Opportunities and Challenges for Alternative Nanoplasmonic Metals: Magnesium and Beyond

Alternative Plasmonic Materials for Fluorescence Enhancement

Noble metals such as gold and silver have been used extensively for a range of plasmonic applications, including enhancing the fluorescence rate of a dye molecule, as evidenced by numerous experiments over the past two decades. Recently, a variety of doped semiconductors have been proposed as alternative plasmonic materials, exhibiting plasmonic resonances from ultraviolet to far-infrared. In this work, we investigate the suitability of these alternative materials for enhancing the fluorescence of a molecule. Considering nanosized spheres, we study their response under plane wave illumination and the resulting enhancement factors when coupled to a quantum emitter. Comparisons with standard plasmonic metals reveal that semiconductor materials lead to a significantly reduced, and often strongly quenched, emission of light caused by their dominant absorption, which hinders fluorescence enhancement. However, we show that enhancement may be obtained when considering poor emitting dyes and high refractive index environments. Our findings demonstrate that these alternative materials result in weaker fluorescence enhancement compared to their plasmonic counterparts. Nonetheless, there are means to compensate for this, and a reasonable enhancement can be achieved for dyes in the infrared spectrum.

Real-time monitoring of 2,5-dimethylpyrazine in solid, liquid, and gas phases during processing of fried skipjack tuna steaks using intestinal villus-like Ag nanoparticles@Au nanowires surface-enhanced Raman substrate chips

The production and distribution of 2,5-dimethylpyrazine (2,5-DMP), a key volatile flavor compound, are associated with the frying of skipjack tuna steaks. In this study, an intestinal villus-like chip was fabricated to quantitatively detect 2,5-DMP in multiphase systems based on surface-enhanced Raman spectroscopy (SERS). The chip exhibited excellent SERS performance with an enhancement factor of 1.16 × 108, excellent uniformity and reproducibility, and low detection limits of 6.49 pg/mL, 0.182, and 0.920 ng/mL for 2,5-DMP solid, liquid, and gas models, respectively. The results indicated the 2,5-DMP content with growth rates in the order of frying steam > frying steaks > frying oil, and the 2,5-DMP content in frying steam was linearly correlated (R2 = 0.992) with the trend of the acid value and total polar compounds in frying oil. Therefore, this strategy for achieving the reliable detection of 2,5-DMP can assist in monitoring the frying process and food safety.

Magnesium Nanoparticles for Surface-Enhanced Raman Scattering and Plasmon-Driven Catalysis

Nanostructures of some metals can sustain localized surface plasmon resonances, collective oscillations of free electrons excited by incident light. This effect results in wavelength-dependent absorption and scattering, enhancement of the incident electric field at the metal surface, and generation of hot carriers as a decay product. The enhanced electric field can be utilized to amplify the spectroscopic signal in surface-enhanced Raman scattering (SERS), while hot carriers can be exploited for catalytic applications. In recent years, cheaper and more earth abundant alternatives to traditional plasmonic Au and Ag have gained growing attention. Here, we demonstrate the ability of plasmonic Mg nanoparticles to enhance Raman scattering and drive chemical transformations upon laser irradiation. The plasmonic properties of Mg nanoparticles are characterized at the bulk and single particle level by optical spectroscopy and scanning transmission electron microscopy coupled with electron energy-loss spectroscopy and supported by numerical simulations. SERS enhancement factors of ∼102 at 532 and 633 nm are obtained using 4-mercaptobenzoic acid and 4-nitrobenzenethiol. Furthermore, the reductive coupling of 4-nitrobenzenethiol to 4,4'-dimercaptoazobenzene is observed on the surface of Mg nanoparticles under 532 nm excitation in the absence of reducing agents, indicating a plasmon-driven catalytic process. Once decorated with Pd, Mg nanostructures display an enhancement factor of 103 along with an increase in the rate of catalytic coupling. The results of this study demonstrate the successful application of plasmonic Mg nanoparticles in sensing and plasmon-enhanced catalysis.

Stability of Plasmonic Mg-MgO Core-Shell Nanoparticles in Gas-Phase Oxidative Environments

Magnesium is a recent addition to the plasmonic toolbox: nanomaterials that efficiently utilize photons' energy due to their ability to sustain localized surface plasmon resonances. Magnesium nanoparticles protected by a native oxide shell can efficiently absorb light across the solar spectrum, making them a promising photocatalytic material. However, their inherent reactivity toward oxidation may limit the number of reactions in which Mg-MgO can be used. Here, we investigate the stability of plasmonic Mg-MgO core-shell nanoplates under oxidative conditions. We demonstrate that the MgO shell stabilizes the metallic Mg core against oxidation in air at up to 400 °C. Furthermore, we show that the reactivity of Mg-MgO nanoplates with water vapor (3.5 vol % in N2) decreases with temperature, with no oxidation of the Mg core detected from 200 to 400 °C. This work unravels the potential of Mg-MgO nanoparticles for a broad range of catalytic transformations occurring in oxidative environments.

Far-field, near-field and photothermal response of plasmonic twinned magnesium nanostructures

Magnesium nanoparticles offer an alternative plasmonic platform capable of resonances across the ultraviolet, visible and near-infrared. Crystalline magnesium nanoparticles display twinning on the (101̄1), (101̄2), (101̄3), and (112̄1) planes leading to concave folded shapes named tents, chairs, tacos, and kites, respectively. We use the Wulff-based Crystal Creator tool to expand the range of Mg crystal shapes with twinning over the known Mg twin planes, i.e., (101̄x), x = 1, 2, 3 and (112̄y), y = 1, 2, 3, 4, and study the effects of relative facet expression on the resulting shapes. These shapes include both concave and convex structures, some of which have been experimentally observed. The resonant modes, far-field, and near-field optical responses of these unusual plasmonic shapes as well as their photothermal behaviour are reported, revealing the effects of folding angle and in-filling of the concave region. Significant differences exist between shapes, in particular regarding the maximum and average electric field enhancement. A maximum field enhancement (|E|/|E0|) of 184, comparable to that calculated for Au and Ag nanoparticles, was found at the tips of the (112̄4) kite. The presence of a 5 nm MgO shell is found to decrease the near-field enhancement by 67% to 90% depending on the shape, while it can increase the plasmon-induced temperature rise by up to 42%. Tip rounding on the otherwise sharp nanoparticle corners also significantly affects the maximum field enhancement. These results provide guidance for the design of enhancing and photothermal substrates for a variety of plasmonic applications across a wide spectral range.

Bimetallic copper palladium nanorods: plasmonic properties and palladium content effects

Cu is an inexpensive alternative plasmonic metal with optical behaviour comparable to Au but with much poorer environmental stability. Alloying with a more stable metal can improve stability and add functionality, with potential effects on the plasmonic properties. Here we investigate the plasmonic behaviour of Cu nanorods and Cu-CuPd nanorods containing up to 46 mass percent Pd. Monochromated scanning transmission electron microscopy electron energy-loss spectroscopy first reveals the strong length dependence of multiple plasmonic modes in Cu nanorods, where the plasmon peaks redshift and narrow with increasing length. Next, we observe an increased damping (and increased linewidth) with increasing Pd content, accompanied by minimal frequency shift. These results are corroborated by and expanded upon with numerical simulations using the electron-driven discrete dipole approximation. This study indicates that adding Pd to nanostructures of Cu is a promising method to expand the scope of their plasmonic applications.

Beyond the Visible: A Review of Ultraviolet Surface-Enhanced Raman Scattering Substrate Compositions, Morphologies, and Performance

The first observation of ultraviolet surface-enhanced Raman scattering (UV-SERS) was 20 years ago, yet the field has seen a slower development pace than its visible and near-infrared counterparts. UV excitation for SERS offers many potential advantages. These advantages include increased scattering intensity, higher spatial resolution, resonance Raman enhancement from organic, biological, and semiconductor analytes, probing UV photoluminescence, and mitigating visible photoluminescence from analytes or substrates. One of the main challenges is the lack of readily accessible, effective, and reproducible UV-SERS substrates, with few commercial sources available. In this review, we evaluate the reported UV-SERS substrates in terms of their elemental composition, substrate morphology, and performance. We assess the best-performing substrates with regard to their enhancement factors and limits of detection in both the ultraviolet and deep ultraviolet regions. Even though aluminum nanostructures were the most reported and best-performing substrates, we also highlighted some unique UV-SERS composition and morphology substrate combinations. We address the challenges and potential opportunities in the field of UV-SERS, especially in relation to the development of commercially available, cost-effective substrates. Lastly, we discuss potential application areas for UV-SERS, including cost-effective detection of environmentally and militarily relevant analytes, in situ and operando experimentation, defect engineering, development of materials for extreme environments, and biosensing.

Scalable Superabsorbers and Color Filters Based on Earth-Abundant Materials

Optical materials based on unconventional plasmonic metals (e.g., magnesium) have lately driven rising research interest for the quest of possibilities in nanophotonic applications. Several favorable attributes of Mg, such as earth abundancy, lightweight, biocompatibility/biodegradability, and its active reactions with water or hydrogen, have underpinned its emergence as an alternative nanophotonic material. Here, we experimentally demonstrate a thin film-based optical device composed exclusively of earth-abundant and complementary metal-oxide semiconductor (CMOS)-compatible materials (i.e., Mg, a-Si, and SiO₂). The devices can exhibit a spectrally selective and tunable near-unity resonant absorption with an ultrathin a-Si absorbing layer due to the strong interference effect in this high-index and lossy film. Alternatively, they can generate diverse reflective colors by appropriate tuning of the a-Si and SiO₂ layer thicknesses, including all the primary colors for RGB (red, green, blue) and CMY (cyan, magenta, yellow) color spaces. In addition, the reflective hues of the devices can be notably altered in a zero power-consumption fashion by immersing them in water due to the resulted dissolution of the Mg back-reflection layer. These compelling features in combination with the lithography-free and scalable fabrication steps may promise their adoption in various photonic applications including solar energy harvesting, optical information security, optical modulation, and filtering as well as structure reuse and recycling.

Tip-Enhanced Raman Imaging of Plasmon-Driven Coupling of 4-Nitrobenzenethiol on Au-Decorated Magnesium Nanostructures

Magnesium nanoparticles (MgNPs) exhibit localized surface plasmon resonances across the ultraviolet, visible, and near-infrared parts of electromagnetic spectrum and are attracting increasing interest due to their sustainability and biocompatibility. In this study, we used tip-enhanced Raman spectroscopy (TERS) to examine the photocatalytic properties of MgNP protected by a thin native oxide layer and their Au-modified bimetallic analogs produced by partial galvanic replacement, Au-MgNPs. We found no reduction of 4-nitrobenzenethiol (4-NBT) to p,p'-dimercaptoazobisbenzene (DMAB) when a Au-coated tip was placed in contact with a self-assembled monolayer of 4-NBT molecules adsorbed on MgNPs alone. However, decorating Mg with Au made these bimetallic structures catalytically active. The DMAB signal signature of photocatalytic activity was more delocalized around AuNPs attached to Mg than around AuNPs on a Si substrate, indicating coupling between the Mg core and Au decorations. This report on photocatalytic activity of a bimetallic structure including plasmonic Mg paves the way for further catalyst architectures benefiting from Mg's versatility and abundance.

Plasmonic magnesium nanoparticles decorated with palladium catalyze thermal and light-driven hydrogenation of acetylene

Bimetallic Pd-Mg nanoparticles were synthesized by partial galvanic replacement of plasmonic Mg nanoparticles, and their catalytic and photocatalytic properties in selective hydrogenation of acetylene have been investigated. Electron probe studies confirm that the Mg-Pd structures mainly consist of metallic Mg and sustain several localized plasmon resonances across a broad wavelength range. We demonstrate that, even without light excitation, the Pd-Mg nanostructures exhibit an excellent catalytic activity with selectivity to ethylene of 55% at 100% acetylene conversion achieved at 60 °C. With laser excitation at room temperature over a range of intensities and wavelengths, the initial reaction rate increased up to 40 times with respect to dark conditions and a 2-fold decrease of the apparent activation energy was observed. A significant wavelength-dependent change in hydrogenation kinetics strongly supports a catalytic behavior affected by plasmon excitation. This report of coupling between Mg's plasmonic and Pd's catalytic properties paves the way for sustainable catalytic structures for challenging, industrially relevant selective hydrogenation processes.