PHOTOELECTRON SPECTROSCOPY (PES)

Transcription

1 PHOTOELECTRON SPECTROSCOPY (PES)

2 NTRODUCTON Law of Photoelectric effect Albert Einstein, Nobel Prize 1921 Kaiser-Wilhelm-nstitut (now Max-Planck- nstitut) für Physik Berlin, Germany High-resolution electron spectroscopy Kai Siegbahn, Nobel Prize 1981 Uppsala university, Sweden

3 BASC CONCEPT E hω E k Detector (E f ) φ E vac Valence band E F E B Discrete core levels Where, E B hω φ E kin E hω φ B E kin Binding energy referred to the Fermi level Photon energy Work function of spectrometer Kinetic energy of ejected photoelectron

4 ENERGY REFERENCE E hω kin E B φ sample e - E E hω kin E B hω kin E B φ φ sample spec. ( φ ). φsample spec Vacuum level Vacuum level φ sample φ spec. Fermi level Same level Fermi level hω E B Spectrometer Core level Sample E hω B E kin φ spec.

5 BASC REQUREMENT OF PES SYSTEM 1. A monochromatic photon source. 2. An electron energy analyser (which can disperse the emitted electrons according to their kinetic energy, and thereby measure the flux of emitted electrons of a particular energy. 3. A high vacuum environment.

6 LGHT SOURCES X-rays Al K α Mg K α 1486 ev 1254 ev XPS, ESCA UV He-lamp Ne-lamp H-lamp 21.2 ev 40.8 ev 16.8 ev 26.9 ev 4 hv 12 ev UPS Synchrotron radiation PES

7 SYNCHROTRON RADATON Atom nucleus Atom Protein Cell 4000 ev 10 ev Wavelength (meter) Hard X-ray Ultraviolet light nfrared light Radio wave Visible light Unique feature of Synchrotron radiation Ultra-bright light, high polarization Highly directional, collimated beam Wide spectrum of wavelength, stretching from R to X-rays Spot size ~1x3 mm 2 High energy resolution

8 BASC REQUREMENT OF PES SYSTEM 1. A monochromatic photon source. 2. An electron energy analyser (which can disperse the emitted electrons according to their kinetic energy, and thereby measure the flux of emitted electrons of a particular energy. 3. A high vacuum environment.

9 ELECTRON ENERGY ANALYSERS

10 ELECTRON ENERGY ANALYSER E k E pass Retarding Voltage (E ret.) E k -E ret E pass

11 ENERGY RESOLUTON N(E k ) N(E k ) E E E E kin E E kin N V f OUT Schematic view of a spherical deflector analyser. Source and exit slit widths s and w. Sphere radii R 1 and R 2 and the optical radius R m (R 1 +R 2 )/2.

12 ENERGY RESOLUTON V f E pass e R ( R 2 1 R R 1 2 ) E pass V f Energy resolution E E pass 2 s R m + QAnalyser give a w 2R m 2 +α broading Choose: s, w and E pass small

13 BASC REQUREMENT OF PES SYSTEM 1. A monochromatic photon source. 2. An electron energy analyser (which can disperse the emitted electrons according to their kinetic energy, and thereby measure the flux of emitted electrons of a particular energy. 3. A high vacuum environment.

14 WHY ULTRA HGH VACUUM (UHV) S NEEDED? Consider an ideal gas. The number of gas atoms/molecules striking a unit area of a surface per second is given by; n But N V ν 8RT where ν 4 πm N N Na pv nrt RT N V R a P T n Na 4R 8R π p MT 2.6x10 24 p MT 1 ( p 2 m s in N m 2 ) 3.5x10 22 p MT 1 ( p 2 m s in torr)

15 AN EXAMPLE Ex. Assume an atom of radius of ca. 3 Å. Then there is room for ca atom on a cm 2. With p 10-6 torr M 28kg/kmol n 3x10 14 (1/cm 2 s) T 300 K f every atom sticks to the surface a monolayer is formed in about 3 sec!. At p torr a monolayer is formed in about 3x10 4 seconds 8 hours. Big differences (in sticking probability) between different materials. (From experience at torr Si, Au etc days, Al, Cu etc. hours, Ti, Mo etc. minutes)

16 HOW UHV CAN BE OBTANED? Good choice of material selected ex. stainless steel Good design of chamber Good pumping systems Long time baking

17 SAMPLE SZE? On a larger scale, the sample surface is hit with photon flux focused in a small spot. Electrons emitted by the photon are from ca Å of the surface layers depending on electron escape depth in the sample investigated.

18 SENSTVTY Electrons : give a high surface sensitivity Since λ mean free path is small in solids λ average distance an electron travels before it is inelastically scattered, i.e. Suffers energy losses λ(e k ) increases monotonically with E kin (for E kin 50 ev)

19 ELECTRON S FLUX The flux of electrons of a given energy and momentum decays exponentially with the distance travelled in a material. x 0 e x λ 0 θ Small λ gives high surface sensitivity x cosθ 0 e x λ cosθ Electrons that have not been inelastically scattered have been generated close to the surface. The surface sensitivity can be enhanced by selecting a larger electron emission angle, θ. Normal emission, θ0 o.

20 A SMPLE LAYER ATENUATON MODEL d d d θ tot tot 0 0 (1 + e 1 1 e d ( ) λ d λ + e 2d ( ) λ + e 3d ( ) λ +...) Normal emission o o o tot 0 1 e 1 d λ cosθ Angle dependent d d d θ d Surf bulk 0 tot e d λ cosθ o o o Surf layer Surf bulk e d λ cosθ 1

21 z 0 d. 1, overlayer from ) (1 overlayer no with i.e. 0 0 d thickness of overlayer with e Thus top on overlayer when e by attenuated is signal substrate The e c dz ce c dz ce d Subst Overl d d d z Overlayer z Clean Substrate λ λ λ λ λ λ λ

22 UNQUE ADVANTAGE FOR PES TECHNQUES Photoelectron Spectroscopy (PES) + Angle resolved photoelectron spectroscopy (ARPES) Examine core-level hv ev Elementals annalysis Quantitative analysis Thickness Location Examine valence levels hv ev Density of state (DOS) Surface band structure

23 ELEMENTALS ANALYSS Binding Energy 2s 117eV Binding Energy 2p 73 ev Binding Energy 2s 149 ev Binding Energy 2p 99 ev KNETC ENERGY (ev)

24 KNETC ENERGY (ev) Chemical shifts E F B exhibits chemical shifts (~1-10 ev) depending on chemical surrounding nformation about the surrounding elements can be obtained. Surface shifts n solids a specific signal from the atoms in the outermost surface layers can be obtained since they feel a different surrounding (potential) than atoms in the bulk. Surface shiftes < 1 ev.

25 QUANTTATVE ANALYSS

26 PHOTOONZATON CROSS SECTONS 4f 5d 5s 5p

27 CHARACTERZATON Carbon? ntensity (relative units) C 1s Graphite like carbon θ e 75 o SiC SiO 2 SiC θ e < 75 o Binding Energy (ev) ( g) ( SiO2 ) C ; Si ( SiC) ( SiC)

28 TECHNQUES C0.05 Å Layers attenuation model 4 SiO 2 SiC 25.5 Å C C C C g C SiC e dox λ cosθ C e ( e λ cosθ C d g e 1) Ratios 2 Si Si C C Si SiO Si SiC d ox 2 λsi cosθ e ( e 1) 0 80 C/Si x Emission angle C Thickness A:oxide, B: bulk SiC, C: graphite g) C ; Si ( SiC) ( SiO ( 2 ) ( SiC) A σ B CB Oxide λ ln + 1 B σ A CA Graphite λ ln C B σ B σ C C C B C e d 1 λ + 1

29 PHOTOELECTRON SPECTROSCOPY ADVANTAGES Good energy resolution (chemical shifts & surface shifts) Semiquantitative analysis, within a few %, fairly easy DSADVANTAGE Poor spatial resolution (in most cases).