In the simulation, the influence of incident light angle on the enhancement factors of electric field and Raman scattering is discussed in Fig. 2. The radius of Au nanospheres is set as 50 nm, and the distance between the two Au nanopheres is set to 2 nm. Selected light source is linearly polarized plane wave, and the measurement was made every 15° from the X-axis to the Z-axis, so as to discuss its influence on the enhancement performance of the system, as shown in Fig. 2. Figure 2 (a) shows that the electric field enhancement factor decreases gradually when the incident angle increases from 0 ° ~ 90 °. It is noticed that, the wavelength of the peak position is kept 635 nm and does not change with the angle. This may be because the light absorption property of the material is independent of the change of the incident angle, but related to the configuration of Au dimer. Figure 2 (b) shows the field distribution of Au dimer with the radius of 50 nm and distance of 2 nm under the excited wavelength of 635 nm. Hot spots on the electric field have been greatly enhanced and localized area to the nanometer gap, which is due to the surface from excimer phenomenon, such as that in the resonance state of electromagnetic field energy is efficiently into the metal surface free electrons collective vibrational energy. The red area represents a relative stronger electric field, which demonstrate that the electromagnetic field is enhanced and limited within the scope of the metal surface. Moreover, it can be seen that the electric field strength of dimer gap at θ = 0° is significantly higher than that at θ = 60° on the same scale,which is because vertical incidence maximizes the vertical field component19.
In order to optimize the local electric field and the Raman enhancement, the radius of Au nanospheres should be considered, as shown in Fig. 3. By fixing the angle of incidence θ = 0° and the distance between nanoparticles d = 2 nm, the radius of Au dimer is calculated in the range from 10 nm to 70 nm. It can be clearly seen in Fig. 3(a) that the electric field enhancement factor is very small when r = 10 nm, and there are the obvious resonance peaks for the electric field enhancement between 20 nm and 70 nm. With the increase of the radius, the peak value of the electric field enhancement factor gradually increases and then decreases. When r = 50 nm and λI = 635 nm, the electric field enhancement reaches the maximum value of around 220, and then the peak begins to decline. It is noticed that the width of the wave peak increases, which make the excitation wavelength broaden when calculating the Raman enhancement factor, and thus the application of Au dimer becomes wider. With the increase of the radius, it is found that the peak value of the electric field enhancement factor is red shifted obviously, and the corresponding resonance wavelength is shifted from 545 nm to 740 nm. The red shift of the peak is mainly due to the Au nuclear on both sides of the opposite sign of surface charge between the restoring force of abate20. For the larger Au nuclear radius, the distance between the surface charge increase results in the decrease of the interaction of the dimer. Therefore, the restoring force between them will be abated, resulting in a red shift phenomenon of the LSPR peak. Figure 3(a) shows that the electric field enhancement factor is the largest with the excited wavelength of 635 nm, which is used as the excitation wavelength to calculate the Raman enhancement factor with different radius according to formula (1), as shown in Fig. 3(b). When r = 50 nm, the Raman enhancement factor achieve 109 at around 535 nm ~ 675 nm, and the maximum Raman enhancement factor is 2.4 × 109 at 635 nm, which will continue to increase with subsequent structural optimization. In the simulation process, it is assumed that the incident wavelength is 635 nm, that is, the Stokes scattering wavelength in Raman scattering should be greater than 635 nm. Therefore, r = 60 nm Raman enhancement factor is larger in the range from 635 nm to 735 nm, and thus the stronger molecular SERS signals can be obtained. However, it is worth noting that when r = 40 nm, due to its maximum enhancement factor around 600 nm, it is conducive to the measurement of An-Stokes scattered waves21. The Au dimer at this excitation wavelength is very conducive to the non-destructive detection of living cells in life sciences22,23.
In order to find the configuration of Au dimer with better enhancement effect, the influence of the distance between Au dimers on the electric field and the Raman enhancement factor is studied by fixing the Au radius of 50 nm and the angle of incident angle of 0°, and the simulation results are shown in Fig. 4. With the decrease of the distance between dimers, the peak value of the electric field enhancement factor appears obvious red shift, and it gradually increases with the decrease of the distance from 3 nm to 1 nm. However, when the distance continues to be reduced to 1 nm, the enhancement factor has a multiple enhancement compared with other distance in the whole simulation range, and reaches the maximum value of around 583 under the excited wavelength of 675 nm. The peak value of enhancement factor is 1.5 ~ 3 times for the other peaks. The intensity of the electric field decreases slightly at 600 nm ~ 650 nm and then peaks, probably due to the interference of incident light with the scattered radiation24. This electric field enhancement factor is a very groundbreaking discovery and provided a reliable data support for SERS development. Figure 4(b) also calculates the Au dimer Raman enhancement factor at excitation wavelength of 635 nm. At d = 3 nm, the Raman enhancement factor is generally lower than other distance enhancement factors, but the peak value of the Raman enhancement factor is also up to 108, which can meet general experimental requirements of molecule characterization and recognition. The peak value of Raman enhancement factor is above 109 when the gap distance is the range from 1 nm to 2 nm, especially for d = 1 nm, the maxiumum value of Raman enhancement factor reaches 7.8 × 1010, which is far beyond the experimental requirements of single molecule characterization and recognition imaging.
For the spherical dimer structure, the influence of radius change on the position of the electric field peak is greater than that of the distance. For Au@Al2O3 core-shell structure, the total control radius is R = 50 nm. Figure 5 (a) (b) shows the curve of electric field enhancement factor and Raman enhancement factor with the change of incident light wavelength in the case of Au@Al2O3 dimer with spacing d = 1 nm and total radius R = 50 nm. With the increase of the thickness of Al2O3, the peak value of the electric field enhancement factor gradually decreases and shows a blue shift, which has certain guiding significance to the selection of SERS substrate material and the selection of excitation wavelength. Under the condition of constant radius, Fig. 5 (c) clearly shows from the side diagram and top view of the electric field distribution that the electric field will gradually decrease with the gradual increase of the thickness of Al2O3. This is mainly because the thickness of the shell increases the distance between Au nuclei25. Depending on Su's26 paper, the resonance spectrum of Al is in the near ultraviolet spectrum, while that of Au is in the visible spectrum. Therefore, it is speculated that the Al2O3 covering the surface of the Au nucleus leads to the blue shift of the electric field enhancement factor. Compared with the Au dimer in Fig. 4 (a), the peak value of the electric field enhancement factor in Au@Al2O3 dimer is relatively concentrated. This led us to speculate whether the core-shell structure of two different materials would cause the resonance peak to move towards the resonance peak of the other material, eventually leading to a concentration of formant positions.
Figure 6 shows the comparison of Raman enhancement factors at different distances of Au and Au@Al2O3 dimers corresponding to different excitation wavelengths. At Au dimers r = 50 nm, the spectral position corresponding to the maximum electric field enhancement factor with distance from 1 nm to 3 nm is selected as the excitation spectrum, i.e. λex = 620 nm, 625 nm, 635 nm, 650 nm, 675 nm and the Raman enhancement factor with distance d = 2 nm, 3 nm is calculated. Figure 6 (b) and (d) respectively represent the spectral position corresponding to the maximum electric field enhancement factor of Au@Al2O3 at R = 50 nm, dimer distance d = 1 nm, shell thickness r0 = 1 nm ~ 4 nm as the excitation spectrum. At λex = 635 nm,640 nm,650 nm,665 nm, the Raman enhancement factors of Au@Al2O3 dimer at D = 2 nm and 3 nm are calculated. It can be seen from Fig. 6 (a) and (b) that the strength of Au@Al2O3 dimer is greater than that of Au dimer at the same distance. With the comparison of Fig. 6 (c) and (d), it is found that with the increase of Al2O3 thickness, the increase times of electric field enhancement factor will gradually increase. Therefore, it is quite meaningful to wrap Al2O3 material on the Au core shell. At the same distance, there is a certain difference in the Raman enhancement factor under different excitation wavelengths, but it is basically in an order of magnitude, which also shows that the Au dimer has a wider excitation spectrum under different distance, making its application scope wider27. Figure 6 shows that the influence of excitation wavelength on the final Raman enhancement factor of the final model with the increase of the thickness of Al2O3, however the influence of excitation wavelength on the final Raman enhancement factor of Au dimer increases with the increase of distance. The reason may be that Al2O3 as shell material makes Au@Al2O3. The resonance absorption peak of dimer is more concentrated, which leads to Au@Al2O3 When spherical dimer is used as SERS substrate configuration, it is easier to select excitation wavelength. It can be found from the Fig. 6 that slight changes in each configuration will change the corresponding optimal excitation wavelength, so the theoretical simulation is very meaningful. The results show that the Raman enhancement of Au@Al2O3 dimer is very sensitive to distance, and the wrapped Al2O3 can transfer the Raman enhancement of Au core well. Currently, the materials used for SERS substrate are generally precious metals, and the preparation cost is relatively high, thus resulting in the appearance of the shell structure. With the progress of subsequent manufacturing methods, ultra-high sensitivity SERS signals were provided for living cell detection and biological imaging28, and further understanding of cell changes is promoted.