Cro

Cr2O3

CrO3

Cr2S3

Cr2(SO4)3

K2CrO4

PbCrO4

CrF3

Cr3C2

CrB

CrN

CrSi

CrSi2

SS-304

Basic XPS Information Section

  The Basic XPS Information Section provides fundamental XPS spectra, BE values, FWHM values, BE tables, overlays of key spectra, histograms and a table of XPS parameters.
The Advanced XPS Information Section is a collection of additional spectra, overlays of spectra, peak-fit advice, data collection guidance, material info,
common contaminants, degradation during analysis, auto-oxidation, gas capture study, valence band spectra, Auger spectra, and more.
Published literature references, and website links are summarized at the end of the advanced section.
→  Periodic Table – HomePage                       XPS Database of Polymers               → Six (6) BE Tables

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Chromium (Cr)

 

Chromite – FeCr2O4	Chromium – Cro	Uvarovite  – Ca3Cr2(SiO4)3

 

Page Index
Pure Element Spectra with Peak-fits IMFP and Cross-sections for Pure Element Native Oxide Spectra with Peak-fits Pure Oxide Spectra with Peak-fits	Valence Band Spectra Six (6) Tables of Chemical State BEs  Histograms of NIST BEs Advanced XPS Information Section	Peak-fits and Overlays of Cr Chemical Compounds Flood Gun Effects on Native Oxide Spectra Study of UHV Gas Capture after Cleaning	Auger Peaks and Spectra Contamination XPS Facts, Guidance, Information Chemical State Spectra from Literature

• Expert Knowledge & Explanations

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Chromium (Cr^o) Metal

 Peak-fits, BEs, FWHMs, and Peak Labels

 

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	.
Chromium (Cro) Metal Cr (2p) Spectrum – raw spectrum ion etched clean	Chromium (Cro) Metal Peak-fit of Cr (2p) Spectrum w/o asymm
→  Periodic Table – HomePage
Chromium (Cro) Metal Cr (2p) Spectrum – extended range	Chromium (Cro) Metal Peak-fit of Cr (2p) Spectrum (w asymm)
.
Chromium (Cro) Metal Cro metal (3p) and (3s) – raw spectrum	Chromium (Cro) Metal Peak-fit of Cro metal (3p) and (3s)
→  Periodic Table – HomePage   Survey Spectrum of Chromium (Cro) Metal with Peaks Integrated, Assigned and Labelled
→  Periodic Table  XPS Signals for Chromium (Cro) Metal Spin-Orbit Term,  BE (eV) Value, and Scofield σ for Aluminum Kα X-rays (1486 eV, 8.33 Ang) Overlaps Spin-Orbit Term BE (eV) Value Scofield σ from 1486 eV X-rays IMFP (TPP-2M) in Å F (1s) overlaps Cr (2s) 695 3.91 14.5   Cr (2p1/2) 584 3.98 16.1 Te (3d) overlaps Cr (2p3/2) 574.29 7.69 16.1 Al (2p), Pt (4f), Cu (3p) overlaps Cr (3s) 74 0.596 22.4   Cr (3p) 43 1.173 22.8 σ:  abbreviation for the term Scofield Photoionization Cross-Section which is used with IMFP and TF to generate TSFs and atom% quantitation Energy Loss Peaks ~20 eV Auger Peaks Energy Loss:  ~20 eV above peak max Expected Bandgap for Cr2O3: ~3 eV  *Scofield Cross-Section (σ) for C (1s) = 1.0 →  Periodic Table   Valence Band Spectrum from Cro Metal  Fresh exposed bulk produced by extensive Ar+ ion etching   Plasmon Peaks from Cro Metal  Fresh exposed bulk produced by extensive Ar+ ion etching Cr (2p) – Extended Range Spectrum Cr (2p) – Extended Range Spectrum – Vertically Zoomed   →  Periodic Table Cr (LMM) Auger Peaks from Cro   Fresh exposed bulk produced by extensive Ar+ ion etching Cro metal Native Cr Oxide Features Observed xx xx xx →  Periodic Table
Artefacts Caused by Argon Ion Etching
Chromium Carbide(s) can form when ion etched Reactive Metal Surfaces capture Residual UHV Gases (CO, H2O, CH4 etc)	Argon Trapped in Cro can form when Argon Ions are used to removed surface contamination
Side-by-Side Comparison of Chromium Native Oxide & Chromium Oxide (Cr2O3) Peak-fits, BEs, FWHMs, and Peak Labels
Cr Native Oxide	Cr2O3  (man-made single crystal, <100>)
Cr (2p) from Cr Native Oxide Flood Gun OFF As-Measured, C (1s) at 285.0 eV	Cr (2p) from Cr2O3 – exposed bulk of crystal Flood Gun ON Charge Referenced to C (1s) at 285.0 eV
→  Periodic Table
	.
Cr Native Oxide	Cr2O3 – (man-made single crystal, <100>)
C (1s) from Cr Native Oxide on Chromium As-Measured, C (1s) at 285.0 eV (Flood Gun OFF)	C (1s) from Cr2O3 – exposed bulk of crystal Flood Gun ON Charge Referenced to C (1s) at 285.0 eV
Cr Native Oxide	Cr2O3 – (man-made single crystal, <100>)
O (1s) from Cr Native Oxide on Chromium As-Measured, C (1s) at 285.0 eV (Flood Gun OFF)	O (1s) from Cr2O3 – exposed bulk of crystal Flood Gun ON Charge Referenced to C (1s) at 285.0 eV
Cr Native Oxide	Cr2O3 – (man-made single crystal, <100>)
Cr (KLL) Auger Peaks from Cr Native Oxide on Chromium As-Measured, C (1s) at 285.0 eV (Flood Gun OFF)	Cr (KLL) Auger Peaks from Cr2O3 – exposed bulk of crystal Flood Gun ON Charge Referenced to C (1s) at 285.0 eV

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Survey Spectrum of Chromium Native Oxide
with Peaks Integrated, Assigned and Labelled

 

→  Periodic Table 

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Survey Spectrum of Chromium Oxide (Cr₂O₃)
with Peaks Integrated, Assigned and Labelled

→  Periodic Table  

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Overlays of Cr (2p) Spectra for
Cr Native Oxide and Chromium Oxide, Cr₂O₃ 

Caution: BEs from Grounded Native Oxides can be Misleading if Flood Gun is ON

Overlay of Cr metal and Cr Native Oxide – Cr (2p) Native Oxide C (1s) = 285.0 Flood gun OFF	Overlay of Cr metal and Cr2O3 – Cr (2p) Pure Oxide C (1s) = 285.0 eV Chemical Shift:  2.0
→  Periodic Table	Copyright ©:  The XPS Library

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Overlay of of Cr (2p)
Cr^o Metal, Cr Native Oxide, & Cr₂O₃ (xtal)

Features Observed

• xx
• xx
• xx

→  Periodic Table 

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Valence Band Spectra
Cr^o, Cr₂O₃

Cro Ion etched clean	Cr2O3 – (single crystal, <100>) Flood gun is ON,  Charge referenced so C (1s) = 285.0 eV

Overlay of Valence Band Spectra for Cr metal and Cr2O3

Features Observed

• xx
• xx
• xx

→  Periodic Table 

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Chromium Minerals, Gemstones, and Chemical Compounds
Lopezite – K2Cr2O7	Eskolaite – Cr2O3	Tarapacaite – K2(CrO4)	Crocite -PbCrO4

→  Periodic Table 

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Six (6) Chemical State Tables of Cr (2p_3/2) BEs

 

• The XPS Library Spectra-Base
• PHI Handbook
• Thermo-Scientific Website
• XPSfitting Website
• Techdb Website
• NIST Website

→  Periodic Table 

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Notes of Caution when using Published BEs and BE Tables from Insulators and Conductors:

• Accuracy of Published BEs
  • The accuracy depends on the calibration BEs used to calibrate the energy scale of the instrument.  Cu (2p3) BE can vary from 932.2 to 932.8 eV for old publications 
  • Different authors use different BEs for the C (1s) BE of the hydrocarbons found in adventitious carbon that appears on all materials and samples.  From 284.2 to 285.3 eV
  • The accuracy depends on when the authors last checked or adjusted their energy scale to produce the expected calibration BEs
• Worldwide Differences in Energy Scale Calibrations
  • For various reasons authors still use older energy scale calibrations 
  • Some authors still adjust their energy scale so Cu (2p3/2) appears at 932.2 eV or 932.8 eV because this is what the maker taught them
  • This range causes BEs in the higher BE end to be larger than expected 
  • This variation increases significantly above 600 eV BE
• Charge Compensation
  • Samples that behave as true insulators normally require the use of a charge neutralizer (electron flood gun with or without Ar+ ions) so that the measured chemical state spectra can be produced without peak-shape distortions or sloping tails on the low BE side of the peak envelop. 
  • Floating all samples (conductive, semi-conductive, and non-conductive) and always using the electron flood gun is considered to produce more reliable BEs and is recommended.
• Charge Referencing Methods for Insulators
  • Charge referencing is a common method, but it can produce results that are less reliable.
  • When an electron flood gun is used, the BE scale will usually shift to lower BE values by 0.01 to 5.0 eV depending on your voltage setting. Normally, to correct for this flood gun induced shift, the BE of the hydrocarbon C (1s) peak maximum from adventitious carbon is used to correct for the charge induced shift.
  • The hydrocarbon peak is normally the largest peak at the lowest BE. 
  • Depending on your preference or training, the C (1s) BE assigned to this hydrocarbon peak varies from 284.8 to 285.0 eV.  Other BEs can be as low as 284.2 eV or as high as 285.3 eV
  • Native oxides that still show the pure metal can suffer differential charging that causes the C (1s) and the O (1s) and the Metal Oxide BE to be larger
  • When using the electron flood gun, the instrument operator should adjust the voltage and the XY position of the electron flood gun to produce peaks from a strong XPS signal (eg O (1s) or C (1s) having the most narrow FWHM and the lowest experimentally measured BE. 

→  Periodic Table 

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Table #1

Cr (2p_3/2) Chemical State BEs from:  “The XPS Library Spectra-Base”

C (1s) BE = 285.0 eV for TXL BEs
and C (1s) BE = 284.8 eV for NIST BEs

Element	Atomic #	Compound	As-Measured by TXL or NIST Average BE	Largest BE	Hydrocarbon C (1s) BE	Source
Cr	24	Cr23C6	574.2 eV		285.0 eV	The XPS Library
Cr	24	Cr-B2	574.3 eV		285.0 eV	The XPS Library
Cr	24	Cr-B2 (N*1)	574.3 eV		284.8 eV	Avg BE – NIST
Cr	24	Cr – element	574.4 eV		285.0 eV	The XPS Library
Cr	24	Cr3C2	574.5 eV		285.0 eV	The XPS Library
Cr	24	Cr2-S3 (N*2)	574.8 eV	575.4 eV	284.8 eV	Avg BE – NIST
Cr	24	Cr-N	575.5 eV		285.0 eV	The XPS Library
Cr	24	Cr-O2 (N*1)	576.3 eV		285.0 eV	The XPS Library
Cr	24	Cr-2O3	576.7 eV		285.0 eV	The XPS Library
Cr	24	Cr-(OH)3 (N*3)	577.0 eV	577.4 eV	284.8 eV	Avg BE – NIST
Cr	24	CuCr2O4 (N*1)	577.1 eV		284.8 eV	Avg BE – NIST
Cr	24	Cr(NO3)3 (N*1)	577.3 eV		284.8 eV	Avg BE – NIST
Cr	24	Cr-Cl3  (N*2)	577.4 eV	577.8b eV	284.8 eV	Avg BE – NIST
Cr	24	Cr-O3 (N*4)	578.9 eV	580.1 eV	285.0 eV	The XPS Library
Cr	24	CrF3 (N*3)	579.3 eV	580.3 eV	284.8 eV	Avg BE – NIST
Cr	24	Li2CrO4 (N*2)	579.8 eV		284.8 eV	Avg BE – NIST
Cr	24	Na2CrO4 (N*2)	579.8 eV	580.5 eV	284.8 eV	Avg BE – NIST
Cr	24	Cr-(OH)6			285.0 eV	The XPS Library
Cr	24	Cr-Ox			285.0 eV	The XPS Library
Cr	24	Cr-Si			285.0 eV	The XPS Library

Charge Referencing Notes

• (N*number) identifies the number of NIST BEs that were averaged to produce the BE in the middle column.
• The XPS Library uses Binding Energy Scale Calibration with Cu (2p_3/2) BE = 932.62 eV and Au (4f_7/2) BE = 83.98 eV.  BE (eV) Uncertainty Range:  +/- 0.2 eV
• Charge Referencing of insulators is defined such that the Adventitious Hydrocarbon C (1s) BE (eV) = 285.0 eV.  NIST uses C (1s) BE = 284.8 eV 
• Note:   Ion etching removes adventitious carbon, implants Ar (+), changes conductivity of surface, and degrades chemistry of various chemical states.
• Note:  Ion Etching changes BE of C (1s) hydrocarbon peak.
• TXL – abbreviation for: “The XPS Library” (https://xpslibrary.com).  NIST:  National Institute for Science and Technology (in USA)

→  Periodic Table 

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Table #2

Cr (2p_3/2) Chemical State BEs from:  “PHI Handbook”

C (1s) BE = 284.8 eV

→  Periodic Table 

Copyright ©:  Ulvac-PHI

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Table #3

Cr (2p_3/2) Chemical State BEs from:  “Thermo-Scientific” Website

C (1s) BE = 284.8 eV

Chemical state	Binding energy Cr (2p3)
Cr metal	574.3
Cr (III) oxide	~576
Cr (VI) oxide	~580

→  Periodic Table 

Copyright ©:  Thermo Scientific 

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Table #4

Cr (2p_3/2) Chemical State BEs from:  “XPSfitting” Website

Chemical State BE Table derived by Averaging BEs in the NIST XPS database of BEs
C (1s) BE = 284.8 eV

 

→  Periodic Table 

Copyright ©:  Mark Beisinger

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Table #5

Cr (2p_3/2) Chemical State BEs from:  “Techdb.podzone.net” Website

 

XPS Spectra – Chemical Shift | Binding Energy
C (1s) BE = 284.6 eV

XPS(X線光電子分光法)スペクトル 化学状態 化学シフト ケミカルシフト Element Level Compound B.E.(eV) min   max Cr 2p3/2 Cr 574.4 ±0.1 574.3 ～ 574.5 Cr 2p3/2 Cr Nitride 576.0 ±0.3 575.7 ～ 576.2 Cr 2p3/2 CrBr3 576.1 ±0.2 575.9 ～ 576.3 Cr 2p3/2 K3Cr(CN)6 576.4 ±0.4 576.0 ～ 576.8 Cr 2p3/2 CrOOH 577.0 ±0.4 576.6 ～ 577.3 Cr 2p3/2 Cr(acac)3 577.0 ±0.9 576.1 ～ 577.8 Cr 2p3/2 CrCl3 577.4 ±0.4 577.0 ～ 577.7 Cr 2p3/2 Cr(OH)3 577.4 ±0.4 577.0 ～ 577.8 Cr 2p3/2 Oxide 578.1 ±1.7 576.4 ～ 579.8 Cr 2p3/2 K2Cr2O7 579.9 ±0.3 579.6 ～ 580.2 Cr 2p3/2 CrF3 580.5 ±0.5 580.0 ～ 580.9

→  Periodic Table 

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Histograms of NIST BEs for Cr (2p_3/2) BEs

Important Note:  NIST Database defines Adventitious Hydrocarbon C (1s) BE = 284.8 eV for all insulators.

Histogram indicates:  574.2 eV for Cro based on 15 literature BEs	Histogram indicates:  576.5 eV for Cr2O3 based on 24 literature BEs

Table #6

NIST Database of Cr (2p_3/2) Binding Energies

NIST Standard Reference Database 20, Version 4.1

Data compiled and evaluated
by
Alexander V. Naumkin, Anna Kraut-Vass, Stephen W. Gaarenstroom, and Cedric J. Powell
©2012 copyright by the U.S. Secretary of Commerce on behalf of the United States of America. All rights reserved.

Important Note:  NIST Database defines Adventitious Hydrocarbon C (1s) BE = 284.8 eV for all insulators.

 

→  Periodic Table 

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Statistical Analysis of Binding Energies in NIST XPS Database of BEs

 

 

→  Periodic Table 

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Advanced XPS Information Section

Expert Knowledge, Spectra, Features, Guidance and Cautions  

for XPS Research Studies on Chromium Materials

 

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Expert Knowledge Explanations

→  Periodic Table 

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Chromium Chemical Compounds

 

Peak-fits and Overlays of Chemical State Spectra

Pure Chromium, Cro:  Cr (2p) Cu (2p3/2) BE = 932.6 eV	Cr2O3:  Cr (2p) C (1s) BE = 285.0 eV	CrF3:  Cr (2p) C (1s) BE = 285.0 eV

Features Observed

• xx
• xx
• xx

→  Periodic Table 

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Overlay of Cr (2p) Spectra shown Above

C (1s) BE = 285.0 eV

 Chemical Shift between Cr vs Cr₂O₃: 2.0 eV
 Chemical Shift between Cr vs CrF₃:  5.4 eV

→  Periodic Table 

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Native Oxide on Chromium, (Cr^o)
Naturally Formed in lab air at 25 C^o 1 atm 

Survey Spectrum from Native Oxide on Cro Flood gun is OFF, C (1s) BE = 286.8 eV	Cr (2p) Chemical State Spectrum from Native Oxide on Cro Flood gun is OFF, C (1s) BE = 286.8 eV
	.
O (1s) Chemical State Spectrum from Native Oxide on Cro Flood gun is OFF, C (1s) BE = 286.8 eV	C (1s) Chemical State Spectrum from Native Oxide on Cro Flood gun is OFF, C (1s) BE = 286.8 eV

Features Observed

• xx
• xx
• xx

→  Periodic Table 

 

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Chromium Oxide (Cr₂O₃)
exposed bulk of single crystal

Survey Spectrum from Cr2O3 Flood gun is ON, C (1s) BE = 285.0 eV	Cr (2p) Chemical State Spectrum from Cr2O3 Flood gun is ON, C (1s) BE = 285.0 eV
	.
O (1s) Chemical State Spectrum from Cr2O3 Flood gun is ON, C (1s) BE = 285.0 eV	C (1s) Chemical State Spectrum from Cr2O3 Flood gun is ON, C (1s) BE = 285.0 eV
	.
Cr (3p) Chemical State Spectrum from Cr2O3 Flood gun is ON, C (1s) BE = 285.0 eV Freshly cleaved to expose bulk	Cr (3s) Chemical State Spectrum from Cr2O3 Flood gun is ON, C (1s) BE = 285.0 eV Freshly cleaved to expose bulk3s Multiplet Splitting = 4.0 eV
	.
Valence Band Spectrum from Cr2O3 Flood gun is ON, C (1s) BE = 285.0 eV Freshly cleaved to expose bulk	Auger Signals from Cr2O3 Flood gun is ON C (1s) BE = 285.0 eV
Shake-up Features for Cr2O3
Multiplet Splitting Features for Chromium Compounds
Cro metal – NO Splitting for Cr (3s)	Cr2O3 Compound – Multiplet Splitting Peaks for Cr (3s)
	.
Cro metal – NO Splitting for Cr (2p3/2)	Cr2O3 Compound – Multiplet Splitting Peaks for Cr (2p3/2)

Features Observed

• xx
• xx
• xx

→  Periodic Table 

 

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Chromium Chemical Compounds

 

→  Periodic Table 

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Quantitation Details and Information

Quantitation by XPS is often incorrectly done, in many laboratories, by integrating only the main peak, ignoring the Electron Loss peak, and the satellites that appear as much as 30 eV above the main peak.  By ignoring the electron loss peak and the satellites, the accuracy of the atom% quantitation is in error.

When using theoretically calculated Scofield cross-section values, the data must be corrected for the transmission function effect, use the calculated TPP-2M IMFP values, the pass energy effect on the transmission function, and the peak area used for calculation must include the electron loss peak area, shake-up peak area, multiplet-splitting peak area, and satellites that occur within 30 eV of the main peak.

→  Periodic Table 

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Flood Gun Effect on Native Oxide of Chromium

 

Native Oxide of Chromium Sheet – Sample GROUNDED
versus
Native Oxide of Chromium Sheet – Sample FLOATING

 

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Native Oxide of Chromium Sheet – Sample Grounded

Electron Flood Gun:  0 Voltage (FG OFF), Min Voltage versus Max Voltage

Cr (2p)	O (1s)	C (1s)
→  Periodic Table

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Native Oxide of Chromium Sheet – Sample Floating

Electron Flood Gun:  0 Voltage (FG OFF), Min Voltage versus Max Voltage

Cr (2p)	O (1s)	C (1s)
→  Periodic Table

→  Peri

 

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AES Study of UHV Gas Captured by Freshly Ion Etched Chromium

Chromium sheet was ion etched and allowed to react with residual UHV gases in ion pumped AES overnight – ~14 hr run.

Cr (LMM) Signal: Cr at front -> CrOx at rear Cr KE = 522.1 eV,    CrOx KE = 522.7 eV	O (KLL) Signal: Cr at front -> CrOx at rear O KE = 508eV

Features Observed

• xx
• xx
• xx

→  Periodic Table 

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AES Chemical State Spectra from
Slow Depth Profile of Native Cr Oxide

Native Cr Oxide on Cr piece was slowly ion etched using High Energy Resolution conditions to measure Chemical States by Auger

Cr (LMM)	O (KLL)

 

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Chromium Alloys

 

XxCu	XxCu
→  Periodic Table
XxCu	XxCu

 

Copyright ©:  The XPS Library 

 

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XPS Facts, Guidance & Information

→  Periodic Table 

		Element		Chromium (Cr)
		Primary XPS peak used for Peak-fitting:		Cr (2p)
		Spin-Orbit (S-O) splitting for Primary Peak:		Spin-Orbit splitting for “p” orbital, ΔBE = 9.2 eV
		Binding Energy (BE) of Primary XPS Signal:		574.4 eV
		Scofield Cross-Section (σ) Value:		Cr (2p3/2) = 7.69       Cr (2p1/2) = 3.98
		Conductivity:		Cr resistivity =   Native Oxide suffers Differential Charing
		Range of Cr (2p) Chemical State BEs:		574 – 580 eV range   (Cro to CrF2)
		Signals from other elements that overlap Cr (2p) Primary Peak:		xx (xx)
		Bulk Plasmons:		~xx eV above peak max for pure
		Shake-up Peaks:		xx
		Multiplet Splitting Peaks:		xx
		General Information about XXX Compounds:		xx
		Cautions – Chemical Poison Warning		xx

Copyright ©:  The XPS Library 

→  Periodic Table 

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Information Useful for Peak-fitting Cr (2p)

• FWHM (eV) of Pure Cr^o :  ~0.9 eV using 50 eV Pass Energy after ion etching:
• FWHM (eV) of Cr₂O₃ xtal:  ~1.1 eV using 50 eV Pass Energy  (before ion etching)
• Binding Energy (BE) of Primary Signal used for Measuring Chemical State Spectra:  574.4 eV for Cr (2p3) with +/- 0.2 uncertainty
• List of XPS Peaks that can Overlap Peak-fit results for Cr (2p):   

→  Periodic Table 

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General Guidelines for Peak-fitting XPS Signals

• Typical Energy Resolution for Pass Energy (PE) setting used to measure Chemical State Spectra on Various XPS Instruments
  • Ag (3d_5/2) FWHM (eV) = ~0.95 eV for PE 50 on Thermo K-Alpha
  • Ag (3d_5/2) FWHM (eV) = ~1.00 eV for PE 80 on Kratos Nova
  • Ag (3d_5/2) FWHM (eV) = ~0.95 eV for PE 45 on PHI VersaProbe
• FWHM (eV) of Pure Elements: Ranges from 0.4 to 1.0 eV across the periodic table
• FWHM of Chemical State Peaks in any Chemical Compound:  Ranges from 1.1 to 1.6 eV  (in rare cases FWHM can be 1.8 to 2.0 eV)
• FWHM of Pure Element versus FWHM of Oxide:  Pure element FWHM << Oxide FWHM  (e.g. 0.8 vs 1.5 eV, roughly 2x)
• If FWHM Greater than 1.6 eV:  When a peak FWHM is larger than 1.6 eV, it is best to add another peak to the peak-fit envelop.
• BE (eV) Difference in Chemical States: The difference in chemical state BEs is typically 1.0-1.3 eV apart.  In rare cases, <0.8 eV.
• Number of Peaks to Use:  Use minimum. Do not use peaks with FWHM < 1.0 eV unless it is a or a conductive compound.
• Typical Peak-Shape:  80% G: 20% L,   or Voigt : 1.4 eV Gaussian and 0.5 eV Lorentzian
• Spin-Orbit Splitting of Two Peaks (due to Coupling):  The ratio of the two (2) peak areas must be constrained.

Notes:

• Other Oxidation States can appear as small peaks when peak-fitting
• Pure element signals normally have asymmetric tails that should be included in the peak-fit.
• Gaseous state materials often display asymmetric tails due to vibrational broadening.
• Peak-fits of C (1s) in polymers include an asymmetric tail when the energy resolution is very high.
• Binding energy shifts of some compounds are negative due to unusual electron polarization.

→  Periodic Table 

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Contaminants Specific to Chromium

• Chromium develops a thick native oxide due to the reactive nature of clean Chromium .
• The native oxide of Cr O_x is 1-2 nm thick.
• Chromium thin films often have a low level of iron (Fe) in the bulk as a contaminant or to strengthen the thin film
• Chromium forms a low level of carbide when the surface is ion etched inside the analysis chamber

 

Commonplace Contaminants

• Carbon and Oxygen are common contaminants that appear on nearly all surfaces. The amount of Carbon usually depends on handling.
• Carbon is usually the major contaminant.  The amount of carbon ranges from 5-50 atom%.
• Carbon contamination is attributed to air-borne organic gases that become trapped by the surface, oils transferred to the surface from packaging containers, static electricity, or handling of the product in the production environment.
• Carbon contamination is normally a mixture of different chemical states of carbon (hydrocarbon, alcohol or ether, and ester or acid).
• Hydrocarbon is the dominant form of carbon contamination. It is normally 2-4x larger than the other chemical states of carbon.
• Carbonate peaks, if they appear, normally appear ~4.5 eV above the hydrocarbon C (1s) peak max BE.
• Low levels of carbonate is common on many s that readily oxidize in the air.
• High levels of carbonate appear on reactive oxides and various hydroxides.  This is due to reaction between the oxide and CO₂ in the air.
• Hydroxide contamination peak is due to the reaction with residual water in the lab air or the vacuum.
• The O (1s) BE of the hydroxide (water) contamination normally appears 0.5 to 1.0 eV above the oxide peak
• Sodium (Na), Potassium (K), Sulfur (S) and Chlorine (Cl) are common trace to low level contaminants
• To find low level contaminants it is very useful to vertically expand the 0-600 eV region of the survey spectrum by 5-10X
• A tiny peak that has 3 or more adjacent data-points above the average noise of the background is considerate to be a real peak
• Carbides can appear after ion etching various reactive s.  Carbides form due to the residual CO and CH₄ in the vacuum.
• Ion etching can produce low oxidation states of the material being analyzed.  These are newly formed contaminants.
• Ion etching polymers by using standard Ar+ ion guns will destroy the polymer, converting it into a graphitic type of carbon

→  Periodic Table 

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Data Collection Guidance

• Chemical state differentiation can be difficult
• Collect principal Cr (2p) peak as well as Cr (3p).
• Long time exposures (high dose) to X-rays can degrade various polymers, catalysts, high oxidation state compounds
• During XPS analysis, water or solvents can be lost due to high vacuum or irradiation with X-rays or Electron flood gun
• Auger signals can sometimes be used to discern chemical state shifts when XPS shifts are very small

→  Periodic Table 

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Data Collection Settings for Chromium (Cr)

• Conductivity:  Chromium readily develops a native oxide that is sensitive to Flood Gun – Differential Charging Possible – float sample recommended
• Primary Peak (XPS Signal) used to measure Chemical State Spectra:  Cr (2p3) at 574 eV
• Recommended Pass Energy for Measuring Chemical State Spectrum:  40-50 eV    (Produces Ag (3d_5/2) FWHM ~0.7 eV)
• Recommended # of Scans for Measuring Chemical State Spectrum:  4-5 scans normally   (Use 10-25 scans to improve S/N)
• Dwell Time:  50 msec/point
• Step Size:  0.1 eV/point   (0.1 eV/step or 0.1 eV/channel)
• Standard BE Range for Measuring Chemical State Spectrum:  565 – 595 eV
• Recommended Extended BE Range for Measuring Chemical State Spectrum:  560 – 610 eV
• Recommended BE Range for Survey Spectrum:  -10 to 1,100 eV   (above 1,100 eV there are no useful XPS signals, except for Ge and Ga)
• Typical Time for Survey Spectrum:  3-5 minutes for newer instruments, 5-10 minutes for older instruments
• Typical Time for a single Chemical State Spectrum with high S/N:  5-10 minutes for newer instruments, 10-15 minutes for older instruments 

→  Periodic Table 

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Effects of Argon Ion Etching

• Carbides appear after ion etching Cr and various reactive s.  Carbides form due to the residual CO and CH₄ in the vacuum.
• Ion etching can produce low oxidation states of the material being analyzed.  These are newly formed contaminants.
• Ion etching polymers by using standard Ar+ ion guns will destroy the polymer, converting it into a graphitic type of carbon

→  Periodic Table 

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