CdTe Self-assembled Quantum Dots Research Articles

All-Optical Reconfigurable Metamaterial Employing the Self-Assembly of CdTe Quantum Dots

Positively and negatively charged polyelectrolytes, namely, Poly(diallyldimethylammonium chloride) and Poly(styrene sulfonate), respectively, were employed to disperse and deploy negatively charged quantum dots on an otherwise passive metamaterial structure with a resonant frequency of 0.62 THz, by employing a layer-by-layer, self-assembly scheme. Upon exposure to a UV source with a wavelength of 365 nm the amplitude modulation was observed to increase with increases in the number of deposited bi-layers, until a modulation maximum of 2.68% was recorded enabling an all-optical, dynamically reconfigurable metamaterial geometry. Furthermore, amplitude modulation was subsequently observed to decrease with further increases in the number of layers employed due to quenching and shadowing effects. The experimental observations reported herein will enable the utilization of all-optical reconfigurable THz devices for communication and data transmission applications.

Surfactant-Induced Self-Assembly of CdTe Quantum Dots into Multicolor Luminescent Hybrid Vesicles.

Here, we report the interaction of mercaptosuccinic acid (MSA)-capped CdTe quantum dots (QDs) with hexadecyltrimethylammonium bromide (CTAB) surfactant and subsequent formation of self-assembled multicolor luminescent vesicles in aqueous medium. A continuous phase sequence from clear (C1) to turbid (T1), precipitate (P), turbid (T2), and clear (C2) has been observed for QD solution upon increasing the concentration of positively charged CTAB, indicating dynamic equilibrium between various self-assembled supramolecular structures. In contrast, no such changes have been observed in the presence of negatively charged sodium dodecyl sulfate and neutral Triton X-100 surfactants, indicating specific electrostatic interactions behind the observed phase separation behavior. Epi-fluorescence imaging in the C1 and C2 regions reveals the presence of surfactant-induced aggregates of QD. The morphologies and photoluminescence properties of self-assembled supramolecular structures in the T1 and T2 region have been explored by using scanning electron microscopy (SEM), atomic force microscopy (AFM), and confocal laser scanning microscopy (CLSM). SEM and AFM images reveal distinct spherical vesicles in the T1 and T2 regions of the binary mixture. Moreover, CLSM results show that these spherical vesicles are inherently luminescent due to the presence of self-assembled QDs. Fabrication of multicolor luminescent vesicles has been demonstrated by tuning the size of CdTe QD. Using CLSM, we have further demonstrated efficient encapsulation of Rhodamine 6G dye into these self-assembled vesicles without any structural disruption. While these luminescent vesicles are quite stable in neutral and basic pH (pH = 6.5-11), they are unstable in acidic pH (pH = 4.5-5.5). Moreover, it has been observed that this pH-responsive structural change is totally reversible. The present findings of self-assembled luminescent vesicles from QD-CTAB binary mixture may open up new opportunities in various applications such as bioimaging, drug delivery, and sensing.

Exciton energy transfer-based fluorescent sensor for the detection of Hg2+ through aptamer-programmed self-assembly of QDs

Herein, an original exciton energy transfer-based sensitive fluorescence sensor for the determination of Hg2+ has been designed through DNA aptamer-programmed self-assembly of CdTe quantum dots (QDs). In this work, CdTe QDs were applied as fluorescence signal source. The two pieces of T-rich aptamer played a role as molecular recognition probes which could bind to the target Hg2+ specifically. The extent of Hg2+-triggered self-assembly of QDs depended on the concentration of Hg2+, which resulted in an exciton energy transfer effect between QDs, giving an obvious fluorescence signal decrease and red-shift of the fluorescent peak. Based on this principle, we could detect the Hg2+ in two different signal modes. The limit of detection (LOD) was 3.33 nM. The proposed sensing method exhibited its application in detecting Hg2+ in real water samples with satisfactory performance. The results indicated that this proposed sensor will be of great potential in biological and analytical fields.

Rapid preparation of self-assembled CdTe quantum dots used for sensing of DNA in urine

In this article, the authors report a systematic study of the self-assembly of CdTe quantum dots (QDs) stabilized by mercaptosuccinic acid (MSA) at laboratory temperature (25 °C) or after thermal treatment (90 °C).

Immunoassay for carcinoembryonic antigen based on the Zn2+-enhanced fluorescence of magnetic-fluorescent nanocomposites

Carbon coated magnetic composite nanoparticles (Fe3O4@C) were prepared and further fabricated into a new kind of the multifunctional core/shell nanoparticles (Fe3O4@C@CdTe) by a self-assembly of CdTe quantum dots. These nanoparticles exhibited good fluorescence (FL) property. In addition, zinc oxide (ZnO) nanowires (NWs) with high surface area were synthesized via solvothermal reaction under a hydrothermal condition. In a sandwich immune-device for the detection of carcinoembryonic antigen (CEA), the Fe3O4@C@CdTe nanoparticles were applied as the solid support for CEA-primary monoclonal antibody (CEA-McAb1) and the ZnO NWs were applied as the label of CEA-secondary monoclonal antibody (CEA-McAb2). The Fe3O4@C@CdTe-McAb1 was collected at the bottom of the quartz cuvette using a magnet. The ZnO NWs bond with Fe3O4@C@CdTe after the immune reaction. The Zn2+ ions are subsequently released from ZnO NWs by dissolution in acidic buffer solution and the enhanced FL intensity is measured. In this way, the proposed FL immune-device displayed excellent analytical performance for the detection of CEA in the range of 0.005–100ngmL−1 with a low detection limit of 1.6pgmL−1, and the assay results of CEA in real serum samples were in a good agreement with the standard enzyme-linked immunosorbent assay (ELISA) tests.

Solvent-Triggered Self-Assembly of CdTe Quantum Dots into Flat Ribbons

Diluting an aqueous colloid containing purified CdTe quantum dots (QD) by injecting into common organic solvents triggered self-assembly into a variety of structures. Nanoribbons formed in methanol with aspect ratios near 1000 containing discrete dots lacking a packing order. The flat ribbons were 30–90 nm (8–22 QDs) wide based on AFM and TEM, about 8–18 nm (2–5 QDs) high based on AFM, and 0.5–10 μm long based on SEM. Passivation of defect sites, likely by S, enhanced the photoluminescence of the ribbons relative to the raw QDs. Multibranched clusters containing fused dots formed in IPA as well as ribbons with pendent nodules. The photoluminescence of the assortment was attenuated compared to the raw QDs. Injecting into acetone not only yielded ribbons and clusters but also dissolved the dots over a period of 20 days, forming flower-like assemblies whose petals consisted of bundles of CdS wires. Diluting in solvents with lower dielectric constants than water initially aggregated the dots by reducing the electrostatic screening between the negatively charged thioglycolic acid (TGA) ligand layers. The solubility of TGA in the solvents determined the superstructure that formed. Extracting the smallest portion of this layer in methanol promoted vectorial growth into ribbons consistent with dipole–dipole attractive and charge–charge repulsive interactions. Removing more of the TGA layer in IPA caused the dots to fuse into webs containing clustered ribbons and branches, and the directional nature of the superstructure was lost. Completely deprotecting the surface in acetone promoted photoetching and dissolved the dots. Control of the ligand surface density by means of the solubility adds another method to direct spontaneous self-organization of QDs.

Hyper-Branched CdTe Nanostructures Based on the Self-Assembling of Quantum Dots and Their Optical Properties

As the priority of interconnects and active components in nanoscale optical and electronic devices, three-dimensional hyper-branched nanostructures came into focus of research. Recently, a novel crystallization route, named as "nonclassical crystallization," has been reported for three-dimensional nanostructuring. In this process, Quantum dots are used as building blocks for the construction of the whole hyper-branched structures instead of ions or single-molecules in conventional crystallization. The specialty of these nanostructures is the inheritability of pristine quantum dots' physical integrity because of their polycrystalline structures, such as quantum confinement effect and thus the luminescence. Moreover, since a longer diffusion length could exist in polycrystalline nanostructures due to the dramatically decreased distance between pristine quantum dots, the exciton-exciton interaction would be different with well dispersed quantum dots and single crystal nanostructures. This may be a benefit for electron transport in solar cell application. Therefore, it is very necessary to investigate the exciton-exciton interaction in such kind of polycrystalline nanostructures and their optical properites for solar cell application. In this research, we report a novel CdTe hyper-branched nanostructures based on self-assembly of CdTe quantum dots. Each branch shows polycrystalline with pristine quantum dots as the building units. Both steady state and time-resolved spectroscopy were performed to investigate the properties of carrier transport. Steady state optical properties of pristine quantum dots are well inherited by formed structures. While a suppressed multi-exciton recombination rate was observed. This result supports the percolation of carriers through the branches' network.

Ultra low density of CdTe quantum dots grown by MBE

This work presents methods of controlling the density of self-assembled CdTe quantum dots (QDs) grown by molecular beam epitaxy. Two approaches are discussed: increasing the deposition temperature of CdTe and the reduction of CdTe layer thickness. Photoluminescence (PL) measurements at low temperature confirm that both methods can be used for significant reduction of QDs density from 1010QD/cm2 to 107–108QD/cm2. For very low QDs density, identification of all QDs lines observed in the spectrum is possible.

Controlled self-assembly of CdTe quantum dots into different microscale dendrite structures by using proteins as templates

The controlled self-assembly of quantum dots (QDs) into complex microscale structures is crucial for the study of their collective properties. Here, we demonstrate that diverse structured microscale dendrites could be formed on glass surfaces assembled from CdTe QDs by using ten different proteins as templates, through evaporation-induced self-assembly (EISA). The proteins could self-assemble into dendritic microstructures by solvent evaporation from solutions, and the dispersed CdTe QDs could grow into various dendritic patterns on the micropatterns of proteins, as a result of a simple combination of CdTe QDs with proteins. The structures of the CdTe dendrites can be tailored by controlling the pH value of the solvent of the protein or the addition of metal ions. Polarization dependent experiments demonstrate that the formed CdTe dendrites possess the properties of crystallinity and birefringence. This work opens a new, convenient avenue for the manipulation of the self-assembly of CdTe QDs into different dendritic microstructures.

Characterization of Self-Assembled CdTe Quantum Dots Grown on ZnTe/GaSb

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Electro-optical characterization of Ti/Au–ZnTe Schottky diodes with CdTe quantum dots

In this study the electric and optical spectroscopy techniques have been applied to investigate ZnTe (p-type)–Ti/Au Schottky diodes containing a layer of CdTe self-assembled quantum dots (SAQDs). The reference sample was the ZnTe–Ti/Au diode without dots. Both samples were grown by molecular beam epitaxy technique. Raman measurements confirmed the presence of the CdTe layer while the photoluminescence proved that CdTe quantum dots were formed in the investigated structure. The PL spectra reveal the CdTe QD electron-hole recombination energy equal to 2.1eV at 10K. Based on the temperature PL measurements the activation energy of PL quenching has been determined to be equal to 22meV. Further confirmation for the QD formation has been obtained from the C–V characteristics which exhibited a step related to the charge accumulation at the QD states. DLTS spectra for the sample with QDs yield three hole-related signals with apparent activation energies equal to EH1=0.16eV, EH2=0.2eV and EH3=0.4eV. For the reference ZnTe–Ti/Au diode solely single signal was observed of signature close to the level H3 in the QD sample. Detailed characterization of the traps as well as the PL studies lead to the conclusion that the level H2 is related to the defects located close to the QDs created during the growth while the other traps are associated with defects present in the ZnTe bulk material.

Temperature dependence of photoluminescence intensity of self-assembled CdTe quantum dots in the ZnTe matrix under different excitation conditions

The temperature dependence of the integrated photoluminescence intensity of nanometer-sized ZnTe/CdTe/ZnTe quantum wells has been investigated under different excitation conditions. It has been shown that the character of thermal decay of the luminescence intensity depends on the frequency of the exciting light and, under the above-barrier excitation, strongly depends on the optical excitation power density. It has been found that an increase in the excitation intensity leads to a saturation of thermal quenching of the luminescence in the low-temperature range. The conclusion has been drawn that this behavior reflects the saturation of nonradiative recombination centers with photoexcited carriers.

Charging Effects in Self-Assembled CdTe Quantum Dots

Charging Effects in Self-Assembled CdTe Quantum Dots

Tuning the inter-shell splitting in self-assembled CdTe quantum dots

We present photoluminescence studies of highly excited single self-assembled CdTe quantum dots under continuous-wave and pulsed excitations. We observe appearance of emission bands related to sequential filling of s-, p- and d-shells. We analyze the inter-shell splitting for five samples, in which the dots were formed from a strained CdTe layer of different width. We find that by increasing the CdTe layer width, the inter-shell splitting increases. In a time resolved measurement, we observe a radiative cascade between transitions involving one, two, and more than two excitons.

Deep Level Transient Spectroscopy of Hole Traps Related to CdTe Self-Assembled Quantum Dots Embedded in ZnTe Matrix

The capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) measurements have been made on a Schottky Ti-ZnTe (p-type) diode containing CdTe self-assembled quantum dots (QD) and control diode without dots. The C-V curve of the QD diode exhibits a characteristic step associated with the QD states whereas the reference diode shows ordinary bulk behavior. A quasistatic model based on the self-consistent solution of the Poisson's equation is used to simulate the capacitance. By comparison of the calculated C-V curve with the experimental one, hole binding energy at the QD states is found to be equal about 0.12 eV. The results of DLTS measurements for the sample containing QDs reveal the presence of a low-temperature peak which is not observed for the control diode. Analysis of its behavior at different bias conditions leads to the conclusion that this peak may be related to the hole emission from the QD states to the ZnTe valence band. Its thermal activation energy obtained from related Arrhenius plot equals to 0.12 eV in accordance with the energy obtained from the Poisson's equation. Thus based on the C-V and DLTS studies it may be concluded that the thermal activation energy of holes from the QD states to the ZnTe valence band in the CdTe/ZnTe QD system is equal about 0.12 eV.

Stark spectroscopy and radiative lifetimes in single self-assembled CdTe quantum dots

We present studies on Coulomb interactions in single self-assembled CdTe quantum dots. We use a field effect structure to tune the charge state of the dot and investigate the impact of the charge state on carrier wave functions. The analysis of the quantum confined Stark shifts of four excitonic complexes allows us to conclude that the hole wave function is softer than electron wave function, i. e. it is subject to stronger modifications upon changing of the dot charge state. These conclusions are corroborated by time-resolved photoluminescence studies of recombination lifetimes of different excitonic complexes. We find that the lifetimes are notably shorter than expected for strong confinement and result from a relatively shallow potential in the valence band. This weak confinement facilitates strong hole wave function redistributions. We analyze spectroscopic shifts of the observed excitonic complexes and find the same sequence of transitions for all studied dots. We conclude that the universality of spectroscopic shifts is due to the role of Coulomb correlations stemming from strong configuration mixing in the valence band.

Optical measurements of single CdTe self-assembled quantum dots grown on ZnTe/GaSb

The CdTe quantum dots (QDs) grown on a ZnTe epilayer deposited directly on a GaSb substrate are studied by transmission electron microscopy (TEM) and near-field scanning optical microscopy. High resolution TEM images show CdTe QD of planoconvex shape. Near-field photoluminescence (NPL) in an applied magnetic field is used to measure the Zeeman splitting and diamagnetic shift of single CdTe QDs. Finally, time resolved NPL measurements suggest that the higher energy lines in the CdTe QD NPL spectra correspond to exciton transitions occurring in larger CdTe QDs and have shorter exciton lifetimes.

Hierarchical self-assembly of CdTe quantum dots into hyperbranched nanobundles: Suppression of biexciton Auger recombination

In this paper, we report a novel nanobundle structure formed by the hierarchical self-assembly of TGA-capped CdTe quantum dots. HR-TEM confirms the polycrystalline phase of the bundle structure, and that pristine quantum dots are the building units. The steady state absorption and luminescence properties of the pristine quantum dots can be well inherited by the nanobundles. In transient state observation, carrier quenching induced by Auger recombination is found to be remarkably suppressed. Electron delocalizing to close building units is considered to be the reason. Suppression of Auger recombination may earn much more time for charge separation, which makes the novel nanobundle structures suitable for the excellent donor material in solar cell applications.

Self-assembly of CdTe QDs into urchin-like microspheres by the assistance of a long-chain ionic liquid monolayer

In the present work, urchin-like CdTe microspheres were prepared through the self-assembly of CdTe quantum dots (QDs) via the monolayer of a long-chain ionic liquid, 1-hexadecyl-3-methylimidazolium bromide (C16mimBr). The TEM and SEM images indicated that the synthesized urchin-like microspheres were about 2.0 μm in diameter and composed of nanorods grown on their surfaces. The effects of the growth time and the concentration of CdTe QDs were investigated, and the results revealed that sufficient growth time and appropriate concentration were both required for the formation of the urchin-like CdTe microspheres. Based on these results, a possible mechanism for the formation of the urchin-like CdTe microspheres is proposed.

Dynamics of charge leakage from self-assembled CdTe quantum dots

We study the leakage dynamics of charge stored in an ensemble of CdTe quantum dots embedded in a field-effect structure. Optically excited electrons are stored and read out by a proper time sequence of bias pulses. We monitor the dynamics of electron loss and find that the rate of the leakage is strongly dependent on time, which we attribute to an optically generated electric field related to the stored charge. A rate equation model quantitatively reproduces the results.