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optical1_lect21 - Optical Properties I: Color Pigments and...

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Unformatted text preview: Optical Properties I: Color Pigments and Gemstones Chemistry 754 Solid State Chemistry Lecture #21 May 16, 2003 Electromagnetic Radiation and the Visible Spectrum UV Violet Blue Green Yellow Orange Red Near IR 100-400 nm 100400-425 nm 400425-492 nm 425492-575 nm 492575-585 nm 575585-647 nm 585647-700 nm 64710,000-700 nm 10,00012.4 - 3.10 eV 3.10 - 2.92 eV 2.92 - 2.52 eV 2.52 - 2.15 eV 2.15 - 2.12 eV 2.12 - 1.92 eV 1.92 - 1.77 eV 1.77 - 0.12 eV Red Orange Yellow Violet Blue Green If absorbance occurs in one region of the color wheel the material material appears with the opposite (complimentary color). For example: a material absorbs violet light Color = Yellow a material absorbs green light Color = Red a material absorbs violet, blue & green Color = Orange-Red Orange a material absorbs red, orange & yellow Color = Blue E = hc/ = {(4.1357 x 10-15 eV-s)(2.998 x 108 m/s)}/ hc/ eVm/s)}/ E (eV) = 1240/(nm) (eV) 1240/ 1 Color in Extended Inorganic Solids Intratomic (Localized) excitations Cr3+ Gemstones (i.e. Cr3+ in Ruby and Emerald) Blue and Green Cu2+ compounds (i.e. malachite, turquoise) Blue Co2+ compounds (i.e. Al2CoO4, azurite) Charge-transfer excitations (metal-metal, anion-metal) Charge(metalanion Fe2+ Ti4+ in sapphire Fe2+ Fe3+ in Prussian Blue O2- Cr6+ in BaCrO4 Valence to Conduction Band Transitions in Semiconductors WO3 (Yellow) CdS (Yellow) & CdSe (Black) HgS (Cinnabar - Red)/ HgS (metacinnabar - Black) Intraband excitations in Metals Strong absorption within a partially filled band leads to metallic lustre metallic or black coloration Band to Band Transitions Optical Properties of Semiconductors We can examine the relationship between bonding (spatial and energetic overlap) and optical properties by considering the bandbandgaps of those compounds which adopt the Sphalerite structure, with all ions in tetrahedral coordination (see figure to the right). Since electronic transitions from the valence to conduction band span a fairly large range of energies, semiconductors act as sort of a long pass filter (only reflecting light with energy less than the band gap). This can give rise to only certain colors. Band Gap (eV) (eV) > 3.0 3.0-2.5 2.3-2.5 1.8-2.3 < 1.8 Color Example White ZnO Yellow CdS Orange GaP Red HgS Black CdSe 2 Spatial Overlap and Band Gap (Eg) Conduction Band (Antibonding) Antibonding) Eg Valence Band (Bonding) Eg What are the effects of increasing the spatial overlap? VanVanVechten & Phillips assumed spatial overlap is proportional to d-2.5 (where d is the bond distance). Primary Effect: Increases the overall energy level of the Effect: conduction band (more highly antibonding) [Eg ] antibonding) [E Secondary Effect: Increases the bandwidth [Eg ] Effect: [E Ionicity and Band Gap (Eg) Conduction Band (Antibonding) Antibonding) Eg Valence Band (Bonding) Eg What are the effects of increasing the electronegativity difference between the elements? Primary Effect: Increases the separation of the valence and Effect: conduction bands (the bonds become more ionic) [Eg ] [E Secondary Effects: Decreases the bandwidth [Eg ] and Effects: [E reduces the covalent antibonding destabilization [Eg ] [E 3 Energetic & Spatial Overlap Constant Spatial Overlap Increasing Ionicity (Eg ) Al Si P Zn Ga Ge As Se Cd In Sn Sb Te ZnSe CdTe d=2.45 =0.90 =0.90 d=2.81 =0.61 =0.61 Si Ge Sn d=2.35 =0 =0 d=2.44 =0 =0 d=2.81 =0 =0 AlP GaAs InSb d=2.37 =0.62 =0.62 d=2.45 =0.43 =0.43 d=2.80 =0.31 =0.31 Decreasing Spatial Overlap (Eg ) (E Slowly Decreasing Ionicity (Eg ) Si Ge Sn Experimental 1.12 eV AlP 0.67 eV GaAs 0.08 eV InSb Band Gaps 2.45 eV 1.42 eV 0.17 eV ZnSe CdTe 2.8 eV 1.4 eV Colored Semiconductors CdS (Eg=2.42 eV) eV) CdTe (Eg=1.50 eV) eV) We see that as the band gap decreases as either electronegativity of the anion decreases (CdS (CdS CdTe, ZnS ZnSe) or the CdTe, ZnSe) overlap decreases (ZnS (ZnS CdS). CdS) Also note that colored compounds only result when 1.8<Eg<3.0. Larger gaps give 1.8<E white/transparent & smaller gaps give black. ZnS (Eg=3.6 eV) eV) ZnSe (Eg=2.58 eV) eV) 4 Bandgap Engineering Heterojunction laser The GaAs (Eg=1.4 eV, a=5.65A) : AlAs (Eg=2.1 eV, a=5.66A) is among the most important for optoelectronic devices, because of the excellent lattice matching. Figures taken from "Semiconductor Optoelectronic Devices", by P. Bhattacharya By forming solid solutions of different semiconductors electrical engineers are electrical able to design materials and junctions with specific band gaps. This approach often called band gap engineering is used to fabricate a number of devices, such as LED's, semiconductor (diode, heterojunction, quantum well) lasers, heterojunction, photodetectors, solar cells, etc. In this field it is important to consider (a) the photodetectors, (a) band gap of the two end members, (b) the lattice match between materials, (c) materials, direct vs. indirect gap materials. Light Emitting Diodes Forward bias at a p-n junction forces pminority carriers across the junction. This causes electrons to fall into vacant holes radiatively, giving off light. Dopants are radiatively, important sites for localizing carriers, thus transitions are often CB acceptor, or donor VB Common LED materials GaAs (Eg=1.43 eV) Near IR eV) GaP:N (Eg = 2.25 eV) Yellow eV) GaP:N (E GaP:Zn,O (Eg = 2.25 eV) Red eV) GaP:Zn,O (E GaN, SiC Blue GaN, 5 Cr3+ Gemstones Excitation of an electron from one d-orbital to another d-orbital on the same ddatom often times gives rise to absorption in the visible region of the spectrum. The Cr3+ ion in octahedral coordination is a very interesting example of this. Slight changes in it's environment lead to changes in the changes splitting of the t2g and eg orbitals, which changes the color the material. orbitals, Hence, Cr3+ impurities are important in a number of gemstones. Ruby Al2O3:Cr3+ Sunlight Alexandrite BeAl2O4:Cr3+ Incandescent light Emerald Be2Al2Si6O18 :Cr3+ Cr3+ Gemstones Excitation of an electron from one d-orbital to another d-orbital on the same ddatom often times gives rise to absorption in the visible region of the spectrum. The Cr3+ ion in octahedral coordination is a very interesting example of this. Slight changes in it's environment lead to changes in the changes splitting of the t2g and eg orbitals, which changes the color the material. orbitals, Hence, Cr3+ impurities are important in a number of gemstones. Ruby Host Corundum Al2O3 2.23 eV Alexandrite Chrysoberyl BeAl2O4 2.17 eV Emerald Beryl Be3Al2Si6O18 2.05 eV t2geg Splitting Color Red Blue-Green (Daylight) Red (Candlelight) Green 6 Cr3+ Gemstones-Host Structures In each of the three gemstones Cr3+ impurities substitute for Al3+ on an octahedral site. Corundum Al2O3 Beryl Be3Al2Si6O18 Term Symbols Term symbols are shorthand notation for representing the electron electron configuration of a localized ion. 2S+1L J S = The total spin quantum # of the atom/ion (keep in mind that each electron has a spin angular momentum of + or -) L = The total angular momentum quantum # J = L+S See any advanced inorganic chemistry text for a description of term term symbols. We will concentrate on the spin value, as it can easily be easily interpreted to determine the number of unpaired electrons. Transitions where the total spin changes require at least one electron to reverse it's spin. Such transitions are called spin forbidden transitions. They are very weak transitions. 7 Tunabe-Sugano Diagram Cr3+ The Tunabe-Sugano diagram below shows the allowed electronic excitations for Tunabefor Cr3+ in an octahedral crystal field (4A2 4T1 & 4A2 4T2). The dotted vertical line shows the strength of the crystal field splitting for Cr3+ in Al2O3. The 4A2 4T1 energy difference corresponds to the splitting between t2g and eg Spin Allowed Transition eg t2g 4T 1 & 4T 2 States eg t2g 2E 1 eg t2g State 4A 2 Ground State Ruby Red This figure (taken from "The Physics and Chemistry of Color, by Kurt Nassau) shows the two spin allowed transitions for Cr3+ in ruby give rise to absorption in the violet and the green. Since the transmission in the red is much more than the transmission in the blue rubies are red. The 2E 4A2 fluorescence adds to the deep red color of a ruby. 8 Emerald Green This figure (also taken from "The Physics and Chemistry of Color", by Kurt Color", Nassau) shows the transitions for Cr3+ in beryl. The crystal field splitting is a little bit smaller than ruby, so that the lower energy absorption is shifted absorption to lower energy taking out much of the reflectance of red light and changing the color to green. The Alexandrite Effect Alexandrite (BeAl2O4:Cr3+) has a crystal field splitting, 2.17 eV, eV, intermediate between ruby (Al2O3 - 2.23 eV) and Beryl (Be3Al2Si6O18 eV) - 2.07 eV). Thus it has some transmittance in the red and a fairly eV). large transmittance in the blue-green region of the spectrum. This blueleads to very interesting color and optical properties. In the presence of reddish light, as from a candle or an incandescent light source, it is deep red in color. Resembling a ruby. In the presence of light with a major component in the blue/UV region of the spectrum, such as sunlight or fluorescent light, the the reflectance in the blue-green region of the spectrum dominates and bluealexandrite resembles an emerald. 9 Cu2+ Transitions The d9 configuration of Cu2+, leads to a Jahn-Teller distortion of the Jahnregular octahedral geometry, and sets up a fairly low energy excitation from dx2-y2 level to a dz2 level. If this absorption falls in the red or orange regions of the spectrum, a green or blue color can result. Some notable examples include: d x2 - y 2 Excited State dz2 Pseudo t2g Malachite (green) Cu2CO3(OH)2 Turquoise (blue-green) (blueCuAl6(PO4)(OH)8*4H2O Azurite (blue) Cu3(CO3)2(OH)2 Ground State d x2 - y 2 dz2 Pseudo t2g Charge Transfer in Sapphire The deep blue color the gemstone sapphire is also based on impurity doping into Al2O3. The color in sapphire arises from the following charge transfer excitation: Fe2+ + Ti4+ Fe3+ + Ti3+ (max ~ 2.2 eV, 570 nm) eV, The transition is facilitated by the geometry of the corundum structure where the two ions share an octahedral face, which allows for favorable overlap of the dz2 orbitals. orbitals. Unlike the d-d transition in Ruby, the chargedchargetransfer excitation in sapphire is fully allowed. Therefore, the color in sapphire requires only ~ 0.01% impurities, while ~ 1% impurity level is needed in ruby. In the mineral ilmenite, FeTiO3, all of the Al3+ ilmenite, has been replaced by Fe2+/Ti4+. The absorption band broadens out and the color becomes black. 10 Anion to Metal Charge Transfer Normally charge transfer transitions from an anion (i.e. O2-) HOMO to a cation LUMO fall in the UV region of the spectrum and do not give rise to color. However, d0 cations in high oxidation states are quite electronegative lowering the energy of the transition metal based LUMO. This moves the transition into the visible region of the spectrum. The strong covalency of the metalmetal oxygen bond also strongly favors tetrahedral coordination, giving giving rise to a structure containing isolated MO4n- tetrahedra. Some tetrahedra. examples of this are as follows: Ca3(VO4)2 (tetrahedral V5+) Color = White 6+) Color = Yellow PbCrO4 (tetrahedral Cr 6+) Color = Yellow CaCrO4 & K2CrO4 (tetrahedral Cr Color = Yellow PbMoO4 (tetrahedral Mo6+) 7+) KMnO4 (tetrahedral Mn Color = Maroon 11 ...
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This note was uploaded on 06/11/2011 for the course CHEM 101 taught by Professor Stegemiller during the Spring '07 term at Ohio State.

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