Single Colloidal Quantum Dots Research Articles

High-dimensional quantum key distribution using orbital angular momentum of single photons from a colloidal quantum dot at room temperature

High-dimensional quantum key distribution (HDQKD) is a promising avenue to address the inherent limitations of basic quantum key distribution (QKD) protocols. However, experimental realizations of HDQKD to date have relied on indeterministic photon sources that limit the achievable key rate. In this paper, we demonstrate a full emulation of a HDQKD system using a single colloidal giant quantum dot (gQD) as a deterministic, compact, and room-temperature single-photon source (SPS). We demonstrate a practical protocol by encoding information in a high-dimensional space (d = 3) of the orbital angular momentum of the photons. Our experimental configuration incorporates two spatial light modulators for encoding and decoding the spatial information carried by individual photons. Our experimental demonstration establishes the feasibility of utilizing high radiative quantum yield gQDs as practical SPSs for HDQKD. We also experimentally demonstrate surpassing the traditional d = 2 QKD capacity with comparable error rates, indicating a significant improvement in performance while maintaining reliability.

Room-Temperature Single-Photon Sources Based on Colloidal Quantum Dots: A Review.

Single-photon sources (SPSs) play a crucial role in quantum photonics, and colloidal quantum dots (CQDs) have emerged as promising and cost-effective candidates for such applications due to their high-purity single-photon emission at room temperature. This review focuses on various aspects of CQDs as SPSs. Firstly, a brief overview of the fundamental optical properties of CQDs is provided, including emission wavelength engineering and fluorescence intermittency, and their single-photon emission properties. Subsequently, this review delves into research concerning CQDs as SPSs, covering topics such as the coupling of single CQDs to microcavities, both in weak and strong coupling regimes. Additionally, methods for localizing and positioning CQDs are explored, which are critical for on-chip SPSs devices.

Single PbS colloidal quantum dot transistors

Colloidal quantum dots are sub-10 nm semiconductors treated with liquid processes, rendering them attractive candidates for single-electron transistors operating at high temperatures. However, there have been few reports on single-electron transistors using colloidal quantum dots due to the difficulty in fabrication. In this work, we fabricated single-electron transistors using single oleic acid-capped PbS quantum dot coupled to nanogap metal electrodes and measured single-electron tunneling. We observed dot size-dependent carrier transport, orbital-dependent electron charging energy and conductance, electric field modulation of the electron confinement potential, and the Kondo effect, which provide nanoscopic insights into carrier transport through single colloidal quantum dots. Moreover, the large charging energy in small quantum dots enables single-electron transistor operation even at room temperature. These findings, as well as the commercial availability and high stability, make PbS quantum dots promising for the development of quantum information and optoelectronic devices, particularly room-temperature single-electron transistors with excellent optical properties.

Time-Resolved Line Shapes of Single Quantum Emitters via Machine Learned Photon Correlations.

Solid-state single-photon emitters (SPEs) are quantum light sources that combine atomlike optical properties with solid-state integration and fabrication capabilities. SPEs are hindered by spectral diffusion, where the emitter's surrounding environment induces random energy fluctuations. Timescales of spectral diffusion span nanoseconds to minutes and require probing single emitters to remove ensemble averaging. Photon correlation Fourier spectroscopy (PCFS) can be used to measure time-resolved single emitter line shapes, but is hindered by poor signal-to-noise ratio in the measured correlation functions at early times due to low photon counts. Here, we develop a framework to simulate PCFS correlation functions directly from diffusing spectra that match well with experimental data for single colloidal quantum dots. We use these simulated datasets to train a deep ensemble autoencoder machine learning model that outputs accurate, noiseless, and probabilistic reconstructions of the noisy correlations. Using this model, we obtain reconstructed time-resolved single dot emission line shapes at timescales as low as 10ns, which are otherwise completely obscured by noise. This enables PCFS to extract optical coherence times on the same timescales as Hong-Ou-Mandel two-photon interference, but with the advantage of providing spectral information in addition to estimates of photon indistinguishability. Our machine learning approach is broadly applicable to different photon correlation spectroscopy techniques and SPE systems, offering an enhanced tool for probing single emitter line shapes on previously inaccessible timescales.

Biexciton Blinking in CdSe-Based Quantum Dots.

Experiments on single colloidal quantum dots (QDs) have revealed temporal fluctuations in the emission efficiency of the single-exciton state. These fluctuations, often termed "blinking", are caused by opening/closing of charge-carrier traps and/or charging/discharging of the QD. In the regime of strong optical excitation, multiexciton states are formed. The emission efficiencies of multiexcitons are lower because of Auger processes, but a quantitative characterization is challenging. Here, we quantify fluctuations of the biexciton efficiency for single CdSe/CdS/ZnS core-shell QDs. We find that the biexciton efficiency "blinks" significantly. The additional electron due to charging of a QD accelerates Auger recombination by a factor of 2 compared to the neutral biexciton, while opening/closing of a charge-carrier trap leads to an increase of the nonradiative recombination rate by a factor of 4. To understand the fast rate of trap-assisted recombination, we propose a revised model for trap-assisted recombination based on reversible trapping. Finally, we discuss the implications of biexciton blinking for lasing applications.

Plasmon-enhanced single photon source directly coupled to an optical fiber

A bright source of fiber-coupled, polarized single photons is an essential component of any realistic quantum network based on today's existing fiber infrastructure. Here, we develop a Purcell-enhanced, polarized source of single photons at room temperature by coupling single colloidal quantum dots to the localized surface plasmon-polariton modes of single gold nanorods, combined on the surface of an optical nanofiber. A maximum enhancement of the photon emission rate of 62 times was measured corresponding to a degree of polarization of $85%$, and a brightness enhancement of four times in the fiber mode. Evanescent coupling of photons to the nanofiber guided modes ensures automatic coupling to a single mode fiber. Our technique opens the way to realizing bright sources of polarized single photons connected to fiber networks using a simple composite technique.

Improved surface passivation of lead sulfide quantum dots using less sterically crowded alkylammonium iodides

Efficient surface passivation of colloidal quantum dots (CQDs) is crucial for improving the photovoltaic performance of CQD-based solar cell (CQDSC) devices. Although several alkylammonium iodides (AMIs), representatively tetra-n-butylammonium iodide (TBAI), have been reported for iodide passivation of lead sulfide (PbS) CQDs, the factors influencing the passivation are still barely understood. In this research, the impacts of steric crowding, acidity and ionic dissociation of seven AMIs with different lengths and numbers of alkyl substituents on the iodide passivation of the PbS CQDs are studied. The oleate ligands on the pristine CQD surface are more effectively exchanged with iodides by less sterically crowded AMIs. The higher acidity of tertiary AMIs compared with quaternary AMIs also enhances the elimination of oleate ligands and the passivation of iodides. The extent of the ligand exchange consequently influences the photovoltaic properties of CQDSCs, including trap density, built-in potential and charge mobility. Thus, the triethylamine hydroiodide (tri-EAHI)-treated CQDSC demonstrates the highest power conversion efficiency of 5.37% for a single CQD layer-based cell and 10.1% for a PbS-tri-EAHI/PbS-PDT-based bilayer cell.

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

Complete Mapping of Interacting Charging States in Single Coupled Colloidal Quantum Dot Molecules

Colloidal quantum dots (CQDs), major building blocks in modern optoelectronic devices, have so far been synthesized with only one emission center where the exciton resides. Recent development of coupled colloidal quantum dots molecules (CQDM), where two core–shell CQDs are fused to form two emission centers in close proximity, allows exploration of how charge carriers in one CQD affect the charge carriers in the other CQD. Cryogenic single particle spectroscopy reveals that while CQD monomers manifest a simple emission spectrum comprising a main emission peak with well-defined phonon sidebands, CQDMs exhibit a complex spectrum with multiple peaks that are not all spaced according to the known phonon frequencies. Based on complementary emission polarization and time-resolved analysis, this is assigned to fluorescence of the two coupled emission centers. Moreover, the complex peak structure shows correlated spectral diffusion indicative of the coupling between the two emission centers. Utilizing Schrödinger-Poisson self-consistent calculations, we directly map the spectral behavior, alternating between neutral and charged states of the CQDM. Spectral shifts related to electrostatic interaction between a charged emission center and the second emission center are thus fully mapped. Furthermore, effects of moving surface charges are identified, whereby the emission center proximal to the charge shows larger shifts. Instances where the two emission centers are negatively charged simultaneously are also identified. Such detailed mapping of charging states is enabled by the coupling within the CQDM and its anisotropic structure. This understanding of the coupling interactions is progress toward quantum technology and sensing applications based on CQDMs.

Electron transport in single colloidal quantum dots in an interelectrode nanogap

Based on the current-voltage characteristics of single colloidal QDs -InSb, -PbS, -HgSe, -CdSe quantum dots, on random samples statistically determined and investigated the mechanisms of barrier and ballistic tunneling, Coulomb confinement, quasi-periodic modulation of electron transport in the model of a deep extended potential well and depending on QDs size and de Broglie wavelength for the electron. The possibility of manifestation of Bloch oscillations has been experimentally confirmed. The parameters of all investigated processes were determined and tabulated. Keywords: Nanoparticle, quantum dot, dimensional quantization, quantum selection, barrier and ballistic tunneling, Coulomb confinement, Coulomb ladder, Bloch oscillations.

All-optical fluorescence blinking control in quantum dots with ultrafast mid-infrared pulses.

Photoluminescence intermittency is a ubiquitous phenomenon, reducing the temporal emission intensity stability of single colloidal quantum dots (QDs) and the emission quantum yield of their ensembles. Despite efforts to achieve blinking reduction by chemical engineering of the QD architecture and its environment, blinking still poses barriers to the application of QDs, particularly in single-particle tracking in biology or in single-photon sources. Here, we demonstrate a deterministic all-optical suppression of QD blinking using a compound technique of visible and mid-infrared excitation. We show that moderate-field ultrafast mid-infrared pulses (5.5 μm, 150 fs) can switch the emission from a charged, low quantum yield grey trion state to the bright exciton state in CdSe/CdS core-shell QDs, resulting in a significant reduction of the QD intensity flicker. Quantum-tunnelling simulations suggest that the mid-infrared fields remove the excess charge from trions with reduced emission quantum yield to restore higher brightness exciton emission. Our approach can be integrated with existing single-particle tracking or super-resolution microscopy techniques without any modification to the sample and translates to other emitters presenting charging-induced photoluminescence intermittencies, such as single-photon emissive defects in diamond and two-dimensional materials.

Heralded Spectroscopy Reveals Exciton-Exciton Correlations in Single Colloidal Quantum Dots.

Multiply excited states in semiconductor quantum dots feature intriguing physics and play a crucial role in nanocrystal-based technologies. While photoluminescence provides a natural probe to investigate these states, room-temperature single-particle spectroscopy of their emission has proved elusive due to the temporal and spectral overlap with emission from the singly excited and charged states. Here, we introduce biexciton heralded spectroscopy enabled by a single-photon avalanche diode array based spectrometer. This allows us to directly observe biexciton–exciton emission cascades and measure the biexciton binding energy of single quantum dots at room temperature, even though it is well below the scale of thermal broadening and spectral diffusion. Furthermore, we uncover correlations hitherto masked in ensembles of the biexciton binding energy with both charge-carrier confinement and fluctuations of the local electrostatic potential. Heralded spectroscopy has the potential of greatly extending our understanding of charge-carrier dynamics in multielectron systems and of parallelization of quantum optics protocols.

Single Photon Source from a Nanoantenna-Trapped Single Quantum Dot.

Single photon sources with high brightness and subnanosecond lifetimes are key components for quantum technologies. Optical nanoantennas can enhance the emission properties of single quantum emitters, but this approach requires accurate nanoscale positioning of the source at the plasmonic hotspot. Here, we use plasmonic nanoantennas to simultaneously trap single colloidal quantum dots and enhance their photoluminescence. The nano-optical trapping automatically locates the quantum emitter at the nanoantenna hotspot without further processing. Our dedicated nanoantenna design achieves a high trap stiffness of 0.6 (fN/nm)/mW for quantum dot trapping, together with a relatively low trapping power of 2 mW/μm2. The emission from the nanoantenna-trapped single quantum dot shows 7× increased brightness, 50× reduced blinking, 2× shortened lifetime, and a clear antibunching below 0.5 demonstrating true single photon emission. Combining nano-optical tweezers with plasmonic enhancement is a promising route for quantum technologies and spectroscopy of single nano-objects.

Near-field modulation of single photon emitter with a plasmonic probe

Single solid-state quantum dots have significant potential as bright single-photon sources for scalable photonic quantum information technologies. Engineering their radiative relaxation properties is of significant importance for their practical applications. In this study, we demonstrate a cavity-free, broadband approach for modulating and collecting the fluorescence of a single-photon emitter using a fiber taper–silver nanowire plasmonic probe. When the plasmonic probe is located above a single colloidal quantum dot at approximately 20 nm, the photon-emitter interaction increased rapidly and a significant decrease, by an average factor of 3.38, in the lifetime of the quantum dot was observed. The fluorescence signal of the quantum dots was collected by the hybrid probe, with significantly higher efficiency than that of the traditional metal-coated near-field probe. The results of the numerical simulation were in good agreement with the experimental results. The proposed near-field modulation method can be applied to other single-photon sources and proved to be a flexible method for manipulating the luminescence of systems based on single-photon emitters.

Photoluminescence Blinking and Biexciton Auger Recombination in Single Colloidal Quantum Dots with Sharp and Smooth Core/Shell Interfaces.

There is an inconsistence on whether a smooth core/shell interface can reduce Auger recombination and suppress photoluminescence (PL) blinking in single colloidal quantum dots (QDs). Here, we investigate the influence of a core/shell interface on PL blinking and biexciton Auger recombination by comparing the single-dot PL spectra of CdxZn1-xSeyS1-y/ZnS core/shell QDs with sharp and smooth interfaces. The inconsistence can be clarified when considering different PL blinking mechanisms. For the single QDs showing Auger blinking, a smooth core/shell interface potential can suppress PL blinking through reducing the Auger recombination. In contrast, we find slightly reduced biexciton Auger recombination rates but increased PL blinking activities in the band-edge carrier (BC)-blinking QDs with the smooth core/shell interface. This is because the smooth interface potential cannot reduce the PL blinking caused by the transfer of electrons to the surface states; however, there is potential to increase electron wave function delocalization for reducing the biexciton Auger recombination rate.

Biexciton Dynamics in Single Colloidal CdSe Quantum Dots.

The investigation of biexciton dynamics in single colloidal quantum dots (QDs) is critical to biexciton-based applications. Generally, a biexciton exhibits an extremely low photoluminescence (PL) quantum yield as well as very fast PL decay due to strong nonradiative Auger recombination, making it difficult to investigate the biexciton dynamics. Here, we develop a quantitative method based on intensity- and time-resolved photon statistics to investigate the biexciton dynamics in single colloidal QDs. This robust method can be used under high-excitation conditions to determine the absolute radiative and Auger recombination rates of both neutral and charged biexciton states in a single QD level, and the corresponding ratios between the two states agree with the theoretical predictions of the asymmetric band structures of CdSe-based QDs. Furthermore, the surface traps are found to provide additional nonradiative recombination pathways for the biexcitons, and their contributions are quantified by the method.

Determination of Statistical Criteria for an Automatic Search for Pairing Objects in Quantum Dot Images

The question of the possibility of determining the distance between two closely located blinking single colloidal quantum dots using the super-resolution far-field luminescence microscopy technique is examined. The results of the numerical simulation of microscopic images of a single pair of point sources of light for various modes of luminescence blinker are presented. Using the developed algorithm with subdiffraction accuracy, the relative position of the emitters in a pair is determined for various types of blinking dynamics of single quantum dots in model and laboratory experiments.

Contribution of electron-phonon coupling to the luminescence spectra of single colloidal quantum dots

Luminescence spectroscopy experiments were realized for single colloidal quantum dots CdSe/ZnS in a broad temperature range above room temperature in a nitrogen atmosphere. Broadening and shifts of spectra due to the temperature change as well as due to spectral diffusion processes were detected and analyzed. A linear correlation between the positions of maxima and the squared linewidths of the spectra was found. This dependence was explained by a model that takes into account the slow variation of the electron-phonon coupling strength.

DNA-Mediated Self-Assembly of Plasmonic Antennas with a Single Quantum Dot in the Hot Spot.

DNA self-assembly is a powerful tool to arrange optically active components with high accuracy in a large parallel manner. A facile approach to assemble plasmonic antennas consisting of two metallic nanoparticles (40 nm) with a single colloidal quantum dot positioned at the hot spot is presented here. The design approach is based on DNA complementarity, stoichiometry, and steric hindrance principles. Since no intermediate molecules other than short DNA strands are required, the structures possess a very small gap (≈ 5 nm) which is desired to achieve high Purcell factors and plasmonic enhancement. As a proof-of-concept, the fluorescence emission from antennas assembled with both conventional and ultrasmooth spherical gold particles is measured. An increase in fluorescence is obtained, up to ≈30-fold, compared to quantum dots without antenna.

Intrinsic blinking characteristics of single colloidal CdSe-CdS/ZnS core-multishell quantum dots

Fluorescence blinking of single colloidal semiconductor quantum dots (QDs) has been extensively studied, and several sophisticated models have been proposed. In this work, we derive Heisenberg equations of motion to carefully study principal transition processes, i.e., photoexcitation, energy relaxation, impact ionization and Auger recombination, radiative and nonradiative recombinations, and tunneling between core states and surface states, of the electron-hole pair in single CdSe-CdS/ZnS core-multishell QDs and show that the on-state probability density distribution of the QD fluorescence obeys the random telegraph signal theory because of the random radiative recombination of the photoexcited electron-hole pair in the QD core, while the off-state probability density distribution obeys the inverse power law distribution due to the series of random walks of the photoexcited electron in the two-dimensional surface-state network after the electron tunnels from the QD core to the QD surface. These two different blinking characteristics of the single QD are resolved experimentally by properly adjusting the optical excitation power and the bin time.