HAXPES reference spectra of MoS₂ with Cr K_α excitation

Recently developed laboratory-based hard x-ray photoelectron spectrometers (HAXPES) allow us to excite deeper core-levels, enabling different depth probing as a function of the studied core-level. In the case of MoS₂, Cr K_α radiation (5414.8 eV) allows us to ionize all Mo and S core-levels of principal quantum numbers N = 2, 3, and 4.

In this work, we report HAXPES spectra recorded on a pure, bulk MoS₂ sample. MoS₂ is widely used in mechanics and wear science, electronics, and phototonics, as well as in chemistry and catalysis. In HAXPES, this material has already been studied with Ag L_α x rays (2984.2 eV) (Ref. 1) and with synchrotron radiation hard x rays (5.9 keV) (Ref. 2) with a focus on the band alignment and the low energy core-levels. The present data include a survey scan, high-resolution Mo 2s, Mo 2p, Mo 3s, Mo 3p, Mo 3d, Mo 4s, Mo 4p, S 1s, S 2p, and S 2s core-levels, and x-ray excited Auger spectra.

Specimen: MoS₂

CAS Registry #: 1317-33-5

Specimen Characteristics: Homogeneous; solid; polycrystalline; semiconductor; inorganic compound; other

Chemical Name: Molybdenum disulfide

Source: HQ Graphene

Composition: MoS₂

Form: Polycrystalline solid

Structure: Hexagonal, layered structure

History and Significance: Air exposed thick MoS₂ sample

As Received Condition: Bulk MoS₂ crystal

Analyzed Region: Same as specimen

Ex Situ Preparation/Mounting: The sample was mounted on the sample holder using a double-sided conductive tape.

In Situ Preparation: None

Charge Control: Low-energy electrons (1 eV, filament 1.1 A) and low-energy Ar⁺ ions (100 eV)

Temp. During Analysis: 300 K

Pressure During Analysis: <2 × 10⁻⁷ Pa

Pre-analysis Beam Exposure: 0 s

Manufacturer and Model: ULVAC-PHI Quantes

Analyzer Type: Spherical sector

Detector: Multichannel plate resistive

Number of Detector Elements: 32

Analyzer Mode: Constant pass energy

Throughput (T = E^N): The energy dependence can be modeled using the following equation: $A E p= ( a 2 a 2 + R 2 ) b$⁠, where a and b are constants, E_p is the pass energy, A is the peak area, and R is the retard ratio equal to E/E_p, where E is the kinetic energy. Three spectral regions are recorded on a sputter cleaned sample (Cu, Ag, and Au) at different pass energies. The values of a and b are then determined by a linear least square fit of the data applying the equation described above.

Excitation Source Window: Al

Excitation Source: Cr K_α monochromatic

Source Energy: 5414.8 eV

Source Strength: 45 W

Source Beam Size: 100 × 100 μm²

Signal Mode: Multichannel direct

Incident Angle: 22°

Source-to-Analyzer Angle: 46°

Emission Angle: 45°

Specimen Azimuthal Angle: 0°

Acceptance Angle from Analyzer Axis: 0°

Analyzer Angular Acceptance Width: 20° × 20°

Manufacturer and Model: ULVAC-PHI Quantes

Energy: 100 and 2000 eV

Current: 0.001 mA

Current Measurement Method: Faraday cup

Sputtering Species and Charge: Ar⁺

Spot Size (unrastered): 100 μm

Raster Size: 3000 × 3000 μm²

Incident Angle: 45°

Polar Angle: 45°

Azimuthal Angle: 45°

Comment: Differentially pumped ion gun used for presputtering of the sample and to prevent reoxidation during analysis.

Energy Scale Correction: The decrease in photoionization cross sections in HAXPES (Refs. 3 and 4) leads to a very low C 1s intensity. Therefore, the binding energy was referenced to the S 2p binding energy position measured with Al K_α radiation after shifting the C 1s peak to 284.8 eV. Doing so, the S 2p binding energy was 162.4 eV. The HAXPES spectra were then rescaled by shifting the S 2p to 162.4 eV. This correction sets the Mo 3d_5/2 binding energy to 228.8 eV, in line with a previous study (Ref. 5) and in the range of reported values by NIST (Ref. 6).

Recommended Energy Scale Shift: −0.4 eV for binding energy

Peak Shape and Background Method: Shirley background was employed for peak area determination. No curve fitting was performed on the spectra. The Auger peaks were identified using previous work on H₂S and PHI Handbook (Refs. 7 and 8).

Quantitation Method: PHI Multipak software version 9.9.0.8 was used to perform quantitation of bulk MoS₂. An XPS quantification conducted on the same sample, considering Mo 3d and S 2p core-levels, gives a 1:1.97 Mo:S ratio. The determined HAXPES intensities were checked for repeatability by comparing, before and after acquisition, the intensity of the Ag 3d and Ag 2p3/2 lines of a sputter-clean, bulk silver sample mounted on the same holder. In all cases, the standard deviation in intensity was below 4%. Empirically determined sensitivity factors (RSFs) were provided by Multipak. The RSFs were derived from the pure-element relative sensitivity factor as defined in ISO 18118:2015 (Ref. 9), which were measured on pure element samples using a Cr K_α source. They, therefore, account for the decrease in cross section and different escape depths of photoelectrons using higher energy photons. RSFs are reported proportional to the RSF of F 1s equal to 1. Corrected RSFs including the spectrometer transmission function and asymmetry parameter of the considered core-line were used to calculate the concentrations. The area used for the quantification and reported in the Spectral Feature Table was measured over the entire energy range of presented spectra. For instance, the S 1s area includes the small peak around 2480 eV. As shown in the Spectral Feature Table, the percentage of deviation relative to the nominal 1:2 Mo:S ratio depends on the considered core-levels and is up to −45%, with some combinations providing the expected figure. A more detailed HAXPES study is necessary to improve quantitation results of this material.

This work was performed at the Platform for NanoCharacterization (PFNC) of CEA-Leti with support from the Recherche Technologique de Base (RTB) and “France 2030-ANR-22-PEEL-0014” programs of the French Research Agency (ANR). The authors acknowledge the support of the PHI-Leti TANDEMS collaboration program.

The authors have no conflict to disclose.

Alexandre Boyer: Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Nicolas Gauthier: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Olivier Renault: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article and its supplementary material.