2 SnI 4 nanosheets, with R = C 18 H 35 . Under specific synthetic conditions, stable mixtures of 3D nanocrystals CsSnI 3 and 2D nanosheets (Ruddlesden-Popper (R-NH 3 + ) 2 Cs n−1 Sn n I 3n+1 with n > 1) are obtained. These results set a path to exploiting the high potential of Sn halide perovskite nanocrystals for opto-electronic applications.

Thermally induced structural transformation of 2D materials opens unique avenues for generating other 2D materials by physical methods. Imaging these transitions in real time provides insight into synthesis routes and property tuning. We have used in situ transmission electron microscopy (TEM) to follow thermally induced structural transformations in layered CoSe2. Three transformation processes are observed: orthorhombic to cubic-CoSe2, cubic-CoSe2 to hexagonal-CoSe, and hexagonal to tetragonal-CoSe. In particular, the unit-cell-thick orthorhombic structure of CoSe2 transforms into cubic-CoSe2 via rearrangement of lattice atoms. Cubic-CoSe2 transforms to hexagonal-CoSe at elevated temperatures through the removal of chalcogen atoms. All nanosheets transform to basal-plane-oriented hexagonal 2D CoSe. Finally, the hexagonal to tetragonal transformation in CoSe is a rapid process wherein the layered morphology of hexagonal-CoSe is broken and islands of tetragonal-CoSe are formed. Our results provide nanoscopic insights into the transformation processes of 2D CoSe2 which can be used to generate these intriguing 2D materials and to tune their properties by modifying their structures for electro-catalytic and electronic applications.

Direct observation of the growth dynamics of 2D transition metal dichalcogenides (TMDs) is of key importance for understanding and controlling the growth modes and for tailoring these intriguing materials to desired orientations and layer thicknesses. Here, various stages and multiple growth modes in the formation of WS 2 layers on different substrates through thermolysis of a single solid-state (NH 4 ) 2 WS 4 precursor are revealed using in situ transmission electron microscopy. Control over vertical and horizontal growth is achieved by varying the thickness of the drop-casted precursor from which WS 2 is grown during heating. First depositing platinum (Pt) and gold (Au) on the heating chips much enhance the growth process of WS 2 resulting in an increased length of vertical layers and in a self-limited thickness of horizontal layers. Interference patterns are formed by the mutual rotation of two WS 2 layers by various angles on metal deposited heating chips. This shows detailed insights into the growth dynamics of 2D WS 2 as a function of temperature, thereby establishing control over orientation and size. These findings also unveil the important role of metal substrates in the evolution of WS 2 structures, offering general and effective pathways for nano-engineering of 2D TMDs for a variety of applications.

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Abstract3D superlattices made of colloidal quantum dots are a promising candidate for the next generation of optoelectronic devices as they are expected to exhibit a unique combination of tunable optical properties and coherent electrical transport through minibands. While most of the previous work was performed on 2D arrays, the control over the formation of these systems is lacking, where limited long‐range order and energetical disorder have so far hindered the potential of these metamaterials, giving rise to disappointing transport properties. Here, it is reported that nanoscale‐level controlled ordering of colloidal quantum dots in 3D and over large areas allows the achievement of outstanding transport properties. The measured electron mobilities are the highest ever reported for a self‐assembled solid of fully quantum‐confined objects. This ultimately demonstrates that optoelectronic metamaterials with highly tunable optical properties (in this case in the short‐wavelength infrared spectral range) and charge mobilities approaching that of bulk semiconductor can be obtained. This finding paves the way toward a new generation of optoelectronic devices.