Colloidal CdTe Research Articles

Lower-Temperature Nucleation and Growth of Colloidal CdTe Quantum Dots Enabled by Prenucleation Clusters with Cd-Te Bond Conservation.

The reason why heating is required remains elusive for the traditional synthesis of colloidal semiconductor quantum dots (QDs) of II-VI metal chalcogenide (ME). Using CdTe as a model system, we show that the formation of Cd-Te covalent bonds with individual Cd- and Te-containing compounds can be decoupled from the nucleation and growth of CdTe QDs. Prepared at an elevated temperature, a prenucleation-stage sample contains clusters that are the precursor compound (PC) of magic-size clusters (MSCs); the Cd-Te bond formation occurs at temperatures higher than 120 °C in the reaction. Afterward, the PC-to-QD transformation appears via monomers at lower temperatures in dispersion. Our findings suggest that the number of Cd-Te bonds broken in the PC reactant is similar to that of Cd-Te bonds formed in the QD product. For the traditional synthesis of ME QDs, heating is responsible for the M-E bond formation rather than for nucleation.

Structure dependent activation of a Co molecular catalyst through photoinduced electron transfer from CdTe quantum dots.

Complexes of quantum dots with molecular catalysts are promising building blocks for photo-catalytic applications. Herein, we report the formation of stable complexes between colloidal CdTe quantum dots (CQDs) and two synthesized structurally different cobalt porphyrin derivatives (CoPp and CoPm, with phenyl and mesityl groups attached at the meso positions, respectively) through a sulfur bridge. Using both spectroscopy and computational methods, we found that the porphyrin adopts a "flat" binding mode on the CQD surface. We observed the coordination of the Co center on the CQD surface. This coordination is stronger for CoPp than for CoPm, resulting in a larger red shift in the absorption band. In addition, we measured a four fold increase in the electron transfer (ET) rate from the CQD to CoPp compared to that with CoPm by a transient absorption study and the charge recombination extended to tens of nanoseconds or longer depending on the structure of the porphyrin periphery. A spectrum measured after the ET points to a loss of coordination between the Co and CQD in a CoP/CQD complex. The experimental results are in agreement with density functional theory calculation results on the CoP complexes on CdTe surfaces, pointing to the porphyrin preferring to align along the CQD surface in the ground state. The change of porphyrin alignment from flat alignment before the excitation to upright alignment after the ET is a likely cause for the extended lifetime of the charge-separated (CS) state, due to an increase in the CS distance. Furthermore, the spectrum of the CS state can be assigned to catalytically active CoIP, proposing the applicability of the complexes in CO2 reduction.

Size-dependent photoinduced transparency in colloidal CdTe quantum dots in the strong confinement regime: an inverse linear relationship.

Nanomaterials have been investigated as saturable absorbers for ultrafast lasers because of their large photoinduced transparency related to ground-state bleaching. However, the quantum dot size effect on the photoinduced transparency in the strong confinement regime has not been explored due to the challenge of accurately measuring the ground state and the excited-state absorption cross-sections. At the same time, these optical properties are essential to calculate several chemical and physical quantities at the nanoscale. In this context, we have employed the photoluminescence saturation method to determine the ground-state absorption cross-section and the femtosecond open-aperture Z-scan technique to investigate the size-dependent ground-state bleaching of glutathione-capped CdTe QDs synthesized in an aqueous medium. The results were modeled using rate equations within the three-energy levels approach. Our results pointed out that the photoinduced transparency rate at the 1S3/2(h) → 1S(e) transition peak presents an inverse linear relationship with the QD diameter (from 2.2 nm up to 3 nm). Otherwise, the larger QDs have a higher ground-state cross-section, which is directly proportional to the ground-state bleaching. To explain this apparent contradiction, we calculate the effective absorption coefficient αeff = σ/V (σ is the absorption cross section and V is the QD volume) for the QDs and observed that the smaller QDs have a higher absorption from the ground to the first excited state, corroborating our results. Finally, our results showed that the saturable absorption effect in CdTe-QDs is slightly higher than that obtained for graphene and other 2D materials and smaller than the black phosphorus in the visible region.

Tuning the photoluminescence of CdTe quantum dots by controllable coupling to plasmonic Au nanoparticles

A controllable variation of the photoluminescence (PL) intensity of semiconductor quantum dots (QDs) via their coupling to plasmonic nanoparticles (NPs) is the potential basis for optoelectronic and sensing applications. In this work, the effect of Au NPs on the PL of colloidal CdTe QDs is investigated in solution and solid films. An PL enhancement for the QDs synthesized in water was observed in case of spectral overlap of the plasmon absorption band of and QD PL band. In case of Au NPs synthesized in dimethyl sulfoxyl the trend is to reduction of the PL intensity. For the reference samples prepared by mixing QDs not with Au NP solutions but with corresponding pure solvent, certain PL enhancement was observed and presumably attributed to reduction of self-absorption or non-radiative interparticle interaction in less concentrated QD solution. However, the contribution of this dilution-related enhancement is expected to be independent of the spectral properties of NPs and QDs. Therefore, the observed in this work different behavior of QD PL in certain combinations of QDs and NPs is attributed to interaction between electronic excitation in the QD and plasmon.

Optical and Scintillation Properties of Record-Efficiency CdTe Nanoplatelets toward Radiation Detection Applications.

Colloidal CdTe nanoplatelets featuring a large absorption coefficient and ultrafast tunable luminescence coupled with heavy-metal-based composition present themselves as highly desirable candidates for radiation detection technologies. Historically, however, these nanoplatelets have suffered from poor emission efficiency, hindering progress in exploring their technological potential. Here, we report the synthesis of CdTe nanoplatelets possessing a record emission efficiency of 9%. This enables us to investigate their fundamental photophysics using ultrafast transient absorption, temperature-controlled photoluminescence, and radioluminescence measurements, elucidating the origins of exciton- and defect-related phenomena under both optical and ionizing excitation. For the first time in CdTe nanoplatelets, we report the cumulative effects of a giant oscillator strength transition and exciton fine structure. Simultaneously, thermally stimulated luminescence measurements reveal the presence of both shallow and deep trap states and allow us to disclose the trapping and detrapping dynamics and their influence on the scintillation properties.

Enhancement of biexciton generation via resonance energy transfer in nanoclusters of colloidal CdTe quantum dots for optical gain applications

Colloidal CdTe quantum dots (QDs) of two different sizes (2.7 nm and 3.7 nm diameter) were self-assembled into mixed quantum dot donor-acceptor nanoclusters (QDANs) in aqueous solution. The ratio of small QDs (SQDs) to large QDs (LQDs) was 10:1, and the assembly was mediated by electrostatic attraction between oppositely-charged ligands (thioglycolic acid, TGA, and mercaptoethylamine, MA) on the QD surfaces. Under low excitation fluence (well below the threshold for biexciton formation), photoluminescence (PL) emission spectra and time-resolved decays in the QDANs revealed quenching in the SQDs and enhanced emission from the LQDs, providing evidence for resonance energy transfer (RET) from the wider band gap SQDs to the narrower band gap LQDs. The observed RET rate constant, determined from comparison of decays of free SQDs and those in the QDANs, was (3.4 ns)−1. Under these low-fluence conditions, the PL emission lifetimes of the LQDs in QDANs were the same as the free LQDs. Under higher excitation intensity, the decay of the LQDs in the QDAN showed a new, fast (sub-nanosecond), fluence-dependent component, suggesting formation of biexcitons and multiexcitons on the LQDs. The free LQDs required significantly (approximately 4-fold) higher excitation intensity than those in the QDANs to show similar evidence of biexciton formation through lifetime shortening. Since the QDANs contained a 10-fold excess of SQDs, the excitation energy that was initially distributed over the full population of QDs was funneled and concentrated into the small number of LQDs. These results support the idea that optical gain through biexciton formation can be enhanced by using RET, even when the rate constant for RET is much lower than that for biexciton decay.

Quenched or alive quantum dots: The leading roles of ligand adsorption and photoinduced protonation

The fluorescence emission of water-soluble CdTe quantum dots (QDs) capped with mercaptocarboxylic acids (MCAs) is known to be pH-dependent. However, this behaviour is quite different from a study to another, so that literature suffers from a lack of coherence. Here we assume that the QD fluorescence efficiency is actually driven by the acid-base equilibrium of MCA thiol groups, and that light-excited QDs open a non-radiative relaxation path through photoinduced protonation. We address this issue by examining colloidal CdTe QDs with (time-resolved) fluorescence spectroscopy under various conditions of acidity and light excitation. It appears that the emission of QDs is quenched below a critical pH value of 6.87, and that light excitation power strengthens this quenching. We thus demonstrate the existence of an additional photochemical process and developed a mathematical modeling accounting for all our experimental results. With only three parameters, it is possible to accurately predict the fluorescence decay of QDs over time, at any pH. Further, we also related the critical pH value of 6.87 to QD surface properties, explaining why observations may differ from a study to another and making the literature much more coherent.

Aqueous CdTe colloidal quantum dots for bio-imaging of Artemia sp

Aqueous Cadmium Telluride (CdTe) colloidal quantum dots was synthesized using 3-mercaptoproponic acid (3-MPA) as a capping agent. CdTe QDs with band gap of 2.26 eV exhibit absorption edge and florescence emission at 502 nm and 569 nm respectively. The shoulder peak appearance in florescence emission owing to state filling effect. The cubic zinc-blend crystal structure of CdTe QDs are confirmed from XRD pattern with crystallite size ~2.12 nm. The morphological analysis carried out through HRTEM show spherical shaped particles with the average particles size of 3-MPA capped CdTe QDs is 3.3 nm. The elemental compositional and capping effect of the 3-MPA caped CdTe QDs are characterized via EDS, FTIR spectrum and Raman analysis. The bioimaging of Artemia sp using CdTe QDs is accomplished by way of dipping the Artemia animal into greenish-yellow colored (λemission 569) CdTe QDs aqueous solution. The fluorescent images showed clear morphology features of dead juvenile Artemia without affecting their internal and external structure. The linking of CdTe QDs to calcium localized in different cytoplasmic compartments of juvenile Artemia sp through MPA ligand lead to obtain the bright images. This study confirm that water soluble colloidal CdTe QDs well suitable for external morphological observation in a lower invertebrate like Artemia, Daphnia and Jellyfishes.

Temperature- and size-dependent photoluminescence in colloidal CdTe and Cd x Zn1−x Te quantum dots

Semiconductor colloidal quantum dots (QDs) of CdTe and alloyed Cd x Zn1−x Te QDs with N-acetyl-L-cysteine capping ligands are synthesized by a reflux method in aqueous solution. Alloying provides a new degree of freedom to tune the optical and electronic properties of the nanocrystals. The photoluminescence (PL) of Cd x Zn1−x Te QDs is sharper and displays a highly enhanced quantum yield (QY) of 65% relative to the 16% of CdTe QDs. The fluorescence of Cd x Zn1−x Te QDs is observed to be highly stable for over 12 months without degradation, while that of CdTe QDs begins to mildly flocculate around 8 months of storage. To characterise the material structure and composition, UV-Vis absorption spectroscopy, x-ray powder diffraction, transmission electron microscopy, and inductively coupled plasma mass spectrometry measurements are carried out. To understand the fundamental processes that play in the luminescence behaviour, temperature- and size-dependent PL spectra are investigated in the range 80–300 K. The Varshni and O’Donnell equations fit well on the PL peak emission energies and the Huang–Rhys parameter indicates the strengthening of exciton–phonon coupling in the nanocrystals upon alloying and with decreasing nanocrystal sizes. PL linewidth analysis reveals that the inhomogeneous broadening is considerably reduced in Cd x Zn1−x Te QDs relative to CdTe. Moreover, the quantum confinement effect of the nanocrystals leads to an increase in exciton–acoustic phonon interactions with the coefficients ranging between 26.9 and 95.6 µeV K−1 compared to the bulk CdTe value of 0.72 µeV K−1. Exciton–longitudinal optical phonon interactions are made stronger by the ZnTe alloying with the coefficients lying in the range between 24.8 and 41.7 meV and also with the effect of increasing crystal size. An Arrhenius plot of PL integrated area is used to calculate the thermal activation energy value E a of the non-radiative recombination channel, which is 132 meV for CdTe QDs and a higher value of 185 meV for Cd x Zn1−x Te QDs. This is consistent with the observed QY enhancement in Cd x Zn1−x Te QDs as a higher E a value indicates reduced generation of non-radiative recombination centres and a decrease in defect densities upon alloying. Cd x Zn1−x Te QDs with enhanced fluorescence properties serve both as a medium for studying fundamental effects of alloying and its properties, and for practical applications such as biomedical labelling and optoelectronics.

Two-Dimensional Layers of Colloidal CdTe Quantum Dots: Assembly, Optical Properties, and Vibroelectronic Coupling

Manufacturing of silica platforms functionalized by CdTe quantum dots (QDs) of 3.4 nm diameter through (3-aminopropyl)triethoxysilane (APTES) aliphatic organosilanes is performed to preserve QD exc...

Metastable CdTe@HgTe Core@Shell Nanostructures Obtained by Partial Cation Exchange Evolve into Sintered CdTe Films Upon Annealing.

Partial Hg2+ → Cd2+ cation exchange (CE) reactions were exploited to transform colloidal CdTe nanocrystals (NCs, 4–6 nm in size) into CdTe@HgTe core@shell nanostructures. This was achieved by working under a slow CE rate, which limited the exchange to the surface of the CdTe NCs. In such nanostructures, when annealed at mild temperatures (as low as 200 °C), the HgTe shell sublimated or melted and the NCs sintered together, with the concomitant desorption of their surface ligands. At the end of this process, the annealed samples consisted of ligand-free CdTe sintered films containing an amount of Hg2+ that was much lower than that of the starting CdTe@HgTe NCs. For example, the CdTe@HgTe NCs that initially contained 10% of Hg2+, after being annealed at 200 °C were transformed to CdTe sintered films containing only traces of Hg2+ (less than 1%). This procedure was then used to fabricate a proof-of-concept CdTe-based photodetector exhibiting a photoresponse of up to 0.5 A/W and a detectivity of ca. 9 × 104 Jones under blue light illumination. Our strategy suggests that CE protocols might be exploited to lower the overall costs of production of CdTe thin films employed in photovoltaic technology, which are currently fabricated at high temperatures (above 350 °C), using post-process ligand-stripping steps.

Colloidal CdxM1-xTe Nanowires from the Visible to the Near Infrared Region: N,N-Dimethylformamide-Mediated Precise Cation Exchange.

Cation exchange has been a successful methodology for tuning the bandgaps of nanomaterials, while the most popular protocol in the toluene/methanol system lacks precise compositional control due to its inherent poor solvent compatibility. We herein report an alternative cation exchange route in N,N-dimethylformamide (DMF) solvent for converting preformed colloidal CdTe nanowires into CdxM1-xTe (M = Pb2+, Zn2+, Ag+, Hg2+) nanowires with good batch-to-batch reproducibility. The resulting CdxM1-xTe nanowires show a tunable bandgap from 2.26 to 0.63 eV, and the energy levels of these nanowires can be finely tuned. Furthermore, a comparative study for the cation exchange of CdTe nanowires with Pb2+ ions in toluene/methanol and DMF illustrated that the reduction of Cd2+ extraction and the Pb2+ introduction barrier accounts for precise compositional control. The cation exchange reaction in the DMF phase provides an efficient way to obtain nanomaterials with precise composition control. Moreover, these available high-quality colloidal semiconductor nanowires also pave the way for near-infrared device exploration.

Optical sensing of ultra-trace As3+ species using 2-mercaptoethanol capped CdTe Quantum Dots in aqueous medium

This work describes the optical response of luminescent colloidal CdTe quantum dots (QDs) stabilized with 2-mercaptoethanol (ME) and thioglycolic acid (TGA) synthesized by a one-pot hydrothermal method for sensing As3+ ions in aqueous solution. The sensing behavior of CdTe QDs was determined by examining the fluorescence spectra with the varying molar concentration of As3+ ions in the solution. The fluorescence peaks of TGA-CdTe QDs (λ = 648 nm) depreciated in the presence of As3+ ions. However, ME-CdTe QDs (λ = 610 nm) peaks got enhanced in the presence of As3+ ions. The quenching responses of TGA-CdTe fluorescence peak is attributed to a non-radiative relaxation process due to the presence of traps states on the surface. On the other hand, luminescence enhancement of ME-CdTe QDs can be ascribed to the active surface related radiative decay, owing to surface passivation of dangling bonds in QDs or due to the formation of higher bandgap AsTe shell structure formed on the ME-CdTe QDs. The detection limit in this method was found to be 10 nM l−1. Hence, establishing ME-CdTe as a suitable probe for ultra-trace detection of As3+ ion in aqueous solution.

Four Types of CdTe Magic-Size Clusters from One Prenucleation Stage Sample at Room Temperature.

Four types of colloidal semiconductor CdTe magic-size clusters (MSCs), each of which is in a single-ensemble form, have been obtained at room temperature from a single induction period (IP) sample in dispersion. The induction period is the prenucleation stage that occurs prior to nucleation and growth of colloidal quantum dots (QDs). Three types display sharp optical absorption peaking at either 371, 417, or 448 nm, and the fourth type exhibits a sharp absorption doublet with peaks at 350 and 371 nm. These MSCs are respectively denoted as sMSC-371, sMSC-417, sMSC-448, and dMSC-371. We show that the evolution of the various MSCs is affected by the nature of their dispersions. We hypothesize that the evolution of MSCs involves their precursor compounds (PCs), which are transparent in optical absorption. The present study explores new avenues for the exclusive synthesis of four types of CdTe MSCs (with each in a single-ensemble form) and provides an improved understanding for their formation.

Investigations on the interaction of water-soluble semiconductor polymer with thioglycolic acid (TGA) capped CdTe quantum dots

Studies of nanocomposites produced by the association of quantum dots (QDs) and polymers, in general, use the nanocrystals as the energy's donor and the polymer's molecules as acceptors. The production of nanocomposites in which a water-soluble polymer is the energy donor species and the quantum dots the acceptors species is particularly difficult due to low luminescence quantum yield and low fluorescence lifetimes of these molecules when compared to the QDs. In this work, we associated colloidal CdTe QDs stabilized with thioglycolic acid (TGA) with water-soluble semiconductor luminescent polymer Poly{[2,5-bis(2-(N,N-diethylamino)ethoxy)-1,4-phenylene]-alt-1,4-phenylene} (PPE-DBE) to produce nanocomposites that favors the polymers as donors and QDs as acceptors. We characterized the composites by absorption, static fluorescence, and dynamic fluorescence. Our results showed the intensification of the QDs emission and its quantum yield on the nanocomposite and the decrease of the polymer lifetime. Thus, we verified the occurrence of energy transfer from the polymer to the quantum dots, especially for samples which the ratio of PPP-DBE monomers per number of QDs in solution is approximately 465, achieving an increase of 11.34% on the QDs' fluorescence quantum yield.

Improved photoluminescence and monodisperse performance of colloidal CdTe quantum dots with Cannula method

Colloidal quantum dots are nano semiconductor materials that have been found in many applications, producing multiple exciton generation, unique optical and electronic properties, adjustable in size and bandwidth. Synthesized QDs are expected to exhibit high photoluminescence quantum yield and monodisperse properties according to their application area. Cannula method was adapted together with the organometallic synthesis method for the first time in the literature to increase the photoluminescence quantum yield of organometallic CdTe QD and minimize the full width at half maximum value of the photoluminescence band. Injection of precursors by the Cannula method is much faster than the injecting with the conventional method of using a glass syringe, which limits the size distribution in the solution during synthesis. In addition, the fastest injection method using Cannula method yields the shortest full width half maximum value of 27.20 nm for CdTe QDs in the literature. The photoluminescence quantum yield value of the CdTe QDs synthesized by the classical method was 8.12±2.1%, while the photoluminescence quantum yield of the CdTe QDs synthesized by the Cannula method was increased to 25.66±2.1%.

Preparation and characterization of colloidal CdTe nanoparticles by laser ablation in liquids

Nanoparticles generation by laser ablation of a solid target in a liquid environment is an easy method. Cadmium Telluride (CdTe) colloidal nanoparticles have been synthesized by laser ablation Nd:YAG with wavelengths of 1064nm and double frequency at 532 nm, number of pulses 50 pulses, with pulse energy= 620mJ, 700mJ of a solid target CdTe is immersed in double distilled deionized water (DDIW) and in methanol liquid. Influences of the laser energy and different solutions on the formation and optical characterization of the CdTe nanoparticles have been studied using atomic force microscope (AFM) and the UV-Vis absorption. As a results, it leads to the absorbance in UV-Vis spectra of samples prepared in water at laser wavelength of 532nm is lower than the absorbance of 1064nm at same laser energy. AFM images show that CdTe NPs have spherical shape in nanometer size.

Tin Domain Growth on Quasi-Two-Dimensional CdTe and CdSe Nanoparticles

Tin domain growth on quasi-two-dimensional colloidal CdSe and CdTe nanoparticles having the zinc blende structure has been studied. The initial quasi-two-dimensional CdSe and CdTe nanoparticles having lateral sizes of 100–200 nm were prepared by a colloidal method. Tin domain growth was accomplished in tetrahydrofuran via the reduction of a tin(II) salt by tetrabutylammonium borohydride. The tin domains had sizes of 10–20 nm as probed by TEM. In case of CdSe nanoparticles, tin domains were grown inside the inner cavities of initially rolled nanoparticles. A β-tin phase was identified by X-ray diffraction. The absorption spectra featured the broadening of exciton bands corresponding to quasi-two-dimensional nanoparticles, the spectral positions of absorption peaks remaining almost unchanged.

Tunable single and double emission semiconductor nanocrystal quantum dots: a multianalyte sensor

We have prepared stable colloidal CdTe and CdTe/ZnS core–shell quantum dots (QDs) using hot injection chemical route. The developed CdTe QDs emit tunable single and dual photoluminescence (PL) bands, originating from the direct band edge and the surface state of QDs, as evident by the steady-state and time-resolved spectroscopy. The developed CdTe and CdTe/ZnS QDs act as optical sensors for the detection of metal ions (e.g., Fe2+ and Pb2+) in the feed water. The PL quenching in the presence of analytes has been examined by both the steady-state and time-resolved PL spectroscopy. The linear Stern–Volmer (S–V) plots obtained for PL intensity and lifetime as a function of metal ion concentration demonstrates the diffusion-mediated collisional quenching as a dominant mechanism together with the possibility of fluorescence resonance energy transfer. Thus, the prepared core and core–shell QDs which cover a broad spectral range of white light with high quantum yield (QY) are highly sensitive to the detection of metal ions in feed water and are also important for biological applications (Ratnesh and Mehata 2017 Spectrochim. Acta A: Mol. Biomol. Spectro. 179 201–10).

Crystal-Phase Control of Catalytically Grown Colloidal CdTe Quantum Wires: Dual Role of n-Tetradecylphosphonic Acid

n-Tetradecylphosphonic acid (TDPA) is an acid and also a source for a strong-binding ligand, n-tetradecylphosphonate (TDPT), for the preparation of the cadmium precursor Cd(DOPT)x(TDPT)1–0.5x (where DOPT = di-n-octylphosphinate). The reaction chemistry of the cadmium precursor and tri-n-octylphosphine telluride (TOPTe) is manipulated to favor the formation of a CdTe solute if the catalytic role of TDPA as an acid is predominant; otherwise the stabilization of the cadmium precursor by TDPT incorporation results in a significant excess of Te in the reaction mixture. These exert opposite consequences on the crystal-phase purity of colloidal CdTe quantum wires (QWs) grown from initial bismuth (Bi) nanoparticles. Primary dissolution of the available CdTe or Te into Bi nanoparticles affords liquid (y ≈ z) or solid (y ≪ z) BixCdyTez catalysts, respectively. The solid catalysts enable the solution–solid–solid growth of nearly phase-pure wurtzite QWs, whereas the liquid catalysts fulfill the solution–liquid–solid ...