Arrangement Of Quantum Dots Research Articles

Stronger Coupling of Quantum Dots in Hole Transport Layer Through Intermediate Ligand Exchange to Enhance the Efficiency of PbS Quantum Dot Solar Cells.

Nowadays, the extensively used lead sulfide (PbS) quantum dot (QD) hole transport layer (HTL) relies on layer-by-layer method to replace long chain oleic acid (OA) ligands with short 1,2-ethanedithiol (EDT) ligands for preparation. However, the inevitable significant volume shrinkage caused by this traditional method will result in undesired cracks and disordered QD arrangement in the film, along with adverse increased defect density and inhomogeneous energy landscape. To solve the problem, a novel method for EDT passivated PbS QD (PbS-EDT) HTL preparation using small-sized benzoic acid (BA) as intermediate ligands is proposed in this work. BA is substituted for OA ligands in solution followed by ligand exchange with EDT layer by layer. With the new method, smoother PbS-EDT films with more ordered and closer QD packing are gained. It is demonstrated stronger coupling between QDs and reduced defects in the QD HTL owing to the intermediate BA ligand exchange. As a result, the suppressed nonradiative recombination and enhanced carrier mobility are achieved, contributing to ≈20% growth in short circuit current density (Jsc) and a 23.4% higher power conversion efficiency (PCE) of 13.2%. This work provides a general framework for layer-by-layer QD film manufacturing optimization.

Spatial-Arrangement-Assisted Emission Energy Fine Tuning of CdSe Quantum Dots (QDs) in QD–Block Copolymer Complexes

Quantum dots (QDs) exhibit size-dependent optical properties, where both the absorption and fluorescence energy levels vary with QD size. However, this dependence results in a discontinuity of intrinsically accessible energy levels for the bandgap, posing challenges in achieving precise energy tuning within a specific range. Herein, we demonstrate emission energy control of QDs with identical absorption energy levels by manipulating the spatial arrangement of QDs within QD–polymer complexes through hydrophobic interactions. The phase behavior of the QD–polymer complexes was modulated by adjusting the mass fraction of hydrophilic and hydrophobic blocks in the block copolymer, utilizing two types of amphiphilic block copolymers and varying temperatures. The QDs were spontaneously trapped within the hydrophobic region of the polymer template in water, resulting in spherical, cylindrical, and vesicle structures of QD–polymer complexes, corresponding to spherical, cylindrical, and layered assemblies of QDs, respectively. Depending on the QDs’ location within the QD–polymer complex, the surface area in contact with water varied, leading to different degrees of oxidation and, consequently, a change in the fluorescence energy level of QDs. This study introduces a novel method to fine tune the emission energy (<15 eV) of QDs by adjusting the polymer form factor without complicated procedures.

Rationally Designed Topological Quantum Dots in Bottom-Up Graphene Nanoribbons.

Bottom-up graphene nanoribbons (GNRs) have recently been shown to host nontrivial topological phases. Here, we report the fabrication and characterization of deterministic GNR quantum dots whose orbital character is defined by zero-mode states arising from nontrivial topological interfaces. Topological control was achieved through the synthesis and on-surface assembly of three distinct molecular precursors designed to exhibit structurally derived topological electronic states. Using a combination of low-temperature scanning tunneling microscopy and spectroscopy, we have characterized two GNR topological quantum dot arrangements synthesized under ultrahigh vacuum conditions. Our results are supported by density-functional theory and tight-binding calculations, revealing that the magnitude and sign of orbital hopping between topological zero-mode states can be tuned based on the bonding geometry of the interconnecting region. These results demonstrate the utility of topological zero modes as components for designer quantum dots and advanced electronic devices.

Impact of electron-phonon coupling on electron transport through T-shaped arrangements of quantum dots in the Kondo regime.

We calculate the conductance through strongly correlated T-shaped molecular or quantum dot systems under the influence of phonons. The system is modelled by the extended Anderson–Holstein Hamiltonian. The finite-U mean-field slave boson approach is used to study many-body effects. Phonons influence both interference and correlations. Depending on the dot unperturbed energy and the strength of electron–phonon interaction, the system is occupied by a different number of electrons that effectively interact with each other repulsively or attractively. This leads, together with the interference effects, to different spin or charge Fano–Kondo effects.

Photoluminescence enhancement by deterministically site-controlled, vertically stacked SiGe quantum dots

The Si/SiGe heterosystem would be ideally suited for the realization of complementary metal-oxide-semiconductor (CMOS)-compatible integrated light sources, but the indirect band gap, exacerbated by a type-II band offset, makes it challenging to achieve efficient light emission. We address this problem by strain engineering in ordered arrays of vertically close-stacked SiGe quantum dot (QD) pairs. The strain induced by the respective lower QD creates a preferential nucleation site for the upper one and strains the upper QD as well as the Si cap above it. Electrons are confined in the strain pockets in the Si cap, which leads to an enhanced wave function overlap with the heavy holes near the upper QD’s apex. With a thickness of the Si spacer between the stacked QDs below 5 nm, we separated the functions of the two QDs: The role of the lower one is that of a pure stressor, whereas only the upper QD facilitates radiative recombination of QD-bound excitons. We report on the design and strain engineering of the QD pairs via strain-dependent Schrödinger-Poisson simulations, their implementation by molecular beam epitaxy, and a comprehensive study of their structural and optical properties in comparison with those of single-layer SiGe QD arrays. We find that the double QD arrangement shifts the thermal quenching of the photoluminescence signal at higher temperatures. Moreover, detrimental light emission from the QD-related wetting layers is suppressed in the double-QD configuration.

Elucidation of the Mechanism of Quantum Dot Arrangement Based on Self-Assembly of an Azobenzene Derivative

Abstract The formation of well-defined nanostructures comprising assembled semiconductor quantum dots (QDs) is a challenging research task. Recently, we found that the introduction of π-conjugated molecules with a self-assembly ability into small CdSe QDs led to the formation of highly ordered QD arrangements. Here, we demonstrate the in-depth coaggregation process of large-sized CdSe QDs and azobenzene derivative 1 possessing an amino group functioning as an adhesive to the QD surface. Upon mixing the above QDs with assembled azobenzene derivative 1 in apolar solvents, linearly arranged QD structures along assembled azobenzene derivative 1 were formed over time. In the formed coaggregates, efficient energy transfer between the arranged QDs occurred, as confirmed by a change in the emission spectra and lifetimes. Analysis of time-dependent emission properties revealed the coaggregation mechanism of QDs and 1.

A Highly Ordered Quantum Dot Supramolecular Assembly Exhibiting Photoinduced Emission Enhancement

AbstractMulticomponent supramolecular assembly systems enable the generation of materials with outstanding properties, not obtained from single‐component systems, via a synergetic effect. Herein, we demonstrate a novel supramolecular coassembly system rendering highly ordered quantum dot (QD) arrangement structures formed via the self‐assembly of azobenzene derivatives, where the photocontrollable photoluminescence (PL) properties of the QDs are realized based on photoisomerization. Upon mixing the assembled azobenzene derivatives and QDs in apolar media, a time‐evolution coaggregation into hierarchical nanosheets with a highly ordered QD arrangement structure occurs. Upon photoirradiation, the nanosheets transform into ill‐defined aggregates without arranged QDs together with enhancing the PL intensity. In days, the photoirradiated coaggregates undergo recovery of the PL properties corresponding to the arranged QDs through thermal isomerization.

A Highly Ordered Quantum Dot Supramolecular Assembly Exhibiting Photoinduced Emission Enhancement.

Multicomponent supramolecular assembly systems enable the generation of materials with outstanding properties, not obtained from single-component systems, via a synergetic effect. Herein, we demonstrate a novel supramolecular coassembly system rendering highly ordered quantum dot (QD) arrangement structures formed via the self-assembly of azobenzene derivatives, where the photocontrollable photoluminescence (PL) properties of the QDs are realized based on photoisomerization. Upon mixing the assembled azobenzene derivatives and QDs in apolar media, a time-evolution coaggregation into hierarchical nanosheets with a highly ordered QD arrangement structure occurs. Upon photoirradiation, the nanosheets transform into ill-defined aggregates without arranged QDs together with enhancing the PL intensity. In days, the photoirradiated coaggregates undergo recovery of the PL properties corresponding to the arranged QDs through thermal isomerization.

Influence of polystyrene ligand length on the spatial arrangement of quantum dots within PS-b-PEO micelles

The spatial arrangement of functional inorganic nanoparticles within polymer micelles is essential to the nanocomposite performances. Polystyrene (PS) of different lengths (PS24, PS91 and PS163) are grafted onto the surface of fluorescent CdSe/CdS core/shell quantum dots (QDs) through ligand exchange procedure, and their grafting density decreases from 2.80 to 0.54, 0.18 chains/nm2 with increase of PS ligand length. Under two competing effects, i.e. wettability between QDs and block copolymer PS120-b-PEO318 and the attraction between QDs, the precise location of PS-capped QDs inside the co-assemblies can be regulated by the length of PS ligands. The low grafting density of PS163 on the QD surface cannot overcome the van der Waals and hydrophobic attraction between QDs and cause the local aggregation of QDs within the co-assemblies. On the contrary, short PS24 ligands with high grafting density can avoid QD aggregation, but exhibit poor wettability with copolymer, which confines the QDs in the central portion of the core of co-assemblies. PS91 ligands with medium grafting density have good wettability with block copolymer and facilitate the homogeneous distribution of QDs inside the cores of co-assemblies. Furthermore, the influence of stirring time and water addition rate on the structure of co-assemblies is also investigated.

Quantum phenomena in nanostructures

Quantum phenomena in nanostructures, especially epitaxial structures, are utilized for the fabrication of quantum devices. Here we discuss self organized growth of epitaxial quantum structures on silicon and their electronic structures, such as electronic density of states, and electronic transport in such systems. We discuss nanodots and nanowires, or an equivalent of zero-dimensional (0D) and one-dimensional (1D) structures in more details, and in general structures in all dimensional cross-over regimes. Further arrangements of quantum dots (QDs) in particular structures offers to form quantum bits and quantum dot molecules (QDMs). This could lead to exploration of new physics and new quantum devices.

A Study of Quantum Dots in a Multigrain Layer of a Planar-End Microstructure

Quantum dots (QDs) of CdSe, PbS, and InSb semiconductors in a multigrain layer of a planar-end microstructure have been studied. A model of an ordered QDs arrangement in a micron gap of the structure and a current flow along the lines of their series-parallel arrangement are considered. At low voltages (less than 8 V), electron transport is determined by thermal and tunnel emission from QDs into the gap, while at high voltages it is determined by charge restriction in QDs according to the model of Coulomb blockade. A strong effect of radiation with IR and UV spectra on the current–voltage characteristics has been found.

Photoluminescence of compact GeSi quantum dot groups with increased probability of finding an electron in Ge

The photoluminescence (PL) of the combined Ge/Si structures representing a combination of large (200–250 nm) GeSi disk-like quantum dots (nanodisks) and four-layered stacks of compact groups of smaller (30 nm) quantum dots grown in the strain field of nanodisks was studied. The multiple increase in the PL intensity was achieved by the variation of parameters of vertically aligned quantum dot groups. The experimental results were analyzed on the basis of calculations of energy spectra, electron and hole wave functions. It was found that the quantum dot arrangement in compact groups provides the effective electron localization in Δx,y-valleys with an almost equal probability of finding an electron in the Si spacer and Ge barrier. As a result, the main channels of radiative recombination in the structures under study correspond to spatially direct optical transitions.

Исследование квантовых точек в мультизеренном слое планарно-торцевой микроструктуры

Quantum dots (QDs) of Cds, PbS, and InSb semiconductors in a multi-grain layer of planar-edge microstructure were studied. A model of the ordered arrangement of CT in the micro-gap structure and the flow of current along the lines of parallel arrangement of QD is proposed. Electronic transport at low voltage values (less than 8 V) is determined by thermal and tunnel emission from the QD into the gap, at high values - by the charge restriction in the QD according to the Coulomb block model. A strong influence of IR and UV radiation on the VAC was found.

Colloidal Quantum Dot Arrangement Assisted by Perylene Bisimide Self-Assembly.

Colloidal semiconductor nanocrystals, so-called quantum dots (QDs), are attractive as molecular-like smart nanomaterials, and their emission and optoelectronic properties in the dispersed state have been actively studied. The construction of supramolecular structures composed of multiple QDs, however, is still challenging. Here, a new strategy to form supramolecular QD structures via self-assembly of perylene bisimide (PBI) dyes is demonstrated. In a mixed solution, QDs and PBI undergo time-dependent fusion to form an isolated colloidal QD-PBI complex or a unique QD-PBI co-aggregate composed of QDs arranged along a sheet-like PBI nanostructure, and these dramatically different supramolecular structures can be controlled by the solvent polarity.

Self-ordering of InAs nanostructures on (631)A/B GaAs substrates

The high order self-organization of quantum dots is demonstrated in the growth of InAs on a GaAs(631)-oriented crystallographic plane. The unidimensional ordering of the quantum dots (QDs) strongly depends on the As flux beam equivalent pressure (PAs) and the cation/anion terminated surface, i.e., A- or B-type GaAs(631). The self-organization of QDs occurs for both surface types along , while the QD shape and size distribution were found to be different for the self-assembly on the A- and B-type surfaces. In addition, the experiments showed that any misorientation from the (631) plane, which results from the buffer layer waviness, does not allow a high order of unidimensional arrangements of QDs. The optical properties were studied by photoluminescence spectroscopy, where good correspondence was obtained between the energy transitions and the size of the QDs.

Backbone Engineered γ-Peptide Amphitropic Gels for Immobilization of Semiconductor Quantum Dots and 2D Cell Culture.

We are reporting a spontaneous supramolecular assembly of backbone engineered γ-peptide scaffold and its utility in the immobilization of semiconductor quantum dots and in cell culture. The stimulating feature of this γ-peptide scaffold is that it efficiently gelates both aqueous phosphate buffers and aromatic organic solvents. A comparative and systematic investigation reveals that the greater spontaneous self-aggregation property of γ-peptide over the α- and β-peptide analogues is mainly due to the backbone flexibility, increased hydrophobicity, and π-π stacking of γ-phenylalanine residues. The hydrogels and organogels obtained from the γ-peptide scaffold have been characterized through field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), FT-IR, circular dichroism (CD), wide-angle X-ray diffraction, and rheometric study. Additionally, the peptide hydrogel has displayed a stimuli-responsive and thixotropic signature, which leads to the injectable hydrogels. 2D cell culture studies using normal and cancer cell lines reveal the biocompatibility of γ-peptide hydrogels. Further, the immobilization of semiconductor core-shell quantum dots in the transparent γ-peptide organogels showed ordered arrangement of quantum dots along the peptide fibrillar network with retaining photophysical property. Overall, γ-peptide scaffolds may serve as potential templates for the design of new functional biomaterials.

A triangular triple quantum dot with tunable tunnel couplings

A two-dimensional arrangement of quantum dots (QDs) with finite inter-dot tunnel coupling provides a promising platform for studying complicated spin correlations as well as for constructing large-scale quantum computers. Here, we fabricate a tunnel-coupled triangular triple QD with a novel gate geometry in which three dots are defined by positively biasing the surface gates. At the same time, the small area in the center of the triangle is depleted by negatively biasing the top gate placed above the surface gates. The size of the small center depleted area is estimated from the Aharonov–Bohm oscillation measured for the triangular channel but incorporating no gate-defined dots, with a value consistent with the design. With this approach, we can bring the neighboring gate-defined dots close enough to one another to maintain a finite inter-dot tunnel coupling. We finally confirm the presence of the inter-dot tunnel couplings in the triple QD from the measurement of tunneling current through the dots in the stability diagram. We also show that the charge occupancy of each dot and that the inter-dot tunnel couplings are tunable with gate voltages.

Spatial-symmetry-induced dark states and charge trapping effects in the coupled quantum dots

In a system of N interacting single-level quantum dots (QDs), we study the relaxation dynamics and the current–voltage characteristics determined by symmetry properties of the QD arrangement. Different numbers of dots, initial charge configurations, and various coupling regimes to reservoirs are considered. We reveal that effective charge trapping occurs for particular regimes of coupling to the reservoir when more than two dots form a ring structure with the CN spatial symmetry. We reveal that the effective charge trapping caused by the CN spatial symmetry of N coupled QDs depends on the number of dots and the way of coupling to the reservoirs. We demonstrate that the charge trapping effect is directly connected with the formation of dark states, which are not coupled to reservoirs due to the system spatial symmetry CN. We also reveal the symmetry blockade of the tunneling current caused by the presence of dark states.

Transport in serial spinful multiple-dot systems: The role of electron-electron interactions and coherences.

Quantum dots are nanoscopic systems, where carriers are confined in all three spatial directions. Such nanoscopic systems are suitable for fundamental studies of quantum mechanics and are candidates for applications such as quantum information processing. It was also proposed that linear arrangements of quantum dots could be used as quantum cascade laser. In this work we study the impact of electron-electron interactions on transport in a spinful serial triple quantum dot system weakly coupled to two leads. We find that due to electron-electron scattering processes the transport is enabled beyond the common single-particle transmission channels. This shows that the scenario in the serial quantum dots intrinsically deviates from layered structures such as quantum cascade lasers, where the presence of well-defined single-particle resonances between neighboring levels are crucial for device operation. Additionally, we check the validity of the Pauli master equation by comparing it with the first-order von Neumann approach. Here we demonstrate that coherences are of relevance if the energy spacing of the eigenstates is smaller than the lead transition rate multiplied by ħ.

Electric states in laterally and vertically arrayed type-II quantum dots

Laterally and vertically arrayed GaSb type-II quantum dots (QDs) in GaAs are theoretically studied to clarify how the electronic states of carriers are affected by the QD arrangement. By solving the Schrödinger equation under appropriate periodic boundary conditions, we numerically evaluate the wave functions of the carriers and show that different QD arrangements strongly vary the electron wave function. When the QDs are densely arranged in a plane, the electron wave function is spread laterally. For the closely stacked QDs in a vertical direction, the electron wave function is located at the sides of the QDs. We also show that the change of the electron wave function leads to the increase of the oscillator strength; the maximum value of the oscillator strength for the laterally arrayed QDs is four times larger than that for the single QD, while the vertical QD arrangement increases the oscillator strength by up to 36 times.