Firstly, gold nanocups were obtained by selectively growing Au on PbS nanooctahedron and then dissolving PbS components. Using gold nanocups as starting substrate, Au/Ag core-shell nanostructures were synthesized by adding growth solution prepared by AgNO3. Using it as a substrate, adding a growth solution configured with HAuCl4 to the solution, with a molar ratio of 1:1 between HAuCl4 and AgNO3. Au/AgAu1 core-shell nanostructures were obtained through electrical substitution and overgrowth processes. Figure 2(a) shows the SEM images of Au/AgAu1 core-shell nanostructures. The gap between the gold nanocup core and the silver gold hybrid nanoshell can be clearly observed under a transmission electron microscope. These Au/AgAu core-shell nanostructures with gaps provide a good foundation for gap enhanced SERS based on magnetic plasmon coupling. Figure 2(d) is the TEM image of the nanostructure, and Fig. 2(g) is the TEM image of a single particle of the nanostructure. Repeat the above process and control the molar ratio of HAuCl4 and AgNO3 added to each layer at 1:1. Au/AgAu2 core-shell nanostructures with two-layer gaps and Au/AgAu3 core-shell nanostructures with three-layer gaps were synthesized, respectively. Figure 2(b) shows the SEM images of Au/AgAu2 core-shell nanostructures, and Fig. 2(c) shows the SEM images of Au/AgAu3 core-shell nanostructures. Under transmission electron microscopy, it can be observed that the Au/AgAu2 core-shell nanostructure contains two layers of gaps. Figure 2(e) shows the TEM image of the nanostructure, and Fig. 2(h) shows the TEM image of a single particle of the nanostructure. It was observed that the Au/AgAu3 core-shell nanostructure contains three layers of gaps, and the width of the gaps decreases as it moves towards the outer layer. Figure 2(f) shows the TEM image of the nanostructure, while Fig. 2(i) shows the TEM image of a single particle of the nanostructure. During the experiment, we also prepared the core-shell structure with notch, but the characterization results showed that the Raman enhancement effect of this structure was far worse than that of the circular core-shell structure, so the subsequent research focused on the circular core-shell structure.
We used a visible ultraviolet spectrophotometer to determine the extinction spectra of the product, and studied the plasmon resonance properties of the gold nanocup/silver gold hybrid nanoshell core-shell nanostructure using extinction spectra. Figure 3(a) shows the extinction spectra of Au/AgAu1-RhB1, Au/AgAu2-RhB1, and Au/AgAu3-RhB1 core-shell nanostructures at the same particle concentration. The plasmon resonance peak of the initial gold nanocup is located at 615 nm. After the gold nanocup is mixed with RhB aqueous solution, the growth solution containing AgNO3 and the growth solution containing HAuCl4 are added, and a layer of silver gold hybrid nanoshell is grown on the surface of the gold nanocup to obtain the Au/AgAu1-RhB1 core-shell nanostructure. Repeat the above experimental operation and regenerate a long layer of silver gold hybrid nanoshell on the surface of the above product to obtain the Au/AgAu2-RhB1 core-shell nanostructure. Continue to repeat the operation, and obtain the product Au/AgAu3-RhB1 with RhB molecule embedded in the first layer gap and three layers of AgAu hybrid nanoshells. Observing the extinction spectra of the three products, we found that with the increase of the number of AgAu hybrid nano shells, the isoplasmon formant gradually redshifted, and the acromion appeared at 555 nm to the left of the main plasma formant. With the increase of the number of shells, the acromion intensity gradually increased and produced a slight redshift. Figure 3(b) shows the extinction spectra of Au/AgAu3-RhB1, Au/AgAu3-RhB2, and Au/AgAu3-RhB3 core-shell nanostructures at the same particle concentration. These three nanostructures are all wrapped with three layers of silver gold hybrid nanoshell on the surface of gold nano cup core, the difference is that RhB molecules are embedded in the first layer gap, the second layer gap and the third layer gap respectively. The extinction spectrum shows that the plasmon resonance peaks of the three nanostructures are all located at 615 nm, and a shoulder peak appears at 910 nm. With the position of RhB molecular embed moving outward, the intensity of the shoulder peak gradually increases, accompanied by a slight red shift. During the process of couple replacement, a very narrow gap is formed between the shell and the shell, which can induce strong plasmon coupling and generate strong localized electromagnetic fields in the nano gap. These Au/AgAu1, Au/AgAu2, and Au/AgAu3 core-shell nanostructures provide reliable ideas for enhancing Raman signals.
By observing the TEM images of the three products, the gaps between shell and core, as well as between shell and shell, can be clearly observed. However, due to the denser shell, the observation effect on the gaps is not satisfactory. In order to display the internal structure of each product more clearly and intuitively, we took photos of the distribution of Au and Ag elements in the products of these three structures, as shown in Fig. 4. The content of gold in Au/AgAu1 core-shell nanostructures is much higher than that of silver, and the distribution is relatively uniform. Silver is mainly distributed on its surface, and there is a clear boundary for concentration changes in both distribution maps, which corresponds to the gaps in the nanostructure, as shown in Fig. 4(a). In the element distribution diagram of Au/AgAu2 core-shell nanostructures, two significant concentration changes can be observed, indicating the existence of two gaps in the structure, as shown in Fig. 4(b). In the element distribution map of Au/AgAu3 core-shell nanostructures, there are three significant concentration variation boundaries, indicating the existence of three gaps in the structure, and the lighter the color towards the outer side, the lower the element distribution concentration, and the smaller the width between the outer boundary and the boundary, indicating a smaller thickness towards the outer shell layer, as shown in Fig. 4(c).
Under the excitation of light with a wavelength of 633 nm, the SERS characteristics of gold nanocup core/silver gold nanoshell plasmon core shell nanostructures with RhB molecules embedded in the first layer gap and wrapped with one layer, two layers and three layers of silver gold shells respectively were tested. The measured results were in good agreement with the plasmon resonance peak, as shown in Fig. 5(a). The Raman spectrum of RhB molecule is clearly visible, indicating that RhB molecule is well embedded in the nano gap. As the number of shell layers increases, the Raman intensity of this nanostructure sharply increases and rapidly decays. The Raman intensity of Au/AgAu2-RhB1 core-shell nanostructures is the strongest, and the peak broadening is the highest at around 1500 nm. This is because there is only one layer gap in the Au/AgAu1-RhB1 structure to enhance the Raman scattering signal, and the effect is poor. There is a double gap electromagnetic field coupling in the Au/AgAu2-RhB1 core-shell nanostructure, which has greatly enhanced the Raman scattering signal. However, the embedded position of RhB in the Au/AgAu3-RhB1 core-shell nanostructure is too deep, the outer shell is too thick, and the shielding effect on excitation is too strong, resulting in poor Raman scattering enhancement effect. Using the same method, we also measured the SERS characteristics of the gold nanocup core/silver gold nanoshell plasmon core shell nanostructure with RhB molecules embedded in the first layer, the second layer, and the third layer gaps, all wrapped with three layers of silver gold shells. The results are shown in Fig. 5(b). The structure showed that the Raman enhancement effect of RhB molecule was the weakest when it was connected to the first gap, and the Raman signal enhancement effect of RhB molecule when it was embedded into the second gap was much greater than that of the other two structures. This was due to the electromagnetic coupling between the first and third nanogaps in the three-gap nanostructure, which further enhanced the electromagnetic field in the second gap.