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

Photon E = hv

Ionization of atom

Emission of photoelectron

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

e-

Photoemission process

Since the kinetic energy of an electron is directly related to the binding energy in the solid:

KE = hv –EB - Φanalyzer

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):

Etransition = Efinal(n-1 + 1) - Einitial (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/SrTiO3)

Filaments (at ground)

e-

e-

e-

e-

Al

Mg+15 KeV

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; Kα, Kβ, etc. + background

User selects one or other anode for use

hv=1483.6eV

hv = 1253.6 eV

X-ray Source

Emits characteristic lines, but also other lines that can broaden spectra

Photoemission Process

Some electrons will reach the analyzer without undergoing inelastic interactions with solid: KE = hv-BE-Φanalyzer. These electrons (Auger or photemission) will occur as elastic, or characteristic peaks in the electron emission spectrum

Others will interact with the solid and lose energy (and chemical information). This contributes to the secondary electron background

KE

N(E)

Background

Elastic PeakNote: Background intensity “step” increase occurs at KE< KEpeak Why?

Why does background increase towards lower KE?

Outer Hemisphere (VO)

Inner Hemisphere VI

e- E = KE

Retarding/focussing lens

Retards Electrons to Epass

KE-Vretard = Epass (Vretard varied, Epass constant)

e- E = Epass

Detector

Pass Energy = C(V0-VI)

Only electrons with E = Epass+/- δE get thru the analyzer

δE increases with Epass

Note: Intensity Increases with Pass energy, resolution decreases!

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 ~ 107

e-

107 e- Vbias

EFermi

Valence Band

3p

3s

2p

2s

1s

EB

hv

Evacuum

Work function, sample surface(Φsurface)

e-

KE ~ hv –EB - Φanalyzer

Photoemission from a core leve

Auger: KE of (KL1L2) transition = EK-EL1-EL2 –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

hv

Because the Fermi levels of the sample and spectrometer are Because the Fermi levels of the sample and spectrometer are aligned, we only need to know the spectrometer work function, aligned, we only need to know the spectrometer work function, specspec, to calculate BE(1s). , to calculate BE(1s).

EE1s1s

SampleSample SpectrometerSpectrometer

ee--

Free Electron EnergyFree Electron Energy

Fermi Level, EFermi Level, Eff

Vacuum Level, EVacuum Level, Evv

sample

KE(1s) KE(1s)

spec

BE(1s)

Sample/Spectrometer Energy Level Diagram- Conducting Sample

19

Why Φanalyzer?

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 104, 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) = kΔqi + Vi ground state characteristics.

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

Exception: Nanoparticles

Exception: Nanoparticles

Exception: Nanoparticles reflect final state screening

Binding energy decreases as Pt particle size increases

Pt(111)71.2 eV

Oxidized Pt

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

ΔEB = 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

Limited charge, small screening

Larger screening response

d

ΔR ~ d See Vamala, et al, and references therein

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 AxBy

To a first approximation: We have the concentration of A (NA) is given byIA = NA FA where F = atomic sensitivity factor

Thus: NA/NB = (IA FB)/IBFA

More accurately, this should be modified by the mean free path λA:

NA/NB = IAFBλB/IBFAλA

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