Magnetic Semiconductor Quantum Dots Research Articles

Alloyed Mn–Cu–In–S nanocrystals: a new type of diluted magnetic semiconductor quantum dots

A new type of Mn–Cu–In–S diluted magnetic semiconductor quantum dots was synthesized and reported for the first time. The quantum dots, with no highly toxic elements, not only show the same classic diluted magnetic behavior as Mn-doped CdSe, but also exhibit tunable luminescent properties in a relatively large window from 542 to 648 nm. An absolute photoluminescence quantum yield up to 20% was obtained after the shell growth of ZnS. This kind of magnetic/luminescent bi-functional Mn–Cu–In–S/ZnS core/shell quantum dot might serve as promising nanoprobes for use in dual-mode optical and magnetic resonance imaging techniques.

Degree of circular polarization in II–VI diluted magnetic semiconductor quantum dots

Degree of circular polarization (DCP) in II–VI diluted magnetic semiconductor quantum dots (QDs) has been studied analytically. Energy levels have been calculated using Luttinger–Kohn Hamiltonian and effective mass approximation. Effects due to application of externa magnetic field have been investigated, followed by calculation of transition dipole moment and DCP. Numerical estimates made for Mn-doped CdSe/ZnSe QDs show that DCP in undoped QDs is negligible while transition metal ion doping yields substantial polarization rotation (≈−2.20%) even at moderate magnetic fields (≈0.5T).

Origin of ferromagnetism in self-assembled Ga1−xMnxAs quantum dots grown on Si

Self-assembled Ga1−xMnxAs magnetic semiconductor quantum dots have been grown on Si (001) substrates using droplet epitaxy by molecular-beam epitaxy. Structural characterization reveals that these dots possess a zinc-blende lattice. The Ga1−xMnxAs quantum dots with 8% Mn content are ferromagnetically soft and slightly anisotropic at low temperature, and show the superparamagnetic behavior with a blocking temperature of 20 K in the in-plane direction. Moreover, the results of x-ray absorption near edge structure provide direct evidences for the substitutional Mn2+ ion with a half-filled d5 configuration for the Ga site, suggesting the hole-mediated ferromagnetism in zero-dimensional Ga1−xMnxAs quantum dots.

Kondo-like resonance at high temperatures in a magnetic semiconductor single-electron transistor

We theoretically study electrical transport in a single-electron transistor (SET) consisting of a magnetic semiconductor quantum dot weakly coupled to non-magnetic source, drain and gate electrodes. Owing to the spin–spin interaction between charge carrier spins and spins of localized magnetic electrons, conductance as a function of gate voltage shows all the specific signatures of Kondo resonance, even at temperatures much higher than the Kondo temperature.

Fabrication of (Ga,Mn)As magnetic semiconductor quantum dots on Si substrates by droplet epitaxy

Abstract(Ga,Mn)As magnetic semiconductor quantum dots were fabricated on Si(001) substrates by droplet epitaxy using molecular‐beam epitaxy. It is found that both the diameter and density of the (Ga,Mn)As quantum dots can be modulated by changing the amount of Ga deposited. The high‐resolution transmission electron microscopy image shows that the dots possess a zincblende structure. The presence of ferromagnetic ordering with a Mn content of 0.08 was observed, and the ferromagnetic transition temperature can be increased from approximately 50 K to 80 K by post‐growth annealing (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic anisotropy in symmetric magnetic colloidal quantum dots doped with fewMn2+impurities

We present numerical exact diagonalization studies of the magnetism in magnetic II-VI colloidal semiconductor quantum dots charged with controlled number of electrons coupled to few magnetic-ion $({\text{Mn}}^{2+})$ dopants. The interplay between various relevant spin interactions and the discrete nature of ${\text{Mn}}^{2+}$ spin distribution is shown to be essential in the magnetism of few ${\text{Mn}}^{2+}$-doped quantum dots. An interesting revealed feature is the existence of pronounced magnetic anisotropies in symmetric magnetic quantum dots, which are related to the orbital quenchings induced by electron-Mn spin interactions. The orbital quenching effects further lead to significant deviations of the magnetizations of randomly Mn-doped dots from the standard Brillouin-function descriptions.

Novel nanocomposites consisting of in vivo-biotinylated bacterial magnetic particles and quantum dots for magnetic separation and fluorescent labeling of cancer cells

Novel nanocomposites consisting of nano-sized bacterial magnetic particles (BacMPs) and semiconductor quantum dots (QD) were developed for use in targeting and identifying cancer cells. BacMPs are synthesized by magnetotactic bacterium, Magnetospirillum magneticumAMB-1 and can display functional proteins using gene fusion techniques. In this study, two functional proteins, biotin carboxyl carrier protein (BCCP), derived from AMB-1 and protein G, derived from Streptococcus were displayed on BacMPs. BCCP was biotinylated in AMB-1 cells by endogenous biotin ligase. After purification of in vivo-biotinylated BacMPs, streptavidin and antibodies were immobilized on protein G-BCCP-displaying BacMPs (protein G-BCCP-BacMPs) viabiotin and protein G, respectively. Furthermore, multi-color labeling of the protein G-BCCP-BacMPs was achieved with streptavidin-labeled QD. Using streptavidin-QD/protein G-BCCP-BacMP nanocomposites, we successfully demonstrated magnetic manipulation and fluorescent labeling of lung cancer cells.

Effects of concentration on magnetization in a diluted magnetic semiconductor quantum dot

The donor bound spin polaron in a Cd 1− x Mn x Te quantum dot is investigated theoretically. Spin polaronic shifts are estimated using a mean field theory. Magnetization is calculated for various concentrations of Mn 2+ ions with the dot sizes. The lowest binding energies in a diluted magnetic semiconductor of a Cd 1− x Mn x Te quantum dot are also estimated. Using the effective mass approximation, calculations are presented with and without spatial dependent effective masses. It is found that (i) the lowest binding energy decreases with the dot radius (ii) position dependent mass gives larger binding energy for smaller dots (iii) the ionization energy becomes more when spin interaction energy is included (iv) variation of increase in ionization energy is sharper for smaller dots with increase in concentration and (v) the magnetization of Mn subsystem increases when concentration of Mn 2+ ions increases and it has appreciable changes for smaller dots.

Coherent Aharonov-Bohm oscillations in type-II (Zn,Mn)Te/ZnSe quantum dots

The magneto-photoluminescence of type-II (Zn,Mn)Te quantum dots is presented. As a result of the type-II band alignment, Aharonov-Bohm (AB) oscillations in the photoluminescence intensity are evident. In addition, an interesting interplay between the AB effect and the spin polarization in these diluted magnetic semiconductor quantum dots is observed. The intensity of the AB oscillations increases with both magnetic field and the degree of optical polarization, indicating that the suppression of spin fluctuations improves the coherence of the system.

Kondo effect and spin-polarized transport through a quantum dot with Rashba spin-orbit interaction

By using the finite-$U$ slave-boson approach, we study theoretically the spin accumulation, spin-dependent transport, and the shot noise in a magnetic semiconductor quantum dot induced by the Rashba spin-orbit interactions (SOI) in the Kondo regime. It is shown that the Rashba SOI causes the splitting of the Kondo peak and more conductance dips appear in the Fano-type conductance. The spin-polarized conductance indicates two peaks locating both sides of ${ϵ}_{d}=\ensuremath{-}U∕2$ (${ϵ}_{d}$ and $U$ represent the dot level and the Coulomb repulsion, respectively). In the large bias limit and ignoring the effect of Rashba SOI, the Fano factor can be tuned by the direct coupling between two leads while not related to the Coulomb repulsion. It is found that the Rashba SOI can increase or decrease the Fano factor, which depends on the dot level and the chemical potentials in both leads. When the dot level is below the chemical potentials, the Fano factor increases in the presence of Rashba SOI; otherwise, the Rashba SOI suppresses the Fano factor.

Electrical Manipulation of Nonvolatile Spin Cell Based on Diluted Magnetic Semiconductor Quantum Dots

In this paper, electrical manipulation of a memory cell based on a semiconductor nanostructure consisting of a diluted magnetic semiconductor (DMS) quantum dot (QD) and a reservoir of itinerant holes separated by an energy barrier is investigated theoretically. The operating principle takes advantage of the paramagnetic-ferromagnetic (PM-FM) phase transition mediated by the itinerant holes in the DMS QD that can lead to electrically controlled write/erase operations. Nonvolatility can be achieved when the structure is properly designed to reach a thermodynamic equilibrium at both the PM and FM configurations (i.e., bistability). Assuming a parabolic confining potential in the QD, the performance characteristics of the proposed nanostructure are analyzed including the scalability and the lifetime. An advantage of this memory concept is the extremely small dissipative energy for write/erase functions due to the open-circuit nature of the process. A readout scheme enabling electrical detection, with the repetition rate up to the 10-100-MHz range, is also explored by utilizing only two contacts. Finally, a potential application of the proposed memory cell is discussed as a rudimentary device for logic AND and OR operations

ELECTRICALLY TUNABLE "RENORMALIZATION" OF s–d EXCHANGE INTERACTION IN A DILUTED MAGNETIC SEMICONDUCTOR QUANTUM DOT

We theoretically demonstrate that how the Rashba spin–orbit interaction induced by the external electric field can be used to manipulate spin degree of freedom of electrons in a diluted magnetic semiconductor quantum dot (DMS QD) in the presence of a weak perpendicular magnetic field. The effects of Rashba spin–orbit interaction, induced by the electrostatic potential, on the apparent total spin splitting in a diluted magnetic semiconductor quantum dot have been studied. We propose that the apparent total spin splitting is the result of electrically tunable "renormalization" of s–d exchange interaction. It could be favorably engineered to electrically tune the spin degree of freedom in the spintronic devices.

Controlled optical switching in DMS quantum dots

AbstractWe have explored an exchange‐induced property of the electronic structure of diluted magnetic semiconductor quantum dots in order to propose an externally controlled optical switching device. Various confinement shapes, dot sizes and host material compositions are used in order to optimize the device operation. This excitonic optical switching can be established within well defined temperature and magnetic composition ranges. We are proposing a method for the implementation of this optical device that can be switched between two well‐defined regimes. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Colloidal magnetic nanocrystals: synthesis, properties and applications

There has been a tremendous research effort in the past few years in colloidal magnetic nanocrystals. New synthetic methods have been developed that enable a wide variety of magnetic materials to be synthesized in nanocrystal form, including ferromagnetic transition metals, intermetallics and metal oxides. These nanocrystals can be obtained with controlled size, shape and composition. Nanocrystal heterostructures, such as core/shell particles and heterodimers can be synthesized. New materials such as doped magnetic semiconductor quantum dots and nanowires have been made. The magnetic properties of these unique nanostructures have been measured and their applications in medicine, information storage and processing and sensing are being developed. This review attempts to provide a summary and a flavor for this exciting research area as it has evolved during the past few years.

Combined effects of electric and magnetic fields on confined donor states in a diluted magnetic semiconductor parabolic quantum dot

A variational formalism for the calculation of the binding energies of hydrogenic donors in a parabolic diluted magnetic semiconductor quantum dot is discussed. Results are obtained for Cd $_{1-x_{in}}$ Mn $_{x_{in}}$ Te/Cd $_{1-x_{out}}$ Mn $_{x_{out}}$ Te structures as a function of the dot radius in the presence of external magnetic and electric fields applied along the growth axis. The donor binding energies are computed for different field strengths and for different dot radii. While the variation of impurity binding energy with dot radii and electric field are as expected, the polarizability values enhance in a magnetic field. However, for certain values of dot radii and in intense magnetic fields the polarizability variation is anomalous. This variation of polarizability is different from non- magnetic quantum well structures. Spin polaronic shifts are estimated using a mean field theory. The results show that the spin polaronic shift increases with magnetic field and decreases as the electric field and dot radius increase.

Giant Excitonic Zeeman Splittings in Colloidal Co2+-Doped ZnSe Quantum Dots

Colloidal Co(2+):ZnSe diluted magnetic semiconductor quantum dots (DMS-QDs) were prepared by the hot injection method and studied spectroscopically. Ligand-field electronic absorption and magnetic circular dichroism (MCD) spectra confirm homogeneous substitutional speciation of Co(2+) in the ZnSe QDs. Absorption spectra collected at various times throughout the syntheses reveal that dopants are absent from the central cores of the QDs but are incorporated at a constant concentration during nanocrystal growth. The undoped cores are associated with dopant exclusion from the ZnSe critical nuclei. Analysis of low-temperature electronic absorption and MCD spectra revealed excitonic Zeeman splitting energies (DeltaE(Zeeman)) of these Co(2+):ZnSe QDs that were substantially smaller than anticipated from bulk Co(2+):ZnSe data. This reduction in DeltaE(Zeeman) is explained quantitatively by the absence of dopants from the QD cores, where dopant-exciton overlap would be greatest. Since dopant exclusion from nucleation appears to be a general phenomenon for DMS-QDs grown by direct chemical methods, we propose that DeltaE(Zeeman) will always be smaller in colloidal DMS-QDs grown by such methods than in the corresponding bulk materials.

Giant Zeeman effects in colloidal diluted magnetic semiconductor quantum dots with homogeneous dopant speciation

Ligand field electronic absorption and magnetic circular dichroism (MCD) spectroscopies are used to probe Co2+ dopant speciation in ZnO, ZnSe, and CdS nanocrystals and confirm the high quality of these colloidal diluted magnetic semiconductor quantum dots (DMS-QDs). Measurements of the QD giant Zeeman splittings using MCD and Zeeman spectroscopies show no enhancement of N0(α−β) relative to bulk. MCD spectroscopy is also presented as an effective method for verifying the successful synthesis of colloidal DMS-QDs when doping cannot be confirmed by ligand field absorption, highly resolved electron paramagnetic resonance, or other rigorous dopant-specific spectroscopic means.

Spin relaxation of excitons in nonmagnetic quantum dots: Effect of spin coupling to magnetic semiconductor quantum dots

The effect of spin coupling on the spin polarization of exciton was investigated in double quantum dot (QD) structures, one layer consisting of self-assembled nonmagnetic CdZnSe quantum QDs, and the other of CdMnSe diluted magnetic semiconductor QDs, both layers embedded in ZnSe. The magneto-photoluminescence (PL) studies on this double-layer structure were carried by detecting a single circular polarization in order to investigate the polarization dependence of the PL intensity. We have observed two well separated PL peaks in double QDs, corresponding to emissions from the CdZnSe and CdMnSe QD layers, respectively. It appears that the degree of the PL polarization emitted by the CdZnSe QDs in this double-layer structure is much larger than that observed in a single layer of nonmagnetic CdZnSe QDs. We discussed this enhancement of polarization in terms of antiparallel spin interaction between excitons localized in the nonmagnetic QDs and in the magnetic QDs.

CONTROLLABLE SPIN POLARIZATION IN A DMS QUANTUM DOT

The effects of electro-magnetic confining potentials and the s–d exchange interaction between substituted Mn ions and carriers on the spin polarization of carriers in an diluted magnetic semiconductor quantum dot are investigated within the framework of the effective-mass theory. The energy eigenvalues and wavefunctions of a single electron in the presence of an external magnetic field are studied by solving the one particle Schrödinger equation including the conventional Zeeman effect, the s–d exchange interaction and the electric confining potential which describes the dot. The eigenenergy structure for low lying states is strongly dependant on the relative sizes of the s–d exchange interactions and the conventional Zeeman energy splitting. When the spin splitting exceeds the cyclotron energy splitting, the Landau level overlappings occur so that the spin polarization of carriers is induced in low lying energy states. This spin polarization of carriers in the diluted magnetic semiconductor quantum dot can be controlled by changing the electro-magnetic confining potentials.

Nonvolatile bistability effect based on the electrically controlled phase transition in scaled magnetic semiconductor nanostructures

We explore the bistability effect in a dimensionally scaled semiconductor nanostruncture consisting of a diluted magnetic semiconductor quantum dot (QD) and a reservoir of itinerant holes separated by a barrier. The bistability stems from the magnetic phase transition in the QD mediated by the changes in the hole population. Our calculation shows that when properly designed, the thermodynamic equilibrium of the scaled structure can be achieved at two different configurations; i.e., the one with the QD in a ferromagnetic state with a sufficient number of holes and the other with the depopulated QD in a paramagnetic state. Subsequently, the parameter window suitable for this bistability formation is discussed along with the the conditions for the maximum robustness/non-volatility. To examine the issue of scaling, an estimation of the bistabiity lifetime is made by considering the thermal fluctuation in the QD hole population via the spontaneous transitions. A numerical evaluation is carried out for a typical carrier-mediated magnetic semiconductor (e.g., GaMnAs) as well as for a hypothetical case of high Curie temperature for potential room temperature operation.