Electrons In Quantum Dots Research Articles

Graphene/III–V Quantum Dot Mixed-Dimensional Heterostructure for Enhanced Radiative Recombinations via Hole Carrier Transfer

Fabrication of high quantum efficiency nanoscale device is challenging due to increased carrier loss at surface. Low dimensional materials such 0D quantum dots and 2D materials have been widely studied to mitigate the loss. Here, we demonstrate a strong photoluminescence enhancement from graphene/III-V quantum dot mixed-dimensional heterostructures. The distance between graphene and quantum dots in the 2D/0D hybrid structure determines the degree of radiative carrier recombination enhancement from 80% to 800% compared to the quantum dot only structure. Time-resolved photoluminescence decay also shows increased carrier lifetimes when the distance decreases from 50 to 10 nm. We propose that the optical enhancement is due to energy band bending and hole carrier transfer, which repair the imbalance of electron and hole carrier densities in quantum dots. This 2D graphene/0D quantum dot heterostructure shows promise for high performance nanoscale optoelectronic devices.

Single-Shot Readout of Multiple Donor Electron Spins with a Gate-Based Sensor

Proposals for large-scale semiconductor spin-based quantum computers require high-fidelity single-shot qubit readout to perform error correction and read out qubit registers at the end of a computation. However, as devices scale to larger qubit numbers integrating readout sensors into densely packed qubit chips is a critical challenge. Two promising approaches are minimizing the footprint of the sensors, and extending the range of each sensor to read more qubits. Here we show high-fidelity single-shot electron spin readout using a nanoscale single-lead quantum dot (SLQD) sensor that is both compact and capable of reading multiple qubits. Our gate-based SLQD sensor is deployed in an all-epitaxial silicon donor spin-qubit device, and we demonstrate single-shot readout of three 31P donor quantum dot electron spins with a maximum fidelity of 95%. Importantly, in our device the quantum dot confinement potentials are provided inherently by the donors, removing the need for additional metallic confinement gates and resulting in strong capacitive interactions between sensor and donor quantum dots. Our results are consistent with a 1/d1.4 scaling of the capacitive coupling between sensor and 31P dots (where d is the sensor-dot distance), compared to 1/d2.5−3.0 in gate-defined quantum dot devices. Due to the small qubit size and strong capacitive interactions in all-epitaxial donor devices, we estimate a single sensor can achieve single-shot readout of approximately 15 qubits in a linear array, compared to 3–4 qubits for a similar sensor in a gate-defined quantum dot device. Our results highlight the potential for spin-qubit devices with significantly reduced sensor densities.Received 19 October 2021Revised 19 August 2022Accepted 4 January 2023DOI:https://doi.org/10.1103/PRXQuantum.4.010319Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasQuantum computationQuantum information with solid state qubitsQuantum metrologyQuantum Information

Interdependence of External Magnetic and Temperature Effect on Thermodynamic Properties of GaAs Two Electron Quantum Dot

The particular interest of this paper is to investigate the impact of various values of temperature exposed to weak and strong magnetic field strength. A thermodynamic property's oscillatory change as a function of magnetic field effect (B) intensifies the quantization of electron orbits in a constant magnetic field intensity and is the primary contributor to the de Haas-van Alphen effects due to cyclotron frequency and its impact on localizing electron at circular region imposed with the magnetic field that is in contrary to the result of the temperature effect. Thus the interdependent effects of external magnetic field and temperature on thermodynamic properties are studied with harmonic oscillator potentials considering material parameters of GaAs quantum dot. The finite energy state is analytically solved using Nikiforov-Uvarov mathematical formalism. Moreover, the direct impact of the external magnetic fields and temperature on thermodynamic properties of the system is analyzed, and numerically simulated using matlab R2017a version. The dominance of temperature over the external magnetic field and vice versa effect is investigated, thus the value specific heat capacity fluctuated, while the equiponderate impact of temperature and magnetic field shows similar steady values of the specific heat capacity. The study clearly shows the interdependence of magnetic field and temperature affect thermodynamic quantities: partition function, mean energy, entropy, and specific heat capacity.

Magnetic and Electronic Properties of Edge-Modified Triangular WS2 and MoS2 Quantum Dots

The magnetic and electronic properties of zigzag-triangular WS2 and MoS2 quantum dots are investigated using density functional theory calculations. The pristine WS2 and MoS2 nanodots hold permanent spin on their edges which originates from the unpaired electrons of the transition metals at the edges. The ferromagnetic spin ordering in zigzag-triangular WS2 and MoS2 can be transformed to antiferromagnetic ordering with S = 0 and to nonmagnetic, respectively, by edge passivation with 2H. The calculations of the Curie Temperature indicate that these magnetic states are stable and withstand room temperature. The paramagnetic susceptibility of these structures significantly decreases by edge sulfuration. Moreover, it can be converted to diamagnetic susceptibility by edge passivation with 2H as found in WS2 nanodots. These structures are semiconductors with energy gaps of ~3.3 eV that decrease unexpectedly by edge passivation due to the existence of lone pairs from S atoms that give a high contribution to the low-energy molecular orbitals. With these preferable magnetic properties and controlled electronic ones, WS2 and MoS2 quantum dots are potential candidates for spintronic applications.

Amine Hole Scavengers Facilitate Both Electron and Hole Transfer in a Nanocrystal/Molecular Hybrid Photocatalyst

A well-known catalyst, fac-Re(4,4'-R2-bpy)(CO)3Cl (bpy = bipyridine; R = COOH) (ReC0A), has been widely studied for CO2 reduction; however, its photocatalytic performance is limited due to its narrow absorption range. Quantum dots (QDs) are efficient light harvesters that offer several advantages, including size tunability and broad absorption in the solar spectrum. Therefore, photoinduced CO2 reduction over a broad range of the solar spectrum could be enabled by ReC0A catalysts heterogenized on QDs. Here, we investigate interfacial electron transfer from Cd3P2 QDs to ReC0A complexes covalently bound on the QD surface, induced by photoexcitation of the QD. We explore the effect of triethylamine, a sacrificial hole scavenger incorporated to replenish the QD with electrons. Through combined transient absorption spectroscopic and computational studies, we demonstrate that electron transfer from Cd3P2 to ReC0A can be enhanced by a factor of ∼4 upon addition of triethylamine. We hypothesize that the rate enhancement is a result of triethylamine possibly altering the energetics of the Cd3P2-ReC0A system by interacting with the quantum dot surface, deprotonation of the quantum dot, and preferential solvation, resulting in a shift of the conduction band edge to more negative potentials. We also observe the rate enhancement in other QD-electron acceptor systems. Our findings provide mechanistic insights into hole scavenger-quantum dot interactions and how they may influence photoinduced interfacial electron transfer processes.

Tunable Mid-Infrared Interband Emission from Tensile-Strained InGaAs Quantum Dots.

We demonstrate the ability to tailor self-assembled growth of In0.5Ga0.5As quantum dots (QDs) on GaSb(111)A surfaces by molecular beam epitaxy. Spontaneous formation via the Volmer-Weber growth mode produces QDs with excellent structural and optical quality. By harnessing tensile strain to reduce their band gap energy, these QDs are characterized by light emission that extends into the midwave infrared wavelength range of 3.2-3.9 μm (0.318-0.388 eV). As we increase QD size, we can tune the band alignment from type-III to type-II, where light emission occurs due to interband recombination between quantum confined electrons in the InGaAs QDs and holes in the GaSb barriers. Of particular interest is an unusual blue-shift in emission wavelength with increasing QD size, which we attribute to the incorporation of Sb into the InGaAs QDs from the GaSb barriers. By expanding this approach to produce tensile-strained QDs from other narrow band gap semiconductors, we anticipate the development of a range of highly tunable mid-infrared light sources.

On-demand electrical control of spin qubits

Once called a 'classically non-describable two-valuedness' by Pauli, the electron spin forms a qubit that is naturally robust to electric fluctuations. Paradoxically, a common control strategy is the integration of micromagnets to enhance the coupling between spins and electric fields, which, in turn, hampers noise immunity and adds architectural complexity. Here we exploit a switchable interaction between spins and orbital motion of electrons in silicon quantum dots, without a micromagnet. The weak effects of relativistic spin-orbit interaction in silicon are enhanced, leading to a speed up in Rabi frequency by a factor of up to 650 by controlling the energy quantization of electrons in the nanostructure. Fast electrical control is demonstrated in multiple devices and electronic configurations. Using the electrical drive, we achieve a coherence time T2,Hahn ≈ 50 μs, fast single-qubit gates with Tπ/2 = 3 ns and gate fidelities of 99.93%, probed by randomized benchmarking. High-performance all-electrical control improves the prospects for scalable silicon quantum computing.

Polarization-independent enhancement of optical absorption in a GaAs quantum well embedded in an air-bridge bull’s-eye cavity with metal electrodes

Electron spins in gate-defined quantum dots (QDs) formed in semiconductor quantum wells (QWs) are promising stationary qubits for implementing large-scale quantum networks in a scalable manner. One key ingredient for such a network is an efficient photon–spin interface that converts any polarization state of a flying photonic qubit to the corresponding spins state of the electron in gate-defined QDs. A bull’s-eye cavity is an optical cavity structure that can enhance the photon absorption of an embedded gate-defined QD without polarization dependence. In this paper, we report the successful fabrication of air-bridge bull’s-eye cavities with metal electrodes and demonstrate the nearly polarization-independent optical absorption of a GaAs QW embedded in the cavities. This work marks an important step toward realizing an efficient photon–spin interface using gate-defined QDs.

Flopping-mode electron dipole spin resonance in the strong-driving regime

Achieving high-fidelity control of spin qubits with conventional electron dipole spin resonance (EDSR) requires large magnetic field gradients of about $1\phantom{\rule{0.16em}{0ex}}{\mathrm{mTnm}}^{\ensuremath{-}1}$, which also couple the qubit to charge noise, and large drive amplitudes of order 1 mV. The flopping mode is an alternative method to drive EDSR of an electron in a double quantum dot, where the large displacement between both dots increases the driving efficiency. We propose to operate the flopping mode in the strong-driving regime to use the full magnetic field difference between the two dots. In simulations, the reduced required magnetic field gradients suppress the infidelity contribution of charge noise by more than two orders of magnitude, while providing Rabi frequencies of up to $60\phantom{\rule{0.16em}{0ex}}\mathrm{M}\mathrm{Hz}$. However, the near degeneracy of the conduction band in silicon introduces a valley degree of freedom that can degrade the performance of the strong-driving mode. This necessitates a valley-dependent pulse optimization and makes operation to the strong-driving regime questionable.

Heat Capacity of Two Interacting Electrons in Coupled Double Quantum Dots

Heat Capacity of Two Interacting Electrons in Coupled Double Quantum Dots

Prolate spheroidal quantum dot: Effect of dot size and eccentricity on confined energy states

We have explored the energy states of an electron trapped in a prolate spheroidal shape CdSe quantum dot (QD) within a finite barrier height, utilizing the Schrodinger equation in a prolate spheroidal coordinate system. The calculation of various eigen energy states have been performed as a function of the QD volume and spheroid eccentricity. Our findings demonstrate that with the increase in both the QD’s volume and the eccentricity, the confinement energy decreases. Furthermore, we discovered that for any given l value, the energy of states corresponding to higher and lower sublevels (m) decreases with an increase in eccentricity. However, the decrease in energy of sublevels with increasing eccentricity varied for different m values. Additionally, a comparison of the energy states of a confined electron in prolate spheroidal and spherical quantum dots revealed that they are located at different positions.

Influence of Magnetic Field on Electron Energy Levels in Semconductor Quantum Dots in the Presence of Spin-Orbit Interactions

Energy levels of electrons in semiconductor quantum dots are obtained within the framework of perturbation theory taking into account the Rashba and Dresselhaus spin-orbit interactions and an external magnetic field. The circular quantum dots are simulated by a new smooth confinement potential of a finite depth and width. The dependence of energy levels on a constant uniform magnetic field and potential parameters is presented.

Wigner molecules in phosphorene quantum dots

We study Wigner crystallization of electron systems in phosphorene quantum dots with confinement of an electrostatic origin with both circular and elongated geometry. The anisotropy of the effective mass allows for the formation of Wigner molecules in the laboratory frame with a confined charge density that has lower symmetry than the confinement potential. We find that in circular quantum dots separate single-electron islands are formed for two and four confined electrons but not for three trapped carriers. The spectral signatures of the Wigner crystallization to be resolved by transport spectroscopy are discussed. Systems with Wigner molecule states are characterized by a nearly degenerate ground state at $B=0$ and are easily spin-polarized by the external magnetic field. In electron systems for which the single-electron islands are not formed, a more even distribution of excited states at $B=0$ is observed, and the confined system undergoes ground state symmetry transitions at magnetic fields of the order of 1 Tesla. The system of five electrons in a circular quantum dot is indicated as a special case with two charge configurations that appear in the ground-state as the magnetic field is changed: one with the single electron islands formed in the laboratory frame and the other where only the pair-correlation function in the inner coordinates of the system has a molecular form as for three electrons. The formation of Wigner molecules of quasi-1D form is easier for the orientation of elongated quantum dots along the zigzag direction with heavier electron mass. The smaller electron effective mass along the armchair direction allows for freezing out the transverse degree of freedom in the electron motion.

Abnormal quantum moment of inertia and structural properties of electrons in 2D and 3D quantum dots: an ab initio path-integral Monte Carlo study

We present extensive new direct path-integral Monte Carlo results for electrons in quantum dots in two and three dimensions. This allows us to investigate the nonclassical rotational inertia (NCRI) of the system, and we find an abnormal negative quantum moment of inertia (2014 Phys. Rev. Lett. 112 235301) under some conditions. In addition, we study the structural properties by computing a re-normalized, angular-resolved center-two particle correlation function. Remarkably, we find no connection between the spatial structure and the NCRI, since the former can be nearly identical for Fermi- and Bose-statistics for parameters where the superfluid fraction is diverging towards negative infinity.

Counterbalancing of electron and hole transfer in quantum dots for enhanced photocatalytic H2 evolution

Counterbalancing of electron and hole transfer in quantum dots for enhanced photocatalytic H2 evolution

Electron–hole interaction in cylindrical quantum dots

The Coulomb interaction of electron and hole in a cylindrical quantum dot in the frame of the first perturbation theory and the effective mass approximation is studied. The analogue of Brus’s equation for Coulomb interaction of electron–hole pair in a spherical quantum dot is obtained for a cylindrical nanocrystal and an expression for the energy band gap shift is derived, which can be used for explaining of some experimental results obtained for MoS2 and 3C-SiC nanocrystals.

Amorphous NiP quantum dots as a robust electrocatalyst for oxygen evolution reaction

• 2.7 nm NiP amorphous quantum dots were prepared by metal-support interaction. • Fe/N/C support tunes the NiP quantum dots electron structure configuration. • Overpotential of 242 mV at 10 mA cm −2 and Tafel slopes of 47 mV dec −1 are achieved. Amorphous nickel phosphorus (NiP) is regarded as one of the most promising efficient catalysts for oxygen evolution reaction (OER). Here, amorphous NiP quantum dots with a diameter of 2.7 nm were successfully prepared by metal-Fe/N/C support interaction. Owing to the high specific surface areas, the abundant defects caused by N-doping, and the charge transfer between Fe single atoms and NiP, amorphous NiP quantum dots achieve a low overpotential of 242 mV at 10 mA cm −2 . This work provides an efficient way to construct amorphous quantum dots by strong metal-support interaction.

Optical Properties of Cylindrical Quantum Dots with Hyperbolic-Type Axial Potential under Applied Electric Field.

In this paper, we have researched the electronic and optical properties of cylindrical quantum dot structures by selecting four different hyperbolic-type potentials in the axial direction under an axially-applied electric field. We have considered a position-dependent effective mass model in which both the smooth variation of the effective mass in the axial direction adjusted to the way the confining potentials change and its abrupt change in the radial direction have been considered in solving the eigenvalue differential equation. The calculations of the eigenvalue equation have been implemented considering both the Dirichlet conditions (zero flux) and the open boundary conditions (non-zero flux) in the planes perpendicular to the direction of the applied electric field, which guarantees the validity of the results presented in this study for quasi-steady states with extremely high lifetimes. We have used the diagonalization method combined with the finite element method to find the eigenvalues and eigenfunction of the confined electron in the cylindrical quantum dots. The numerical strategies that have been used for the solution of the differential equations allowed us to overcome the multiple problems that the boundary conditions present in the region of intersection of the flat and cylindrical faces that form the boundary of the heterostructure. To calculate the linear and third-order nonlinear optical absorption coefficients and relative changes in the refractive index, a two-level approach in the density matrix expansion is used. Our results show that the electronic and, therefore, optical properties of the structures focused on can be adjusted to obtain a suitable response for specific studies or goals by changing structural parameters such as the widths and depths of the potentials in the axial direction, as well as the electric field intensity.

Small Charging Energies and g-Factor Anisotropy in PbTe Quantum Dots

PbTe is a semiconductor with promising properties for topological quantum computing applications. Here, we characterize electron quantum dots in PbTe nanowires selectively grown on InP. Charge stability diagrams at zero magnetic field reveal large even–odd spacing between Coulomb blockade peaks, charging energies below 140 μeV and Kondo peaks in odd Coulomb diamonds. We attribute the large even–odd spacing to the large dielectric constant and small effective electron mass of PbTe. By studying the Zeeman-induced level and Kondo splitting in finite magnetic fields, we extract the electron g-factor as a function of magnetic field direction. We find the g-factor tensor to be highly anisotropic with principal g-factors ranging from 0.9 to 22.4 and to depend on the electronic configuration of the devices. These results indicate strong Rashba spin–orbit interaction in our PbTe quantum dots.

Proposal for a cavity-induced measurement of the exchange coupling in quantum dots

In spin qubit arrays the exchange coupling can be harnessed to implement two-qubit gates and to realize intermediate-range qubit connectivity along a spin bus. In this work, we propose a scheme to characterize the exchange coupling between electrons in adjacent quantum dots. We investigate theoretically the transmission of a microwave resonator coupled to a triple quantum dot occupied by two electrons. We assume that the right quantum dot (QD) is always occupied by one electron while the second electron can tunnel between the left and center QDs. If the two electrons are in adjacent dots they interact via the exchange coupling. By means of analytical calculations we show that the transmission profile of the resonator directly reveals the value of the exchange coupling strength between two electrons. From perturbation theory up to second order we conclude that the exchange can still be identified in the presence of magnetic gradients. A valley splitting comparable to the interdot tunnel coupling will lead to further modifications of the cavity transmission dips that also depend on the valley phases.