Advanced Lab Course X-Ray Photoelectron Spectroscopy M210 As of: 2015-04-01 Aim: Chemical analysis of surfaces. Content 1 INTRODUCTION 1 2 BASICS 1 3 EXPERIMENT 3 3.1 Qualitative analysis 6 3.2 Chemical Shifts 7 3.3 Quantitative Analysis 7 4 TEST PROCEEDING AND EVALUATION 9 4.1 Test 9 5 LITERATURE 9
1 Introduction Today X-Ray photoelectron spectroscopy is the most common method for chemical analysis of surfaces. It was developed in the research group of k. Siegbahn (Nobel Prize 1981). Siegbahn named his new method ESCA (Electron Spectroscopy Chemical analysis) and used it mostly for the chemical analysis of gas molecules. With the development of ultra-high vacuum (UHV) technology in the late sixties XPS or ESCA became suitable for the analysis of solid surfaces and commercial instruments became available. 2 Basics When a solid surface is irradiated with UV light or X-rays electrons are emitted due to the photoelectric effect. The energy spectrum of these electrons yields information about the initial state of those electrons and therefore about the chemical composition of the irradiated solid. This process is shown in Figure 1. The electron is excited by photon with the energy hv. If the excitation energy is sufficient, the electron is ejected from the atom with a well-defined kinetic energy E kin (external photoelectric effect). The atom is left in an excited, ionized state with a hole in its electron shell. From the kinetic energy of the photoelectron we can calculate the binding energy E B of the electron with respect to the vacuum level: E B = hv E kin 1 Usually electrons from the inner shell of an atom (core electrons) are evaluated in an XPS experiment, because the spectra of valence electrons are too complicated to retrieve chemical information. Following Koopmans Theorem the binding energy equals the negative orbital energy ε K of the electrons prior to emission: E B = ε K 2 1
The theorem is only a estimation, because it is based on a single electron approximation. It assumes that the ionized atom does not relax. This assumption is not justified: The remaining electrons now have a lower potential energy because the positive charge of the nucleus is shielded less efficiently. The excess energy is transferred to the emitted electrons. In addition relative δε rel and correlation effects δε korr due to electron-electron interaction have to be evaluated. For the binding energy we calculate: E B = - ε K - δε relax + δε rel + δε Korr 3 Besides the photoelectron peaks we find the peaks in the electron energy spectra which are due to the Auger effect, which is illustrated in Figure 2. During the Auger process an electron from an outer shell fills the hole produced by an earlier XPS process and transfers the energy difference to another electron, which is emitted from the atom. Therefore the auger process results in a doubly ionized atom. The kinetic energy of the auger electron does other than the energy of the photoelectron not depend on the energy of the absorbed photon but only on energy differences between the involved electrons. The kinetic energy is approximately: E kin = ( E 3 - E 1 ) E 4 After the emission the electrons have to travel through the solid until they are emitted from surface. On their way to the surface the electrons can suffer energy losses e.g due to inelastic scattering. Those scattered electrons do not contribute to the photo electrons peaks but to the signal background. For very small kinetic energies (10 100 ev) the background increases dramatically due to secondary electrons. These are electrons which have suffered multiple energy loss. 2
3 Experiment In an X-ray photoemission experiment the sample is irradiated with soft X-rays of certain energy. These x-rays are generated by bombarding a water cooled anode with 10-15 kev electrons. This generates characteristic x-rays depending on the anode material and a background due to Bremsstrahlung. Typical anode materials are Magnesium or Sodium, because the energy width of the characteristic x-ray lines are small for light elements. The photon energies are Mg K α1,2 hv = 1253.6 ev Al K α1,2 hv = 1486.6 ev The energy width for these lines are 0.7 ev and 0.85 ev respectively. These lines width allow sufficient energy resolution for most applications, even without x-ray monochromator. 3
Beside the main K α1,2 x-rays line we also find less intensive lines e.g. K α 3.4 - lines, which cause the so called satellite peaks in the photoelectron spectra. Those satellite peaks have intensities of up to 10% and are usually shifted by 10ev to lower energies. The kinetic energy E kin of the emitted electrons is measured with a photoelectron spectrometer and depends on the binding energy of the electrons as follows: E kin = h.v E B S W With: hv = x-ray energy E B = Binding energy of emitted electron S = correction for charged samples (S = 0 for conducting, grounded samples) W = work function of the spectrometer. Usually the samples are electrically connected to the spectrometer to balance the charge of the emitted electrons and to adjust the two Fermi levels. Therefore the binding energies are usually referenced to Fermi level: E B F = hv E Kin W spcetrometer The work function W of the spectrometer has to be calibrated with metallic samples where the binding energies are known ( e.g Cu, Ag, Au) To obtain a spectrum of the emitted electrons an interval de has to be set around the energy E where electrons are detected and E is scanned over the range of interest. The energy filter here is hemispherical capacitor or hemispherical electron energy analyzer. Two electrodes which are formed as concentric hemispheres form an electrostatic field which can only be passed by electrons with the so called pass energy E pass. Slits at the entrance and exit of the analyzer define the energy resolution de. In addition we have an electrostatic electron lens at the analyzer entrance which can decelerate the electrons before they enter the analyzer and define the area on the samples from where the electrons are collected. This combination allows for two different modes to acquire spectra: 4
FAT (fixed analyzer transmission), the analyzer transmission is fixed and the deceleration by the entrance lens is scanned over the energy range of interest. For this mode the energy resolution is also fixed over the entire spectrum. FRR (fixed retarding ratio), pass energy and deceleration are scanned to obtain a spectrum. Here ΔE / E is constant. Usually photoelectron spectra are acquired using the FAT mode because the energy resolution is constant for the whole spectrum. The FRR mode is advantageous for peaks with low intensities and large width. The photoelectrons are detected by a photomultiplier or channeltrons. Which count electrons passing the analyzer? The computerized electronic gives the photoelectron spectrum with photoelectron counts versus binding energy. The photoelectrons are collected from an area of 3 by 5 millimeters. The XPS results are therefore averaged over a sample area of 15 mm 2. The XPS facility is built from the following components: Introduction chamber to transfer a sample from the air, preparation chamber to clean or to treatment a sample and analysis chamber. After placing the sample into the introduction chamber, the chamber is evacuated to pressure below 10-6 mbar by a turbomolecular pump. The adjacent preparation chamber is equipped with a copper evaporator and mass spectrometer. In the analysis chamber we fixed the x-rays source with a Al-K α / Mg-K α twin anode and the hemispherical energy analyzer. Additionally we have an ion sputter gun for sample cleaning. Base pressure in the preparation and analysis chamber are less than 1.10-10 mbar. 5
3.1 Qualitative analysis The element in your sample can be identified directly from a survey scan from the characteristics photoemission lines. Since the radiation is known it is a trivial matter to transform the spectrum so that it is plotted against Binding Energy as opposed to Kinetic Energy. Closer inspection of the spectrum shows that emission from some levels (most obviously 3p, 3d etc.) does not give rise to a single photoemission peak, but closely space doublet. The removal of an electron from 3d sub-shell by photo-ionization leads to a new configuration for the final state which has non-zero orbital angular moment. There will be coupling between the unpaired spin and orbital angular moment. At low binding energy the valence band emission occurs. 6
3.2 Chemical Shifts For more information about the chemical state of those elements at high resolution scan of the most intensive line of each element is needed. The exact binding energy of an electron depends not only upon the level from which photoemission is occurring, but also upon: a) the formal oxidation state of the atom b) the local chemical and physical environment These give rise to small shifts (a few ev) in the peak positions in the spectrum, so-called chemical shifts. Assuming that the relativistic and correlation effects are negligible and that all atoms of an element show the same relaxation, independent of their chemical state or environment, shifts can be interpreted as chemical shifts. Usually species with a higher electron density or valence state are shifted towards lower binding energy. 3.3 Quantitative Analysis The ionization probability of a core electron is almost independent from the valence state of the element. Therefore the intensity of a photoemission line is proportional to the number of atoms of this element in the detected volume and allows a quantitative analysis by XPS. The soft x-ray penetrates a few micrometer s into the solid and cause the emission of electrons. The emitted electrons are scattered and inelastically while moving through the solid. In a homogeneous solid the number of electrons N which do not suffer any energy loss decreases exponentially with path length. N = N o Exp ( -d / λ ) (E kin ) cos Θ 4 The parameters for the probe depth of XPS are the material dependent Inelastic free path λ of the electrons, which is of order of few nm. The mean free path of the electrons scales to a good approximation with E 0.7 with increasing kinetic energy E k of the electrons. Θ is the exit angle of the electrons with respect to the sample normal. Only those electrons contribute to a photoemission line which do not suffer any energy loss contribute to the background which has to be subtracted for a quantitative analysis. 7
The intensity of a photoemission peak from element A is I A = б A N A G A ( E A ) λ A (E A ) X A 5 With: б A = is the relative excitation probability N A = number of atoms A in the probe volume G A ( E A ) = transmission of spectrometer at kinetic energy E A λ A (E A ) = the inelastic mean free path of electron with Kinetic energy in the sample X A = accounts for various instrument and geometry related parameters Photon flux of X-ray source Orientation of the sample Depth profile of A in the sample Orientation of the X-ray source towards the spectrometer The area of a peak after background subtraction yields the relative intensity to be measured. From Equation 5 Na = I A /( б A. G A ( E A ). λ A (E A ).X A ) 6 the denominator in Eq.6 can be defined as the atomic sensitivity factor S a of element A. By assumption that a sample is homogeneous and S 1 /S 2 ratio is matrix independent, a general expression for determining the atom fraction of any constituent in a sample, C x, can be written as: C x = (I x / S x ) : Σ (I i / S i ) Values of S as well as I based on peak area measurements. 8
4 Test proceeding and evaluation This test is run under permanent supervision; do not try to operate any part of the XPS equipment by yourself. The UHV equipment is expensive and even simple repairs are extremely time consuming. The sample has been placed in the introduction chamber earlier without any sample treatment. 4.1 Test Acquire a survey scan and identify the elements in your sample. You will find a XPS handbook with all necessary information in the lab. Try to identify all the lines in your survey scan except the lines from valence states (E B < 30 ev). Take high resolution scans of the 3 most intensive elements lines. (Remove the surface adsorbed layer by ion sputtering if it is necessarily and take a survey scans again). Calculate the concentrations of elements at the sample surface. Your supervisor will have all the necessary data. Estimate the chemical shifts and try to understand local chemical environment for elements which were found. 5 Literature Method of surface analysis, j.m. Walls (ed), Cambridge University press, 1989 (UB: Da 4566) Surface analysis methods in materials science, D.J. O Connor, B.A. Sexton, R.St.C. Smart ( EDS), Springer verlag 1992 (UB : Pd 138-23) Practical surface analysis by auger and x-ray photoelectron spectroscopy, D. Briggs and M.P. Seah (Eds), John wiley and sons, 1983 (UB: DA 3173) 9