Atomic layer deposition transforms SnS 2 into SnS
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d performance with recyclability and less chemical waste. “The synthesis procedures of such nanomaterials are very simple … while using ordinary equipment and nontoxic solvents,” Wang says. Hortense Le Ferrand
of solar cells,” say Michel Nasilowski and Dane deQuilettes from the GridEdge Solar research program at the Massachusetts Institute of Technology. Sub-1-nm inorganic materials also have the potential to achieve multifunctionality
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Atomic layer deposition transforms SnS2 into SnS
Reaction of SnS2 with Sn(dmamp)2
S
eong Keun Kim of the Korea Institute of Science and Technology, Republic of Korea, and co-workers have devised an in situ transformation as a new strategy to synthesize metal chalcogenides. They specifically demonstrated that exposing SnS2 nanosheets to the vapor of bis(1-dimethylamino-2-methyl-2-propoxy)tin(II) (Sn(dmamp)2) reduced SnS2 to SnS with a high purity yield. This finding has been published in Chemistry of Materials (doi: 10.1021/acs.chemmater.9b04387). The SnS2-to-SnS transformation was accomplished using atomic layer deposition (ALD). ALD deposits thin films onto a substrate by stacking one atomic layer per reaction cycle. The researchers first grew a 4-nm-thick SnS2 film onto a SiO2 substrate, using Sn(dmamp)2 and H2S as the sources of Sn and S, respectively. Subsequently, H2S gas was eliminated, and they exposed the SnS2 thin film to only the vapor of Sn(dmamp)2 (1.06 Torr) at 270°C. In each reaction cycle, the exposure lasted for 2 s followed by purging for 10 s. After at least 200 cycles, the researchers confirmed that SnS2 was fully converted to SnS according to the lattice spacing measured by transmission electron microscopy, as well as a characteristic vibrational peak of SnS detected using Raman spectroscopy. The discovery of this phase transformation occurred by serendipity. Kim, the corresponding author, says that the research team initially “... tried to fabricate vertical p–n junction diodes by stacking p-type SnS layers on n-type SnS2 layers using ALD.” However, instead of a double-layered SnS2/SnS heterostructure, they discovered that they had obtained a pure SnS thin film.
SnS
SnS2
Sn S
Sn(dmamp)2
Substrate Reaction terminated
Diffusion through grain boundary SnS2 SnS
SnS
Diffusion through vdW layers
c
b
SnS2
SnS
(a) The process of the transformation from SnS2 to SnS, where (dmamp)2 is bis(1-dimethylamino-2methyl-2-propoxy)tin(II); transmission electron microscope images of (b) SnS2 and (c) SnS. Scale bars represent 10 nm. Credit: Chemistry of Materials.
The mechanism of the transformation is associated with Sn2+ adsorption and S diffusion. Upon contacting SnS2, Sn(dmamp)2 adsorbed onto SnS2, and the dmamp ligands decomposed into volatile products, including acetone. The adsorbed Sn2+ then combined with the S atoms within the surface. With the reaction cycle repeated, the S atoms on the surface were mainly consumed, and those buried inside the SnS2 thin film started to diffuse upward to replenish the surface through grain boundaries. The transformation
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