Nanopattern formation in self-assembled monolayers of thiol-capped Au nanocrystals

The structure and the stability of the transferred monolayers of gold-thiol nanoparticles, formed at air-water interface at different surface pressure, on to silicon surface have been studied using two complementary techniques, x-ray reflectivity and atomic force microscopy (AFM). Networklike nanopatterns, observed through AFM, of the in-plane aggregated nanoparticles can be attributed to the late stage drying of the liquid trapped in the islands formed by nanoparticles. During drying process the trapped liquid leaves pinholes in the islands which by the process of nucleation and growth carry the mobile nanoparticles on their advancing fronts such that the nanoparticles are trapped at the boundaries of similar adjacent holes. This process continues bringing about in-plane as well as out-of-plane restructuring in the monolayer until the liquid evaporates completely rendering a patterned structure to the islands and instability in the monolayer is then stabilized.

Figure 1

(Color online) Left panel: typical TEM micrograph with particle size distribution in the inset. The average size of the metallic core of the nanocrystal is 2.9 nm. Right panel: UV-VIS absorption spectra showing plasmon band at around 515 nm.

Figure 2

(Color online) $\left(\pi -A\right)$ isotherm recorded at the Langmuir trough showing various phases. Inset: corresponding derivative curve to emphasize the changes.

Figure 3

(Color online) AFM topographs of the transferred films (a1-d1 for scan size $10\times 10\mu {\text{m}}^2$ and a2-d2 for scan size $1\times 1\mu {\text{m}}^2$) deposited at the four different pressures corresponding to different phases in the isotherm. a1 and a2 are the AFM images for LE-G phase, b1 and b2 for LE phase, c1 and c2 for LC-LE phase and d1 and d2 for LC phase. Insets of (a1-d1) show typical line profiles.

Figure 4

: (Color online) (a)–(h) Successive AFM images (scan size $4\times 4\mu {\text{m}}^2$) of the film deposited at $\pi =12\text{mN}/\text{m}$ (corresponding to the LC-LE phase) taken every 30 min. (a) is after 30 min of deposition; (b)–(h) show images after 1, 1.5, 2, 2.5, 3, 3.5, and 4 h respectively. The small voids grow in dimension with time to give a patterned appearance to the island. (i-l) Typical height histograms of the images (b), (d), (f), and (h). The white boxes in b, d, f and h show the area from which the histograms have been calculated. The average height of the monolayer is seen to increase with time.

Figure 5

(Color online) Time-evolution XRR profiles (scattered graphs) and the respective fits (solid lines) for the film transferred at $\pi =2\text{mN}/\text{m}$. Lower panel shows the EDPs (obtained from convolution of AED of stratified layers with interfacial roughness) corresponding to the fits.

Figure 6

(Color online) Time-evolution XRR profiles (scattered graphs) and the respective fits (solid lines) for the film transferred at $\pi =5\text{mN}/\text{m}$. Lower panel shows the EDPs corresponding to the fits.

Figure 7

(Color online) Time-evolution XRR profiles (scattered graphs) and the respective fits (solid lines) for the film transferred at $\pi =10\text{mN}/\text{m}$. Lower panel shows the EDPs corresponding to the fits.

Figure 8

(Color online) Time-evolution XRR profiles (scattered graphs) and the respective fits (solid lines) for the film transferred at $\pi =14\text{mN}/\text{m}$. Lower panel shows the EDPs corresponding to the fits. The peaks in the XRR profiles of the sample (marked by arrow) may be due to some multilayered stacks formed at the edges of the film after drying.

Figure 9

(Color online) The schematics for the initial and the final positions (fluctuation in the monolayer) of the clusters in the film transferred at different pressures, derived from the respective EDPs. The relative fluctuation is very small in case of films corresponding to the mixed LE phases (a) and (b) but it is rather large for the films corresponding to the mixed LC phases (c) and (d).