Controlled Spreading Rates to Distribute Nanoparticles as Uniform Langmuir Films

Abstract

We report on using a controlled spreading rate to create Langmuir films of nanoparticles with more uniform, macroscale packing. A dispersion of hydrophobic quantum dots in n-hexane was deposited on subphase solutions containing various compositions of water and glycerol. Fluorescence images were captured as the film spread radially. An average spreading rate was defined using film radius and time at maximum expansion. On water with the highest spreading rate, films have an open region surrounded by a coffee ring. At a well-defined slower spreading rate, a distinct inner compact region appears between the open film and coffee ring, now called an outer compact region. As the spreading rate decreases further, the relative position for the open film boundary moves inward while the relative areas for the inner and outer compact regions increase. Films are the smallest in size at the slowest spreading rate on glycerol. The patterns are button-like with a central depleted region (open film), compact inner and outer regions, and a less-dense outer edge region. Normalized radial profiles were used to generate a partition map for the relative radial positions marking each film region at different spreading rates. Area number densities were calculated in the highest-packed regions. The values give no conclusive evidence that nanoparticles stack as multilayers, even the most compactly covered regions. Films spreading on glycerol form the most uniform, circular-shaped, densely packed arrangement of nanoparticles as their final pattern.

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

“…[87,88] While Langmuir-based techniques remain popular to some extent, they require sophisticated surface functionalization or coassembly with polymers [89] and it is difficult to obtain uniform nanoparticle monolayers using them. [90] Figure 5. The combination of a,g,h,k) ex situ TEM images and FT images and b,e,i,l) in situ GISAXS and c,f,j,m) GIWAXS measurements at different stages of PbSe nanocrystal self-assembly allows for a full characterization and mechanistic understanding of the hexagonal to square phase transformation.…”

“…[ 87,88 ] While Langmuir‐based techniques remain popular to some extent, they require sophisticated surface functionalization or coassembly with polymers [ 89 ] and it is difficult to obtain uniform nanoparticle monolayers using them. [ 90 ] Instead, the spontaneous self‐assembly of a broad range of particles (with respect to their composition or shape) at different interfaces has gained attraction in the past decade. The self‐assembly of CdSe particles at the water–air interface into 2D domains has been first (passingly) reported in 2005, [ 91 ] but it was not until 2010 that interfacial nanoparticle self‐assembly became a method of choice.…”

“…Once the dispersion is placed on the subphase, it quickly spreads on the liquid surface. [ 90 ] The shape and size of the resulting “film” follows different expansion and retraction phases. First, it spreads radially with a spreading rate depending on the interfacial tension between solvent and subphase.…”

“…First, it spreads radially with a spreading rate depending on the interfacial tension between solvent and subphase. [ 90 ] At this phase, which has been termed “colloidal gas,” [ 77 ] the nanoparticle concentration is low and solvent–particle interactions dominate ( Figure a). A thin ordered film of densely packed particles precedes the bulk dispersion, which dissolves again within several seconds if the solvent's spreading rate exceeds its evaporation rate.…”

“…At this stage, the floating film shrinks in size and may ultimately display different regions like a coffee‐ring or an open film (Figure 6c). [ 90 ] In the end, the thin film floating on the subphase is transferred to a substrate either by removal of the subphase or “scooping” of the film. The whole self‐assembly process is routinely finished within 10s of minutes to a few hours.…”