Metallic and Non-Metallic Plasmonic Nanostructures for LSPR Sensors

Abstract

Localized surface plasmonic resonance (LSPR) provides a unique scheme for light management and has been demonstrated across a large variety of metallic nanostructures. More recently, non-metallic nanostructures of two-dimensional atomic materials and heterostructures have emerged as a promising, low-cost alternative in order to generate strong LSPR. In this paper, a review of the recent progress made on non-metallic LSPR nanostructures will be provided in comparison with their metallic counterparts. A few applications in optoelectronics and sensors will be highlighted. In addition, the remaining challenges and future perspectives will be discussed.

Keywords: 2D materials; LSPR; SERS; graphene; high sensitivity; plasmonic nanostructures.

Figure 3

(a) Schematic description of R6G molecules attached on Au nanoparticles/graphene SERS substrate. Reproduced with permission [39]. Copyright 2015, Elsevier. (b) Schematic description of the SERS process with graphene-enhanced CM effect.

Figure 8

(a) Device structure of a CH₃NH₃PbI₃/graphene/AuNPs hybrid photodetector. Reproduced with permission from Ref. [89]. Copyright 2009, Royal Society of Chemistry. (b) Device configuration of a CH₃NH₃PbI₃/AuNPs hybrid photodetector. Reproduced with permission from Ref. [90]. Copyright 2018, Wiley. (c) Schematic diagrams of a metal NP embed in a perovskite thin film with a direct contact to perovskite, and a metal NP embedded in a SiO₂ metafilm with a perovskite thin film atop. The yellow arrows represent incident light on the perovskite. The charge, energy and heat transfer effects are illustrated by green, blue and orange arrows, respectively, which can be effectively suppressed using the AgNPs embedded silica metafilm. (d) Schematic structure of a perovskite/graphene heterojunction photodetector on AgNPs-silica metafilm. Reproduced with permission from Ref. [94]. Copyright 2019, American Chemical Society. (e) Schematic image of AuCu/CsPbCl₃ core/shell/graphene hybrid photodetectors with LSPR enhanced photoresponsivity. Reproduced with permission from Ref. [95]. Copyright 2020, Wiley. (f) Schematic illustration of the graphene/WS₂-ND/AgNP metafilm heterostructure photodetector Reproduced with permission from Ref. [96]. Copyright 2019, American Chemical Society.

Figure 1

Schematic description of LSPR in metallic NPs through oscillation of free electrons on the NP surface.

Figure 2

Localized surface plasmon resonance (LSPR) frequency dependence on free carrier density and doping constraints. The bottom panel shows the modulation of the LSPR frequency (ω_sp) of a spherical nanoparticle by control of its free carrier concentration (N). LSPR frequency is estimated as: 1/2π $\sqrt{({Ne}^2/({\epsilon }_o{m}_e({\epsilon }_{\infty }+2{\epsilon }_m)}$. The high frequency dielectric constant ${\epsilon }_{\infty }$ is assumed to be 10, the medium dielectric constant ${\epsilon }_m$ is set as 2.25 for toluene, and the effective mass of the free carrier $m_e$ is assumed to be that of a free electron. The e is the electronic charge and ${\epsilon }_o$ is the permittivity of free space. The top panel shows a calculation of the number of dopant atoms required for nanoparticle sizes ranging from 2 to 12 nm to achieve a free carrier density between 10¹⁷ and 10²³ cm⁻³. For LSPRs in the visible region, a material in which every atom contributes a free carrier to the nanoparticle, as for metals, is required. For LSPRs in the infra-red, carrier densities of 10¹⁹–10²² cm⁻³ are required. Below 10¹⁹ cm⁻³, the number of carriers (for a 10 nm nanocrystal) may be too low (<10) to support an LSPR mode. The brown diamond indicates the region of interest in the present study. Reprint with permission of [13], Copyright 2011, Springer Nature.

Figure 4

Schematic depiction of the synthesis process: (a) wet transfer of graphene on SiO₂/Si substrates (b,c) MoS₂ synthetization on graphene via the vapor transport process. (d,e) Synthesis of the MoS₂ nanostructures: N-disc at low (f) and N-donut at high (g) (NH4)₂MoS₄ precursor concentration with the hypothetical growth mechanism where triangles denote MoS₂ nuclei produced during the vapor transport annealing procedure. Reproduced with permission [43]. Copyright 2021, MDPI.

Figure 5

(a) Raman map of MoS₂ (using A_1g mode). (b) Representative AFM images ($5 \mathsf{\mu }\mathrm{m}\times 5 \mathsf{\mu }\mathrm{m}$) of the sample with zoom in view (c). Reproduced with permission [43]. Copyright 2021, MDPI. (d) Vertical MoS₂/graphene bilayer heterostructure (top) and the corresponding Electron Localization Function (ELF) (bottom) showing the localized electron concentration underneath the sulfur atom demonstrating the charge transfer occurrence. Reproduced with permission [41]. Copyright 2019, Wiley-VCH. (e) Raman spectra of the R6G molecules with different concentrations in the range of 5 × 10⁻⁵ M to 5 × 10⁻⁹ M (c), and (f) 5 × 10⁻¹⁰ M to 2 × 10⁻¹² M on the MoS₂ N-donut/graphene vdW heterostructures substrates. All spectra were collected using 532 nm excitation. Reproduced with permission [43]. Copyright 2021, MDPI.

Figure 6

(a) 3D ELF plot of the stack of Au/WS₂ heterostructure. Reproduced with permission [54]. Copyright 2019, ACS. (b) Graphene Raman spectra taken on four samples: graphene only, WS₂NDs/graphene, AuNPs/graphene and AuNPs/WS₂-NDs/graphene. (c) Raman map of WS₂. (d) AFM image of the WS₂-NDs; (e,f) an SEM image of an AuNPs/WS₂-NDs/graphene sample and EDS maps of W, Au and C. (g) Raman spectra of R6G molecules with a concentration of 5 × 10⁻⁵ M on different substrates. Reproduced with permission [53]. Copyright 2020, ACS.

Figure 7

(a) Schematic description of AuNP/graphene SERS substrate with R6G probe molecules attached. (b) Comparison of Raman spectra of R6G probe molecules attached on four different substrates: AuNPs/graphene/SiO₂/Si, AuNPs/SiO₂/Si, graphene/SiO₂/Si and bare SiO₂/Si, respectively. Reproduced with permission [39]. Copyright 2015, Elsevier. (c) Raman spectra of R6G molecules at different concentrations in the range of 5 × 10⁻⁵ to 8 × 10⁻⁷ on AuNP/graphene SERS substrate. Reproduced with permission [24]. Copyright 2017, Elsevier. (d) Schematic description of graphene/AuNPs SERS substrates (top-left inset), and the Raman spectra of R6G molecules on the graphene/AuNP SERS substrates of concentrations in the range of 10⁻⁷ to 10⁻¹¹. Reproduced with permission [52]. Copyright 2016, Elsevier. (e) Schematic diagram of MoS₂/AgNPs hybrid system (f) Raman spectra of R6G molecules of different concentrations from 5 × 10⁻⁵ to 8 × 10⁻⁹_. Reproduced with permission [58]. Copyright 2016, Elsevier. (g) Schematic showing the Au-WS₂ nanohybrid SERS platform, (h) Raman spectra of R6G molecules using concentrations from 5 × 10⁻⁴ to 8 × 10⁻⁸ on Au-WS₂ nanohybrid SERS. Reproduced with permission [59]. Copyright 2018, Microchimica Acta.