**4. SERS substrates**

To be used in a sensor system, a SERS substrate should enhance the Raman effect sufficiently to enable consistent and uniform chemical detection sensitivity across the surface, maintaining its properties as long as possible in the time and to provide a high number of sites for molecular adsorption.

In principle, all systems possessing free carriers show the surface enhancement effect. However, the plasmon properties—such a wavelength and width of its resonance—depend on the nature of the metal surface and on its geometry and affect the enhancement factor (EF) of the surface [4, 5, 19]. The width of plasmon resonance resulted to be: w = γ(ε<sup>b</sup> + 3), where γ is the electron scattering rate and ε<sup>b</sup> is the contribution to the inter-band transitions to the dielectric constant. Smaller conductivity and a large number of inter-band transitions in the region of plasmon resonance give resonance peak with large width and hence smaller amplification of electric field [19]. To this respect, the coin metals (Ag, Au, and Cu) resulted to be the most appropriate to be used for SERS with their amplification factors much larger than unity. Differently, the enhancement factor of good conductors as Al, Pt, and In is larger but not much larger than unity and is only slightly greater than unity for most other metals. The other advantage of using coin metals is that their plasmon resonance wavelength is in the visible–near infrared.

Early SERS experiments used gold colloids in solution. Nowadays, by exploiting semiconductor lithographic fabrication technology, periodic patterns on Si surface can be reproducibly fabricated over large areas. Ordered geometry provides uniform SERS signals from anywhere on the active surface, avoiding that only small uncontrolled areas of the total metal surface

#### 208 Raman Spectroscopy


**Table 1.** Enhancement factors of gold coated substrates with different geometry.

have the correct geometry for surface enhancement, as occurred by using substrates with random nanosized roughening or nanoparticle separation and sharp metallic features, produced with the previous techniques. Usually, the SERS substrates are gold coated because, although the gold has a smaller EF than silver, it is more resistant against oxidation in air.

We used three variations of SERS active gold substrates: an ordered array of inverted squarebased pyramids, (KlariteTM, D3 Technologies Ltd3), an electrochemically nano-roughened surface (Nanova), and an array of nanopillars, 50–80 nm in width, 600 nm in height, fabricated by the Department of Micro and Nanotechnology of the University of Denmark [21–23].

We measured the EFs of all the above cited substrates by measuring the Raman signal of the same molecule, in this way only electromagnetic enhancement due to the geometry of the used structure was considered, avoiding the effect of chemical enhancement [20, 24]. The obtained results are reported in **Table 1**. The regularity of nanostructure guaranteed a uniform enhancement factor across the surface and a high reproducibility of Raman signal in spite of the differences among the EFs.

> When the number of analyte molecules is too large, a sample layer is formed covering the molecules with surface-enhanced Raman signal. To this respect, it is of prime importance that the substrate geometry provides enhancement region accessible for molecular adsorption.

Surface-Enhanced Raman Spectroscopy Characterization of Pristine and Functionalized Carbon…

http://dx.doi.org/10.5772/intechopen.74065

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**Figure 4.** Variation in the intensity of surface-enhanced Raman signal with the amount of analyte mass probed by the

laser.

**Figure 3.** (a) Sequential steps of the drop and dry technique for films deposition and (b) top view of dried film.
