X-ray photoelectron spectroscopy was performed on a freshly exposed thorium tetrafluoride (ThF4) granule surface. The sample surface exhibits stoichiometric ThF4 with hydrocarbon contamination and an oxide impurity which is tentatively attributed to thorium oxyfluoride.

Topics
Sample handling, History of science, Optical communications, Crystal structure, Associated liquids, Polycrystalline material, X-ray photoelectron spectroscopy, Nuclear reactors, Chemical elements

  • Accession #: 01649

  • Technique: XPS

  • Host Material: ThF4

  • Instrument: PHI VersaProbe III

  • Major Elements in Spectra: Th and F

  • Minor Elements in Spectra: C and O

  • Published Spectra: 5

  • Spectra in Electronic Record: 5

  • Spectral Category: Comparison

Thorium tetrafluoride (ThF4) has been widely used as a low-index, mechanically stable material for optical coating applications (Ref. 1). In recent years, LiF–ThF4 also becomes one of the most promising blanket salt candidates for molten salt nuclear reactors (Ref. 2). However, there are only a few studies presenting the characteristics of thorium oxidation state in ThF4, particularly by x-ray photoelectron spectroscopy (XPS) (Refs. 3–7). In this work, ThF4 is analyzed by XPS, and core-level spectra features of Th and F, as well as O and C from contamination and impurities, are presented.

Host Material: ThF4

CAS Registry #: 13709-59-6

Host Material Characteristics: Homogeneous; solid; polycrystalline; dielectric; and inorganic compound

Chemical Name: Thorium tetrafluoride

Source: Materion Corporation

Host Composition: ThF4

Form: Clear polycrystalline solid

Structure: Monoclinic

History and Significance: 3–6 mm ThF4 granules (99.99%) were received in a sealed stainless steel container and stored in a glove box filled with argon. In the glovebox, a tweezer tip was used to firmly press on a granule to break it apart, resulting in a multifaceted specimen surface with a size of ∼4 × 2 mm2.

As Received Condition: Clear granule

Analyzed Region: Same as host material

Ex Situ Preparation/Mounting: The specimen with a freshly exposed surface was mounted on the sample holder by double-sided adhesive tape in the glove box and transferred to XPS chamber via a transfer vessel filled with argon within 30 min.

In Situ Preparation: None

Charge Control: Low energy electron flood and low energy Ar+ flood.

Temp. During Analysis: 300 K

Pressure During Analysis: <8 × 10−7 Pa

Preanalysis Beam Exposure: 0 s.

Manufacturer and Model: PHI VersaProbe III

Analyzer Type: Spherical sector

Detector: Multichannel resistive plate

Number of Detector Elements: 32

Analyzer Mode: Constant pass energy

Throughput (T = EN): N = 0

Excitation Source Window: None

Excitation Source: Al Kα monochromatic

Source Energy: 1486.6 eV

Source Strength: 50 W

Source Beam Size: 200 × 200 μm2

Signal Mode: Multichannel direct

Incident Angle: 0°

Source-to-Analyzer Angle: 45°

Emission Angle: 45°

Specimen Azimuthal Angle: 0°

Acceptance Angle from Analyzer Axis: 0°

Analyzer Angular Acceptance Width: 20° × 20°

Energy Scale Correction: The binding energy scale was referenced to C 1s = 284.8 eV.

Recommended Energy Scale Shift: 01649-01, +1.788 eV; 01649-02, 03, 04, 05, and +1.841 eV

Peak Shape and Background Method: A Shirley background and pure Gaussian peak shape were used for O 1s curve fitting, and all fitted components were constrained to have the same peak width FWHM. A Shirley background was used to calculate the peak area for quantification.

Quantitation Method: Quantification was done using phi multipak Software Version 9.9.0.8. Corrected sensitivity factors were provided by the software.

SPECTRAL FEATURES TABLE

Spectrum ID #Element/TransitionPeak Energy
(eV)		Peak Width FWHM
(eV)		Peak Area
(eV × counts/s)		Sensitivity FactoraConcentration
(at. %)		Peak Assignment
01649-02 Th 4f7/2 336.3 1.6 36959 292.200 11.5 ThF4 
01649-02 Th 4f5/2 345.6 1.7 31441 … … ThF4 
01649-02 Th satellite 353.5 3.8 5981 … … ThF4 
01649-02 Th satellite 361.7 3.8 4463 … … ThF4 
01649-03 F 1s 684.9 1.7 15800 26.000 55.0 ThF4 
01649-03 Th 4d5/2 677.5 4.4 13444 … … ThF4 
01649-04 O 1s 532.2 2.2 577 19.058 1.3 ThOF2 
01649-04 O 1s 533.6 2.2 110 19.058 0.5 contamination 
01649-05 C 1s 284.8 1.7 2859 8.164 31.7 contamination 
Spectrum ID #Element/TransitionPeak Energy
(eV)		Peak Width FWHM
(eV)		Peak Area
(eV × counts/s)		Sensitivity FactoraConcentration
(at. %)		Peak Assignment
01649-02 Th 4f7/2 336.3 1.6 36959 292.200 11.5 ThF4 
01649-02 Th 4f5/2 345.6 1.7 31441 … … ThF4 
01649-02 Th satellite 353.5 3.8 5981 … … ThF4 
01649-02 Th satellite 361.7 3.8 4463 … … ThF4 
01649-03 F 1s 684.9 1.7 15800 26.000 55.0 ThF4 
01649-03 Th 4d5/2 677.5 4.4 13444 … … ThF4 
01649-04 O 1s 532.2 2.2 577 19.058 1.3 ThOF2 
01649-04 O 1s 533.6 2.2 110 19.058 0.5 contamination 
01649-05 C 1s 284.8 1.7 2859 8.164 31.7 contamination 
a

Corrected sensitivity factors were provided by phi multipak Software Version 9.9.0.8.

Footnote to Spectrum 01649-02: The Th 4f7/2 binding energy of 336.3 eV is in agreement with the values of 336.5–337.1 eV reported in the literature for ThF4 (Refs. 5, 7, and 8). Two satellite features are present on the higher binding energy side of the Th 4f5/2 peak, with the binding energy separation from the Th 4f5/2 peak (ΔE) of ∼7 and ∼15 eV, respectively. The former one is attributed to a shake-up satellite of the Th 4f5/2 peak, while the latter one is assigned to an energy loss peak of the Th 4f7/2 photoemission line (Ref. 5). The shake-up satellite of the Th 4f7/2 peak is masked by the Th 4f5/2 component. The asymmetric shape of the satellite features is possibly due to multiple shake-up components for each ThF4 satellite.

Footnote to Spectrum 01649-04: Two peaks have been deconvoluted from the O 1s signals with the peak position at 532.2 and 533.6 eV, respectively. The peak at 532.2 eV is tentatively attributed to ThOF2, similar to the peak assignment to UOF2/UO2F2 (532.1 eV) in our previous XPS analysis of UF4 (Ref. 9). The Th 4f binding energies of ThOF2 are similar to those of ThF4 and could not be separated in the Th 4f region (01649-02). A ThO2 peak at ∼530.5 eV (Ref. 10) is absent on ThF4, in contrast to the presence of the UO2 peak (530.7 eV) on UF4.

View Large

ANALYZER CALIBRATION TABLE

Spectrum ID #Element/TransitionPeak Energy
(eV)		Peak Width FWHM
(eV)		Peak Area
(eV × counts/s)		Sensitivity FactorConcentration
(at. %)		Peak Assignment
… Au 4f7/2 83.94 0.77 51 634 … … … 
… Cu 2p3/2 932.64 0.86 63 237 … … … 
… Ag 3d5/2 368.21 0.64 67 234 … … … 
Spectrum ID #Element/TransitionPeak Energy
(eV)		Peak Width FWHM
(eV)		Peak Area
(eV × counts/s)		Sensitivity FactorConcentration
(at. %)		Peak Assignment
… Au 4f7/2 83.94 0.77 51 634 … … … 
… Cu 2p3/2 932.64 0.86 63 237 … … … 
… Ag 3d5/2 368.21 0.64 67 234 … … … 
View Large

GUIDE TO FIGURES

Spectrum (Accession) #Spectral RegionVoltage ShiftaMultiplierBaselineComment #
01649-01 Survey −1.788 1 … … 
01649-02 Th 4f −1.841 1 … … 
01649-03 F 1s −1.841 1 … … 
01649-04 O 1s −1.841 1 … … 
01649-05 C 1s −1.841 1 … … 
Spectrum (Accession) #Spectral RegionVoltage ShiftaMultiplierBaselineComment #
01649-01 Survey −1.788 1 … … 
01649-02 Th 4f −1.841 1 … … 
01649-03 F 1s −1.841 1 … … 
01649-04 O 1s −1.841 1 … … 
01649-05 C 1s −1.841 1 … … 
a

Voltage shift of the archived (as-measured) spectrum relative to the printed figure. The figure reflects the recommended energy scale correction due to a calibration correction, sample charging, flood gun, or other phenomenon.

View Large
. Accession #:01649-01Host Material:ThF4Technique:XPSSpectral Region:SurveyInstrument:PHI VersaProbe IIIExcitation Source:Al Kα monochromaticSource Energy:1486.6 eVSource Strength:50 WSource Size:0.2 × 0.2 mm2Analyzer Type:Spherical sector analyzerIncident Angle:0°Emission Angle:45°Analyzer Pass Energy:280 eVAnalyzer Resolution:1.6 eVTotal Signal Accumulation Time:1106 sTotal Elapsed Time:1254 sNumber of Scans:20Effective Detector Width:1.0 eV
View largeDownload slide

Accession #:01649-01
Host Material: ThF4 
Technique: XPS 
Spectral Region: Survey 
Instrument: PHI VersaProbe III 
Excitation Source: Al Kα monochromatic 
Source Energy: 1486.6 eV 
Source Strength: 50 W 
Source Size: 0.2 × 0.2 mm2 
Analyzer Type: Spherical sector analyzer 
Incident Angle: 0° 
Emission Angle: 45° 
Analyzer Pass Energy: 280 eV 
Analyzer Resolution: 1.6 eV 
Total Signal Accumulation Time: 1106 s 
Total Elapsed Time: 1254 s 
Number of Scans: 20 
Effective Detector Width: 1.0 eV 
Accession #:01649-01
Host Material: ThF4 
Technique: XPS 
Spectral Region: Survey 
Instrument: PHI VersaProbe III 
Excitation Source: Al Kα monochromatic 
Source Energy: 1486.6 eV 
Source Strength: 50 W 
Source Size: 0.2 × 0.2 mm2 
Analyzer Type: Spherical sector analyzer 
Incident Angle: 0° 
Emission Angle: 45° 
Analyzer Pass Energy: 280 eV 
Analyzer Resolution: 1.6 eV 
Total Signal Accumulation Time: 1106 s 
Total Elapsed Time: 1254 s 
Number of Scans: 20 
Effective Detector Width: 1.0 eV 
View Large

. Accession #:01649-01Host Material:ThF4Technique:XPSSpectral Region:SurveyInstrument:PHI VersaProbe IIIExcitation Source:Al Kα monochromaticSource Energy:1486.6 eVSource Strength:50 WSource Size:0.2 × 0.2 mm2Analyzer Type:Spherical sector analyzerIncident Angle:0°Emission Angle:45°Analyzer Pass Energy:280 eVAnalyzer Resolution:1.6 eVTotal Signal Accumulation Time:1106 sTotal Elapsed Time:1254 sNumber of Scans:20Effective Detector Width:1.0 eV
View largeDownload slide

Accession #:01649-01
Host Material: ThF4 
Technique: XPS 
Spectral Region: Survey 
Instrument: PHI VersaProbe III 
Excitation Source: Al Kα monochromatic 
Source Energy: 1486.6 eV 
Source Strength: 50 W 
Source Size: 0.2 × 0.2 mm2 
Analyzer Type: Spherical sector analyzer 
Incident Angle: 0° 
Emission Angle: 45° 
Analyzer Pass Energy: 280 eV 
Analyzer Resolution: 1.6 eV 
Total Signal Accumulation Time: 1106 s 
Total Elapsed Time: 1254 s 
Number of Scans: 20 
Effective Detector Width: 1.0 eV 
Accession #:01649-01
Host Material: ThF4 
Technique: XPS 
Spectral Region: Survey 
Instrument: PHI VersaProbe III 
Excitation Source: Al Kα monochromatic 
Source Energy: 1486.6 eV 
Source Strength: 50 W 
Source Size: 0.2 × 0.2 mm2 
Analyzer Type: Spherical sector analyzer 
Incident Angle: 0° 
Emission Angle: 45° 
Analyzer Pass Energy: 280 eV 
Analyzer Resolution: 1.6 eV 
Total Signal Accumulation Time: 1106 s 
Total Elapsed Time: 1254 s 
Number of Scans: 20 
Effective Detector Width: 1.0 eV 
View Large

Close modal
. . Accession #: 01649-02. Host Material: ThF4. Technique: XPS. Spectral Region: Th 4f. Instrument: PHI VersaProbe III. Excitation Source: Al Kα monochromatic. Source Energy: 1486.6 eV. Source Strength: 50 W. Source Size: 0.2 × 0.2 mm2. Analyzer Type: Spherical sector. Incident Angle: 0°. Emission Angle: 45°. Analyzer Pass Energy: 26 eV. Analyzer Resolution: 0.65 eV. Total Signal Accumulation Time: 1803 s. Total Elapsed Time: 7374 s. Number of Scans: 10. Effective Detector Width: 0.1 eV
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  • Accession #: 01649-02

  • Host Material: ThF4

  • Technique: XPS

  • Spectral Region: Th 4f

  • Instrument: PHI VersaProbe III

  • Excitation Source: Al Kα monochromatic

  • Source Energy: 1486.6 eV

  • Source Strength: 50 W

  • Source Size: 0.2 × 0.2 mm2

  • Analyzer Type: Spherical sector

  • Incident Angle: 0°

  • Emission Angle: 45°

  • Analyzer Pass Energy: 26 eV

  • Analyzer Resolution: 0.65 eV

  • Total Signal Accumulation Time: 1803 s

  • Total Elapsed Time: 7374 s

  • Number of Scans: 10

  • Effective Detector Width: 0.1 eV

. . Accession #: 01649-02. Host Material: ThF4. Technique: XPS. Spectral Region: Th 4f. Instrument: PHI VersaProbe III. Excitation Source: Al Kα monochromatic. Source Energy: 1486.6 eV. Source Strength: 50 W. Source Size: 0.2 × 0.2 mm2. Analyzer Type: Spherical sector. Incident Angle: 0°. Emission Angle: 45°. Analyzer Pass Energy: 26 eV. Analyzer Resolution: 0.65 eV. Total Signal Accumulation Time: 1803 s. Total Elapsed Time: 7374 s. Number of Scans: 10. Effective Detector Width: 0.1 eV
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  • Accession #: 01649-02

  • Host Material: ThF4

  • Technique: XPS

  • Spectral Region: Th 4f

  • Instrument: PHI VersaProbe III

  • Excitation Source: Al Kα monochromatic

  • Source Energy: 1486.6 eV

  • Source Strength: 50 W

  • Source Size: 0.2 × 0.2 mm2

  • Analyzer Type: Spherical sector

  • Incident Angle: 0°

  • Emission Angle: 45°

  • Analyzer Pass Energy: 26 eV

  • Analyzer Resolution: 0.65 eV

  • Total Signal Accumulation Time: 1803 s

  • Total Elapsed Time: 7374 s

  • Number of Scans: 10

  • Effective Detector Width: 0.1 eV

Close modal
. . Accession #: 01649-03. Host Material: ThF4. Technique: XPS. Spectral Region: F 1s. Instrument: PHI VersaProbe III. Excitation Source: Al Kα monochromatic. Source Energy: 1486.6 eV. Source Strength: 50 W. Source Size: 0.2 × 0.2 mm2. Analyzer Type: Spherical sector. Incident Angle: 0°. Emission Angle: 45°. Analyzer Pass Energy: 26 eV. Analyzer Resolution: 0.65 eV. Total Signal Accumulation Time: 1326 s. Total Elapsed Time: 7374 s. Number of Scans: 20. Effective Detector Width: 0.1 eV
View largeDownload slide

  • Accession #: 01649-03

  • Host Material: ThF4

  • Technique: XPS

  • Spectral Region: F 1s

  • Instrument: PHI VersaProbe III

  • Excitation Source: Al Kα monochromatic

  • Source Energy: 1486.6 eV

  • Source Strength: 50 W

  • Source Size: 0.2 × 0.2 mm2

  • Analyzer Type: Spherical sector

  • Incident Angle: 0°

  • Emission Angle: 45°

  • Analyzer Pass Energy: 26 eV

  • Analyzer Resolution: 0.65 eV

  • Total Signal Accumulation Time: 1326 s

  • Total Elapsed Time: 7374 s

  • Number of Scans: 20

  • Effective Detector Width: 0.1 eV

. . Accession #: 01649-03. Host Material: ThF4. Technique: XPS. Spectral Region: F 1s. Instrument: PHI VersaProbe III. Excitation Source: Al Kα monochromatic. Source Energy: 1486.6 eV. Source Strength: 50 W. Source Size: 0.2 × 0.2 mm2. Analyzer Type: Spherical sector. Incident Angle: 0°. Emission Angle: 45°. Analyzer Pass Energy: 26 eV. Analyzer Resolution: 0.65 eV. Total Signal Accumulation Time: 1326 s. Total Elapsed Time: 7374 s. Number of Scans: 20. Effective Detector Width: 0.1 eV
View largeDownload slide

  • Accession #: 01649-03

  • Host Material: ThF4

  • Technique: XPS

  • Spectral Region: F 1s

  • Instrument: PHI VersaProbe III

  • Excitation Source: Al Kα monochromatic

  • Source Energy: 1486.6 eV

  • Source Strength: 50 W

  • Source Size: 0.2 × 0.2 mm2

  • Analyzer Type: Spherical sector

  • Incident Angle: 0°

  • Emission Angle: 45°

  • Analyzer Pass Energy: 26 eV

  • Analyzer Resolution: 0.65 eV

  • Total Signal Accumulation Time: 1326 s

  • Total Elapsed Time: 7374 s

  • Number of Scans: 20

  • Effective Detector Width: 0.1 eV

Close modal
. . Accession #: 01649-04. Host Material: ThF4. Technique: XPS. Spectral Region: O 1s. Instrument: PHI VersaProbe III. Excitation Source: Al Kα monochromatic. Source Energy: 1486.6 eV. Source Strength: 50 W. Source Size: 0.2 × 0.2 mm2. Analyzer Type: Spherical sector. Incident Angle: 0°. Emission Angle: 45°. Analyzer Pass Energy: 26 eV. Analyzer Resolution: 0.65 eV. Total Signal Accumulation Time: 2184 s. Total Elapsed Time: 7374 s. Number of Scans: 80. Effective Detector Width: 0.1 eV
View largeDownload slide
. . Accession #: 01649-04. Host Material: ThF4. Technique: XPS. Spectral Region: O 1s. Instrument: PHI VersaProbe III. Excitation Source: Al Kα monochromatic. Source Energy: 1486.6 eV. Source Strength: 50 W. Source Size: 0.2 × 0.2 mm2. Analyzer Type: Spherical sector. Incident Angle: 0°. Emission Angle: 45°. Analyzer Pass Energy: 26 eV. Analyzer Resolution: 0.65 eV. Total Signal Accumulation Time: 2184 s. Total Elapsed Time: 7374 s. Number of Scans: 80. Effective Detector Width: 0.1 eV
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  • Accession #: 01649-04

  • Host Material: ThF4

  • Technique: XPS

  • Spectral Region: O 1s

  • Instrument: PHI VersaProbe III

  • Excitation Source: Al Kα monochromatic

  • Source Energy: 1486.6 eV

  • Source Strength: 50 W

  • Source Size: 0.2 × 0.2 mm2

  • Analyzer Type: Spherical sector

  • Incident Angle: 0°

  • Emission Angle: 45°

  • Analyzer Pass Energy: 26 eV

  • Analyzer Resolution: 0.65 eV

  • Total Signal Accumulation Time: 2184 s

  • Total Elapsed Time: 7374 s

  • Number of Scans: 80

  • Effective Detector Width: 0.1 eV

. . Accession #: 01649-04. Host Material: ThF4. Technique: XPS. Spectral Region: O 1s. Instrument: PHI VersaProbe III. Excitation Source: Al Kα monochromatic. Source Energy: 1486.6 eV. Source Strength: 50 W. Source Size: 0.2 × 0.2 mm2. Analyzer Type: Spherical sector. Incident Angle: 0°. Emission Angle: 45°. Analyzer Pass Energy: 26 eV. Analyzer Resolution: 0.65 eV. Total Signal Accumulation Time: 2184 s. Total Elapsed Time: 7374 s. Number of Scans: 80. Effective Detector Width: 0.1 eV
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. . Accession #: 01649-04. Host Material: ThF4. Technique: XPS. Spectral Region: O 1s. Instrument: PHI VersaProbe III. Excitation Source: Al Kα monochromatic. Source Energy: 1486.6 eV. Source Strength: 50 W. Source Size: 0.2 × 0.2 mm2. Analyzer Type: Spherical sector. Incident Angle: 0°. Emission Angle: 45°. Analyzer Pass Energy: 26 eV. Analyzer Resolution: 0.65 eV. Total Signal Accumulation Time: 2184 s. Total Elapsed Time: 7374 s. Number of Scans: 80. Effective Detector Width: 0.1 eV
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  • Accession #: 01649-04

  • Host Material: ThF4

  • Technique: XPS

  • Spectral Region: O 1s

  • Instrument: PHI VersaProbe III

  • Excitation Source: Al Kα monochromatic

  • Source Energy: 1486.6 eV

  • Source Strength: 50 W

  • Source Size: 0.2 × 0.2 mm2

  • Analyzer Type: Spherical sector

  • Incident Angle: 0°

  • Emission Angle: 45°

  • Analyzer Pass Energy: 26 eV

  • Analyzer Resolution: 0.65 eV

  • Total Signal Accumulation Time: 2184 s

  • Total Elapsed Time: 7374 s

  • Number of Scans: 80

  • Effective Detector Width: 0.1 eV

Close modal
. . Accession #: 01649-05. Host Material: ThF4. Technique: XPS. Spectral Region: C 1s. Instrument: PHI VersaProbe III. Excitation Source: Al Kα monochromatic. Source Energy: 1486.6 eV. Source Strength: 50 W. Source Size: 0.2 × 0.2 mm2. Analyzer Type: Spherical sector. Incident Angle: 0°. Emission Angle: 45°. Analyzer Pass Energy: 26 eV. Analyzer Resolution: 0.65 eV. Total Signal Accumulation Time: 1359 s. Total Elapsed Time: 7374 s. Number of Scans: 30. Effective Detector Width: 0.1 eV
View largeDownload slide

  • Accession #: 01649-05

  • Host Material: ThF4

  • Technique: XPS

  • Spectral Region: C 1s

  • Instrument: PHI VersaProbe III

  • Excitation Source: Al Kα monochromatic

  • Source Energy: 1486.6 eV

  • Source Strength: 50 W

  • Source Size: 0.2 × 0.2 mm2

  • Analyzer Type: Spherical sector

  • Incident Angle: 0°

  • Emission Angle: 45°

  • Analyzer Pass Energy: 26 eV

  • Analyzer Resolution: 0.65 eV

  • Total Signal Accumulation Time: 1359 s

  • Total Elapsed Time: 7374 s

  • Number of Scans: 30

  • Effective Detector Width: 0.1 eV

. . Accession #: 01649-05. Host Material: ThF4. Technique: XPS. Spectral Region: C 1s. Instrument: PHI VersaProbe III. Excitation Source: Al Kα monochromatic. Source Energy: 1486.6 eV. Source Strength: 50 W. Source Size: 0.2 × 0.2 mm2. Analyzer Type: Spherical sector. Incident Angle: 0°. Emission Angle: 45°. Analyzer Pass Energy: 26 eV. Analyzer Resolution: 0.65 eV. Total Signal Accumulation Time: 1359 s. Total Elapsed Time: 7374 s. Number of Scans: 30. Effective Detector Width: 0.1 eV
View largeDownload slide

  • Accession #: 01649-05

  • Host Material: ThF4

  • Technique: XPS

  • Spectral Region: C 1s

  • Instrument: PHI VersaProbe III

  • Excitation Source: Al Kα monochromatic

  • Source Energy: 1486.6 eV

  • Source Strength: 50 W

  • Source Size: 0.2 × 0.2 mm2

  • Analyzer Type: Spherical sector

  • Incident Angle: 0°

  • Emission Angle: 45°

  • Analyzer Pass Energy: 26 eV

  • Analyzer Resolution: 0.65 eV

  • Total Signal Accumulation Time: 1359 s

  • Total Elapsed Time: 7374 s

  • Number of Scans: 30

  • Effective Detector Width: 0.1 eV

Close modal

This work was performed in part at the Surface Analysis Laboratory in Department of Chemistry at Virginia Tech, which is supported by the National Science Foundation (NSF) under Grant No. CHE-1531834.

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