Conduction electrons in metal or metal-like nanomaterials can be coherently excited by incident light to collectively oscillate at metal/dielectric interfaces. The collective oscillating mode of electrons and the nanomaterials that support are referred to as surface plasmons and plasmonic materials, respectively. There are two types of surface plasmons: (i) localized surface plasmons (LSP), in which coherent electrons oscillate around the NP surfaces or nanoscale crevices, and (ii) propagating surface plasmons (that is, surface plasmon polaritons (SPPs)), in which the coherent electrons oscillate as a longitudinal wave at extended metal surfaces. Nano-optics, especially the sub-field plasmonics is the study of light at nanoscale and the study of the interaction between light and nano-scale objects supporting surface plasmons.
    Surface plasmon supports the characteristic extinction energy and strong electromagnetic field in the vicinity of the nanostructures, which has great importance for a wide variety of applications ranging from biosensors, spectroscopies and microscopy, optoelectronics and solar cells.
    Our research interests in nano-optics and plasmonics are to rational design novel nano-devices for nano-spectroscopies, nano-sensor and nano-imaging based on the comprehensive understanding of the mechanism of plasmon-enhanced spectroscopies and its correlation to the plasmonic properties of nanostructures, by means of new theories, concepts and simulation methods. The relation of our plasmonic researches to other researches in our group could be described in Figure 1.

Plasmonic properties and SERS activity from transition metal nanostructures

1. SERS from transition-metal cauliflower-like structures prepared by electrochemical roughening methods

Figure 2. STM (a) and AFM (b) images of electrochemically roughened Pt and Rh electrodes, respectively. FDTD simulated electric field distribution for spherical (c) and cauliflower-like (d) Rh nanoparticles. The incident beam illuminates along the y direction with x-polarization[1].

2. SERS from gold@transition-metal core@shell nanostructures

Figure 3. Schematic illustration of the electromagnetic field distribution around Au@Pt core–shell nanoparticles under laser irradiation (left). The dependence of the electromagnetic field strength (normalized with the strength on the Pt surface) on the distance from the Pt shell is shown in the right plot, indicating a substantially strong field enhancement can still be obtained on the surface[2].

Plasmonic properties of a hybrid structures including plasmonic nanoparticles on probing material surfaces

1. The first and second generations of hotspots
    The first generation of hotspots were generated from single nanostructures such as nanospheres and nanocubes (FIG. 4a,b), or nanorods freely suspended in a homogeneous medium. These hotspots exhibit moderate SERS-activity; however, some rationally designed single nanoparticles with sharp corners and/or with intra-particle gaps, such as Au and Ag nanostars, nanoflowers and mesocages exhibit much higher SERS-activity.
    The second generation of SERS hotspots are generated from coupled nanostructures with controllable inter-particle nanogaps (for example, the NP dimers shown in FIG. 4c,d or oligomers and NP arrays shown in Fig. 4e,f) or inter-unit nanogaps in nano-patterned surfaces (FIG. 4g,h). Such hotspots exhibit excellent SERS activity. Because the average SERS intensities from coupled plasmonic nanostructures are typically 2–4 orders of magnitude greater than that from single nanostructures, they are more commonly used for trace-molecule detection. SERS hotspots from coupled nanostructures are extremely small (1–5 nm), but the Raman signals of probe molecules at the hotspots contribute significantly to the total Raman signal.

Figure 4. First- and second-generation SERS hotspots for trace-molecule detection.

2. The third-generation hotspots
    In general, first- and second-generation hotspots are not well suited for surface analysis of many materials. For example, widely used materials such as silicon wafer or ceramics cannot be squeezed into the extremely tiny and narrow regions of the hotspots formed by coupled nanostructures, such as those shown in FIG. 4c-h. Therefore, it is highly desirable to design plasmonic nanostructures that can have hotspots right on the surface of the materials to be probed. The goal can be realized by taking into account of the effect of the materials to be probed in the design of the plasmonic nanostructures. Because, the localized SPR response and local EM field distribution not only depends on the shape, size of plasmonic nanostructures, but also depends sensitively on the dielectric properties of probe materials close to the plasmonic nanostructures. The hotspot generated from the hybrid structures consisting of plasmonic nanostructures and the probe materials can be considered third-generation hotspots. Representative examples of such hybrid structures are shown in FIG. 5.

Figure 5. Third-generation SERS hotspots for surface analysis of materials.

3. The plasmonic nature of SHINERS from general materials
    SHINERS is a representative technique supporting third-generation hotspots. The working principle of SHINERS for gold, silver, and more general metal single-crystal surfaces have been extensively studied in our group[4-8].

Figure 6. Fano Resonance nature of a Au@SiO2 nanoparticle dimer on a flat gold surface. Coupled dipole of Nanoparticles induce the imaginary dipole on the metal or dielectric surfaces, to form a magnetic-dipole-like mode. It is the plasmonic dark mode with less irradiative efficiency, but with strong near field.

References

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2. Tian, Z.-Q., Ren, B., Li, J.-F. & Yang, Z.-L. Expanding generality of surface-enhanced raman spectroscopy with borrowing sers activity strategy. Chem. Commun., 3514-3534 (2007).
3. Ding, S.-Y. et al. Nanostructure-based plasmon-enhanced raman spectroscopy for surface analysis of materials. Nature Reviews Materials 1, 16021 (2016).
4. Li, J.-F. et al. Extraordinary enhancement of raman scattering from pyridine on single crystal au and pt electrodes by shell-isolated au nanoparticles. J. Am. Chem. Soc. 133, 15922-15925 (2011).
5. Li, J.-F. et al. Shell-isolated nanoparticle-enhanced raman spectroscopy. Nature 464, 392-395 (2010).
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9. Chen, S. How to Light Special Hot Spots in Multiparticle-Film Configurations. ACS Nano 10, 581-587 (2016).