Real-time studies of evaporation-induced colloidal self-assembly by optical microspectroscopy

Real-time monitoring of the whole growth process of evaporation-induced colloidal self-assembly has been conducted using an optical microspectroscopy setup. Our observations suggest that the assembly process can be divided into three different growth stages as evidenced by the variations detected in the reflectance spectra. The thickness variation of the growing colloidal crystal was monitored by examining the Fabry-Perot fringes in the reflectance spectra. Furthermore, the scalar wave approximation was utilized to analyze the evolution of optical properties with growth. More detailed information, including the time dependence of number of layers and volume fraction of water, has been revealed by comparing the experimental and calculated reflectance spectra. The present work demonstrates that in situ real-time microspectroscopy is a promising technique for monitoring and investigating the dynamic process of colloidal self-assembly.

Figure 1

Schematic diagram of the experimental setup for reflectance measurement.

Figure 2

Real time evolution of the reflectivity during EISA at room temperature. The labels indicate the elapsed time in minutes.

Figure 3

(a) The peak position (red circles) and stop-band attenuation (blue squares) as functions of the elapsed time. These data points are obtained from the reflectance spectra recorded in the real-time monitoring experiment at room temperature (partially shown in Fig. 2). Schematic diagram for the EISA growth process of different stages: (b) only the growth front, (c) both the growth front and the drying front, and (d) only the drying front, under the monitored region.

Figure 4

Normal incidence reflectance spectra recorded at the elapsed time of: (a) 200 min and (b) 505 min. The insets, which are obtained from magnifying the rectangle regions, show the closely spaced Fabry-Perot fringes. (c) Calculated thickness of the growing CPC as a function of the elapsed time by using Eq. (3) with $n_{\text{eff}}=1.46$ (red circles) and $n_{\text{eff}}=1.53$ (blue squares). The polynomial fitting curve indicates the actual thickness variations.

Figure 5

The post-growth heights of CPC relative to the substrate measured by a profilometer: (a) along the central axis and (b) along the horizontal direction at 8.77 and 6.52 mm, respectively. The inset shows the photograph of a post-growth film deposited on the front wall of the cuvette.

Figure 6

Normal-incidence reflectance spectra for the growing CPC under a hot air stream at different times (circles). The red solid curves are calculated using the SWA with parameters: (a) $D=246$ nm, $n_{\text{eff}}=1.53$; (b) $D=242$ nm, $n_{\text{eff}}=1.53$; (c) $D=240$ nm, $n_{\text{eff}}=1.53$; (d) $D=240$ nm, $n_{\text{eff}}=1.52$; and (e) $D=240$ nm, $n_{\text{eff}}=1.49$. The dashed lines highlight the peak position variations.

Figure 7

Real-time studies of the faster growth process of EISA introduced by a hot air stream. (a) The peak position (red squares) and stop-band attenuation (blue circles) vs elapsed time. (b) Experimental full-with at half-maximum and calculated number of layers as functions of the elapsed time. The inset shows the stop-band width for a single 20-layer colloidal crystal of 240 nm PS spheres, as a function of dielectric contrast ${\psi }_0$, calculated by the SWA. (c) Volume fraction of water in the crystal as a function of the elapsed time. The open circles are extracted from the SWA, while the red solid curve represents the best fit obtained using Eq. (7).