Effect of surface ligands on the optical properties of aqueous soluble CdTe quantum dots (original) (raw)
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Nanotechnology, 2008
The influence of thioalkyl acid ligand was evaluated during aqueous synthesis at 100 • C and under hydrothermal conditions (150 • C) of CdTe and CdSe quantum dots (QDs). Experiments performed with 3-mercaptopropionic acid (MPA), 6-mercaptohexanoic acid (MHA) and 11-mercaptoundecanoic acid (MUA) demonstrated that the use of MHA and MUA allowed for the preparation of very small nanoparticles (0.6-2.5 nm) in carrying out the reaction under atmospheric pressure or in an autoclave and that the photophysical properties of QDs were dependent on the ligand and on the synthesis conditions. The influence of various experimental conditions, including the Te-to-Cd ratio, temperature, and precursor concentration, on the growth rate of CdTe or CdSe QDs has been systematically investigated. The fluorescence intensities of CdTe QDs capped with MPA, MHA, or MUA versus pH were also found to be related to the surface coverage of the nanoparticles.
Effect of chemical composition on luminescence of thiol-stabilized CdTe nanocrystals
Nanoscale Research Letters, 2007
Judicious selection of the amount of surfactant during synthesis enables a drastic increase in the photoluminescence efficiency of aqueous CdTe nanocrystals (NCs) stabilized by thioglycolic acid (TGA). Elemental determination of the NCs was undertaken to identify the origin of this effect. The molar ratio of (Te + S) to Cd approached unity when the optimum amount of TGA was used during synthesis, whereas the number of S atoms originating from TGA molecules in one NC (2.6 nm of diameter) remained unchanged at 90 ± 3. This indicates that the core lattice composition at the beginning of synthesis, rather than the surface conditions, affects the photoluminescence efficiency of the NCs even after prolonged refluxing.
Physical Chemistry Chemical Physics, 2016
The optical properties of semiconductor nanocrystals (SC NCs) are largely controlled by their size and surface chemistry, i.e., the chemical composition and thickness of inorganic passivation shells and the chemical nature and number of surface ligands as well as the strength of their bonds to surface atoms. The latter is particularly important for CdTe NCs, which-together with alloyed Cd x Hg 1Àx Te-are the only SC NCs that can be prepared in water in high quality without the need for an additional inorganic passivation shell. Aiming at a better understanding of the role of stabilizing ligands for the control of the application-relevant fluorescence features of SC NCs, we assessed the influence of two of the most commonly used monodentate thiol ligands, thioglycolic acid (TGA) and mercaptopropionic acid (MPA), on the colloidal stability, photoluminescence (PL) quantum yield (QY), and PL decay behavior of a set of CdTe NC colloids. As an indirect measure for the strength of the coordinative bond of the ligands to SC NC surface atoms, the influence of the pH (pD) and the concentration on the PL properties of these colloids was examined in water and D 2 O and compared to the results from previous dilution studies with a set of thiol-capped Cd 1Àx Hg x Te SC NCs in D 2 O. As a prerequisite for these studies, the number of surface ligands was determined photometrically at different steps of purification after SC NC synthesis with Ellman's test. Our results demonstrate ligand control of the pH-dependent PL of these SC NCs, with MPA-stabilized CdTe NCs being less prone to luminescence quenching than TGA-capped ones. For both types of CdTe colloids, ligand desorption is more pronounced in H 2 O compared to D 2 O, underlining also the role of hydrogen bonding and solvent molecules.
Synthesis of CdTe Quantum Dots in Aqueous Solution and Their Optical and Structural Characterization
Science of Advanced Materials, 2012
We describe the optical and structural characterization of highly luminescent thioglycolic acid-stabilized CdTe quantum dots (CdTe-QD) synthesized in water. Samples prepared under different reflux times were characterized using transmission electron microscopy (TEM), X-ray diffraction, Raman spectroscopy, absorbance and photoluminescence techniques. TEM measurements revealed CdTe-QD having a nearly spherical shape (2.5 nm in size), so as tellurium nanorods in the same sample. The corresponding Raman spectrum displays a characteristic peaks of CdTe (broad bands at 141 cm −1 and 162 cm −1 , respectively) and tellurium nanorods (121 cm −1). Three extra peaks at 228, 252 and 277 cm −1 could be associated to the TO 1 and LO 2 modes of CdS nanoparticles. X-ray diffraction measurements done on dried sample indicated the face-centered cubic structure for CdTe and testified the presence of CdS nanoparticles, crystallizing in the hexagonal phase. All refluxed samples exhibit high luminescence that increases with reflux time. Their absorbance spectra display a well resolved excitonic peak in the 350-510 nm range, whilst photoluminescence peaks shift red (500-600 nm) due to the Stokes shift. To explain the observed strong luminescence, it is assumed that a CdS shell develops at the CdTe surface due to the thioglycolic acid decomposition, providing the surface passivation.
Chemistry of Materials, 2004
A strong photoluminescence enhancement effect of illumination on CdTe nanocrystals stabilized by thioglycolic acid (TGA) was observed. Under optimized conditions, the fluorescence quantum yield of CdTe nanocrystals in aqueous solution reached 85% at room temperature. Optical measurements indicated that the photodegradation of TGA rather than NCs was the main reason for the PL enhancement effect since sulfide ions released from TGA during illumination coordinated with cadmium ions on the surface of CdTe and formed a shell structure of CdS there. X-ray photoelectron spectroscopic results further proved the formation of CdS shell on the CdTe core during illumination. The colloidal solution of CdTe/ CdS core-shell nanocrystals with photoluminescence quantum yield higher than 80% turned out to be very stable. Therefore, the current investigations not only reveal the PL enhancement effect of illumination, but also provide a useful synthetic route for producing water-soluble and highly fluorescent CdTe/CdS core-shell nanocrystals.
Preparation of Highly Luminescent Cdte Quantum Dots and Its Probes
We have prepared CdTe QDs in the size range from 2.8 to 4.4 nm with different organic ligands bonded on the QD surface. The organic ligands usually contain thiol, carboxyl or amino groups. These ionic groups provide QDs water solubility and should serve as linkers for conjugation of QD with important molecules to create selective luminescent probes. We have prepared QDs with mercaptopropionic acid, thioglycolic acid, cystein, mercaptoundecanoic acid, mercaptoethanol and cysteamine on the surface. Various synthesis procedures were also tested. The luminescence quantum yields of prepared QDs were compared. The secondary method was used for the determination of quantum yield with fluorescein as a standard. The determined quantum yields vary from 0.02 to 12% depending on synthesis conditions and organic ligand used. The highest quantum yield have been obtained for QD with cysteamine on the surface prepared at pH = 3. Preparation conditions of luminescent probes via conjugation of QDs wi...
Journal of Physical Chemistry B, 2004
Highly luminescent CdSe and CdTe quantum dots (QDs) are prepared in a hot solvent of capping molecules (TOP/TOPO/HDA for CdSe and TOP/DDA for CdTe). The influence of exchange of the capping molecules with different types of thiol molecules (amino ethanethiol, (3-mercaptopropyl)trimethoxysilane, hexanethiol, 2-propenethiol, and 4-mercaptophenol) is investigated for both CdSe and CdTe QDs. A remarkable difference is observed: capping exchange with thiol molecules results in an increased luminescence efficiency for CdTe QDs but induces quenching of the excitonic emission of CdSe QDs. The striking difference between the two types of II-VI QDs is explained by the difference in the energy of the valence band top. The lower energetic position of the valence band for CdSe results in hole trapping of the photogenerated hole on the thiol molecule, thus quenching the luminescence. For CdTe the valence band is situated at higher energies with respect to the redox level of most thiols, thus inhibiting hole trapping and maintaining a high luminescence efficiency.
Physica B: Condensed Matter, 2010
Thiol-capped CdTe nanoparticles were synthesized in aqueous solution by wet chemical route. CdTe nanoparticles with bifunctional molecule mercaptoacetic acid as a stabilizer were synthesized at pH E 11.2 and using potassium tellurite as tellurium source. The effect of refluxing time on the preparation of these samples was measured using UV-vis absorption and photoluminescence analysis. By increasing the refluxing time the UV-vis absorption and photoluminescence results show that the band edge emission is redshifted. The synthesized thiol-capped CdTe were characterized with FT-IR, TEM and TG-DTA. The particle size was calculated by the effective mass approximation (EMA). The role of precursors, their composition, pH and reaction procedure on the development of nanoparticles are analyzed.
International Nano Letters, 2013
Water-soluble cadmium telluride (CdTe) quantum dots (QDs) were synthesized using thioglycolic acid as capping agent; this reaction was carried out at pH = 10.2 and refluxed at 100°C for 18 h. The CdTe QDs prepared at various reflux times from 1 to 18 h were coated on the glass substrates to obtain thin films of the CdTe QDs with the same thickness. The CdTe thin films were characterized by X-ray diffraction, ultraviolet-vis spectroscopy, and photoluminescence spectroscopy. The absorption thresholds of the CdTe thin films are blueshifted by about 0.65 eV with respect to the bulk value (1.5 eV), due to the quantum size effect as expected from the nanocrystalline nature of the CdTe QDs. X-ray diffraction showed that the films consisted of small CdTe nanocrystallites, 2.91 to 3.57 nm in size, showing quantum size effects. The effects of temperature on the electrical properties of the films were studied in detail. Electrical resistivity measurements were carried out for different films in the temperature range from 343 to 463 K. It is shown that activation energy increases by increasing the reflux time.
ChemistrySelect, 2019
The nature of the capping ligands used and the strength of their interactions with the surface atoms of the nanoparticles (NPs) impact greatly on the material properties and the stability of the resulting NPs. The structural analysis revealed that all the as-prepared CdTe NPs samples had a zinc blende crystal structure of different phases. The glutathione and L-cysteine capped CdTe NPs had the same zinc blende phase (JCPDS no. 75-2086) while thioglycolic acid capped and co-capped CdTe NPs possessed another zinc blende phase (JCPDS no. 75-2083). The calculated crystallite sizes were in the range 2-26 nm for all the samples. The optical spectroscopy studies showed various properties when different capping ligands were used. The band gap energies for all the as-prepared CdTe NPs confirmed the results obtained from the X-ray diffraction. The band gap energies were in the range of 2.39-3.34 eV for all the as-prepared CdTe NPs.