Growth Control of InP/ZnSe Heterostructured Nanocrystals.

The morphology of heterostructured semiconductor nanocrystals (h-NCs) dictates the spatial distribution of charge carriers and their recombination dynamics and/or transport, which are the main performance indicators of photonic applications utilizing h-NCs. The inability to control the morphology of heterovalent III-V/II-VI h-NCs composed of heavy-metal-free elements hinders their practical use. As a case study of III-V/II-VI h-NCs, the growth control of ZnSe epilayers on InP NCs is demonstrated here. The anisotropic morphology in InP/ZnSe h-NCs is attributed to the facet-dependent energy costs for the growth of ZnSe epilayers on different facets of InP NCs, and effective chemical means for controlling the growth rates of ZnSe on different surface planes are demonstrated. Ultimately, this article capitalizes on the controlled morphology of InP/ZnSe h-NCs to expand their photophysical characteristics from stable and pure emission to environment-sensitive one, which will facilitate their use in a variety of photonic applications.

Synthesis of Ternary and Quaternary Group III-Arsenide Colloidal Quantum Dots via High-Temperature Cation Exchange in Molten Salts: The Importance of Molten Salt Speciation.

Colloidal semiconductor nanocrystals are an important class of materials which have many desirable optoelectronic properties. In their bulk phases, gallium- and aluminum-containing III-V materials such as GaAs, GaP, and Al1-xGaxAs represent some of the most technologically important semiconductors. However, their colloidal synthesis by traditional methods is difficult due to the high temperatures needed to crystallize these highly covalent materials and the extreme reactivity of Ga- and Al- precursors toward organic solvents at such high temperatures. A recently developed paradigm shift in the synthesis of these materials is to use molten inorganic salts as solvents to prepare Ga- containing III-V colloidal nanocrystals by cation exchange of the corresponding indium pnictide (InPn) colloidal nanocrystals. There have been several successful applications of molten salt solvents to prepare III-phosphide colloidal nanocrystals. However, little is known about the nature of these reaction environments at the relevant reaction conditions and synthesis of III-arsenide colloidal nanocrystals remains challenging. Herein we report a detailed study on cation exchange of InPn nanocrystals using nominally Lewis basic molten salt solvents with added gallium halides. Surprisingly, these salt systems phase separate into two immiscible phases, and the nanocrystals preferentially segregate to one of the phases. Using a suite of in situ spectroscopy tools, we identify the phase the nanocrystals segregate to as Lewis neutral alkali tetrahalogallate molten salts. We apply in situ high-temperature Raman spectroscopy to identify the chemical species present in several molten salt compositions at experimentally relevant reaction conditions to elucidate a molecular basis for the reactivity observed. We then employ Lewis neutral KGaI4 molten salts to prepare high-quality In1-xGaxAs and In1-xGaxP nanocrystals and demonstrate that deviation from Lewis neutral conditions accelerate nanocrystal decomposition in the case of III-arsenide materials. Further, we expand to KAlI4-based molten salts to prepare In1-x-yGaxAlyAs nanocrystals which represent an example of solution-synthesized quaternary III-V nanocrystals. These insights provide a molecular basis for the rational development of molten salt solvents, thus allowing the preparation of a diverse array of multicomponent III-V colloidal nanocrystals.

Amygdala resting-state functional connectivity alterations in patients with chronic insomnia disorder: correlation with electroencephalography beta power during sleep.

STUDY OBJECTIVES: This study investigated alterations in resting-state functional connectivity (RSFC) and hyperarousal biomarkers in patients with chronic insomnia disorder (CID), compared with good sleepers (GS). We also examined the relationships between altered RSFC and hyperarousal biomarkers. METHODS: Fifty patients with CID and 52 GS completed self-reporting questionnaires, and then underwent polysomnography and resting-state functional magnetic resonance imaging. We analyzed RSFC in the amygdala (AMG) and anterior insula (aINS), which are core regions of the salience network that are likely to be involved in hyperarousal. We also analyzed electroencephalography (EEG) relative beta power and heart rate variability (HRV) parameters (e.g. low and high frequency) during sleep. We then tested between-group differences in the RSFC and hyperarousal biomarkers; we examined correlations of RSFC with EEG beta power and HRV. RESULTS: Compared with GS, patients with CID showed more negative RSFC between the right amygdala (R.AMG) and left supramarginal gyrus (L.SMG), but less positive RSFC between the left aINS and bilateral lateral prefrontal cortex. The R.AMG-L.SMG RSFC was negatively correlated with EEG beta power in central regions (C3: r = -0.336, p = 0.012; C4: r = -0.314, p = 0.024). CONCLUSIONS: Decreased RSFC between the R.AMG and L.SMG in patients with insomnia may reflect the difficulty in cortical top-down regulation of the AMG, indicating daytime hyperarousal. Individuals who experience hyperarousal during the daytime may also exhibit cortical hyperarousal during sleep, as indicated by increased EEG beta power.

Coherent heteroepitaxial growth of I-III-VI2 Ag(In,Ga)S2 colloidal nanocrystals with near-unity quantum yield for use in luminescent solar concentrators.

Colloidal Ag(In,Ga)S2 nanocrystals (AIGS NCs) with the band gap tunability by their size and composition within visible range have garnered surging interest. High absorption cross-section and narrow emission linewidth of AIGS NCs make them ideally suited to address the challenges of Cd-free NCs in wide-ranging photonic applications. However, AIGS NCs have shown relatively underwhelming photoluminescence quantum yield (PL QY) to date, primarily because coherent heteroepitaxy has not been realized. Here, we report the heteroepitaxy for AIGS-AgGaS2 (AIGS-AGS) core-shell NCs bearing near-unity PL QYs in almost full visible range (460 to 620 nm) and enhanced photochemical stability. Key to the successful growth of AIGS-AGS NCs is the use of the Ag-S-Ga(OA)2 complex, which complements the reactivities among cations for both homogeneous AIGS cores in various compositions and uniform AGS shell growth. The heteroepitaxy between AIGS and AGS results in the Type I heterojunction that effectively confines charge carriers within the emissive core without optically active interfacial defects. AIGS-AGS NCs show higher extinction coefficient and narrower spectral linewidth compared to state-of-the-art heavy metal-free NCs, prompting their immediate use in practicable applications including displays and luminescent solar concentrators (LSCs).

Impact of Morphological Inhomogeneity on Excitonic States in Highly Mismatched Alloy ZnSe1-XTeX Nanocrystals.

ZnSe1-XTeX nanocrystals (NCs) are promising photon emitters with tunable emission across the violet to orange range and near-unity quantum yields. However, these NCs suffer from broad emission line widths and multiple exciton decay dynamics, which discourage their practicable use. Here, we explore the excitonic states in ZnSe1-XTeX NCs and their photophysical characteristics in relation to the morphological inhomogeneity of highly mismatched alloys. Ensemble and single-dot spectroscopic analysis of a series of ZnSe1-XTeX NC samples with varying Te ratios coupled with computational calculations shows that, due to the distinct electronegativity between Se and Te, nearest-neighbor Te pairs in ZnSe1-XTeX alloys create localized hole states spectrally distributed approximately 130 meV above the 1Sh level of homogeneous ZnSe1-XTeX NCs. This forms spatially separated excitons (delocalized electron and localized hole in trap), accounting for both inhomogeneous and homogeneous line width broadening with delayed recombination dynamics. Our results identify photophysical characteristics of excitonic states in NCs made of highly mismatched alloys and provide future research directions with potential implications for photonic applications.

Direct patterning of colloidal quantum dots with adaptable dual-ligand surface.

Colloidal quantum dots (QDs) stand at the forefront of a variety of photonic applications given their narrow spectral bandwidth and near-unity luminescence efficiency. However, integrating luminescent QD films into photonic devices without compromising their optical or transport characteristics remains challenging. Here we devise a dual-ligand passivation system comprising photocrosslinkable ligands and dispersing ligands to enable QDs to be universally compatible with solution-based patterning techniques. The successful control over the structure of both ligands allows the direct patterning of dual-ligand QDs on various substrates using commercialized photolithography (i-line) or inkjet printing systems at a resolution up to 15,000 pixels per inch without compromising the optical properties of the QDs or the optoelectronic performance of the device. We demonstrate the capabilities of our approach for QD-LED applications. Our approach offers a versatile way of creating various structures of luminescent QDs in a cost-effective and non-destructive manner, and could be implemented in nearly all commercial photonics applications where QDs are used.

Interface polarization in heterovalent core-shell nanocrystals.

The potential profile and the energy level offset of core-shell heterostructured nanocrystals (h-NCs) determine the photophysical properties and the charge transport characteristics of h-NC solids. However, limited material choices for heavy metal-free III-V-II-VI h-NCs pose challenges in comprehensive control of the potential profile. Herein, we present an approach to such a control by steering dipole densities at the interface of III-V-II-VI h-NCs. The controllable heterovalency at the interface is responsible for interfacial dipole densities that result in the vacuum-level shift, providing an additional knob for the control of optical and electrical characteristics of h-NCs. The synthesis of h-NCs with atomic precision allows us to correlate interfacial dipole moments with the NCs' photochemical stability and optoelectronic performance.

Steering Interface Dipoles for Bright and Efficient All-Inorganic Quantum Dot Based Light-Emitting Diodes.

The state-of-the-art quantum dot (QD) based light-emitting diodes (QD-LEDs) reach near-unity internal quantum efficiency thanks to organic materials used for efficient hole transportation within the devices. However, toward high-current-density LEDs, such as augmented reality, virtual reality, and head-up display, thermal vulnerability of organic components often results in device instability or breakdown. The adoption of a thermally robust inorganic hole transport layer (HTL), such as NiO, becomes a promising alternative, but the large energy offset between the NiO HTL and the QD emissive layer impedes the efficient operation of QD-LEDs. Here, we demonstrate bright and stable all-inorganic QD-LEDs by steering the orientation of molecular dipoles at the surfaces of both the NiO HTL and QDs. We show that the molecular dipoles not only induce the vacuum level shift that helps alleviate the energy offset between the NiO HTL and QDs but also passivate the surface trap states of the NiO HTL that act as nonradiative recombination centers. With the facilitated hole injection into QDs and suppressed electron leakage toward trap sites in the NiO HTL, we achieve all-inorganic QD-LEDs with high external quantum efficiency (6.5% at peak) and brightness (peak luminance exceeding 77 000 cd/m2) along with prolonged operational stability. The approaches and results in the present study provide the design principles for high-performance all-inorganic QD-LEDs suited for next-generation light sources.

A Bioinspired Stretchable Sensory-Neuromorphic System.

Conventional stretchable electronics that adopt a wavy design, a neutral mechanical plane, and conformal contact between abiotic and biotic interfaces have exhibited diverse skin-interfaced applications. Despite such remarkable progress, the evolution of intelligent skin prosthetics is challenged by the absence of the monolithic integration of neuromorphic constituents into individual sensing and actuating components. Herein, a bioinspired stretchable sensory-neuromorphic system, comprising an artificial mechanoreceptor, artificial synapse, and epidermal photonic actuator is demonstrated; these three biomimetic functionalities correspond to a stretchable capacitive pressure sensor, a resistive random-access memory, and a quantum dot light-emitting diode, respectively. This system features a rigid-island structure interconnected with a sinter-free printable conductor, which is optimized by controlling the evaporation rate of solvent (≈160% stretchability and ≈18 550 S cm-1 conductivity). Devised design improves both areal density and structural reliability while avoiding the thermal degradation of heat-sensitive stretchable electronic components. Moreover, even in the skin deformation range, the system accurately recognizes various patterned stimuli via an artificial neural network with training/inferencing functions. Therefore, the new bioinspired system is expected to be an important step toward implementing intelligent wearable electronics.

Tailoring the Electronic Landscape of Quantum Dot Light-Emitting Diodes for High Brightness and Stable Operation.

The charge injection imbalance into the quantum dot (QD) emissive layer of QD-based light-emitting diodes (QD-LEDs) is an unresolved issue that is detrimental to the efficiency and operation stability of devices. Herein, an integrated approach to harmonize the charge injection rates for bright and stable QD-LEDs is proposed. Specifically, the electronic characteristics of the hole transport layer (HTL) is delicately designed in order to facilitate the hole injection from the HTL into QDs and confine the electron overflow toward the HTL. The well-defined exciton recombination zone by the engineered QDs and HTL results in high performance with a peak luminance exceeding 410 000 cd/m2, suppressed efficiency roll-off characteristics (ΔEQE < 5% between 200 and 200 000 cd/m2), and prolonged operational stability. The electric and optoelectronic analyses reveal the charge carrier injection mechanism at the interface between the HTL and QDs and provides the design principle of QD heterostructures and charge transport layers for high-performance QD-LEDs.

Direct Photolithographic Patterning of Colloidal Quantum Dots Enabled by UV-Crosslinkable and Hole-Transporting Polymer Ligands.

Quantum dot (QD)-based displays call for nondestructive, high-throughput, and high-resolution patterning techniques with micrometer precision. In particular, self-emissive QD-based displays demand fine patterns of conductive QD films with uniform thickness at the nanometer scale. To meet these requirements, we functionalized QDs with photopatternable and semiconducting poly(vinyltriphenylamine-random-azidostyrene) (PTPA-N3-SH) ligands in which hole-transporting triphenylamine and UV-crosslinkable azide (-N3) groups are integrated. The hybridized QD films undergo chemical crosslinking upon UV irradiation without loss in the luminescence efficiency, enabling micrometer-scale QD patterns (pitch size down to ∼10 µm) via direct photolithography. In addition, the conjugated moieties in the ligands allow the crosslinked QD films to be used in electrically driven light-emitting diodes (LED). As the ultimate achievement, a patterned QD-LED was prepared with a maximum luminance of 11 720 cd m-2 and a maximum external quantum efficiency (EQE) of 6.25%. The present study offers a simple platform to fabricate conductive nanoparticle films with micrometer-scale patterns, and thus we anticipate that this system will expedite the realization of QD-based displays and will also be applicable to the manufacture of nanoparticles for other electronic devices.

High-resolution patterning of colloidal quantum dots via non-destructive, light-driven ligand crosslinking.

Establishing multi-colour patterning technology for colloidal quantum dots is critical for realising high-resolution displays based on the material. Here, we report a solution-based processing method to form patterns of quantum dots using a light-driven ligand crosslinker, ethane-1,2-diyl bis(4-azido-2,3,5,6-tetrafluorobenzoate). The crosslinker with two azide end groups can interlock the ligands of neighbouring quantum dots upon exposure to UV, yielding chemically robust quantum dot films. Exploiting the light-driven crosslinking process, different colour CdSe-based core-shell quantum dots can be photo-patterned; quantum dot patterns of red, green and blue primary colours with a sub-pixel size of 4 µm × 16 µm, corresponding to a resolution of >1400 pixels per inch, are demonstrated. The process is non-destructive, such that photoluminescence and electroluminescence characteristics of quantum dot films are preserved after crosslinking. We demonstrate that red crosslinked quantum dot light-emitting diodes exhibiting an external quantum efficiency as high as 14.6% can be obtained.

"Positive Incentive" Approach To Enhance the Operational Stability of Quantum Dot-Based Light-Emitting Diodes.

Balanced charge injection promises high efficiency of quantum dot-based light-emitting diodes (QD-LEDs). The most widely used approach to realize charge injection balance impedes the injection rate of the dominant charge carrier with energetic barriers. However, these approaches often accompany unwanted outcomes (e.g., the increase in operation voltage) that sacrifice the operational stability of devices. Herein, a "positive incentive" approach is proposed to enhance the efficiency and the operational stability of QD-LEDs. Specifically, the supply of hole, an inferior carrier than its counterpart, is facilitated by adopting a thin fullerene (C60) interlayer at the interface between the hole injection layer (MoOX) and hole transport layer (4,4'-bis(9-carbazolyl)-1,1'-biphenyl). The C60 interlayer boosts the hole current by eliminating the universal energy barrier, lowers the operation voltage of QD-LEDs, and enhances the charge balance in the QD emissive layer within the working device. Consequently, QD-LEDs benefitting from the adoption of the C60 interlayer exhibit significantly enhanced device efficiency and operation stability. Grounded on the quantitative assessment of the charge injection imbalance within the QD emissive layer, the impact of electrical parameters of QD-LEDs on their optoelectronic performance and operational stability is also discussed.

III-V colloidal nanocrystals: control of covalent surfaces.

Colloidal quantum dots (QDs) are nanosized semiconductors whose electronic features are dictated by the quantum confinement effect. The optical, electrical, and chemical properties of QDs are influenced by their dimensions and surface landscape. The surface of II-VI and IV-VI QDs has been extensively explored; however, in-depth investigations on the surface of III-V QDs are still lagging behind. This Perspective discusses the current understanding of the surface of III-V QDs, outlines deep trap states presented by surface defects, and suggests strategies to overcome challenges associated with deep traps. Lastly, we discuss a route to create well-defined facets in III-V QDs by providing a platform for surface studies and a recently reported approach in atomistic understanding of covalent III-V QD surfaces using the electron counting model with fractional dangling bonds.

Unraveling the Origin of Operational Instability of Quantum Dot Based Light-Emitting Diodes.

We investigate the operational instability of quantum dot (QD)-based light-emitting diodes (QLEDs). Spectroscopic analysis on the QD emissive layer within devices in chorus with the optoelectronic and electrical characteristics of devices discloses that the device efficiency of QLEDs under operation is indeed deteriorated by two main mechanisms. The first is the luminance efficiency drop of the QD emissive layer in the running devices owing to the accumulation of excess electrons in the QDs, which escalates the possibility of nonradiative Auger recombination processes in the QDs. The other is the electron leakage toward hole transport layers (HTLs) that accompanies irreversible physical damage to the HTL by creating nonradiative recombination centers. These processes are distinguishable in terms of the time scale and the reversibility, but both stem from a single origin, the discrepancy between electron versus hole injection rates into QDs. Based on experimental and calculation results, we propose mechanistic models for the operation of QLEDs in individual quantum dot levels and their degradation during operation and offer rational guidelines that promise the realization of high-performance QLEDs with proven operational stability.

Ligand-Asymmetric Janus Quantum Dots for Efficient Blue-Quantum Dot Light-Emitting Diodes.

We present ligand-asymmetric Janus quantum dots (QDs) to improve the device performance of quantum dot light-emitting diodes (QLEDs). Specifically, we devise blue QLEDs incorporating blue QDs with asymmetrically modified ligands, in which the bottom ligand of QDs in contact with ZnO electron-transport layer serves as a robust adhesive layer and an effective electron-blocking layer and the top ligand ensures uniform deposition of organic hole transport layers with enhanced hole injection properties. Suppressed electron overflow by the bottom ligand and stimulated hole injection enabled by the top ligand contribute synergistically to boost the balance of charge injection in blue QDs and therefore the device performance of blue QLEDs. As an ultimate achievement, the blue QLED adopting ligand-asymmetric QDs displays 2-fold enhancement in peak external quantum efficiency (EQE = 3.23%) compared to the case of QDs with native ligands (oleic acid) (peak EQE = 1.49%). The present study demonstrates an integrated strategy to control over the charge injection properties into QDs via ligand engineering that enables enhancement of the device performance of blue QLEDs and thus promises successful realization of white light-emitting devices using QDs.

Multifunctional Dendrimer Ligands for High-Efficiency, Solution-Processed Quantum Dot Light-Emitting Diodes.

We present multifunctional dendrimer ligands that serve as the charge injection controlling layer as well as the adhesive layer at the interfaces between quantum dots (QDs) and the electron transport layer (ETL) in quantum dot light-emitting diodes (QLEDs). Specifically, we use primary amine-functionalized dendrimer ligands (e.g., a series of poly(amidoamine) dendrimers (PADs, also referred to PAMAM)) that bind to the surface of QDs by replacing the native ligands (oleic acids) and also to the surface of ZnO ETL. PAD ligands control the electron injection rate from ZnO ETL into QDs by altering the electronic energy levels of the surface of ZnO ETL and thereby improve the charge balance within QDs in devices, leading to the enhancement of the device efficiency. As an ultimate achievement, the device efficiency (peak external quantum efficiency) improves by a factor of 3 by replacing the native ligands (3.86%) with PAD ligands (11.36%). In addition, multibranched dendrimer ligands keep the QD emissive layer intact during subsequent solution processing, enabling us to accomplish solution-processed QLEDs. The approach and results in the present study emphasize the importance of controlling the ligands of QDs to enhance QLED performance and also offer simple yet effective chemical mean toward all-solution-processed QLEDs.

Colloidal Spherical Quantum Wells with Near-Unity Photoluminescence Quantum Yield and Suppressed Blinking.

Thick inorganic shells endow colloidal nanocrystals (NCs) with enhanced photochemical stability and suppression of photoluminescence intermittency (also known as blinking). However, the progress of using thick-shell heterostructure NCs in applications has been limited due to the low photoluminescence quantum yield (PL QY ≤ 60%) at room temperature. Here, we demonstrate thick-shell NCs with CdS/CdSe/CdS seed/spherical quantum well/shell (SQW) geometry that exhibit near-unity PL QY at room temperature and suppression of blinking. In SQW NCs, the lattice mismatch is diminished between the emissive CdSe layer and the surrounding CdS layers as a result of coherent strain, which suppresses the formation of misfit defects and consequently permits ∼100% PL QY for SQW NCs with a thick CdS shell (≥5 nm). High PL QY of thick-shell SQW NCs is preserved even in concentrated dispersion and in film under thermal stress, which makes them promising candidates for applications in solid-state lightings and luminescent solar concentrators.