Energy Levels Of Quantum Dots Research Articles

Boosting energy levels in graphene magnetic quantum dots through magnetic flux and inhomogeneous gap

We study the effects of a magnetic flux and an inhomogeneous gap on the energy spectrum of graphene magnetic quantum dots (GMQDs). By considering the Dirac equation in the infinite mass framework, we can analytically obtain eigenspinor expressions. By applying boundary conditions, we obtain an energy spectrum equation in terms of system parameters such as radius, magnetic field, energy, flux, and gap. In the infinite limit, we recover Landau levels for graphene in a magnetic field. We show that the energy spectrum increases significantly in the presence of flux and a gap inside the GMQDs, which prolongs the lifetime of the trapped electron states. We show that higher flux also produces new Landau levels of negative angular momentum. Meanwhile, we find that the gap increases the separation between the electron and hole energy bands. As shown in the radial probability analysis, flux and gap emerge as influential factors in controlling electron mobility, affecting confinement, and prolonging the presence of quasi-bound states.

Cross-correlations between currents and tunnel magnetoresistance in interacting double quantum dot-Majorana wire system

We theoretically investigate the spin and charge transport properties of a double quantum dot coupled to distinct edges of the nanowire hosting Majorana zero-energy modes. The focus is on the analysis of the currents flowing through the left and right junctions and their cross-correlations. We show that the system reveals very different transport properties depending on the detuning protocol of the quantum dot energy levels. For the symmetric detuning, the current dependencies reveal only two maxima associated with resonant tunneling, and currents in the left and right arms of the system reveal weak positive cross-correlations. On the other hand, for antisymmetric detuning, the flow of electrons into drains is maximized and strongly correlated in one bias voltage direction, while for the opposite bias direction a spin blockade is predicted. Furthermore, we observe a suppression of the current cross-correlations at a highly symmetric detuning point, indicating the involvement of the Majorana zero-energy modes in the transport processes. To gain insight into the role of the spin polarization of the Majorana edge states, we analyze the spin-dependent transport characteristics by considering the relationship between the spin canting angle, which describes the coupling of the Majorana modes to the spin of the quantum dots, and the magnetic configurations of the ferromagnetic drains. Moreover, we examine the non-local zero bias anomaly in the differential conductance, detailed analysis of which revealed a specific operational mode of the device that can facilitate the identification of the Majorana presence in the quantum dot-Majorana wire system. Finally, we also consider the transport properties in different magnetic configurations of the system and discuss the behavior of the associated tunnel magnetoresistance.

The magneto thermoelectric coefficients of double quantum dots in series connected to ferromagnetic electrodes

In this paper, the charge and spin magneto coefficients of a double quantum dots system coupled in series connected to ferromagnetic electrodes with collinear magnetic alignments at finite bias voltage are theoretically investigated based on Green’s functions in a linear response regime. Magneto and spin magneto thermopower are estimated as a function of the thermal gradient and quantum dot energy levels. Magneto conductance and spin magneto conductance are also studied. The results showed that the positions of the peaks can be controlled for each value of polarization, and the magneto conductance is negative where the charge conductance for parallel alignment is lesser than that for antiparallel alignment except in the case of high spin polarization and ferromagnetic spin exchange. Enhancing the magneto thermoelectric coefficients of double quantum dots is a promising step in creating adaptable magnetic switching for thermoelectric generations, which could find use in thermal energy-based logic circuits or heat control applications.

Optimizing Energy Levels and Improving Film Compactness in PbS Quantum Dot Solar Cells by Silver Doping.

PbS quantum dot (QD) solar cells harvest near-infrared solar radiation. Their conventional hole transport layer has limited hole collection efficiency due to energy level mismatch and poor film quality. Here, how to resolve these two issues by using Ag-doped PbS QDs are demonstrated. On the one hand, Ag doping relieves the compressive stress during layer deposition and thus improves film compactness and homogeneity to suppress leakage currents. On the other hand, Ag doping increases hole concentration, which aligns energy levels and increases hole mobility to boost hole collection. Increased hole concentration also broadens the depletion region of the active layer, decreasing interface charge accumulation and promoting carrier extraction efficiency. A champion power conversion efficiency of 12.42% is achieved by optimizing the hole transport layer in PbS QD solar cells, compared to 9.38% for control devices. Doping can be combined with compressive strain relief to optimize carrier concentration and energy levels in QDs, and even introduce other novel phenomena such as improved film quality.

Generating Long-Lived Charge Carriers in CdS Quantum Dots by Cu-Doping for Photocatalytic CO2 Reduction.

Converting CO2 into high-value-added chemicals has been recognized as a promising way to tackle the fossil fuel crisis. Quantum dots (QDs) have been extensively studied for photocatalytic CO2 reduction due to their excellent optoelectronic properties. However, most of the photogenerated charge carriers recombine before they participate in the photocatalytic reaction. It is crucial to regulate the charge carriers to minimize undesired charge recombination, thus, promoting surface photocatalysis. Herein, we report a copper-doped CdS (Cu:CdS) QD photocatalyst for CO2 reduction. Density functional theory simulations and experimental results demonstrate that Cu dopants create intermediate energy levels in CdS QDs that can extend the lifetime of exciton charge carriers. Furthermore, the long-lived charge carriers can be harnessed for the photocatalytic reaction on Cu:CdS QDs. The resultant Cu:CdS QDs exhibited a significantly enhanced photocatalytic activity toward CO2 reduction compared to the pristine CdS QDs. This work highlights the importance of charge regulation in photocatalysts and opens new pathways for the exploration of efficient QD photocatalysts.

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.

Modelling and experimental characterization of double layer InP/AlGaInP quantum dot laser

Spectrum of an InP/AlGaInP self- assembled double-layer quantum dot (QD) laser fabricated by metal–organic vapor-phase epitaxy is theoretically and experimentally investigated. A bimodal QD size distribution (small and large QD groups) was detected which is formed during the fabrication. A model is proposed based on rate equations accounting for the superposition of two inhomogeneous QD groups. The total output power and the power spectral density (PSD) of the fabricated QD laser are experimentally characterized at room temperature. The output spectrum is segmented into the sum of two Gaussians curves (super Gaussian) belonging to the small and large QD groups. The peak PSD and the spectral width of each group are extracted and their dependency on the injected current density is analysed. The peak of the large QDs is found to be dominant at small current while the peak of the small QDs dominated at high current alongside a reduction in its spectral width leading to lasing based on them. This behaviour is attributed to the saturation of the large QDs energy levels due to its relatively long radiative lifetime. The experimental analysis is in a good agreement with the theoretical results.

Remarks on a quantum dot described by scalar exponential potentials

Based on the power-exponential potential model for quantum dots, we study the confinement of a particle to a spherical quantum dot from a long-range potential. We show that analytical solutions to the Schrödinger equation can be achieved for s-waves. We also show that the energy levels of the spherical quantum dot are influenced by the topology of a global monopole. Further, we show that a non-null revival time exists and it can be influenced by the global monopole topology.

Synergy of Front-Surface Energy-Level Gradient and Lattice Anchoring Effect for Enhancing Perovskite Solar Cell Performance.

A front surface gradient of the absorber valence band can effectively reduce the open-circuit voltage (VOC) loss of perovskite solar cells by suppressing the minority carrier concentration near the front surface. However, the existing method is limited to the one-step fabrication process, resulting in underachieved photon harvesting and power conversion efficiency (PCE). To solve the problem, ZnCd-based alloy quantum dots (QDs) are utilized to create a valence-band-maximum gradient at the front surface of a two-step processed FAPbI3 absorber. This design significantly enhances VOC without requiring surface passivation. Furthermore, it is demonstrated that reducing the QD-perovskite lattice mismatch while maintaining QD's energy levels mitigates nonradiative recombination without compromising the front surface gradient effect. As a result, normal-structured perovskite solar cells achieve a VOC equivalent to 93% of the Schockley-Queisser limit and a PCE of 24.37%.

Josephson current across a Double Quantum Dot Josephson junction in T-Shape Configuration

By implementing the Keldysh non-equilibrium Green's function equation of motion approach, Josephson current has been examined across a T-shaped uncorrelated double quantum dot Josephson junction. The behavior of the Josephson current as a function of the main quantum dot energy level for varied interdot tunneling and different dot-lead coupling strengths is examined. With this configuration, we illustrate that the side-attached quantum dot offers an alternative route for electron transmission, which modifies the Josephson current by varying interdot tunneling. Further, we also investigate how the dot-lead coupling strengths affect the Josephson current.

Energy Levels of Quantum Dots in Monolayer of Molybdenum Disulfide MoS$$_2$$

Energy Levels of Quantum Dots in Monolayer of Molybdenum Disulfide MoS$$_2$$

Effect of inhomogeneous gap on energy levels in graphene magnetic quantum dots

We study the energy levels of fermions magnetically confined in graphene through an electrostatic potential and an inhomogeneous gap, including Δ2 inside and Δ1 outside of the generated quantum dots. Our findings illustrate that the energy levels exhibit either symmetric or asymmetric behavior, contingent upon the signs of τ and the quantum angular momentum number m. It is discovered that Δ1 acts by breaking the symmetry and generating a new set of energies.

Application of Scanning Tunneling Microscopy and Spectroscopy in the Studies of Colloidal Quantum Qots.

Colloidal quantum dots display remarkable optical and electrical characteristics with the potential for extensive applications in contemporary nanotechnology. As an ideal instrument for examining surface topography and local density of states (LDOS) at an atomic scale, scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) has become indispensable approaches to gain better understanding of their physical properties. This article presents a comprehensive review of the research advancements in measuring the electronic orbits and corresponding energy levels of colloidal quantum dots in various systems using STM and STS. The first three sections introduce the basic principles of colloidal quantum dots synthesis and the fundamental methodology of STM research on quantum dots. The fourth section explores the latest progress in the application of STM for colloidal quantum dot studies. Finally, a summary and prospective is presented.

A robust and unique approach for tuning the energy level of Ag2Se quantum dots via “on-surface” manipulation of nitrogen-containing groups of surface-coordinated ligands

A robust and unique approach for tuning the energy level of Ag2Se quantum dots via “on-surface” manipulation of nitrogen-containing groups of surface-coordinated ligands

Phase and Thermal-Driven Transport Across T-Shaped Double Quantum Dot Josephson Junction

The phase and thermal driven transport properties of the T-shaped uncorrelated double quantum dot Josephson junction are analyzed by using Keldysh non-equilibrium Green's function equation of motion technique. In this setup, we have shown that the side-attached quantum dot provides an additional route for electron transmission which is affecting the transport properties by adjusting the interdot hopping between the main dot and the side dot. We began with investigating the impact of interdot hopping on Andreev bound states and Josephson supercurrent. When a small thermal bias is applied across the superconducting leads, the system exhibits a finite thermal response which is primarily due to the, thermally induced, quasi-particle current. The behavior of the Josephson supercurrent and the quasi-particle current flowing through the quantum dots is examined for various interdot hopping and thermal biasing. Finally, the system is considered in an open circuit configuration where the thermally driven quasi-particle current is compensated by the phase-driven Josephson supercurrent and the thermophase effect is observed. The effect of interdot hopping and the position of quantum dot energy level on the thermophase Seebeck coefficient is investigated

Calibration‐Free and High‐Sensitivity Microwave Detectors Based on InAs/InP Nanowire Double Quantum Dots

AbstractAt the cutting‐edge of microwave detection technology, novel approaches which exploit the interaction between microwaves and quantum devices are rising. In this study, microwaves are efficiently detected exploiting the unique transport features of InAs/InP nanowire double quantum dot‐based devices, suitably configured to allow the precise and calibration‐free measurement of the local field. Prototypical nanoscale detectors are operated both at zero and finite source‐drain bias, addressing and rationalizing the microwave impact on the charge stability diagram. The detector performance is addressed by measuring its responsivity, quantum efficiency and noise equivalent power that, upon impedance matching optimization, are estimated to reach values up to ≈2000 A W−1, 0.04 and ≈, respectively. The interaction mechanism between the microwave field and the quantum confined energy levels of the double quantum dots is unveiled and it is shown that these semiconductor nanostructures allow the direct assessment of the local intensity of the microwave field without the need for any calibration tool. Thus, the reported nanoscale devices based on III‐V nanowire heterostructures represent a novel class of calibration‐free and highly sensitive probes of microwave radiation, with nanometer‐scale spatial resolution, that may foster the development of novel high‐performance microwave circuitries.

Effect of energy level configuration on storage of optical solitons in InAs/GaAs quantum dot electromagnetically induced transparency medium

Based on the current growth technology of quantum dot in the experiment, considering that the probe fields and control fields at different frequencies are coupled between different energy levels of the InAs/GaAs quantum dot, the ladder-type, Λ-type and V-type energy level configurations can be formed. The linear and nonlinear properties of these energy level configurations of InAs/GaAs quantum dots are studied by using semiclassical theory combined with multiple scale method. It is shown that in the linear case, electromagnetic induction transparency windows can be formed among ladder-type, Λ-type and V-type energy level configurations. And the width of the transparent window increases with the strength of the control pulse increasing. For the nonlinear case, under the current experimental condition, optical solitons can be formed and stored in ladder-type configuration and <inline-formula><tex-math id="M6">\begin{document}$ {{\Lambda }} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20221965_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20221965_M6.png"/></alternatives></inline-formula>-type energy level configuration. However, optical solitons cannot be formed in the V-type energy level configurations, which is because the nonlinear effect of the system is very weak. Furthermore, it is demonstrated that the fidelity of the storage and retrieval of the optical solitons is higher than that of linear optical pulse and strongly nonlinear optical pulse. Interestingly, it is also found that the amplitude of stored optical solitons in <inline-formula><tex-math id="M8">\begin{document}$ {{\Lambda }} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20221965_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20221965_M8.png"/></alternatives></inline-formula>-type energy level configuration is higher than that in ladder-type energy level configuration. This study provides a theoretical basis for semiconductor quantum dot devices to modulate the amplitude of the stored optical solitons.

Novel ultra-wideband fluorescence material: Defect state control based on nickel-doped semiconductor QDs embedded in inorganic glasses

In recent years, the development of an environmentally friendly quantum dots (QDs) embedded luminous solid by a simple method has attracted considerable attention. In this study, semiconductor ZnS QDs were successfully prepared in an inorganic matrix of amorphous glass, which yielded beneficial broad-band emission in the long-wavelength region of the visible range. The strong red emission belonged to the defect state energy level of the ZnS QDs, which could be enhanced by incorporation of nickel ions into the fixed matrix to regulate the defects state. The novel material had a small self-absorption, wide excitation and emission ranges, and thus potential applications in light-conversion devices, luminescent solar concentrators, and solar cell cover glasses.

Quantum Mechanical Analysis Based on Perturbation Theory of CdSe/ZnS Quantum-Dot Light-Emission Properties.

A simulation of quantum dot (QD) energy levels was designed to reproduce a quantum mechanical analytic method based on perturbation theory. A Schrödinger equation describing an electron–hole pair in a QD was solved, in consideration of the heterogeneity of the material parameters of the core and shell. The equation was solved numerically using single-particle basis sets to obtain the eigenstates and energies. This approach reproduced an analytic solution based on perturbation theory, while the calculation was performed using a numerical method. Owing to the effectiveness of the method, QD behavior according to the core diameter and external electric field intensity could be investigated reliably and easily. A 9.2 nm diameter CdSe/ZnS QD with a 4.2 nm diameter core and 2.5 nm thick shell emitted a 530 nm green light, according to an analysis of the effects of core diameter on energy levels. A 4 nm redshift at V/cm electric field intensity was found while investigating the effects of external electric field on energy levels. These values agree well with previously reported experimental results. In addition to the energy levels and light emission wavelengths, the spatial distributions of wavefunctions were obtained. This analysis method is widely applicable for studying QD characteristics with varying structure and material compositions and should aid the development of high-performance QD technologies.

Electrochemically Determining Electronic Structure of ZnO Quantum Dots with Different Surface Ligands.

The electronic structure of quantum dots (QDs) including band edges and possible trap states is an important physical property for optoelectronic applications. The reliable determination of the energy levels of QDs remains a big challenge. Herein we employ cyclic voltammetry (CV) to determine the energy levels of three types of ZnO QDs with different surface ligands. Coupled with spectroscopic techniques, it is found that the onset potential of the first reductive wave is likely related to the conduction band edges while the first oxidative wave originates from the trap states. The determined specific energy levels in CV further demonstrates that the ZnO QDs without surface ligands mainly have oxygen interstitial defects whilst the ZnO QDs covered with ligands contain oxygen vacancies. The present electrochemical method offers a powerful and effective way to determine the energy levels of wide bandgap ZnO QDs, which will boost their device performance.