3.1. Structural analysis of monometallic (Ag and Cu) and bimetallic (Ag-Cu and Cu-Ag) thin films
Fig.1 (A) shows the XRD pattern of silver thin films of two different lengths. The peaks at 2θ values of 38.1°, 44.09° and 77.29° correspond to (111), (200), (311) planes of silver, respectively. Thus, the XRD spectrum confirmed the crystalline structure of silver films. No other peak corresponds to any impurity is seen. All the peaks in XRD pattern can be readily indexed to a face-centered cubic structure of silver as per available literature (JCPDS, File No. 4-0783). By using the Scherer equation on the above peaks, the average size of the crystallites on the silver thin films is determined and tabulated in Table.1.
The lattice constant calculated from this pattern has been found to be a = 0.4085 nm, which is consistent with the standard value a = 0.4086 nm. Fig.1 (B) shows the XRD pattern of copper thin films of different thicknesses. The presence of peaks for 50 nm thick film at 2θ of 43.25°, 50.37° and 74.05°, corresponds to (111), (200), (311) planes of copper, respectively. The 30 nm thick copper film exhibits amorphous nature. All the peaks in XRD pattern can be readily indexed to a face-centered cubic structure of copper as per available literature (JCPDS, File No.89-2838). By using the Scherrer equation on the above peaks, the average crystallite size in the copper thin films is determined and shown in Table 1. Fig.1(C) and (D) Shows the XRD patterns of Ag-Cu and Cu-Ag bimetallic thin films.
Fig 1 (A) and (B) shows the presence of silver and copper peaks at 2θ values 38.1°, 44.09°, 77.29°, corresponds to (111), (200), (311) planes of silver and copper respectively. The dominant peak of Ag (111) confirms the Ag prior to the copper. Similarly, in Fig1 (C) copper is having a dominant peak (111) at 2θ value of 44.09°. A low intensity silver peak at 2θ value 38.1° is also seen. The grain sizes were calculated using the Scherer equation and tabulated in Table 2.
3.2. FE-SEM Analyses of monometallic (Ag and Cu) and bimetallic (Ag-Cu and Cu-Ag) nanostructured thin films
Fig.2 shows the FESEM images of silver and copper thin films of 30 and 50 nm thicknesses respectively prepared using thermal evaporation technique. The fig 2. (a) and (b) shows the morphology of randomly distributed silver nanoballs. Films with higher thickness show larger silver spheres in the size range of tens of nanometers. Similarly in copper films with smaller thickness, nanobeads with diameters of a few nanometers are seen (Fig.2 (c)). When the thickness is increased, not surprisingly, more of these small copper beads developed into clusters. The copper clusters show highly dense beads with no gaps between two nanobeads.
The FE-SEM images in Fig.2 (e)shows that the Ag-Cu thin film has a highly aggregated nanobeads like structure and Fig.2 (f)the Cu-Ag thin film has aggregated nanoclusters like structure. Apart from this, no other clear or useful information is obtained from the FESEM analysis. FESEM images in Fig.2 (e and f) shows the aggregated nanobeads and nanoclusters like structures of Ag-Cu and Cu-Ag bimetals. In these bimetallic thin films, SERS intensity is found to be stronger for adsorbed molecules on the surfaces covered with aggregated nanoparticles. This shows the electric field is due to the aggregation in between the nanobeads and clusters results into higher order SERS (Yi et al. 2011). As seen in earlier sections, pure metallic thin films of copper and silver didn’t show much high order enhancement for R6G 10-10 and 10-8 molar; but the bimetallic coating could able to detect dye concentration up to 10-10 molar with clear spectrum. Hence it is evidenced that the aggregates structures gives room for the formation of more hot spots, that in turn help the detection of dye molecule even in their very low concentration level.
The Enhancement factor was also calculated according to the equation,
whereIsurf and Ibulkare the integrated intensities of R6G molecules adsorbed on Ag-Cu nanoclustersthin films for 10-10 M and 10-3 M of R6G bulk, respectively. NsurfandNbulkare the corresponding numbers of R6G molecules adsorbed respectively on the SERS substrate and in the bulk solution effectively illuminated by the laser beam, Nbulk=AhCbulkNA, where A is the area of the laser focal spot, h is the focal depth of the laser, and h is13 µm.Cbulkis the concentration of R6G bulksolution, here Cbulk=10-3 M, NA is the Avogadro constant for the below equation.
The above calculations were done using Fig.6 for the 20 μl R6G solution (1x10-3 M) and the R6G - absorbed on bimetallic thin films (Ag-Cu) )(1x10-10 M, 20 μl), which were spread on the 1x1 cm2 prepared substrate. A laser spot area of 5 µm and power of 15mW was applied with an accumulation number of 1sec for all recordings.
3.3 SERS Spectra for monometallic Ag and Cu and bimetallic Ag-Cu R6G and Cu-Ag R6G thin films
The particle size, shape and specific adsorption sites are the important factors in surface plasmon resonance (SPR). The junction between two Ag nano beads and Cu nano clusters acts as hotspot for the detection of low-density dye molecules. Fig 3 shows SERS spectra of R6G dye with different concentrations ranging from 10-5 to 10-10 M adsorbed on Ag, Cu films. The Raman peaks are sharper for silver film with thickness 30 nm and show detection limit up to 10-8 molar concentration. In case of 50 nm thick silver film, the R6G peaks are sharper but the detection limit is low compared to the other film. Due to the external electrical field applied on the nanobeads, they are polarized and consequently an enormous electrical field generate around it.
The electrical SPR is influenced by the thickness and roughness of the film or by random arrangement of the nanobeads. This effect results in higher order SERS intensity and hence the detection low of R6G dye molecule with concentration. Similar results are observed with copper films with thickness of 30 nm. Nanoscale structure and sharp points in metal nanostructures are the primary features needed to produce the highest possible enhancement in Raman scattering. Here individual Ag nanoballs structure with size of around 100 nm (Fig.2) which results in strong localized plasmon resonance and offers an opportunity to realize notable SERS enhancement in the case of silver thin films.
3.3.1.Influence of Ag and Cu film thickness on SERS response
The effect of the thickness of the silver and copper films on the SERS response was studied using the 1365 cm-1 peak R6G dye of Fig 3. It is seen that the sensitivity of the silver substrate to adsorbed R6G dye is improved with higher thick metal film. Further increase of the film thickness (not shown in the figure) resulted in a decline in the sensitivity. When the thickness is increased from a minimal value, the number of larger silver clusters increased. Since these clusters are in the size range for optimal SERS enhancement, the response is improved. When the thickness is further increased, the clusters merged together which leads to the decrease in the SERS signal. Silver film with 50 nm thickness shows maximum response. In the case of copper, some SERS response is seen in film with 30 nm thick, whereas 50 nm and higher thickness films show very little SERS response.
From Fig. 4, Ibulk(1365cm-1) and Isurf(1361 cm-1) are determined as 24.10 and 48.10 cps respectively. Here the incident laser power is the same for normal Raman spectrum and SERS spectrum acquisition. Hence Isurf /Ibulkis calculated as 48.10 / 24.10. The Nbulk/Nsurfvalue is calculated to be 32.656×105 and ISERS/ Ibulk is about 7.79 for the vibration peak at 1367 cm-1. Finally, the EF of this Ag-Cu SERS substrate is determined as 6.5×105.