Effect of surface ligands on the optical properties of aqueous soluble CdTe quantum dots
© Silva et al.; licensee Springer. 2012
Received: 20 July 2012
Accepted: 16 September 2012
Published: 27 September 2012
We investigate systematically the influence of the nature of thiol-type capping ligands on the optical and structural properties of highly luminescent CdTe quantum dots synthesized in aqueous media, comparing mercaptopropionic acid (MPA), thioglycolic acid (TGA), 1-thioglycerol (TGH), and glutathione (GSH). The growth rate, size distribution, and quantum yield strongly depend on the type of surface ligand used. While TGH binds too strongly to the nanocrystal surface inhibiting growth, the use of GSH results in the fastest growth kinetics. TGA and MPA show intermediate growth kinetics, but MPA yields a much lower initial size distribution than TGA. The obtained fluorescence quantum yields range from 38% to 73%. XPS studies unambiguously put into evidence the formation of a CdS shell on the CdTe core due to the thermal decomposition of the capping ligands. This shell is thicker when GSH is used as ligand, as compared with TGA ligands.
Over the past two decades, semiconductor nanocrystals have attracted great attention of researchers due to their unique optical properties. In particular, luminescent quantum dots (QDs) are defined as semiconductor structures with physical dimensions that are smaller than the exciton Bohr radius[1, 2]. These materials exhibit a strong quantum confinement effect, and this effect causes the appearance of size-dependent optical properties, which has attracted great attention for application of QDs in different technological areas, including biological labeling, light-emitting diodes, and photovoltaic devices.
QDs can be produced via a number of synthetic methods. These techniques have great advances in recent years, which have enabled the synthesis of monodisperse nanocrystals with good optical properties as well as different compositions and morphologies. Up to now, the most successful method to prepare highly luminescent II-VI colloidal semiconductors is the organometallic synthetic route, which uses trioctylphosphine oxide and trioctylphosphine as surface ligands, in order to avoid nanocrystal growth and aggregation. Alternatively, organic metal salts such as Cd carboxylates or phosphonates can be reacted with the chalcogenide source in 1-octadecene. However, these methods require high temperatures, and the resulting nanoparticles are insoluble in water, which makes the final product incompatible with the biological systems[5–7]. More recently, QDs have been prepared in aqueous medium because this synthetic approach is simpler, less toxic, and generates water-soluble nanocrystals that are directly biologically compatible. Nevertheless, this method generally produces nanoparticles with lower fluorescence quantum yields, when compared to the synthesis in organic media. The lower florescence quantum yield is attributed to the defects and traps on the surface of the nanocrystals. Therefore, researchers have investigated the influence of the surface ligands so as to remove these defects and improve the optical properties[4, 9, 10].
Surface ligands consist of a polar anchoring group and either an apolar hydrocarbon chain (synthesis in organics) or a charged group (synthesis in water). These ligands must dynamically adsorb on/desorb from the surface of the nanocrystals at the synthesis temperature in order to allow for growth while the nanoparticles are stabilized against aggregation. Peng et al. reported on the effect of amine ligands on the growth of CdSe QDs and proved that the ligand dynamics on the nanocrystal surface depends on the reaction temperature and on the concentration and chain length of the stabilizers. Earlier studies showed that ligands play an important role during the formation of nanocrystals, exerting a strong effect on both the nucleation and growth stages. Hence, surface ligands can control the size, shape, growth kinetics, and optical properties of the QDs[4, 10–14].
Also, in the case of water-soluble CdTe nanocrystals, the influence of some types of surface ligands on the structural and optical properties has been studied[15, 16]. The most often used surface ligands are thioglycolic acid (TGA) or mercaptopropionic acid (MPA). The growth kinetics of TGA-coated CdTe has been analyzed quantitatively by means of dynamic light scattering (DLS) measurements, and the growth rates, size distributions, critical radii, and diffusion constants have been calculated in the framework of the theoretical Sugimoto model. There are two distinct regimes of kinetics: (1) slow increase in the hydrodynamic radius and (2) faster growth of nanoparticles compared with the previous regime. These two kinetic regimes allow for a certain control of nanocrystal size and size distribution, with no need for post-preparative fractionation techniques.
In 2010, Lesnyak et al. described a novel ligand, 5-mercaptomethyltetrazole, for the aqueous synthesis of CdTe nanocrystals. Tetrazoles are five-membered cyclic compounds containing four nitrogen atoms of different types (pyrrole and pyridine type). CdTe nanocrystals obtained via the ‘standard method’ but using mercaptomethyltetrazole instead of TGA as the stabilizer exhibited fluorescence in the 510- to 610-nm range, depending on the reflux time, and fluorescence quantum yields reaching up to 60%. Upon addition of a solution of Cd2+ ions, the CdTe nanocrystals irreversibly formed hydrogels, i.e., highly porous 3D networks. The interaction of MPA-capped CdTe nanocrystals, synthesized in aqueous media with cysteine and homocysteine, has already been described. Glutathione (GSH), a thiol-containing tripeptide, has been shown to be able to provide improved biocompatible capping for semiconductor nanocrystals as compared with many other water-soluble ligands. Moreover, GSH appears to work best with CdTe in terms of promoting high photoluminescence[10, 21].
In the present work, we conducted a comparative study of the synthesis of CdTe QDs prepared in aqueous media using four different surface ligands: MPA, TGA, 1-thioglycerol (TGH), and GSH. The influence of these ligands on the surface of the nanocrystals was evaluated on the basis of the changes in their optical properties and their sizes.
Tellurium powder (200 mesh, 99.8%), TGH (98%), reduced l-glutathione (GSH, 99%), 3-MPA (99%), and rhodamine 101 (100%) were obtained from Sigma-Aldrich (St. Louis, MO, USA). TGA (99%) and thiourea (99%) were purchased from Synth (Diadema, São Paulo, Brazil). Sodium borohydride (NaBH4, 97%) was acquired from Nuclear (São Paulo, Brazil), and cadmium chloride monohydrate (CdCl2·H2O, 99%) was provided by Vetec (Rio de Janeiro, Brazil). All the chemicals were used without additional purification. Milli-Q ultrapure water (Millipore Co., Billerica, MA, USA) was employed in the synthesis of nanoparticles.
Syntheses of MPA-, TGA-, TGH- and GSH-coated CdTe QDs
The experimental procedure was performed according to, but different stabilizing ligands were utilized. Briefly, 0.8 mmol of tellurium powder and 1.6 mmol of sodium borohydride were diluted in 20 mL of Milli-Q water in a 25-mL three-neck flask. The reaction mixture was heated to 80°C under argon flow in order to obtain a clear deep red solution. The resulting NaHTe was used as tellurium source. Next, 0.4 mmol of Cd solution and 1.4 mmol of MPA were mixed in 80 mL of Milli-Q water, and the pH value was adjusted to 10.0 by the addition of NaOH (1.0 molL−1). This solution was heated at 100°C under argon bubbling and then 4.0 mL of freshly prepared NaHTe was added with the aid of a syringe. The resulting solution was refluxed at 100°C for different times in order to obtain CdTe QDs of different sizes. Aliquots were taken at defined time intervals, and their UV-vis absorption and photoluminescence (PL) spectra were recorded. Samples were precipitated by addition of acetone and dried in vacuum prior to characterization by X-ray diffration (XRD), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The same procedure was followed for the synthesis involving the other surface ligands investigated in this work. For acquisition of the TEM images, the nanocrystals were submitted to a phase-transfer procedure based on the partial exchange of the MPA stabilized by dodecanethiol (DDT). To this end, 1 mL of an aqueous solution of CdTe was placed in a test tube, and 1 mL of 1-dodecanethiol and 2 to 3 mL of acetone were added to this solution. The test tube was vigorously shaken and heated to the boiling point of acetone. The transfer of the nanocrystals to the organic phase was detected from the change in the color of the latter phase.
UV-vis absorption and PL spectra were acquired on a Varian Cary 50 spectrophotometer (Varian Inc., Palo Alto, CA, USA) and Shimadzu RF-5301 PC spectrofluorophotometer (Shimadzu Corporation, Nakagyo-ku, Kyoto, Japan), respectively. The spectrofluorophotometer is equipped with a xenon lamp of 150 W. The absorption and fluorescence measurements were typically performed with 10-mm quartz cuvettes (Shimadzu) using air-saturated solutions at room temperature. The fluorescence quantum yield (ϕf) of the nanocrystals was estimated by comparing the integrated emission of the QD samples, obtained at one excitation wavelength, with that of a standard fluorescent dye, rhodamine 101[24, 25]. We have used the wavelength excitation of 355 nm. Essentially, stock solutions of the standard and QD samples with similar absorbance (no higher than 0.02) at the same excitation wavelength can be assumed to be absorbing the same number of photons. Hence, a simple ratio of the integrated fluorescence intensities of the two solutions (recorded under identical conditions) yielded the ratio of the quantum yield values. Since the quantum yield for the standard sample rhodamine 101 is known (ϕf = 1.0 in water[24, 25]), it is trivial to calculate the quantum yield for the QDs. Identical instrument settings for the sample and standard solutions were carefully checked, and the solvent adsorption and emission spectra were subtracted from the absorption and emission spectra of the sample and standard solutions. This was done directly in the software of the equipment used. In addition, the measurements were repeated for at least three different concentrations of the sample and the reference dye. Powder X-ray diffraction (XRD) patterns were recorded on a Shimadzu XRD-6000 using CuKα radiation. FTIR spectra of the materials were obtained by the conventional KBr pellet technique, in a GXI spectrum Perkin Elmer spectrometer (PerkinElmer, Waltham, MA, USA), operating between 4,000 and 400 cm−1. A minimum of 32 scans were recorded with a resolution of 2 cm−1. The KBr salt and the nanoparticles were dried for 2 h under 110°C and 40°C, respectively. Both samples were kept under vacuum until the moment of the analyses. The sizes of the nanocrystals were determined by DLS measurements using a HeNe source (λ = 632.8 nm) and a photomultiplier as detector. The correlation functions were calculated by the BI9000-AT correlator board Brookhaven Inst. Co. (Holtsville, NY, USA). A scattering angle of 30.0° was used during these measurements, and the samples were maintained in a thermal bath at 25.0°C with a precision of 0.1°C. TEM was performed on a TEM-FEG JEM 2100F microscope (JEOL Ltd., Akishima, Tokyo, Japan) operating at 200 kV. XPS was conducted on an ultrahigh vacuum system (base pressure of 3.0 × 10−10 mbar) equipped with a standard non-monochromatic Mg Kα X-ray source (hν = 1,253.6 eV) and a concentric hemispherical electron-energy analyzer (CLAM2, VG Microtech, East Sussex, UK). The binding energy (BE) scale was calibrated using the carbon peak from the surface contamination as reference (C 1s at 284.6 eV).
Results and discussion
Comparison of ϕ f and Stokes shift of CdTe QDs
Stokes shift (meV)
Stokes shift (meV)
Stokes shift (meV)
Stokes shift (meV)
The intensity correlation function C(q,t) is proportional to the squared dynamic structure factor S(q,t) (). In the case of bidispersions, S(q,t) becomes the sum of two exponentials corresponding to each species. The measurements can then be well fitted by an expression given by the sum of a fast decaying exponential and a slower one. The decay rate Γ is computed from this fitting of the experimental results, and the diffusion coefficient is calculated through Equation 1. This coefficient can be replaced directly in Equation 2 yielding the hydrodynamic radius Rh.
In this work, we carried out a systematic investigation of different thiol-stabilizing ligands on the properties of CdTe QDs synthesized in aqueous solution. The growth rate, size distribution, and quantum yield strongly depend on the type of surface ligand. Under the same refluxing conditions, the highest growth rate is obtained in the presence of the GSH ligand. However, TGH ligands bind to the QD surface too strongly, hindering particle growth and yielding broad size distribution. TGA and MPA ligands furnish comparable results due to their similar molecular structures. The exception is the size dispersion at short reaction times, which is much broader for TGA. No size focusing is observed in any case. With the exception of TGH-coated CdTe QDs, for which a slight decrease in the PL linewidth is observed with reflux time, the QDs exhibit broader size distributions and lower quantum yields for prolonged reaction times. The infrared spectra indicate that the ligands are connected to the nanocrystal surface via the SH group. XPS results clearly evidence the formation of a CdS shell on the CdTe core due to the thermal decomposition of the surface ligands. This CdS shell is thicker when GSH is used as the capping ligand as compared with TGA. In most cases, the shell thickness is directly correlated to the photostability of the QDs, and therefore, GSH-coated CdTe QDs are the most promising candidates for applications relying on good photoluminescence properties such as biological labeling or displays and lighting.
This work was supported by CAPES, CNEN, CNPq, and FAPEMIG. The authors would like to acknowledge the Brazilian Nanotechnology National Laboratory LNNano and LNLS for providing the equipment and technical support for the experiments involving transmission electron microscopy. We thank D.L. Ferreira and P. Licínio for the support and the use of the DLS.
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