Controlling surface morphology and sensitivity of granular and porous silver films for surface-enhanced Raman scattering, SERS

Abstract

The design of efficient substrates for surface-enhanced Raman spectroscopy (SERS) for large-scale fabrication at low cost is an important issue in further enhancing the use of SERS for routine chemical analysis. Here, we systematically investigate the effect of different radio frequency (rf) plasmas (argon, hydrogen, nitrogen, air and oxygen plasma) as well as combinations of these plasmas on the surface morphology of thin silver films. It was found that different surface structures and different degrees of surface roughness could be obtained by a systematic variation of the plasma type and condition as well as plasma power and treatment time. The differently roughened silver surfaces act as efficient SERS substrates showing greater enhancement factors compared to as prepared, sputtered, but untreated silver films when using rhodamine B as Raman probe molecule. The obtained roughened silver films were fully characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron (XPS and Auger) and ultraviolet–visible spectroscopy (UV–vis) as well as contact angle measurements. It was found that different morphologies of the roughened Ag films could be obtained under controlled conditions. These silver films show a broad range of tunable SERS enhancement factors ranging from 1.93 × 102 to 2.35 × 105 using rhodamine B as probe molecule. The main factors that control the enhancement are the plasma gas used and the plasma conditions, i.e., pressure, power and treatment time. Altogether this work shows for the first time the effectiveness of a plasma treatment for surface roughening of silver thin films and its profound influence on the interface-controlled SERS enhancement effect. The method can be used for low-cost, large-scale production of SERS substrates.

SERS Activity of Silver Nanosphere, Triangular Nanoplates, Hexagonal Nanoplates and Quasi-Spherical Nanoparticles: Effect of Shape and Morphology

“…Nevertheless, the Raman scattering is not very high, and this limitation prevents its use for the detection of low analyte concentrations [1]. For this reason, Surface Enhanced Raman Scattering (SERS) is a powerful vibrational spectroscopic technique that can extend the sensitivity of conventional Raman spectroscopy to the single molecule level [2][3][4].…”

Label‐Free Mycotoxin Raman Identification by High‐Performing Plasmonic Vertical Carbon Nanostructures

“…In contrast, O 2 as a source of highly reactive species, led to plasma chemical interactions with the analyte, resulting in a breakdown of the important bonds between analyte and substrate. A combination of kinetic and chemical cleaning mechanisms takes much less time than observed for a single gas plasma reported by Okeil et al [44] and Liu et al [45] The mechanism that causes the SERS enhancement factor to be largest for the S1 sample is most likely due to several geometrical parameters of the film that were found to be optimum in similar SERS substrates prepared by the glancing angle deposition technique called nanosculptured thin films (nSTFs): [46,47] • Thickness of around 300 nm, which is determined by the wavelength used and the carbon structure underneath • Vertical columnar structure caused the formation of sharp Au needle heads after Au deposition • Porosity around 30% was found, not only in nSTFs [41] but also in other systems with a large number of hot spots to be the optimum, for example, Au nanoparticles spread on a substrate. [48] The porosity effect is a result of compromise between the density of hot spots and the optimum gap between the nanofeatures.…”

“…[43] For instance, Okeil et al used a low-pressure Ar plasma to remove analyte residuals from granular and porous Ag films. [44] Because of Au film's better oxidation resistance than Ag, we were able to use a combination of Ar and O 2 gases to obtain a synergetic effect of cleaning. Noble gas argon is known to act as a kinetic remover due to a preferable ion bombardment mechanism.…”

Carbon‐Assisted Stable Silver Nanostructures

“…The reactive plasma of O 2 or air destroys the nanostructured morphology (Figure 2), while an inert Ar plasma ablates organic residues in a nonreactive manner. [ 28,43 ] This not only maintains the Ag nanostructure during the etching process but also makes it highly resistant against exposure to environments that typically require additional surface passivation. The amount of silver oxide that is detected by XPS is negligible and probably localized in individual patches rather than a conformal layer.…”