Chap 2 Energy Absorption of Luminescent Materials

2-1 General considerations

2-2 Effect of Host Lattice 主體晶格效應主體晶格效應主體晶格效應主體晶格效應(a) 共價效應 covalence effect

(b) 結晶場效應 crystal field effect

2-3 Energy level diagrams of Individual Ions 個別離子之個別離子之個別離子之個別離子之能階圖簡介能階圖簡介能階圖簡介能階圖簡介

(a) 過渡金屬離子 (dn, d0: W6+, Mo6+, Nb5+,V5+, d10: Zn2+,Cd2+, Ga3+, Sb3+)

(b) 稀土金屬離子 (fn→fn, fn→ fn-15d與電荷遷移)

(c) 主族金屬離子 (s2)

(d) 其他電荷遷移 (LMCT, MLCT, MMCT)

(e) 色心 (F-center)

2-4 Host lattice absorption 主體晶格對能量之吸收主體晶格對能量之吸收主體晶格對能量之吸收主體晶格對能量之吸收- 兩類光學躍遷機制

(a) 能產生電子電洞等自由載子者 (ZnS, ZnSe,ZnTe, CdS, CdTe)

(b) 能產生自由載子者 (紫外照射CaWO4, 激子存在於WO42-中; NaCl)

f-block

d-block

s-block p-block

No. f e- R3+ 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Elements Used for Formation of Inorganic Phosphors

3+ 5+ 3+ 2+

4+

HL: Host lattice

CT: Eu3+→O2- Charge

transfer

f – f absorption

Y2O3: Solid state crystal structureGeometry of Y3+: 6 coordinate: octahedral

Prototypical structure:

Symmetrical Stretching Mode (breathing mode)

The frequency of vibrational modes (聲子聲子聲子聲子) is determined by

host lattice (H) in which a luminescent center (A) is located !

Vibration motion – harmonic

*F: Restoring force

F = -k (R-Ro)

*Potential energy

E = 1/2k (R-Ro)2

*Energy of vibrational level

En = (n+1/2)hν

Crossing point: possible to return to ground state non-radiatively.

E

R

Calculated energy level terms for free Rare Earth Ions (4fn)

Incompletely filled 4f shell, 4f is shielded by 5s2, 5p6 orbitals.

1 2 3 4 5 6 7 8 9 10 11 12 13 n =

5D0

Host Lattice effect on optical transitions within 4f n configuration is small (but essential)

自由態稀土自由態稀土自由態稀土自由態稀土3+離子理論能階圖離子理論能階圖離子理論能階圖離子理論能階圖

Highest

Lowest

How to Understand the Probability of Optical Transitions

between v = 0 and v =1 vibrational states?

The probability (P) is proportional to

P ∞∞∞∞ <e││││r││││g> <χχχχv’││││χχχχ0>

1.When one considers the absorption bands, ∑ χv (v = 0 → n)must be considered

2.上式中 <e│r│g>: electronic matrix elements, independent of vibration. 決定 ground → excited state 的intensity

3. <χ1│χo>: vibrational overlap of Ψ‘s決定absorption band 的形狀

e: excited state wavefunction (ΨΨΨΨe) g: ground state wavefunction (ΨΨΨΨg)

r: electric-dipole operator (電雙極運算子電雙極運算子電雙極運算子電雙極運算子) that drives the transition

χχχχv’,χχχχo: vibrational states wavefunctions

一般而言一般而言一般而言一般而言, Electronic-vibrational coupling係指係指係指係指

electronic與vibration of optical centers之間的交互作用之間的交互作用之間的交互作用之間的交互作用, 可以用可以用可以用可以用 ∆∆∆∆R ( = Ro’’’’–––– Ro)表示表示表示表示

∆∆∆∆R大大大大, 則為強交互作用則為強交互作用則為強交互作用則為強交互作用; ∆∆∆∆R 小小小小, 則為弱交互作用則為弱交互作用則為弱交互作用則為弱交互作用

(1)(1)(1)(1) ∆∆∆∆R = 0 時時時時, the vibrational overlap is maximized for v = 0 and v’= 0

∴∴∴∴ absorption spectrum 由line所組成(線光譜),代表v = 0 → v’= 0之遷移(其中無振

動遷移牽涉其中)，故又稱之為zero-vibrational或non-phonon transition (i.e., zero-

phonon lines)

(2) ∆∆∆∆R ≠≠≠≠0 時時時時 v = 1 level與與與與 v’’’’ > 0 level之間有之間有之間有之間有maximal overlap

∴ absorption spectrum is broad, ∆∆∆∆R越大越大越大越大,則激發光譜中吸收峰越寬則激發光譜中吸收峰越寬則激發光譜中吸收峰越寬則激發光譜中吸收峰越寬, 此時兩個此時兩個此時兩個此時兩個states,所對應之化學鍵性質差異越大所對應之化學鍵性質差異越大所對應之化學鍵性質差異越大所對應之化學鍵性質差異越大

上述上述上述上述coupling係指電子與發光中心係指電子與發光中心係指電子與發光中心係指電子與發光中心(optical center)之振動兩者交互作用之振動兩者交互作用之振動兩者交互作用之振動兩者交互作用。。。。

高溫時基態可能為高溫時基態可能為高溫時基態可能為高溫時基態可能為 v> 0 而非而非而非而非 v = 0, 此時會造成此時會造成此時會造成此時會造成

光譜中吸收峰變寬光譜中吸收峰變寬光譜中吸收峰變寬光譜中吸收峰變寬!!!!

通常稱通常稱通常稱通常稱 ∆∆∆∆R = 0 weak coupling 弱偶合作用弱偶合作用弱偶合作用弱偶合作用

∆∆∆∆R > 0 intermediate coupling

∆∆∆∆R >> 0 strong coupling 強偶合作用強偶合作用強偶合作用強偶合作用

決定band 寬度寬度寬度寬度

Coupling 係指電子與係指電子與係指電子與係指電子與vibration center之間的交互作用之間的交互作用之間的交互作用之間的交互作用,而而而而∆∆∆∆R決決決決定此交互作用之強弱定此交互作用之強弱定此交互作用之強弱定此交互作用之強弱

∆∆∆∆R = 0 the weakest coupling scheme

∆∆∆∆R > 0 the intermediate coupling scheme

∆∆∆∆R >> 0 the strongest coupling

溫度升高溫度升高溫度升高溫度升高

absorption band broadened

高溫

Intensity of Optical Transition (absorption) related to <e︱︱︱︱r︱︱︱︱g> matrix elements

In solids, selection rules NOT absolute, some forbidden transitions slightly

relaxed. Why?

It is related to

(a) Wavefunctions admixture into the original, unperturbed wavefunctions.

(b) Spin-orbit coupling, electronic-vibrational coupling uneven crystal field terms.

Selection Rules (光學躍遷選擇律光學躍遷選擇律光學躍遷選擇律光學躍遷選擇律)1. The spin selection rule which forbids electronic transition between levels

with different spin states (∆∆∆∆S = 0, forbidden).

2. Parity selection rule which forbids electronic (electric-dipole) transitions between levels with the same parity. (Same parity forbidden, 如如如如: d →→→→ d or f →→→→ f)

Reference

1. Henderson (1989) “Optical Spectroscopy of Inorganic Solids”. Oxford University Press.2. DiBartolo (1968)”Optical Interaction in Solids”, Wiley.

HL: Host lattice

CT: Eu3+-O2- Charge transfer

f – f absorption

Highest occupied level of ground state: 2p, 3s of oxygen

Lowest unoccupied level of exc. state: mixture of 3s of oxygen and 4d of Y.顯顯顯顯

示基態與激發態兩者有極大之差異示基態與激發態兩者有極大之差異示基態與激發態兩者有極大之差異示基態與激發態兩者有極大之差異,兩者之間躍遷牽涉相當大兩者之間躍遷牽涉相當大兩者之間躍遷牽涉相當大兩者之間躍遷牽涉相當大∆∆∆∆R值及值及值及值及bonding差異差異差異差異

- Higher covalency: 組成元素離子間組成元素離子間組成元素離子間組成元素離子間 electronegativity 差異小差異小差異小差異小,故故故故 charge transfer (CT)

transition between these ions shifted to lower energy, CT absorption band of Eu3+ in YF3 is at higher energy than that in Y2O3

- In Y2O2S:Eu3+, Eu tends to be present in 2+ and 3+ states, thus, the maximal Eu2+

CT is much lower than the rest .

The Influence of Host Lattice on Absorption of Luminescent Materials(a) Covalency effect

當共價性增加時當共價性增加時當共價性增加時當共價性增加時,,,,由於電子密度更分散於軌域中由於電子密度更分散於軌域中由於電子密度更分散於軌域中由於電子密度更分散於軌域中,,,,電子間斥力減弱電子間斥力減弱電子間斥力減弱電子間斥力減弱,,,,不同能階不同能階不同能階不同能階

間電子躍遷對應能量減小間電子躍遷對應能量減小間電子躍遷對應能量減小間電子躍遷對應能量減小, , , , 故產生紅位移故產生紅位移故產生紅位移故產生紅位移....

(b) Crystal field effect結晶場係指發光中心離子周圍環境的電場強度結晶場係指發光中心離子周圍環境的電場強度結晶場係指發光中心離子周圍環境的電場強度結晶場係指發光中心離子周圍環境的電場強度，，，，某些光學躍遷對結晶場顯某些光學躍遷對結晶場顯某些光學躍遷對結晶場顯某些光學躍遷對結晶場顯

示高度依存度示高度依存度示高度依存度示高度依存度。。。。

Examples:

- Host:Bi3+ (6s2, 6s2→→→→ 6s16p1), 6s, 6p electrons residing on the surfaceof Bi3+ ion, the repulsion effect is large.

- Host:Gd3+ (4f7, 4f→→→→ 4f), f electrons lie inside the Gd3+ ion, the effect is small

- Covalency Y2O3 > YF3, thus, Eu3+ f →→→→ f energy is lower (longer labs)

- Higher covalency: 組成元素離子間組成元素離子間組成元素離子間組成元素離子間 electronegativity 差異小差異小差異小差異小,故故故故 charge transfer (CT) transition between these ions shifted to lower energy, CT

absorption band of Eu3+ in YF3 is at higher energy than that in Y2O3

- In Y2O2S:Eu3+, Eu tends to be present in 2+ and 3+ states, thus, the

maximal Eu2+ CT is much lower than the rest .

(b) Crystal field (C.F.) effect- the electric field at the site of luminescent center due to surrounding,

some optical transitions are highly dependent on strength of C.F. – Cr3+

例如例如例如例如: Al2O3:Cr3+ is red while Cr2O3:Cr3+ is green. Both hosts are isostructural.

In Al2O3:Cr3+ - Cr3+ occupies Al3+ site, stronger crystal field is expectedthan in Cr2O3.

∴∴∴∴optical transition in ruby(紅寶石紅寶石紅寶石紅寶石)is at higher energy than that in Cr2O3

((((即紅寶石吸收較短波長可見光而所顯示波長較長之紅色互補光即紅寶石吸收較短波長可見光而所顯示波長較長之紅色互補光即紅寶石吸收較短波長可見光而所顯示波長較長之紅色互補光即紅寶石吸收較短波長可見光而所顯示波長較長之紅色互補光))))

此外此外此外此外, , , , 由於結晶場強度決定由於結晶場強度決定由於結晶場強度決定由於結晶場強度決定optical transition 之劈裂之劈裂之劈裂之劈裂 ((((splitting))))

∵∵∵∵不同主體晶格不同主體晶格不同主體晶格不同主體晶格 造成不同晶場強度造成不同晶場強度造成不同晶場強度造成不同晶場強度 導致不同導致不同導致不同導致不同splitting,因此因此因此因此optical center的光譜的光譜的光譜的光譜可被利用以探討可被利用以探討可被利用以探討可被利用以探討site symmetry....

另一方面另一方面另一方面另一方面,,,,若有非對稱結晶場存在若有非對稱結晶場存在若有非對稱結晶場存在若有非對稱結晶場存在,,,,則往往可以不遵守則往往可以不遵守則往往可以不遵守則往往可以不遵守selection rules....

K2CrO4 (NH4)2Cr2O7

Ruby

Cr2O3Cr2(CO3)3

祖母綠祖母綠祖母綠祖母綠

CrCl3

紫翠玉紫翠玉紫翠玉紫翠玉

Cd(S1-xSex) mixed crystals

Nephelauxetic Effect (電子雲擴散效應電子雲擴散效應電子雲擴散效應電子雲擴散效應)

共價性增加共價性增加共價性增加共價性增加, 電子間作用力降低電子間作用力降低電子間作用力降低電子間作用力降低, electronic transition between E-levels with an energy difference shifted to lower energy

1.YF3:Eu中主體吸收峰消失可能出現在更短波長中主體吸收峰消失可能出現在更短波長中主體吸收峰消失可能出現在更短波長中主體吸收峰消失可能出現在更短波長.

2.波長為波長為波長為波長為250nm之之之之Y2O3:Eu CT band在在在在YF3:Eu中偏移至中偏移至中偏移至中偏移至150nm.

3. YF3:Eu中中中中, 4f-5d之躍遷出現了之躍遷出現了之躍遷出現了之躍遷出現了! 而在而在而在而在YF3:Eu中則無中則無中則無中則無.

4.兩者中兩者中兩者中兩者中Eu3+之吸收峰為尖銳之吸收峰為尖銳之吸收峰為尖銳之吸收峰為尖銳weak lines (因為因為因為因為4f 軌域遮蔽良好軌域遮蔽良好軌域遮蔽良好軌域遮蔽良好)

相對於相對於相對於相對於O2- →→→→ Eu3+ charge transfer, F- →→→→ Eu3+CT 吸收更難吸收更難吸收更難吸收更難,

應出現在更高能量應出現在更高能量應出現在更高能量應出現在更高能量)

Example:

表面與內部之表面與內部之表面與內部之表面與內部之Eu3+的的的的optical transition energy 不同不同不同不同,如如如如:

1. Y2O3:Eu3+中中中中, Y3+離子具有離子具有離子具有離子具有2種結晶學上獨立的格位種結晶學上獨立的格位種結晶學上獨立的格位種結晶學上獨立的格位(lattice sites), 故產生兩種故產生兩種故產生兩種故產生兩種Eu3+之放射之放射之放射之放射/吸收峰吸收峰吸收峰吸收峰.

2.當主體物質為玻璃態時當主體物質為玻璃態時當主體物質為玻璃態時當主體物質為玻璃態時, 因為缺乏因為缺乏因為缺乏因為缺乏translational symmetry,因此因此因此因此放放放放射射射射////吸收峰產生寬化現象吸收峰產生寬化現象吸收峰產生寬化現象吸收峰產生寬化現象((((broadening).).).).

Spectral Inhomogeneous Broadening

Parity(宇稱宇稱宇稱宇稱): The molecular orbitals of homonuclear molecules (同核分子)are

labelled with a subscript g or u that specifies their parity, that is, their behavior

under inversion symmetry operation.

The parity designation applies only to homonuclear diatomic molecules, since

heteroatomic molecules do not have an inversion center (i).

H2+: 2ΣΣΣΣg Neutral H2: 1ΣΣΣΣg O2: 3ΣΣΣΣg

Term symbol

Spin multiplicityXg,u

The total angular momentum of all electrons around the

internuclear axis is denoted by symbols Σ, Π,

∆…..corresponding to the S, P, D, F, G, H, I, ……of atoms.

Overall parity: g x g = g, u x u = g, g x u = u

Spectroscopic Terminology

Optically active cations having electrons external to the closed shell which can be excited by a photon of proper energy, e.g., Sn2+

(5s25p0)is optically active, Sn4+ (5s0) is NOT

For crystal field levels: 2S+1 X

X: A, no degeneracy E, two fold degeneracy T, three fold degeneracy

Russell-Sanders Coupling (L=S coupling)- valid for light atoms

L = ΣΣΣΣ li S = ΣΣΣΣ si J = L + S (J can also be obtained as J = ΣΣΣΣi ji

is used because it agrees well with experimental values most of the time

L = 0 1 2 3 4 5 6 7

2S+1 LJ Term S P D F G H I J

(S: total spin quantum no.; L: total orbital angular quantum no.; Degeneracy (2L+1) and maybe lifted by crystal field)

R-S coupling adequately describes the optically active E states for nearly all atoms and ion, with the exception of the rare earths

(a) d-d transitions (e.g., [Ti(OH2)6]3+)

- The transitions are responsible for many colors – characteristic d-complexes

(b) Vibronic transitions: a forbidden g→ g transition

can become allowed if the center of symmetry is

eliminated by an asymmetrical vibration, and so the

t2g → eg transition becomes weakly allowed (with f =

0.0001) if the complex vibrates asymmetrically.

分子之不對稱分子之不對稱分子之不對稱分子之不對稱 electronic transition稱之為稱之為稱之為稱之為

vibronic transition

∆o

20,000 cm-1 = 2.5 eV = 500 nm

Specific types of transitionsThe integrated absorption coefficient (A) is expressed as oscillator strength (f) and f is defined as f = kA. 而以下依而以下依而以下依而以下依f值之大小值之大小值之大小值之大小,探討幾種常見的吸收躍遷探討幾種常見的吸收躍遷探討幾種常見的吸收躍遷探討幾種常見的吸收躍遷:

(c) Charge transfer transitions (electrons move through a considerable distance –

the dipole moment is large and parity allowed, MnO4- Intense purple color!! )

-transfer of an electron from the ligand into the d-orbitals of Mn atom,

λλλλmax = 530 nm, f (oscillator strength) = 0.03

- Energy absorption lifting π electrons into an anti-bonding π* orbitals. The energy is 7 eV (吸收波長180 nm) for an un-conjugated C=C double bonds

The C=C double bond as a chromophore

ππππ →→→→ ππππ* (7eV, 180 nm) The C = O as a chromophore

n →→→→ ππππ* (4 eV, 290 nm)

Oxygen

nonbonding

lone-pair e-

Antibonding

C-O π* orbital

(d) ππππ →→→→ ππππ* (7eV, 180 nm) and n →→→→ ππππ* (4 eV, 290 nm) transitions

d1 – Octahedral symmetry

而在而在而在而在 tetrahedraL d1 complexes中並不具有中並不具有中並不具有中並不具有“iiii ”的反轉對稱的反轉對稱的反轉對稱的反轉對稱,,,,因此因此因此因此

selection rule is relaxed by mixing small amount of opposite-parity wave function into the d wavefunction

因此因此因此因此 Color of tetrahedral transition-metal ions is stronger, as compared to octahedral cases. f (oscillator strength) = kA.

Metal Complexes with d1 configuration

∆∆∆∆t

∆∆∆∆o

Cr3+ ground state: 4A2

2S + 1 = 2(3/2) + 1 = 4

L = 2+1+0 = 3 →→→→ 光譜項光譜項光譜項光譜項 F

4F

4A2

Absorption transitions from 4A2

obey spin selection rules.

4T2

4T1(F)

4T1(P)

Free ion d3, 3 absorption bands with low intensity observed

Energy Level Diagrams of Individual Ions – the transition metal ions (dn)

Transition Metals: incompletely filled d-shell; d1, d3, d5 ions with incompletely filled shells (0<n<10); Energy levels originated from dn

configurations 可以用可以用可以用可以用Tanabe-Sugano diagram 加以計算加以計算加以計算加以計算

The energy of transition is determined by crystal field,其實其實其實其實2T2 →→→→ 2E (d →→→→d) 為為為為

parity-forbidden, less intense color 之之之之transition with energy determined by strength of crystal field.

Tanabe-Sugano diagram for d3

Mn2+ (d5): parity forbidden d →→→→ d (基態為基態為基態為基態為6A1)

For d5 ions (基態為基態為基態為基態為6A1) all possible transitions are spin-forbidden and parity-forbidden.

E (eV) = 1240/ λλλλ(nm)

1 eV = 8064.5 cm-1

∆∆∆∆ = 2E – 2T2 (crystal field strength)

∆∆∆∆ = 20000 cm-1 for M3+ visible

Strength of crystal field determines ∆∆∆∆values

400 nm 750 nm

~25,000 cm-1 13,225 cm-1

3.10 eV 1.65 eV

水溶液中水溶液中水溶液中水溶液中Ti3+之之之之UV-Vis吸收光譜吸收光譜吸收光譜吸收光譜2T2 →→→→ 2E

Charge transfer

MnF2, MnCl2: light rose color

Molar absorption coefficient for Mn2+ is lower than that of Cr3+ (d3) by 102; molar abs. coefficient of Cr3+ is three order of magnitude weaker than parity allowed transitions.

narrow

broadbroad

Mn2+ (d5) Ground state: 6A1

(上述上述上述上述band width is due to coupling with vibrations)

MnF2(aq)之UV-Vis吸收光譜

Mn2+:

- 源自於源自於源自於源自於t23e2 excited state的的的的 4A1, 4E

∆∆∆∆R is vanishing, thus, transitions from 6A1→→→→4A1, 4E should appear as lines

- 然而然而然而然而 4T1, 4T2係源自於係源自於係源自於係源自於t24e1之電子組態之電子組態之電子組態之電子組態, 6A1→→→→ 4T1 or 4T2

∆∆∆∆R ≠≠≠≠ 0, observed as a band in the absorption spectra

Observations and Rationalizations of Band Width in Mn2+ Absorption Spectra

In the U.V. region transition metal ions 通常顯示通常顯示通常顯示通常顯示broad and strong absorption due to

(a) Ligand-to-metal charge transfer (LMCT)- d0 ions: Cr6+, Mn7+

strongly colored in oxides due to charge transfer transitions; 如如如如: MnO4

- and Cr2O72- are deeply colored

(b)其他比較重要的其他比較重要的其他比較重要的其他比較重要的LMCT為為為為V5+, Nb5+, W6+ (d0) d0: YVO4, YNbO4, CaWO4

Molecular energy levels for the tetrahedral VO43- group

excitation

Excitation: 3t2 →→→→ 3a1

Emission: 4t2 →→→→ 3t2

Excited state consists of an increased e density in the vicinity of V ion, along the Td bonds →→→→ Charge Transfer (CT)

Absorption transition之之之之E或或或或λλλλ取決於取決於取決於取決於

(a) d1→→→→d0之之之之ionization energy, (b) number and nature of the ligands, (c) lattice 中中中中ion-ion interaction

CaWO4(with WO42-) CaMoO4 Ca3WO6 (with isolated WO6

6-)

CT νννν (cm-1) 40,000 34,000 35,000 (286 nm)

(CT: M6+ + e- (氧氧氧氧----鉬鉬鉬鉬)→→→→M5+)

WO3結構中具有結構中具有結構中具有結構中具有WO66- groups sharing O2-則吸收出現在可見光範圍則吸收出現在可見光範圍則吸收出現在可見光範圍則吸收出現在可見光範圍

Ca3WO6結構中具有結構中具有結構中具有結構中具有isolated WO66- groups ,此為此為此為此為optical center與與與與CT bands交互交互交互交互

作用所形成作用所形成作用所形成作用所形成

Transition Metal Ions with d0 configuration

YVO4, YNbO4, CaWO4

- Strong, broad bands in the Ultraviolet region.

- Charge transfer from Oxygen to Mn+ (d0 ion)- Electrons are excited from a non-bonding orbital to antibondingorbital 造成造成造成造成 Metal-Oxygen bonding 減弱減弱減弱減弱

- M-O Bonding is strongly weakened after optical absorption

此時造成此時造成此時造成此時造成∆∆∆∆R >> 0 and band width is large

鑭系鑭系鑭系鑭系((((lanthanide))))自由離子之電子組態基態能級與光譜項之比較自由離子之電子組態基態能級與光譜項之比較自由離子之電子組態基態能級與光譜項之比較自由離子之電子組態基態能級與光譜項之比較

Calculated energy level terms for free Rare Earth Ions (4fn)

Incompletely filled 4f shell, 4f is shielded by 5s2, 5p6 orbitals.

1 2 3 4 5 6 7 8 9 10 11 12 13 n =

5D0

Host Lattice effect on optical transitions within 4f n configuration is small (but essential)

自由態稀土自由態稀土自由態稀土自由態稀土3+離子理論能階圖離子理論能階圖離子理論能階圖離子理論能階圖

Experimentally observed 4f n energy level diagrams for trivalent

lanthanides in the range 40,000-70,000 cm-1(250 - 142 nm). Levels

from which emission is observed are marked with a semicircle.

Rare earth 4fn ions (f→→→→ f transitions)

R3+電子組態 4fn..5s2..5p6.. 4fn electrons are well shielded by 5s, 5p..

Crystal field splitting強度之數量級遠較dn之過渡金屬為小

- R2O3: white (∵∵∵∵ parity forbidden)，其中僅 Nd2O3: faint violet;

Tb4O7, Pr4O7, Pr6O11 dark color(∵∵∵∵ R3+/R4+共存之緣故)

-Eu3+(aq)具有 low molar absorptivity, sharp lines f-f transition and

∆R = 0

此時parity selection rule並非因振動摻入而產生relaxation,而是因

uneven component of crystal field (如R3+ ions位於一不具反轉對稱中心的格位)

Uneven component 將具有將具有將具有將具有opposite parity之之之之5d混入混入混入混入R3+之之之之4f 波函波函波函波函數之中數之中數之中數之中, 因此造成因此造成因此造成因此造成4f - 4f 躍遷產生躍遷產生躍遷產生躍遷產生。。。。因此光譜學家說因此光譜學家說因此光譜學家說因此光譜學家說: 4f-4f transitions steal some intensity from allowed 4f-5d transitions。。。。

Rare earth ions (interconfiguration 4f-5d CT transitions)

1. CT transitions (4fn-4fn+1L-1)

2. 4fn-4fn-15d transitions (allowed by selection rule)

稀土離子能階的劈裂稀土離子能階的劈裂稀土離子能階的劈裂稀土離子能階的劈裂(splitting)強度順序強度順序強度順序強度順序

Crystal field (102-103cm-1)< Spin-orbit (103-104 cm-1) < Interelectronic

repulsion (>105 cm-1)

Eu3+ in aqueous solution - Sharp peaks are due to

intraconfigurational (f→→→→f) transitions with ∆∆∆∆R = 0

Low molar absorptivity!!

在在在在Eu3+(aq) 溶液吸收光譜中溶液吸收光譜中溶液吸收光譜中溶液吸收光譜中

1. Parity selection rule 之之之之relaxation係因為係因為係因為係因為uneven component of crystal field (即即即即Eu3+位於一位於一位於一位於一不具有反轉對稱不具有反轉對稱不具有反轉對稱不具有反轉對稱

的格位中的格位中的格位中的格位中)所造成所造成所造成所造成.

2.此此此此项项项项uneven component 將具有將具有將具有將具有opposite parity之之之之5d軌域軌域軌域軌域

波函數混入波函數混入波函數混入波函數混入Eu3+之之之之4f 波函數中波函數中波函數中波函數中,因此造成因此造成因此造成因此造成4f – 4f 吸收峰產生吸收峰產生吸收峰產生吸收峰產生!!

3.光譜學家認為光譜學家認為光譜學家認為光譜學家認為: forbidden 4f – 4f absorption steals some intensity from allowed 4f – 5d absorption transitions.

Rare earth 4fn ions (Intraconfigurational f→→→→ f transitions)R3+電子組態 4fn..5s2..5p6.. 4fn electrons are well shielded by 5s, 5p..

Crystal field splitting強度之order of magnitude遠較dn之過渡金屬為小

Rare earth ions (interconfiguration 4f-5d CT transitions)

1. CT transitions (4fn-4fn+1L-1)

2. Interconfigurational 4fn-4fn-15d transitions (allowed by selection rule)

- R2O3: white (∵∵∵∵ parity forbidden)其中僅 Nd2O3: faint violet; Tb4O7, Pr4O7, Pr6O11 dark color(∵∵∵∵ R3+/R4+共存之緣故)

- Eu3+(aq) - low molar absorptivity, sharp lines f-f transition and ∆R = 0, 此時parity selection rule並非因振動摻入而產生relaxation,而是因uneven component of crystal field (如：R3+ ions位於一不具反轉對稱中心的格位)

Uneven component 將具有將具有將具有將具有opposite parity之之之之5d混入混入混入混入R3+之之之之4f 波函數之波函數之波函數之波函數之

中中中中, 因此造成因此造成因此造成因此造成4f - 4f 躍遷產生躍遷產生躍遷產生躍遷產生。。。。因此光譜學家說因此光譜學家說因此光譜學家說因此光譜學家說: 4f-4f transitions steal some intensity from allowed 4f-5d transitions。。。。

E (eV) = 1240/ λλλλ(nm) 1 eV = 8064.5 cm-1

(a)CT transitions 4fn-4fn+1L-1 (由由由由LLLL至至至至4f4f4f4f的遷移的遷移的遷移的遷移))))

allowed and ∆∆∆∆R ≠≠≠≠ 0, broad absorption bands

發生於容易被還原之發生於容易被還原之發生於容易被還原之發生於容易被還原之R4+離子離子離子離子 (R4+ + e-→→→→ R3+), 如如如如: Ce4+,Pr4+,Tb4+ –CT broad absorption bands; orange Y2O3:Tb3+ (CT absorption band in visible)

另一方面另一方面另一方面另一方面, R2+亦易發生亦易發生亦易發生亦易發生4f-5d transitions (Sm2+ -可見光可見光可見光可見光; Eu2+,Yb2+-長波長之紫外範圍長波長之紫外範圍長波長之紫外範圍長波長之紫外範圍)

(b) 4fn-4fn-15d transitions

1)發生於傾向被氧化之發生於傾向被氧化之發生於傾向被氧化之發生於傾向被氧化之R2+離子離子離子離子( R = Sm, Eu,Yb,傾向被氧化成傾向被氧化成傾向被氧化成傾向被氧化成R3+離子離子離子離子); 2)另一方面另一方面另一方面另一方面,傾向被氧化成傾向被氧化成傾向被氧化成傾向被氧化成R4+ ( R = Ce, Pr,Tb)之離子則常在紫外光範之離子則常在紫外光範之離子則常在紫外光範之離子則常在紫外光範

圍產生圍產生圍產生圍產生4f – 5d吸收帶吸收帶吸收帶吸收帶; 3)傾向被還原成傾向被還原成傾向被還原成傾向被還原成Sm2+, Eu2+, Yb2+之之之之R3+離子離子離子離子,則則則則在紫外範圍產生在紫外範圍產生在紫外範圍產生在紫外範圍產生CT absorption bands.

(Ref. Structure and Bonding, vol. 26, p. 43(1976))

CaSO4(s):Ce3+ 由於由於由於由於crystal field splitting 造成造成造成造成4f – 5d 吸收躍遷吸收躍遷吸收躍遷吸收躍遷, 因此吸收光譜中出現因此吸收光譜中出現因此吸收光譜中出現因此吸收光譜中出現5 peaks

Ce3+: 4f1 (ground state) 5d1 (excited state)

(a)CT transitions 4fn-4fn+1L-1 (由由由由LLLL至至至至4f4f4f4f的遷移的遷移的遷移的遷移))))

allowed and ∆∆∆∆R ≠≠≠≠ 0, broad absorption bands

發生於容易被還原之發生於容易被還原之發生於容易被還原之發生於容易被還原之R4+離子離子離子離子 (R4+ + e-→→→→ R3+), 如如如如: Ce4+,Pr4+,Tb4+ – CT broad absorption bands; orange Y2O3:Tb3+ (CT absorption band in visible)

另一方面另一方面另一方面另一方面, R2+亦易發生亦易發生亦易發生亦易發生4f-5d transitions (Sm2+ -可見光可見光可見光可見光; Eu2+,Yb2+-長波長之紫外範圍長波長之紫外範圍長波長之紫外範圍長波長之紫外範圍)

(b) 4fn-4fn-15d transitions

1)發生於傾向被氧化之發生於傾向被氧化之發生於傾向被氧化之發生於傾向被氧化之R2+離子離子離子離子( R = Sm, Eu,Yb,傾向被氧化成傾向被氧化成傾向被氧化成傾向被氧化成

R3+離子離子離子離子); 2)另一方面另一方面另一方面另一方面,傾向被氧化成傾向被氧化成傾向被氧化成傾向被氧化成R4+ ( R = Ce, Pr,Tb)之離子之離子之離子之離子

則常在紫外光範圍產生則常在紫外光範圍產生則常在紫外光範圍產生則常在紫外光範圍產生4f – 5d吸收帶吸收帶吸收帶吸收帶; 3)傾向被還原成傾向被還原成傾向被還原成傾向被還原成Sm2+, Eu2+, Yb2+之之之之R3+離子離子離子離子,則在紫外範圍產生則在紫外範圍產生則在紫外範圍產生則在紫外範圍產生CT absorption bands.

(Ref. Structure and Bonding, vol. 26, p. 43(1976))

Ions with S2 configurations (Sb3+, Bi3+, Sn2+, Pb2+)

Tl+: 6s2 (基態基態基態基態), 1S0

6s16p1(激發態激發態激發態激發態) 3P1,2 1P1

Strong optical s2 →sp transition in ultraviolet, 1S0→3P0, 3P1, 2P2,

1P1, usually 1S0→

1P1 dominates the spectrum, 1S0 – 3P1 is also observed.

事實上spin-forbidden

transition becomes more intense in As3+ (4s2), Sb3+

(5s2), Bi3+ (6s2)

1S0 →3P0 forbidden (因∆J =

0, total angular momentum)

1S0→3P2 forbidden (因∆J =

2)

但若與vibration coupling之後會有部分強度band width of s2 ions depends on the host lattice

KI:Tl+吸收光譜吸收光譜吸收光譜吸收光譜

Ions with d10 Configuration (e.g., Zn2+, Ga3+, Sb5+)

- Luminescence of d10 ions – the exact nature is unclear, but mainly consisting of a CT transition from the ligands (oxygen

2p orbital) to an anti-bonding orbital (partly on d10 ions and

partly on ligands)

(Blasse, G.: Chem Phys. Lett. (1990) 175, 237)

Examples: ZnGa2O4

Zn2GeO4

M2SnO4

Oxygen vacancies, cation defects,

Self-activated luminescence

-Showing intense and broad absorption bands in the short

wavelength UV (高強度寬吸收帶高強度寬吸收帶高強度寬吸收帶高強度寬吸收帶 !!)

Other charge Transfer Transitions

除了除了除了除了LMCT之外之外之外之外, , , , MLCT (Metal-to-ligand Charge Transfer)雖然在氧化物中不常見雖然在氧化物中不常見雖然在氧化物中不常見雖然在氧化物中不常見,,,,但在配位化合物中但在配位化合物中但在配位化合物中但在配位化合物中,,,,卻十分常見卻十分常見卻十分常見卻十分常見,,,,

MMCT (Metal-to-metal charge transfer)

An electron is transferred from one metal ion to another.

Prussian Blue Fe4[Fe(CN)6]3: Fe2+/Fe3+

BaBiO3: (with Bi3+/Bi5+charge transfer)

Blue saphire: Al2O3:Fe2+,Ti4+ ; the color is due to Fe2+→→→→ Ti4+

charge transfer)

YVO4:Bi3+ (with Bi3+→→→→ V5+ charge transfer)

CaWO4:Pb2+ (with Pb2+→→→→ W6+ charge transfer)

Color Centers (色心色心色心色心) F-Center

-Consisting of an electron in a halide vacancy in KCl.

-The first optical transition is parity-allowed 1s →→→→ 2p (similar to that of hydrogen atom).

In KCl its absorption maximum is red, so KCl crystals containing F-center is strongly and deeply blue colored

Smoky quartzIrradiated glass

Century-old desert

amethyst

Synthetic quartz

紫水晶紫水晶紫水晶紫水晶

不同鹼金屬氯化物中所形成色心之吸收光譜之比較不同鹼金屬氯化物中所形成色心之吸收光譜之比較不同鹼金屬氯化物中所形成色心之吸收光譜之比較不同鹼金屬氯化物中所形成色心之吸收光譜之比較

主體晶格之能量吸收大致依所產生之帶電載子可區分為主體晶格之能量吸收大致依所產生之帶電載子可區分為主體晶格之能量吸收大致依所產生之帶電載子可區分為主體晶格之能量吸收大致依所產生之帶電載子可區分為:

(a) Those can produce electrons and holes 能產生電子電洞者能產生電子電洞者能產生電子電洞者能產生電子電洞者

(b) Those can NOT. 無法無法無法無法產生電子電洞者產生電子電洞者產生電子電洞者產生電子電洞者

Absorption of ZnS can create an e- in the CB and hole in the VB – Charge transfer type optical transition.

Positions of transitions can be shifted by replacing Zn or S

UV excitation of CaWO4 creating an exciton (e- and hole remaining together) in the WO4

2- band, 由於束縛甚強又稱之為由於束縛甚強又稱之為由於束縛甚強又稱之為由於束縛甚強又稱之為

Frenkel exciton

First absorption band of NaCl – at 8 eV (155 nm真空紫外範圍真空紫外範圍真空紫外範圍真空紫外範圍) due to 3p6→→→→ 3p54s1 transition on Chloride ion

Zinc Blende (cubic ZnS)

Wurtzite (hexagonal ZnS)

能隙中含有施體能隙中含有施體能隙中含有施體能隙中含有施體,受體與陷坑能階的螢光體之中受體與陷坑能階的螢光體之中受體與陷坑能階的螢光體之中受體與陷坑能階的螢光體之中

可能的吸收與放射躍遷示意圖可能的吸收與放射躍遷示意圖可能的吸收與放射躍遷示意圖可能的吸收與放射躍遷示意圖

主體晶格之吸收主體晶格之吸收主體晶格之吸收主體晶格之吸收

Levels with S character

Levels with Zn character

Delocalized luminescence center

donor-acceptor pair ZnS:Ag,Cl or

ZnS:Cu,Al

luminescence

Donor (Cl, Al)

Acceptor (Cu, Ag)

Conduction band

Valence band

0.1eV

1eV

3.7eV

electron

hole

Zn

Cu

AlS

luminescence

The representation of dopants (Cu,Ag, Cl) levels for ZnS-based phosphors.