Chapter 6： X-Ray Photoelectron Spectroscopy (XPS) Electron Spectroscopy for Chemical Analysis (ESCA) The Nobel Prize in Physics 1981 Kai M. Siegbahn b. 1918 d. 2007 "for his contribution to the development of high-resolution electron spectroscopy" 1 Advanced Photonics Laboratory 2 What is XPS? X-ray Photoelectron spectroscopy, based on the photoelectric effect,1,2 was developed in the mid-1960’s by Kai Siegbahn and his research group at the University of Uppsala, Sweden.3 1. H. Hertz, Ann. Physik 31,983 (1887). 2. A. Einstein, Ann. Physik 17,132 (1905). 1921 Nobel Prize in Physics. 3. K. Siegbahn, Et. Al.,Nova Acta Regiae Soc.Sci., Ser. IV, Vol. 20 (1967). 1981 Nobel Prize in Physics. Advanced Photonics Laboratory 1 3 Advanced Photonics Laboratory 4 Comparison of Sensitivities H Ne Co Zn Zr Sn Nd Yb Hg Th AESa nd XPS 1% 5E19 RBS PIXE PIXE 1ppm 5E16 SIMS 1ppb 0 20 40 60 ATOMIC NUMBER 80 5E13 100 Advanced Photonics Laboratory 2 5 X-ray Photoelectron Spectroscopy Photon energies: 100-2000eV, Electron energies: 0-1000eV Advanced Photonics Laboratory 6 Photoemission, what’s it used for ? A) What elements are present in the surface region. Different elements have different binding energies of the inner (core) levels. B) Often, also the chemical state of the elements can be determined, eg. Al-metal can be distinguished from Al-oxide. The exact binding energy of a core level depends on the chemical state. Chemical shifts. C) The surface geometry can be qualitatively determined. Using diffraction effects and/or the chemical shifts of the binding energies (and imagination) D) The band-structure of the solid can be measured. Measuring the emission from the valence band in an angle resolved manner E) Chemically sensitive microscopy is possible. (Note: Chemically, not just element specific) Combine the above with either focusing of the incoming photons or magnifying electron optics. Some 10 nanometers to micrometers are typical values for the spatial resolution. In other words, chemical composition, geometrical structure and electronic structure. Quite complete information ! Advanced Photonics Laboratory 3 7 Introduction to the Solid State In solids, atomic and molecular energy levels broaden into bands that in principle contain as many states as there are atoms/molecules in the solid. Bands may be separated by a band gap with energy Eg Advanced Photonics Laboratory Energy Bands in the Solid State 8 Bands are continuous and delocalized over the material Band “widths” are determined by size of orbital overlap The highest-energy filled band (which may be only partially filled) is called the valence band The lowest-energy empty band is called the conduction band Advanced Photonics Laboratory 4 9 The Workfunction: A Barrier to Electron Emission How does the electronic arrangement in solids affect surfaces? In particular, how can an electron be removed ? Free electron! For some electron being removed, its energy just as it gets free is EV The energy required to remove the electron is the workfunction φ (typically several eV) Advanced Photonics Laboratory 10 The Workfunction: A Barrier to Electron Emission Workfunctions vary from <2 eV for alkali metals to >5 eV for transition metals. Material Na Cu Ag Au Pt W W(111) W(100) W(110) W(112) Crystal State polycrystalline polycrystalline polycrystalline polycrystalline polycrystalline polycrystalline single crystal single crystal single crystal single crystal Workfunction (eV) 2.4 4.4 4.3 4.3 5.3 4.5 4.39 4.56 4.68 4.69 The workfunction is the ‘barrier” to electron emission – like the wall in the particle-in-a-box concept. Advanced Photonics Laboratory 5 11 Basic Considerations for Surface Spectroscopy Common sampling “modes” Spot sampling (點) Raster scanning (二維掃描) Depth profiling (縱深) Surface contamination: The obvious contamination/alteration of surfaces that can be the result of less-than careful sample preparation Solid surfaces can adsorb gases: y At 10-6 torr, a complete monolayer of a gas (e.g. CO) takes just 3 seconds to form. y At 10-8 torr, monolayer formation takes 1 hour. Most studies are conducted under vaccuum – although there are newer methods that don’t require this. Advanced Photonics Laboratory 12 Surface Spectrometric Analysis Surface spectrometric techniques: X-ray fluorescence (from electron microscopy) Auger electron spectrometry X-ray photoelectron spectrometry (XPS/UPS) Secondary-ion mass spectrometry (SIMS) Depth profiling – if you are going to study surfaces with high lateral resolution (e.g. using microscopy), then wouldn’t it be nice to obtain information from various depths within the sample? Advanced Photonics Laboratory 6 13 Depth sensitivity: E = hν KE = 1 mv 2 2 ejected from } Electrons depth < ~ 1 Mean-free path created deep in } Electrons material scanner and eventually X-rays penetrate deep into material recombine with holes Simple analysis of XPS: KE = 1 mv 2 = hν − Ebinding 2 Advanced Photonics Laboratory 14 The Basic Idea Behind Surface Spectrometry Photons, electrons, ions: they can go in and/or out!!! Leads to lots of techniques, and lots of acronyms! Primary photon electron ion Secondary photon electron ion Surface Primary Secondary Name of Technique photon (X-ray/UV) electron XPS (ESCA) and UPS photon (X-ray) or electron electron Auger electron spec. (AES) ion ion SIMS (secondary ion MS) photon ion LMMS (laser microprobe MS) electron Photon (X-ray) SEM “electron microprobe” Advanced Photonics Laboratory 7 15 X-ray Photoelectron Spectrometry (XPS) Photoelectron spectroscopy is used for solids, liquids and gases, but has achieved prominence as an analytical technique for solid surfaces XPS:“soft” x-ray photon energies of 200-2000 eV for analysis of core levels UPS: vacuum UV energies of 10-45 eV for analysis of valence and bonding electrons Photoelectric effect: Proposed by A. Einstein (1905), harnessed by K. Siegbahn (1950-1970) to develop XPS Advanced Photonics Laboratory 16 XPS: Basic Concepts Like in AES, photoelectrons can not escape from depths greater than 10-50 A inside a material Schematically, the photoelectron process is: A + hν → A+* + e − atom or molecule cation Like in AES, the kinetic energy of the emitted electron is measured in a spectrometer Advanced Photonics Laboratory 8 17 XPS Basic Principle Advanced Photonics Laboratory 18 Energy Diagram 簡化起見 EK~EK’ Advanced Photonics Laboratory 9 19 XPS Energy Scale 橫軸的表示法 The XPS instrument measures the kinetic energy of all collected electrons. The electron signal includes contributions from both photoelectron and Auger electron lines. Advanced Photonics Laboratory 20 XPS Energy Scale- Kinetic energy 橫軸的表示法-動能 KE = hv - BE - Φspec Where: BE= BE= Electron Binding Energy KE= KE= Electron Kinetic Energy Φspec= Spectrometer Work Function 區分 Photoelectron line energies: Dependent on photon energy. Auger electron line energies: Not Dependent on photon energy. If XPS spectra were presented on a kinetic energy scale, one would need to know the XX-ray source energy used to collect the data in order to compare the chemical states in the sample with data collected using another source. Advanced Photonics Laboratory 10 21 XPS Energy Scale- Binding energy 橫軸的表示法 – 束縛能 BE = hv - KE - Φspec Where: BE= BE= Electron Binding Energy KE= KE= Electron Kinetic Energy Φspec= Spectrometer Work Function Photoelectron line energies: Not Dependent on photon energy. Auger electron line energies: Dependent on photon energy. The binding energy scale was derived to make uniform comparisons of chemical states straight forward. Advanced Photonics Laboratory 22 Fermi Level Referencing Free electrons (those giving rise to conductivity) find an equal potential which is constant throughout the material. FermiFermi-Dirac Statistics: T=0 K kT<< Ef kT<<E f(E) f(E) = 1 exp[(E)/kT] + 1 exp[(E-Ef)/kT] 1.0 0.5 0 1. At T=0 K: f(E)=1 for E<E E<Ef f(E)=0 for E>E E>Ef Ef 2. At kT<< Ef (at room temperature kT=0.025 kT<<E kT=0.025 eV) eV) f(E)=0.5 for E=E E=Ef Advanced Photonics Laboratory 11 23 Fermi Level Referencing E f Fermi Edge of N(E)/E TiN, room tem perture 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 Binding energy (eV) Advanced Photonics Laboratory 24 Sample/Spectrometer Energy Level Diagram- Conducting Sample 待測物為導體時 e- Sample Spectrometer Free Electron Energy Vacuum Level, Ev hv KE(1s) KE(1s) Φsample Φspec 功函數的值跟動能比起來好像很小 Fermi Level, Ef BE(1s) E1s Because the Fermi levels of the sample and spectrometer are aligned, we only need to know the spectrometer work function, function, Φspec, to calculate BE(1s). 不需知道待測物 Advanced Photonics Laboratory 12 25 Sample/Spectrometer Energy Level Diagram- Insulating Sample 待測物為絕緣體時 eSample Spectrometer Free Electron Energy KE(1s) Φspec Vacuum Level, Ev Ech hv Fermi Level, Ef BE(1s) E1s A relative buildbuild-up of electrons at the spectrometer raises the Fermi level of the spectrometer relative to the sample. A potential Ech will develop. 介面電荷產生內建電位 Advanced Photonics Laboratory 26 Binding Energy Referencing BE = hv - KE - Φspec- Ech Where: BE= BE= Electron Binding Energy KE= KE= Electron Kinetic Energy Φspec= Spectrometer Work Function Ech= Surface Charge Energy Ech can be determined by electrically calibrating the instrument to a spectral feature. 還是可以校正出內建電位值 C1s at 285.0 eV Au4f7/2 at 84.0 eV Advanced Photonics Laboratory 13 27 XPS: X-ray Processes Advanced Photonics Laboratory 28 XPS: Photoelectron Emission and Binding Energy The kinetic energy of the emitted electron can be related to the “binding energy”, or the energy required to remove an electron from its orbital. Higher binding energies mean tighter binding – e.g. as atomic number goes up, binding energies get tighter because of increasing number of protons. 動能表示 Advanced Photonics Laboratory Ebinding = hν − IP (gas) Ebinding = hν − BE − w (solid) 束縛能表示 http://www.chem.qmw.ac.uk/surfaces/scc/scat5_3.htm 14 29 XPS: Binding Energy The workfunction w is usually linked to the spectrometer (if the sample is electrically connected) In gases, the BE is directly related to IP Ionization potential – the energy required to take an electron out of its orbital all the way to the “vacuum” (i.e. far away!) PE spectroscopy on gases is used to check the accuracy of modern quantum chemical calculations In conducting solids the workfunction is involved Koopman’s Theorem: binding energy = -(orbital energy) Orbital energies can be calculated from Hartree-Fock Another definition for XPS binding energy: the minimum energy required to move an inner electron from its orbital to a region away from the nuclear charge. Absorption edges result from this same effect Advanced Photonics Laboratory 30 X-ray source (without monochromator) + Electron energy analyzer Advanced Photonics Laboratory 15 31 X-ray monochromators: Use similar principles to optical monochromators. But, some important differences: Problem: With conventional (Czerny-Turner) monochromators, need to collimate the light (entrance slit to diffraction grating) and focus the light (diffraction grating to exit slit). But, X-ray mirrors are very difficult to make, very expensive, and not very efficient (reflectivity <100%). Czerny-Turner (conventional) Roland Circle Solution: Use Roland Circle Monochromator design. Must bend crystals (usually quartz) onto a curved surface. Want to match Bragg condition and focus the X-rays simultaneously with one “grating”. Note: X-ray diffraction occurs both parallel and perpendicular to grating surface. If angle is “off”, then no diffraction at all (loss of intensity) Advanced Photonics Laboratory X-ray monochromator: Roland Circle (Calc’s for quartz) 32 彎曲的晶體 Advanced Photonics Laboratory 16 33 X-ray monochromator Advanced Photonics Laboratory 34 Principle of kinetic energy determination - I F= qE =m(V²/R) F --- Force V --- speed R --- trajectory radius E --- electrical fields established by U potential m --- electron mass q --- electron charge Advanced Photonics Laboratory 17 35 ri F= mv 2 2 E kinetic = r r Ekinetic = eΔV ⎛1 1⎞ 2 ⎜ − ⎟r 兩個力相等 ⎜⎝ ri ro ⎟⎠ Centrifugal Must be the same, so ro F= 2 Ekinetic eΔV = r ⎛1 1⎞ 2 ⎜⎜ − ⎟⎟r ⎝ ri ro ⎠ Centripetal (Electrostatic) Ekinetic = eΔV ⎛1 1⎞ 2⎜⎜ − ⎟⎟r ⎝ ri ro ⎠ eΔV eΔV eΔV eΔV = = = ⎛ 1 1 ⎞⎛ ri + ro ⎞ ⎛ ri + r0 ri + ro ⎞ ⎛ ro ri ⎞ ⎛ ro ri ⎞ ⎟ ⎜ 1 + − − 1⎟ ⎜ − ⎟ 2⎜⎜ − ⎟⎟⎜ − ⎟ ⎜⎜ ro ⎟⎠ ⎜⎝ ri ro ⎟⎠ ⎜⎝ ri ro ⎟⎠ ⎝ ri ro ⎠⎝ 2 ⎠ ⎝ ri Ekinetic = eΔV ⎛ ro ri ⎞ ⎜⎜ r − r ⎟⎟ o ⎠ ⎝ i Advanced Photonics Laboratory 36 Resolution of the hemispherical analyzer: E0 E ∂Ekinetic eΔV eΔV = = 0 = = 2 ∂r ⎛ ri + ro ⎞ rmean ⎛1 1⎞ ⎛1 1 ⎞ r +r ⎜ ⎟ 2⎜⎜ − ⎟⎟r 2 2⎜⎜ − ⎟⎟⎛⎜ i o ⎞⎟ 2 ⎝ ⎠ r r 2 r r ⎝ ⎠ ⎝ i o⎠ ⎝ i o⎠ Es ⎞ ⎛ ∂E Effective resolution = ΔE = ⎜ kinetic ⎟Δr = 0 rmean ⎝ ∂r ⎠ Effective slit width To get high resolution, need to decrease Eo (“pass energy”) or increase rmean (use a bigger analyzer). Phi 360: rmean = 14.0 cm. Effective slit width = 2 mm ΔE = 0.015 * E0 Advanced Photonics Laboratory 18 37 Principle of kinetic energy determination - II Dispersive analysis of the kinetic energy spectrum n(KE) A field is applied between 2 parallel plates, distance s apart. The lower plate has slits a distance r apart (entrance and exit slits). The photoelectrons with kinetic energy KE are transmitted to the detector. By varying Vd the spectrum of electron kinetic energies can thus be obtained. KE is proportional to Vd therefore the plot of electron flux at the detector against V is the photoemission spectrum. The parallel plate electrostatic analyzer Advanced Photonics Laboratory 38 CMA (Cylindrical Mirror Analyzer) Advanced Photonics Laboratory 19 39 Single pass double pass (類似雙光柵也可降低動能) 優點 High collection efficiency. Easily constructed and maintained. Can be made compact in size. 缺點 Energy calibration depends on sample position in front of analyzer. Resolution depends on Ep and size of analyzer. Advanced Photonics Laboratory 40 Advantages The retarding grid slows down the incoming electrons to one given value of Ep regardless of their initial KE. The absolute resolution ΔE is therefore independent of initial KE. Advanced Photonics Laboratory 20 41 CHA Channel Plate Concentric Hemispherical Analyser (CHA) operates on principle similar to CMA, but with a different geometry. Channel Plate Analyzer spatially “images” electron intensity from the sample at each given energy. Advanced Photonics Laboratory 42 HSA (Hemispherical Sector Analyzer) Advanced Photonics Laboratory 21 43 CMA vs. HAS • CMA: most effective in AES due to high transmission (signal electron /total electron generated). • HSA: always preferred for XPS because it can maintain adequate luminosity at high energy, resolution with the addition of a lens. HSA CMA resolution 0.8- 1.0 V >1.4 V transmission high higher luminosity energetic position angular dependence measurements machining high w/lens independent suitable high precision required irrelevant dependent unsuitable less precision required Advanced Photonics Laboratory 44 XPS: Sources Monochromatic sources using electrons fired at elemental targets that emit x-rays. Can be coupled with separate post-source monochromators containing crystals, for high resolution (x-ray bandwidth of <0.3 Å) XPS Sources (hit core electrons): Mg Kα radiation: hν = 1253.6 eV Al Kα radiation: hν = 1486 eV Ag Lα radiation: hν = 2984.3 eV UPS Sources (hit valence electrons): He(I) radiation: hν = 21.2 eV (~58.4 nm) A Thermo-Electron Dual-anode (Al/Mg) XPS source hν = 23 eV (~53.7 nm) He(II) radiation: hν = 41 eV (~30.4 nm) Focusing the spot and lateral resolution 10-μm diameter spots are now possible X-ray不易聚焦 Advanced Photonics Laboratory 22 45 XPS: Spectral Interpretation Orbital binding energies can be interpreted based on correlation tables, empirical trends and theoretical analysis. Peaks appear in XPS spectra for distinguishable atomic and molecular orbitals. Auger peaks also appear in XPS spectra – they are easily distinguished by comparing the XPS spectra from two sources (e.g. Mg and Al Kα lines). The Auger peaks remain unchanged w.r.t. kinetic energy, while the XPS peaks shift. 要分辨是不是Auger訊號就用不同的X-ray波長打 Advanced Photonics Laboratory 46 XPS: Binding Energy Ranges XPS Photoelectron Binding Energies versus Atomic Number (Z) *Data from C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Muilenberg, Eds., "Handbook of X-ray Photoelectron Spectroscopy," Perkin-Elmer Corp., Flying Cloud, MN, 1979. Image from http://www.cem.msu.edu/~cem924sg/BindingEnergyGraph.html (accessed 12-Nov-2004) Advanced Photonics Laboratory 23 47 A Typical XPS spectrum 1. Sharp peaks due to photoelectrons created within the first few atomic layers (elastically scattered). 2. Multiplet splitting (occurs when unfilled shells contain unpaired electrons). Spin的影響 3. A broad structure due to electrons from deeper in the solid which are ineslastically scattered (reduced KE) forms the background. 4. Satellites (shake-off and shake-up) are due to a sudden change in Coulombic potential 5. Plasmons which are created by collective excitations of the valence band as the photoejected electron passes through the valence band. ‧Extrinsic Plasmon: excited as the energetic PE propagates through the solid after the photoelectric process ‧Intrinsic Plasmon: screening response of the solid to the sudden creation of the core hole in one of its atom The two kinds of Plasmon are indistinguishable. 6. Auger peaks produced by X-rays (transitions from L to K shell: O KLL or C KLL). Advanced Photonics Laboratory 48 Sharp Peak (core level) Advanced Photonics Laboratory 24 49 Multiplet Splitting Multiplet splitting occurs when there are unfilled shells containing unpaired electrons. For instance, transition metals with unfilled d orbitals and rare earths with unfilled f orbitals all show multiplet splitting. Advanced Photonics Laboratory 50 Multiplet Splitting When an additional vacancy is made by photoionization coupling occurs between the unpaired electron left behind (after ionization) and the unpaired electron in the originally incompletely filled shell. Example: Fe2O3 has 5 unpaired electrons in the 3d shell as shown below. Following photoionization in the 3s shell, there are 2 possible final states. Schematic of Multiplet Splitting following photoionization in Fe3+ and XPS spectrum 能量的差異其實是看final與initial，因為spin的影響造成分裂 Advanced Photonics Laboratory 25 51 Satellites 光電子往外跑的過程中激發了外層電子至高能階的能量損耗 Arise when a core electron is removed by a photoionization. There is a sudden change in the effective charge due to the loss of shielding electrons. (This perturbation induces a transition in which an electron from a bonding orbital can be transferred to an anti-bonding orbital simultaneously with core ionization. Two types of satellite are detected. Shake-up: The outgoing electron interacts with a valence electron and excites it (shakes it up) to a higher energy level. As a consequence the energy core electron is reduced and a satellite structure appears a few eV below (KE scale) the core level position. Shake-off: The valence electron is ejected from the ion completely (to the continuum). Appears as a broadening of the core level peak or contribute to the inelastic background. Advanced Photonics Laboratory 52 Shake-up satellites distinct peaks a few eV below the main line Shake-off satellites broad feature at lower energy w.r.t. to main line 打出去打上去 Advanced Photonics Laboratory 26 53 Plasmons 表面夠乾淨才看得到 This feature is specific to clean surfaces. The photoelectron excites collective oscillations in the conduction band (free-electron gas), so called Plasmons. (discrete energy loss). Surface plasmon: bulk plasmon / 1.414. For Al, Mg, Na etc…the energies are 15.3 eV, 10.6 eV and 5.7 eV, respectively. Advanced Photonics Laboratory 54 Chemical shift Chemical shift arises in the initial state from the displacement of the electronic charge from the atom towards its ligands, reducing the electrostatic potential at the atom. There is a final state shift due to the polarization of the ligand by the core on the central atom. Core electron BE in molecular systems exhibits chemical shifts which are simply related to various quantitative measures of covalency. Greater the electronegativity of the ligands, the greater the BE of the core electron of the ligated atom. 電負度差異越大束縛能越大 Basic concept: The core electrons feel an alteration in the chemical environment when a change in the potential (charge distribution) of the valence shell occurs. For example assume that the core electrons are inside a hollow spherical charged shell. Each core electron then sees a potential. A change in Q by ΔQ gives a change in V. where ΔV is the chemical shift Advanced Photonics Laboratory 27 55 Oxidized metal surfaces Oxidized and clean Cr 2p spectra (left). Oxidized and clean Cu 2p spectra (right). The oxide layer resulted in extra peaks (shoulder at higher BE—left of the main line). Satellites are also seen on the Cu 2p spectra. Advanced Photonics Laboratory 56 Angular Distributions The inelastic mean free paths (IMFP) are in the range of 5 -100Å falling within 5 –40 Å for inorganic materials. To enhance the surface signal we can vary the photon energy—closer to the attenuation length minimum or decrease the angle of electron emission relative to a solid surface. 增強表面XPS訊號重點是到偵測器的距離可以穿透的長度可能一樣不過深度就不同了 For example: for Au 4f the IMFP (d) is about 22 Å at angle of 90°using AlKα. The depth (d) probed by XPS becomes about 4-5 Å at angle = 10°. Advanced Photonics Laboratory 28 Background: Spin-Orbit Energy Levels Spin-Orbit Zeeman mj = 3/2 mj = 1/2 mj = –1/2 mj = –3/2 j = 3/2 l = 1 (p) s = 1/2 57 mj = 1/2 mj = –1/2 j = l + s, l − s j = 1/2 m j = − j , − j + 1,... j − 1, j mj = 1/2 l = 0 (s) s = 1/2 mj = –1/2 j = 1/2 Advanced Photonics Laboratory XPS: Energy Level Basics 58 j 1S1/2 1/2 2S1/2 1/2,3/2 2P1/2, 2P3/2 n=1 n=2 Spin-Orbit Splitting n=3 Advanced Photonics Laboratory 29 Remember → 59 Spin-Orbit (l-s) Coupling 與DOS有關 (2j -+1) / (2j ++1) Degeneracy = 2j+1 or Initial state Final state without spin-orbit interaction 4f7/2 4 4f5/2 3 Spin-orbit interaction lifts degeneracy The degeneracy determines the probability of the state being the final state of the system! W f7 4f7/2 4f5/2 Ratio of intensities: 4:3 4f5/2 4f7/2 4f7/2= 31.4 Δ=2.18 eV ΔE – spin-orbit splitting 5p3/2 Binding Energy 45 靠近原子核 Advanced Photonics Laboratory 35 分開的能量就是LS耦合的大小 25 強度就是能態密度 60 Static Charging XPS: It arises as a consequence of the build-up of a positive charge at the surface of non-conducting specimens. The rate of photo-electron loss is greater than that of their replacement from within the specimen. It produces a retarding field at the surface that will shift the peaks (reduce the KE of the ejected electrons). 不導電試片會有內建電場產生 Suggestions: 1) Use of the adventitious Carbon line to correct any shift (most materials exhibit a C 1s line). 參考準位 2) Deposition of a very thin layer of gold (as use in SEM). 3) Use of a “flood gun” low energy electrons (0-5 eV). 有點像補電子中性化的動作 Advanced Photonics Laboratory 30 61 Static Charging Advanced Photonics Laboratory 62 XPS: Typical Spectra An XPS survey spectrum of stainless steel: Spectrum image from http://www.mee-inc.com/esca.html Advanced Photonics Laboratory 31 63 XPS: Typical Spectra An expanded XPS spectrum of the C1s region of PET: Spectrum image from http://www.mee-inc.com/esca.html Advanced Photonics Laboratory 64 XPS spectra for clean Al(100) Spin-orbit splitting and surface splitting Advanced Photonics Laboratory 32 65 XPS: Chemical Shifts Peaks appear in XPS spectra for distinguishable atomic and molecular orbitals. Effects that cause chemical shifts in XPS spectra: Oxidation states Covalent structure Neighboring electron withdrawing groups Anything else that can affect ionization/orbital energies 任何會影響週遭組態的都算 Advanced Photonics Laboratory 66 Chemical shifts and why they exist The core level binding energies are A simple (and not entirely sufficient) explanation : found to depend on the chemical state of the atom under investigation. WHY ? Strange as the core levels do NOT take part in the bonding 想像這是 Ti 想像這是 Ti+ Advanced Photonics Laboratory 33 67 XPS: Depth Profiling Option 1: Sputtering techniques Disadvantage – can change the surface normal Option 2: Angle-resolved XPS (AR-XPS) electron Reducing the photoelectron take-off angle (measured from the sample surface) reduces the depth from which the XPS information is obtained. XPS is more surface sensitive for grazing take-off angles than for angles close to the surface normal (longer PE paths). The most important application of angle resolved XPS (AR-XPS) is in the estimation of the thickness of thin films e.g. contamination, implantation, sputtering-altered and segregation layers. grazing Sample Advanced Photonics Laboratory XPS: Applications 68 A modern application of XPS – study the nature of PEG as a surface coating to prevent biofouling in biosensors 生物淤積 Biofouling: the tendency of proteins to adsorb to silicon-based surfaces XPS can be used, with AFM, to observe the coating of PEG onto silicon surfaces (PEG-silane coupling) - Increased C 1s C-O signal indicates greater grafting density 移植 Advanced Photonics Laboratory 34 69 XPS: Quantitative Applications Quantitative XPS is not as widely used as the qualitative version of the technique. 定量分析不容易 Variations in instrument parameters and set-up have traditionally caused problems with reproducibility Using internal standards, XPS can achieve quantitative accuracies of 3-10% in most cases (and getting better every year, as more effort is put into this type of analysis) Advanced Photonics Laboratory 70 AES and XPS: Combined Systems Dual Auger/XPS systems are very common, also combined with a basic SEM Note - SAM = scanning Auger microprobe Auger is seen as complementary to XPS with generally better lateral resolution Both are extreme surface sensitive techniques: AES better elemental quantitative analysis XPS contains more chemical information Also, remember that Auger peaks are often seen in XPS spectra (and are hence useful analytically) – they can be identified by changing source, so that the X-ray peaks shift (the Auger peaks do not). Advanced Photonics Laboratory 35 Comparison of XPS, AES and Other Techniques Characteristic AES XPS SEM/X-ray EM SIMS Elemental range Li and higher Z Li and higher Z Na and higher Z All Z Specificity Good Good Good Good Quantification With calibration With calibration With calibration Correction necessary Detection limits (atomic fraction) 10-2 to 10-3 10-2 to 10-3 10-3 to 10-8 10-3 to 10-8 Lateral resolution (um) 0.05 ~1000 0.05 1 Depth resolution (nm) 0.3-2.5 1-3 1000-50000 0.3-2 Organic samples No Yes Yes* Yes Insulator samples Yes* Yes Yes* Yes* Structural information Elemental Elemental and Chemical Elemental Elemental and Chemical Destructiveness Low Very Low Medium Medium 71 * = yes, with compensation for the effects of sample charging SIMS = secondary ion mass spectrometry, discussed in the “Ion and Particle Spectrometry” Lectures. SeeLaboratory Strobel and Heineman, Chemical Instrumentation, A Systematic Approach, Wiley, 1989, pg. 832. Advanced Photonics 72 XPS: New Applications A recent report in Chem. Commun. (2005) by Peter Licence and co-workers describes the use of XPS to study ionic liquids Normal liquids evaporate under ultrahigh vacuum (UHV), ionic liquids do not (they have a vapor pressure of nearly zero!) Why? Ionic liquids have become important for electrochemistry, catalysis, etc… Advanced Photonics Laboratory 36 73 其他跟x-ray相關的實驗 XAS: X-ray Absorption Spectroscopy Apparatus photon in, photon out Energy Diagram XAS is a photon in / photon out process. Cross sections for XAS vs. Auger (photon in / electron out) depend on nuclear charge Z (Auger dominates for lower Z). Advanced Photonics Laboratory 74 XAS: Survey & High Resolution Scans XAS Spectrum (survey scan) K-level XAS + EXAFS Spectrum (high-resolution scan) L-levels Advanced Photonics Laboratory 37 75 Need a tunable X-ray source: The only practical source is a synchrotron Advanced Photonics Laboratory 76 EXAFS: (Extended X-ray Absorption Fine Structure) EXAFS is periodic structure near XAS edge due to electron scattering. Fourier analysis of periodicities yields information about atomic spacings. Advanced Photonics Laboratory 38 77 EXAFS: Spectrum and Fourier Transform XAS of Ge shows sharp rise at 11 keV (K-edge), where modulation above edge is EXAFS data. Fourier transform shows nearest neighbor and the second nearest neighbor distances. Advanced Photonics Laboratory 78 X-ray fluorescence (XRF) Theory NX-rays = Natoms∫σ(E)φ'(E)dE . [ω(exp(-∑iμiδxi))εΔT] (1) For a monoenergetic beam of energy, Eo , NX-rays = Natoms σ(Eo)φ(Eo)ω(exp(-∑iμiδxi))εΔT (2) where NX-rays Natoms σ ω φ μi δxi ε ΔT = = = = = = = = = number of background–subtracted X-rays number of target atoms seen by the beam photoelectric cross section fluorescence yield flux of incident X-rays linear attenuation coefficient(s) Secondary X-ray pathlength(s) photopeak detection efficiency detector live time (cm2) (#/cm2.s) (/cm) (cm) (s) Advanced Photonics Laboratory 39 Incident wave 1 Incident wave 2 2 a Scattered wave Scattered by atom 1 and atom 2 Phase shift = 2ka+2δ1+δ2 Phase shift due to scattering at atom 1, reflection back from atom 2, and then back to atom 1 is 2ka+2δ1+δ2 Phase shift = ka+δ1 Sum of incident planewave + scattered wave = 1+expi(2ka+2δ1+δ2) Total absorption intensity ~ E*E ~ 1+sin(2ka+2δ1+δ2) 79 Advanced Photonics Laboratory 80 “EXAFS Equation” Scattering intensity of the jth atom Number of atoms j “disorder” term (decreases intensity of oscillations) 2 χ (k ) = ∑ 2 N j f j (k ) exp −2 k σ exp kR j 2 Electron scattering (λ=inelastic mean free path) −2 R j / λ ( k ) sin (2kR j + δ j (k ) ) Interference factor Sum over atoms Advanced Photonics Laboratory 40 X-ray Absorption Spectroscopy Excitation of a specific core electron with monochromatic X-ray source Events prior to and subsequent to ejection of electron provide useful information about the system XANES EXAFS (X-ray Absorption Near-Edge Spectroscopy) Information Provided Presence or absence of specific bonds Oxidation state Orientation (Extended X-ray Absorption Fine Structure) Information Provided Identity of neighbors Neighbor coordination numbers Interatomic distances Thermal or static disorder 81 Advanced Photonics Laboratory XANES Theory A visual representation Outgoing photoelectron wave Backscattered wave Interaction of waves from absorber and backscattering neighbor yield information on the system. 82 Advanced Photonics Laboratory 41 X-ray Absorption Spectrum μ(E) EXAFS XANES 9000 10000 Photon Energy 11000 83 Advanced Photonics Laboratory XANES Theory •The near edge region is dominated by multiplescattering of the low kinetic energy photoelectron by neighboring atoms •Edge position is a function of oxidation state •Coordination and molecular orbitals influence peak shapes •Comparison with known spectra allows for ‘fingerprinting’ of samples 84 Advanced Photonics Laboratory 42 85 Electronic and atomic structures study of BaxSr1-xTiO3 by XAS spectroscopy Advanced Photonics Laboratory 86 Introduction Kuo et. al. reported that tetragonal to cubic phase transition occurs at x=0.75 and a new structure ordering transition around x=0.4 in BaxSr1-xTiO3 (BST) system when samples were prepared by sol-gel technique. X-ray Raman Shou-Yi Kuo et. al., Phys. Rev. B 64, 224103 (2001) Advanced Photonics Laboratory 43 87 Experimental BaxSr1-xTiO3 prepared by sol-gel technique x=0, 0.2, 0.4, 0.5, 0.6, 0.75 and 1 Samples: Ba(Sr)-O-Ti : O K-edge Side view O-Ti-O : Ti K,L3,2-edge Ba,Sr Ti Ba-O Ba-Ti : Ba,Sr K-edge Ba-Ba(Sr) O Data Collection: Facilities BeamLine S.R.R.C (Taiwan) HSGM Edge O K-edge Ti L3,2-edge Wiggler-C Ti K-edge BL12B2 Ba K-edge Sr K-edge Spring-8 (Japan) Shou-Yi Kuo et. al., Phys. Rev. B 64, 224103 (2001) Advanced Photonics Laboratory 88 Detector Storage ring I0 monochromator Detector Detector Sr K-edge spectrum (A) XANES (A) (A) Normailze Absorption (eV) ~50eV Detector Sample X-ray absorption near edge spectrum -Electronic Configuration*Valence, site symmetry… (B) (B) EXAFS (B) Extend x-ray absorption fine structure ~50-1000eV -Structural information16000 16200 16400 Photon Energy (eV) 16600 16800 *Distance, Debye-Waller Factor, number of neighboring atoms… Advanced Photonics Laboratory 44 89 Results and Discussion A1 D1 C1 B1 E1 X=1 X=0.75 X=0.6 A1 B1 O2p-Ti3d C1 D1 O2p-Ba5d/Sr4d E1 O3p-Ti4sp O K-edge after subtracting a Gaussian X=0.5 X=0.4 X=0 X=0.4 X=0.2 X=0.6 X=0.75 X=0.5 X=1 O K-edge X=0.2 Normalized Absorption (Arb. Units) Normalized Absorption (Arb. Units) O K-edge Gaussian X=0 TiO2 530 535 540 545 0.8 0.4 0.0 Photon Energy (eV) 529 530 531 532 Photon Energy (eV) R. Brydson et. al., J. Phys. Condens. Matter 1, 797 (1989) Advanced Photonics Laboratory 90 Normalized Absorption (Arb. Units) Ti L3,2-edge D2 B2 C2 A2 X=1 L3 : A2 B2 O2p3/2-Ti3dπ O2p3/2-Ti3d σ L2 : C2 D2 O2p1/2-Ti3dπ O2p1/2-Ti3d σ X=0.75 X=0.6 X=0.5 X=0.4 X=0.2 X=0 TiO2 455 460 465 470 Photon Energy (eV) Arctangent G. Van der Laan et. al., Phys. Rev. B41, 12366 (1990) Advanced Photonics Laboratory 45 91 X=0.75 Sr K-edge X=0 X=0.2 X=0.4 X=0.5 X=0.6 X=0.75 8 X=0.6 X=0.5 3 B5 kx Magnitude of FT (Arb. Units) 10 X=0.4 A5 X=0.2 6 X=0 4 C5 4 D5 6 8 k( 2 0 0 2 4 Radial Distance ( ) 6 A5 B5 C5 D5 -1 10 12 ) Sr-O Sr-Ti Sr-Sr Sr-Ba Advanced Photonics Laboratory 92 Li - U 1. Particle identification 2. Surface impurity 3. Film impurity 4. Chemical structure identification Li - U 1. Particle identification 2. Surface impurity 3. Film impurity Advanced Photonics Laboratory 46 93 Auger electron 2p1/2, 2p3/2 L2,3 2s L1 Photoelectron hν Incident electron (or hν) 1s K Ekin = EK - EL - EL Ekin = hν - EB 1 3 Advanced Photonics Laboratory 94 Ekin = EK - EL - EL 1 N(E) Intensity N(E) Ekin = hν - EB KLL Auger peak 3 Energy loss Elastic Peak peak Binding Energy (eV) EF Core levels Valence Conduction Band Band dN(E)/dE 0 eV Kinetic energy (eV) Ep Al Kα → hν = 1486.6 eV λ = 8.34 Å Advanced Photonics Laboratory 47 95 Electron Microprobes and X-ray Emission Electron microscopy (usually SEM) can also be used to perform X-ray emission analysis in a manner similar to X-ray fluorescence analysis see the X-ray spectrometry lecture for details on the spectra The electron microprobe (EM) is the commonly used name for this type of X-ray spectrometry Both WDS and EDS detectors are used (as in XRF), elemental mapping Not particularly surface sensitive! Advanced Photonics Laboratory Electron Microprobes: X-ray Emission 96 Advanced Photonics Laboratory 48 Electron Spectroscopy 97 Electron spectroscopy – measuring the energy of electrons. Major forms: Auger electron spectroscopy X-ray/UV photoelectron spectroscopy Electron energy loss spectroscopy (EELS) Advanced Photonics Laboratory 98 Electron Spectroscopy: Surface Sensitivity Electrons can only escape from shallow depths in the surface of a sample, because they will undergo collisions and lose energy. Advanced Photonics Laboratory 49 99 Auger Electron Spectrometry (AES) The Auger process can also be a source of spectral information. Auger electrons are expelled from atomic/molecular orbitals and their kinetic energy is characteristic of atoms/molecules However, since it is an electron process, analysis of electron energy is necessary! This is unlike the other techniques we have discussed, most of which measure photon wavelengths or energy Auger electron emission is a three-step (three electron) process, that leaves an atom doubly-ionized Advanced Photonics Laboratory 100 AES: Basic Mechanism Advanced Photonics Laboratory 50 101 AES: Basic Mechanism Auger electrons are created from outer energy levels (i.e. less-tightly bound electrons, possibly valence levels). This example would be called a LMM Auger electron. Other Common types are denoted KLL and MNN. Advanced Photonics Laboratory 102 AES: Efficiency of Auger Electron Production Two competing processes: X-ray fluorescence Auger electron emission Auger electrons predominate at lower atomic number (Z) Photoelectron emission does not compete! ωK = number of K photons produced number of K shell vacancies created ω Auger = 1 − ω K 激發出電子後不是直接放出x-ray 就是產生Auger電子 Advanced Photonics Laboratory 51 103 AES: Spectrometer Design AES instruments are designed like an SEM – often they are integrated with an SEM/EDXA system Unlike an SEM, AES instruments are designed to reach higher vacuum (10-8 torr) Electron detector Electron Gun Energy analyzer from adsorbed gases, etc… Basic components: Electron source/gun Electron energy analyzer Electron detector Control system/computer Ion gun (for depth profiling) Io Gu n n Helps keep surfaces clean and free Auger electrons Sample Advanced Photonics Laboratory 104 AES (and XPS): Electron Energy Analyzers Two types of electron energy analyzers (also used in XPS): Electrons only pass if their KE is: KEelectron = ke ΔV k= R1 R2 R22 − R12 Concentric hemispherical analyzer (HSA higher resolution) – better resolution, mostly for XPS/UPS Cylindrical mirror analyzer (CMA higher efficiency) More common for AES 易於聚焦與調整 Advanced Photonics Laboratory 52 105 AES: Detectors More sophisticated detectors are needed to detect low numbers of Auger electrons. Two types of electronmultiplier detectors: Discrete dynode Continuous dynode Both types of detector are also used in XPS/UPS!!! Advanced Photonics Laboratory 106 AES: Surface Analysis AES is very surface sensitive (10-50 Ǻ) and its reliance on an electron beam results in excellent lateral resolution Electron beam does not have to be monochromatic Note: an X-ray beam can also be used for AES, but is less desirable b/c it cannot currently be focused as tightly (as is the case in XPS) Auger electrons typically have energies of < 1000 eV, so they are only emitted from surface layers. 能量太小無法從深處跑出來 Advanced Photonics Laboratory 53 107 AES: Spectral Interpretation AES Electron Kinetic Energies* versus Atomic Number (Most intense peaks only. Valid for CMA-type analyzers.) *Data is from J.C. Vickerman (Ed.), "Surface Analysis: The Principal Techniques“, John Wiley and Sons, Chichester, UK, 1997 . Image from http://www.cem.msu.edu/~cem924sg/KineticEnergyGraph.html (accessed 12-Nov-2004) Advanced Photonics Laboratory AES: Typical Spectra 108 Advanced Photonics Laboratory 54 AES: Elemental Surface Analysis 109 Very common application of AES - elemental surface analysis For true surface analysis, AES is better than SEM/X-ray emission (electron microprobe) because it is much more surface sensitive AES can be easily made quantitative using standards. 適合做表面成份的定量分析 Image from http://www.cem.msu.edu/~cem924sg/ (accessed 12-Nov-2004) Advanced Photonics Laboratory AES: Chemical Shifts Chemical information (i.e. on bonding, oxidation states) should be found in Auger spectra because the electron energy levels are sensitive to the chemical environment. In practice, it is not (usually) there because too many electron energy levels are involved – it is difficult to calculate and simulate Auger spectra. 110 不太適合做表面chemical shift的分析 Advanced Photonics Laboratory 55