Ryan Atkins

Suppressing a charge density wave by changing dimensionality in the ferecrystalline compounds ([SnSe]1.15)1(VSe2)n with n = 1, 2, 3, 4.

The compounds, ([SnSe]1.15)1(VSe2)n with n = 1, 2, 3, and 4, were prepared using designed precursors in order to investigate the influence of the thickness of the VSe2 constituent on the charge density wave transition. The structure of each of the compounds was determined using X-ray diffraction and scanning transmission electron microscopy. The charge density wave transition observed in the resistivity of ([SnSe]1.15)1(VSe2)1 was confirmed. The electrical properties of the n = 2 and 3 compounds are distinctly different. The magnitude of the resistivity change at the transition temperature is dramatically lowered and the temperature of the resistivity minimum systematically increases from 118 K (n = 1) to 172 K (n = 3). For n = 1, this temperature correlates with the onset of the charge density wave transition. The Hall-coefficient changes sign when n is greater than 1, and the temperature dependence of the Hall coefficient of the n = 2 and 3 compounds is very similar to the bulk, slowly decreasing as the temperature is decreased, while for the n = 1 compound the Hall coefficient increases dramatically starting at the onset of the charge density wave. The transport properties suggest an abrupt change in electronic properties on increasing the thickness of the VSe2 layer beyond a single layer.

Insights from STEM and NBED studies into the local structure and growth mechanism of misfit layered compounds prepared using modulated reactants

Abstract X-ray diffraction and transmission electron microscopy were used to probe the structure of the misfit compound [(SnSe)1.15]1(VSe2)1 grown using an elementally modulated precursor. The specular X-ray diffraction pattern contained only 00l reflections, which yielded a c lattice parameter of 1.203(1) nm. Cross-section STEM revealed alternating layers of SnSe and VSe2, in agreement with the structure model refined from the X-ray diffraction pattern using Rietveld refinement. Plan-view transmission electron microscopy revealed the in-plane grain structure of the films, yielding grain sizes in agreement with previously reported in plane X-ray diffraction studies and the cross-section STEM images. The plan view images also contained Moiré fringes resulting from grains with different relative tilting on both sides of interfaces as well as Moiré fringes resulting from different relative rotations between domains. An energy-filtered nano-beam electron diffraction pattern obtained from at least one domain in the [(SnSe)1.15]1(VSe2)1 sample investigated in cross section contained a series of resolvable supercell reflections along the c axis that indicated that the supercell c-axis lattice parameter was a multiple of three times that determined using X-ray diffraction. Energy filtered NBED of plan-view samples showed diffraction patterns from select regions with 12-fold symmetry, indicating that the arrangement of the layers is not rotationally random from layer to layer. This suggests that during the self-assembly of the amorphous modulated elemental precursor, the SnSe and VSe2 constituent layers must nucleate off the adjacent interfaces of the growing crystal, yielding layers that are locally rotationally aligned with growing crystal. Different processing conditions during the precursor to crystal self-assembly might enable the domain size and/or the extent of order to be controlled.

Raman spectroscopy insights into the size-induced structural transformation in SnSe nanolayers.

Raman spectroscopy is used to probe the structural changes in [SnSe]m[MoSe2]n ferecrystal thin films as a function of m, the number of bilayers of SnSe. In spite of the interleaved structure in the intergrowths, Raman spectra can be described as a superposition of spectra from the individual components, indicating that the interaction at the interface between the components is relatively weak. Analysis of room-temperature Raman spectra indicate that the MoSe2 layers separating the SnSe layers are nanocrystalline in all of the samples studied, with little change as the number of Se-Mo-Se trilayers (n) or SnSe bilayers (m) increases, reflecting the rotational disorder between adjacent trilayers. A thickness-dependent, continuous transition occurs in the SnSe layer as m is increased, from a pseudotetragonal structure when the layers are thin to a bulk-like orthorhombic SnSe structure when the SnSe layer thickness is increased. Polarization analysis of the Raman scattering from these materials allows the symmetry evolution of the SnSe layers through this transition to be determined.

Detection of nanoscale embedded layers using laboratory specular X-ray diffraction

Unusual specular X-ray diffraction patterns have been observed from certain thin film intergrowths of metal monochalcogenide (MX) and transition metal dichalcogenide (TX2) structures. These patterns exhibit selective “splitting” or broadening of selected (00l) diffraction peaks, while other (00l) reflections remain relatively unaffected [Atkins et al., Chem. Mater. 24, 4594 (2012)]. Using a simplified optical model in the kinematic approximation, we illustrate that these peculiar and somewhat counterintuitive diffraction features can be understood in terms of additional layers of one of the intergrowth components, MX or TX2, interleaved between otherwise “ideal” regions of MX-TX2 intergrowth. The interpretation is in agreement with scanning transmission electron microscope imaging, which reveals the presence of such stacking “defects” in films prepared from non-ideal precursors. In principle, the effect can be employed as a simple, non-destructive laboratory probe to detect and characterize ultrathin laye...

Methods of Electron Crystallography as Tools for Materials Analysis

Abstract. New methods of electron crystallography, particularly modern methods of electron diffraction have opened new strategies for the structure analysis of nanostructured materials and materials systems. The possibilities and limitations of the combined use of electron crystallography methods will be demonstrated for a semi-automatic orientation determination of MnAs clusters in a GaAs matrix and structural investigations of ferecrystals.

Structural investigations of ferecrystals [(SnSe) 1+δ ] m [TSe 2 ] n (T = Mo, Ta) by means of transmission electron microscopy

The nanostructure of ferecrystals [(SnSe)<inf>1+δ</inf>]<inf>m</inf> [TSe<inf>2</inf>]<inf>n</inf> with T = Mo, Ta, synthesized by the modulated elemental reactants (MER) method with 1 ≤ m, n ≤ 30, was investigated by various methods of transmission and scanning transmission electron microscopy (TEM/STEM), including precession electron diffraction (PED). The influence of the layer composition, sequence, and thickness, on the resulting intergrowth structure and disorder is elucidated, including various aspects of intra- and inter-layer stacking disorder.

Synthesis of new ferecrystals (SnSe) y (TSe 2 ) where T = V and Ta

(SnSe)<inf>1.09</inf>(VeSe<inf>2</inf>) and (SnSe)<inf>1.16</inf>(TaSe<inf>2</inf>) ferecrystals were synthesized using the modulated elemental reactant (MER) technique. The materials composition, in-plane crystalline structure, and layering characteristics were characterized through electron microprobe, x-ray diffraction, and transmission electron microscope analysis.