Formation Of CdSe Quantum Dots Research Articles

Surface Defect-Driven Chemical Transformation of MoSe2 Nanosheets

The chemistry of atomically thin two-dimensional layered transition metal dichalcogenides (TMDs) largely depends upon the presence of defect sites in their structures. Here, we demonstrate the role of defects of MoSe2 nanosheets (NSs) in two different transformative reactions for the synthesis of MoSe2–CdS nano-heterostructures (NHSs) and CdSe quantum dots (QDs). MoSe2–CdS NHSs are synthesized via passivating the defects of MoSe2 using thiol moieties, followed by the growth of CdS over the surface of MoSe2 NSs. Furthermore, in the absence of thiols, we observe that defects in MoSe2 NSs significantly accelerate the cation-exchange process upon direct exposure to the cadmium (Cd) precursor, resulting in the formation of CdSe QDs through cation displacement reaction (CDR). We have probed the transformation of MoSe2 NSs into CdSe QDs through Raman and optical spectroscopic measurements. Furthermore, theoretical calculations under the framework of density functional theory (DFT) suggest that CDR is feasible on the surface of defect-rich NSs, which becomes unfavorable in the case of defect-free or thiol-passivated MoSe2 NSs. These results provide new insights into understanding the multifaceted role of defects of TMDs in different transformative reactions.

CdSe quantum dot formation induced by electron beam irradiation

The formation of quantum dots (QDs) induced by electron beam irradiation onto a highly strained CdSe layer on ZnSe (001) is investigated. In situ reflection high-energy electron diffraction is used for dual purposes: QD formation by electron beam irradiation and monitoring the progress of surface morphology. QD formation is confirmed by observation of nanoscale three-dimensional islands using transmission electron microscopy. It is also verified by imaging of an isolated single QD emission at room temperature by photoluminescence microscopy. Electron beam irradiation increases QD size both in height and lateral directions, resulting in the shift of emission from green to yellow spectral region.

Synthesis, Structural and Optical Investigation of Cdse Semiconductor Quantum Dots

In this paper, we present a simple synthetic route for the development of high quality CdSe quantum dots TGA capped in aqueous solutions. CdSe semiconducting Quantum dots were prepared by a chemical method at room temperature. The crystal structure, morphology, spectral and compositional properties were analyzed from X-ray Diffraction, High resolution transmission electron microscopy (HRTEM), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), UV-VIS absorbance and Photoluminescence (PL), Fourier transform infrared (FTIR) and Fourier transform RAMAN (FT-RAMAN) techniques. The XRD results depicted the formation of CdSe quantum dots with an average crystallite size of 3 nm and 4 nm, which is in accordance with observations from HRTEM and TEM micrographs with an average particle diameter range between 2-3 nm. The UV-VIS absorbance peaks indicate a broad absorption behavior of CdSe quantum dots. The calculated band gap energy is found to be 2.55 eV and further it is found the particle size estimated from the maximum excitonic peaks for CdSe 2.7 nm. The photoluminescence studies evidenced that the luminesce property of quantum dots can be varied by systematically tuning precursor concentrations and the PL confirms the strong excitation peaks for the synthesized CdSe QDs with peak centered at 603 nm. The capping of TGA was confirmed from FTIR and FT-RAMAN studies. From the FTIR spectra it is observed that, for the synthesized CdSe QDs almost all the peaks are common with slightly shifting behavior towards larger and shorter wave numbers. FT-Raman analysis was performed to study the sample qualities such as micro crystallinity, homogeneity and surface conditions for the as synthesized CdSe quantum dot. The resulting colloids are very stable, and no precipitate is observed over a period of 6 months.

Formation of CdSe quantum dots from single source precursor obtained by thermal and laser treatment

The synthesis of CdSe quantum dots (QDs) from a single source precursor is a known way to form a nanocomposite by its thermal decomposition. Recently, some of them have been used to grow QDs by direct laser patterning. Here, the authors report the study of the formation of the CdSe QDs starting from the decomposition of the 2-(N,N-dimethylamino)ethylselenolate of cadmium dispersed in a polymethylmethacrylate film induced both by heating and laser patterning. The formation of the CdSe QDs under annealing at 150 °C is studied as a function of the precursor concentration and molar ratio with respect to two different QD ligands such as oleic acid and oleylamine. The photoluminescent spectra of the formed nanocomposite show that the oleic acid induces the formation of the direct bandgap, while oleylamine shows only a broadband emission. The laser patterning of the film was carried out with a UV laser (355 nm) with a pulse frequency of 40 kHz and a fluence of 1.06 J/cm2. The resulting patterned areas were characterized by a fluorescence microscope showing the formation of a photoluminescence path only when the ligands were present, suggesting the formation of the functional (photoluminescent) QDs.

Synthesis, Structural and Optical Investigation of CdSe semiconductor Quantum Dots

In this paper, we present a simple synthetic route for the development of high quality CdSe quantum dots TGA capped in aqueous solutions. CdSe semiconducting Quantum dots were prepared by a chemical method at room temperature. The crystal structure, morphology, spectral and compositional properties were analyzed from powder X-ray Diffraction, High resolution transmission electron microscopy (HRTEM), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), UV-Vis absorbance and Photoluminescence (PL), Fourier transform infrared (FTIR) and Fourier transform RAMAN (FT-RAMAN) techniques. The XRD results depicted the formation of CdSe quantum dots with the average crystallite size of 3 nm and 4 nm, which is in accordance with observations from HRTEM and TEM micrographs predict an average particle diameter of the QDs range between 2-3 nm. The UV-Vis absorbance peaks indicate the broad absorption behavior of the synthesized CdSe quantum dots. The calculated band gap energy was 2.55 eV and further it is found the particle size estimated from the maximum excitonic peaks for CdSe 2.7 nm. The photoluminescence studies evidenced that the luminesce property of quantum dots can be varied by systematically tuning precursor concentrations. The PL emission spectrum depicts the strong excitation peaks for the synthesized CdSe QDs with peaks centered at 603 nm. The capping of TGA was confirmed from FTIR and FT-RAMAN studies. From the FTIR spectra it is observed that, for the synthesized CdSe QDs almost all the peaks are common with slightly shifting behavior towards larger and shorter wave numbers. FT-Raman analysis was performed to study the sample qualities such as microcrystallinity, homogeneity and surface conditions for the as synthesized CdSe quantum dot. The resulting colloids are very stable, and no precipitate is observed over a period of 6 months.

Dynamics of radiolytic formation of CdSe quantum dots in aqueous solution containing different alcohols: A pulse radiolysis study

The dynamics of radiolytic formation of CdSe quantum dots in aqueous solution containing equimolar ammoniated cadmium sulphate, Cd(NH3)4SO4 and sodium selenosulphate, Na2SeSO3 as precursors for Cd and Se respectively, have been investigated in the presence of different aliphatic alcohols, namely methanol, ethanol, 1-propanol, 2-propanol or 1-butanol by electron pulse radiolysis studies. The mechanism of CdSe quantum dots formation was studied in N2O saturated aqueous solutions to investigate the formation through reactions with different aliphatic alcohols. The formation of the CdSe quantum dots was found to proceed via the formation of short lived transient intermediate species having and absorption at around 500 nm, which are formed through the reaction of different aliphatic alcohol radicals with the cadmium and selenium precursors. The rate constants for the formation and decay of these intermediate species have been determined. Optical properties of these quantum dots were found to be different in the presence of different alcohols.

Studying the influence of substrate conductivity on the optoelectronic properties of quantum dots langmuir monolayer

The formation of CdSe quantum dots (QDs) monolayers was studied by Langmuir Blodgett method. The fluorescence (PL) spectra of QD monolayers were investigated at different substrate type (glass, silicon and ITO glass) and the influence of graphene sheets layer (as a conductive surface) on the QDs properties has also been studied. The optoelectronic properties of QDs can be tuned by deposition of insulating nano-size layers of the liquid crystal between QDs and conductive substrate. The monolayer of QDs transferred on conductive surface (glass with ITO) has lowest intensity of PL spectra due to quenching effect. The PL intensity of QDs could be tuned by using various type of substrates or/and by transformed high conductive layer. Also the photooxidation processes of CdSe QDs monolayer on the solid surface can be controlled by selection of suitable substrate. The current-voltage (I–V) characteristics of QDs thin film on ITO surface was studied using scanning tunneling microscope (STM).

Proficient surface modification of CdSe quantum dots for highly luminescent and biocompatible probes for bioimaging: A comparative experimental investigation

The cytotoxicity of Cd based Quantum dots (QDs) is major concern that hinders their practical use in bioimaging. Therefore, the objective of first of its kind report is to overcome this hindrance where the comparative cytotoxicity study of poly CdSe, CdSe QDs, CdSe/ZnS QDs and CdSe QDs encapsulated by TEOS, polyvinylalcohal and polyethylene glycol being presented. All the QD were synthesized and encapsulated by wet chemical method and studied. Prepared QDs were monodispersed, spherical in shape and exhibited size range of 1.8–2.2 nm. Elemental analysis confirms the formation of CdSe QDs and other mentioned structures. Surface functional groups notify the biocompatibility of core-shell and encapsulated QDs with respect to poly and bare CdSe QDs. Cytotoxicity of QDs was analyzed using human embryonic kidney cell line. Encapsulated QDs were tested even at very high concentrations and for long incubation time of 24 h and even upto 72 h of incubation. Core-shell and encapsulated QDs acquire excellent monochromatic luminescence intensity and negligible cytotoxicity as compared to prepared poly CdSe and bare CdSe QDs thus can be used as luminescent probes for bioimaging.

CdSe/ZnS core–shell QDs: Synthesis and investigating optical properties

In this work, CdSe quantum dots (QDs) were synthesized using a microwave activated reaction between NaHSe and CdSO4 in the presence of thioglycolic acid (TGA) as capping molecule and then using a one-pot method, ZnS shell was grown subsequently around CdSe cores by a room temperature reaction based on the photo-sensitivity of Na2S2O3 dissociation. Synthesized QDs were characterized by means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), UV-visible (UV-Vis) and photoluminescence (PL) spectroscopy. All these analyses confirmed the formation of CdSe QDs and successful growth of ZnS shell around CdSe cores.

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.

Real-time in situ probing of high-temperature quantum dots solution synthesis.

Understanding the formation mechanism of colloidal nanocrystals is of paramount importance in order to design new nanostructures and synthesize them in a predictive fashion. However, reliable data on the pathways leading from molecular precursors to nanocrystals are not available yet. We used synchrotron-based time-resolved in situ small and wide-angle X-ray scattering to experimentally monitor the formation of CdSe quantum dots synthesized in solution through the heating up of precursors in octadecene at 240 °C. Our experiment yields a complete movie of the structure of the solution from the self-assembly of the precursors to the formation of the quantum dots. We show that the initial cadmium precursor lamellar structure melts into small micelles at 100 °C and that the first CdSe nuclei appear at 218.7 °C. The size distributions and concentration in nanocrystals are measured in a quantitative fashion as a function of time. We show that a short nucleation burst lasting 30 s is followed by a slow decrease of nanoparticle concentration. The rate-limiting process of the quantum dot formation is found to be the thermal activation of selenium.

Luminescent properties of fluorophosphate glasses with molecular cadmium selenide clusters

It is experimentally shown that, prior to the formation of CdSe quantum dots in fluorophosphate glasses with cadmium and selenium ions in the process of synthesis, subnanosized molecular clusters (CdSe)n are formed, which exhibit luminescence in the visible spectral region upon UV excitation. Heat treatment of the glasses increases the size of molecular clusters and makes their optical properties closer to the optical properties of CdSe semiconductor quantum dots. An increase in the sample temperature from 20 to 250°C leads to reversible thermal quenching of the luminescence.

Compositional Dependence of CdSe Quantum Dot Formation on Silicate Host Glass Composition

Silicate glasses were synthesized with additional CdO and ZnSe of varying content. When CdO was added up to 3 mol% with the amount of ZnSe fixed at 1 mol%, absorption due to Se2− color centers significantly decreased and the color of glasses was bleached without any evidence of CdSe quantum dots (QDs). However, characteristic absorption due to CdSe QDs appeared without additional heat treatment when the ZnSe content exceeded 1.5 mol%, whereas the CdO content was fixed at 1 mol%. Raman spectra and transmission electron microscopy verified the formation of CdSe QDs. The role of CdO and ZnSe in CdSe QD formation has been discussed in relation to their structural contribution within silicate glasses.

Mechanism-Oriented Controllability of Intracellular Quantum Dots Formation: The Role of Glutathione Metabolic Pathway

Microbial cells have shown a great potential to biosynthesize inorganic nanoparticles within their orderly regulated intracellular environment. However, very little is known about the mechanism of nanoparticle biosynthesis. Therefore, it is difficult to control intracellular synthesis through the manipulation of biological processes. Here, we present a mechanism-oriented strategy for controlling the biosynthesis of fluorescent CdSe quantum dots (QDs) by means of metabolic engineering in yeast cells. Using genetic techniques, we demonstrated that the glutathione metabolic pathway controls the intracellular CdSe QD formation. Inspired from this mechanism, the controllability of CdSe QD yield was realized through engineering the glutathione metabolism in genetically modified yeast cells. The yeast cells were homogeneously transformed into more efficient cell-factories at the single-cell level, providing a specific way to direct the cellular metabolism toward CdSe QD formation. This work could provide the foundation for the future development of nanomaterial biosynthesis.

Effect of Cd(ii) and Se(iv) exposure on cellular distribution of both elements and concentration levels of glyoxal and methylglyoxal in Lepidium sativum

In this work, the effect of cadmium (0-5.0 mg L(-1) as cadmium chloride, Cd(II)) and selenium (0-2.0 mg L(-1) as sodium selenite, Se(IV)) was studied in Lepidium sativum with specific focus on glyoxal (GO) and methylglyoxal (MGO) and on the cellular distribution of both elements under different exposure conditions. The concentrations of two reactive α-ketoaldehydes present as natural metabolites and as by-products of lipid peroxidation, were increased in plants treated with Cd(II), providng complementary experimental evidence on element phytotoxicity in garden cress, in terms of oxidative damage. Even though for higher than 1.0 mg L(-1) Se in medium similar adverse effect was found, under simultaneous exposure to both elements the changes in GO and MGO concentrations were clearly attenuated as compared to a single stressor treatment. This effect was accompanied by lower uptake of the two elements, significant decrease of their relative distribution in the fraction containing polar compounds and their increase in fraction corresponding to insoluble cell fragments/components, suggesting that the direct in vivo interaction between two element forms might be involved in the favorable effects of simultaneous treatment with Cd(II) + Se(IV). The fluorescence spectra obtained for biomass extracts corresponding to different exposure conditions suggested possible in vivo formation of CdSe quantum dots; however further studies are needed for ultimate identification and characterization of such nanoparticulate species.

Optimized Synthesis and Fluorescence Spectrum Analysis of CdSe Quantum Dots

An optimized synthesis route was applied for controlling the preparation of CdSe quantum dots (QDs) in an aqueous solution. Some key factors which influencing the properties of CdSe QDs, such as initial pH, stabilizers, ratio of precursor, etc. were investigated. The size, shape, crystal structure, and optical property of CdSe QDs were also characterized by TEM, XRD, UV-Vis, and fluorescence (FL) spectra. The result showed that high-quality cubic CdSe QDs with 3 nm were obtained. The experiments also confirmed that thioglycolic acid (TGA), under the conditions of weak acid, is a better stabilizer than others. The ratio of [Cd2+] to [SeSO3 2−] played an important role in the formation of CdSe QDs. The mechanisms about the influence factors were also presented.

CdSe/ZnSe quantum dot structures grown by molecular beam epitaxy with a CdTe submonolayer stressor

A procedure for formation of CdSe quantum dots (QDs) in a ZnSe matrix is suggested. The procedure is based on the introduction of a CdTe submonolayer stressor deposited on the matrix surface just before deposition of the material of the QDs. (For CdTe/ZnSe structure, the relative lattice mismatch is Δa/a ≈ 14%.) The stressor forms small strained islands at the ZnSe surface, thus producing local fields of high elastic stresses controlling the process of the self-assembling of the QDs. According to the data of transmission electron microscopy, this procedure allows a considerable increase in the surface density of QDs, with a certain decrease in their lateral dimensions (down to 4.5 ± 1.5 nm). In the photoluminescence spectra, a noticeable (∼150 meV) shift of the peak to longer wavelengths from the position of the reference CdSe/ZnSe QD structure is observed. The shift is due to some transformation of the morphology of the QDs and an increase in the Cd content in the QDs. Comprehensive studies of the nanostructures by recording and analyzing the excitation spectra of photoluminescence, the time-resolved photoluminescence spectra, and the cathodoluminescence spectra show that the emission spectra involve two types of optical transitions, namely, the type-I transitions in the CdSeTe/ZnSe QDs and the type-II transitions caused mainly by the low cadmium content (Zn,Cd)(Se,Te)/ZnSe layer formed between the QDs.

Analysis of lattice dynamics and magnetic properties of II‐VI layers and interfaces by Raman spectroscopy

AbstractWe report on the lattice dynamics of abrupt and intermixed interfaces of II‐VI‐layers and quantum dots, studied by Raman spectroscopy from embedded CdSe monolayers in BeTe, and from CdSe quantum dots in ZnSe. It is demonstrated that the best CdSe/BeTe interface quality is achieved, when CdTe interface bonds are applied. The corresponding vibration modes are described very well by density functional calculations, and they also reveal the onset of CdSe quantum dot formation. For these interfaces, the reduced symmetry (i.e. non‐equivalence of [011] and [01‐1] in‐plane axes) is analyzed from the polarization dependent Raman scattering intensity. Similar results are presented for Raman scattering from Te surface dimers on BeTe(100). Furthermore, the (s,p)‐d exchange energy of diluted magnetic II‐VI semiconductors is studied by spin flip Raman spectroscopy. For n‐doped (Zn,Mn)Se, a doping‐induced reduction of the exchange energy and enhancement of the spin relaxation time T is demonstrated. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

CdSe quantum dot formation: alternative paths to relaxation of a strained CdSe layer andinfluence of the capping conditions

CdSe/ZnSe quantum dot formation is investigated by studying different steps of the growth.To precisely control the critical thickness of CdSe grown on a ZnSe buffer layer, the CdSeself-regulated growth rate in atomic layer epitaxy growth mode is determined by reflectionhigh-energy electron diffraction (RHEED) measurements for a temperature range between 180 and280 °C. Then, the two-dimensional–three-dimensional (2D–3D) transition of a strainedCdSe layer on (001)-ZnSe induced by the use of amorphous selenium is studied.The formation of CdSe islands is found when 3 monolayers (ML) of CdSe aredeposited. When only 2.5 ML of CdSe are deposited, another relaxation mechanism isobserved, leading to the appearance of strong undulations on the surface. We alsostudied the evolution of the surface morphology when 2.7 ML are deposited, tostudy the boundary between those two phenomena. The influence of capping onquantum dot morphology is investigated. It is found that cadmium is redistributedwithin the layer during capping. Our results show that the cadmium distributionafter capping depends on the capping temperature and on the strain of the CdSelayer. Cadmium incorporation after capping is also studied. It is found that theamount of incorporated cadmium depends on the strain of the CdSe layer beforecapping.

In situ observation of Zn-induced etching during CdSe quantum dot formation using time-resolved ellipsometry

A combined segregation and desorption process has been observed in situ by ellipsometry in real-time during overgrowth of a CdSe layer by a ZnSe cap layer using migration enhanced epitaxy. This segregation enhanced etching of CdSe during Zn deposition is known to play an important role in the formation process of CdSe quantum dots. The time-resolved ellipsometry data can be fitted assuming a rapid thickness reduction of about 68% of the CdSe layer, consistent with results obtained by high-resolution x-ray diffraction after growth. Furthermore, a significant change in growth rate during deposition of CdSe has been observed.