X-ray Photoemission Spectroscopy (XPS - Ma4)

Transcription

1 Master Laboratory Report X-ray Photoemission Spectroscopy (XPS - Ma4) Supervisor: Andrew Britton Students: Dachi Meurmishvili, Muhammad Khurram Riaz and Martin Borchert Date: November 17th

2 Contents 1 Introduction Emission of electrons and energy levels including the Fermi level Photoemission Auger electron emission Secondary effects Surface sensitivity Experimental setup Production of X-rays Electron energy analyser Elaboration Silver sample Samarium sample Unknown material: 10 Deutsche Mark coin Conclusion 17 2

3 1 Introduction In this experiment we use X-rays to probe the band structure (energy levels) of different known and unknown materials by analysing the emitted electrons. For this we will learn and cover the basic concepts of photo emission. When the photon hits the surface of the solid it ionises the atoms and induces the emission of core electrons as photo electrons. Also due to secondary processes more electrons with various distinct kinetic energies are emitted. There are 2 main types of photo electron spectroscopy, depending on the energy of the radiation used to eject electrons. For low energy (3eV < hν < 150eV ) spectroscopy we can use ultraviolet Photo electron spectroscopy(ups) and for higher energy spectroscopy (up to eV ) we can use X-ray photo electron spectroscopy(xps). We will be dealing with energies around 1keV. 1.1 Emission of electrons and energy levels including the Fermi level The entire field of electron emission spectroscopy is based on the fact that electrons are on discrete (or in case of bands: separated quasi discrete) energy levels with respect to their nucleus. This energies correspond the the electron orbitals and are filled via the Aufbau-principle based on the Hund s rules. The first ten energy levels are 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d. (1) An illustration of this (together with photoemission discussed below) can be seen in figure 1. When we expose the sample to X-rays we can detect electrons with certain various energies that originate from different physical processes. The core electrons (inner-shell electrons) require more energy to leave the atom than the valance-shell electron as the inner-shell electrons are much closer to the nucleus and bounded stronger. We will cover these now and later talk about how the experimental setup is and how we detect the electrons and measure their kinetic energy Photoemission When a photon with frequency ν and thus energy E photon = hν hits an electron all this energy is transferred to the electron. If the energy is exactly equal to the difference to another non-fully occupied energy level the electron will just move there. We define the energy needed to reach the most upper electron shell a.k.a. Fermi level (E F ) as the binding energy E kin. We further define the energy needed to leave the atomic potential completely (reach the vacuum level) starting from the Fermi level as the work function φ. The leftover energy after leaving the atomic potential is of course kinetic energy which we can detect for example by the detector, described below. Our interest lies in determining the binding energies of the electrons as this will give us a lot of insight of the structure of the atom. 3

4 From the energy conservation we have: E(A) + hν = E(A ) + E kin + ϕ (2) E kin = hν (E(A ) E(A)) ϕ (3) Here we introduced definitions E(A) energy of an atom. E(A ) energy of the ionized atom, and the (E(A ) E(A)) E bind binding energy. Which is to be understood the following way: By Ionizing atom we mean, making a vacant energy level in a inner shell by ejecting one electron by X-ray, so that hole creates opportunity for other electrons from upper level to jump down in order to fill the missing spot and by doing, so they emit energy. Which means that ionized atom has some kind of potential energy, that fully filled atom has not. So binding energy defined this way will be positive and (4) hν = E bind + ϕ + E kin (5) E kin = hν ϕ E bind (6) = E F E bind (7) An illustration of above described photoelectric effect can be seen in figure (1). (8) SAMPLE SPECTROMETER Photoelectron with E kin = hν - (E bind+ φ) Work function φ Difference in work functions Δφ Vacuum level Fermi level Kinetic energy Binding energy 3p orbital 3s 2p 2s 1s Figure 1: Photoelectric effect. (Energy level diagram for a sample.) We see different energy levels of the occupied orbitals and the fermi level at the top. If an electron is given the energy to reach the fermi level plus an additional energy (called the work function ϕ) then it can escape the atomic potential (to the vacuum, this level is called the vacuum energy). The left over energy is transferred to the electron as kinetic energy. As the spectrometer has a different work function, we only measure an effective work function ϕ. 4

5 1.1.2 Auger electron emission The Auger process is a surface specific phenomena that involves the emission of a low (kinetic) energy electrons from the sample surface. It is used for (further) determining the composition of the energy levels of a given sample. Auger emission is a multi-step process. 1) Atomic ionization 2) Electron relaxiation 3) Emittance of a photon within the atom 4) Emittance of the auger electron 1. Atomic ionisation The auger process is initiated by the creation of a core hole by the above described photoemission: An X-ray removes one electron from the lower energy levels of an atom and thus creates a core hole. 2. Electron relaxation This core hole is filled by a relaxing electron from a higher energy level. 3. The energy emitted in this process as a photon can either escape the atom or hit another electron and transfer its energy onto it. 4. Auger-Electron emission If the hit electron is bound weakly enough, so that (its binding energy plus the work function is smaller than the energy of the photon) E bind + ϕ hν (9) it can escape. With the leftover energy transferred to the electron as kinetic energy E kin. An example of this is shown in 2. 5

6 Kinetic energy Binding energy Photoelectron with E = hν - (E + φ) kin 1) Emission of a photoelectron bind 4) Emission of a third electron with E = E(3p) - E(2p) - (E(3p) + φ) => Augerelectron Work function φ 3) Emission of a photon 2) relaxation of another electron Vacuum level Fermi level 3p orbital 3s 2p 2s Figure 2: Auger Process in 4 steps. First a Photoelectron is emitted due to incoming radiation, such as x-ray. This creates a core hole which is filled by relaxation of a higher electron. The energy difference between the initial state and the final state of this electron is transferred to a third electron via a photon. This difference is independent of the energy of the incoming x-ray. 1s The Auger electrons are emitted with kinetic energies that depends only on the difference between energy levels of the relaxing electron and the emitted electron. That is to say, unlike the photo emission lines, changing the X-ray characteristic energy does not alter the position of the Auger lines in the recorded spectra with respect to a kinetic energy scale. The Auger peaks always appear at the same kinetic energy and it is the photoelectric lines that move when a different X-ray source is used Secondary effects Free electrons can interact with the valance electrons and excite them or receive energy from them. The same phenomenon can also happen when an electron, which is released due to a primary process (photo emission or an Auger process), interacts with a valence electron. The effects from this that we see is a background and several shake-up peaks from additionally released electrons by these secondary effects. A primary electron can sometimes transfer enough energy to a secondary electron for it to leave the atom. Higher energy primary electrons can of course excite more electrons and electrons of different energies. Because of that the amount of low (kinetic) energy secondary electrons is 6

7 a lot higher than high energy electrons. We see this in the measurements as well as background noise is higher on the left side with lower (kinetic) energy Surface sensitivity Electrons generated by either the photoelectric effect or the mechanisms described by Auger, leave the surface with a characteristic energy provided they have not undergone some secondary energy loss process. The probability of these electrons emerging from the surface without some energy loss is related to the inelastic mean-free-path λ, which is in turn a function of the kinetic energy of the ejected electrons. The sampling depth for a given photoelectric line recorded with different X-ray anodes is therefore different, while the sampling depth for the same Auger line is unaffected by similar considerations. 1.2 Experimental setup In the ultra high vacuum chamber we have an X-ray source, an electron detector and different samples. We can choose between two different X-ray energies as discussed below Production of X-rays There is two way to produce X-ray radiation: 1) Bremsstrahlung Radiation and 2) Characteristic Radiation. 1) Bremsstrahlung - which means braking radiation - electrons are decelerated, braked when they hit the metal object after the bombardment. Accelerated electron emits E-M radiation, and when the kinetic energy of electron is high enough the radiation is in X-ray spectrum. The characteristic radiation of Bremsstrahlung is continuous spectra which shifts to higher frequencies when the energy of the electrons increase. The bombarding electrons which hit the metal object can also, admitting enough energy, eject the electrons from the inner shells of the atoms. This creates vacancies inside the atomic electron shells which are quickly filled by electrons dropping down from higher energy levels emitting sharply peaked Characteristic X-rays. 2) Characteristic Radiation - characterises atoms - from emitted X-rays, which have sharply defined frequencies that are associated with the difference between the atomic energy levels of the target atoms. The electron shells in atom are labelled as K, L, M, N, O, P, and Q; or 1, 2, 3, 4, 5, 6, and 7 which describe different levels of energy. Higher number means bigger energy, so outer shell electrons have greater energy then inner. However the coulomb pull of the 7

8 nucleus is weaker for outer shell electrons so they can be easily teared away from the atom. Transition to 1st level is denoted by K, to 2nd level by L. and for the transition which happens from different upper levels to the same lower level we use Greek subscripts α, β... denoting the first upper energy level, second upper energy level and so on... For example: The X-rays produced by transitions from the n=2 to n=1 levels are called K α X-rays, and those for the n = 3ß1 transition are called K β X-rays. Transitions to the n=2 energy level (or L-shell) are designated as L X-rays: n = 3ß2 is L α, n = 4ß2 is L β, etc. The frequencies of the characteristic X-rays for different atoms can be predicted from the Bohr model. In our case we use 9kV-12kV voltage in an X-ray tube and its construction allows us to choose between Mg and Al as a material of the anode with a different characteristic emission spectrum. Normally the spectrum will include Bremsstrahlung and characteristic radiation, however we make use of a 1µm thick Al window to make sure that the large portion of continuous Bremsstrahlung will get absorbed Electron energy analyser The detection system in XPS consist of three parts 1) Electrical lens 2) Hemispherical electron energy 3) Electron detector 1) Electrical lens system The photoelectron from the sample passes through the electrical lens which focusses the energy and motion of the electrons towards the entrance to the hemisphere. 2) Hemispherical electron energy analyser The hemispherical analyser consist of two concentric hemispheres with different potential applied to them and the electrons which have the energy that is set to it can pass through. The energy of the electrons is changed to the pass energy E pass. All elctrons with a higher or lower kinetic energy than the set one hit the hemisphere. 3) Electron detector Those electrons which have an energy equal to the set energy cross the sphere and reach the other end where an electron detector is placed. E pass will affect the resolution and sensitivity of the spectrometer. If E pass is higher the signal and the sensitivity are higher but the resolution is lower. The electrons which hit 8

9 the wall of the analyser have different energies and these electrons are detected as noise in the electron detector. 2 Elaboration In this experiment we probed different samples using X-ray spectroscopy: A silver surface, a silver surface with additional layers of Samarium that has been evaporated and settled on the sample and a 10 Deutsche Mark coin of which the composition is unknown. We can switch between an aluminium anode with an X-ray energy of hν = 1486eV and a magnesium anode with hν = 1256eV. It is relatively easy to switch between the anodes but it takes time so we rather probed different samples with the same anode and then switch to the other anode. 2.1 Silver sample To get a general overview of how X-ray emission spectroscopy works, we took an overview spectrum of silver using the Al anode. The fermi energy was found to be at E F = 1479eV, the spectrum can be seen in Sm overview PE = 40eV - Al Intensity in arbitrary units Spectrum for the x-ray from Al Fermilevel at E = 1480eV for Al fitted peaks Kinetic Energy in ev Figure 3: Overview of silver with Al anode After that we took an overview spectrum of the silver using the Mg anode. The Fermi energy was found to be at E F = 1247eV, the spectrum can be seen in 4. 9

10 14000 Sm overview PE = 40eV - Mg Spectrum for the x-ray from Mg Fermilevel at E = 1247eV for Mg fitted peaks Figure 4: Overview of silver with Mg anode Using python we fitted peaks into the spectra which can be seen in the graphs as thin lines. Then we took the difference between the peaks and the Fermi energy and identified these as the binding energies. We compared them to the tables provided in the Wiki and identified several photo peaks. We also wrote a python script to go through all possible auger peaks for silver and then compared the list with the auger peak we found. It turned out to be equivalent to the transition between the 3s and 4s orbitals. E Kin (Mg) E Bind (Mg) E Kin (Al) E Bind (Al) Lit. val. Orbital K1s L 1 2s L 2 2p 1/ L 3 2p 3/ M 1 3s Auger (3s/4s) M 2 3p 1/ M 3 3p 3/ M 4 3d 3/ M 5 3d 5/ Shake-up Shake-up N 1 4s N 2 4p 1/ N 3 4p 3/2 Table 1: Identified peaks of Silver (Ag) 10

11 2.2 Samarium sample After studying the silver sample we rotated the sample holder in such a way that we could evaporate samarium onto another silver sample. We then probed the samarium with the aluminium anode. We first took a closeup spectrum of samarium around the Fermi edge. The sum of two Gaussian peaks was fitted into the spectrum to identify the 4f and 5p peaks. The Fermi energy was found to be E F = (1481 ± 1)eV. From that the binding energies of the to peaks were found to be E 4f = (6.1 ± 1)eV and E 5p = (21.1 ± 1)ev. This fits the expected peaks very well. Further splitting due to nuclear hyper fine splitting could not be observed. The spectrum can be seen in 5. Intensity in arbitrary units High resolution (and high noise) spectrum around the fermi level for P = 25eV of Sm two single gaussian curves that in sum were fited to the data. data produced with the Al annode Fermilevel Kinetic Energy in ev Figure 5: Fermiedge We then looked at the 3d peaks at binding energies of 1100eV. As the energy of the X-ray of Mg is only at 1256eV, it is better to use the Al X-ray with hν = 1458eV. We used different values for the pass energies at E P = {15eV, 20eV, 25eV }. The measurements including the fit as a sum of two Gaussians are plotted into the spectrum at figure 6. 11

12 Intensity in arbitrary units Sm 3d orbital comparison of different value for PE - Al PE = 25 ev 25 ev fitted sum of two gaussians PE = 20 ev 20 ev fitted sum of two gaussians PE = 15 ev 15 ev fitted sum of two gaussians Kinetic Energy in ev Figure 6: 3d For each pass energy we could identify the left peak as the 3d 3/2 at E bind = eV and the right peak as the 3d 5/2 at E bind = eV (literature values). The experimental values can be seen in 2. All found values are within three σ of the literature value. E P inev E kin in ev E bind in ev ± ± ± ± ± ± ± ± ± ± ± ± 3.5 Table 2: Table Lastly we also took an overview spectrum of the samarium sample. The two regions that we looked at before are now at the extreme ends of the energy scale of this spectrum. The spectrum and the peaks we were able to identify are shown in 7. All peaks for samarium are shown in table 3. Again the auger peaks are found from all possible transitions inside the samarium. Surely it is to note that the level that an electron relaxes to has to be unoccupied, which means the binding energy of that level has to be less than the energy of the incoming X-ray. 12

13 2 Intensity in arbitrary units Sm overview PE = 40eV - Al 20eV fitted peaks Al annode Kinetic Energy in ev Figure 7: Overview of Sm with Al anode E Kin Mg in ev E Bind Mg in ev Literature in ev Type / Orbital see prev. task M4 3d 3/2 see prev. task M5 3d 5/ Auger M5 / O3 / N Auger M5 / N4 / O Auger M5 / N5 / O M4 3d 3/2 265 N2 4p 1/ N3 4p 3/ N4 4d 3/ N5 4d 5/ O1 5s see prev. task 21.3 O2 5p 1/2 see prev. task 21.3 O2 5p 3/2 see prev. task 5.2 N6 4f 5/2 see prev. task 5.2 N6 4f 7/2 Table 3: Identified peaks of Samarium (Sm) 2.3 Unknown material: 10 Deutsche Mark coin We now want to determine the composition of an unknown material. We only know that it is a 10 Deutsche Mark special edition coin. We again took overview spectra of the coin with the two different x-ray energies. The results can be seen in figures (8) and (9). 13

14 2 Intensity in arbitrary units Coin spectrum from Al anode with pass energy = 40eV Spectrum fitted peaks Fermi level at E=1479 ev Kinetic Energy in ev Figure 8: Overview of the coin with Al anode Coin spectrum from Mg anode with pass energy = 40eV PE = 40 ev Mg fitted peaks Fermi level at E=1258 ev Intensity in arbitrary units Kinetic Energy in ev Figure 9: Overview of the coin with Mg anode To determine the fermi level we took spectra around the regions that correspond to the two different X-ray energies. The results can be seen in figures (10) and (11). 14

15 2 Intensity in arbitrary units Coin spectrum around the fermi level - Al anode fit spectrum Fermi level Kinetic Energy in ev Figure 10: Higher resolution spectrum around the fermi level with Al anode 2 Intensity in arbitrary units Coin spectrum around the fermi level - Mg Anode fit spectrum Fermi level Kinetic Energy in ev Figure 11: Higher resolution spectrum around the fermi level with Mg anode We were able to find quite a few peaks, but unfortunately most of them seem to be auger peaks as their kinetic energy is the same in both spectra. This made finding the materials very hard as we had to go through all possible combinations of many possible candidate materials. We do not think that the coin is made of a rare material but rather a common one. Therefore we only considered tin, zinc, copper, iron and silver as base materials for the coin We also considered pollution from oxygen in the air as well as carbon and of course samarium from previous vaporisations. The peaks we could idenify are listed in table (4). Most of transitions with a kinetic energy close to one of our peaks are forbidden due to the conservation of angular momentum which can be found in the transition rules for dipole transitions within the material. Unfortunately we were only clearly able to identify Silver and Samarium for sure with one photo peak close to one 15

16 of copper. The huge background of the spectrum was the biggest reason why we could not identify more peaks. E Kin (Mg) E Bind (Mg) E Kin (Al) E Bind (Al) Lit. val. Orbital Photop. L1 2s 1/2 (Cu) Auger M2/N1/N1 (Ag) Auger M3/N1/N2 (Ag) Auger L2/N1/N3 (Sm) No available Augerp Auger M5/O3/N2 (Sm) Auger M5/N3/O1 (Sm) Auger M4/N2/O1 (Sm) No available Photop No available Photop No available Photop No available Photop Photop. M1 3s 3/2 (Cu) Table 4: Identified peaks of the 10 DM Coin s spectrum 16

17 3 Conclusion We probed three different samples with two different X-ray energies. The samples are a silver surface, a silver surface with additional samarium on top of it and a 10 DM coin. With the two X-ray energies we were able to distinguish between photo emission peaks and auger peaks. We saw a lot of background noise which made it very hard to distinguish between some of the peaks. The results for silver are summarised in table (1), the ones for the samarium surface in table (3) and the ones for the coin in table (4). Every time we first probed the right end of the spectra with low binding energies to find the fermi edge, which was not easy but possible every time. In case of silver we only took an overview spectrum to see what information we could get from it and then applied this knowledge together with better resolution measurements for samarium where we, for example probed the electrons from the 3d orbitals a lot better. In case of the coin we did not know what material it was made of so we had to compare its spectrum to a lot of tables. We were able to identify mostly silver as the material that the coin is made of which matches with the information of[1] where it is stated that the coin is made of 925 parts (out of 1000) of silver. We also found samarium from the previous evaporations. In this experiment we learned a lot about energy levels in the atom, the definition of the work function, the fermi edge and core electrons and understood the basics of emission spectroscopy. We also got a deeper insight in the difference between Auger emission versus photoemission and were able to distinguish between the two by probing the samples with two different photon energies. References [1] MDM Deutsche Münze. Angaben zu den Münzen (Material). url: mdm.de/10-dm-silbermuenzen-1987-bis-2001-komplett-pp-st. 17