Surface enhanced Raman scattering improvement of gold triangular nanoprisms by a gold reflective underlayer for chemical sensing

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Abstract

We report on an improvement way of the SERS signal of Au triangular nanoprisms for a highly sensitive detection of chemical molecules. This improvement is obtained by a simple addition of a gold reflective layer under Au nanoprisms. Using the same Au triangular nanoprisms obtained by nanosphere lithography, we studied experimentally the thickness effect of this gold underlayer on the SERS intensity of the triangular nanoprisms. We demonstrated that this SERS intensity increased with the thickness of the gold reflective underlayer, and this is due to the increment of the Au underlayer reflectivity. Thus, we showed that the metallic reflective underlayer has an important key for SERS enhancement. Indeed, enhancement factors of 108 were found for the most important thickness of the gold underlayer.

Introduction

Since these last years, the fabrication of Surface Enhanced Raman Scattering (SERS) substrates having high enhancement factors (EF) is one of most important works in this domain. Indeed, the SERS sensitivity is strongly dependent on the optical properties of the SERS active surface (electromagnetic field [1]). Thus, a critical point of SERS is to finely control the surface properties and the nanostructure geometries in order to achieve high, stable and reproducible enhancement factors [2], [3]. Several lithographic techniques, which allow to tune the size, shape, and the organization of nanostructures, exist to fabricate the SERS substrates like X-ray Interference Lithography (XIL) [4], Nanoimprint Lithography (NIL) [5], [6], [7], NanoSphere Lithography (NSL) [8], [9], [10], [11], [12], [13], Deep UV lithography [14], and Electron Beam Lithography (EBL) [15], [16], [17], [18], [19]. In addition, structures with metallic multilayers can improve SERS signal such as a three-layer SERS substrate consisting of an Ag film over Ag-clad colloidal Au submonolayer developed by Mulvaney et al. [20]. This SERS substrate has reached an EF of 7 × 104. Other groups have studied Au nanostructures on Au or Ag underlayer and they demonstrated larger EFs than those obtained in the absence of metallic underlayer [17], [19], [21]. Pérez-Mayen et al. have also showed SERS substrates based on star-like gold nanoparticles, which are deposited on silicon substrates or gold underlayer coated on silicon substrates, where the highest EFs (up to 1012) are obtained with the presence of the gold underlayer [22]. However, the explanation of these higher EFs in presence of metallic underlayer is not very well-understood, and the thickness effect of this metallic underlayer was little studied at our knowledge. In this paper, we present a simple way to improve the SERS signal of gold triangular nanoprisms, which consists of a gold reflective underlayer under the gold nanoprisms. The thickness effect of this gold underlayer on the sensitivity of Au triangular nanoprism SERS substrates is investigated. To do that, we used the NSL technique to realize gold triangular nanoprisms, because NSL is a simple, low cost and versatile fabrication tool. Then, we used gold underlayers with 3 different thicknesses.

Section snippets

SERS substrate fabrication

The fabrication process of Au triangular nanoprisms on gold underlayer is as follows: (i) cleaning of the glass substrate, (ii) deposit of titanium and gold layers under vacuum, (iii) deposit of colloidal beads of polystyrene on the gold underlayer, (iv) second Au layer deposition by evaporation under vacuum, (vi) lift-off process.

Firstly, the following protocol: ethanol, acetone, and Piranha solution ((3:1) H2SO4/H2O2 30%), is used to clean the glass substrates. Then, a 2 nm titanium layer,

Results and discussion

Firstly, the surface plasmon resonance of gold triangular nanoprisms was measured, and we found a plasmon resonance at λplasmon = 812 nm (see Fig. 2). Then, Au nanoprisms were functionalized with the thiophenol molecules following the protocol described in Section 2.2, and were characterized by SERS measurements. Fig. 3 represents the SERS spectra of Au triangular nanoprisms obtained for three different thicknesses of the gold underlayer at two excitation wavelengths: (a) λexc = 660 nm and (b) λexc = 

Conclusion

In this paper, we demonstrated the importance of a metallic underlayer under gold triangular nanoprisms on the SERS intensity and sensitivity for 3 different thicknesses (30, 50 and 100 nm). The highest EF values were obtained for a thickness of 100 nm (98% of reflectivity at 785 nm, EF = 1.3 × 108). We also showed that the SERS intensity and sensitivity varied linearly with the reflectivity of gold underlayer. Moreover, independently of the excitation wavelength (660 or 785 nm), a gain of the SERS

Acknowledgements

The authors acknowledge ANR P2N (ANR-12-NANO-0016) and the support of the French Government for partial funding of the project in which this work takes place. This work was partly supported by the French RENATECH network. IOGS/CNRS is also part of the European Network of Excellence in BioPhotonics, Photonics for Life, P4L.

Jean-François Bryche, 25 years old, he received his Master degree in applied physics at University of Paris-Sud in 2013. Currently, he is a PhD student at Institute of Fundamental Electronics (Orsay, France) and at the Charles Fabry Laboratory from the Institute d’Optique Graduate School (Palaiseau, France). He works on the development of SPR and SERS sensors by nanostructuring the surface with several lithography techniques.

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    Jean-François Bryche, 25 years old, he received his Master degree in applied physics at University of Paris-Sud in 2013. Currently, he is a PhD student at Institute of Fundamental Electronics (Orsay, France) and at the Charles Fabry Laboratory from the Institute d’Optique Graduate School (Palaiseau, France). He works on the development of SPR and SERS sensors by nanostructuring the surface with several lithography techniques.

    Anna Tsigara, studied Physics (B.Sc.) and Photonics (M.Sc.) at the University of Ioannina, Greece, and received her Ph.D. in Materials Science from the National Technical University of Athens, Greece. She has worked on microsensor, photonic sensor and Near Field Optical Microscopy projects at the Micra Institute at ITT Dublin, at TPCI of NHRF in Athens, at the University of Montpellier II and University Paris Sud, and she is currently Staff engineer at Seagate Technology.

    Benoit Bélier, 45 years old, he received his Master degree in applied physics at the University of Limoges in 1994. Then, he earned a fifth year diploma (DEA) from the University of Montpellier II in microelectronics and optoelectronics in 1996. He received his Ph.D. in electronics and optoelectronics from the University of Montpellier II in 1999. In 2000, he was post-doctoral fellow at the Laboratory for Analysis and Architecture of Systems (Toulouse, France). His research involves development and microfabrication of MEMS for biological applications. Currently, he is a CNRS research engineer at the Institute of Fundamental Electronics (Orsay, France).

    Marc Lamy de la Chapelle is professor at Université Paris 13, France, in the Laboratory of Chemistry, Properties and Structure of Biomaterials and Therapeutics Agents. He received his Ph.D. in science physics in 1998 from the University of Nantes, France. His research activities are focused on nano-optics and Raman spectroscopy. His research subject is the application of surface-enhanced Raman spectroscopy and tip-enhanced Raman spectroscopy to biological issues, especially to disease diagnosis. He is head of the Spectroscopies of Biomolecules and Biological Media research team and director of the CNRS national research network on Molecular Plasmonics and Enhanced Spectroscopies.

    Michael Canva, born in 1963, received his Ph.D. degree in 1992, in Orsay University, France. Since then, he is a researcher for CNRS. He was visiting scientist at CREOL, Orlando, FL, USA for 2 years, 1996–1998 and at Duke University, NC, USA, for 1 year, 2009–2010. Currently, his activity focuses on plasmonics, especially surface plasmons resonance imaging systems for dynamic biochip applications. He heads the Biophotonic group at the Charles Fabry Laboratory, Institut d’Optique Graduate School (Palaiseau, France), and he is currently based at Laboratoire Nanotechnologies et Nanosystèmes, LN2, located at Université de Sherbrooke in Canada.

    Bernard Bartenlian is a CNRS researcher, physicist, at the Institute of Fundamental Electronics of Université Paris-Sud / Université Paris-Saclay. In 1991, he obtained his Ph.D. in Materials Science for the MBE growth and characterization of Ga(Al)As on silicon. From 1992 to the mid 2000 years, he worked on the study of magnetic nanostructure properties for the storage of ultra-high density recording, using top-down and bottom-up approaches. Since, he works to the adaptation of non-conventional lithographies to biological applications. He also collaborates with jurist researchers on the nanotechnologies regulation. He is involved in the teaching of his research in Master degree.

    Grégory Barbillon completed his Ph.D. in Physics (2007) at the University of Technology of Troyes (France). Then, he obtained his Habilitation (HDR) in Physics (2013) at the University of Paris Sud (Orsay, France). Actually, he is contractual researcher at the Institute of Fundamental Electronics (Orsay, France) in the group of Nanobiophotonics and Nanobioelectronics. His research interests were focused on Plasmonics, Nanophotonics, Surface Enhanced Raman Scattering, Biosensors, Near-field Optics, Biophotonics, Nanotechnology and Fluorescence.

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