Optical Properties and Materials

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1 1 Optical Properties and Materials Chemistry 754 Solid State Chemistry Lecture #27 June 3, 2002 Electromagnetic Radiation and the Visible Spectrum UV nm ev Red Violet Violet nm ev Blue nm ev Green nm ev Orange Blue Yellow nm ev Orange nm ev Red nm ev Near IR 10, nm ev Yellow Green If absorbance occurs in one region of the color wheel the 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 a material absorbs red, orange & yellow Color = Blue E = hc/λ = {( x ev-s)(2.998 x 10 8 m/s)}/λ E (ev( ev) ) = 1240/λ(nm)

2 2 Color in Extended Inorganic Solids Intratomic (Localized) excitations Cr 3+ Gemstones (i.e. Cr 3+ in Ruby and Emerald) Blue and Green Cu 2+ compounds (i.e. malachite, turquoise) Blue Co 2+ compounds (i.e. Al 2 CoO 4, azurite) Charge-transfer excitations (metal-metal, anion-metal) Fe 2+ Ti 4+ in sapphire Fe 2+ Fe 3+ in Prussian Blue O 2- Cr 6+ in BaCrO 4 Valence to Conduction Band Transitions in Semiconductors WO 3 (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 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 band-gaps of those compounds which adopt the Sphalerite/ Diamond 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) Color Example > 3.0 White ZnO Yellow CdS Orange Red HgS < 1.8 Black CdSe

3 3 Spatial Overlap and Band Gap (E( g ) Conduction Band (Antibonding) E g E g Valence Band (Bonding) What are the effects of increasing the spatial overlap? Van- Vechten & 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 conduction band (more highly antibonding) ) [E[ g ] Secondary Effect: : Increases the bandwidth [E[ g ] Si Ge Sn Decreasing Overlap (Covalency) AlP AlAs AlSb Decreasing Overlap Bond Distances Constant Overlap (Covalency) 2.35 A AlP 2.44 A GaAs 2.81 A InSb 2.37 A 2.45 A ZnSe 2.80 A CdTe Constant Overlap Al Ga Decreasing Overlap Ga In Zn Cd Al Ga In Si Ge Sn 2.45 A 2.81 A 2.37 A GaP 2.36 A InP 2.54 A 2.45 A GaAs 2.45 A InAs 2.62 A 2.66 A GaSb 2.64 A InSb 2.80 A P As Se Sb Te

4 4 Ionicity and Band Gap (E( g ) Conduction Band (Antibonding) E g E g Valence Band (Bonding) What are the effects of increasing the electronegativity difference between the elements? Primary Effect: : Increases the separation of the valence and conduction bands (the bonds become more ionic) [E[ g ] Secondary Effects: : Decreases the bandwidth [E[ g ] ] and reduces the covalent antibonding destabilization [E[ g ] Electronegativity Difference Increasing Ionicity Si χ=0 AlP χ=0.62 Ge χ=0 GaAs χ=0.43 ZnSe χ=0.90 Sn χ=0 InSb χ=0.31 CdTe χ=0.61 Decreasing Ionicity Zn Cd Al Ga In Si Ge Sn P As Se Sb Te Decreasing Ionicity Al Ga Increasing Ionicity Ga In AlP χ=0.62 GaP χ=0.06 InP χ=0.28 AlAs χ=0.99 GaAs χ=0.43 InAs χ=0.65 AlSb χ=0.65 GaSb χ=0.09 InSb χ=0.31 Increasing Ionicity P As Decreasing Ionicity As Sb

5 5 Band Gap Si 1.12 ev Ge 0.67 ev Sn 0.08 ev Decreasing Overlap (E( g ) Decreases Ionicity (E g ) Increasing Ionicity (E g ) Constant Covalency AlP 2.45 ev GaAs 1.42 ev InSb 0.17 ev Zn Cd Al Ga In Si Ge Sn P As Sb Te ZnSe 2.8 ev CdTe 1.4 ev Se AlP 2.45 ev AlAs 2.15 ev AlSb 1.61 ev Decreasing Ionicity/Constant Overlap Al Ga (E g ) Increasing Ionicity/Decreasing Overlap Ga In (E g ) GaP 2.27 ev GaAs 1.42 ev GaSb 0.73 ev InP 1.35 ev InAs 0.36 ev InSb 0.17 ev Increasing Ionicity/Decreasing Overlap P As (E g ) Decreasing Ionicity/Decreasing Overlap As Sb (E g ) When ionicity and overlap effects are in opposite directions, the overlap effect dominates Direct & Indirect Gap Semiconductors Ge Si GaAs Figure taken from Fundamentals of Semiconductor Theory and Device Physics, by S. Wang Direct Gap Semiconductor: : Maximum of the valence band and minimum of the conduction band fall at the same place in k-space. α α (hν-e g ) 1/2 Indirect Gap Semiconductor: : Maximum of the valence band and minimum of the conduction band fall different points in k-space. Lattice vibration (phonon) is involved in electronic excitations, much smaller absorption efficiency. α α (hν-e g ) 2

6 6 Bandgap Engineering Heterojunction laser The GaAs (E g =1.4 ev, a=5.65a) : AlAs (E g =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 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, photodectors,, solar cells, etc. In this field it is important to consider (a) the band gap of the two end members, (b) the lattice match between materials, (c) direct vs. indirect gap Chemistry materials Solid State Chemistry Light Emitting Diodes Forward bias at a p-n junction forces minority carriers across the junction. This causes electrons to fall into vacant holes radiatively, giving off light. Dopants are important sites for localizing carriers, thus transitions are often CB acceptor, or donor VB Common LED materials GaAs (E g =1.43 ev) Near IR GaP:N (E( g = 2.25 ev) Yellow GaP:Zn,O (E( g = 2.25 ev) Red GaN, SiC Blue

7 7 Cr 3+ Gemstones Excitation of an electron from one d-orbital to another d-orbital on the same atom often times gives rise to absorption in the visible region of the spectrum. The Cr 3+ ion in octahedral coordination is a very interesting example of this. Slight changes in it s environment lead to changes in the splitting of the t 2g and e g orbitals,, which changes the color the material. Hence, Cr 3+ impurities are important in a number of gemstones. Host Ruby Alexandrite Emerald Corundum Chrysoberyl Beryl Al 2 O 3 BeAl 2 O 4 Be 3 Al 2 Si 6 O 18 t 2g e g Splitting 2.23 ev 2.17 ev 2.05 ev Color Red Blue-Green (Daylight) Red (Candlelight) Green Cr 3+ Gemstones-Host Structures In each of the three gemstones Cr 3+ impurities substitute for Al 3+ on an octahedral site. Corundum Al 2 O 3 Chrysoberyl BeAl 2 O 4 Beryl Be 3 Al 2 Si 6 O 18

8 8 Term Symbols Term symbols are shorthand notation for representing the electron configuration of a localized ion. 2S+1 L J S = The total spin quantum # of the atom/ion (keep in mind that each electron has a spin angular momentum of +½ or -½) L = The total angular momentum quantum # J = L+S See any advanced inorganic chemistry text for a description of term symbols. We will concentrate on the spin value, as it can easily be 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. Tunabe-Sugano Diagram Cr 3+ The Tunabe-Sugano diagram below shows the allowed electronic excitations for Cr 3+ in an octahedral crystal field ( 4 A 2 4 T 1 & 4 A 2 4 T 2 ). The dotted vertical line shows the strength of the crystal field splitting for Cr 3+ in Al 2 O 3. The 4 A 2 4 T 1 energy difference corresponds to the splitting between t 2g and e g Spin Allowed Transition e g t 2g 4 T 1 & 4 T 2 States e g t 2g e g 2 E 1 State t 2g 4 A 2 Ground State

9 9 Ruby Red This figure (taken from The Physics and Chemistry of Color, by Kurt Nassau) shows the two spin allowed transitions for Cr 3+ 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 2 E 4 A 2 fluorescence adds to the deep red color of a ruby. Emerald Green This figure (also taken from The Physics and Chemistry of Color, by Kurt Nassau) shows the transitions for Cr 3+ in beryl. The crystal field splitting is a little bit smaller than ruby, so that the lower energy absorption is shifted to lower energy taking out much of the reflectance of red light and changing the color to green.

10 10 The Alexandrite Effect Alexandrite (BeAl 2 O 4 :Cr 3+ ) has a crystal field splitting, 2.17 ev, intermediate between ruby (Al 2 O ev) ) and Beryl (Be 3 Al 2 Si 6 O ev). Thus it has some transmittance in the red and a fairly large transmittance in the blue-green region of the spectrum. This leads 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 reflectance in the blue-green region of the spectrum dominates and alexandrite resembles an emerald. Cu 2+ Transitions The d 9 configuration of Cu 2+, leads to a Jahn-Teller distortion of the regular octahedral geometry, and sets up a fairly low energy excitation from d dx 2 -y 2 level to a d dz 2 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: Malachite (green) Cu 2 CO 3 (OH) 2 Turquoise (blue-green) CuAl 6 (PO 4 )(OH) 8 *4H 2 O Azurite (blue) Cu 3 (CO 3 ) 2 (OH) 2 Excited State Pseudo t 2g Ground State Pseudo t 2g dx 2 -y 2 dz 2 dx 2 -y 2 dz 2

11 11 Charge Transfer in Sapphire The deep blue color the gemstone sapphire is also based on impurity doping into Al 2 O 3. The color in sapphire arises from the following charge transfer excitation: Fe 2+ + Ti 4+ Fe 3+ + Ti (λ max max ~ 2.2 ev,, 570 nm) 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 dz 2 orbitals. Unlike the d-d transition in Ruby, the charge-transfer 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,, FeTiO 3, all of the Al 3+ has been replaced by Fe 2+ /Ti 4+. The absorption band broadens out and the color becomes black. Anion to Metal Charge Transfer Normally charge transfer transitions from an anion (i.e. O 2- ) to a cation fall in the UV region of the spectrum and do not give rise to color. However, d 0 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 metal-oxygen bond also strongly favors tetrahedral coordination, giving rise to a structure containing isolated MO n- 4 tetrahedra.. Some examples of this are as follows: Ca 3 (VO 4 ) 2 (tetrahedral V ) Color = White PbCrO 4 (tetrahedral Cr ) Color = Yellow CaCrO 4 & K 2 CrO 4 (tetrahedral Cr ) Color = Yellow PbMoO 4 (tetrahedral Mo ) Color = Yellow KMnO 4 (tetrahedral Mn ) Color = Maroon

12 12 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) Electroluminescence: Electrical injection of carriers (LED s) Sensitizers and Activators Sensitizer Absorbs the incident energy (photon or excited electron). Often times the host lattice acts as the sensitizer. Activator The site where the electron radiatively relaxes. Some common ions which act as activators: Absorbed photon Activator Emitted photon Sensitizer Energy Transfer Host Lattice Typically, the host lattice should have the following properties: Large Band Gap So as not to absorb the emitted radiation. Stiff Easily excited lattice vibrations can lead to non-radiative relaxation, which decreases the efficiency.

13 Ion Common Luminescent Ions Excited State Ground State λ max Emission Mn 2+ (3d 5 ) t 2 4 e 1 ( 4 T 1 ) t 2 3 e 2 ( 6 A 1 ) Green-Orange-Red* Sb 3+ (5s 2 ) 5s 1 5p 1 5s 2 Blue* Ce 3+ (4f 1 ) 4f 0

13 13 Ion Common Luminescent Ions Excited State Ground State λ max Emission Mn 2+ (3d 5 ) t 2 4 e 1 ( 4 T 1 ) t 2 3 e 2 ( 6 A 1 ) Green-Orange-Red* Sb 3+ (5s 2 ) 5s 1 5p 1 5s 2 Blue* Ce 3+ (4f 1 ) 4f 0 5d 1 4f 1 5d 0 Near UV to Red* Eu 2+ (4f 7 ) 4f 6 5d 1 4f 7 5d 0 Near UV to Red* Tm 3+ (4f 12 ) Er 3+ (4f 11 ) Tb 3+ (4f 8 ) Pr 3+ (4f 2 ) Eu 3+ (4f 6 ) 1 G 4 3 H nm (Blue) 4 S 3/2 4 I 15/2 545 nm (Green) 5 D 4 7 F nm (Green) 3 P 0 3 H 5 ( 3 F 2 ) 605 (635) nm (Red) 5 D 0 7 F nm (Red) *The energy of transitions involving d, p or s orbitals is very sensitive to the crystal field splitting induced by the lattice. Conventional Fluorescent Lights Excitation Source Hg gas discharge UV Light (photoluminescence) Sensitizer/Host Lattice Fluoroapatite Ca 5 (PO 4 ) 3 F Activators Blue = Sb 3+ (5s 1 5p 1 5s 2 ) λ max = 480 nm Orange-Red = Mn 2+ (t 2g4 e 1 g t 2g3 e g2 ) λ max = 580 nm

14 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.

14 14 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. Red Phosphor Host Lattice = (Y 2-x Eu x )O 3 x = (Bixbyite( structure) Sensitizer = O 2p Eu 5d charge transfer (λ( max ~ 230 nm) Activator = 5 D 0 7 F 2 transition on Eu [f 6 ion] (λ max ~ 611 nm) Green Phosphor Host Lattice = (La 0.6 Ce 0.27 Tb 0.13 )PO 4 (Monazite structure) Sensitizer = 4f 1 5d 1 excitation on Ce [f 1 ion] (λ( max ~ 250 nm) Activator = 5 D 4 7 F 5 transition on Tb [f 8 ion] (λ max ~ 543 nm) Blue Phosphor Host Lattice = (Sr( Sr,Ba,Ca) 5 (PO 4 ) 3 Cl (Halophosphate( structure) Sensitizer = 4f 7 5d 0 4f 6 5d 1 transition on Eu Activator = 4f 6 5d 1 4f 7 5d 0 transition on Eu 2+ max ~ 450 nm) 2+ (λ max For a detailed yet very readable description of fluorescent light phosphors see: electrochem.org/publications/interface/summer98/if6-98-page org/publications/interface/summer98/if6-98-page28-31.pdfpdf Cathode Ray Tube Excitation Source Electron beam (cathodoluminescence( cathodoluminescence) Red Sensitizer/Host - YVO 4 Activator Eu 3+ Green Sensitizer/Host ZnS (CB) Activator Ag + Blue Sensitizer/Host ZnS (CB) Activator Cu + Image taken from

15 15 Luminscence in ZnS The electronic transition in ZnS (E g = ev) ) phosphors is from the conduction band to an impurity level in the band gap. Typically either Ag + or Cu +, which are substitutional impurities. hν Conduction Band e - hν e - Cu + Ag + Valence Band Self Luminescence in AWO 4 The AWO 4 (A= Ca, Sr, Ba) tungstates,, based upon the scheelite structure with isolated tetrahedra,, are self luminescent. Luminescence in these materials can be described by the following process. 1. WO 2-4 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 photon. hν hν e g t 2g e g t 2g O 2p O 2p

16 16 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 = Y 3 Al 5 O 12 (Garnet) Activator = Nd 3+ (4f 3 ) Lifetime = 10-4 s λ = 1064 nm Ruby laser Host = Al 2 O 3 Activator = Cr 3+ Lifetime = 5 ms λ = nm Transparent Conducting Oxides Characteristics of a transparent conducting oxide (TCO) High transparency in the visible (E( g > 3.0 ev) High electrical conductivity (σ( > 10 3 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) Smart windows and displays based on electrochromics Defrosting windows and antistatic coatings TCO Materials In 2 O 3 :Sn (ITO) SnO 2 :Sb ZnO:F & ZnO:M (M=Al, In, B, Ga) Cd 2 SnO 4 CuAlO 2 & CuGaO 2

17 TCO Structure Types I In 2 O 3 Bixbyite (Ia3)

Cd(CdSn)O 4 Inverse Spinel (Fd3m) 6-coordinate Sn 4+ /Cd 2+

(R-3) 6-coordinate Sn 4+ 6-coordinate Zn 2+ 4-coordinate O

(P6 3 mc) 4-coordinate Zn 2+ 4-coordinate O 2- Corner

17 17 TCO Structure Types I In 2 O 3 Bixbyite (Ia3) 6-coordinate In 3+ 4-coordinate O 2- Edge & Corner Sharing Cd(CdSn)O 4 Inverse Spinel (Fd3m) 6-coordinate Sn 4+ /Cd 2+ 4-coordinate O 2- Edge & Corner Sharing ZnSnO 3 Ilmenite (R-3) 6-coordinate Sn 4+ 6-coordinate Zn 2+ 4-coordinate O 2- Edge & Face Sharing TCO Structure Types II ZnO Wurtzite (P6 3 mc) 4-coordinate Zn 2+ 4-coordinate O 2- Corner Sharing SnO 2 Rutile (P4 2 /mnm) 6-coordinate Sn 4+ 3-coordinate O 2- Edge & Corner Sharing

18 18 Electronic Structure 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. (*See Freeman, et al. in MRS Bulletin, August 2000, pp ) A A highly disperse single s-band at the bottom of the conduction band. in order to give the carriers (electrons) high mobility To achieve this condition we need main group s 0 ions (Sn 2+, In 3+, Cd 2+, Zn 2+ ) 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 band falls 3-4 ev above the O 2p band (Bi 5+ & Pb 4+ are too electronegative, In 3+, Zn 2+, Cd 2+, Sn 4+ are of roughly the proper energy) A 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.

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