Samarium thin films molecular plated from N,N-dimethylformamide characterized by XPS

Oak Ridge National Laboratory’s (ORNL) Radiochemical Engineering Development Center (REDC) produces actinide thin films for a variety of scientific programs around the world (Refs. 1–4). The Californium Rare Isotope Breeder Upgrade (CARIBU) program at Argonne National Laboratory utilizes a ²⁵²Cf thin film as a fission fragment source produced at REDC to provide the Argonne Tandem Linac Accelerator System with beams of neutron-rich nuclei. In order to achieve a high fission fragment flux for the experiments run under the CARIBU program, the ²⁵²Cf films must be as thin and uniform as possible to minimize the self-attenuation of fission fragments in the thin film itself. A slight increase in thickness can lead to dramatically reduced fission fragment fluxes (Ref. 3). It is important to understand how current actinide thin film production techniques can be optimized to yield high-purity thin films with the desired morphologies. The most recent CARIBU sources have been produced by molecular plating californium from ammonium acetate solutions (Refs. 1–4). Previous work at ORNL using samarium as a surrogate for californium showed that molecular plating from ammonium acetate produced depositions with thicknesses of roughly 5 μm and only ∼50% area coverages of the substrate (Ref. 3). Characterization of the samarium thin films using an x-ray photoelectron spectrometer (XPS) gave evidence that the depositions were composed of samarium acetate rather than pure samarium oxide/hydroxide, which the literature suggests the samarium should deposit as (Refs. 3 and 4).

N,N-dimethylformamide (DMF) was recently investigated as an alternative molecular plating solvent by Vascon et al.^5,6 They reported that DMF produced higher quality lanthanide thin films in terms of morphology compared with typical molecular plating solvents. They also noted that the surface roughness of the deposition substrates had a noticeable effect on the morphology of the thin films produced. For these reasons, it was of interest to investigate DMF as a potential molecular plating solvent for CARIBU source production. Although the effect DMF had on the morphology of deposition was investigated by Vascon, whether the elemental and chemical composition were also impacted by the surface roughness was not. XPS measurements were performed to answer this question.

Initial studies were conducted with samarium as a surrogate for californium since it is nonradioactive and easier to handle. Samarium itself has applications as a component of magnetic, catalytic, and optical materials. It is also an important element in the nuclear fuel cycle since it is a major fission product and neutron poison.

Two different surface finishes, mirrorlike (#8 finish) and brushed (#4 finish), were utilized for thin film molecular plating with DMF to evaluate the effect of surface finish on the morphology and chemical composition of the depositions.

XPS results from the samarium molecular plating in DMF exhibited noticeable differences in surface composition for the two surfaces. Specifically, the relative amounts of different carbon species were different. Although the two samples had carbon peaks at ∼285 eV (C-to-C bond) and ∼290 eV (carbonate-type C bond), the mirrorlike finish sample had a significantly higher peak at ∼285 eV compared with its peak at ∼290 eV. The brushed finish sample had the opposite effect with the peak at ∼290 eV being of higher intensity than the one at ∼285 eV. It was also found that the mirrorlike finish sample had higher ratios of carbon to samarium for total carbon as well as each peak. It then appeared that the mirrorlike finish produces samarium thin films with higher amounts of trapped solvent and/or degradation products.

The survey scans for both samples confirm that the major elemental constituents present are samarium, carbon, and oxygen along with minor amounts of sodium. The sodium trace contamination could arise from a variety of sources, such as generally handling, the hydrochloric acid solutions that are used to dissolve SmCl₃ prior to the molecular plating. Samarium is added to the DMF solution in the form of SmCl₃ dissolved in dilute hydrochloric acid.

The Sm 3d regions for both samples show that samarium is deposited as Sm(III). The C 1s regions for both samples have a carbonate-type carbon peak and carbon-to-carbon-type peak. The carbonate-type carbon is likely from chemisorbed species from the DMF and/or solvent degradation products. Similar results have been seen by Vascon et al.^5,6 The XPS results suggest that the physisorbed and chemisorbed carbon-containing species are more stable on the mirrorlike-finish substrate than on the brushed-finish substrate. The binding energies of the O 1s peaks for both samples are consistent with oxygen being present in O/C-type to metal-type bonds and/or metal-to-hydroxide bonds.

Host Material: Molecular Sm thin film on mirrorlike stainless steel

CAS Registry #: Unknown

Host Material Characteristics: Homogeneous; solid; unknown crystallinity; unknown conductivity; inorganic compound; thin film

Chemical Name: Sm₂O₃

Source: Molecular-plated thin film

Host Composition: Sm₂O₃ thin film, as deposited

Form: Thin film deposited onto the stainless steel substrate with a mirrorlike finish

Structure: Unknown

History and Significance: Samarium was molecular plated as a thin film onto stainless steel substrates from a DMF solution containing 0.444 mM SmCl₃. Stainless steel plates with two different surface finishes, either mirrorlike or brushed, were used as the cathode and deposition substrate material. A constant current of 0.7 mA cm⁻² was applied for 15 min. A platinum plate was used as the anode. The deposition solution was removed from the electrochemical cell immediately after the deposition was completed. The electrochemical cell was then rinsed twice with DMF. The deposition substrate was then removed, stored in a Petri dish, and allowed to dry in a fume hood.

As Received Condition: As deposited onto a stainless steel plate. The sample was stored in a Petri dish under ambient conditions.

Analyzed Region: 400 μm × 400 μm

Ex Situ Preparation/Mounting: The specimen was examined with a scanning electron microscope prior to analysis.

In Situ Preparation: None

Charge Control: Dual-beam low-energy electron/ion source was used for charge neutralization (Thermo Scientific FG-03). The ion gun current was 150 μA, and the voltage was 45 V.

Temp. During Analysis: 300 K

Pressure During Analysis: <2 × 10⁻⁵ Pa

Preanalysis Beam Exposure: 0 s

Host Material: Molecular Sm thin film on brushed stainless steel

CAS Registry #: Unknown

Host Material Characteristics: Homogeneous; solid; unknown crystallinity; unknown conductivity; inorganic compound; thin film

Chemical Name: Sm₂O₃

Source: Molecular-plated thin film

Host Composition: Sm₂O₃ thin film, as deposited

Form: Thin film deposited onto the stainless steel substrate with a brushed finish

Structure: Unknown

History and Significance: Samarium was molecular plated as a thin film onto stainless steel substrates from a DMF solution containing 0.444 mM SmCl₃. Stainless steel plates with two different surface finishes, either mirrorlike or brushed, were used as the cathode and deposition substrate material. A constant current of 0.7 mA cm⁻² was applied for 15 min. A platinum plate was used as the anode. The deposition solution was removed from the electrochemical cell immediately after the deposition was completed. The electrochemical cell was then rinsed twice with DMF. The deposition substrate was then removed, stored in a Petri dish, and allowed to dry in a fume hood.

As Received Condition: As deposited onto a stainless steel plate. The sample was stored in a Petri dish under ambient conditions.

Analyzed Region: 400 μm × 400 μm

Ex Situ Preparation/Mounting: The specimen was examined with a scanning electron microscope prior to analysis.

In Situ Preparation: None

Charge Control: Dual-beam low-energy electron/ion source was used for charge neutralization (Thermo Scientific FG-03). The ion gun current was 150 μA, and the voltage was 45 V.

Temp. During Analysis: 300 K

Pressure During Analysis: <2 × 10⁻⁵ Pa

Preanalysis Beam Exposure: 0 s

Manufacturer and Model: Thermo Scientific K-Alpha XPS

Analyzer Type: Spherical sector

Detector: Multichannel resistive plate

Number of Detector Elements: 128

Analyzer Mode: Constant pass energy

Throughput (T = E^N): N = −1

Excitation Source Window: 0.25 m Rowland circle monochromator with microfocused x-ray source

Excitation Source: Al K_α monochromatic

Source Energy: 1486.68 eV

Source Strength: 72 W

Source Beam Size: 5 μm × 5 μm

Signal Mode: Analog 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: 45° × 0°

Manufacturer and Model: Thermo Scientific EX-06

Energy: 1000 eV

Current: 10 mA

Current Measurement Method: Faraday cup

Sputtering Species: Ar

Spot Size (unrastered): 400 μm

Comment: The ion gun was utilized to clean the surfaces of the calibration standards.

Energy Scale Correction: The sample was not charging, and no energy-scale correction was needed.

Recommended Energy Scale Shift: 0

Peak Shape and Background Method: thermo avantage software version 4.61 was used to carry out the background subtraction using the Shirley function as well as the determination of the peak positions and full width at half maximum (FWHM) values. The peaks were fitted using Gaussians.

Quantitation Method: The atomic concentrations were calculated using the Al Scofield sensitivity factors in the thermo avantage software version 4.61.

The authors thank Harry Meyer for the use of his lab to perform the measurements. This research was supported by the U.S. Department of Energy, Office of Nuclear Physics, Isotope Development and Production for Research and Applications Program. This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy (DOE). The U.S. government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for U.S. government purposes. The DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).