10964 | J. Mater. Chem. C, 2015, 3, 10964--10967 This journal is©The Royal Society of Chemistry 2015

Cite this: J.Mater. Chem. C, 2015,

3, 10964

Super-stable centimetre-scale inverse opal beltsintegrated with CdTe QDs for narrow bandfluorescence optical waveguiding†

Xianyong Lu,*a Lingmei Ni,‡a Shimin Wu,‡a Yanzi Wu,a Hongyan Cai,a Ke Ding,b

Lihong Jing,b Yi Hou,b Ying Zhu,*a Mingyuan Gaob and Lei Jiangab

Super-stable centimetre-scale SiO2 inverse opal belts integrated

with CdTe QDs have been prepared through a synergetic collaboration

between materials and structures. It was found that SiO2/CdTe

inverse opal belts present excellent narrow band fluorescence

optical waveguiding properties.

Optical waveguides have received great interest due to theirpotential applications in the high speed data transmission ofnext-generation processing systems.1,2 As the performance ofprocessors increases, waveguide-based optical interconnectsbetween processors, chips and even gates is becoming vital tothe development of high performance digital systems.3,4

Inverse opals with well-ordered periodic structures can providean opportunity to manipulate photons within small volumesfor photonic applications.5,6 However, the optical properties ofinverse opals are often limited by the relatively low refractiveindex of materials adopted, such as silica.7 In contrast, II–VIsemiconductor materials have much higher refractive indices.Semiconductor nanomaterials such as nanowires and nano-belts have been reported to be used as optical integrationdevices, due to their 1D feature and advantageous photonicproperties, such as optical waveguiding and nanolaser sourcecapabilities,8 which require not only structurally stable wave-guiding materials, but also effective coupling of the photonicconfinement of the reverse opal structure and the electronicstructure of the fluorescent semiconductor materials inte-grated, for achieving a high speed optical interconnect with alarge transmission distance. Nevertheless, the performance ofabsorption–emission–absorption of waveguiding devices madeup of semiconductors largely determines the loss of energy andthe PL band structure of the outlight.8,9 Therefore, a rational

integration of a reverse opal structure with fluorescent semi-conductors becomes essential for fulfilling the above-mentionedrequirements. CdTe QDs are of high refractive index showingwide excitation spectra, high emission efficiency, electron trans-port efficiency, and excellent photochemical stability.10 Dopingfluorescent CdTe QDs in colloidal crystals has been demon-strated to be an effective approach for coupling the fluorescenceproperties of QDs with the photonic confinement of opalstructures.11 However, integrating CdTe QDs into an inverseopal structure to achieve a waveguiding device with stablespontaneous emission has not been reported yet.

In the current work, we report an easy and simple approachfor fabricating centimetre-scale SiO2/CdTe inverse opal beltsshowing highly reproducible spontaneous emission. The schematicillustration of the preparation of a SiO2 inverse opal integrated withCdTe QDs is shown in Fig. 1a. Firstly, centimetre-scale beltscomposed of cross-linked polyacrylic acid (c-PAA) colloidal particleswere prepared via a curvature substrate and the negative pressurecontrolled vertical deposition (CSNPVD) method, as reported in ourprevious work.12 PAA colloidal crystals were subsequently employedas a template for forming an inverse opal through a chemicalvapour deposition (CVD) of tetraethoxysilane in a closed desiccatorin the presence of ammonia.13 Similar to a typical Stober reaction,SiO2 as an infiltration material is formed by hydrolysis andcondensation of tetraethoxysilane catalyzed by ammonia.A typical CVD was performed in 24 hours, and PAA colloidal beltswere coated by a uniform layer of silica shell. Then, SiO2 inverseopal belts were obtained by removing the PAA colloidal crystaltemplate under a temperature of 600 1C in an air atmosphere.Finally, SiO2 inverse opal belts were immersed into a thioglycolicacid (TGA)-capped CdTe QDs aqueous solution for 8 hours,14

washed with water (pH = 9.1) three times and then vacuum dried.The shell thickness of the SiO2 inverse opal can be tuned by theduration of the CVD. Subsequently, centimetre-scale SiO2 inverseopal belts decorated with CdTe QDs exhibit a strong fluorescenceemission under UV-vis irradiation (seen Fig. S2, ESI†).

The structures of the PAA colloidal crystal belts, PAA@SiO2

colloidal crystals belts, SiO2 inverse opal belts and SiO2 inverse

a Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of

Ministry of Education, Beijing Key Laboratory of Bio-inspired Energy Materials

and Devices, School of Chemistry and Environment, Beihang University, Beijing

100191, P. R. China. E-mail: zhuying@buaa.edu.cnb Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5tc02568k‡ These authors contributed equally to this work.

Received 18th August 2015,Accepted 28th September 2015

DOI: 10.1039/c5tc02568k

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opal belts integrated with CdTe QDs were characterized usingscanning electron microscope (SEM) and transmission electronmicroscopy (TEM). The low-magnification SEM images shownin Fig. 1b–h indicate the internal structure changes in theprocess of forming SiO2 inverse opal integrated with CdTeQDs. Fig. 1b and c are typical SEM images of PAA colloidalcrystal belts which were obtained via a CSNPVD strategy. Theaverage length of a PAA colloidal belt is about 1 cm, which iscomposed of submicron c-PAA microspheres with an averagediameter of 159 � 15 nm.15 After CVD, the mean size of silica-coated c-PAA microspheres increased to 177 � 14 nm. It wasalso clearly observed that the silica-coated c-PAA microsphereshave a more compact arrangement among submicrospheresthan the original c-PAA colloids. After removing the c-PAAmaterials, SiO2 inverse opals are porous structures in whichthe pore arrangement is well ordered and interconnected with-out any cracks, as shown in Fig. 1g. The well-arranged porous,uniform silica wall and interconnected pores are attributed tothe chemical vapour reaction, providing easy reach between thevoids in the c-PAA colloidal crystal (Fig. 1g). The thickness ofthe silica wall is 18.0 � 0.6 nm, from a static of the SEM images.The interconnected porous structure (Fig. 1g, right image)shows that the majority of connected points of the PAA colloidshave connected pores. The average diameter of pores on thehollow SiO2 microspheres is 55.2 � 8.6 nm. When SiO2 inverseopal belts were immersed in a CdTe QDs colloid solution, CdTeQDs were introduced into the voids by capillary force.16,17 Thecross-sectional TEM images taken from thin slices of SiO2

inverse opals decorated with CdTe QDs show that the QDswere coated with a surface of SiO2 hollow spheres (Fig. 1i). Theoptical properties of SiO2 inverse opal belts integrated withCdTe QDs could be tuned by the size of the CdTe nanocrystals,

because CdTe QD nanocrystals have strong size-dependentoptical properties.18 Herein, differently sized TGA-capped CdTeQDs with a central photoluminescence peaking at 540 nm(green CdTe QDs), 575 nm (yellow CdTe QDs) and 636 nm(red CdTe QDs) were employed as decorations for SiO2 inverseopal belts. Both fluorescence and absorption spectra of thesethree samples are shown in Fig. 2a. Due to the interconnectedpores in the SiO2 inverse opal, there are strong capillaryforces between the CdTe QDs solution and the inverse opal.Therefore, CdTe QDs were easily introduced into the pores ofthe SiO2 inverse opal belts. SiO2 inverse opal belts integratedwith three differently sized CdTe QDs were also characterizedusing conventional fluorescence microscopy. The results shownin Fig. 2b–d prove that CdTe QDs have successfully decoratedthe inner wall of the porous SiO2 microspheres which presentsa perfect one-to-one correspondence between the bright-fieldimage and the fluorescence image.

Each TGA molecule bears two functional groups, i.e., amercapto group and a carboxyl group. The mercapto group isbound on the surface of the CdTe QDs,19 while the pendantcarboxyl group provides not only aqueous solubility, but thepossibility for coupling the CdTe QDs with matrix materials(Fig. 3b). The SiO2 hollow submicrospheres have a large numberof hydroxyl groups. Therefore, CdTe QDs were expected to havea strong interaction with the SiO2 inverse opal via hydrogenbonding interactions (shown in Fig. 3b).20 This interaction isbased on the hydroxyl groups from the SiO2 inverse opal and thecarboxyl groups on the surface of the TGA-capped CdTe QDs.This type of hydrogen bonding interaction was well characterizedfrom FT-IR spectra (Fig. 3c). The peak located at 1605 cm�1 canbe attributed to the stretching vibrations of the carbonyl groups(CQO) of the TGA-capped CdTe QDs.21 However, when theCdTe QDs were decorated on the surface of the SiO2 inverseopal, the carbonyl group absorbing peak had a 36 cm�1 red-shift. These results indicated that strong interactions existbetween the hydroxyl groups and carbonyl groups as hydrogenbonds,22 which enable the firm attachment of CdTe QDs on the

Fig. 2 (a) Absorption and PL spectra of the TGA-capped CdTe QDs withdifferent sizes. The excitation wavelength for the fluorescence measurementswas 400 nm. Microscopic images of the SiO2 inverse opal belts decoratedwith (b) red-emitting CdTe QDs, (c) yellow-emitting CdTe QDs and (d) red-emitting CdTe QDs. The left-hand photographs in (b)–(d) are bright-fieldimages, whereas the right-hand photographs are fluorescence images. Thescale bars in the bright-field images correspond to 200 mm.

Fig. 1 (a) Schematic illustration of SiO2 inverse opal belts integrated withCdTe QDs. (b) and (c) SEM images with different magnifications of PAAcolloidal crystal belts. (d) and (e) PAA@SiO2 colloidal crystal belts. (f) and (g)SiO2 inverse opal belt. (h) and (i) SEM image and cross-sectional TEMimages (inset is a high-resolution TEM image) taken from thin slices ofsilica inverse opal belts decorated with CdTe QDs, respectively.

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10966 | J. Mater. Chem. C, 2015, 3, 10964--10967 This journal is©The Royal Society of Chemistry 2015

surface of the SiO2 inverse opal. It should be mentioned thatSiO2 inverse opal belts dotted with CdTe QDs are very stableunder ambient conditions and retain their fluorescence stabilityover 2 years (seen Fig. S3 of ESI†).

The optical waveguiding properties of the as-prepared SiO2

inverse opal belts decorated with red CdTe QDs as a typical samplewere studied using near-field scanning optical microscopy (NSOM),as schematically shown in Fig. 4a. When a SiO2 inverse opal belt

decorated with CdTe QDs was excited with a focused laser(l = 405 nm) at the centre, fluorescence could be observed atthe end. Generally, light emission can only be observed at thelocal area of the excited point in the micro-area of fluorescenceimages.23 The spectra of the output light at the end of the SiO2

inverse belts integrated with CdTe QDs suggests that these beltsabsorb the excitation light and propagate the photolumines-cence (PL) emission towards the end without light scattering atthe edge of belts. It is a typical optical waveguide phenomenon.In order to gain further insights into the optical waveguidingbehavior within the 3D nanocomposite inverse opal belts, thespatially resolved near-field image of the propagation light atthe output light point was obtained (Fig. 4c). The PL spectra ofthe pure TGA-capped CdTe QDs is also located at 636 nm withthe full width at half maximum (FWHM) of 54.1 nm. In contrast,the SiO2 inverse opal belts integrated with CdTe QDs revealed adecreasing of the FWHM (10.4 nm) of the PL spectrum of theoutput light. It indicates that the light propagates in the beltswith a stable PL band in the SiO2/CdTe inverse opal belts. ThePL band with a narrow FWHM in the inverse opal structureplays an important role in integrating the PL band of the CdTeQDs. Only the terminal end of the SiO2/CdTe inverse opal belt inthe same direction of light has an output light, from the NSOWimage, which proves that the photon is confined into the inverseopal belts during light propagation. SiO2 inverse opal belts withother sizes of CdTe QDs were also studied in Fig. S4 of the ESI.†From the above-mentioned results, SiO2 inverse opal beltsintegrated with CdTe QDs exhibit an excellent stable and narrowPL band in the output light during light waveguiding.

Conclusions

In summary, we have successfully prepared super-stablecentimetre-scale SiO2 inverse opal belts integrated with CdTeQDs via hydrogen bonding interactions between the carboxylgroups of TGA and hydroxyl groups of silica. SiO2 inverse opalbelts integrated with CdTe QDs serve as a directional opticalwaveguide material with excellent stability and a significantlynarrow PL band in the output light. This new type of opticalwaveguide material has potential applications in novel opticalinterconnects with controlled electrical and photonic confines.

Acknowledgements

The authors thank the financial support of the Beijing NaturalScience Foundation (2132030), the National Natural ScienceFoundation of China (21103006), China Scholarship Council(201303070263), 863 Program (2012AA030305), and the FundamentalResearch Funds for the Central Universities (YWF-10-01-B16,YWF-13-DX-XYJL-004). The authors thank Prof. Dr Jin Zhai andDr K. Wang for their kind help with the NSOW characterization.

Notes and references

1 L. Tong, R. R. Gattass, J. B. Ashcom, S. He, J. Lou, M. Shen,I. Maxwell and E. Mazur, Nature, 2003, 426, 816–819.

Fig. 3 (a) Detailed schematic illustration of the SiO2 inverse opal dottedwith CdTe QDs. (b) The hydrogen bonding effect between the SiO2 hollowmicrosphere and TGA-capped CdTe QDs. (c) FT-IR spectra of TGA-cappedCdTe NPs (black line) and the SiO2 inverse opal dotted with TGA-cappedCdTe NPs (red line).

Fig. 4 (a) Schematic illustration of optical waveguiding in SiO2 inverseopal belts decorated by CdTe QDs. (b) PL spectra detected with the NSOMcollection tip held at the terminus of the nanocomposite inverse opal beltsand with the excitation laser beam at the side with an angle of 451.(c) Optical waveguiding in inverse opal belts detected using NSOM.

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