Cadmium Selenide Nanocrystals Research Articles

Growth kinetics of CdSe nanocrystals synthesized in liquid paraffin via one-pot method

The cadmium selenide (CdSe) nanocrystals are synthesized in liquid paraffin via one-pot method. The growth kinetics of the nanocrystals is investigated by the measurements of their ultraviolet–visible absorption and fluorescence emission. The results show that very fast growth occurs linearly for the nanoparticles regardless of the molar ratio of Cd to Se until a reaction time of approximately 60 s, followed by essential growth rate decrease. The nanocrystals thereafter possess a two-stage kinetic feature consisting of a rapid growth within approximately 30 min and a slow growing up to 10 h. The particles size is not governed by the reaction temperature but by the molar ratio of Cd to Se. From the viewpoint of both strong fluorescence emission and saving in raw materials, a reaction temperature of 200 °C and a molar ratio of Cd to Se of 4/1 are appropriate for the synthesis of the CdSe nanocrystals.

Poly-(3)hexylthiophene nanowire networks for versatile fabrication of bulk heterojunctions with increased active volume

Spontaneous demixing of poly-(3)hexylthiophene (P3HT) and polystyrene (PS) generates a dense array of crystalline P3HT nanowires embedded in a PS matrix. The PS phase is subsequently removed, to form a network of P3HT nanowires with a greatly enhanced surface area-to-volume ratio relative to a planar P3HT film. Photovoltaic devices fabricated from these networks backfilled with either organic ([6,6]-phenyl-C61-butyric acid methyl ester (PCBM)) or inorganic (cadmium selenide (CdSe) nanocrystals) material have dramatically increased short-circuit current and power conversion efficiencies relative to planar-bilayer-prepared devices, with increases of ∼8× and ∼3× for PCBM and CdSe nanocrystals, respectively, suggesting that these devices have increased P3HT active volume. This two-step backfill technique can produce nanostructured all-organic and hybrid organic/inorganic bulk heterojunction structures that cannot be formed with traditional one-step casting methods.

Expeditious Stochastic Calculation of Random-Phase Approximation Energies for Thousands of Electrons in Three Dimensions.

A fast method is developed for calculating the random phase approximation (RPA) correlation energy for density functional theory. The correlation energy is given by a trace over a projected RPA response matrix, and the trace is taken by a stochastic approach using random perturbation vectors. For a fixed statistical error in the total energy per electron, the method scales, at most, quadratically with the system size; however, in practice, due to self-averaging, it requires less statistical sampling as the system grows, and the performance is close to linear scaling. We demonstrate the method by calculating the RPA correlation energy for cadmium selenide and silicon nanocrystals with over 1500 electrons. We find that the RPA correlation energies per electron are largely independent of the nanocrystal size. In addition, we show that a correlated sampling technique enables calculation of the energy difference between two slightly distorted configurations with scaling and a statistical error similar to that of the total energy per electron.

Synthesis and characterization of CdSe nanocrystals in the presence of butylamine as a capping agent

TOPO-capped cadmium selenide (CdSe) nanocrystals of sizes between 3 and 8 nm have been synthesized, and the surface-capping molecule, trioctylphosphine oxide, was replaced by butylamine. The effects of changing the surface ligands of the synthesized CdSe nanocrystals on the structural, optical, and electrical properties were investi- gated. The shift toward shorter wavelength (higher energy) in the visible range of the optical absorption band edge was observed by UV-Vis spectroscopy, and a blue-shift of the photoluminescence peaks was observed with luminescent quenching. Surface modification was found to cause an increase in the surface energy of nanocrystals, resulting in the improvement in charge carrier separation and cell performance in applications towards bulk hetero-junction solar cells.

Semiconductor nanocrystals dispersed in imidazolium-based ionic liquids: a spectroscopic and morphological investigation

A growing interest is devoted to the study of imidazolium-based ionic liquids as innovative materials to combine with functional elements for advanced technological applications. Materials based on semiconductor and oxide nanocrystals in ionic liquids can be promising for their integration in lithium batteries, as well as in innovative solar cells. Although the physical chemical properties and the solvation dynamics of bare ionic liquids have been extensively studied, their combination with colloidal nanocrystals still remains almost unexplored. Here, the optical properties of organic-capped luminescent cadmium selenide nanocrystals coated by a shell of zinc sulfide (CdSe(ZnS)) dispersed in 1,3-dialkyl imidazolium ionic liquids have been investigated, also in dependence of the alkyl chain length on the imidazolium ring and of the anion nature, by using both time-integrated and time-resolved optical spectroscopy. The observed variations in decay profiles of the ionic liquid in presence of colloidal nanocrystals suggest that the dispersion of the nanostructures induces modifications in the ionic liquid structural order. Finally, atomic force microscopy analysis has provided insight into the topography of the investigated dispersions deposited as film, confirming the organization of the ionic liquids in super-structures, also upon nanocrystal incorporation.

Rational design on n-type organic materials for high performance organic photovoltaics

The demand for further optimization of the photovoltaic efficiency has stimulated an intensive research effort both for new low-band-gap polymeric materials as electron donors and for new efficient electron-accepting materials. As regards the latter, less attention has been observed on the optimization of the electron acceptor material compared to the extensive studies on the electron donor polymer in organic photovoltaic (OPV) cells. The majority of the acceptor materials used so far in solar cells are organic (carbon based) materials, however other traditional acceptor materials include inorganic semiconductors, such as cadmium selenide (CdSe) nanocrystals, titanium oxide (TiO2) and zinc oxide (ZnO) nanoparticles. From the implemented organic materials high power conversion efficiencies PCEs (>9.0%) are observed in OPVs utilizing fullerene derivatives and especially [6,6]-phenyl-C61 butyric acid methyl ester (PC60BM) as the acceptor. Recently, very promising efficiencies of ∼5.0% have been also obtained by Polyera corporation utilizing soluble conjugated polymers as both the donor and the acceptor components. It is therefore expected that through improved materials design for enhancing the electron mobility, better tuning of the energy levels and absorption profile of organic materials, significant improvements in the device performance can still be expected. In this work, the current trends on the most promising solution processable n-type organic materials (fullerene derivatives, small molecules and conjugated polymers) will be presented in detail, emphasizing on the correlation between structure/optoelectronic properties/morphology characterization and device performances.

Anisotropic Formation and Distribution of Stacking Faults in II–VI Semiconductor Nanorods

Nanocrystals of cadmium selenide exhibit a form of polytypism with stable forms in both the wurtzite and zinc blende crystal structures. As a result, wurtzite nanorods of cadmium selenide tend to form stacking faults of zinc blende along the c-axis. These faults were found to preferentially form during the growth of the (001) face, which accounts for 40% of the rod's total length. Since II-VI semiconductor nanorods lack inversion symmetry along the c-axis of the particle, the two ends of the nanorod may be identified by this anisotropic distribution of faults.

In Situ Formation of Pyridyl-Functionalized Poly(3-hexylthiophene)s via Quenching of the Grignard Metathesis Polymerization: Toward Ligands for Semiconductor Quantum Dots

The synthesis of well-defined, end-functional poly(3-hexylthiophene)s (P3HTs) by in situ quenching of the Grignard metathesis (GRIM) polymerization is complicated by the extreme tendency to favor difunctional products in all but a few cases. A facile one-pot method for preparing 2-pyridyl and 3-pyridyl P3HTs with high abundance of monofunctional products is established via an examination of the kinetics of the end-functionalization quenching reaction with lithium chloride complexes of 2- and 3-pyridyl Grignard reagents. Density functional theory calculations guide the selection of pyridine as the end group, which provides the capacity to ligate cadmium selenide (CdSe) nanocrystals and arrests aggregation upon thermal annealing when dispersed in a P3HT matrix. The relative abundances of various end-functional products, as ascertained by high-resolution matrix assisted laser desorption ionization time-of-flight mass spectrometry, can be altered through the use of 1-pentene as an additive: GRIM polymerizations quenched with 3-pyridyl and 2-pyridyl Grignard reagents show 5% and 18% abundances of difunctional, pyridyl-capped P3HTs, respectively, when 1-pentene is present at 1000:1 relative to the nickel catalyst. This represents a significant improvement compared to quenching with aryl Grignard reagents, where difunctional products predominate. The ability to manipulate end group compositions coupled with the propensity of pyridyl-functionalized P3HTs to ligate semiconductor quantum dots (SQDs) opens new possibilities for tuning the morphology of conjugated polymer/SQD blends.

CdSe Nanocrystal Assemblies on Anodized TiO2 Nanotubes: Optical, Surface, and Photoelectrochemical Properties

The in situ preparation of high-quality organically synthesized cadmium selenide (CdSe) nanocrystals on TiO2 nanotubes (T_NT) prepared by anodization is presented. The deposition of CdSe nanocrystals has been facilitated under a high-pressure and low-viscosity solvothermal process. The formation, growth, and assembly of CdSe nanocrystals in the form of a dense film on T_NT have been studied using thermal (thermogravimetry and calorimetry) and optical (UV–vis and microscopy) techniques. It has been concluded that an organic treatment, followed by annealing under a nitrogen atmosphere at reduced temperatures, helps control CdSe nanocrystal morphology without causing significant particle size growth. Photoelectrochemical measurements indicate that the electrode assembly consisting of T_NT and CdSe can achieve a stable photocurrent density of 6.7 mA/cm2 and a charge-separation efficiency of 35%.

Synthesis and characterisation of CdSe nanocrystals using NaHSeO 3 as selenium source

Sodium hydrogen selenite was used as a novel selenium source in the synthesis of 3-mercaptopropionic acid-stabilised cadmium selenide (CdSe) nanocrystals (NCs) in an aqueous medium. The size, crystalline structure and optical properties of these NCs were characterised via high-resolution transmission electron microscopy, X-ray powder diffraction (XRD), ultraviolet-visible light absorption and photoluminescence (PL) spectroscopy. The factors affecting the Se:Cd molar ratio were also discussed. The CdSe NC grains are roughly spherical, and their average size is ~4-5-nm. The XRD patterns of the synthesised CdSe NCs are characteristic of zinc-blended structures. The obtained CdSe NCs exhibit a relatively narrow PL band at a full-width at half-maximum of ~125-44-nm, as well as a high PL peak in the yellow spectral range. The quantum yields of the corresponding CdSe NCs at Cd-to-Se molar ratios of 0.1, 0.2 and 0.3 are 2.38, 3.70 and 9.32%, respectively, indicating the strong dependence of the NC size on the precursor concentration in the period immediately following the precursor injection.

Erratum to: Quenching of photoluminescence in cadmium selenide nanocrystals in external electric fields for different excitation photon energies

Erratum to: Quenching of photoluminescence in cadmium selenide nanocrystals in external electric fields for different excitation photon energies

Quenching of photoluminescence in cadmium selenide nanocrystals in external electric fields for different excitation photon energies

The effect of external electric fields on the photoluminescence of quantum-sized nanocrystals of cadmium selenide excited by photons of various energies is studied. Photoluminescence quenching by external electric fields is found to be different for nanoparticles with different shapes (quantum dots and nanorods) and does not depend on the exciting photon energy. The relationship between the strength of the external electric field and the degree of quenching is determined empirically for both types of nanoparticles. A possible mechanism for the effect of an external electric field on the excitation and quenching of photoluminescence in quantumsized nanoparticles is discussed.

Dynamic Microscopy Study of Ultrafast Charge Transfer in a Hybrid P3HT/Hyperbranched CdSe Nanoparticle Blend for Photovoltaics.

We present a spectroscopic investigation on a new hyperbranched cadmium selenide nanocrystals (CdSe NC)/poly(3-hexylthiophene) (P3HT) blend, a potentially good active component in hybrid photovoltaics. Combined ultrafast transient absorption spectroscopy and morphological investigations by means of an ultrafast confocal microscope reveal a strong influence of the complex local structure on the photogenerated carrier dynamics. In particular, we map the electron-transfer process across the hybrid NC/polymer interface, and we reveal that charge separation occurs through a preferential pathway from the CdSe nanobranches to the P3HT chains. Efficient charge generation at the distributed heterojunction is also confirmed by scanning kelvin probe force microscopy measurements.

Photoinduced processes in nanocrystals of cadmium selenide in an external electric field

The dependence of photoinduced processes in cadmium selenide nanocrystals in an external electric field on the energy of the exciting photons is established. A relationship between the photoinduced buildup of photoluminescence and its quenching in an external electric field in quantum sized cadmium selenide nanocrystals is demonstrated. The mechanisms for the quenching of the photoluminescence and for the influence of the exciting photon energy on the photoinduced processes are discussed.

CdSe Nanocrystal/C60-liquid composite material with enhanced photoelectrochemical performance

The unique optoelectronic properties of a nanocomposite material comprised of cadmium selenide (CdSe) nanocrystals (NCs) embedded in a photoconductive liquid C60 have been investigated. CdSe NCs are shown to sensitize the composite material to visible light, resulting in charge transfer from optically excited CdSe NCs to the solvent-free liquid C60 phase.

“Green” Synthesis of Cadmium Selenide Nanocrystals: The Scope of 1,2,3-Selendiazoles in the Synthesis of Magic-Size Nanocrystals and Quantum Dots

A range of cadmium selenide nano-particles (the quantum dots (QDs), the magic-size nano-crystals (MSNCs) or the mixture of two) have been synthesized by the use of organoselenium reagents viz. 1,2,3-selenadiazole and metal salts in an appropriate choice of long chain fatty acids e.g., oleic acid with or without a solvent. These different types of nanocrystals can be easily obtained simply by variation in reaction temperature. The employed approach does not use any hazardous reagents and is typically non-aqueous and can be considered "green." The temperature at which the reaction is carried out along with the ratio of reactants and surfactant affect the nature of products and have led to understanding of some parameters affecting the formation of either "thermodynamic" or "kinetically" stabilized products. We have obtained sub-nanosized particles (magic-sized nanocrystals), zero-dimensional quantum dots and self-assembled structures by these methods. The materials have been studied by UV-Visible spectroscopy (UV-Vis), Photoluminescence spectroscopy (PL), X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersion X-ray analysis (EDAX), and Transmission electron microscopy (TEM).

Band-like transport, high electron mobility and high photoconductivity in all-inorganic nanocrystal arrays

Flexible, thin-film electronic and optoelectronic devices typically involve a trade-off between performance and fabrication cost. For example, solution-based deposition allows semiconductors to be patterned onto large-area substrates to make solar cells and displays, but the electron mobility in solution-deposited semiconductor layers is much lower than in semiconductors grown at high temperatures from the gas phase. Here, we report band-like electron transport in arrays of colloidal cadmium selenide nanocrystals capped with the molecular metal chalcogenide complex In(2)Se(4)(2-), and measure electron mobilities as high as 16 cm(2) V(-1) s(-1), which is about an order of magnitude higher than in the best solution-processed organic and nanocrystal devices so far. We also use CdSe/CdS core-shell nanoparticles with In(2)Se(4)(2-) ligands to build photodetectors with normalized detectivity D* > 1 × 10(13) Jones (I Jones = 1 cm Hz(1/2) W(-1)), which is a record for II-VI nanocrystals. Our approach does not require high processing temperatures, and can be extended to different nanocrystals and inorganic surface ligands.

“Yellow emitting” magic-size cadmium selenide nanocrystals via a simplified spray pyrolysis method

Magic-size nanocrystals of cadmium selenide showing yellow-light emission were synthesized in a single-step spray pyrolysis process at 150 °C. An organoselenium precursor dissolved in N,N-dimethyl formamide was introduced via a syringe pump and the droplets were decomposed in hot solution of cadmium oleate. The ratio of Cd:Se precursor, the process temperature at which the decomposition is carried out and the co-formation of quantum dots play critical role in unique optical property of yellow-light emission.

Precursor Conversion Kinetics and the Nucleation of Cadmium Selenide Nanocrystals

The kinetics of cadmium selenide (CdSe) nanocrystal formation was studied using UV-visible absorption spectroscopy integrated with an automated, high-throughput synthesis platform. Reaction of anhydrous cadmium octadecylphosphonate (Cd-ODPA) with alkylphosphine selenides (1, tri-n-octylphosphine selenide; 2, di-n-butylphenylphosphine selenide; 3, n-butyldiphenylphosphine selenide) in recrystallized tri-n-octylphosphine oxide was monitored by following the absorbance of CdSe at λ = 350 nm, where the extinction coefficient is independent of size, and the disappearance of the selenium precursor using {(1)H}(31)P NMR spectroscopy. Our results indicate that precursor conversion limits the rate of nanocrystal nucleation and growth. The initial precursor conversion rate (Q(o)) depends linearly on [1] (Q(o)(1) = 3.0-36 μM/s) and decreases as the number of aryl groups bound to phosphorus increases (1 > 2 > 3). Changes to Q(o) influence the final number of nanocrystals and thus control particle size. Using similar methods, we show that changing [ODPA] has a negligible influence on precursor reactivity while increasing the growth rate of nuclei, thereby decreasing the final number of nanocrystals. These results are interpreted in light of a mechanism where the precursors react in an irreversible step that supplies the reaction medium with a solute form of the semiconductor.

Nonmonotonic dependence of the photoluminescence peak position of cadmium selenide nanocrystals as a function of the excitation photon energy

The photoluminescence properties of colloidal CdSe quantum dots with a mean size of 3 nm were studied. The experimental results revealed the nonmonotonic dependence of the photoluminescence peak’s position as a function of the excitation photon energy being varied from 2.41 to 3.68 eV. This effect can be explained by the size distribution of the nanocrystals and the spectral dependence of their absorption coefficients.