Determination of the Insulation Gap of Uranium Oxides by Spectroscopic Ellipsometry and Density Functional Theory

Abstract

Detecting and understanding the complex signatures of species for attribution of highly enriched uranium, HEU, is challenging even though these compounds have been intensively studied for 65 years. Attempts to obtain, for example, chemical speciation signatures on uranium oxides are frustrated by the presence of extremely diverse phases, complex structures, and their tendency to form solid solutions with the coexistence of many nonstoichiometric oxides. More importantly, the spectroscopic signatures of many of these oxides, using common techniques such as X-ray diffraction or Raman scattering, are remarkably similar with each other. On the other hand, the effort to understand the U−O system also exhibits some of the most intriguing and challenging properties in theoretical and computational chemistry. This is due to the spatial extent between localization and delocalization of the 5f orbitals of the uranium atom. In this article, spectroscopic ellipsometry (SE) measurements and a comparison of six fitting methods as well as theoretical calculations are combined to examine the intrinsic electronic structure and the corresponding band gap of uranium oxides to determine the chemical speciation in a ∼102 nm thick reactively sputtered uranium oxide film. The SE results reveal that the UO x film exhibits two absorption edges, a primary absorption edge slightly above 2.6 eV and a secondary absorption at 1.7−1.8 eV. The optical band gaps compared with the theoretical calculations performed on UO 2 , U 4 O 9 , U 3 O 7 , α-U 3 O 8 , α-UO 3 , δ-UO 3 , and γ-UO 3 suggest that the UO x film is composed of at least two components; the primary absorption is caused by the α-UO 3 sublayer, which is superimposed on top of an adjacent α-U 3 O 8 sublayer that is hypothesized to be heteroepitaxial growth of α-U 3 O 8 along the UO x /substrate interface. Comparison to the ellipsometry measurements shows that the DFT+U and hybrid (HSE) calculations predict the correct trend for band gaps as a function of oxidation state and crystallography but they fail to capture the exact gaps. However, they provide important information for interpretation of the experimental results and highlight some of the structural complexity that prevails in the UO x compounds. The combination of theoretical and experimental methods to examine the intrinsic electronic structure and the band gap of the corresponding uranium oxides could benefit from the development of new methods for better distinguishing chemical speciation in uranium oxides. In addition, the experimental measurement of the indirect band gap of α-U 3 O 8 , is, to our knowledge, reported for the first time.

Modified Tauc–Lorentz dispersion model leading to a more accurate representation of absorption features below the bandgap

“…Using the TLLG model, the value of energy bandgap parameter was determined to be E g = 2.59 eV which is slightly smaller than the value obtained by the Tauc plot method (E g = 2.64 ± 0.02 eV, Ref. [91]) and very close to the band-gap energy of 2.61 eV determined in Ref. [92] for α-UO 3 , the α-phase of uranium trioxide, a "non-traditional" semiconducting material.…”

“…The optical properties of the uranium oxide films deposited by magnetron sputtering were determined over different spectral ranges by spectroscopic ellipsometry in Refs. [90,91]. The UO x dielectric functions in these references were obtained by applying the Drude-Lorentz model [90] and point-by-point data inversion [91] (the latter uses six different methods of the band gap determination based on linear extrapolation of various functional dependences for ε 2 and the absorption coefficient α).…”

“…Here we used the TLLG model to fit ε 2 spectrum extracted by He et al [91]. A combination of two TL oscillators with common E g and Lorentz function was used to fit the UO x optical properties (Fig.…”

“…Imaginary part of the dielectric function ε 2 for~1000 Å thick UO x film (from Ref. [91]) and its parameterization by the TLLG model with two TL oscillators and common E g parameter plus single Lorentz oscillator. Note that in this case the Lorentzian line shape worked better than Gaussian one in fitting the sub-band feature below E g .…”

Closure of the Mott gap and formation of a superthermal metal in the Fröhlich-type nonequilibrium polaron Bose-Einstein condensate in UO2+x

“…The polarons on the apical O in cuprates tunnel between the two halves of a symmetric, degenerate, double well potential because the two structures involved are identical 24,25,101,102 (although it is possible that the polarons in the CuO 2 planes are more complicated 87,103 ). An asymmetric double well would have been superfluous because of the assumption that the functionality originates in the ground state phase at all temperatures.…”