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Hybrid SiO2-coated nanocrystal-based heterostructures: Assembly, morphology transition, and photoluminescence at room temperature
Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (2011) 289–296
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Hybrid SiO2 -coated nanocrystal-based heterostructures: Assembly, morphology transition, and photoluminescence at room temperature Ping Yang, Masanori Ando, Norio Murase ∗ Health Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Midorigaoka, Ikeda-city, Osaka 563-8577, Japan
a r t i c l e
i n f o
Article history: Received 12 January 2011 Received in revised form 13 March 2011 Accepted 5 April 2011 Available online 12 April 2011 Keywords: Nanocomposites Fibers Sol–gel methods Functional composite Morphology
a b s t r a c t Coating CdTe nanocrystals (NCs) with a hybrid SiO2 shell with CdS-like clusters drastically increased their photoluminescence (PL) efficiency and substantially red-shifted their PL peak. The coating was done either by a two-step synthesis including a sol-gel process and a subsequent reflux with a short reaction time or by a sol–gel procedure with a long reaction time at room temperature. When the solution of the coated NCs containing thiolglycolic acid (TGA), Cd2+ , and SiO2 monomers was left at room temperature, the coated NCs self-assembled into heterostructures. The self-assembly behavior of the NCs is ascribed to an electrostatic force that led to the attachment of the NCs to a composite of TGA, Cd2+ , and SiO2 monomers. The PL of the resulting assemblies depended strongly on the hybrid nanostructure, i.e., CdSlike clusters formed near the CdTe core. Experimental conditions such as the concentration of the hybrid NCs in solution and the growth time were adjusted to control the assembly morphology. When the NC concentration was sufficiently high, assemblies with a sheaf-like morphology were created at room temperature. Systematic reduction of the NC concentration resulted in the creation, in turn, of belt-like fibers, solid fibers, and long nanowires. When an aqueous solution with a low concentration of hybrid NCs was kept at room temperature for six months, the assemblies exhibited tubal and helical morphologies. This morphological evolution is ascribed to the concentration dependence of the growth kinetics. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor nanocrystals (NCs) can be exploited as building blocks for photonic, electronic, and magnetic devices as well as for use in sensing and optical applications [1]. NCs assembled into matrices are an important class of materials for field-effect transistors, photodetectors, light-emitting diodes, metamaterials, and solar cells [2–7] due to the advanced properties of the NCs. For example, silica nanofibers functionalized with CdS NCs exhibit interesting mesostructural transformations with simultaneous changes in gas adsorption behavior and CdS NC emissions [8]. The self-assembly of nanomaterials has attracted a great deal of attention, bridging various areas of science and engineering, that has resulted in the design and development of advanced materials, innovative methodologies, and general theories [9,10]. Various approaches have been used to assemble NCs in an orderly manner and to investigate the scope of potential applications. For example, template methods have been used to study the self-assembly of semiconductor NCs [11]. Various organic and bio-
∗ Corresponding author. Tel.: +81 72 751 9647; fax: +81 72 751 9637. E-mail address: [email protected] (N. Murase). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.04.002
chemical molecules have been used to drive the self-assembly of chemically produced semiconductor NCs. Previous efforts have concentrated on using such scaffolds to spatially arrange nanoelements as a strategy for tailoring the electrical, magnetic, or photonic properties of the material [12]. Although many strategies are promising, it is still necessary to develop straightforward and controllable methods for self-assembly of nanostructures, especially structures with unique properties. The assembly of NCs is usually driven by the interactions between the individual building blocks, so the control of the NC surface properties is an important factor in their assembly [13]. The simplest interacting NC system is composed of two nearby NCs. The main interactions between the NCs are electron (or hole) tunneling and Förster resonant energy transfer due to dipole–dipole interaction. These interactions cause the NCs to assemble into a one-dimensional (1D) nanowire in solution [14]. The self-assembly of NCs at a two-phase interface (gas–solid, liquid–solid, gas–liquid, or liquid–liquid) has been investigated, and NCs with a one- or two- dimensional nanostructure have been produced at a two-phase interface, with the interface acting as a template [15]. The formation force of assemblies produced by hydrogen bonding, charge transfer, covalent bonding, lateral capillary force, and hydrophobic interactions has been studied [16–20]. Self-assembly based on coordinative or hydrogen bonding
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interactions is a particularly useful method for creating superstructures because of its low cost, easy controllability, and simplicity. Organic nanowires and nanotubes have been assembled using noncovalent interactions, including hydrogen-bonding and intermolecular p–p stacking [21]. One of the major challenges in the self-assembly of NCs based on noncovalent interactions is clarifying the interaction between the NCs and the resulting nanostructure. Functional 1D nanomaterials have attracted much attention in the field of nanotechnologies because such materials hold great promise for advanced catalysis, electronics, sensors, energy storage/conversion, and photonic applications due to their unique morphology and their optical, magnetic, and electronic properties [22,23]. Luminescent CdTe NCs have been encapsulated within cadmium thiolate composite fibers with a tunable morphology and luminescence [24,25]. Thus, filling 1D nanomaterials with functional NCs is highly promising for nanotechnology applications. However, this potential self-assembly mechanism is still not well understood. Previously, hybrid SiO2 -coated CdTe NCs were prepared by using a simple reflux procedure [26,27]. The hybrid NCs revealed red-shifted PL spectra and high PL efficiency. In this paper, we found out such hybrid SiO2 -coated CdTe NCs can be created at room temperature with a long reaction time. In contrast, the red-shift on the PL spectra was small compared with that prepared by reflux. Further, we investigate the self-assembly of CdTe NCs at room temperature using Cd-TGA composites as a template. The assembly exhibited different morphology (sheef-like) and different PL properties compared with that created using CdTe NCs by reflux [24]. In addition, hybrid SiO2 -coated CdTe NCs can be assembled into various morphological heterostructures through a long sol–gel process at room temperature. However, the heterostructure did not exhibit any change on their PL spectra compared with the initial hybrid NCs. 2. Material and methods 2.1. Chemicals and materials All chemicals were obtained from Sigma–Aldrich and used as received. All chemicals were of analytical grade or of the highest purity available. The pure water was obtained from a Milli-Q synthesis system (resistivity of 18.2 M cm).
Fig. 1. Spectral and morphological properties of hybrid SiO2 -coated CdTe NCs (Sample 1, Table 1) with reflux time of 2.5 h: (a) absorption and PL spectra; (b) TEM image. Absorption and PL spectra of initial CdTe NCs are shown for comparison. Luminescent properties of NCs are summarized in Table 1.
a 0.2 m filter to remove the composite fibers that were created simultaneously during the reflux. 2.3. Self-assembly of hybrid SiO2 -coated CdTe NCs We investigated the concentration effect of these starting materials on the assembly morphology since the concentration of these starting materials determined the growth speed of fibers. The NC concentration was increased by using a 3000-MWCO-filter to condense the aqueous solution of hybrid SiO2 -coated CdTe NCs
2.2. Preparation of hybrid SiO2 -coated CdTe NCs TGA-capped CdTe NCs with green (d ∼ 2.6 nm) emission in aqueous solution were prepared using a procedure including the use of cadmium perchlorate and hydrogen telluride, as described elsewhere [28]. The hybrid SiO2 -coated CdTe NCs were prepared using a two-step synthesis process. In step 1, CdTe NCs were coated with a thin SiO2 layer by stirring them in an aqueous solution containing Cd2+ , TGA, CdTe NCs, TEOS, and NH3 . Briefly, CdTe colloidal solution (2 mL) was precipitated by 2-propanol and redispersed in a 2 mL aqueous solution of Cd2+ and TGA (pH ∼ 10, adjusted using an NaOH solution of 1 M). Diluted ammonia (0.1 mL, 6.25 wt.%) and TEOS (20 L) were mixed with the redispersed CdTe colloidal solution in a beaker sealed to reduce ammonia evaporation during incorporation. After being stirred for 3 h, the CdTe NCs were coated with a thin SiO2 layer containing Cd2+ and TGA. The solution then became homogenous and transparent. In step 2, a reflux process using this solution caused CdS-like clusters to nucleate and grow in the SiO2 shell. The reflux time was for 2.5 h. The TGA concentration and TGA/Cd2+ molar ratio of the solution were 0.01 M and 2, respectively. The resulting samples were finally filtered using
Fig. 2. Absorption and PL spectra of hybrid SiO2 -coated CdTe NCs (Asprepared = Sample 2; After 40 days = Sample 3). Peak was observed around 355 nm in absorption spectrum for Sample 3.
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Table 1 PL properties and preparation conditions of hybrid SiO2 -coated CdTe NCs. Sample
PL peak wavelength (nm)
FWHM (nm)
PL efficiency (%)
Preparation
1 2 3 CdTe NCsa
613 547 566 555
50 52 47 54
61 28 38 24
Reflux Room temperature Room temperature, 40-day storage Initial colloidal NCs
a
Properties of CdTe NCs are shown for comparison.
Table 2 Preparation parameters and final morphologies of assemblies with hybrid SiO2 -coated CdTe NCs. Sample
Concentration of NCs (×10−5 M)
Initial samplea
Growth time
Final morphology
4 5 6 7 8 9
2.01 0.97 0.49 0.24 0.12 2.43
1 1 1 1 1 2
7 days 1 month 1.5 months 1.5 months 6 months 40 days
Sheaf-like Belt-like Thick rod Thin rod Rod, tubal, belt-like Sheaf-like
a
Sample number shown in Table 1.
prepared by the two-step synthesis or by the sol–gel process. The TGA, Cd2+ ions, and NH3 in the solutions were partially removed by repeated condensation and dilution. Solutions with various concentrations of hybrid NCs were kept in glass bottles with covers at room temperature for different periods to enable us to investigate the morphological evolution during assembly. Finally, solution samples were removed from the bottles with pipettes and placed on glass slides for observation with a fluorescence microscope. 2.4. Apparatus Observations by TEM were carried out using an FEI Tecnai G2 F20 (200 kV) electron microscope. Morphological observation of assemblies was done using a field emission scanning electron microscope (Hitachi, S-5000). Photoluminescence and optical images of sample fibers were obtained with a fluorescent microscope (Nikon, Eclipse 80i). The absorption and PL spectra were recorded using conven-
tional spectrometers (Hitachi U-4000 and F-4500, respectively). Both of excitation and emission slits are 5 nm for the measurement of PL spectra. The size distributions of samples in solution were obtained using a dynamic light-scattering particle analyzer (Nanotrac, Nikkiso). The PL efficiencies of samples in solution were estimated using a method previously reported [29,30]. Briefly, the PL and absorption spectra of a standard quinine solution (quinine in 0.1 N H2 SO4 solution; PL efficiency 0 of 55%) were measured in a 1 cm quartz cell as a function of the concentration. The emission intensity (in units of the number of photons) is expressed as P0 ≈ K0 a0 10−0.5a0 , where a0 is absorbance at the excitation wavelength (365 nm), and K is the apparatus function. After measurement of the absorbance and PL intensity of the sample using the same apparatus parameters, the PL efficiency of the sample was derived by comparing its PL intensity with that of the standard quinine solution. The error in the PL efficiency was estimated to be within 10% by comparing the results using two standards: quinine and R6G.
Fig. 3. Photoluminescence images taken under 365 nm UV light showing evolution of hybrid SiO2 -coated CdTe NCs (2.01 × 10−5 M concentration) into assemblies with sheaf-like morphology (Sample 4, Table 2) after (a) one day, (b) two days, (c) four days, (d) seven days. Insets in (a) and (d) show fiber morphologies.
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Fig. 4. Photoluminescence images taken under 365 nm UV light showing evolution of hybrid SiO2 -coated CdTe NCs (9.7 × 10−6 M concentration) into assemblies with belt-like morphology (Sample 5, Table 2) after (a) two days, (b) seven days, (c) ten days, (d) one month. Insets show fiber morphologies.
3. Results and discussion 3.1. Evolution of PL properties of CdTe NCs coated with hybrid SiO2 shell A two-step synthesis including a sol–gel process (Step 1) and a subsequent reflux (Step 2) was used to create hybrid SiO2 -coated
CdTe NCs. Briefly, colloidally prepared green-emitting CdTe NCs (2.6 nm in diameter) were coated with a thin SiO2 layer by adapting the use of tetraethyl orthosilicate (TEOS), ammonia, Cd2+ , and thiolglycolic acid (TGA) to a sol–gel reaction. A subsequent reflux process including a further sol–gel reaction was then used to fabricate the hybrid NCs. During reflux, CdS-like clusters nucleated and grew in the SiO2 shell. At the same time, the SiO2 shell became
Fig. 5. Optical (left) and PL (right) images of hybrid SiO2 -coated CdTe NCs that assembled (after 1.5 months) into fibers with thick solid morphology [(a) and (b), Sample 6] and thin solid morphology [(c) and (d), Sample 7]. Insets in (b) and (d) show fiber morphologies.
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Fig. 6. SEM images of hybrid SiO2 -coated CdTe NCs assembled into morphology-tunable fibers (Samples 4–7, Table 2): (a) Sample 4 with sheaf-like morphology,(b) Sample 4 at high magnification; (c) Sample 5 with belt-like morphology, (d) Sample 6 with solid morphology, (e) Sample 6 at high magnification, (f) Sample 7 with long nanowire morphology.
thicker due to the deposition of SiO2 monomers. The procedure used to prepare the hybrid SiO2 -coated CdTe NCs is described in more detail in Section 2. Fig. 1 shows the absorption and PL spectra and a transmission electron microscopy (TEM) image of hybrid SiO2 -coated CdTe NCs (Sample 1 in Table 1) prepared using the two-step synthesis with a reflux time of 2.5 h. After reflux (Step 2), the hybrid SiO2 coated CdTe NCs were ∼6 nm in diameter (from TEM observation) while the initial CdTe NCs were ∼2.6 nm in diameter. Compared with the initial CdTe NCs, the hybrid SiO2 -coated ones exhibited a red-shifted PL peak wavelength (555 nm for initial NCs, 613 nm for hybrid NCs) and higher PL efficiency (24% for initial NCs, 61% for hybrid NCs). These changes are ascribed to the CdS-like clusters in the SiO2 shell that formed close to the CdTe core. This is supported by the observation of an absorption shoulder around 380 nm for the hybrid NCs (Fig. 1a). The preparation-condition dependence of the PL properties, including the PL peak wavelength, the full-width at half maximum (FWHM) of the PL spectra, the lifetimes, and PL efficiency, of hybrid SiO2 -coated CdTe NCs are described in detail elsewhere [27]. We also used a sol–gel procedure with a slow reaction to create hybrid SiO2 -coated CdTe NCs at room temperature. The CdS-like clusters formed in the SiO2 shell through the reaction of Cd2+ ions in solution and S2− ions generated by the decomposition of free TGA molecules (not connected to Cd2+ ions) in solution at room temperature. When the precursor solution prepared using the sol–gel process was stored at room temperature for a long time (ca. 40 days) at room temperature, CdS-like clusters nucleated and grew in the
SiO2 shell through a slow sol–gel reaction. The thickness of the SiO2 shell increased with the storage time due to partially hydrolyzed TEOS being retained in the solution. Fig. 2 shows the absorption and PL spectra of hybrid SiO2 -coated CdTe NCs for Sample 2 (prepared using the sol–gel process) and Sample 3 (prepared using the sol–gel process and then stored for 40 days after partially removing the NH3 from the solution) (see Table 1). Sample 3 had a peak around 355 nm in the absorption spectrum, indicating that CdS-like clusters formed during the slow sol–gel reaction. Due to the formation of CdS-like clusters in the SiO2 shell, a red-shifted PL peak wavelength (from green to yellow) and increased PL efficiency (from 28 to 38%) for Sample 3 were observed. The PL properties and preparation conditions of the hybrid SiO2 -coated CdTe NCs (Samples 1–3) are summarized in Table 1. Dynamic light scattering measurement showed that Samples 2 and 3 were respectively 3.9 ± 0.9 and 4.6 ± 1.1 nm in diameter. The smaller red shift of the PL peak wavelength of the hybrid NCs prepared at room temperature means the reaction that formed the hybrid nanostructure near the CdTe core was slow. 3.2. Self-assembly of hybrid SiO2 -coated CdTe NCs into different morphologies Previous studies have demonstrated that some mercaptocarboxylic acids can form complicated complexes with cadmium ions, with primary coordination of cadmium ions to the thiol groups and a secondary coordination to the carboxylic groups. This dual coor-
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Fig. 7. SEM (a–e) and PL (f) images at different magnifications of hybrid SiO2 -coated CdTe NCs assembled into fibers (Sample 8). White dots in SEM images are hybrid SiO2 -coated NCs.
dination affects the formation of 1D CdTe nanowires [31]. In the current experiment, the molar ratio of TGA to Cd2+ in solution was 2, so the Cd-TGA complex contained one or two ligands. The carboxyl group could link to another Cd-TGA chain-like structure through the electrostatic adsorption between Cd2+ and carboxyl groups. The connection of several Cd-TGA complexes resulted in the formation of longer Cd-TGA clusters. These clusters grew into nanowires, which acted as seeds for the growth of morphology-tunable assemblies. Namely, the SiO2 monomers generated by the hydrolysis of TEOS were deposited on the clusters through the –OH groups on their surface. The hybrid SiO2 -coated NCs could easily attach to the nanowire formed by the Cd-TGA clusters because the hybrid SiO2 shells contained –OH groups. Subsequently, the nanowire grew into assemblies with different morphologies because of the deposition of SiO2 monomers, Cd-TGA complex, and the hybrid NCs. The growth of assemblies with hybrid NCs at room temperature differed from that of fibers prepared by reflux. This growth was driven by the slow evaporation of solvent when a precursor solution with hybrid NCs was kept in a bottle with a cover at room temperature. According to our previous work, the concentration of starting materials plays an important role in determining the morphology of fibers created by reflux. In the experiment reported here, we systematically investigated the concentration dependence of the morphology by changing the experimental conditions during growth at room temperature. Aqueous solutions of hybrid SiO2 coated CdTe NCs (Sample 1) were condensed or diluted into target concentrations. The preparation conditions and final morphologies of the assemblies are summarized in Table 2. We experimentally
observed the evolution in the morphology of assembly during growth. Fig. 3 shows the evolution of hybrid SiO2 -coated CdTe NCs (Sample 4; NC concentration of 2.01 × 10−5 M) into assemblies after one, two, four, and seven days. The PL images reveal bright red-emission, and the PL spectra remained unchanged over time. The morphology was bayberry-like after one day. It became larger after two days. The morphology was sheaf-like after four days. It became larger after seven days. Careful examination of a typical sheaf-like morphology image revealed that each sheaf was composed of many compressed nanobelts (as shown later). Although the formation of 1D fibers is generally ascribed to anisotropic aggregation behaviors, a high starting-material concentration results in more crystal splitting, which leads to a sheaf-like morphology [32,33]. The morphology shown in Fig. 3 is attributed to both crystal nucleation and growth and to crystal splitting. When the concentration of the hybrid SiO2 -coated CdTe NCs was reduced, the morphology changed: the assemblies became larger and there was less splitting, as shown in Fig. 4. After two days, the NCs had assembled into a pearl-chain morphology, as shown in Fig. 4a. After seven days, they had a nanowire morphology, as shown in Fig. 4b. The morphology was straw-like after ten days, as shown in Fig. 4c. Finally, after one month, fibers with a belt-like morphology were observed, as shown in Fig. 4d. The formation of crystals with nonthermodynamic equilibrium shapes is driven kinetically. The formation includes the initial formation of nuclei immediately after supersaturation and subsequent growth of the nuclei. The formation of such hierarchical structures requires fast crystal growth. Reducing the concentration of hybrid
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To investigate the NC concentration dependence of morphology of assembly, we further reduced the NC concentration in solution. Fig. 7 shows the SEM and PL images of Sample 8, the sample with the lowest concentration (1.2 × 10−6 M) after storage at room temperature for six months. As expected, belt-like, tubal, helical, and solid morphologies were observed. The observation of several morphologies in one sample means that the fiber morphology depends strongly on the growth process as well as the NC concentration. Fig. 7b shows a typical tubal morphology, and Fig. 7c shows a typical helical morphology. These morphologies are ascribed to the nucleation and subsequent growth kinetics although the mechanism is not clear. The reason for the difference in morphologies is uncertain, but the difference could be due to anisotropic aggregation in different directions. The fibers were created using Cd-TGA complexes as a scaffold, and the connection of the complexes resulted in the formation of longer Cd-TGA clusters. These clusters grew into a structure with a two-dimensional sheet-like morphology. At a low NC concentration, the flat sheets tended to roll so as to reduce the surface energy in water. The rolling of flat sheets into curled ones reduces both the surface energy and the tension caused by the asymmetry of the sheet. The curled sheets seamed into tubes due to ring-closure of the curled chains through the formation of new electrostatic connections between the Cd-TGA chain-like structures at the two edges. When the concentration of hybrid NCs was high, the hollow spaces were quickly filled by the hybrid NCs and SiO2 monomers. Therefore, a hollow morphology was not observed for Samples 4–7. The helical morphology shown in Fig. 7c is ascribed to 1D CdTGA chains because a similar helical morphology was observed in a Cd-TGA nanobelt [34]. However, at a high concentration, hybrid
Fig. 8. Photoluminescence images taken under 365 nm UV light showing evolution of hybrid SiO2 -coated CdTe NCs prepared by sol–gel process into assemblies with sheaf-like morphology (Sample 9, Table 2): (a) after 2 days, (b) after 40 days.
SiO2 -coated CdTe NCs reduces the growth rate, which reduces the rate of crystal splitting. Therefore, by tuning the concentration of hybrid NCs, we were able to prepare structures with either a sheaf-like or belt-like morphology with little crystal splitting. The splitting disappeared completely when the concentration of hybrid SiO2 -coated CdTe NCs was reduced sufficiently due to the concentration effect of crystal growth kinetics. Fig. 5 shows optical and PL images after 1.5 months of fibers prepared using a lower concentration of hybrid SiO2 -coated CdTe NCs (4.9 × 10−6 M, Sample 6; 2.4 × 10−6 M, Sample 7) than for Sample 5 (9.7 × 10−6 M). Crystal splitting during growth was not evident for Samples 6 and 7. Solid fibers (Sample 6, Fig. 5a) with high PL, and nanowires (Sample 7, Fig. 5c) were observed. The growth speeds of these samples were substantially lower than those of Samples 4 and 5. The precise morphologies of Samples 4–7 were determined using scanning electron microscopy (SEM) imaging. The images in Fig. 6a and b clearly show that Sample 4 had a sheaf-like morphology and that each sheaf consisted of compressed nanobelts. Moreover, the flat nanobelts had widths of ∼150 nm and lengths of ∼5 m. The image in Fig. 6c shows that Sample 5 had a belt-like morphology with little splitting. The homogeneity of color image of this sample indicates that the hybrid NCs were aligned in the fibers. The images in Fig. 6d and e show that Sample 6 had a thick solid morphology. The image in Fig. 6f shows that Sample 7 had a thin solid morphology. These results indicate that the concentration of hybrid NCs in the solution is the key to controlling the morphology of the assembly.
Scheme 1. Formation process of hybrid NC-based heterostructures. (a), By hybrid SiO2 -coated CdTe NCs (after reflux); (b), By CdTe NCs (after step 1).
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SiO2 -coated CdTe NCs cross-linked the 1D Cd-TGA chains in perpendicular directions by introducing additional Cd2+ ions and SiO2 monomers into the gaps of the chains. Consequently, the helical structure formed by pure Cd-TGA complexes was lost. For comparison, we observed the self-assembly of hybrid SiO2 coated CdTe NCs prepared at room temperature (Step 1). Fig. 8 shows the morphological evolution of the NCs into fibers (Sample 9; hybrid NC concentration of 2.4 × 10−5 M). After two days, a small bayberry-like morphology was observed, as shown in Fig. 8a. After one month, a yellow-emitting sheaf-like morphology was observed, as shown in Fig. 8b. The morphological evolution of Sample 9 was very similar to that of Sample 4. Therefore, the growth mechanism was similar because they had similar NC concentrations. Moreover, the emission color of these assemblies was yellow, which is same as that of the hybrid SiO2 -coated CdTe NCs (Sample 3). These results differ from those for the assemblies obtained using hybrid NCs prepared through reflux. The PL wavelength of those assembles remained unchanged over time, as shown in Figs. 3 and 4, and this is ascribed to the small number of free TGA molecules in the solution. After reflux, the number of free TGA molecules in the solution was very low because of the heat treatment process. Therefore, almost no S2− ions were generated during assembly of the hybrid NCs at room temperature. The size of the CdS-like clusters close to the CdTe core remained unchanged during the self-assembly of the hybrid NCs. These observations showed that the self-assembly of hybrid NCs in sheaf-like morphology occurred together with the nucleation and growth of CdS-like clusters in the SiO2 shell and that the hybrid NCs prepared at room temperature exhibited the same self-assembly behavior as the hybrid SiO2 -coated CdTe NCs prepared by reflux. To illustrate the different mechanism of assemblies created by two kinds of NCs, Scheme 1 shows the formation process of hybrid NC-based heterostructures: (a) by hybrid SiO2 -coated CdTe NCs (after reflux) and (b) by CdTe NCs (after step 1). In the case of (a), the hybrid NCs were assembled into fibers without any change for their structure. Therefore, their PL properties remained unchange. In the case of (b), the red-shift of PL spectra occurs because of the hybrid structure created during assembly. Similar phenomenon was observed when luminescent fibers prepared using CdTe NCs by reflux [24]. 4. Conclusion Hybrid SiO2 -coated CdTe NCs were fabricated by a two-step synthesis (sol–gel process and a subsequent reflux with a short reaction time) or by a sol–gel procedure with a long reaction time at room temperature. The NCs self-assembled into heterostructures with different morphologies through the electrostatic force between the hybrid CdTe NC and the composite of Cd2+ ions, TGA molecules, and SiO2 monomers generated by the hydrolysis of TEOS at room temperature. The morphology of the resulting assemblies depended strongly on the concentration of hybrid NCs. At a higher concentration, the NCs prepared by the two-step synthesis self-assembled into a structure with a sheaf-like morphology because the large amount of starting materials resulted in crystal splitting during growth. At a lower concentration, the NCs self-assembled into beltlike fibers with little splitting. At even lower concentrations, the crystal splitting disappeared completely, and the NCs assembled into solid fibers or nanowires. At the lowest concentration, the
NCs assembled into fibers with a helical or tubal morphology. This assembly behavior was also observed for hybrid SiO2 -coated CdTe NCs prepared using the sol–gel process with a long reaction time at room temperature. A yellow-emitting structure with a sheaf-like morphology was created by storing green-emitting hybrid NCs prepared using the sol–gel process for 40 days. The strategy described here can serve as a guide for fabricating nanostructured materials with hierarchical order. Acknowledgements We are grateful to Drs. Tomoki Kato and Kazunori Kawasaki (AIST) for SEM observation. References [1] J. Chen, W. Liao, X. Chen, T. Yang, S.E. Wark, D.H. Son, J.D. Batteas, P.S. Cremer, ACS Nano 3 (2009) 173–180. [2] J.M. Luther, M. Law, Q. Song, C.L. Perkins, M.C. Beard, A.J. Nozik, ACS Nano 2 (2008) 271–280. [3] B.A. Ridley, B. Nivi, J.M. Jacobson, Science 286 (1999) 746–749. [4] D.V. Talapin, C.B. Murray, Science 310 (2005) 86–89. [5] G. Konstantatos, I. Howard, A. Fischer, S. Hoogland, J. Clifford, E. Klem, L. Levina, E.H. Sargent, http://www.nature.com/nature/ journal/v442/n7099/full/nature04855.htmlNature 442 (2006) 180–183. [6] C. Bertoni, D. Gallardo, S. Dunn, N. Gaponik, A. Eychmüller, Appl. Phys. Lett. 90 (2007) 034107-1–034107-3. [7] E.V. Shevchenko, D.V. Talapin, N.A. Kotov, S. O’Brien, Nature 439 (2006) 55–59. [8] W. Chae, S. Lee, M. An, K. Choi, S. Moon, W. Zin, J. Jung, Y. Kim, Chem. Mater. 17 (2005) 5651–5657. [9] P. Yang, C. Li, N. Murase, Langmuir 21 (2005) 8913–8917. [10] J. Ge, Y. Hu, T. Zhang, Y.J. Yin, J. Am. Chem. Soc. 129 (2007) 8974–8975. [11] N. Oh, J.H. Kim, C.S. Yoon, Adv. Mater. 20 (2008) 3404–3409. [12] Y. Lin, A. Böker, J. He, K. Sill, H. Xiang, C. Abetz, X. Li, J. Wang, T. Emrick, S. Long, Q. Wang, A. Balazs, T.P. Russell, http://www.nature.com/nature/journal/ v434/n7029/full/nature03310.htmlNature 434 (2005) 55–59. [13] F. Gao, Q. Lu, X. Meng, S. Komarneni, J. Phys. Chem. C 112 (2008) 13359–13365. [14] Z. Tang, N.A. Kotov, M. Giersig, Science 297 (2002) 237–240. [15] K.M. Gatta’s-Asfura, C.A. Constantine, M.J. Lynn, D.A. Thimann, X. Ji, R.M. Leblanc, J. Am. Chem. Soc. 127 (2005) 14640–14646. [16] Y. Shimazaki, M. Mitsuishi, S. Ito, M. Yamamoto, Langmuir 13 (1997) 1385–1387. [17] E. Brynda, M. Houska, J. Colloid Interface Sci. 183 (1996) 18–25. [18] J. Sun, T. Wu, Y. Sun, Z. Wang, Z. Xi, J. Shen, W. Cao, Chem. Commun. (1998) 1853–1854. [19] J. Anzai, Y. Kobayashi, N. Nakamura, M. Nishimura, T. Hoshi, Langmuir 15 (1999) 221–226. [20] P. Kralchevsky, K. Nagayama, Particles at fluid interfaces and membranes Attachment of Colloid Particles and Proteins to Interfaces and Formation of Two-Dimensional Arrays (Studies in Interface Science), vol. 10, Elsevier Science B. V., Amsterdam, 2001. [21] C. O’Dwyer, D. Navas, V. Lavayen, E. Benavent, M.A. Sanata Ana, G. González, S.B. Newcomb, C.M. Sotomayor Torres, Chem. Mater. 18 (2006) 3016–3022. [22] Y.S. Zhao, H. Fu, F. Hu, A. Peng, W.S. Yang, J. Yao, Adv. Mater. 20 (2008) 79–83. [23] M. Liusar, C. Sanchez, Chem. Mater. 20 (2008) 782–820. [24] P. Yang, M. Ando, N. Murase, Adv. Mater. 21 (2009) 4016–4019. [25] B. Law, R. Weissleder, C.H. Tung, Bioconjug. Chem. 18 (2007) 1701–1704. [26] N. Murase, P. Yang, Small 5 (2009) 800–803. [27] P. Yang, N. Murase, Adv. Funct. Mater. 20 (2010) 1258–1265. [28] C. Li, N. Murase, Chem. Lett. 34 (2005) 92–93. [29] N. Murase, C. Li, J. Lumin. 128 (2008) 1896–1903. [30] M. Grabolle, M. Spieles, V. Lesnyak, N. Gaponik, A. Eychmüller, U. Resch-Genger, Anal. Chem. 81 (2009) 6285–6294. [31] N. Gaponik, D.V. Talapin, A.L. Rogach, K. Hoppe, E.V. Shevchenko, A. Kornowski, A. Eychmüller, H. Weller, J. Phys. Chem. B 106 (2002) 7177–7185. [32] K. Liu, H. You, Y. Zheng, G. Jia, Y. Huang, M. Yang, Y. Song, L. Zhang, H. Zhang, Cryst. Growth. Des. 10 (2010) 16–19. [33] H. Deng, C. Liu, S. Yang, S. Xiao, Z. Zhou, Q. Wang, Cryst. Growth. Des. 8 (2008) 4432–4439. [34] H. Niu, M. Gao, Angew. Chem. Int. Ed. 45 (2006) 6462–6466.
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Hybrid SiO2 -coated nanocrystal-based heterostructures: Assembly, morphology transition, and photoluminescence at room temperature Ping Yang, Masanori Ando, Norio Murase ∗ Health Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Midorigaoka, Ikeda-city, Osaka 563-8577, Japan
a r t i c l e
i n f o
Article history: Received 12 January 2011 Received in revised form 13 March 2011 Accepted 5 April 2011 Available online 12 April 2011 Keywords: Nanocomposites Fibers Sol–gel methods Functional composite Morphology
a b s t r a c t Coating CdTe nanocrystals (NCs) with a hybrid SiO2 shell with CdS-like clusters drastically increased their photoluminescence (PL) efficiency and substantially red-shifted their PL peak. The coating was done either by a two-step synthesis including a sol-gel process and a subsequent reflux with a short reaction time or by a sol–gel procedure with a long reaction time at room temperature. When the solution of the coated NCs containing thiolglycolic acid (TGA), Cd2+ , and SiO2 monomers was left at room temperature, the coated NCs self-assembled into heterostructures. The self-assembly behavior of the NCs is ascribed to an electrostatic force that led to the attachment of the NCs to a composite of TGA, Cd2+ , and SiO2 monomers. The PL of the resulting assemblies depended strongly on the hybrid nanostructure, i.e., CdSlike clusters formed near the CdTe core. Experimental conditions such as the concentration of the hybrid NCs in solution and the growth time were adjusted to control the assembly morphology. When the NC concentration was sufficiently high, assemblies with a sheaf-like morphology were created at room temperature. Systematic reduction of the NC concentration resulted in the creation, in turn, of belt-like fibers, solid fibers, and long nanowires. When an aqueous solution with a low concentration of hybrid NCs was kept at room temperature for six months, the assemblies exhibited tubal and helical morphologies. This morphological evolution is ascribed to the concentration dependence of the growth kinetics. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor nanocrystals (NCs) can be exploited as building blocks for photonic, electronic, and magnetic devices as well as for use in sensing and optical applications [1]. NCs assembled into matrices are an important class of materials for field-effect transistors, photodetectors, light-emitting diodes, metamaterials, and solar cells [2–7] due to the advanced properties of the NCs. For example, silica nanofibers functionalized with CdS NCs exhibit interesting mesostructural transformations with simultaneous changes in gas adsorption behavior and CdS NC emissions [8]. The self-assembly of nanomaterials has attracted a great deal of attention, bridging various areas of science and engineering, that has resulted in the design and development of advanced materials, innovative methodologies, and general theories [9,10]. Various approaches have been used to assemble NCs in an orderly manner and to investigate the scope of potential applications. For example, template methods have been used to study the self-assembly of semiconductor NCs [11]. Various organic and bio-
∗ Corresponding author. Tel.: +81 72 751 9647; fax: +81 72 751 9637. E-mail address: [email protected] (N. Murase). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.04.002
chemical molecules have been used to drive the self-assembly of chemically produced semiconductor NCs. Previous efforts have concentrated on using such scaffolds to spatially arrange nanoelements as a strategy for tailoring the electrical, magnetic, or photonic properties of the material [12]. Although many strategies are promising, it is still necessary to develop straightforward and controllable methods for self-assembly of nanostructures, especially structures with unique properties. The assembly of NCs is usually driven by the interactions between the individual building blocks, so the control of the NC surface properties is an important factor in their assembly [13]. The simplest interacting NC system is composed of two nearby NCs. The main interactions between the NCs are electron (or hole) tunneling and Förster resonant energy transfer due to dipole–dipole interaction. These interactions cause the NCs to assemble into a one-dimensional (1D) nanowire in solution [14]. The self-assembly of NCs at a two-phase interface (gas–solid, liquid–solid, gas–liquid, or liquid–liquid) has been investigated, and NCs with a one- or two- dimensional nanostructure have been produced at a two-phase interface, with the interface acting as a template [15]. The formation force of assemblies produced by hydrogen bonding, charge transfer, covalent bonding, lateral capillary force, and hydrophobic interactions has been studied [16–20]. Self-assembly based on coordinative or hydrogen bonding
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interactions is a particularly useful method for creating superstructures because of its low cost, easy controllability, and simplicity. Organic nanowires and nanotubes have been assembled using noncovalent interactions, including hydrogen-bonding and intermolecular p–p stacking [21]. One of the major challenges in the self-assembly of NCs based on noncovalent interactions is clarifying the interaction between the NCs and the resulting nanostructure. Functional 1D nanomaterials have attracted much attention in the field of nanotechnologies because such materials hold great promise for advanced catalysis, electronics, sensors, energy storage/conversion, and photonic applications due to their unique morphology and their optical, magnetic, and electronic properties [22,23]. Luminescent CdTe NCs have been encapsulated within cadmium thiolate composite fibers with a tunable morphology and luminescence [24,25]. Thus, filling 1D nanomaterials with functional NCs is highly promising for nanotechnology applications. However, this potential self-assembly mechanism is still not well understood. Previously, hybrid SiO2 -coated CdTe NCs were prepared by using a simple reflux procedure [26,27]. The hybrid NCs revealed red-shifted PL spectra and high PL efficiency. In this paper, we found out such hybrid SiO2 -coated CdTe NCs can be created at room temperature with a long reaction time. In contrast, the red-shift on the PL spectra was small compared with that prepared by reflux. Further, we investigate the self-assembly of CdTe NCs at room temperature using Cd-TGA composites as a template. The assembly exhibited different morphology (sheef-like) and different PL properties compared with that created using CdTe NCs by reflux [24]. In addition, hybrid SiO2 -coated CdTe NCs can be assembled into various morphological heterostructures through a long sol–gel process at room temperature. However, the heterostructure did not exhibit any change on their PL spectra compared with the initial hybrid NCs. 2. Material and methods 2.1. Chemicals and materials All chemicals were obtained from Sigma–Aldrich and used as received. All chemicals were of analytical grade or of the highest purity available. The pure water was obtained from a Milli-Q synthesis system (resistivity of 18.2 M cm).
Fig. 1. Spectral and morphological properties of hybrid SiO2 -coated CdTe NCs (Sample 1, Table 1) with reflux time of 2.5 h: (a) absorption and PL spectra; (b) TEM image. Absorption and PL spectra of initial CdTe NCs are shown for comparison. Luminescent properties of NCs are summarized in Table 1.
a 0.2 m filter to remove the composite fibers that were created simultaneously during the reflux. 2.3. Self-assembly of hybrid SiO2 -coated CdTe NCs We investigated the concentration effect of these starting materials on the assembly morphology since the concentration of these starting materials determined the growth speed of fibers. The NC concentration was increased by using a 3000-MWCO-filter to condense the aqueous solution of hybrid SiO2 -coated CdTe NCs
2.2. Preparation of hybrid SiO2 -coated CdTe NCs TGA-capped CdTe NCs with green (d ∼ 2.6 nm) emission in aqueous solution were prepared using a procedure including the use of cadmium perchlorate and hydrogen telluride, as described elsewhere [28]. The hybrid SiO2 -coated CdTe NCs were prepared using a two-step synthesis process. In step 1, CdTe NCs were coated with a thin SiO2 layer by stirring them in an aqueous solution containing Cd2+ , TGA, CdTe NCs, TEOS, and NH3 . Briefly, CdTe colloidal solution (2 mL) was precipitated by 2-propanol and redispersed in a 2 mL aqueous solution of Cd2+ and TGA (pH ∼ 10, adjusted using an NaOH solution of 1 M). Diluted ammonia (0.1 mL, 6.25 wt.%) and TEOS (20 L) were mixed with the redispersed CdTe colloidal solution in a beaker sealed to reduce ammonia evaporation during incorporation. After being stirred for 3 h, the CdTe NCs were coated with a thin SiO2 layer containing Cd2+ and TGA. The solution then became homogenous and transparent. In step 2, a reflux process using this solution caused CdS-like clusters to nucleate and grow in the SiO2 shell. The reflux time was for 2.5 h. The TGA concentration and TGA/Cd2+ molar ratio of the solution were 0.01 M and 2, respectively. The resulting samples were finally filtered using
Fig. 2. Absorption and PL spectra of hybrid SiO2 -coated CdTe NCs (Asprepared = Sample 2; After 40 days = Sample 3). Peak was observed around 355 nm in absorption spectrum for Sample 3.
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Table 1 PL properties and preparation conditions of hybrid SiO2 -coated CdTe NCs. Sample
PL peak wavelength (nm)
FWHM (nm)
PL efficiency (%)
Preparation
1 2 3 CdTe NCsa
613 547 566 555
50 52 47 54
61 28 38 24
Reflux Room temperature Room temperature, 40-day storage Initial colloidal NCs
a
Properties of CdTe NCs are shown for comparison.
Table 2 Preparation parameters and final morphologies of assemblies with hybrid SiO2 -coated CdTe NCs. Sample
Concentration of NCs (×10−5 M)
Initial samplea
Growth time
Final morphology
4 5 6 7 8 9
2.01 0.97 0.49 0.24 0.12 2.43
1 1 1 1 1 2
7 days 1 month 1.5 months 1.5 months 6 months 40 days
Sheaf-like Belt-like Thick rod Thin rod Rod, tubal, belt-like Sheaf-like
a
Sample number shown in Table 1.
prepared by the two-step synthesis or by the sol–gel process. The TGA, Cd2+ ions, and NH3 in the solutions were partially removed by repeated condensation and dilution. Solutions with various concentrations of hybrid NCs were kept in glass bottles with covers at room temperature for different periods to enable us to investigate the morphological evolution during assembly. Finally, solution samples were removed from the bottles with pipettes and placed on glass slides for observation with a fluorescence microscope. 2.4. Apparatus Observations by TEM were carried out using an FEI Tecnai G2 F20 (200 kV) electron microscope. Morphological observation of assemblies was done using a field emission scanning electron microscope (Hitachi, S-5000). Photoluminescence and optical images of sample fibers were obtained with a fluorescent microscope (Nikon, Eclipse 80i). The absorption and PL spectra were recorded using conven-
tional spectrometers (Hitachi U-4000 and F-4500, respectively). Both of excitation and emission slits are 5 nm for the measurement of PL spectra. The size distributions of samples in solution were obtained using a dynamic light-scattering particle analyzer (Nanotrac, Nikkiso). The PL efficiencies of samples in solution were estimated using a method previously reported [29,30]. Briefly, the PL and absorption spectra of a standard quinine solution (quinine in 0.1 N H2 SO4 solution; PL efficiency 0 of 55%) were measured in a 1 cm quartz cell as a function of the concentration. The emission intensity (in units of the number of photons) is expressed as P0 ≈ K0 a0 10−0.5a0 , where a0 is absorbance at the excitation wavelength (365 nm), and K is the apparatus function. After measurement of the absorbance and PL intensity of the sample using the same apparatus parameters, the PL efficiency of the sample was derived by comparing its PL intensity with that of the standard quinine solution. The error in the PL efficiency was estimated to be within 10% by comparing the results using two standards: quinine and R6G.
Fig. 3. Photoluminescence images taken under 365 nm UV light showing evolution of hybrid SiO2 -coated CdTe NCs (2.01 × 10−5 M concentration) into assemblies with sheaf-like morphology (Sample 4, Table 2) after (a) one day, (b) two days, (c) four days, (d) seven days. Insets in (a) and (d) show fiber morphologies.
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Fig. 4. Photoluminescence images taken under 365 nm UV light showing evolution of hybrid SiO2 -coated CdTe NCs (9.7 × 10−6 M concentration) into assemblies with belt-like morphology (Sample 5, Table 2) after (a) two days, (b) seven days, (c) ten days, (d) one month. Insets show fiber morphologies.
3. Results and discussion 3.1. Evolution of PL properties of CdTe NCs coated with hybrid SiO2 shell A two-step synthesis including a sol–gel process (Step 1) and a subsequent reflux (Step 2) was used to create hybrid SiO2 -coated
CdTe NCs. Briefly, colloidally prepared green-emitting CdTe NCs (2.6 nm in diameter) were coated with a thin SiO2 layer by adapting the use of tetraethyl orthosilicate (TEOS), ammonia, Cd2+ , and thiolglycolic acid (TGA) to a sol–gel reaction. A subsequent reflux process including a further sol–gel reaction was then used to fabricate the hybrid NCs. During reflux, CdS-like clusters nucleated and grew in the SiO2 shell. At the same time, the SiO2 shell became
Fig. 5. Optical (left) and PL (right) images of hybrid SiO2 -coated CdTe NCs that assembled (after 1.5 months) into fibers with thick solid morphology [(a) and (b), Sample 6] and thin solid morphology [(c) and (d), Sample 7]. Insets in (b) and (d) show fiber morphologies.
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Fig. 6. SEM images of hybrid SiO2 -coated CdTe NCs assembled into morphology-tunable fibers (Samples 4–7, Table 2): (a) Sample 4 with sheaf-like morphology,(b) Sample 4 at high magnification; (c) Sample 5 with belt-like morphology, (d) Sample 6 with solid morphology, (e) Sample 6 at high magnification, (f) Sample 7 with long nanowire morphology.
thicker due to the deposition of SiO2 monomers. The procedure used to prepare the hybrid SiO2 -coated CdTe NCs is described in more detail in Section 2. Fig. 1 shows the absorption and PL spectra and a transmission electron microscopy (TEM) image of hybrid SiO2 -coated CdTe NCs (Sample 1 in Table 1) prepared using the two-step synthesis with a reflux time of 2.5 h. After reflux (Step 2), the hybrid SiO2 coated CdTe NCs were ∼6 nm in diameter (from TEM observation) while the initial CdTe NCs were ∼2.6 nm in diameter. Compared with the initial CdTe NCs, the hybrid SiO2 -coated ones exhibited a red-shifted PL peak wavelength (555 nm for initial NCs, 613 nm for hybrid NCs) and higher PL efficiency (24% for initial NCs, 61% for hybrid NCs). These changes are ascribed to the CdS-like clusters in the SiO2 shell that formed close to the CdTe core. This is supported by the observation of an absorption shoulder around 380 nm for the hybrid NCs (Fig. 1a). The preparation-condition dependence of the PL properties, including the PL peak wavelength, the full-width at half maximum (FWHM) of the PL spectra, the lifetimes, and PL efficiency, of hybrid SiO2 -coated CdTe NCs are described in detail elsewhere [27]. We also used a sol–gel procedure with a slow reaction to create hybrid SiO2 -coated CdTe NCs at room temperature. The CdS-like clusters formed in the SiO2 shell through the reaction of Cd2+ ions in solution and S2− ions generated by the decomposition of free TGA molecules (not connected to Cd2+ ions) in solution at room temperature. When the precursor solution prepared using the sol–gel process was stored at room temperature for a long time (ca. 40 days) at room temperature, CdS-like clusters nucleated and grew in the
SiO2 shell through a slow sol–gel reaction. The thickness of the SiO2 shell increased with the storage time due to partially hydrolyzed TEOS being retained in the solution. Fig. 2 shows the absorption and PL spectra of hybrid SiO2 -coated CdTe NCs for Sample 2 (prepared using the sol–gel process) and Sample 3 (prepared using the sol–gel process and then stored for 40 days after partially removing the NH3 from the solution) (see Table 1). Sample 3 had a peak around 355 nm in the absorption spectrum, indicating that CdS-like clusters formed during the slow sol–gel reaction. Due to the formation of CdS-like clusters in the SiO2 shell, a red-shifted PL peak wavelength (from green to yellow) and increased PL efficiency (from 28 to 38%) for Sample 3 were observed. The PL properties and preparation conditions of the hybrid SiO2 -coated CdTe NCs (Samples 1–3) are summarized in Table 1. Dynamic light scattering measurement showed that Samples 2 and 3 were respectively 3.9 ± 0.9 and 4.6 ± 1.1 nm in diameter. The smaller red shift of the PL peak wavelength of the hybrid NCs prepared at room temperature means the reaction that formed the hybrid nanostructure near the CdTe core was slow. 3.2. Self-assembly of hybrid SiO2 -coated CdTe NCs into different morphologies Previous studies have demonstrated that some mercaptocarboxylic acids can form complicated complexes with cadmium ions, with primary coordination of cadmium ions to the thiol groups and a secondary coordination to the carboxylic groups. This dual coor-
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Fig. 7. SEM (a–e) and PL (f) images at different magnifications of hybrid SiO2 -coated CdTe NCs assembled into fibers (Sample 8). White dots in SEM images are hybrid SiO2 -coated NCs.
dination affects the formation of 1D CdTe nanowires [31]. In the current experiment, the molar ratio of TGA to Cd2+ in solution was 2, so the Cd-TGA complex contained one or two ligands. The carboxyl group could link to another Cd-TGA chain-like structure through the electrostatic adsorption between Cd2+ and carboxyl groups. The connection of several Cd-TGA complexes resulted in the formation of longer Cd-TGA clusters. These clusters grew into nanowires, which acted as seeds for the growth of morphology-tunable assemblies. Namely, the SiO2 monomers generated by the hydrolysis of TEOS were deposited on the clusters through the –OH groups on their surface. The hybrid SiO2 -coated NCs could easily attach to the nanowire formed by the Cd-TGA clusters because the hybrid SiO2 shells contained –OH groups. Subsequently, the nanowire grew into assemblies with different morphologies because of the deposition of SiO2 monomers, Cd-TGA complex, and the hybrid NCs. The growth of assemblies with hybrid NCs at room temperature differed from that of fibers prepared by reflux. This growth was driven by the slow evaporation of solvent when a precursor solution with hybrid NCs was kept in a bottle with a cover at room temperature. According to our previous work, the concentration of starting materials plays an important role in determining the morphology of fibers created by reflux. In the experiment reported here, we systematically investigated the concentration dependence of the morphology by changing the experimental conditions during growth at room temperature. Aqueous solutions of hybrid SiO2 coated CdTe NCs (Sample 1) were condensed or diluted into target concentrations. The preparation conditions and final morphologies of the assemblies are summarized in Table 2. We experimentally
observed the evolution in the morphology of assembly during growth. Fig. 3 shows the evolution of hybrid SiO2 -coated CdTe NCs (Sample 4; NC concentration of 2.01 × 10−5 M) into assemblies after one, two, four, and seven days. The PL images reveal bright red-emission, and the PL spectra remained unchanged over time. The morphology was bayberry-like after one day. It became larger after two days. The morphology was sheaf-like after four days. It became larger after seven days. Careful examination of a typical sheaf-like morphology image revealed that each sheaf was composed of many compressed nanobelts (as shown later). Although the formation of 1D fibers is generally ascribed to anisotropic aggregation behaviors, a high starting-material concentration results in more crystal splitting, which leads to a sheaf-like morphology [32,33]. The morphology shown in Fig. 3 is attributed to both crystal nucleation and growth and to crystal splitting. When the concentration of the hybrid SiO2 -coated CdTe NCs was reduced, the morphology changed: the assemblies became larger and there was less splitting, as shown in Fig. 4. After two days, the NCs had assembled into a pearl-chain morphology, as shown in Fig. 4a. After seven days, they had a nanowire morphology, as shown in Fig. 4b. The morphology was straw-like after ten days, as shown in Fig. 4c. Finally, after one month, fibers with a belt-like morphology were observed, as shown in Fig. 4d. The formation of crystals with nonthermodynamic equilibrium shapes is driven kinetically. The formation includes the initial formation of nuclei immediately after supersaturation and subsequent growth of the nuclei. The formation of such hierarchical structures requires fast crystal growth. Reducing the concentration of hybrid
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To investigate the NC concentration dependence of morphology of assembly, we further reduced the NC concentration in solution. Fig. 7 shows the SEM and PL images of Sample 8, the sample with the lowest concentration (1.2 × 10−6 M) after storage at room temperature for six months. As expected, belt-like, tubal, helical, and solid morphologies were observed. The observation of several morphologies in one sample means that the fiber morphology depends strongly on the growth process as well as the NC concentration. Fig. 7b shows a typical tubal morphology, and Fig. 7c shows a typical helical morphology. These morphologies are ascribed to the nucleation and subsequent growth kinetics although the mechanism is not clear. The reason for the difference in morphologies is uncertain, but the difference could be due to anisotropic aggregation in different directions. The fibers were created using Cd-TGA complexes as a scaffold, and the connection of the complexes resulted in the formation of longer Cd-TGA clusters. These clusters grew into a structure with a two-dimensional sheet-like morphology. At a low NC concentration, the flat sheets tended to roll so as to reduce the surface energy in water. The rolling of flat sheets into curled ones reduces both the surface energy and the tension caused by the asymmetry of the sheet. The curled sheets seamed into tubes due to ring-closure of the curled chains through the formation of new electrostatic connections between the Cd-TGA chain-like structures at the two edges. When the concentration of hybrid NCs was high, the hollow spaces were quickly filled by the hybrid NCs and SiO2 monomers. Therefore, a hollow morphology was not observed for Samples 4–7. The helical morphology shown in Fig. 7c is ascribed to 1D CdTGA chains because a similar helical morphology was observed in a Cd-TGA nanobelt [34]. However, at a high concentration, hybrid
Fig. 8. Photoluminescence images taken under 365 nm UV light showing evolution of hybrid SiO2 -coated CdTe NCs prepared by sol–gel process into assemblies with sheaf-like morphology (Sample 9, Table 2): (a) after 2 days, (b) after 40 days.
SiO2 -coated CdTe NCs reduces the growth rate, which reduces the rate of crystal splitting. Therefore, by tuning the concentration of hybrid NCs, we were able to prepare structures with either a sheaf-like or belt-like morphology with little crystal splitting. The splitting disappeared completely when the concentration of hybrid SiO2 -coated CdTe NCs was reduced sufficiently due to the concentration effect of crystal growth kinetics. Fig. 5 shows optical and PL images after 1.5 months of fibers prepared using a lower concentration of hybrid SiO2 -coated CdTe NCs (4.9 × 10−6 M, Sample 6; 2.4 × 10−6 M, Sample 7) than for Sample 5 (9.7 × 10−6 M). Crystal splitting during growth was not evident for Samples 6 and 7. Solid fibers (Sample 6, Fig. 5a) with high PL, and nanowires (Sample 7, Fig. 5c) were observed. The growth speeds of these samples were substantially lower than those of Samples 4 and 5. The precise morphologies of Samples 4–7 were determined using scanning electron microscopy (SEM) imaging. The images in Fig. 6a and b clearly show that Sample 4 had a sheaf-like morphology and that each sheaf consisted of compressed nanobelts. Moreover, the flat nanobelts had widths of ∼150 nm and lengths of ∼5 m. The image in Fig. 6c shows that Sample 5 had a belt-like morphology with little splitting. The homogeneity of color image of this sample indicates that the hybrid NCs were aligned in the fibers. The images in Fig. 6d and e show that Sample 6 had a thick solid morphology. The image in Fig. 6f shows that Sample 7 had a thin solid morphology. These results indicate that the concentration of hybrid NCs in the solution is the key to controlling the morphology of the assembly.
Scheme 1. Formation process of hybrid NC-based heterostructures. (a), By hybrid SiO2 -coated CdTe NCs (after reflux); (b), By CdTe NCs (after step 1).
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SiO2 -coated CdTe NCs cross-linked the 1D Cd-TGA chains in perpendicular directions by introducing additional Cd2+ ions and SiO2 monomers into the gaps of the chains. Consequently, the helical structure formed by pure Cd-TGA complexes was lost. For comparison, we observed the self-assembly of hybrid SiO2 coated CdTe NCs prepared at room temperature (Step 1). Fig. 8 shows the morphological evolution of the NCs into fibers (Sample 9; hybrid NC concentration of 2.4 × 10−5 M). After two days, a small bayberry-like morphology was observed, as shown in Fig. 8a. After one month, a yellow-emitting sheaf-like morphology was observed, as shown in Fig. 8b. The morphological evolution of Sample 9 was very similar to that of Sample 4. Therefore, the growth mechanism was similar because they had similar NC concentrations. Moreover, the emission color of these assemblies was yellow, which is same as that of the hybrid SiO2 -coated CdTe NCs (Sample 3). These results differ from those for the assemblies obtained using hybrid NCs prepared through reflux. The PL wavelength of those assembles remained unchanged over time, as shown in Figs. 3 and 4, and this is ascribed to the small number of free TGA molecules in the solution. After reflux, the number of free TGA molecules in the solution was very low because of the heat treatment process. Therefore, almost no S2− ions were generated during assembly of the hybrid NCs at room temperature. The size of the CdS-like clusters close to the CdTe core remained unchanged during the self-assembly of the hybrid NCs. These observations showed that the self-assembly of hybrid NCs in sheaf-like morphology occurred together with the nucleation and growth of CdS-like clusters in the SiO2 shell and that the hybrid NCs prepared at room temperature exhibited the same self-assembly behavior as the hybrid SiO2 -coated CdTe NCs prepared by reflux. To illustrate the different mechanism of assemblies created by two kinds of NCs, Scheme 1 shows the formation process of hybrid NC-based heterostructures: (a) by hybrid SiO2 -coated CdTe NCs (after reflux) and (b) by CdTe NCs (after step 1). In the case of (a), the hybrid NCs were assembled into fibers without any change for their structure. Therefore, their PL properties remained unchange. In the case of (b), the red-shift of PL spectra occurs because of the hybrid structure created during assembly. Similar phenomenon was observed when luminescent fibers prepared using CdTe NCs by reflux [24]. 4. Conclusion Hybrid SiO2 -coated CdTe NCs were fabricated by a two-step synthesis (sol–gel process and a subsequent reflux with a short reaction time) or by a sol–gel procedure with a long reaction time at room temperature. The NCs self-assembled into heterostructures with different morphologies through the electrostatic force between the hybrid CdTe NC and the composite of Cd2+ ions, TGA molecules, and SiO2 monomers generated by the hydrolysis of TEOS at room temperature. The morphology of the resulting assemblies depended strongly on the concentration of hybrid NCs. At a higher concentration, the NCs prepared by the two-step synthesis self-assembled into a structure with a sheaf-like morphology because the large amount of starting materials resulted in crystal splitting during growth. At a lower concentration, the NCs self-assembled into beltlike fibers with little splitting. At even lower concentrations, the crystal splitting disappeared completely, and the NCs assembled into solid fibers or nanowires. At the lowest concentration, the
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