Colloidal Semiconductor Quantum Dots Research Articles

Advancing the Coupling of III–V Quantum Dots to Photonic Structures to Shape Their Emission Diagram

AbstractThe development of optoelectronic devices based on III–V semiconductor colloidal quantum dots (CQDs) is highly sought after due to their reduced toxicity. While devices based on conventional CQDs (II–VI semiconductors, halide perovskites) have achieved impressive technological leaps since their discovery, the most mature of these compounds contain toxic heavy metal elements (Cd, Hg, or Pb), which are highly undesirable for safe industrial scale applications. The strong covalent bonds of III–V compounds like InP, InAs, or InSb prevent the release of their toxic atoms, making them safer. However, these same bonds create severe material constraints. Namely, their harsher reaction conditions and increased sensitivity to oxidation have kept most of the research focused on material development. Meanwhile, their integration into devices and their coupling to photonic structures lag behind. Here, the integration of InAs/ZnSe core‐shell CQDs is advanced. First, the material parameters necessary to design plasmonic gratings coupled to the CQDs are elucidated and those gratings are fabricated. Angle‐resolved spectroscopy shows that the plasmon modes successfully couple to the CQD layer's emission leading to a tunable directivity with a 15° linewidth. A 3‐fold increase of the PL signal is achieved at normal incidence, thus advancing toward the goal of efficient outcoupling in LEDs.

(Digital Presentation) Cooperative Quantum Dynamics of Excitons in Colloidal Quantum Dots for Novel Optoelectronic Applications

Colloidal semiconductor quantum dots (QDs) are excellent materials for combining photonics and nanoscience. They have been extensively studied to understand the photophysics of quantum confined excitons and to develop efficient optoelectronic devices such as QD solar cells and light-emitting diodes. The photophysical processes of QD devices are governed by a large number of QDs constructing the devices. Therefore, cooperative quantum dynamics of QDs, i.e., synchronous responses of QD ensembles, have a great potential for boosting optoelectronic device performances.Before the use of cooperative quantum dynamics in QD devices, the understanding of quantum dynamics of excitons, i.e., coherent properties of multiexcitons, is required to draw out quantum cooperativity from QD ensembles. In order to elucidate coherent dynamics of multiexcitons during resonant laser-pulse excitation, we performed a quantum interference spectroscopy of QD responses. We found that multiexcitons generate the high-frequency coherent oscillations with integer multiples of the exciton resonance frequency, which is called harmonic quantum coherence, and that the coherent responses govern the microscopic processes in the resonant multiphoton absorption [1-3]. Furthermore, it was demonstrated that the coherent responses were detectable as photocurrent signals in thin films of QD ensembles [4]. These developments build up an essential base of quantum cooperativities emerging in optoelectronics.Here, we report recent investigations of cooperative quantum dynamics in QDs. In optoelectronic devices, electronic couplings between QDs are expected to generate a new type of quantum cooperativities. To generate electronic coupling, we fabricated closely packed PbS QDs by using a ligand exchange method. We found that nonlinear photocurrent signals are strongly enhanced in the QD solids. The comparison analysis of different ligands clarified that the enhancement ratio increases monotonically with the decrease of the inter-QD distance [5]. This means that the signal enhancement originates from the electronic coupling and cooperative behavior of QDs. These findings show that the cooperative quantum dynamics provide a new route to advanced optoelectronic technology such as nonlinear amplifiers.Part of this work was supported by JSPS KAKENHI (JP19H05465, JP22H01990, and JP23K17877), JST PRESTO (JPMJPR23H3), and JST CREST (JPMJCR21B4).

Random lasers with controlled emission modes by colloidal semiconductor quantum dots coupled to plasmonic nanorod arrays

Random lasers based on scattering for light amplification have gained extensive attention due to their simple structures. Colloidal semiconductor quantum dots (CSQDs) are excellent candidates as active media for random lasing. However, achieving CSQDs-based random lasers is challenging because the non-radiative Auger recombination produces a remarkable carrier loss. Here, we report random lasers with low thresholds based on the CSQDs coupled to the plasmonic nanorod arrays. The random lasers result from the enhancing excitation and emission rates of the CSQDs through plasmon resonance energy transfer induced by the spectral overlap between the resonant cavity modes of the plasmonic nanorod arrays and the excitons of the CSQDs, which reduces the carrier loss and establishes the optical gain in the CSQDs. Moreover, the incoherent and coherent emission modes can be controlled by varying the filling configurations (i.e., full-filling and partial-filling) of the plasmonic nanorods embedded in anodic aluminum oxide templates. Consequently, the incoherent random laser with a threshold of 19.3 μJ/mm2 and coherent random lasers with a threshold of 13.5 μJ/mm2 are performed, respectively. The proposed system provides a feasible method for low-threshold CSQDs-based random lasers with controlled emission modes, showing a promising avenue for potential applications.

Strain-graded quantum dots with spectrally pure, stable and polarized emission

Structural deformation modifies the bandgap, exciton fine structure and phonon energy of semiconductors, providing an additional knob to control their optical properties. The impact can be exploited in colloidal semiconductor quantum dots (QDs), wherein structural stresses can be imposed in three dimensions while defect formation is suppressed by controlling surface growth kinetics. Yet, the control over the structural deformation of QDs free from optically active defects has not been reached. Here, we demonstrate strain-graded CdSe-ZnSe core-shell QDs with compositionally abrupt interface by the coherent pseudomorphic heteroepitaxy. Resulting QDs tolerate mutual elastic deformation of varying magnitudes at the interface with high structural fidelity, allowing for spectrally stable and pure emission of photons at accelerated rates with near unity luminescence efficiency. We capitalize on the asymmetric strain effect together with the quantum confinement effect to expand emission envelope of QDs spanning the entire visible region and exemplify their use in photonic applications.

Bismuth sulfide quantum dots-CsPbBr3 perovskite nanocrystals heterojunction for enhanced broadband photodetection

Colloidal semiconductor nanocrystals (NCs) or quantum dots (QDs) have shown great potential for solution-processable photodetector due to their exceptional optical and electronic properties. However, broadband and sensitive photodetection from single QDs- based devices is quite challenging. Nano-heterojunction with proper band alignment based on two different materials offers significant advantages for developing broadband photodetector. Herein, we report ultraviolet–visible (UV–Vis) to near-infrared (NIR) light-responsive photodetector based on solution-processed nano-heterojunction of visible light absorber CsPbBr3 perovskite NCs and wide absorption range, environment-friendly Bi2S3 QDs. Our results demonstrate that the CsPbBr3–Bi2S3 nano-heterojunction-based photodetector has higher responsivity (380 μA/W at a wavelength of 532 nm) and higher specific detectivity (1.02 × 105 Jones), as compared to the individual CsPbBr3 or Bi2S3 QDs based devices. Interestingly, the detection wavelength range of our heterojunction device is further extended to the near-infrared region (1064 nm) due to the broadband absorption range of Bi2S3 QDs, which is not observed in the visible light absorber CsPbBr3 devices. Remarkably, the responsivity of the heterojunction device is 90 μA W−1. The enhanced specific detectivity and the broadband response of hybrid devices are attributed to the improved charge carrier generation, efficient charge separation and transfer at the interface between CsPbBr3 and Bi2S3 QDs.

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

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

The influence of solvent refractive index on the photoluminescence decay of thick-shell gradient-alloyed colloidal quantum dots investigated in a wide range of delay times.

Colloidal semiconductor quantum dots have many potential optical applications, including quantum dot light-emitting diodes, single-photon sources, or biological luminescent markers. The optical properties of colloidal quantum dots can be affected by their dielectric environment. This study investigated the photoluminescence (PL) decay of thick-shell gradient-alloyed colloidal semiconductor quantum dots as a function of solvent refractive index. These measurements were conducted in a wide range of delay times to account for both the initial spontaneous decay of excitons and the delayed emission of excitons that has the form of a power law. It is shown that whereas the initial spontaneous PL decay is very sensitive to the refractive index of the solvent, the power-law delayed emission of excitons is not. Our results seem to exclude the possibility of carrier self-trapping in the considered solvents and suggest the existence of trap states inside the quantum dots. Finally, our data show that the average exciton lifetime significantly decreases as a function of the solvent refractive index. The change in exciton lifetime is qualitatively modeled and discussed.

Pathway of Room-Temperature Formation of CdSeS Magic-Size Clusters from Mixtures of CdSe and CdS Samples.

The synthetic application of prenucleation-stage samples of colloidal semiconductor quantum dots (QDs) is in its infancy. It is shown that when two prenucleation-stage samples of binary CdSe and CdS are mixed, ternary CdSeS magic-size clusters (MSCs) grow at room temperature in dispersion. As the amount of the CdS sample increases, the optical absorption of the CdSeS MSCs blueshifts from ≈380 to ≈360 nm. It is proposed that the cluster in the CdSe sample reacts with the CdS monomer from the CdS sample. The monomer substitution reaction of CdSe by CdS can proceed continuously; thus, CdSeS MSCs with tunable compositions are obtained. The present study provides compelling evidence that clusters formed in the prenucleation stage of QDs. The clusters are precursor compounds (PCs) of MSCs, transforming at room temperature with the thermoneutrality principle of isodesmic reactions. The nucleation and growth of QDs follows a multi-step non-classical instead of one-step classical nucleation model.

Emission behaviour of CdTe/SiO2 core/shell quantum dots in external electric field

Optical properties of CdTe/SiO2 core/shell colloidal quantum dots were investigated in external electric field in the range of 0–140 kV/cm. Maximum of photoluminescence intensity was centered at 2.39 eV with the full width at half maximum of 0.26 eV. It was found that the photoluminescence intensity of quantum dots tends to decrease by 22% and the integrated luminescence intensity tends to decrease by 23% with increasing electric field to the maximum value. However, an increase in the integrated luminescence intensity in the region of 60 kV/cm was observed. We show that both interband and trap-state luminescence is affected by an external electric field with no Stark shift observed. Based on these results, we propose that relaxation of the excited states in CdTe/SiO2 core/shell colloidal quantum dots exposed to an external electric field is affected by two field-dependent mechanisms, i.e. field-induced quenching of luminescence and blocking of charge carrier trapping. The obtained results can be useful to gain better understanding of the electric field effect on the optical properties of CdTe/SiO2 colloidal semiconductor quantum dots and, in particular, on the operation of hybrid organic-inorganic LEDs with emitting layers based on the studied nanocrystals.

Optical Activity and Excitonic Characteristics of Chiral CdSe Quantum Dots.

Introduction of chirality to colloidal semiconductor quantum dots (QDs) triggers a chiroptical effect. However, there remains a knowledge gap in the mechanism of chirality transfer and amplification from molecules to QDs. By time-dependent density functional theory calculations combined with a correlated electron-hole picture, we explored the chiroptical activity of CdSe QDs decorated with different chiral monocarboxylic acids from an excitonic perspective. Our calculations showed strong circular dichroism (CD) signals in the visible region for the chiral CdSe QDs. The excitonic states with large CD originate from QDs, while the chiral molecules break the orthogonality between electric and magnetic transition dipoles, which synergistically facilitates the prominent dissymmetric effect. The considered monocarboxylic acid chiral molecules all favor the bidentate adsorption configuration of the carboxyl group on the CdSe surface, endowing an identical CD signature but distinct excitonic characteristics. These findings are crucial for the regulation of chirality and excitons in semiconductor QDs to develop excitonic devices.

Direct Visualization of Confinement and Many‐Body Correlation Effects in 2D Spectroscopy of Quantum Dots

AbstractThe size tunable color of colloidal semiconductor quantum dots (QDs) is probably the most elegant illustration of the quantum confinement effect. As explained by the simple “particle‐in‐a‐box” model, the transition energies between the levels increase when the “box” becomes smaller. To investigate quantum confinement effects, typically a well‐defined narrow size distribution of the nanoparticles is needed. In this contribution, how coherent electronic two‐dimensional spectroscopy (2DES) can directly visualize the quantum size effect in a sample with broad size distribution of QDs is demonstrated. The method is based on two features of the 2DES – the ability to resolve inhomogeneous broadening and the capability to reveal correlations between the states. In QD samples, inhomogeneous spectral broadening is mainly caused by the size distribution and leads to elongated diagonal peaks of the spectra. Since the cross peaks correlate the energies of two states, they allow drawing conclusions about the size dependence of the corresponding states. It is also found that the biexciton binding energy changes between 3 and 8 meV with the QD size. Remarkably, the size dependence is non‐monotonic with a clear minimum.

Resonant tunneling in a colloidal CdS semiconductor quantum-dot single-electron transistor based on heteroepitaxial-spherical Au/Pt nanogap electrodes.

Semiconductor quantum dots (QDs) have unique discrete energy levels determined by the particle size and material. Therefore, they have potential applications as novel optical and electronic devices. Among those, colloidal group II-VI semiconductor quantum dots stand out for their facile synthesis and band gaps aligned with the visible light spectrum. However, the electrical characterization studies of an individual quantum dot necessitate the size of nanogap electrodes being equal to the size of the quantum dot, which has conventionally been evaluated using techniques such as scanning tunneling microscopy (STM) and nanogaps fabricated by electromigration. The complexity of device fabrication has restricted research in this area. Here, we present a pioneering approach for the electrical characterization of single-QD: heteroepitaxial-spherical (HS) Au/Pt nanogap electrodes. We fabricated transistors through chemisorption, an anchoring colloidal CdS QD (3.8 nm) between the HS-Au/Pt nanogap electrodes (gap separation: 4.5 nm). The resulting device functions as a quantum-dot single-electron transistor (QD-SET), showing resonant tunneling-an inherent characteristic of the QD. A steep current increase was observed at a negative voltage, apart from the theoretical single-electron tunneling current by Coulomb blockade phenomena, which agreed with the theoretical resonant tunneling current through a discrete energy level of the QD. This underscores the promise of HS-Au/Pt nanogap electrodes in realizing single-QD devices, offering a pathway toward unlocking their full potential.

Rational ligand design for enhanced carrier mobility in self-powered SWIR photodiodes based on colloidal InSb quantum dots.

Solution-processed colloidal III-V semiconductor quantum dot photodiodes (QPDs) have potential applications in short-wavelength infrared (SWIR) imaging due to their tunable spectral response range, possible multiple-exciton generation, operation at 0-V bias voltage and low-cost fabrication and are also expected to replace lead- and mercury-based counterparts that are hampered by reliance on restricted elements (RoHS). However, the use of III-V CQDs as photoactive layers in SWIR optoelectronic applications is still a challenge because of underdeveloped ligand engineering for improving the in-plane conductivity of the QD assembled films. Here, we report on ligand engineering of InSb CQDs to enhance the optical response performance of self-powered SWIR QPDs. Specifically, by replacing the conventional ligand (i.e., oleylamine) with sulfide, the interparticle distance between the CQDs was shortened from 5.0 ± 0.5 nm to 1.5 ± 0.5 nm, leading to improved carrier mobility for high photoresponse speed to SWIR light. Furthermore, the use of sulfide ligands resulted in a low dark current density (∼nA cm-2) with an improved EQE of 18.5%, suggesting their potential use in toxic-based infrared image sensors.

Thermally-Induced Isomerization of Prenucleation Clusters During the Prenucleation Stage of CdTe Quantum Dots.

The evolution of prenucleation clusters in the prenucleation stage of colloidal semiconductor quantum dots (QDs) has remained unexplored. With CdTe as a model system, we show that substances form and isomerize prior to the nucleation and growth of QDs. Called precursor compounds (PCs), the prenucleation clusters are relatively optically transparent and can transform to absorbing magic-size clusters (MSCs). When a prenucleation-stage sample at 25, 45, or 80 °C is dispersed in a mixture of cyclohexane (CH) and octylamine (OTA) at room temperature, either MSC-371, MSC-417, or MSC-448 evolves with absorption peaking at 371, 417, or 448 nm, respectively. We propose that PC-371 forms at 25 °C, and isomerizes to PC-417 at 45 °C and to PC-448 at 80 °C. The PCs and MSCs are quasi isomers. Relatively large and small amounts of OTA favor PC-371 and PC-448 in dispersion, respectively. The present findings suggest the existence of PC-to-PC isomerization in the QD prenucleation stage.

Thermally‐Induced Isomerization of Prenucleation Clusters During the Prenucleation Stage of CdTe Quantum Dots

AbstractThe evolution of prenucleation clusters in the prenucleation stage of colloidal semiconductor quantum dots (QDs) has remained unexplored. With CdTe as a model system, we show that substances form and isomerize prior to the nucleation and growth of QDs. Called precursor compounds (PCs), the prenucleation clusters are relatively optically transparent and can transform to absorbing magic‐size clusters (MSCs). When a prenucleation‐stage sample at 25, 45, or 80 °C is dispersed in a mixture of cyclohexane (CH) and octylamine (OTA) at room temperature, either MSC‐371, MSC‐417, or MSC‐448 evolves with absorption peaking at 371, 417, or 448 nm, respectively. We propose that PC‐371 forms at 25 °C, and isomerizes to PC‐417 at 45 °C and to PC‐448 at 80 °C. The PCs and MSCs are quasi isomers. Relatively large and small amounts of OTA favor PC‐371 and PC‐448 in dispersion, respectively. The present findings suggest the existence of PC‐to‐PC isomerization in the QD prenucleation stage.

Dipolar Ligands Tune Plasmonic Properties of Tin-Doped Indium Oxide Nanocrystals.

Surface functionalization with dipolar molecules is known to tune the electronic band alignment in semiconductor films and colloidal quantum dots. Yet, the influence of surface modification on plasmonic nanocrystals and their properties remains little explored. Here, we functionalize tin-doped indium oxide nanocrystals (ITO NCs) via ligand exchange with a series of cinnamic acids with different electron-withdrawing and -donating dipolar characters. Consistent with previous reports on semiconductors, we find that withdrawing (donating) ligands increase (decrease) the work function caused by an electrostatic potential shift across the molecular layer. Quantitative analyses of the plasmonic extinction spectra reveal that varying the ligand molecular dipole affects the near-surface depletion layer, with an anticorrelated trend between the electron concentration and electronic volume fraction, factors that are positively correlated in as-synthesized NCs. Electronic structure engineering through surface modification provides access to distinctive combinations of plasmonic properties that could enable optoelectronic applications, sensing, and hot electron-driven processes.

Electric-field-induced colour switching in colloidal quantum dot molecules at room temperature.

Colloidal semiconductor quantum dots are robust emitters implemented in numerous prototype and commercial optoelectronic devices. However, active fluorescence colour tuning, achieved so far by electric-field-induced Stark effect, has been limited to a small spectral range, and accompanied by intensity reduction due to the electron-hole charge separation effect. Utilizing quantum dot molecules that manifest two coupled emission centres, we present a unique electric-field-induced instantaneous colour-switching effect. Reversible emission colour switching without intensity loss is achieved on a single-particle level, as corroborated by correlated electron microscopy imaging. Simulations establish that this is due to the electron wavefunction toggling between the two centres, induced by the electric field, and affected by the coupling strength. Quantum dot molecules manifesting two coupled emission centres may be tailored to emit distinct colours, opening the path for sensitive field sensing and colour-switchable devices such as a novel pixel design for displays or an electric-field-induced colour-tunable single-photon source.

Nanoscale LiZnN - Luminescent Half-Heusler Quantum Dots.

Colloidal semiconductor quantum dots are a well-established technology, with numerous materials available either commercially or through the vast body of literature. The prevalent materials are cadmium-based and are unlikely to find general acceptance in most applications. While the III-V family of materials is a likely substitute, issues remain about its long-term suitability, and other earth-abundant materials are being explored. In this report, we highlight a nanoscale half-Heusler semiconductor, LiZnN, composed of readily available elements as a potential alternative system to luminescent II-VI and III-V nanoparticle quantum dots.

Highly polarized light emission from hybrid of quantum dots and plasmons in the intermediate coupling regime.

Colloidal semiconductor quantum dots (QDs), with a size tunable bandgap and remarkably high quantum efficiency, have been recognized as ideal light sources in quantum information and light emitting devices. For light sources, besides the emission intensity and spectral profile, the degree of polarization (DoP) is an essential parameter. Here, by embedding a monolayer of QDs inside the nanogap between a bottom Au mirror and a top Ag nanowire, we have demonstrated highly polarized light emission from the QDs with an average DoP of 0.89. In addition to the anisotropic photoluminescence (PL) intensity, the PL spectra are distinct at different polarizations, with an asymmetric spectral shape or even two-peak features. Such an anisotropic emission behavior arises from the coupling between the QDs and the largely confined and polarization-dependent gap-plasmons in the Au/QD/Ag nanocavities in the intermediate coupling regime. Our results demonstrate the possibility of achieving highly polarized light sources by coupling spherical QDs to single anisotropic plasmonic nanocavities, to provide new opportunities in the future design of polarized QD-based display devices.

Tetrameric Antibody Complexes and Affinity Tag Peptides for the Selective Immobilization and Imaging of Single Quantum Dots.

Colloidal semiconductor quantum dots (QDs) are of widespread interest as fluorescent labels for bioanalysis and imaging applications. Single-particle measurements have proven to be a very powerful tool for better understanding the fundamental properties and behaviors of QDs and their bioconjugates; however, a recurring challenge is the immobilization of QDs in a solution-like environment that minimizes interactions with a bulk surface. Immobilization strategies for QD-peptide conjugates are particularly underdeveloped within this context. Here, we present a novel strategy for the selective immobilization of single QD-peptide conjugates using a combination of tetrameric antibody complexes (TACs) and affinity tag peptides. A glass substrate is modified with an adsorbed layer of concanavalin A (ConA) that binds a subsequent layer of dextran that minimizes nonspecific binding. A TAC with anti-dextran and anti-affinity tag antibodies binds to the dextran-coated glass surface and to the affinity tag sequence of QD-peptide conjugates. The result is spontaneous and sequence-selective immobilization of single QDs without any chemical activation or cross-linking. Controlled immobilization of multiple colors of QDs is possible using multiple affinity tag sequences. Experiments confirmed that this approach positions the QD away from the bulk surface. The method supports real-time imaging of binding and dissociation, measurements of Förster resonance energy transfer (FRET), tracking of dye photobleaching, and detection of proteolytic activity. We anticipate that this immobilization strategy will be useful for studies of QD-associated photophysics, biomolecular interactions and processes, and digital assays.