Planar Quantum Dots Research Articles

Conformal SnO2 and NiOx charge transport layers for pragmatic air-processed perovskite solar cells

The industrialization of perovskite (PVK) solar cells (PSCs) necessitates simple manufacturing, cheapness, nontoxicity, high efficiency, and stability. We report an air-processed lead halide PSCs with cheap, nontoxic, stable NiOx and SnO2 as the hole and electron transport layers separately, to meet all the above commercial requests. By preparing the NiOx (water-dispersible) and SnO2 (oil-dispersible) quantum dots (QDs) with good conductivity and energy band matched well with that of PVK, we fabricate inverted PSCs (ITO/NiOx/PVK/SnO2/Ag) with efficiency up to 20.1/14.6% for active areas of 0.0707/1 cm2, under 1 sun simulated illumination with enhanced UV light ratio. The UV-light resistant PSCs reported here will impel their application in the high altitude areas with strong UV light. Importantly, the monodisperse size and dominant (110) plane of SnO2 QDs achieved by controlling the solvothermal parameters, realize the conformal deposition and defect prevention of SnO2, respectively, thus improving the PSC efficiency and stability.

Fabrication of a novel microflower BiOCl composite incorporated with rosemary powder for enhanced oxygen vacancies and visible-light photocatalytic efficiency

Incorporating metallic or nonmetallic compounds is a promising method for enhancing the oxygen vacancies (OVs), charge transport, and visible light absorption of a bismuth oxychloride (BiOCl) semiconductor. This research aims to improve the properties of BiOCl by incorporating rosemary powder (RPC). BiOCl photocatalysts with enhanced OVs were synthesized through the addition of 1–4 wt% of RPC using a one-pot solvothermal process. The addition of 3 % RPC resulted in the formation of very small particles on the BiOCl lamellar structures, which were aligned with the (002) plane of carbon quantum dots (CQDs). The uniformly distributed CQDs transformed the BiOCl microstructure from a two-dimensional lamellar structure to a three-dimensional floral structure, leading to an increased surface area and pore volume. The incorporation of RPC enhanced the visible light absorption capacity of BiOCl. The carbon atoms in RPC readily attach to oxygen atoms in the Bi-O bond, disrupting the Bi-O lattice bond and forming OVs. The addition of RPC contributed to a higher number of OVs, which impacted both the energy difference between the valence and conduction bands and their locations. This resulted in better charge transfer and enhanced active species formation. The 3 %-RPC-BOC photocatalyst had the highest efficiency, achieving a 98 % degradation of rhodamine B (RhB) and a 45 % removal of organic carbon within 48 min under visible light. This composite also had excellent stability and reusability, as its crystal structure remained unaltered even after conducting 7 cycles. The primary active species of 3 %-RPC-BOC system is the superoxide radical (·O2-), while the hydroxyl radical (·OH) and hole (h+) operate as minor active species. Additionally, the 3 %-RPC-BOC achieved a high tetracycline (TC) removal rate of 82.1 %, which is 2.21 times higher than that of pure BiOCl.

The effect of hydrostatic pressure, temperature and impurity factor on linear and nonlinear optical properties of InxGa1-xAs tuned quantum dot with modified Gaussian potential under the action of a vertical magnetic field

This study presents a detailed theoretical analysis of the effects of hydrostatic pressure, temperature, impurity factors, and external electric fields on the optical properties of InxGa1-xAs tuned quantum dot. A uniform magnetic field, directed perpendicular to the quantum dot plane, is applied. The system is excited using a monochromatic electromagnetic field, considering electron intersubband transitions. Energy levels and wave functions are determined using the parabolic band approximation and effective mass approximation. The linear and nonlinear optical absorption coefficients and refractive index changes are computed using the compact-density matrix approach and the iterative method. Numerical results indicate that the peaks of the linear and nonlinear optical absorption coefficients and refractive index changes shift towards lower energies (red shift) and higher energies (blue shift) under varying hydrostatic pressure, temperature, and impurity factor strength. Additionally, the peak values of the absorption coefficients and refractive index changes increase or decrease based on these parameter variations. These results have significant implications for the design and optimization of quantum dot-based optoelectronic devices.

Perfect Zeeman Anisotropy in Rotationally Symmetric Quantum Dots with Strong Spin-Orbit Interaction.

In nanoscale structures with rotational symmetry, such as quantum rings, the orbital motion of electrons combined with a spin-orbit interaction can produce a very strong and anisotropic Zeeman effect. Since symmetry is sensitive to electric fields, ring-like geometries provide an opportunity to manipulate magnetic properties over an exceptionally wide range. In this work, we show that it is possible to form rotationally symmetric confinement potentials inside a semiconductor quantum dot, resulting in electron orbitals with large orbital angular momentum and strong spin-orbit interactions. We find complete suppression of Zeeman spin splitting for magnetic fields applied in the quantum dot plane, similar to the expected behavior of an ideal quantum ring. Spin splitting reappears as orbital interactions are activated with symmetry-breaking electric fields. For two valence electrons, representing a common basis for spin-qubits, we find that modulating the rotational symmetry may offer new prospects for realizing tunable protection and interaction of spin-orbital states.

A first-principles study of the electronic, vibrational, and optical properties of planar SiC quantum dots

With the reported synthesis of a fully planar 2D silicon carbide (SiC) allotrope, the possibilities of its technological applications are enormous. Recently, several authors have computationally studied the structures and electronic properties of a variety of novel infinite periodic SiC monolayers, in addition to the honeycomb one. In this work, we perform a systematic first-principles investigation of the geometry, electronic structure, vibrational, and optical absorption spectra of several finite, but, fully planar structures of SiC, i.e. 0D quantum dots (QDs). The sizes of the studied structures are in the 1.20–2.28 nm range, with their computed HOMO(H)-LUMO(L) gaps ranging from 0.66 eV to 4.09 eV, i.e. from the IR to the UV region of the spectrum. The H-L gaps in the SiC QDs are larger as compared to the band gaps of the corresponding monolayers, confirming the quantum confinement effects. In spite of covalent bonding in the QDs, Mulliken charge analysis reveals that Si atoms exhibit positive charges, whereas the C atoms acquire negative charges, due to the different electron affinities of the two atoms. Furthermore, a strong structure property relationship is observed with fingerprints both in the vibrational and optical spectra. The wide range of H-L gaps in different SiC QDs makes them well-suited for applications in fields such as photocatalysis, light-emitting diodes, and solar cells.

3D two-electron double quantum dot: Comparison between the behaviour of some physical quantities under two different confinement potentials in the presence of a magnetic field

We have considered a system consisting of two coupled quantum dots containing two electrons, i.e., two quantum dots next to each other with one excess electron each, subjected to an uniform magnetic field perpendicular to the quantum dots plane. The effect of different confinement potential profiles under which the electrons are subjected is studied. This study has been performed in the light of interest in fundamental logical quantum-gate operations: we have been concerned in analysing the behaviour of the main physical quantities which should be involved in a two-qubit quantum-gate operation for two different profiles of confinement found in literature. Our purpose was to establish how sensitive the physical quantities are to the confinement profile and in which range of magnetic field this issue can be critical.

Coherent control of electron spin qubits in silicon using a global field

Silicon spin qubits promise to leverage the extraordinary progress in silicon nanoelectronic device fabrication over the past half century to deliver large-scale quantum processors. Despite the scalability advantage of using silicon technology, realising a quantum computer with the millions of qubits required to run some of the most demanding quantum algorithms poses several outstanding challenges, including how to control many qubits simultaneously. Recently, compact 3D microwave dielectric resonators were proposed as a way to deliver the magnetic fields for spin qubit control across an entire quantum chip using only a single microwave source. Although spin resonance of individual electrons in the globally applied microwave field was demonstrated, the spins were controlled incoherently. Here we report coherent Rabi oscillations of single electron spin qubits in a planar SiMOS quantum dot device using a global magnetic field generated off-chip. The observation of coherent qubit control driven by a dielectric resonator establishes a credible pathway to achieving large-scale control in a spin-based quantum computer.

Quantum‐Confined Dodecahedron CsPbBr3 Quantum Dots by A Sequential Post‐Treatment Strategy for Efficient Blue PeLEDs

AbstractCsPbBr3 quantum dots (QDs) prepared through traditional hot‐injection and room temperature reprecipitation methods are usually hexahedral (nanoplate/cube) and green‐emitting. These methods severely obstruct the development of blue perovskite‐based light‐emitting diodes (PeLEDs) because the resultant QDs always suffer unachievable morphology regulation. Here, highly efficient blue‐emitting rhombic dodecahedron CsPbBr3 QDs (CsPbBr3 <12> QDs) are obtained by regulating morphological transformation for the first time through a novel and facile sequential post‐treatment strategy. Hydrobromide (‐NH3+) with stronger adsorption energy promotes the morphology transform of QDs from nanoplate to cube by substituting oleylamine in the first step. Then, polar solvent vapor eliminates PbX64− and led to the (112) crystal plane of the cube QDs being exposed, which is effectively stabilized by ‐NH3+ to realize morphology transition from cube to the dodecahedron. Highly efficient 12‐faceted dodecahedrons are endowed with significantly enhanced photoluminescence quantum yield of 89.8% at 476 nm as well as uniform size distribution and mono‐dispersity. The target blue PeLEDs achieve the maximum external quantum efficiency of 6.6% (480 nm) and excellent spectrum stability. This study provides a new strategy to prepare blue‐emitting CsPbBr3 <12> QDs by delicate control toward the growth and polyhedron process of crystal.

Dynamics of Hole Singlet-Triplet Qubits with Large g-Factor Differences.

The spin-orbit interaction permits to control the state of a spin qubit via electric fields. For holes it is particularly strong, allowing for fast all electrical qubit manipulation, and yet an in-depth understanding of this interaction in hole systems is missing. Here we investigate, experimentally and theoretically, the effect of the cubic Rashba spin-orbit interaction on the mixing of the spin states by studying singlet-triplet oscillations in a planar Ge hole double quantum dot. Landau-Zener sweeps at different magnetic field directions allow us to disentangle the effects of the spin-orbit induced spin-flip term from those caused by strongly site-dependent and anisotropic quantum dot g tensors. Our work, therefore, provides new insights into the hole spin-orbit interaction, necessary for optimizing future qubit experiments.

Kerr nonlinearity in asymmetric disc-like quantum dot and its controllability

Nonlinear Kerr materials have the potential to be used as principal components for developing optical quantum computers. Among these materials, quantum dots have high nonlinearity and high Kerr-dispersion. This paper investigated the Kerr effect in a doped asymmetric quantum dot. The results showed that third-order nonlinear optical property such as the Kerr effect can be controlled by Rashba coupling. Experimentally, the strength of Rashba coupling can be changed and thus manipulated by applying a voltage to the electrostatic gate installed on the two-dimensional plane of quantum dots. The numerical calculations illustrated that the presence of attractive impurity and enhancement of the strength of Rashba coupling in an asymmetric quantum dot increase the Kerr nonlinearity. Furthermore, the results demonstrated that the magnitude of the Kerr effect of a symmetric doped quantum dot is larger than an asymmetric doped quantum dot.

A two-dimensional array of single-hole quantum dots

Quantum dots fabricated using methods compatible with semiconductor manufacturing are promising for quantum information processing. In order to fully utilize the potential of this platform, scaling quantum dot arrays along two dimensions is a key step. Here, we demonstrate a two-dimensional quantum dot array where each quantum dot is tuned to single-charge occupancy, verified by simultaneous measurements using two integrated radio frequency charge sensors. We achieve this by using planar germanium quantum dots with low disorder and a small effective mass, allowing the incorporation of dedicated barrier gates to control the coupling of the quantum dots. We measure the hole charge filling spectrum and show that we can tune single-hole quantum dots from isolated quantum dots to strongly exchange coupled quantum dots. These results motivate the use of planar germanium quantum dots as building blocks for quantum simulation and computation.

Single-particle and collective excitations in quantum wires comprised of vertically stacked quantum dots: Finite magnetic field

A theoretical investigation has been made of the magnetoplasmon excitations in a quasi-one-dimensional electron system composed of vertically stacked, self-assembled InAs/GaAs quantum dots. The smaller length scales involved in the experiments impel us to consider a perfectly periodic system of two-dimensionally confined InAs quantum dot layers separated by GaAs spacers. Subsequent system is subjected to a two-dimensional confining (harmonic) potential in the [Formula: see text]–[Formula: see text] plane and an applied magnetic field (B) in the symmetric gauge. This scheme defines virtually a system of quantum wire comprised of vertically stacked quantum dots (VSQD). We derive and discuss the Dyson equation, the generalized (nonlocal and dynamic) dielectric function, and the inverse dielectric function for investigating the single-particle and collective (magnetoplasmon) excitations within the framework of (full) random-phase approximation (RPA). As an application, we study the influence of the confinement potential and the magnetic field on the component eigenfunctions, the density of states (DOS), the Fermi energy, the collective excitations, and the inverse dielectric functions. How the B-dependence of DOS validate the VSQD mimicking the realistic quantum wires, the Fermi energy oscillates as a function of the Bloch vector, the intersubband single-particle continuum bifurcates at the origin, a collective excitation emerges and propagates within the gap of the split single-particle continuum, and the alteration in the well- and barrier-widths allows to customize the excitation spectrum in the desired energy range are some of the remarkable features of this investigation. These findings demonstrate, for the very first time, the significance of investigating the system of VSQD subjected to a quantizing magnetic field. Given the edge over the planar quantum dots and the foreseen applications in the single-electron devices and quantum computation, investigating the system of VSQD is deemed vital. The results suggest exploiting magnetoplasmon qubits to be a potential option for implementing the solemn idea of quantum state transfer in devising quantum gates for the quantum computation and quantum communication networks.

Terahertz photo-generated current in a two-dimensional quantum dot system

The photo-generated current due to electronic transitions in a semiconductor planar quantum dot attached to outgoing leads is theoretically investigated. An electron is confined in the dot by a pure quantum mechanical effect, which is due to the higher ground state energy of the quantum wells forming the leads, as compared to the one in the dot. The dynamics of such a confined electron interacting with a light pulse is investigated by numerically solving a time-dependent Schrödinger equation within the effective mass approximation and goes beyond the lowest order perturbative approach. Our results show the coexistence of both linear and non-linear contributions to the photo-generated current in this system, sharply peaked at frequencies in the terahertz range, which are further tunable by the quantum dot radius. The peaks can be made even sharper as one adds a narrow constriction in the dot–leads connection. The details of the dependence of the peaks’ frequency, intensity, and sharpness on system parameters are discussed.

Semiclassical modeling of coupled quantum-dot–cavity systems: From polaritonlike dynamics to Rabi oscillations

Semiconductor quantum dots in photonic cavities are strongly coupled light-matter systems with prospective applications in optoelectronic devices and quantum information processing. Here we present a theoretical study of the coupled exciton--light field dynamics of a planar quantum dot ensemble, treated as two-level systems, embedded in a photonic cavity modeled by Maxwell's equations. When excited by coupling an external short laser pulse into the cavity, we find an exciton-polariton-like behavior for weak excitation and Rabi oscillations for strong excitation with a sharp transition between these regimes. In the transition region we find highly non-linear dynamics involving high harmonics of the fundamental oscillation. We perform a numerical study based on the Finite-Difference-Time-Domain method for the solution of Maxwell's equations coupled to Bloch equations for the quantum dots and also derive an analytical model to describe the coupled cavity-quantum dot system, which allows us to describe the light field dynamics in terms of a Newton-like dynamics in an effective anharmonic potential. From the shape of this potential combined with the initial conditions the transition can be well understood. The model is then extended to a broadened ensemble of quantum dots. For weak excitation the polariton spectrum broadens and the lines slightly shift, however, the sharp transition to the Rabi oscillation regime is still present. Furthermore, we find a second, lower threshold with additional lines in the spectra which can be traced back to Rabi oscillations driven by the polariton modes. Our approach provides new insights in the dynamics of both quantum dot and light field in the photonic structure.

The magneto-optics in quantum wires comprised of vertically stacked quantum dots: A calling for the magnetoplasmon qubits

A deeper sense of advantages over the planar quantum dots and the foreseen applications in the single-electron devices and quantum computation have given vertically stacked quantum dots (VSQD) a width of interest. Here, we embark on the collective excitations in a quantum wire made up of vertically stacked, self-assembled InAs/GaAs quantum dots in the presence of an applied magnetic field in the symmetric gauge. We compute and illustrate the influence of an applied magnetic field on the behavior characteristics of the density of states, Fermi energy, and collective (magnetoplasmon) excitations (obtained within the framework of random-phase approximation (RPA)). The Fermi energy is observed to oscillate as a function of the Bloch vector. Remarkably, the intersubband single-particle continuum splits into two with a collective excitation propagating within the gap. This is attributed to the (orbital) quantum number owing to the applied magnetic field. Strikingly, the alteration in the well and barrier widths can enable us to customize the excitation spectrum in the desired energy range. These findings demonstrate, for the very first time, the viability and importance of studying the VSQD subjected to an applied magnetic field. The technological promise that emerges is the route to devices exploiting magnetoplasmon qubits as the potential option in designing quantum gates for the quantum communication networks.

Tuning the carrier tunneling in a single quantum dot with a magnetic field in Faraday geometry

We report on an increase in the carrier tunneling time in a single quantum dot (QD) with a magnetic field in Faraday geometry using photocurrent spectroscopy. A nearly 60\% increase in hole tunneling time is observed with an applied magnetic field equal to 9 T. For a truncated pyramid QD, hole tunnels out faster at the lateral edge of the QD due to the reduced barrier height. The magnetic field in Faraday geometry shrinks the hole wave function at the center of QD plane, which weakens the tunneling at lateral edge and increases the average tunneling time. This mechanism also works for electron but the effect is smaller. The electron wave function is more localized at the center of the QD due to the uniform confining potential, therefore the relatively weak shrinkage caused by the magnetic field does not reduce the tunneling rate significantly.

Gate defined quantum dot realized in a single crystalline InSb nanosheet

A single crystalline InSb nanosheet is an emerging planar semiconductor material with potential applications in electronics, infrared optoelectronics, spintronics, and topological quantum computing. Here, we report on the realization of a quantum dot device from a single crystalline InSb nanosheet grown by molecular-beam epitaxy. The device is fabricated from the nanosheet on a Si/SiO2 substrate, and quantum dot confinement is achieved by the top gate technique. Transport measurements of the device are carried out at a low temperature in a dilution refrigerator. It is found that the measured charge stability diagram is characterized by a series of small Coulomb diamonds at high plunger gate voltages and a series of large Coulomb diamonds at low plunger gate voltages, demonstrating the formation of a gate-tunable quantum dot in the InSb nanosheet. Gate-defined planar InSb quantum dots offer a renewed platform for developing semiconductor-based quantum computation technology.

How the inter-electronic potential Ansätze affect the bound state solutions of a planar two-electron quantum dot model

The model of a two-electron quantum dot, confined to move in a two dimensional flat space, in the presence of an external harmonic oscillator potential, is revisited for a specific purpose. Indeed, eigenvalues and eigenstates of the bound state solutions are obtained for any oscillation frequency considering both the 1∕r and ln r Ansätze for inter-electronic Coulombic-like potentials in 2D. Then, it is pointed out that the significative difference between measurable quantities predicted from these two potentials can shed some light on the problem of space dimensionality as well as on the physical nature of the potential itself.

Characterization of CuCl quantum dots grown in NaCl single crystals via optical measurements, X-ray diffraction, and transmission electron microscopy

We evaluated the crystal size, shape, and alignment of the lattice planes of CuCl quantum dots (QDs) embedded in NaCl single crystals by optical measurements, X-ray diffraction (XRD) patterns, and transmission electron microscopy (TEM). We obtained, for the first time, an XRD pattern and TEM images for CuCl QDs in NaCl crystals. The XRD pattern showed that the lattice planes of the CuCl QDs were parallel to those of the NaCl crystals. In addition, the size of the QDs was estimated from the diffraction width. It was apparent from the TEM images that almost all CuCl QDs were polygonal, although some cubic QDs were present. The mean size and size distribution of the QDs were also obtained. The dot size obtained from optical measurements, XRD, and TEM image were almost consistent. Our new findings can help to reveal the growth mechanism of semiconductor QDs embedded in a crystallite matrix. In addition, this work will play an important role in progressing the study of optical phenomena originating from assembled semiconductor QDs.

Planar Is Better: Monodisperse Three-Layered MoS2 Quantum Dots as Fluorescent Reporters for 2,4,6-Trinitrotoluene Sensing in Environmental Water and Luggage Cases.

In this study, we present a simple but effective fluorescent system for highly sensitive and versatile sensing of 2,4,6-trinitrotoluene (TNT) using few layered planar MoS2 quantum dots (QDs) as reporters. Excitation-independent emitting MoS2 QDs were first fabricated by using the proposed ultrasonic-hydrothermal-based top-down method assisted by carbon-free hydroxylamine hydrochloride. The obtained pristine MoS2 QDs were then modified with cysteine for introducing amino groups as TNT binding sites. The as-prepared MoS2 QDs possess a planar structure, which can more adequately interact with flat aromatic TNT molecules due to π-π attraction and decreased steric effects, compared with traditional spherical/quasi-spherical QDs. As a result, they exhibit extremely high sensitivity for TNT sensing (1 nM and 2 ng for solution and substrate assay, respectively). The common ions containing in environmental water samples do not interfere with the sensing. Furthermore, the QDs-decorated test paper shows an instantaneous (within 1 min) response to trace amounts of deposited TNT, and the fluorescence quenching can even be well-visualized by the naked eye. Because of favorable analytical performances, the proposed MoS2 QDs-based TNT sensing system has potential applications in both environmental water monitoring and security screening.