Photoelectron Spectroscopy applied to molecular surface science

Rodrigo M. Petoral Jr.

November 2005

Table of Contents

1. Introduction……………………………………………………………….. 3 2. Principle……………………………………………………...……………. 4 3. Instrumentation for photoelectron spectroscopy……………………..… 8

3.1. Photon sources………………………………………………….… 8 3.2. Energy Analyzer………………………………………………….. 10 3.3. Electron detector………………………………………………….. 11 3.4 Vacuum System…………………………………………………… 12 Laboratory XPS system set-up……………………………. 13

4. Spectral Analysis……….…………………………….…………….……. 16 4.1. Origin of Photoelectron peaks……………………………………. 16

4.1.1. Primary XPS structures………………………….. ………… 16 Inelastic background……………………………………… 16

Spin-orbit splitting………………………………………… 16 Core-level chemical shift…………………………….…. .. 17

4.1.2. Secondary XPS structures…………………………………. 19 X-ray satellites…………………………………………… 19 Final state effects (intrinsic satellites)……………………. 20 Extrinsic satellites………………………………………… 21

4.2.Qualitative analysis……………………………………………….. 22 4.3 Quantitative analysis……………………………………………… 22 Spectra Quantification and surface sensitivity …………… 23

5. Angle dependent XPS……………….…………………………………… 26 6. References………………………………………………………………. 27

2

1. Introduction

Photoelectron spectroscopy is an analytical technique in which a sample is

irradiated with photons and the emitted photoelectrons are studied. The basis of the

technique lies on the photoelectric effect discovered by Hertz in 1887 and theoretically

described by Einstein in 1905. Photons can induce electron emission from a solid

provided that the photon energy (hν) is greater than the work function (φ), defined as the

minimum energy required to remove an electron from the highest occupied energy level

in the solid to the ‘vacuum level’. The first X-ray induced photoelectron spectroscopy

was developed by Robinson in 1913 [Robinson & Rawlins, 1914]. The work was

continued to the 1930’s but problem with energy resolution of electron spectrometers

gave scientists difficulties to analyze results during that time. Thus, the method was not

useful until high-resolution electron spectrometers became available so as to give unique

information of the energy distribution of the emitted electrons. High-resolution

spectrometers were then developed in the mid-1950’s by Kai Siegbahn and his research

group at the University of Uppsala in Sweden. The method is since then called ESCA

(Electron Spectroscopy for Chemical Analysis) [Nordling, 1957; Sokolowski, 1957;

Siegbahn, 1967]. Precise determination of the binding energy of the core electrons was

achieved and that small chemical shifts in the binding energies were detected [Hagström,

1964]. It was in association with chemical shifts, related to the chemical state, the term

ESCA was first used. The technique uses high-energy photons (x-rays) for irradiation.

The method is also known as XPS (X-ray Photoelectron Spectroscopy) and the name is

now favored due to the development of many electron spectroscopies. A related method

called UPS (Ultraviolet Photoelectron Spectroscopy) uses low-energy photons

(ultraviolet radiation). The use of x-rays for excitation permits both core and valence

electrons to be probed while ultraviolet radiation probes the outer valence electrons.

3

2. Principle Photoelectron spectroscopy is in principle a simple process. A photon with certain

energy hits and excites an electron so it can escape the atom. Applying the principle of

energy conservation (from Einstein’s photoelectric equation), the kinetic energy of the

emitted photoelectrons can be estimated: ikin

if EEEh +−=ν (2.1)

where νh =incoming photon energy

fE = total energy of ion (final state)

= total energy of electron (initial state) iE

= photoelectron’s kinetic energy ikinE

The final state, E, will affect the energy of the photoelectron from Eq.2.1. If the final

state is an ion, the binding energy of the photoelectron can be defined as: ifV

B EEE −= (2.2)

where = photoelectron’s binding energy with reference to the vacuum level VBE

of the sample

The photoelectron’s binding energy is affected by the element’s initial- and final-state

configurations as previously shown in Eq. 2.2. Rewriting Eq. 2.1 for an N-electron

system,

)1(),1()( kinfi EKNEhvNE +−=+ (2.3)

where K= one electron orbital K from which photoionization occurs. It is known that,

)1()( kinVB EhKE −= ν (2.4)

It follows that Eq. 2.2 can be rewritten as,

)(),1()( NEKNEKE ifVB −−= (2.5)

The binding energy is therefore not just the binding energy of the electrons in the ground

state, but corresponds also to the final state.

Normally, a starting point to theoretically calculate the approximations to N-

electron wave functions is by using non-relativistic Hartree Fock (HF) self-consistent-

consistent field (SCF) method. Difficulties associated with hole-state calculations in

determining binding energies can be avoided by approximation called “frozen orbital” or

4

Koopman’s theorem. The theorem assumes that when an electron is removed from an N-

electron system, the wavefunctions of all the other electrons are unchanged, i.e. the

remaining N-1 electrons are frozen in their original distribution. However, in reality, the

remaining electrons relax to a different energy state after photoemission and the core hole

will influence the final state of the photoemitted electrons such that the Koopman’s

theorem seldom applies. Taking this relaxation energy (dErelax) into account, the binding

energies estimated with Koopman’s theorem (KT) are therefore greater than the value

measured in XPS:

relaxKTV

BVB dEKEKE −= )()( (2.6)

For solid samples, the Fermi level is the reference energy level used. The Fermi

level is often considered the highest occupied level in solid. A work function term is

entered into the equation, defined as the energy needed to move the electron from the

Fermi level to the vacuum level (See Fig.1). Thus, ikin

FB EEh ++= φν (2.7)

where = binding energy of electron relative to the Fermi level FBE

φ = work function for the sample

Figure 1 sumarizes the XPS process, illustrating the X-ray excitation of a 1s core level.

The kinetic energy measured by the spectrometer is related to the vacuum level of the

spectrometer material. The work function for the spectrometer material must be

introduced in the final equation:

spkinFB EEh φν ++= (2.8)

where = kinetic energy measured kinE

= work function for the spectrometer spφ

5

EF

EV

Ekin

φsp

φ

EbF

EbV

e-

0

Kinetic E

nergy

N(E)

Sample

Spectrometerhν

hν

Eikin

EF

EV

Ekin

φsp

φ

EbF

EbV

e-

0

Kinetic E

nergy

N(E)

Sample

Spectrometerhν

hν

Eikin

Figure 1. Energy diagram showing the photoioniztion process of a core-electron.

During the measurement, if the photon energy and the spectrometer work function is

known, the binding energy of the photoelectrons can be derived from the kinetic energy

of the emitted electrons relative to the Fermi level. The binding energy scale was derived

to make uniform comparisons of the chemical states straightforward. Figure 1 also

diagrams the photoelectric equation and the effect of a contact potential between a

conducting sample (e.g, metals) and the electron spectrometer.

Both core and valence level electrons can be probed using X-rays as an excitation

radiation. Core levels are defined as the inner quantum shells that do not participate in

chemical bonding, while valence levels are electrons in the more weakly bound, partially

filled outer quantum shells. The explanation to chemical identification is that core

electrons are insensitive to their surroundings when condensed into a solid phase and

retains the binding energies that are signatures of the atom type. The outermost

electrons, which are directly involved in chemical bonding in a solid, are broadened in a

FBE

6

‘valence band’ and is effectively probed using the UPS technique mentioned in the

introduction. Except for hydrogen, all elements can be detected and studied. The reason is

that all elements except hydrogen have a characteristic core level electrons even when

combined with other elements to form a compound. The binding energy value of an

electron in the hydrogen atom (13.6 eV) falls in the valence band region for other

elements.

Binding energy (BE) represents the strength of interaction between electrons and

the nuclear charge. BE follows the energy of different levels, i.e.

BE(1s)>BE(2s)>BE(2p)>BE(3s) and so on (See Fig 2). The binding energy of an orbital

also increases with increasing atomic number. The higher the nuclear charge of an

adatom, the higher the binding energy of a given core level (e.g. BE (O 1s) >BE(N 1s)

>BE (C 1s)) ( Fig. 2 ) Also, the BE of an orbital is not affected by isotopes [e.g. BE(7Li

1s)=BE(6Li 1s)].

02004006008001000

Inte

nsity

(arb

itrar

y un

its)

Binding Energy (eV)

4f7/2

4f5/2

5p 5d

4d5/24d

3/2

4p3/2

4p1/24s

C 1s

O1sN1s

Figure 2. XP spectra of an organic adsorbate on gold substrate illustrating the electron lines coming from the Au signal at different levels and the presence of the carbon, oxygen and nitrogen signals from the adsorbate.

7

3. Instrumentation for photoelectron spectroscopy

Surface analysis by photoelectron spectroscopy requires irradiating a solid in an

Ultra-high vacuum (UHV) chamber with monoenergetic soft X-rays and analyzing the

energies of the emitted electrons. Thus, there are four essential parts of an XPS

equipment namely: photon source, energy analyzer, electron detector and a vacuum

system.

3.1. Photon sources: X-ray gun, UV lamp and synchrotron radiation

X-rays are invisible, highly penetrating electromagnetic radiation of much shorter

wavelength (higher frequency) than visible light. The wavelength range for X-rays is

from about 10-8 m to about 10-11 m. A way of producing x-rays from an x-ray gun is by

thermionic emission, i.e. by heating a filament (e.g. tungsten) thus creating a high-density

electron cloud. The accelerated electrons hit the anode, usually made up of Al or Mg,

and holes are created in the core shells of the anode material. The holes result in de-

excitations from higher energy levels emitting characteristic x-rays. The Kα1,2 lines,

associated with decays from 2p1/2 →1s and 2p3/2 →1s, are usually used because of its

dominating and intense characteristics. For Al, the Kα 1,2 line is at 1486.6 eV, line width

of 0.85 eV at full width at half maximum (FWHM), and at 1253.6 eV with line width of

0.7 eV FWHM for Mg. It is possible to reduce the width to about 0.2 eV by

monochromatizing the X-ray. A way to monochromate the X-ray is by single or multiple

Bragg refelections from suitable crystals to pick out just a part of the dominant Kα 1,2 line.

Monochromatizing the light also eliminates X-ray satellites. Unmonochromated sources

produces doubly ionized (Kα 3,4) emmision giving rise to photoelectron satellites at

kinetic energies10 eV higher. Also, the emission lines produced are superimposed on

broad background (Bremsstrahlung). A thin (~10-30 μm) foil of Al or Be is placed in

between the sample and the X-ray source. This is done to prevent detection of secondary

electrons from the X-ray source and to isolate the pumping of the outgassing X-ray target

from the UHV of the sample target. Cooling of the X-ray anode is also another important

aspect to consider. The process of creating X-ray photons is very ineffective with

8

efficiency of about 1% and the rest is converted to heat. The cooling is done by water so

as not to destroy the anode.

Ultraviolet form of radiation can be acquired using UV discharged lamps. Sources

could be HeI (21.2 eV) or HeII (42 eV). These low energy photons allow the excitation of

valence electrons (with binding energy <30 eV) that are directly involved on chemical

bonds of compounds. This technique will not be covered extensively in this manual.

Synchrotron radiation has excellent and unique features that so far can be

regarded as a better light source for photoelectron spectroscopy studies. The radiation

produced has characteristics i.e. ultra-bright light, high polarization, highly directional,

collimated beam and wide spectrum of wavelength from IR to X-rays. With the presence

of a monochromator, the light can easily be tuned depending on the type of measurement

in mind to achieve spectra of better resolution and better surface sensitivity. The output

spectral curve for the different type of photon sources is shown in Figure 2.3, clearly

demonstrates the broad continuum.

B R I L L I A N C E

Figure 3. Spectral curve for different type of photon sources.

9

3.2. Energy analyzer

The electron energy distribution [the number of electrons detected ‘N(E)’ as a

function of their kinetic energy, Ekin) can be measured using an electrostatic energy

analyzer. There are different methods to characterize the energy, Ekin, of a charged

particle: (1) time-of-flight method used for time-resolved studies, (2) deceleration by

retarding electric field, and (3) change of orbit by electric or magnetic field whose

principle is used by deflection analyzers. The most commonly used electron energy

analyzer in XPS is an electrostatic deflection-type analyzer called concentric

hemispherical analyzer (CHA) or spherical sector/deflection analyzer. The analyzer

consists of two concentric hemispherical electrodes where voltage, Vf, is applied between

the electrodes. The sphere voltage determines the pass energy of the analyzer (Epass), i.e.

the kinetic energy of the electrons needed to reach the electron detector. It is defined as,

fpass keVE = (3.1)

where k is called the spectrometer constant. Right before the entrance slit of the CHA, a

multi-element electrostatic lens system is found. The functions of the lens system are the

following: (1) helps collect large distribution of electrons (larger flux); (2) focuses

electrons at entrance of slit; and, (3) retards or slows down electrons to pass energy by

applying retardation potential, Vr. The sphere potential is kept constant during the

measurements, i.e. Epass is also fixed while Vr is swept. With a constant sphere voltage,

the resolution [ΔE(FWHM)/E], defined as the ability to separate closely spaced

photoemission peaks, is also constant during the measurement. An electron with kinetic

energy EO is retarded by the electron lens so that when it enters the half spherical

analyzer the kinetic energy of the electron is

fropass ekVeVEE =−= (3.2)

The spectrometer constant k considers the geometry of the analyzer and will be a term in

the photoelectric equation (Eq. 2.8). The equation will then be,

spfrFB ekVeVEh φν +++= (3.3)

10

To determine k and φsp , measurement on photoelectron lines of known energies must be

made at different sphere voltages. Common calibration lines can be seen in Table 1,

together with X-ray energy lines for Al and Mg.

Table 1. Reference Binding Energies (eV) [Seah, 1989]

Calibration level FBE in eV

using Al Kα (hν=1486.6 eV)

using Mg Kα (hν=1253.6 eV)

Cu 3p 75.14 75.13 Au 4f7/2 83.98 94.00 Ag 3d5/2 368.26 368.27

Cu L3MM 567.96 334.94 Cu 2p3/2 932.67 932.66

Ag M4NN 1128.78 895.75 The spectrometer constant k can be determined by applying the photoelectric equation

when measuring with different Vf and Vr:

21

21

ff

rr

VVVV

k−−

= (3.4)

An expression for φsp can be derived from Eq. 3.3 and 3.4. The values of φsp is equal to

4.7 eV for the instrument that will be used in the laboratory exercise.

3.3. Electron detector

A way to amplify the current (in the order of 10-16 to 10-13 A) coming from the

spherical analyzer is to use a Channel electron Multiplier (Channeltron). The multiplier is

a small spiral glass tube coated internally with a resistive material across which potential

is applied. When electrons enter the multiplier, they will strike this highly resistive and

emissive multiplier wall and cause secondary emissions. The secondary electrons

accelerate along the wall of the tube and the repeated collisions cause an avalanche

effect. A single electron input may give rise to an output pulse of 108 electrons. The gain

of the Channeltron is dependent upon the resistance and capacitance in the anode circuit

of the multiplier, and also the voltage applied to the device.

11

3.4 Vacuum System

UHV system, with pressure ranging from 10-8 to 10-11 Torr, is needed for this type

of surface analysis. There are two main reasons why UHV is required: (1) To enable

atomically clean surfaces to be prepared for study, and such surfaces to be maintained in

a contamination-free state for the duration of the experiment and, (2) To permit the use of

low energy electron and ion-based experimental techniques without undue interference

from gas phase scattering. These reasons can be placed into context with regards to

variables related to pressure. One of the crucial factors in determining how long a surface

can be maintained clean (or, alternatively, how long it takes to build-up a certain surface

concentration of adsorbed species) is the number of gas molecules impacting on the

surface from the gas phase. From the elementary kinetic theory, the surface impact rate

(r) of a gas at a fixed pressure (P) and temperature (T) is expressed as:

( ) 2/12 mkTPr

π= (3.5)

For nitrogen at 300 K and a pressure of 10-8 Torr, the surface impact rate is about

5 x 1012/(cm2s1). If every molecule that strikes the surface sticks, a ‘clean’ surface would

be covered with monolayer of nitrogen in three minutes; thus, experiments on clean

surfaces require UHV conditions.

12

CLAM 2 electron energy analyzer

Analysis chamber Preparation

chamber

CLAM 2 electron energy analyzer

Analysis chamber Preparation

chamber

Figure 4. XPS system designed for organic and biomolecular studies

Laboratory XPS system set-up

The UHV system [Uvdal, 1991] that this laboratory exercise will take place is

especially designed for organic molecular studies. A photograph of the system in shown

in Figure 4. It consists of three chambers, the introduction chamber, the preparation

chamber and the analysis chamber. The introduction system or the load-lock system

consists of a heatable/coolable sample holder placed on a sample shaft where a sample

from air can be quickly mounted. The intro chamber can be pumped down from

atmosphere to 1 x 10-8 Torr. The sample can be transferred to the preparation chamber

having a base pressure of about 2 x 10-10 Torr. Facilities such as ion sputtering, mass

spectroscopy and sample preparation by vapor deposition of molecular films from solid,

liquid or gas are available. A special cryogenic pump system can be found inside the

prep chamber. The cryo-system consists of a copper cylinder, mounted on a cold-head of

a two-stage, closed cycle, He-refrigerator. The copper cylinder surrounds the sample

shaft and the sample holder with a surface temperature of about 20 K. The third chamber

is the analysis chamber consisting of mainly the XPS spectrometer system. The

spectrometer system is a VG instrument consists of a CLAM 2 electron energy analyzer

and a twin Mg/Al anode as X-ray source (XR3E2 X-ray source). Schematic figure of the

13

geometry of the sample holder, the photon source and the entrance to the energy analyzer

is shown in Figure 5.

Electron lens

Electron energy analyzer

Electron detector

Photon source

Electron lens

Electron energy analyzer

Electron detector

Photon source

Figure 5. The X-ray source operates at a constant power of about 300 W. The electron analyzer system consists of a retarding 4-element electron lens, a hemispherical electron analyzer and a multi-channel electron detector. A VGX 900 data system is interfaced to serve as the data collection and analysis system.

Another feature of the spectrometer is that it is designed [Salaneck, 1988] to provide

optimum angle dependent XPS, XPS(θ). The unpolarized (essentially radially polarized)

photons strike the surface of the sample with their momentum vector kx at an angle of 30°

to the surface as illustrated in Figure 6. The sample can be rotated about the x-axis, which

goes through the center of the sample. When the rotation angle of the sample is zero, the

electrons leave the sample with an exit angle of 30° w.r.t the nomal to the surface

(considered to be the normal exit). Electrons can have an exit angle closer to grazing exit

by rotating the sample about the x-axis. The rotation is equivalent to moving the analyzer

in an arc in the y-z plane. A rotation angle of 90° corresponds to electrons leaving the

surface exactly parallel to the surface. Normally, angle dependent measurements were

14

made using photoelectron take-off angle (TOA) of 30° and 80° with respect to the surface

normal of the sample and the two angles correspond respectively to the Bulk and Surface

mode terms of the measurement.

e -

x

z

y (θ=90°)kx

30°

θ

e -

x

z

y (θ=90°)kx

30°

θ

Figure 6. Sample holder and geometry for XPS (θ).

15

4. Spectral Analysis

An XPS spectrum is a graphical picture of the binding energy for the electron

versus the number of electrons in a small energy interval. A typical XP spectrum is

earlier shown in Figure 2. The well-defined peaks basically came from electrons that

have not lost any energy on their way out from the sample. The XP spectrum is

composed of primary and secondary structures originating from different effects.

Photoemission process is often envisage as three steps:

(i) absorption and ionization (initial state effects),

(ii) response of atom and creation of photoelectron (final state effects),

(iii) transport of electron to surface and escape (extrinsic losses).

All contribute structure to XP spectrum.

4.1 Origin of Photoelectron peaks

4.1.1 Primary Structures in XPS

Inelastic background

A big contribution to the background comes from electrons that have lost some of

their energy in inelastic scattering. Continuous background is due to a random energy

loss process and that the electron can lose energy in several steps. XP spectra show

characteristic “stepped” background, i.e. the intensity of background to high binding

energy of photoemission peak is always greater than the low binding energy. Due to the

inelastic process (extrinsic losses) from deep in the bulk, only electrons close to surface

can, on the average, escape without energy loss. Electrons deep on the surface loose

energy and emerge with reduced kinetic energy (increased binding energy) , while

electrons situated very deep in surface loose all energy and cannot escape the surface.

Spin-orbit splitting

Spin–orbit splitting is an initial state effect. Coupling between the magnetic field

of spin and angular momentum, occurs in any electron in orbital. Depending on the

16

angular momentum of an electron, spin angular momentum (s) and orbital angular

momentum (l), an XP signal could give rise to either single or double peaks in an XP

spectrum. The total angular momentum, j, is defined as

slj ±= (4.1)

Double-peaks arise due to the spin-orbital coupling and the number of spin-orbit split

levels at each j values is defined by the degeneracy = 2j +1. The degeneracy defines the

intensity ratio of the two peaks. A table below defines the degeneracy values for each

sub-shell. It has to be noted that s-orbitals are not spin-orbit split (singlet in XPS). The

binding energy of the lower j value in a doublet is higher, e g. BE 2p1/2 > BE 2p3/2, and

the magnitude of spin-orbit splitting increases with atomic number. See Table 2 below.

Table 2. Subshell j values Degeneracy

s 1/2 - p 1/2, 3/2 1, 2 d 3/2, 5/2 2, 3 f 5/2, 7/2 3, 4

An example of spin-orbit splitting can be seen on Au 4f signal. The intensity ratio of Au

4f7/2: Au 4f5/2 is 4:3.

Core-level chemical shift

The binding energy for the core level electrons of an element is modified when

constituted in a molecule. Its precise binding energy will depend critically on the species

to which it is bonded. These chemical shifts of the core level energy signify an important

source of information regarding the electronic structure of bonded atoms, and these can

be studied with XPS measurements with high-resolution electron spectrometers.

It has been shown from theoretical models that the chemical shift for the binding

energies of the electrons has a close connection to the effective charge for an atom in a

molecule. The simplest model of chemical shift is an ion. Charge transfer may leave

atoms with partial positive (or negative) charges, leading to a shift in core levels to higher

(or lower) binding energies associated with increased (or decreased) Coulombic attraction

17

between core electrons and the nucleus. Consequently, atoms in a high formal oxidation

state will yield XPS peaks at high binding energy relative to the same atom in a low

oxidation state. The extent of the chemical shift is dependent on the local chemical and

physical environment surrounding the concerned atom and the shift can be as large as 10

eV. Other effects such as changes in crystal potential, polarizability and Fermi level may

also cause considerable shifts in core level binding energies.

An example of chemical shift due to electronegativity effects of the surrounding

atoms is shown in Figure 8. The C 1s gas phase XPS spectrum of ethyl trifluroacetate

[Siegbahn, 1967] shows a clearly resolved four carbon core levels. Typical C 1s and O 1s

binding energies for organic sample [Beamson and Briggs, 1992 ] is listed in Table 3.

Table 3. Typical C 1s and O 1s binding energies for organic samples* Functional group Binding Energy

(eV) hydrocarbon C-H; C-C 285.0

amine C-N 286.0 alcohol; ether C-O-H; C-O-C 286.5

Cl bound to carbon C-Cl 286.5 F bound to carbon C-F 287.8

carbonyl C=O 288.0 amide N-C=O 288.2

acid; ester O-C=O 289.0 urea N-C(=O)-N 289.0

carbamate O-C(=O)-N 289.6 carbonate O-C(=O)-O 290.3

2F bound to carbon -CH2CF2- 290.6 3F bound to carbon -CF3 293-294

carbonyl C=O; O-C=O 532.2 Alcohol; ether C-O-H; C-O-C 532.8

ester C-O-C=O 533.7 * The observed binding energies will depend on the specific environment where the functional groups are located. Most ranges are ± 0.2 eV, but some (e.g. flurocarbon samples) can be larger.

18

Chemical shift (eV)

-20246810

Inte

nsity

(Arb

. uni

ts) C C O C C

O

FF

F

H

H

H

H

H

Chemical shift (eV)

-20246810

Inte

nsity

(Arb

. uni

ts) C C O C C

O

FF

F

H

H

H

H

H

Figure 8. C 1s XP spectrum of ethyltrifluroacetate (gas phase) showing four different chemical environments of carbon atoms in the molecule [Siegbahn, 1967]. 4.1.2 Secondary Structure in XPS

X-ray satellites

The emission spectra from the X-ray source also consist of smaller components

with higher energy, aside from just having a characteristic energy. A family of smaller

peaks with lower binding energy can be found for every photoelectron peak that comes

from a Kα photon. These peaks have intensity and distance that are characteristics for the

anode material. An example of X-ray satellites for Mg is shown in Fig. 9.

Sharp photoemission lines in XPS can be achieved when using a monochromatic

x-ray source. Emission from non-monochromatic x-ray sources involves presence of x-

ray fluorescence emission lines superimposed on broad background (Bremsstrahlung).

The emission also produces “ghost” peaks in the XP spectrum at lower binding energy.

19

Figure 9. Mg X-ray satellites observed in the C 1s spectrum of graphite.

Final State Effects (intrinsic satellites)

Final state effects occur during atom relaxation and creation of photoelectron

following core-hole creation. Removal of a core electron causes a large perturbation of

the electronic structure that can cause the ionization, to couple with both valence

electronic and vibrational excitations. There are two types of mechanisms in screening

the hole created upon photoemission: intermolecular and intramolecular relaxation.

These relaxation events can give rise to binding energy shifts of the photoelectron

spectrum. The intermolecular screening is caused by electronic polarization of the

surrounding molecules. The intramolecular relaxation is caused by the electrons or nuclei

within a molecule. These relaxation events are very fast processes, occurring at about 10-

100 femtoseconds. Different excitation events as shake-up, shake-off and Auger

emission, might occur during photoionization that originate from different screening

effects. The emitting processes are illustrated in Figure 10.

20

Core levels

Occupied Valence levels

Unoccupied Valence levels

Ground state

Continuos kinetic energy

Core ionization

Shake-up Shake-off Auger decay

Core levels

Occupied Valence levels

Unoccupied Valence levels

Ground state

Continuos kinetic energy

Core ionization

Shake-up Shake-off Auger decay

Figure 10. Different photoelectron emission process.

Extrinsic satellites

Photoelectrons, in some material, can lose energy from cooperation of other

electrons in the surface region. Peaks with energies above the binding energy of the

original peak appear. These peaks are easily seen on metals as compared to insulators.

Plasmon excitations in metal occur as energy losses to the valence band electrons in well-

defined quanta. Figure 11 shows spectrum of aluminum showing energy loss (plasmon)

lines associated with the 2s line of aluminum. The energy interval between the primary

peak from the energy loss is called the plasmon energy.

21

Figure 11. Peaks originating from plasmon excitations

4.2. Qualitative Analysis

The chemical composition of the sample can be identified using XPS. Elemental

analysis of the components can be carried out through identification of electron line/s

associated with the specific elements present in the sample. All elements can be identified

except for hydrogen, and that the elements that are close to each other in the periodic

table gives electron lines that are well separated. Some lines that originate from Auger

processes are also visible.

4.3 Quantitative analysis

The intensity of the electron line basically depends on the amount of material.

XPS has the possibility to quantify the amount of material present by estimating the

relative area under the curve of a particular peak specific to chemical specie that is of

interest. The intensity of the signal not only depends on the concentration of the elements,

but also on other factors such as the inelastic mean free path and absorption cross section

of different elements.

22

Spectra Quantification and Surface sensitivity

The intensity of the photoelectron line excited from a semi-infinite homogenous

specimen, with an atomically clean surface, is described by [Briggs, 1994]:

dxxknjdI ⎟⎠⎞

⎜⎝⎛ −

=θλ

σcos

exp (4.2)

where I is the intensity, n is the number of atoms per unit area of the element of interest, j

is the flux of X-ray photons, σ is the photoionization cross-section for a particular

transition, k is a factor characteristic to the instrument performance, λ is the inelastic

mean free path (IMFP), θ is the angle between the surface normal and the ejected

electron and x is the distance from the sample surface. The IMFP is defined as average

distance traveled by an electron between inelastic collisions [Powell, 1999]. IMFP is

dependent on the kinetic energy of the electron. The exact relationship between the IMFP

and kinetic energy depends on the detailed electronic structure of the element or

compound of interest. In Figure 12, IMFP is plotted as a function of kinetic energy where

the general features are similar for all elements. The curve is characterized by IMFPs for

lower kinetic energies, a minimum at 100 eV and an increase again toward high energy.

This curve is commonly called the “universal curve” [Seah and Dench, 1979].

igure 12. The mean free path of electron (λ) as a function of kinetic energy.

10 100 1000

100

50

10

0

Electron kinetic energy (eV)

Inel

east

ic m

ean

free

path

(Å)

10 100 1000

100

50

10

0

Electron kinetic energy (eV)

Inel

east

ic m

ean

free

path

(Å)

F

23

The exponential character of Eq. 4.2 reflects the decrease in integral intensity of

electron signal coming from the deeper layer. For sample with an overlayer of thickness,

d, the total intensity of the surface layer (s) and bulk layer (b) can be determined by

evaluating the integral of Eq. 4.2 from 0 to d and from d to infinity, respectively,

resulting to:

⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛ −−=

θλθλσ

cosexp1cos

sss

s dkjnI (4.3)

⎟⎟⎠

⎞⎜⎜⎝

⎛ −=

θλθλσ

cosexpcos

sbb

b dkjnI (4.4)

where λs and λb are the IMFP of the surface layer and the bulk, respectively. In practice,

tion (ni) of atom i, the total intensity for that

particu

intensity ratios are used instead of absolute intensity; thus, the energy independent parts

of the equations (jkcosθ) can be neglected.

To determine the relative concentra

lar atom, Ii is divided by the relative atomic sensitivity factor, Si:

i

iIn = (4.5) i S

iiiiiii TAyS λβσ= (4.6)

where σ = photoe

ent ( i.e. angle of the

mation of photoelectrons of the

m which photoelectrons can be detected,

ample, and

n of two photoelectron

lectric cross section for the particular transition,

β = angular efficiency factor for the instrumental arrangem

photon path and emitted photoelectron that is detected),

y = efficiency in the photoelectron process for for

normal photoelectron energy ,

A = area of the sample fro

T = efficiency of detection of the photoelectrons emerging from the s

λ = inelastic mean free path of the electrons in the sample

The equation below is used to estimate the relative concentratio

signals from a homogenous sample,

22

111 / SIn= (4.7)

2 / SIn

24

The atomic sensitivity values used to calculate the relative intensity ratios of elements

concerned in this work are based upon established cross-sections corrected for the kinetic

energy dependence of the spectrometer detection efficiency and an average value

dependence of λ on kinetic energy (Note: S values are either provided by the

manufacturer of the spectrometer or can be measured using reference samples).

Figure 13. Schematic illustration of XPS(θ) on an ordered monolayer of DOPA derived thiol molecules. Bulk mode in this XP spectra is measured at TOA = 20° while surface mode is measured at TOA = 80°. Enhancement of the surface sensitivity is exhibited by changing the electron TOA. In the surface sensitive mode, the electron intensity from the outermost part (e.g. hydroxyl groups) is enhanced relative to the deeper lying groups (e.g. amide group).

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5. Angle dependent X-ray Photoelectron Spectroscopy

Angle-dependent XPS, XPS(θ), is a non-destructive technique which can be used

to estimate the composition depth profile of samples [Tyler, 1989]. This type of

experiment defines the angle θ to be the angle between the surface normal and the

entrance to the electron energy analyzer, often referred to as the take-off angle (TOA) of

an electron. XPS(θ) is a powerful tool in studying molecular overlayer on semi-infinite

flat substrate. The basic principle producing surface enhancement for low electron exit

angles is illustrated in Figure 13. The intensity of the signal from the electrons coming

from the substrate is attenuated by inelastic scattering of electrons in the overlayer. The

signal from the substrate will be weaker because of the effective distance through which

unscattered electrons must travel in order to leave the sample surface at an angle such

that they can be collected in the analyzer situated at an angle from the surface normal. At

grazing electron incidence, the overlayer/substarte ratio, ns/nb, is predicted to increase

strongly as θ increases. XPS(θ) can be used to estimate the orientation of molecules on

the surface from well-ordered (but not necessarily crystalline) overlayers. The shadowing

effect, illustrated in Fig. 13, describes the situation for the organic adsorbates investigated

in this work. Enhancement of the intensity from the core levels of the outermost species

(near the vacuum interface) will then be observed when measuring in a surface sensitive

mode, i.e. θ is close to 90°. The signal coming from the hydroxl group attached to the

phenyl ring in DOPA-PT adsorbates is observed to be enhanced as compared to the

amide group, as exemplified in the O (1s) XPS spectra in Fig. 13.

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6. References

Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers: The Scienta ESCA300 Database: John Wiley and Sons, New York, 1992. Briggs, D.; Seah, M. P. Practical Surface Analysis Vol. 1. Auger and X-ray Photoelectron spectroscopy (2nd edition): John Wiley and Sons, Chichester, England; 1994. Hagstrom, S.; Nordling, C.; Siegbahn, K. Zeitschrift fur Fysik 1964, 178, 439. Nordling, C.; Sokolowski, E.; Siegbahn, K. Phys. Rev. 1957, 105, 1976. Powell, C. J.; Jablonski, A.; Tilinin, I. S.; Tanuma, S.; Penn, D. R. J. Elect. Spectrosc. 1999, 98–99, 1. Robinson H.; Rawlinson, W. F. Phil. Mag. 1914, 28, 277. Salaneck, W. R.; Bergman, R.; Sundgren, J. E.; Rockett, A.; Greene, J. E. Surf. Sci. 1988, 198, 461. Seah, M. P. Surf. Interf. Anal. 1989, 14, 488.

Seah, M. P.; Dench, W. A. Surf. Interf. Anal. 1979, 1, 2. Siegbahn, K.; Nordling, C. N.; Fahlman, A.; Norberg, R.; Hamrin, K.; Hedman, J.; Johansson, G.; Karlsson, S. E.; Lindgre, I.; Lindberg, B. ESCA: Atomic, molecular and Solid State Structure by Means of Electron Spectroscopy: Almqvist and Wiksells, Sweden: 1967. Sokolowski, E.; Nordling, C.; Siegbahn, K. Ark. Fys. 1957, 12, 301. Tyler, B. J.; Castner, D. G.; Ratner, B. D. J. Vac. Sci. Technol. A 1989, 7, 1646. Uvdal, K. Linköping Studies in Science and Technology Dissertation No. 248, 1991.

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APPENDIX: Handling of XP spectra obtained from XPS measurement To download the spectra from XPS PC to diskette:

1. Always be reminded to save the measured spectra first. 2. Exit the VGX900 program (CTRL F10). 3. Look for your file in directory C:/VGX900/ and save it to your floppy disk

(C:\VGX900\copy filename.* a: ) 4. Copy also the file vg2kal.exe to directory A use to ”unzip” the saved file to

different files containing different XPS regions measured. (C:\VGX900\copy vg2kal.exe a: ) 5. Unzip the file in diskette. ( A:\vg2kal filename.* filename2 ).You will get several

files corresponding to the different XPS regions you measured. The filename of the unzipped files will be: filename2.1, filename2.2, ... , filename2.n, where n is the number of regions measured.

6. To go back to the window/screen used in XPS measurement, just type: C:\VGX900\vgx900

To upload the XPS spectra to the XPSPEAK program for curve-fitting:

1. First, copy your files downloaded in the diskette to directory C:\TB2006/Group#, where # is your group number.

2. Double click the XPSPEAK41 icon on the desktop then click on the DATA menu. Import your data by clicking the Import (ASCII) and choose the desired file you wanted to do curve fitting. You normally will first choose the first file (filename2.1) and set it as your first region (see the 2nd open window) to do curve fitting.

3. To import a file to the next region, click on the region number first and then import it by clicking again Import (ASCII) and choose the desired file you wanted to do curve fitting.

4. Now you are ready to do curve fitting. Curve-fitting using XPSPEAK41 program:

1. Draw a background to your desired spectrum. Click the Background menu and choose a linear or shirley type of background. You can decide which end-points to use to set the background. Click Accept and close after drawing the background.

2. Add peak/s you needed to fit the best curve. Vary the peak position, FWHM and Area that would satisfy the best fit curve. Note: The FWHM of S (2p) peaks should have FWHM ≤ 1.5 eV while the rest (Au 4f, C 1s, O 1s, N 1s, etc.) could have FWHM ≤ 2.0 eV. As much as possible, the FWHM of the peaks fitted in a certain region should be about the same.

3. You have the option to optimize/fix the peak’s position, FWHM or Area (or all of the parameters at the same time) provided that the peaks fitted have followed the necessary right conditions (eg. Ratio of area and distances of spin-orbit split peak, %Lorentzian-Gaussian of the peak is fixed, etc.).

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4. Save the XPS spectra with the fitted peaks having ”.xps” file extension (eg. samplename.xps). The saved file will contain all the regions with curve fitted peaks.

Viewing the curve-fitted spectra in KALEIDAGRAPH:

1. After peak-fitting the spectrum for a desired region, click on Export in the DATA menu to convert the ASCII file to data file. (filename2.1 filename21.dat)

2. Open the data file on notepad and replace all ”,” to ”.”. (Note: if only necessary) 3. Double click the KALEIDAGRAPH icon and open the data file you want to plot. 4. In the Gallery menu, click linear then line. Then click the button in column X for

the Binding energy and the buttons in column Y for the rest of the remaining buttons except for the difference.

5. You can modify the plot by clicking the plot style or axis options on the Plot menu. You are free to change the type and size of the

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