6 X-ray Photoelectron Spectroscopy – 2

Dr. Ajit K. Mahapatro

Learning Objectives

From this module students may get to know about the following:

i. Peak identification

ii. Chemical shift and quantitative analysis of X-ray Photoelectron Spectroscopy (XPS).

iii. Depth profile of XPS spectrum.

1. Introduction

X-ray Photoelectron Spectroscopy (XPS) counts number of electrons ejected from the materials when X- ray is irradiated onto the surface. The XPS spectrum represents the number of electrons recorded in certain range of energies. XPS is a powerful tool for chemical analysis at the surface of the specimen materials. It identifies the presence of elements in the layer within several nanometers from the surface. Importantly, the positions of peaks in the XPS spectra are sensitive to their chemical compositions. For example, carbon peak positions in carbon dioxide (CO2) are different from those in saturated hydrocarbons. This phenomenon, is represented as chemical shift in XPS spectra and provides extra information for chemical analysis. Also, XPS instruments are able to generate depth profiles of the chemistry of the surface layer through the bulk materials and reveal the spatial distributions of elements on the surface and inside layers of the bulk materials. The XPS spectra are quantified in terms of peak positions and peak intensities. Peak position indicates the elemental and chemical composition, and peak intensities measures quantity of materials at the surface. Other values such as full width half maxima (FWHM) are useful indicators of chemical state changes and physical influence. This module focuses on the qualitative and quantitative analysis of XPS spectrum.

2. Spectrum Analysis

The XPS spectra is analyzed by identifying the peaks and locating the shift in peak positions due to presence of elemental compositions.

Peak identification

The binding energies of various core level electrons for peak identification in XPS spectra are tabulated in the reference section of this module. These energies are the primary source for identifying the presence of elemental components. Peak identifications in XPS spectra, however, are more complicated because Auger peaks may be present. We use two different X-ray sources to distinguish the Auger peaks from photoelectron peaks. An Auger peak represents the kinetic energy of an Auger electron that changes with energy of primary X-rays. Thus, an Auger peak will shift in apparent binding energy in an XPS spectrum when we change the X-ray source. For example, an Auger peak shifts by 233 eV in the XPS spectrum when we change the radiation from Mg Kα (1253.6 eV) to Al Kα (1486.6 eV).

Peak positions in an XPS spectrum are likely to be affected by spectrometer conditions and the sample surface. Before XPS peak identification, we need to calibrate the binding energy. Calibration is particularly important for samples with poor electrical conductivity. Calibration can be done with an internal standard that has a peak showing little or no chemical shift (Viz., elemental Si). The most common method is use of the C1S peak at 285 eV appearing from the carbon adsorbed on the surface of the sample. The carbon from organic debris (as C- H or C-C) in air is found on all samples exposed to the environment. The peaks of core level binding energies as listed in Table in the reference section attached with this module are sufficiently unique for element identification. The binding energy increases with the degree of oxidation. The binding energy of aluminum atom is different in different chemical environment. The binding energy for pure aluminum metal is 72-73 eV and for Al oxide is 74 -75 eV [1-2].

Chemical Shifts

Chemical shifts of binding energy peaks for an element are caused by the surrounding chemical states of the element. Knowledge of the possible chemical shifts is necessary to identify the peaks accurately. We can use the features of the chemical shifts to identify the elements and chemical compounds. For example, Figure 1 shows the XPS spectrum of poly-vinyl trifluoroacetate (PVTFA). The carbon atoms in different environments generate distinctive peaks as shown in Figure 1a. Similarly, oxygen atoms in two different environments also generate two peaks as shown in Figure 1b. The XPS spectra shown in Figure 1 with defined peak positions and relative peak intensities can also be used as a fingerprint to identify PVTFA. The small shifts in binding energy is caused peak overlap as shown in Figure 1 b. Hence, we might need to carefully resolve the overlapped peaks with assistance from other computer software.

Figure 1: XPS spectrum of PVTFA. (a) C_1S and (b) O_1S spectra [1]
 

The chemical shifts of the C1S peak are summarized in Figure 2. The range of the C1S shift extends over 12 eV. Quantifying the amount of chemical shifts in the spectrum can identify different carbon bonds. The binding energy difference between C – C and C – O is about 3 eV. This is extremely useful for identifying polymers by revealing various carbon bonds. Also, the chemical shifts reveal the degree of oxidation in molecular solids. Higher chemical shifts occur for larger number of electrons transferred. Chemical shifts decrease with increasing atomic number and suggests less possibility in metallic and semiconducting elements. For example, the shift range for Si2p is less than 6 eV. Chemical shifts also occur in AES spectra, and the chemical shifts can be significantly larger than the shifts in XPS. For example,  the  shift  between  metallic  and  oxide  Al  peaks Al 2,3 (AES)  is  more  than  5  eV  and  the corresponding shift for Al2p (XPS) binding energy is only about 1 eV [1, 3-4 ].

Figure 2: Chart of carbon chemical shifts in XPS spectra [Materials Characterization Book, Yang Leng, 2008 John Wiley and Sons (Asia) Pvt. Ltd.].

Issues with Insulating Materials

The peak positions in XPS spectra for insulating materials do not agree well with the standards provided in handbooks. To understand this phenomenon, let us start with the basic equation for binding energy calculation, as

EK = hν – EB – Ф,

where, ‘Ф’ is the work function of the material and represents the energy required for an electron to escape from the surface of the material, ‘h’ is the Planck’s constant, and ν is the frequency. When a conducting material is electrically contacted to a spectrometer, in Equation above, a work function of the spectrometer and its value is constant. However, for an insulating sample, it is not certain and is related to the surface potential of the material. Photoemission from an insulating material builds-up positive charges on the surface of the sample. This build-up changes the surface potential of the material during the XPS data aquisition. In insulating materials, the incident electrons cannot be removed by electrical conduction, leading to charge the surface negatively in AES. Change of surface potential makes the kinetic energy or binding energy measured tens of eV different from the standard value mentioned in published table.

Figure 3: The compensation of electron loss due to photoelectron escape using low-energy electron flux from a flood gun.

The charge neutralization method is used to resolve the surface potential issues, by providing stable and uniform surface potential on solids. In XPS, the surface of the material is provided with flux of low- energy electrons using electron gun as shown in Figure 3. The electron flux compensates charge loss due to photoelectrons that are escaping from the surface. The flux helps maintaining stable surface potential. Performing XPS in insulating materials, the surface charge issue is prominent with monochromatic compared with non-monochromatic X-ray radiation, because the non-monochromatic X-rays illuminate the window of the X-ray gun as well as other surfaces inside the vacuum chamber, producing sufficient secondary electron flux to neutralize the positively charged surface of the material. Hence, uses’ of non- monochromatic X-rays has  the identical effect as using an electron gun.

3. Quantitative Analysis

3.1 Peaks and sensitivity factors

Quantitative elemental analysis in X-ray spectroscopy is identical to the electron spectroscopy. This quantifies the concentrations of chemical elements of the sample, measured from the peak intensities of the spectra. Most of the parameters required for quantification of the atomic fractions of elements from the intensities of electron signals in both XPS and AES, are not known, and the following empirical equation is generally used.

Xi = I_i/S_i / ∑ I_j/S_j                                                                                 ( 1 )

Where, Xi is the atomic fraction of a surface element, Ii is the peak intensity, and Si is the sensitivity factor. The Si for each element is experimentally determined and varies with conditions of the instrument and sample surface. It is desirable to measure the sensitivity factor for a given spectrometer and for a specific sample type. Generally, the sensitivity factors are used from tables in handbooks with certain corrections according to the instrument characteristics, due to it’s difficulty in compiling all the sensitivity factors. The quantitative calculations using Equation (1) has assumptions and approximations. The intensities of XPS peaks can be calculated from the peak areas after nullifying the background. Sensitivity factors are varied for different materials and for considering developments in instruments. For example, The chemical contamination on the surface of the sample affects the accuracy for Equation (1).

4. Composition depth profile:

The compositional change with position in the material can be understood by measuring the depth profile XPS starting from the surface plane. XPS is a surface sensitive technique, a depth profile XPS for materials can be recorded by combining a sequence of ion-gun etching steps with XPS measurements starting from the top surface. An ion gun is used to etch the material for a period of time and XPS spectra are acquired. After each ion gun etch, the material exposes a new surface and the XPS spectra is recorded and provides elemental details for analyzing the composition of these newly etched surfaces. Generally, the depth profile of composition is determined using sputtered depth profiling using an ion-gun. The ion gun provides a flux of positively charged argon ions with current density of 1–50 μA mm−2 and energy of 0.5–5 keV. The ion beam bombards a sample, causing it to eject atoms from the surface. The schematics for the depth profile experiments are sketched in Figure 4. The material is dug for different depths of d1, d2, d3 and d4 as in Figure 4.

A set of XPS spectra corresponding to the O1S peaks of TiO2 is displayed in Figure 5. The objective of the experiments is to observe the trend in the elemental quantification values as a function of etch-time. The actual depth for each XPS analysis is dependent on the etch-rate of the material being etched by ion-gun at any given depth. For example, the data in Figure 5 derives from a multilayer sample consisting of alternating layers of silicon oxide and titanium oxide on top of a silicon substrate. In another example, Figure 6(a, b, c) provides information of the XPS spectra of C1s at various depth positions of d1, d2 and d3 (where, d1 < d2 < d3) performed for a graphene oxide (GO) coated indium tin oxide (ITO) layers (GO/ITO). Here, the appearance of peaks at binding energy (BE) ~ 284.8 eV is assigned to C1s core level. In Addition, at the GO surface (d1), all the structures show a hump at ~286.8 eV and 288.6eV that corresponds to C-O and C=O, respectively. The intensities of the C1s peak decreases and peak corresponds to the C-O vanishes as digging depths are increased to d2 and d3 within the GO layers. In figure 6(d, e, f) the peak at BE of ~530.1 eV is assigned to the O1s core level of the oxygen for all the structures [47]. At the surface (d1) of GO/ITO a hump is observed at BE of ~532.3 eV indicates the presence of C-O bonding. A prominent increase in the intensity of O1s peak and diminishing C-O peak is observed with further removal of oxygen, from the GO surface layers (d1) towards the GO/ITO (d3).

Figure 4: Schematic of an XPS depth profile.

Figure 5: The set of O1S spectra measured during the depth profile experiment [www.casaxps.com].

Figure 6: XPS of graphene oxide on ITO substrate with different depth (a) C_1S   spectra with d1 (b) with d₂ (c) d3 (e) O_1S   spectra with _d1 (f) d₂ (g) d₃

Quantitative analysis of a depth profile, in addition to quantify concentrations, requires accurate measurements. There are several factors for resolution of issues raised in depth measurements. First, the etching rate for each type of atom is not constant even though the ion beam energy and ion density are exactly the same. The consequence of this phenomenon is that the preferential etching occurs in samples containing multiple elements. In samples comprising several elements which are not uniformly distributed, certain areas are etched faster than other areas because of preferential etching. This results a rough crater surface, generated with increasing depth of etching [1, 5]. The depth scale of the profile is based on the average etching rates. Thus, the measured concentration of elements with higher etching rates (higher sputter yield) will be higher than the real concentration. Second, the incident ions can smear the interface in the depth profiles. Figure 7 schematically illustrates the interface smearing during depth profiling. This is also the case shown in Figure 8, in which the smooth interface between the coating and titanium substrate becomes diffuse in the profile. The main reason for interface smearing is that the incident ions produce coating of atoms and/or substrate atoms to cross the interface. The atom displacements caused by the ion bombardment could be understood, considering the energy brought into the sample by the ions.

Figure 7: Smearing of the interface in a depth profile due to ion bombardment.

Figure 8: XPS Depth profile of a titanium surface coated with calcium phosphate which contains Ca, P, and O. C in the profile is from surface contamination [Materials Characterization Book, Yang Leng, 2008 John Wiley and Sons (Asia) Pvt. Ltd.].

5. Summary:

1. The introduction and, qualitative analysis of XPS spectrum has been discussed.

2. The  peak  identification,  chemical  shift  in  XPS  spectrum  and  issue  with insulating materials are discussed with schematic diagram.

3. The quantitative analysis of XPS and sensitivity calculation has been discussed.

4. The possibility of the depth profile in XPS samples has been studied.

References:

1.Materials Characterization Book, Yang Leng, 2008 John Wiley and Sons (Asia) Pte Ltd.

2.C. J. Corcoran, H. Tavassol, M. A. Rigsby, P. S. Bagus, and A. Wieckowski, Journal of Power Sources 195 (2010) 7856-7879.

3.P.K. Ghosh, Introduction to photoelectron spectroscopy , Wiley, 1983

4.J. Chastain, Handbook of X-ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra for Identification and Interpretation of XPS data, Perkin Elmer Corporation, 1992.