Donor Quantum Dots Research Articles

Evidence of resonant energy transfer between CdSe/ZnS quantum dots and Neutral Red dye molecules

Förster resonant energy transfer (FRET) process can be monitored by observing the quenching of the donor luminescence in the presence of an acceptor particle. In a different way, the present study demonstrates such process in a pair system represented by CdSe/ZnS quantum dots (QDs) and Neutral Red dyes through a spatially resolved micro-photoluminescence (μ-PL) technique. The possibility of their use in biological applications can be explored, such as mimicking sensitive chemical-biological sensors. In order to obtain the signature of the nonradiative transfer FRET, the donor-acceptor intermolecular distance was changed by the acceptors concentration. A macroscopic point of view of the system was obtained by measures of the photon diffusion lengths as a function of the donor-acceptor distances. Both the QD donor fluorescence quenching and the fall of the spatial photon distribution with the acceptor concentration evidenced the FRET process. The results also confirmed that the chloroform solvent was clearly more appropriate for enhancing the FRET in the QD-NR system.

Ultrafast Charge Transfer at a Quantum Dot/2D Materials Interface Probed by Second Harmonic Generation.

Hybrid quantum dot (QD)/transition metal dichalcogenide (TMD) heterostructures are attractive components of next generation optoelectronic devices, which take advantage of the spectral tunability of QDs and the charge and exciton transport properties of TMDs. Here, we demonstrate tunable electronic coupling between CdSe QDs and monolayer WS2 using variable length alkanethiol ligands on the QD surface. Using femtosecond time-resolved second harmonic generation (SHG) microscopy, we show that electron transfer from photoexcited CdSe QDs to single-layer WS2 occurs on ultrafast (50 fs to 1 ps) time scales. Moreover, in the samples exhibiting the fastest charge transfer rates (≤50 fs) we observed oscillations in the time-domain signal corresponding to an acoustic phonon mode of the donor QD, which coherently modulates the SHG response of the underlying WS2 layer. These results reveal surprisingly strong electronic coupling at the QD/TMD interface and demonstrate the usefulness of time-resolved SHG for exploring ultrafast electronic-vibrational dynamics in TMD heterostructures.

Fluorescence Quenching Immunoaffinity Test Column with Quantum Dots as Fluorescence Donors for the Quick Detection of Malachite Green and Crystal Violet in Aquatic Products

A visual fluorescence quenching immunoaffinity test column (FQ-ITC) assay was developed for the simultaneous detection of malachite green (MG) and crystal violet (CV) residues in aquatic products. The proposed FQ-ITC assay was based on fluorescence resonance energy transfer from the fluorescence donor (quantum dot) to the fluorescence acceptor (MG or CV) and the specific identification between the analytes and their polyclonal antibodies. The process can be completed within 10 min, and the results can be observed by the naked eye. After the evaluation of the specificity and sample matrix effect, the established FQ-ITC was found to be successfully applied in the detection of MG or CV in grass carp, striped bass, carp, and shrimp, with a visual limit of detection of 2 μg/kg. The accuracy of the FQ-ITC assay was verified by comparison with liquid chromatography-tandem mass spectrometry. The simple, time-saving, sensitive, and visual observation characteristics make the established FQ-ITC assay an effective analytical tool for the simultaneous detection of MG and CV in aquatic products.

Long-Range Resonant Energy Transfer Using Optical Topological Transitions in Metamaterials

The control and enhancement of resonance energy transfer is highly desirable for a variety of applications ranging from solar cells to spectroscopic rulers. However, the process of direct resonance energy transfer is distance dependent and limited to ∼10 nm for typical donor–acceptor pairs. Here we demonstrate long-range (∼160 nm) direct energy transfer between donor quantum dots and acceptor dye molecules through the use of an optical topological transition (OTT) in a metamaterial. The OTT in a metamaterial, modifies the density of states between the donor and acceptor, resulting in the long-range energy transfer with transfer efficiency of ∼32%. Theoretical calculation based on master-equation formalism is used to model the system and is found to be in good agreement with the experimental observation. The use of OTTs in metamaterials to enhance and control energy transfer process can have wide array of potential applications ranging from organic solar cells to quantum entanglement.

Relaxation times measured for singlet and triplet electron spin states between two donor qubits

Two single-electron spin qubits based on single and double donor quantum dots controllably produce singlet and triplet states transitions in the combined system of two electron spins. The lifetimes were measured using single spin readout.

A Quantum Dot-Protein Bioconjugate That Provides for Extracellular Control of Intracellular Drug Release.

The ability to control the intracellular release of drug cargos from nanobioconjugate delivery scaffolds is critical for the successful implementation of nanoparticle (NP)-mediated drug delivery. This is particularly true for hard NP carriers such as semiconductor quantum dots (QDs) and gold NPs. Here, we report the development of a QD-based multicomponent drug release system that, when delivered to the cytosol of mammalian cells, is triggered to release its drug cargo by the simple addition of a competitive ligand to the extracellular medium. The ensemble construct consists of the central QD scaffold that is decorated with a fixed number of maltose binding proteins (MBPs). The MBP binding site is loaded with dye or drug conjugates of the maltose analogue beta-cyclodextrin (βCD) to yield a QD-MBP-βCD ensemble conjugate. The fidelity of conjugate assembly is monitored by Förster resonance energy transfer (FRET) from the QD donor to the dye/drug acceptor. Microplate-based FRET assays demonstrated that the βCD conjugate was released from the MBP binding pocket by maltose addition with an affinity that matched native MBP-maltose binding interactions. In COS-1 cells, the microinjected assembled conjugates remained stably intact in the cytosol until the addition of maltose to the extracellular medium, which underwent facilitated uptake into the cell. Live cell FRET-based confocal microscopy imaging captured the kinetics of realtime release of the βCD ligand as a function of extracellular maltose concentration. Our results demonstrate the utility of the self-assembled QD-MBP-βCD system to facilitate intracellular drug release that is triggered extracellularly through the simple addition of a well-tolerated nutrient and is not dependent on the use of light, magnetic field, ultrasound, or other traditional methods of stimulated drug release. We expect this extracellularly triggered drug release modality to be useful for the in vitro characterization of new drug candidates intended for systemic delivery/actuation and potentially for on-demand drug release in vivo.

Nanometer-Scale Resolution Achieved with Nonradiative Excitation

Nonradiative excitation fluorescence microscopy (NEFM) is a promising technique allowing the observation of biological samples beyond the diffraction limit. By coating a substrate with an homogeneous monolayer of quantum dots (QDs), NEFM is achieved through a nonradiative energy transfer from QDs (donors) to dye molecules located in the sample (acceptors). The excitation depth of the sample is then given by the Forster radius, which corresponds to a few nanometers above the surface. The powerful axial resolution of NEFM is highlighted by observing the adhesion of giant unilamellar vesicles (GUVs) on strong interaction with coated surfaces. In this paper, we demonstrate that the QD-quenching level is valuable to calculate and map the distance between the membrane and the surface with a high precision along the optical axis. By tuning the electrostatic interactions between the membrane and the substrate, we have been able to measure a height displacement of ≈1 nm of the lipid membrane. The experimental resu...

Improving Solar Cell Performance Using Quantum Dot Triad Charge-Separation Engines

We use kinetic modeling to explore the current–voltage, power–voltage, and power conversion efficiency characteristics of quantum dot dyads and triads as possible light absorption and charge separation engines in quantum dot, bulk heterojunction solar cells. The external and internal power conversion quantum efficiencies are significantly enhanced by introducing a third quantum dot between the donor and acceptor quantum dots. Given the constraint of comparable charge-recombination and charge-separation rates, open-circuit voltages for triads are predicted to be about 10%–17% larger than those for dyads, and short-circuit currents for triads are about 400% larger than those for dyads. These improvements in the efficiencies can be further enhanced by tuning the band-edge energy offset of the middle-position quantum dot from its neighbors. The band-edge energies of the middle quantum dot should be tuned so that they form a cascading band-edge energy alignment from the band-edge energies of the left CdTe QD to the right CdSe QD. To produce the most favorable solar cell performance, the middle quantum dot’s conduction (valence) band edge should be closer to the right quantum dot’s band edge when the charge recombination rates are low (high) and near the conduction (valence) band edge of the left quantum dot when the charge recombination rates are high (low). This analysis identifies important strategies to design multi-QD assemblies for solar energy harvesting and conversion.

Optical and magneto optical responses assigned to probable processes of formation of exciton bound to an ionized donor in quantum dot

The system formed by an electron and a hole coupled by Coulomb interactions with an ionized donor is the least known among the excitonic complexes. This complex is usually described as an exciton bound to an ionized donor. Strictly speaking, the exact mechanism of formation of this complex remains unclear. Two processes of formation giving rise to this system can be imagined. In a first process labeled A, the complex may be regarded as an exciton trapped by an ionized donor while in the second mechanism labeled B, this complex can be derived from the binding of a neutral donor and a hole. From a theoretical point of view both protocols can occur, with different probabilities and different binding energies, and consequently lead to different lines in the optical absorption spectra. In our hypothesis, we assume that the statistical mixture contains the two species coming from different origins and which were formed randomly. In this context of uncertainty and mixing processes, we propose to determine the absorption coefficient in the framework of the two possible hypotheses. In the aim to contribute with a valid description of the absorption spectrum, we report in this paper a full theoretical analysis of the optical and magneto-optical phenomena accompanying the two possible processes of formation, taking into account the dot sizes, the magnetic field strength, and the effect of the dielectric constant of the host material.

Non Radiative Excitation Fluorescence Microscopy: A New Method for Studying Membrane Adhesion at the Nanoscale

Non-radiative Excitation Fluorescence Microscopy (NEFM) is a promising technique allowing the observation of biological samples beyond the diffraction limit. By coating a substrate with an homogenous monolayer of quantum dots (QDs), Förster Resonance Energy Transfer (FRET) could be achieved from QDs (donors) to dye molecules located in the sample (acceptors). Therefore, the excitation depth of the sample is then given by the Förster radius, which corresponds to few nanometers above the surface. Here, we present this original method to probe the adhesion of Giant Unilamellar Vesicles (GUVs), negatively charged, in strong interaction with a positively charged surface ( QDs layer is coated with Poly-L-Lysine ). Distances between the surface and GUVs are lower than 5 nm. We used the QD-quenching level to calculate and map the absolute distance between the membrane and the surface with a nanometer resolution. By tuning the electrostatic interactions between the surface and the membrane, we have been able to measure a displacement of about 1 nm of the lipid membrane height.

Quantification of Single-Molecule FRET between Quantum Dots and Organic Dyes

Single-molecule Förster resonance energy transfer (FRET) is a useful technique for studying inter- and intra-molecular dynamics in biophysics. FRET efficiency is highly sensitive to distance, with half-maximal energy transfer occurring at an inter-dye distance on the order of 5 nm, which makes it possible to quantify distances changes on the molecular scale. FRET efficiency also depends on the spectral overlap and quantum yield of the donor and acceptor molecules, and therefore the choice of dye pair is critical to the success of any experiment. Most organic dyes that are used for single-molecule imaging have bleaching lifetimes in the tens of seconds at most, which limits their use to relatively fast processes. Quantum dot nanocrystals can emit for thousands of seconds without entering a long-lived dark state, which should make them good candidates for use as a FRET donor for longer single-molecule experiments, but there has been little quantitative study of the energy transfer efficiency at the single-molecule level. In this work, we use dsDNA to couple individual quantum dot donors to fluorescent dye acceptors. We then immobilize these FRET pairs on a functionalized glass coverslip and image them using total internal fluorescence (TIRF) microscopy with additional optics that allow us to image the donor and acceptor simultaneously. The length of the DNA between the pair can be varied from 11 to 32 base pairs, allowing us to observe and quantify the energy transfer efficiency over a range of distances on the nanometer scale. Our goal is to develop the use of quantum dots as donors for quantitative single-molecule FRET experiments.

Doping mechanism in novel semiconductor materials

Doping is a key technology in modern semiconductor industry. With reduced size of semiconductor devices, quantum effect becomes significant, and the classic theory for device design will no longer be valid. Thus the traditional doping technology is facing a great challenge. On the other hand, the third generation wide-gap semiconductors like GaN, ZnO and TiO2 have great application potential in optoelectronics and microelectronics due to their wide band gap, direct gap character and good stability. However, there is a doping bottleneck on the p-type doping of these materials. In the field of application of semiconductor technology, doping mechanism in photocatalysis, spintronic materials, and novel two-dimensional (2D) semiconductors are also the frontier of research. The research in this project focuses on the above key topics. The primary discoveries include the follows: (i) Based on first-principles band structure calculations and analysis of the band-edge wave function characters, we proposed that passivated (Mo+C)-doped TiO2 is a strong candidate for hydrogen production through water splitting. The results received highly attention from the experts in the field of photocatalysis, and the feasibility was confirmed by recent experimental studies. (ii) We discovered that the presence of spontaneous magnetization in nitrides and oxides with sufficient holes is an intrinsic property of these first-row d0 semiconductors. The origin and enhancement of such hole-induced ferromagnetism is studied, which suggested new possibility of preparing non-magnetically doped spintronic semiconductor devices. (iii) We systematically studied the doping mechanism and bottleneck of donor and acceptor impurities in quantum dots and quantum wires. Due to the effect of quantum confinement effect, the energy of the conduction band minimum (CBM) increases whereas that of the valence and maximum (VBM) decreases. We found that defect formation energy and transition energy level increase when the size of the quantum dot (QD) decreases, leading to the self-purification effect and degrade the performance of devices. Focused on this problem, we proposed the approach of overcoming the doping bottleneck in nanostructures, and developed a model to describe the quantum Stark effect in hole impurity state of quantum dots. These works promoted the research in the application of nano-devices. (iv) For the first time, the band alignment between two-dimensional transition-metal dichalcogenides MX2 (M=Mo, W; X=S, Se, Te) is calculated. We predicted the type-II band alignment in MoX2/WX2 heterostructures. In addition, direct-indirect gap transition and semiconductor-metal transition in strained MoS2 is observed. Moreover, we studied the mechanical, optical properties and band gap modulation of graphyne, a new allotrope of carbon, and discovered that Ca-doped graphyne had high capacity of hydrogen storage. The project has attracted great attentions, and the related publications are highly cited by researchers all over the world. These works bring better understanding of semiconductor doping theory, and have important scientific value for the device design and property prediction of new generation semiconductors and nano-devices.

Electronic states and optical properties of single donor in GaN conical quantum dot with spherical edge

In this paper we present a theoretical investigation of quantum confinement effects on the electron and single donor states in GaN conical quantum dot with spherical edge. In the framework of the effective mass approximation, the Schrödinger equations of electron and donor have been solved analytically in an infinite potential barrier model. Our calculations show that the energies of electron and donor impurity are affected by the two characteristic parameters of the structure which are the angle Ω and the radial dimension R. We show that, despite the fact that the reduction of the two parameters Ω and R leads to the same confinement effects, the energy remains very sensitive to the variation of the radial part than the variation of the angular part. The analysis of the photoionization cross-section corresponding to optical transitions between the conduction band and the first donor energy level shows clearly that the reduction of the radius R causes a shift in resonance peaks towards the high energies. On the other hand, the optical transitions between 1s−1p, 1p−1d and 1p−2s show that the increment of the conical aperture Ω (or reduction of R) implies a displacement of the excitation energy to higher energies.

Accelerating FRET between Near-Infrared Emitting Quantum Dots Using a Molecular J-Aggregate as an Exciton Bridge.

Fast energy transfer (EnT) among quantum dots (QDs) with near-infrared (NIR) emission is essential for fully exploiting their light harvesting and photon downconversion (multiexciton generation) abilities. This paper demonstrates a relayed EnT mechanism that accelerates the migration of NIR excitons between PbS QDs by a factor of 20 from that of one-step EnT through a polyelectrolyte and even a factor of ∼2 from that of one-step EnT between QDs in direct contact, by employing a J-aggregate (J-agg) of a cyanine dye as an exciton bridge. The donor QDs, acceptor QDs, and J-agg are electrostatically assembled into a sandwich structure with layer-by-layer deposition. Estimates of EnT rate and yield from transient and steady-state absorption and photoluminescence spectroscopies show that the rate-limiting step in the relay is EnT from the donor QD to the J-agg, while EnT from the J-agg to the acceptor QD occurs in <10 ps. A comparison of this system to the analogous solution-phase system suggests that the overall donor-to-acceptor EnT yield in the relay (18%) can be improved by depositing the J-agg with more intermolecular order. This work demonstrates the viability of relayed EnT through a molecular bridge as a strategy for accelerating long-distance exciton migration in assemblies of QDs, in particular in the near-infrared.

Magnetic field and dielectric environment effects on an exciton trapped by an ionized donor in a spherical quantum dot

Magnetic field and host dielectric environment effects on the binding energy of an exciton trapped by an ionized donor in spherical quantum dot are investigated. In the framework of the effective mass approximation and by using a variational method, the calculations have been performed by developing a robust ten-terms wave function taking into account the different inter-particles correlations and the distortion of symmetry induced by the orientation of the applied magnetic field. The binding and the localization energies are determined as functions of dot size and magnetic field strength. It appears that the variation of magnetic shift obeys a quadratic law for low magnetic fields regime while, for strong magnetic fields, this shift tends to be linear versus the magnetic field strength. The stability of this complex subjected to a magnetic field is also discussed according to the electron-hole ratio and the dielectric constant of the surrounding medium. A last point to highlight is that the Haynes' rule remains valid even in the presence of an applied magnetic field.

Resonant electronic excitation energy transfer by exchange mechanism in the quantum dot system

A microscopic theory of nonradiative resonance energy transfer between spherical A3B5 semiconductor quantum dots by the exchange mechanism is suggested. The interdot Coulomb interaction is taken into consideration. It is assumed that the quantum dot-donor and the quantum dot-acceptor are made from the same A3B5 compound and are embedded in the matrix of another material that produces potential barriers for electrons and holes. The dependences of the energy transfer rate on the quantum-dot system parameters are found in the frame of the Kane model that provides the most adequate description of the real spectra of A3B5 semiconductors. The analytical treatment is carried out with using the density matrix method, which enabled us to perform an energy transfer analysis both in the weak-interaction approximation and in the strong-interaction approximation. The numerical calculations showed the saturation of the energy transfer rate at the distances between the donor and the acceptor approaching the contact one. The contributions of the exchange and direct Coulomb intractions can be of the same order at the small distances and can have the same value in the saturation range.

Quantum Dot-Peptide-Fullerene Bioconjugates for Visualization of in Vitro and in Vivo Cellular Membrane Potential.

We report the development of a quantum dot (QD)-peptide-fullerene (C60) electron transfer (ET)-based nanobioconjugate for the visualization of membrane potential in living cells. The bioconjugate is composed of (1) a central QD electron donor, (2) a membrane-inserting peptidyl linker, and (3) a C60 electron acceptor. The photoexcited QD donor engages in ET with the C60 acceptor, resulting in quenching of QD photoluminescence (PL) that tracks positively with the number of C60 moieties arrayed around the QD. The nature of the QD-capping ligand also modulates the quenching efficiency; a neutral ligand coating facilitates greater QD quenching than a negatively charged carboxylated ligand. Steady-state photophysical characterization confirms an ET-driven process between the donor-acceptor pair. When introduced to cells, the amphiphilic QD-peptide-C60 bioconjugate labels the plasma membrane by insertion of the peptide-C60 portion into the hydrophobic bilayer, while the hydrophilic QD sits on the exofacial side of the membrane. Depolarization of cellular membrane potential augments the ET process, which is manifested as further quenching of QD PL. We demonstrate in HeLa cells, PC12 cells, and primary cortical neurons significant QD PL quenching (ΔF/F0 of 2-20% depending on the QD-C60 separation distance) in response to membrane depolarization with KCl. Further, we show the ability to use the QD-peptide-C60 probe in combination with conventional voltage-sensitive dyes (VSDs) for simultaneous two-channel imaging of membrane potential. In in vivo imaging of cortical electrical stimulation, the optical response of the optimal QD-peptide-C60 configuration exhibits temporal responsivity to electrical stimulation similar to that of VSDs. Notably, however, the QD-peptide-C60 construct displays 20- to 40-fold greater ΔF/F0 than VSDs. The tractable nature of the QD-peptide-C60 system offers the advantages of ease of assembly, large ΔF/F0, enhanced photostability, and high throughput without the need for complicated organic synthesis or genetic engineering, respectively, that is required of traditional VSDs and fluorescent protein constructs.

Determination of cDNA encoding BCR/ABL fusion gene in patients with chronic myelogenous leukemia using a novel FRET-based quantum dots-DNA nanosensor

In the present study, we developed a sensitive method based on fluorescence resonance energy transfer (FRET) for the determination of the BCR/ABL fusion gene, which is used as a biomarker to confirm the clinical diagnosis of both chronic myelogenous leukemia (CML) and acute lymphocytic leukemia (ALL). For this purpose, CdTe quantum dots (QDs) were conjugated to amino-modified 18-mer oligonucleotide ((N)DNA) to form the QDs-(N)DNA nanosensor. In the presence of methylene blue (MB) as an intercalator, the hybridization of QDs-(N)DNA with the target BCR/ABL fusion gene (complementary DNA), brings the MB (acceptor) at close proximity of the QDs (donor), leading to FRET upon photoexcitation of the QDs. The enhancement in the emission intensity of MB was used to follow up the hybridization, which was linearly proportional to concentration of the target complementary DNA in a range from 1.0 × 10−9 to 1.25 × 10−7 M. The detection limit of the proposed method was obtained to be 1.5 × 10−10 M. Finally, the feasibility and selectivity of the proposed nanosensor was evaluated by the analysis of derived nucleotides from both mismatched sequences and clinical samples of patients with leukemia as real samples.

Ag colloids and arrays for plasmonic non-radiative energy transfer from quantum dots to a quantum well

Non-radiative energy transfer (NRET) can be an efficient process of benefit to many applications including photovoltaics, sensors, light emitting diodes and photodetectors. Combining the remarkable optical properties of quantum dots (QDs) with the electrical properties of quantum wells (QWs) allows for the formation of hybrid devices which can utilize NRET as a means of transferring absorbed optical energy from the QDs to the QW. Here we report on plasmon-enhanced NRET from semiconductor nanocrystal QDs to a QW. Ag nanoparticles in the form of colloids and ordered arrays are used to demonstrate plasmon-mediated NRET from QDs to QWs with varying top barrier thicknesses. Plasmon-mediated energy transfer (ET) efficiencies of up to ∼25% are observed with the Ag colloids. The distance dependence of the plasmon-mediated ET is found to follow the same d−4 dependence as the direct QD to QW ET. There is also evidence for an increase in the characteristic distance of the interaction, thus indicating that it follows a Förster-like model with the Ag nanoparticle-QD acting as an enhanced donor dipole. Ordered Ag nanoparticle arrays display plasmon-mediated ET efficiencies up to ∼21%. To explore the tunability of the array system, two arrays with different geometries are presented. It is demonstrated that changing the geometry of the array allows a transition from overall quenching of the acceptor QW emission to enhancement, as well as control of the competition between the QD donor quenching and ET rates.

Efficient Förster Resonance Energy Transfer Donors of In(Zn)P/ZnS Quantum Dots

We demonstrate a detailed investigation of the effect of the type and concentration of zinc precursor on the optical properties of In(Zn)P/ZnS quantum dots. We achieved up to 87% quantum yield along with 54 nm emission bandwidth for the green emitters with changing the concentration of the Zn precursors. Employing efficient green emitters as the donor species, we demonstrated an efficient Förster resonance energy transfer (FRET) couple of green and red emitting InP-based quantum dots. With a FRET efficiency level of 70.3% achieved (analyzed from the donor lifetime with and without an acceptor), we further demonstrated the enhancement of the acceptor emission nearly twofold due to the energy transfer. Our results provide new insights toward the understanding of the excitonic interactions among donor and acceptor quantum dots of the III–V family for light harvesting applications.