Deposition of Wafer‐Scale Single‐Component and Binary Nanocrystal Superlattice Thin Films Via Dip‐Coating

Abstract

thickness have the potential to be tuned by varying the solvent, NC concentration, substrate surface treatment, and withdrawal speed. Dip-coating has been utilized previously for deposition of high-quality NC-based electronics, [26][27][28][29][30] but the work to date that explores dip-coating as a method for control over NC superlattice assembly is limited. [ 31,32 ] Here, we demonstrate that dipcoating can be used to deposit ordered, polycrystalline superlattices of a variety of NC materials over 4 in (100 mm) wafer scales. We show that fi lm thickness can be controlled by varying the withdrawal speed to deposit fi lms from sub-monolayer to several multilayers. The combination of transmission electron microscopy (TEM), scanning electron microscopy (SEM), and grazing incidence small-angle X-ray scattering (GISAXS) shows that the ordered superlattice structure observed at the nanoscale by microscopy extends over the entire dip-coated area. Finally, we present the fi rst demonstration that NC assembly via dipcoating can be extended to form several binary NC superlattices (BNSLs) including AlB 2 , NaZn 13 , CaCu 5 , and MgZn 2 structures, exhibiting the versatility of dip-coating for depositing a variety of NC materials and superlattice structures.Lead selenide (PbSe) [ 33,34 ] was chosen as the pilot material for optimizing the dip-coating conditions because of its particularly narrow size distribution. Different dispersing solvents and substrate treatments commonly used in NC deposition were explored (see Section S1.1 and S1.2 of the Supporting Information). We focused primarily on assemblies deposited from toluene dispersions, which were found to favorably wet n-octyltriethoxysilane (OTS)-treated surfaces, facilitating longrange, planar order. Additionally, we studied how dip-coating withdrawal speed affects the fi lm thickness and coverage on the substrate. Figure 1 shows a series of PbSe NC thin fi lms deposited by dip-coating from a NC solution in toluene onto OTS-treated substrates at withdrawal rates of 1, 5, 15, 25, 50, and 100 mm min -1 . The 100 mm min -1 withdrawal rate is rapid in comparison to typical dip-coating processes. The fi lm color shows the optical uniformity and the color change refl ects thickness-dependent interference as the fi lms become thicker at increasing withdrawal speeds (Figure 1 a), which is quantifi ed by a shift in the energy of the fi lm refl ectance spectrum (Figure 1 b). The proportional relationship of withdrawal speed to fi lm thickness is likely due to higher withdrawal speeds wicking up a thicker solvent fi lm along the substrate surface. The slowest withdrawal speed of 1 mm min -1 , is an exception to this trend and results in a fi lm thicker than that formed at 5 mm min -1 . At the slowest withdrawal rates, the solvent evaporates across the substrate faster than the withdrawal speed as illustrated in Figure 1 c. Atomic force microscopy (AFM) ( Figure S1, Supporting Information) measurements were performed to analyze Monodisperse nanocrystals (NCs) provide an opportuni...

Tolerance to structural disorder and tunable mechanical behavior in self-assembled superlattices of polymer-grafted nanocrystals

“…Our understanding of nanocrystal superlattice mechanical behavior would also be enriched by obtaining a full, tensile stress-strain curve for this material, to understand properties such as strength and fracture toughness, and by studying the time dependence of superlattice mechanical properties. To accomplish these goals, it is desirable to fabricate selfassembled nanocrystal arrays with bulk length scales, which remains a challenge in the field (8,12,49). Nevertheless, selfassembled nanocrystal superlattices are a rich model system for understanding mechanical behavior in materials under nanoscale confinement, and the properties of polymer nanocomposites.…”

Designed Assembly and Integration of Colloidal Nanocrystals for Device Applications

“…Although dip‐coated films are often interrupted by process‐related edge effects (for example, the Marangoni effect results in zones with irregular transitions at the upper margin during the immersion step, which is caused by variations in the surface tension), the problem can be overcome by scale‐up. For example, Murray's group reported the formation of a superlattice on the wafer scale, based on the dip‐coating process (Figure c–e). In addition, one important benefit of the dip‐coating process is that it can be conducted as a continuous roll‐to‐roll process, which is easily adaptable for the large‐scale continuous production.…”

Assembly and Electronic Applications of Colloidal Nanomaterials

“…[154,[207][208][209] Thicker QD films, such as PbSe or PbS, can be deposited by repeating the dipping process to achieve final thicknesses of 30-40 nm, allowing them to be used in photodetectors or solar cells. [211] Furthermore, dip coating can be used to create densely aligned SWCNTs by withdrawal-deposition due to confinement effects at the air-water interface. Slow evaporation during substrate withdrawal has enabled wafer-scale PbSe and PbS nanocrystal superlattices.…”

“…[305] In these glassy thin films, each nanocrystal is surrounded tightly with neighboring nanocrystals with close-packed spacing, but this order is only short-ranged. [87] Nanocrystal superlattices have recently been achieved at the wafer scale by a dip-coating method that exploits slow withdrawing and evaporation rates, [211] as well as blade coating with relatively fast drying. [90] Long-range ordering of nanocrystals in thin films, often referred as superlattices or supercrystals, can be realized by controlled drop casting with slow solvent evaporation, which allows nanocrystals to have sufficient time to find their equilibrium sites.…”