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Oct 9, 2013 - Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA. ABSTRACT: High quality epitaxial thin films of ...
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Preparation of Epitaxial Uranium Dicarbide Thin Films by PolymerAssisted Deposition Robert E. Jilek,† Eve Bauer,† Anthony K. Burrell,† Thomas M. McCleskey,‡ Quanxi Jia,*,† Brian L. Scott,*,† Miles F. Beaux,† Tomasz Durakiewicz,† John J. Joyce,† Kirk D. Rector,‡ Jie Xiong,† Krzysztof Gofryk,† Filip Ronning,† and Richard L. Martin§ †

Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA § Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA ‡

ABSTRACT: High quality epitaxial thin films of cubic UC2 were synthesized using a solution based technique. The films were characterized using XRD, UPS, Raman, and resistivity. The substrate lattice is yttrium stabilized zirconia and serves to stabilize the high temperature cubic phaseof UC2 (>1765 °C) at room temperature. The resistivity and UPS data indicate that UC2 has relatively low electrical conductivity consistent with HSE hybrid DFT calculations showing a narrow band gap. In situ XRD measurements show that the UC2 films oxidize to U3O8 above 200 °C. KEYWORDS: actinide carbide, epitaxial thin film, nuclear energy

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have been used to prepare UC at much lower temperature.3b In spite of the wealth of knowledge on these materials, little is known about uranium carbide thin films. In 2004, researchers used sputter codeposition to generate films that were found to have compositions UC1.2 and UC1.6. X-ray diffraction studies revealed the presence of polycrystalline UC and UC2 in these films; however, the diffraction peaks were broadened possibly due to stress, inhomogeneities, or the presence of amorphous material.6 Herein, we report the first preparation of singlecrystal quality UC2 by polymer-assisted deposition (PAD). In this process, metal polymer solutions are used as film precursors, where the polymer not only controls solution viscosity but also binds to uranium to form a homogeneous aqueous solution of metal ions.7 The metal polymer solutions are spin coated onto single crystal substrates of proper lattice match and then heated in the appropriate atmosphere (ethylene for carbides) to depolymerize the polymer and form the film.

INTRODUCTION The carbides of uranium have received much attention in recent years as potential fuel sources for Generation IV nuclear reactors. While at least six different Generation IV reactor concepts are currently being explored, the consensus is that new nuclear fuels must be developed for these advanced systems to maintain reasonable operating temperatures.1 Two such possibilities are uranium carbide (UC) and uranium dicarbide (UC2), which have much higher thermal conductivities than conventional nuclear fuels such as UO2, mixed-metal oxides, and ThO2. This would have the effect of lowering fuel centerline temperatures, which would increase operational safety, fuel lifetime, and efficiency. In addition to their potential as nuclear fuels, uranium carbide has been used for many years as a target material in the production of neutron-rich isotopes.2 In order to gain a more in-depth understanding of the properties of uranium carbide materials, we have synthesized and characterized epitaxial thin films of UC2. These thin films are of high purity and are nearly single crystal in quality and thus facilitate high quality measurements to validate computational models. Herein, we report the synthesis and characterization of these thin films including valence band photoemission, Raman spectroscopy, electrical conductivity, and in situ XRD studies of oxidation. Uranium carbides have been known for quite some time, with U2C3 and UC2 first mentioned over a century ago.3 These bulk materials were originally prepared by treating U3O8 with graphite in an electric arc furnace.3a,c,4 While carbothermic methods are still used to prepare uranium carbides,5 other strategies such as the reaction of uranium metal with methane © 2013 American Chemical Society

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EXPERIMENTAL SECTION

Sample Preparation. The precursor solution for growth of UC2 films was prepared by adding aqueous uranyl nitrate (300 mM solution in deionized (18 MΩ) H2O) to a mixture of polyethyleneimine (PEI) polymer and ethylenediaminetetraacetic acid (EDTA) in water. The polymer solution was prepared by adding 2.0 g of EDTA (Aldrich 99.995% pure) and 2 g of BASF polyethyleneamine polymer without further purification to deionized (18 MΩ) H2O. The polymer solution Received: August 5, 2013 Revised: September 20, 2013 Published: October 9, 2013 4373

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was adjusted to pH 7 with concentrated hydrochloric acid. This solution was spin-coated onto yttria-stabilized zirconia substrates at 3000 rpm for 30 s. The films were heated to 120 °C (1 °C/min) and held at this temperature for 1 h. The temperature was then ramped to 350 °C (1 °C/min) and held at this temperature for 1 h. Finally, the temperature was ramped to 1000 °C (1 °C/min) and annealed at this temperature for 4 h. During the entire heating cycle, the sample was under a flow of 97 sccm mixture of H2(6%)/Ar and 3 sccm ethylene. This provided UC2 films with thicknesses in the range of 40−50 nm. Caution: The synthetic procedure involved the use of depleted uranium (238U), which is a radioactive alpha emitter and toxic metal. Precautions should be used to prevent exposure of workers and the public including radiation monitoring, fume hoods, and personal protective equipment. XRD Characterization. XRD was used to characterize the crystallographic orientation and degree of crystallinity of the films. XRD analyses were performed using a Philips X’pert high resolution Xray diffractometer equipped with a two bounce hybrid monochromator and an open three-circle Eulerian cradle. To evaluate the epitaxial growth of the UC2 film on (100) YSZ substrate, both normal θ−2θ and ϕ-scans were carried out. Photoemission. The photoemission measurments we conducted at the Synchrotron Radiation Center (SRC) and at Los Alamos National Laboratory (LANL). The SRC experiments covered the photon energy of 98 and 92 eV (U 5f resonanace and antiresonance) while the LANL experiments covered the photon energy of 40.8 and 21.2 eV. Using the resonanace and antiresonance techniques in photoemission we can essentially turn on and off the cross sections for uranium 5f emission in PES. The samples were cleaned in situ by thermal desorption or Ar ion sputtering. The measurement temperatures were room temperature or 8 K. The data shown in Figure 2b is taken with a variable polarization undulator set to vertical photon polarization. Data were also collected in the horizontal configuration which showed a substantially smaller U 5f compared to the C 2p emission. The difference in the horizontal and vertical polarizations gives solid indication of the high quality of the PAD samples. In Situ XRD of UC2 Oxidation. X-ray diffraction data were collected on a Bruker D8 Discover diffractometer with Cu Kα (λ = 1.54059 Å) radiation that was monochromatized with Göbel mirror and Ge accentric cut channel (ACC) monochromator. The reflections were collected with a NaI(Tl) scintillation detector. The sample was heated in air in an Anton Paar domed hot stage to 900 °C, and data were collected at 100 °C intervals. Raman. The Raman data were collected on a Witec Alpha300R AFM/Raman microscope using 532 nm excitation. The spectra were acquired using a 600 g/mm grating (blazed at 500 nm) and detected with an 16 bit EMCCD camera cooled to −60 °C with no gain. The integration time was 2 s for 25 accumulations. Electrical Conductivity. Resistance measurements were performed using a standard four point technique with platinum wires attached to the sample with silver paint. To convert the measured resistance into resistivity, one needs to consider the geometric factor. A value of 20 ± 10 × 10−6 cm has been estimated based on the sample width, length, and film thickness.

Figure 1. X-ray diffraction 2θ-scan pattern (blue) and ϕ-scans (red, inset) of UC2 film on yttrium stabilized zirconia single crystal substrate. The 2θ-scan demonstrates the plane of the film corresponds to (00l) since only the (002) and (004) reflections are observed. The ϕ-scan shows that the (111) reflections of the UC2 crystal lattice are aligned with the (111) reflections of the YSZ substrate, thus, demonstrating epitaxy.

We originally attempted to grow heteroepitaxial thin films of uranium carbides on substrate materials such as c-cut sapphire and lanthanum aluminum oxide (LAO). This resulted in black, amorphous materials which were highly conductive. When yttria-stabilized zirconia (YSZ) was used as a substrate, epitaxial UC2 films were produced. This was particulary surprising since YSZ (a = 0.512 nm) and the monocarbide UC (a = 0.4951 nm) are both cubic with a lattice mismatch of only 3.3%. Moreover, we did not obtain low temperature, tetragonal UC2, but rather we achieved high temperature, cubic UC2 at significantly milder temperatures than previously reported.9,11 As with uranium oxides, whose oxidation states could be crystallographically pinned through heteroepitaxy,12 the ability to prepare cubic UC2 at 1000 °C is attributed to epitaxial stabilization of the crystalline lattice. Finally, the cubic UC2 film is stable at room temperature. The X-ray diffraction patterns from the θ−2θ scan and ϕscans of UC2 and YSZ are shown in Figure 1. The UC2 is preferentially oriented along the c-axis as only the (002) and (004) reflections are visible in the XRD pattern (Figure 1). The calculated lattice parameters of the UC2 film vary slightly in comparison with the lattice parameter of bulk UC2, indicating a pseudocubic structure (a = 0.5478, b = 0.5581, c = 0.5423 nm). The in-plane orientation of the film was determined by ϕ-scans of (111) UC2 and (111) YSZ. The scans are nearly identical with four peaks separated by 90°, which is indicative of excellent in-plane orientation (Figure 1, inset). The epitaxial relationship between UC2 and YSZ can be described as (002)UC2∥(200)YSZ and ⟨111⟩UC2∥⟨111⟩YSZ based on both the θ−2θ scan and ϕ-scans. Figure 2 shows the ultraviolet photoemission (UPS) valence band spectrum for UC2. The spectrum is dominated by a manifold most likely composed of primarily C 2p weight extending from approximately 3 to 10 eV below the Fermi energy (EF). The U 5f weight is centered at 1 eV below EF but extending up to the Fermi energy. The assignment of this feature as being U 5f was determined by Fano resonance− antiresonance performed at the SRC as shown in Figure 2b. From the resonance−antiresonance data, it is observed that there is not a large component of C 2p in the first 2 eV from the Fermi energy so hybridization in UC2 is small. It is also observed that the U 5f line width is ∼1 eV wide while the C 2p

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RESULTS AND DISCUSSION Of the uranium carbides, UC2 is particularly interesting because there are at least three known polymorphs. The low temperature polymorph has a body-centered tetragonal CaC2type structure with space group I4/mmm,3b,8 while the high temperature polymorph is cubic with space group Pa3̅.9 This transformation has been studied for a number of metal dicarbides, and the transition temperature varies from 100 °C for BaC2 to 1765 °C for UC2.12,13 A third, high pressure polymorph of UC2 also exists, which requires 17.6 GPa to transition from the low temperature tetragonal structure to a high pressure hexagonal structure.10 4374

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Figure 3. Raman spectrum of UC2 on YSZ.

Figure 4. Temperature dependent resistance of an epitaxial UC2 film on YSZ substrate.

moiety. C2, which is an unstable gas phase molecule, can be stabilized in solids when associated with metals. The archetypical example is CaC2, where the anion is C22− and has a C−C bond distance of 1.19 Å; this bond distance is indicative of an aceytlenic triple bond. In the compound ThC2, the bond distance is elongated to 1.48 Å, which is consistent with a double bond and C24−. The central question with regards to C2 in uranium dicarbide is to what extent it is reduced by uranium, which can exist in U(III) to U(VI) oxidation states. A symmetric stretching frequency of the C−C bond as measured by Raman spectroscopy should be indicative of whether the bond is single, double, or triple in nature. The Raman spectrum is shown in Figure 3, and two bands are observed at 1337 cm−1 and 1575 cm−1. These are consistent with the D and G bands observed in carbide containing structures, with the G band near 1600 cm−1 assigned to graphene sp2 −sp2 carbon atom stretching frequencies and the D band associated with sp2−sp3 defect vibrational modes. In our UC2 films, the latter probably results from defects occurring at the surface and substrate interface of the film and also between crystalline mosaics. The 1600 cm−1 C22− band is consistent with recent theoretical predictions,14 namely, that UC2 is formally U4+(C2)4‑. Electrical conductivity measurements were performed on the UC2 epitaxial films using a standard four probe technique and represent the first resistance measurements performed on the cubic phase. For UC2 the resistance of the film increases with

Figure 2. Valence band and resonance photoemission. Panel a (top) shows the valence band photoemission spectrum for UC2 obtained using He IIa 40.8 eV photons and HeIa 21.2 eV photons (inset shows an enlargement of the EF region). Panel b (bottom) shows the 5d−5f resonance (98 eV) and antiresonance (92 eV) spectra indicating the majority of 5f spectral weight residing in the peak centered around −1.5 eV.

width is probably 4 eV wide indicating substantially more 2p bonding compared to 5f bonding. An enlargement of the near EF region in the inset of Figure 2 shows a metallic 5f feature cut by EF, consistent with previously measured conductivity in UC2.13 The data in the Figure 2a inset was taken at 21.2 eV photon energy and 8 K so that the details of the Fermi energy are apparent. In this inset, one can see that the tail of the spectral weight extends through the Fermi energy but is mostly attributed to U 5f intensity which is consistent with the metallic but limited conductivity of the UC2 material. Raman spectroscopy was performed in order to obtain information regarding the nature of the C−C bonding in the C2 4375

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In order to understand the oxidative properties of the UC2 films we employed in situ X-ray diffraction to measure the oxidation in air of a film as it was heated from room temperature to 900 °C. The (002) peak of the UC2 was observed at room temperature and was eliminated by 500 °C as the film was fully oxidized to U3O8 (Figure 5). Intermediate temperatures showed phases corresponding to UC2−xOx as evidenced by a shift in the (002) peak of the UC2 and a color change in the films from shiny black (UC2) to metallic (UC2−xOx) to brown (U3O8) (Figure 6).

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CONCLUSIONS We have successfully prepared epitaxial uranium dicarbide thin films by polymer-assisted deposition. These examples represent the first epitaxial uranium carbide films of any composition and, most notably, we generate the high temperature polymorph (1765 °C) of UC2 at 1000 °C that is stable at room temperature. This serves as a remarkable example of the types of materials that can be accessed through epitaxial stabilization. Our UC2 films have been well-characterized by XRD, UPS, Raman spectroscopy, and temperature dependent resistivity measurements. The resistivity and UPS data indicate that UC2 has relatively low electrical conductivity consistent with HSE hybrid DFT calculations showing a narrow band gap semiconductor. In depth PES experiments are currently underway to determine the exact nature of the Fermi surface and conductivity. The Raman spectroscopy indicated a C22− carbide moiety implying a U4+ oxidation state The in situ oxidation as a function of temperature of the UC2 film as followed by XRD shows intermediate oxidized films of composition UC2−xOx before final oxidation to U3O8. This work adds to the growing list of PAD materials and demonstrates that the technique can be applied throughout the periodic table and the chemistry reported herein could be used as a template for the preparation of epitaxial carbide films of the transuranic elements.

Figure 5. (Top) X-ray diffraction pattern of UC2 film and YSZ substrate. The (002) peak of UC2 shifts to higher 2θ as the film is heated and oxidized from room temperature (RT) to 400 °C. At 500 °C the (002) peak vanishes indicating full oxidation of the UC2 film. (Bottom) X-ray diffraction pattern of the fully oxidized UC2 film showing the U3O8 peak growing in and annealing. The dome peaks are from the containment dome on the XRD heating stage.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.J.). *E-mail: [email protected] (B.L.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Laboratory Directed Research and Development program at Los Alamos National Laboratory. The work was also performed, in part, by the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy Office of Science.

Figure 6. UC2 film with camera lens reflection (left image); partially oxidized UC2 film, UC2−xOx (middle image); fully oxidized UC2 film, U3O8 (right image).

decreasing temperature, indicating semiconductor like behavior (Figure 4). This is consistent with our HSE screened hybrid DFT calculations that show cubic UC2 to be a semiconductor, with a narrow gap of 0.40 eV.14 This result is also in agreement with our PES measurements (vide supra) showing limited conductivity. The exact nature of the Fermi level will need to be probed further with more detailed PES measurements in order to determine if cubic UC2 is a poor metal or semiconductor. Previous resistivity measurements on UC2 were made on the tetragonal phase13 and showed metallic behavior in line with our HSE screened hybrid DFT calculations. The tetragonal phase resistivity was reported to be 70 μΩ·cm, which is 4 times smaller than the resistivity we measured for our cubic phase (300 μΩ·cm). This is consistent with the cubic phase being on the boundary between a poor metal and a semiconductor.

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ABBREVIATIONS PAD, polymer assisted deposition; YSZ, yttrium stabilized zirconia; LAO, lanthanum aluminum oxide; EDTA, ethylenediaminetetraacetic acid; XRD, X-ray diffraction; UPS, ultraviolet photoemission spectroscopy; PES, photoelectron spectroscopy; EMCCD, electron multiplying charge coupled device; AFM, atomic force microscopy; HSE, Heyd−Scuseria− Ernzerhof; DFT, density functional theory

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