Figure 2a shows the cross-sectional SEM image of PSi layer obtained after the etching process. Thickness of the PSi layer was approximately 4 µm. The top view SEM image presented in figure 2b reveals that the PSi layer is composed of a network of nano-pores with sponge-like structure. It is clear that the pores are uniformly distributed and have nearly circular shapes. The pore size distribution ranges from 5 nm to 45 nm with the peak distribution has the maximum value within the range from 20 nm to 25 nm as shown in figure 2c, and figure 2d depicts the reflectance spectrum of PSi structure with several peaks like Fabry-Perot resonant spectrum in the wavelength range from 500 nm to 1000 nm.
Figure 3 presents the morphology of nano-silver coating layer on the silicon surface after 2 minutes immersed into 4.8 M HF and 10-2 M AgNO3 solution. The silver coating layers can be generated from nano-particle, nano flower-like (figure 3a) and nano-dendrite (figure 3b) forms. The average sizes are of 50 nm for nano-particles, of 200 nm for nano flower-like, and the length of nano-dendrites is of 1500 nm.
Figures 4a and 4c illustrate the SEM images of the AgNPs growths on the PSi surface and on the PSi-nanopillars, respectively, in which the samples prepared by immersion plating of the bare PSi into 1 mM AgNO3 solution for 20 min at room temperature.
The AgNP size distribution on the PSi surface is homogeneous with an average size of 25 nm, but the AgNP size on the nanopillars may be smaller. The existences of AgNP deposited on the PSi surface and on the nanopillars measured by the energy dispersive X-ray spectroscopy (EDX) are demonstrated in figures 4b and 4d. The appearance of the Si and O elements is obvious in EDX spectra because of the Si substrate and the oxidation of Si35,36. The evidence of nano-silver deposited on the nano-pillars inside the porous silicon structures shows that the hybrid AgNPs/PSi substrates would be to have larger active surface area for SERS measurement as quasi-three-dimension (quasi-3D) structure in comparison with two-dimension (2D) structure of AgNPs/Si. Figure 4e shows the XRD analysis of AgNPs deposited on PSi structure. There are two diffraction peaks at 38.5o and 44.7o corresponding to the reflection of 111 and 200 planes of the face-centred cubic (FCC) silver nanoparticles, respectively. These two peaks were labelled according to the standards of diffraction card of the Joint Committee on Power Diffraction Standards (JCPDS) file No. 01-073-6859.
From various parameters influenced on the morphology of AgNPs, the concentration of metallic salt solution and the deposition duration play key roles for deposition of nano-Ag on the PSi surface23,37. Figure 5 presents homogenous growth of nano-silver at different immersion times of (5, 10, 30) minutes. Figures 5a and 5b show that only an Ag partial coverage of the porous surface of 70-80% is obtained for immersion times of 5 and 10 minutes. In these cases, the particles are partially conjugated, forming a fractal-like network of large islands of Ag at early stages (5 and 10 minutes). In between, much smaller particles are observed on the underlying PSi substrate. This duration suggests that the nucleation of Ag nanoparticles started almost simultaneously at numerous sites across the whole PSi structure. Nevertheless, the existence of small Ag particles demonstrates that the nucleation and growth process are going on. Using the immersion time of 30 min, the PSi surface is covered of 98% with a dense layer composed of nanometer-size Ag particles as illustrated in figure 5c. The size is related to the number and the gap between the AgNPs, where at the early soaking times the large islands of Ag are formed and the gap between AgNPs is in the range from 50 to 160 nm. After dipping for 30 minutes, many overlapping AgNPs are observed as shown in figure 5c. For investigation on how overlapping AgNPs influenced to SERS effect, we used the samples with immersion time of 20 minutes as shown in figure 4a.
In order to verify the SERS activity of AgNPs deposited on the silicon and PSi substrates in the trace detection of toxic substances, DPA was selected as a model analyte due to its high toxicity and widespread use in agriculture. In addition, the DPA is a highly chemical reactive compound due to the imine hydrogen atom, which can easily be replaced electrophilically25, a reaction can be used for adsorbing DPA into PSi, where the molecular structures are of SiOx (x<2) and/or Si-O-H.
Figure 6 shows Raman spectra of DPA molecules absorbed on the AgNPs/PSi substrate excited at the wavelength of 532 nm with a DPA concentration of 10-3M (line a), and of DPA in the solid form (line b). The change of Raman spectrum from SERS measurement in the spectral peaks compared to common Raman measurement can be explained by chemical interaction between nano-silvers and DPA molecules. Some peaks with typical strong intensity at 517 cm-1, 913 cm-1, 1177 cm-1 and 1620 cm-1 correspond to vibration modes of C=C stretching vibrations, C-H out of plane ring wagging, C-H in plane outer ring angle bending and C-C/C-N in plane stretching38,39. Two primary characteristic Raman peaks of DPA at 339 cm-1 and 1297 cm-1 are assigned to C-N-C in plane angle bending and C-N in plane stretching38. The strong other ones at 807 cm-1, 1373 cm-1 and 1588 cm-1 are attributed to C-H out of plane ring wagging, C-C/C-N in plane stretching and C=C in plane stretching, respectively40. The intensity of the peak at 1620 cm-1 was chosen as the parameter to characterize the SERS signals.
Figures 7a and 7c present the SERS signals of DPA excited at 532 nm wavelength with varying concentrations from 10-9 M to 10-3 M adsorbed on the AgNPs/Si and AgNPs/PSi substrates, respectively. It is shown that SERS is attained at all DPA concentrations of the range from 10-9 M to 10-3 M and the basic SERS signals of DPA molecules can still be identified at the extremely low concentration of 10-9 M. A linear dependence between the logarithmic concentration of DPA and the intensity of the peak at 1620 cm-1 was observed for SERS substrates based on AgNPs/Si (figure 7b), and on AgNPs/PSi (figure 7d). These results show a good linearity (R2 = 0.9979) and the limit of detection (LOD) is of 10-9 M.
Figure 8a demonstrates the SERS intensities for DPA with a concentration of 10-9 M adsorbed on the AgNPs/Si and AgNPs/PSi substrates. The Raman signal obtained from AgNPs/PSi is 5 times higher than that from AgNPs/Si. Figure 8b shows the dependence of Raman intensities upon DPA concentration in the range of 10-3-10-9 M. The Raman intensity from the hybrid nano-Ag/PSi structure is several times higher than that from nano-Ag/Si substrate. This phenomenon can be explaind by the large active surface area of the nano-Ag/PSi as a quasi-three-dimension (quasi-3D) structure in comparison with 2D structure from the nano-Ag/Si, because the nano-Ag can be simultaneously deposited on the surface of the PSi substrate and on the nano-pillars of the PSi structures. Further work is needed in order to control the nano-pillar sizes for obtaining the optimum active surface of hybrid nano-Ag/porous silicon structural SERS substrates.
Table 1 shows the limit of detection (LOD) for DPA measuring by different methods. The electrochemical sensors based on different substrates possess the LOD of 10-4–10-8 M, and the portable Raman spectrometer without the SERS effect has the LOD of 10-3 M. Our result of LOD obtained from SERS substrates based on the hybrid AgNPs/PSi structures is below 10-9 M. Thus, the hybrid AgNPs/PSi SERS substrate exhibits an ultrasensitive detection of DPA down to 10-9 M, which indicates a good method for toxic molecular sensing.
Table 1. Summary of limit of detection (LOD) for DPA obtained from different technics.
Techniques
|
Detection Limit (M)
|
Ref&Year
|
Thin-film Electrochemical sensor
|
3.9 x 10-6 M
|
[23] 2014
|
Portable Raman spectrometer
|
14.43 x 10-3 M
|
[41] 2019
|
Molecularly imprinted polymers (MIP) technique based Carbon paste electrochemical sensor
|
10-4 M
|
[42] 2017
|
Electrochemical sensor based on reduced Graphene Oxide/Fe3O4-molecularly imprinted Polymer
|
5x 10-8 M
|
[43] 2018
|
Surface-enhanced Raman scattering based on AgNPs/PSi
|
Below 10-9 M
|
This work
|