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Application of Infrared Spectroscopy MatSci 571 2016 U. Mazur

Application of Infrared Spectroscopy MatSci 571 2016 U. Mazur. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS, DRIFT). advantages : easy sample preparation , surface technique (1-10  depth ), non-destructive

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Application of Infrared Spectroscopy MatSci 571 2016 U. Mazur

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  1. Application of Infrared Spectroscopy MatSci 571 2016 U. Mazur

  2. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS, DRIFT) advantages: easysamplepreparation, surface technique (1-10depth), non-destructive disadvantages: requires special accessories, surface technique (1-10 depth), not universal Praying Mantis accessory powders, liquids, and suspensions (dyed textiles, cosmetic compounds, printed papers, catalytic materials, agricultural and botanical specimen, etc.) materials can be used neat and diluted in non-absorbing powders such as KBr, KCl, Ge, and Si or ZnSe, ZnS, CdTe, and MgO smaller particle size yields improved spectra Diffuse reflectance can be used though out the IR-VIS-UV spectral range

  3. DRIFT Spectra of powders and rough surfaces can be recorded by illuminating these surfaces and collecting scattered light for spectral analysis. In the UV-VIS integrating spheres and in the infrared ellipsoidal and parabolic mirrors are used to collect the scattered radiation. Reaction chambers for diffuse reflectance allow the study of catalysis and oxidation reactions in situ. DRIFT spectra may exhibit features of transmission, external reflection, and/or internal reflection. Sample particle size and angle of illumination and/or observation can affect the observed spectra. Schematic Diagram of Light Scattering from a Powder Sample

  4. Quantitative DRIFT The Kubelka-Monk (K-M) transform is a mathematical transfrom for converting diffuse reflectance intensity into absorbance. K-M diffuse reflectance spectra can be used as one would use absorbance for spectra measured in transmittance. Where, R∞ is the absolute reflectance, K is the absorption coefficient, and S is the scattering coefficient. Due to the difficulty in measuring the absolute reflectance R∞, the comparative reflectance r∞ with respect to a standard powder such as KBr or KCl (for which K is near zero 0) is employed.

  5. Quantitative DRIFT Light emitted from the powder contains specular reflected light and diffuse reflected light. The specular reflection must be reduced (ideally elliminated) to obtain accurate diffuse reflectance spectra. To achieve this, the particle size must be reduced. Reducing the particle size to a size  to the wavelength studied decreases the proportion of specular reflection and maximizes the scattering efficiency. The particle shape and packing status are other important factors, in addition to the particle size. Samples are normally not measured directly but in a form diluted to a suitable concentration (1 % to 10 %) in a KBr or KCl standard powder. After Kubelka-Monk (K-M) transformation the spectral intensities vary linearly with concentration (obey Beer’s Law)

  6. Kubelka Munk Transform The Kubelka-Munk equation creates a linear relationship for spectral intensity relative to sample concentration. It assumes • infinite sample dilution in a non-absorbing matrix, • a constant scattering coefficient (S), and • an “infinitely thick” sample layer. These conditions can be achieved for highly diluted, small particle samples (the scattering coefficient is a function of sample size and packing) and a sample layer of at least 1.5 mm. With proper sample preparation diffuse, reflectance spectroscopy can provide ppm sensitivity and high quality results.

  7. Quantitative DRIFT Diagram of the DRIFT optical path. IR light comes from right hits M1 bounces of M2 then M3 and shines infrared light on the sample. M4 collects the diffuse reflected light, which passes via M5 and M6 to the detector. The sample plate is 6 mm in diameter and 1.5 mm in depth. Highly sensitive measurements on a small volume of sample can be made by filling the sample plate with KBr powder and putting the sample powder mixed with a small quantity of KBr powder onto it. Diffuse Reflectance Accessory

  8. The Praying Mantis™ (Harrick) This accessory incorporates two 90° off-axis ellipsoidal mirrors. One ellipsoid focuses the incident beam on the sample while the second collects the diffusely reflected radiation from the sample. Both ellipsoidal mirrors are tilted forward so the diffusely reflected radiation is collected at an azimuthal angle of 120°. Optical geometry permits collection of the diffusely reflected radiation, making the DRA quite practical for routine measurements. It is ideal for reliable diffuse reflectance studies of powders and other rough surface solid samples. Low T chamber operates at 1-2 atm from 150ºC to 600ºC. High T, low pressure chamber up to 910ºC (under vacuum) and 133 μPa (10-6 torr) to 133 kPa (1 ktorr). High pressure chamber can withstand pressures up to 3.44MPa (25.8 ktorr).

  9. Comparison of DRIFT with Transmission IR Spectroscopy 1,2-Bis(diphenylphosphino)ethane (dppe) is a commonly used bidentate ligand in coordination chemistry. Transmission spectrum of 1,2-bis(diphenyl phosphino)ethane, KBr pellet, at 298 K. DRIFT spectrum of 1,2-bis(diphenyl phosphino)ethane, pure powder, at 298 K. The diffuse reflection technique combines the advantage of faster and simpler sample preparation with the absence of the spurious water band in the spectrum. Oil & Gas Science and Technology – Rev. IFP, 2004, 59, 215

  10. Boron Nitride Nanotubes (BNNT) BNNT are great interest to biomedicine, material science, and electronics. BNNT were synthesized using the Laser Vaporization Plume Chemistry approach, otherwise known as the Pressurized Vapor/ Condenser (PVC) Method. Tube Length≤ 200 µm band gap is 5.7 eV. ATR FTIR spectrum of BNNT. Bands at 1355 cm-1 and 804 cm-1, correspond to B–N stretching and B–N bending, respectively. 3220 cm-1 transition is assigned to the stretching vibration of OH groups of boric acid and was found to be sensitive to ambient environmental conditions. http://www.nanointegris.com/en/bnnt

  11. FTIR for Quantitative Assessment of Bone Bioapatite Diagenesis* DRIFT, ATR, and Transmission are used to evaluate diagenesis by anthropological geochemists and by examining the crystallinity (IR-SF factor) and CO3:PO4 ratio of bone material. Peak heights at 565 and 605 cm-1 and the height of the minimum trough between them gives the IR-SF factor: Modern fresh bone have IR-SF of 2.5 to 3.25. Archaeological bone samples that contain a measurable amount of collagen tend to have IR-SF values less than fresh bone. High IR-SF ratios indicate large ordered crystals, low IR-SF values are consistent with smaller sized crystals having irregular structure. *Diagenesis is a complex process involving physical and chemical post-mortem alterations to bones and teeth that is influenced by the burial environment. Spectra modern bone sample obtained by different IR methods. Journal of Archaeological Science 46 (2014) 16

  12. Atmospheric Ageing of Nanosized Silicon Nitride Powders Ambient oxidation of nanosized silicon nitride powder was studied by DRIFT. S2N3 powder samples were stored in Ar, in dry air and in air of 80% humidity. No significant changes were detected on storage in argon gas for 90 days. Samples stored in dry air and humid air become oxidized and have N-H bands. The nitride powder tends to agglomerate in both dry and humid air, and results in changes in the particle size and morphology. Nanosized silicon nitride powders, therefore, should be processed by excluding their contact with the ambient atmosphere. 540 days in humid (80%) air 540 days in dry air 90 days in Ar DRFT spectra and TEM images of Si2N3 samples subjected to aging. J. Mater. Chem., 2001, 11, 859

  13. DRIFT Application to the in situ Analysis of Catalysts Zeolite sample with Si/Al ratio of 13.6. Intense band at 3741 cm–1 is due to the free terminal silanols and the bands at 3630 and 3566 cm–1 are due to the bridged Al-OH-Si acid hydroxyls located respectively in the supercages and hexagonal prisms. A weak signal at 3670 cm–1 is due to Al-OH of extra-lattice species, as well as two shoulders at 3552 and 3526 cm–1. The baseline shows a slope resulting from the size of the particles causing diffuse scattering. Compared with the transmission spectrum, the DRIFT spectrum exhibits a better baseline. The 3670 band is more clearly defined. Oil & Gas Science and Technology – Rev. IFP, 2004, 59, 215

  14. DRIFT Application to the in situ Analysis of Catalysts Surface species on the catalyst surface (Pt/Al2O3) formed during 1 h of NO dosing at 423 K. In first few minutes, a series of bands appears, increasing in intensity during the experiment up to 1hr of nitration. The bands at 1548 and 1584 cm−1 can be attributed to N = O vibrations of the nitrate species and the intense band at 1836 cm−1 to linear NO on the oxidized platinum. The band at 1993 cm−1 could result from the adsorption of NO linearly on Pt2+. At 423 K,a small quantity of CO was present. The bands at 2128 and 2222 cm−1, respectively, are attributed to COlin/Pt and AlNCO. Oil & Gas Science and Technology – Rev. IFP, 2004, 59, 215

  15. DRIFT Study of CuO−CeO2−TiO2 Oxides for NOx Reduction with NH3 at Low T NOx are air pollutants contributing to acid rain, photochemical smog and ozone depletion. A CuO−CeO2−TiO2 catalyst is used for selective catalytic reduction of NOx with NH3 at low T. DRIFT spectra of CuCeTi catalyst after dosing with NH3+NO+O2 at various T. In N−H region, bands assigned to coordinated NH3 on Lewis acid sites (3373, 3254 and 3161 cm−1) 1610, 1580, 1280 and 1225 cm−1 are assigned to nitrate species along with σasNH3 on Lewis acid sites at 1600 cm−1. 1880 cm-1 weakly adsorbed NO on the surface of the CuCeTi. Studies indicate that most of the adsorbed NH3 species react with NOx. Oxygen oxidizes the Cu2+−NO species to nitrates, which are stored on cerium sites or can be directly reduced to N2. ACS Appl. Mater. Interfaces 2014, 6, 8134

  16. Attenuated Total Reflectance (ATR) advantages: for solids, liquids, semisolids, thin films, easy sample preparation, surface technique (1-5 depth), non-destructive disadvantages: requires special accessories, surface technique (1-10 depth), not universal For ATR, light is introduced into a suitable prism at an angle exceeding the critical angle for internal reflection. This produces an evanescent wave at the reflecting surface. From the interaction of the evanescent wave with the sample, a spectrum can be recorded with little or no sample preparation. For thin films, ATR spectra are the same as transmission spectra. For thick films, absorption bands are more intense at longer wavelengths. As the angle of incidence approaches the critical angle, the bands tend to broaden on the long wavelength side and the minima are displaced to longer wavelengths (lower wavenumbers). Dispersion type spectra are observed very close to and below critical angle.

  17. ATR Materials Note: KRS 5 (Thallium Bromide-Iodide)

  18. 2= 1.00 1= 1.33 Total internal reflection can be seen at the air-water boundary. Total Internal Reflection: Interface Between Water and Air When light hits an interface between two different media, it can behave in two different ways: the light partially refracts (bends) and partially reflects. The refractive index of the material is what determines how fast light travels in a material. Light travels about one and a third times faster in air than in water. Water has a higher refractive index than air. When light is propagating from water to air, for all angles less than a certain critical angle, the light gets transmitted through. But once the angle is larger than the critical angle; all of the light gets reflected. This is what is called total internal reflection. The refractive index of air is 1.0 and the refractive index of water is 1.33 so the critical angle for total internal reflection for a water-air interface is 48.7°.

  19. ATR Crystal of IR transparent materials (KRS-5, ZnS, Ge) • solids, liquids, semisolids, and thin films • evanescent wave - it is slightly bigger than the crystal, so it penetrates a small distance beyond the crystal surface into space Reflectivity (fraction of radiation reflected) R=(n2-n1)2/(n2+n1)2 n2: refractive index of air or thin sample n1: refractive index of crystal Snell’s law: n1sin  = n2sin  c: critical angle  : angle of incidence for c internal reflectance occurs c = sin-1n2/n1 when light enters material 2 (n2) from 1 (n1) n2 n1 (glass to air) internal reflection (total reflection for 90c) For a typical ATR crystal in contact with air, c = 19

  20. n2 n1 ATR Depth of penetration (DP): depth at which the evanescent wave is attenuated to 13.5% of its total intensity DP = 1/[2WNc(sin2 - 2sc)1/2] W: wavenumber c: crystal refractive index : angle of incidence sc:sample/crystal average DP is 0.1-5 microns common  is 45o and 60o Dp is defined as the distance between the sample surface and the position where the intensity of the penetrating Evanescent wave dies off to (1/e)2 or 13.5%, or its amplitude has decayed to 1/e. n2 > n1

  21. ATR Wavelength of light: the longer the wavelength of the incident light (lower wavenumber), the greater the depth of penetration into the sample. This yields an ATR spectrum that differs from the analogous transmission spectrum, where band intensities are higher in intensity at longer wavelength. However, the ATR spectrum is readily converted to absorbance units

  22.  = 60  = 45  = 30 transmission spectrum in mineral oil ATR and Angle of Radiation ATR of ascorbic acid on KRS-5 prism. Top  = 45 light entrance. Bottom  = 60 deg entrance. ATR of NaClO3 on KRS-5 prism. Applied Infrared Spectroscopy. J. Wiley & Sons.

  23. s and p Polarization The electric field vector for p polarized light lies in the plane of incidence. The electric field vector for s polarized light is perpendicular to the plane of incidence (s-polarization, from German senkrecht perpendicular) Plane of incidence is the plane containing the surface normal and the propagation vector (k vector) p-polarization vector s-polarization vector plane of incidence

  24. FTIR-ATR of Self-assembled (SAM) Thiol Monolayers on Gold  A 1-tetradecanethiol, TDT on a gold surface. Note that the CH2-plane is perpendicular to the alkyl chain axis, and that both sym(CH2) and asym(CH2) are in that plane, where sym(CH2) and asym(CH2) are orthogonal to each other. ATR of C-H stretching region of TDT adsorbed on gold with different angles of incidence 25 to 60. Self-assembled monolayers of TDT were prepared by immersing the clean gold-coated substrate into a freshly prepared solution of TDT (10-3 M in ethanol) for 20 h. Supramolecular Science 1998, 5, 607

  25. FTIR-ATR of SAM Thiol Monolayers on Gold The tilt angle, , based on the interaction of a polarized electric field with a pair of orthogonal dipole moments of sym and asym CH2 vibrations . The following relates the dichroic ratio, D, with : Polarized FTIR-ATR spectra for TDT SAMs: (1) parallel polarized beam; (2) perpendicular polarized beam The orientation of the molecular vibrational plane is determined by formalisms which relate this plane to the surface coordinates via Euler angles.  was calculated to be 34. Supramolecular Science 1998, 5, 607

  26. Polarization Measurement of Stretched PET Film Using ATR Spectra of unstretched PET film. There are hardly any differences between any of the four spectra, indicating that there is no molecular orientation. Spectra of PET film stretched by a factor of 3. 1,340 cm-1 corresponds to CH2 wagging and the mode in which the molecules oscillate in the direction of their long axes. In (a), this peak is intense for p polarized light and weak for s polarized light. This is reversed in (b), indicating that the molecules are oriented in the stretching direction. 1,410 cm-1 is an in-plane bending vibration of benzene rings.

  27. IR Combined with SPM AFM-ATR and spectra of a polystyrene In AFM-IR IR light absorbed by the sample is converted to heat, causing a rapid thermal expansion pulse under the AFM tip, in turn exciting resonant oscillation of the AFM cantilever. IR absorption spectra result by measuring the cantilever oscillation amplitude (proportional to to the sample absorption coefficient) as a function of the wavelength of the incident radiation. AFM-IR technique combines an atomic force microscope with an IR spectrometer as shown in this schematic. After illumination with a pulsed OPO IR source, the cantilever ringdown characteristics from the sample provide both spectral and mechanical data about the sample.

  28. Characterization of a polyethylene–polyamide multilayer film using nanoscale infrared spectroscopy and imaging Tapping mode AFM and AFM-IR spectra from seven layers (color coded) of a multilayer film consisting of polyethylene (PE) and polyamide (PA). IR spectra collected from layers labeled A and F are consistent with polyethylene. Spectra collected from layers C, E, and G are consistent with a polyamide. The IR spectrum recorded from layer D, which can be considered as the barrier layer in the film, is consistent with polyethylene-co-(vinyl alcohol) (EVOH). Vibrational Spectroscopy 82, 2016, 10

  29. AFM-IR of a Biomaterial AFM-IR spectra of a wood cross-section; blue markers in the middle lamella region where there is a higher content of lignin (~1504 cm-1 ) than the layer with red markers.

  30. ATR VideoMVP™ (Harrick) ATR Spectra from two locations on a credit card. The inset photographs are bordered by the color of the corresponding spectrum. The VideoMVP™ has high performance diamond ATR accessory plus video imaging and optional force sensing capabilities. It has a a sampling area of less than 500 μm diameter on its monolithic diamond. Also a heatable sampling plates for operation up to 200°C with diamond, Si or ZnSe ATR crystals, 100°C with Ge. Good for hard samples, abrasive powders, fibers, beads, and even corrosive materials. The video imaging system provides a real-time magnified view of the sample through the ATR crystal. This image can be seen on the built-in display or digitally captured. Solid samples are compressed against the ATR crystal using the built-in pressure applicator. A force sensor measures force applied to the sample.

  31. Specular Reflectance Reflection Absorption Infrared Spectroscopy (RAIRS) advantages: easysamplepreparation, surface technique, non-destructive disadvantages: requires special accessories, sample must have smooth reflective surface (Al, Au), not universal Light is reflected from a smooth (mirror-like) sample. Non-destructive, non-contact technique (Francis and Elison in 1959); particularly useful for film thickness and refractive index measurements, as well as recording spectra of thin films on metal substrates. Spectra may look different from transmission spectra in many ways, e.g. bands may be shifted to higher wavenumbers, spectra may follow the dispersion in the refractive index, and spectral contrast may not depend linearly on sample thickness.

  32. RAIRS Since it is an optical (photon in/photon out) technique it is not necessary for such studies to be carried out in vacuum. The technique is not inherently surface-specific. The surface signal is readily distinguishable from gas-phase or solute absorptions using polarization effects. Sensitivity (i.e. the signal is usually very weak owing to the small number of adsorbing molecules). Typically, the sampled area is ca. 1 cm2 with less than 1015 adsorbed molecules. Small signals (0.01% - 2% absorption) can still be recorded at relatively high resolution (ca. 1 cm-1). Low frequency modes ( < 600 cm-1 ) are not generally observable - this means that it is not usually possible to see the vibration of the metal-adsorbate bond and attention is instead concentrated on the intrinsic vibrations of the adsorbate species in the range 600 - 3600 cm-1. This is because of the low sensitivity of detectors in this region, NOT an inherent characteristic of the technique.

  33. RAIRS To detect sub-monolayer coverages RAIRS, uses near grazing incidence geometry Optimum angle of incidence for RAIRS is 75 to 86 parallel polarization, depending on the properties of the surface. Greenler RG. 1966.J. Chem. Phys.44:310–15 RAIRS of PNBA as a function of angle of incidence. Fresenius J Anal Chem (1998) 362 : 15–20

  34. RAIRS Surface Selection Rule The observation of vibrational modes of adsorbates on metallic substrates is subject to the surface dipole selection rule. This states that only those vibrational modes which give rise to an oscillating dipole perpendicular (normal) to the surface are IR active and give rise to an observable absorption band. Vibration of a polar molecule oriented parallel to the substrate surface induces an image charge in the surface that cancels out the dipole, whereas it is enhanced for vibrations perpendicular to the surface.

  35. s and p Polarization The electric field vector for p polarized light lies in the plane of incidence. The electric field vector for s polarized light is perpendicular to the plane of incidence (s-polarization, from German senkrecht perpendicular) polarization vector surface normal surface normal polarization vector IR ray IR ray   plane of incidence plane of incidence plane of perpendicular polarized light plane of parallel polarized light

  36. RAIRS: Choosing Angle of Incidence RAIRS spectra of 70 nm thick SiO2 on an AL substrate at various angles of incidence with parallel (p) polarized light. RAIRS spectra of 70 nm thick SiO2 on an Al substrate at various angles of incidence with perpendicular (s) polarized light.

  37. Adsorption of CO on Cu(100) Surface FT-RAIRS spectra of CO/Cu(100). A. Adsorption at 23 K showing both monolayer chemisorbed CO peak and multilayered physisorbed CO. B. Effect of warming to 26 K to desorb multilayers, leaving a single physisorbed monolayer. C. Effect of warming to 35 K and recooling to 23 K. D. Surface described in c following further adsorption of CO at 23 K. 128 co-added scans. J. Chem. Soc. Faraday Trans. 1997, 93, 2315.

  38. Nitric Oxide (NO) Adsorption on a Pt Surface The sequence of spectra demonstrate how IR spectroscopy can clearly reveal changes in the adsorption geometry of chemisorbed molecules. In this particular instance, all the bands are due to the stretching mode of the N-O bond in NO molecules adsorbed on a Pt surface, but the vibrational frequency is sensitive to changes in the coordination and molecular orientation of the adsorbed NO molecules. ν(N-O) spectra obtained from a Pt surface subjected to a fixed exposure of NO at various temperatures.Bridged NO species, bonded to the step edge, is the most stable species on the surface, a 1610-1620 cm-1 band. 1801 cm-1 is tentatively assigned to the formation of an O-NO complex. This species forms when a NO molecule bonds on top of an O atom, which results from the dissociation of NO on the Pt surface at RT. Note - the surface coverage of adsorbed NO molecules decreases as the temperature is raised and little NO remains adsorbed at temperatures of 450 K and above. J. Phys. Chem. B, 2004, 108, 289

  39. CO Chemisorbed on a Pt Surface RAIRS spectrum shown below was obtained for a saturation coverage of CO on a Pt surface at 300 K. The reduction in the stretching frequency of terminally-bound CO from the value observed for the gas phase molecule ( 2143 cm-1 ) can be explained in terms the bonding of CO to metals: A : σ-bonding -- overlap of a filled  -"lone pair" orbital on C with empty metal orbitals (of the correct symmetry) - charge transfer from the CO molecule to the metal center. B : π bonding -- overlap of filled metal dπ (and pπ) orbitals with the π* antibonding MO of the CO molecule. Leads to charge transfer into the CO antibonding orbital and reduction in the CO bond strength and its vibrational frequency (relative to the isolated molecule). The majority of the CO molecules are bound terminally to a single Pt surface atom

  40. HCN adsorption on a Pt Surface The RAIRS spectra of HCN adsorption on Pt at sub-ambient temperatures; the surface species give rise to much weaker absorptions than NO, and signal/noise is important. Spectra also illustrate the surface normal selection rule for metallic surfaces. a) 0.15 L HCN at 100 K: HCN is weakly coordinated to the surface in a linear end-on fashion via the nitrogen; the ν(H-CN) mode is seen at 3302 cm-1 but the ν(C-N) mode is too weak to be seen and the δ(HCN) mode expected at ca. 850 cm-1 is forbidden by the surface selection rule. The overtone of the bending mode, 2δ(HCN), is however allowed and is evident at ca. 1580 cm-1. (b) 1.50 L HCN at 100 K: Higher exposures lead to the physisorption of HCN molecules into a second layer. These molecules are inclined to the surface normal and the HCN bending mode (820 cm-1) of these second layer molecules is no longer symmetry forbidden. Hydrogen bonding between molecules in the first and second layers also leads to a noticeable broadening of the ν(H-CN) band to lower wavenumbers.(c) 30 L HCN at 200 K: At the higher temperature of 200 K only a small amount of molecular HCN remains bound in an end-on fashion to the surface. The relatively strong band at 2084 cm-1 suggests that some dissociation has also occurred to give adsorbed CN groups, which give rise to a markedly more intense ν(C-N) band than the HCN molecule itself. Langmuir, 1 L = 1x10-6 mbar s.

  41. Infrared Spectroscopy from Phonons Phonons = lattice vibrations total number of phonon modes: dn N N: number of unit cells in crystal n: number of atoms in unit cell d: dimensionality arranged in dn branches: d acoustic branches d (n-1) optical branches IR spectroscopy couples only to optical phonons Example: diatomic chain optical phonon, “internal” vibration of unit cell In 3-dimensional crystals, one distinguishes between transverse and longitudinal optical phonons: TO phonons usually have lower energy. Phonons in metals are difficult to see, because free electrons screen E-field from interior of sample in IR spectral region (R1) IR phonon spectroscopy mostly in insulators and semiconductors.

  42. Comparison of Phonons & Photons PHOTONS Quantized normal modes of electromagnetic waves. The energies & momenta of photons are quantized PHONONS Quantized normal modes of lattice vibrations. The energies & momenta of phonons are quantized Phonon wavelength: λphonon≈ 10-10 m Photon wavelength (visible): λphoton≈ 10-6 m

  43. Infrared Spectra of AIN Films Prepared by Ion-Beam Deposition N-H and the C-H stretches appear near 3250 and 2900 cm-1, respectively. Band near 2200 cm-1 is due to Al-N=N stretch. Al-O stretch is at 950 cm-1. Below 1000 cm-1, only the LO AlN mode can be observed in the reflectance spectrum. This result is expected for near grazing incidence reflectance data. TO component is near 640 cm-1. Broadness of the LO and TO bands is due to the amorphous or nanocrystalline nature of the AlN film. The tunneling vibrational bands in (c) near 730 and 1820 cm-1 are associated with AI-H motions in a hydrogen-implanted alumina barrier. Vibrational spectra of AIN films synthesized with 25% H2-enriched N2: (a) absorbance spectrum of a 25-nm-thick film deposited on a KBr window; (b) RAIRS spectrum (at 80 incidence) of 8-nm-thick film on an Al mirror; (c) IET spectrum of a -0.4-nm-thick film. J. Phys. Chem. 1990, 94, 189

  44. Photoacoustic Spectroscopy (PAS) advantages: can work on any gas, liquid, or solid, nondestructive disadvantages: requires special accessories, not very sensitive Spectra are not dependent on sample size or shape. The signal to noise ratio may increase with sample surface area. Sample loading and unloading can be tedious. A final advantage of PAS is that there is little sample preparation, simple place the sample in the accessory and start taking spectra. APS is particularly sensitive to interfering spectra from evolved gases. Extreme caution must be exercised to minimize vibration of the apparatus.

  45. PAS • Sample literally 'talks' to the observer. Light entering the photoacoustic cell passes through undetected if the sample is non-absorbing, but heats and expands the gas in the cell if the light is absorbed. This expansion (and subsequent contraction) of the gas makes an audible sound whenever absorption occurs. Sound is detected by a microphone. • APS is particularly sensitive to interfering spectra from evolved gases. Extreme caution must be exercised to minimize vibration of the apparatus. • Spectrum is a direct measure of the absorption of radiation by the sample • Detection of pressure waves is related to properties such as thermal diffusivity which can vary between samples. Thermal diffusivity is not necessarily constant with sample depth. • Background spectra are obtained from carbon black. Normally, one avoids taking infrared spectra of carbon black because it has broad, featureless absorbance bands that are not very useful. However, since carbon black absorbs at all infrared wavenumbers, it has a PAS signal at all wavenumbers making it useful as a background material.

  46. PAS PAS drops into the sample compartment of most FTIRs. Samples are placed in a sample cup (~1/2 inch across and 1/2 inch deep), and KBr window is sealed above the sample. Helium (or air) is the gas used. PAS has optics to direct the radiation through the IR window and focus it on the sample  sample heats up. Heat from different layers in the sample diffuses towards the sample's surface. This movement of heat is known as a thermal wave. Once a thermal wave reaches the surface of the sample, the gas above the sample is heated and expands. This expansion causes a pressure wave to propagate through the gas, literally, the wind blows inside the sample cell. PAS accessory contains a sensitive microphone (originally developed for hearing aids) to hear the pressure wave. A diagram of a photoacoustic accessory.

  47. PAS: Depth Profiling The sampling depth in a PAS experiment is defined as the depth from which 63% of the thermal wave that reaches the surface originates. The equation used to calculate the sampling depth is: where: L = sampling depth in cm D = sample thermal diffusivity, in cm2/sec F = frequency of modulation of radiation When one needs to know the sampling depth in a PAS experiment, this equation can be used to calculate the value. Now, the frequency with which infrared radiation is modulated by an interferometer is given as follows: where: f = frequency of modulation  = moving mirror velocity, in cm/sec  = wavenumber of infrared radiation Then

  48. PAS: Depth Profiling The sampling depth is dependent on the moving mirror speed of the interferometer. It is possible to obtain spectra from different depths within the same sample by simply changing the scan speed. The ability of PAS to obtain spectra from different depths within a sample is known as depth profiling. Depth profiling can produce interesting results if a sample's composition changes with depth. It is important to realize that PAS cannot do a layer by layer spectra within a sample (unlike NMR imaging). Instead, when L is increased, the entire depth from which the signal is obtained increases. Thus, when L is small, the spectrum of the top layer of the sample is obtained. When L is large, the top layer and lower layers contribute to the infrared spectrum.

  49. PAS of Nanomaterials The PA technique offers a unique method to study the spectra of powdered rare earth materials. Their spectra are atomic in nature due to the shielding of the electrons in the unfilled f orbitals by a closed electronic 5s25p6 shell. Well-defined peaks corresponding to transitions from the ground state (7F0) to various excited states are obtained, and the observed peaks are assigned based on the data available in the literature. Although some transitions such as 5D1 ← 7F0 and 5D3 ← 7F0 are not allowed, the proximity of 7F1 level to the 7F0 level causes mixing of levels making these transitions allowed. PA spectrum of Eu2O3 nanopowder (~60 nm diameter). J. Chem. Ed. 2009, 86,1238

  50. PAS of Nanomaterials PA spectra of bulk and as-prepared CdS (semiconductor) nanowires are shown in Figure 2. The band gap of bulk CdS powder occurs at 2.39 ± 0.04 eV, which agrees with the literature value of 2.42 eV. The absorption edge of CdS nanowires is much steeper and occurs at a slightly larger value of 2.49 ± 0.04 eV. The increased steepness might be attributed to the relatively well-ordered structure and size distribution. PA spectra of bulk and CdS nanowires. The average diameter of the nanowires was about 50 nm. J. Chem. Ed. 2009, 86,1238

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