Observation of negative surface and interface energies of quantum dots.

Surface energy is a fundamental property of materials and is particularly important in describing nanomaterials where atoms or molecules at the surface constitute a large fraction of the material. Traditionally, surface energy is considered to be a positive quantity, where atoms or molecules at the surface are less thermodynamically stable than their counterparts in the interior of the material because they have fewer bonds or interactions at the surface. Using calorimetric methods, we show that the surface energy is negative in some prototypical colloidal semiconductor nanocrystals, or quantum dots with organic ligand coatings. This implies that the surface atoms are more thermodynamically stable than those on the interior due to the strong bonds between these atoms and surfactant molecules, or ligands, that coat their surface. In addition, we extend this work to core/shell indium phosphide/zinc sulfide nanocrystals and show that the interfacial energy between these materials is highly thermodynamically favorable in spite of their large lattice mismatch. This work challenges many of the assumptions that have guided thinking about colloidal nanomaterial thermodynamics, investigates the fundamental stability of many technologically relevant colloidal nanomaterials, and paves the way for future experimental and theoretical work on nanocrystal thermodynamics.

Boosting exciton dissociation and charge transfer in CsPbBr3 QDs via ferrocene derivative ligation for CO2 photoreduction.

Photo-catalytic CO2 reduction with perovskite quantum dots (QDs) shows potential for solar energy storage, but it encounters challenges due to the intricate multi-electron photoreduction processes and thermodynamic and kinetic obstacles associated with them. This study aimed to improve photo-catalytic performance by addressing surface barriers and utilizing multiple-exciton generation in perovskite QDs. A facile surface engineering method was employed, involving the grafting of ferrocene carboxylic acid (FCA) onto CsPbBr3 (CPB) QDs, to overcome limitations arising from restricted multiple-exciton dissociation and inefficient charge transfer dynamics. Kelvin Probe Force Microscopy and XPS spectral confirmed successfully creating an FCA-modulated microelectric field through the Cs active site, thus facilitating electron transfer, disrupting surface barrier energy, and promoting multi-exciton dissociations. Transient absorption spectroscopy showed enhanced charge transfer and reduced energy barriers, resulting in an impressive CO2-to-CO conversion rate of 132.8 µmol g-1 h-1 with 96.5% selectivity. The CPB-FCA catalyst exhibited four-cycle reusability and 72 h of long-term stability, marking a significant nine-fold improvement compared to pristine CPB (14.4 µmol g-1 h-1). These results provide insights into the influential role of FCA in regulating intramolecular charge transfer, enhancing multi-exciton dissociation, and improving CO2 photoreduction on CPB QDs. Furthermore, these findings offer valuable knowledge for controlling quantum-confined exciton dissociation to enhance CO2 photocatalysis.

Molecular surface programming of rectifying junctions between InAs colloidal quantum dot solids.

Heavy-metal-free III-V colloidal quantum dots (CQDs) show promise in optoelectronics: Recent advancements in the synthesis of large-diameter indium arsenide (InAs) CQDs provide access to short-wave infrared (IR) wavelengths for three-dimensional ranging and imaging. In early studies, however, we were unable to achieve a rectifying photodiode using CQDs and molybdenum oxide/polymer hole transport layers, as the shallow valence bandedge (5.0 eV) was misaligned with the ionization potentials of the widely used transport layers. This occurred when increasing CQD diameter to decrease the bandgap below 1.1 eV. Here, we develop a rectifying junction among InAs CQD layers, where we use molecular surface modifiers to tune the energy levels of InAs CQDs electrostatically. Previously developed bifunctional dithiol ligands, established for II-VI and IV-VI CQDs, exhibit slow reaction kinetics with III-V surfaces, causing the exchange to fail. We study carboxylate and thiolate binding groups, united with electron-donating free end groups, that shift upward the valence bandedge of InAs CQDs, producing valence band energies as shallow as 4.8 eV. Photophysical studies combined with density functional theory show that carboxylate-based passivants participate in strong bidentate bridging with both In and As on the CQD surface. The tuned CQD layer incorporated into a photodiode structure achieves improved performance with EQE (external quantum efficiency) of 35% (>1 µm) and dark current density < 400 nA cm-2, a >25% increase in EQE and >90% reduced dark current density compared to the reference device. This work represents an advance over previous III-V CQD short-wavelength IR photodetectors (EQE < 5%, dark current > 10,000 nA cm-2).

A de novo protein catalyzes the synthesis of semiconductor quantum dots.

De novo proteins constructed from novel amino acid sequences are distinct from proteins that evolved in nature. Construct K (ConK) is a binary-patterned de novo designed protein that rescues Escherichia coli from otherwise toxic concentrations of copper. ConK was recently found to bind the cofactor PLP (pyridoxal phosphate, the active form of vitamin B6). Here, we show that ConK catalyzes the desulfurization of cysteine to H2S, which can be used to synthesize CdS nanocrystals in solution. The CdS nanocrystals are approximately 3 nm, as measured by transmission electron microscope, with optical properties similar to those seen in chemically synthesized quantum dots. The CdS nanocrystals synthesized using ConK have slower growth rates and a different growth mechanism than those synthesized using natural biomineralization pathways. The slower growth rate yields CdS nanocrystals with two desirable properties not observed during biomineralization using natural proteins. First, CdS nanocrystals are predominantly of the zinc blende crystal phase; this is in stark contrast to natural biomineralization routes that produce a mixture of zinc blende and wurtzite phase CdS. Second, in contrast to the growth and eventual precipitation observed in natural biomineralization systems, the CdS nanocrystals produced by ConK stabilize at a final size. Future optimization of CdS nanocrystal growth using ConK-or other de novo proteins-may help to overcome the limits on nanocrystal quality typically observed from natural biomineralization by enabling the synthesis of more stable, high-quality quantum dots at room temperature.

A prototype protein nanocage minimized from carboxysomes with gated oxygen permeability.

Protein nanocages (PNCs) in cells and viruses have inspired the development of self-assembling protein nanomaterials for various purposes. Despite the successful creation of artificial PNCs, the de novo design of PNCs with defined permeability remains challenging. Here, we report a prototype oxygen-impermeable PNC (OIPNC) assembled from the vertex protein of the ß-carboxysome shell, CcmL, with quantum dots as the template via interfacial engineering. The structure of the cage was solved at the atomic scale by combined solid-state NMR spectroscopy and cryoelectron microscopy, showing icosahedral assembly of CcmL pentamers with highly conserved interpentamer interfaces. Moreover, a gating mechanism was established by reversibly blocking the pores of the cage with molecular patches. Thus, the oxygen permeability, which was probed by an oxygen sensor inside the cage, can be completely controlled. The CcmL OIPNC represents a PNC platform for oxygen-sensitive or oxygen-responsive storage, catalysis, delivery, sensing, etc.

Memristive Switching Mechanism in Colloidal InP/ZnSe/ZnS Quantum Dot-Based Synaptic Devices for Neuromorphic Computing.

Quantum dots (QDs) have garnered a significant amount of attention as promising memristive materials owing to their size-dependent tunable bandgap, structural stability, and high level of applicability for neuromorphic computing. Despite these advantageous properties, the development of QD-based memristors has been hindered by challenges in understanding and adjusting the resistive switching (RS) behavior of QDs. Herein, we propose three types of InP/ZnSe/ZnS QD-based memristors to elucidate the RS mechanism, employing a thin poly(methyl methacrylate) layer. This approach not only allows us to identify which carriers (electron or hole) are trapped within the QD layer but also successfully demonstrates QD-based synaptic devices. Furthermore, to utilize the QD memristor as a synapse, long-term potentiation/depression (LTP/LTD) characteristics are measured, resulting in a low nonlinearity of LTP/LTD at 0.1/1. On the basis of the LTP/LTD characteristics, single-layer perceptron simulations were performed using the Extended Modified National Institute of Standards and Technology, verifying a maximum recognition rate of 91.46%.

Metal Fluorides Passivate II-VI and III-V Quantum Dots.

Quantum dots (QDs) with metal fluoride surface ligands were prepared via reaction with anhydrous oleylammonium fluoride. Carboxylate terminated II-VI QDs underwent carboxylate for fluoride exchange, while InP QDs underwent photochemical acidolysis yielding oleylamine, PH3, and InF3. The final photoluminescence quantum yield (PLQY) reached 83% for InP and near unity for core-shell QDs. Core-only CdS QDs showed dramatic improvements in PLQY, but only after exposure to air. Following etching, the InP QDs were bound by oleylamine ligands that were characterized by the frequency and breadth of the corresponding ν(N-H) bands in the infrared absorption spectrum. The fluoride content (1.6-9.2 nm-2) was measured by titration with chlorotrimethylsilane and compared with the oleylamine content (2.3-5.1 nm-2) supporting the formation of densely covered surfaces. The influence of metal fluoride adsorption on the air stability of QDs is discussed.

Superexchange Mechanism in Coupled Triangulenes Forming Spin-1 Chains.

We show that the origin of the antiferromagnetic coupling in spin-1 triangulene chains, which were recently synthesized and measured by Mishra et al. ( Nature 2021, 598, 287-292), originates from a superexchange mechanism. This process, mediated by intertriangulene states, opens the possibility to control parameters in the effective bilinear-biquadratic spin model. We start from the derivation of an effective tight-binding model for triangulene chains using a combination of tight-binding and Hartree-Fock methods fitted to hybrid density functional theory results. Next, correlation effects are investigated within the configuration interaction method. Our low-energy many-body spectrum for NTr = 2 and NTr = 4 triangulene chains agree well with the bilinear-biquadratic spin-1 chain antiferromagnetic model when indirect coupling processes and superexchange coupling between triangulene spins are taken into account.

Improving Three-Photon Fluorescence of Near-Infrared Quantum Dots for Deep Brain Imaging by Suppressing Biexciton Decay.

Three-photon fluorescence microscopy (3PFM) is a promising brain research tool with submicrometer spatial resolution and high imaging depth. However, only limited materials have been developed for 3PFM owing to the rigorous requirement of the three-photon fluorescence (3PF) process. Herein, under the guidance of a band gap engineering strategy, CdTe/CdSe/ZnS quantum dots (QDs) emitting in the near-infrared window are designed for constructing 3PF probes. The formation of type II structure significantly increased the three-photon absorption cross section of QDs and caused the delocalization of electron-hole wave functions. The time-resolved transient absorption spectroscopy confirmed that the decay of biexcitons was significantly suppressed due to the appropriate band gap alignment, which further enhanced the 3PF efficiency of QDs. By utilizing QD-based 3PF probes, high-resolution 3PFM imaging of cerebral vasculature was realized excited by a 1600 nm femtosecond laser, indicating the possibility of deep brain imaging with these 3PF probes.

Manipulations of Electronic and Spin States in Co-Quantum Dot/WS2 Heterostructure on a Metal-Dielectric Composite Substrate by Controlling Interfacial Carriers.

Charge and spin are two intrinsic attributes of carriers governing almost all of the physical processes and operation principles in materials. Here, we demonstrate the manipulation of electronic and spin states in designed Co-quantum dot/WS2 (Co-QDs/WS2) heterostructures by employing a metal-dielectric composite substrate and via scanning tunneling microscope. By repeatedly scanning under a unipolar bias, switching the bias polarity, or applying a pulse through nonmagnetic or magnetic tips, the Co-QDs morphologies exhibit a regular and reproducible transformation between bright and dark dots. First-principles calculations reveal that these tunable characters are attributed to the variation of density of states and the transition of magnetic anisotropy energy induced by carrier accumulation. It also suggests that the metal-dielectric composite substrate is successful in creating the interfacial potential for carrier accumulation and realizes the electrically controllable modulations. These results will promote the exploration of electron-matter interactions in quantum systems and provide an innovative way to facilitate the development of spintronics.

Competition of Moiré Network Sites to Form Electronic Quantum Dots in Reconstructed MoX2/WX2 Heterostructures.

Twisted bilayers of two-dimensional semiconductors offer a versatile platform for engineering quantum states for charge carriers using moiré superlattice effects. Among the systems of recent interest are twistronic MoX2/WX2 heterostructures (X = Se or S), which undergo reconstruction into preferential stacking domains and highly strained domain wall networks, determining the electron/hole localization across moiré superlattices. Here, we present a catalogue of options for the formation of self-organized quantum dots and wires in lattice-reconstructed marginally twisted MoX2/WX2 bilayers with a relative lattice mismatch δ ≪ 1 for twist angles ranging from perfect alignment to θ ∼ 1°. On the basis of multiscale modeling taking into account twirling of domain wall networks, we analyze bilayers with both parallel and antiparallel orientations of their unit cells and describe crossovers between different positioning of band edges for electrons and holes across moiré superlattices when θ < δ and θ > δ.

Synergism in Binary Nanocrystals Enables Top-Illuminated HgTe Colloidal Quantum Dot Short-Wave Infrared Imager.

Thanks to their tunable infrared absorption, solution processability, and low fabrication costs, HgTe colloidal quantum dots (CQDs) are promising for optoelectronic devices. Despite advancements in device design, their potential for imaging applications remains underexplored. For integration with Si-based readout integrated circuits (ROICs), top illumination is necessary for simultaneous light absorption and signal acquisition. However, most high-performing traditional HgTe CQD photodiodes are p-on-n stack and bottom-illuminated. Herein, we report top-illuminated inverted n-on-p HgTe CQD photodiodes using a robust p-type CQD layer and a thermally evaporated Bi2S3 electron transport layer. The p-type CQD solid is achieved by exploring the synergism in binary HgTe and Ag2Te CQDs. These photodetectors show a room-temperature detectivity of 3.4 × 1011 jones and an EQE of ∼44% at ∼1.7 µm wavelength, comparable to the p-on-n HgTe CQD photodiodes. A top-illuminated HgTe CQD short-wave infrared imager (640 × 512 pixels) was fabricated, demonstrating successful infrared imaging.

Coherently and Incoherently Pumped Telecom C-Band Single-Photon Source with High Brightness and Indistinguishability.

Long-range, terrestrial quantum networks require high-brightness single-photon sources emitting in the telecom C-band for maximum transmission rates. For solid-state quantum emitters, the underlying pumping process, i.e., coherent or incoherent excitation schemes, impacts several photon properties such as photon indistinguishability, single-photon purity, and photon number coherence. These properties play a major role in quantum communication applications, the latter in particular for quantum cryptography. Here, we present a versatile telecom C-band single-photon source that is operated coherently and incoherently using two complementary pumping schemes. The source is based on a quantum dot coupled to a circular Bragg grating cavity, whereas coherent (incoherent) operation is performed via the novel SUPER scheme (phonon-assisted excitation). In this way, high end-to-end-efficiencies (ηend) of 5.36% (6.09%) are achieved simultaneously with a small multiphoton contribution g(2)(0) of 0.076 ± 0.001 [g(2)(0) of 0.069 ± 0.001] for coherent (incoherent) operation.

Electric Dipole Coupling of a Bilayer Graphene Quantum Dot to a High-Impedance Microwave Resonator.

We implement circuit quantum electrodynamics (cQED) with quantum dots in bilayer graphene, a maturing material platform that can host long-lived spin and valley states. Our device combines a high-impedance (Zr ≈ 1 kΩ) superconducting microwave resonator with a double quantum dot electrostatically defined in a graphene-based van der Waals heterostructure. Electric dipole coupling between the subsystems allows the resonator to sense the electric susceptibility of the double quantum dot from which we reconstruct its charge stability diagram. We achieve sensitive and fast detection of the interdot transition with a signal-to-noise ratio of 3.5 within 1 µs integration time. The charge-photon interaction is quantified in the dispersive and resonant regimes by comparing the resonator response to input-output theory, yielding a coupling strength of g/2π = 49.7 MHz. Our results introduce cQED as a probe for quantum dots in van der Waals materials and indicate a path toward coherent charge-photon coupling with bilayer graphene quantum dots.

Designable Nanoadaptor for Enhanced Recognition of Natural Killer Cell to Tumor via Bio-orthogonal Click Reaction.

Highly efficient recognition of cancer cells by immune cells is important for successful therapeutic-cell-based cancer immunotherapy. Herein, we present a facile NIR-II nanoadaptor [hyaluronic acid (HA)/dibenzocyclooctyne (DBCO)-Au:Ag2Te quantum dots (QDs)] for enhancing the tumor recognition and binding ability of natural killer (NK) cells via a bio-orthogonal click reaction in vivo. The Nanoadaptor possesses superior tumor-targeting capacity, facilitating the accumulation of the chemical receptor DBCO at the tumor sites. Subsequently, the enrichment of DBCO on tumor cell surfaces provides multivalent recognition sites for capturing pretreated azide engineered NK92 cells (NK92-N3) through an efficient click reaction, thereby significantly enhancing the therapeutical efficiency. The dynamic process of nanoadaptor-mediated recognition of NK cells to tumor cells could be vividly observed using multiplexed NIR-II fluorescence imaging in a mouse model of lung cancer. Such a nanoadaptor strategy can be extended to other therapeutic cellular systems and holds promise for future clinical applications.

AFM-IR of Electrohydrodynamically Printed PbS Quantum Dots: Quantifying Ligand Exchange at the Nanoscale.

Colloidal quantum dots (cQDs), semiconductor materials with widely tunable properties, can be printed in submicrometer patterns through electrohydrodynamic printing, avoiding aggressive photolithography steps. Postprinting ligand exchange determines the final optoelectronic properties of the cQD structures. However, achieving a complete bulk exchange is challenging, and the conventional vibrational analysis lacks the required spatial resolution. Infrared nanospectroscopy enables quantitative analysis of vibrational signals and structural topography on the nanometer scale upon ligand substitution on lead sulfide cQDs. A solution of ethanedithiol led to rapid (∼60 s) exchange of ≤90% of the ligands, in structures up to ∼750 nm thick. Prolonged exposures (>1 h) caused the degradation of the microstructures, with a systematic removal of cQDs regulated by surface:bulk ratios and solvent interactions. This study establishes a method for the development of devices through a combination of tunable photoactive materials, additive manufacturing of microstructures, and their quantitative nanometer-scale analysis.

Unveiling the 3D Morphology of Epitaxial GaAs/AlGaAs Quantum Dots.

Strain-free GaAs/AlGaAs semiconductor quantum dots (QDs) grown by droplet etching and nanohole infilling (DENI) are highly promising candidates for the on-demand generation of indistinguishable and entangled photon sources. The spectroscopic fingerprint and quantum optical properties of QDs are significantly influenced by their morphology. The effects of nanohole geometry and infilled material on the exciton binding energies and fine structure splitting are well-understood. However, a comprehensive understanding of GaAs/AlGaAs QD morphology remains elusive. To address this, we employ high-resolution scanning transmission electron microscopy (STEM) and reverse engineering through selective chemical etching and atomic force microscopy (AFM). Cross-sectional STEM of uncapped QDs reveals an inverted conical nanohole with Al-rich sidewalls and defect-free interfaces. Subsequent selective chemical etching and AFM measurements further reveal asymmetries in element distribution. This study enhances the understanding of DENI QD morphology and provides a fundamental three-dimensional structural model for simulating and optimizing their optoelectronic properties.

Polarity-Mediated Antisolvent Control Enables Efficient Lanthanide-Based near-Infrared Perovskite LEDs.

Alloying lanthanide ions (Yb3+) into perovskite quantum dots (Yb3+:CsPb(Cl1-xBrx)3) is an effective method to achieve efficient near-infrared (NIR) luminescence (>950 nm). Increasing the Yb3+ alloying ratio in the perovskite matrix enhances the luminescence intensity of Yb3+ emission at 990 nm. However, high Yb3+ alloying (>15%) results in vacancy-induced inferior material stability. In this work, we developed a polarity-mediated antisolvent manipulation strategy to resolve the incompatibility between a high Yb3+ alloying ratio and inferior stability of Yb3+:CsPb(Cl1-xBrx)3. Precise control of solution polarity enables increased uniformity of the perovskite matrix with fewer trap densities. Employing this strategy, we obtain Yb3+:CsPb(Cl1-xBrx)3 with the highest Yb3+ alloying ratio of 30.2% and a 2-fold higher electroluminescence intensity at 990 nm. We lever the engineered Yb3+:CsPb(Cl1-xBrx)3 to fabricate NIR-LEDs, achieving a peak external quantum efficiency (EQE) of 8.5% at 990 nm: this represents the highest among perovskite NIR-LEDs with an emission wavelength above 950 nm.

Packaging Quantum Dots in Viral Particles via a Strep-tag II/Streptavidin System for Single-Virus Tracking.

Single-virus tracking provides a powerful tool for studying virus infection with high spatiotemporal resolution. Quantum dots (QDs) are used to label and track viral particles due to their brightness and photostability. However, labeling viral particles with QDs is not easy. We developed a new method for labeling viral particles with QDs by using the Strep-tag II/streptavidin system. In this method, QDs were site-specifically ligated to viral proteins in live cells and then packaged into viral-like particles (VLPs) of tick-borne encephalitis virus (TBEV) and Ebola virus during viral assembly. With TBEV VLP-QDs, we tracked the clathrin-mediated endocytic entry of TBEV and studied its intracellular dynamics at the single-particle level. Our Strep-tag II/streptavidin labeling procedure eliminates the need for BirA protein expression or biotin addition, providing a simple and general method for site-specifically labeling viral particles with QDs for single-virus tracking.

Polarized and Bright Telecom C-Band Single-Photon Source from InP-Based Quantum Dots Coupled to Elliptical Bragg Gratings.

Bright, polarized, and high-purity single-photon sources in telecom wavelengths are crucial components in long-distance quantum communication, optical quantum computation, and quantum networks. Semiconductor InAs/InP quantum dots (QDs) combined with photonic cavities provide a competitive path, leading to optimal single-photon sources in this range. Here, we demonstrate a bright and polarized single-photon source operating in the telecom C-band based on an elliptical Bragg grating (EBG) cavity. With a significant Purcell enhancement of 5.25 ± 0.05, the device achieves a polarization ratio of 0.986, a single-photon purity of g2(0) = 0.078 ± 0.016, and a single-polarized photon collection efficiency of ∼24% at the first lens (NA = 0.65) without blinking. These findings suggest that C-band QD-based single-photon sources are potential candidates for advancing quantum communication.