Fabrication of Si/Au Core/Shell Nanoplasmonic Structures with Ultrasensitive Surface-Enhanced Raman Scattering for Monolayer Molecule Detection

Abstract

Nanoparticles of noble metals are typically plasmonic materials, but the pure metals have high non-irradiative ohmic losses at optical frequencies, leading to large absorption and unwanted heating effects. Additionally, spherical silicon nanoparticles (NPs) have a unique optical system with a high-refractive-index dielectric nanostructure. Combining these two components in one functional core/shell NP nanostructure of Si/M, where M represents a noble metal, to produce resonant optical electronic and magnetic responses has been rare. Herein, an approach for the assembly of Si/M core/shell NPs is developed based upon double-beam laser ablation in liquid, thereby fabricating a series of Si/M (M=Au, Ag, Pd and Pt) core/shell NPs and characterizing the intense visible-light scattering modes from the as-fabricated Si/Au NPs as a nanoplasmonic structure. Dark-field optical measurements show that these Si/Au NPs have a strong electromagnetic response in the visible-light region, and the resonant frequencies can be modulated by NP size and morphology. The dispersed NPs are used as a highly-sensitive surface-enhanced Raman scattering (SERS) active probe for detecting monolayer molecules, as proven herein using 4-MBT molecules. A SERS plasmonic enhancement factor of ~10 8 is found for these Si/Au NPs by correlating the SERS measurement with scanning electron microscopy analysis. These findings present the possibility of confining light in ubiquitous silicon-based semiconductor technologies and manipulating the optical properties of nontraditional plasmonic nanostructures, while also opening up new perspectives.This technique can be extended to other noble metals to form similar structures and fabricate low-loss metamaterials and nanophotonic devices.

One-step femtosecond laser ablation synthesis of sub-3 nm gold nanoparticles stabilized by silica

“…Due to the significantly different atomic numbers between gold and silicon, it is possible to visually identify the two elements: the higher atomic number element has higher contrast (gold = white) than the lower atomic number element (silicon = grey) [47]. The EDX spectrum in Fig.…”

“…Due to its proximity to the neutral Si 0 species, the peak at 98.0 eV is most likely in the form of Si 1− , since the Si 1+ is shifted up by ∼1 eV from the Si 0 peak [69][70][71]. The presence of this putative Si 1− species is surprising due to the electropositive nature of Si, but the accessibility of metastable phases in the nonequilibrium environment formed at the plasma-water interface [3,14,47,72] could enable the formation of such exotic species. The large peak centered around 103.5 eV in the silica-Au NPs was deconvoluted into two peaks, which are likely two types of silica species with different electronic structures [68,73], or silicon species with different oxide layer thicknesses [69,74].…”

“…Recently, the approaches of PLAL and laser photochemical reduction have been combined into a single process through ablation of a solid target either immersed in or subsequently exposed to metal salt solution, in what is termed reactive laser ablation in liquid (RLAL) [10,[45][46][47][48][49][50][51][52]. RLAL has been widely used to generate metal-silica nanostructures, including Si/M (M = Ag, Au, Pd, Pt) core/shell NPs [47], Ag-core/silica-shell NPs [48], and large silica particles decorated with Au or Ag NPs [49]. While these RLAL reports have demonstrated the generation of hybrid semiconductor-metal nanocomposite materials in a single step, the metal-silica products are too large for catalysis applications such as CO oxidation that typically requires sub-5 nm metal NPs [53][54][55][56].…”

“…In this work, we report a RLAL approach where a Si wafer immersed in a KAuCl 4 solution is ablated using sub-50 femtosecond (fs) laser pulses (fs-RLAL), producing significantly smaller silica-Au nanostructures than in previous reports using nanosecond lasers [47][48][49]. Using a high surface area support such as silica is commonly used to stabilize metal nanoparticles during catalytic reactions [60,61].…”

The Synthesis of Hybrid Gold-Silicon Nano Particles in a Liquid

“…Here we report on the synthesis of silicon NP by the continuous laser irradiation of a thin crystalline monolithic silicon target placed in ethanol. The possibility of formation of hybrid silicon-gold structures by laser ablation in colloidal solutions 5 is demonstrated here. The increase of the optical near field magnitude in the emission of the NPs is observed and interpreted in terms of the redistribution of the near-field scattering intensity due to the nanoantenna action by gold nanoparticles.…”