Active Stratification of Colloidal Mixtures for Asymmetric Multilayers

Abstract

Stratified films offer high performance and multifunctionality, yet achieving fully stratified films remains a challenge. The layer-by-layer method, involving the sequential deposition of each layer, has been commonly utilized for stratified film fabrication. However, this approach is time-consuming, labor-intensive, and prone to leaving defects within the film. Alternatively, the self-stratification process exploiting a drying binary colloidal mixture is intensively developed recently, but it relies on strict operating conditions, typically yielding a heterogeneous interlayer. In this study, an active interfacial stratification process for creating completely stratified nanoparticle (NP) films is introduced. The technique leverages NPs with varying interfacial activity at the air-water interface. With the help of depletion pressure, the lateral compression of NP mixtures at the interface induces individual desorption of less interfacial active NPs into the subphase, while more interfacial active NPs remain at the interface. This simple compression leads to nearly perfect stratified NP films with controllability, universality, and scalability. Combined with a solvent annealing process, the active stratification process enables the fabrication of stratified films comprising a polymeric layer atop a NP layer. This work provides insightful implications for designing drug encapsulation and controlled release, as well as manufacturing transparent and flexible electrodes.

Keywords: colloidal particles; depletion pressure; functional films; interfaces; interfacial activity; multilayers; stratification.

Figure 1

Concept of the active stratification process proposed in this work. a) Schematic illustration of the active stratification process proposed in this work. b) Compressional Langmuir isotherms of the PS (diameter = 960 nm) and SiO₂ (diameter = 700 nm) NP monolayers at the air–water (containing 0.8 wt% PEG) interface. The SiO₂ and PS NP monolayers withstand Π up to ≈20 and ≈50 mN m⁻¹, respectively, and then collapse by further compression. The compressional modulus (E _comp) is calculated as E _comp = ‐A·(dΠ/dA), where A is the trough area, when the close‐packed monolayers are compressed (indicated by the dashed lines). c,d) Schematic illustrations and corresponding scanning electron microscope (SEM) or optical microscope images of the collapsing PS NP monolayer c) and SiO₂ NP monolayer d) according to lateral compression at the interface. The collapsing PS NP monolayer exhibits out‐of‐plane deformations in the order of wrinkling, buckling, and folding as compression proceeds. The images of the wrinkling and buckling are top‐view optical microscope images, and the image of the folding is a cross‐sectional SEM image. The asterisk markers indicate the crest points of the wrinkles. The SEM image shows the cross‐section of the SiO₂ NP bilayer that results from the collapsing SiO₂ NP monolayer at the interface.

Figure 2

Compressional Langmuir isotherm analysis. a) Compressional Langmuir isotherms of the NP mixtures [PS (diameter = 960 nm) and SiO₂ (diameter = 700 nm)] at the air–water (containing 0.8 wt% PEG) interface according to the various values of α. The regime 1 represents where Π increases gently after reaching ≈20 mN m⁻¹, and the regime 2 represents where Π rises sharply again from Π ≈ 25 mN m⁻¹. The star markers indicate the point at which the regime 1 ends (i.e., the regime 2 starts). b) SEM images of the well‐packed NP mixture monolayers at the interface with distinction of each type of NP. Each type of NP is distinguished according to the size (PS NP: red circle, SiO₂ NP: blue circle) using MATLAB with a customized code, and counted to obtain the values of α (Figure S2, Supporting Information). All scale bars are 10 µm. c) Correlation between the values of α obtained from the isotherms and SEM images. The values of α from the isotherms are extracted based on the fact that the regime 2 starts at the trough area (marked by the stars markers) corresponding to the fraction of the area occupied by the PS NPs, 1/(1+α).

Figure 3

Structural characterization of the NP mixture [PS (diameter = 960 nm) and SiO₂ (diameter = 700 nm) NPs] layers at the air–water (containing 0.8 wt% PEG) interface. a) SEM images with schematic illustrations as a function of TA at α ≈ 2. As the TA is reduced, the number of the SiO₂ NPs underneath the PS NPs increases, and then stratification into a PS NP monolayer atop a double layer of the SiO₂ NPs is achieved at the end of regime 1 (TA = 0.33). b) The number of layers extracted from the surface profiles (Figure S3, Supporting Information) as a function of TA at α ≈ 2. The error bars represent the standard deviation of the number of layers obtained from 10 different profiles. c) A photographic image of the NP mixture layer (α ≈ 2) at the interface when the TA is 0.33. d) Top‐view and cross‐sectional element‐mapped SEM images of the NP mixture films at a TA of 0.33. e) Cross‐sectional SEM images of the stratified NP films with various values of α, deposited at the end of regime 1 (Figure 2a). The stratified NP films are composed of a PS NP monolayer atop α layers of the SiO₂ NPs. All scale bars are 1 µm unless otherwise noted.

Figure 4

Universality of the active stratification process. a) SEM images of the stratified NP films of various NP combinations manufactured at the air–water (containing 0.8 wt% PEG) interface by the active stratification process. The compositions of the stratified films are described by the following nomenclature: type and diameter of more interfacial active NP/type and diameter of less interfacial active NP. The Cu NW has a diameter of 100 nm and a length of 10–20 µm. All scale bars are 1 µm unless otherwise noted. In (a), the surfaces of the SiO₂ NPs with diameters of 380 and ≈100 nm are treated with dichlorodimethylsilane to increase interfacial activity. b) Electrical conductivity of the stratified films composed of Cu NW monolayers atop SiO₂ NP monolayers depending on the material loading‐compression cycle. The top panel presents a representative cross‐sectional SEM image of the stratified film obtained with one loading‐compression cycle. The middle panel shows a photographic image of the electrical conductivity measurement of the stratified film. 200 data points were measured, and their frequencies are shown in the form of a Gaussian function (bottom panel).

Figure 5

Schematic illustrations and cross‐sectional SEM images of the stratified NP mixture [PS (diameter = 960 nm) and SiO₂ (diameter = 700 nm) NPs] layers at the air–water (containing 0.8 wt% PEG) interface upon the interfacial solvent annealing. Toluene floats on the air–water interface and anneals the PS NPs only at/above the interface, resulting in a clearly stratified film composed of a thin PS film atop the sparse SiO₂ NPs or SiO₂ NP bilayers at α ≈ 0.5 or ≈2, respectively. In contrast, the stratified NP layer is annealed after deposition, which leads to a PS‐infiltrated SiO₂ NP film. All scale bars are 1 µm.