Photoelectron Spectroscopy. Xiaozhe Zhang 10/03/2014

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1 Photoelectron Spectroscopy Xiaozhe Zhang 10/03/2014

2 A conception last time remain Secondary electrons are electrons generated as ionization products. They are called 'secondary' because they are generated by other radiation (the primary radiation). This radiation can be in the form of ions, electrons, or photons with sufficiently high energy, i.e. exceeding the ionization potential. Photoelectrons can be considered an example of secondary electrons where the primary radiation are photons.

3 What is photoelectron spectroscopy? Photoelectron spectroscopy utilizes photo-ionization and analysis of the kinetic energy distribution of the emitted photoelectrons to study the composition and electronic state of the surface region of a sample

4 Auger Electron Auger electron and photoelectron e - released to analyze Free e e - of high energy that will occupy the vacancy of the core level 1 e - gun e - Vacancy 1, 2, 3 and 4 are the order of steps in which the e - s will move in the atom when hit by the e - gun.

5 Auger electron and photoelectron

6 Photoelectron spectroscopy XPS, also known as ESCA, is the most widely used surface analysis technique because of its relative simplicity in use and data interpretation. XPS ESCA UPS PES X-ray Photoelectron Spectroscopy Electron Spectroscopy for Chemical Analysis Ultraviolet Photoelectron Spectroscopy Photoemission Spectroscopy

7 Analytical Methods

8 Equation KE=hν-E B -Ø KE Hν Kinetic Energy (measure in the XPS spectrometer) photon energy from the X-Ray source (controlled) Ø spectrometer work function. It is a few ev, it gets more complicated because the materials in the instrument will affect it. Found by calibration. E B Binding energy(be), is the unknown variable

9 Equation KE=hv-E B -Ø The equation will calculate the energy needed to get an e - out from the surface of the solid. Knowing KE, hv and Ø the (BE)E B can be calculated.

10 # of electrons KE versus BE(E B ) KE can be plotted depending on BE Each peak represents the amount of e - s at a certain energy that is characteristic of some element. BE increase from right to left 1000 ev 0 ev E E E Binding energy (ev) KE increase from left to right

11 # of electrons Interpreting XPS Spectrum: Background The X-Ray will hit the e - s in the bulk (inner e - layers) of the sample N = noise e - will collide with other e - from top layers, N4 decreasing its energy to contribute to the N3 noise, at lower kinetic energy than the N2 peak. N1 The background noise increases with BE because the SUM of all noise is taken from the beginning of the analysis. Binding energy N tot = N1 + N2 + N3 + N4

12 Analytical Methods

13 Orbital splitting

14 Orbital splitting

15 XPS Sampling Depth

16 XPS Sampling Depth Sampling Depth is defined as the depth from which 95% of all photoelectrons are scattered by the time they reach the surface ( 3λ) Most λ s are in the range of 1 ~ 3.5 nm for Al Kα radiation So the sampling depth (3λ) for XPS under these conditions is 3 ~ 10 nm

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18 XPS spectrum example The XPS peaks are sharp. In a XPS graph it is possible to see Auger electron peaks. The Auger peaks are usually wider peaks in a XPS spectrum. Aluminum foil is used as an example on the next slide.

19 XPS Spectrum O 1s O Auger O because of Mg source C Al Al O 2s Sample and graphic provided by William Durrer, Ph.D. Department of Physics at the Univertsity of Texas at El Paso

20 Auger Spectrum Characteristic of Auger graphs The graph goes up as KE increases. Sample and graphic provided by William Durrer, Ph.D. Department of Physics at the Univertsity of Texas at El Paso

21 Identification of XPS Peaks The plot has characteristic peaks for each element found in the surface of the sample. There are tables with the KE and BE already assigned to each element. After the spectrum is plotted you can look for the designated value of the peak energy from the graph and find the element present on the surface.

22 XPS Imaging

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26 Thank you for your time!