Trevor Keiber

Atomic layer deposition of zinc sulfide with Zn(TMHD)2

The atomic layer deposition (ALD) of ZnS films with Zn(TMHD)2 and in situ generated H2S as precursors was investigated, over a temperature range of 150–375 °C. ALD behavior was confirmed by investigation of growth behavior and saturation curves. The properties of the films were studied with atomic force microscopy, scanning electron microscopy, energy-dispersive x-ray spectroscopy, ultraviolet–visible–infrared spectroscopy, and extended x-ray absorption fine structure. The results demonstrate a film that can penetrate a porous matrix, with a local Zn structure of bulk ZnS, and a band gap between 3.5 and 3.6 eV. The ZnS film was used as a buffer layer in nanostructured PbS quantum dot solar cell devices.

Resolution of the discrepancy between the variation of the physical properties of Ce1-xYbxCoIn5 single crystals and thin films with Yb composition

Extraordinary electronic phenomena including an Yb valence transition, a change in Fermi surface topology, and suppression of the heavy fermion quantum critical field at a nominal concentration 0.2 have been found in the system. These phenomena have no discernable effect on the unconventional superconductivity and normal-state non-Fermi liquid behaviour that occur over a broad range of up to 0.8. However, the variation of the coherence temperature and the superconducting critical temperature with nominal Yb concentration for bulk single crystals is much weaker than that of thin films. To determine whether differences in the actual Yb concentration of bulk single crystals and thin film samples might be responsible for these discrepancies, we employed Vegard’s law and the spectroscopically determined values of the valences of Ce and Yb as a function of to determine the actual composition of bulk single crystals. This analysis is supported by energy-dispersive X-ray spectroscopy, wavelength-dispersive X-ray spectroscopy, and transmission X-ray absorption edge spectroscopy measurements. The actual composition is found to be about one-third of the nominal concentration up to 0.5, and resolves the discrepancy between the variation of the physical properties of single crystals and thin films with Yb concentration.

Room‐Temperature Ba(Fe1−xCox)2As2 is not Tetragonal: Direct Observation of Magnetoelastic Interactions in Pnictide Superconductors

Lattice distortions corresponding to Ba displacements with respect to the FeAs sublattice are revealed to break the room-temperature tetragonal symmetry in Ba(Fe1-x Cox)2 As2. The displacements yield twin domains of the size of ≈10 nm. The domain size correlates with the magnitude of the local Fe magnetic moment and its non-monotonic dependence on Co concentration.

Structure in multilayer films of zinc sulfide and copper sulfide via atomic layer deposition

Multilayer film stacks of ZnS and CuxS (x ∼ 2) were made via atomic layer deposition. The precursors were bis(2,2,6,6-tetramethyl-3,5-heptanedionato)zinc, bis(2,2,6,6-tetramethyl-3,5-heptanedionato)copper, and H2S generated in situ for sulfur. Samples were deposited at 200 °C, in layers ranging from approximately 2 to 20 nm thick, based on binary growth rates. The properties of the film stacks were studied with atomic force microscopy, ultraviolet–visible spectroscopy, and extended x-ray absorption fine structure. The results demonstrate that the structure of films with the thinnest layers is dominated by CuxS, whereas in the thicker films, the structure is determined by whichever material is first deposited. This can be attributed to the crystal structure mismatch of ZnS and CuxS.

Comparison of local distortions in Ba8Ga16X30 (X = Si, Ge, Sn): an EXAFS study

We report an extended X-ray fine structure (EXAFS) analysis of the type-I clathrates Ba8Ga16X30 (X = Si, Sn) and compare the results with the results of previous studies on X = Ge. The distribution of Ga on the three crystallographic cage sites is not random, with the Ga preferentially having X as the nearest neighbor. Our results show that for X = Si, Sn the average pair distances within the cages (Ga–Sn, Ga–Ga, Ga–Si, Sn–Sn) are significantly different from the distances found in X-ray diffraction, with some much shorter bonds and some much longer bonds. These results suggest a substantial buckling of the cages, particularly for Ba8Ga16Sn30. The environment about Ba, extracted from Ba K edge EXAFS, becomes increasingly disordered from Ge to Si to Sn, and for Ba8Ga16Sn30 the nearest Ba neighbor distance is very short, consistent with severe buckling. This buckling contributes to the increased local disorder for Ba8Ga16Si30 and Ba8Ga16Sn30, and provides an explanation for a higher resistivity and a lower ZT than for Ba8Ga16Ge30.

Structure and magnetism in LaCoO3

The temperature dependence of the hexagonal lattice parameter c of single crystal LaCoO3(LCO) with H = 0 and 800 Oe, as well as LCO bulk powders with H = 0, was measured using high-resolution x-ray scattering near the transition temperature To ≈35 K. The change of c(T) is well characterized by a power law in T − To for T>To and by a temperature independent constant for T<To when convoluted with a Gaussian function of width 8.5 K. This behavior is discussed in the context of the unusual magnetic behavior observed in LCO as well as recent generalized gradient approximation calculations.

Anisotropic distortions in Sb doped CePt4Ge12

We investigated the local structure of the Sb doped skutterudite CePt4Ge12-xSbx using the Extended X-ray Absorption Fine Structure Technique (EXAFS). As the concentration of Sb is increased the disorder around Ce increases rapidly, and for x = 3, the peak for the nearest neighbor (Ce-Ge) is no longer observed. In contrast, for the Pt site, the disorder of the nearest neighbors is small even for x = 3. Thus the distortions are anisotropic and appear to be mainly in the plane of the Ce-Ge bonds and Ge4 rings. The increased disorder about Ce will decrease the lattice thermal conductivity at low temperatures, and likely is part of the reason for improved thermoelectric properties for the x = 1 sample.

Structure and magnetism in LaCoO 3

The temperature dependence of the hexagonal lattice parameter c of single crystal LaCoO3(LCO) with H = 0 and 800 Oe, as well as LCO bulk powders with H = 0, was measured using high-resolution x-ray scattering near the transition temperature To ≈35 K. The change of c(T) is well characterized by a power law in T − To for T>To and by a temperature independent constant for T<To when convoluted with a Gaussian function of width 8.5 K. This behavior is discussed in the context of the unusual magnetic behavior observed in LCO as well as recent generalized gradient approximation calculations.