L32145s09 - Lecture 32: Optical Properties Reading...

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Lecture 32, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University Reading assignment : C&R §19.1-19.4, §19.7; §19.9; pp. 776-777; §19.13; OGC §20.3, §22.8 Learning objectives: In terms of band structure, understand … … why metals are reflective … use of semiconductors as solar cells and light-emitting diodes … optical transparency in insulators How trends in lattice parameter and band-gap energy determine color in solid-solution pigments and LEDs How lasers work Lecture 32: Optical Properties 1
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Lecture 32, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University Review: Electromagnetic Spectrum Visible light is only a small part of the EM spectrum Approximate color scale: 400-450 nm: violet 450-500 nm: blue 500-550 nm: green 550-600 nm: yellow 600-650 nm: orange 650-700 nm: red (OGC Fig. 4.3; C&R Fig. 19.2) 2
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Lecture 32, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University Review: Electronic Transitions in Atoms (OGC §4.2) Emission & absorption spectra from “free” atoms (e.g. monatomic vapors): Transitions between allowed energy states for electrons Are characteristic of the element emitting the spectrum Discrete energy levels narrow, sharp emissions/absorptions OGC Figure 4.9 3
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Lecture 32, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University Energy and Wavelength E = h ν= h c λ E , ν , λ : energy, frequency, and wavelength of photon h : Planck’s constant, 6.626×10 –34 J s = 4.136×10 –15 eV s c : speed of light in vacuum, 2.998×10 8 m s –1 For energy in eV and wavelength in nm, E [ in eV ] = 1240 [ in nm ] visible range: 400-700 nm; 3.1-1.8 eV 4
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Lecture 32, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University Interaction of EM Radiation with Metals (C&R pp. 764-5) C&R Figure 19.4 Band structure of metals: Continuum of empty states from Fermi level … … to “vacuum level” (electron removed from atom) E < Φ (the work function) : absorption & reemission — reflection Recall if E > Φ : photoelectric effect 5
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Lecture 32, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University Interaction of EM Radiation with Metals (C&R pp. 764-5) Metals:
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L32145s09 - Lecture 32: Optical Properties Reading...

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