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SiO2 monomer-triggered self-assembly of hybrid CdTe quantum dots
Accepted Manuscript Title: SiO2 Monomer-Triggered Self-Assembly of Hybrid CdTe Quantum Dots Authors: Yue Zhao, Simin Lu, Katarzyna Matras-Postolek, Ping Yang PII: DOI: Reference:
S0255-2701(16)30395-6 http://dx.doi.org/doi:10.1016/j.cep.2017.02.009 CEP 6929
To appear in:
Chemical Engineering and Processing
Received date: Revised date: Accepted date:
13-9-2016 14-1-2017 21-2-2017
Please cite this article as: Yue Zhao, Simin Lu, Katarzyna Matras-Postolek, Ping Yang, SiO2 Monomer-Triggered Self-Assembly of Hybrid CdTe Quantum Dots, Chemical Engineering and Processing http://dx.doi.org/10.1016/j.cep.2017.02.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
SiO2 Monomer-Triggered Self-Assembly of Hybrid CdTe Quantum Dots Yue Zhao,a Simin Lu,a Katarzyna Matras-Postolek,b Ping Yanga* a
School of Material Science and Engineering, University of Jinan, Jinan, 250022, China.
b
Faculty of Chemical Engineering and Technology, Cracow University of Technology,
Krakow, 31-155, Poland *Corresponding Author has to be addressed. E-mail: [email protected] (P. Yang).
1
Graphic Abstract
Highlight 1. SiO2 monomer-triggered self-assembly makes hybrid CdTe QDs into various nanostructures. 2. The assemblies retained high photoluminescence. 3. The hybrid QDs retained high stability and assembled in PBS solutions.
Abstract The self-assembly activity of quantum dots (QDs) including thioglycolic acid (TGA)-capped CdTe and hybrid SiO2-coated CdTe QDs into one- to three-dimensional (3D) nanostructures was reported using a slow growth process. SiO2 monomers from a sol-gel process and TGA play vital roles for the self-assembly. The hybrid SiO2-coated QDs were assembled into 1D nanotube, 2D nanosheets, and 3D nanoflowers by adjusting QD concentrations. It is found that the assemblies actually are CdTe@Cd-TGA-SiO2 complex hybrid nanostructures in which many well-separated CdTe QDs are uniformly distributed. Such assembly also occurred in an initial CdTe QD solution or in PBS and NaCl solutions. The assembly in the initial CdTe QD solution is accompanied by a hybrid SiO2 layer which 2
resulted in red-shifted photoluminescence (PL) from green to yellow. Because of the domain growth of NaCl to form fractal structures through tip splitting and side branching dynamics, the hybrid SiO2-coated QDs were assembled into leaf morphology. As a result, the CdTe QDs with their in effective protection by hybrid Cd-TGA-SiO2 complex, these nanostructures show visually bright PL and retain the size-quantized properties of the QDs. These QD-based assemblies may be suitable for subsequent processing into quantum-confined materials and devices. Keywords：QDs, SiO2, Hybrid, Assembly, CdTe
Introduction In recent years, the self-assembly of nanoparticles as an important fabrication method in nanochemistry and nanotechnology has been bridging various areas of science and engineering and led to the design and development of advanced materials, innovative methodologies, and general theories [1, 2]. It shows great potential for directing the organization of nanoparticles and opens new avenues of nanotechnology through controlling the fabrication of one- to three-dimensional (3D) nanostructures with unique properties. The self-assembly of semiconductor quantum dots (QDs) has attracted a great deal of attention because assembled spontaneously could create new functional elements for the application of devices [3-5]. The QDs can be exploited as building blocks for electronic, photonic, and magnetic devices as well as for use in sensing and optical applications through self-assembling. The assembly of the QDs could bridge the gap between nanometer scale building blocks and micrometer scale patterning processes. Therefore, the study of QD self-assembly has been attractive in various application fields. Several techniques have been developed to assemble QDs in an orderly manner and to investigate their potential applications. Combining low-cost methods for making superstructures containing QDs with standard patterning techniques are now leveraged 3
for the application of nanomaterials. For example, template methods have been used to study the self-assembly of semiconductor QDs [6]. Previous efforts have concentrated on using such scaffolds to spatially arrange nanoelements as a strategy to tailor their properties [7]. Nanowire, nanosheets, nanochains, 2D networks, and superlattices have been fabricated in solutions or at interfaces by intermolecular or interparticle interaction force fields [8-13]. Although strategies are promising, it is still expected to develop straightforward and controllable methods for self-assembly of the QDs, especially, simplicity, versatility, and low cost. Because QD cores dictate their properties, whereas the surface-bound ligands define the interactions among QDs with their surroundings, ligand-stabilized colloidal QDs are ideally suited to hierarchically self-assemble [14]. To create QD nanostructures with different dimensions and shapes and containing both fused and nonfused nanocrystals, the self-assembly triggered by removing some or all of the ligand has been used. The assembly of QDs is usually driven by the interactions between the individual building blocks, thus, the control of the QD surface properties plays an important role for their assembly [15]. For example, basic units with sheet morphology were assembled into 6-membered rings with QDs interacted in different orientations [16]. The authors found that the dipole moment and small positive charge, both of which are due to the anisotropy of the QDs and the directional hydrophobic attraction drove self-organization. Thus, the QD self-assembly mechanism is affected by various factors which provide opportunities to explore the design and fabrication of advanced materials. Thioglycolic acid (TGA)-capped CdTe is an ideal system for the assembly because of the strong anisotropic properties of nanocrystals or quantum dots (QDs) [17]. The assembly of CdTe QDs occurred because of the interaction of particles. In fact, partially removing the TGA ligand from the surface caused the QDs to aggregate spontaneously in water. 1D CdTe QD nanowires was grew parallel to the (001) direction of the lattice. For low surface charge and strong face-to-face attraction, the QDs form 2D sheets and for much high surface charge, the long-range electrostatic 4
repulsion makes a 2D sheet unfavorable, and instead the QDs assembled into ribbons [16]. Several kinds of techniques have been reported to assemble CdTe QDs. One of the pioneering works by Kotov’s group indicated self-assembly of CdTe QDs into freely suspended nanowires under tightly controlled experimental conditions [8]. Namely, these nanowires just existed in solutions. Little literature reported on stable assembly after solutions. It is still desirable to research a method to create highly stable QD assemblies. Because of unique luminescence and absorption properties, aqueous CdTe QDs are presumably the most studied nanomaterials in last several ten years for applications [18]. Since the QDs are sensitive for their surroundings, retaining initial photoluminescence (PL) properties in QD assembly is important for general theory study and applications. After assembly, the properties of QDs could be changed. The ligand controlling is a key factor for the assembly. For example, hydrazine removed TGA from the surface of CdTe QDs allowing dipole−dipole interactions to form aggregates that organized into wires [17]. QD twisted nanoribbons were formed by starting the synthesis with a relatively low ligand to Cd ratio rather than removing ligands [19]. As a consequence, approaches that enable facilely highly luminescent QD assembly into functional mesoscopic architectures and nanoscale devices that can be harnessed in the emerging fields of optoelectronics and nanobiotechnology have still garnered significant attention [16]. In this paper, initial and hybrid SiO2-coated CdTe QDs were used to investigate the self-assembly. A Cd-TGA-SiO2 hybrid layer was used to reduce the ligand surface density, producing particles that assemble either anisotropically into 1D nanotubes and 2D flat sheets, or isotropically into 3D flowers. In this case, the QDs revealed high PL efficiency and stability. Compared with the reports referenced previously, our results have obvious advantages. The QDs still retain their high luminescence and stabile in the self-assembly process. The self-assembly occurred at room temperature in different solution including water, a PBS buffer solution and a NaCl solution. The detailed
5
process and mechanism of the assembly were discussed. Current method may be utilizable for the fabrication of other heterostructures and applications.
Experimental Chemicals. Chemicals were obtained from Sinopharm Chemical Reagent Company, Tianjin Chemical Reagent Institute, and 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. The phosphate-buffered saline (PBS, 10X) buffer solutions were prepared in our laboratory. Preparation of QDs. TGA-capped CdTe QDs with green emitting (average d = 2.7 nm) in aqueous solution were prepared using a procedure including the use of cadmium perchlorate and hydrogen telluride, as described in a previous paper [20，21]. Hybrid SiO2-coated CdTe QDs were prepared using a two-step synthesis process as reported in our papers [22, 23]. Firstly, CdTe QDs were coated with a thin SiO2 layer using a sol-gel process by stirring them in an aqueous solution containing Cd2+, TGA, CdTe QDs, tetraethyl orthosilicate (TEOS), and ammonia for 3h. Secondly, a reflux process using this solution caused CdS-like clusters to nucleate and grow within the SiO2 shell. The resulting samples were deal with for further characteristics and self-assembly. Self-assembly of QDs. All self-assembly processes were carried out at room temperature. Because the concentration of QDs is vital for the morphology, the QD concentration was controlled by using a 3000-MWCO-filter to condense the aqueous solution of initial and hybrid SiO2-coated CdTe QDs. The TGA, Cd2+ ions, and NH3 in the solutions were partially removed by repeated condensation and dilution.
Because a reflux did not change the
number of QDs, the sizes and concentrations of CdTe QDs were estimated by the absorbance of CdTe QDs using a known procedure in the literature as bellow [17]. 6
The absorbance (A) of CdTe QD suspension was measured as a function of wavelength of light using UV-visible spectroscopy. According to Beer-Lambert’s law, the absorbance of a solution is directly proportional to the concentration of the solution (c) and the path length traversed by the incident beam of light (L) [17]. A = εcL where, ɛ is the extinction coefficient which scales with the particle diameter. The particle diameter (d in nm) and concentration (c in M) were calculated from the below two equations [17]. d = (9.8127×10-7)ελEP3 - (1.7147×10-3)λEP2 + (1.0064)λEP - 194.84 λEP = 3450 × d2.4 × c × L where, λEP (nm) is the wavelength of the first excitonic absorption peak and AEP is the absorbance at λEP. To carry out the self-assembly process, solutions with different QD concentrations were kept in glass bottles with covers at room temperature for different periods (several days to months) to enable us to investigate the morphological evolution during assembly. Finally, resulting samples were taken out from the bottles with pipettes and placed on glass slides for observation with a fluorescence microscope. Table 1 illustrates the preparation conditions and properties of assembly samples. The reaction time was selected to be sure no further evolution after this reaction time. Characterization. Morphology observations by transmission electron microscopy (TEM) were carried out using an FEI Tecnai G2 F20 electron microscope. The high resolution transmission electron microscopy (HRTEM) observation was made on a Tecnai T20 microscope operating at 200 kV (FEI). Morphological observation of assemblies was also done using a field emission scanning electron microscope (SEM, Hitachi, S-5000). The absorption and PL spectra of solution samples were taken using conventional spectrometers Hitachi U-4100 and F-4600, respectively. The PL and optical images of samples were obtained with an Olympus IX 71 fluorescence microscope (Olympus Optical Co.). The PL efficiency of QD samples was
7
calculated by comparison with a standard Quine solution with same light path as indicated in our previous papers [22, 23].
Results and discussion Preparation and properties of QDs. Fig. 1 shows the PL and adsorption spectra of initial and hybrid SiO 2-coated CdTe QDs. TGA-capped CdTe QDs revealed a PL peak wavelength of 551nm, the full-width at half maximum (FWHM) of PL spectrum of 50 m, an average diameter of 2.7 nm, and a PL efficiency of 18%. For the preparation of hybrid SiO 2-coated CdTe QDs, colloidally prepared green-emitting CdTe QDs were coated with a thin SiO2 layer by adding TEOS in an alkaline CdTe colloidal solution with Cd 2+ ions and TGA. Because of CdTe QDs have a Cd-rich surface, TGA was attached with the Cd2+ ions. Because of SiO2 monomers generated via a sol-gel process connected with TGA-Cd composite, no ligand exchange occurred during the growth of SiO2 layer. In addition, the Cd2+ ions and TGA were dispersed in the SiO2 layer. A subsequent reflux process including a sol-gel reaction was then used to form hybrid QDs. The hybrid QDs revealed red-shifted PL and absorption spectra (a PL peak wavelength of 624nm), decreased FWHM of PL spectrum (46 nm), increased size (5.8 nm in diameter according to TEM observation) and increased PL efficiency of 62%.
The PL properties shown in Fig. 1 indicate that the formation of hybrid SiO2-coated CdTe QDs is really different from traditional semiconductor QDs. The spectral change in the hybrid SiO2-coated QDs compared with the spectra of initial CdTe QDs was due to the formation of small CdS-like clusters (1 nm or less in size) in the vicinity of the CdTe QDs in the hybrid SiO2 shell during reflux [21]. These CdS-like clusters could form a core-shell-like semiconductor structure with CdTe cores. During reflux, CdS-like clusters nucleated and grew in the SiO2 shell. The S2- ions were generated from the decomposition of free TGA in the solution with heat treatment. At the same time, the SiO2 shell became thicker due to the SiO2 monomers generated from the 8
hydrolysis and condensation of TEOS. The clusters can passivate the surface of the cores instead of TGA ligands. The role of the clusters is similar with a semiconductor shell on CdTe cores. Because this core-shell-like semiconductor structure was fixed within a hybrid SiO2 shell, the hybrid QDs became stable compared with traditional semiconductor core-shell QDs. The PL spectra of the hybrid QDs is different from other semiconductor core-shell QDs because their PL peak wavelengths just depended on the size of CdS-like clusters rather than the thickness of the hybrid SiO 2 shell. These clusters were incorporated in the shell during reflux because of ion diffusion. Since Cd2+ and TGA form dimers [23], the number of S2- ions in the solution is related to the concentration of TGA and Cd2+ in the solution and the reflux time. Therefore, the PL properties of the hybrid QDs depended strongly on the concentration and molar ratio of TGA and Cd2+ in the solution which affect the size of the CdS-like clusters in the hybrid SiO2 shell. Self-assembly of hybrid SiO2-coated CdTe QDs. Fig. 2 shows the images of CdTe@Cd-TGA-SiO2 complex nanotubes prepared via the self-assembly of hybrid SiO2-coated CdTe QDs in a CdTe QD concentration of 1.2 10-5 M in an aqueous solution for 3 months, (a & d) SEM images, (b) color image under 365 UV light irradiation, and (c) TEM image. The TEM image in Fig. 2d clearly shows tubular morphology. The tubes have an average outside diameter of 500 nm and inside diameter of 150 nm. After assembling into nanotubes, QDs still retained bright red-emitting as shown in Fig. 2b, Their PL spectra remained unchanged compared with their initial one. The TEM image of the nanotubes in Fig. 2c shows the QDs homogeneously distributed in the nanotubes. To investigate the effect of concentration on the morphology, the QD concentration in solutions was decreased to 6 10-6 M. Fig. 3 shows the TEM images of CdTe@Cd-TGA-SiO2 complex nanosheets prepared via the self-assembly of hybrid QDs with a CdTe QD concentration of 6 10-6 M in an aqueous solution for 3 months, (a) with low magnification and (b) with high magnification. The inset in (b) shows the image of a single QD with well-developed lattice fringes. The typical nanosheet 9
structure was formed shown in Fig. 3a and the QDs distributed homogeneously in the sheet as shown in Fig. 3b. Compared with the CdTe@Cd-TGA-SiO2 complex nanotubes shown in Fig. 2, CdTe@Cd-TGA-SiO2 complex nanosheets were created in a low QD concentration. This confirms the QD concentration is vital for the morphology of the assembly. We discuss the reason bellow. TGA is commonly used as a capping agent for the synthesis of hydrophilic CdTe QDs [24]. It is well known, the formation of TGA and Cd2+ composition depended on the ratio of TGA to Cd2+ in solutions [25]. The formation of a Cd-thiol layer on the surface of aqueous CdTe QDs effectively passivates the surface, resulting in high PL efficiency. In an aqueous solution, Cd2+ ions could link with the mercapto group in the TGA and form a complex. Eychmüller’s group reported that the composition of the Cd-TGA complex depends on the initial concentrations of Cd2+ and TGA in the water and that the concentrations of three types of Cd-TGA complexes (Cd2+ connected with 1, 2, and 3 ligands, respectively) depend on the pH of the solution [25]. In current experiments with an alkaline condition, the TGA/Cd2+ molar ratio in the solution is 1.5. Thus, Cd-TGA complexes formed in solutions with 1 to 2 ligands in the solution. The carboxyl group could link to another Cd-TGA chain-like structure through the electrostatic adsorption between Cd2+ and carboxyl groups. The overlap of Cd-TGA complexes resulted in the formation of longer Cd-TGA clusters. These longer clusters grew into nanowires, which acted as seeds for the growth of morphology-tuneable assemblies. Furthermore, the SiO2 monomers generated from the hydrolysis of TEOS were deposited on the clusters through the –OH groups on their surface. Thin SiO2 layer coated QDs could easily attach to the nanowire formed from 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 QDs. This means that the assembly derived from SiO2 monomers. For a self-assembly process, the nanowires grown from Cd-TGA clusters were taken as a soft template to grow CdTe@Cd-TGA-SiO2 complex assemblies with 10
various morphologies. The attachment of hybrid SiO2-coated QDs to the nanowire is ascribed to the hybrid SiO2 shell with Cd2+ and TGA complex. This attachment was carried out either via the connection of ≡Si–OH+Cd–TGA or via Cd–TGA complex within the hybrid SiO2 shell [26, 27]. In the low QD concentration, the nanowire was grown into a 2D sheet-like morphology because of low surface charge and strong face-to-face attraction. In the case of the amount of building blocks is not enough, the nanosheets were created as shown in Fig. 3. With increasing the amount of building blocks, increased surface charge and the long-range electrostatic repulsion make the flat sheet with a tendency to roll in order to reduce the surface energy in water. The rolling of the flat sheet into a curled one is favorable. Finally, the curled sheet will seam into nanotubes by ring-closure of the curled chains through the formation of new hydrogen bonding between the Cd–TGA chain-like structures at the two edges [27]. Thus, the CdTe@Cd-TGA-SiO2 complex nanotubes formed as shown in Fig. 2. A similar mechanism has been used to explain the formation of microtubes by using small organic molecules [28]. If the amount of building blocks, solid fibers and flower-like assembly will be created because of the fast filling of the hybrid SiO2-coated QD, SiO2 monomers, Cd-TGA complex. The detail will be explained as follows. In a high QD concentration, we prepared a flower-like CdTe@Cd-TGA-SiO2 complex assembly constructed of solid nanofibers. The hybrid SiO 2-coated CdTe QDs were re-dispersed in a TGA and Cd2+ solution with a concentration of 2.4 10-4 M. The solution was kept in a bottle with a cover for 7 days. Fig. 4 shows the color (a) and TEM (b) images of CdTe@Cd-TGA-SiO2 complex flowers prepared via the self-assembly of hybrid QDs in an aqueous solution. The average size of flower assembly is about 2-6 μm.
The average diameter of the solid fiber in the flower
assembly is about 35 nm. Same with the CdTe@Cd-TGA-SiO2 complex nanotubes and nanosheets, the PL peak wavelength remained unchanged. Except for forming 1D nanostructure, mercapto carboxylic acids can form complicated complexes with cadmium ions, with primary coordination of cadmium 11
ions to the thiol groups and a secondary coordination to the carboxylic groups. 29 The formation of crystals with nonthermo dynamic equilibrium shapes is driven kinetically while the formation of 1D fibers is generally ascribed to anisotropic aggregation behaviour [30]. For a high starting-material concentration, crystal splitting resulted in hierarchical structures requires fast crystal growth. Because of a high QD concentration together with much more Cd2+ ions, TGA, and SiO2 monomers, the initial formation of nuclei immediately after supersaturation and subsequent growth of the nuclei. Subsequently quick growth of the nuclei led to crystal splitting. Further growth resulted in the formation of a flower-like assembly constructed of fibers. Because of the amount of starting materials, the fibers are solid as shown in Fig. 4. The hybrid QDs were prepared using a two-step synthesis. In the first step, CdTe QDs were coated with a thin SiO2 shell via a sol-gel process in a TGA and Cd2+ solution. Because such SiO2 shell has similar structure and composition with finally one (after reflux), we investigated their self-assembly in an aqueous solution solution for comparison. Fig. 5 shows the microscopy images of CdTe@Cd-TGA-SiO2 complex sheaf-like assembly prepared via the self-assembly of CdTe QDs coated with a thin SiO2 shell by step 1 with a CdTe QD concentration of 2.4 10-4 M in a PBS buffer solution, (a) color images under 365 UV light irradiation and (b) image under bright field. This sample was prepared using a solution after step 1. The concentration of CdTe QDs was increased into 10-4 and kept at room temperature for 40 days (with covered). We observed the sheaf-like morphology as shown in Fig. 5. The insert in in Fig. 5b shows TEM image of a flower. The
self-assembly
mechanism
of
these
QDs
is
very
similarly
to
CdTe@Cd-TGA-SiO2 complex flowers shown in Fig. 4. Because of TGA and Cd2+ ions in the solution, we are sure the morphology is similar with CdTe@Cd-TGA-SiO2 complex flowers assembly. However, the average increased because of a slow growth rate. The PL color became into yellow. This is ascribed to the formation of core-shell-like structure.
12
The self-assembly results in Fig. 5 differ from those for the assemblies obtained using hybrid SiO2-coated CdTe QDs. In that case, no PL peak shift was observed. This is ascribed to the little amount of free TGA molecules in the solution after reflux. Therefore, no CdS-like clusters formed near the CdTe cores during assembly. In the case of using the QDs prepared after step 1, large amount of free TGA molecules decomposed to generate S2- ions. Thus, a red-shifted PL peak was observed. QD self-assembly in PBS or NaCl solutions As we mentioned before, hybrid SiO2-coated CdTe QDs revealed a high stability because of protection from the shell. The biological application of QDs is an important field which has been attractive. We therefore dispersed the CdTe QDs coated with a thin SiO2 shell prepared after step 1 in a PBS buffer solution for half year. Because of TGA and Cd2+ ions in solution were removed from the solution by filtering, the amount of free TGA is rare. Fig. 6 shows the microscopy images of CdTe@Cd-TGA-SiO2 complex flowers prepared via the self-assembly of CdTe QDs coated with a thin SiO2 shell prepared by step 1 with a CdTe QD concentration of 2.4 10-5 M in a PBS buffer solution, (a) image under bright field and (b) color image under 365 UV light irradiation. After half year, the QDs in the solution still retained their PL properties including PL peak wavelength and efficiency. On the surface, we observed very little amount of QD assembly as shown in Fig. 6. These assemblies with an average size of 200 μm revealed bright green emitting. This result indicates the QDs are stable in the case of no TGA in the solution. This stability is related to the thin SiO2 shell. In addition, no CdS clusters formed in the case of without Cd 2+ ions and free TGA molecules in the solution. Thus, no red-shifted PL was observed. The assembly is ascribed to the TGA and SiO2 components on the QDs. Because of the amount of TGA and SiO2 monomers, the yield of the assembly is very low. Because of the main component of PBS buffer solution is NaCl, we think, NaCl may be useful for the assembly of the QDs because no assembly obtained when these QDs were re-dispersed in pure water in same conditions.
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To further indicate above experimental phenomenon, we re-dispersed hybrid SiO2-coated CdTe QDs (after step 2) in a 0.15 M NaCl solution. The assembly was carried out via a droplet dewetting process. The solution sample was dropped on a slide glass with hydrophilic surface (the bottom of an as-discharged Petri dish which has a glass bottom and a plastic cover). To control the evaporation speed of solvent, several holes were made on the plastic cover. Fig. 7 shows the microscopy images of CdTe@Cd-TGA-SiO2 complex sample (sample 6 shown in Table 1) prepared via the self-assembly of hybrid SiO2-coated CdTe QDs coated CdTe QDs with a concentration of 2.4 10-5 M in a NaCl solution, (a) image under bright field and (b) color images under 365 UV light irradiation. The samples reveal bright red-emitting and pretty dendritic fractal morphology. No individual QDs were observed in the visual field. This means all the QDs were assembled during NaCl growth. This is ascribed to the domain growth of NaCl to form fractal structures through tip splitting and side branching dynamics. For this operation, slow evaporation of solvent plays an important role for the incorporation of the QDs. To explain the genesis of the fractal alignment of hybrid SiO 2-coated CdTe QDs on a hydrophilic glass surface using NaCl molecules as scaffolds, two models were proposed. Sodium chloride crystals have face-centered cubic symmetry, where, the chloride ions are arranged in a cubic close-packing fashion while the sodium ions fill all the cubic gaps between them. Each ion is surrounded by six ions of the other kind; the surrounding ions are located at the vertices of a regular octahedron. For the assembly of QDs via the growth of NaCl, a model is domain growth which has occurred in a Langmuir monolayer to create a fractal structure because of a hydrodynamic mechanism where concentration gradients produced by supersaturation generate a hydrodynamic flow through the Marangoni effect [31]. The tip splitting growth of NaCl gives rise to dense branched morphologies. A morphology transition from tip-splitting to side branching may be existed, in which, structures with pronounced dendrites. In addition, the capillary dewetting is an origin of these fractal
14
morphologies. These two models can be used to explain the assembly mechanism of the hybrid SiO2-coated CdTe QDs in this paper. A droplet dewetting technique using NaCl as a scaffold was used to create those pretty dendritic fractal morphology because the assembly of hybrid SiO 2-coated CdTe QDs into fractal alignment are the following two reasons: (i) NaCl crystals grow quickly in an aqueous solution and without any crystalline H2O molecules in their structure and (ii) the carboxyl group on the surface of the hybrid QDs links with Na + ions through hydrogen bonding. Namely, when hybrid SiO2-coated CdTe QDs were re-dispersed in a NaCl solution, they were covered with a layer of Na+ ions due to the hydrogen bonding between the Na+ ions in solution and the –COO- or –OH group on the surface of the hybrid QDs. The further deposition of Cl - ions resulted in the growth of NaCl crystal, in which the self-assembly of the hybrid SiO2-coated CdTe QDs occurred. On a hydrophilic slide glass surface, because of exterior liquid evaporation, the contact line of the drying drop was pinned on the substrate, which contained almost all the solute. The exterior liquid evaporation governs the morphology of the assembly. The amount of solvent and the ration of solute are key factors. The liquid evaporation from the edge was replenished by liquid from the interior, producing the outward capillary flow of the solvent and leading to highly selective deposition [30]. The evaporation resulted in the crystal growth process of NaCl which including nucleation and growth. The subsequent tip-splitting and side branching dynamics made the assemblies of hybrid SiO2-coated CdTe QDs with different fractal alignments.
Conclusions Hybrid SiO2-coated CdTe QDs with red-emitting (624 nm) were prepared via a reflux process using green-emitting (551 nm) CdTe QDs to investigate QD self-assembly activity for the fabrication of 1D to to 3D nanostructures via a slow growth process at room temperature. Because of CdS clusters near the CdTe cores, the hybrid SiO2-coated QDs revealed a red-shifted PL peak and increased PL efficiency to 15
62 % from 18%. SiO2 monomers from a sol-gel process and TGA play vital roles for QD self-assembly as well as retaining QD initial PL properties and high stability. Using optimal preparation conditions, the hybrid SiO2-coated QDs were assembled into 1D nanotubes, 2D nanosheets, and 3D nanoflowers. It is found that the assemblies actually are CdTe@Cd-TGA-SiO2 complex hybrid nanostructures in which many well-separated CdTe QDs are uniformly distributed.
The hybrid nanotubes can reach
several tens micrometers in length. Similar assembly activity was also observed for CdTe QDs coated with a thin SiO2 shell by step 1 or QDs in PBS or NaCl solutions. The assembly in the initial CdTe QD solution is accompanied by a hybrid SiO 2 layer which resulted in red-shifted PL from green to yellow. CdTe QDs coated with a thin SiO2 shell revealed high stability because they retained their initial PL properties for half year. Because of the domain growth of NaCl to form fractal structures through tip splitting and side branching dynamics, the hybrid SiO2-coated QDs were assembled into leaf morphology.
Because of the CdTe QDs with their in effective protection by
hybrid Cd-TGA-SiO2 complex, these nanostructures exhibited visually bright PL and retain the size-quantized properties of the QDs. Current method will be utilizable for the assembly of nanoparticles and applications in various devices.
Acknowledgements This work was supported in part by the project from National Research Program of China (973 Program, 2013CB632401), the program for Taishan Scholars, the projects from National Natural Science Foundation of China (Grant no. 51572109, 51501071, 51302106, 51402123, and 51402124).
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1645. 18 H. Park, G. Yang, S. Chun, D. Kim, J. Kim, Appl. Phys. Lett., 2013, 103, 51906. 19 J. Liu, P. Raveendran, G. Qin, and Y. Ikushima, Chem. Comm. 2005, 2005, 2972. 20 N. Gaponik, D. V. Talapin, A. L. Rogach, K. Hoppe, E. V. Shevchenko, A. Kornowski, A. Eychmüller, and H. Weller, J. Phys. Chem. B, 2002, 106, 7177; 21 P. Yang, A. Zhang, H. Sun, F. Liu, Q. Jiang, and X. Cheng, J. Colloid Interface Sci., 2010, 345, 222. 17
22 N. Murase, and P. Yang, Small, 2009, 5, 800. 23 P. Yang and N. Murase, Adv. Funct. Mater. 2010, 20, 1258. 24 D. V. Talapin, S. Haubold, A. L. Rogach, A. Kornowski, M. Haase, and H. Weller, J. Phys.Chem. B 2001, 105, 2260 25 A. Shavel, N. Gaponik, and A. Eychmüller, J. Phys. Chem. B 2006, 110, 19280. 26 H. Bao, Y. Gong, Z. Li, and M. Gao, Chem. Mater. 2004, 16, 3853 27 P. Yang, N. Murase, Q. Ma, Y. Cao, A. Zhang, R. Shi, Y. Zhu, and J. Wang, CrystEngComm. 2011, 13, 7276-. 28 Y. S. Zhao, W. Yang, D. Xiao, X. Sheng, X. Yang, Z. Shuai, Y. Luo, and J. Yao, Chem. Mater., 2005, 17, 6430. 29 K. Liu, H. You, Y. Zheng, G. Jia, Y. Huang, M. Yang, Y. Song, L. Zhang, and H. Zhang, Cryst. Growth. Des. 2010, 10. 16. 30 H. Deng, C. Liu, S. Yang, S. Xiao, Z. Zhou, and Q. Wang, Cryst. Growth. Des. 2008, 8. 4432. 31 A. Gutierrez–Campos, G. Diaz–Leines, and R. Castillo, J. Phys. Chem. B, 2010, 114, 5034.
Fig. 1. PL and absorption spectra of initial and hybrid SiO2-coated CdTe QDs. The inset in Fig. shows the color images of QD samples under 365 uv light irradiation.
18
Fig. 2. Images of CdTe@Cd-TGA-SiO2 complex nanotubes (sample 1 shown in Table 1) prepared via the self-assembly of hybrid QDs with CdTe QD concentration of 1.2 10-5 M in aqueous solution. (a & d), SEM images. (b), Color image under 365 UV light irradiation. (c), TEM image.
19
Fig. 3. TEM images of CdTe@Cd-TGA-SiO2 complex nanosheets (sample 2 shown in Table 1) prepared via the self-assembly of hybrid QDs with CdTe QD concentration of 6 10-6 M in aqueous solution. (a), Low magnification. (b), High magnification. The inset in (b) shows the image of a single QD with well-developed lattice fringes. The typical nanosheet structure was formed shown in (a) and the QDs distributed homogeneously in the sheet as shown in (b).
Fig. 4. Color microscopy (a) and TEM (b) images of CdTe@Cd-TGA-SiO2 complex flowers (sample 3 shown in Table 1) prepared via the self-assembly of hybrid QDs with CdTe QD concentration of 2.4 10-4 M in aqueous solution. 20
Fig. 5. Microscopy images of CdTe@Cd-TGA-SiO2 complex sheaf (sample 4 shown in Table 1) prepared via the self-assembly of CdTe QDs coated with thin SiO2 shell with a CdTe QD concentration of 2.4 10-4 M in aqueous solution. (a), Color image under 365 UV light irradiation. (b), Image under bright field. Insert in (b) shows TEM image of a flower.
Fig. 6. Microscopy images of CdTe@Cd-TGA-SiO2 complex flowers (sample 5 shown in Table 1) prepared via the self-assembly of CdTe QDs coated with thin SiO2 21
shell with a CdTe QD concentration of 2.4 × 10−5 M in a PBS buffer solution. (a), Image under bright field. (b), Color image under 365 UV light irradiation.
Fig. 7. Color microscopy images of CdTe@Cd-TGA-SiO2 complex sample (sample 6 shown in Table 1) prepared via the self-assembly of hybrid SiO2-coated CdTe QDs with a concentration of 2.4 10-5 M in in a NaCl solution. (a), Image under bright field. (b), Color image in 365 UV light irradiation.
Table 1. Preparation conditions and properties of samples QD concentration Sample
QDs used
Solvent*
(10-5 M)
Reaction time Morphology (day)
1
Step 2
1.2
H2O
90
Nanotube
2
Step 2
0.6
H2O
90
Nanosheet
3
Step 2
2.4
H2O
7
Flower
4
Step 1
2.4
H2O
40
Sheaf
5
Step 1
2.4
PBS
180
Sheaf
6
Step 2
2.4
NaCl
short
Leaf
*Samples 5 and 6 were prepared in PBS and NaCl solutions, respectively.
22
S0255-2701(16)30395-6 http://dx.doi.org/doi:10.1016/j.cep.2017.02.009 CEP 6929
To appear in:
Chemical Engineering and Processing
Received date: Revised date: Accepted date:
13-9-2016 14-1-2017 21-2-2017
Please cite this article as: Yue Zhao, Simin Lu, Katarzyna Matras-Postolek, Ping Yang, SiO2 Monomer-Triggered Self-Assembly of Hybrid CdTe Quantum Dots, Chemical Engineering and Processing http://dx.doi.org/10.1016/j.cep.2017.02.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
SiO2 Monomer-Triggered Self-Assembly of Hybrid CdTe Quantum Dots Yue Zhao,a Simin Lu,a Katarzyna Matras-Postolek,b Ping Yanga* a
School of Material Science and Engineering, University of Jinan, Jinan, 250022, China.
b
Faculty of Chemical Engineering and Technology, Cracow University of Technology,
Krakow, 31-155, Poland *Corresponding Author has to be addressed. E-mail: [email protected] (P. Yang).
1
Graphic Abstract
Highlight 1. SiO2 monomer-triggered self-assembly makes hybrid CdTe QDs into various nanostructures. 2. The assemblies retained high photoluminescence. 3. The hybrid QDs retained high stability and assembled in PBS solutions.
Abstract The self-assembly activity of quantum dots (QDs) including thioglycolic acid (TGA)-capped CdTe and hybrid SiO2-coated CdTe QDs into one- to three-dimensional (3D) nanostructures was reported using a slow growth process. SiO2 monomers from a sol-gel process and TGA play vital roles for the self-assembly. The hybrid SiO2-coated QDs were assembled into 1D nanotube, 2D nanosheets, and 3D nanoflowers by adjusting QD concentrations. It is found that the assemblies actually are CdTe@Cd-TGA-SiO2 complex hybrid nanostructures in which many well-separated CdTe QDs are uniformly distributed. Such assembly also occurred in an initial CdTe QD solution or in PBS and NaCl solutions. The assembly in the initial CdTe QD solution is accompanied by a hybrid SiO2 layer which 2
resulted in red-shifted photoluminescence (PL) from green to yellow. Because of the domain growth of NaCl to form fractal structures through tip splitting and side branching dynamics, the hybrid SiO2-coated QDs were assembled into leaf morphology. As a result, the CdTe QDs with their in effective protection by hybrid Cd-TGA-SiO2 complex, these nanostructures show visually bright PL and retain the size-quantized properties of the QDs. These QD-based assemblies may be suitable for subsequent processing into quantum-confined materials and devices. Keywords：QDs, SiO2, Hybrid, Assembly, CdTe
Introduction In recent years, the self-assembly of nanoparticles as an important fabrication method in nanochemistry and nanotechnology has been bridging various areas of science and engineering and led to the design and development of advanced materials, innovative methodologies, and general theories [1, 2]. It shows great potential for directing the organization of nanoparticles and opens new avenues of nanotechnology through controlling the fabrication of one- to three-dimensional (3D) nanostructures with unique properties. The self-assembly of semiconductor quantum dots (QDs) has attracted a great deal of attention because assembled spontaneously could create new functional elements for the application of devices [3-5]. The QDs can be exploited as building blocks for electronic, photonic, and magnetic devices as well as for use in sensing and optical applications through self-assembling. The assembly of the QDs could bridge the gap between nanometer scale building blocks and micrometer scale patterning processes. Therefore, the study of QD self-assembly has been attractive in various application fields. Several techniques have been developed to assemble QDs in an orderly manner and to investigate their potential applications. Combining low-cost methods for making superstructures containing QDs with standard patterning techniques are now leveraged 3
for the application of nanomaterials. For example, template methods have been used to study the self-assembly of semiconductor QDs [6]. Previous efforts have concentrated on using such scaffolds to spatially arrange nanoelements as a strategy to tailor their properties [7]. Nanowire, nanosheets, nanochains, 2D networks, and superlattices have been fabricated in solutions or at interfaces by intermolecular or interparticle interaction force fields [8-13]. Although strategies are promising, it is still expected to develop straightforward and controllable methods for self-assembly of the QDs, especially, simplicity, versatility, and low cost. Because QD cores dictate their properties, whereas the surface-bound ligands define the interactions among QDs with their surroundings, ligand-stabilized colloidal QDs are ideally suited to hierarchically self-assemble [14]. To create QD nanostructures with different dimensions and shapes and containing both fused and nonfused nanocrystals, the self-assembly triggered by removing some or all of the ligand has been used. The assembly of QDs is usually driven by the interactions between the individual building blocks, thus, the control of the QD surface properties plays an important role for their assembly [15]. For example, basic units with sheet morphology were assembled into 6-membered rings with QDs interacted in different orientations [16]. The authors found that the dipole moment and small positive charge, both of which are due to the anisotropy of the QDs and the directional hydrophobic attraction drove self-organization. Thus, the QD self-assembly mechanism is affected by various factors which provide opportunities to explore the design and fabrication of advanced materials. Thioglycolic acid (TGA)-capped CdTe is an ideal system for the assembly because of the strong anisotropic properties of nanocrystals or quantum dots (QDs) [17]. The assembly of CdTe QDs occurred because of the interaction of particles. In fact, partially removing the TGA ligand from the surface caused the QDs to aggregate spontaneously in water. 1D CdTe QD nanowires was grew parallel to the (001) direction of the lattice. For low surface charge and strong face-to-face attraction, the QDs form 2D sheets and for much high surface charge, the long-range electrostatic 4
repulsion makes a 2D sheet unfavorable, and instead the QDs assembled into ribbons [16]. Several kinds of techniques have been reported to assemble CdTe QDs. One of the pioneering works by Kotov’s group indicated self-assembly of CdTe QDs into freely suspended nanowires under tightly controlled experimental conditions [8]. Namely, these nanowires just existed in solutions. Little literature reported on stable assembly after solutions. It is still desirable to research a method to create highly stable QD assemblies. Because of unique luminescence and absorption properties, aqueous CdTe QDs are presumably the most studied nanomaterials in last several ten years for applications [18]. Since the QDs are sensitive for their surroundings, retaining initial photoluminescence (PL) properties in QD assembly is important for general theory study and applications. After assembly, the properties of QDs could be changed. The ligand controlling is a key factor for the assembly. For example, hydrazine removed TGA from the surface of CdTe QDs allowing dipole−dipole interactions to form aggregates that organized into wires [17]. QD twisted nanoribbons were formed by starting the synthesis with a relatively low ligand to Cd ratio rather than removing ligands [19]. As a consequence, approaches that enable facilely highly luminescent QD assembly into functional mesoscopic architectures and nanoscale devices that can be harnessed in the emerging fields of optoelectronics and nanobiotechnology have still garnered significant attention [16]. In this paper, initial and hybrid SiO2-coated CdTe QDs were used to investigate the self-assembly. A Cd-TGA-SiO2 hybrid layer was used to reduce the ligand surface density, producing particles that assemble either anisotropically into 1D nanotubes and 2D flat sheets, or isotropically into 3D flowers. In this case, the QDs revealed high PL efficiency and stability. Compared with the reports referenced previously, our results have obvious advantages. The QDs still retain their high luminescence and stabile in the self-assembly process. The self-assembly occurred at room temperature in different solution including water, a PBS buffer solution and a NaCl solution. The detailed
5
process and mechanism of the assembly were discussed. Current method may be utilizable for the fabrication of other heterostructures and applications.
Experimental Chemicals. Chemicals were obtained from Sinopharm Chemical Reagent Company, Tianjin Chemical Reagent Institute, and 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. The phosphate-buffered saline (PBS, 10X) buffer solutions were prepared in our laboratory. Preparation of QDs. TGA-capped CdTe QDs with green emitting (average d = 2.7 nm) in aqueous solution were prepared using a procedure including the use of cadmium perchlorate and hydrogen telluride, as described in a previous paper [20，21]. Hybrid SiO2-coated CdTe QDs were prepared using a two-step synthesis process as reported in our papers [22, 23]. Firstly, CdTe QDs were coated with a thin SiO2 layer using a sol-gel process by stirring them in an aqueous solution containing Cd2+, TGA, CdTe QDs, tetraethyl orthosilicate (TEOS), and ammonia for 3h. Secondly, a reflux process using this solution caused CdS-like clusters to nucleate and grow within the SiO2 shell. The resulting samples were deal with for further characteristics and self-assembly. Self-assembly of QDs. All self-assembly processes were carried out at room temperature. Because the concentration of QDs is vital for the morphology, the QD concentration was controlled by using a 3000-MWCO-filter to condense the aqueous solution of initial and hybrid SiO2-coated CdTe QDs. The TGA, Cd2+ ions, and NH3 in the solutions were partially removed by repeated condensation and dilution.
Because a reflux did not change the
number of QDs, the sizes and concentrations of CdTe QDs were estimated by the absorbance of CdTe QDs using a known procedure in the literature as bellow [17]. 6
The absorbance (A) of CdTe QD suspension was measured as a function of wavelength of light using UV-visible spectroscopy. According to Beer-Lambert’s law, the absorbance of a solution is directly proportional to the concentration of the solution (c) and the path length traversed by the incident beam of light (L) [17]. A = εcL where, ɛ is the extinction coefficient which scales with the particle diameter. The particle diameter (d in nm) and concentration (c in M) were calculated from the below two equations [17]. d = (9.8127×10-7)ελEP3 - (1.7147×10-3)λEP2 + (1.0064)λEP - 194.84 λEP = 3450 × d2.4 × c × L where, λEP (nm) is the wavelength of the first excitonic absorption peak and AEP is the absorbance at λEP. To carry out the self-assembly process, solutions with different QD concentrations were kept in glass bottles with covers at room temperature for different periods (several days to months) to enable us to investigate the morphological evolution during assembly. Finally, resulting samples were taken out from the bottles with pipettes and placed on glass slides for observation with a fluorescence microscope. Table 1 illustrates the preparation conditions and properties of assembly samples. The reaction time was selected to be sure no further evolution after this reaction time. Characterization. Morphology observations by transmission electron microscopy (TEM) were carried out using an FEI Tecnai G2 F20 electron microscope. The high resolution transmission electron microscopy (HRTEM) observation was made on a Tecnai T20 microscope operating at 200 kV (FEI). Morphological observation of assemblies was also done using a field emission scanning electron microscope (SEM, Hitachi, S-5000). The absorption and PL spectra of solution samples were taken using conventional spectrometers Hitachi U-4100 and F-4600, respectively. The PL and optical images of samples were obtained with an Olympus IX 71 fluorescence microscope (Olympus Optical Co.). The PL efficiency of QD samples was
7
calculated by comparison with a standard Quine solution with same light path as indicated in our previous papers [22, 23].
Results and discussion Preparation and properties of QDs. Fig. 1 shows the PL and adsorption spectra of initial and hybrid SiO 2-coated CdTe QDs. TGA-capped CdTe QDs revealed a PL peak wavelength of 551nm, the full-width at half maximum (FWHM) of PL spectrum of 50 m, an average diameter of 2.7 nm, and a PL efficiency of 18%. For the preparation of hybrid SiO 2-coated CdTe QDs, colloidally prepared green-emitting CdTe QDs were coated with a thin SiO2 layer by adding TEOS in an alkaline CdTe colloidal solution with Cd 2+ ions and TGA. Because of CdTe QDs have a Cd-rich surface, TGA was attached with the Cd2+ ions. Because of SiO2 monomers generated via a sol-gel process connected with TGA-Cd composite, no ligand exchange occurred during the growth of SiO2 layer. In addition, the Cd2+ ions and TGA were dispersed in the SiO2 layer. A subsequent reflux process including a sol-gel reaction was then used to form hybrid QDs. The hybrid QDs revealed red-shifted PL and absorption spectra (a PL peak wavelength of 624nm), decreased FWHM of PL spectrum (46 nm), increased size (5.8 nm in diameter according to TEM observation) and increased PL efficiency of 62%.
The PL properties shown in Fig. 1 indicate that the formation of hybrid SiO2-coated CdTe QDs is really different from traditional semiconductor QDs. The spectral change in the hybrid SiO2-coated QDs compared with the spectra of initial CdTe QDs was due to the formation of small CdS-like clusters (1 nm or less in size) in the vicinity of the CdTe QDs in the hybrid SiO2 shell during reflux [21]. These CdS-like clusters could form a core-shell-like semiconductor structure with CdTe cores. During reflux, CdS-like clusters nucleated and grew in the SiO2 shell. The S2- ions were generated from the decomposition of free TGA in the solution with heat treatment. At the same time, the SiO2 shell became thicker due to the SiO2 monomers generated from the 8
hydrolysis and condensation of TEOS. The clusters can passivate the surface of the cores instead of TGA ligands. The role of the clusters is similar with a semiconductor shell on CdTe cores. Because this core-shell-like semiconductor structure was fixed within a hybrid SiO2 shell, the hybrid QDs became stable compared with traditional semiconductor core-shell QDs. The PL spectra of the hybrid QDs is different from other semiconductor core-shell QDs because their PL peak wavelengths just depended on the size of CdS-like clusters rather than the thickness of the hybrid SiO 2 shell. These clusters were incorporated in the shell during reflux because of ion diffusion. Since Cd2+ and TGA form dimers [23], the number of S2- ions in the solution is related to the concentration of TGA and Cd2+ in the solution and the reflux time. Therefore, the PL properties of the hybrid QDs depended strongly on the concentration and molar ratio of TGA and Cd2+ in the solution which affect the size of the CdS-like clusters in the hybrid SiO2 shell. Self-assembly of hybrid SiO2-coated CdTe QDs. Fig. 2 shows the images of CdTe@Cd-TGA-SiO2 complex nanotubes prepared via the self-assembly of hybrid SiO2-coated CdTe QDs in a CdTe QD concentration of 1.2 10-5 M in an aqueous solution for 3 months, (a & d) SEM images, (b) color image under 365 UV light irradiation, and (c) TEM image. The TEM image in Fig. 2d clearly shows tubular morphology. The tubes have an average outside diameter of 500 nm and inside diameter of 150 nm. After assembling into nanotubes, QDs still retained bright red-emitting as shown in Fig. 2b, Their PL spectra remained unchanged compared with their initial one. The TEM image of the nanotubes in Fig. 2c shows the QDs homogeneously distributed in the nanotubes. To investigate the effect of concentration on the morphology, the QD concentration in solutions was decreased to 6 10-6 M. Fig. 3 shows the TEM images of CdTe@Cd-TGA-SiO2 complex nanosheets prepared via the self-assembly of hybrid QDs with a CdTe QD concentration of 6 10-6 M in an aqueous solution for 3 months, (a) with low magnification and (b) with high magnification. The inset in (b) shows the image of a single QD with well-developed lattice fringes. The typical nanosheet 9
structure was formed shown in Fig. 3a and the QDs distributed homogeneously in the sheet as shown in Fig. 3b. Compared with the CdTe@Cd-TGA-SiO2 complex nanotubes shown in Fig. 2, CdTe@Cd-TGA-SiO2 complex nanosheets were created in a low QD concentration. This confirms the QD concentration is vital for the morphology of the assembly. We discuss the reason bellow. TGA is commonly used as a capping agent for the synthesis of hydrophilic CdTe QDs [24]. It is well known, the formation of TGA and Cd2+ composition depended on the ratio of TGA to Cd2+ in solutions [25]. The formation of a Cd-thiol layer on the surface of aqueous CdTe QDs effectively passivates the surface, resulting in high PL efficiency. In an aqueous solution, Cd2+ ions could link with the mercapto group in the TGA and form a complex. Eychmüller’s group reported that the composition of the Cd-TGA complex depends on the initial concentrations of Cd2+ and TGA in the water and that the concentrations of three types of Cd-TGA complexes (Cd2+ connected with 1, 2, and 3 ligands, respectively) depend on the pH of the solution [25]. In current experiments with an alkaline condition, the TGA/Cd2+ molar ratio in the solution is 1.5. Thus, Cd-TGA complexes formed in solutions with 1 to 2 ligands in the solution. The carboxyl group could link to another Cd-TGA chain-like structure through the electrostatic adsorption between Cd2+ and carboxyl groups. The overlap of Cd-TGA complexes resulted in the formation of longer Cd-TGA clusters. These longer clusters grew into nanowires, which acted as seeds for the growth of morphology-tuneable assemblies. Furthermore, the SiO2 monomers generated from the hydrolysis of TEOS were deposited on the clusters through the –OH groups on their surface. Thin SiO2 layer coated QDs could easily attach to the nanowire formed from 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 QDs. This means that the assembly derived from SiO2 monomers. For a self-assembly process, the nanowires grown from Cd-TGA clusters were taken as a soft template to grow CdTe@Cd-TGA-SiO2 complex assemblies with 10
various morphologies. The attachment of hybrid SiO2-coated QDs to the nanowire is ascribed to the hybrid SiO2 shell with Cd2+ and TGA complex. This attachment was carried out either via the connection of ≡Si–OH+Cd–TGA or via Cd–TGA complex within the hybrid SiO2 shell [26, 27]. In the low QD concentration, the nanowire was grown into a 2D sheet-like morphology because of low surface charge and strong face-to-face attraction. In the case of the amount of building blocks is not enough, the nanosheets were created as shown in Fig. 3. With increasing the amount of building blocks, increased surface charge and the long-range electrostatic repulsion make the flat sheet with a tendency to roll in order to reduce the surface energy in water. The rolling of the flat sheet into a curled one is favorable. Finally, the curled sheet will seam into nanotubes by ring-closure of the curled chains through the formation of new hydrogen bonding between the Cd–TGA chain-like structures at the two edges [27]. Thus, the CdTe@Cd-TGA-SiO2 complex nanotubes formed as shown in Fig. 2. A similar mechanism has been used to explain the formation of microtubes by using small organic molecules [28]. If the amount of building blocks, solid fibers and flower-like assembly will be created because of the fast filling of the hybrid SiO2-coated QD, SiO2 monomers, Cd-TGA complex. The detail will be explained as follows. In a high QD concentration, we prepared a flower-like CdTe@Cd-TGA-SiO2 complex assembly constructed of solid nanofibers. The hybrid SiO 2-coated CdTe QDs were re-dispersed in a TGA and Cd2+ solution with a concentration of 2.4 10-4 M. The solution was kept in a bottle with a cover for 7 days. Fig. 4 shows the color (a) and TEM (b) images of CdTe@Cd-TGA-SiO2 complex flowers prepared via the self-assembly of hybrid QDs in an aqueous solution. The average size of flower assembly is about 2-6 μm.
The average diameter of the solid fiber in the flower
assembly is about 35 nm. Same with the CdTe@Cd-TGA-SiO2 complex nanotubes and nanosheets, the PL peak wavelength remained unchanged. Except for forming 1D nanostructure, mercapto carboxylic acids can form complicated complexes with cadmium ions, with primary coordination of cadmium 11
ions to the thiol groups and a secondary coordination to the carboxylic groups. 29 The formation of crystals with nonthermo dynamic equilibrium shapes is driven kinetically while the formation of 1D fibers is generally ascribed to anisotropic aggregation behaviour [30]. For a high starting-material concentration, crystal splitting resulted in hierarchical structures requires fast crystal growth. Because of a high QD concentration together with much more Cd2+ ions, TGA, and SiO2 monomers, the initial formation of nuclei immediately after supersaturation and subsequent growth of the nuclei. Subsequently quick growth of the nuclei led to crystal splitting. Further growth resulted in the formation of a flower-like assembly constructed of fibers. Because of the amount of starting materials, the fibers are solid as shown in Fig. 4. The hybrid QDs were prepared using a two-step synthesis. In the first step, CdTe QDs were coated with a thin SiO2 shell via a sol-gel process in a TGA and Cd2+ solution. Because such SiO2 shell has similar structure and composition with finally one (after reflux), we investigated their self-assembly in an aqueous solution solution for comparison. Fig. 5 shows the microscopy images of CdTe@Cd-TGA-SiO2 complex sheaf-like assembly prepared via the self-assembly of CdTe QDs coated with a thin SiO2 shell by step 1 with a CdTe QD concentration of 2.4 10-4 M in a PBS buffer solution, (a) color images under 365 UV light irradiation and (b) image under bright field. This sample was prepared using a solution after step 1. The concentration of CdTe QDs was increased into 10-4 and kept at room temperature for 40 days (with covered). We observed the sheaf-like morphology as shown in Fig. 5. The insert in in Fig. 5b shows TEM image of a flower. The
self-assembly
mechanism
of
these
QDs
is
very
similarly
to
CdTe@Cd-TGA-SiO2 complex flowers shown in Fig. 4. Because of TGA and Cd2+ ions in the solution, we are sure the morphology is similar with CdTe@Cd-TGA-SiO2 complex flowers assembly. However, the average increased because of a slow growth rate. The PL color became into yellow. This is ascribed to the formation of core-shell-like structure.
12
The self-assembly results in Fig. 5 differ from those for the assemblies obtained using hybrid SiO2-coated CdTe QDs. In that case, no PL peak shift was observed. This is ascribed to the little amount of free TGA molecules in the solution after reflux. Therefore, no CdS-like clusters formed near the CdTe cores during assembly. In the case of using the QDs prepared after step 1, large amount of free TGA molecules decomposed to generate S2- ions. Thus, a red-shifted PL peak was observed. QD self-assembly in PBS or NaCl solutions As we mentioned before, hybrid SiO2-coated CdTe QDs revealed a high stability because of protection from the shell. The biological application of QDs is an important field which has been attractive. We therefore dispersed the CdTe QDs coated with a thin SiO2 shell prepared after step 1 in a PBS buffer solution for half year. Because of TGA and Cd2+ ions in solution were removed from the solution by filtering, the amount of free TGA is rare. Fig. 6 shows the microscopy images of CdTe@Cd-TGA-SiO2 complex flowers prepared via the self-assembly of CdTe QDs coated with a thin SiO2 shell prepared by step 1 with a CdTe QD concentration of 2.4 10-5 M in a PBS buffer solution, (a) image under bright field and (b) color image under 365 UV light irradiation. After half year, the QDs in the solution still retained their PL properties including PL peak wavelength and efficiency. On the surface, we observed very little amount of QD assembly as shown in Fig. 6. These assemblies with an average size of 200 μm revealed bright green emitting. This result indicates the QDs are stable in the case of no TGA in the solution. This stability is related to the thin SiO2 shell. In addition, no CdS clusters formed in the case of without Cd 2+ ions and free TGA molecules in the solution. Thus, no red-shifted PL was observed. The assembly is ascribed to the TGA and SiO2 components on the QDs. Because of the amount of TGA and SiO2 monomers, the yield of the assembly is very low. Because of the main component of PBS buffer solution is NaCl, we think, NaCl may be useful for the assembly of the QDs because no assembly obtained when these QDs were re-dispersed in pure water in same conditions.
13
To further indicate above experimental phenomenon, we re-dispersed hybrid SiO2-coated CdTe QDs (after step 2) in a 0.15 M NaCl solution. The assembly was carried out via a droplet dewetting process. The solution sample was dropped on a slide glass with hydrophilic surface (the bottom of an as-discharged Petri dish which has a glass bottom and a plastic cover). To control the evaporation speed of solvent, several holes were made on the plastic cover. Fig. 7 shows the microscopy images of CdTe@Cd-TGA-SiO2 complex sample (sample 6 shown in Table 1) prepared via the self-assembly of hybrid SiO2-coated CdTe QDs coated CdTe QDs with a concentration of 2.4 10-5 M in a NaCl solution, (a) image under bright field and (b) color images under 365 UV light irradiation. The samples reveal bright red-emitting and pretty dendritic fractal morphology. No individual QDs were observed in the visual field. This means all the QDs were assembled during NaCl growth. This is ascribed to the domain growth of NaCl to form fractal structures through tip splitting and side branching dynamics. For this operation, slow evaporation of solvent plays an important role for the incorporation of the QDs. To explain the genesis of the fractal alignment of hybrid SiO 2-coated CdTe QDs on a hydrophilic glass surface using NaCl molecules as scaffolds, two models were proposed. Sodium chloride crystals have face-centered cubic symmetry, where, the chloride ions are arranged in a cubic close-packing fashion while the sodium ions fill all the cubic gaps between them. Each ion is surrounded by six ions of the other kind; the surrounding ions are located at the vertices of a regular octahedron. For the assembly of QDs via the growth of NaCl, a model is domain growth which has occurred in a Langmuir monolayer to create a fractal structure because of a hydrodynamic mechanism where concentration gradients produced by supersaturation generate a hydrodynamic flow through the Marangoni effect [31]. The tip splitting growth of NaCl gives rise to dense branched morphologies. A morphology transition from tip-splitting to side branching may be existed, in which, structures with pronounced dendrites. In addition, the capillary dewetting is an origin of these fractal
14
morphologies. These two models can be used to explain the assembly mechanism of the hybrid SiO2-coated CdTe QDs in this paper. A droplet dewetting technique using NaCl as a scaffold was used to create those pretty dendritic fractal morphology because the assembly of hybrid SiO 2-coated CdTe QDs into fractal alignment are the following two reasons: (i) NaCl crystals grow quickly in an aqueous solution and without any crystalline H2O molecules in their structure and (ii) the carboxyl group on the surface of the hybrid QDs links with Na + ions through hydrogen bonding. Namely, when hybrid SiO2-coated CdTe QDs were re-dispersed in a NaCl solution, they were covered with a layer of Na+ ions due to the hydrogen bonding between the Na+ ions in solution and the –COO- or –OH group on the surface of the hybrid QDs. The further deposition of Cl - ions resulted in the growth of NaCl crystal, in which the self-assembly of the hybrid SiO2-coated CdTe QDs occurred. On a hydrophilic slide glass surface, because of exterior liquid evaporation, the contact line of the drying drop was pinned on the substrate, which contained almost all the solute. The exterior liquid evaporation governs the morphology of the assembly. The amount of solvent and the ration of solute are key factors. The liquid evaporation from the edge was replenished by liquid from the interior, producing the outward capillary flow of the solvent and leading to highly selective deposition [30]. The evaporation resulted in the crystal growth process of NaCl which including nucleation and growth. The subsequent tip-splitting and side branching dynamics made the assemblies of hybrid SiO2-coated CdTe QDs with different fractal alignments.
Conclusions Hybrid SiO2-coated CdTe QDs with red-emitting (624 nm) were prepared via a reflux process using green-emitting (551 nm) CdTe QDs to investigate QD self-assembly activity for the fabrication of 1D to to 3D nanostructures via a slow growth process at room temperature. Because of CdS clusters near the CdTe cores, the hybrid SiO2-coated QDs revealed a red-shifted PL peak and increased PL efficiency to 15
62 % from 18%. SiO2 monomers from a sol-gel process and TGA play vital roles for QD self-assembly as well as retaining QD initial PL properties and high stability. Using optimal preparation conditions, the hybrid SiO2-coated QDs were assembled into 1D nanotubes, 2D nanosheets, and 3D nanoflowers. It is found that the assemblies actually are CdTe@Cd-TGA-SiO2 complex hybrid nanostructures in which many well-separated CdTe QDs are uniformly distributed.
The hybrid nanotubes can reach
several tens micrometers in length. Similar assembly activity was also observed for CdTe QDs coated with a thin SiO2 shell by step 1 or QDs in PBS or NaCl solutions. The assembly in the initial CdTe QD solution is accompanied by a hybrid SiO 2 layer which resulted in red-shifted PL from green to yellow. CdTe QDs coated with a thin SiO2 shell revealed high stability because they retained their initial PL properties for half year. Because of the domain growth of NaCl to form fractal structures through tip splitting and side branching dynamics, the hybrid SiO2-coated QDs were assembled into leaf morphology.
Because of the CdTe QDs with their in effective protection by
hybrid Cd-TGA-SiO2 complex, these nanostructures exhibited visually bright PL and retain the size-quantized properties of the QDs. Current method will be utilizable for the assembly of nanoparticles and applications in various devices.
Acknowledgements This work was supported in part by the project from National Research Program of China (973 Program, 2013CB632401), the program for Taishan Scholars, the projects from National Natural Science Foundation of China (Grant no. 51572109, 51501071, 51302106, 51402123, and 51402124).
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Fig. 1. PL and absorption spectra of initial and hybrid SiO2-coated CdTe QDs. The inset in Fig. shows the color images of QD samples under 365 uv light irradiation.
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Fig. 2. Images of CdTe@Cd-TGA-SiO2 complex nanotubes (sample 1 shown in Table 1) prepared via the self-assembly of hybrid QDs with CdTe QD concentration of 1.2 10-5 M in aqueous solution. (a & d), SEM images. (b), Color image under 365 UV light irradiation. (c), TEM image.
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Fig. 3. TEM images of CdTe@Cd-TGA-SiO2 complex nanosheets (sample 2 shown in Table 1) prepared via the self-assembly of hybrid QDs with CdTe QD concentration of 6 10-6 M in aqueous solution. (a), Low magnification. (b), High magnification. The inset in (b) shows the image of a single QD with well-developed lattice fringes. The typical nanosheet structure was formed shown in (a) and the QDs distributed homogeneously in the sheet as shown in (b).
Fig. 4. Color microscopy (a) and TEM (b) images of CdTe@Cd-TGA-SiO2 complex flowers (sample 3 shown in Table 1) prepared via the self-assembly of hybrid QDs with CdTe QD concentration of 2.4 10-4 M in aqueous solution. 20
Fig. 5. Microscopy images of CdTe@Cd-TGA-SiO2 complex sheaf (sample 4 shown in Table 1) prepared via the self-assembly of CdTe QDs coated with thin SiO2 shell with a CdTe QD concentration of 2.4 10-4 M in aqueous solution. (a), Color image under 365 UV light irradiation. (b), Image under bright field. Insert in (b) shows TEM image of a flower.
Fig. 6. Microscopy images of CdTe@Cd-TGA-SiO2 complex flowers (sample 5 shown in Table 1) prepared via the self-assembly of CdTe QDs coated with thin SiO2 21
shell with a CdTe QD concentration of 2.4 × 10−5 M in a PBS buffer solution. (a), Image under bright field. (b), Color image under 365 UV light irradiation.
Fig. 7. Color microscopy images of CdTe@Cd-TGA-SiO2 complex sample (sample 6 shown in Table 1) prepared via the self-assembly of hybrid SiO2-coated CdTe QDs with a concentration of 2.4 10-5 M in in a NaCl solution. (a), Image under bright field. (b), Color image in 365 UV light irradiation.
Table 1. Preparation conditions and properties of samples QD concentration Sample
QDs used
Solvent*
(10-5 M)
Reaction time Morphology (day)
1
Step 2
1.2
H2O
90
Nanotube
2
Step 2
0.6
H2O
90
Nanosheet
3
Step 2
2.4
H2O
7
Flower
4
Step 1
2.4
H2O
40
Sheaf
5
Step 1
2.4
PBS
180
Sheaf
6
Step 2
2.4
NaCl
short
Leaf
*Samples 5 and 6 were prepared in PBS and NaCl solutions, respectively.
22