Previously, Park and colleagues could explore that the shape of the deposited CVD-GM spaced between Au NP-Cu foil was not always flat and deduced that it might be composed of a partially corrugated CVD-GM as a result of interaction between z-polarized excitation field and the corresponding parallel z-directional phonon mode of CVD-GM, similar to the radial breathing mode (RBM) from single walled carbon nanotubes (SWCNTs). Hence, this specific out-of-plane phonon mode could be identified as RBLM modes [14–16]. In addition, it was also simulated by finite difference time-domain (FDTD) electrodynamics calculation that the origin of RBLM mode could be identified as a result of simple dipole-image-dipole interaction model and it had a pivotal role to generate a z-polarized excitation electromagnetic (EM) field [16]. By virtue of this theoretical result, we can find out the motivation of this work in terms of providing a straightforward nanoscale metrology to quantitatively and optically evaluate the degree of z-directional morphological corrugation of CVD-GM with corresponding electrical properties (lateral carrier mobility or sheet resistance value) and the availability toward biomedical application fields, emphasizing the importance of morphological surface status of CVD-GM, resulting in different values of the I[out-of-plane, RBLM] / I[in-plane, 2D] at certain domain due to probably insertion of biomaterials likewise Au NP / biomaterials / CVD-GM / Au TF formation. Figure 1 shows the representative four pairs of SERS spectra of CVD-GM spaced in our NPoM system, highlighting the RBLM (A) and the corresponding 2D peak region (B).
Previously, our plasmonic NPoM system showed a slight increase in the Full Width at Half-Maximum (FWHM) of the 2D peak with corresponding increase of intensity of RBLM signal. At that time, it was still vague to determine the definite relationship between the RBLM intensity and the corresponding local structure [15]. In contrast to the previous report, we can now try to establish a new and more straightforward metrology concept than before. From top to bottom in each Figure 1(A) and (B), we can exhibit four sets SERS spectra of CVD-GM spaced at four different NPoMs [15, 17]. At this time, it is worthwhile to note that the RBLM mode from (A) and 2D mode from (B) can be represented as an out-of-plane and an in-plane phonon mode, respectively, as mentioned earlier. Therefore, the intensity ratio between out-of-plane and in-plane phonon mode likewise I[RBLM] / I[2D] maybe one of the meaningful and straightforward value beyond optical diffraction to limit to quantitatively estimate among the degree of z-directional morphological corrugation status, the corresponding lateral carrier mobility and bio-medical application fields utilizing of high specific surface area and ultrahigh flexibility of a CVD-GM surface, leading to variable I[RBLM] / I[2D] values depending on the degree of pressure-strain, strain-to-electricity and chemical-coupled interaction with the very surrounded CVD-GM [18].
Table 1
(I[RBLM] / I[2D] values at each NPoM)
|
Number of
NPoM
|
I[RBLM] /
I[2D]
|
|
#1
|
1.67
|
|
#2
#3
#4
|
1.61
1.33
0.75
|
Table 1 shows the calculated I[RBLM] / I[2D] values at each NPoM system. As we anticipated, the significant difference values are clearly observed, indicating that the degree of z-directional morphological corrugation of CVD-GM from #1 NPoM case exhibits the highest among these four sets. Hence, we can quantitatively assess the degree of z-directional morphological corrugation of CVD-GM domains as follows #1 > #2 > #3 > #4, shown below in Figure 2.
Moreover, we can also readily anticipate that higher I[RBLM] / I[2D] value indicates the lower lateral carrier mobility (or higher sheet resistance value) due to different CVD-GM shape, especially, along z-direction (ex: mountain or plain), which could be expressed as the relationship between Raman 2D peak maximum position of the CVD-GM and the corresponding electrostatic force microscope (EFM) results [19, 20]. In detail, the compressive strain effect (relatively blue-shift of a 2D peak maximum position) was investigated in relatively complex and surrounded CVD-GM ripple domains with lower amplitude voltages, indicating more charged domains through the EFM experiment while relatively higher EFM amplitude voltage was observed in threading (or relatively non-complex) CVD-GM ripple domains, explaining less charged domains. At that time, the local structure revealed by AFM topography could be employed in estimating the correlation study between optical Raman microscopy results regarding the strain effect as well as the degree of surface charge effect revealed by EFM (Please see the Figure S1 in supplementary material).
Overall, a relatively higher surface charged domain obtained via higher EFM amplitude imaging technique is in sync with a relatively higher and complex domains as a result of an AFM topography or the degree of the correlated blue-shifted Raman in-plane (G or 2D) maximum peak position [8, 19, 20]. Moreover, the relationship between higher degree of blue-shifted Raman in-plane (G or 2D) maximum peak positions and the resultant higher degree of sheet resistance (lower lateral carrier mobility) values was clearly substantiated in this work [8].
Furthermore, it is undergoing to establish the correlation between I[RBLM] / I[2D] and real root mean square (rms) roughness by atomic force microscope (AFM) measurement for the purpose of precisely correlating with I[RBLM] / I[2D] obtained via optical Raman microscopy. Once we establish the correlation between I[RBLM] / I[2D] and rms roughness using the AFM system, we may not have some complexities such as elaborate operation and many times of replacement of expensive AFM tips.
Figure 3(A) describes schematic relationship between relatively higher and lower degree of z-directional structure of the CVD-GM. It gives us an important meaning the 3(A) shows relatively less lateral carrier mobility due to higher degree of z-protrusion, whereas 3(B) deciphers relatively lower degree of z-directional structure of CVD-GM, indicating higher lateral carrier mobility. Accordingly, we can now readily understand that the higher value of I[RBLM] / I[2D] is corresponding with Figure 3(A), whereas the lower value of I[RBLM] / I[2D] is corresponding with Figure 3(B). Furthermore, the simultaneous observation of a sequential blue-shift of the RBLM peak from bottom to top in Figure 1(A) may exhibit that a degree of n-doping caused by a high electron phonon coupling (EPC) along z-direction at the Au NP / CVD-GM / Au TF may be observed and we can claim that the degree of EPC and n-doping can be ascribed to the relatively high degree of z-polarized EM and resultant higher contribution of out-of-plane phonon movement in our NPoM systems [21].