Investigating the Formation of MoSe₂ and TiSe₂ Films from Artificially Layered Precursors

Abstract

The reaction of ultrathin layers of Mo and Ti with Se was investigated, and significantly different reaction pathways were found. However, in both systems postdeposition annealing results in smooth dichalcogenide films with specific thicknesses determined by the precursor. X-ray diffraction (XRD) patterns of as-deposited Mo|Se films around a 1:2 ratio of Mo to Se contain weak, broad reflections from small and isolated MoSe2 crystallites that nucleated during deposition and a sharper intensity maximum resulting from the composition modulation created from the alternating deposition of Mo and Se layers. In contrast, as-deposited Ti|Se films around a 1:2 ratio of Ti to Se contain narrow and intense 00l reflections from TiSe2 crystallites and do not contain a Bragg reflection from the sequence of deposited Ti|Se layers. The as-deposited TiSe2 crystallites have a larger c-axis lattice parameter than was previously reported for TiSe2, however, which suggests a poor vertical interlayer registry and/or high defect densities including interstitial atoms. In-plane XRD patterns show the nucleation of both TiSe2 and Ti2Se during deposition, with the Ti2Se at the substrate. For both systems, annealing the precursors decreases the peak width and increases the intensity of reflections from crystalline TiSe2 and MoSe2. Optimized films consist of a single phase after the annealing and show clear Laue oscillations in the specular XRD patterns, which can only occur if a majority of the diffracting crystallites in the film consist of the same number of unit cells. The highest quality films was obtained when an excess of ∼10% Se was deposited in the precursor, which presumably acts as a flux to facilitate diffusion of metal atoms to crystallite growth fronts and compensates for Se loss to the open system during annealing.

Synthesis and Electrical Properties of a New Compound (BiSe)0.97(Bi2Se3)1.26(BiSe)0.97(MoSe2) Containing Metallic 1T-MoSe2

“…Due to reactions with the substrate and/or oxidation at the surface, it is common to form one or more fewer unit cells of the intended heterostructure than the number of layers deposited. 34 We observed this in the precursors studied here, as the Laue 3). The specular XRD pattern of the as-deposited precursor contains reflections from two different sources.…”

“…In reactions of layered precursors, the local compositions and nanoarchitecture become important parameters as they control what nucleates and the diffusion lengths required for growth. Due to reactions with the substrate and/or oxidation at the surface, it is common to form one or more fewer unit cells of the intended heterostructure than the number of layers deposited . We observed this in the precursors studied here, as the Laue oscillations around the (002) reflections in samples 1–4 indicate that 10 unit cells formed from the 11 repeating sequences deposited.…”

Defects in Layered van der Waals Heterostructures: Implications for Thermoelectrics

“…Nucleation occurs at multiple sites within the precursors because any amorphous region formed during deposition onto the nominally room temperature substrates is supersaturated with respect to the crystalline constituents. Several recent reports discuss methods to control nucleation rates/sites, which could be used to reduce domain boundary and layer step defects. ,, Local variations in composition will always occur; therefore, controlling layer step defects would require controlling processing conditions and diffusion rates. , Our results also suggest that it should be possible to control the type and density of defects in these heterostructures by controlling the precursor structure and processing conditions to influence properties. Defects are desirable in some cases, such as reduced lattice thermal conductivity, which can be improved by increasing dislocation densities and decreasing grain size .…”

“…There are thin (nanometer) impurity regions at the top and bottom of the film that are distinct from the repeating pattern of SnSe and TiSe 2 layers and can result from excess Sn, Ti, or Se and can form additional compounds such as SnSe 2 at the surface . The bottom layer of the film may also contain impurity compounds, such as Ti 2 Se or Ti silicides, that form when the initial layer of Ti reacts with the subsequently deposited Se layer or with the substrate during the deposition or annealing processes …”

“…13 The bottom layer of the film may also contain impurity compounds, such as Ti 2 Se or Ti silicides, that form when the initial layer of Ti reacts with the subsequently deposited Se layer or with the substrate during the deposition or annealing processes. 18 The majority of the higher magnification image of ([SnSe] 1+δ ) 1 (TiSe 2 ) 1 in Figure 1c contains the targeted repeating layer sequence, which is marked with blue (SnSe) and orange (TiSe 2 ) rectangles to easily identify the difference in constituents for several repeat units. This region contains few obvious defects despite the low temperature of the annealing (350 °C, 30 min) 13 to self-assemble.…”