optical2_lect22 - Optical Properties II Emission of Light...

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Unformatted text preview: Optical Properties II: Emission of Light, Displays and Transparent Conductors Chemistry 754 Solid State Chemistry Lecture #22 May 21, 2003 Emission of Light The optical properties of extended solids are utilized not only for their color, but also for the way in which they emit light. Luminescence Emission of light by a material as a consequence of it absorbing energy. There are two categories: Fluorescence: Emission involves a spin allowed transition (short excited state lifetime) Phosphorescence: Emission involves a spin forbidden transition (long lived excited state). Luminescence can also be classified according to the method of excitation: Photoluminescence: Photon excitation (i.e. fluorescent lights) Cathodoluminescence: Cathode rays (TV & Computer displays) Cathodoluminescence: Electroluminescence: Electrical injection of carriers (LED's) Electroluminescence: 1 Sensitizers and Activators Sensitizer Absorbs the incident energy (photon or excited electron). Often times the host lattice acts as the sensitizer. sensitizer. Activator The site where the electron radiatively relaxes. Some common ions which act as activators: Absorbed photon Activator Emitted photon Sensitizer Energy Transfer properties: Host Lattice Typically, the host lattice should have the following Large Band Gap So as not to absorb the emitted radiation. Stiff Easily excited lattice vibrations can lead to non-radiative nonrelaxation, which decreases the efficiency. Common Luminescent Ions Ion Mn2+ (3d5) Sb3+ (5s2) Ce Eu 3+ Excited State t24e1 (4T1) 5s15p1 4f 5d 4f 5d 1 4 6 0 1 1 Ground State t23e2 (6A1) 5s2 4f 5d 7 1 0 0 max Emission Green-Orange-Red* Blue* Near UV to Red* Near UV to Red* 450 nm (Blue) 545 nm (Green) 545 nm (Green) 605 (635) nm (Red) 611 nm (Red) (4f ) (4f ) 7 1 2+ 4f 5d 3 4 Tm3+ (4f12) Er 3+ G4 D4 P0 3 H6 F5 3 (4f ) (4f ) (4f ) 6 2 11 S3/2 3 I15/2 7 Tb3+ (4f8) Pr 3+ 3+ 5 H5 ( F2) 7 Eu 5 D0 F2 *The energy of transitions involving d, p or s orbitals is very sensitive to the crystal field splitting induced by the lattice. 2 Conventional Fluorescent Lights Excitation Source Hg gas discharge UV Light (photoluminescence) Sensitizer/Host Lattice Sensitizer/Host Fluoroapatite Ca5(PO4)3F Activators Blue = Sb3+ (5s15p1 5s2) max = 480 nm Orange-Red = Mn2+ (t2g4eg1 t2g3eg2) max = 580 nm Orange- Tricolor Fluorescent Lights Tricolor fluorescent lights are more commonly used today because they give off warmer light, due to more efficient luminescence in the red region of the spectrum. Such lights contain a blend of at least three phosphors. phosphors. Red Phosphor Green Phosphor Host Lattice = (Y2-xEux)O3 x = 0.06-0.10 (Bixbyite structure) 0.06(Bixbyite Sensitizer = O2- 2p Eu3+ 5d charge transfer (max ~ 230 nm) ( 5D 7F transition on Eu3+ [f6 ion] ( Activator = max ~ 611 nm) 0 2 Blue Phosphor Host Lattice = (La0.6Ce0.27Tb0.13)PO4 (Monazite structure) Sensitizer = 4f1 5d1 excitation on Ce3+ [f1 ion] (max ~ 250 nm) ( Activator = 5D4 7F5 transition on Tb3+ [f8 ion] (max ~ 543 nm) Host Lattice = (Sr,Ba,Ca)5(PO4)3Cl (Halophosphate structure) (Sr,Ba,Ca) (Halophosphate Sensitizer = 4f75d0 4f65d1 transition on Eu2+ Activator = 4f65d1 4f75d0 transition on Eu2+ (max ~ 450 nm) For a detailed yet very readable description of fluorescent light phosphors see: light http://www.electrochem.org/publications/interface/summer98/IF6-98-Page28-31.pdf http://www.electrochem.org/publications/interface/summer98/IF6-98-Page28-31.pdf 3 Cathode Ray Tube The cathode ray tube is the technology used in most computer monitors and TV's. The details of a typical commercial CRT's are as are follows. Excitation Source Electron beam (cathodoluminescence) (cathodoluminescence) Red Sensitizer/Host - YVO4 Sensitizer/Host Image taken from Activator Eu3+ http://www.howstuffworks.com/tv2.htm Green Sensitizer/Host ZnS (CB) Sensitizer/Host Activator Ag+ Blue Sensitizer/Host ZnS (CB) Sensitizer/Host Activator Cu+ Luminscence in ZnS While the insulating red phosphor (Y1-xEux)VO4 operates on a principle very similar to fluorescent light phosphors (with a different excitation source of course), the blue and green phosphors employ a different scheme for electronic excitation and luminescence. The electronic excitation in ZnS (Eg = 3.6 eV) is from the valence band eV) to the conduction band. While the relaxation that leads to the luminescence is from the conduction band to an impurity level in the band gap. Typically either Ag+ or Cu+, which are substitutional impurities. The energy of the emitted light can be tuned by changing impurities or changing the band gap of the semiconductor. h Conduction Band eeh Cu+ Ag+ Valence Band 4 Electroluminescence Flat Panel Displays In electroluminescence an electron is directly injected into the phosphor (in the excited state) and it relaxes giving off a photon. This diagram shows how by running current through a single row (absorbant back (absorbant electrode) and a single column (transparent front electrode) it is possible to light up a single pixel. Taken from the Planar systems website. http://www.planar.com/technology/el.asp Self Luminescence in AWO4 The AWO4 (A= Ca, Sr, Ba) tungstates, based upon the scheelite structure Sr, Ba) tungstates, with isolated tetrahedra, are self luminescent. Luminescence in these tetrahedra, materials can be described by the following process. 1. WO42- group absorbs a UV photon via a charge transfer from oxygen to tungsten. 2. Excited state electron is in an antibonding state, weakens/lengthens the bond lowering the energy of the excited state. 3. Electron returns to the ground state giving off a longer wavelength wavelength photon. h t2g h O 2p eg O 2p t2g eg 5 Solid State Lasers A Laser gives off light at a single wavelength. To achieve this the activator needs to have the following properties A long lived excited state A very narrow emission spectrum (localized luminescence centers) Nd:YAG laser Host = Y3Al5O12 (Garnet) Activator = Nd3+ (4f3) Lifetime = 10-4 s = 1064 nm Ruby laser Host = Al2O3 Activator = Cr3+ Lifetime = 5 ms = 693.4 nm Transparent Conducting Oxides Characteristics of a transparent conducting oxide (TCO) High transparency in the visible (Eg > 3.0 eV) (E eV) High electrical conductivity ( > 103 S/cm) ( Applications of transparent conductors Optoelectronic devices (LED's, Semiconductor lasers, photovoltaic cells, etc.) Flat Panel Displays (liquid crystals, electroluminescent displays) Heat efficient windows (reflect IR, transparent to visible light) light) Smart windows and displays based on electrochromics Defrosting windows and antistatic coatings TCO Materials In2O3:Sn (ITO) SnO2:Sb & SnO2-xFx ZnO:F & ZnO:M (M=Al, In, B, Ga) ZnO:F ZnO:M Ga) Cd2SnO4 CuAlO2 & CuGaO2 6 TCO Structure Types I In2O3 Bixbyite (Ia3) 6-coordinate In3+ 4-coordinate O2Edge & Corner Sharing Cd(CdSn)O4 Cd( CdSn)O Inverse Spinel (Fd3m) 6-coordinate Sn4+/Cd2+ 4-coordinate O2Edge & Corner Sharing ZnSnO3 Ilmenite (R-3) 6-coordinate Sn4+ 6-coordinate Zn2+ 4-coordinate O2Edge & Face Sharing TCO Structure Types II ZnO Wurtzite (P63mc) 4-coordinate Zn2+ 4-coordinate O2Corner Sharing SnO2 Rutile (P42/mnm) mnm) 6-coordinate Sn4+ 3-coordinate O2Edge & Corner Sharing 7 Desirable Properties-TCO's The following guidelines were put forward* as guidelines for the most desirable features of the electronic band structure for a TCO material. material. (*See Freeman, et al. in MRS Bulletin, August 2000, pp. 45-51) 45A highly disperse single s-band at the bottom of the conduction band. s in order to give the carriers (electrons) high mobility To achieve this condition we need main group s0 ions (Sn2+, In3+, Cd2+, Zn2+) Separation of this band from the valence band by at least 3 eV in order to be transparent across the visible spectrum We need the appropriate level of covalency so that the conduction conduction band falls 3-4 eV above the O 2p band (Bi5+ & Pb4+ are too 3electronegative, In3+, Zn2+, Cd2+, Sn4+ are of roughly the proper energy) A splitting of this band from the rest of the conduction band in order to keep the plasma frequency in the IR range Direct M ns - M ns interactions across the shared octahedral edge are useful to stabilize the s-band wrt the rest of the CB. s- Electronic Structure In2O3 Taken from Freeman, et al. in MRS Bulletin, August 2000, pp. 45-51 45- 8 ...
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