Surfactant-dependent photoluminescence of CdTe/CdS nanocrystals

ABSTRACT

The photoluminescence of aqueously synthesised core/shell CdTe/CdS quantum dots (QDs) was investigated. Two molar ratios (2.4 and 1.3) of thioglycolic acid (TGA) to Cd²⁺ were compared to determine the best synthesis conditions for high photoluminescent quantum yield (PLQY) and photostability. A difference in the PLQY of the CdTe/CdS QDs was observed when CdS shells were grown with different TGA/Cd²⁺ ratios. The difference in the observed PLQY was attributed to the quality of the passivation of the CdTe during the CdS shell growth. At TGA/Cd²⁺ ratio of 1.3, the CdS shell forms through homogeneous nucleation, which is limited by diffusion of growth material from the solution onto the QDs surface. Due to the lattice mismatch of CdTe and CdS, the core will experience coherence strain resulting in dislocation sites and surface defects between nucleation sites which can result in non-radiative trap states. When the TGA/Cd²⁺ ratio is 2.0, the CdS shell grows epitaxially, minimising the number of surface trap states. Finally, we observed that the fluorescence intermittency was supressed for CdTe QDs after UV light illumination, attributed to annealing of deep surface trap states by UV light.     

A one-pot aqueous synthesis of high-luminescent thiol-capped CdTe and its bioapplication

Our study described a synthesis of thiol-capped high-luminescent (quantum yield as high as 80%) CdTe quantum dots (QDs) with a facile method in aqueous phase. The fluorescence of the as-prepared CdTe QDs could be tuned from 500 nm to 650 nm. More importantly, after beta-actin conjugation, the CdTe QDs were successfully conjugated with live cells to observe their configurations, demonstrating their potentially broad application as biolabels. The crystal structure, morphology and optical property of the products were investigated by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and fluorescence spectrophotometer.     

Effect of thioglycolic acid on the stability and photoluminescence properties of colloidal solutions of CdTe nanocrystals

We have studied the physicochemical properties of cadmium telluride nanostructures prepared by colloidal synthesis and analyzed the photoluminescence characteristics of CdTe nanocrystals stabilized with thioglycolic acid. The results demonstrate that the relationship between Cd and Te precursors and the synthesis time influence the nanocrystal size and the optical properties of the nanocrystal-based disperse system.     

Aqueous synthesis of highly monodispersed thiol-capped CdTe quantum dots through electrochemical approach

A facile green approach has been developed to control the growth regime in the aqueous synthesis of CdTe semiconductor Quantum dots (QDs) via the electrochemistry method. The Low growth temperature and slow injection of Te precursors are used to prolong the diffusion controlled stage and thus suppress Ostwald ripening during nanocrystal growth. The experimental results showed that a low concentration of Te precursor would definitely influence the growth procedure. The narrow absorption peaks in the UV-visible absorption spectra, as well as transmission electron microscopy images indicated that the as-prepared CdTe QDs had a good monodispersity. The high-resolution transmission electron microscopy (HRTEM) images and powder X-ray diffraction (XRD) pattern suggested that the as-prepared QDs have high crystallinity and cubic structure. The QDs exhibited high fluorescence QYs about 50% and the best of QY 67% without any postpreparative treatment over a broad spectral range of 516-609 nm, which could be further broadened by long-term refluxing. The current work suggested that electrochemical method was an attractive approach to the synthesis of high-quality II-VI semiconductor QDs at a large scale.     

The effect of photo-irradiation on the optical properties of thiol-capped CdTe quantum dots

In this report, the effect of photo-irradiation on the optical properties of thioglycolic acid (TGA) capped CdTe QDs was investigated. The photo-irradiation led to an increase in the photoluminescence (PL) efficiency of TGA-capped CdTe QDs with a low quantum yield under both the open air and nitrogen atmosphere. The photo-irradiation caused a blue-shift of PL peak under the open air and almost no change of PL peak position under the nitrogen atmosphere. The XPS study revealed the oxidation of the unpassivated surface of the Te atoms which mainly contributed to the observed optical property changes for CdTe QDs photo-irradiated under the open air. While for the CdTe QDs photo-irradiated under the nitrogen atmosphere, the decomposition of TGA led to the release of sulphur which formed a CdS shell on the CdTe core, resulting in an enhanced PL efficiency.     

具有狭窄荧光光谱的红色荧光CdSe纳米晶合成研究

采用胶体化学方法合成了红色荧光的CdSe纳米晶，具有较大的粒径和狭窄的荧光峰。其制备方法是以粒径较小的CdSe纳米晶（3-4nm）作为晶种，再通过相应前驱体的缓慢加入来得到晶种各向同性的生长，最终生成较大粒径的CdSe纳米晶，颗粒直径可达8-9nm，且荧光峰值在红光波长范围。所得的材料具有很好的单分散性和均匀的粒径分布，表现出半峰宽≤30nm的狭窄而对称的荧光光谱。     

Colloidal PbS nanocrystals with narrow photoluminescence spectra

Luminescent PbS nanocrystals have been synthesized by the colloidal method. PVA has been employed to modify the surface of prepared PbS nanocrystals and improve their optical properties. Optical and morphological characteristics of lead sulfide nanocrystals have been studied by high resolution electron microscopy and spectrophotometer. Optical absorption and photoluminescence (PL) studies of the PbS nanocrystals have shown the strong quantum confinement effects. For the first time, the prepared lead sulfide nanocrystals have emitted at 608 nm wavelength with narrow band width (21 nm) and high Stokes shift. Experimental results have shown that the surface charge traps have higher contribution to the optical properties of colloidal PbS nanocrystals and in our sample photoluminescence was due to hole relaxation. These properties made them a material with potential application in nanophotonics.     

On the photoluminescence emission of ZnO nanocrystals

Photoluminescence emission (PL) behaviour of ZnO nanocrystals capped by different organic molecules and dispersed in solvents with increasing dielectric constant is studied. PL energy is observed to red shift or blue shift or even remains invariant with increasing dielectric constant of the dispersing medium depending on the capping agent. The effective dielectric constant of the dispersing medium shifts the surface defect levels energetically and governs the interfacial charge re-organisation. Further, solvent relaxation effects also cause lowering of the energy levels. Defects located at the surface of nanocrystals profoundly influence the luminescence behaviour.     

Time-resolved photoluminescence spectroscopy of ligand-capped PbS nanocrystals

PbS nanocrystals are synthesized using colloidal techniques and have their surfaces capped with oleic acid. The absorption band edge of the PbS nanocrystals is tuned between 900 and 580?nm. The PbS nanocrystals exhibit tuneable photoluminescence with large non-resonant Stokes shifts of up to 500?meV. The magnitude of the Stokes shift is found to be dependent upon the size of PbS nanocrystals. Time-resolved photoluminescence spectroscopy of the PbS nanocrystals reveals that the photoluminescence has an extraordinarily long lifetime of 1?μs. This long fluorescence lifetime is attributed to the effect of dielectric screening similar to that observed in other IV-VI semiconductor nanocrystals.     

Cluster synthesis of branched CdTe nanocrystals for use in light-emitting diodes

Highly luminescent cadmium telluride (CdTe) nanocrystals were synthesized using Li(2)[Cd(4)(SPh)(10)] as a reactive Cd cluster compound at relatively low temperature, making it a safe precursor for the large scale synthesis of CdTe nanocrystals. Transmission electron microscopy (TEM) showed that the shape of the CdTe nanocrystals changes from nanorods to branched structures with increasing reaction time. The nanocrystals show high luminescent quantum yields up to 37% for CdTe branched nanostructures, and as high as 52% for CdTe/CdS core-shell heterostructures. CdTe/CdS nanocrystals were used to make light-emitting diodes in combination with organic layers for electron and hole injection. The devices show a maximum luminance efficiency of 0.35?cd?A(-1).     

Introducing CTAB into CdTe/PVP nanofibers enhances the photoluminescence intensity of CdTe nanoparticles

The major objective of this work is focused on the preparation and characterization of the photoluminescence (PL) property of poly(vinyl pyrrolidone) (PVP) embedding CdTe nanoparticles. The CdTe nanoparticles were generated via the reaction of Cd²⁺ with NaHTe and then stabilized by thiolglycolic acid (TGA). In the process of preparing CdTe/PVP nanofibers by electrospinning, a surfactant, cetyltrimethylammonium bromide (CTAB), was introduced to prevent CdTe nanoparticles from congregating inside the PVP nanofibers. Then the results of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron diffraction (ED) showed that the average diameter of CdTe/PVP nanofibers was 300 (± 61) nm, in which the CdTe nanoparticles were incorporated into the PVP nanofibers homogeneously. Finally, the PL spectra proved that the photoluminescence intensity of CdTe/PVP nanofibers was enhanced by the addition of CTAB.     

二氧化钛纳米晶的制备及其光致发光的研究

以Ti（SO4）2为前驱体,采用沉淀法制备了二氧化钛纳米晶,通过X射线衍射（XRD）、透射电子显微镜（TEM）和光致发光（PL）光谱对微晶进行表征。研究表明,制备的TiO2纳米晶呈类球形颗粒且分散性好,平均粒径最小约为4nm,热处理温度升高到750℃时,样品仍为单一的锐钛矿相;PL谱分析表明,室温下,用高于带隙的能量激发,在370～550nm范围内纳米TiO2粒子呈现出强而宽的发光带,分别对应于价带和导带间的电子跃迁以及表面态的发光;另外发现随焙烧温度升高,粒径增大的同时,发光峰的强度呈无规则变化,分析可能与表面态和晶粒内部缺陷的数量有关。     

Facile synthesis of amine-functionalized graphene quantum dots with highly pH-sensitive photoluminescence

Graphene quantum dots (GQDs) as promising materials have gained increasing attention due to their optical and chemical properties, but the complicated synthetic procedures restrict their large-scale application. Here we report a novel one-pot hydrothermal method to synthesize the GQDs with bifunctional groups (-OH and -NH₂) using a polycyclic aromatic hydrocarbon as the precursor, in which 1,5-dinitronaphthalene was graphitized in aqueous ammonium solution. The as-synthesized GQDs with single-layer structures have a size distribution of 1–3 nm and an average value of 1.5 nm. The alkaline species play a crucial role in the formation of amine-functionalized GQDs. More importantly, the as-synthesized GQDs exhibit a distinct pH-sensitive photoluminescence and the fluorescence colors vary from bluish green to yellow with pH value from 5 to 10, which can be attributed to the protonation and deprotonation of the amine-groups in acid or alkaline solution.     

Investigation of the excitonic luminescence band of CdTe solar cells by photoluminescence and photoluminescence excitation spectroscopy

The excitonic luminescence band of polycrystalline cadmium telluride layers has been investigated by Photoluminescence (PL) and Photoluminescence excitation spectroscopy (PLE). CdTe was deposited by means of close space sublimation and the samples were activated by different chlorine containing compounds, i.e. cadmium chloride, hydrochloric acid, and sodium chloride as well as by simple air activation or received no post deposition treatment. In the PL spectra, four different peaks within the excitonic luminescence band were resolved. These include the free-exciton peak and two transitions of excitons bound to defects. Furthermore, free excitons and band to band transitions were detected by means of PLE. The PL and PLE spectra are discussed with respect to the post deposition treatments.     

Synthesis of highly stable dihydrolipoic acid capped water-soluble CdTe nanocrystals

To prevent the dissociation of organic ligands from the inorganic core and accordingly enhance colloidal stability, is the prerequisite for the further application of nanocrystals in different areas. Dihydrolipoic acid (DHLA) appears very promising for the stabilization and the further functionalization of CdTe nanocrystals because DHLA is characterized by a carboxylic acid group and two thiol functions, through which simultaneous anchorage onto a nanoparticle's surface is possible. For the first time, we synthesized highly luminescent CdTe nanocrystals (maximum quantum yield of?42%) in aqueous solution by the reaction of Cd(2+) and NaHTe using DHLA as capping reagent. The influence of various experimental variables, including reaction temperature, ligand-to-Cd ratio and pH value as well as Te-to-Cd ratio, on the growth rate and optical properties of the obtained CdTe nanocrystals have been systematically investigated. Furthermore, the stability of the dithiol DHLA capped CdTe nanocrystals was compared with the monothiol ligands thioglycolicacid (TGA) and mercaptopropionic acid (MPA) capped nanocrystals. Experimental results indicate that the DHLA capped nanoparticles show better chemical and thermal stability than those capped by TGA or MPA. The mechanism for the high-quality optical properties and the good stability of the resulting CdTe nanocrystals is also primarily elucidated.     

Influence of atmosphere on photoluminescence properties of Eu-doped ZnO nanocrystals

We have fabricated Eu³⁺-doped ZnO (ZnO:Eu) nanocrystals (NCs) by a reverse micelle method, and have studied their photoluminescence (PL) properties in vacuum, nitrogen gas, and oxygen gas atmospheres. The ZnO:Eu NCs exhibit the exciton, defect and Eu³⁺ PL under the inter-band photoexcitation of the ZnO host NCs. The intensity ratio among the three PL peaks is sensitive to the atmosphere for the PL measurements. We discuss the influence of the surrounding gas atmosphere to the PL properties.     

A photoluminescence study of film structure in CdTe nanoparticle thin films

The layer-by-layer deposition of thin films of CdTe nanoparticles and three different polyelectrolytes has been investigated. Photoluminescence spectra were used to monitor the energy transfer properties within the films. As the number of bilayers in a thin film was increased a decrease in the energy of the light emitted was observed. The wavelength change is a two-stage process. Deposition of the first one to two bi-layers of a thin film produced a sharp energy change (626 nm to 637 nm with the addition of a single bi-layer) whereas deposition of subsequent bi-layers produced a more gradual energy change (642 nm-646 nm with the addition of 5 bi-layers). A space-filling mechanism is suggested to account for these changes; smaller nanoparticles penetrate the earlier levels of a thin film and increase the inter-particle energy transfer opportunities within the layers.