Growth Of CdS Shell Research Articles

High luminescence of CdTe/CdSe/CdS core/shell/shell QDs: synthesis via a simple photochemical approach and gamma dosimetry application

In this research CdTe/CdSe/CdS core/shell/shell quantum dots (QDs) were synthesized by a photochemical approach using thioglycolic acid as capping agent and S2- source for CdS shell growth. X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform-infrared spectroscopy (FT-IR), and Raman results confirmed formation of CdTe/CdSe/CdS core/shell/shell structure. Photoluminescence (PL) quantum yield (QY) of the CdTe QDs was 23% and 50 and 70% for CdTe/CdSe and CdTe/CdSe/CdS QDs, respectively. Effect of 60Co gamma irradiation on optical and structural properties of CdTe and CdTe/CdSe/CdS core/shell/shell QDs was investigated. The obtained results indicated stability of crystalline structure of QDs after gamma irradiation but PL intensity of the CdTe/CdSe/CdS QDs decreased gradually after receiving different doses of gamma irradiation with a red shift in PL peak position. The obtained results suggested these luminescence QDs as potential materials for gamma dosimetry.

Optically Active CdSe/CdS Nanoplatelets Exhibiting Both Circular Dichroism and Circularly Polarized Luminescence

AbstractLigand‐induced chirality in colloidal semiconductor nanocrystals attracts attention because of their tunable chiroptical properties. Here, the induced chirality and circularly polarized luminescence (CPL) are investigated as a function of the CdS shell growth in a range of 2D CdSe/CdS nanoplatelets (NPLs) capped with chiral ligands. Five samples of CdSe/CdS NPLs are synthesized by a one‐pot approach to vary the island‐like shell on a four‐monolayer (4 ML) CdSe NPLs core, which effectively reduces the interfacial strain energy. The successful preparation of L‐/D‐Cysteine‐capped CdSe/CdS NPLs with both tunable circular dichroism (CD) and CPL behaviors and a maximum anisotropic luminance factor (glum) of 5.29 × 10−4 is described. The induced chiroptical response shows a direct relationship with the formation of island‐like shell in the first and second stages and shows a clear signal evolution. In the third stage with a full coating shell, the CD and CPL signals are inversely proportional to the CdS shell thickness. The island‐like shell gives birth to the CPL signal, while the formation of full coating shell decreases the induced chirality. Such chiral and emissive NPLs provide an ideal platform for the rational design of semiconductor nanocrystals with chiroptical properties in areas of biomedicine, polarizers, and new generation of display devices.

Colloidal-ALD-Grown Core/Shell CdSe/CdS Nanoplatelets as Seen by DNP Enhanced PASS-PIETA NMR Spectroscopy.

Ligand exchange and CdS shell growth onto colloidal CdSe nanoplatelets (NPLs) using colloidal atomic layer deposition (c-ALD) were investigated by solid-state nuclear magnetic resonance (NMR) experiments, in particular, dynamic nuclear polarization (DNP) enhanced phase adjusted spinning sidebands–phase incremented echo-train acquisition (PASS–PIETA). The improved sensitivity and resolution of DNP enhanced PASS–PIETA permits the identification and study of the core, shell, and surface species of CdSe and CdSe/CdS core/shell NPLs heterostructures at all stages of c-ALD. The cadmium chemical shielding was found to be proportionally dependent on the number and nature of coordinating chalcogen-based ligands. DFT calculations permitted the separation of the the 111/113Cd chemical shielding into its different components, revealing that the varying strength of paramagnetic and spin–orbit shielding contributions are responsible for the chemical shielding trend of cadmium chalcogenides. Overall, this study points to the roughening and increased chemical disorder at the surface during the shell growth process, which is not readily captured by the conventional characterization tools such as electron microscopy.

High efficiency sandwich structure luminescent solar concentrators based on colloidal quantum dots

Luminescent solar concentrators (LSCs) have received significant attention because of their low cost, large-area and high efficiency sunlight energy harvesting. Colloidal core/shell quantum dots (QDs) are promising candidates as absorbers/emitters in LSCs. However, due to the limitation of QDs properties and device architectures, LSCs fabricated using QDs still face the challenges of low optical efficiency and limited long-term stability for the large-area LSCs. In this work, we synthesized CdSe/CdS QDs, and found that higher CdS shell growth temperature results in improved uniformity in structure and morphology and more suitable optical properties. Based on the CdSe/CdS QDs, a large-area (∼100 cm2) sandwich structure luminescent solar concentrator (LSC) was fabricated. By laminating the QDs layer between two sheets of optical clear glass, the reabsorption losses of the device can be reduced due to the decrease of photon escape. The as-fabricated sandwich structure device exhibits an external optical efficiency of ∼ 2.95% under natural sunlight illumination, which represents a 78% enhancement in efficiency over the single layer film LSCs based on CdSe/CdS QDs. More importantly, the sandwich structure can protect the QDs interlayer from the impact of the ambient environment (e.g. oxygen, moisture and alkalinity) and enhance the long-term stability of LSCs. Our work shows that the use of suitably tuned core-shell QDs and the sandwich structure in LSC architecture can dramatically enhance the external optical efficiency of LSC devices based on CdSe/CdS QDs.

Impact of Shell Growth on Recombination Dynamics and Exciton-Phonon Interaction in CdSe-CdS Core-Shell Nanoplatelets.

We investigate the impact of shell growth on the carrier dynamics and exciton-phonon coupling in CdSe-CdS core-shell nanoplatelets with varying shell thickness. We observe that the recombination dynamics can be prolonged by more than one order of magnitude, and analyze the results in a global rate model as well as with simulations including strain and excitonic effects. We reveal that type I band alignment in the hetero platelets is maintained at least up to three monolayers of CdS, resulting in approximately constant radiative rates. Hence, observed changes of decay dynamics are not the result of an increasingly different electron and hole exciton wave function delocalization as often assumed, but an increasingly better passivation of nonradiative surface defects by the shell. Based on a global analysis of time-resolved and time-integrated data, we recover and model the temperature dependent quantum yield of these nanostructures and show that CdS shell growth leads to a strong enhancement of the photoluminescence quantum yield. Our results explain, for example, the very high lasing gain observed in CdSe-CdS nanoplatelets due to the type I band alignment that also makes them interesting as solar energy concentrators. Further, we reveal that the exciton-LO-phonon coupling is strongly tunable by the CdS shell thickness, enabling emission line width and coherence length control.

Completely quenching of the trap states emission of CdSe QDs by CdS/ZnS shell growth using a one pot photochemical approach and application for dye photo-degradation

In this research CdSe QDs was grown by microwave activated approach. Then using UV-sensitivity of TGA, CdS shell was grown on the CdSe cores and subsequently ZnS was shelled on the CdSe/CdS QDs. Synthesized QDs were characterized by means of XRD, FTIR, EDAX, UV–Vis and PL analysis. CdSe QDs indicated a broad band surface trap states emission between 500 and 800 nm with a peak at about 600 nm. This emission was decreased after CdS shell growth and was completely quenched after ZnS shell growth and a narrow band edge emission confirming successfully growth of the shell. Photocatalyst activity of the synthesized QDs was investigated by using methyl orange (MO) and methylene blue (MB) as pollutant. The CdSe indicated good photocatalyst activity for photo-degradation and it׳s activity was considerably increased after CdS shell and CdS/ZnS shell growth.

A simple UV-assisted, room temperature approach for synthesis of water soluble PbS and PbS/CdS core-shell QDs

In this work PbS and PbS/CdS core-shell QDs, were synthesized via a simple UV-assisted approach. Prepared QDs were characterized by means of XRD, FTIR, SEM, TEM, UV–vis and PL analysis. Synthesized QDs indicated cubic phase with a spherical morphology and the mean particle size was obtained about 4 nm. PbS QDs showed an emission with a peak at about 850 nm which it was tunable with the UV-illumination time. CdS shell growth resulted in the shift of the XRD pattern peaks to the higher angles, blue shift of the PL peak and considerably enhancement of the PL intensity. PbS QDs indicated strong photocatalyst activity for Methylene blue (MB) photo-degradation which it was increased significantly after CdS shell growth.

Photovoltaic performance of CdS/CdTe junctions on ZnO nanorod arrays

One-dimensional nanostructures, such as nanorod (NR) arrays, are expected to improve the photovoltaic (PV) response of solar cells with an ultrathin absorber due to an increased areal (junction) density and light trapping. We report on the deposition of CdS and CdTe:As semiconductor thin films on ZnO NR arrays by means of metalorganic chemical vapour deposition (MOCVD). The change in optical properties of the ZnO NRs upon the growth of CdS shell was monitored and compared to the simulated data, which confirmed the presence of strong light scattering effects in the visible and near infrared regions. The PV performance of nanostructured vs. planar CdS/CdTe solar cells (grown using the material from the same MOCVD run) showed similar conversion efficiencies (~ 4%), despite the current density being lower for the nanostructured cell due to its thicker CdS window. A clear improvement in the quantum efficiency was however observed in the near infrared region, resulting from the light trapping by the ZnO/CdS core-shell NR structure. We also showed that reduction of surface defects and use of high absorber carrier density would boost the efficiency beyond that of planar CdTe solar cells. The reported device performance and the direct observation of light trapping are promising towards optimisation of extremely-thin-absorber CdTe PV devices.

Electron Stability and Negative-Tetron Luminescence in Free-Standing Colloidal n-Type CdSe/CdS Quantum Dots.

We examine the effects of CdS shell growth on photochemical reduction of colloidal CdSe quantum dots (QDs) and describe the spectroscopic properties of the resulting n-type CdSe/CdS QDs. CdS shell growth greatly slows electron trapping. Because of this improvement, complete two-electron occupancy of the 1Se conduction-band orbital is achieved in CdSe/CdS QDs and found to be much more stable than in past experiments. Simultaneous photoluminescence at two different energies is now observed from QDs possessing two excess conduction-band electrons, reflecting competing recombination of discretized 1Se and 1Pe conduction-band electrons within photogenerated four-carrier negative tetrons (three electrons and one hole). Stable occupancy of the 1Pe level is not achievable under these conditions, and possible reasons are discussed. The stability and accessibility of these multielectron configurations, and the facile spectroscopic detection of negative tetrons, both make photodoped core/shell QDs attractive for exploring the physical properties of free-standing heavily n-doped colloidal CdSe-based QDs.

Preparation of fluorescent CdTe@CdS core@shell quantum dots using chemical free gamma irradiation method

This study investigates pH dependent gamma radiation effects on Mercaptopropionic Acid (MPA) capped CdTe quantum dots (QDs). Gamma radiation, as fast and chemical free catalyser, causes CdS Shell formation on MPA capped CdTe QDs at very high alkaline environment. UV–vis absorption spectroscopic results showed that, with increase in gamma radiation doses, the absorption peak of CdTe QDs decreased to lower wavelength (decrease in particle size) in both acidic & neutral medium (as synthesised), and it increased towards longer wavelength in basic medium indicating the possibility of growth of CdS shell over CdTe core QDs. X-ray diffractogram studies have revealed that the peaks of CdTe QDs shift towards higher angles after irradiation, supporting the notion of the core@shell particle structure of CdTe@CdS. TEM images confirmed the extra stretch around core particles from CdS material resulted in increase of average size of QDs. EDX study revealed the chemical composition of material system. X-ray photoelectron spectroscopic results revealed the shift in the binding energy of both Cd and S thereby indicating the change in the coordination condition after irradiation promoted the particles’ formation of core@shell in nature. The systematic photoluminescence spectroscopic investigation at all possible pH values against ascended gamma radiation doses revealed the formation of CdS shell over CdTe core at optimum pH value (i.e. only at basic pH 12). Contrarily, photo-oxidation of QDs’ was observed at neutral (and below) pH values towards gamma irradiation. The quantum yield is increased only after introducing external shell source material at highest alkaline media confirming the formation of CdS shell on CdTe core.

CdSe and CdSe/CdS core-shell QDs: New approach for synthesis, investigating optical properties and application in pollutant degradation.

In this work, CdSe quantum dots (QDs) were synthesized by a simple and rapid microwave activated approach using CdSO4 , Na2 SeO3 as precursors and thioglycolic acid (TGA) as capping agent molecule. A novel photochemical approach was introduced for the growth of CdS QDs and this approach was used to grow a CdS shell around CdSe cores for the formation of a CdSe/CdS core-shell structure. The core-shells were structurally verified using X-ray diffraction, transmission electron microscopy and FTIR (Fourier-transform infrared (FTIR)) spectroscopy. The optical properties of the samples were examined by means of UV-Vis and photoluminescence (PL) spectroscopy. It was found that CdS QDs emit a broad band white luminescence between 400 to 700nm with a peak located at about 510nm. CdSe QDs emission contained a broad band resulting from trap states between 450 to 800nm with a peak located at 600nm. After CdS shell growth, trap states emission was considerably quenched and a near band edge emission was appeared about 480nm. Optical studies revealed that the core-shell QDs possess strong ultraviolet (UV)-visible light photocatalytic activity. CdSe/CdS core-shell QDs, showed an enhancement in photodegradation of Methyl orange (MO) compared with CdSe QDs.

Surfactant-dependent photoluminescence of CdTe/CdS nanocrystals

ABSTRACTThe 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.

PbS/CdS Core/Shell Quantum Dots by Additive, Layer-by-Layer Shell Growth

We demonstrate additive shelling of PbS core quantum dots by CdS using room-temperature colloidal atomic layer deposition. Combining in-depth electron microscopy, absorption spectroscopy and elemental analysis, we present a structural model of the PbS/CdS core/shell quantum dots after each of the successive colloidal ALD cycles. In particular, we find that such core/shell quantum dots are cation-rich and we argue that this nonstoichiometry should be attributed to a Cd-excess at the outer surface. Analyzing various PbS/CdS core/shell QDs, we demonstrate that CdS shells have no effect on the energy of the PbS band-edge transition for PbS core QDs larger than ≈4 nm, a finding we support by effective mass calculations. In spite of this, CdS shell growth generally decreases the photoluminescence quantum yield of PbS/CdS core/shell QDs as compared to PbS core QDs. This suggests that photoluminescence quenching is mainly caused by defects at the PbS/CdS interface.

A computational ab initio study of surface diffusion of sulfur on the CdTe (111) surface

In order to discern the formation of epitaxial growth of CdS shell over CdTe nanocrystals, kinetics related to the initial stages of the growth of CdS on CdTe is investigated using ab-initio methods. We report diffusion of sulfur adatom on the CdTe (111) A-type (Cd-terminated) and B-type (Te-terminated) surfaces within the density functional theory (DFT). The barriers are computed by applying the climbing Nudge Elastic Band (c-NEB) method. From the results surface hopping emerges as the major mode of diffusion. In addition, there is a distinct contribution from kick-out type diffusion in which a CdTe surface atom is kicked out from its position and is replaced by the diffusing sulfur atom. Also, surface vacancy substitution contributes to the concomitant dynamics. There are sites on the B- type surface that are competitively close in terms of the binding energy to the lowest energy site of epitaxy on the surface. The kick-out process is more likely for B-type surface where a Te atom of the surface is displaced by a sulfur adatom. Further, on the B-type surface, subsurface migration of sulfur is indicated. Furthermore, the binding energies of S on CdTe reveal that on the A-type surface, epitaxial sites provide relatively higher binding energies and barriers than on B-type.

One-pot microwave assisted approach for synthesis of CdSe/CdS core-shell quantum dots (QDs) and investigating optical properties

In this work, CdSe QDs were synthesized using a microwave assisted method and chemical reaction between NaHSe, CdSO4 at the presence of TGA as capping molecule. Thereafter without CdSe extraction, CdS shell was grown subsequently around CdSe cores by a reaction based on the heat sensitivity of Na2S2O3 dissociation. Synthesized QDs were characterized by means of X-ray diffraction spectroscopy (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), UV–Vis and photoluminescence (PL) spectroscopy. All of these analyzes confirmed formation of CdSe QDs and successfully growth of CdS shell on surface of CdSe to forming CdSe/CdS core-shell structure.

Biexciton Auger Recombination in CdSe/CdS Core/Shell Semiconductor Nanocrystals.

A theoretical study of the positive and negative trion channels in the nonradiative Auger recombination of band-edge biexcitons (BXs) in CdSe/CdS core/shell nanocrystals (NCs) is presented. The theory takes into account the BX fine-structure produced by NC asymmetry and hole-hole exchange interaction. The calculations show that growth of CdS shell upon CdSe core suppresses the rate of the Auger recombination via negative trion channel, while the more efficient Auger recombination via positive trion channel shows much weaker dependence on the shell thickness. The demonstrated oscillatory dependence of the BX Auger rate on the core and shell sizes is explained qualitatively in terms of overlap of the ground and excited carrier wave functions. The calculations show that raise of temperature accelerates the Auger recombination in CdSe/CdS NCs due to reduction of the bulk energy gaps of CdSe and CdS.

Near-Room Temperature Synthesis of Core/Shell-Structured Quantum Dots.

Core/shell-structured quantum dots (QDs) are considered as important active materials for optoelectronic devices. There have been a lot of synthesis procedures developed so far. Real epitaxial growth of shell layer, however, has not been reported yet. Here, a simple method for the synthesis of CdSe/CdS core/shell QDs is presented. Epitaxial growth of CdS shell on CdSe core is carried out by near room temperature successive ion layer adsorption and reaction (RT-SILAR). Our method for shell formation is conducted at room temperature, which facilitates the separation of resulting products from shell growth solution. After full coverage with one monolayer (ML) of CdS on CdSe QDs surface, photoluminescence (PL) quantum yield (QY) reaches up to 60%. Produced CdSe/CdS QDs have an elongated morphology, implying that CdS layers are formed in an epitaxial manner without etching or deteriorating CdSe QDs surface. We believe that our synthesis method for core/shell-structured QDs would be an ideal model for practical implication of QDs as well as fundamental studies.

Controlling Charge Carrier Overlap in Type-II ZnSe/ZnS/CdS Core-Barrier-Shell Quantum Dots.

We describe the synthesis and spectroscopic characterization of colloidal ZnSe/ZnS/CdS nanocrystals, which exhibit a type-II electronic structure and wave function overlap that is strongly dependent on the thickness of the ZnS barrier. Barrier thickness is controlled by both the amount of deposited material and the reaction and annealing temperature of CdS shell growth. The results show that a single monolayer of ZnS mitigates the overlap significantly, while four and more monolayers effectively suppress band edge absorption and emission. Transient absorption spectra reveal a broad distribution of excitons with mixed S and P symmetry, which become allowed due to alloy formation and contribute to charge carrier relaxation across the barrier. We present a model of the core/shell interface based on cation diffusion, which allows one to estimate the extent of the diffusion layer from optical spectra.

Effect of shell thickness on electrochemical property of wurtzite CdSe/CdS core/shell nanocrystals

A series of core/shell CdSe/CdS nanocrystals (NCs) with different shell thicknesses were prepared, and the electrochemical band gap (EgCV), which was characterized with the cyclic voltammogram, was increased with the growth of CdS shell whereas the optical band gap (EgOP), which was gotten by optical absorption/photoluminescence, was decreased with the increase of the shell thickness. For comparison, a series of CdSe NCs with different diameter were prepared also, and both the EgCV and EgOP values exhibit similar dependence on the NC's size. It is suggested that the EgCV value is affected by not only the electronic energy level of the CdSe core but also the CdS shell surface.

Synthesizing and investigating photoluminescence properties of CdTe and CdTe@CdS core-shell quantum dots (QDs): A new and simple microwave activated approach for growth of CdS shell around CdTe core

In this work, photoluminescence (PL) properties of CdTe and CdTe@CdS core-shell quantum dots (QDs) were investigated. CdTe and CdTe@CdS QDs were synthesized using a microwave activated method. CdS shell grown using a microwave activated method which was based on the heat sensitivity of Na2S2O3. X-ray diffraction (XRD) analysis demonstrated cubic crystalline structure for both of the synthesized QDs, but for CdTe@CdS QDs, XRD diffraction peaks shifted to the longer angle. Sizes of CdTe and CdTe@CdS QDs were obtained as about 2.5 and 3.5 nm from transmission electron microscopy (TEM) images. UV-Vis and PL spectra showed that in this method both of the absorption and emission of CdTe QDs shifted from near blue to near red. By the growth of CdS shell around CdTe QDs, a red shift was observed in absorption and PL spectra. PL QY was obtained as about 23% and 38% for CdTe and CdTe@CdS core-shell QDs, respectively. Increase in the thickness of the CdS shell was possible only by the increase in the microwave irradiation time in the CdS shell growth step. Growth of the CdS shell around CdTe core, resulted in the red shift in band edge and PL spectra to the longer wavelengths.