Shell-Controlled Photoluminescence in CdSe/CNT Nanohybrids
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A new type of nanohybrids containing carbon nanotubes (CNTs) and CdSe quantum dots (QDs) was prepared using an electrostatic self-assembly method. The CdSe QDs were capped by various mercaptocarboxylic acids, including thioglycolic acid (TGA), dihydrolipoic acid (DHLA) and mercaptoundecanoic acid (MUA), which provide shell thicknesses of ~5.2, 10.6 and 15.2 Å, respectively. The surface-modified CdSe QDs are then self-assembled onto aridine orange-modified CNTs via electrostatic interaction to give CdSe/CNT nanohybrids. The photoluminescence (PL) efficiencies of the obtained nanohybrids increase significantly with the increase of the shell thickness, which is attributed to a distance-dependent photo-induced charge-transfer mechanism. This work demonstrates a simple mean for fine tuning the PL properties of the CdSe/CNT nanohybrids and gains new insights to the photo-induced charge transfer in such nanostructures.
KeywordsCarbon nanotubes CdSe Nanohybrids Photoluminescence Charge transfer
Recently, nanohybrids containing both semiconductor quantum dots (QDs) and carbon nanotubes (CNTs) have been the subject of great interest as a consequence of the development of methods for the chemical modification of CNTs and the seeking for novel functional materials [1–4]. Attachment of QDs onto conducting CNTs would place a metallic wire in direct chemical contact with the QDs surface. The metallic CNTs could then promote direct charge transport and efficient charge transfer from the QDs [5–8]. This system has the potential to significantly increase the efficiency of photovoltaic devices [9, 10]. Besides, there has been an increasing interest on using CNTs in biological system . Attaching QDs onto CNT can afford fluorescent labels and be utilized for detection, imaging and cell sorting in biological applications .
For all the above applications, the understanding to how the nanohybrid structure affects the charge transfer and energy transfer behaviors between the QDs and CNT is crucial. The interactions between II–VI QDs (such as CdS, CdSe and CdTe) and CNTs have been investigated by several groups [8, 13–15]. The charge-transfer efficiencies were evaluated by studying the changes in the photoluminescence (PL) [13, 16, 17] or photo-electrochemical properties of such hybrid materials [8, 14, 15, 18]. The PL behaviors of the nanohybrids were found to be strongly dependent on how the QDs are attached to the CNTs. For example, strong PL quenching by charge-transfer mechanism were reported for the CdS/TOAB/CNT , CdSe/pyridine/CNT  and CdSe/pyrene/CNT systems . Partial emission quenching was observed on nanohybrids consisting of dendron-modified CdS QDs on CNT . In contrast, Giersig [22, 23] reported preserved CdTe PL by insulating the CNTs using silica coating. The previous works have shown that the PL properties of the QD/CNT nanohybrids are strongly dependent on QD-CNT separation, but precise control to the distance between QDs and CNT was difficult to achieve in the available systems.
Mercaptocarboxylic Acids Modified CdSe QDs
Trioctylphosphine oxide (TOPO)-capped CdSe QDs were prepared as reported previously . The QDs were then modified by mercaptocarboxylic acids through a ligand-exchange reaction as described below. The thioglycolic acid (TGA) was diluted to 1 M with PBS (pH = 7.4) buffer. The TGA solution (3 mL) was added to the solution TOPO/CdSe, which was dissolved in 1 mL of chloroform. The mixture was stirred for 2 in dark and then separated into two layers spontaneously. The water layer was extracted and centrifuged to produce a pellet of TGA-capped QDs (TGA-CdSe). The supernatant, which contains free TGA, was discarded. Dihydrolipoic acid (DHLA)-capped CdSe QDs (DHLA-CdSe) were prepared as reported previously . The mercaptoundecanoic acid (MUA)-capped CdSe QDs (MUA-CdSe) was prepared in a similar way as the TGA-CdSe, except that MUA was dissolved in ethanol. The mercaptocarboxylic acids modified CdSe QDs were purified by three cycles of filtration using membrane separation filters followed by washing with MilliQ ultra-pure water. Small amount of potassium tert-butoxide was added to the CdSe QDs solution to improve the solubility in water and also turn the QDs into a negatively charged state.
Aridine Orange-Modified CNT
Multiwall CNTs were purified in 37% HCl. The purified CNTs were then noncovalently modified with aridine orange (AO), following the processes described in our previous work . The molecular structure of AO is shown in Scheme 1. In a typical process, 2.0 mg purified CNTs were added to an aqueous solution containing 2 mg AO and sonicated at room temperature for at least 2 h to reach a maximum adsorption. The AO-modified CNTs (AO/CNTs) were separated from the solution by filtration using a 0.22-μm diameter cellulose acetate–cellulose nitrate membrane, and then thoroughly rinsed to remove access AO.
The aqueous solutions of CdSe QDs were then titrated with AO/CNT solution and gently stirred to give the CdSe/CNT nanohybrids. The produced nanohybrids were subjected to three washing cycles to remove the excess CdSe QDs. In each washing cycle, the samples were centrifuged at 4,000 rpm for 3 min then redispersed in PBS buffer after removing the supernatant.
Results and Discussions
The changes in the CdSe/CNT absorption and the PL spectra can be understood by the photo-induced charge transfer within the nanohybrids (Scheme 1). The valence band and conduction band energy levels for the as-prepared CdSe QDs are taken to be 6.2 and 3.9 eV, respectively . The semiconducting CNTs normally have an energy gap ranging from 0 to 1.1 eV, and the Fermi level of the metallic CNTs is taken to be 4.5–5.0 eV [31, 32]. The PL of the QDs is due to the radiative decay path from their excited state to ground state . Given that the nanotubes quenched the CdSe QDs, the QD-CNT interaction must provides an alternative nonradiative decay path. It is believed that this nonradiative decay path occurs because the difference in electron affinity between the CdSe QDs and the CNTs is sufficient to allow electron transfer from the QDs to the CNTs . Based on the energy diagram, the formation of CdSe/CNT conjugates favors the electron transfer from the QDs (donor) to the CNTs (acceptor) such that the excited electrons moved to the CNTs rather than be emitted as the PL peak. Thus, the electrons of the excitons could be partially transferred to CNTs by an electron-injection mechanism, while the remaining electrons exhibit a reduced emission by an electron-hole recombination process.
As mentioned above, the different mercaptocarboxylic acid coatings provide different shell thicknesses for the CdSe QDs. The PL spectra show that the intensity of the QD emission peaks in the CdSe/CNT nanohybrids is strongly dominated by the shell thickness. The PL of the TGA-CdSe QDs is nearly completely vanished after binding with the CNTs. The PL of the DHLA-CdSe QDs is dramatically reduced to a moderate intensity, indicating partial quenching occurred. In contrast, the PL intensity of the MUA-CdSe QDs remains nearly unchanged in the presence of the CNTs (Fig. 5d) and showing no obvious quenching occurs. The different PL intensities of the CdSe/CNT nanohybrids suspensions are visible to naked eyes in the fluorescent images of the nanohybrid solution. As seen in Fig. 5e–g, under UV irradiation, the PL brightness increases with the chain length of the capping mercaptocarboxylic acids on the CdSe QDs.
As demonstrated in Fig. 5, the fluorescence of the CdSe/CNT is well controlled by the length of the organic spacers. In a similar fashion, Kulakowich et al.  studied the optical effects of tuning the separation between gold colloids and QDs on planar substrates. A clear increase of the PL has been observed as the separation increased. In our strategy, the separation between the QDs and CNTs is adjusted by the lengths of alkyl chains of the capping agents. The fully extended lengths of TGA, DHLA and MUA are 5.2, 10.6 and 15.2 Å, respectively, as estimated from semi-empirical (PM3) calculations. Assuming that the AO molecules are adsorbed on the CNT sidewalls with a coplanar configuration and using the normal van der Waals diameter values for the AO and carboxylic acids, the separation between the edges of the CdSe QDs and CNTs are estimated to be around 14, 19 and 24 Å for the TGA-CdSe/CNT, DHLA-CdSe/CNT and MUA-CdSe/CNT systems, respectively. As the average distance between the CdSe QDs and CNTs increases within 10 Å ranges, the PL properties of the CdSe/CNT change from total quenching to nearly no quenching. This result clearly demonstrates that a precise control to the QD-CNT spacing at angstrom level is essential for controlling the PL properties of the CdSe/CNT nanohybrids.
We report a new and facile method for attaching CdSe QDs onto CNT surface by electrostatic self-assembly. By using different mercaptocarboxylic ligands, the shell thicknesses of the CdSe QDs are well controlled within angstrom-level precision. The formation of various CdSe/CNT nanohybrids has been confirmed by TEM, XRD, FT-IR, UV–vis absorption and PL spectroscopies. The absorption and PL spectra of the hybrid materials suggested that photo-induced electron transfer from CdSe QDs to CNTs took place in the nanohybrids. The efficiency of the PL quenching decreases upon increasing the shell thickness due to the distance-dependent electron transfer efficiency. This work demonstrates that the shell thickness control to the QDs opens up a straightforward methodology for investigating the interaction between fluorescent nanomaterials coupled with CNTs. Since there is large number of bifunctional ligands readily available for researchers, this method can be used as a general approach to tailor the QD-CNT separation within molecular size range. The ability to allow or prevent charge transfer from photoexcited QDs to CNT provides a useful technique in developing QDs/CNT nanomaterials to become flexible, facile building blocks for many practical applications, including electrooptic devices, biological nanoprobes for cytological investigation and in situ fluorescently detectable microsensors.
The authors are grateful to the financial support from the program for New Century Excellent Talents in University (NCET), National Natural Science Foundation of China (NSFC. 20503011, 20621091), Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP. 20050730007), and Key Project for Science and Technology of the Ministry of Education of China (106152).