Surface-Enhanced Electronic Raman Scattering from Self-Assembled Alkanethiol Monolayers on Gold Surfaces

Abstract

A preliminary model is presented that describes the observed surface-enhanced electronic Raman scattering spectrum of self-assembled n-alkanethiol monolayers on roughened Au surfaces. At room temperature, a series of intense resonance Raman spectral lines, superimposed on the normal surface-enhanced Raman vibrational spectrum, are produced when monolayers of CH 3 (CH 2 ) m SH (m ) 8 to 17) are illuminated with laser radiation between 630 and 740 nm. We describe the effects of isotopic substitution, and of varying both the alkane chain length and the temperature of the monolayer films.

Electrons in image states near roughened metal surfaces

“…Virtually identical alkanethiol SEERS spectra are obtained for roughened Ag and Au electrodes, 11,12 aggregated Au colloidal monolayer films, 12 or aggregated Au colloid. 13 Unlike surfaces cleaved to yield specific terrace and step edge orientations, roughened surfaces have an irregular distribution of edges and terraces.…”

“…2. 11 With the exception of intensity, spectra are identical ͑within the limit of resolution͒ for R(CH 2 ) m SH SAM's ͑RϭCH 3 HC, or HOOC͒ whose alkyl chain lengths extend from mϭ8 to 17 for 32 S, 34 S, or Se headgroups, and for perdeuterated alkyl chains. There is also no observable change when spectra are recorded with the films in air or aqueous solution.…”

“…[8][9][10] Image states associated with roughened metal surfaces have been observed for chemisorbed self-assembled monolayers ͑SAM's͒ of alkanethiols and alkaneselenols on roughened gold and silver surfaces with a resolution of approximately 0.1 meV via surface enhanced Raman scattering ͑SERS͒. 11 Consequently, these systems provide an opportunity to explore the character of image states in the presence of roughened surfaces and chemisorbed dielectric overlayers. A sketch of a portion of an alkanethiol coated roughened gold substrate is shown in Fig.…”

“…The use of SERS has permitted the observation of spectra produced from scattering between eight states whose binding energies range from Ϫ0.155 eV to Ϫ0.006 eV relative to the vacuum energy. Since the Raman spectra obtained via SERS involve electronic transitions between image state energy levels, the process is referred to as SEERS, 11 where the second E signifies the electronic nature of the scattering. Image states associated with smooth surfaces are usually probed using twophoton photoelectron emission spectroscopy ͑2PPE͒ with a typical resolution of 50 meV.…”

Plasmon-Enabled Study of Self-Assembled Alkanethiol Ordering on Roughened Ag Substrates

“…Many studies have examined the formation and interfacial properties of SAMs on monocrystalline Au(111), flat Ag films, or assembled colloidal films. − Carron and co-workers examined some properties of alkanethiols on chemically roughened Ag foil and found lower monolayer wetability for SAMs on roughened Ag compared to smooth Ag, and Pemberton and co-workers saw increases in gauche band intensity in the SERS spectra of short chain alkanethiols on roughened Ag electrodes compared to monolayers formed on smooth Au or Ag . While the substrates examined in these literature precedents presented some roughness, most commonly used SERS substrates have significantly larger roughness, likely influencing the SAM properties.…”