UV-Vis Spectroscopy Class Notes PDF

Course Content 1. X-Ray Methods of Analysis - TEST I 2. UV, Vis, IR and THz spectroscopy 3. Nuclear Magnetic resonance ,Thermal Analysis Techniques 4. Raman Spectroscopy...

Course Content 1. X-Ray Methods of Analysis - TEST I 2. UV, Vis, IR and THz spectroscopy 3. Nuclear Magnetic resonance ,Thermal Analysis Techniques 4. Raman Spectroscopy - TEST II 4. Electron Methods of Analysis 5. STM, TEM, AFM Techniques 6. Mass spectroscopy - TEST III 7. Polarimetry Techniques IN201-Analytical Instrumentation:Aug-Dec 2024 1  Electromagnetic Spectrum Frequency(Hertz) Name of the Spectrum/Radiation Photon energy Wavelength (eV) (Angstroms) (Hard X-rays) (Soft X-rays) THz 2 https://halas.rice.edu/unit-conversions  Ultraviolet-visible (UV-VIS) SPECTROSCOPY Fluorescence spectroscopy Pump-probe spectroscopy Near infrared (NIR) spectroscopy Fourier-transform infrared (FTIR) spectroscopy Terahertz spectroscopy 3  NON-Ionization processes Absorption: Interaction of Resonant light with Atoms /molecules. Spectroscopy, Imaging, Sensors, Detectors, Emitters… NMR, UV-Vis, THz, Interferometry, Optical Microscopy Reflection/Transmission: Imaging, Scattering phenomena, Low-energy neutron scattering experiments Electromagnetic Field Meters: Cell phones, Wi-Fi routers, and power lines Thermal Imaging: Infrared cameras, Temperature sensors, … 4 UV-VIS SPECTROSCOPY First method for exploring the atomic and molecular structure through the absorption lines from the sun light History of UV-Vis spectroscopy 5  FEATURES UV-vis spectroscopy: Analytical Techniques using the EM band of UV-Vis radiation: 200-400nm (UV) and 400-700nm (Visible) Non-destructive: Does not damage/alter the sample Qualitative and Quantitative analysis High sensitivity: nanomolar-picomolar concentration Determination of purity of sample Essential tool in analytical chemistry and atomic/molecular spectroscopy to provide valuable information about composition, concentration, and electronic properties of the substances/materials 6 SIMPLISTIC SCHEMATIC OF UV-VIS ABSORPTION SPECTROSCOPY Source of radiant energy: UV and VISIBLE Collimating/Slit system Monochromator system: Grating/Prisms/Filters Sample: Materials/Solutions Wavelength selective slits Detectors: Photovoltaic cell (Semiconductors)/Photomultiplier tubes 7 SPECTROPHOTOMETER An Instrument that measures the ratio of the radiant power of two EM beams over a large wavelength region. It utilizes dispersing element (Prisms/Gratings) instead of filters to scan large wavelength region Analytical Instrumentation 2023 8 SOURCE OF RADIATION ENERGY Analytical Instrumentation 2023 9 SOURCE FOR VISIBLE RADIATION TUNGSTEN HALOGEN LAMP Its construction is simple similar to a house hold lamp The bulb contains a filament of Tungsten fixed in evacuated condition and then filled with inert gas The filament can be heated up to 3000K, beyond this Tungsten starts sublimating It is used when polychromatic light is required. To prevent this along with inert gas some amount of halogen (Iodine) is used Analytical Instrumentation 2023 10 Analytical Instrumentation 2023 11  Source for UV-Vis Radiation They are stable and robust Hydrogen lines: 410nm, 434nm, 486nm, 656nm 12 Mercury Discharge Lamp: Shows characteristic sharp lines: Not suitable for continuous spectral studies Analytical Instrumentation 2023 13 Xenon Discharge Lamp: Possesses two tungsten electrodes separated by some distance and filled with Xenon gas under pressure An intense arc is formed between the electrodes by applying the high voltage Intensity is higher than hydrogen discharge lamp and lamp operates at very high voltages Wavelengths: 250nm-800nm Analytical Instrumentation 2023 14 SLITS Choose range of Wavelengths at the exit slit Analytical Instrumentation 2023 15 MONOCHROMATORS Analytical Instrumentation 2023 16 FILTERS ✓ Colors in the glass filters ae produced by incorporating metal oxides like V, Cr, Mn, Fe, Ni, Co, Cu etc.. 17 FILTERS Analytical Instrumentation 2023 18 INTERFERENCE FILTERS Analytical Instrumentation 2023 19 INTERFERENCE FILTERS Analytical Instrumentation 2023 20 PRISM 21 GRATINGS Analytical Instrumentation 2023 22 DIFFRACTION GRATINGS Analytical Instrumentation 2023 23 DIFFRACTION GRATINGS Analytical Instrumentation 2023 24 TRANSMISSION GRATINGS Analytical Instrumentation 2023 25 ADVANTAGES OF GRATINGS Analytical Instrumentation 2023 26 COMPARISION OF PRISM VS GRATINGS Analytical Instrumentation 2023 27 DETECTORS : UV-VIS SPECTROSCOPY Analytical Instrumentation 2023 28 DETECTORS : REQUIREMENTS Analytical Instrumentation 2023 29 BARRIER LAYER CELL / PHOTOVOLTAIC CELLS Analytical Instrumentation 2023 30 BARRIER LAYER CELL / PHOTOVOLTAIC CELLS Analytical Instrumentation 2023 31 Silicon and InGaAs Charge coupled Devices Liquid-Nitrogen cooled Si or InGaAs based CCD array detectors with monochromator CCD detector Monochromator Silicon: 300nm-950nm (UV-VIS) InGaAs: 950-1700nm (NIR) IN201-Analytical Instrumentation:Aug-Dec 2024 32 INSTRUMENNT DESIGNS Analytical Instrumentation 2023 33 Uv-vis spectroscopy - instrumentation SINGLE BEAM PHOTOSPECTROMETER Analytical Instrumentation 2023 34 DOUBLE BEAM PHOTOSPECTROMETER Analytical Instrumentation 2023 35 DOUBLE CHANNEL (BEAM) MONOCHROMATIC UV-VIS SPECTROPHOTOMETER I0 I0 Entrance slit Exit slit I0 I 36 Analytical Instrumentation 2023 37 Analytical Instrumentation 2023 38 Analytical Instrumentation 2023 39 Analytical Instrumentation 2023 40 Analytical Instrumentation 2023 41 Analytical Instrumentation 2023 42 43 Monochromator with components Analytical Instrumentation 2023 44 IN201-Analytical Instrumentation:Aug-Dec 2024 45 ABSORPTION SPECTROSCOPY: TYPE OF TRANSITIONS Type of Radiation Type of Transition X-rays Inner shell Electron UV-Visible-NIR Outer shell Electron IR and THz Molecular Vibrational THz-Microwave Molecular Rotational Excitation frequencies Analytical Instrumentation 2023 46 MOLECULAR TRANSITIONS Energy levels are formed by the Hybridization of multi-atom molecular systems Forms Bonding and Antibonding Hybridized states Energy Energy Antibonding: Higher energy (Less stable) > > Bonding state: Lowest energy state (More Stable) 47 ELECTRONIC TRANSITIONS Analytical Instrumentation 2023 48 ENERGY LEVELS IN SEMICONDUCTORS Hamiltonian for 1-D system V(x) = 0 for free electron: Free electron model (No perturbation) Bulk Semiconductor ℏ2 2 2 2 (𝑖𝑘𝑥 𝑥+𝑖𝑘𝑦 𝑦+𝑖𝑘𝑧 𝑧) 𝐸 𝑘𝑥 𝑘𝑦 𝑘𝑧 = 𝑘𝑥 + 𝑘𝑦 + 𝑘𝑧 , 𝜓 = 𝜓 0 𝑒 2𝑚∗ Confinement along z-axis Confinement along z and y -axis Confinement along z, y and x -axis 49 ABSORPTION SPECTRUM OF QUANTUM DOTS 𝑛 2 ℏ2 𝜋 2 𝐸𝑛 = 2𝑚𝐿2 Quantum/nanodots show size dependent Optical properties Smaller the Dot-Size, higher the Absorption and Emission Energies 50 SINGLE-WALL CARBON NANOTUBES (SWCNTS): NANO(QUANTUM)WIRES (n, m): Indices for CNTs 1-D material Tunable optical and electronic properties Semiconducting and Metal Anisotropic properties PL in the NIR (10,5) Synthesis: CVD grown and Electric Arc discharge Suspension: Tip sonication in a surfactant and centrifugal separation Animation by Prof. S. Maruyama (Univ. of Tokyo) BAND STRUCTURE OF CNTS At higher density of states: energy levels become sharp (Discrete): Van-hove singularities Electron excitation from VB to CB creates formation of excitons with allowed transition of E11, E22 and E33 R. B. Weisman and J. Kono ‘ Handbook of Carbon nanomaterials’ (2019). ABSORPTION SPECTRUM: UV-VIS-NIR SPECTROMETER Chirality Mixed CNTs (CoMoCAT) soln. Depending on the Diameter, (n,m) species of carbon nanotubes, the absorption spectrum varies Larger the (n,m) number, higher the diameter 0.20 E11 (6,5) and longer the absorption wavelength for E11 CoMoCAT-No defects 982 nm transition 0.16 UV-VIS-NIR absorption spectroscopy helps in identifying the nanotube species in a mixed Absorbance 0.12 solution 570nm E11 (6,4) 0.08 E (6,5) 583nm 22 E22 (6,4) E22 (7,5) and (7,6) 0.04 500 600 700 800 900 1000 Wavelength (nm) BEER LAMBERT’S LAW: ABSORBANCE Statement: The quantity of light absorbed by the substance is directly proportional to the concentration (C) of the substance and the path length (dx) of the light through the substance Integrating from I0 to I and 0 to L 𝐼 𝐿 𝑑𝐼 𝑑𝐼 𝑑𝐼 − ∝ 𝐶 𝑑𝑥 − = 𝐾 𝐶 𝑑𝑥 −න = න 𝐾 𝐶 𝑑𝑥 𝐼 𝐼 𝐼0 𝐼 0 𝐼0 𝐾 𝐼 𝐼 𝑙𝑜𝑔 = 𝐶𝐿 2.303 𝑙𝑜𝑔 = −𝐾 𝐶 𝐿 Ln = −𝐾 𝐶 𝐿 𝐼 2.303 𝐼0 𝐼0 𝐼𝑜 𝐾 𝐴 = ɛ ∙ 𝐶 ∙ 𝐿 = 𝑙𝑜𝑔 ɛ= Molar Absorption/extinction co- 𝐼 2.303 efficient 54 Absorbance BEER LAMBERT’S LAW Beer Lambert’s law quantifies the process of absorption 𝐼 = 𝐼0 10− ɛ𝐶𝐿 = 𝐼0 10−𝐴 Intensity of light decreases exponentially as it passes through an absorbing medium. The absorbance (A) is defined as log (Io/I) = ɛ C L Where, ɛ is the Molar Extinction coefficient. It is a constant for the absorbing species and defines the absorption of the species at a particular wavelength. It is determined by the number and type of chromophores present in each molecule of the absorbing species. C is the concentration of the sample in the cuvette L is the length of the light path through the sample Absorbance is an additive property. The absorbance of the mixture of n species is given by 𝑛 𝐴𝑚𝑖𝑥𝑡𝑢𝑟𝑒 = ෍ 𝐴𝑖 55 𝑖=0 CALIBRATION CURVE According to the Beer Lamberts Law, Absorbance in a sample is proportional to the concentration and the Absorbance v/s concertation plot gives a straight line 𝐴 =ɛ∙𝐶 ∙𝐿 𝐴∝𝐶 Rhodamine B solutions with different concentrations in water measured using the Dual Beam Spectrophotometer This calibration curve is determined from the standard/known solution Quantitative analysis: By knowing the absorbance of the sample, one can determine the concentration 56 UV-VIS ABSORPTION SPECTROSCOPY OF CARBON NANOTUBES CNTs in Aqueous solution 𝐴∝𝐶 CNTs on Polyamide film The linearity in the Beer-Lambert’s law deviates under non-ideal Deviates from linearity conditions Analytical Instrumentation 2023 57 DEVIATION FROM THE BEER-LAMBERT’S LAW Chemical factors in the solutions Chemical interactions: Association/dissociation of for molecules at high concertation Dilution: Role of refractive index Speed of light in dense/light medium will be different and multiple scattering may change the real absorption Fluorescence or phosphorescence emissions 𝑆𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑣𝑎𝑐𝑢𝑢𝑚 𝑟𝑒𝑓𝑟𝑎𝑐𝑡𝑖𝑣𝑒 𝑖𝑛𝑑𝑒𝑥 𝑛 = 𝑆𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑚𝑒𝑑𝑖𝑢𝑚 At very low concentrations, n  1 At very high concentrations: n > 1 Analytical Instrumentation 2023 58 DEVIATION FROM THE BEER-LAMBERT’S LAW Instrumental factor: Beer-Lamberts law is ideal for absorption of monochromatic light Selection of wavelengths: Spectral deviations Deviation due to polychromatic light The molar absorptivity changes with wavelength of light The slit width of the monochromator should be at least 1/10th of the natural absorption bandwidth of the 59 sample DEVIATION FROM THE BEER-LAMBERT’S LAW ▪ Effect of Slit width Spectra bandwidth (SBW) : The width at half the maximum intensity of the band of light leaving the monochromator) SBW = Slit width * Dispersion of monochromator Natural bandwidth (NBW) : The width of the absorption spectrum of the sample at half the absorption maximum Condition : SBW ~ 1/10th NBW ▪ Contribution from the stary light ▪ Mismatched Cuvettes size/thickness Analytical Instrumentation 2023 60 MEASURING BANDGAP USING UV-VIS ABSORPTION SPECTROSCOPY Direct Bandgap Indirect Bandgap A two-particle interaction between an A three-particle interaction (photon, electron, electron and a photon phonon) to ensure momentum conservation The shape of the UV-vis absorption spectrum can distinguish between direct, indirect, allowed/forbidden transitions Much stronger and sharper absorption for Direct bandgap materials 61 MEASURING BANDGAP USING UV-VIS ABSORPTION SPECTROSCOPY ln 10 × 𝐴 I = I0 10-l 𝛼(𝑐𝑚−1 ) = 𝑙 (𝑐𝑚) Tauc Relation: Finding the absorption edge, optical bandgap for the semiconductors (𝛼ℎ𝜐)𝛾 = 𝐴 (ℎ𝜐 − 𝐸𝑔 ) : denotes the nature of electronic transition Where, : is absorption coefficient and can take following 4 values h: plank’s constant  = 2 : Direct allowed transitions : photon frequency  = ½ : Indirect allowed transitions A: Absorbance  = 2/3 : Direct forbidden transitions Eg: Energy bandgap  = 1/3 : Indirect forbidden transitions The resulting plot has a linear behavior that extrapolated to the energy axis, estimates the optical bandgap of the material 62 CORRELATION BETWEEN THE TAUC PLOT AND THE ENERGY BANDGAP (𝛼ℎ𝜐)𝛾 = 𝐴 (ℎ𝜐 − 𝐸𝑔 ) Consider this as a functional form 𝑦 =𝑚𝑥+𝑐 : Slope-intercept form of a line Y = 0 : interception on energy X-axis 0 = 𝐴 (ℎ𝜐 − 𝐸𝑔 ) 𝐸𝑔 = ℎ𝜐 Energy Bandgap By extrapolating the linear region in the Tauc-plot, the energy bandgap of the material can be determined at the Energy axis intercept point. 63 DETERMINING THE ENERGY BANDGAP FOR TIO2 FILM USING UV-VIS ABSORBANCE DATA UV-VIS absorbance data (h)2 vs Energy Calculate (h)2 (eV cm-1)2 (h)2 = (2.303  A  1240/λ)2 Absorbance (A) units: (eV cm-1)2 and plot it v/s Energy (eV) Energy (eV) Wavelength, λ (nm) Perform the linear (line) fit to the raising edge of the curve to get the line intercept on the energy axis The intercept point determines the energy bandgap (Eg) of the material 64  1 (𝛼ℎ𝜐)2 = 𝐴 (ℎ𝜐 − 𝐸𝑔 ) (𝛼ℎ𝜐)2 = 𝐴 (ℎ𝜐 − 𝐸𝑔 ∓ 𝐸𝑃 ) The clue for the identification of the direct/indirect bandgap nature is the linear behavior of the (h) plot 65  More examples ZnO thin film Silicon thin film (Direct bandgap) (Indirect bandgap) MoS2 MoS2 –Direct bandgap –Indirect bandgap Analytical Instrumentation 2023 66