Silver nanoclusters decked diamond thin film as a substrate for surface-enhanced Raman scattering

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Since the advent of surface-enhanced Raman scattering (SERS) there has been a continuous effort to develop stable and reproducible SERS active substrates that not only provide a large enhancement factor but are also useful in analytical applications like bio-sensing. 1 However, most of the substrates lack long-term stability due to instability of metal nanostructures deposited on the substrate surface and/or the substrate itself. 1On the other hand, owing to its diverse intrinsic properties, diamond thin film has established itself as an exceptionally stable electro-chemical transducer. 2 Diamond thin film's dielectric properties 3 are anticipated to help in the enhancement of surface plasmons resonance 4 and thereby enabling itself to detect bio-molecular interactions in real time based on SERS. 4 Moreover, over the past decade, diamond thin film has been developed as an attractive bio-sensoric material. 5In this work, a typical polycrystalline diamond thin film surface decked with silver (Ag) nanoclusters is shown as a suitable substrate for SERS.Standard Rhodamine 6G (R6G) probe molecules are used to access the SERS activity of the test surface. 6This work is expected to benefit SERS based bio-sensing using selective SERS metal (like Ag)/diamond/silicon (Si)/back contact (like Aluminum) structured metal-insulator-semiconductor devices.
Polycrystalline diamond thin film considered in this study was deposited on p-type (100) Si substrate using microwave plasma enhanced chemical vapor deposition technique.Submicron sized diamond grains constituted the microstructure of the thin film.The detailed deposition procedure and other characteristics of this diamond thin film have been previously reported. 7,8 he polycrystalline diamond thin film used in this work is a typical one and has been proven to have bio-sensoric activity. 9,10 g nanoclusters are deposited on the diamond thin film surface using a magnetron cluster deposition system (NANODEP 60 from Oxford Applied Research, UK) following a previously reported procedure. 11Ag nanoclusters deposition was carried out for only 2 min in order to avoid formation of any thick coating of Ag which may be unfavorable for SERS activity.SERS data obtained from R6G molecules adsorbed on as-deposited and Ag nanoclusters decked diamond thin film surfaces are shown in Fig. 2. For convenience Raman spectra obtained from asdeposited (Fig. 2(c)) and Ag nanoclusters decked (Fig. 2(b)) diamond thin film surfaces are also included in Fig. 2. Raman bands at 1135, 1332, 1483, and 1550 cm −1 in Fig. 2(c) are often seen in the Raman spectrum of a typical diamond thin film.The broad 1332 cm −1 Raman peak indicates the presence of small diamond crystallites in the diamond film. 12,13 he appearance of Raman band at 1135 cm −1 (with an accompanied band at 1480 cm −1 ) is due to the presence of trans-polyacetylene segments in the grain boundary regions of the diamond thin film. 145][16] Before discussing further about SERS signals from R6G molecules, it is important to note (Fig. 2) that Ag nanoclusters decked diamond thin film did not result in any SERS signal from the underlying diamond phase.Therefore, SERS data obtained from R6G molecules adsorbed Ag nanoclusters deposited diamond thin film surface have no interference from Raman signals corresponding to the underlying diamond phase.
SERS signals corresponding to R6G molecules immobilized on as-deposited diamond thin film surface are not observed (Fig. 2(a)); the reasons for this is the absence of any interaction between R6G molecules and the test surface at the atomic level and too low concentration of R6G molecules on the test surface to scatter Raman signals.On the other hand, SERS signals corresponding to R6G molecules adsorbed on Ag nanoclusters decked diamond thin film are highly amplified and can be observed (Fig. 2(d)) at ∼612, ∼772, ∼920, ∼1130, ∼1188, ∼1312, ∼1362, ∼1506, ∼1572, and ∼1649 cm −1 . 11The reason for this is the creation of an apt condition for SERS.It is evident that the presence of Ag nanoclusters has resulted in SERS signals from R6G molecules.This is attributed to the surface plasmon resonance of the Ag nanoclusters.Ag has been commonly used to study trace amounts of Raman active materials using SERS methodology.Raman scattering resonance region depends upon the plasmon energy of the metal particles used to create an apt condition for SERS.For Ag particles, the resonance region is in the range 1300-1500 cm −1 .The presence of two very high intensity SERS signals corresponding to R6G molecules at 1312 and 1362 cm −1 , which fall in the resonance region of Ag confirms that R6G molecules are adsorbed on the test surface.Additionally, the enhancement of R6G vibrational modes at ∼612 and ∼1649 cm −1 suggests that the enhancement originates due to the charge transfer from the Fermi level of the Ag metal to the lowest unoccupied molecular orbital of the adsorbed molecules, which changes the effective polarizability of the molecules, in-turn resulting in the observed SERS activity. 11In the present context, this electromagnetic effect can originate from the localized surface plasmon resonance of the Ag nanocluster aggregates 17 on the diamond films.These aggregates can generate a huge electromagnetic field at the junction sites.If there is Raman signal enhancement mainly through any chemical mechanism, 1 R6G molecules immobilized on the as-deposited diamond thin film could have been detected using Raman scattering.
Fig. 3 shows high quality Raman spectra (only for comparison purposes) obtained from R6G molecules adsorbed on Ag nanoclusters decked on diamond film, p-type (100) Si surface and on as-deposited diamond film.It is evident that Si also shows SERS activity.However, it should be noted that Si cannot replace diamond/Si structure for any further bio-sensing device fabrication.The comparison of the spectra reveals the existence of different SERS enhancements.A simple quantitative intensity comparison is shown in Fig. 3 for a selected Raman band at ∼1650 cm −1 .It can be clearly observed that the Raman signal enhancement in the case of Ag nanoclusters decked diamond film is better than that in Si.Furthermore, reproducibility (refers to the ability to produce SERS signal at various locations of the SERS active surface with as minimum as possible intensity variation) of the SERS activity of Ag nanoclusters decked diamond thin film has been checked.SERS spectra obtained from R6G molecules adsorbed at several positions on Ag nanoclusters decked diamond film are shown in Fig. 4. The intensities of Raman bands at ∼613, ∼773, ∼1186, ∼1362, ∼1509, ∼1533, ∼1576, and ∼1649 cm −1 are quantified using Loretzian fit.The results showed that the intensity variations of all the Raman bands are less than 5%, which is an acceptable value.This in turn shows that the test surface is homogeneous.Here, it is also very important to mention that the fabricated Ag nanoclusters decked diamond thin film showed the same reproducibility even after leaving it in ambient conditions for days together.This shows that neither Ag nanoclusters degraded nor the underlying diamond.In another simple experiment, to check the reusability of the SERS substrate under consideration, the substrate was ultrasonicated in methanol to remove the adsorbed R6G molecules.Then, freshly prepared R6G solution was dropped and allowed to dry naturally following the procedures described in the beginning.Raman signals with the similar intensity from R6G could be again detected.It was found that the fabricated SERS substrate is reusable for at least two times.Thereafter, the Raman signal intensity would reduce.In conclusion, a stable Ag nanoclusters decked diamond film demonstrates an excellent and reproducible SERS activity which is explained in terms of the well-known electromagnetic enhancement mechanism.The diamond film used in the present experiments is a typical one and can be deposited on a variety of substrates with ease.Similarly, Ag nanoclusters deposition procedure used in this work is not only very easy but also not time consuming.Even though the R6G molecules' concentration used in the present study is good enough for practical applications, the detection limits have to be further determined.The influence of Ag nanoclusters size, morphology, distribution etc., on any further enhancement and detection limits is the subject of future investigation.
FIG. 1.(a) Low magnification plane view secondary electron micrographs of Ag nanoclusters decked diamond thin film; (b) the corresponding high magnification micrograph.As-deposited diamond thin film surface is shown in the inset.

FIG. 2 .
FIG. 2. Raman spectra obtained from (a) R6G on diamond thin film (b) Ag deposited diamond thin film (c) as-deposited diamond thin film, and (d) R6G adsorbed on Ag deposited diamond thin film.