X-ray Photoelectron Spectroscopy/ Electron spectroscopy for chemical analysis (ESCA), By Francis Chindeka

Transcription

1 X-ray Photoelectron Spectroscopy/ Electron spectroscopy for chemical analysis (ESCA), By Francis Chindeka

2 X-ray photoelectron spectroscopy (XPS) or Electron spectroscopy for chemical analysis (ESCA), Surface analysis technique Elemental composition, Empirical formula, Chemical and electronic states of the elements that exist within a material. Requires ultra high vacuum (P ~ 10 8 to 10 9 millibar)

3 XPS Background XPS is a technique that relies on the photoemission/ photoelectric effect discovered by Heinrich Rudolf Hertz in 1887 It is based on Einstein s idea about the photoelectric effect, developed around It was later developed in the mid 1960 s by Dr. Siegbahn and his research group.

4 The photoemission process BE (ev) = hν - KE - φ (Energy hν) binding energy (BE) is the energy with which the electron was bound to the nucleus φ represents the combined electron spectrometer and sample work functions and is an instrument dependent factor normally derived for each instrument as part of a calibration procedure

5 wikipedia.org Key components of the XPS instrument

6 XPS setup (cylindrical mirror analyser)

7 Overview of typical XPS Instrument (Kratos axis ultra DLD)

8 DATA AQUISITION Depth profiling Optical imaging XPS imaging Wide /survey scans High resolution scans

9 General Requirements Wear powder free gloves Tools: clean and demagnetised. Samples must be stable in an Ultra High Vacuum environment. reduce sample size to avoid cross contamination Pumping the samples overnight in the STC may also be advisable,

10 VISION MANAGER SOFTWARE

11 Constant height bar tool SAMPLE HOLDERS SAMPLE STUB Sample mounting clips Plane sample bar Constant height bar The standard sample bar has a flat surface 130 x 15 mm. Samples up to 4 mm thick may be secured on the standard bar using the sample mounting clips or double sided, conductive adhesive tape.

12 Loading samples Samples loaded into Sample Treatment Chamber (SAC) SAC

13 STAGE: sample positioning Vacuum bellows Autostage fork Stepper motors Categories of XPS: 1. X-ray source defines the area of analysis (have variable xray beam), 2. and those which use the electron transfer lens (have fixed xray beam=1mm). Spot diameters as small as 10 μm

14 Sample Analysis Chamber (SAC) UHV Detector Ion gun Sample Sample holder

15 Energy levels, Why? XPS spectral peaks are identified by the BE of the shell from which the electron was ejected (1s, 2s, 2p, etc.) Binding energy (ev) of electrons on different atomic level : K L M 1s 1/2 2s 1/2 2p 1/2 2p 3/2 3s 1/2 3p 1/2 3p 3/2 3d 3/2 3d 5/2 6 C O Si Cl

16 Depth profiling acceptance cone of spectrometer rastering ion beam ion beam etch pit sample surface Specimen Uses ion gun for etching in order to investigate the composition of a sample as a function of depth. As XPS is a surface sensitive technique the depth to which information is gathered from is usually approximately 5 nm. Kratos instrument manual

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18 Optical imaging Takes sample pictures similar to smart phones Also used for alignment in conjunction with XPS Imaging to align Xray beam, analysis spot and energy analyser entry

19 XPS Imaging (x ray beam takes a picture of the elemental composition of sample surface)

20 Wide /survey scans C1s O 1s BE/eV XPS BE Lookup table Literature Atomic conc C1s 93.10% O1s 6.90%

21 High resolution scans Data analysis Chemical bonding causes a shift in BE thus it can be used for identification of chemical bonds e.g. «an atom A linked to a more electronegative atom B emits photoelectrons with lower kinetic energy (higher binding energy) than the atom A linked to itself, and conversely.» BE (ev) = hν - KE - φ Thus the peak position (BE) is modified The ability to discriminate between different chemical environments is one of the major strength of the XPS technique.

22 Chemical shift and electronegativity The chemical shift (few ev) is the same for all electronic level of an atom E > 0 if B is more electronegative than A ; E < 0 if B is less electronegative than A the effect of the different neighbour atoms are additive. E > 0 B is more electronegative than A E < 0 B is less electronegative than A Simple atom (ref) Increasing BE BE ref BE ref

23 C 1s High resolution scans C1s Peak synthesis and curve fitting Identify the underlying peaks CPS CPS BE/ ev BE/eV 284.4eV, C=C 285.5eV, C-O,C=O 286.6eV, C-O 287.9eV, C=O Series5 Series6 Envelope Quantification report XPS BE Lookup table Literature

24 Surface modification characterization N H 2 N 3 HCl, NaNO 2 (i) N 2 N 3 N 3 N 3 N 3 (ii) electrochemical grafting N 3 Au Au-PAz N 3 O O MTPrOPhOPc CuI, DMF/ACN, N 3 N N N 3 N O O N N N N M N N N N O N 3 O O M= H 2 (5), Co (6) or Mn (7) N N N 3 N O Au-PAz-MPrOPhOPc

25 XPS Characterization: Survey Spectra XPS survey spectra of (a) (i) bare Au and (ii) Au-PAz and (b) click reaction with (i) H 2 TPrOPhOPc (5), (ii) CoTPrOPhOPc (6) and (iii) MnTPrOPhOPc (7). Electrodes Elements Au (At %) N 1s (At %) O 1s (At %) C 1s (At %) Si 2p (At %) Au Au-PAz Au-PAz-H2TPrOPhOPc Au-PAz-CoTPrOPhOPc Cr 4p (At %) Au-PAz-MnTPrOPhOPc Origin of elements Au (gold surface) N 1s, C 1s and O 1s (PAz and Pcs) Si and Cr (gold surface)

26 XPS Characterization: High res. Carbon Atom Component Positions (ev) Assignments Component Percent (%) N 1s C 1s N 1s C 1s C 1s high resolution spectra of (a) Au-PAz monolayer and (b) Au-PAz-H 2 TPrOPhOPc N=N + =N N=N + =N Au-C C-H, C-C, C=C C-N N-H C-N, -N=N + =N C=N, N=N + =N Au-C C-H, C-C, C=C C-N Pro (C-O) Ph(C-O-C)Pc 3.11

27 THANK YOU