Distance dependence of SERS enhancement factor from Langmuir-Blodgett monolayers on metal island films: evidence for the electromagnetic mechanism

Abstract

Langmuir-Blodgett (LB) films of arachidic acid have been used as spacer layers to control the separation distance between evaporated In and Ag metal island films and LB monolayers of a lightly substituted phthalocyanine. From relative intensity measurements, the distance dependence of the surface-enhanced Raman scattering (SERS) enhancement factor for the phthalocyanine was determined and found to be long range. In addition the magnitude of the enhancement was determined by measurement of the normal Raman intensity of phthalocyanine monolayers deposited directly onto unmetalized substrates. The results are compared with calculations of the enhancement factor based on an electromagnetic field enhancement mechanism. Good quantitative agreement is found between calculation and experiment for both the distance dependence and magnitude of the enhancement. We conclude that our SERS from island films is due to the electromagnetic mechanism and rule out short-range chemical mechanisms.

Surface-Enhanced Raman Scattering: Metal Nanostructures Coated With Langmuir-Blodgett Films

“…63 In 1985 Aroca et al 64 reported the SERS effect of a metalfree tetra-tert-butylphthalocyanine ((t-bu) 4 H 2 Pc) LB monolayer deposited onto evaporated In and Ag island films, while Chen et al 65 presented SERS results for LB monolayers and bilayers of polydiacetylene deposited onto evaporated Ag island films. The SERS signal was 10 2 times the RRS signal for both In and Ag islands [56], consistent with enhancement factors for other molecules. Chen et al 66 also applied RRS and SERS to study structural and electronic properties of polydiacetylene forming LB monolayer and multilayers.…”

“…In general, not only SERS benefits from this electromagnetic effect but also other processes such as emission, absorption and non-linear effects 54,55 . The SERS effect is highly distance dependent, and is eliminated when the distance d in Figure 5 reaches around 150 Å 56 . Although the single nanoparticle is used to give a simple explanation of the EM enhancement mechanism, in practice, SERS/SERRS is observed thanks to the contribution from the highest field or hot spot found in small nanoparticle dimers and aggregates in which interstitial sites are the super-enhancing locations.…”

“…The distance dependence of the moleculemetallic substrate for SERS was studied by Kovacs et al 56 with LB films of arachidic acid as spacer layers to control the distance between evaporated In and Ag island films and LB monolayers of a highly substituted phthalocyanine. This approach was possible due to the large SERS cross section of phthalocyanine and the low SERS cross section of arachidic acid.…”

“…However, there is a wide scope of molecules that have been incorporated into LB films for SERS/SERRS studies. They include mono and bis phthalocyanines 56,64,71,80,85 , polydiacetylene 65 , cyanides 69 110 , rhodamine and derivatives 25,111 , poly-p-phenylenevinylene derivative 88 , poly(vinyl) pyrrolidone 95 , xanthene dye 112 . A detailed discussion of molecule specificity for SERS and a list of molecules studied using SERS or SERRS are found in 10 .…”

Food additives characterization by infrared, Raman, and surface‐enhanced Raman spectroscopies

“…Not all the vibrational modes are intensified equally in the SERS spectra. In this technique, the orientation 23,24 and the distance 25,26 of functional groups (responsible for the adsorption process of the molecule) with respect to the surface are critical for enhancement of particular Raman bands. Thus, only the molecular fragments on the metal surface or very close to it can contribute to the SERS signal.…”