Curvature and self-assembly of semi-conducting nanoplatelets

Abstract

Semi-conducting nanoplatelets are two-dimensional nanoparticles whose thickness is in the nanometer range and controlled at the atomic level. They have come up as a new category of nanomaterial with promising optical properties due to the efficient confinement of the exciton in the thickness direction. In this perspective, we first describe the various conformations of these 2D nanoparticles which display a variety of bent and curved geometries and present experimental evidences linking their curvature to the ligand-induced surface stress. We then focus on the assembly of nanoplatelets into superlattices to harness the particularly efficient energy transfer between them, and discuss different approaches that allow for directional control and positioning in large scale assemblies. We emphasize on the fundamental aspects of the assembly at the colloidal scale in which ligand-induced forces and kinetic effects play a dominant role. Finally, we highlight the collective properties that can be studied when a fine control over the assembly of nanoplatelets is achieved.

Layer-by-Layer Deposition of 2D CdSe/CdS Nanoplatelets and Polymers for Photoluminescent Composite Materials

“…After the LbL deposition, we find a shorter PL lifetime τ average of 0.9 ± 0.1 ns in both the PDDA/NPL and PEI/NPL nanocomposites with an additional fast recombination process, which is fitted with the time constants τ short‑PDDA = 0.5 ± 0.1 ns and τ short‑PEI = 0.5 ns ±0.1 ns. Possible explanations for the PL lifetime changes include homo-Förster resonance energy transfer (homo-FRET) between individual NPLs in different layers − and fast excitonic decay to quenching sites in CdSe/CdS core–crown NPLs, which lack full passivation by MUA ligands. Similar PL quenching and lifetimes have been observed for CdSe/CdS core–shell NPLs studied by time-resolved photoluminescence, transient absorption, and time-resolved terahertz spectroscopy .…”

“…The PLQY of the NPLs decreases by the phase transfer (from 70 to 35%), induced by hole traps originating from the MUA thiolate ligand. , The nanocomposite coatings exhibit a PLQY of 4.7 and 5.3% for PDDA/NPL and PEI/NPL films, respectively. Compared to aqueous dispersions, interactions such as homo-FRET and energy transfer between individual CdSe/CdS NPLs have to be considered and have been observed for closely packed stacks of NPLs. − , However, e.g., homo-FRET is most efficient in NPL stacks of reduced disorder and NPLs arranged next to each other in a single layer have shown significantly less efficient FRET. , With a large estimated Förster radius of 13.5 nm, homo-FRET between CdSe/CdS NPLs could be responsible for decreased PLQY and photoluminescence lifetimes even through a polymer layer. ,, On the other hand, resonant energy transfer to the polymer in our case is very unlikely due to a lack of spectral overlap between the NPL photoluminescence and the PEL absorption . Saidzhonov et al have reported on the influence of incorporating hydrophobic CdSe NPLs stabilized with oleic acid by a PMMA matrix .…”

“…Compared to aqueous dispersions, interactions such as homo-FRET and energy transfer between individual CdSe/CdS NPLs have to be considered and have been observed for closely packed stacks of NPLs. [66][67][68]73 However, e.g., homo-FRET is most efficient in NPL stacks of reduced disorder and NPLs arranged next to each other in a single layer have shown significantly less efficient FRET. 74,75 With a large estimated Forster radius of 13.5 nm, homo-FRET between CdSe/CdS NPLs could be responsible for decreased PLQY and photoluminescence lifetimes even through a polymer layer.…”

Assembly, Properties, and Application of Ordered Group II–VI and IV–VI Colloidal Semiconductor Nanoparticle Films

“…Furthermore, the 2D assembly into thin films on a solid substrate has not been summarized as thoroughly. For recent examples, see the reviews of Barad et al [39] for an in-depth summary on selected 2D-assembly strategies with a focus on metallic, metal oxide, and polymer nanoparticles, Cimada daSilva et al [40] for the interfacial self-assembly of lead chalcogenide QDs into 2D superstructures, or Guillemeney et al [41] for the assembly of semiconductor nanoplatelets. This stands in contrast to the high importance assigned to thin films consisting of colloidal semiconductor nanocrystals in many different applications like photovoltaics, (photo-)sensing, or (opto-)electronics.…”

The Crucial Role of Solvation Forces in the Steric Stabilization of Nanoplatelets

“…Among the different shapes of nanoparticles, platelets have extraordinarily large facets in relation to their particle size . Additionally, CdSe nanoplatelets are synthesized typically with very dense ligand shells. ,− …”