iii. analytical aspects photoelectron spectroscopy cheetham & day, chapter 3 surface technique:...

III. Analytical Aspects Photoelectron Spectroscopy Cheetham & Day, Chapter 3

• Surface Technique: cannot provide completely reliable analysis for bulk samples

• Alternatives include

(a) Electron Microprobe

(b) Scanning Electron Microscopy (SEM)

(c) Inductively-Coupled Plasma/Mass Spectrometry (ICP-MS)

(d) Atomic Emission

(e) X-ray/Neutron Diffraction

III. Analytical Aspects Photoelectron Spectroscopy: Background Cheetham & Day, Chapter 3

Vacuum Level

Measured Energy of a photon: E = h E 200-2000 eV (X-rays): XPS (core and valence e) E 10-45 eV (Vacuum Ultraviolet): UPS (valence e)

Minimum energy needed (threshold frequency): hc = e; = work function (2-6 eV)

Maximum kinetic energy of the photoelectron is Ekin

max = h e (electrons close to EF) Ekin = h EB e (electrons in states EB below EF)

EF = “Fermi level” = Energy of “HOMO” in metal; EB = Core electron “Binding Energy”

Hand-Outs: 23


Characteristic core electron binding energies for each element: Electron Spectroscopy for Chemical Analysis(ESCA; P. Siegbahn)

• Chemical Shifts

• Multiplet Structures

• Satellites XPS

Hand-Outs: 24


Characteristic core electron binding energies for each element: Electron Spectroscopy for Chemical Analysis(ESCA; P. Siegbahn)

• Chemical Shifts

• Multiplet Structures

• Satellites UPS

Hand-Outs: 24

(1) Source of radiation

(a) X-rays: high photoelectron flux (I 3.5) and good resolution (line-widths)K of low-Z elements: L(2p) K(1s)Mg (1254 eV) and Al (1487 eV)

Al: K1,2 (2p 1s) shows 0.4 eV; lifetime width of 1s hole state 0.47 eV resolution 0.9 eV

Improve resolution by focusing using Bragg reflections, but sacrifice intensity.

(b) UV: gas discharge lampsHe(I): 21.21 eV; 1s12p1 1s2

He(II): 40.8 eV; 2p1 1s1

Better resolution (ca. 0.05 eV) but distorted backgrounds due to “degraded” electrons.

III. Analytical Aspects Photoelectron Spectroscopy: The Experiment Cheetham & Day, Chapter 3

Hand-Outs: 25

(2) Spectrometer (XPS): Deflection spectrometer


Hand-Outs: 25

(3) Sample Preparation: how stable is sample toward decomposition at low p (< 107 torr)

XPS (ESCA) provides information about a thin surface layer.

15 Å

1000-10000 Å

e Inelastic Scattering

Lower EKE ("Higher" EB)

• Electrons from the bulk are inelastically scattered and produce broad structure extending

toward greater binding energy (lower kinetic energy).• Sample often requires careful surface preparation: cleaving, Ar-ion sputtering,

deposition on a substrate.• Often have surface oxide, CO2, grease (C can be a “reference”).• Nonmetallic samples: Au film provides metallic surface.


For Al K: h = 1487 eVMean Free Path ~ 7-18 Å

2

1/ 21 2 O

1MO (s) MO (s) O (g);

2n n pK p

Hand-Outs: 26

III. Analytical Aspects Photoelectron Spectroscopy: The Experiment Cheetham & Day, Chapter 3(4) Spectrum Analysis (Deceptively simple; traditionally complex)

Binding energies: reference level and calibration (?); metal vs. nonmetal (charging effects); multiplets, satellites, shake-off; …

Differences in PE cross-sections

M(s) + h M+(s) + e

h = EKE(e) + E(M+) E(M) = EKE(e) + EB

Spectrum plotted as Intensity (Counting rate) vs. Binding Energy

EB = h EKE(e)

Hand-Outs: 26

Mg K

III. Analytical Aspects Photoelectron Spectroscopy: XPS Spectrum of Pd(s)

http://www.chem.qmul.ac.uk/surfaces/scc/scat5_3.htm

330 eV

690 eV720 eV

910 eV920 eV

581 eV

Hand-Outs: 27

Mg K



330 eV

690 eV720 eV

910 eV920 eV

Transform EKE (KE) to EB (BE = h KE)

343 eV333 eV534 eV

561 eV

581 eV

673 eV

920 eV

0-8 (4-12) eV(4d, 5s)

54, 88 eV(4s, 4p)

Hand-Outs: 27

Mg K



330 eV

690 eV720 eV

910 eV920 eV

Transform EKE (KE) to EB (BE = h KE)

343 eV333 eV534 eV

561 eV

581 eV

673 eV

920 eV

0-8 (4-12) eV(4d, 5s)

54, 88 eV(4s, 4p)

Tail: E loss from solid

Hand-Outs: 27

Spin-orbit splitting of 3d peak

Mg K



330 eV

690 eV720 eV

910 eV920 eV

Transform EKE (KE) to EB (BE) = h KE

343 eV333 eV534 eV

561 eV

581 eV

673 eV

920 eV

Pd+: 2D5/2 (6 levels); 2D3/2 (4 levels) (3d)9 > ½-filled: E(J=5/2) < E(J=3/2)

Hand-Outs: 27

Pd(s) Pd+(s) + e

Mg K



330 eV

690 eV720 eV

910 eV920 eV

Transform EKE (KE) to EB (BE) = h KE

343 eV333 eV534 eV

561 eV

581 eV

673 eV

920 eV

Auger Process (2-electron process) Hole created in M (n = 3) shell; Relaxation from N (n = 4) shell into M; N M (E released) E < E overcomes binding from N shell

(Auger Electron)

K (1s)

L1 (2s)L23 (2p)

M1 (3s)M23 (3p)

N1 (4s)N23 (4p)

Vacuum Level

Relaxation (Hole in M shell)

Auger Electron

First Hole

Second Hole Third Hole

MNN Auger Transition

Hand-Outs: 27-28

Chemical Shifts (Binding Energies) Measured binding energy of core electrons Calculated energy of the core state

ObservedKoopman's Theorem(I.E. = EB)

Ground State of Neutral Atom

Ground State of Ion

Relaxation Processes

Z (core/valence) Z+ (core1/valence): (lower observed EB; higher measured EKE)

• valence electrons in final state feel lower screening from the nucleus, behaves like Z+1 element

(in metals, this is only seen for elements with states in the conduction band near EF)• Intra-atomic: single-ion shift to core electron if valence electron is missing (constant potential, ca. 10 eV for free atoms)• Inter-atomic: response of the environment to ionization of atom. Polarization of neighboring ions in insulators; of conduction electrons in metals.

III. Analytical Aspects Photoelectron Spectroscopy Inorg. Chem. 1984, 23, 2625-2632.

Hand-Outs: 29

The Binding Energy of an electron depends on:(1) Formal oxidation state of the atom;(2) Local chemical and physical environment

Typically, EB increases with oxidation state e.g., compare Ti (metallic) with TiO2 (insulating)


2p1/2 2p3/2

ca. 5 eV

Hand-Outs: 30


Examine Madelung potential and local environment: PbO vs PbO2 (ca. 2.3 eV shift in 4f binding energies from PbO (Pb2+) to PbO2 (Pb4+).

Pb

O

Pb

O

Pb:4 n.n. O4 n.n. Pb

PbO PbO2

Pb:6 n.n. O

Hand-Outs: 30


Changes of relative position of Fermi level can lead to “negative” chemical shifts (especially in valence band spectra): e.g. CdCl2

Cd CdCl2

4d 4d

5s

5p

Cl 3p

Conduction BandEF

EF

Energy

Metals

EB relative to EF

SemiconductorsInsulators

Build-up of + chargeat surface; dipole layer;

EF depends ondopant concentrations

Hand-Outs: 30

III. Analytical Aspects Examples of Valence Band Spectra Cheetham & Day, Chapter 3

Au: metallic

No states at theFermi level

7 eV broad 5d Band;Satellites (shoulders);6s contributions

Narrow 5d Band --Weak interatomic overlap in CsAu

EB increases as Au is “reduced”Au0 “Au1”

CsAu: transparent, red insulator

Au

Cs

2.89 Å

4.26 Å

Hand-Outs: 31


(2-electron processes)Ni Cu Zn

• Increasing EB

• 3d band narrows

(Nuclear charge increases)

3d 3d

3d

4s + 4p

Hand-Outs: 31


ReO3

Calculated Density of States Curve

Valence Band XPS SpectrumMg K

Re 5d (t2g orbitals)Re-O antibonding

O 2p nonbonding

O 2p Re-O bonding

States with Re contributionshave stronger intensities than O states

Different cross-sections for emissionof photoelectrons from Re vs. O

WO3

Hand-Outs: 31


VO2

Rutile-type(Tetragonal);Metallic

Monoclinic;Insulating

Hand-Outs: 31

Occupied MOs in LixNb3Cl8 x = 0: 7 electrons x = 1: 8 electrons

Nb3Cl8 LixNb3Cl8

Mg K

He(II)

He(I)


Mg K

He(II)

He(I)

Nb MOs

HOMO

Hand-Outs: 32

III. Analytical Aspects Related Spectroscopies Cheetham & Day, Chapter 3

• Photoelectron Spectroscopy – occupied electronic states; • Bremsstrahlung Isochromat Spectroscopy (BIS) – empty electronic states;

• Auger Spectroscopy – surface chemical composition; • Rutherford Backscattering – chemical composition (heavy elements);

• Extended X-Ray Absorption Fine Structure (EXAFS) – local structure (SRO)

• Electron Energy Loss Spectroscopy (EELS) – excitation spectra

iii. analytical aspects photoelectron spectroscopy cheetham & day, chapter 3 surface technique:...

Documents

valence e e

experiment cheetham

background cheetham

s e h

binding energy eb

ev resolution

h eb e electrons

high photoelectron