pure g-C3N4 and the annealing nanohybrid under visible light irradiation. Even under light irradiation with wavelength longer than 590 nm, the hybrid photocatalyst displays a H2 generation rate as high as 22.8 µmol h−1 due to the plasmonic sensitization effect. The result in our work can provide an alternative for fabricating Z-scheme heterostructures that take advantages of Z-scheme-induced charge carrier separation, accompanied with plasmon-enhanced light harvesting of semiconductor to advance the solar energy conversion efficiency in photocatalysis.

A controlled growth of triangular AuCu alloy nanostars and high photocatalytic activities of AuCu@CdS heterostars

Metallic nanostars have tunable plasmon resonances and high photocatalytic activities; however, controlling the number and length uniformity of branches for the colloidal nanostars is still a challenge.

Rationally designed dual-plasmonic gold nanorod@cuprous selenide hybrid heterostructures by regioselective overgrowth for in vivo photothermal tumor ablation in the second near-infrared biowindow.

NIR-II plasmonic materials offer multiple functionalities for in vivo biomedical applications, such as photothermal tumor ablation, surface-enhanced Raman scattering biosensing, photoacoustic imaging, and drug carriers. However, integration of noble metals and plasmonic semiconductors is greatly challenging because of the large lattice-mismatch. This study reports the regioselective overgrowth of Cu2-xSe on gold nanorods (GNRs) for preparation of dual-plasmonic GNR@Cu2-xSe hybrid heterostructures with tunable NIR-II plasmon resonance absorption for in vivo photothermal tumor ablation.Methods: The regioselective deposition of amorphous Se and its subsequent conversion into Cu2-xSe on the GNRs are performed by altering capping agents to produce the GNR@Cu2-xSe heterostructures of various morphologies. Their photothermal performances for NIR-II photothermal tumor ablation are evaluated both in vitro and in vivo.Results: We find that the lateral one- and two-side deposition, conformal core-shell coating and island growth of Cu2-xSe on the GNRs can be achieved using different capping agents. The Cu2-xSe domain size in these hybrids can be effectively adjusted by the SeO2 concentration, thereby tuning the NIR-II plasmon bands. A photothermal conversion efficiency up to 58-85% and superior photostability of these dual-plasmonic hybrids can be achieved under the NIR-II laser. Results also show that the photothermal conversion efficiency is dependent on the proportion of optical absorption converted into heat; however, the temperature rise is tightly related to the concentration of their constituents. The excellent NIR-II photothermal effect is further verified in the following in vitro and in vivo experiments.Conclusions: This study achieves one-side or two-side deposition, conformal core-shell coating, and island deposition of Cu2-xSe on GNRs for GNR@Cu2-xSe heterostructures with NIR-II plasmonic absorption, and further demonstrates their excellent NIR-II photothermal tumor ablation in vivo. This study provides a promising strategy for the rational design of NIR-II dual-plasmonic heterostructures and highlights their therapeutic in vivo potential.

Tunable Plasmonic Resonances in TiN Nanorod Arrays

In this work, titanium nitride (TiN) nanorod arrays were fabricated using glancing angle deposition in a magnetron sputtering system. The deposition parameters, including the bias on the substrate and the flow rate of nitrogen, were varied to deposit various TiN nanorod arrays. Before glancing angle deposition was conducted, uniform TiN films were deposited and their permittivity spectra, for various deposition parameters, were obtained. The effect of the deposition parameters on the morphology of the nanorods is analyzed here. The polarization-dependent extinctance spectra of TiN nanorod arrays were measured and compared. Extinction, which corresponds to the longitudinal mode of localized surface plasmon resonance, can be significantly changed by tuning the N2 flow rate and substrate bias voltage during deposition.

Hollow Porous Gold Nanoshells with Controlled Nanojunctions for Highly Tunable Plasmon Resonances and Intense Field Enhancements for Surface-Enhanced Raman Scattering.

Plasmonic metal nanostructures with nanogaps have attracted great interest owing to their controllable optical properties and intense electromagnetic fields that can be useful for a variety of applications, but precise and reliable control of nanogaps in three-dimensional nanostructures remains a great challenge. Here, we report the control of nanojunctions of hollow porous gold nanoshell (HPAuNS) structures by a facile oxygen plasma-etching process and the influence of changes in nanocrevices of the interparticle junction on the optical and sensing characteristics of HPAuNSs. We demonstrate a high tunability of the localized surface plasmon resonance (LSPR) peaks and surface-enhanced Raman scattering (SERS) detection of rhodamine 6G (R6G) using HPAuNS structures with different nanojunctions by varying the degree of gold sintering. As the neck region of the nanojunction is further sintered, the main LSPR peak shifts from 785 to 1350 nm with broadening because the charge transfer plasmon mode becomes more dominant than the dipolar plasmon mode, resulting from the increase of conductance at the interparticle junctions. In addition, it is demonstrated that an increase in the sharpness of the nanojunction neck can enhance the SERS enhancement factor of the HPAuNS by up to 4.8-fold. This enhancement can be ascribed to the more intense local electromagnetic fields at the sharper nanocrevices of interparticle junctions. The delicate change of nanojunction structures in HPAuNSs can significantly affect their optical spectrum and electromagnetic field intensity, which are critical for their practical use in a SERS-based analytical sensor as well as multiple-wavelength compatible applications.

Tailoring thermo-optical properties of eosin B dye using surfactant-free gold-silver alloy nanoparticles.

Surfactant free gold, silver and gold-silver alloy nanoparticles were synthesized using a laser mediated method for localized surface plasmon resonance tuning. The effect of these nanoparticles on the thermo-optical properties of eosin B dye was investigated. Dual beam mode matched thermal lens method was implemented to evaluate the thermal diffusivity of the eosin B with gold, silver and gold-silver alloy nanoparticles. Concentration and composition dependant changes in thermo-optical properties of the eosin B-nanoparticle systems were quantified. As the concentration of nanoparticles incorporated into the dye solution increased, the thermal diffusivity and fluorescence emission intensity of the samples were found to be decreased. At the same time an enhancement of the thermal lens signal was observed with the introduction of nanoparticles into the system. Further enhancement in signal and reduction in thermal diffusivity and fluorescence intensity can be obtained with fine tuning of surface plasmon resonance wavelength by gold, silver and gold-silver alloy nanoparticles.

Comparative Investigation of Plasmonic Properties between Tunable Nanoobjects and Metallized Nanoprobes for Optical Spectroscopy

In order to evaluate the optical efficiency of tip-based probes for future tip-enhanced optical spectroscopy applications, we developed an experimental setup based on the coupling of an achromatic inverted microscope equipped with a total internal reflection objective and an atomic force microscopy (AFM) head. This spectroscopic tool has been validated using individual nanofabricated antennas (gold nanodisks/nanocones) on a glass substrate which act as nanoresonators based on localized surface plasmons. Spectrally tunable transverse electric and magnetic plasmonic resonances are identified and are in excellent agreement with numerical calculations performed as a function of the nanoantenna geometry and size. We investigated a series of state-of-the-art gold-coated AFM probes, which are commonly used for tip-enhanced (Raman spectroscopy) optical experiments. Their scattering spectrum consists of resonances depending on the tip sharpness or granularity superimposed on a broad emission spectrum due to a semi-infinite metal layer acting as a nonresonant antenna. From the comparison between the plasmonic response of both types of optical antennas, a new generation of probes for tip-enhanced optical spectroscopy is proposed in which single plasmonic nanoantennas are engineered at the apex of a nonmetallic AFM tip. As from numerical simulation results, such tips would ensure a spectral tunability as a function of the material, size, and geometry, together with expected high enhancement factors. Such features would allow the design of spectrally tunable surface-enhanced Raman spectroscopy substrates and should be a reliable and efficient alternative to tips commonly used in tip-enhanced optical spectroscopy experiments such as tip-enhanced Raman spectroscopy.

Graphene Antidot Terahertz Plasmonic Metasurfaces Employing Self-Aligned Metal Cores for Sensing Applications

Surface plasmons in graphene have great potential for molecular sensing applications thanks to their exceedingly high sensitivity to environmental changes. Here, we demonstrate a type of hybrid graphene–metal metasurface that supports strong graphene plasmonic resonances in the terahertz range. Each unit cell of such a hybrid metasurface consists of a graphene antidot enclosing a metal disk realized using a self-aligned photolithography process. This hybrid design combines the advantages of both graphene- and metal-based photonic structures, leading to ∼3 times stronger tunable plasmonic resonances and an order of magnitude larger near-field intensity enhancement with respect to those of bare graphene antidot metasurfaces.

Ultrahighly Enhanced Performance of Single Cadmium Selenide Nanobelt by Plasmonic Gold Particles

Noble metal nanoparticles have been demonstrated by a huge application prospect for photodetector (PD) due to their unique and tunable optical properties. Herein, a simple strategy is presented by a combination of gold nanoparticles (Au NPs) and broadband photoresponse cadmium selenide nanobelts (CdSe NBs) to get ultrasensitive, broadband photoresponse (300–720 nm) and tunable photoresponse PD. Concretely, the Au NPs are fabricated on CdSe NBs via ion sputtering and annealing, and the morphology of Au NPs is systematically adjusted by simply tuning the sputter time from 60 to 140 s. Compared with the pure CdSe NB PD, the Au NPs hybrid CdSe NB PD exhibits a high responsivity, especially in the range of 525–575 nm with low light intensity (enhancement of ≈2894% at 550 nm with 79.6 μW cm−2). The response time of the hybrid PD is decreased substantially from 0.8 to 0.2 ms. More importantly, the hybrid PD possesses a tunable absorption in the range of 538–580 nm, which is benefited from tunable plasmon resonance of Au NPs. These results are confirmed by the theoretical simulation. It is believed that this strategy offers new opportunities to design ultrasensitive, broadband spectra, and tunable wavelength PD.

Tuning of localized plasmon resonance in colloidal gold nano-particles by ultrafast interband photoinjection of free carriers: Superplasmonic states?

Colloidal solutions of gold nanoparticles in water were pumped by filamentary 800-nm, 100-fs laser pulses at variable supercritical peak powers and probed by the accompanying ultrashort white-light supercontinuum pulses. Their broadband (UV-near-IR) spectra revealed the localized plasmon resonance blue-shifting, broadening, and intensity reduction vs the incident peak laser power as a result of the increasing concentration of s-band free carriers additionally photoinjected from high-density d-bands.

Dynamic Tuning of Gap Plasmon Resonances Using a Solid-State Electrochromic Device.

Plasmonic antennas and metasurfaces can effectively control light-matter interactions, and this facilitates a deterministic design of optical materials properties, including structural color. However, these optical properties are generally fixed after synthesis and fabrication, while many modern-day optics applications require active, low-power, and nonvolatile tuning. These needs have spurred broad research activities aimed at identifying materials and resonant structures capable of achieving large, dynamic changes in optical properties, especially in the challenging visible spectral range. In this work, we demonstrate dynamic tuning of polarization-dependent gap plasmon resonators that contain the electrochromic oxide WO3. Its refractive index in the visible changes continuously from n = 2.1 to 1.9 upon electrochemical lithium insertion and removal in a solid-state device. By incorporating WO3 into a gap plasmon resonator, the resonant wavelength can be shifted continuously and reversibly by up to 58 nm with less than 2 V electrochemical bias voltage. The resonator can remain in a tuned state for tens of minutes under open circuit conditions.

Hybrid magnetic plasmon resonance induced tunable half-wave plate based on graphene-dielectric-metal structure

We propose a terahertz half-wave plate composed of a periodic array of rectangular-shaped metal-dielectric-graphene (RMDG) sandwich structure on a dielectric substrate supported by a thick metallic film. The role of graphene sheet is to combine with the rectangular-shaped metal to provide tunable anisotropic hybrid magnetic plasmon resonance. The results reveal that about 90% of energy of the incident linearly polarized light at a certain frequency is converted to the perpendicular polarization direction after reflection from the wave plate. The high energy polarization conversion efficiency is attributable to the fact that the electric vector of the hybrid magnetic plasmon resonance in RMDG is mainly perpendicular to the graphene sheet and the absorption loss of graphene is suppressed. The half-wave plate we demonstrate here, consisting of nanostructured metal and non-structured graphene, utilizes mature metal nanostructure preparation process and avoids the graphene processing, which consequently facilitates the fabrication and promotes the application of half-wave plate.

High-Responsivity MSM Solar-Blind UV Photodetector Based on Annealed ITO/Ag/ITO Structure Using RF Sputtering

In this paper, a novel ITO/Ag/ITO (IAI) multilayer UV photodetector was successfully prepared via RF magnetron sputtering technique to achieve the dual-benefit of improved photoresponse and outstanding visible blindness properties. The influence of the annealing process on the performance of the elaborated IAI UV photodetector was carried out. Moreover, the structural and photoelectrical properties of the sputtered tri-layered structures with and without annealing effects were also analyzed. It was found that the elaborated UV -photodetector provides near perfect UV absorbance of 96%. This achievement is attributed to the improved light trapping capability and tunable plasmonic resonance effects induced by the efficient optical coupling between free-space UV photons and Ag plasmons. It was also revealed that the heat treatment paves a new path toward preparing efficient visible-blind UV photodetectors, where the annealed sample demonstrated a superior rejection ratio of 225 as compared to that of the unannealed counterpart. Besides, electrical characterization underlined the ability of the annealed IAI UV photodetector at 500°C for offering exciting opportunities, where it exhibits a high responsivity of 3.8mA/W at self-powered condition. Therefore, by well optimizing both IAI multilayer geometry and annealing conditions, we were able to elaborate an ultrathin photodetector suitable for high-performance and flexible UV sensing applications.

Morphology-Tailored Gold Nanoraspberries Based on Seed-Mediated Space-Confined Self-Assembly.

Raspberry-like structure, providing a high degree of symmetry and strong interparticle coupling, has received extensive attention from the community of functional material synthesis. Such structure constructed in the nanoscale using gold nanoparticles has broad applicability due to its tunable collective plasmon resonances, while the synthetic process with precise control of the morphology is critical in realizing its target functions. Here, we demonstrate a synthetic strategy of seed-mediated space-confined self-assembly using the virus-like silica (V-SiO2) nanoparticles as the templates, which can yield gold nanoraspberries (AuNRbs) with uniform size and controllable morphology. The spikes on V-SiO2 templates serve dual functions of providing more growth sites for gold nanoseeds and activating the space-confined effect for gold nanoparticles. AuNRbs with wide-range tunability of plasmon resonances from the visible to near infrared (NIR) region have been successfully synthesized, and how their geometric configurations affect their optical properties is thoroughly discussed. The close-packed AuNRbs have also demonstrated huge potential in Raman sensing due to their abundant “built-in” hotspots. This strategy offers a new route towards synthesizing high-quality AuNRbs with the capability of engineering the morphology to achieve target functions, which is highly desirable for a large number of applications.

Nonlinear Graphene Nanoplasmonics.

Nonlinear optics is limited by the weak nonlinear response of available materials, a problem that is generally circumvented by relying on macroscopic structures in which light propagates over many optical cycles, thus giving rise to accumulated unity-order nonlinear effects. While this strategy cannot be extended to subwavelength optics, such as in nanophotonic structures, one can alternatively use localized optical resonances with high quality factors to increase light-matter interaction times, although this approach is limited by inelastic losses partly associated with the nonlinear response. Plasmons-the collective oscillations of electrons in conducting media-offer the means to concentrate light into nanometric volumes, well below the light-wavelength-scale limit imposed by diffraction, amplifying the electromagnetic fields upon which nonlinear optical phenomena depend. Due to their abundant supply of free electrons, noble metals are the traditional material platform for plasmonics and have thus dominated research in nanophotonics over the past several decades, despite exhibiting large ohmic losses and inherent difficulties to actively modulate plasmon resonances, which are primarily determined by size, composition, and morphology. Highly doped graphene has recently emerged as an appealing platform for plasmonics due to its unique optoelectronic properties, which give rise to relatively long-lived, highly confined, and actively tunable plasmon resonances that mainly appear in the infrared and terahertz frequency regimes. Efforts to extend graphene plasmonics to the near-infrared and visible ranges involve patterning of graphene into nanostructured elements, thus facilitating the optical excitation of localized resonances that can be blue-shifted through geometrical confinement while maintaining electrical tunability. Besides these appealing plasmonic attributes, the conical electronic dispersion relation of graphene renders its charge carrier motion in response to light intrinsically anharmonic, resulting in a comparatively intense nonlinear optical response. The combined synergy of extreme plasmonic field enhancement and large intrinsic optical nonlinearity are now motivating intensive research efforts in nonlinear graphene plasmonics, the recent progress of which we discuss in this Account. We start with a description of the appealing properties of plasmons in graphene nanostructures down to molecular sizes, followed by a discussion of the unprecedented level of intrinsic optical nonlinearity in graphene, its enhancement by resonant coupling to its highly confined plasmons to yield intense high harmonic generation and Kerr nonlinearities, the extraordinary thermo-optical capabilities of this material enabling large nonlinear optical switching down to the single-photon level, and its strong interaction with quantum emitters.

Graphene-covered sandwich nanostructure for enhanced light absorption

At present, the combination of graphene and metal nanostructures is explored for strong light–matter interaction. In this study, the optical properties of a composite nanostructure are studied experimentally by simply coating a monolayer graphene on a sandwich nanostructure (Ag nanoparticles–SiO2 film–Ag nanoparticles) fabricated via oblique angle deposition. Results indicate that graphene coverage leads to enhanced light absorption in the visible range. The simulation results show that the interaction between graphene and nanoparticles plasmons increase the light absorption originating from the strong localized electric field of graphene. In addition, the light–graphene interaction can be tuned via plasmon resonance by changing the size of nanoparticles and the thickness of the SiO2 film. The simplicity of the fabrication process and the tunability of plasmon resonance makes the application of graphene-covered nanostructures in diverse areas promising.

Shape-engineered silver nanocones for refractive index plasmonic nanosensors.

Silver nanocones with tunable plasmon resonances and high refractive index (RI) sensitivity have attracted much attention. Herein, through systematic measuring of the RI sensitivities of silver nanocones with different geometric parameters, the size and shape effects are investigated. The results show that RI sensitivities increase as silver nanocones become longer and the widths of their heads become smaller. Through engineering of the outline symmetry, the silver nanocones exhibit RI sensitivity as high as 910 nm/RIU (RI unit) and the figure of merit arrives at 3.8.

Localized surface plasmon resonance tunability of VO2/Au/VO2 composite nanotriangle sandwich array

A vanadium dioxide (VO2) and Gold nanotriangle composite sandwich structure (VO2/Au/VO2) was fabricated via nanosphere lithography technique. It exhibits reversible localized surface plasmon resonance tunability through external thermal stimuli VO2 phase transition by tuning the near-field permittivity of Au nanotriangle. Compared optical response to the Au/VO2 (bottom Au top VO2) and VO2/Au (bottomVO2 top Au) composite nanotriangle arrays, VO2/Au/VO2 structure shows a largest wavelength shift of 42 nm and a largest transmittance modulation of 13.2% between the hot-metallic and cold-insulating states. And the wavelength shift of Au/VO2 nanotriangle array is larger than that of VO2/Au array for the higher electric intensity on top surface of Au nanotriangle. The results suggest that this sandwich architecture is one path for designing a plasmonic structure with tunable resonance responses that can be tailored by varying the top and bottom phase materials properties of the hybrid structures.

Low-Temperature Photocatalytic Hydrogen Addition to Two-Dimensional MoO3 Nanoflakes from Isopropyl Alcohol for Enhancing Solar Energy Harvesting and Conversion

Recently, research has been focused on the plasmon resonances in semiconductors (e.g., metal oxides) with tunable resonance absorption covering visible, near-infrared, and mid-infrared regions to enhance the efficiency in the utilization of solar energy. Two-dimensional (2D) molybdenum trioxide (MoO3) nanoflakes were studied to form substoichiometric oxides with strong tunable plasmon resonance via photocatalytic hydrogen addition. Despite the classic concept that the hydrogen addition to MoO3 from pure alcohols requires elevated temperatures more than 200 °C, we report a low-temperature hydrogen addition reaction of 2D MoO3 nanoflakes with pure isopropyl alcohol under visible light irradiation. The nanoflakes were produced by a facile liquid exfoliation of the MoO3 whiskers from chemical vapor deposition. Using alcohol–water solutions at different mixing ratios, the exfoliation process can tune the dimensions of the nanoflakes, as well as their defect structures. For the photocatalytic hydrogen addition ...

Dynamic Plasmon Resonance Tuning for Surface Enhanced Sensing.

We report on fabricating plasmonic nanorod crystals using focused ion beam lithography. We first demonstrate manipulating the profiles of nanorods perpendicularly aligned with the substrate. Then we show accurate control of nanorod outlines can be achieved. We also show that it is feasible to manufacture nanorods obliquely aligned with the substrate. Tunable plasmon resonance can be realized with different tilting angles and geometries. Our approach may find important applications in plasmon-assisted sensing and surface enhanced spectroscopy.