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Uri Banin

Publications and source records attributed to Uri Banin.

34 records · Page 2Linked to original sources
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↗
arXiv2021

Perspective on Coupled Colloidal Quantum Dot Molecules

Electronic coupling and hence hybridization of atoms serve as the basis for the rich properties of the endless library of naturally occurring molecules. Colloidal quantum dots (CQDs) manifesting quantum strong confinement, possess atomic like characteristics with s and p electronic levels, which popularized the notion of CQDs as artificial atoms. Continuing this analogy, when two atoms are close enough to form a molecule so that their orbitals start overlapping, the orbitals' energies start to split into bonding and anti-bonding states made out of hybridized orbitals. The same concept is also applicable for two fused core-shell nanocrystals in close proximity. Their band-edge states, which dictate the emitted photon energy, start to hybridize changing their electronic and optical properties. Thus, an exciting direction of artificial molecules emerges leading to a multitude of possibilities for creating a library of new hybrid nanostructures with novel optoelectronic properties with relevance towards diverse applications including quantum technologies. In a model fused core-shell homodimer molecule, the hybridization energy is strongly correlated with the extent of structural continuity, the delocalization of the exciton wavefunction, and the barrier thickness as calculated numerically. The hybridization impacts the emitted photon statistics manifesting a faster radiative decay rate, photon bunching effect, and modified Auger recombination pathway compared to the monomer artificial atoms. Future perspectives for the nanocrystals chemistry paradigm are highlighted.

cond-mat.mes-hall↗
arXiv2020

Material Challenges for Colloidal Quantum Nanostructures in Next Generation Displays

The recent technological advancements have greatly improved the quality and resolution of displays. Yet, issues like full color gamut representation and long lasting durability of the color emitters require further progression. Colloidal quantum dots manifest an inherent narrow spectral emission with optical stability, combined with various chemical processability options which will allow for their integration in display applications. Apart from their numerous advantages, they also present unique opportunities for the next technological leaps in the field.

cond-mat.mes-hall↗
arXiv2019

Coupled Colloidal Quantum Dot Molecules

Coupling of atoms is the basis of chemistry, yielding the beauty and richness of molecules. We utilize semiconductor nanocrystals as artificial atoms to form nanocrystal molecules that are structurally and electronically coupled. CdSe/CdS core/shell nanocrystals are linked to form dimers which are then fused via constrained oriented attachment. The possible nanocrystal facets in which such fusion takes place are analyzed with atomic resolution revealing the distribution of possible crystal fusion scenarios. Coherent coupling and wavefunction hybridization are manifested by a red shift of the band gap, in agreement with quantum mechanical simulations. Single nanoparticle spectroscopy unravels the attributes of coupled nanocrystal dimers related to the unique combination of quantum mechanical tunneling and energy transfer mechanisms. This sets the stage for nanocrystals chemistry to yield a diverse selection of coupled nanocrystal molecules constructed from controlled core/shell nanocrystal building blocks. These are of direct relevance for numerous applications in displays, sensing, biological tagging and emerging quantum technologies.

physics.app-ph↗
arXiv2019

Colloidal Quantum Nanostructures: Emerging Materials for Display Applications

Colloidal semiconductor nanocrystals (SCNCs) or, more broadly, colloidal quantum nanostructures constitute outstanding model systems for investigating size and dimensionality effects. Their nanoscale dimensions lead to quantum confinement effects that enable tuning of their optical and electronic properties. Thus, emission color control with narrow photoluminescence spectra, wide absorbance spectra, and outstanding photostability, combined with their chemical processability through control of their surface chemistry leads to the emergence of SCNCs as outstanding materials for present and next-generation displays. In this Review, we present the fundamental chemical and physical properties of SCNCs, followed by a description of the advantages of different colloidal quantum nanostructures for display applications. The open challenges with respect to their optical activity are addressed. Both photoluminescent and electroluminescent display scenarios utilizing SCNCs are described.

physics.app-ph↗
arXiv2019

Electronic coupling in colloidal quantum dot molecules; The case of CdSe/CdS core/shell homodimers

Coupled colloidal quantum dot molecules composed of two fused CdSe/CdS core/shell sphere monomers were recently presented. Upon fusion, the potential energy landscape is changing into two quantum dots separated by a pre-tuned potential barrier with energetics dictated by the conduction and valence band offsets of the core/shell semiconductors, and width controlled by the shell thickness and the fusion reaction conditions. In close proximity of the two nanocrystals, orbital hybridization occurs, forming bonding and anti-bonding states in analogy to the hydrogen molecule. In this study, we examine theoretically the electronic and optical signatures of such a quantum dot dimer compared to its monomer core/shell building blocks. We examine the effects of different core sizes, barrier widths, different band offsets and neck sizes at the interface of the fused facets, on the system wave-functions and energetics. Due to the higher effective mass of the hole and the large valence band offset, the hole still essentially resides in either of the cores breaking the symmetry of the potential for the electron as well. We found that the dimer signature is well expressed in a redshift of the bandgap both in absorption and emission, in slower radiative lifetimes and in an absorption cross-section which is significantly enhanced relative to the monomers at energies above the shell absorption onset, while remains essentially at the same level near the band-edge. This study provides essential guidance to pre-design of coupled quantum dot molecules with specific attributes which can be utilized for various new optoelectronic applications.

cond-mat.mes-hall↗
arXiv2019

Mesophase Formation Stabilizes High-Purity Magic-Sized Clusters

Magic-sized clusters (MSCs) are renowned for their identical size and closed-shell stability that inhibit conventional nanoparticle (NP) growth processes. Though MSCs have been of increasing interest, understanding the reaction pathways toward their nucleation and stabilization is an outstanding issue. In this work, we demonstrate that high concentration synthesis (1000 mM) promotes a well-defined reaction pathway to form high-purity MSCs (greater than 99.9 percent). The MSCs are resistant to typical growth and dissolution processes. Based on insights from in-situ X-ray scattering analysis, we attribute this stability to the accompanying production of a large, hexagonal organic-inorganic mesophase (greater than 100 nm grain size) that arrests growth of the MSCs and prevents NP growth. At intermediate concentrations (500 mM), the MSC mesophase forms, but is unstable, resulting in NP growth at the expense of the assemblies. These results provide an alternate explanation for the high stability of MSCs. Whereas the conventional mantra has been that the stability of MSCs derives from the precise arrangement of the inorganic structures (i.e., closed-shell atomic packing), we demonstrate that anisotropic clusters can also be stabilized by self-forming fibrous mesophase assemblies. At lower concentration (less than 200 mM or greater than 16 acid-to-metal), MSCs are further destabilized and NPs formation dominates that of MSCs. Overall, the high concentration approach intensifies and showcases inherent concentration-dependent surfactant phase behavior that is not accessible in conventional (i.e., dilute) conditions. This work provides not only a robust method to synthesize, stabilize, and study identical MSC products, but also uncovers an underappreciated stabilizing interaction between surfactants and clusters.

physics.app-ph↗
arXiv2019

Chemically reversible isomerization of inorganic clusters

Structural transformations in molecules and solids have generally been studied in isolation, while intermediate systems have eluded characterization. We show that a pair of CdS cluster isomers provides an advantageous experimental platform to study isomerization in well-defined atomically precise systems. The clusters coherently interconvert over an est. 1 eV energy barrier with a 140 meV shift in their excitonic energy gaps. There is a diffusionless, displacive reconfiguration of the inorganic core (solid-solid transformation) with first order (isomerization-like) transformation kinetics. Driven by a distortion of the ligand binding motifs, the presence of hydroxyl species changes the surface energy via physisorption, which determines phase stability in this system. This reaction possesses essential characteristics of both solid-solid transformations and molecular isomerizations, and bridges these disparate length scales.

physics.chem-ph↗
arXiv2012

Periodic negative differential conductance in a single metallic nano-cage

We report a bi-polar multiple periodic negative differential conductance (NDC) effect on a single cage-shaped Ru nanoparticle measured using scanning tunneling spectroscopy. This phenomenon is assigned to the unique multiply-connected cage architecture providing two (or more) defined routes for charge flow through the cage. This, in turn, promotes a self- gating effect, where electron charging of one route affects charge transport along a neighboring channel, yielding a series of periodic NDC peaks. This picture is established and analyzed here by a theoretical model.

cond-mat.mes-hall↗
arXiv2010

Enhancement of two photon processes in quantum dots embedded in subwavelength metallic gratings

We show a large enhancement of two-photon absorption processes in nanocrystal quantum dots and of light upconversion efficiency from the IR to the near-IR spectral regime, using a hybrid optical device in which near-IR emitting InAs quantum dots were embedded on top a metallic nanoslit array. The resonant enhancement of these nonlinear optical processes is due to the strong local electromagnetic field enhancements inside the nanoslit array structure at the extraordinary transmission resonances. A maximal two-photon absorption enhancement of more than 20 was inferred. Different high field regions were identified for different polarizations, which can be used for designing and optimizing efficient nonlinear processes in such hybrid structures. Combining nanocrystal quantum dots with subwavelength metallic nanostructures is therfore a promising way for a range of possible nonlinear optical devices.

physics.optics↗
arXiv2005

Interaction of scanning probes with semiconductor nanocrystals; Physical mechanism and basis for near field optical imaging

We investigate the modification of photoluminescence (PL) from single semiconductor nanocrystal quantum dots (NCs) in proximity of metal and semiconducting Atomic Force Microscope (AFM) tips. The presence of the tip alters the radiative decay rate of an emitter via interference and opens efficient non radiative decay channels via energy transfer to the tip material. These effects cause quenching (or enhancement) of the emitter's PL intensity, as a function of its distance from the interacting tip. We take advantage of this highly distance dependent effect to realize a contrast mechanism for high resolution optical imaging. AFM tips are optimized as energy acceptors by chemical functionalization with InAs NCs to achieve optical resolution down to 30 nm. The presented experimental scheme offers high resolution optical information while maintaining the benefits of traditional AFM imaging. We directly measure the PL intensity of single NCs as a function of the tip distance. Our results are in good agreement to calculation made by a classical theoretical model describing an oscillating dipole interacting with a planar mirror.

physics.chem-ph↗
arXiv2005

Optical Gain from InAs Nanocrystal Quantum Dots in a Polymer Matrix

We report on the first observation of optical gain from InAs nanocrystal quantum dots emitting at 1.55 microns based on a three-beam, time resolved pump-probe technique. The nanocrystals were embedded into a transparent polymer matrix platform suitable for the fabrication of integrated photonic devices.

cond-mat.other↗
arXiv2002

Size dependent tunneling and optical spectroscopy of CdSe quantum rods

Photoluminescence excitation spectroscopy and scanning tunneling spectroscopy are used to study the electronic states in CdSe quantum rods that manifest a transition from a zero dimensional to a one dimensional quantum confined structure. Both optical and tunneling spectra show that the level structure depends primarily on the rod diameter and not on length. With increasing diameter, the band-gap and the excited state level spacings shift to the red. The level structure was assigned using a multi-band effective-mass model, showing a similar dependence on rod dimensions.

cond-mat.mes-hall↗
arXiv2001

Imaging and spectroscopy of artificial-atom states in core/shell nanocrystal quantum dots

Current imaging scanning tunneling microscopy is used to observe the electronic wavefunctions in InAs/ZnSe core/shell nanocrystals. Images taken at a bias corresponding to the s conduction band state show that it is localized in the central core region, while images at higher bias probing the p state reveal that it extends to the shell. This is supported by optical and tunneling spectroscopy data demonstrating that the s-p gap closes upon shell growth. Shapes of the current images resemble atom-like envelope wavefunctions of the quantum dot calculated within a particle in a box model.

cond-mat.mes-hall↗
arXiv2001

Control of charging in resonant tunneling through InAs nanocrystal quantum dots

Tunneling spectroscopy of InAs nanocrystals deposited on graphite was measured using scanning tunneling microscopy, in a double-barrier tunnel-junction configuration. The effect of the junction symmetry on the tunneling spectra is studied experimentally and modeled theoretically. When the tip is retracted, we observe resonant tunneling through the nanocrystal states without charging. This is in contrast to previous measurements on similar nanocrystals anchored to gold by linker molecules, where charging took place. Charging is regained upon reducing the tip-nanocrystal distance, making the junctions more symmetric. The effect of voltage distribution between the junctions on the measured spectra is also discussed.

cond-mat.mes-hall↗