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

Abstract

semiconductor nanoplatelets (NPLs) are strongly photoluminescent materials with interesting properties for optoelectronics. Especially their narrow photoluminescence paired with a high quantum yield is promising for light emission applications with high color purity. However, retaining these features in solid-state thin films together with an efficient encapsulation of the NPLs is a challenge, especially when trying to achieve high-quality films with a defined optical density and low surface roughness. Here, we show photoluminescent polymer-encapsulated inorganic−organic nanocomposite coatings of 2D CdSe/CdS NPLs in poly(diallyldimethylammonium chloride) (PDDA) and poly(ethylenimine) (PEI), which are prepared by sequential layer-by-layer (LbL) deposition. The electrostatic interaction between the positively charged polyelectrolytes and aqueous phase-transferred NPLs with negatively charged surface ligands is used as a driving force to achieve self-assembled nanocomposite coatings with a well-controlled layer thickness and surface roughness. Increasing the repulsive forces between the NPLs by increasing the pH value of the dispersion leads to the formation of nanocomposites with all NPLs arranging flat on the substrate, while the surface roughness of the 165 nm (50 bilayers) thick coating decreases to R a = 14 nm. The photoluminescence properties of the nanocomposites are determined by the atomic layer thickness of the NPLs and the 11-mercaptoundecanoic acid ligand used for their phase transfer. Both the full width at half-maximum (20.5 nm) and the position (548 nm) of the nanocomposite photoluminescence are retained in comparison to the colloidal CdSe/CdS NPLs in aqueous dispersion, while the measured photoluminescence quantum yield of 5% is competitive to state-of-the-art nanomaterial coatings. Our approach yields stable polymer-encapsulated CdSe/CdS NPLs in smooth coatings with controllable film thickness, rendering the LbL deposition technique a powerful tool for the fabrication of solid-state photoluminescent nanocomposites.

Highly Transparent, Yet Photoluminescent: 2D CdSe/CdS Nanoplatelet‐Zeolitic Imidazolate Framework Composites Sensitive to Gas Adsorption

“…The ligand exchange to MUA is an established route to retain a high PLQY of the NPLs in aqueous solution (35% compared to 70% for NPLs in hexane) and is accompanied by a red-shift (from 513 nm to 542 nm) and slight broadening of the PL (FWHM: 9 nm to 22 nm). [29,36,37] We find that in the composite film, the PL position and FWHM is nearly retained, only shifting (from 542 nm to 546 nm; FWHM from 22 nm to 25 nm) due to the change of the dielectric constant of the matrix. [38] A rather broad PL background can be attributed to the ZIF-90.…”

“…The PLQY of the composite film is 8%, well in line with values of 5-10% typically reported for solid-state thin films encapsulating pre-polymerized core-only and core-crown NPLs and NCs. [6,28,29,31,39] Figure 3. Optical properties of the composite thin films.…”

“…[23,[25][26][27] Using colloidal 2D CdSe/CdS NPLs for solid-state application calls for their stable encapsulation inside a matrix. [28,29] Detrimental interactions which lead to a decreased PLQY and nonradiative charge carrier recombination such as charge transfer or FRET can occur between individual NPLs and between the NPLs and the matrix. [6] Suitable matrices must therefore prevent NPL agglomeration while staying inert.…”

Perpendicular Alignment of 2D Nanoplatelet Emitters in Electrospun Fibers: A Result of the Barus Effect?

“…To characterize the emission and absorption of single NPLs with known orientation, a sample of polymer/NPL nanocomposites with the NPLs oriented parallel to the surface of a microscope cover slip was prepared by layer‐by‐layer deposition. [ 41 ] This sample was used as a control and the orientation was reconfirmed by AFM (Figure 1c, for details see Experimental Section).…”

“…Preparation of NPLs Monolayer: The monolayer of NPLs on a microscope coverslip was prepared by a layer-by-layer (LbL) deposition process using a dip robot (DR-3, Riegler & Kirstein) as described by Li, Klepzig et al [41] Since the LbL process is based on electrostatic attraction between two oppositely charged molecules in aqueous conditions, NPLs were first transferred to aqueous phase via a ligand exchange approach. For this, 11mercaptoundecanoic acid (MUA) was used to replace oleic acid ligands on NPLs surface, in order to make the NPLs hydrophilic and capable of being deprotonated.…”