Single Quantum Dot Research Articles

Measuring Non-Destructively the Total Indium Content and Its Lateral Distribution in Very Thin Single Layers or Quantum Dots Deposited onto Gallium Arsenide Substrates Using Energy-Dispersive X-ray Spectroscopy in a Scanning Electron Microscope.

The epitaxial deposition of a precise number, or even fractions, of monolayers of indium (In)-rich semiconductors onto gallium arsenide (GaAs) substrates enables the creation of quantum dots based on InAs, InGaAs and indium phosphide (InP) for infrared light-emitting and laser diodes and the formation of indium antimonide (InSb)/GaAs strained layer superlattices. Here, a facile method based on energy-dispersive X-ray spectroscopy (EDXS) in a scanning electron microscope (SEM) is presented that allows the indium content of a single semiconductor layer deposited on a gallium arsenide substrate to be measured with relatively high accuracy (±0.7 monolayers). As the procedure works in top-down geometry, where any part of a wafer can be inspected, measuring the In content of the surface layer in one location without destroying it can also be used to map the lateral indium distribution during quantum dot formation and is a method suitable as an in-situ quality control tool for epitaxy.

Hydroxymethylation-Specific Ligation-Mediated Single Quantum Dot-Based Nanosensors for Sensitive Detection of 5-Hydroxymethylcytosine in Cancer Cells.

5-Hydroxymethylcytosine (5hmC) modification is a key epigenetic regulator of cellular processes in mammalian cells, and its misregulation may lead to various diseases. Herein, we develop a hydroxymethylation-specific ligation-mediated single quantum dot (QD)-based fluorescence resonance energy transfer (FRET) nanosensor for sensitive quantification of 5hmC modification in cancer cells. We design a Cy5-modified signal probe and a biotinylated capture probe for the recognition of specific 5hmC-containing genes. 5hmC in target DNA can be selectively converted by T4 β-glucosyltransferase to produce a glycosyl-modified 5hmC, which cannot be cleaved by methylation-insensitive restriction enzyme MspI. The glycosylated 5hmC DNA may act as a template to ligate a signal probe and a capture probe, initiating hydroxymethylation-specific ligation to generate large amounts of biotin-/Cy5-modified single-stranded DNAs (ssDNAs). The assembly of biotin-/Cy5-modified ssDNAs onto a single QD through streptavidin-biotin interaction results in FRET and consequently the generation of a Cy5 signal. The nanosensor is very simple without the need for bisulfite treatment, radioactive reagents, and 5hmC-specific antibodies. Owing to excellent specificity and high amplification efficiency of hydroxymethylation-specific ligation and near-zero background of a single QD-based FRET, this nanosensor can quantify 5hmC DNA with a limit of detection of 33.61 aM and a wider linear range of 7 orders of magnitude, and it may discriminate the single-nucleotide difference among 5hmC, 5-methylcytosine, and unmodified cytosine. Moreover, this nanosensor can distinguish as low as a 0.001% 5hmC DNA in complex mixtures, and it can monitor the cellular 5hmC level and discriminate cancer cells from normal cells, holding great potential in biomedical research and clinical diagnostics.

Topologically‐Protected Single‐Photon Sources with Topological Slow Light Photonic Crystal Waveguides

AbstractSlow light waveguides are advantageous for implementing high‐performance single‐photon sources required for scalable operation of integrated quantum photonic circuits (IQPCs), though such waveguides are known to suffer from propagation loss due to backscattering. A way to overcome the drawback is to use topological photonics, in which robust waveguiding in topologically‐protected optical modes has recently been demonstrated. Here, single‐photon sources are reported using single quantum dots (QDs) embedded in topological slow light waveguides based on valley photonic crystals. Purcell‐enhanced single‐photon emission is demonstrated from a QD into a topological slow light mode with a group index over 20 and its robust propagation even under the presence of sharp bends. These results pave the way for the realization of robust and high‐performance single‐photon sources indispensable for IQPCs.

Enhanced Luminance of CdSe/ZnS Quantum Dots Light-Emitting Diodes Using ZnO-Oleic Acid/ZnO Quantum Dots Double Electron Transport Layer.

The widely used ZnO quantum dots (QDs) as an electron transport layer (ETL) in quantum dot light-emitting diodes (QLEDs) have one drawback. That the balancing of electrons and holes has not been effectively exploited due to the low hole blocking potential difference between the valence band (VB) (6.38 eV) of ZnO ETL and (6.3 eV) of CdSe/ZnS QDs. In this study, ZnO QDs chemically reacted with capping ligands of oleic acid (OA) to decrease the work function of 3.15 eV for ZnO QDs to 2.72~3.08 eV for the ZnO-OA QDs due to the charge transfer from ZnO to OA ligands and improve the efficiency for hole blocking as the VB was increased up to 7.22~7.23 eV. Compared to the QLEDs with a single ZnO QDs ETL, the ZnO-OA/ZnO QDs double ETLs optimize the energy level alignment between ZnO QDs and CdSe/ZnS QDs but also make the surface roughness of ZnO QDs smoother. The optimized glass/ITO/PEDOT:PSS/PVK//CdSe/ZnS//ZnO-OA/ZnO/Ag QLEDs enhances the maximum luminance by 5~9% and current efficiency by 16~35% over the QLEDs with a single ZnO QDs ETL, which can be explained in terms of trap-charge limited current (TCLC) and the Fowler-Nordheim (F-N) tunneling conduction mechanism.

Immunoprofiling of Severity and Stage of Bacterial Infectious Diseases by Ultrabright Fluorescent Nanosphere-Based Dyad Test Strips

Bacterial infectious diseases are common clinical diseases that seriously threaten human health, especially in countries and regions with poor environmental hygiene. Due to the lack of characteristic clinical symptoms and signs, it is a challenge to distinguish a bacterial infection from other infections, leading to misdiagnosis and antibiotic overuse. Therefore, there is an urgent need to develop a specific method for detection of bacterial infections. Herein, utilizing ultrabright fluorescent nanospheres (FNs) as reporters, immunochromatographic dyad test strips are developed for the early detection of bacterial infections and distinction of different stages of bacterial infectious diseases in clinical samples. C-reactive protein (CRP) and heparin-binding protein (HBP) are quantified and assayed because their levels in plasma are varied dynamically and asynchronously during the progression of the disease. The detection limits of CRP and HBP can reach as low as 0.51 and 0.65 ng/mL, respectively, due to the superior fluorescence intensity of each FN, which is 570 times stronger than that of a single quantum dot. The assay procedure can be achieved in 22 min, fully meeting the needs of rapid and ultrasensitive detection in the field. This constructed strip has been successfully used to profile the stage and severity of bacterial infections by monitoring the levels of CRP and HBP in human plasma samples, showing great potential as a point-of-care biosensor for clinical diagnosis. In addition to bacterial infections, the developed ultrabright FN-based point-of-care testing can be readily expanded for rapid, quantitative, and ultrasensitive detection of other trace substances in complex systems.

Double quantum dots decorated layer structure CeCO3OH for improved N2 photo-fixation

The moderate N2 photofixation catalyzed by CeO2-based materials has been attracted much attention. Herein, double quantum dots of carbon and Co(OH)2 doped layer structure CeCO3OH are demonstrated firstly, in which double quantum dots can provide much more reaction active sites and improve the energy band structures. The double quantum dots decorated catalyst representsthe better performances for the N2 fixation than the single quantum dots. NH3 fixation rate of the co-doped catalyst can reach above 1.1 mmol·gcal−1·h−1 and 0.65 mmol·gcal−1·h−1 with full spectrum and visible light irradiation, respectively. The apparent quantum efficiency at 380 nm has achieved to about 1%. This research affords a fabrication strategy to develop noble metal-free photocatalysts by introducing the composite quantumdotson the samples.

Multiple-Route Exciton Recombination Dynamics and Improved Stability of Perovskite Quantum Dots by Plasmonic Photonic Crystal.

We have studied the excited-state exciton recombination dynamics of perovskite quantum dots (QDs) through time-resolved photoluminescence (PL), PL blinking, PL intensity-dependent lifetime modulation, and long-term photostability tests. The various spectroscopic characterizations elucidate that the perovskite QDs have multiple intrinsic exciton recombination routes even in a single QD, i.e., exciton, biexciton, and positive/negative trions, which are dissimilarly contributed to ON and OFF state emissions. We also find that the enhanced radiative recombination from placing green QDs on a photonic Ag nanotip array induces notably improved long-term PL stability. We consider that the accelerated radiative recombination of QDs by strong coupling with the plasmonics of the photonic Ag nanotip array, while eliminating nonradiative pathways, is proven to be a critical factor for improved long-term stability.

Cooling of nanomechanical vibrations by Andreev injection

A nanoelectromechanical weak link composed of a carbon nanotube suspended between two normal electrodes in a gap between two superconducting leads is considered. The nanotube is treated as a movable single level quantum dot in which the position-dependent superconducting order parameter is induced due to the Cooper pair tunneling. We show that electron tunneling processes significantly affect the state of the mechanical subsystem. We found that at a given direction of the applied voltage between the electrodes, the stationary state of the mechanical subsystem has a Boltzmann form with an effective temperature dependent on the parameters of the device. As this takes place, the effective temperature can reach significantly small values (cooling effect). We also demonstrate that nanotube fluctuations strongly affect the dc current through the system. The latter can be used to probe the predicted effects in an experiment.

Room-Temperature Strong Coupling Between a Single Quantum Dot and a Single Plasmonic Nanoparticle.

A single quantum dot (QD) strongly coupled with a plasmonic nanoparticle yields a promising qubit for scalable solid-state quantum information processing at room temperature. However, realizing such a strong coupling remains challenging due to the difficulty of spatial overlap of the QD excitons with the plasmonic electric fields (EFs). Here, by using a transmission electron microscope we demonstrate for the first time that this overlap can be realized by integrating a deterministic single QD with a single Au nanorod. When a wedge nanogap cavity consisting of them and the substrate is constructed, the plasmonic EFs can be more effectively "dragged" and highly confined in the QD's nanoshell where the excitons mainly reside. With these advantages, we observed the largest spectral Rabi splitting (reported so far) of ∼234 meV for a single QD strong coupling with plasmons. Our work opens a pathway to the massive construction of room-temperature strong coupling solid qubits.

The role of surface charges in the blinking mechanisms and quantum-confined Stark effect of single colloidal quantum dots

The role of surface charges in the blinking mechanisms and quantum-confined Stark effect of single colloidal quantum dots

Self-assembled InAs/GaAs single quantum dots with suppressed InGaAs wetting layer states and low excitonic fine structure splitting for quantum memory

Abstract Epitaxial semiconductor quantum dots (QDs) have been demonstrated as on-demand entangled photon sources through biexciton–exciton (XX-X) cascaded radiative processes. However, perfect entangled photon emitters at the specific wavelengths of 880 nm or 980 nm, that are important for heralded entanglement distribution by absorptive quantum memories, remain a significant challenge. We successfully extend the QD emission wavelength to 880 nm via capping Stranski–Krastanow grown In(Ga)As/GaAs QDs with an ultra-thin Al x Ga1−x As layer. After carefully investigating the mechanisms governing the vanishing of wetting-layer (WL) states and the anisotropy of QDs, we optimize the growth conditions and achieve a strong suppression of the WL emission as well as a measured minor fine structure splitting of only ∼(3.2 ± 0.25) μeV for the exciton line. We further extend this method to fabricate In(Ga)As QDs emitted at 980 nm via introducing InGaAs capping layer, and demonstrate a two-photon resonant excitation of the biexciton without any additional optical or electrical stabilized source. These QDs with high symmetry and stability represent a highly promising platform for the generation of polarization entanglement and experiments on the interaction of photons from dissimilar sources, such as rare-earth-ion-doped crystals for solid quantum memory.

Temperature dependent activation energies of exciton states on a single GaAs laterally coupled-quantum-dot molecule and quantum dot

The objective of this study was to perform spectroscopy measurements of a single GaAs laterally coupled quantum dot molecule and quantum dot. Compared to photoluminescence spectra between a single GaAs laterally coupled quantum dot molecule and a single quantum dot, the presence of optical coupling was confirmed through dipole-dipole interactions in a single coupled quantum dot molecule. Based on temperature dependence measurements, weak exciton-phonon interactions (25 meV) were observed from the ground exciton state of a single GaAs laterally coupled quantum dot molecule in comparison with a strong exciton-phonon interaction (49 meV) from the ground exciton state of a single GaAs QD. Such weak interactions were due to reduced confinement at exciton states in a single laterally coupled quantum dot molecule. In addition, weaker activation energy (∼15 meV) was observed in terms of confined electrons in a single laterally coupled quantum dot molecule, where a confinement effect became reduced due to enlarged dimensional area in one direction along [1 1‾ 0].

Band Alignment Engineering of Semiconductor Nanocrystal Heterostructures Toward Emerging Applications

Due to the superb tunable properties of quantum dots (QDs), they have been largely studied in the past few decades for many promising applications. However, for a single nanocrystal QD, its properties are largely limited due to limited tunability of the composition, charge carrier dynamics, band alignment, and so on. Thus, to solve the problem, and enhance their physical, chemical, electrical, and optical properties, the heterostructure nanocrystals (NCs), which combine single NCs with other materials of either semiconductor or metal, are proposed. In these heterostructures, their properties can be modified through tailoring the band alignment, and therefore providing a great potential for widespread applications. Here, an overview of strategies for band alignment engineering, synthesis methods, and the corresponding applications, such as lighting devices, lasers, photocatalysis, biological applications are offered.

Higher-Order Photon Statistics as a New Tool to Reveal Hidden Excited States in a Plasmonic Cavity

Among the best known quantities obtainable from photon correlation measurements are the g(m) correlation functions. Here, we introduce a new procedure to evaluate these correlation functions based on higher-order factorial cumulants CF,m that integrate over the time dependence of the correlation functions, that is, summarize the available information at different time spans. In a systematic manner, the information content of higher-order correlation functions as well as the distribution of photon waiting times is taken into account. Our procedure greatly enhances the sensitivity for probing correlations and, moreover, is robust against a limited counting efficiency and time resolution in experiment. It can be applied even in case g(m) is not accessible at short time spans. We use the new evaluation scheme to analyze the photon emission of a plasmonic cavity coupled to a single quantum dot. We derive criteria that must hold if the system can be described by a generic Jaynes–Cummings model. A violation of the criteria can be explained by the presence of an additional excited quantum dot state.

Ultra-narrow room-temperature emission from single CsPbBr3 perovskite quantum dots

Semiconductor quantum dots have long been considered artificial atoms, but despite the overarching analogies in the strong energy-level quantization and the single-photon emission capability, their emission spectrum is far broader than typical atomic emission lines. Here, by using ab-initio molecular dynamics for simulating exciton-surface-phonon interactions in structurally dynamic CsPbBr3 quantum dots, followed by single quantum dot optical spectroscopy, we demonstrate that emission line-broadening in these quantum dots is primarily governed by the coupling of excitons to low-energy surface phonons. Mild adjustments of the surface chemical composition allow for attaining much smaller emission linewidths of 35−65 meV (vs. initial values of 70–120 meV), which are on par with the best values known for structurally rigid, colloidal II-VI quantum dots (20−60 meV). Ultra-narrow emission at room-temperature is desired for conventional light-emitting devices and paramount for emerging quantum light sources.

Photovoltaic efficiency at maximum power of a quantum dot molecule

In this work, the behavior of the efficiency at the maximum power of a quantum dot molecule, acting as a device for photovoltaic conversion, is investigated. A theoretical approach using a master equation, considering the effect of the energy offsets, and the width of the quantum barrier, identify realistic physical conditions that enhance the photovoltaic response of the photocell. By mapping the effect of the control of the energy offsets of the nanostructure, a condition for gain in 30% of maximum power delivered per molecule if compared with a single quantum dot is demonstrated. Studying the behavior as a function of temperature, the physical system exhibits gain when compared to the Chambadal-Novikov-Curzon-Ahlborn efficiency at maximum power, without exceeding Carnot's efficiency, as expected from the second law of thermodynamics.

Dynamic transport characteristics of side-coupled double-quantum-impurity systems

A systematic study is performed on time-dependent dynamic transport characteristics of a side-coupled double-quantum-impurity system based on the hierarchical equations of motion. It is found that the transport current behaves like a single quantum dot when the coupling strength is low during tunneling or Coulomb coupling. For the case of only tunneling transition, the dynamic current oscillates due to the temporal coherence of the electron tunneling device. The oscillation frequency of the transport current is related to the step voltage applied by the lead, while temperature T, electron–electron interaction U and the bandwidth W have little influence. The amplitude of the current oscillation exists in positive correlation with W and negative correlation with U. With the increase in coupling t 12 between impurities, the ground state of the system changes from a Kondo singlet of one impurity to a spin singlet of two impurities. Moreover, lowering the temperature could promote the Kondo effect to intensify the oscillation of the dynamic current. When only the Coulomb transition is coupled, it is found that the two split-off Hubbard peaks move upward and have different interference effects on the Kondo peak at the Fermi surface with the increase in U 12, from the dynamics point of view.

Strong Coupling between a Single Quantum Emitter and a Plasmonic Nanoantenna on a Metallic Film.

The strong coupling between single quantum emitters and resonant optical micro/nanocavities is beneficial for understanding light and matter interactions. Here, we propose a plasmonic nanoantenna placed on a metal film to achieve an ultra-high electric field enhancement in the nanogap and an ultra-small optical mode volume. The strong coupling between a single quantum dot (QD) and the designed structure is investigated in detail by both numerical simulations and theoretical calculations. When a single QD is inserted into the nanogap of the silver nanoantenna, the scattering spectra show a remarkably large splitting and anticrossing behavior of the vacuum Rabi splitting, which can be achieved in the scattering spectra by optimizing the nanoantenna thickness. Our work shows another way to enhance the light/matter interaction at a single quantum emitter limit, which can be useful for many nanophotonic and quantum applications.

Thermal release tape-assisted semiconductor membrane transfer process for hybrid photonic devices embedding quantum emitters

The ability to combine different materials enables a combination of complementary properties and device engineering that cannot be found or exploited within a single material system. In the realm of quantum nanophotonics, one might want to increase device functionality by, for instance, combining efficient classical and quantum light emission available in III–V semiconductors, low-loss light propagation accessible in silicon-based materials, fast electro-optical properties of lithium niobate, and broadband reflectors and/or buried metallic contacts for local electric field application or electrical injection of emitters. However, combining different materials on a single wafer is challenging and may result in low reproducibility and/or low yield. For instance, direct epitaxial growth requires crystal lattice matching for producing of defect-free films, and wafer bonding requires considerable and costly process development for high bond strength and yield. We propose a transfer printing technique based on the removal of arrays of free-standing membranes and their deposition onto a host material using a thermal release adhesive tape-assisted process. This approach is versatile, in that it poses limited restrictions on the transferred and host materials. In particular, we transfer 190 nm-thick GaAs membranes that contain InAs quantum dots and which have dimensions up to about 260 μm × 80 μm onto a gold-coated silicon substrate. We show that the presence of a back reflector combined with the etching of micropillars significantly increases the extraction efficiency of quantum light from a single quantum dot line, reaching photon fluxes exceeding 8 × 105 photons per second. This flux is four times higher than the highest count rates measured from emitters outside the pillars on the same chip. Given its versatility and ease of processing, this technique provides a path to realising hybrid quantum nanophotonic devices that combine virtually any material in which free-standing membranes can be made onto any host substrate, without specific compatibility issues and/or requirements.

Local spectroscopic imaging of a single quantum dot in photoinduced force microscopy

Analysis of environmentally sensitive materials is essential for developing and optimizing nanostructured photochemical materials and devices. Photoinduced force microscopy (PiFM) is a promising local spectroscopic technique to visualize nanoscale local optical responses by measuring the optical forces between the scanning tip and sample. In this study, we examined isolated single quantum dots (QDs) with ligands on a gold substrate via PiFM under ultra-high vacuum to characterize the QD adsorption state on the basis of the optical force. The strong self-consistent optical interaction through the tip-substrate plasmonic gap induced by laser light modulates the PiFM image depending on QD crystal existence in the gap. This observation clarified the QD absorption situation on the substrate, and the crystal position in the QDs was determined even though the ligand walls covered the crystal. This insight concerning force spectroscopy can aid further research on the photochemistry of nanostructured materials and molecular spectroscopy.