Dr. Tim Nunney Thermo Fisher Scientific, East Grinstead, UK Dr. Nick Bulloss Thermo Fisher Scientific, Madison, WI, USA Dr. Harry Meyer III Oak Ridge

Transcription

1 Dr. Tim Nunney Thermo Fisher Scientific, East Grinstead, UK Dr. Nick Bulloss Thermo Fisher Scientific, Madison, WI, USA Dr. Harry Meyer III Oak Ridge National Laboratory, TN, USA

2 Introduction New materials create new characterization challenges Full characterization requires a variety of experimental techniques 2

3 Introduction Techniques need to be chosen that provide complimentary information Elemental and chemical composition Molecular fingerprinting Composition with increasing depth Surface structure Morphology XPS Surface Analysis Microanalysis 3

4 Introduction to X-ray Microanalysis In an electron column, electrons are accelerated through an electric field, acquiring kinetic energy. The energy is transferred to the sample Yields a variety of signals for analysis 4

5 EDS Basic Overview Energy Dispersive X-Ray Spectroscopy (EDX / EDS) Analysis of elemental content of micro-volumes using non-destructive techniques. Based on inner electron transitions between inner atomic shells. Energetic electron from an electron column dislodges an orbital electron. Electron from a higher energy shell fills the vacancy, losing energy in the process. The lost energy appears as emitted radiation of energy (characteristic X-Ray). 5

6 Electron Beam & Sample Interaction Interaction Volume The volume in which the primary electrons interact with a sample As electrons interact with the sample, they are scattered and spread. 6

7 Properties of X-Rays The relationship between Energy, Frequency, and Wavelength may be expressed by one formula E is the Energy of the radiation is the Wavelength of the radiation F is the Frequency of the radiation c is the speed of light 3 x cm/second h is Planck s constant x ev-sec = x erg-sec 7

8 Detectors Energy-dispersive detectors (EDS) Wavelength-dispersive detectors (WDS) Collect the full range of x-ray energies simultaneously Collect one element at a time via a diffracting crystal 8

9 Definition of a surface XPS surface analysis What is a surface? XPS measures Surfaces using XPS and angle resolved XPS Ultra-thin films using XPS and angle resolved XPS (ARXPS) Thin films using XPS in combination with sputter profiling Surface (1 nm) 3 atomic layers The modified layer is often far too thin to be characterized with most techniques. The extreme surface sensitivity of XPS ensures that only the top few nanometers of the sample are analyzed. Bulk Ultra-thin film (1 to 10 nm) 3-30 atomic layers Thin Film (10 nm to 2µm) atomic layers Note: Approximate layer thickness only. Actual values depend upon materials 9

10 XPS instrumentation UHV System Ultra-high vacuum keeps surfaces clean Allows longer photoelectron path length X-ray source Typically Al Ka radiation Monochromated using quartz crystal Low energy electron flood gun Low energy e - (and possibly Ar + ) Analysis of insulating samples Ion gun Typically noble gas ions Sample cleaning Depth profiling Electron transfer lens Flood gun Hemispherical analyser Detector Ion gun Mono crystal X-ray source 10

11 Basic XPS theory X-rays Photoelectron ejected Binding Kinetic Energy (ev) NaCl Surface is composed of atoms Electrons surround the nucleus of an atom, occupying orbitals at different energies Surface is irradiated with X-rays from a photon source X-rays cause electrons to be ejected (photoelectrons) Kinetic energy, KE, of photoelectrons is measured by an analyser The binding energy, BE, of the electrons is deduced from the kinetic energy and photon energy BE = h -KE Binding energy depends upon Element Orbital from which electron was ejected Chemical state of the element 11

12 Surface analysis and XPS O Elemental identification Which elements are present? Elemental identification Can detect all elements except for H Elemental quantification How much of an element is present? Detection limit >0.05% for most elements Allows determination of stoichiometry Zn 1200 Zn Fe F Ca C Cl S P Element At% P 0.29 S 0.29 Cl 0.22 C Ca O F 1.50 Fe 6.74 Zn 3.03 Mg 0.12 Binding energy / ev Elemental identification of tribology sample 12

13 Surface analysis and XPS Poly(ethylene terephthalate), PET Carbon O C C O C C O O n Elemental identification Which elements are present? Can detect all elements except for H Elemental quantification How much of an element is present? Detection limit >0.05% for most elements Allows determination of stoichiometry Chemical state identification and quantification Bonding states for each element Chemical structure Chemical state identification Binding energy / ev High energy resolution spectroscopy

14 Surface analysis and XPS Poly(ethylene terephthalate), PET Oxygen O C C O C C O O n Elemental identification Which elements are present? Can detect all elements except for H Elemental quantification How much of an element is present? Detection limit >0.05% for most elements Allows determination of stoichiometry Chemical state identification and quantification Bonding states for each element Chemical structure Chemical state identification Binding energy / ev High energy resolution spectroscopy

15 Case Study 1: Membrane electrode assembly

16 MEA fuel cell Introduction Schematic of polymer electrolyte membrane Pt/C Nafion K-Alpha optical image of ULAMprepared MEA fuel cell MEA fuel cell Energy/environmental application Fuel cells for clean conversion of hydrogen 1 Proton exchange membrane fuel cell (PEMFC) High conversion efficiency and low/zero pollution Low operating temperature and relatively quick start-up Membrane Electrode Assembly (MEA) is important component of PEMFC Active layer of platinum/carbon black catalyst with polymeric adjacent layers Practical problem Diffusion of Pt into adjacent polymer layers would reduce Pt/C catalytic effect Solution XPS imaging of sample ULAM (ultra low angle microtomy) sample preparation to allow imaging of nanoscale layers Quantitative elemental and chemical state mapping 16 1 Korean J Chem Eng, 23(4), (2006)

17 MEA fuel cell Chemical analysis of Nafion Backscatter image of cross-section of MEA fuel cell component Anode Anode Membrane Cathode Membrane Data points MEA fuel cell Chemical analysis of Nafion Membrane Electrode Assembly (MEA) consists of layers microns thick Thin Pt/C layers around a thicker Nafion layer Nafion is an effective proton exchange membrane Permits hydrogen ion transport while preventing electron conduction Requirement to analyze profile of Pt in Nafion layer Layers are too thick for XPS sputter profiling Not enough data points per layer with standard cross-sectioning Use ultra-low angle microtomy to create crosssection with layers on micron scale 2 Enables XPS mapping and imaging analysis Data points Cathode Backscatter image of ULAM-prepared MEA fuel cell component 17 2 Journal of Materials Science 40 (2005)

18 MEA fuel cell Chemical analysis of Nafion Pt/C Pt/C Nafion ULAM-prepared MEA fuel cell Nafion MEA fuel cell Chemical analysis of Nafion XPS can distinguish carbon bonding states Epoxy (used in making the ULAM cross-section) C-C, C-O and C=O bonding observed Nafion (part of the MEA) CF 2 bonding observed Surface contamination of Nafion gives additional C-C and C-O peaks Epoxy 18

19 MEA fuel cell Elemental quantification of MEA-ULAM sample Pt/C Nafion MEA fuel cell Elemental quantification of MEA-ULAM sample XPS can elementally quantify small areas on a sample Quantification of Pt/C, Nafion and Epoxy zones below Elemental quantification of MEA-ULAM sample ULAM-prepared MEA fuel cell Fluorine observed in Pt/C zones Nafion intentionally mixed with Pt/C during manufacture of catalyst layer Sulfur detected in all regions, principally in Nafion and Pt/C zones XPS is able to detect elements at low concentration (S<0.5at% in Pt/C zone) Pt detected in catalytically active layer At% Element At% Pt S 0.44 C O F Atomic concentration of elements in Pt/C layer 0.00 Pt (x50) S (x50) C O F Element Nafion Pt/C Epoxy 19

20 MEA fuel cell Large area XPS mapping No Pt in Nafion zone Pt in catalytically active layers MEA fuel cell Large area XPS mapping Possible to quantify elements/chemical states over a wide sample area Map of Pt/C and Nafion zones shown below, overlaid on optical image of MEA-ULAM sample Linescan from map confirms no large scale diffusion of Pt 0% Pt in Nafion, in agreement with spectroscopic result on previous slide Quantified Pt at% linescan taken from large area XPS map Pt/C Nafion Pt/C Pt/C Linescan from mapping data Pt/C Nafion Principal components phase map of MEA sample Large area XPS map of Pt / Nafion layers and interfaces in ULAM-MEA fuel cell sample 20

21 Membrane Electrode Assembly Analyses performed at 10kV on a FESEM Pt/C Nafion Spectral Image net count maps (background subtracted) 21

22 Membrane Electrode Assembly Linescan analyses (50 points) Pt Si C Pt F No apparent migration from the Pt/C electrode into the Nafion 22

23 Case Study 2: Thin film solar cell

24 Delaminated CIGS solar cell 3 SEM of a Cu(In,Ga)Se 2 solar cell (cross-section) and its mode of operation 3 D. Abou-Ras et al. Elemental distribution profiles across Cu(In,Ga)Se 2 solar-cell absorbers acquired by various techniques. In: M. Luysberg, K. Tillmann, T. Weirich (Eds.), EMC 2008, Vol 1: Instrumentation and Methods, Proceedings of the 14th European Microscopy Congress 2008, Aachen, Germany, September 1-5, 2008 (Springer, 2008) p Energy/environmental application Solar cells based on Cu(In, Ga)Se 2 (CIGS) Thin-film stack on glass (or in this case steel) Mo and Zn oxide layer form electrical contacts p-type CIGS film (sunlight absorber) and n-type CdS film form p-n junction Excellent efficiency Low cost compared to thicker silicon-based solar cells Practical problem Delamination of device Controlling film composition and interfacial chemistry between layers (affects electrical properties) Solution EDS imaging of good & delaminated areas High resolution elemental information XPS imaging & sputter depth profiling Elemental and composition information as a function of depth Identify delamination layer 24

25 Delaminated CIGS solar cell EDS Point Analysis Delamination zone Some of the CIGS material had delaminated from the substrate. Measurements were taken from the three distinct areas of the sample: (1 & 2) the top layers of the sample (3) the metallic surface electrical contact material (4) the underlying metallic substrate. 25

26 Delaminated CIGS solar cell EDS Point Analysis 26

27 Delaminated CIGS solar cell EDS Point Analysis Locations 1 and 2 on the film are the same material consisting of a majority of In and Se with a small amount of Cu and a small amount of Ga. Only small amounts of Zn, O, Cd and S are measured due to the thin nature of these layers. Location 3 on the metallic contact layer is silver. Location 4 where the film is removed shows the base Mo substrate that the layers are grown on. 27

28 Delaminated CIGS solar cell XPS Point Analysis XPS point analysis at the same locations as the EDS analysis 28

29 Delaminated CIGS solar cell XPS Elemental analysis Se3d Counts / s Ag3d C1s Mo3d In3d 1 & 2 Zn2p3 O1s Sn3d5 4 Ga2p3 3 Red spectrum is average of points 1 & Binding Energy (ev) 29

30 Area 1 XPS Chemical analysis 3.40E+04 Sn3d Scan 1.80E+05 In3d Scan 3.20E E+05 Counts / s 3.00E E E E E+04 SnO Element Counts / s 1.40E E E E E E+04 Element InOx 2.00E E E E Binding Energy (ev) Binding Energy (ev) 6.00E+04 O1s Scan 7000 C1s Scan Counts / s 5.00E E E E+04 Metal CO3 Metal oxide Counts / s carbonate C-C or C-H carbide Cl2s (Metal ClO4) Chemical state analysis suggests that points 1 & 2 are indium-tin-oxide (ITO) 1.00E Binding Energy (ev) Binding Energy (ev) 30

31 Area 3 XPS Chemical analysis 3000 S2p Scan 1.40E+04 C1s Scan 1.60E+05 Ag3d Scan Counts / s Metal SO4 organic SO2 Metal SO3 Element Metal S Counts / s 1.20E E E E E+03 carbonate C-C or C-H carbide Cl2s (Metal ClO4) Counts / s 1.40E E E E E E E+04 Element Ag2O AgO E E Binding Energy (ev) Binding Energy (ev) Binding Energy (ev) 2900 Cl2p Scan 1.50E+04 O1s Scan Counts / s Metal ClO4 Metal Cl Counts / s 1.40E E E+04 Metal CO3 Metal oxide E E Binding Energy (ev) Binding Energy (ev) 31

32 Delaminated CIGS solar cell Point analysis comparison EDS Ga-K Zn-K O-K Cr-K In-L Cd-L Ag-L Mo-L Se-K Fe-K Cu-K Area Area Area Area XPS Ga2p3 Zn2p3 O1s Sn3d5 In3d Ag3d Mo3d Se3d C1s S2p Cl2p Area 1/ Area Area EDS shows the presence of the expected stack components. XPS shows the presence of small contaminants, and the nature of the outer layer. 32

33 Delaminated CIGS solar cell XPS Mapping 33

34 Delaminated CIGS solar cell XPS Phase Analysis Phase 1 Phase 2 Phase 3 Phase 4 Phase 5 Se3d S2p Mo3d C1s Ag3d Cd3d In3d Sn3d O1s A O1s B Zn2p Ga2p

35 CIGS solar cell Cross-section of CIGS film stack Low Magnification A Spectral Imaging mapping analysis was performed at low magnification on the cross-section sample to understand all of the layers that constitute the material. The thickness of all of the layers was approximately 1/3 mm. 35

36 CIGS solar cell Cross-section of CIGS film stack 36

37 CIGS solar cell Cross-section of CIGS film stack 37

38 Delaminated CIGS solar cell XPS Depth Profile 3keV Ar+ ion beam 60s etch time Compucentric rotation Depth scale calibrated to EDS result 38

39 Delaminated CIGS solar cell XPS Depth Profile ITO CIGS Mo Cr Steel Atomic percent (%) ZnO CdS Se3d S2p Mo3d C1s Cd3d5 In3d5 Sn3d5 O1s A O1s B Cr2p3 Fe2p3 Cu2p3 Zn2p3 Ga2p Etch Depth (nm) 39

40 Delaminated CIGS solar cell XPS Depth Profile 1.40E E+04 In3d E+04 Atomic percent (%) Counts / s 8.00E E E E E Binding Energy (ev) Se3d S2p Mo3d C1s Cd3d5 In3d5 Sn3d5 O1s A O1s B Cr2p3 Fe2p3 Cu2p3 Zn2p3 Ga2p Etch Depth (nm) 40

41 Delaminated CIGS solar cell XPS Depth Profile CIGS Mo Cr Steel Atomic percent (%) Se3d S2p Mo3d C1s In3d5 O1s A O1s B Cr2p3 Fe2p Etch Depth (nm) 41

42 Delaminated CIGS solar cell XPS Depth Profile ITO CIGS Mo Cr Steel ZnO CdS 42

43 Delaminated CIGS solar cell XPS Depth Profile CIGS Mo Cr Steel 43

44 Delaminated CIGS solar cell XPS Depth Profile Delamination zone Ga/In gradient Ag contact 44

45 Acknowledgements Oak Ridge National Laboratory (HTML) RJ Lee Group Application specialists in East Grinstead, UK & Madison, USA 45