Quantification of the Relative z-Polarized Electromagnetic Field Contribution and Associated Investigation of Asymmetric Shape of Layer Breathing Mode from Au Nanoparticle–Graphene–Au Thin Film Junctions

Abstract

Simultaneous utilization of both in-plane and out-of-plane phonon vibration of graphene sandwiched between a Au nanoparticle (NP) and Au TF junction can have a significant role in quantifying the z-polarized electromagnetic (EM) field contribution. The layer breathing mode (LBM), which involves the relative displacement of the individual graphene layers in the graphene sheet in the normal directions, can be exclusively observed at Au NP–Au TF junctions. By employing the I LBM/I 2D value at each junction, the author can show that a nearly 50–70-times stronger z-polarized EM contribution is present at Au NP–Au TF junctions. Additionally, the author can also determine that a Au NP with a truncated shape shows a relatively ∼1.3-times higher z-polarized EM contribution than a Au NP with a spherical shape because of the higher density of the pointed part from the truncated Au NP. Furthermore, the author reveals the evolution of the asymmetric shape of the LBM peak with increasing z-polarized EM strength. The increase of the full width at half-maximum (fwhm) from the major LBM and the correlated blue shift of the minor LBM with increasing z-polarized EM contribution imply that the degree of coupling between highly localized electrons and low-energy phonons from graphene along the z axis might be reflected and that of the additional n-doped status of graphene stemming from the relatively higher electron density at truncated sites within Au NP might also be reflected. The author is currently developing this surface-enhanced-Raman-scattering- (SERS-) based nanometrology as a more facile optical characterization tool, especially for exploring out-of-plane phonon vibration of various 2D materials.

Nano-Physical Characterization of Chemical Vapor Deposition-Grown Monolayer Graphene for High Performance Electrode: Raman, Surface-Enhanced Raman Spectroscopy, and Electrostatic Force Microscopy Studies

“…Optical characterization of CVD-MG by upright-type Raman spectroscopy is extensively used as a conventional method to understand the degree of doping [35], number of layers [36], and the influence of mechanical strain [6,34,35,37] predominantly employing in-plane phonon vibrations. Park and colleagues readily recognized the z-directional out-of-plane phonon vibrational modes, such as the RBLM and transverse nano-scroll-type defect modes of CVD-MG at each of the Au NP/Au film junctions via the SERS signal obtained using linearly polarized HeNe laser (633 nm) irradiation [20,21,25,26]. Generally, a 633 nm laser beam is utilized as the excitation source since it closely matches the localized surface plasmon (LSP) resonance of Au NPs and the coupled resonance surface plasmon polariton (SPP) of the Au NP-Au film junction system [18][19][20][21][22][23][24][25][26].…”

“…Park and colleagues readily recognized the z-directional out-of-plane phonon vibrational modes, such as the RBLM and transverse nano-scroll-type defect modes of CVD-MG at each of the Au NP/Au film junctions via the SERS signal obtained using linearly polarized HeNe laser (633 nm) irradiation [20,21,25,26]. Generally, a 633 nm laser beam is utilized as the excitation source since it closely matches the localized surface plasmon (LSP) resonance of Au NPs and the coupled resonance surface plasmon polariton (SPP) of the Au NP-Au film junction system [18][19][20][21][22][23][24][25][26]. Based on these studies, the un-polarized SERS signals identified along the z-axis were attributed to the xy-plane axially symmetric SERS signal distribution from a z-directional dipole emitter (corresponding to the Au NP-Au film coupling orientation).…”

“…This results in a 5-6 orders of magnitude variation in the SERS intensity across hot spots even within identical specimens. Owing to the reproducibility of the SERS signal intensity, a nanoparticle-on-mirror (NPoM) system composed of Au NP/CVD-MG/Au films could be employed to comparatively explore the contribution of both in-plane and out-of-plane vibration modes at individual NPoMs according to the SERS intensity ratio [18][19][20][21][22][23][24][25][26]. It is worthwhile to note the significance of previous work by Park and coworkers pertaining to utilization of the Au NP/molecule/Au Film system [17,18].…”

“…As the EM enhancement factor of the Au NP/molecule/Au film was within ~106 and exhibited a nearly uniform intensity distribution, the NPoM-SERS system was utilized in this article. Due to the dipole-dipole interaction between Au NPs and the Au film, the SERS signal revealed the presence of the radial breathing-like mode (RBLM) associated with the normal-to-surface direction (z-axis) from spaced CVD-MG in the NPoM system, which was present in the low frequency region along with three nano-scroll-type defect peaks [20,21]. The peak intensity and position of the transverse 1, 2, 3 peaks below 700 cm −1 were also useful indicators of the tilted z-directional CVD-MG protrusion.…”

“…Electrical characteristics of CVD-MG obtained by examining the degree of the tilted formation of out-of-pane modes is also useful in anticipating carrier mobility [24]. Moreover, it was shown that the zpolarized EM field was almost a factor of 50-70 times stronger at each NPoM compared to the xy-plane EM field, which significantly enhanced the I(RBLM)/I(2D) [20]. (3) To explore and make use of the key characteristics of CVD-MG, it is vital to study their chemical and atomic structures.…”