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Scripta Materialia 66 (2012) 646–649

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Self-assembled quantum dots–polyhedral oligomeric silsesquioxanenanohybrids with enhanced photoluminescence

Qi Li,a Lijie Dong,a Xiang Wang,a Jing Huang,a Haian Xiea and Chuanxi Xionga,b,⇑aState Key Laboratory of Advanced Technology for Materials Synthesis and Processing and School of Materials Science

and Engineering, Wuhan University of Technology, Wuhan 430070, People’s Republic of ChinabSchool of Materials Science and Engineering, Wuhan Textile University, Wuhan 430073, People’s Republic of China

Received 21 December 2011; accepted 23 January 2012Available online 28 January 2012

We propose and demonstrate a novel family of polyhedral oligomeric silsesquioxane-modified quantum dots (QDs) with remark-ably enhanced photoluminescence, which breaks the convention that the optical properties of semiconductor nanocrystals oftendecrease after surface modification. The result suggests that molecules with cubic nanostructure can enhance the fluorescence inten-sity of QDs, and the resultant hybrid material could be a very promising candidate for various applications, including optoelectronicdevices and biological fluorescence labeling and imaging.� 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Surface modification; Self-assembly; Quantum dots; Luminescence; Silicon

Semiconductor quantum dots (QDs) with di-verse chemical functionalities and flexible processabilitythrough surface engineering have attracted much atten-tion in recent years and hold great promise in diverseprospective applications, such as light-emitting diodes[1], photovoltaic solar cells [2], sensors [3] and biologicalfluorescence imaging [4,5]. Ligand exchange with vari-ous thiol-containing or amine-containing molecules,including mercaptopropionic acid [6], dihydrolipoic acid[7], dithiothreitol [8] and pyridine [9], has already beenextensively investigated, where the original surface li-gands of QDs are replaced by different functionalgroups for the purpose of bioconjugation. However,the fluorescence intensity of QDs decreases upon suchligand exchange. In other cases, QDs are encapsulatedin amphiphilic polymers [10–12] or inorganic silica shells[13–16] without disturbing their native ligands in orderto promote the chemical stabilization of QDs, which ishighly favorable for averting fluorescence quenching.Nevertheless, the photoluminescence intensity of QDsalso declines after encapsulation.

1359-6462/$ - see front matter � 2012 Acta Materialia Inc. Published by Eldoi:10.1016/j.scriptamat.2012.01.047

⇑Corresponding author at: State Key Laboratory of AdvancedTechnology for Materials Synthesis and Processing and School ofMaterials Science and Engineering, Wuhan University of Technol-ogy, Wuhan 430070, People’s Republic of China. Tel./fax: +86 2787652879; e-mail: cxx@live.whut.edu.cn

In this contribution, we propose and demonstrate anovel family of polyhedral oligomeric silsesquioxane(POSS)-modified QDs (QDs–POSS) nanohybrids withenhanced photoluminescence, which breaks the conven-tion that the optical properties often decrease aftersurface modification. The result suggests that moleculeswith cubic nanostructure can enhance the fluorescenceintensity of QDs. Additionally, this method is generaland can be extended to other semiconductor nanocrys-tals (ZnO, PbS, Mn-doped ZnSe) and noble metal nano-particles (Au, Ag). By employing POSS with a specificstructure, the fluorescence intensity of QDs can increaseby 30%, which endows QDs–POSS nanohybrids withthe following advantages: (a) they can be easily intro-duced into polymers to obtain luminescent materialsdue to the fine compatibility between the POSS andthe polymers [17–19]; this will be beneficial to theconstruction of optoelectronic devices based on QDs;(b) the combination of highly luminescent QDs andcytocompatible POSS will make it suitable for biomedi-cal applications; and (c) it is expected to realize low-costapplications of QDs because only a small amount ofQDs is needed to retain the optical properties of the ori-ginal QDs.

Figure 1 shows the strategy for preparing the firstQDs–POSS nanohybrid (termed QDs–POSS1), startingfrom hydrophobic CdSe/CdS/ZnS QDs. Typically,highly fluorescent QDs dispersed in chloroform with

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Figure 2. TEM images of red-emitting (a, b) QDs and (c, d) QDs–POSS1 nanohybrid. (e) Fluorescence spectra of QDs and QDs–POSS1nanohybrid with two different emission wavelengths. Scale bar: (a)20 nm, (b) 10 nm, (c) 5 nm, (d) 1 nm.

Figure 1. Schematic illustration of the preparation of QDs–POSS1nanohybrid through ligand exchange.

Q. Li et al. / Scripta Materialia 66 (2012) 646–649 647

emission wavelengths of 580 nm (yellow emitting) and630 nm (red emitting) were synthesized according tothe literature, with a small modification [20]. Due tothe thiol–metal affinity interaction, QDs–POSS1 with acore–shell structure was obtained by the facile ligand ex-change between QDs and mercaptopropylisobutyl POSS(POSS-SH).

Fourier transform infrared spectroscopy (FTIR) wasapplied to verify the surface modification of the QDs(Fig. S1). The presence of an Si–O stretching band lo-cated at 1110 cm�1 indicates the interaction betweenQDs and POSS-SH. Transmission electron microscopy(TEM) images show that red-emitting QDs in chloro-form are highly monodisperse, with an average particlediameter of about 5.5 nm (Fig. 2a and b). After ligandexchange, the size of QDs–POSS1 is approximately7.5 nm, as shown in Figure 2c and d. This value suggeststhat each POSS molecule is about 1 nm in size, a value ingood accordance with previous reports [21,22]. Theoret-ically, an individual QD can be densely coated by about20 POSS-SH molecules on the equatorial plane accord-ing to a calculation based on the circumference formula.Figure 2d clearly shows that a single QD is surroundedby 16 POSS-SH molecules, a little less than the expectedvalue, which is probably due to the steric effect of POSS-SH. As demonstrated by the results of energy-dispersiveX-ray analysis (EDX; Fig. S2), the exsting elements ex-plain the FTIR results well, confirming the successfulsurface modification of QDs.

Ultraviolet (UV)–visible absorbance spectroscopyshows that the optical properties of QDs and QDs–POSS1 are characterized at the same concentration ofQDs (Fig. S3); the QDs–POSS1 has the same profile asthe corresponding spectra of the original QDs. Even attheir exciting wavelength region (365 nm), no change inthe absorbance could be detected. The similar tail shapeof the two traces also implies a comparable solubility ofthe QDs before and after POSS incorporation. Remark-ably, QDs–POSS1 presents a higher fluorescence inten-sity, increased by 10% compared to QDs (Fig. 2e).Considering the UV–visible profile, we attribute thisenhancement of fluorescence intensity to the features ofthe attached POSS moieties. POSS-SH is a cubic-shapedmolecule that contains an inorganic silica-like core, sur-rounded by seven non-reactive isobutyl groups and onereactive thiol group. Unlike inorganic silica, the POSS-SH molecule, with its regular cubic nanostructure, playsan important role in changing the UV light path (Fig. S4).

Upon UV radiation, QDs are excited by some incidentalUV lights, giving rise to the emission of light; others arereflected. When the reflected lights reach the surface ofPOSS-SH, they are reflected again, and thus the QDsare excited again. Thus, the QDs will receive multipleexcitations derived from multiple reflections, resultingin a cumulative light effect. Therefore, the optical proper-ties of the QDs are enhanced.

By taking advantage of highly luminescent QDs–POSS1 and the fine compatibility between the POSSand the polymers, QDs–POSS1 can be easily incorpo-rated into hydrophobic polymers to obtain luminescentmaterials. Upon UV excitation (k = 365 nm), the filmsemit bright, homogeneous light (Fig. S5), which corre-sponds to the high fluorescence intensity of films (Fig. S6).

In the QDs–POSS1 nanohybrid, monofunctional POSS-SH enhances the fluorescence intensity of the QDs. We alsodesigned another QDs–POSS nanohybrid (termed QDs–POSS2) with an assembly structure where POSS withmultifunctional groups is used (Fig. 3). It is achieved viathe electrostatic force between polyethylenimine (PEI)-coated QDs (QDs–PEI) and octa(tetramethylammonium)polyhedral oligomeric silsesquioxane (TMA-POSS).Surface modification of QDs with PEI renders the result-ing nanoparticles cationic, which then react with nega-

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Figure 4. TEM images of (a, b) QDs–PEI and (c, d) QDs–POSS2nanohybrid. (e) EDX spectrum of the QDs–POSS2 nanohybrid shownin (c). (f) Fluorescence spectra of QDs–PEI and the QDs–POSS2nanohybrid. Scale bar: (a) 5 nm, (b) 5 nm, (c) 10 nm, (d) 5 nm.

Figure 3. Schematic illustration of the preparation of QDs–POSS2nanohybrid through electrostatic force.

648 Q. Li et al. / Scripta Materialia 66 (2012) 646–649

tively charged TMA-POSS through a cation–anion pro-cess to obtain the corresponding QDs–POSS2 nanohy-brid. In this system, TMA-POSS with eight oxygenanion groups on each corner acts as a cubic linker be-tween QDs–PEI through electrostatic force, and this en-ables the spontaneous formation of an assemblystructure of QDs in aqueous solution.

The QDs–POSS2 nanohybrid was determined by thewell-resolved infrared absorption bands (Fig. S7). Thepresence of an N–H stretching band located at3414 cm�1 indicates the modification of QDs by PEI.The electrostatic interaction between QDs–PEI andTMA-POSS was confirmed by the presence of an Si–Ostretching band at 1082 cm�1.

TEM images show that PEI-coated QDs are nearlymonodisperse, with no evidence of aggregation (Fig. 4aand b). After the electrostatic interaction with TMA-POSS, QDs are self-assembled, resulting in a shorter dis-tance between the QDs (Fig. 4c and d). The EDX resultshows that the QDs–POSS2 nanohybrid contains Cd,Se, Zn, S, Si, O, C and N, which suggests the existenceof both QDs–PEI and TMA-POSS (Fig. 4e). In theQDs–POSS2 nanohybrid, the TMA-POSS was function-alized as nanobridges between QDs–PEI, which facili-tates the electrostatic stabilization of QDs in aqueoussolution.

Fluorescence spectra show that the fluorescence inten-sity of QDs increases by 130% compared with QDs–PEIafter the electrostatic interaction with TMA-POSS, asshown in Figure 4f. Even when compared with the initialQDs, a 30% increase in the fluorescence intensity couldstill be found. In the QDs–POSS2 nanohybrid, TMA-POSS is employed as a cubic linker between QDs–PEI,leading to the formation of an assembly structure thatcomprises many units resembling a QDs–POSS1 core–shell structure. The collective effect of all the units con-tributes to the significant enhancement of the opticalproperties.

In conclusion, we have reported that the photolumi-nescence properties of QDs are enhanced through mod-ification with POSS. By employing monofunctionalPOSS-SH, the fluorescence intensity of QDs increasesby 10%, due to a cumulative light effect. The three-dimensional structure of POSS-SH can reflect the lightemitted, so that QDs will receive multiple excitations,which enhances their QDs. Importantly, QDs–POSS2with an assembly structure is designed where the fluores-cence intensity of the QDs is increased by 30%, which isascribed to the collective effect of all the units. Thesehighly luminescent QDs–POSS nanohybrids are ex-pected to be promising candidates for various applica-tions, including optoelectronic devices and biologicalfluorescence labeling and imaging.

This work was financially supported by the 973Program (No. 2010CB227105) and the National NaturalScience Foundation of China (No. 51173139, 51072151).

Supplementary data associated with this article canbe found, in the online version, at doi:10.1016/j.scriptamat.2012.01.047.

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