SERS study on the synergistic effects of electric field enhancement and charge transfer in an Ag₂S quantum dots/plasmonic bowtie nanoantenna composite system

Abstract

Localized surface plasmon resonance (LSPR) of nanostructures and the interfacial charge transfer (CT) of semiconductor materials play essential roles in the study of optical and photoelectronic properties. In this paper, a composite substrate of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m1"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>Ag</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> <mml:mi mathvariant="normal">S</mml:mi> </mml:mrow> </mml:math> quantum dots (QDs) coated plasmonic Au bowtie nanoantenna (BNA) arrays with a metal–insulator–metal (MIM) configuration was built to study the synergistic effect of LSPR and interfacial CT using surface-enhanced Raman scattering (SERS) in the near-infrared (NIR) region. The Au BNA array structure with a large enhancement of the localized electric field (E-field) strongly enhanced the Raman signal of adsorbed p-aminothiophenol (PATP) probe molecules. Meanwhile, the broad enhanced spectral region was achieved owing to the coupling of LSPR. The as-prepared Au BNA array structure facilitated enhancements of the excitation as well as the emission of Raman signal simultaneously, which was established by finite-difference time-domain simulation. Moreover, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m2"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>Ag</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> <mml:mi mathvariant="normal">S</mml:mi> </mml:mrow> </mml:math> semiconductor QDs were introduced into the BNA/PATP system to further enhance Raman signals, which benefited from the interfacial CT resonance in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m3"> <mml:mrow> <mml:mi>BNA</mml:mi> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>Ag</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> <mml:mi mathvariant="normal">S</mml:mi> <mml:mtext>-</mml:mtext> <mml:mi>QDs</mml:mi> <mml:mo>/</mml:mo> <mml:mi>PATP</mml:mi> </mml:mrow> </mml:math> system. As a result, the Raman signals of PATP in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m4"> <mml:mrow> <mml:mi>BNA</mml:mi> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>Ag</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> <mml:mi mathvariant="normal">S</mml:mi> <mml:mtext>-</mml:mtext> <mml:mi>QDs</mml:mi> <mml:mo>/</mml:mo> <mml:mi>PATP</mml:mi> </mml:mrow> </mml:math> system were strongly enhanced under 785 nm laser excitation due to the synergistic effect of E-field enhancement and interfacial CT. Furthermore, the SERS polarization dependence effects of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m5"> <mml:mrow> <mml:mi>BNA</mml:mi> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>Ag</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> <mml:mi mathvariant="normal">S</mml:mi> <mml:mtext>-</mml:mtext> <mml:mi>QDs</mml:mi> <mml:mo>/</mml:mo> <mml:mi>PATP</mml:mi> </mml:mrow> </mml:math> system were also investigated. The SERS spectra indicated that the polarization dependence of the substrate increased with decreasing polarization angles <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m6"> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>θ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>pola</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> of excitation from p-polarized <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m7"> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>θ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>pola</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>90</mml:mn> <mml:mo>°</mml:mo> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> excitation to s-polarized <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m8"> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>θ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>pola</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>0</mml:mn> <mml:mo>°</mml:mo> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> excitation. This study provides a strategy using the synergistic effect of interfacial CT and E-field enhancement for SERS applications and provides a guidance for the development of SERS study on semiconductor QD-based plasmonic substrates, and can be further extended to other material-nanostructure systems for various optoelectronic and sensing applications.