Scanning Electron Microscopy & FTIR spectroscopy characterization of Polystyrene Colloidal Crystals  on various substrates

5. Scanning Electron Microscopy & FTIR spectroscopy characterization of Polystyrene Colloidal Crystals on various substrates

 Simona Badilescu, Yong-Hong Ye*, 
Georges Bader and Vo-Van Truong

Thin Films and Photonics Research Group
Département de physique et d`Astronomie
Université de Moncton,
Moncton, N.B., Canada E1A 3E9

* Department of Physics,
Nanjing Normal University, P.R.China

Introduction

Polystyrene (PS) colloidal crystals are well-ordered structures formed by the spontaneous ordering of monodisperse colloids. As the individual PS spheres are large enough to be tracked by microscopic methods, colloidal crystals are useful as model systems for crystal ordering (for example in noble gas solids, important for matrix isolation techniques), as well as for solid-liquid phase transitions [1,2]. More recently, self-organizing systems such as PS colloidal crystals have been used as template material for the fabrication of photonic crystals and optical components [3,4]. Using microscopy images from reflected light, Gigault and Dutcher [5] have observed that the interaction between the PS spheres is different when they are ordered on various substrates. We have recently studied the self-assembly of spheres on patterned polymer substrates with different properties [6]. Our results showed that the ordering of spheres, that is, the quality of the template, depend not only on the topology of the substrate but foremost on the properties of the substrate material.

Infrared work on PS spheres was directed mostly toward the analysis of chemically-altered surfaces or on the effects of sphere size on the diffuse reflectance infrared spectra [7]. In spite of the importance of PS colloidal crystals, none of the papers reported on their infrared characteristics. The aim of this paper is to study the morphology and the FTIR spectra of PS spheres self-assembled on various substrates.

Experimental

Monodisperse polystyrene (PS) microspheres with diameters between 50 and 500 nm were provided in the form of an aqueous suspension (5.0%w/v) by Bands Inc., and Polyscience Inc. These spheres contain linear polystyrene and their aqueous suspensions are stabilized against flocculating with a large number of ionizable carboxyl groups chemically bonded to their surfaces. Before use, the suspension was diluted with bidistilled and deionized water and sonicated. The substrates were carefully cleaned using common procedures. The Si wafer, which was used for all experiments, was first sonicated to remove the PS spheres from the previous sample deposition, then in an acid bath (a mixture of hydrochloric and nitric acids) and finally in distilled water.

To prepare a homogeneous silica coating on the Si wafer, a sol-gel process involving the acid catalyzed hydrolysis and condensation of an organic precursor, tetraethylorthosilicate (TEOS), was used [8]. A dip-coating apparatus made in our laboratory was used for the deposition. The Si substrate was lowered into the coating solution and then withdrawn at speeds of 3-4 mm/s. As a result of its exposure to the moisture in air, TEOS hydrolyzed into the oxide. The film was dried at 80°C for 1 hour and then heat-treated at 300°C to remove the residual organic material.

PS spheres were deposited on the glass microslides and the IR transparent substrates, either by the vertical [9,10], or by the horizontal convective self-assembly method [11], respectively.

For the vertical deposition, the substrate is placed vertically into a vial containing a diluted aqueous suspension of spheres. The vial is kept at 50°C in a temperature-controlled oven for 2-3 days. To deposit the sample horizontally, the aqueous suspension of spheres is spread on the substrate and allowed to dry slowly over a period of 24 hours.

The ordered structures of PS microspheres on glass and silicone substrates were studied with a JSM-5200 scanning electron microscope (SEM). The spectra were recorded with a Bomem M100 FTIR instrument in both the transmission and ATR modes. A SPECAC wire polarizer (Kent-England) was used to measure the polarized spectra.

Results and Discussion

All the samples deposited by the “vertical” method have the same appearance regardless of the substrate material. Due to Bragg diffraction, they are iridescent with a predominant color determined by the size of the spheres. Low magnification SEM images (Figure 1a and 1b) revealed the presence of a large number of close spaced lines (~ 15-20 mm) running perpendicular to the direction of the deposition. As shown in the high magnification image (Figure 1c), the ordered arrays of PS spheres extend over the lines. The close spaced lines visible in the images are cracks (average width of 1-2 mm), due to the stresses developing in the film during the drying process. The pattern of cracks seems more regular on glass than on the silicon substrate but the overall picture is the same. As long as the preparation mode of the sample is the same, there are no significant changes in the self-organization of the spheres on different substrates. However, as we have observed experimentally, the properties of the substrate may facilitate the formation of the ordered structures. Indeed, as the PS spheres are deposited from aqueous suspensions, samples useful for infrared measurements (8-10 layers) could be deposited more easily on hydrophilic substrates such as Si and SiO2 than, for example, on a ZnSe ATR crystal or a polystyrene film substrate. To deposit the ordered array of spheres on polymer substrates (polystyrene or polyester films) we had to use the horizontal self-assembly method.

Figure 1. SEM micrograph of PS microspheres (300 nm) 
self-assembled onto a glass substrate, x750 
(a) on a silicon substrate, x1000 
(b), and same as b at a x10.000 magnification.

Table 1 shows the position of the major bands of the PS colloidal crystal on various substrates and a typical spectrum of the spheres (100 nm) assembled on a Si substrate is given in Figure 2. The bands belonging to the ordered spheres are extremely narrow and their position is independent on the size of the spheres. They appear at slightly higher frequencies in the case of the ZnSe crystal (ATR measurements) than on Si or SiO2/Si substrates, respectively. The bare Si wafer and the SiO2 / Si substrates, both have hydroxylated surfaces, favoring the spontaneous sticking of PS spheres coming from the water suspension. No bands belonging to water molecules can be seen in the spectra as water is completely removed from the sample during the long drying process. In the spectra corresponding to the dense PS multilayers, a weak band can be observed at 1556 cm-1, belonging to carboxyl groups (nas COO ) linked with alkyl groups to the surface of spheres. 

 

Zinc Selenide Silicon Silicon Dioxide* Poly(acetate) Polystyrene Assignment
3029 3026 3027 3030 3020 nCH (ring)
2922 2923 2922 2918 2915 nCH (CH2)
1605 1601 1603 1598 1599 nC=C (ring)
1497 1493 1495 1500 1495
1457 1452 1453 1459 1448 d CH (CH2)
760 754 762 758 763 gCH (5H)
707 700 703 705 709 gCH (CH2)

Table 1.  Position of the major IR bands (cm-1) of 
PS microspheres self-assembled on various substrates.
SiO2 on Si

Figure 2. IR spectrum of PS microspheres (100 nm) 
self-assembled onto a silicone substrate by the vertical method.

Table 1 shows the position of the major bands of the PS colloidal crystal on various substrates and a typical spectrum of the spheres (100 nm) assembled on a Si substrate is given in Figure 2. The bands belonging to the ordered spheres are extremely narrow and their position is independent on the size of the spheres. They appear at slightly higher frequencies in the case of the ZnSe crystal (ATR measurements) than on Si or SiO2/Si substrates, respectively. The bare Si wafer and the SiO2 / Si substrates, both have hydroxylated surfaces, favoring the spontaneous sticking of PS spheres coming from the water suspension. No bands belonging to water molecules can be seen in the spectra as water is completely removed from the sample during the long drying process. In the spectra corresponding to the dense PS multilayers, a weak band can be observed at 1556 cm-1, belonging to carboxyl groups (nas COO ) linked with alkyl groups to the surface of spheres. 


The presence of a regular pattern of drying cracks introduces a strong anisotropy in the samples prepared by the vertical thermal convection method. This becomes evident in polarized IR spectra. The first measurements with the polarizer set in orthogonal directions showed a strong dependence of the spectra on the position of the samples. More consistent results were obtained by keeping the polarizer set in the same position and rotating the sample. Figure 3 shows the spectra corresponding to the sample oriented in 4 different positions with respect to the polarizer. Some of the deep cracks are visible without any magnification and they allow the orientation of the sample. For the first spectrum (3a), the strips were parallel to the vector of the polarized light (see the inset). Then, the sample was rotated to 90 (3b), 180 (3c) and 270
°(3d), respectively. The spectra show that, when the polarizer is aligned parallel to the strips (3a and 3c), the bands appear much stronger than in the two spectra corresponding to the horizontal strips (3b and 3d). While close, the two pairs of spectra are not entirely the same, probably due to the slightly different positions of the sample. The band the most sensitive to the position of the sample is the 700 cm-1 band belonging to the “rocking” of CH2 groups (gCH2). To account for this particular behaviour, we have considered the “grating effect” of the crack pattern. Indeed, due to the close spaced lines, the sample acts as a second polarizer: when parallel to the first, the bands appear strongly enhanced, and when horizontal, the two “polarizers” are crossed and the intensity of the bands is considerably reduced. Because of its wavelength (around 14.3 m) close to the width of the ordered arrays of spheres between two cracks, the effect could be especially marked on the 700 cm-1 band. The effect seems to be less important on the 760 cm-1 band probably due to the origin of this band. Indeed, this band belongs to the out-of-plane vibration of the hydrogen atoms in the aromatic ring, and contains also a slight contribution from the Si substrate. Due to the stress, the order of the microspheres could be disrupted along the cracks, resulting in an orientation of the polymer chains. Unfortunately, the “grating effect” of the sample is too strong to allow any measure of the molecular orientation.

Figure 3. Polarized IR spectra (|| ) of PS microspheres
on a Si substrate (vertical method) corresponding to 4 positions of the sample
(see inset): vertical strips (a) and (c) and horizontal strips (b) and (d).

In order to prove the “grating effect”, we prepared a sample by the horizontal self-assembly method and have studied its morphology and the polarized spectrum. To prepare an ordered sample, the aqueous suspension of PS microspheres is dropped onto the center of the substrate and the spheres spread all over the surface moving along the radial directions. The AFM image of samples prepared by this method (Figure 4) show the presence of concentric drying cracks. As expected, due to the circular pattern of drying cracks, the spectra of samples deposited by the horizontal method are independent on the position of the sample. Figure 5 shows that parallel and perpendicular polarized spectra of the sample are sensibly the same.

Figure 4. SEM micrographs of PS microspheres (320 nm)
deposited by the horizontal method.

Figure 5. Polarized IR spectra of PS microspheres 
self-assembled onto a silicon substrate by the horizontal method.

Conclusions

Methods to prepare PS ordered structures suitable for infrared measurements have been investigated. We have found that, the deposition of ordered layers on hydrophilic substrates such as the Si wafer or SiO2/ Si, is possible with both, the vertical and the horizontal method, respectively. Under the same conditions, the self-assembly of the microspheres on hydrophobic substrates could be accomplished only by using the horizontal method.

AFM images revealed that, regardless of the substrate material, the spontaneous ordering of monodisperse PS microspheres leads to the formation of similar close-packed lattices. However, low-magnification images show that the domains of ordered microspheres are separated by close-spaced drying cracks. The pattern of cracks strongly depends on the method of deposition: the vertical method leads to linear cracks perpendicular to the direction of deposition while the horizontal deposition forms circular cracks. By using polarized IR spectra, we have proved the “grating effect” of drying cracks formed through the vertical deposition method.

The results of our study can be used in the fabrication of high quality colloidal crystal multilayers on various substrates for applications in the areas of photonic optical components. Polarized infrared spectra can indeed be an appropriate tool for characterizing samples prepared by different methods.

References

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Received 24th May 2001,received in revised format 14th June, 
accepted 14th June 2001.

REF:  Simona Badilescu, Yong-Hong Ye, Georges Bader & Vo-Van Truong
Internet J. Vib. Spec.[www.irdg.org/ijvs] 5, 3, 5 (2001)