Raman Scattering And Surface Enhanced Raman Scattering
In order to develop complex materials with controlled properties we need to know the nature of chemical and physical processes at interfaces. To achieve this it is necessary to develop techniques for controlling and characterizing the molecular structure of interfaces in composite materials. Existing Surface analysis techniques such as UHV, XPS can detect a monolayer of most elements. They cannot provide direct information regarding molecular structures of adsorbed molecules on surfaces. They cannot be used to perform in situ analysis as require conditions of high vacuum
Raman spectroscopy is an easy sampling technique, since bulk materials varying in shapes (films, powders, fibers, plates, pellets) may be studied without any modification. It works well in moist environments and can be used on samples having small areas. Thus Raman spectroscopy has its own advantages over other surface analysis techniques such as XPS, AES, HREELS, and IR.
Principle Of Raman Scattering
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When a photon of energy hvo collides with a molecule two different types of light scattering can occur.
1) Rayleigh scattering
2) Raman scattering
Rayleigh scattering is an elastic collision between the incident photon and the molecule in which the photon neither loses nor gains energy. Raman scattering is an inelastic collision where the photon either gains energy from or loses energy to the molecule. The energy of the scattered light is h (v0+v1) or h (v0-v1). The energy gained or lost hv1 corresponds to the vibrational energy of the molecule. Thus the energy of the scattered light depends on the frequency of the incident light and the shift hv1from the Rayleigh scattering line is a constant corresponding to the vibrational energy. I.e. the scattered light contains a spectrum of wavelengths in which the intensity peaks are shifted from the excitation wavelength by energy equivalent amounts corresponding to that for the excitation of the molecular vibration modes or crystal phonons. These vibrational spectra are highly specific for the material studied and can be used for unambiguous identification of substances. .The Raman lines occurring at frequency (v0-v1) are called the Stokes lines and the lines occurring at frequency (v0+v1) as anti-Stokes lines. The stokes lines are of higher intensity than anti stokes lines due to the Boltzmann distribution - i.e., due to higher population of molecules in the ground state than in the excited state. Because of this feature of stoke lines we use them in the study of Raman scattering.
In 1928 C.V. Raman observed the scattering effect, using sunlight and complementary filters . Later mercury vapor lamps were used for illumination of samples. Current Raman spectrometers use lasers as monochromatic light sources and the type of laser depends on the wavelength, sensitivity and spectral resolution required. Sample optics are used to focus the laser on to the sample and collect the Raman scattered light. In the wide spread backscattering or 180° arrangement a single objective serves for both, focusing the laser and collecting the scattered light.
Elastically scattered light must be removed in front of the spectrometer as it has more intensity than the Raman scattered light and would produce stray radiation at the detector and holographic notch filters are used for this purpose. Spectral analysis of the Raman scattered light is done by dispersive or Fourier Transform (FT) spectrometers.
Different accessories are available for Raman spectroscopy. Confocal microscopes afford lateral resolution of approximately 1μm and depth resolution down to approximately 2μm. The depth resolution is especially important for surface and interface spectroscopy, because it helps to eliminate Raman intensities from the bulk phases. Fiber optics can be used to guide the exciting laser light to the sample and the scattered light to the spectrometer. This makes changing the samples easier and eliminates the hazards of freely propagating laser light. Special sample cells have been constructed for measurement at high or low temperatures and at high pressures, and for laser multi-pass through gaseous samples.
For large molecules, Raman spectra contains numerous bands which cannot always be assigned to particular vibrational modes. However the large number of bands when measured with appropriate spectral resolution enables unambiguous identification of substances by comparing the spectra. Vibrations of weakly polar and even symmetrically bonded atoms usually result in intense Raman bands as Raman activity is related to changes in molecular polarizability during vibration. Structural isomers usually have different spectra and can be easily distinguished. Polarization effects are another feature of Raman spectroscopy that improves the assignment of bands and enables the determination of molecular orientation. Analysis of the polarized and non-polarized bands of isotropic phases enables determination of the symmetry of respective vibrations. For aligned molecules in crystals or at surfaces it is possible to measure the dependence of upto six independent Raman spectra on the polarization and direction of propagation of incident and scattered light relative to the molecular or crystal axes.
The intensity of the scattered radiation depends not only on the polarizability and concentration of the analyte molecules, but also on the optical properties of the sample and adjustment of the instrument. Absolute Raman intensities are therefore not an accurate measure of concentration. These intensities are useful for quantification under well defined experimental conditions and for well characterized samples.
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One limitation of Raman Spectroscopy for studying thin organic films adsorbed onto metal surfaces is that the low intensity of the Raman scattering must be detected in the presence of a large background. The problem of low Raman cross section has been dealt with SERS (Surface enhanced Raman scattering).
Background Of Surface Enhanced Raman Scattering
Surface Enhanced Raman Scattering (SERS), is a surface sensitive technique that results in the enhancement of Raman scattering signals by molecules adsorbed on nanometer sized rough metal surfaces. The signal enhancement compared with Raman signal can be as much as 1014-1015, which allows the technique to be sensitive enough to detect single molecules .
As in Fig 2 Alumina nanoparticles deposited on the glass slide provide the required roughness. Silver layer evaporated on to the nanoparticles provides the enhancement. Organic molecules adsorbed on the Silver surface can be detected by irradiation with a laser and collecting the Raman scattered light.
SERS is more complex compared Raman scattering. When we consider the Raman vibrational spectrum of a molecule in gas phase, the basic components involved in this are just the molecule and the incident radiation. In contrast, in SERS, the basic components involved are a molecule, a metal nanostructure and electromagnetic radiation. This difference introduces greater complexity to the SERS measurements. The important challenges to the interpretation of SERS spectra are:
1) The molecule interacts with a metal nanostructure. The adsorption on solid surfaces can be divided according to the strength of bonding between the particle and the substrate into two categories, physisorption and chemisorption. Physisorption refers to the weak interactions arising from van der Waals forces, with adsorption energies well below those of normal chemical bonds. Physical adsorption may alter the surface structure of molecular solids but not that of metals. When the adsorption energy is large enough and comparable to chemical bond energies, chemisorption is used.
2) Incident photons can induce substrate excitations such as electron hole pairs, surface plasmons or surface phonons that may be involved in the enhancement of photo-induced processes. The most important one is that absorption of light by nanostructures can create strong local electric fields at the location of the adsorbed species. This enhanced local field will strongly affect the optical properties of the adsorbate and is the main factor for the SERS effect.
3) Interaction of incident light with adsorbed molecules may lead to photodissociation, photoreactions or photodesorption. All these processes can leave their own effects in SERS spectra.
4) Interaction of light with metallic nanostructure depends on the value of complex dielectric function at the excitation wavelength and this will determine the enhancement observed at a particular frequency of excitation. Particle absorption and scattering depend on the shape and size of the metal nanostructure, SERS intensities are also influenced by these. In addition, the excitations in nanostructures are strongly influenced by the dielectric constant if the medium.
5) The dynamics of the interaction of light with the adsorbate leads to a pattern of Raman intensities determined by the selection rules. A distinction between the selection rules for vibrational transitions of a molecule in the gas phase and the surface selection rules for a fixed spatially oriented molecule at the surface of an enhancing nanostructure. Surface selection rule encompass the symmetry properties of the dipole transitions and the modification of the intensities due to the components of local electric field vector at the surface. They apply to molecules anchored on nanostructures where Raman and infrared intensities are further modulated by the spatial orientation of the local electric field interacting with the polarizability derivative tensor. Since the adsorbed molecule generally belongs to a different symmetry point group than that of the parent molecule, the corresponding allowed modes and their polarization are also different.
6) SERS is obtained by excitation with visible or near infrared light. The presence of metal nanostructures may permit new excitations in the molecule-nanostructure complex, such as charge transfer transitions from the Fermi to LUMO level of the molecule. Since the excitation is in resonance with the electronic transition of the adsorbed metal complex, the inelastic scattering is due to resonance Raman scattering (SERRS).
7) Small amounts of impurities may sometimes be responsible for the sudden signals that further complicate the interpretation of the observed SERS spectra.
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In 1974 Martin Fleischman observed SERS from pyridine adsorbed on electrochemically roughened silver electrode and their analysis of the large signal was due increased surface area of the rough substrate, which is due to number of molecules scattering on the surface , and they did not recognize the major enhancement. Three years later two groups independently noted that the concentration of scattering species could not account for the enhanced signal and each proposed a mechanism for the enhancement. These formed the underlying principles for the SERS effect. Jeanmaire and Van Duyne proposed a charge-transfer effect , while Albrecht and Creighton proposed an electromagnetic effect . There is a disagreement over the absolute and relative enhancement factors among the Electromagnetic effect and the charge transfer effect even today. It is because both Electromagnetic and Charge transfer methods are critical roughness based models. These two mechanisms arise because the intensity of Raman scattering is directly proportional to the square of the induced dipole moment, μind, which, in turn, is the product of the Raman polarizability, α, and the magnitude of the incident electromagnetic field, E.
When optimizing for the SERS activity, the surfaces are very rough, consisting of various roughness components ranging from atomic to sub micron size. Therefore it is difficult to distinguish the contribution from the EM and CT models.
The increase in intensity of the Raman signal for adsorbates on particular surfaces occurs because of an enhancement in the electric field provided by the surface. When the incident light in the experiment strikes the surface, localized surface plasmons are excited. The field enhancement is greatest when the plasmon frequency,ωp, is in resonance with the radiation. Furthermore, in order for scattering to occur, the plasmon oscillations must be perpendicular to the surface; if they are in-plane with the surface, no scattering will occur. It is because of this requirement that roughened surfaces or arrangements of nanoparticles are typically employed in SERS experiments as these surfaces provide an area on which these localized collective oscillations can take place.
The light incident on the surface can excite a variety of phenomena in the surface, yet the complexity of this situation can be minimized by surfaces with features much smaller than the wavelength of the light, as only the dipolar contribution will be recognized by the system. The dipolar term contributes to the plasmon oscillations, which leads to the enhancement. The reason that the SERS effect is so pronounced is because the field enhancement occurs twice. Initially, the field enhancement magnifies the intensity of incident light which will excite the Raman modes of the molecule being studied, therefore increasing the signal of the Raman scattering. The Raman signal is then further magnified by the surface by the same mechanism as the incident light was, resulting in a greater increase in the total output signal of the experiment. The enhancement factor E at each molecule is given by
Where Eω is the local electric field enhancement factor at the incident frequency ω and Eω' is the corresponding factor at the stoke shifted frequency ω'. In conventional SERS, E is averaged over the surface area of the particles where molecules can adsorb to generate the observed enhancement factor <E>, while in single-molecule SERS (SMSERS) it is the maximum enhancement Emax that is of interest.
The enhancement is not equal for all frequencies. For those frequencies for which the Raman signal is only slightly shifted from the incident light, both the incident laser light and the Raman signal can be near resonance with the plasmon frequency, leading to the E4 enhancement. When the frequency shift is large, the incident light and the Raman signal cannot both be on resonance with ωp, thus the enhancement at both stages cannot be maximal.
The choice of surface metal is also dictated by the plasmon resonance frequency. Visible and near-infrared radiation (NIR) is used to excite Raman modes. Silver and gold are typical metals for SERS experiments because their plasmon resonance frequencies fall within these wavelength ranges, providing maximal enhancement for visible and NIR light. Copper is another metal whose absorption spectrum falls within the range acceptable for SERS experiments . Platinum and palladium nanostructures also display plasmon resonance within visible and NIR frequencies.
While the electromagnetic theory of enhancement can be applied regardless of the molecule being studied, it does not fully explain the magnitude of the enhancement observed in many systems. For many molecules, often those with a lone pair of electrons, in which the molecules can bond to the surface, a distinctly different mechanism of enhancement has been described which does not involve surface plasmons. This chemical mechanism involves charge transfer between the chemisorbed species and the metal surface. The chemical mechanism only applies in specific cases and probably occurs in concert with the electromagnetic mechanism.
The HOMO to LUMO transition for many molecules requires much more energy than the infrared or visible light typically involved in Raman experiments. When the HOMO and LUMO of the adsorbate fall symmetrically about the Fermi level of the metal surface, light of half the energy can be employed to make the transition, where the metal acts as a charge-transfer intermediate. Thus, a spectroscopic transition that might normally take place in the UV can be excited by visible light.
SERS is sensitive to the surface on which the experiment is taking place. The first experiments, and some modern experiments, took place on electrochemically roughened silver . Now surfaces are often prepared using a distribution of metal nanoparticles on the surface .
The shape and size of the metal nanoparticles strongly affects the strength of the enhancement because these factors influence the ratio of absorption and scattering events . There is an ideal size for these particles—not just any small particles will have the same impact on the Raman intensity—as well as an ideal surface thickness for each experiment . For the SERS to occur metal particles or metal features responsible for its operation should be small with respect to the wavelength of the exciting light i.e. the SERS active systems should possess structure in the range of 5-100nm range. Likewise, the dimensions of the active structure cannot be smaller than the molecule and the upper bound is determined by the excitation wavelength. Particles which are too large allow the excitation of multipoles, which are nonradiative. As only the dipole transition leads to Raman scattering, the higher-order transitions will cause a decrease in the overall efficiency of the enhancement. Particles which are too small, however, lose their electrical conductance and cannot enhance the field. Furthermore, when the particle size approaches a few atoms, the definition of a plasmon does not hold, as there must be a large collection of electrons to oscillate together.
SERS enhancement of silver is more than gold followed by copper. The reason can be explained with a simple rudimentary model [SERS a retrospective]. The polarizability of a small metal sphere with dielectric function ε(λ) and radius R surrounded by a vacuum is given by
 α= R3ε-1ε+2
Combining this expression with the expression for dielectric function of Drude metal slightly modified by interband transitions is given by
Where εb is the contribution of interband transitions to the dielectric function
ωp is the metal plasmon resonance
γ is the electron scattering rate, inversely proportional to the metal's DC conductivity
Combining the above two equations yields
Hence, when is γ large, either because of the inherent poor conductivity of the metal or due to the fact that the metal nano features are so small that electronic scattering at the particles surfaces become the dominant electron-scattering process, the quality of the resonance is reduced, and with it the SERS enhancement. And for metals with high εb , the width of the resonance is increased and hence the SERS enhancement decreases. Most transition metals are poor SERS-enhancing systems because, for them, the two effects combine to reduce their SERS enhancement ability, i.e. their conductivity is low (γ is large) and the interband contribution to the dielectric function is great (εb is large).
The name surface enhanced Raman spectroscopy implies that it provides the same information that traditional Raman spectroscopy does, simply with a greatly enhanced signal. While the spectra of most SERS experiments are very similar to the non-surface enhanced spectra, there are often differences in the number of modes present. Additional modes not found in the traditional Raman spectrum can be present in the SERS spectrum, while other modes can disappear.
The modes observed in any spectroscopic experiment are dictated by the symmetry of the molecules and are usually summarized by selection rules. When molecules are adsorbed to a surface, the symmetry of the system can change, slightly modifying the symmetry of the molecule, which can lead to differences in mode selection.
One common way in which selection rules are modified arises from the fact that many molecules that have a center of symmetry lose that feature when adsorbed to a surface. The loss of a center of symmetry eliminates the requirements of the mutual exclusion rule, which dictates that modes can only be either Raman or Infrared active. Thus, modes that would normally appear only in the infrared spectrum appear of the free molecule can appear in the SERS spectrum.
The symmetry of a molecule can be changed in different ways depending on the orientation in which the molecule is attached to the surface. In some experiments, it is possible to determine the orientation of adsorption to the surface from the SERS spectrum, as different modes will be present depending on the way in which the symmetry is modified.
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