Bismuth, by high-sensitivity low energy ion scattering

Ion beams are effectively used to prepare and characterize surfaces and thin films. Scattering of low energy inert gas ions (He⁺, Ne⁺) or alkali ions (Li⁺, Na⁺) enables elemental surface analysis sensitive to the outermost atomic layer.^1–3 Incident ions with energies from 1 to 10 keV are usually used in low energy ion scattering (LEIS) spectroscopy.⁴ Atoms of the outer surface play a crucial role in many processes such as catalysis, adhesion, or electron emission. LEIS analysis can be applied to various materials including conductors, semiconductors, insulators, crystalline and amorphous materials, organics, or polymers.¹ 

Qualitative analysis is straightforward as the energy of backscattered projectiles is characteristic of specific projectile-target interaction.¹ However, the quantification process is complicated by still not fully understood and theoretically described charge-exchange processes between incident ion and target surface atoms. The quantification of a given element is based on comparing the measured signal with this received from a well-defined references (Ref. 5). Such an approach is straightforward in many cases when the matrix effect is absent.⁶ 

For some elements, the backscattered ion yield oscillates with the change of primary beam energy. This can be represented by the scattering of ⁴He⁺ ions on the Pb atoms where the (quasi-)resonant charge-exchange process between the He projectile 1s level and the target Pb atom d-levels is possible during the close approach. In 1978, Zartner et al. systematically studied these oscillations and confirmed their atomic nature.⁷ Similar oscillatory behavior was found for many projectile and surface atom combinations. The same year, Christensen et al. presented the oscillations for ⁴He⁺ ion scattering on Bi atoms. This was measured with a primary ion beam angle of incidence of 45° and a scattering angle of 90° within the energy range from 165 to 2500 eV.⁸ 

Here, we provide experimental data for a wider energy range and different scattering geometry, both defined by dedicated and commercially available high-sensitivity low energy ion scattering spectrometer (HS-LEIS) instrument (Qtac¹⁰⁰, GmbH IONTOF). In the energy range from 0.50 to 6.00 keV, spectra of 52 primary ion beam energies were obtained. For this article, eight of them were selected to represent an oscillatory pattern of Bi backscattered yields (all 52 spectra are available in the accession data files). The scattering angle is fixed at 145° and the incident beam is perpendicular to the surface plane.

Bismuth is a metal with a possible difference between the bulk and surface characteristics as the surface exhibits better metallic properties than the bulk material.⁹ Bismuth analysis is of high interest because of its potential applications p.a. in photonics and catalysis or fabrication of various types of bismuth nanoparticles.¹⁰ Nanosized forms of bismuth-containing materials are used in optical, chemical, electronic, or biomedical fields because of their non-toxicity, stability, and cost-effective fabrication.¹¹ Because of the electronic structure and binding energy of the atom, bismuth, together with other elements from the 13th to the 15th group, exhibits oscillations of scattered ion yields within a wide energy range of primary helium ions.¹² It is ascribed to a single electron transfer between two (quasi-)resonant states. Generally, observed oscillatory behavior is rather regular and these oscillations are equidistant as a function of a reciprocal velocity.^1,13 Different behavior was reported for bismuth, as its yields follow a specific oscillatory pattern.⁸ Characterization of bismuth can be provided by x-ray diffraction, Auger electron spectroscopy, x-ray photoelectron spectroscopy, and electron microscopy.^14–16 

This work aims to deliver bismuth spectra for a broad range of primary ion beam energies and to define them as a bismuth reference. The oscillatory behavior of absolute ion yields of ⁴He⁺ scattered on the Bi target is demonstrated by the joint plot of all 52 spectra (Fig. 1). Moreover, a table containing spectral features of all spectra can be found in the form of a supplementary document.

The reciprocal velocity plot of the modified ion yield becomes a standard characteristic of the charge-exchange processes between the projectile and the surface of the target or individual target atoms. The plot is linear whenever the single charge-exchange process is dominant, and no oscillatory behavior is presented. The slope of the linear fit is given by characteristic velocity, and it is a measure of overall neutralization.¹ In our case (Fig. 2), the yield is assumed to be modified by the (quasi-)resonant charge-exchange process between the He projectile 1s level and the target atom d-levels, similar to the scattering of ⁴He⁺ on Bi described by Christensen et al.⁸ Such interaction is possible during the close approach and, thus, the reciprocal velocities of incoming and outgoing trajectories are used in the definition of the horizontal scale. In this case, as Auger neutralization plays a minor role, our data are presented as a function of inverse initial velocity 1/v_i as it is suggested by Goebl et al.¹⁷ On the vertical scale, S represents the signal in counts/nC, N_Bi is the number of bismuth atoms per unit area (atomic surface concentration) and (dσ/dΩ) states for the differential scattering cross section in Ų/sr that was calculated based on the Thomas–Fermi–Molière potential.

Spectra of bismuth were obtained by analysis of high-purity Bi foil (99.999%). Also, thin bismuth films deposited on Si wafer by molecular beam epitaxy (MBE) were analyzed. It was found that the intensity of the yield backscattered on bismuth atoms deposited on silicon shows an identical oscillatory pattern to those obtained by scattering on bismuth atoms within the thin foil. The fitted signals are a little lower for the deposited film with an average decrease of about 12%. This is probably related to the quality of the deposited bismuth layer. Therefore, pure Bi foil was selected as a suitable bismuth reference and the spectra of thin Bi film obtained by MBE are not included nor further discussed in this contribution. High-purity Cu foil (99.999%) was used to define the instrument sensitivity.

Both Bi and Cu foils were cleaned by argon and neon sputtering in the analytical chamber of the Qtac¹⁰⁰ (base pressure below 5 × 10⁻⁸ Pa) to minimize oxygen and carbon contamination. The presence of the contaminants was followed together with the evaluation of the surface peak to obtain the maximum Bi signal. Bismuth foil was sputtered with Ar⁺ (5 keV) with a fluence of 3 × 10¹⁶ ions cm⁻² (1.7 × 1.7 mm²) and Ne⁺ (5 keV) with a fluence of 2 × 10¹⁶ ions cm⁻² (1.6 × 1.6 mm²). The same procedure was applied for Cu foil sputtered with Ar⁺ (5 keV) with a fluence of 2 × 10¹⁷ ions cm⁻² (1.7 × 1.7 mm²) and Ne⁺ (5 keV) with a fluence 2 × 10¹⁶ ions cm⁻² (1.6 × 1.6 mm²). In both cases, a defocused ion beam was used. The pass energy (PE) of the analyzer was settled as 3000 eV for the primary ion beam energies from 0.50 to 4.05 keV and 4700 eV for the energies from 4.05 to 6.00 keV. The difference in signal connected to the change of PE was 1.4% so the signals measured with PE = 4700 eV were treated accordingly. In the Supplemental dataset table, fitted peak integrals of as-measured spectra are presented. Moreover, as the values of the peak widths (FWHMs) are also included, it is appropriate to mention that the step in the FWHM values between 4.05 and 4.27 keV is ascribed to the change of the PE.

Projectiles of ⁴He⁺ in a wide energy range from 0.50 to 6.00 keV were used to obtain spectra of the bismuth surface. A single collision peak between projectile and target atoms can be observed in the spectrum followed by background at lower energies. This background results from backscattering in deeper layers and its intensity is given by the reionization process which requires some minimum energy so-called reionization threshold energy.¹ The intensity of the background is very low for energies below 1.00 eV and slowly rises at higher energies. Thus, our experimental data suggest that the energy threshold for reionization can be found between 1.00 and 1.25 eV for He-Bi interaction.

The large set of the 52 spectra for the He ion scattering on Bi atoms together with polycrystalline Cu reference is presented for the commercially available dedicated LEIS instrument with the scattering geometry of 145°. While the selection of the eight spectra representing the oscillatory behavior of Bi yield is presented, the rest is available in the attached files. The fit of experimental Bi peaks by an error function and a Gaussian function is displayed in the form of a table with peak areas for all measured spectra.¹⁹ 

Host Material: Bi

CAS Registry #: 7440-69-9

Host Material Characteristics: Homogeneous; solid; polycrystalline; conductor; metal; other

Chemical Name: Bismuth

Source: abcr GmbH, Germany

Host Composition: Bismuth

Form: High-purity foil

Structure: rho

History & Significance: Bismuth is an important heavy metal because of its non-toxicity and stability. Its different (nano)forms have potential applications in industry and biomedicine.

As Received Condition: High-purity (99.999%) foil, 1.0 mm thick

Analyzed Region: Same as host material

Ex Situ Preparation/Mounting: As received, the sample was mounted on a conducting sample holder.

In Situ Preparation: Sputtering by 5 keV Ar⁺, fluence of 3 × 10¹⁶ ions cm⁻² (1.7 × 1.7 mm²) and by 5 keV Ne⁺, fluence of 2 × 10¹⁶ ions cm⁻² (1.6 × 1.6 mm²).

Charge Control: None

Temp. During Analysis: 300 K

Pressure During Analysis: 4.4 × 10⁻⁷ Pa

Partial Pressure of Reactive Gases During Analysis: N/A

Preanalysis Beam Exposure: None

Chemical Name: Copper

Source: Alfa Aesar Chemicals, USA

Homogeneity: Homogeneous

Form: Solid

Specific Surface Area: N/A

Comment: High-purity foil (99.999%), 0.025 mm thick

Ex Situ Preparation/mounting: As received, the sample was mounted on conducting sample holder

In Situ Preparation: Sputtering with 5 keV Ar⁺, fluence 2 × 10¹⁷ ions cm⁻² (1.7 × 1.7 mm²) and by 5 keV Ne⁺, fluence 2 × 10¹⁶ ions cm⁻² (1.6 × 1.6 mm²).

Charge Control: none

Temp. During Analysis: 300 K

Pressure During Analysis: 4.6 × 10⁻⁷ Pa

Partial Pressure of Reactive Gases During Analysis: N/A

Manufacturer and Model: IONTOF GmbH (Münster, Germany), Qtac100

Analyzer Type: Double Toroidal

Detector: Other

Number of Detector Elements: Continuous

Analyzer Mode: Constant pass energy

Energy Dependence of Detection: Constant

Charge Compensation Energy: Not used

Time of Flight Filter Used: No

Time of Flight Filter Comment: N/A

Purpose of this Ion Source: Analysis beam

Manufacturer and Model: IONTOF GmbH (M�, Germany), electron impact ion source

Energy: 500–6000 eV

Current: (0.990–5.12) ×10⁻⁶ mA

Current Measurement Method: Faraday cup

Species: ⁴He

Spot Size (unrastered): 50 μm

Raster Size: 1500 × 1500 μm

Incident Angle: 0°

Polar Angle: N/A

Azimuthal Angle: 0°–360°

Scattering Angle: 145°

Comment: Qtac¹⁰⁰ instrument is equipped with a single ion source used either for sputtering or for analysis. For Bi analysis, the lowest and highest value of primary ion beam energy and its current are stated.

Ion Source 2 of 2

Purpose of this Ion Source: Sputtering beam

Manufacturer and Model: IONTOF GmbH, electron impact ion source

Energy: 5000 eV

Current: (50–60) × 10⁻⁶ mA

Current Measurement Method: Faraday cup

Species: ²⁰Ne, ⁴⁰Ar

Spot Size (unrastered): 50 μm

Raster Size: 1700 × 1700 (1600 × 1600) μm

Incident Angle: 0°

Polar Angle: N/A

Azimuthal Angle: 360°

Scattering Angle: 145°

Comment: Ion beam settings for sputtering

Energy Scale Correction: No correction

Peak Shape and Background Method: Peak energies, widths (FWHMs), and areas were obtained after subtraction of LEIS background by an error function and a Gaussian function fit as reported in Ref. 19.

Quantitation Method: None

Sensitivity Factor (source): Relative sensitivity factor of Bi to polycrystalline Cu is 0.58 (E_i = 3000 eV).

CzechNanoLab project LM2023051 funded by MEYS CR is gratefully acknowledged for the financial support of the measurements/sample fabrication at CEITEC Nano Research Infrastructure.

The authors have no conflicts to disclose.

Elena Vaníčková: Data curation (lead); Investigation (equal); Writing – original draft (lead). Stanislav Průša: Investigation (equal); Supervision (lead). Tomáš Šikola: Funding acquisition (lead).

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