Article pubs.acs.org/JPCC
Out-of-Plane Directional Charge Transfer-Assisted Chemical Enhancement in the Surface-Enhanced Raman Spectroscopy of a Graphene Monolayer Won-Hwa Park*,†,§ and Myunghee Jung‡ †
Department of Chemistry, College of Natural Sciences, Seoul National University, Seoul, 08826, South Korea New Materials Team, LG Electronics Advanced Research Institute, Seoul, 06763, South Korea
‡
S Supporting Information *
ABSTRACT: We present an out-of-plane directional charge transfer (CT)-assisted chemical enhancement (CM) in the surface-enhanced Raman scattering (SERS) of a graphene monolayer (GM) sandwiched at an individual Au prism-Au thin film (TF) plasmonic junction. By comparing previous reports, in which the enhancement of out-of-plane phonon modes of a GM was mostly governed by z-directional local field formed at a Au nanoparticle (NP)−Au TF junction, we can reveal that a CT-assisted CM along the z-direction has a pivotal role by enhancing radial breathing-like mode (RBLM) intensity at face-to-face junction types. The anticorrelation between 2D and RBLM intensity from ridge to flat domains of an Au prism indicates that a plasmonic EM effect is not a critical element at flat domain, whereas the RBLM intensity is strongly enhanced at face-to-face junction. Furthermore, the closest GM from a z-axis has a significant role based on vector-based investigation in enhancing CT-assisted CM effect between Au prism and TF. These phenomena may be interpreted by (1) introducing the coupling of the plasmon modes of a metallic nanostructure, especially interaction between dipolar and quadrupolar modes and their interference with a large contact area of flat compared to ridge from an Au prism adjacent to the dielectric GM at a face-to-face junction by hot (or ballistic) electrons (Fano-like resonance) and (2) the electrons present at flat Au faces may have a high probability to generate CT in terms of relatively large portion of homogeneous electron distribution than sphere, leading to secure a large amount of CT electrons. Moreover, (3) close to the surface-to-normal direction of GM, we can reveal that CT along the z-axis is more favorable than the tilted. These interpretations offer an opportunity to study the mechanism of CT-assisted CM in detail and for optimizing the sensitivity of nanostructures in terms of Fano resonance and the resulting high performance localized surface plasmon resonance (LSPR) sensors.
1. INTRODUCTION Surface-enhanced Raman scattering (SERS) is a metal surfacesensitive technique that enhances Raman scattering signals by molecules adsorbed on rough metal nanostructures.1−4 Recently, a lot of attempts for maximizing and generating reproducible SERS signals have been mostly carried out on the basis of plasmonic electromagnetic (EM) field enhancement. However, the SERS-active junctions (hotspots) from metal nanoparticle aggregates gave us unpredictable SERS signals varying 5−6 orders in magnitude from hotspot to hotspot, which prevents analysis in a quantitative way.5,6 Moreover, single molecule level SERS (SM-SERS) has been investigated via charge transfer (CT) effect between metal nanoparticle (NP) and adsorbed molecules.7−9 The photoinduced metal−molecule CT is believed to offer an extra chemical enhancement in SERS, but its microscopic mechanism and relative importance still remain a challenging issue.9−20 Nevertheless, the critical questions can be summarized as follows: (1) How does one quantify CT enhancement factors? (2) Which of the vibrational bands are CT active? (3) What are the influences of molecular structures and metal−molecule contacts on the CT enhancement? and (4) How does one © XXXX American Chemical Society
maximize the overall Raman enhancement combining between EM and CT enhancement? To focus on issues (2) and (3) primarily, we can employ an individual Au prism−graphene monolayer (GM)−Au thin film (TF) junction model case in terms of elucidating CT influence in this work. The GM fabricated via chemical vapor deposition (CVD) method,21,22 is primarily recognized as a promising material to apply for CT effect in SERS as in previous reports.23,24 Especially, Park and co-workers carried out SERS of CVD-GM sandwiched between a Au NP and Au TF plasmonic gap junction.25−31 From these investigations, they could observe the out-of-plane phonon modes at low frequency region ( #3 > #4 ≥ #1 based on mainly the length scale.30
Interestingly, the highest length (highest RBLM and lowest 2D intensity) is also anticorrelated to the lowest angle from a RBLM axis, implying that #2 is the closest to surface-to-normal direction, resulting in the highest CT-assisted CM along the zaxis being anticipated as well. Figure 6 shows the schematic picture illustrating how to employ a CT-assisted CM. The significant role in CT in this work may be ascribed to forming a junction by face-to-face formation of Au nanostructures between prism and thin film, and Fano-like resonance influence may be present considering a large contact area of flat domain of Au prism adjacent to the dielectric GM such as face-to-face junction, although a subtle zprotrusion of GM is still present.
subsequent peaks from the lower-frequency, called transverse acoustic mode series (TA1, TA2, TA3), can be efficiently utilized as an indicator of subtly tilted out-of-plane phonon movement of GM from the z-axis. The RBLM, TA1, TA2, and TA3 are displayed as black, red, blue, and green solid arrows, respectively. The different length of each peak is reflected as different intensities and the different angle from surface-tonormal direction (RBLM axis) can also be described based on the difference of maximum peak positions between RBLM and each TA peak. From this investigation, we can also show that #2, which exhibits the highest RBLM intensity, has the closest position to the RBLM axis. Overall, the degree of tilted formation of GM can also follow positive correlation with
4. CONCLUSIONS In summary, we study a CT-assisted CM in the SERS of a GM sandwiched at an individual Au prism−Au TF plasmonic junction. In this work, we can reveal that the CT-assisted CM effect along the z-direction has a pivotal role in enhancing the RBLM of a GM. The anticorrelated relationship between RBLM and 2D intensity distribution from ridge-to-face to faceD
DOI: 10.1021/acs.jpcc.6b07674 J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
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to-face junctions cannot be explained by plasmonic EM field regime. From this point of view, the CT effect of GM exhibited at “face-to-face” junction may be attributed to (1) introducing the coupling of the plasmon modes of a metallic nanostructure, especially, interaction between dipolar and quadrupolar modes and their interference (Fano-like Resonance) with a large contact area adjacent to the dielectric GM such as face-to-face junction by hot (or ballistic) electrons and (2) the electrons present at flat Au faces may have a high probability to generate CT in terms of relatively large portion of homogeneous electron distribution than sphere, leading one to secure a large amount of CT electrons. Furthermore, (3) at close to surfaceto-normal direction of a tilted GM, we can reveal that CT along the z-axis is more favorable than the tilted. It means that the closest GM from a RBLM axis has a significant role in enhancing CT effect between Au prism and TF. Although the quantification of CM still remains, we can give important signatures in CT effect in terms of securing a spectral clue in dealing with CT-active vibrational mode as a RBLM intensity distribution of a GM at face-to-face junction. This analysis offers an opportunity for optimizing the sensitivity of nanostructures in terms of CT and the resulting high performance LSPR sensors by precise examination of RBLM intensity and shape in this case. Maximizing Raman enhancement combined with both EM and CM is also underway.
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07674. The raw spectral data and AFM image, and related SERS image of 2D and RBLM of the sample (PDF)
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Figure 6. Schematic picture describing how to employ CT-assisted CM. The significant role in CT may be ascribed to the coupling of the plasmon modes of a metallic nanostructure, especially, interaction between dipolar and quadrupolar modes and their interference with a large contact area adjacent to the dielectric GM at face-to-face position between prism and thin film by hot (or ballistic) electrons, although a subtle z-protrusion of GM is resent. Compared to the face-to-face case, the ridge-to-face domain shows marginal gap distance so that a charge capacitive EM plasmonic field can be formed, leading to enhance 2D peak intensity.
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Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]; Tel: +41 44 63 54422. Present Address §
(W.-H.P.) Department of Chemistry, University of Zürich Winterthurerstrasse 190, CH-8057 Zürich Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the LG Advanced Research Institute (LGARI) for doing this research. E
DOI: 10.1021/acs.jpcc.6b07674 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jpcc.6b07674 J. Phys. Chem. C XXXX, XXX, XXX−XXX
