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Chemical Physics Letters 421 (2006) 1–4

Fluorophores-modified silica sphere as emission probein photonic crystals

Kai Song, Renaud Vallee, Mark Van der Auweraer, Koen Clays *

Department of Chemistry, University of Leuven, Celestijnenlaan 200, B-3001 Leuven, Belgium

Received 11 July 2005; in final form 12 December 2005Available online 8 February 2006

Abstract

The spontaneous emission of fluorophores embedded in a photonic crystal has been studied. By nano-engineering a sandwich-likephotonic structure, such that fluorescent spheres constitute a middle layer between the photonic crystals, we can precisely control thelocation of fluorophores in photonic crystals and exclude the presence of fluorophores near the crystal surface. It has been found thatthe stopband in the transmission spectrum is deeper than the stopband in the emission spectrum. We conjecture that the omni-directionalpropagation of the emission from a point source in an incomplete photonic bandgap is the cause of the shallower stopband in emission.� 2005 Elsevier B.V. All rights reserved.

1. Introduction

Photonic crystals have received a wealth of attentionrecently, because of their potential of controlling light,i.e., streams of photons, in such nano-engineered opticalstructures, in much the same way as electrical currents,i.e., streams of electrons, are controlled nowadays in semi-conductors or in (electrical) integrated circuits [1,2]. Forthe light propagation from light sources inside or outsideof photonic crystals, at the certain frequency and underthe certain direction, there will be bandgap due to Braggdiffraction. This bandgap is similar to the bandgap of elec-tric semiconductors and represents a range of energies, orwavelengths, that are not allowed in the photonic crystal.

There are a number of techniques to engineer periodicdielectric properties for the realization of a photonic band-gap. These techniques can be divided in the mostly physical‘top-down’ techniques, such as micromachining and lithog-raphy, and the mostly chemical ‘bottom-up’ techniques ofself-assembly. The latter approach has inherent advantagestowards three-dimensional photonic structures and two-

0009-2614/$ - see front matter � 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2005.12.043

* Corresponding author. Fax: +32 16 327 982.E-mail addresses: songkai75@gmail.com (K. Song), koen.clays@

fys.kuleuven.be (K. Clays).

dimensional structural features. Convective self-assemblyallows for the facile deposition of colloidal particles froma suspension [3,4]. Langmuir–Blodgett deposition is a sur-face technique for the deposition of monolayers of amphi-philic particles [5–10]. A disadvantage of the self-assemblyis that it is largely restricted to materials with relatively lowrefractive index and that it results in a hexagonal closedpacking (HCP) or a face-centered cubic (FCC) structure.Such a structure with insufficient dielectric contrast doesnot allow for a complete bandgap in all directions. In thisstudy, we use self-assembled photonic crystals, made of sil-ica particles, hence, also a photonic crystal with an incom-plete bandgap.

With the prime aim as light localization [2], photoniccrystal structures are also studied for the manipulation ofthe emission of fluorophores, embedded in the crystal.The periodic dielectric properties can either provide feed-back for improved lasing, or result in a forbidden region,the bandgap, where emission is suppressed and the fluores-cence lifetime is enhanced. These effects have been studiedby use of either excited atoms, quantum dots, fluorescentmolecules, or thermal radiation as point sources for pho-tons [11–16]. However, in most of the previous references,chromophores were introduced into photonic crystals justby simple infiltration or homogeneous embedding. As aresult, it is difficult to control the location of chromophore

Fig. 1. Schematic representation of the surface structure of RITC-modified silica sphere.

2 K. Song et al. / Chemical Physics Letters 421 (2006) 1–4

and discuss about some light propagating properties inphotonic crystals. In this study, we will focus on a newmethod to embed fluorophore into photonic crystal andthe bandgap effect on its spontaneous emission with theprecise control of location of fluorophore.

2. Experimental

The nano-engineering of sandwich-like structure wasfinished by self-assembly of monodispersed silica spheres.Pure silica spheres were synthesized according to the Sto-ber–Fink–Bohn method [17]. The size of spheres was easilycontrolled by adjusting the reaction temperature with fixedconcentrations of starting materials. In order to learn thebandgap effect on emission of fluorophore, the proper sizeof silica spheres was chosen to match the bandgap withemission peak of fluorophore. In our case, the hydrolysisof TEOS was carried out at 21 �C with fixed concentrationof starting materials, respectively, NH3 (Riedel–de Haen,25% NH3, 0.51 M), H2O (milli-Q, 4.52 M) and TEOS(Aldrich, 0.23 M).

The laser dye we chose was rhodamine isothiocyanate(RITC, ICN Biomedicals Inc.), which can be easily con-nected to the N-end of aminopropyltriethoxysilane (APS,Fluka) via coordination bond and then fixed on the surfaceof silica sphere during the hydrolysis of silicate [18]. Briefly,at room temperature, the mixture of 12 mg RITC and14 mg APS in 3 ml EtOH was stirred for 12 h. After adding4.0 ml TEOS into the mixture of 67 ml EtOH, 3.0 ml NH3

and 4.0 ml H2O, the reaction product of RITC and APSwas added at 25 �C. After stirring overnight and centrifu-gation in EtOH four times, light pink RITC-modified silicasphere was obtained. By controlling the concentrations ofstarting materials, the diameter of final spheres was alsoaround 260 nm.

By convective self-assembly method, successive deposi-tion of 25 layers of the bare silica particles on the glass sub-strate, of 5 layers of fluorescent particles on top of these 25layers bare silica, and of another 25 layers of bare silicaparticles on top of the fluorescent layers, resulted in a pho-tonic crystal with sandwich-like structure. As a referencesample, a photonic crystal with a bandgap spectrallylocated away from the emission spectrum of the fluoro-phores, was also fabricated.

The sample was characterized by scanning electronmicroscopy (SEM), light transmission and photolumines-cence spectroscopy. Fig. 1 shows a schematic of the sand-wich-like structure with all the fluorophores located inthe middle and surrounded by photonic crystals (a); anSEM image of this sandwich-like structure, with the twostacks of 25 layers clearly discernible (b); and an SEMimage of the uniformly infiltrated sample (c). The lighttransmission experiment (in a commercial UV/Vis/NIRabsorption spectrometer, Perkin–Elmer Lambda 900) is adirectional measurement, with the collimated incident lightof the spectrometer impinging perpendicular to the sub-strate, and hence, also, perpendicular to the (111) plane

of the photonic crystal. The spectral position of the (incom-plete) bandgap of a colloidal photonic crystal has beenshown to be a sensitive function of the incidence angle.The photoluminescence experiments were carried out witha confocal scanning fluorescence microscope (Olympus,IX70) at ambient temperature and atmosphere. The exper-imental setup has been described elsewhere [19]. A circu-larly polarized beam consisting of 2 ps pulses at awavelength of 543 nm (Spectra Physics, Argon ion laser,Tsunami titanium–sapphire laser, optical parametric oscil-lator, pulse picker, frequency doubler) was focused perpen-dicularly on the sample using an oil immersion objectivelens (Olympus NA 1.4, 100·). The emission was collectedthrough the same objective lens and directed to theentrance of the CCD camera (Princeton Instruments) fittedwith a polychromator (Acton Spectra Pro 150) in order toacquire fluorescence emission spectra.

3. Results and discussion

3.1. Morphology of spheres and photonic crystals

Fig. 1 shows the surface structure of RITC-modified sil-ica sphere. During the centrifugation of RITC-modified sil-ica spheres, it could be observed that the supernatant wascolorless in the last several rounds, which means that allfluorophores were fixed on sphere and not allowed to leavethe sphere.

The morphology of silica spheres modified by RITC wascharacterized by SEM observations. Both bare and fluoro-phore-modified silica colloids were uniform spheres, withan average diameter of �265 nm. The size polydispersitywas estimated to be less than 7% by SEM analysing.Fig. 2a is an SEM image of RITC-modified silica spheres,and Fig. 2b illustrates the sandwich-like structure of thesample. During the convective deposition, the thicknesscan be well controlled by changing the concentration ofcolloids suspension. In order to get enough bandgap effect,we deposited both top and bottom slabs of bare silicaspheres to �25 layers. Since the need of precisely control-ling of fluorophore inside photonic crystals, fluorescentslab just contains about 5 layers of RITC-modified silicaspheres. The schematic illustration and SEM image ofsandwich-like sample are shown in Fig. 2b,c.

Fig. 2. SEM image of RITC-modified silica spheres (a); schematicrepresentation of a sandwich-like structure (b) and SEM image ofsandwich-like sample (c).

Fig. 3. Emission spectra of RITC in the sandwich-like sample (solid) andin a reference sample (dash).

Fig. 4. Relative emission (solid line) and transmission (dotted line) spectraof the sandwich-like sample.

K. Song et al. / Chemical Physics Letters 421 (2006) 1–4 3

3.2. Emission spectra of fluorophore embedded in photonic

crystals

Fig. 3 shows the emission spectrum of the sample withsandwich-like structure (solid line). For comparison, theemission spectrum of the same fluorophores, but embedded

in the reference photonic crystal, is also shown (dashedlines). This spectrum is not affected by the bandgap. Thespectra are normalized at wavelengths longer than650 nm, where there is no bandgap. Decided by the sizeof silica spheres and the refractive index, the bandgaplocates at around 580 nm, which overlaps the emissionpeak of the fluorophore. The emission spectra clearly showthe photonic bandgap effect with a substantial dip in theemission spectrum with respect to the reference.

Fig. 4 shows the transmission spectra (dotted lines) ofsandwich-like sample. For correct comparison with theemission spectra (solid lines), relative spectra were pre-sented. For the emission spectra, the relative spectra wereobtained by dividing the emission spectra of both struc-tures with the emission spectrum of the reference sample(with the bandgap out of the emission region). For thetransmission spectra, spectra were divided by the transmis-sion spectrum of the reference. The suppression in trans-mission experiments is larger than the suppression in theluminescence experiments. This was observed earlier inthe case of uniformly infiltrated photonic crystals, where

Fig. 5. Schematic illustration of the omni-directional propagation andbackscattering of emission from photon point sources embedded in asandwich-like sample. L: lens; D: diode.

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the presence of fluorophores near the surface of the crystalwas invoked as a possible cause [20]. Point source photonemitters at the surface do not experience the photonicbandgap, and thereby diminishing the bandgap effectobservable in the emission spectra.

However, here, we also observe the same reduced band-gap effect in luminescence spectra from fluorophores thatwere positioned in the middle layer of the sandwich-likephotonic crystal. Since no fluorophores are located nearthe surface of the photonic crystal in our samples, theabove mentioned cause cannot be invoked to explain theeffect. Here, we suggest a rather different explanation, interms of the omni-directional emission by a photon pointsource and backscattering in an incomplete bandgap mate-rial. Fig. 5 shows schematically the position of the fluoro-phores (in the middle layer) and the incomplete bandgap(stopband). The bandgap is effective only for a small solidangle of emission (30� with respect to the normal of the(111) plane of the photonic crystal). For a solid angleexceeding this value there is no forbidden bandgap effect.In these directions, the photoluminescence can freely prop-agate, and even be scattered in all directions by surface orother defects. This is in strong contrast with the transmis-sion experiment, where only light propagating along thenormal to the (111) plane is impinging on the photoniccrystal. The effect of the bandgap along C1 1 1 is strongestin this case.

4. Conclusions

In conclusion, we have prepared fluorophore-modifiedsilica spheres and nano-engineered a photonic crystal withphoton point sources located only in the middle and notnear the surface of the crystal. We have compared the

bandgap as observed in emission spectra with the oneobtained from transmission experiments. The differencein intensity of the bandgap is attributed to the omni-direc-tional emission of the point sources, together with theincomplete bandgap associated to self-assembled photoniccrystals of silica particles.

Acknowledgments

R.V. is a postdoctoral fellow of the Fund for ScientificResearch-Flanders (FWO-V). This research was supportedby this Fund for Scientific Research-Flanders (FWO-V,G.0261.02), by the Belgian Government (IUAP/5/3) andthe University of Leuven (GOA/2000/03).

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