Layered Chalcogenide Materials Research Articles

Substrate Mediation in Vapor Deposition Growth of Layered Chalcogenide Nanoplates: A Case Study of SnSe2

The two-dimensional (2D) nanostructure of layered chalcogenide materials (LCMs) is very interesting, but to make it a useful material platform necessiates the development of sophisticated synthetic control. We present the synthesis of single-crystalline SnSe2 nanoplates by a noncatalytic vapor deposition process and demonstrate that the growth is subject to strong influences from substrates. The effect of substrates is evidenced by a temperature-dependent morphological difference in the nanoplates grown on mica and silicon substrates. At a growth temperature of 280 °C, the diameter of the nanoplates grown on mica is larger than those grown on silicon by a ratio of 6.2. This ratio substantially decreases to be unity (1.04) at a higher temperature of 420 °C. Additionally, different from the nanoplates grown on silicon, which always show a well-defined hexagonal shape, the nanoplates grown on mica exhibit an irregular shape at low temperature and can gradually evolve into the hexagonal shape with increasing temperature. Our dynamics analysis suggests that the observed temperature-dependent morphological difference can be specifically correlated to the migration of SnSe2 adatoms on the surface of substrates. SnSe2 adatoms may migrate on mica more easily than on silicon substrates because of the presence of dangling bonds at the surface of silicon. The role of the facile migration on mica in the nanoplate growth may be further confirmed by a correlation between the nanoplate diameter and the distance between neighboring nanoplates. While the focus is on SnSe2, we believe that this understanding can apply to the growth of two-dimensional (2D) nanostructures of other layered chalcogenide materials (LCMs). It may pave the way for the development of a general synthetic paradigm that leverages the effect of substrates to control the growth of 2D LCM nanostructures.

Epitaxial Nanosheet–Nanowire Heterostructures

We demonstrate synthesis of a new type of heterostructures that comprise two-dimensional (2D) nanosheets (NSs) epitaxially grown at one-dimensional (1D) nanowires (NWs). The synthesis involves materials with a graphite-like layered structure in which covalently bonded layers are held by weak van der Waals forces. GeS was used as a prototype material in this work. The synthesis also involves a seeded-growth process, where GeS NWs are grown first as seeds followed by a seeded growth of NSs at the pre-grown NWs. We observe that exposing the pre-grown NWs to air prior to the seeded growth is critical for the formation of NSs to yield NS-NW heterostructures. Our experimental results suggest that this might be due to a mild oxidation at the NW surface caused by the air exposure, which could subsequently facilitate the nucleation of NSs at the NWs. It also suggests that the surface oxidation needs to be controlled in a proper range in order to achieve optimized NS growths. We believe that this synthetic strategy may generally apply to the growth of NS-NW heterostructures of other layered chalcogenide materials. NS-NW heterostructures provide capabilities to monolithically integrate the functionality of 1D NWs and 2D NSs into a 3D space. It holds great potential in applications that request complex nanomaterials with multiple functionality, high surface area, and efficient charge transport, such as energy storage, chemical sensing, solar energy conversion, and 3D electric and photonic devices.

Transport measurements in lithium-intercalated In2Se3

Abstract The effects of intercalation on the transport properties of the layered chalcogenide material In2Se3 are reported and discussed. Transport measurements have been carried out using both Hall effect and far-infrared reflectivity spectroscopy from which conductivity and Hall mobility are deduced. Spectra of intercalated samples clearly show the Drude edge shift and are analysed using the single-carrier Drude model. The charge transfer between the intercalant lithium atoms and the electron bands of the host upon intercalation is discussed.

Lithium insertion in layered materials as battery cathodes

Results are presented for discharge processes of lithium electrochemical cells containing layered chalcogenide materials. Systematic and precise studies of the voltage-composition relation were made of lithium insertion in InSe, In 2Se 3, MoS 2, TiS 2 and NiPS 3. The stability of the intercalated phases is discussed on the basis of electrochemical potential spectroscopy measurements, and the inverse derivative composition-voltage is analysed using the lattice gas model. Using galvanostatic experiments, the chemical diffusion coefficient D(x) and the Darken factor K 1(x) of the solid solution electrodes are determined.