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Electromagnetic Enhancement of Graphene Raman Spectroscopy by Ordered and Size-Tunable Au Nanostructures.

Zhang S, Zhang X, Liu X - Nanoscale Res Lett (2015)

Bottom Line: The size-controllable and ordered Au nanostructures were achieved by applying the self-assembled monolayer of polystyrene microspheres.Few-layer graphene was transferred directly on top of Au nanostructures, and the coupling between graphene and the localized surface plasmons (LSPs) of Au was investigated.We found that the LSP resonance spectra of ordered Au exhibited a redshift of ~20 nm and broadening simultaneously by the presence of graphene.

View Article: PubMed Central - PubMed

Affiliation: State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510641, China. mssgzhang@scut.edu.cn.

ABSTRACT
The size-controllable and ordered Au nanostructures were achieved by applying the self-assembled monolayer of polystyrene microspheres. Few-layer graphene was transferred directly on top of Au nanostructures, and the coupling between graphene and the localized surface plasmons (LSPs) of Au was investigated. We found that the LSP resonance spectra of ordered Au exhibited a redshift of ~20 nm and broadening simultaneously by the presence of graphene. Meanwhile, the surface-enhanced Raman spectroscopy (SERS) of graphene was distinctly observed; both the graphene G and 2D peaks increased induced by local electric fields of plasmonic Au nanostructures, and the enhancement factor of graphene increased with the particle size, which can be ascribed to the plasmonic coupling between the ordered Au LSPs and graphene.

No MeSH data available.


Typical SEM images of the Au nanostructures with initial thicknesses of a 20 nm b 30 nm covered by the graphene film
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Fig3: Typical SEM images of the Au nanostructures with initial thicknesses of a 20 nm b 30 nm covered by the graphene film

Mentions: Figure 2 shows the AFM images of Au nanostructures with different sizes and shapes after removing the PS microspheres. Obviously, the long-range hexagonal order inserted from the original PS template is conserved. When the initial thickness of the Au film is 15 nm, the shape of the Au nanostructures is sphere-like nanoparticles, with an average width of ~100 ± 4.5 nm. Another point we have to notice from the SEM images is that the shape of the Au nanostructures gradually changed from more sphere-like nanodots to sharp triangles with the increase of the initial thickness of the Au film from 15 to 20 nm. The effect of the shape changes was also quantified by evaluating the circularities of the individual nanostructures, defined as the ratio of the square of the perimeter to 4πA, where A is the area of the particular nanostructure. The circularity should be 1.654 for a regular triangle and should approach 1 for a perfect circle [13]. The resultant values of the Au nanostructures with and without graphene coverage are listed in Table 1. From the table, we can distinctly see that with the deposition time increasing from 10 to 30 min (corresponding initial thickness from 15 to 40 nm), the circularity of the Au nanostructures first increases and then decreases, and the average width changes from 100 ± 4.5 to 140 ± 7.8 nm. As the diameter of the PS sphere is 500 nm, the gap (inscribed circle) among the PS spheres is approximately 77 nm. When the initial thickness of the Au film is only 15 nm, the Au film cannot cover the whole gap surface. Due to the different thermal expansion coefficients between the Au films and the substrate, when the initial thickness of the Au film is ~15 nm, the compressive stress induced by the Ostwald ripening mechanism would cause the Ag films to form isolated nanoparticles. With the increase of the initial thickness of the Au film to 20 nm, the whole gap among the PS nanospheres can be approximately filled, leading to shape transformation of the Au nanostructures from nanodots to triangles. When the initial thickness of Au was further increased to 40 nm, the whole gap of the PS spheres can be fully filled and the shape of the Au nanostructures changes to nanospheres due to the larger thickness of Au. The typical SEM images of 15- and 20-nm Au nanostructures covered with graphene are presented in Fig. 3. It is clear that the continuous graphene film has been successfully transferred on the surface of Au nanostructures, and the electron beam can easily penetrate through the atomically thin graphene to display the underlying Au nanostructures. The ridges and cracks formed on the graphene surface during the wet transfer processes can be also distinctly observed.Fig. 2


Electromagnetic Enhancement of Graphene Raman Spectroscopy by Ordered and Size-Tunable Au Nanostructures.

Zhang S, Zhang X, Liu X - Nanoscale Res Lett (2015)

Typical SEM images of the Au nanostructures with initial thicknesses of a 20 nm b 30 nm covered by the graphene film
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4595411&req=5

Fig3: Typical SEM images of the Au nanostructures with initial thicknesses of a 20 nm b 30 nm covered by the graphene film
Mentions: Figure 2 shows the AFM images of Au nanostructures with different sizes and shapes after removing the PS microspheres. Obviously, the long-range hexagonal order inserted from the original PS template is conserved. When the initial thickness of the Au film is 15 nm, the shape of the Au nanostructures is sphere-like nanoparticles, with an average width of ~100 ± 4.5 nm. Another point we have to notice from the SEM images is that the shape of the Au nanostructures gradually changed from more sphere-like nanodots to sharp triangles with the increase of the initial thickness of the Au film from 15 to 20 nm. The effect of the shape changes was also quantified by evaluating the circularities of the individual nanostructures, defined as the ratio of the square of the perimeter to 4πA, where A is the area of the particular nanostructure. The circularity should be 1.654 for a regular triangle and should approach 1 for a perfect circle [13]. The resultant values of the Au nanostructures with and without graphene coverage are listed in Table 1. From the table, we can distinctly see that with the deposition time increasing from 10 to 30 min (corresponding initial thickness from 15 to 40 nm), the circularity of the Au nanostructures first increases and then decreases, and the average width changes from 100 ± 4.5 to 140 ± 7.8 nm. As the diameter of the PS sphere is 500 nm, the gap (inscribed circle) among the PS spheres is approximately 77 nm. When the initial thickness of the Au film is only 15 nm, the Au film cannot cover the whole gap surface. Due to the different thermal expansion coefficients between the Au films and the substrate, when the initial thickness of the Au film is ~15 nm, the compressive stress induced by the Ostwald ripening mechanism would cause the Ag films to form isolated nanoparticles. With the increase of the initial thickness of the Au film to 20 nm, the whole gap among the PS nanospheres can be approximately filled, leading to shape transformation of the Au nanostructures from nanodots to triangles. When the initial thickness of Au was further increased to 40 nm, the whole gap of the PS spheres can be fully filled and the shape of the Au nanostructures changes to nanospheres due to the larger thickness of Au. The typical SEM images of 15- and 20-nm Au nanostructures covered with graphene are presented in Fig. 3. It is clear that the continuous graphene film has been successfully transferred on the surface of Au nanostructures, and the electron beam can easily penetrate through the atomically thin graphene to display the underlying Au nanostructures. The ridges and cracks formed on the graphene surface during the wet transfer processes can be also distinctly observed.Fig. 2

Bottom Line: The size-controllable and ordered Au nanostructures were achieved by applying the self-assembled monolayer of polystyrene microspheres.Few-layer graphene was transferred directly on top of Au nanostructures, and the coupling between graphene and the localized surface plasmons (LSPs) of Au was investigated.We found that the LSP resonance spectra of ordered Au exhibited a redshift of ~20 nm and broadening simultaneously by the presence of graphene.

View Article: PubMed Central - PubMed

Affiliation: State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510641, China. mssgzhang@scut.edu.cn.

ABSTRACT
The size-controllable and ordered Au nanostructures were achieved by applying the self-assembled monolayer of polystyrene microspheres. Few-layer graphene was transferred directly on top of Au nanostructures, and the coupling between graphene and the localized surface plasmons (LSPs) of Au was investigated. We found that the LSP resonance spectra of ordered Au exhibited a redshift of ~20 nm and broadening simultaneously by the presence of graphene. Meanwhile, the surface-enhanced Raman spectroscopy (SERS) of graphene was distinctly observed; both the graphene G and 2D peaks increased induced by local electric fields of plasmonic Au nanostructures, and the enhancement factor of graphene increased with the particle size, which can be ascribed to the plasmonic coupling between the ordered Au LSPs and graphene.

No MeSH data available.