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.
Highlights
The great enhancement of Raman signals obtained from molecules when they are in close vicinity to a rough noble-metal surface has attracted a great deal of interest in the last decades [1]
When these highly energetic electrons collide with the feed gas atoms or molecules this results into a series of successive complex processes, namely ionization (1 and 2), excitation (3) and relaxation (4) as well as dissociation in case of diatomic species (5) [75]: controlled rf plasma treatment is a very interesting technique for the modification of surfaces as a number of different chemical species can be generated at low temperature in the gas phase, which can strongly interact with the exposed surface leading to distinct physical and chemical changes [63,76]
The SERS enhancement of the as-sputtered 200 nm silver film can be greatly enhanced by an appropriate plasma treatment leading to a 30-fold enhancement compared to a commercial SERS substrate and to an about 1200-fold enhancement when compared with an untreated 200 nm sputtered silver film
Summary
The great enhancement of Raman signals obtained from molecules when they are in close vicinity to a rough noble-metal surface (e.g., gold, silver and copper) has attracted a great deal of interest in the last decades [1]. (d) A single plasma treatment (hydrogen, nitrogen or argon plasma) mainly results in an increase of the surface roughness of the sputtered silver films and to an increase of the SERS activity. Using AFM (Figure S2, Supporting Information File 1) it shows that surface roughness increases from 2.02 nm for an untreated silver film to 5.03 nm for 5 min, 6.29 nm for 15 min and 6.85 nm for 45 min hydrogen plasma treatment time.
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