Ultrathin 2-dimensional nanomaterials (nanosheets) are attracting increasing attention due to their unique physical, electronic and structural properties [1]. Colloidal semiconductor nanosheets are of particular interest, since they combine the extraordinary properties of 2D nanomaterials with versatility in terms of composition, size, shape and surface, and the prospects of solution processability. To date, colloidal NSs of a variety of binary compositions have been prepared (e.g. CdX with X = S, Se, Te; PbS; SnX, with X = S, Se; In₂S₃; Cu2–xS; Cu2–xSe; WS₂) [2], but reports on nanosheets of multinary semiconductors are scarce, despite the interesting properties of this class of materials. For instance, CuInS₂ (CIS) is a direct semiconductor with a bandgap of 1.45 eV and large absorption coefficients, which yields nanocrystals with photoluminescence tunable in the visible to the near-infrared spectral range. In this work we developed a new two-stage method to synthesize colloidal CIS nanosheets, in which CIS nanocrystals (NCs) are used as building blocks.  

In the first stage, dodecanethiol-capped chalcopyrite CIS NCs with a tetrahedral shape and 2.5 nm in size are synthesized. These NCs are purified and subsequently resuspended in the reaction mixture and heated to the reaction temperature. This process yields ~3 nm thick single crystalline wurtzite Cu_xIn_yS_z (x/y = 1.5-3) nanosheets with triangular or hexagonal shapes, and lateral dimensions ranging from 50 nm to ~1 μm. The non-stoichiometry of the nanosheets is also reflected in the appearance of a broad absorption band in the NIR, which is assigned to a localized plasmon resonance involving excess charge carriers.  

We propose that the formation of the nanosheets occurs by two-dimensional self-organization of the CIS seed NCs, followed by sintering and recristalization from the original cubic chalcopyrite structure to the hexagonal wurtzite structure. The driving force for the self-organization is the extraction of indium ions from the CIS seed NCs by sulfur present in solution, which is assumed to create a transient charge imbalance that favors the attachment of the NCs. The formation of three-dimensional assemblies is prevented by the ligand coverage of the cation rich facets, which become the top and bottom facets of the nanosheets. This also constrains further growth of the nanosheets in thickness, allowing growth only in the lateral dimensions by attachment of NC building blocks.  

[1] Narita, A.; et al. Nature Chem. 6 (2014) 126-132.

[2] van der Stam, W. et al. Chem. Mater. 27 (2015) 283−291.