Effect of capping agents on optical and antibacterial properties of cadmium selenide quantum dots

Cadmium selenide quantum dots (CdSe QDs) were synthesized in aqueous phase by the freezing temperature injection technique using different capping agents (viz. thioglycolic acid, 1-thioglycerol, L-cysteine). Absorption spectra of CdSe QDs exhibited a blue shift as compared to its bulk counterpart, which is an indication of quantum confinement effect. The photoluminescence spectra of CdSe QDs confirmed that the particles are poly-dispersed and possess enhanced luminescent property, depending upon the chemical nature of capping agents. The QDs have been characterized by Fourier-transform infrared spectroscopy, atomic absorption spectroscopy and transmission electron microscopy. Further, antimicrobial activity of as-prepared QDs has also been investigated using the disk diffusion method.     

Ligand and precursor effects on the synthesis and optical properties of PbS quantum dots

We study how the as-received chemical reagents of a commonly used ligand oleylamine (C18-amine) and precursor PbCl2, each at two different purity statuses, affect the growth of PbS quantum dots in a solventless, relatively green, constant reaction-temperature synthesis system. It is found that the growth behavior of PbS quantum dots reflected from their absorption and photoluminescence spectra is quite sensitive to the purity status of the ligand and precursor under certain circumstances, while the lifetime and quantum yield of quantum dots exhibiting a monomodal or nearly monomodal photoluminescence band are not considerably affected. For instance, the effect of the ligand purity status is particularly evident when a higher PbCl2/S ratio is applied. The use of lower purity C18-amine leads to the growth showing much stronger temperature dependence and also facilitates the earlier entry of Ostwald process highlighted by a bimodal photoluminescence structure. Consistently, a 2 wt% increase in the PbCl2 purity from 98 wt% to 100 wt% (or the absence of 2 wt% of impurities) largely postpones the start of Ostwald process and thus significantly improves both absorption and photoluminescence spectra. These results imply that in order to produce PbS quantum dots with narrow absorption and photoluminescence peaks, one needs to optimize reaction parameters as well as select chemicals of appropriate purities. Moreover, the unintentional involvement of chemicals of different purity status may partially account for the irreproducibility problem often encountered in quantum dot synthesis.     

Optical and photocatalytic properties of arginine-stabilized cadmium sulfide quantum dots

Currently, environmental pollution caused by organic compounds leads to severe negative consequences in the human society. Therefore, the removal of these pollutants from aqueous media has become one of the most important issues in environmental science. In the present study, CdS QDs were successfully prepared under aqueous conditions using l-arginine as the stabilizing agent. Optical property determination results reveal that the CdS QDs exhibited strong absorption and photoluminescence in a visible wavelength region. Moreover, the CdS QDs could effectively degrade two organic dyes under visible light irradiation. This suggested that the CdS QDs prepared in this work might be used as the potential photocatalyst to effectively treat the organic pollutants under visible light irradiation.     

Cu-In-S三元量子点的合成及其性能调控

采用前驱体分解法制备了Cu-In-S量子点,研究了制备工艺对Cu-In-S量子点的形貌以及光学性能的影响。实验结果表明,反应时间和反应温度可影响Cu-In-S纳米颗粒的尺寸和光学性能。随时间增加,Cu-In-S粒径变大,同时会伴随着棒状晶体的出现,荧光发射谱的峰位发生红移。随反应温度升高,纳米晶的形核速率和长大速率增加,并且粒径也有增大,纳米晶的形状可以由单一的球形变为球形与棒状的混合,荧光谱峰位亦会发生红移。X射线光电子能谱分析表明,所制备颗粒为CuInS2纳米晶。为进一步制备无毒量子点发光器件(QLEDs)奠定了基础。     

Near-field optical properties of quantum dots, applications and perspectives

Recent years have witnessed tremendous research in quantum dots as excellent models of quantum physics at the nanoscale and as excellent candidates for various applications based on their optoelectronic properties. This review intends to present theoretical and experimental investigations of the near-field optical properties of these structures, and their multimodal applications such as biosensors, biological labels, optical fibers, switches and sensors, visual displays, photovoltaic devices and related patents.     

Surfactant mediated optical properties of cytosine capped CdSe quantum dots

This letter demonstrates the use of one of the nucleobases, ‘cytosine’ as a new capping agent in controlling the size of the nanoparticles. A size dependent blue shift in optical absorption with enhanced luminescence is observed. Since the calculated density of states do not show any change in the band gap of as-prepared quantum dots after capping, the observed blue shift of the absorption peak can solely be attributed to the so-called size-effect whereas the enhancement in luminescence to surfactant mediated defect passivation. It is expected that the observed properties of the cytosine capped CdSe quantum dots would facilitate a better bio-compatibility of tailor-made nanoparticles for bio-imaging applications.     

Tunable optical properties of colloidal quantum dots in electrolytic environments

The absorption spectra of colloidal cadmium sulfide quantum dots in electrolytic solutions are found to manifest a shift in the absorption threshold as the concentration of the electrolyte is varied. These results are consistent with a shift in the absorption threshold that would be caused by electrolytic screening of the field caused by the intrinsic spontaneous polarisation of these würtzite structured quantum dots. These electrolyte-dependent absorption properties provide a potential means of gaining insights on the variable extracellular and intracellular electrolytic concentrations that are present in biological systems.     

Improved optical properties of CdS quantum dots by ligand exchange

High-quality CdS nanocrystals with highly efficient and narrow band edge emission have been prepared by means of decomposition of suitable precursors in non-coordinating solvents at high temperature. The role played by capping ligands, solvents and their interactions has been investigated by optical measurements. Capping layer exchange treatments with different amines have been carried out suggesting a possible influence of alkylic chain length in modifying the optical properties of CdS nanocrystals. Enhancement of band edge emission has been demonstrated with octylamine ligand.     

Facile covalent immobilization of cadmium sulfide quantum dots on graphene oxide nanosheets: preparation, characterization, and optical properties

A facile approach for the preparation of a novel hybrid material containing graphene and an inorganic semiconducting material, cadmium sulfide quantum dots (CdS QDs), is demonstrated for the first time. First, amino-functionalized CdS QDs were prepared by modifications of the kinetic trapping method. Then, pristine graphite was oxidized and exfoliated to obtain graphene oxide nanosheets (GONS), which were then acylated with thionyl chloride to introduce acyl chloride groups on their surface. Subsequently, immobilization of the CdS QDs on the GONS surface was achieved through an amidation reaction between the amino groups located on the CdS QDs surface and the acyl chloride groups bound to the GONS surface. Fourier transform infrared spectroscopy (FT-IR), (1)H nuclear magnetic resonance ((1)H-NMR), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and energy dispersive x-ray (EDX) spectroscopy were employed to investigate the changes in the surface functionalities, while high resolution transmission electron microscopy (HR-TEM) and field emission scanning electronic microscopy (FE-SEM) were used to study the morphologies and distribution of the CdS QDs on the GONS surface. Thermogravimetric analysis (TGA) was employed to characterize the weight loss of the samples on heating. Photoluminescence (PL) measurements were used to study the optical properties of the prepared CdS QDs and the CdS-graphene hybrid material.     

Thermal-induced intermixing effects on the optical properties of long wavelength low density InAs/GaAs quantum dots

The effect of post-growth rapid thermal annealing on the photoluminescence properties of long wavelength low density InAs/GaAs (001) quantum dots (QDs) with well defined electronic shells has been investigated. For an annealing temperature of 650 °C for 30 s, the emission wavelength and the intersublevel spacing energies remain unchanged while the integrated PL intensity increases. For higher annealing temperature, blue shift of the emission energy together with a decrease in the intersublevel spacing energies are shown to occur due to the thermal activated In–Ga interdiffusion. While, this behaviour is commonly explained as a consequence of the enrichment in Ga of the QDs, the appearance of an additional exited state for annealing temperatures higher than 650 °C suggests a variation of the intermixed QDs's volume/diameter ratio toward QDs's enlargement.     

Surface functionalized cadmium telluride quantum dots for the optical detection and determination of herbicides

An optical sensor for detection of herbicides was developed through the functionalization of CdTe quantum dots (CdTe-QDs) with cysteamine hydrochloride. The functionalized CdTe-QDs was characterized with various physicochemical methods such as X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, energy dispersive X-ray analysis, Ultraviolet–visible and photoluminescence spectroscopies. The optical band gap of the functionalized CdTe-QDs as calculated by using Tauc plot was 3.75 eV. It was found that the fluorescence intensity of the functionalized CdTe-QDs quenched linearly in the presence of different herbicides according to the Stern–Volmer equation. Thus, the functionalized CdTe-QDs can be used as simple, rapid, inexpensive, and optical sensitive sensor for practical detection of herbicides.     

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.     

Nanostructure assembly of indium sulphide quantum dots and their characterization

Nanocrystals (approximately 5 nm) of the semiconducting wide band gap material beta-In2S3 obtained by chemical synthesis through a hydrothermal route were characterized for phase and compositional purity. These nanoparticles exhibited quantum confinement characteristics as revealed by a blue-shifted optical absorption. These quantum dots of beta-In2S3 were electrically driven from a monodisperse colloidal suspension on to conducting glass substrates by Electophoretic Deposition (EPD) technique and nanostructural thin films were obtained. The crystalline and morphological structures of these deposits were investigated by X-ray diffraction and nanoscopic techniques. We report here that certain interesting nanostructural morphologies were observed in the two-dimensional quantum dot assemblies of beta-In2S3. The effect of the controlling parameters on the cluster growth and deposit integrity was also systematically studied through a series of experiments and the results are reported here.     

Investigation on one-pot hydrothermal synthesis,structural and optical properties of ZnS quantum dots

The advantage of hydrothermal synthesis of semiconductor quantum dots (QDs) over the control of particles size, morphology and stability is reported here. In a typical synthesis procedure, the zinc and sulfur precursor molar ratio of 1:3 was used in an aqueous solution at 150 °C. The cubic phase of ZnS with average particles size of 5 nm was confirmed and estimated from the X-ray diffraction (XRD) analysis. The composition and purity of the sample were analyzed from (energy dispersive-ray analysis) EDAX and (X-ray photoelectron spectroscopy analysis) XPS spectra. The absorption spectrum shows the large shift in the absorption band over 90 nm due to the quantum confinement of carriers. The emission spectrum of quantum dots carry more evidence on the presence of shallow trap, deep trap in the band gap of the material responsible for weak emission in the spectral region of 450–500 nm. High resolution transmission electron microscope and scanning electron microscope studies reveal the structural and morphological features of ZnS with slightly distorted spherical morphology. We found that the coordinating ability of solvent strongly influences the reaction process and morphology of the products.     

Three-dimensional optical control of individual quantum dots

We show that individual colloidal CdSe-core quantum dots can be optically trapped and manipulated in three dimensions by an infrared continuous wave laser operated at low laser powers. This makes possible utilizing quantum dots not only for visualization but also for manipulation, an important advantage for single molecule experiments. Moreover, we provide quantitative information about the magnitude of forces applicable to a single quantum dot and of the polarizability of an individual quantum dot.     

Effects of multiple organic ligands on size uniformity and optical properties of ZnSe quantum dots

The effects of multi-ligands on the formation and optical transitions of ZnSe quantum dots have been investigated. The dots are synthesized using 3-mercapto-1,2-propanediol and polyvinylpyrrolidone ligands, and have been characterized by X-ray diffraction, transmission electron microscopy (TEM), UV–visible absorption spectroscopy, photoluminescence spectroscopy, and Fourier transform infrared spectroscopy. TEM reveals high monodispersion with an average size of 4 nm. Polymer-stabilized, organic ligand-passivated ZnSe quantum dots exhibit strong UV emission at 326 nm and strong quantum confinement in the UV–visible absorption spectrum. Uniform size and suppressed surface trap emission are observed when the polymer ligand is used. The possible growth mechanism is discussed.     

Effects of different modified underlayer surfaces on growth and optical properties of InGaN quantum dots

InGaN/GaN quantum dots were grown on the sapphire (0 0 0 1) substrate in a metalorganic chemical vapor deposition system. The morphologies of QDs deposited on different modified underlayer (GaN) surfaces, including naturally as grown, Ga-mediated, In-mediated, and air-passivated ones, were investigated by atomic force microscopy (AFM). Photoluminescence (PL) method is used to evaluate optical properties. It is shown that InGaN QDs can form directly on the natural GaN layer. However, both the size and distribution show obvious inhomogeneities. Such a heavy fluctuation in size leads to double peaks for QDs with short growth time, and broad peaks for QDs with long growth time in their low-temperature PL spectra. QDs grown on the Ga-mediated GaN underlayer tends to coalesce. Distinct transform takes place from 3D to 2D growth on the In-mediated ones, and thus the formation of QDs is prohibited. Those results clarify Ga and In's surfactant behavior. When the GaN underlayer is passivated in the air, and together with an additional low-temperature-grown seeding layer, however, the island growth mode is enhanced. Subsequently, grown InGaN QDs are characterized by a relatively high density and an improved Gaussian-like distribution in size. Short surface diffusion length at low growth temperature accounts for that result. It is concluded that reduced temperature favors QD's 3D growth and surface passivation can provide another promising way to obtain high-density QDs that especially suits MOCVD system.     

Formation mechanism and optical properties of InAs quantum dots on the surface of GaAs nanowires

Formation mechanism and optical properties of InAs quantum dots (QDs) on the surface of GaAs nanowires (NWs) were investigated. This NW-QDs hybrid structure was fabricated by Au-catalyzed metal organic chemical vapor deposition. We found that the formation and distribution of QDs were strongly influenced by the deposition time of InAs as well as the diameter of GaAs NWs. A model based on the adatom diffusion mechanism was proposed to describe the evolution process of the QDs. Photoluminescence emission from the InAs QDs with a peak wavelength of 940 nm was observed at room temperature. The structure also exhibits a decoupling feature that QDs act as gain medium, while NW acts as Fabry-Perot cavity. This hybrid structure could serve as an important element in high-performance NW-based optoelectronic devices, such as near-infrared lasers, optical detectors, and solar cells.