Formation of Colloidal Copper Indium Sulfide Nanosheets by Two-Dimensional Self-Organization

Abstract

Colloidal 2D semiconductor nanosheets (NSs) are an interesting new class of materials due to their unique properties. However, synthesis of these NSs is challenging, and synthesis procedures for materials other than the well-known Pb- and Cd-chalcogenides are still underdeveloped. In this paper, we present a new approach to make copper indium sulfide (CIS) NSs and study their structural and optical properties. The CIS NSs form via self-organization and oriented attachment of 2.5 nm chalcopyrite CuInS2 nanocrystals (NCs), yielding triangular- and hexagonal-shaped NSs with a thickness of ∼3 nm and lateral dimensions ranging from 20 to 1000 nm. The self-organization is induced by fast cation extraction, leading to attractive dipolar interactions between the NCs. Primary amines play a crucial role in the formation of the CIS NSs, both by forming in situ the cation extracting agent, and by preventing the attachment of NCs to the top and bottom facets of the NSs. Moreover, DFT calculations reveal that the amines are essential to stabilize the covellite crystal structure of the product CIS NSs. The NSs are indium-deficient and the off-stoichiometry gives rise to a plasmon resonance in the NIR spectral window.

Interplay between Surface Chemistry, Precursor Reactivity, and Temperature Determines Outcome of ZnS Shelling Reactions on CuInS2 Nanocrystals

“…This process converts the CIS seed NCs into In-poor CIS NCs, which subsequently undergo 2D self-organization and oriented attachment, yielding In-poor CIS nanosheets. 43 This demonstrates that S-ODE/OLAM is capable of quickly extracting In 3+ (and to a lesser extent Cu + ) from CIS NCs, thereby leading to pronounced etching, which is characterized by large blue-shifts accompanied by large reductions in the In/Cu ratio and low Zn-content. The extent of the etching is directly related to the reactivity of the Zn-precursor: an unreactive Zn-precursor, such as ZnSt 2 , is unable to react with the S-precursors fast enough to outcompete the In-extraction reaction, thereby resulting in pronounced etching, followed by 2D self-organization.…”

Near-Infrared-Emitting CuInS2/ZnS Dot-in-Rod Colloidal Heteronanorods by Seeded Growth

“…The strong and broad NIR absorption bands observed in the spectra of the product CIS/ZnS HNCs obtained by using injection methods II and III ( Figure S3 ) are assigned to localized surface plasmon resonances (LSPR), which are often observed in Cu-chalcogenide NCs. 9 , 24 , 25 LSPR bands involve excess charge carriers, which are due to stoichiometry deviations, typically Cu-vacancies, which lead to excess holes in the valence band. 9 , 24 However, LSPRs due to In-vacancies have also been reported.…”

“… 9 , 24 However, LSPRs due to In-vacancies have also been reported. 25 These observations imply that methods II and III lead to product HNCs that are rich in cation (Cu or In) vacancies, in contrast to method IV.…”

“…The etching induced by S/ODE can be ascribed to side-reactions with the Sulfur precursor, since elemental sulfur is known to react with ODE/alkylamines forming a number of reactive S-species (e.g., H 2 S), 25 , 45 which have been shown to extract In 3+ (and to a lesser extent also Cu + ) from CIS NCs. 25 The etching induced by Zn(oleate) 2 can be attributed to a reaction through which Zn 2+ in solution and In 3+ /Cu + on the surface of CIS NCs exchange coordinating molecules, resulting in In/Cu oleate complex and S/S 2– in solution. 46 Cation exchange is also likely in Protocol III, since the CIS NC seeds are directly injected in a hot solution containing only Zn-precursors.…”

Anisotropic 2D Cu2–xSe Nanocrystals from Dodecaneselenol and Their Conversion to CdSe and CuInSe2 Nanoparticles

“…The role of ligands in stabilizing facets is a well-known phenomenon. 19 For example, it has been shown that alkylamines stabilize the high energy (001) top and bottom facets of covellite CIS nanosheets, 52 oleic acid directs the 2D-growth of PbS by stabilizing the (100) facet, 53 and amines and DDT stabilize different djurleite Cu 2– x S facets, thereby providing control over where CE reactions take place and consequently over the morphology of Cu 2– x S/I–III–VI 2 hetero-nanorods. 54 We thus conclude that the driving force for the internal reconstruction process that accompanies the structural transformation of the nanoplatelets from tetragonal umangite Cu 2– x Se to hexagonal wurtzite CdSe and CuInSe 2 is likely the minimization of both the total surface free energy and the reconstruction strain during the structural reorganization process, in such a way that the total volume expansion work is kept to a minimum ( V Cu 2 Se = 1.7 × 10 3 nm 3 , V CdSe = 2.1 × 10 3 nm 3 , V CuInSe 2 = 1.8 × 10 3 nm 3 ).…”