title: Plasmonic nano-protrusions: hierarchical nanostructures for single-molecule Raman spectroscopy.
abstract: Classical methods for enhancing the electromagnetic field from substrates for spectroscopic applications, such as surface-enhanced Raman spectroscopy (SERS), have involved the generation of hotspots through directed self-assembly of nanoparticles or by patterning nanoscale features using expensive nanolithography techniques. A novel large-area, cost-effective soft lithographic technique involving glancing angle deposition (GLAD) of silver on polymer gratings is reported here. This method produces hierarchical nanostructures with high enhancement factors capable of analyzing single-molecule SERS. The uniform ordered and patterned nanostructures provide extraordinary field enhancements that serve as excitatory hotspots and are herein interrogated by SERS. The high spatial homogeneity of the Raman signal and signal enhancement over a large area from a self-assembled monolayer (SAM) of 2-naphthalenethiol demonstrated the uniformity of the hotspots. The enhancement was shown to have a critical dependence on the underlying nanostructure via the surface energy landscape and GLAD angles for a fixed deposition thickness, as evidenced by atomic force microscopy and scanning electron microscopy surface analysis of the substrate. The nanostructured surface leads to an extremely concentrated electromagnetic field at sharp nanoscale peaks, here referred to as 'nano-protrusions', due to the coupling of surface plasmon resonance (SPR) with localized SPR. These nano-protrusions act as hotspots which provide Raman enhancement factors as high as 108over a comparable SAM on silver. Comparison of our substrate with the commercial substrate Klarite™ shows higher signal enhancement and minimal signal variation with hotspot spatial distribution. By using the proper plasmon resonance angle corresponding to the laser source wavelength, further enhancement in signal intensity can be achieved. Single-molecule Raman spectra for rhodamine 6G are obtained from the best SERS substrate (a GLAD angle of 60°). The single-molecule spectrum is invariant over the substrate, due to the patterned ordered nanostructures (nano-protrusions).
authors.affiliation: Department of Electrical and Computer Engineering, 349 Engineering Building West, University of Missouri-Columbia, Columbia, MO 65211, USA. Department of Chemical, Biological and Environmental Engineering, Tiernan Hall, New Jersey Institute of Technology, Newark, NJ 07102, USA.
authors.forename: Sagnik
authors.initials: S
authors.lastname: Basuray
authors.forename: Avinash
authors.initials: A
authors.lastname: Pathak
authors.forename: Sangho
authors.initials: S
authors.lastname: Bok
authors.forename: Biyan
authors.initials: B
authors.lastname: Chen
authors.forename: Steven C
authors.initials: SC
authors.lastname: Hamm
authors.forename: Cherian J
authors.initials: CJ
authors.lastname: Mathai
authors.forename: Suchismita
authors.initials: S
authors.lastname: Guha
authors.forename: Keshab
authors.initials: K
authors.lastname: Gangopadhyay
authors.forename: Shubhra
authors.initials: S
authors.lastname: Gangopadhyay