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Uv vis and raman spectroscopy

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UV-Vis Spectroscopy

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Uv vis and raman spectroscopy

  2. 2. ▪ The study of molecular structure and dynamics through the absorption, emission and scattering of light What is Spectroscopy?
  3. 3. Principle of Spectroscopy • The principle is based on is the measurement of the spectrum of sample containing atom/molecule • Spectrum is a graph of intensity of absorbed or emitted radiation by sample verses frequency or wavelength • Spectrophotometer is an instrument design to measure the spectrum of a compound.
  5. 5. Consider the collision between a photon and a molecule ν0 νs
  6. 6. Raman Effect ▪ Scattering can be: – Elastic- Scattered photon have the same energy and frequency as the incident photons; it is called as Rayleigh scattering – Inelastic- A small fraction of light [approx. 1 in 107] is scattered at optical frequencies different than the frequency of incident photons. This process of scattering is termed the Raman effect ▪ Stokes Raman Scattering: Emitted photon is of lower frequency than incident photon ▪ Anti-stokes Raman Scattering: Emitted photon is of higher frequency than incident photon
  7. 7. Interaction between electric field of incident photon and molecule ▪ Light, with incident frequency ‘n0’, has an oscillating electric field (E) : E = E0 cos (2pn0t) ▪ Induces molecular electric dipole (µ): µ =  E =  E0 sin (2pn0t) ▪ Proportional to molecular polarizability () →Ease with which the electron cloud around a molecule can be distorted ▪ In molecular vibrations, the normal coordinate Q varies periodically with the vibrational frequency ‘nvib’ Q = Qo cos (2pnvibt) where Qo is the magnitude of the given normal vibration E
  8. 8. ▪ α = αo + (δα/δQ)0 Q = αo + (δα/δQ)0 Qo cos (2pnvibt) where (αo) is the equilibrium value of (α), and (δα/δQ)0 is the change in polarizability with external vibration. ▪ Resultant dipole ▪ Raman scattering occurs only when the molecule is ‘polarizable’ change in polarizability, ▪ Routine energy range: 200 - 4000 cm–1 Rayleigh scattering µ = [α0E0 cos(2πνt)] + ½ (δα/δQ)0 E0[cos(2π(ν- νvib)t) - cos(2π(ν+ νvib)t)] Raman scattering Anti-Stokes Raman ScatteringStokes Raman Scattering
  9. 9. ▪ Which modes will have a change in polarizability? asymmetric stretch Vibrational spectroscopy spectrum rules symmetric stretch bend
  10. 10. • Gross selection rule in IR spectroscopy: vibration must lead to an oscillating dipole 4000 2000 0 Infrared spectrum of CO2 • Gross selection rule in Raman spectroscopy: vibration must lead to a change in polarizability Vibrational spectroscopy spectrum rules
  11. 11. 4000 2000 0  Only the symmetric stretch is observed. What happened to the other two vibrations? Vibrational spectroscopy spectrum rules Raman spectrum of CO2
  12. 12. Symmetric stretch Q  d dQ> 0 
  13. 13. Bend Q  d dQ = 0 
  14. 14. Asymmetric stretch d dQ = 0  Q 
  15. 15. Types of Raman Spectroscopy ▪ Coherent Anti-Stokes Raman Spectroscopy (CARS) ▪ Resonance Raman (RR) Spectroscopy ▪ Surface-Enhanced Raman Spectroscopy (SERS) ▪ Surface-Enhanced Resonance Raman Spectroscopy (SERRS)
  16. 16. Information from Raman Spectroscopy characteristic Raman frequencies changes in frequency of Raman peak polarisation of Raman peak width of Raman peak intensity of Raman peak composition of material stress/strain state crystal symmetry and orientation quality of crystal amount of material e.g. Si 10 cm-1 shift per % strain e.g. thickness of transparent coating e.g. MoS2, MoO3 e.g. orientation of CVD diamond grains e.g. amount of plastic deformation
  17. 17. Identification of single atomic layers of graphene A. C. Ferrari, et al., Phys. Rev. Lett. (2006), Vol. 97, 187401 Graphite Graphene
  18. 18. Carbon Nanotube Typical Raman spectra for SWCNT
  19. 19. Raman spectra of the CNTs after different times of nitrogen plasma treatment Comparison of Raman spectra of SWCNTs, DWCNTs, and MWCNTs
  20. 20. Effect of high-pressure on octahedra tilts: LaAlO3 P. Bouvier & J. Kreisel, J. Phys.: Condens. Matter (2002), Vol. 14, pp. 3981 ‘Tilted’ perovskites (ABX3)
  21. 21. Raman imaging in nano-technology Contacts on a Si wafer Do the contacts induce strain ? Raman (strain) image Strain underneath & at corner of contacts Potential effect on dopants … Change in band position Strain !
  23. 23. Introduction 23 UV visible spectroscopy is also known as electronic spectroscopy in which, the amount of light absorbed at each wavelength of UV and visible regions of electromagnetic spectrum is measured. This absorption of electromagnetic radiations by the molecules leads to molecular excitations.
  24. 24. Principle of UV-VIS Spectrometry Ultraviolet light and visible light cause an electronic Transition of electron from one filled orbital to another of higher Energy unfilled orbital. These transition occur between the electronic energy levels. As molecule absorbs energy , an electron is promoted from occupied orbital to an unoccupied orbital of greater potential energy. Generally the most probable transition is from (HOMO) to (LUMO).
  25. 25. Continued…  Ultraviolet absorption spectra arise from transition of electron within a molecule from a lower level to a higher level.  A molecule absorb ultraviolet radiation of frequency (𝜗), the electron in that molecule undergo transition from lower to higher energy level. The energy can be calculated by the equation, E=h𝜗 25
  26. 26. E₁-Eₒ= h𝜗 Etotal=Eelectronic+Evibrational+Erotational The energies decreases in the following order: Electronic >Vibrational > Rotational Continued…
  27. 27. Types of Transitions In U.V spectroscopy molecule undergo electronic transition involving σ, π and n electrons. Four types of electronic transition are possible.  σ ⇾ σ* transition  n ⇾ σ* transition  n ⇾ π* transition  π ⇾ π* transition
  28. 28. Transition’s Characteristics ~ 400–700 nm ~ 150-250 nm ~ 200 – 400 nm ~ 115 nm
  29. 29. When a sample is exposed to light energy that matches the energy difference between a electronic transition within the molecule, the light energy would be absorbed by the molecule and the electrons would be promoted to the higher energy orbital. A spectrometer records the degree of absorption by a sample at different wavelengths and the resulting plot of absorbance (A) versus wavelength (λ) is known as a spectrum. The significant features:  λmax (wavelength at which there is a maximum absorption)  Emax (The intensity of maximum absorption) The Absorption Spectrum
  30. 30. Continued… UV-Vis Spectrum of Silver Nanoparticles UV-visible spectrum of Silver Nanoparticles showing maximum absorption at 420 nm. λmax Emax
  31. 31. Lambert’s Law ▪ When a monochromatic radiation is passed through a solution, the decrease in the intensity of radiation with thickness of the solution is directly proportional to the intensity of the incident light. ▪ Let I be the intensity of incident radiation. x be the thickness of the solution. Then I dx dI  KI dx dI  Lambert’s Law Where, , A is AbsorbanceA I I 0 log (ε is Absorption coefficient)A = ε ℓ
  32. 32. Beer’s Law • When a monochromatic radiation is passed through a solution, the decrease in the intensity of radiation with thickness of the solution is directly proportional to the intensity of the incident light as well as concentration of the solution. • Let I be the intensity of incident radiation. x be the thickness of the solution. C be the concentration of the solution. Then IC dx dI . Beer’s Law E is Molar extinction coefficient 0I I T  T is transmittance A = ε C ℓ
  33. 33. Applications of UV-Vis spectroscopy Detection of functional groups Detection of extent of conjugation Identification of an unknown compound Determination of configurations of geometrical isomers Determination of the purity of a substance
  34. 34. Stability of Nanoparticle The optical properties of silver nanoparticles change when particle agglomerate When nanoparticle aggregate, the plasmon resonance will be red- shifted to a longer wavelength The peak will broaden or a secondary peak will form at longer wavelengths
  35. 35. Identification of Size & Shape Magnitude, peak wavelength and spectral bandwidth of the SPR of nanoparticles are dependent on particle size, shape and material composition  Different shape have characteristic peak in spectra like triangular shaped particles appear red, pentagon appear green, and blue particles are spherical.
  36. 36. Determination of Concentration With increase in the concentration of silver nanoparticle the SPR peak show bathochromic shift ie. shift towards red. Concentration of silver nanoparticle solutions can be calculated using the Beer-Lambert’s law as it correlates the optical density with concentration
  37. 37. In situ Nanoparticle Assesment Use in determinate the changes that occur during the synthesis of the nano particle in the in situ process Increase in no. of nano particle show Hypsochromic shift
  38. 38. References ▪ Yoon D., Moon H. and Cheong H. Variations in the Raman Spectrum as a Function of the Number of Graphene Layers. Journal of the Korean Physical Society (2009), Vol. 55 (3), pp. 1299-1303 ▪ Ahmed Jamal G. R., Mominuzzaman S. M. Different Techniques for Chirality Assignment of Single Wall Carbon Nanotubes. Journal of Nanoscience and Nanoengineering (2015), Vol. 1 (2), pp. 74-83. ▪ Hooijdonk E. V., Bittencourt C., Snyders R.,Colomer J-F. Functionalization of vertically aligned carbon nanotubes. Beilstein J. Nanotechnol. (2013), Vol. 4, pp. 129–152. ▪ Bouvier P., Kreisel J. Pressure-induced phase transition in LaAlO3. J. Phys.: Condens. Matter (2002), Vol. 14, pp. 3981–3991 ▪ Joshi M, Bhattacharyya A, Wazid A S, Characterization techniques for nanotechnology application in textile, Indian Journal of Fibre & Textile Research (2008), Vol. 33, pp. 304-317. ▪ Zook J M, Long S E, Cleveland D, Geronimo C A, MacCuspie R I, Measuring silver nanoparticle dissolution in complex biological and environmental matrices using UV–visible absorbance, Anal Bioanal Chem, (2011), Vol. 98, 1993-2002.
  39. 39. THANK YOU!