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Plasmonic Metal Displacement Deposition on Porous Silicon for SERS Substrate Fabrication
Hanna Bandarenka

Last modified: 2012-01-06


Abstract-Displacement technique was applied to form silver and copper deposits. Using porous silicon as a substrate gave a metal film nanoroughness. Morphology of the obtained samples was studied with scanning electron microscopy. Raman spectra of samples impregnated with rhodamin 6G solution were taken and their dependences on porous silicon morphology and metal deposition regimes were determined. Enhancement of Raman signal was detected from silver films deposited on meso- and macroporous silicon, and copper porous film obtained due to complete mesoporous silicon displacement by copper atoms.  


Since its discovery surface enhanced Raman spectroscopy (SERS) has been attracting a great attention of scientific community because of possibility to detect small amount of substances. This phenomenon takes a place due to an enhancement in the electric field provided by the surface. When the incident light in the experiment strikes the nanostructured surface of Cu or nobel metal films, localized surface plasmons are excited.  SERS provides information on biologically or medically relevant species and qualifies for trace analysis [1]. SERS detection requires substrates and fabrication process improvement. Recently, it has been revealed that rough surfaces obtained by displacement deposition of Ag and Cu on porous silicon (PS) are perspective as SERS-active substrates [2-4]. Simplicity and economy of technology are among the advantages of such substrates. Ag is known as best SERS metal provided high level of signal rising. Cu has low cost in comparison with other plasmonic metals. In the present work we have reported results of the development and optimization of SERS from Ag and Cu films formed by displacement deposition on PS.

PS was prepared by an electrochemical etching of p- and n-type monocrystalline Si wafers in HF-based solutions. Different etching regimes were applied to vary morphological properties of PS. After that PS samples were immersed into metal salt (AgNO3, CuSO4) solution with small additions of HF, ethanol or isopropyl alcohol for periods from several minutes to hours. Samples for the SERS measurements were impregnated for 2 hours in the rhodamin 6G (R6G) solution. The surface morphology was examined by SEM (Hitachi 5500). Raman spectra were obtained with (a) SpectraPro 500 I spectrometer (diode laser - 532 nm radiation) or (b) self-made Raman spectrometer (512 nm radiation from He-Cd laser).

Our previous study [5] has revealed that variation of PS morphology and metal displacement deposition regimes allowed fabrication of several principally different structures: (i) PS dotted with separated metal



Figure 1. SEM top view of mesoPS before (a) and after (b) Ag deposition


Figure 2. Dependence of  SERS intensity on Ag deposition time

nanoparticles (NPs), (ii) PS covered with quasi continuous metal films, and (iii) porous metal layers got by prolonged displacement process. SERS activity of all mentioned structural types was checked for both metals and some of them have promoted increasing of Raman signal. Figure 1 presents SEM plan view images of mesoPS formed at 30 mA/cm2 current density before (a), after (b) immersion in Ag salt solution. Initial PS was of 1 mm thickness and had pore diameter of several tens of nanometers. Figure 1(b) represents (ii) type of Ag/PS structures. Diameter of Ag NPs is about 110±40 nm. An average distance between large Ag clusters is in the range 100-200 nm. Meanwhile the gaps between clusters are filled by smaller NPs (~15-25 nm), many of them almost touch each other that should promote the plasmon coupling. Therefore, strong local field enhancement areas (so-called “hot spots”) appears. The intensity of the SERS signal was estimated at 532 nm laser radiation from the amplitude of the 1650 cm–1 line in the spectrum of R6G and the maximum was found from PS after 5 min of Ag deposition (Fig.2). Successful results were also obtained from p-type macroPS covered with partially connected Ag NPs and p-type mesoPS fully converted into porous Cu during 4 h deposition. The intensity of the SERS signal was estimated from the amplitude of the 1366 cm–1 line in the spectrum of R6G. For macroPS the maximum was detected from sample obtained after 120 min of Ag deposition.


Acknowledgements. The present research was funded under the followed projects: F11MC-019, T10M-089 and T11OB-057.



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