Structural properties of some conducting polymers and carbon nanotubes investigated by SERS spectroscopy
Introduction
The study of conjugated-bond compounds has lately given a particular attention to polymer conductors owing to their high potential for technological applications. The electrical conduction of polymers depends on the stereo-regularity of their skeleton, which in turn depends on the position of carbon atoms to which radicals can attach during polymerization. Consequently, the investigation of their vibrational properties, which depend on their structure and doping state, is of particular interest. To better understand these properties, optical methods such as infrared absorption and Raman spectroscopy are widely used. In both fundamental research and a broad range of applications, polymeric conductors are used in the form of thin films, for which specific experimental techniques have to be adapted and developed. Within this framework, the Surface Enhanced Raman Scattering (SERS) has lately proved to be a very useful technique that provides high quality spectra and allows the observation of fine structural details of films with very small thickness 1, 2, 3. Even though many details of the process have yet to be understood, it is generally admitted that the SERS exaltation has a double origin: an electromagnetic one and a chemical one [4]. The electromagnetic enhancement is however the dominant mechanism. This mechanism consists in the excitation of localized and delocalized surface plasmons (SP) and is effective in copper, silver and gold.
It is well known that the delocalized surface plasmons, also called propagating surface plasmons, may be produced by optical excitation when the component parallel to the interface of the electric wave vector of the incident light equals the real part of the wave-vector of the surface plasmons [5]. In practice, this condition is realized by means of an optical coupler, which can be a prism 6, 7or a diffraction grating [8]. The intense electromagnetic field that exists at the interface between the metal and the surrounding medium excites Raman transitions in the molecules adsorbed on the metal surface. Part of the Raman radiation thus produced excites in turn surface plasmons at the Stokes frequency, and then decouples as a propagating Raman radiation inside the prism [9]or emerges as a diffracted radiation along a well-defined direction depending on the grating periodicity when the coupler is a diffraction grating [10]. To do this in practice, i.e., using a prism or a grating as optical couplers, is rather cumbersome in SERS spectroscopy. If the metal support has a rough microstructure with dimensions in the range 10–100 nm, the enhancement mechanism is the same. As a result, such a technique is much more attractive. Qualitatively, the rough metallic support may be considered a “superposition of diffraction gratings with different periods, each period being smaller than the wavelength of the exciting radiation”. In this case, the morphology of the metal surface becomes very important for SERS spectroscopy. The magnitude of the enhancement factor depends on the properties of the metal used (the dielectric constant and the state of the surface) and on the wavelength of the exciting radiation. The second enhancing mechanism for SERS, of chemical origin, is harder to identify or assess in quantitative terms. The chemical mechanism contributing to Raman scattering exaltations is based on an increased polarizability of the molecules that are adsorbed on metal surface under the influence of incident radiation, as a result of which new chemical bonds with the metal surface are formed. In this case, polarizability becomes much higher than that of free molecules [11]. When electronic transitions are involved and only certain bands associated with entirely symmetrical vibrations are exalted, this process can be considered similar to resonant Raman scattering. This mechanism is, as a rule, accompanied by a charge transfer metal–molecule or molecule–metal, which explains in part the success of the SERS studies on materials with empty π* orbitals.
The way how polymer thin films are prepared is yet another cause for SERS spectrum disturbances. Three types of samples are customarily used: (i) thin films deposited directly on the rough metal (Ag or Au) support by vacuum evaporation or sublimation; (ii) thin films obtained by evaporating the solvent, in vacuum or a controlled atmosphere, from a polymer solution of known concentration uniformly distributed on the metal surface, (iii) thin films electrochemically deposited on metal electrodes by cyclic voltammetry. This last procedure is also used for in situ SERS studies. If the last two procedures are used, the SERS spectra are often influenced by the appearance of an interfacial reaction at the metal surface, because the deposition of the polymer film is preceded by the oxidation of the electrode 12, 13, 14, 15. The present work is a synthesis of our research on the SERS spectroscopy of conducting polymer thin films. Special attention has been given now to the dependence of the SERS spectra on: (i) the type of the metal support and its roughness characteristics, by analogy with the use of a diffraction grating as an optical coupler, (ii) the type and thickness of the polymer film, (iii) the appearance of interfacial chemical reactions between the metal and the solvent. For some polymers, in our case poly(3-hexyl thiophene) (3-PHT), the doping process by means of the Lewis acid (FeCl3) generates the establishment of a state of disorder in the molecular chain, which is observed in the SERS spectra by the modification of the Lorentzian profile of the Raman lines. This modification consists in the appearance of a contribution of Gaussian type in the low energy wings of the Raman lines. The paper also reports for the first time results on the SERS spectroscopy of nanometric structures, particularly SERS spectra of carbon nanotubes.
Section snippets
Roughness of the metal support
The polymer thin films were deposited on rough Ag, Au or Cu supports. The strong dependence of the SERS spectrum intensity on the metal support roughness 4, 5imposes certain restrictions on the preparation of the metal films. We used the vacuum evaporation technique with a deposition rate of l nm/s at a pressure lower than 10−5 Torr, with the atomic beam at almost grazing incidence (θin=80°) on a microscope slide used as a target support [16]. The decrease of the incidence angle θin of the
Variation of the SERS spectrum intensities with roughness parameters
At this point, we have to indicate what is meant by roughness and its characteristics from the perspective of SERS. Scanning microscopy images of rough, SERS-active, surfaces of Ag, Au and Cu were already presented in an earlier paper 15, 17. In Fig. 1 is shown a SERS active surface of Ag, Au and Cu. In qualitative terms, such rough surfaces may be likened to a bird's-eye view of a multitude of columns of nearly the same diameter and at a certain tilt from the vertical. Based on this image, we
Conclusions
The SERS, a most useful technique for thin film investigation, has lately been increasingly applied to the study of conducting polymers. Some details of the exaltation process have yet to be elucidated. It is, however, widely accepted that the exaltation of the surface Raman scattering stems from a double origin: electromagnetic and chemical. The chemical contribution to the exaltation process remains a controversial issue. It depends on the chemical interactions at metal/dielectric (polymer)
References (29)
- et al.
Spectrochim. Acta
(1992) - et al.
Electrochimica Acta
(1992) - et al.
Electrochimica Acta
(1990) - et al.
Electrochimica Acta
(1990) - et al.
Chem. Phys. Lett.
(1993) - et al.
J. Chem. Phys.
(1992) - et al.
Macromol. Rapid Commun.
(1994) - A. Otto, Surface enhanced Raman scattering, in: R.K. Chang, T.E. Furtak (Eds.), Topics in Applied Physics, Light...
- H. Raether, Surface Plasmons on Smooth and Rough Surfaces and Gratings, Springer Tracts in Modern Physics, Spinger,...
Z. Physik.
(1968)
Z. Naturf.
Opt. Commun.
J. Phys. Chem.
J. Chem. Phys.
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