Langmuir-Blodgett Transfer of Nanocrystal Monolayers: Layer Compaction, Layer Compression, and Lattice Stretching of the Transferred Layer

Abstract

Grazing incidence small angle X-ray scattering (GISAXS) was used to study the structure and interparticle spacing of monolayers of organic ligand-stabilized iron oxide nanocrystals floating at the air-water interface on a Langmuir trough, and after transfer to a solid support via the Langmuir-Blodgett technique. GISAXS measurements of the nanocrystal arrangement at the air-water interface showed that lateral compression decreased the interparticle spacing of continuous films. GISAXS also revealed that Langmuir-Blodgett transfer of the nanocrystal layers to a silicon substrate led to a stretching of the film, with a significant increase in interparticle spacing.

Keywords: GISAXS; Langmuir–Blodgett transfer; monolayers; nanocrystals; self-assembly.

Figure 1

Side view (A) and top view (B) illustrations of the experimental configuration used for the GISAXS measurements of nanocrystal monolayers floating at the air–water interface of a Langmuir trough. Please note that the effective incident angle of the X-ray beam is exaggerated in panel (A).

Figure 2

Nanocrystal monolayer characterization. (A) TEM of a Fe₂O₃ nanocrystal monolayer. (B) Schematic of a hexagonal lattice of oleic acid coated nanocrystals with edge-to-edge spacing (δ), center-to-center spacing (a), and crystalline core (D_c) defined. (C,D) Solution phase SAXS of Fe₂O₃ nanocrystals in hexane. (C) Scattering intensity vs. q and (D) Associated Porod plot of for Fe₂O₃ nanocrystals. The experimental data (■) and best fit (solid line) are shown. From the best fit, the core diameter and standard deviation was determined to be 7.14 ± 0.54 nm.

Figure 3

Nanocrystal LB films deposited at different surface pressures. (A) Surface pressure–area isotherm of 7.14 ± 0.54 nm oleic acid stabilized Fe₂O₃ nanocrystals on a Langmuir–Blodgett trough. The film was compressed at 10 mm/min. A total of 300 µL of a 0.5 mg/mL dispersion of Fe₂O₃ nanocrystals dispersed in chloroform was deposited onto an area of 250 cm². Vertical dashed lines mark the regions of the isotherm corresponding to the gas (1), liquid (2), solid (3), and collapse (4) phases. The upper x-axis denotes the surface coverage of Fe₂O₃ nanocrystals. (B) GISAXS performed on Fe₂O₃ nanocrystals films at the air–water interface over a surface coverage range of 0.18–0.72 which correspond to gas and liquid phases (regions 1 and 2) of the isotherm. (C) GISAXS scattering performed on Fe₂O₃ nanocrystals films at the air–water interface during compression over a surface coverage range of 0.72 to 0.95 which correspond to solid phase (region 3) of the isotherm. The graphs are projection integrations of GISAXS scattering images taken at various surface coverages on the air–water interface. The edge-to-edge separation between nanocrystals remains at 2.1 nm as the surface coverage increases from 0.18 to 0.72 during nanocrystal deposition and decreases from 2.1 nm to 1.3 nm over a surface coverage of 0.72 to 0.95. (D–G) are schematic illustrations of nanocrystal assembly at the air–water interface within the regions denoted in panel (A). A top-down view of the LB trough is depicted. Black circles correspond to Fe₂O₃ nanocrystals and vertical black bars correspond to the trough barriers. For simplicity, the ligands on the surface of the nanocrystal were not drawn. (H–K) SEM images of vertically transferred films at surface pressures of (H) 0 mN/m, (I) 1 mN/m, (J) 12 mN/m, and (K) 16 mN/m. SEM images are placed adjacent to corresponding film morphologies depicted in images (D–G).

Figure 4

Comparison of GISAXS scattering images for Fe₂O₃ nanocrystals on a silicon substrate vs. at the air–water interface. (A) GISAXS scattering pattern (inset) for a monolayer of 7.14 nm Fe₂O₃ nanocrystals on a silicon substrate transferred at a surface coverage of 0.89. (B) GISAXS scattering pattern (inset) for a monolayer of 7.14 nm Fe₂O₃ nanocrystals at the air–water interface taken at a surface coverage of 0.72. Both images are plotted on a logarithmic false-color scale where blue corresponds to an intensity of 10 and red to 10,000 in arbitrary units. The associated plots are projection integrations onto the q_x-axis of the scattering image. The lattice row indices of a hexagonal monolayer are indicated on the projection integration.

Figure 5

Contour plots of GISAXS scattering projection integrations taken during first compression (A), first decompression (B), and second compression (C) cycles of 7.14 nm diameter Fe₂O₃ nanocrystals at the air–water interface. The film was compressed and then decompressed at 0.5 mm/min with scattering images taken approximately every 10 sec. The film was compressed from a surface coverage of 0.72 to 1.2, however scattering beyond a surface coverage of 0.95 (not shown) resulted in blocking of the X-ray beam by the moving barrier. (D) Plot of edge-to-edge separation vs. surface coverage during the first compression (■), first decompression (♦), and second compression (▲) cycles. Calculation of the edge-to-edge separation was determined from the peak position of the {10} Bragg rod.

Figure 6

Projection integrations of GISAXS scattering images for 7.14 nm Fe₂O₃ nanocrystal monolayer films deposited onto silicon substrates at various nanocrystal surface coverage with the X-ray beam oriented both perpendicular (A) and parallel (B) to the substrate retrieval direction. (C) Plot of the edge-to-edge spacing vs. surface coverage for nanocrystal films transferred to silicon substrates with the X-ray beam (■) perpendicular and (♦) parallel to the dipping direction. For comparison, the edge-to-edge spacing for nanocrystals compressed at the (▲) air–water interface is also plotted.

Figure 7

Langmuir–Blodgett vertical transfer of a nanocrystal monolayer from the air–water interface onto a solid substrate.