Growth Of CdS Shell Research Articles

Characterization of Photocurrent Generation Dynamics in Polymer Solar Cells Based on ZnO/CdS-Core/Shell Nanoarrays by Intensity Modulated Photocurrent Spectroscopy: Theoretical Modeling

A theoretical model is developed for the dynamic characterization of hybrid polymer-based solar cells (HPSCs) based on vertically aligned ZnO/CdS-core/shell nanorod arrays (ZC-NAs) by intensity modulated photocurrent spectroscopy (IMPS). The model describes the effects of CdS shell formation on charge generation and transport dynamics. Particularly, an analytical expression for the ineffective polymer phase model in nanoarray solar cells is developed and introduced into IMPS model for the first time. The main expectations of the IMPS model are confirmed by the experimental data of the polymer/ZC-NA cells with the CdS shell thickness (L) of 3–8 nm. It is shown that the contributions from CdS absorption (f1) and polymer absorption (f2) to charge generation are determined by the core/shell nanoarray structure and the intrinsic polymer property, while the optimal CdS shell thickness (Lopt) depends on the interspacing between ZnO core nanorods and the exciton diffusion length of the polymer. The photocurrent generation is dominantly the competitive results of f1 and f2 contributions subjected to the change in L, with the polymer as a dominant absorption material. Fittings of the measured IMPS responses to the IMPS model reveal that the L-dependence of photocurrent generation dominantly originates from f1, f2, and the polymer exciton dissociation rate S at the polymer/CdS interface. Moreover, the first-order rate constants for the surface defects to trap and detrap the injected electrons in ZnO core nanorods are found to decrease with CdS shell growth and become saturated at Lopt. Furthermore, it is demonstrated that the effective electron diffusion coefficient De in the ZnO nanorods reaches a peak value at Lopt as the result of the largest photogenerated electron density in conduction band. Those results provide a guide to the design of efficient HPSCs based on the core/shell nanoarrays with complementary properties.

Sample-averaged biexciton quantum yield measured by solution-phase photon correlation.

The brightness of nanoscale optical materials such as semiconductor nanocrystals is currently limited in high excitation flux applications by inefficient multiexciton fluorescence. We have devised a solution-phase photon correlation measurement that can conveniently and reliably measure the average biexciton-to-exciton quantum yield ratio of an entire sample without user selection bias. This technique can be used to investigate the multiexciton recombination dynamics of a broad scope of synthetically underdeveloped materials, including those with low exciton quantum yields and poor fluorescence stability. Here, we have applied this method to measure weak biexciton fluorescence in samples of visible-emitting InP/ZnS and InAs/ZnS core/shell nanocrystals, and to demonstrate that a rapid CdS shell growth procedure can markedly increase the biexciton fluorescence of CdSe nanocrystals.

Structural Profiles of Nanocrystals from ASAXS and Crystallographic Techniques

The chemical synthesis of core/shell colloidal nanocrystals (NCs) have lead to an pronounced improvement in the optical properties and the chemical stability of semiconducting NCs [1]. The main topic of this work is the structural characterisation of core/shell NCs with anomalous small angle x-ray scattering (ASAXS) in combination with diffraction techniques (XRD) at laboratory- and synchrotron sources (HZB-BESSY and ESRF). Furthermore we complete these findings with complementary microscopy techniques (TEM). The detailed knowledge of the structural properties of the core/shell NCs allows to study the impact of the nanometer sized dimensions on their optical properties. The infrared emission of lead chalcogenide nanocrystals (NCs) in the size range of 5 - 10 nm can be drastically increased stabilising the core with a hard protective shell [1,2]., e.g., PbS/CdS NCs shows a higher efficiency and stability [2] with respect to pure PbS-NCs. In contrast to a shell growth on top of a core, we investigated in this study the CdS-shell growth on PbS NCs driven by Cd for Pb cation exchange [2]. Especially, we studied three different final shell thicknesses of 0.9, 1.5 and 2 nm. The chemical composition profile of the CdS-shell as a function of reaction time are derived from ASAXS experiments in sub-nanomter resolution. The crystal structure of the shell was derived by XRD combined with TEM measurements, respectively. We relate the chemical and structural information to the measured PL intensities of the core/shell NCs. We reveal the existence of two different crystalline phases, i.e. the metastable rock salt and the equilibrium zinc blende phase within the chemically pure CdS-shell. The highest improvement in the PL emission was achieved for 0.9 nm shells depicting a large metastable rock salt phase fraction matching the crystal structure of the PbS core. These results could be only achieved using ASAXS that gieves a mean chemical profile of a large ensemble of single core/shell NCs, but in sub-nanometer resolution [3].

Achieving enhanced visible-light-driven photocatalysis using type-II NaNbO3/CdS core/shell heterostructures.

Expanding the light-harvesting range and suppressing the quick recombination of photogenerated charge carriers are of paramount significance in the field of photocatalysis. One possible approach to achieve wide absorption range is to synthesize type-II core/shell heterostructures. In addition, this system also shows great promise for fast separation of charge carriers and low charge recombination rate. Herein, following the surface functionalization method using 3-mercaptopropionic acid (MPA) as a surface functionalizing agent, we report on designing NaNbO3/CdS type-II core/shell heterostructures with an absorption range extending to visible range and explore the opportunity toward degradation of methylene blue (MB) dye as a model pollutant under visible light irradiation. Characterizations including X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), UV-vis diffuse reflectance spectrum (DRS), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and Raman spectroscopy support the growth of CdS shell onto NaNbO3 nanorods. The resulting core/shell heterostructures unveiled high surface areas, enhanced light harvesting, and appreciably increased photocatalytic activity toward MB degradation compared to individual counterparts and the photocatalytic standard, Degussa P25, under visible light irradiation. The remarkably enhanced photocatalytic activity of core/shell heterostructures could be interpreted in terms of efficient charge separation owing to core/shell morphology and resulting type-II band alignment between NaNbO3 and CdS, which creates a step-like radial potential favoring the localization of one of the carriers in the core and the other in the shell. A plausible mechanism for the degradation of MB dye over NaNbO3/CdS core/shell heterostructures is also elucidated using active species scavenger studies. Our findings imply that hydroxyl radicals (OH(•)) play a crucial role in dictating the degradation of MB under visible light. This work highlights the importance of core/shell heterostructures in leading toward new paradigms for developing highly efficient and reusable photocatalysts for the destructive oxidation of recalcitrant organic pollutants.

Shell Thickness Modulation in Ultrasmall CdSe/CdSxSe1–x/CdS Core/Shell Quantum Dots via 1-Thioglycerol

In this study, we report on the synthesis of CdSe/CdS core-shell ultrasmall quantum dots (CS-USQDs) using an aqueous-based wet chemistry protocol. The proposed chemical route uses increasing concentration of 1-thioglycerol to grow the CdS shell on top of the as-precipitated CdSe core in a controllable way. We found that lower concentration of 1-thioglycerol (3 mmol) added into the reaction medium limits the growth of the CdSe core, and higher and increasing concentration (5-11 mmol) of 1-thioglycerol promotes the growth of CdS shell on top of the CdSe core in a very controllable way, with an increase from 0.50 to 1.25 nm in shell thickness. The growth of CS-USQDs of CdSe/CdS was confirmed by using different experimental techniques, such as optical absorption (OA) spectroscopy, fluorescence spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and Raman spectroscopy. Data collected from OA were used to obtain the average values of the CdSe core diameter, whereas Raman data were used to assess the average values of the CdSe core diameter and CdS shell thicknesses.

Influence of the core size on biexciton quantum yield of giant CdSe/CdS nanocrystals

We present a systematic study of photoluminescence (PL) emission intensity and biexciton (BX) quantum yields (QYBX) in individual "giant" CdSe/CdS nanocrystals (g-NCs) as a function of g-NC core size and shell thickness. We show that g-NC core size significantly affects QYBX and can be utilized as an effective tuning parameter towards higher QYBX while keeping the total volume of the g-NC constant. Specifically, we observe that small-core (2.2 nm diameter) CdSe/CdS NCs with a volume of ∼200 nm(3) (shell comprises 4 CdS monolayers) show very low average and maximum QYBX's of ∼3 and 7%, respectively. In contrast, same-volume medium-core (3 nm diameter) NCs afford higher average values of ∼10%, while QYBX's of ∼30% are achieved for same-volume large-core (5.5 nm diameter) CdSe/CdS NCs, with some approaching ∼80%. These observations underline the influence of the g-NC core size on the evolution of PL emissive states in multi-shell NCs. Moreover, our study also reveals that the use of long anneal times in the growth of CdS shells plays a critical role in achieving high QYBX.

Size tuning of MAA capped CdSe and CdSe/CdS quantum dots and their stability in different pH environments

The present work reports synthesis of mercaptoacetic acid capped CdSe nanoparticles soluble in water at different temperatures and with different precursor ratios. This enabled to tune the particle size of QDs from 2.7 to 5.8 nm. The particles consist of nanocrystals; with mixed phase, hexagonal wurtzite as well as sphalerite cubic and are luminescent with quantum yield 10%. The quantum yield up to 20% has been obtained by growing a shell of CdS over the CdSe. HR-TEM images, XRD patterns and the photoluminescence excitation spectra shows epitaxial growth of CdS shell over CdSe and with average size 3.2 ± 1.2 and 4.7 ± 1.2 nm for CdSe and CdSe/CdS quantum dots respectively. FT-IR spectrum and the negative zeta potential value together confirms the attachment of mercaptoacetic acid to the QD surface, where the carboxylic acid group is facing towards solvent and provides stability due to electrostatic hindrance. Further, the QDs are checked for their stability and the luminescence in environments of different pH (4–11 pH). Both CdSe and CdSe/CdS agglomerate with total elimination of fluorescence for 4 pH medium, and no shift in the fluorescence emission peak observed for the 6–9 pH, therefore QDs can be applicable as the fluorescence tags in this specific range of pH.

Controlled Alloying of the Core–Shell Interface in CdSe/CdS Quantum Dots for Suppression of Auger Recombination

The influence of a CdSexS1-x interfacial alloyed layer on the photophysical properties of core/shell CdSe/CdS nanocrystal quantum dots (QDs) is investigated by comparing reference QDs with a sharp core/shell interface to alloyed structures with an intermediate CdSexS1-x layer at the core/shell interface. To fully realize the structural contrast, we have developed two novel synthetic approaches: a method for fast CdS-shell growth, which results in an abrupt core/shell boundary (no intentional or unintentional alloying), and a method for depositing a CdSexS1-x alloy layer of controlled composition onto the CdSe core prior to the growth of the CdS shell. Both types of QDs possess similar size-dependent single-exciton properties (photoluminescence energy, quantum yield, and decay lifetime). However the alloyed QDs show a significantly longer biexciton lifetime and up to a 3-fold increase in the biexciton emission efficiency compared to the reference samples. These results provide direct evidence that the structure of the QD interface has a significant effect on the rate of nonradiative Auger recombination, which dominates biexciton decay. We also observe that the energy gradient at the core-shell interface introduced by the alloyed layer accelerates hole trapping from the shell to the core states, which results in suppression of shell emission. This comparative study offers practical guidelines for controlling multicarrier Auger recombination without a significant effect on either spectral or dynamical properties of single excitons. The proposed strategy should be applicable to QDs of a variety of compositions (including, e.g., infrared-emitting QDs) and can benefit numerous applications from light emitting diodes and lasers to photodetectors and photovoltaics.

Highly efficient and well-resolved Mn2+ ion emission in MnS/ZnS/CdS quantum dots

We demonstrate a strategy for the growth of Mn2+ ion doped cadmium based II–VI semiconductor quantum dots (QDs) with a designed buffer layer of ZnS (MnS/ZnS/CdS or Mn:CdS QDs), which aims to meet the challenge of obtaining highly efficient and well-resolved Mn2+ ion emission. First, small, high quality MnS cores are obtained by using thiols to replace conventional alkyl amines as capping ligands. Then a buffer layer of ZnS with a tailored thickness is introduced to the QDs before the growth of CdS shells to reduce the size mismatch between the Mn2+ (dopant) and Cd2+ (host) ions. The fabricated MnS/ZnS/CdS core/shell QDs exhibit a high PL QY of up to 68%, which is the highest ever reported for any type of Mn2+ ion doped cadmium based II–VI semiconductor QD. The photoluminescence (PL) of the QDs consists of well-resolved Mn2+ ion emission without any detectable emission from the CdS band edge or surface defects. In addition, our MnS/ZnS/CdS QDs cannot only be made water-soluble, but can also be coated by ligands with short carbon chain lengths, nearly without cost to the PL QY, which could make them strong candidates for practical applications in biology/biomedicine and opto/electronic devices.

Effects of Core Size and Shell Thickness on Luminescence Dynamics of Wurtzite CdSe/CdS Core/Shell Nanocrystals

Colloidal CdSe nanocrystals (NCs), whose size is 2.8, 3.8, and 4.9 nm, respectively, were successively overcoated with CdS monolayers (MLs). The X-ray diffraction patterns indicated that the stress in the wurtzite CdSe core was increased with the epitaxial growth of CdS shell, and the CdSe lattice contraction, which was sensitive to core size, did not release with the CdS shell toward 5 MLs. The effects of the CdSe core size and the CdS shell thickness on the temperature-dependent photoluminescence (PL) lifetime were investigated. The PL lifetime undulation with temperature is indicative of the spatial distribution of trap states, and the strong interplay between intrinsic excitons and surface traps can be activated even in the case of the NCs with 5 ML CdS shell.

Preparation of water soluble CdSe and CdSe/CdS quantum dots and their uses in imaging of cell and blood capillary

A novel method for the synthesis of water-soluble CdSe and CdSe/CdS quantum dots (QDs) under the assistance of high-intensity ultrasonic irradiation is reported. As-prepared CdSe/CdS QDs were characterized by X-ray powder diffraction and high-resolution transmission electronmicroscopy. The absorption and fluorescence emission spectra were measured to investigate the effect of CdS passivation on the electronic structure of the quantum dots. After the growth of CdS shell, the photoluminescence quantum yields of CdSe/CdS core–shell quantum dots increased three times more than that of the original CdSe QDs. The QDs were successfully used for the fluorescence imaging of cells and blood capillary.

Facile synthesis and observation of discontinuous red-shift photoluminescence of CdTe/CdS core/shell nanocrystals

A simple method was introduced to synthesize CdTe/CdS core/shell nanocrystals (NCs) by using a new kind of “green” tellurium precursor. Both absorption and photoluminescence (PL) spectra of CdTe/CdS NCs exhibited a discontinuous shift as follows: slight (<10 nm), large (∼30 nm), and slight (<10 nm) red-shift during the growth of CdS shell. Different sized CdTe cores were introduced to synthesize CdTe/CdS core/shell NCs and similar discontinuous red-shift phenomena in the PL spectra were observed. These characteristics indicated that the structure of CdTe/CdS NCs gradually evolved from type-I to type-II, in agreement with transmission electron microscopy (TEM), the tuning of PL quantum yields (QYs), and PL lifetime results.

Effect of shell thickness on two-photon absorption and refraction of colloidal CdSe/CdS core/shell nanocrystals

Colloidal CdSe nanocrystals (NCs) are successively overcoated with CdS monolayers (MLs), and the effective two-photon absorption (TPA) coefficient and refractive index of this series of CdS/CdSe core/shell NCs are measured by a Z-scan technique at 800 nm wavelength. The TPA and nonlinear refraction are enhanced dramatically with the CdS shell growth towards 3 MLs, but decreased with the further shell growth. The effects of surface states, local field, and intrinsic piezoelectric polarization are discussed to explain the optical nonlinearity of the colloidal core/shell NCs.

Synthesis, morphology, and optical properties of colloidal CdTe/CdSe and CdTe/CdS nanoheterostructures based on CdTe tetrapods

We report the synthesis of colloidal CdTe/CdSe and CdTe/CdS nanoheterostructures based on CdTe tetrapods with a CdSe or CdS shell. The shell growth takes place on the lateral faces of the tetrapod legs, whereas the leg length remains essentially unchanged. Both the CdSe and CdS shell growth shifts the luminescence of the nanoheterostructures to the near-IR range, up to 850 nm, with a quantum yield of up to 20%. Analysis of the kinetics of the shift of the excitonic absorption band during nanoheterostructure growth suggests that the growth rate of CdS exceeds that of CdSe. The variation of the luminescence wavelength with shell thickness is compared to numerical modeling results.

Long Electron-Hole Separation of ZnO-CdS Core-Shell Quantum Dots.

The tunability of electronic and optical properties of semiconductor nanocrystal quantum dots (QDs) has been an important subject in nanotechnology. While control of the emission property of QDs in wavelength has been studied extensively, control of the emission lifetime of QDs has not been explored in depth. In this report, ZnO-CdS core-shell QDs were synthesized in a two-step process, in which we initially synthesized ZnO core particles, and then stepwise slow growth of CdS shells followed. The coating of a CdS shell on a ZnO core increased the exciton lifetime more than 100 times that of the core ZnO QD, and the lifetime was further extended as the thickness of shell increased. This long electron-hole recombination lifetime is due to a unique staggered band alignment between the ZnO core and CdS shell, so-called type II band alignment, where the carrier excitation holes and electrons are spatially separated at the core and shell, and the exciton lifetime becomes extremely sensitive to the thickness of the shell. Here, we demonstrated that the emission lifetime becomes controllable with the thickness of the shell in ZnO-CdS core-shell QDs. The longer excitonic lifetime of type II QDs could be beneficial in fluorescence-based sensors, medical imaging, solar cells photovoltaics, and lasers.

Preparation and spectroscopic investigation of colloidal CdSe/CdS/ZnS core/multishell nanostructure

Colloidal CdSe/CdS/ZnS core/multishell nanostructure with the different thicknesses of CdS and ZnS shell was prepared by chemical method using CdO and ZnO. The absorption, photoluminescence and Raman scattering spectra of CdSe core, CdSe/CdS core/shell and CdSe/CdS/ZnS core/multishell structure have been comparatively studied. The photoluminescence full width at half maximum of CdSe core is less than 20 nm, indicating the monodisperse colloidal CdSe nanocrystals. It was found that the reaction temperature needed for the formation of ZnS shell is higher than that for the growth of CdS shell. The strong increase in the emission intensity by successively coating the CdS and ZnS shell around the CdSe core has been discussed. The effect of shell thickness on the spectroscopic characteristics of CdSe/CdS/ZnS core/multishell nanostructure has been investigated.

Synthesis of CuInS2@CdS Core‐Shell Nanocrystals

CuInS2@CdS core‐shell nanocrystals were prepared in a wet chemical process. Transmission electron microscope (TEM), x‐ray energy dispersive spectroscopy (EDAX), x‐ray diffraction (XRD), absorption, and photoluminescence (PL) spectra were used to confirm the formation of the CuInS2@CdS core‐shell structure. The growth of CdS shell not only increased the PL intensity, but also restrained the transformation of CuInS2 from nanoparticles to nanorods after annealing, which was attributed to an effective chemical passivation of the CuInS2 core by the CdS shell.