arXiv subjects

Uri Banin

Publications and source records attributed to Uri Banin.

At least 19 recordsLinked to original sources

arXiv2025

Heralded Emission Detection in InAs/ZnSe Quantum Dot Solids Using Time-Correlated Photons

Harnessing quantum correlations between photons is an emerging frontier in optical spectroscopy, yet experimental demonstrations have largely remained limited to molecular systems at room temperature. Here, we investigate heralded emission detection (HED) under continuous-wave entangled-photon excitation of near-infrared (NIR)-emitting colloidal III-V quantum dot (QD) solids at low temperatures. We demonstrate the advantages of superconducting nanowire single-photon detectors (SNSPDs) for high time resolution ($\sim$72 ps) and large-area NIR avalanche photodiodes (APDs) for high emission count rates ($\sim$2000 cps). Second-order photon-correlation analysis reveals exciton lifetimes and fine-structure energy splittings. These results establish NIR colloidal QDs as a bright, tunable model system for quantum-light spectroscopy and highlight their compatibility with optical cavities as a further experimental control parameter.

physics.optics↗

arXiv2025

Unraveling Size Dependent Bi- and Tri-exciton Characteristics in CdSe/CdS Core/Shell Quantum Dots via Ensemble Time Gated Heralded Spectroscopy

Multiexcitons (MXs) in quantum dots (QDs) manifest many body interactions under quantum confinement. Beyond this fundamental interest, MXs are of importance in numerous optoelectronic applications including QD lasing, light emitting diodes and photocatalysis. Yet, the strong interactions between MXs leading to rapid non-radiative decay introduce challenges for their characterization. While so far, the measurement techniques rely either on indirect methods or on single particle studies, herein we introduce a new method to study MXs in QD ensembles utilizing spectrally resolved time-gated heralded spectroscopy. With this approach we extract the biexciton binding energies in a series of CdSe/CdS QD ensembles of several core/shell sizes, manifesting a transition between attractive and repulsive exciton-exciton interactions. Additionally, for triexcitons, which involve occupation of two excitons in the 1s energy levels, as well as one exciton in the 1p energy levels, we address the open issues of isolating the spectra of the two triexciton pathways from one another and from high-order MXs, and extract the MX lifetimes. The measurements on ensembles provide high photon counts and low noise levels, and alongside the time-gated heralded approach thus enable the observation of MX characteristics that are difficult to resolve in single particle studies. The approach can be further implemented in the characterization of the energies and lifetimes of MXs in other QD systems to enable rapid characterization and understanding of the MX properties. Such insight bears relevance to optoelectronic applications ranging from lasing to electroluminescent devices to quantum light sources.

cond-mat.mes-hall↗

arXiv2024

The Role of Superlattice Phonons in Charge Localization Across Quantum Dot Arrays

Understanding charge transport in semiconductor quantum dot (QD) assemblies is important for developing the next generation of solar cells and light-harvesting devices based on QD technology. One of the key factors that governs the transport in such systems is related to the hybridization between the QDs. Recent experiments have successfully synthesized QD molecules, arrays, and assemblies by directly fusing the QDs, with enhanced hybridization leading to high carrier mobilities and coherent band-like electronic transport. In this work, we theoretically investigate the electron transfer dynamics across a finite CdSe-CdS core-shell QD array, considering up to seven interconnected QDs in one dimension. We find that, even in the absence of structural and size disorder, electron transfer can become localized by the emergent low-frequency superlattice vibrational modes when the connecting neck between QDs is narrow. On the other hand, we also identify a regime where the same vibrational modes facilitate coherent electron transport when the connecting necks are wide. Overall, we elucidate the crucial effects of electronic and superlattice symmetries and their couplings when designing high-mobility devices based on QD superlattices.

physics.chem-ph↗

arXiv2023

Zn-doped P-type InAs Nanocrystal Quantum Dots

Doped heavy metal-free III-V semiconductor nanocrystal quantum dots are of great interest both from the fundamental aspects of doping in highly confined structures, and from the applicative side of utilizing such building blocks in the fabrication of p-n homojunction devices. InAs nanocrystals, that are of particular relevance for short wave IR detection and emission applications, manifest heavy n-type character poising a challenge for their transition to p-type behavior. We present p-type doping of InAs nanocrystals with Zn-enabling control over the charge carrier type in InAs QDs field effect transistors. The post-synthesis doping reaction mechanism is studied for Zn precursors with varying reactivity. Successful p-type doping was achieved by the more reactive precursor, diethylzinc. Substitutional doping by Zn2+ replacing In3+ is established by X-ray absorption spectroscopy analysis. Furthermore, enhanced near IR photoluminescence is observed due to surface passivation by Zn as indicated from elemental mapping utilizing high resolution electron microscopy corroborated by X-ray photoelectron spectroscopy study. The demonstrated ability to control the carrier type, along with the improved emission characteristics, paves the way towards fabrication of optoelectronic devices active in the short wave IR region utilizing heavy-metal free nanocrystal building blocks.

cond-mat.mes-hall↗

arXiv2023

Photon Correlations in Colloidal Quantum Dot Molecules Controlled by the Neck Barrier

We investigate the charge re-distribution upon optical excitation of various necked homodimer CQDMs using single particle emission spectroscopy. By tuning the hybridization of the electron wavefunction at a fixed center-to-center distance through controlling the neck girth, we reveal two coupling limits. On one hand a connected-but-confined situation where neighbouring CQDs are weakly fused to each other manifesting a weak coupling regime, and on the other hand, a connected-and-delocalized situation, where the neck is filled beyond the facet size leading to a rod-like architecture manifesting strong-coupling. Either coupling regimes entrust distinct optical signatures clearly resolved at room temperature in terms of photoluminescence quantum yield, intensity time traces, lifetimes, and spectra of the neutral-exciton, charged-exciton, and biexciton states. The interplay between the radiative and non-radiative Auger decays of these states, turns emitted photons from the CQDMs in the weak-coupling regime highly bunched unlike CQD monomers, while the antibunching is regained at the strong-coupling regime. This behavior correlates with the hybridization energy being smaller than the thermal energy (kT approx. 25meV) at the weak-coupling limit (delta E approax.5-10meV), leading to exciton localization suppressing Auger decay. In the neck-filled architectures, the larger hybridization energy (delta E approx.20-30meV) leads to exciton delocalization while activating the fast charged and multi-exciton Auger decay processes. This work sets an analogy for the artificial molecule CQDMs with regular molecules, where the two distinct regimes of weak- and strong-coupling correspond to ionic- or covalent- type bonding, respectively.

cond-mat.mes-hall↗

arXiv2023

Spontaneous Patterning of Binary Ligand Mixtures on CdSe Nanocrystals: from Random to Janus Packing

Binary compositions of surface ligands are known to improve the colloidal stability and fluorescence quantum yield of nanocrystals (NCs), due to ligand-ligand interactions and surface organization. Herein, we follow the thermodynamics of a ligand exchange reaction of CdSe NCs with alkylthiols mixtures. The effects of ligand polarity and length difference on ligand packing were investigated using isothermal titration calorimetry (ITC). The thermodynamic signature of the formation of mixed ligand shells was observed. Correlating the experimental results with thermodynamic mixing models has allowed us to calculate the inter-chain interactions and infer the final ligand shell configuration. Our findings demonstrate that the small dimensions of the NCs and the subsequent increased interfacial region between dissimilar ligands, in contrast to macroscopic surfaces, allow the formation of a myriad of clustering patterns, controlled by the inter-ligand interactions. This work provides a fundamental understanding of the parameters determining the ligand shell structure and should help guide smart surface design toward NC-based applications.

cond-mat.soft↗

arXiv2023

Two Biexciton Types Coexisting in Coupled Quantum Dot Molecules

Coupled colloidal quantum dot molecules are an emerging class of nanomaterials, introducing new degrees of freedom for designing quantum dot-based technologies. The properties of multiply excited states in these materials are crucial to their performance as quantum light emitters but cannot be fully resolved by existing spectroscopic techniques. Here we study the characteristics of biexcitonic species, which represent a rich landscape of different configurations, such as segregated and localized biexciton states. To this end, we introduce an extension of Heralded Spectroscopy to resolve different biexciton species in the prototypical CdSe/CdS coupled quantum dot dimer system. We uncover the coexistence and interplay of two distinct biexciton species: A fast-decaying, strongly-interacting biexciton species, analogous to biexcitons in single quantum dots, and a long-lived, weakly-interacting species corresponding to two nearly-independent excitons separated to the two sides of the coupled quantum dot pair. The two biexciton types are consistent with numerical simulations, assigning the strongly-interacting species to two excitons localized at one side of the quantum dot molecule and the weakly-interacting species to excitons segregated to the two quantum dot molecule sides. This deeper understanding of multiply excited states in coupled quantum dot molecules can support the rational design of tunable single- or multiple-photon quantum emitters.

cond-mat.mes-hall↗

arXiv2023

Electric field induced color 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 color 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 centers, we present a novel electric-field induced instantaneous color switching effect. Reversible emission color 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 centers dictated by the electric-field and affected by the coupling strength. The quantum dot molecules manifesting two coupled emission centers may be tailored to emit distinct colors, opening the path for sensitive field sensing and color switchable devices such as a novel pixel design for displays or an electric field color tunable single photon source.

cond-mat.mes-hall↗

arXiv2022

Nonadiabatic to Adiabatic Transition of Electron Transfer in Colloidal Quantum Dot Molecules

Electron transfer is an important and fundamental process in chemistry, biology and physics, and has received significant attention in recent years. Perhaps one of the most intriguing questions concerns with the realization of the transitions between nonadiabatic and adiabatic regimes of electron transfer, as the coupling (hybridization) energy, $J$, between the donor and acceptor is varied. Here, using colloidal quantum dot molecules, a new class of coupled quantum dot dimers, we computationally demonstrate how the hybridization energy between the donor and acceptor quantum dots can be tuned by simply changing the neck dimensions and/or the quantum dot size. This provides a handle to tune the electron transfer from the nonadiabatic over-damped Marcus regime to the coherent adiabatic regime in a single system, without changing the reorganization energy, $λ$, or the typical phonon frequency, $ω_c$. We develop an atomistic model to account for several donor and acceptor states and how they couple to the lattice vibrations, and utilize the Ehrenfest mean-field mixed quantum-classical method to describe the charge transfer dynamics as the nonadiabatic parameter, $γ$, is varied. We find that charge transfer rates increase by several orders of magnitude as the system is driven to the coherent, adiabatic limit, even at elevated temperatures, and delineate the inter-dot and torsional acoustic modes that couple most strongly to the charge transfer reaction coordinate.

cond-mat.mes-hall↗

arXiv2022

Semiconductor Bowtie Nanoantenna from Coupled Colloidal Quantum Dot Molecules

Top-down fabricated nanoantenna architectures of both metallic and dielectric materials demonstrated powerful functionalities for Raman and fluorescence enhancement with relevance to single molecule sensing, while inducing directionality of chromophore emission with implications for single photon sources. Herein, we synthesize the smallest bowtie nanoantenna by selective tip-to-tip fusion of two tetrahedral colloidal quantum dots (CQDs) forming a dimer. While the tetrahedral monomers emit non-polarized light, the bowtie architecture manifests nanoantenna functionality of enhanced emission polarization along the bowtie axis as predicted theoretically and revealed by single particle spectroscopy. Theory also predicts the formation of an electric-field hotspot at the bowtie epicenter. This is utilized for selective light induced photocatalytic metal growth at that location, unlike growth on the free tips in dark conditions thus demonstrating the bowtie dimer functionality as a photochemical reaction center. Our findings pave a path for additional bottom-up bowtie architectures applicable in optics, sensing and photocatalysis.

cond-mat.mes-hall↗

arXiv2022

Complete mapping of interacting charging states in single coupled colloidal quantum dot molecules

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

cond-mat.mes-hall↗

arXiv2021

Neck Barrier Engineering in Quantum Dot Dimer Molecules via Intra-Particle Ripening

Coupled colloidal quantum dot (CQD) dimers represent a new class of artificial molecules composed of fused core/shell semiconductor nanocrystals. The electronic coupling and wavefunction hybridization is enabled by the formation of an epitaxial connection with a coherent lattice between the shells of the two neighboring quantum dots where the shell material and its dimensions dictate the quantum barrier characteristics for the charge carriers. Herein we introduce a colloidal approach to control the neck formation at the interface between the two CQDs in such artificial molecular constructs. This allows the tailoring of the neck barrier in pre-linked homodimers formed via fusion of multifaceted wurtzite CdSe/CdS CQDs. The effects of reaction time, temperature and excess ligands is studied. The neck filling process follows an intraparticle ripening mechanism at relatively mild reaction conditions while avoiding inter-particle ripening. The degree of surface ligand passivation plays a key role in activating the surface atom diffusion to the neck region. The degree of neck filling strongly depends also on the initial relative orientation of the two CQDs, where homonymous plane attachment allows for facile neck growth, unlike the case of heteronymous plane attachment. Upon neck-filling, the observed red-shift of the absorption and fluorescence measured both for ensemble and single dimers, is assigned to enhanced hybridization of the confined wavefunction in CQD dimer molecules, as supported by quantum calculations. The fine tuning of the particle interface introduced herein provides therefore a powerful tool to further control the extent of hybridization and coupling in CQD molecules.

cond-mat.mes-hall↗

arXiv2021

High-Sensitivity, High-Resolution Detection of Reactive Oxygen-Species Concentration Using NV Centers

Nitrogen-vacancy (NV) color centers in diamond have been demonstrated as useful magnetic sensors, in particular for measuring spin fluctuations, achieving high sensitivity and spatial resolution. These abilities can be used to explore various biological and chemical processes, catalyzed by Reactive Oxygen Species (ROS). Here we demonstrate a novel approach to measure and quantify Hydroxyl radicals with high spatial resolution, using the fluorescence difference between NV charged states. According to the results, the achieved NV sensitivity is $11 \pm 4 \frac{nM}{\sqrt Hz}$, realized in-situ without spin labels and localized to a volume of $\sim 10$ picoliter.

cond-mat.mes-hall↗

arXiv2021

InAs nanocrystals with robust p-type doping

Robust control over the carrier type is fundamental for the fabrication of nanocrystal-based optoelectronic devices, such as the p-n homojunction, but effective incorporation of impurities in semiconductor nanocrystals and its characterization is highly challenging due to their small size. Herein, InAs nanocrystals, post-synthetically doped with Cd, serve as a model system for successful p-type doping of originally n-type InAs nanocrystals, as demonstrated in field-effect transistors (FETs). Advanced structural analysis, using atomic resolution electron microscopy and synchrotron X-ray absorption fine structure spectroscopy reveal that Cd impurities reside near and on the nanocrystal surface acting as substitutional p-dopants replacing Indium. Commensurately, Cd-doped InAs FETs exhibited remarkable stability of their hole conduction, mobility, and hysteretic behavior over time when exposed to air, while intrinsic InAs NCs FETs were easily oxidized and their performance quickly declined. Therefore, Cd plays a dual role acting as a p-type dopant, and also protects the nanocrystals from oxidation, as evidenced directly by Xray photoelectron spectroscopy measurements of air-exposed samples of intrinsic and Cd doped InAs NCs films. This study demonstrates robust p-type doping of InAs nanocrystals, setting the stage for implementation of such doped nanocrystal systems in printed electronic devices.

cond-mat.mtrl-sci↗

arXiv2021

Surface versus Impurity Doping Contributions in InAs Nanocrystals Field Effect Transistor Performance

The electrical functionality of an array of semiconductor nanocrystals depends critically on the free carriers that may arise from impurity or surface doping. Herein, we used InAs nanocrystals thin films as a model system to address the relative contributions of these doping mechanisms by comparative analysis of as-synthesized and Cu-doped nanocrystal based field-effect transistor (FET) characteristics. By applying FET simulation methods used in conventional semiconductor FETs, we elucidate surface and impurity-doping contributions to the overall performance of InAs NCs based FETs. As-synthesized InAs nanocrystal-based FETs show n-type characteristics assigned to the contribution of surface electrons accumulation layer that can be considered as an actual electron donating doping level with specific doping density and is energetically located just below the conduction band. The Cu-doped InAs NCs FETs show enhanced n-type conduction as expected from the Cu impurities location as an interstitial n-dopant in InAs nanocrystals. The simulated curves reveal the additional contribution from electrons within an impurity sub-band close to the conduction band onset of the InAs NCs. The work therefore demonstrates the utility of the bulk FET simulation methodology also to NC-based FETs. It provides guidelines for control of doping of nanocrystal arrays separately from surface contributions and impurity doping in colloidal semiconductor NCs towards their future utilization as building blocks in bottom-up prepared optoelectronic devices.

cond-mat.mes-hall↗

arXiv2021

From Impurity Doping to Metallic Growth in Diffusion Doping: Properties and Structure of Ag Doped InAs Nanocrystals

Tuning of the electronic properties of pre-synthesized colloidal semiconductor nanocrystals (NCs) by doping plays a key role in the prospect of implementing them in printed electronics devices such as transistors, and photodetectors. While such impurity doping reactions have already been introduced, the understanding of the doping process, the nature of interaction between the impurity and host atoms, and the conditions affecting the solubility limit of impurities in nanocrystals are still unclear. Here, we used a post-synthesis diffusion based doping reaction to introduce Ag impurities into InAs NCs. Optical absorption spectroscopy along with analytical inductively coupled plasma mass-spectroscopy (ICP-MS) were used to present a two stage doping model consisting of a "doping region" and a "growth region", depending on the concentration of the impurities in the reaction vessel. X-ray absorption fine-structure (XAFS) spectroscopy was employed to determine the impurity location and correlate between the structural and electronic properties for different sizes of InAs NCs and dopant concentrations. The resulting structural model describes a heterogeneous system where the impurities initially dope the NC, by substituting for In atoms near the surface of the NC, until the "solubility limit" is reached, after which the rapid growth and formation of metallic structures are identified.

cond-mat.mtrl-sci↗

arXiv2021

Synthesis of InAs/CdSe/ZnSe Core/Shell1/Shell2 Structures with Bright and Stable Near-Infrared Fluorescence

A complex InAs/CdSe/ZnSe Core/Shell1/Shell2 (CSS) structure is synthesized, where the intermediate CdSe buffer layer decreases strain between the InAs core and the ZnSe outer shell. This structure leads to significantly improved fluorescence quantum yield as compared to previously prepared core/shell structures and enables growth of much thicker shells. The shell growth is done using a layer-by-layer method in which the shell cation and anion precursors are added sequentially allowing for excellent control and a good size distribution is maintained throughout the entire growth process. The CSS structure is characterized using transmission electron microscopy, as well as by X-ray diffraction and X-ray-photoelectron spectroscopy which provide evidence for shell growth. The quantum yield for CSS with small InAs cores reaches over 70% - exceptional photoluminescence intensity for III-V semiconductor nanocrystals. In larger InAs cores there is a systematic decrease in the quantum yield, with a yield of ~40% for intermediate size cores down to a few percent in large cores. The CSS structures also exhibit very good photostability, vastly improved over those of organically coated cores, and transformation into water environment via ligand exchange is performed without significant decrease of the quantum yield. These new InAs/CdSe/ZnSe CSS nanocrystals are therefore promising near-IR chromophores for biological fluorescence tagging and optoelectronic devices.

physics.app-ph↗

arXiv2021

Heavily Doped Semiconductor Nanocrystal Quantum Dots

Doping of semiconductors by impurity atoms enabled their widespread technological application in micro and opto-electronics. For colloidal semiconductor nanocrystals, an emerging family of materials where size, composition and shape-control offer widely tunable optical and electronic properties, doping has proven elusive. This arises both from the synthetic challenge of how to introduce single impurities and from a lack of fundamental understanding of this heavily doped limit under strong quantum confinement. We develop a method to dope semiconductor nanocrystals with metal impurities providing control of the band gap and Fermi energy. A combination of optical measurements, scanning tunneling spectroscopy and theory revealed the emergence of a confined impurity band and band-tailing. Successful control of doping and its understanding provide n- and p-doped semiconductor nanocrystals which greatly enhance the potential application of such materials in solar cells, thin-film transistors, and optoelectronic devices.

cond-mat.mtrl-sci↗