Magnetic Semiconductor Quantum Dots Research Articles

Unraveling the physical properties of Mn-doped CdS diluted magnetic semiconductor quantum dots for potential application in quantum spintronics

Unraveling the physical properties of Mn-doped CdS diluted magnetic semiconductor quantum dots for potential application in quantum spintronics

Investigation of the Physical Properties of Mn-Doped CDS Diluted Magnetic Semiconductor Quantum Dots: Non-Linear Band Gap Variation with Downwards Bowing

Investigation of the Physical Properties of Mn-Doped CDS Diluted Magnetic Semiconductor Quantum Dots: Non-Linear Band Gap Variation with Downwards Bowing

Room temperature CNOT operation in a diluted magnetic semiconductor quantum dot

Population evolution in a magnetic impurity doped semiconductor quantum dot has been studied by applying a sequence of pulses of chosen pulse area. By optical excitation mechanism, the population in (J_z=+3/2), heavy hole state of valence band is carried over to (J_z=-3/2), valance band state, via the (J=+1/2) conduction band states. The injected microwaves entangle conduction band states. This arrangement is successfully employed to ascertain quantum CNOT operation, and the calculation predicts maximum fidelity of 80\% for the CNOT operation.

The Influence of Electron-Electron Interaction on RKKY Magnetic Coupling between Local Spins of Diluted Magnetic Semiconductor Quantum Dots

Regulated magnetic interactions in semiconductor nanometer structure have a strong application background in spin electronics and quantum information processing. This paper mainly studies the influence of electron interaction on the magnetic coupling between diluted magnetic semiconductor quantum dots embedded in semiconductors. Taking into account the interaction between electrons, we will calculate the RKKY Magnetic interaction between the local spins of two diluted magnetic semiconductor quantum dots embedded in the semiconductor applying the Keldysh Green’s function method. For the electronic interaction, we use the simple self-consistent mean-field approximation.

Spin Dynamics in Mn:ZnSe Quantum Dots: A Pulsed High-Frequency EPR Study

Dilute magnetic semiconductor quantum dots (DMSQDs) have emerged as potential materials for future spin-based technologies. A critical parameter for the development of this technology is a relative...

Tuning Paramagnetic effect of Co-Doped CdS diluted magnetic semiconductor quantum dots

Abstract Diluted magnetic semiconductor quantum dots (DMS-QDs) are known for their outstanding optical and magnetic properties. II–VI DMS-QDs, in particular, are interesting for spintronics, nonvolatile memory, and magneto-optical devices. Therefore, studying the optical and magnetic properties of different II-VI semiconductors doped with transition metal atoms is of great importance. Tuning II-VI QDs optical properties can be mastered by changing the QDs particle size and/or structure. However tuning the magnetic properties of DMS-QDs is still within trial and error verification, although it is crucial in targeting different applications in spintronics. We hereby demonstrate, the ability to tune the paramagnetic effect of homogeneous Co-doped CdS QDs following a co-precipitation synthesis route with different Co2+ concentrations. The structural, optical and magnetic properties have been comprehensively studied. The dopant cobalt atoms concentration and chemical-configuration were precisely tracked by x-ray photoemission spectroscopy. Excitingly, the different Co-concentrations of 2, 5 and 10% significantly improve the magnetic properties of the CdS QDs, which exhibit a paramagnetic concentration-dependent effect. With 10% of Co atoms, we were able to achieve ∼ 200 × 10 − 6 molar susceptibility, that is, the same value to that of pure CoS. Thus we could obtain the highest possible paramagnetic effect in the CdS semiconducting matrix exhibiting 2.76 eV band gap, i.e. in the visible range. This efficacious result encourages the use of the present method in preparing DMS-QDs materials targeting various spintronics applications.

О доминирующем механизме безызлучательного возбуждения ионов марганца в II-VI полумагнитных полупроводниках

Doping of group II-VI semiconductors and low-dimensional structures based on them with manganese leads to effective quenching of electro-and photoluminescence provided that the electron excitation energy of the crystal exceeds the energy of the intracenter transition of Mn2+ EMn и 2.1 eV. Quenching involves efficient energy transfer from the photoexcited crystal to Mn2 + ions. There are three possible mechanisms of this non-radiative energy transfer: dipole-dipole, exchange and related mechanism associated with sp-d mixing. Although the dipole-dipole mechanism is thought to be ineffective due to the forbidden intracenter transition in Mn2+, and the spin-dependent exchange mechanism is dominant, not all experimental facts accumulated to date supports this conclusion. The article discusses two experimental approaches to identify the dominant mechanism of energy transfer to Mn2 + ions and to evaluate the partial contributions of the mechanisms. One of these approaches is related to optically detectable magnetic resonance at single diluted magnetic semiconductor quantum dots (QD), the second - to plasmon amplification of energy transfer to Mn2 + ions by means of dipole-dipole interaction.

Volume and Concentration Scaling of Magnetism in Dilute Magnetic Semiconductor Quantum Dots

Investigation of the magnetism of dilute magnetic semiconductors (DMSs) by changing particle dimensionality and doping concentration and its complete understanding is a major step toward their application in multifunctional devices. The importance of effects, such as magnetization reversal with size and magnetic invariance with respect to doping concentration for an appropriate functioning of DMS systems, is empirically well-known. However, explicit demonstration of these effects, specifically in nanomaterials, has so far not been studied mainly due to the lack of synthetic handle. In this work, we have demonstrated the prerequisites of DMS materials by isolating origins of magnetism arising due to clustering of magnetic dopant ions as well as sp–d exchange interaction with the host. We have studied magnetism with varying concentrations of dopant ions and have shown that the magnetism arising due to exchange interaction with the host is invariant up to 10% doping concentration, demonstrating the concentra...

Current-Induced Magnetic Polarons in a Colloidal Quantum-Dot Device

Electrical spin manipulation remains a central challenge for the realization of diverse spin-based information processing technologies. Motivated by the demonstration of confinement-enhanced sp-d exchange interactions in colloidal diluted magnetic semiconductor (DMS) quantum dots (QDs), such materials are considered promising candidates for future spintronic or spin-photonic applications. Despite intense research into DMS QDs, electrical control of their magnetic and magneto-optical properties remains a daunting goal. Here, we report the first demonstration of electrically induced magnetic polaron formation in any DMS, achieved by embedding Mn2+-doped CdSe/CdS core/shell QDs as the active layer in an electrical light-emitting device. Tracing the electroluminescence from cryogenic to room temperatures reveals an anomalous energy shift that reflects current-induced magnetization of the Mn2+ spin sublattice, that is, excitonic magnetic polaron formation. These electrically induced magnetic polarons exhibit an energy gain comparable to their optically excited counterparts, demonstrating that magnetic polaron formation is achievable by current injection in a solid-state device.

Doping-Concentration-Induced Ferromagnetism and Antiferromagnetism in In2S3:Dy3+ Quantum Dots

Diluted magnetic semiconductor (DMS) quantum dots have been researched extensively due to their potential applications in next-generation spin-based devices. Herein, the cubic In2S3:Dy3+ DMS quantum dots (3–5 nm) with different doping concentrations were synthesized via a gas–liquid phase chemical deposition method. The effect of Dy3+ content on the photoluminescence (PL) and ferromagnetism was investigated. The PL emission spectra exhibit a blue-shift compared with those reported previously due to the increased quantum size confinement and enhanced intensity attributed to the Dy3+ doping. The distinct and stronger room-temperature ferromagnetism is observed from vibrating sample magnetometer (VSM) measurement. The coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) phases and antiferromagnetic interation plays a dominant position after a certain doping concentration value can be further confirmed according to the zero field cooling/field cooling (ZFC/FC) curves. As revealed in the magnetic origin study from first-principles calculations, the ferromagnetism obtained arises not only from the Dy atoms but also from the In vacancies. In addition, we also proposed a spontaneous mechanism based on the bound magnetic polaron theory to explain the change of saturation magnetizations along with Dy3+ doping concentration. This work provides experimental and theoretical guidance for designing and synthesizing unique spintronic materials, which can promote development of spintronic applications.

Recent Advances in Magnetic Ion-Doped Semiconductor Quantum Dots

Recent Advances in Magnetic Ion-Doped Semiconductor Quantum Dots

Synthesis and investigations of In2S3:Ho3+quantum dots on doping induced changes

Diluted magnetic semiconductor (DMS) quantum dots have been investigated extensively because of their potential applications in spintronic devices that combine the charge and spin properties of electrons in a material. Herein, the In2S3:Ho3+ quantum dots (3–5 nm) with different doping concentrations have been synthesized using the facility built by us and the corresponding growth mechanism is proposed and discussed in detail. We investigated the effect of different Ho3+ content on the optical and magnetic properties. The absorption and emission spectra exhibit a strong quantum size confinement effect and a large Stokes shift. The band gap energy can be tuned by doping concentration varying from 3.19 to 3.62 eV. The emission processes are assigned to the excitonic recombination and near-doping/near-defect excitonic luminescence, respectively. Excess Ho3+ content reduces the intensity of photoluminescence as a result of concentration quenching. Room temperature ferromagnetism is observed from VSM measurement. The change of saturation magnetizations with various Ho3+ doping concentrations is explained and discussed based on BMP theory. In addition, the band gap distribution and the chemical bonding mechanism are clearly illustrated using VASP first-principles calculations. This study provides experimental and theoretical guidance for designing and synthesizing unique spintronic materials, which can promote the development of spintronic applications.

Diffusion doping in quantum dots: bond strength and diffusivity.

Semiconducting materials uniformly doped with optical or magnetic impurities have been useful in a number of potential applications. However, clustering or phase separation during synthesis has made this job challenging. Recently the "inside out" diffusion doping was proposed to be successful in obtaining large sized quantum dots (QDs) uniformly doped with a dilute percentage of dopant atoms. Herein, we demonstrate the use of basic physical chemistry of diffusion to control the size and concentration of the dopants within the QDs for a given transition metal ion. We have studied three parameters; the bond strength of the core molecules and the diffusion coefficient of the diffusing metal ion are found to be important while the ease of cation exchange was not highly influential in the control of size and concentration of the single domain dilute magnetic semiconductor quantum dots (DMSQDs) with diverse dopant ions M2+ (Fe2+, Ni2+, Co2+, Mn2+). Steady state optical emission spectra reveal that the dopants are incorporated inside the semiconducting CdS and the emission can be tuned during shell growth. We have shown that this method enables control over doping percentage and the QDs show a superior ferromagnetic response at room temperature as compared to previously reported systems.

Long-lived emission in Mn doped CdS, ZnS, and ZnSe diluted magnetic semiconductor quantum dots

Slow luminescence is studied in Mn doped CdS, ZnS, and ZnSe quantum dots. Because of the high degeneracy of Mn d-orbitals, we employ the multi-determinant SAC–CI computational method to determine the spin-forbidden transition from the 4T1 first excited to 6A1 ground state. We find that the transition energies for each material are in the excellent agreement with the experimental data. The computations reveal that the absorption spectra are independent of the presence of Mn impurities in quantum dots. The calculations show that the Mn impurity levels are located inside the QD gaps and the slow emission energies are independent of QD sizes. These features allow us to conclude that there are two luminescence peaks in the spectrum with fast (the higher energy) and slow (the lower energy) relaxations. In experiments sometimes the fast luminescence band disappears. This effect depends on Mn concentrations and a doping method. For different QD crystal structures the Mn–S (Se) bond lengths can vary. Therefore we find that the slow luminescence energy is very sensitive to a bond length. Indeed if we change the Mn–S bond length by 0.1Å, the energy increases by 0.2eV within the calculated range of bond lengths.

Spin-Polarized Scanning Tunneling Spectroscopy of Diluted Magnetic Semiconductor Quantum Dots

We have formed ultrathin films of cobalt-doped ZnO quantum dots (QDs), aligned their magnetic domains, and recorded spin-polarized scanning tunneling spectroscopy (SP-STS) with a nickel tip whose magnetization vector was also aligned. From the SP-STS, we have calculated normalized density of states (NDOS) of the diluted magnetic semiconductor (DMS) QDs. We have observed that the intensity of peaks in NDOS spectra has depended on the mutual orientation of the magnetization vectors of the tip of the scanning tunneling microscope (STM) and the domains of the QDs. A higher intensity in the NDOS spectrum is observed in tip–QD systems having their domains aligned parallel to each other as compared to their antiparallel configuration. We have performed a range of control experiments to validate the observation. We hence infer that the orientation of the magnetization vector or the magnetic state of ferromagnetic QDs relative to that of the tip of a STM can be read or probed by recording SP-STS.

Graphene activating room-temperature ferromagnetic exchange in cobalt-doped ZnO dilute magnetic semiconductor quantum dots.

Control over the magnetic interactions in dilute magnetic semiconductor quantum dots (DMSQDs) is a key issue to future development of nanometer-sized integrated "spintronic" devices. However, manipulating the magnetic coupling between impurity ions in DMSQDs remains a great challenge because of the intrinsic quantum confinement effects and self-purification of the quantum dots. Here, we propose a hybrid structure to achieve room-temperature ferromagnetic interactions in DMSQDs, via engineering the density and nature of the energy states at the Fermi level. This idea has been applied to Co-doped ZnO DMSQDs where the growth of a reduced graphene oxide shell around the Zn(0.98)Co(0.02)O core turns the magnetic interactions from paramagnetic to ferromagnetic at room temperature, due to the hybridization of 2p(z) orbitals of graphene and 3d obitals of Co(2+)-oxygen-vacancy complexes. This design may open up a kind of possibility for manipulating the magnetism of doped oxide nanostructures.

Biphasic quantum dots of cubic and hexagonal Mn doped CdS; necessity of Rietveld analysis

Mn2+ doped CdS quantum dots (QDs) were prepared via a simple chemical synthesis method. Incorporation of Mn ions into CdS QDs was monitored by structural, magnetic and optical spectroscopy analysis. Although the diffraction patterns seemed to be single wurtzite hexagonal structure, analysis revealed biphasic composite of CdS QDs hexagonal wurtzite and cubic zinc blende. The band gap, crystal phase and the morphology of CdS QDs were found not to be greatly affected by Mn2+ doping, however there was an optimal Mn2+ doping content of 10% where the magnetism is maximum. The recorded photoluminescence (PL) emission spectra, excited at 370nm, depict emission lines in the UV and blue-shift regions. The emission band reveals that the band gap of pure CdS QDs is around 3eV which is greater than that of bulk CdS (2.42eV). The band gap of Mn:CdS QDs is almost independent of Mn content (x) and the crystallite size remains almost the same for all values of x. The observed structural, magnetic and optical properties have been explained on the basis of formation of two phases of CdS doped with Mn. This work aims to highlight recent advances in the synthesis of Mn:CdS diluted magnetic semiconductor quantum dots and their wonderful performance.

Realizing Ferromagnetic Coupling in Diluted Magnetic Semiconductor Quantum Dots

Manipulating the ferromagnetic interactions in diluted magnetic semiconductor quantum dots (DMSQDs) is a central theme to the development of next-generation spin-based information technologies, but this remains a great challenge because of the intrinsic antiferromagnetic coupling between impurity ions therein. Here, we propose an effective approach capable of activating ferromagnetic exchange in ZnO-based DMSQDs, by virtue of a core/shell structure that engineers the energy level of the magnetic impurity 3d levels relative to the band edge. This idea has been successfully applied to Zn(0.96)Co(0.04)O DMSQDs covered by a shell of ZnS or Ag2S. First-principles calculations further indicate that covering a ZnS shell around the Co-doped ZnO core drives a transition of antiferromagnetic-to-ferromagnetic interaction, which occurs within an effective depth of 1.2 nm underneath the surface in the core. This design opens up new possibility for effective manipulation of exchange interactions in doped oxide nanostructures for future spintronics applications.

Nonequilibrium spin transport through a diluted magnetic semiconductor quantum dot system with noncollinear magnetization

The spin-dependent transport through a diluted magnetic semiconductor quantum dot (QD) which is coupled via magnetic tunnel junctions to two ferromagnetic leads is studied theoretically. A noncollinear system is considered, where the QD is magnetized at an arbitrary angle with respect to the leads’ magnetization. The tunneling current is calculated in the coherent regime via the Keldysh nonequilibrium Green’s function (NEGF) formalism, incorporating the electron–electron interaction in the QD. We provide the first analytical solution for the Green’s function of the noncollinear DMS quantum dot system, solved via the equation of motion method under Hartree–Fock approximation. The transport characteristics (charge and spin currents, and tunnel magnetoresistance (TMR)) are evaluated for different voltage regimes. The interplay between spin-dependent tunneling and single-charge effects results in three distinct voltage regimes in the spin and charge current characteristics. The voltage range in which the QD is singly occupied corresponds to the maximum spin current and greatest sensitivity of the spin current to the QD magnetization orientation. The QD device also shows transport features suitable for sensor applications, i.e., a large charge current coupled with a high TMR ratio.

Ruderman-Kittel-Kasuya-Yosida interaction between diluted magnetic semiconductor quantum dots embedded in semiconductor

Using the Keldysh Green's function method, we calculated the Ruderman-Kittel-Kasuya-Yosida (RKKY) magnetic interaction between the localized spins of two diluted magnetic semiconductor (DMS) quantum dots embedded in a two-dimensional semiconductor. It was shown that the RKKY interaction is strongly dependent on the hybridization between the quantum-confined states in DMS quantum dots and the band in the semiconductor, the discrete energy levels in DMS quantum dots and the carrier density of the semiconductor. Since the carrier energy levels in DMS quantum dots may be adjusted by the applied voltage gates, the RKKY interaction of the present system is gate-controllable. It provides an alternate way to get the controllable RKKY magnetic interaction in semiconductor nanostructures for potential application in quantum information processing.