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.

Controlling the Phase Distribution of Single Bromide Quasi-2-Dimensional Perovskite Crystals via Solvent Engineering for Pure-Blue Light-Emitting Diodes.

To achieve pure-blue emission (460-470 nm), we manipulate the crystallization process of the quasi-2D perovskite, (PBA)2Csn-1PbnBr3n+1, prepared by a solution process. The strategy involves controlling the distribution of "n" phases with different bandgaps, solely utilizing changes in the precursor's supersaturation to ensure that the desired emission aligns with the smallest bandgap. Adjustments in photoluminescence (PL) wavelength are made by changing the solute concentration and solvent polarity, as these factors heavily influence the diffusion of cations, a crucial determinant for the value of "n". Subsequently, we enhance the PL quantum yield from 31 to 51% at 461 nm using trioctylphosphine oxide (TOPO) as an additive of antisolvent, which passivates halide vacancy and promotes orderly crystal growth, leading to faster carrier transfer between phases. With these strategies, we successfully demonstrate pure-blue LEDs with a turn-on voltage of 3.3 V and an external quantum efficiency of 5.5% at an emission peak of 470 nm with a full-width at half-maximum of 31 nm.

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.

Oligomeric Zinc Thiolates Tethering Multidentate Carboxylates for Nondestructive Aqueous Phase Transfer of Quantum Dots.

Functionalization of quantum dots (QDs) via ligand exchange is prone to debase their photoluminescence quantum yield (PL QY) owing to the unavoidable surface damage by excess reactants, and even worse in aqueous medium. Herein, the oligomeric zinc thiolate as the multidentate hydrophilic ligand featuring facile synthetic protocol is proposed. A simple reaction between ZnCl2 and 3-mercaptopropionic acid produces oligomeric ligands containing 3-6 zinc thiolate units, where the terminal moieties provide multidentate anchoring to the surface as well as hydrophilicity. 2D proton nuclear Overhauser effect spectroscopy (2D 1 H NOESY) and X-ray photoelectron spectroscopy (XPS) reveal that the oligomeric zinc thiolate ligands adsorb on the surface via multidentate metal carboxylate bindings without destruction of molecular structure, regardless of partial dissociation of thiolate branches in aqueous phase. Enhanced binding affinity granted by the multidentate nature allows for the effective exchange of original surface ligands without considerable surface deterioration. The zinc thiolate-capped Cd-free aqueous QDs exhibit a high photoluminescence quantum yield of ≈90% and extended stability against long-term storage and photochemical stress.

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).

Smart Materials with Dual Functionality: Repeatable Damage-Detection and Self-Healing.

Polymer materials are extensively used because of their excellent performance; however, when used for a long time, they break and eventually lose their original properties. Thus, smart polymer materials that can repeatedly detect and repair damage must be urgently developed to increase their durability and lifespan. In this study, a smart material with dual functionality (damage-detection and self-healing) is developed via a facile method of incorporating spiropyran (SP) beads, which exhibit changes in color and fluorescence when damaged, into a Diels-Alder (DA)-based self-healing matrix. When polyurethane (PU) is added to the DA-based matrix, the dual functionality exhibits a strong dependence on the proportion of PU. Because the PU ratio affects two opposing factors (damaged area and load-bearing capacity), the damage-detecting ability exhibits the best performance at 40 wt % PU, where both factors are optimized. A high healing efficiency of 96% is achieved via a dynamic DA reaction. In particular, the repeatability of the dual-functionality is successfully attained through the reversibility of the SP beads and DA networks, where the detection and healing efficiencies are reduced by 15 and 23%, respectively, after 10 cycles. Furthermore, the reprocessed fractured specimens exhibit excellent recyclability.

Linearly polarized emission from CdSe/CdS core-in-rod nanostructures: Effects of core position.

Semiconductor nanocrystals with an anisotropic morphology exhibit unique properties, most notably their linear polarization. The colloidal growth of semiconductor nanorods with core dots inside, also referred to as dot-in-rod (DIR) structure, has enabled the synthesis of anisotropic nanocrystals with better stability and controllable fluorescence polarization. In this study, we synthesize CdSe/CdS DIR nanocrystals, in which the position of the CdSe core particle can be controlled by using different ligand compositions during the CdS growth. Varying the core position within the DIR structure, e.g., from the center to the end of the DIR particles, results in a change in the degree of linear polarization. When the core is positioned at the center of the nanorod, the linear polarization turns out to be higher compared with tip-core DIRs. Time-resolved photoluminescence analysis reveals that the center-core DIRs have higher electron-hole interaction than tip-core DIRs because of weak uniaxial strain in center-core DIR that arises from lattice dislocations at the interface to relieve accumulated strain.

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.

Photodynamic treatment of multidrug-resistant bacterial infection using indium phosphide quantum dots.

Infections caused by multidrug-resistant (MDR) bacteria pose an impending threat to humanity, as the evolution of MDR bacteria outpaces the development of effective antibiotics. In this work, we use indium phosphide (InP) quantum dots (QDs) to treat infections caused by MDR bacteria via photodynamic therapy (PDT), which shows superior bactericidal efficiency over common antibiotics. PDT in the presence of InP QDs results in high-efficiency bactericidal activity towards various bacterial species, including Staphylococcus aureus, Bacillus cereus, Escherichia coli and Pseudomonas aeruginosa. Upon light absorption, InP QDs generate superoxide (O2˙-), which leads to efficient and selective killing of MDR bacteria while mammalian cells remain intact. The cytotoxicity evaluation reveals that InP QDs are bio- and blood-compatible in a wide therapeutic window. For the in vivo study, we drop a solution of InP QDs at a concentration within the therapeutic window onto MDR S. aureus-infected skin wounds of mice and perform PDT for 15 min. InP QDs show excellent therapeutic and prophylactic efficacy in treating MDR bacterial infection. These findings show that InP QDs have great potential to serve as antibacterial agents for MDR bacterial infection treatment, as an effective and complementary alternative to conventional antibiotics.

Nondestructive Photopatterning of Heavy-Metal-Free Quantum Dots.

Electroluminescence from quantum dots (QDs) is a suitable photon source for futuristic displays offering hyper-realistic images with free-form factors. Accordingly, a nondestructive and scalable process capable of rendering multicolored QD patterns on a scale of several micrometers needs to be established. Here, nondestructive direct photopatterning for heavy-metal-free QDs is reported using branched light-driven ligand crosslinkers (LiXers) containing multiple azide units. The branched LiXers effectively interlock QD films via photo-crosslinking native aliphatic QD surface ligands without compromising the intrinsic optoelectronic properties of QDs. Using branched LiXers with six sterically engineered azide units, RGB QD patterns are achieved on the micrometer scale. The photo-crosslinking process does not affect the photoluminescence and electroluminescence characteristics of QDs and extends the device lifetime. This nondestructive method can be readily adapted to industrial processes and make an immediate impact on display technologies, as it uses widely available photolithography facilities and high-quality heavy-metal-free QDs with aliphatic ligands.

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.

Efficient, Selective CO2 Photoreduction Enabled by Facet-Resolved Redox-Active Sites on Colloidal CdS Nanosheets.

Advances in nanotechnology have enabled precise design of catalytic sites for CO2 photoreduction, pushing product selectivity to near unity. However, activity of most nanostructured photocatalysts remains underwhelming due to fast recombination of photogenerated electron-hole pairs and sluggish hole transfer. To address these issues, we construct colloidal CdS nanosheets (NSs) with the large basal planes terminated by S2- atomic layers as intrinsic photocatalysts (CdS-S2- NSs). Experimental investigation reveals that the S2- termination endows ultrathin CdS-S2- NSs with facet-resolved redox-catalytic sites: oxidation occurs on S2--terminated large basal facets and reduction happens on side facets. Such an allocation of redox sites not only promotes spatial separation of photoinduced electrons and holes but also facilitates balanced extraction of holes and electrons by shortening the hole diffusion distance along the (001) direction of the ultrathin NSs. Consequently, the CdS-S2- NSs exhibit superb performance for photocatalytic CO2-to-CO conversion, which was verified by the isotope-labeled experiments to be a record-breaking performance: a CO selectivity of 99%, a CO formation rate of 2.13 mol g-1 h-1, and an effective apparent quantum efficiency of 42.1% under the irradiation (340 to 450 nm) of a solar simulator (AM 1.5G). The breakthrough performance achieved in this work provides novel insights into the precise design of nanostructures for selective and efficient CO2 photoreduction.

Light-Driven Ammonia Production by Azotobacter vinelandii Cultured in Medium Containing Colloidal Quantum Dots.

There is an evergrowing demand for environment-friendly processes to synthesize ammonia (NH3) from atmospheric nitrogen (N2). Although diazotrophic N2 fixation represents an undeniably "green" process of NH3 synthesis, the slow reaction rate makes it less suitable for industrially meaningful large-scale production. Here, we report the photoinduced N2 fixation using a hybrid system composed of colloidal quantum dots (QDs) and aerobic N2-fixing bacteria, Azotobacter vinelandii. Compared to the case where A. vinelandii cells are simply mixed with QDs, NH3 production increases significantly when A. vinelandii cells are cultured in the presence of core/shell InP/ZnSe QDs. During the cell culture of A. vinelandii, the cellular uptake of QDs is facilitated in the exponential growth phase. Experimental results as well as theoretical calculations indicate that the photoexcited electrons in QDs within A. vinelandii cells are directly transferred to MoFe protein, the catalytic component of nitrogenase. We also observe that the excess amount of QDs left on the outer surface of A. vinelandii disrupts the cellular membrane, leading to the decrease in NH3 production due to the deactivation of nitrogenase. The successful uptake of QDs in QD-A. vinelandii hybrid with minimal amount of QDs on the outer surface of the bacteria is key to efficient photosensitized NH3 production. The comprehensive understanding of the QD-bacteria interface paves an avenue to novel and efficient nanobiohybrid systems for chemical production.

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.

Surface state-induced barrierless carrier injection in quantum dot electroluminescent devices.

The past decade has witnessed remarkable progress in the device efficiency of quantum dot light-emitting diodes based on the framework of organic-inorganic hybrid device structure. The striking improvement notwithstanding, the following conundrum remains underexplored: state-of-the-art devices with seemingly unfavorable energy landscape exhibit barrierless hole injection initiated even at sub-band gap voltages. Here, we unravel that the cause of barrierless hole injection stems from the Fermi level alignment derived by the surface states. The reorganized energy landscape provides macroscopic electrostatic potential gain to promote hole injection to quantum dots. The energy level alignment surpasses the Coulombic attraction induced by a charge employed in quantum dots which adjust the local carrier injection barrier of opposite charges by a relatively small margin. Our finding elucidates how quantum dots accommodate barrierless carrier injection and paves the way to a generalized design principle for efficient electroluminescent devices employing nanocrystal emitters.

Polarized Electroluminescence Emission in High-Performance Quantum Rod Light-Emitting Diodes via the Langmuir-Blodgett Technique.

Due to their anisotropic structure, quantum rods (QRs) feature unique properties that differ from quantum dots, such as suppression of non-radiative Auger recombination and linearly polarized light emission. Despite many potential advantages, the progress of QR-based light-emitting diodes (QR-LEDs) is left behind due to the difficulty in aligning QRs. In this study, polarized electroluminescence emission is reported in high-performance QR-LEDs by employing the Langmuir-Blodgett (LB) technique. The adoption of the LB technique successfully produces a highly dense and smooth QR film with a high degree of alignment. As a result, the aligned QR films exhibit polarized photoluminescence emission with a degree of linear polarization of 2.1. Advantageous features of the LB technique, such as nondestructiveness, precise thickness control, and the nonnecessity of an additional matrix material, allow to fabricate QR-LEDs with the same procedure as the standard spin coating-based scheme. The device is fabricated via the LB technique, which shows excellent device performance, such as the low turn-on voltage of 1.8 V, peak luminance of 56 287 cd m-2 , and peak external quantum efficiency (EQE) of 10.33%. Furthermore, these devices clearly exhibit an indication of polarized electroluminescence emission, which opens new opportunities for QRs in display technologies.

Fluorescence Switchable Block Copolymer Particles with Doubly Alternate-Layered Nanoparticle Arrays.

The precise self-assembly of block copolymers (BCPs) and inorganic nanoparticles (NPs) under 3D confinement offers microparticles with programmable nanostructures and functionalities. Here, fluorescence-switchable hybrid microspheres are developed by forming doubly alternating arrays of Au NPs and CdSe/ZnS quantum dots (QDs) within polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) BCP domains. These doubly alternating arrays afford controlled nonradiative energy transfer (NRET) between the QDs and Au NPs that is dependent on the layer-to-layer distance. Solvent-selective swelling of the hybrid particles tunes the distance between layers, modulating their NRET behavior and affording switchable fluorescence. The particle fluorescence is "OFF" in water through strong NRET from the QDs to Au NPs, but is "ON" in alcohols due to the increased distance between the Au NP and QD arrays in the swollen P4VP domains. The experimentally observed NRET intensity as a function of interparticle distance shows larger quenching efficiencies than those theoretically predicted due to the enhanced quenching within a 3D-confined system. Finally, the robust and reversible fluorescence switching of the hybrid particles in different solvents is demonstrated, highlighting their potentials for bioimaging, sensing, and diagnostic applications.

Softness- and Size-Dependent Packing Symmetries of Polymer-Grafted Nanoparticles.

Achieving ordered arrays of nanoparticles (NPs) with controlled packing symmetry and interparticle spacing is of great importance to design complex metamaterials. Herein, we report softness- and size-dependent self-assembly behavior of polystyrene-grafted Au NPs (Au@PS NPs). We varied the core size of Au NPs from 1.9 to 9.6 nm and the number-average molecular weight (Mn) of thiol-terminated polystyrene from 1.8 to 7.9 kg mol-1. The optimal packing model based on an "effective softness" parameter λeff that accounts for close-packed and semidilute brush regimes could predict the effective radius of Au@PS NPs (within ±9%) for a wide range of PS Mn, grafting density, and Au core size. With increasing λeff, the self-assembled Au@PS NP superlattices undergo a symmetry transition from hexagonal close packed (hcp) to body-centered tetragonal (bct) to body-centered cubic (bcc). This work demonstrates the effective softness model as a simple but robust tool for the design of NP superlattices with precisely controlled interparticle distance and packing symmetry, both of which are critical for the development of sophisticated materials through control of nanoscale structure.