Photoemission Fundamentals of Data Acquisition and Analysis J. A. Kelber, June 12 2007 Texts: PHI handbook, Briggs and Seah

Outline:

I.

Photoemission process II.

How an xray source works III. How electrons enter the analyzer IV. What do we mean by Pass Energy?

V.

Atomic Sensitivity Factors

Some slides adopted from…

ACRONYMS

Photoemission process

Emission of photoelectron KE = hv-BE, where BE= binding energy of electron in that atom

e -

Photon E = hv Ionization of atom

Since the kinetic energy of an electron is directly related to the binding energy in the solid: KE = hv –E

B Φ

analyzer

We can use core level photoemission for: (1) Quantitative analysis of surface/near surface compositions (2) Bonding environment of a given atom (small changes in KE, the “chemical shift” (3) Electronic structure of the valence band

3 step model of photoemission

: Originally due to Spicer (e.g., Lindau and Spicer, J. El. Spect. and Rel. Phen. 3 (1974) 409) 1. Step 1: Excitation of photoelectron (cross sections, rel. intensities) 2. Step. 2. Response of the system to the core hole (final state effects, like screening of the core hole, shakeup) 3. Step. 3. Transport of the photoelectron to the surface and into the vacuum . (Inelastic mean free path considerations).

Caution: Note that rigorously, the energy of a photoelectron transition is the difference in energy between the initial (ground state of the system with n electrons, and the final state, with n-1 electrons around the atom (ion) and an electron in the vacuum (n-1 + 1): E transition = E final (n-1 + 1) - E initial (n) Therefore, the energy of the transition therefore reflects screening of the core hole in the final state. This is generally not a factor in most uses of XPS, but can be important in, e.g., determining the size of metal nanoparticles. (see publication for Pt/SrTiO 3 )

X-ray Source

e e -

Al

+15 KeV

Mg

e e -

Electrons emitted from one of two filaments (depending on source selected) Electrons at 15 KeV strike Al or Mg anode, causing emission of characteristic x-rays; K α, Kβ, etc. + background User selects one or other anode for use

Emits characteristic lines, but also other lines that can broaden spectra hv=1483.6eV

hv = 1253.6 eV

Filaments (at ground)

Photoemission Process

Some electrons will reach the analyzer without undergoing inelastic interactions with solid: KE = hv-BE Φ analyzer . These electrons (Auger or photemission) will occur as

elastic, or characteristic peaks in the electron emission spectrum

N(E) Others will interact with the solid and lose energy (and chemical information). This contributes to the

secondary electron background

Background Elastic Peak

Note: Background intensity “step” increase occurs at KE< KE peak Why?

Why does background increase towards lower KE?

KE 

Pass Energy = C(V 0 -V I ) Only electrons with E = E pass +/ get thru the analyzer δE δE increases with E pass

Retards Electrons to E pass Outer Hemisphere (V O ) e E = E pass Inner Hemisphere V I

Note: Intensity Increases with Pass energy, resolution decreases!

Retarding/focussing lens

Detector

KE-V retard = E pass (V retard varied, E pass constant)

e E = KE

Sweeping the retarding voltage allows one to sweep out the electron distribution curve (photoemission spectrum)

Detecting Photoelectrons: The Channeltron Horned-shape device Lined with low workfunction phosphor Electron in  many electrons out (cascade) Gain ~ 10 7 e 10 7 e V bias

hv Valence Band

Photoemission from a core leve

e KE ~ hv –E

B Φ

analyzer

E vacuum

Work function, sample surface( Φ surface )

E Fermi

E B

3p 3s 2p 2s 1s

Auger: KE of (KL 1 L 2 ) transition = E K -E L1 -E L2 –U(final state) Is independent of excitation source energy However, when plotting BE (along with XPS data), the peak position depends on hv.

More on Auger later on

Why

Φ

analyzer

?

Sample/Spectrometer Energy Level Diagram- Conducting Sample

e Sample Spectrometer Free Electron Energy KE(1s) KE(1s) Vacuum Level, E v hv



sample



spec Fermi Level, E f BE(1s) E 1s Because the Fermi levels of the sample and spectrometer are aligned, we only need to know the spectrometer work function,



spec , to calculate BE(1s).

19

Binding Energy:

The binding energy is

calculated

:

BE = hv-KE φ

where φ = detector work function (normally 3-5 eV) φ is typically used as “fudge factor’ to align a calibration peak with accepted literature values prior to the start of the experiment

Why do we use constant pass energy?

1. Resolution Constant, with kinetic Energy 2. Easier to quantitatively compare peaks at different energies

Why do we retard electrons?

1. If we did not retard electrons: ΔE = 0.1 eV would require resolution of 1 part in 10 4 , very difficult With retardation, ΔE = 0.1 eV requires resolution of 1 part in 100 (much easier!)

Conclusion: Practical experience shows initial state effects dominate in XPS (with exceptions):

ΔE(Binding) = kΔq i + V i



ground state characteristics.

Thus, careful analysis of the XPS spectrum typically yields info regarding chemical bonding in the ground state.

Exception: Nanoparticles

Exception: Nanoparticles reflect final state screening Exception: Nanoparticles Oxidized Pt Binding energy decreases as Pt particle size increases Pt(111) 71.2 eV

Shift in BE reflects enhanced final state screening with increased particle size.

d Limited charge, small screening Larger screening response ΔR ~ d

See Vamala, et al, and references therein

ΔE B = ΔE(in.state) – ΔR + other effects (e.g., band bending) where ΔR = changes in the relaxation response of the system to the final state core hole (see M.K. Bahl, et al., Phys. Rev. B 21 (1980) 1344

Pass Energy and Analyzer Resolution

Quantitation: 1. Cross sections, transmission functions, and intensities 2. Attenuation

Includes instrumental transmission function, lens factors, etc.

Transmission Functions (T) T = T(KE)  probability of an electron of KE going thru the analyzer to the detector Typically, T~KE -1/2 , but this can be analyzer dependent.

Atomic sensitivity factors typically “adjusted by some manufactures —e.g., PHI has adjusted spot size (lens ) to change with KE. For other manufacturers, can use Scofield cross-sections

Alloy A x B y To a first approximation: We have the concentration of A (N A ) is given by I A = N A F A where F = atomic sensitivity factor Thus: N A /N B = (I A F B )/I B F A More accurately, this should be modified by the mean free path λ A : N A /N B = I A F B λ B /I B F A λ A

Summary: XPS typically done with laboratory-based Al or Mg anode sources Quantitative surface region analysis possible Hemispherical Analyzer, Retarding mode is the preferred laboratory tool Still to come: Chemical Shift Mean free path and attenuation, Auger and final state effects