Fundamentals of Sputter Deposition

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Short communication

Microscopy evidence of the face-centered cubic arrangementof monodisperse polystyrene nanospheres

Hui Zhang a,*, Ren-guan Duan b, Fan Li c, Qing Tang d, Wen-chao Li c

a School of Science, Beijing Jiaotong University, Beijing 100044, PR Chinab Los Alamos National Laboratory, Los Alamos, NM 87545, USA

c Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, PR Chinad Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, PR China

Received 17 May 2005; accepted 30 September 2005Available online 22 November 2005

Abstract

This paper reports a scanning electron microscopy (SEM) investigation of polystyrene artificial opal achieved through self-assemblyof monodisperse polystyrene nanospheres with a diameter of 250 nm from colloidal suspension after being ambient dried. A detailedanalysis of the SEM images verifies that the face-centered cubic (fcc) phase is the most stable one for the polystyrene opal prepared. Thisfinding provides a strong support for, by using polystyrene opal as template, fabricating a photonic crystal with inverse fcc structure of full band gap if the refractive index contrast is higher than 2.8 and the filling fraction of the high index materials is between 0.2 and 0.3.Ó 2005 Elsevier Ltd. All rights reserved.

Keywords: Polystyrene opal; FCC structure; SEM; Photonic crystal

1. Introduction

Artificial opals [1,2] are very attractive candidates foruse in fabrication of optical photonic crystals, which areperiodic dielectric structures. Over the last few years, byself-assembly of colloidal particles, several groups [3–6]have successfully synthesized opal photonic crystals at vis-ible wavelengths and investigated their photonic effects.These opal photonic crystals mainly involve silica and

polystyrene artificial opals. Both transmission spectrumand SEM observation have confirmed that the face-cen-tered cubic (fcc) phase is the only one present in silica opal[7,8]. However, for polystyrene opal, up to date, only opti-cal measurements are presented, which are insufficient toillustrate the packed structure of polystyrene spheres, buta thorough microscopy characterization is still missing.

The structure of polystyrene opal is, therefore, not quiteclear.

Theoretical models [9,10] have indicated that the inversefcc structure can have a full photonic band gap if therefractive index contrast is higher than 2.8 and the fillingfraction of the high index materials is between 0.2 and0.3. Recently, using the opal template technology, inversestructures have been obtained by various groups[5,6,11,12]. Polystyrene opals are normally employed as

such templates to create inverse structures as well as silicaopals. The index contrast of these inverse structures can beimproved by using such background materials as oxidenanoparticles, polymers, metals, etc. For the inverse struc-ture photonic crystals made from polystyrene opal tem-plate, one of the existing problems is whether theirstructures are inverse fcc ones. The stacked type of spheresin original opal template controls that of air voids ininverse structure. This means that we can determine theclose-packed way of air voids in inverse structure by anal-ysis of the template structure.

0261-3069/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.matdes.2005.09.024

* Corresponding author. Tel.: +86 010 51683627; fax: +86 01051688433.

E-mail address: [email protected] (H. Zhang).

www.elsevier.com/locate/matdes

Materials and Design 28 (2007) 1045–1049

Materials& Design

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In this present work, we have prepared highly orderedpolystyrene opal template materials using homemademonodisperse polystyrene nanospheres and performed itsmicroscopy morphology characterization by SEM, whichstrongly favors that the fcc phase is the most stable one.

2. Experimental

The method of seeded emulsion polymerization was used tosynthesize monodisperse polystyrene nanospheres [13]. A polysty-rene latex that was used as seeds to grow polystyrene nanosphereswas poured into a reactor, stirred, and heated to desired temper-ature under the protection of nitrogen atmosphere. The well stir-red mixture of deionized water, styrene monomer, potassiumpersulfate that was used as initiator, and emulsifier that containedabout 30 wt% nonylphenol polyglycol ether sulphate sodium salt,was slowly dropped into the above latex in reactor. Polymeriza-tion reaction took place from the surface of the existing seedsand homogeneous polystyrene nanospheres were formed. As theemulsifier used was anionic type, the polystyrene nanospheres pre-pared were negatively charged. The polystyrene latex finally syn-thesized was diluted to a desired concentration with deionizedwater before use.

The dilute polystyrene colloidal suspension with a volume frac-tion of 4% was spread onto an ordinary glass substrate that wascleaned, put into a clean chamber and allowed to dry in air atroom temperature. With the slow evaporation of water and thegradual increase of colloidal concentration, monodisperse poly-styrene nanospheres lastly transformed to an ordered crystallinestructure by self-assembly.

The polystyrene nanospheres were characterized by a HIT-ACHI H-8100 transmission electron microscope (TEM) and thestructure of polystyrene opal formed was evaluated by a HIT-

ACHI S-4300 SEM. The colour photographs of top surface of opal sample obtained were taken with a SONY P8 digital camera.

3. Results and discussion

3.1. Self-assembly of polystyrene nanospheres for

polystyrene opal

TEM image (Fig. 1) clearly shows that the synthesizedpolystyrene nanospheres are monodisperse, with a diame-ter of around 250 nm and a dispersion smaller than 5%.An excellent monodispersivity of polystyrene nanospheresplays a key role in constructing highly ordered polystyreneopal. During drying, monodisperse polystyrene nano-spheres, as a result of electrostatic repulsion betweenspheres in latex, self-assemble into an ordered crystallinestructure, in which the spheres are interconnected by Vander Waals force. The formed polystyrene opal crystal pos-sesses a thickness of around 0.3 mm, which requires a dry-ing period of about 1 week, and its top surface shows aquite uniform color under white light. Also the diffractioncolor transforms from green to pink when observationangle changes (Fig. 2). These mean that there is a goodalignment of nanospheres in the formed polystyrene opal.

The mechanism of self-assembly of polystyrene nano-

spheres can be described as follows [14]. When polystyrene

latex was dried at room temperature and its concentrationwas increased to certain critical value, the polystyrene nan-ospheres at the suspension surface projected from the sur-face. The lateral capillary force between the projectingspheres would drive the spheres at the suspension surfaceclose to each other and to arrange in the optimum sites intoordered layers. The ordered multilayer was developedgradually toward interior colloid, and an ordered crystal-line structure was formed after complete drying.

3.2. Stacking ways of polystyrene nanospheres

As the polystyrene colloidal suspension was driedslowly, the disordered colloidal structure increasinglytransformed to the ordered crystalline structure and thevolume-filling fraction, i.e. the packing fraction of polysty-rene spheres, changed to about 74%. Therefore, the floa-tage and Coulomb interaction became smaller andsmaller. The crystallization arrangement of polystyrenespheres can be considered as a stacking of hard spheres.It has been known extensively that there are two ways of

packing spheres of equal size together so that they occupy

Fig. 1. TEM image of monodisperse polystyrene nanospheres with adiameter of around 250 nm and a dispersion smaller than 5%.

Fig. 2. Color photographs of top surface of prepared polystyrene opal

taken at different observation angles. (For interpretation of the referencesto color in this figure, the reader is referred to the web version of thisarticle.)

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the minimum volume: one is fcc close-packed and anotheris hexagonal close-packed (hcp). These two stacking struc-tures have the same filling fraction and nearest neighbors.Therefore, the possible close-packed structure of polysty-rene nanospheres can be fcc, hcp, the mixture of fcc andhcp, or random stacking. It is very important to determine

the resulting packing type of polystyrene nanospheresbecause this will have a direct effect on the photonic prop-erties of both polystyrene opal and its inverse structurephotonic crystals.

3.3. Microscopy characterization of the ordered structure of

polystyrene nanospheres

The top surface of the formed polystyrene opal in thisinvestigation reveals a hexagonal alignment (Fig. 3), whichcan correspond to (1 1 1) plane of fcc or (0 0 1) plane of hcp. In order to clearly determine the structure of polysty-rene opal attained, it is necessary to further investigate the

internal arrangements of polystyrene spheres through thecharacterization of side facets from sample cleaved edges.The SEM images of different side facets observed in thecleaved edges are shown in Fig. 4, where square(Fig. 4(a)) and triangle (Fig. 4(b)) arrangements of nano-spheres are obviously shown, respectively. A rectangularalignment with one sphere in the center and four spheresat the corners, which corresponds to hcp stacking, is neverfound in this investigation. These mean that the resulting

Fig. 3. SEM image of top surface of polystyrene opal prepared.

Fig. 4. SEM images of different side facets observed in the cleaved edgesof sample: (a) square arrangement corresponding to {1 0 0} type planes;(b) triangular arrangement corresponding to {1 1 1} type planes; (c)

different crystalline planes.

Fig. 5. SEM images of the long-range ordered arrangement of polystyrenenanospheres observed in the cleaved edges of sample: (a) a large {1 0 0}

type domain; (b) a large internal {1 1 1} type domain.

H. Zhang et al. / Materials and Design 28 (2007) 1045–1049 1047



close-packed structure of polystyrene nanospheres is onlycompatible with the fcc packing. A square arrangementin Fig. 4(a) corresponds to {1 0 0} type plane of fcc struc-ture, while a triangular arrangement in Fig. 4(b) corre-sponds to {1 1 1} type plane in fcc structure. A typicalterrace view is shown in Fig. 4(c), where several differentcrystalline planes are clearly revealed. A corner is formedby two triangular arrangement planes and one squarearrangement plane, that is, two {1 1 1} type facets andone {1 0 0} type facet of fcc structure, respectively.

Two long-range ordered domains, {1 0 0} (Fig. 5(a))and {1 1 1} (Fig. 5(b)) domains, in the side facets can be

seen in the low magnification images (Fig. 5). And alsothe interleavings among {1 0 0} facets and among {1 1 1}facets can be observed in Figs. 5(a) and (b), respectively.The slopes connecting the terraces are either {1 0 0} typeor {1 1 1} type planes. Some of the dislocations and vacan-cies can also be found, which are only like those in anordinary solid state crystal. The combination of observa-tions in Figs. 4 and 5 further proves that the polystyrenenanosphere stacking in the formed opal possesses an fccstructure.

In addition, an fcc crystal shows that the possiblecleaved facets are the boundary planes of a truncated octa-hedron (Fig. 6(a)), where there are 8 hexagonal planes,

(1 1 1), ð1 1 1Þ, ð1 1 1Þ, ð1 1 1Þ, ð1 1 1Þ, ð1 1 1Þ, ð1 1 1Þ,ð1 1 1Þ, and 6 square planes, (1 0 0), (0 1 0), (0 0 1),ð1 0 0Þ, ð0 1 0Þ, ð0 0 1Þ, respectively [8]. There are twoangles of 110° and 126°, between two neighboring {1 1 1}planes, and neighboring {1 1 1} and {1 0 0} planes, respec-tively. Two different {1 1 1} planes and one {1 0 0} planeare connected by one lattice point (Fig. 6(a)). All SEMimages of cleaved edges in this investigation illustrate a sat-isfied relationship as described above. A part of the trun-cated octahedron is drawn to describe such a relationshipin Fig. 6(b). This directly confirms that the polystyrenenanospheres self-assemble in an fcc structure in the result-

ing opal crystal in this work. Furthermore, it is well known

that in fcc structure, the interstices in the square arrange-ment plane of spheres are bigger than those in the hexago-nal arrangement plane. If employing a template with fccsymmetry to construct inverse structure, the filling fractionof higher dielectric background will be increased when infil-trating background materials from the side of squarearrangement plane (as shown arrowhead direction inFig. 6). At present, in most studies, the filling fraction of higher dielectric background is still quite low (less than20%).

As discussed above, the polystyrene nanospheres inaqueous suspension can be considered as the hard spheres

when vaporization rate is slow. For the packing of hardspheres, after two layers are stacked as AB packing, thespheres of the next layer can be stacked either in A typesites, resulting in an ABA packing, or in C type sites, lead-ing to an ABC packing. The gravity energy of A site isidentical as that of C site. So, the either stacking sequencepresents a same probability, which finally results in the for-mation of hcp structure when the growth sequence is ABA-BAB . . . , or in the formation of fcc structure in the casethat the growth sequence is ABCABC . . . . However, arecent computer simulation reveals that the fcc crystallinestructure of hard spheres is more stable than the hcp one[15]. Therefore, the C sites should be favored with regard

to the A sites. Our experimental result that the fcc orderingof polystyrene nanospheres is formed in the resultant opalcrystal is consistent with this computer simulation.

4. Conclusion

In brief, a highly ordered arrangement of polystyrenenanospheres has been produced by self-assembly of mono-disperse polystyrene nanospheres through ambient dryingof colloidal suspension. SEM observations and crystallog-raphy analysis verify that the nanosphere arrangement inthe resultant polystyrene opal in this investigation presents

an fcc structure. This result is also consistent with recent

Fig. 6. (a) A truncated octahedron showing the possible boundaries of an fcc crystal, where hard spheres locate at the corners and the center of everyhexagon. (b) Closer view of the typical boundaries of the cleaved facets, where a part of the truncated octahedron is drawn to describe the boundaryrelationship that is schematic in (a).

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computer simulation that fcc structure is more stable thanhcp one. The resulting fcc arranged polystyrene nano-spheres could be used as a template to fabricate inversefcc photonic crystals with full photonic band gap in anappropriate index contrast.

Acknowledgements

We thank Dr. Zhongwei Niu and Prof. ZhenzhongYang for their help in this research work. This work hasbeen financed by both National Science Foundation of China (Grant No. 59974026) and Science and TechnologyFoundation of Beijing Jiaotong University (Grant No.2003LSM003).

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