Synthesis Of CdSe Quantum Dots Research Articles

Electron beam induced and microemulsion templated synthesis of CdSe quantum dots: tunable broadband emission and charge carrier recombination dynamics

CdSe quantum dots (QDs) were synthesized by a rapid and one step templated approach inside the water pool of AOT (sodium bis(2-ethylhexyl) sulfosuccinate) based water-in-oil microemulsions (MEs) via electron beam (EB) irradiation technique with high dose rate, which favours high nucleation rate. The interplay of different experimental parameters such as precursor concentration, absorbed dose and values (aqueous phase to surfactant molar ratio) of MEs were found to have interesting consequences on the morphology, photoluminescence (PL), surface composition and carrier recombination dynamics of as-grown QDs. For instance, highly stable ultrasmall (∼1.7 nm) bluish-white light emitting QDs were obtained with quantum efficiency (η) of ∼9%. Furthermore, QDs were found to exhibit tunable broadband light emission extending from 450 to 750 nm (maximum FWHM ∼180 nm). This could be realized from the CIE (Commission Internationale d’Eclairage) chromaticity co-ordinates, which varied across the blue region to the orange region thereby, conferring their potential application in white light emitting diodes. Additionally, the average PL lifetime values could be varied from 18 ns to as high as 74 ns, which reflect the role of surface states in terms of their density and distribution. Another interesting revelation was the self-assembling of the initially formed QDs into nanorods with high aspect ratios ranging from 7 to 20, in correspondence with the values. Besides, the fundamental roles of the chemical nature of water pool and the interfacial fluidity of AOT MEs in influencing the photophysical properties of QDs were investigated by carrying out a similar study in CTAB (cetyltrimethylammonium bromide; cationic surfactant) based MEs. Surprisingly, very profound and contrasting results were observed wherein and η of the QDs in case of CTAB MEs were found to be at least three times lower as compared to that in AOT MEs.

Functionalization of CdSe semiconductor nanocrystals with organic charge-transporting ligands

[1,1′-Biphenyl]-4,4′-diamine-N,N′-bis(3-methylphenyl)-N,N′-diphenyl (TPD), a well-studied hole transporting material, has been sulfonated using acetyl sulfate and subsequently used as a passivating ligand in the synthesis of CdSe quantum dots (QDs).

Phase transformations in CdSe quantum dots induced by reaction time

This paper reports the synthesis of high-quality TOP/TOPO capped CdSe nanocrystals using a simple colloidal method. The effects of varying the reaction time on the size and crystallinity of the synthesized CdSe quantum dots were studied in detail. Powder X-ray diffraction (PXRD) analysis showed a dependence of the crystallite phases as a function of reaction time, from cubic zinc-blende to hexagonal wurtzite and back again to the cubic phase. PXRD data was found to be consistent with high resolution transmission electron microscopy (HRTEM) images of the particles, whilst optical spectroscopy demonstrated the quantum dot nature of the particles. The reaction time-dependent phase behavior is best accounted for by rapid growth along the {111} crystallite facets, after which the facial facets then ‘catch-up’, resulting once again in cubic symmetry.

Facile and Green Synthesis of CdSe Quantum Dots in Protein Matrix: Tuning of Morphology and Optical Properties

Herein, we have demonstrated a facile and green approach for the synthesis of Cadmium selenide (CdSe) quantum dots (QDs). The process was mediated by bovine serum albumin (BSA) and it was found that BSA plays the dual role of reducing agent as well as a stabilizing agent. The QDs exhibited sharp excitonic absorption features at ~500 nm and subsequently showed reasonably good photoluminescence (PL) at room temperature. The PL is seen to be strongly dependent on the concentration of the precursors and hence, the luminescence of these QDs could be conveniently tuned across the visible spectrum simply by varying molar ratio of the precursors. It can be envisaged from the fact that a red-shift of about 100 nm in the PL peak position was observed when the molar ratio of the precursors ([Cd2+]:[Se2-], in mM) was varied from 10:5 to 10:40. Subsequently, the charge carrier relaxation dynamics associated with the different molar ratio of precursors has been investigated and very interesting information regarding the energy level structures of these QDs were revealed. Most importantly, in conjunction with the optical tuning, the nanomorphology of these nanoparticles was found to vary with the change in molar ratios of Se and Cd precursors. This aspect can provide a new direction of controlling the shape of CdSe nanoparticles. The possible mechanism of the formation as well as for the shape variation of these nanoparticles with the molar ratios of precursors has been proposed, taking into account the role of amino acid residues (present in BSA). Moreover, the QDs were water soluble and possessed fairly good colloidal stability therefore, can have potential applications in catalysis and bio-labeling. On the whole, the present methodology of protein assisted synthesis is relatively new especially for semiconducting nanomaterials and may provide some unique and interesting aspects to control and fine tune the morphology vis-à-vis, their optical properties.

EFFECT OF OXYGEN ON SIZE-CONTROLLED SYNTHESIS OF CdSe QDs

Unique properties of quantum dots (QDs) are controlled by their customizable particle sizes which can be engineered to suit local need. One-pot organometallic injection synthesis of cadmium selenide (CdSe) QDs is reported. The operation started with the injection of a week old selenium (Se) precursor into a boiling non-coordinating octadecene (ODE) at 195oC resulting in the formation of monodispersed size tunable CdSe QDs with discrete homogeneous nucleation. Differences in the injection and withdrawal time of the sample resulted to the dissimilarity in the shape, size and the opto-electronic properties of the QDs. The effect of oxygen on the synthesized CdSe QDs was studied by exposing freshly prepared sample to atmospheric oxygen for 206 days. The samples were characterized using optical absorption spectroscopy (UV-vis), optical photoluminescence (PL), X-ray diffraction (XRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM), electron dispersive X-ray (EDX) X-ray diffraction spectroscopy (XRD), and X-ray photoelectron spectroscopy (XPS). Noticeable deformation on the size, shape and the crystalline orientation of the CdSe QDs were observed on the oxygen-interacted sample.Although the synthesis method is safe and produced good quality CdSe QDs, the interaction of the sample with oxygen degrades their opto-electronic quality.

Influence of nanocrystal size on the quantum dots sensitized solar cells’ performance with low temperature synthesized CdSe quantum dots

The CdSe quantum dots (QDs) were synthesized with ethanol and glycerol under 50–160°C in the present work. The reaction temperature is lower than the traditional solvent thermal method to fabricate the CdSe QDs around 250–350°C. What’s more, the avoiding of the use of phosphine contained solvent make the whole reaction environmental friendly. Then 3-Mercaptopropionic (MPA) was used to exchange the CdSe QDs’ ligand of olic acid. After that, the MPA-capped CdSe QDs were tethered onto the TiO2’s substrate, which guarantee the high coverage of the CdSe QDs on the TiO2 photoanode and the absorption range of 450–560nm of light harvest. Furthermore, through the results of the CdSe QD’s size for the cells’ performance, we compared the significance of the QDs’ absorption range, the electron injection from the conduction band of CdSe QDs’ to TiO2’s and polysulfide to the valance band of the CdSe QDs in the cells’ performance and discuss the mechanism to improve the efficiency of the QDSCs. Finally, the QDSCs with polysulfide solution and Cu2S as counter electrode show excellent cell performance with open-circuit voltage (Voc) of 0.6V, short-circuit current density (Jsc) of 10.58mA/cm2, fill factor (FF) of 0.564, and PCE of 3.7%.

A novel and green method to synthesize CdSe quantum dots-modified TiO2 and its enhanced visible light photocatalytic activity

In this paper, a facile and green method was employed to synthesize CdSe quantum dots (QDs)-modified TiO2. It exhibited much higher photocatalytic activities than both TiO2 and bulk CdSe-modified TiO2 on degrading malachite green (MG) aqueous solution under visible light irradiation. The vital role that quantum size effect of CdSe QDs plays in enhancing photocatalytic properties of CdSe QDs-modified TiO2 is discussed in detail for the first time. Due to the more negative conduction band potentials of CdSe QDs than that of bulk CdSe, the electron transfer rate from CdSe QDs to TiO2 is much faster than that from bulk CdSe to TiO2, which would be beneficial for an efficient charge carrier's separation. As a result, more quantities of active species (O2•−, H2O2, •OH) are generated during the photocataltytic reaction in CdSe QDs-modified TiO2 system. This work could not only provide a facile and green strategy to fabricate semiconductor QDs-modified TiO2 photocatalyst but also contribute to a deeper understanding of the specific role of semiconductor QDs during the photocatalytic reaction process.

Green synthesis of highly efficient CdSe quantum dots for quantum-dots-sensitized solar cells

Green synthesis of CdSe quantum dots for application in the quantum-dots-sensitized solar cells (QDSCs) is investigated in this work. The CdSe QDs were prepared with glycerol as the solvent, with sharp emission peak, full width at half maximum around 30 nm, and absorption peak from 475 nm to 510 nm. The reaction is environmental friendly and energy saving. What's more, the green synthesized CdSe QDs are coherence to the maximum remittance region of the solar spectrum and suitable as sensitizers to assemble onto TiO2 electrodes for cell devices application. What's more, the dynamic procedure of the carriers' excitation, transportation, and recombination in the QDSCs are discussed. Because the recombination of the electrons from the conduction band of TiO2's to the electrolyte affects the efficiency of the solar cells greatly, 3-Mercaptopropionic acid capped water-dispersible QDs were used to cover the surface of TiO2. The resulting green synthesized CdSe QDSCs with Cu2S as the electrode show a photovoltaic...

Tandem structured quantum dot/rod sensitized solar cell based on solvothermal synthesized CdSe quantum dots and rods

The quantum dots (QD) and quantum rods (QR) of different sizes, shapes, and crystalline phases are synthesized by modified solvothermal method spontaneously employed stirring system and controlled internal applied pressure. The tandem structure of QDs and QRs as well as tetrapods is formed on hierarchical porous titania photoanode by means of electrophoretic deposition. A tremendous enhancement in efficiency of the cell is obtained in samples synthesized at 220 °C for 24 h due to the formation of tandem structure, utilization of Cu2S/CNT composite cathode, co-sensitization of CdS and CdSe, and beneficial role of QRs in electron lifetime. Smaller size QDs with higher band gaps penetrate deeper through the macro-channels of the hierarchical porous structure, while the QRs and tetrapods with lower band gaps are placed on upper layers. Although the charge injection is improved in smaller QDs, the electron lifetime in QRs is longer mainly due to the higher absorption cross section, proper charge separation, introduction of quasi-one dimensional route for charge transport through QRs, and higher surface area available for regeneration with electrolyte. The cell shows the efficiency of 1.05% with JSC of 4.48 mA cm−2, VOC of 0.45 V, and fill factor of 0.52.

A Critical Investigation on the Existence of Selective Microwave Absorption in the Synthesis of CdSe Quantum Dots

The existence of selective microwave absorption phenomena in the synthesis of CdSe quantum dots has been investigated. These types of microwave effects involving selective microwave absorption by specific reagents have recently been proposed in the microwave-assisted synthesis of various nanoparticles. In the present study, the microwave synthesis of CdSe quantum dots was investigated according to a protocol published by Washington and Strouse to clarify the presence of selective microwave heating. Importantly, control experiments involving conventional conductive heating were executed under otherwise (except for the heating mode) identical conditions, ensuring the same heating and cooling profiles, stirring rates, and reactor geometries. Comparison of powder X-ray diffraction, UV-vis, photoluminescence, and transmission electron microscopy data of the obtained CdSe quantum dots reveals that identical types of nanoparticles are obtained independently of the heating mode. Therefore, no evidence for a selective microwave absorption phenomenon could be obtained.

A Novel Approach to the Fabrication of CdSe Quantum Dots in Aqueous Solution: Procedures for Controlling Size, Fluorescence Intensity, and Stability over Time

This paper report a straightforward approach for the synthesis of CdSe quantum dots (CdSe QDs) in aqueous solution. This method, performed in homogeneous phase, affords optimal sizes and high quantum yields for each application desired. It is an a la carte procedure for the synthesis of nanoparticles aimed at their later application. By controlling the experimental conditions, CdSe QDs of sizes ranging between 2 and 6 nm can be obtained. The best results were achieved in an ice‐bath thermostated at 4°C, using mercaptoacetic acid as dispersant. Under these conditions, a slow growth of quantum nanocrystals was generated and this was controlled kinetically by the hydrolysis of to generate Se2− in situ, one of the forming species of the nanocrystal. The organic dispersant mercaptoacetate covalently binds to the Cd2+ ion, modifying the diffusion rate of the cation, and plays a key role in the stabilization of CdSe QDs. In optimum conditions, when kept in their own solution CdSe QDs remain dispersed over 4 months. The NPs obtained under optimal conditions show high fluorescence, which is a great advantage as regards their applications. The quantum efficiency is also high, owing to the formation under certain conditions of a nanoshell of Cd(OH)2, values of 60% being reached.

Rapid phosphine-free synthesis of CdSe quantum dots: promoting the generation of Se precursors using a radical initiator

Trace amounts of AIBN substantially accelerate the homolytic cleavage of the Se–Se bond, facilitating the phosphine-free synthesis of CdSe quantum dots.

Confined synthesis of CdSe quantum dots in the pores of metal–organic frameworks

Ultra-small CdSe nanoparticles (∼2 nm) were synthesised in the pores of a metal–organic framework, MIL-101(Al)-NH2, using a stepwise injection-reaction protocol.

Hydrothermal Synthesis of CdSe Quantum Dots and Their Photocatalytic Activity on Degradation of Cefalexin

In this paper, CdSe quantum dots (QDs) photocatalysts were successfully synthesized by a hydrothermal method. The factors affecting synthesized CdSe QDs were investigated under different experimental conditions. CdSe QDs were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), ultraviolet–visible diffuse reflectance spectrophotometry (UV–vis DRS), photoluminescence (PL) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The photocatalytic activities of CdSe QDs were evaluated on the decomposition of cefalexin under UV irradiation. The results showed that the degradation ratio of Cefalexin could reach 70.34%. Based on the results, the possible mechanism of the photocatalytic reaction on cefalexin with CdSe QDs was discussed.

Continuous‐Flow Synthesis of CdSe Quantum Dots: A Size‐Tunable and Scalable Approach

In recent years, continuous-flow/microreactor processing for the preparation of colloidal nanocrystals has received considerable attention. The intrinsic advantages of microfluidic reactors have opened new opportunities for the size-controlled synthesis of nanocrystals either in the laboratory or on a large scale. Herein, an experimentally simple protocol for the size-tunable continuous-flow synthesis of rather monodisperse CdSe quantum dots (QDs) is presented. CdSe QDs are manufactured by using cadmium oleate as cadmium source, selenium dioxide as selenium precursor, and 1-octadecene as solvent. Exploiting selenium dioxide as selenium source and 1-octadecene as solvent allows execution of the complete process in open air without any requirement for air-free manipulations using a glove box or Schlenk line. Continuous-flow processing is performed with a stainless steel coil of 1.0 mm inner diameter pumping the combined precursor solution through the reactor by applying a standard HPLC pump. The effect of different reaction parameters, such as temperature, residence time, and flow rate, on the properties of the resulting CdSe QDs was investigated. A temperature increase from 240 to 260 °C or an extension of the residence time from 2 to 20 min affords larger nanocrystals (range 3-6 nm) whereas the size distribution does not change significantly. Longer reaction times and higher temperatures result in QDs with lower quantum yields (range 11-28 %). The quality of the synthesized CdSe QDs was confirmed by UV/Vis and photoluminescence spectroscopy, small-angle X-ray scattering, and high-resolution transmission electron microscopy. Finally, the potential of this protocol for large-scale manufacturing was evaluated and by operating the continuous-flow process for 87 min it was possible to produce 167 mg of CdSe QDs (with a mean diameter of 4 nm) with a quantum yield of 28 %.

Synthesis of CdSe Quantum Dots Using Various Long-Chain Fatty Acids and Their Phase Transfer

Monodispersed colloidal photoluminescent CdSe quantum dots (QDs) were synthesized via an organic approach by using cadmium oxide and elemental selenium as precursors, and long-chain fatty acids as surface ligands. The hydrocarbon chain length of the fatty acid was adjusted to investigate the effect on CdSe QDs. The fatty acid ligands with different hydrocarbon chain lengths showed an apparent effect on the nanocrystal nucleation and growth which is the key controlling the size, size distribution and crystal structure of resulting CdSe QDs. This effect was attributable to the steric hindrance of different hydrocarbon length of the fatty acids, which affected the reactivity of the monomers and nanocrystals during the nanocrystal nucleation and growth. The water-soluble CdSe QDs were obtained by encapsulating the CdSe ODs in oil phase with amphiphilic poly(styrene-co-maleic anhydride) (PSMA)-ethanolamine (EA) polymers, which made it possible for further applications of the CdSe QDs in aqueous environment such as surface functionalization for biological labeling and application in photocatalysis and photosensitization.

SIZE-DEPENDENT PHOTOLUMINESCENCE AND ELECTROLUMINESCENCE OF COLLOIDAL CdSe QUANTUM DOTS

Here we adopt a convenient green chemical route for synthesis of CdSe quantum dots, their characterization by UV/Vis absorption spectroscopy, X-ray diffraction study and transmission electron microscopy. We carry out photoluminescence and electroluminescence spectroscopy to investigate the variation in electro-optical property with size. By UV/Vis spectroscopy, blue shift is revealed and bandgap is also calculated. X-ray diffraction spectrum reveals cubic structure and transmission electron micrographs show quantum dots of different size distributions (in the range 2–8 nm). Both the luminescence spectroscopies reveal green-orange luminescence depending upon the size distribution and indicate the possibility of using CdSe quantum dots as light emitting devices with better compatibility and faster response.

Characterizing the Influence of TOPO on Exciton Recombination Dynamics in Colloidal CdSe Quantum Dots

Surface ligands play a significant role in the carrier recombination dynamics of colloidal quantum dots (QDs). Trioctylphosphine oxide (TOPO) is a ligand used during CdSe QD synthesis, which binds to surface cadmium atoms, but, despite its widespread use, its effect on exciton relaxation rates is poorly understood. In this work, we extract TOPO–CdSe binding equilibria from isothermal titration calorimetry experiments and use them to study the effect of TOPO concentration on CdSe time-resolved photoluminescence. We develop a quantitative kinetic model of CdSe QD exciton relaxation that is based upon an ensemble probability distribution of bound ligands and show that bound TOPO gives rise to band-edge states that participate in carrier recombination dynamics. Optoelectronic applications of QDs necessitate a good understanding of interfacial dynamics so that charge separation, carrier transport, and other phenomena can be optimized. This technique is a promising step toward that goal that is useful for any l...

Polyhedral Oligomeric Silsesquioxane as a Ligand for CdSe Quantum Dots

We report the synthesis of CdSe quantum dots (QDs) using a mercapto-substituted polyhedral oligomeric silsesquioxane (SH-POSS). The bulky siloxane cage-like core of the ligand makes this an ideal steric stabilizer, and comparison with conventional branched alkyl phosphonic acid capped CdSe QDs shows SH-POSS capped QDs to have superior optical properties including photoluminescence quantum efficiencies and fluorescence lifetimes. The POSS shell allows for the access of small electrolyte ions and electron transport from the surface of the QDs, evidenced by better performance as a photosensitizer in conjunction with a titania nanotube array electron acceptor in comparison to the conventionally stabilized QDs.

Carrier transport characteristics in CdSe/CdS/Thioglycolic acid ligand quantum dots with a core-shell structure

In the present paper, we synthesize CdSe quantum dots (QDs) that are stabilized by thioglycolic acid according to the water-phase synthesis. The X-ray diffraction and HRTEM results confirm that the samples prepared each possess a sphalerite structure. The EDS and FT-IR spectra of the samples show that a core-shell structure is formed between the CdSe nanoparticles and the ligand. The fine band structures and the characteristics of the surface states in a connection with the structures are identified by the surface photovoltage (SPV) spectrum of the samples. Two SPV response peaks, located at 475 nm (2.61 eV) and 400 nm (3.1 eV), are closely related to the band-band transitions of the core-CdSe and the shell-CdS, respectively; the SPV response at 370 nm (3.35 eV) is correlated with the n → π* transition between the hydroxyl and sulfydryl (or hydroxyl). It is because of an obvious quantum size effect of the samples that both PL line broadens and SPV response intensity increases with the decrease of the grain size of the sample. The change trend of the surface photoacoustic signal intensity is contrary to that of the SPV response intensity of the samples synthesized at varying pH. Moreover, the fine band structures at surfaces and grain boundaries of CdSe QDs prepared are probed by the SPV spectra of the samples at varying pH values. The relationship between the grain size and the photo-generated carrier transport behavior is discussed according to the detected EFISPV results of the QDs.