Birck Nanotechnology Center XPS: X-ray Photoelectron Spectroscopy ESCA: Electron Spectrometer for Chemical Analysis Dmitry Zemlyanov Birck Nanotechnology Center, Purdue University
Outline Introduction A History of the Photoelectric Effect and ESCA Basic Physics Photoemission from solids Binding Energy and Chemical Shift Information Depth Spectral Features Advanced Features Angle Resolved XPS, Depth Profiling, X-Y Mapping Conclusions 2
Introduction to XPS: Brief Historical Review The process of using photons (light) to remove electrons from a bulk material is called photoemission. Hertz (1880) Spark enhancement Hallwachs (1888) Negatively charge Zn plate discharged J.J. Tomson (1899) Light induced electron emission Einstein (1905) Photoelectric effect explained (Nobel prize rewarded in 1921) Steinhardt and Serfass (1951) Photoemission was applied as analytic tool (ESCA) (Nobel prize rewarded in 1981) 3
Introduction to XPS: General Concepts Type of samples: any vacuum compatible materials (no gas or liquid).* * A specially designed high pressure XPS instruments can analyze gas and liquid ( see for instance: ESCA applied to free molecules by K. Siegbahn, et.al, American Elsevier Publishing Company, Inc., New York, 1967 4
Introduction to XPS: General Concepts Energy ω E B hν E ϕ = K 1s 2s 2p E K (The Einstein equation) Vacuum level Fermi level ϕ hν 1s 2s 2p 1s E B E E B B = E ( n 1) E ( n) ε final k initial (Koopmans Theorem) 5
Introduction to XPS: General Concepts Energy of photons and photoelectrons: 1 ev 1.60 10-19 J ev -1 Intensity: electron Volt, ev count per second, cps The energy of a photon is given by the Einstein relation : E = hν h - Planck constant ( 6.62 x 10-34 J s ) ; ν- frequency (Hz) of the radiation. Frequency of 1 ev photon is 2.4 10 15 Hz ; for 1000 ev photon 2.4 10 18 Hz. Wave length of 1 ev photon is 1.23 μm, for 1000 ev photon 1.2 nm. X-ray Photoelectron Spectroscopy (XPS) using soft x-ray (50-5000 ev) radiation to examine core-levels. Ultraviolet Photoelectron Spectroscopy (UPS) using vacuum UV (10-50 ev) radiation to examine valence levels. 6
Introduction to XPS: Units and Spectroscopic Notations 7
Introduction to XPS: Units and Spectroscopic Notations n l j X-Ray Level Electron Level 1 0 1/2 K 1s 2 0 1/2 L 1 2s 2 1 1/2 L 2 2p 1/2 2 1 3/2 L 3 2p 3/2 3 0 1/2 M 1 3s 3 1 1/2 M 2 3p 1/2 3 1 3/2 M 3 3p 3/2 3 2 3/2 M 4 3d 3/2 3 2 5/2 M 5 3d 5/2 8
Introduction to XPS: Qualitative Analysis ELEMENT ANALYSIS Every chemical element has an unique electronic structure, thereby the electrons are emitted with specific kinetic energies. The emission lines for almost all elements are well tabulated. 1 Intensity(CPS) 300 250 200 150 100 50 x10 3 Survey Spectrum or Wide Scan C KLL 1200 O KLL 800 O 1s N 1s = K hν E ϕ 1 See, for instance, NIST X-ray Photoelectron Spectroscopy Database (the National Institute of Standards and Technology, http://srdata.nist.gov/xps/) 400 Binding Energy (ev) C 1s Si 2p Si 2s 0 E B 1s2s 2p (The Einstein equation) hν =1486. 6 ev for Al Kα radiation 9
Introduction to XPS: Quantitative Analysis 45 x 10 3 50 2 x 10 70 x 10 2 40 35 Fe 2p 45 Cr 2p 65 60 O 1s 30 55 25 40 50 CPS 20 CPS CPS 45 15 10 5 35 30 40 35 30 735 730 725 720 715 710 705 Binding Energy (ev) 590 580 570 Binding Energy (ev) 536 534 532 530 528 526 Binding Energy (ev) Normalised Area i = Area of Photoemission Peaki Re lativesensetivity Factor Transmission Fanction( E i kin ) IMFP C ( atomic %) i = N j Normalised Normalised Area i Area j 10
Introduction to XPS: General Concepts The diagram below shows a real XPS spectrum obtained from a Ag foil using Al Kα radiation (1486.6 ev) The main peaks occur at kinetic energies of ca. 350, 770, 880, 915, 1110, 1120, 1390, 1430 and 1480 ev. Since the energy of the radiation is known it is a trivial matter to transform the spectrum BE= hν-ke so that it is plotted against BE as opposed to KE. The most intense peak is now seen to occur at a binding energy of ca. 370 ev. 11
Introduction to XPS: General Concepts Note: In the case of X-ray induced Auger emission it is really meaningless to refer to an associated binding energy. Working downwards from the highest energy levels (closest to Fermi edge): the valence band (4d) emission occurs at a binding energy of ca. 5 ev. the emission from the 4p and 4s levels gives rise to very weak peaks at 60 and 97 ev respectively the most intense peak at ca. 368 ev is due to emission from the Ag 3d levels, whilst the 3p and 3s levels give rise to the peaks at ca. 572/605 ev and 720 ev respectively. the remaining peak is not an XPS peak at all! It is an Auger peak arising from X-ray induced Auger emission. It occurs at a kinetic energy of ca. 350 ev. 12
Introduction to XPS: Spin-Orbit Splitting Closer inspection of the spectrum shows that emission from some levels (most obviously 3p and 3d ) does not give rise to a single photoemission peak, but a closely spaced doublet. The 3d photoemission is in fact split between two peaks, one at 368.3 ev and the other at 374.3 ev, with an intensity ratio of 3:2. This arises from spin-orbit coupling effects in the initial state. 13
Introduction to XPS: Spin-Orbit Splitting Intensity ratio for spin-orbit coupling doublets: Orbital momentum Doublets ratio p p 3/2 : p 1/2 2 : 1 d d 5/2 : d 3/2 3 : 2 f f 7/2 : f 5/2 4 : 3 14
Introduction to XPS: Chemical Shift E E B B = E ( n 1) E ( n) ε final k initial (Koopmans Theorem) INITIAL STATE EFFECT If the energy of the atom s initial state changed, for example by formation of chemical bond with other atoms, the E B of the electrons in that atom will change. E +δ E Β (+δ) Fermi level E Β ( δ) Original level δ E E B = ε k 15
Introduction to XPS: Chemical Shift C*-C Type 1 C*-N Type 2 C*-O Type 3 O=C*-N (amide) Type 4 O=C*-OH (carboxyl) Type 5 RGD silane 20 13 7 18 2 1 C*-N 3 (arginine) Type 6 16
Introduction to XPS: Chemical Shift C -N & C -O * * Intensity, arb. units C * F x OH-C * =O C * N 3 Amide C * -C Residual Hydrocarbons Experimental data Curve-fitting result Peptide constraint 295 290 285 280 Binding Energy, ev The C 1s spectrum obtained from the powder RGD peptide C*-C Type 1 C*-N Type 2 C*-O Type 3 O=C*-N (amide) Type 4 O=C*-OH (carboxyl) Type 5 RGD silane 20 13 7 18 2 1 C*-N 3 Type 6 17
Introduction to XPS: Final State E E B B = E ( n 1) E ( n) ε final k initial (Koopmans Theorem) FINAL STATE EFFECT Final state effects are those factors that influence the states of the atom after the photon has hit it or affect the photoelectron while it is leaving. 18
Introduction to XPS: Final State FINAL STATE EFFECTS: Relaxation The photoemission event has left a hole in a core level. Delocalization of this localized hole due to inflow ( diffusion ) of charge is called relaxation. Intra-atomic relaxation The core hole is delocalized due to rearrangement of electrons in the orbitals of the excited atom. Inter-atomic (extra-atomic) relaxation The core hole is delocalized due to movement of electrons from the surrounding atoms in the material. Localized core hole 1s2s 2p Consequences the leaving electron can escape at higher kinetic energy: the binding energy of the electron to decrease. Relaxation will NOT cause an extra peak to appear in the spectra. 19
Introduction to XPS: Final State FINAL STATE EFFECTS: Satellites Shake-up and shake-off satellites arise when the photoelectron imparts energy to another electron of the atom. Shake-up satellite Shake-off satellite Consequences The photoelectron loses kinetic energy and appears at higher binding energy in the spectrum. Extra peak might appear in the spectrum. 20
Introduction to XPS: Final State 21
Introduction to XPS: Final State FINAL STATE EFFECTS: Plasmons Extrinsic satellites occur during transport of electron to surface. Discrete loss structure is observed. Consequences The photoelectron loses kinetic energy and appears at higher binding energy in the spectrum. Extra peaks might appear in the spectrum. Energy loss (plasmon) lines associated with the 2s line of aluminium (a = 15.3 ev; note surface plasmon at b) 22
Introduction to XPS Photoemission consists of three steps: Absorption and ionization (initial state effects) Response of atom on creation of photoelectron (final state effects) Transport of electron to surface and escape (extrinsic losses) 23
Introduction to XPS: Information depth X-ray can readily travel through solids. Actually, X-rays of 1KeV, will penetrate 1000 nm (1µm) or more into material. Can we say the same about electrons? NO!!! Electrons of 1KeV energy will only penetrate approximately 5nm (0.005µm). 24
Introduction to XPS: Information depth "Universal curve" of electron inelastic mean free path l (IMFP) versus KE (ev) IMFP is average distance between inelastic collisions (Å) 25
Introduction to XPS: Angle Resolved XPS N elastic = { d } N 0 ω ( { }) 1 exp d λ cosθ For normal takeoff angle, cosθ = 1 When d = λ, 63.3 % of electrons come from within 1λ of surface. When d = 2 λ, 86.4 % of electrons come from within 2 λ of surface. When d = 3 λ, 95.0 % of electrons come from within 3 λ of surface Typical λ varies from 10 Å to 35 Å. P = exp λ cosθ Probability of electron escaping without loss N elastic is the number of electron escaping without loss N 0 is the total number of electron generated 26
Introduction to XPS: Angle Resolved XPS ω 27
Introduction to XPS: Angle Resolved XPS Intensity, a.u. θ Photoemission Sample O=C-OH PEG features C-O C-C θ = 75 θ = 60 θ = 45 θ = 30 Relative contribution, % 80 0 36.7 53.1 66.4 78.5 70 80 60 70 60 50 50 40 40 30 Angle, θ S bound to Au S bound S to non-bound Au to Au S non-bound to Au 20 20 1.0 0.8 0.6 0.4 0.2-10 0 10 20 30 40 50 60 70 80 Angle cosθ θ Photoemission Sample S non-bound to Au S bound to Au θ = 75 θ = 60 θ = 45 θ = 30 θ = 0 Residual hydrocarbons 290 285 Binding Energy, ev OH O O O 7 S O O O SH O 7 O O S 7 O O S 7 OH HS O O 7 O OH OH OH O O O O O S 7 θ = 0 166 164 162 160 Bindimg Energy, ev Au The cleaned gold substrates were then immersed into 0.01 M thiolated polyethylene glycol (PEG) acid (HSC 2 H 4 (OC 2 H 4 ) 8 COOH) in ethanol. 28
Introduction to XPS: Angle Resolved XPS Angle Resolved XPS is non-destructive depth profiling of 5nm topmost layer!!! 29
Introduction to XPS: Depth Profiling Sputtering by Ar + + + ( He, Ne etc) X-ray gun Photoelectrons ω Sample Sample Depth profiling by ion sputtering is destructive methods!!! 30
Introduction to XPS: Depth Profiling SiO 2 SiO SiN Si 0 PECVD SiO 2 SiO SiN Intensity, a.u. SiO x interface 1 st layer of SiO 2 Si layer 2 nd layer of SiO 2 105 100 95540 535 530 525 Binding Energy, ev 405 400 395 31
Introduction to XPS: Depth Profiling 32
Introduction to XPS: Depth Profiling Depth profiling by ion sputtering is destructive methods!!! 33
Introduction to XPS: XPS Imaging Some spectrometer can image by scanning the analysis area, also described as the virtual probe of emitted photoelectrons, across sample sequentially to form a map of elemental distribution at the surface with special resolution as low as 5 µm. The mapped area is typically 1.8x1.8 mm. 34
Introduction to XPS: XPS Imaging 35
Introduction to XPS: XPS Imaging Survey Spectrum Si 0 SiO x Intensity (cps) SiO 2 105 102 99 Oxygen Carbon Silicon 1200 1000 800 600 400 200 0 Binding Energy (ev) Surface composition of Silicon Cantilever was studied with 55µm spot. Only oxygen, silicon and carbon were found. The XPS image was obtained using O1s emission. 36
Introduction to XPS: Conclusion Information derived from an XPS experiment (the data obtained from the outermost ~5 nm) Element composition (except H and He) with sensitivity >0.1 at.% Molecular environment (oxidation state, bonding atoms, etc.) Non-destructive depth profiles ~5 nm into the sample using angular-resolved XPS (AR XPS) Destructive depth profiles several hundred nanometers into the sample using ion sputtering Lateral variation in surface compounds (X-Y mapping with spatial resolution of 10-150 µm) Identification of bonding orbital using valence band spectra 37
The Kratos patented magnetic immersion lens A charge neutralization system Spherical mirror and concentric hemispherical analyzers combined with the newly developed delay-line detector (DLD) Fast load lock with cryo/heating options A catalytic cell to facilitate substrate treatment and preparation Kratos Ultra DLD Imaging XPS (Birck 1077) Contact to Dmitry Zemlyanov (dzemlian@purdue.edu ) Monochromized Al and Ag anodes External ports for user-supplied facilities 38