Material Analysis. What do you want to know about your sample? How do you intend to do for obtaining the desired information from your sample?

Transcription

1 Material Analysis What do you want to know about your sample? How do you intend to do for obtaining the desired information from your sample? Why can you acquire the proper information? Symmetrical stretching Antisymmetrical stretching Scissoring Rocking Wagging Twisting 1

2 Raman Scattering When light is scattered from an atom or molecule, most photons are elastically scattered (Rayleigh scattering However, a small fraction of the scattered light (about 1 in 10 million photons) is scattered by an excitation, with the scattered photons having a frequency different from the frequency of the incident photons. The Raman effect was first reported by C. V. Raman and K. S. Krishnan, and independently by G. Landsberg and L. Mandelstam, in Raman received the Nobel Prize in 1930 for his work on the scattering of light. In 1998 the Raman Effect was designated an ACS National Historical Chemical Landmark in recognition of its significance as a tool for analyzing the composition of liquids, gases, and solids 2

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4 The graphic shows a Raman Spectrum of Acetone and Ethanol. Different types of Carbon Hydrides can be directly identified by the Raman shifts of signals. Optical detection is very powerful since the measurement can be performed over distances without the need for any direct contact. _spectroscopy/carbon_hydride.html 4

5 Raleigh & Raman Scattering High resolution Raman imaging of an etched silicon chip over a 1mm 2 area. The image illustrates the presence of crystalline silicon (red), stressed silicon (yellow) and amorphous silicon (green). The blue areas correspond to a metal coating on the chip, which results in zero Raman signal. 5

6 Raman imaging of an off-the-shelf painkiller tablet, illustrating the distribution of aspirin (red), caffeine (green) and paracetamol (blue). A polished granite section was analysed using both Raman and micro-xrf. The top Raman image was acquired with 2 s per point, and shows the distribution of FeS (red), SiO 2 (green) and (K,Na)AlSi 3 O 8 (blue)

7 An incident electromagnetic wave induces a dipole moment during the light-material interaction. The strength of the induced dipole moment, P, is given by P = αe where α is the polarizability and E is the strength of electric field of the incident EM wave. For the incident EM wave, the electric field may be expressed as E = E o cos(2πν o t) where ν o is the frequency of the incident EM (ν o = c/λ). This yields the time-dependent induced dipole moment P = α E o cos(2πν o t) For any molecular bond, the individual atoms are confined to specific vibrational modes, in which the vibrational energy levels are quantized in a manner similar to electronic energies. The physical displacement dq of the atoms about their equilibrium position due to the particular vibrational mode may be expressed as dq = Q o cos(2πν vib t) 7

8 For such small displacements, the polarizability may be approximated by a Taylor series expansion α α = α o + dq Q Based on the vibrational displacement, the polarizability may be given as P = αe = α E α α = α o + Q Q o o o cos( 2πν α cosωt + QoE Q o vib t) cosωt cosω vib t Using a trigonometric identity, the above relation may be recast as P = α E cosωt o o α QoE + Q 2 o [ cos( ω + ω ) t + cos( ω ω ) t] This reveals that induced dipole moments are created at three distinct frequencies The scattered frequency corresponds to the incident frequency is elastic scattering (Rayleigh) The latter two are inelastic processes which is referred to as Raman scattering, with the down-shifted frequency (Stokes scattering) and the up-shifted frequency (anti-stokes scattering) vib vib 8

9 From lecture note of David W. Hahn, Univ. of Florida Symmetric Stretch 1340 cm -1 Raman active Asymmetric Stretch Symmetric Bend 2350 cm cm -1 IR active IR active 9

10 Wavenumber The phonon energy can be obtained from the frequency (wavelength) shift of the captured photons as compared to the incident light. E = hν hν E = hν Although the energy shift can be obtained by calculating the frequency difference, people preferred to use the equivalent wavelength to evaluate the energy. E / hc = hν / hc = ν / c = 1/ λ The common unit for wavenumber (which is considered as a way of evaluate the energy shift) is the cm -1, and generally the value of energy shift is ~ 200 to 4000 cm -1. E / hc hν / hc = 1/ λ = ( λ λ) / λλ R R It is not difficult to tell from above that different excitation laser wavelength will result in different spectrum resolution. 10

11 In addition, the Raman shift is constant with regard to vibrational energy, hence constant in wavenumber, but not wavelength. Consider again the N 2 vibrational mode of ν~ 2331 cm -1. An incident wavelength of 355 nm would be shifted to a wavelength of 387 nm, for a difference of 32 nm. Alternatively, an incident wavelength of 632 nm would be shifted to a wavelength of nm, for a difference of nm. The Raman differential scattering cross-section varies inversely with the fourth power of the excitation wavelength by the proportionality formula: For example, N 2 has a fundamental vibrational mode corresponding to ν ~ 2331 cm -1. The cross-section of 5.5x10-31 cm 2 /sr at 488 nm would be reduced to 1.7x10-31 cm 2 /sr at 632 nm, a reduction of more than a factor of 3. 11

12 * cm-1: X-H stretching (X = C, N, O, S) * cm-1: CX stretching (X = C or N) * cm-1: CX stretching (X = C, N, O) * cm-1: C-X stretching (X = C, N, O) 12

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15 Surface enhanced Raman Scattering SERS The cross section for Raman is much smaller than that of the fluorescence. The cross section for fluorescence is about cm 2 per molecule, but Raman only has ~10-26 cm 2 per molecule. Therefore, Raman was not widely used for a long time until the surface enhanced Raman effect was discovered. In this sense, we can call SERS Surface Enhancement Rescued Spectroscopy. From Shan Jiang, University of Illinois, Urbana, 61801, IL SERS was accidentally discovered while people tried to do Raman on the electrode in 1974 The original idea was to generate a high surface area on the roughened metal. People realize surface area is not the key point in this phenomenon. In 1977, People found the rough silver electrode produce a Raman spectrum that is a million fold more intense than what was expected. SERS overcome the disadvantage of the small cross section of Raman spectroscopy, and could be used to study the single molecule spectroscopy. From Shan Jiang, University of Illinois, Urbana, 61801, IL 15

16 The key features are summarized briefly as follows: SERS occurs when molecules are brought to the surface of metals in a variety of morphologies. The smooth surface is not active for the enhancement. Large enhancements are observed from silver, gold and copper. If the metal nano-particles are used in the system, the particle size for enhancement of Raman to happen ranges from 20nm~300nm. Molecules adsorbed in the first layer on the surface show the largest enhancements. However, the enhancement also has long-range effect of about tens of nanometers. The excitation profile (scattering intensity vs. exciting frequency) deviates from the fourth-power dependence of normal Raman scattering. Surface plasmon excitation When incident light is directed on to the roughened surface it leads to the excitation of the surface plasmons and consequently the electromagnetic field of the light at the surface becomes greatly increased. Due to this, the Raman scattering of the molecule adsorbed on the surface is amplified. 16

17 SERS of Semiconducting Hybrid Nanoparticles Techniques routinely used in the identification of inorganic pigments are generally not applicable to dyes: X-ray fluorescence because of the lack of an elemental signature, Raman spectroscopy because of the generally intense luminescence of dyes Fourier trans-form infrared spectroscopy because of the interference of binders and extenders. Traditionally, the identification of dyes has required relatively large samples(0.5-5 mm in diameter) for analysis by high-performance liquid chromatography. In this Account, samples as small as 25 µm in diameter were identified with surface-enhanced Raman scattering (SERS) 17

18 Resonance Raman Scattering Resonance Raman Scattering An. Acad. Bras. Ciênc. vol.78 no.3 Rio de Janeiro Sept

19 Resonance Raman Scattering 19

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22 TERS Images 22

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24 Coherent Anti-Stokes Raman Spectroscopy CARS CARS is sensitive to the same vibrational signatures of molecules as seen in Raman spectroscopy CARS involves process of multiple photons to address the molecular vibrations which produce signal in coherent with the emitted waves CARS is capable of generating orders of magnitude stronger signal than spontaneous Raman emission. From Wikipedia CARS is a third-order nonlinear optical process involving three laser beams: a pump beam of frequency ω p, a Stokes beam of frequency ω S and a probe beam at frequency ω pr. These beams interact with the sample and generate a coherent optical signal at the anti-stokes frequency (ω p -ω S +ω pr ). The latter is resonantly enhanced when the frequency difference between the pump and the Stokes beams (ω p -ω S ) coincides with the frequency of a Raman resonance, which is the basis of the technique's intrinsic vibrational contrast mechanism From Wikipedia 24

25 The Figure shows a lipid contrast CARS image of an atherosclerotic lesion in the aorta of a mouse model, where the CARS (orange) is combined with the second harmonic signal from collagen (blue) _microscopy.html CARS Setup Diagram 25

26 CARS Images of Living Cells (1/2) CARS Images of Living Cells (2/2) An alternative interpretation is that differences in the environment of the fluorophores, i.e., of the lipids, account for the failure of Nile red to fluoresce in the hypodermal collection. 26