Quantum Nanostructures Research Articles

Electron-donor-impurity polarizability in a semiconductor nanostructure under electric field: Geometrical effects in ellipsoidal quantum dots

The features of electron-impurity polarizability in a quantum nanostructure are investigated from the knowledge of allowed electron states coupled to a shallow-donor impurity. The influence of geometry on the carrier confinement and the presence of external probes such as electric fields are explicitly considered. For the sake of illustration, the example of CdS/ZnSe-based prolate ellipsoidal core/shell quantum dot (PECSQD) configuration is chosen. This problem allows to consider a finite confinement potential model, employing both the finite element method and the effective mass approximation. Within this framework, impurity-related energies and associated polarizability for the lowest states were determined as functions of the shell thickness, core dimensions, impurity position, and applied electric field strength - while keeping the shell dimensions fixed. Electron-impurity energies are largely affected by electric field values. Under thinner shell conditions, polarizability shows variations with field strength that include “anomalous” negative values for certain states. Then, the increase in shell thickness causes a significant increment in the polarizabilities, regardless impurity position.

Role of Quantum Dots and Nanostructures in Photovoltaic Energy Conversion

Nanostructures and quantum dots have substantial effects on enhancing photovoltaic energy conversion efficiency, as evidenced in this comprehensive study. Materials that are nanostructured and nanosized particles are commonly used to address the urgent issues related to energy conversion. The use of nanostructured substances to address issues with energy and natural resources has garnered a lot of interest lately. Directional nanostructures in particular show promise for the conversion, collection, and storage of energy. Due to their unique properties, such as electrical conductivity, mechanical energy, and photoluminescence, quantum dots made from carbon (CQDs) and graphene quantum dots (GQDs) have been integrated into hybrid photovoltaic-thermoelectric systems (PV-TE). It evaluates the effects of nanostructures on solar energy technologies, in particular how they can improve power conversion and light absorption in solar cells. Optical light detectors, which transform photonic energy into signals that are electrical, are among the many optoelectronic uses of CQDs that have drawn attention because they are essential components of contemporary imaging and communication systems, such as visible light cameras, machine vision, medical X-ray and near-infrared image processing, and visible light detection devices. Besides supercapacitors, the study investigates how nanostructures could play a crucial role in contributing to addressing the global energy crisis sustainably, by working as photocatalysts for hydrogen synthesis and supercapacitors.

Physics-informed reduced-order learning from the first principles for simulation of quantum nanostructures

Multi-dimensional direct numerical simulation (DNS) of the Schrödinger equation is needed for design and analysis of quantum nanostructures that offer numerous applications in biology, medicine, materials, electronic/photonic devices, etc. In large-scale nanostructures, extensive computational effort needed in DNS may become prohibitive due to the high degrees of freedom (DoF). This study employs a physics-based reduced-order learning algorithm, enabled by the first principles, for simulation of the Schrödinger equation to achieve high accuracy and efficiency. The proposed simulation methodology is applied to investigate two quantum-dot structures; one operates under external electric field, and the other is influenced by internal potential variation with periodic boundary conditions. The former is similar to typical operations of nanoelectronic devices, and the latter is of interest to simulation and design of nanostructures and materials, such as applications of density functional theory. In each structure, cases within and beyond training conditions are examined. Using the proposed methodology, a very accurate prediction can be realized with a reduction in the DoF by more than 3 orders of magnitude and in the computational time by 2 orders, compared to DNS. An accurate prediction beyond the training conditions, including higher external field and larger internal potential in untrained quantum states, is also achieved. Comparison is also carried out between the physics-based learning and Fourier-based plane-wave approaches for a periodic case.

285 nm AlGaN-BASED DEEP-ULTRAVIOLET LED WITH HIGH INTERNAL QUANTUM EFFICIENCY: COMPUTATIONAL DESIGN

In this paper, the systematic computational design process of AlGaN-based multiple quantum-well (QW) deep-ultraviolet (DUV) light-emitting diode (LED) grown on sapphire (Al2O3) substrate was investigated. An optimization was held to increase internal quantum efficiency (IQE) handling the LED parameters such as doping percentage of the n- and the p-type layers of these devices. The structure parameters of the best design were determined through a customized genetic algorithm integrated into the nanostructure quantum electronic simulation (nextnano). As a determining factor, IQE was obtained to be 24% for the devised 285 nm LED. It has been demonstrated that this result can be increased up to a remarkably high value of 70% by a low threading dislocation density (TDD) and reduced Auger recombination. In addition, the operation input power and potential difference were successfully kept below 0.1 W/mm2 and 5.05 V, respectively.

Quantum element method for multi-dimensional nanostructures enabled by a projection-based learning algorithm

A projection-based learning method developed previously based on proper orthogonal decomposition (POD), together with the quantum element method (QEM), is investigated for a 2D multi-element quantum nanostructure, where an element denotes a generic subdomain of a group of nanostructures. Unlike many other projection-based models, the basis functions for the POD approach are trained via solution data of the electron wave functions in the selected quantum state (QS) derived from direct numerical simulation of the Schrödinger equation for the nanostructure. This learning process minimizes the least square error with a small set of basis functions to reduce computational effort. Based on the QEM, the nanostructures are first partitioned into smaller generic elements (i.e., building blocks), and each of the elements is projected onto the POD space and stored in a database. For a large nanostructure, several generic elements can then be selected and glued together to perform simulation of the selected large nanostructure with the interface continuity imposed by the discontinuous Galerkin method. It has been shown that the QEM offers a reduction in numerical degrees of freedom (DoF) by 3 to 4 orders of magnitude for the trained quantum states with a high accuracy compared to direct numerical simulation. For some untrained quantum states above the trained states, a reasonably accurate prediction can be achieved with a few more DoF.

Physics-driven proper orthogonal decomposition: A simulation methodology for partial differential equations

A simulation methodology derived from a learning algorithm based on Proper Orthogonal Decomposition (POD) is presented to solve partial differential equations (PDEs) for physical problems of interest. Using the developed methodology, a physical problem of interest is projected onto a functional space described by a set of basis functions (or POD modes) that are trained via the POD by solution data collected from direct numerical simulations (DNSs) of the PDE. The Galerkin projection of the PDE is then performed to account for physical principles guided by the PDE. The procedure to construct the physics-driven POD-Galerkin simulation methodology is presented in detail, together with demonstrations of POD-Galerkin simulations of dynamic thermal analysis on a microprocessor and the Schrödinger equation for a quantum nanostructure. The physics-driven methodology allows a reduction of several orders in degrees of freedom (DoF) while maintaining high accuracy. This leads to a drastic decrease in computational effort when compared with DNS. The major steps for implementing the methodology include:•Solution data collection from DNSs of the physical problem subjected to parametric variations of the system.•Calculations of POD modes and eigenvalues from the collected data using the method of snapshots.•Galerkin projection of the governing equation onto the POD space to derive the model.

A Review on Low-Dimensional Nanomaterials: Nanofabrication, Characterization and Applications.

The development of modern cutting-edge technology relies heavily on the huge success and advancement of nanotechnology, in which nanomaterials and nanostructures provide the indispensable material cornerstone. Owing to their nanoscale dimensions with possible quantum limit, nanomaterials and nanostructures possess a high surface-to-volume ratio, rich surface/interface effects, and distinct physical and chemical properties compared with their bulk counterparts, leading to the remarkably expanded horizons of their applications. Depending on their degree of spatial quantization, low-dimensional nanomaterials are generally categorized into nanoparticles (0D); nanorods, nanowires, and nanobelts (1D); and atomically thin layered materials (2D). This review article provides a comprehensive guide to low-dimensional nanomaterials and nanostructures. It begins with the classification of nanomaterials, followed by an inclusive account of nanofabrication and characterization. Both top-down and bottom-up fabrication approaches are discussed in detail. Next, various significant applications of low-dimensional nanomaterials are discussed, such as photonics, sensors, catalysis, energy storage, diverse coatings, and various bioapplications. This article would serve as a quick and facile guide for scientists and engineers working in the field of nanotechnology and nanomaterials.

Asymmetric comb waveguide for strong interactions between atoms and light.

Coupling quantum emitters and nanostructures, in particular cold atoms and optical waveguides, has recently raised a large interest due to unprecedented possibilities of engineering light-matter interactions. In this work, we propose a new type of periodic dielectric waveguide that provides strong interactions between atoms and guided photons with an unusual dispersion. We design an asymmetric comb waveguide that supports a slow mode with a quartic (instead of quadratic) dispersion and an electric field that extends far into the air cladding for an optimal interaction with atoms. We compute the optical trapping potential formed with two guided modes at frequencies detuned from the atomic transition. We show that cold Rubidium atoms can be trapped as close as 100 nm from the structure in a 1.3-mK-deep potential well. For atoms trapped at this position, the emission into guided photons is largely favored, with a beta factor as high as 0.88 and a radiative decay rate into the slow mode 10 times larger than the free-space decay rate. These figures of merit are obtained at a moderately low group velocity of c/50.

Catalyst-free synthesis of sub-5 nm silicon nanowire arrays with massive lattice contraction and wide bandgap

The need for miniaturized and high-performance devices has attracted enormous attention to the development of quantum silicon nanowires. However, the preparation of abundant quantities of silicon nanowires with the effective quantum-confined dimension remains challenging. Here, we prepare highly dense and vertically aligned sub-5 nm silicon nanowires with length/diameter aspect ratios greater than 10,000 by developing a catalyst-free chemical vapor etching process. We observe an unusual lattice reduction of up to 20% within ultra-narrow silicon nanowires and good oxidation stability in air compared to conventional silicon. Moreover, the material exhibits a direct optical bandgap of 4.16 eV and quasi-particle bandgap of 4.75 eV with the large exciton binding energy of 0.59 eV, indicating the significant phonon and electronic confinement. The results may provide an opportunity to investigate the chemistry and physics of highly confined silicon quantum nanostructures and may explore their potential uses in nanoelectronics, optoelectronics, and energy systems.

Deciphering quantum fingerprints in electric conductance

When the electric conductance of a nano-sized metal is measured at low temperatures, it often exhibits complex but reproducible patterns as a function of external magnetic fields called quantum fingerprints in electric conductance. Such complex patterns are due to quantum–mechanical interference of conduction electrons; when thermal disturbance is feeble and coherence of the electrons extends all over the sample, the quantum interference pattern reflects microscopic structures, such as crystalline defects and the shape of the sample, giving rise to complicated interference. Although the interference pattern carries such microscopic information, it looks so random that it has not been analysed. Here we show that machine learning allows us to decipher quantum fingerprints; fingerprint patterns in magneto-conductance are shown to be transcribed into spatial images of electron wave function intensities (WIs) in a sample by using generative machine learning. The output WIs reveal quantum interference states of conduction electrons, as well as sample shapes. The present result augments the human ability to identify quantum states, and it should allow microscopy of quantum nanostructures in materials by making use of quantum fingerprints.

Recent Advances in Luminescent Downconversion: New Materials, Techniques, and Applications in Solar Cells

AbstractNumerous investigations have been done in pursuing phosphors with quantum yield (QY) greater than unity in terms of downconversion (DC) strategies, as well as applications in display, lighting, and particularly in novel solar cells with efficiency exceeding the Shockley–Queisser limit (≈30%). It is of significant interest that: i) DC of one high‐energy photon to two or more low‐energy photons is widely found in lanthanide and/or transition‐metal ions activated materials; ii) broadening of absorption spanning ultraviolet/blue range is achieved by introducing sensitizers with spin‐allowed transitions and especially by combining the advancing perovskite quantum dots, nanostructures, and organic dye molecules; iii) internal QY ≫ 1 and even measured QY > 1 are claimed in many near‐infrared (NIR) DC systems; iv) applications of downconverting layer atop commercial solar cells are performed to increase the photon conversion efficiency. In this review, the development of DC in theories, concepts, and experiments is first summarized, then the breakthrough in visible‐DC is briefly highlighted, NIR‐DC with latest progresses in models, materials, techniques as well as the corresponding optimizations and practices as effective downconverting layers is elaborated, and finally concluding remarks and perspectives in advancing NIR‐DC and applications in novel solar cells are given.

Tunneling theory for a bilayer graphene quantum dot’s single- and two-electron states

The tuneability and control of quantum nanostructures in two-dimensional materials offer promising perspectives for their use in future electronics. It is hence necessary to analyze quantum transport in such nanostructures. Material properties such as a complex dispersion, topology, and charge carriers with multiple degrees of freedom, are appealing for novel device functionalities but complicate their theoretical description. Here, we study quantum tunnelling transport across a few-electron bilayer graphene quantum dot. We demonstrate how to uniquely identify single- and two-electron dot states’ orbital, spin, and valley composition from differential conductance in a finite magnetic field. Furthermore, we show that the transport features manifest splittings in the dot’s spin and valley multiplets induced by interactions and magnetic field (the latter splittings being a consequence of bilayer graphene’s Berry curvature). Our results elucidate spin- and valley-dependent tunnelling mechanisms and will help to utilize bilayer graphene quantum dots, e.g., as spin and valley qubits.

The comb waveguide: a new tool for strong interaction between atoms and light

Coupling quantum emitters and nanostructures, in particular cold atoms and waveguides, has recently raised a large interest due to unprecedented possibilities of engineering light-matter interactions. However, the implementation of these promising concepts has been hampered by various theoretical and experimental issues. In this work, we propose a new type of periodic dielectric waveguide that provides strong interactions between atoms and guided photons with an unusual dispersion. We design an asymmetric comb waveguide that supports a slow mode with a quartic (instead of quadratic) dispersion and an electric field that extends far into the air cladding for an optimal interaction with atoms. We compute the optical trapping potential formed with two guided modes at frequencies detuned from the atomic transition. We show that cold Rubidium atoms can be trapped as close as 100 nm from the structure in a 1.3-mK-deep potential well. For atoms trapped at this position, the emission into guided photons is largely favored, with a beta factor as high as 0.88 and a radiative decay rate into the slow mode 10 times larger than the free-space decay rate.

Resistively Detected NMR Lineshapes in a Local Filling ν<1 Quantum Hall Breakdown

Resistively‐detected nuclear magnetic resonance (RDNMR) is a unique characterization method enabling highly sensitive NMR detection for a single quantum nanostructure, such as a quantum point contact (QPC). In many studies, dynamic nuclear polarization and RDNMR detection are used in a quantum Hall breakdown regime of a local QPC filling factor of 1 (νqpc = 1). However, the RDNMR lineshapes are complicated and still not fully understood yet. Herein, the nuclear spins are systematically polarized by current pumping from the close vicinity of the νqpc = 1 conductance plateau all the way down to pinch‐off point, providing clear evidence that the spin‐flip scattering between two edge channels at the lowest Landau level still occurs in the constriction even when it is close to the pinch off point. The collected RDNMR spectra reveal two sets of distinguished features. First, in a strong to intermediate tunneling regime, we observe an ordinary resistance dip lineshape with snake‐like transition frequencies, indicative of spatial modulation of electron density in the QPC. Second, in a weak tunneling regime, the spectrum turns into a dispersive lineshape, which is interpreted due to the build‐up of two sets of nuclear spin polarization that are in contact with different electron spin polarization.

Visualizing designer quantum states in stable macrocycle quantum corrals

Creating atomically precise quantum architectures with high digital fidelity and desired quantum states is an important goal in a new era of quantum technology. The strategy of creating these quantum nanostructures mainly relies on atom-by-atom, molecule-by-molecule manipulation or molecular assembly through non-covalent interactions, which thus lack sufficient chemical robustness required for on-chip quantum device operation at elevated temperature. Here, we report a bottom-up synthesis of covalently linked organic quantum corrals (OQCs) with atomic precision to induce the formation of topology-controlled quantum resonance states, arising from a collective interference of scattered electron waves inside the quantum nanocavities. Individual OQCs host a series of atomic orbital-like resonance states whose orbital hybridization into artificial homo-diatomic and hetero-diatomic molecular-like resonance states can be constructed in Cassini oval-shaped OQCs with desired topologies corroborated by joint ab initio and analytic calculations. Our studies open up a new avenue to fabricate covalently linked large-sized OQCs with atomic precision to engineer desired quantum states with high chemical robustness and digital fidelity for future practical applications.

A Review of Carbon Nanotubes Electrical Properties for Future Nanotechnology Applications

Interest in the field of carbon nanotubes is due to unique physical properties and to significant prospects for applied applications. In this review, the quantum nanostructures with outstanding electrical properties are studied on electron emission performance optimization of carbon nanotubes with the factors affecting aligning as mechanical elasticity in bending related to electronic structure. The phenomenon of field emission producing current when an electric field is applied to a conductor is also discussed. The carbon nanotubes applications in nanotechnology are clarified in this work and it can be metallic or semiconductor depending on diameter and chirality with high emission and conductivity properties to create electronic devices.

Quantum billiards with correlated electrons confined in triangular transition metal dichalcogenide monolayer nanostructures

Forcing systems through fast non-equilibrium phase transitions offers the opportunity to study new states of quantum matter that self-assemble in their wake. Here we study the quantum interference effects of correlated electrons confined in monolayer quantum nanostructures, created by femtosecond laser-induced quench through a first-order polytype structural transition in a layered transition-metal dichalcogenide material. Scanning tunnelling microscopy of the electrons confined within equilateral triangles, whose dimensions are a few crystal unit cells on the side, reveals that the trajectories are strongly modified from free-electron states both by electronic correlations and confinement. Comparison of experiments with theoretical predictions of strongly correlated electron behaviour reveals that the confining geometry destabilizes the Wigner/Mott crystal ground state, resulting in mixed itinerant and correlation-localized states intertwined on a length scale of 1 nm. The work opens the path toward understanding the quantum transport of electrons confined in atomic-scale monolayer structures based on correlated-electron-materials.

Quantum network approach to spin interferometry driven by Abelian and non-Abelian fields

We present a theory of conducting quantum networks that accounts for Abelian and non-Abelian fields acting on spin carriers. We apply this approach to model the conductance of mesoscopic spin interferometers of different geometry (such as squares and rings), reproducing recent experimental findings in nanostructured InAsGa quantum wells subject to Rashba spin-orbit and Zeeman fields (as, e.g., the manipulation of Aharonov-Casher interference patterns by geometric means). Moreover, by introducing an additional field-texture engineering, we manage to single out a previously unnoticed spin-phase suppression mechanism. We notice that our approach can also be used for the study of complex networks and the spectral properties of closed systems.

Dark-level trapping, lateral confinement, and built-in electric field contributions to the carrier dynamics in c-plane GaN/AlN quantum dots emitting in the UV range

c-plane GaN/AlN quantum dots (QDs) are promising zero-dimensional quantum nanostructures that exhibit single photon emission properties up to room temperature and even above. In this context, it is of prime interest to gain a deeper insight into the recombination dynamics of photogenerated electron–hole pairs captured by such dots. Hence, in this work, we study the time-resolved photoluminescence (PL) properties in the low injection regime and at cryogenic temperatures of c-plane GaN/AlN QD ensembles emitting above the bulk GaN bandgap in order to properly understand the nature of the recombination channels behind the observed non-exponential decay time profiles. Such decays reveal the existence of a relaxation channel competing with the radiative recombination one. It is thus observed that for the former process the dynamics is independent of the dot height, which is attributed to a reversible nonradiative transfer that could be mediated by a spin-flip process to a dark-level state. The radiative recombination process is recognizable thanks to the characteristic dependence of its lifetime with the emission energy, which is well accounted for by the built-in electric field inherent to quantum nanostructures grown along the c axis and the variations in the lateral confinement at play in such QDs. Those conclusions are drawn from the analysis of the time evolution of the PL spectra by means of a simple analytical model that enables to exclude any screening of the built-in electric field.

Temperature Dependence of Carrier Extraction Processes in GaSb/AlGaAs Quantum Nanostructure Intermediate-Band Solar Cells.

From the viewpoint of band engineering, the use of GaSb quantum nanostructures is expected to lead to highly efficient intermediate-band solar cells (IBSCs). In IBSCs, current generation via two-step optical excitations through the intermediate band is the key to the operating principle. This mechanism requires the formation of a strong quantum confinement structure. Therefore, we focused on the material system with GaSb quantum nanostructures embedded in AlGaAs layers. However, studies involving crystal growth of GaSb quantum nanostructures on AlGaAs layers have rarely been reported. In our work, we fabricated GaSb quantum dots (QDs) and quantum rings (QRs) on AlGaAs layers via molecular-beam epitaxy. Using the Stranski–Krastanov growth mode, we demonstrated that lens-shaped GaSb QDs can be fabricated on AlGaAs layers. In addition, atomic force microscopy measurements revealed that GaSb QDs could be changed to QRs under irradiation with an As molecular beam even when they were deposited onto AlGaAs layers. We also investigated the suitability of GaSb/AlGaAs QDSCs and QRSCs for use in IBSCs by evaluating the temperature characteristics of their external quantum efficiency. For the GaSb/AlGaAs material system, the QDSC was found to have slightly better two-step optical excitation temperature characteristics than the QRSC.