The Controlling of Relative Humidity (RH) in the Self-Assembled Polystyrene Micro-Spheres Structures Formation
D. S. Raimundo1, F. J. R. Fernandez, W. J. Salcedo
Laboratório de Microeletrônica, Engenharia Elétrica, Escola Politécnica, Universidade de São Paulo, Av Professor Luciano Gualberto, 158 – travessa 3 sala C2-62, São Paulo, SP, 05508-900, Brazil
1[email protected], +55 11 3091-5310 extension 228
Abstract
The present work reports the fabrication and characterization of self-assembled polystyrene micro-sphere structures obtained by the controlling of relative humidity (RH) during the growth of sphere films on vertical configuration. The possibility of these structures to be applied as photonic crystals was studied. The existence of band gap in the transmittance spectra showed the possibility of application in photonic crystal structures. The quality of structures was analyzed by transmittance spectra and SEM (Scanning Electron Microscopy) images.
Keywords: self-assembling, photonic crystal, polystyrene, relative humidity (RH), band gap.
Introduction
Self-assembly method has been currently utilized by the scientific community in the nanofabrication of materials and porous materials [1]. In the recent years, the systems composed of self-assembled nanometric beads have become important for different applications as patterned array in lithographic techniques, microlenses and templates for inverse opal structures for sensors and optical applications and photonic crystal structures [2].
Since the early pioneering work, photonic crystals have attracted much attention, but large-scale crystalline quality, especially in three-dimensional (3D) materials, has been a challenge [2]. Kuai et al. [3] extended the method that uses capillary forces that assembles monolayer colloidal films to a method that fabricates colloidal crystal multilayer.
Their work showed the possibility of using vertical deposition to produce high-quality colloidal crystal with controlled thickness. Nowadays the influence of parameters in polystyrene deposition process has been studied to obtain high quality structures [4-6].
In the present work, the vertical deposition technique was used to fabricate polystyrene micro- sphere structures from aqueous solutions, and we studied the effect of relative humidity (RH) on the structure quality. The optimal conditions for crystal growth are found based on the results obtained by the variation of RH parameter.
Experimental Procedure
In the present work polystyrene spheres were assembled onto glass and oxidized silicon substrates by vertical deposition to fabricate 3-D self-assembled structures (multilayers). The solutions that were used consisted of polystyrene micro-spheres monodispersion in aqueous solution.
The diameter of polystyrene spheres was 660±3%nm. Before the self-assembling process, the glass substrates were cleaned in extran solution at 60oC and sonicated during 10 minutes. After it, the glass substrates were rinsed using deionized water during 10 minutes. Finally, these structures were immersed into an ethanol solution and heated until completely dry to finish the cleanness. Silicon substrates, before the self-assembling process, were cleaned using conventional chemical microelectronics cleanness and were oxidized at 1000oC to form a 300nm SiO2 thickness. A 515 Philips Scanning Electron Microscope (SEM) was used for direct morphological characterization of 3- D lattice of polystyrene structures using the secondary electrons technique with 30KeV energy.
A Varian Cary 500 UV-VIS-NIR Spectrophotometer was used for optical characterization to verify the band gap existence.
Results and Discussion
The self-assembled polystyrene structures were obtained on glass and oxidized substrates using the controlling of relative humidity (RH) influence.
The structures were obtained at 50oC, using 2.13 wt% monodispersion aqueous solutions and RHs of 50, 60, 70, 80 and 90%.
With the increasing of monodispersion concentration, the increasing of the number of deposited polystyrene sphere layers was verified. It is related to results reported by the literature. It was verified the existence of up to ten layers of spheres.
The figures 1(a) and 1(b) show structures obtained by 50% and 60% of relative humidity. Both samples presented large areas with self-organized
structure (approximately 600µm2 and 800µm2, respectively). The films obtained without the controlling of RH were obtained in an oven at 50oC, with RH lower than 50%. Then, the dry environment promoted a solvent evaporation faster than the relaxing time of the structure. The films obtained without the controlling of RH showed organized areas in the range of 200µm2 approximately. In the current case, relative humidity values of 50% and 60% promoted slow evaporation velocities of the adhered solvent in substrate surface and, consequently, the films presented larger areas of organized structures.
The film deposition rate generally decreases with the increasing of the relative humidity value, obtaining larger thickness. With low velocity of deposition it is possible to obtain a better rearrange of spheres, becoming possible the formation of higher number of layers. Then, a lower rate deposition possibilities the formation of better organized films due to higher equilibrium between the solvent evaporation rate and the spheres hydrodynamic flux. Even with the controlling of RH, the existance of extended defects shows that it is necessary a better contolling during the deposition process, especially in the temperature parameter.
The figure 1(c) shows the structure obtained on glass substate using a RH of 70%. The film presented organized areas in the range of 300µm2. With the increasing of RH (compared to figures 1(a) and 1(b)), it was expected an increasing of organized regions. The decreasing may have occurred because with the increasing of RH, a lateral capillary force increases and the deposition rate decreases. Analyzing the film thickness uniformity, it was verified that the structure obtained with a RH of 70% presented a non- uniform structure thickness (with some steps).
The figure 1(d) shows the structure obtained using a RH of 80%. This film presented organized areas lower than 600µm2. The increasing of RH (compared to 70% RH situation) did not result in significative changes in film structure, confirming the effect of H2O vapor saturated by capillar effect that promoted the existence of higher extended defects density. In some regions the film presented non-uniform thickness. The non-uniformity is due to high monodispersion concentration (2.13wt%) that can have promoted the unstable increasing of film thickness due to high quantity flux of spheres towards the meniscus, resulting in non-uniform film in some specific regions. The hydrophylic condition of surface helps to mantain the equilibrium between the solvent evaporation rate
and the particle flux towards the deposition meniscus, presenting a uniformity of film even in large areas (1200µm2 approximately).
The figure 1(e) shows the polystyrene structure obtained under RH of 90% condition. This film presented similar extended defects to that obtained at 70% and 80 of RH. Self-organized domains presented high quantity of extended defects. It is important to mention that the dimension of cracks in this structure presented lower dimension if compared to previous ones. It is due to, during this RH value the velocity of solvent evaporation was slower than the previous ones.
Analyzing the influence of the five different conditions of RH (50, 60, 70, 80 and 90%) in the quality of films, it was verified that the 50% and 60% RH conditions at 50oC permited the formation of better quality films. In the case of films obtained in conditions of RH≥70%, they presented high density of extended defects formed by the water vapor saturation by capillar effect in the interstitial regions of structures.
The figure 2 (a, b, c, d and e) shows the polystyrene self-assembled structures obtained on silicon oxidized substrates by the controlling of relative humidity (RH) at 50oC, using a (660±3%nm) polystyrene monodispersion of 2.13wt%. The 50, 60, 70, 80 and 90% of RH conditions were applied.
In this case it was also verified that with the increasing of monodispersion concentration occurs the increasing of the number of polystyrene deposited layers.
The figure 2(a) shows the structure obtained on oxidized silicon substrate using a RH of 50%. This structure presented high level of organization, forming films with continuous areas in the range of 800µm2 approximately.
The figure 2(b) shows the polystyrene structure using a RH of 60%. This film presented self- assembled areas with similar values to previous structure (obtained by RH of 50%), 800µm2 approximately. This structure presented lower density of defects in organized domains. The cracks are also presented in this film due to the final drying process that causes an abrupt relaxing of the system, applying high tension to the film. Then, a better controlling in the system temperature is required. At this film the thickness homogeneity is maintained along the entire substrate surface like showed in sample 2(a).
The figure 2(c) shows the polystyrene structure obtained using a RH of 70%. This film presented self-organized areas in the range of 1200µm2 with extended defects with large width. This behavior is due to the low rate of solvent evaporation.
Analyzing the thickness uniformity it was verified non-uniformity (steps on surface) because the high concentration promoted unstable increasing due to high quantity of spheres flux. The films obtained on glass surface presented better quality than the films obtained on oxidized silicon substrates, considering the condition of RHs of 50% and 60%. In the present case (oxidized silicon substrate), the film with better quality was obtained under RH of 70%
condition.
(a)
(b)
(c)
(d)
(e)
Figure 1. SEM images of 3-D self-assembled polystyrene structures obtained on glass substrate at 50oC, using a 2.13wt% polystyrene monodispersion and relative humidity of
(a) 50%, (b) 60%, (c) 70%, (d) 80% and (e) 90%. .
This different behavior is due to different surface condition between glass (hydrophilic) and oxidized silicon (hydrophobic).
(a)
(b)
(c)
(d)
(e)
Figure 2. SEM images of 3-D self-assembled polystyrene structures obtained on oxidized silicon substrate at 50oC, using a 2.13wt% polystyrene monodispersion and relative humidity of (a) 50%, (b) 60%, (c) 70%, (d) 80% and (e) 90%.
The figure 2(d) shows the structure obtained on oxidized silicon surface under RH of 80%
condition. This film presented organized area in the range of 400µm2. The increasing of RH value compared to previous situation (RH of 70%) didn’t increase the organized area. In this case, the organized domains presented low density of
400 800 1200 1600 2000 defects. With the increasing of RH the lateral capillary force and deposition rate decreases, resulting in better crystalline quality. It was verified that in this condition it didn’t occur unstable deposition of film, resulting in very uniform structures. This film obtained on oxidized silicon presented lower density of defects than the film obtained in the same conditions on glass sustrate.
Maybe it is due to the hydrophobic condition of oxidized silicon substrate.
0 20 40 60 80 100
reference V50 V60 V70 V80
Transmittance (%) V90
Wavelength (nm)
Figure 3.Transmittance spectra of 3-D polystyrene structures obtained on glass substrates using RH values of 50% (V50),
60% (V60), 70% (V70), 80% (V80) and 90% (V90).
The figure 2(d) shows the structure obtained on oxidized silicon surface under RH of 80%
condition. This film presented organized area in the range of 400µm2. The increasing of RH value compared to previous situation (RH of 70%) didn’t increase the organized area. In this case, the organized domains presented low density of defects. With the increasing of RH the lateral capillary force and deposition rate decreases, resulting in better crystalline quality. It was verified that in this condition it didn’t occur unstable deposition of film, resulting in very uniform structures. This film obtained on oxidized silicon presented lower density of defects than the film obtained in the same conditions on glass sustrate.
Maybe it is due to the hydrophobic condition of oxidized silicon substrate.
The figure 2(e) shows the structure obtained under RH of 90% condition. This film presented continuous areas in the range of 800µm2. In the organized areas it was formed high quantity of line and localized defects.The hydrophobic condition of the surface can have contributed to the equilibrium between the rate of solvent evaporation and the spheres flux towards meniscus because it was formed a non-uniform thickness film.
Analyzing the five conditions of RH (50, 60, 70, 80 and 90%) in the formation of polystyrene films on oxidized silicon substrate, it was verified that the
condition of RH of 70, 80 and 90% are the best ones to form continuous films with large areas, even with the presence of specific density of defects (in case of RH of 90%). In the condition of RH of 90% the organized area is large and it presented higher density of defects, whereas in the condition of RH of 80% the organized area is smaller but it presented lower density of defects.
The RH controlling promoted the formation of films with extended self-organized areas on glass and oxidized silicon substrates. In the case of glass substrate, the hydrophilic surface condition permited that the RHs of 50% and 60% resulted in the best conditions to form high quality films. In the case of oxidized silicon substrate, films that presented high quality were obtained using RH≥70%. These results showed that the initial condition of the surface is an important parameter to be controlled in the formation of self-organized polystyrene structures. On the other hand, the existence of defects showed that it is necessary a better controlling in the final cooling system in the final phase of the deposition process.
Analyzing the structures obtained on both substrates (glass and oxidized silicon substrates), it was verified that the structures obtained in both substrates presented high structural quality. The structures obtained on oxidized silicon substates are more advantageous because they can be applied in silicon technology making possible their integration in this technology. Then, these organized polystyrene structures can be widely utilized in microelectronic process to attainment of optical, photonic, electronic, optoeletronic and chemical devices.
The figure 3 shows the transmittance spectra of self-organized polystyrene structures obtained at 50oC, using a 2.13wt% polystyrene monodispersion in aqueous solution, under condition of RH of 50, 60, 70, 80 and 90% (spectrum V50, V60, V70, V80 and V90 respectively). It was verified that the obtained bands by the structures presented high degree of definition with width of approximately 130nm. All the structures presented a stop band centered at 1400 nm. The non-organization distorts the profile of the band, widening the depth and decreasing the depth. It was also verified that it has a trend in the increase of the depth of band gap promoted by the increasing of polysturene concentration in the monodispersion and by the controlling of RH conditions. The RH parameter is responsible for the formation of very extended organized areas as verified in figure 1(a-c).
Conclusion
The present experiment showed that the controlling of RH in the formation of self-organized polystyrene structures is very important to the attainment of high quality structures.
The RH controlling promoted the formation of films with extended self-organized areas on glass and oxidized silicon substrates. In the case of glass substrate, the hydrophilic surface condition permited that the RHs of 50% and 60% resulted in the best conditions to form high quality films. In the case of oxidized silicon substrate, films that presented high quality were obtained using RH≥70%. These results showed that the initial condition of the surface is an important parameter to be controlled in the formation of self-organized polystyrene structures.
It was verified that the obtained bands by the structures presented high degree of definition with width of approximately 130nm (in transmittance spectra). All the structures presented a stop band centered at 1400nm. The non-organization distorts the profile of the band, widening the depth and decreasing the depth. It was also verified that it has a trend in the increase of the depth of band gap promoted by the increasing of polysturene concentration in the monodispersion and by the controlling of RH conditions. The RH parameter is responsible for the formation of very extended organized areas.
Then, it was verified that the structures obtained in both types of substrate presented high structural quality. The structures obtained on oxidized silicon substates are more advantageous because they can be applied in silicon technology making possible their integration in this technology. This way, these organized polystyrene structures can be widely utilized in microelectronic process to attainment of optical, photonic, electronic, optoelectronic and chemical devices.
Acknowledgments
The authors thank CNPq and Fapesp for financial support; the Laboratório de Microeletrônica and Laboratório de Sistemas Integráveis at Escola Politécnica, and Laboratório de Espectroscopia Molecular at Instituto de Química, from Universidade de São Paulo, for technical support.
References
[1] O. D. Velev and A. M. Lenhoff, Colloidal crystals as templates for porous materials. Current Opinion in Colloid and Interface Science, Vol.05 (2000), 56-63.
[2] M. Egen, R. Voss, B. Griesebock and R. Zentel, Heterostructures of Polymer Photonic Crystal Films.
Chemical Mater, Vol.15 (2003), 3786-3792.
[3] S. Kuai, X. Hu, A. Haché and V. Truong, High- quality colloidal photonic crystals obtained by optimizing growth parameter in a vertical deposition technique. Journal of Crystal Growth, Vol.267 (2004), 317-324.
[4] S. Rakers, L. F. Chi and H. Fuchs, Influence of the Evaporation Rate on the Packing Order of Polydisperse Latex Monofilms. Langmuir, Vol.13 (1997), 7121-7124.
[5] Y. Li, T. Kunitake and S. Fujikawa, Efficient fabrication of large, robust films of 3D-ordered polystyrene latex. Colloids and surfaces A:
Physicochemical Engineering Aspects, Vol.275 (2006), 209-217.
[6] D. S. Raimundo, A. B. Stelet, F. J. Ramirez- Fernandez and W. J. Salcedo, Self-assembled systems obtained by chemical and electrochemical techniques for photonic crystal fabrication. Microelectronics Journal, Vol.36 (2005), 207-211.