Synthesis and characterization of thiol-stabilized CdTe nanocrystals (original) (raw)
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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.
The Journal of Physical Chemistry B, 1999
As an expansion to the wet chemical route for the preparation of quantum-sized II-VI semiconductor materials, a series of thiol-capped crystalline CdSe nanoparticles has been synthesized in aqueous solution using mercaptoalcohols (2-mercaptoethanol, 1-thioglycerol), and mercapto acids (thioglycolic acid, thiolactic acid) as stabilizers. The smaller (app. 1.4-2.2 nm diameter) CdSe particles were obtained using thioalcohols as capping agents; the use of thioacids as stabilizers produced larger (2.1-3.2 nm diameter) CdSe particles. CdSe nanoparticles were separated from the crude solutions as redissolvable powder samples with narrow size distributions using a size-selective fractionation and have been characterized by UV-vis absorption and photoluminescence spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy, and energy-dispersive X-ray analysis. A calculation of the HOMO-LUMO gap of CdSe particles as a function of their size has been done using an extended effective mass approximation.
Optical properties of thiol-stabilised CdTe nanoparticles
… Journal of Nanoparticles, 2009
Thiol-capped CdTe nanocrystals with cubic zinc blende structure were synthesised in aqueous solution by wet chemical route and microwave assisted method. A series of cadmium telluride (CdTe) nanoparticles capped with mercaptoacetic acid were prepared using different pH values. The synthesised nanoparticles have been characterised with Raman, FT-IR, ultraviolet visible (UV-Vis) absorption, fluorescence spectroscopy and X-ray powder diffraction (XRD) studies.
Thiol-Capping of CdTe Nanocrystals: An Alternative to Organometallic Synthetic Routes
The Journal of Physical Chemistry B, 2002
New approaches to synthesize photostable thiol-capped CdTe nanocrystals are reported. Post-preparative sizeselective precipitation and selective photochemical etching have been developed as methods providing an increase of photoluminescence quantum efficiency of the nanocrystals of up to 40%. Some advantages of thiol-capping in comparison to conventional organometallic syntheses of quantum dots are discussed.
physica status solidi (a), 2010
We have investigated the evolution of thiol‐capped cadmium telluride nanocrystals prepared in aqueous solutions at low and room temperature followed by heating at 50–100 °C. The UV–visible absorption spectra, as well as transmission electron microscopy, atomic force microscopy, and dynamic light scattering observations made it possible to reveal differences in size and structure originating from different heating temperatures. It was shown that minimization of the nucleation process temperature provided formation of larger nanoparticles than at higher temperatures.
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.
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.
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.
Optically detected magnetic resonance of thiol-capped CdTe nanocrystals
Israel Journal of Chemistry, 2001
The optical properties of thiol-stabilized CdTe nanocrystals have been examined. The thiol groups-SR generate a CdS shell at the interface, leading to a CdTe/CdS core-shell structure. The present paper describes our efforts to identify the influence of the CdTe-SR interface on the optical properties of the nanocrystals, utilizing photoluminescence and optically detected magnetic resonance (ODMR) spectroscopy. The photoluminescence spectrum consists of an excitonic peak, overlapped by a broad band at lower energies. The ODMR spectrum, in the spectral regime of the broad band, showed two resonance signals. They are associated with a trapped hole at an anisotropic site of a cadmium vacancy at the Cd-SR interface and an electron in the conduction band.