Vij D.R. Handbook of Applied Solid State Spectroscopy

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11. X-Ray Photoelectron Spectroscopy

The linewidths of the XP spectral peaks, ǻE, is generally on the order of 1 eV

and is defined at half maximum and may be roughly approximated by

equation (11.3) [8].

222

apn

EEEE ''' ' . (11.3)

Here, ǻE

of the X-ray source,

and

ǻE

is the energy resolution of the electron analyzer.

Invoking the

uncertainty

principle, the ǻE

may be defined as shown in

equation (11.4) [8].

4.1 10



(in eV). (11.4)

left behind

by the ejected photoelectron. The lifetime of the hole state is

determined

by the rate for core-hole elimination as illustrated in Figure 11.10.

Figure 11.10 (a) Schematic representations of the photoelectric process, (b) elimination of

core-hole through X-ray fluorescence, and (c) elimination of core-hole through ejection of an

Auger electron.

15

s [8].

11.5.3 Elemental Analysis: Qualitative and Quantitative

Generally, qualitative elemental identification is readily achieved by

comparing the peak binding energies to tabulated values. Numerous

sources of tabulated XPS binding energies are available both in text and on-

line [11, 27]. In many cases elemental identification can be accomplished with

11.5.2 Linewidths

500

is the natural linewidth, ǻE is the linewidth

Here, h is Planck’s constant and Ĳ is the lifetime of the hole state

depicted in Figure 11.10c occurs

The emission of the Auger electron through the Coster-Konig process as

within approximately

11.5 Spectral Analysis

The relative intensities of the spectral peaks, typically obtained by

integrating peak areas from curve-fitting, are directly related to the relative

amount of the corresponding atom in the sample. This direct relationship

allows for facile quantification of different types of atoms after background

elimination [8]. In the simplest case of a homogenous solid, the concentration

of any chosen atom A may be found using equation (11.5) [8].

§·

¨¸

©¹

(11.5)

A A

spectrometer to A. The sensitivity of a spectrometer varies for different atoms

atoms found in the XPS survey.

Li B N F Na Al P Cl K Sc V M Co Cu G As Br Rb Y Nb Tc Ag In Sb IRh Cs La Pr P Eu Tb Ho T Lu Ta Re Ir Au Tl Bi

C O Ne M Si S Ar Ca

Cr Fe Ni Zn G

Kr Sr Zr M Ru Pd Cd Sn

Xe Pb

PtOsWHfYbErBa Ce Nd S G Dy

Elemental Symbol

Relative Sensitivity

Figure 11.11 Relative sensitivity for different elements in XPS. Trends may vary depending on

the spectrometer. Adapted from data in [11].

11.5.4 Secondary Structure

Exchange splitting, also referred to as electrostatic splitting, occurs when

electrons in the valence levels are unpaired [8]. In the simplest case of an s

orbital, the unpaired electron in the valence level interacts with the unpaired

electron left in the core level following photoemission. Thus, the final state of

photoemission, which determines the kinetic energy that will be measured,

the acquisition of survey spectra. For chemical state analysis, high resolution

spectra are acquired and fit.

501

Here C is the concentration of A in the volume sampled, I is the intensity

of the spectra l line attributed to atom A, and S is the sensitivity of the

as illustrated in Figure 11.11. The I and S notations are used to represent all

11. X-Ray Photoelectron Spectroscopy

depends on whether the spins of the aforementioned unpaired electrons are

parallel or antiparallel. This exchange interaction results in splitting of the

photoelectron peak into a doublet separated by the exchange energy. The

exchange energy is defined as the difference between the binding energy of

the photoelectron and the ground state orbital energy in the final state

configuration. The exchange energy may be written as shown in

Equation 11.6 [8].

CO,VO

(2 1) .ESK  (11.6)

Here S represents the sum of the unpaired electron spin and K

CO,VO

denotes

the exchange integral of the core orbital and the valence orbital. When dealing

with non s-orbitals, angular momentum coupling in addition to the exchange

splitting causes the photoelectron peak to be split into multiplets.

Furthermore, the exchange splitting is most dominant when the valence level

and the core level from which the photoelectron originates are of the same

principal quantum number n [8].

“Shake-up lines” are important secondary spectral structures. Following

photoemission, the valence level electrons experience a greater effective

nuclear charge as compared to prior photoemission [28]. The change in the

nuclear attraction causes valence electron relaxation and reorganization,

which may cause valence electrons to be excited to a higher unfilled orbital,

resulting in an overall electronically excited system as its final state

configuration. In this case the ejected photoelectron experiences a reduction

of its kinetic energy to compensate for the excited final state. The

photoelectrons that undergo the kinetic energy reduction as mentioned above

are recorded as shake-up lines in XPS, appearing a few eV above its

characteristic peak on the binding energy scale. The shake-up lines may be

used in differentiating atoms of the same element in various oxidation states.

These lines are particularly strong and therefore valuable with highly

conjugated organic species and certain transition and rare earth metals [8]. An

example of shake-up satellite lines is shown in Figure 11.12.

11.6 XPS IMAGING

Imaging or mapping surface composition in two dimensions is common to

many surface analysis methods such as Auger electron spectroscopy and

secondary ion mass spectrometry. On the other hand, XPS has long suffered

difficulties in constructing and utilizing X-ray optics. However, significant

decades [14, 30–33]. Two different methods have been employed to produce

spatially resolved images [1]. The first involves focusing the X-ray source to

the smallest possible spot size. With this method, images are collected in one

of two ways, either by scanning the X-ray beam across the sample surface or

502

from poor lateral resolution relative to these other techniques because of

advances in spatially-resolved XPS have occurred over the last several

moving the sample under a fixed position X-ray beam. The second method to

achieve spatially resolved images utilizes position-sensitive electron

detection. Generally, the best spatial resolution, which is on the order of a few

Pm, is achieved with this latter method. The XPS imaging technique provides

information for facile understanding of surface species and morphology.

Figure 11.13 is an XPS image of a gold substrate upon which copper has been

deposited in a spatially controlled fashion [34].

Figure 11.12 XP spectrum of C 1s region for benzoperylene-(1,2)-dicarboxylic acid anhydride,

an aromatic organic molecule [29]. Carbon peaks are fitted with dotted lines and the shake-up

satellite peaks are fitted with solid lines. Reused with permission from A. Schöll, Y. Zou, M.

Jung, Th. Schmidt, R. Fink, and E. Umbach, (2004). Journal of Chemical Physics, 121, 10,

Figure 11.13 XPS image showing a Au substrate on which Cu has been deposited with a

potential gradient method. The region labeled (a) is high in Au content and that labeled (b) is

high in Cu [34]. Reprinted with permission from K.M. Balss, B.D. Coleman, C.H. Lansford,

American Chemical Society.

11.6 XPS Imaging

503

11. X-Ray Photoelectron Spectroscopy

Angle-resolved XPS offers the ability to tune the surface specificity of the

XPS signal over the top 0–10 nm of the sample by varying the emission angle

of the ejected photoelectrons. Note that the emission angle is measured with

respect to the surface normal—differentiating it from the take-off angle,

which is measured with respect to the surface plane. As the emission angle T

is altered the escape depth of photoelectrons changes as described in

equation (11.7), where O

is the inelastic mean free path for electrons in the

material [35].

depth cos O T (11.7)

Therefore, the least surface-specific information is obtained when the

emission angle for the photoelectron escape is along the surface normal,



= 0̓. This orientation maximizes the depth from which photoelectrons are

able to escape from the sample and be collected by the electron optics while

retaining their characteristic energies. Changing the emission angle toward the

surface as illustrated in Figure 11.14 reduces the absolute depth along the

surface normal from which electrons can escape with their characteristic

energies. A change from 0q to 75q results in an approximately four-fold

reduction in the probe depth along the surface normal. Recording and

analyzing spectra as a function of depth into the sample provides a glimpse of

the near-surface compositional variation without creating the need to

destructively remove material from the surface [16]. However, care must be

taken with data interpretation for several reasons [35]. First, particularly in

layered materials, O

is dependent on the identity of the material through

which the photoelectrons pass. This phenomenon results in deviations from

the simple cos T dependence predicted by equation (11.7). Additionally, the

use of grazing angles—high emission angles—increases photoelectron energy

lost to surface plasmon bands. Finally, the high surface specificity at low

collection angle can reduce the expected width for peaks due to a decrease in

the coordination number for the surface atoms [35].

Angle-resolved XPS allows for depth-profiling of many samples and has

been used extensively for the analysis of self-assembled monolayers on

various substrates [36–38]. Additionally, angle-resolved XPS combined with

XPS imaging enables the acquisition of three-dimensional information from

the sample. This method is referred to as “3D XPS”.

11.8 RECENT ADVANCES AND APPLICATIONS

A wide variety of advancements in XPS instrumentation and application has

taken place recently. Many of the instrumental advances have focused on the

aforementioned improvements afforded by the use of synchrotron radiation.

The spatial resolution attainable with XPS imaging continues to improve as

11.7 ANGLE-RESOLVED XPS

504

11.8 Recent Advances And Applications

the influence of

XPS.

Two additional areas in which advancement and novel

application have

been made are fast or time-resolved XPS and high pressure

XPS, as described

below.

The ability to monitor chemical reactions is incredibly important to the

in studying catalysts, but recent efforts toward real-time monitoring of surface

reactions have changed that role [39, 40]. The advent of third generation

synchrotron X-ray sources and advances in high speed electronics have

helped reduce the time needed for acquisition of high resolution spectra from

the minute to second time range. Now reactions such as molecular adsorption

and desorption [40], the trimerization of ethylene to form benzene [41], and

the epoxidation of alkenes can be monitored in real time [42]. Although the

temporal resolution for fast XPS does not rival that of other analytical

methods, the type of information XPS provides—namely, chemical state

information—is very useful in these surface reaction studies. A new method

using the application of a pulsed voltage to the sample may enable the study

of millisecond timescales for specific applications [43].

Figure 11.14 Illustration of the variation in photoelectron escape depth as a function of

emission angle.

back decades [44–46]. Recently, however, researchers presented a new

system for XPS analysis in environments up to the mbar pressure range [47].

The system relies on a combination of differential pumping and electrostatic

focusing of the photoelectrons. The authors speculate that this method may

find extension up to a pressure near 100 mbar. This high pressure XPS system

was recently used to investigate the enhancement of halide concentrations at

the liquid/vapor interface [48]. Use of this technique may advance the

understanding of the interaction of water with solid interfaces.

X-ray and electron

optics,

analyzers, and detectors will likely serve to broaden

505

described previously. Continued efforts on engineering

High pressure systems for XPS studies have an extensive history, dating

understanding and designing of catalysts; historically, XPS has played a role

11. X-Ray Photoelectron Spectroscopy

11.9 CONCLUSIONS

Since its proliferation as an analysis method, XPS has been applied to study

medical engineering and semiconductor processing to archeology. The

main strength of XPS lies in its ability not to simply identify elements but also

to provide information regarding chemical environments, oxidation states, and

morphology. Combine this wealth of materials information with a rapidly

progressing ability to spatially map composition across the surface plane and

normal to the plane and XPS promises to retain its place as a vital

spectroscopic tool for those working in a wide range of fields in both industry

and academia.

ACKNOWLEDGMENTS

The authors thank Wesley R. Nieveen, William F. Coleman, John F. Moulder,

and Craig M. Teague for their assistance in review of the manuscript. The

authors also thank Lin Zhu for assistance with preparation of figures and

Richard T. Haasch for the use of Figures 11.3, 11.8, and 11.11.

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507

CHAPTER 12

LUMINESCENCE SPECTROSCOPY

Baldassare Di Bartolo and

John Collins

Department of Physics, Boston College, Chestnut Hill, MA 02467, USA

Department of Physics, Wheaton College, Norton, MA 02766, USA

12.1 INTRODUCTION

12.1.1 Basic Concepts

A material, after absorbing some energy, can become a source of light by two

processes:

1. The absorbed energy is converted into low quantum energy heat that

diffuses through the material, which then emits thermal radiation.

2. The absorbed energy is in part localized as high quantum energy of

atoms, which then emits radiation called luminescence radiation.

We note the following:

1. The quantity and quality of thermal radiation depend primarily on the

temperature rather than on the nature of the body.

2. The quality and quantity of luminescence radiation are strongly

dependent on the nature of the emitting material.

Therefore, the results of investigations in the field of luminescence are

relevant to the study of material bodies. The process of luminescence is started

by exciting the material with UV radiation, X-rays, electrons, alpha particles,

electric fields, or energy that is liberated in chemical reactions. Accordingly,

luminescence is qualified as photoluminescence, Roentgen luminescence,

electroluminescence, and so on. We shall limit ourselves to examining the

process of photoluminescence in solids, i.e., the luminescence emission that

follows the absorption of UV or optical radiation.

We have found it convenient from a pedagogical point of view to subdivide

the luminescent systems into two categories, localized and delocalized. For the

first category, the absorption and emission processes are associated with

quantum states of optically active centers that are spatially localized to particular

12. Luminescence Spectroscopy

510

sites in the solid. For the second category, these processes are associated with

quantum states of the entire solid.

Regardless of the type of excitation or the nature of the system, the common

character of the phenomenon of luminescence resides in the fact that, once

excited, the system is left alone, without any external influence and, in this

condition, it emits radiation that we may appropriately call spontaneous.

The purpose of this chapter is to describe the basic themes that motivate the

investigations in luminescence spectroscopy, the techniques that are used to

conduct these investigations and the challenges that are presented by this field of

research.

In the introduction we consider the nature of luminescence research and

emphasize its material-related character. We also recount briefly the history of

the research in this field. In Section 12.2, we set up the theoretical bases for the

treatment of the interaction of radiation with material systems. In Section 12.3,

we consider the measurements that are conducted in lumine-scence spectroscopy

and examine their simplest and fundamental aspects, apart from their more

advanced technical features. We dedicate the next two Sections, 12.4 and 12.5,

to the treatment of localized luminescent systems in which the radiative

processes are associated with optically active centers. In the following two

processes are associated with quantum states of the entire solid. Finally, in

Section 12.8 we consider the challenges presented by this field of research,

suggesting directions of future efforts and recognizing the fact that luminescence

spectroscopy is an inexhaustible source of problems for future investigations and

educational applications.

12.1.2 History

The first reported observation of luminescent light from glowworms and fireflies

is in the Chinese book Shih-Ching or “Book of Poems” (1200–1100 B.C.).

Aristotle (384–322 B.C.) reported the observation of light from decaying fish.

The first inquiry into luminescence dates circa. 1603 and was made by Vincenzo

Cascariolo. Cascariolo’s interests were other than scientific: He wanted to find

the so-called “philosopher’s stone” that would convert any metal into gold. He

found some silvery white stones (barite) on Mount Paderno, near Bologna,

which, when pulverized, heated with coal, and cooled, showed a purple-blue

glow at night. This process amounted to reduction of barium sulfate to give a

weakly luminescent barium sulfide:

BaSO +2C BaS+2CO

News of this material, the “Bologna stone,” and some samples of it reached

Galileo, who passed them to Giulio Cesare Lagalla. Lagalla called it “lapis

solaris” and wrote about it in his book De phenomenis in orbe lunae (1612).

Sections, 12.6 and 12.7, we treat the delocalized systems in which the radiative