Monolayer Topography of Quantum Dots at the Interfaces

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The surface-pressure isotherm (Figure 1, curve 1) indicated the beginning of the liquid-expanded state of QD monolayers (Figure 2, Scheme I) at low pressures (about 2mN/m) and high areas (starting from 370cm²). The so-called “gaseous-like” phase (Figure 2, Scheme II) at very low pressures (below 0.5mN/m) did not shown because of extremely large areas (above 1000cm²) could not be reached in the standard experiment. In this state two types of structures can be proposed: the aggregates of QDs covered with TOPO (Figure 2, Scheme I) and monolayers of individual TOPO molecules (Figure 2, Scheme II). The gradual increase in the surface pressure until 15-20mN/m (Figure 1, curve a) can be attributed to various structural changes in the QD aggregates in the liquid-expanded state (Figure 2, Scheme I) of QD monolayers, where the TOPO molecules squeezed from individual monolayer back to the surface of QD particles. The relatively sharp increase in the surface pressure above 15-20mN/m can be attributed to the liquid-condensed state of QD monolayers (Figure 2, Scheme III), where TOPO molecules from different layers of the neighboring QD particles are strongly interpenetrating. On the other hand the surface potential values are increasing gradually by monolayer compression (Figure 1, curve b). So, we did not expect high orientation of individual particles in the interfaces. Although the isotherms of these QDs look like typical π-A isotherms of standard amphiphilic molecules (for example TOPO surfactant) the topography of these films displayed a unique behavior (the transitions between liquid-expanded and liquid-condensed states) that was observed by BAM. In spite of the absence of such distinct transitions (Figure 3 a and b) the BAM measurements revealed aggregation of QDs in large domains with higher brightness as compared to initial one. The relatively sharp decrease in the area by increasing surface pressure from 10 to 30mN/m (Figure 1) can be attributed to the liquid-condensed state of QD monolayers. The further monolayer compression above 30mN/m leads to collapse state at around 35-37mN/m (Figure 1). In this state (Figure 2, Scheme IV) densely packed QDs domains can grow only in Z-direction and become 3-D (3-dimentional or net-like structures), but still highly stable as compared to standard amphiphilic molecules.

Figure 2. Schematic representation of the QD-TOPO monolayer at the interfaces: I) QDs liquid-expanded state, II) TOPO liquid-expanded state, III) QDs liquid-condensed state, IV) QDs monolayer collapse state

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In order to proof our suggestion on QD monolayer topography inspired of obtained isotherms (discussed above) the BAM method was used. The dynamic BAM method is the specially attractive to observe the growing process of QD aggregates at the interfaces (Figure 3a, b) that can be indicated by domain brightness intensity etc.

It is a specially pronounced in the case of visualization of phase separation between liquid-expanded and liquid-condensed phases (Figure 3c). For example, from 10 to 30 mN/m the transition observed not in particular point and consist of two phases (Figure 3c), where these two phases liquid-expanded (left) and liquid-condensed (right) are clearly seen on the same image but divided by sharp and bright border curve. By monolayer collapse one can observe the very bright zones (Figure 3d) that can be attributed to the 3-D growth of the QDs domains (Figure 2, Scheme IV). It is important to underline that dark zones are not the pure water or TOPO surface, but it is another layer of QD domains (Figure 3). Because the brightness is increasing in a few times as compared to Figures 3a and b.

The particular areas per QD in 2-D film can be calculated 3-D as follows: I) A=π*(R₁+R₂)² for liquid-expanded monolayer state, II) A=π*(R₁+R₃)² for liquid-condensed monolayer state, III) A=π*(R₁)² for monolayer collapse state, where R₁ is QD radius in nm; R₂ is the thickness of the TOPO monolayer on QD surface (about 0.7nm ^{[9, 11]}), R₃ is about half of the thickness of the TOPO monolayer on QD surface, because of the strong interpenetration of TOPO layer from different particles. Thus, the respective theoretical values of the area per QD in 2-D film are follows: I) 25.8nm²; II) 140.5nm²; III) 198.4nm² in the case of QD.

BAM images of these QDs monolayers have been reported for the first time (Figure 3). Already at the experiments beginning (just after spreading at the air-water interface) these QDs formed monolayers that can be composed of two components (Figure 3a): TOPO molecules (homogeneous black areas) and QD 2-D aggregates (micrometer range light domains). The QD aggregates are growing strongly during monolayer compression and developing so called “net-like structures” of the numerous light domains. It is important to underline that QDs monolayer on 0.1M KCl have almost the same structures as those on the distilled water at low pressures. Further QDs monolayer compression leads to the formation of the rigid 3-D structures in collapse range (pressures about 30-35mN/m) of higher and lower brightness that are in any case have more intensity as initial 2-D monolayers.

The UV-VIS spectra of multilayer film composed 10 QDs monolayer transfer were recordered with the strong band as observed for QD in solution and monolayers reflection spectroscopy. These data prove the structural integrity of QD monolayer films.

As can be seen from the micrographs (Figure 4), obtained by confocal fluorescent microscopy, the QDs formed large bright aggregates. They are mainly combined in the large plain structures with dark holes about 2.0-2.5μm (small bar on the Figure 4a). The large bar of about 10 μm is the general scale for the whole picture, placed in a dark area (showing homogenous TOPO layer). Thus, the confocal fluorescent microscopy data are in agreement with BAM.

The observed QD structures have been enlarged in the Figure 4b in order to observe homogeneous 2-D aggregates as large bright domains (on the right and left side of the picture 4b) in contrast to small dark areas of TOPO layer (at the top, in the middle and at the bottom of this picture). These aggregates have minimal diameter about 159nm (Figure 4b). There are some small extremely bright regions (almost white regions) where the increased fluorescence is due to the increased amount of QDs forming 3-D structures.

This approach is a promising way to fabricate and visualize the monolayer films of colloidal nanocrystals (QDs) that can be useful for design of nanobiohybride structures. Such structures (QDs with natural biomembrane fragments and components ^{[1, 2, 3, 4]}) will be the main objects of the future publication.

4. Conclusion

Fabrication and visualization of the monolayer films of colloidal nanocrystals (QDs) by Langmuir and Brewster angle microscopy techniques are performed. These films can be promising elements of the nanohybrid structures (QDs with natural biomembrane fragments and components) for photovoltaic cells and other“nanoscale” objects for the special tasks in bio- and nanotechnology.

Figure 3. BAM images of the QD monolayers at water-air interface by increasing surface pressures from 0.1mN/m (a) to 35mN/m (d).

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Figure 4. Images of the QD monolayers, obtained by confocal fluorescent microscopy: a) standard picture, b) enlarged picture

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