Mn Se Research Articles

Absorption and Magnetic Circular Dichroism Analyses of Giant Zeeman Splittings in Diffusion-Doped Colloidal Cd(1-x)Mn(x)Se Quantum Dots.

Impurity ions can transform the electronic, magnetic, or optical properties of colloidal quantum dots. Magnetic impurities introduce strong dopant-carrier exchange coupling that generates giant Zeeman splittings (ΔEZ) of excitonic excited states. To date, ΔEZ in colloidal doped quantum dots has primarily been quantified by analysis of magnetic circular dichroism (MCD) intensities and absorption line widths (σ). Here, we report ΔEZ values detected directly by absorption spectroscopy for the first time in such materials, using colloidal Cd(1-x)Mn(x)Se quantum dots prepared by diffusion doping. A convenient method for decomposing MCD and absorption data into circularly polarized absorption spectra is presented. These data confirm the widely applied MCD analysis in the low-field, high-temperature regime, but also reveal a breakdown at low temperatures and high fields when ΔEZ/σ approaches unity, a situation not previously encountered in doped quantum dots. This breakdown is apparent for the first time here because of the extraordinarily large ΔEZ and small σ achieved by nanocrystal diffusion doping.

Mn 掺杂ZnSe 量子点变温发光性质研究

Thermal stability of quantum dot( QD) luminescence is considered as an important factor for their applications in luminescent devices because of the Joule heat caused by inevitable current. The temperaturedependent photoluminescence( PL) properties of Mn-doped Zn Se( Mn∶ Zn Se) QDs with different shell thickness in the temperature range from 80 to 500 K were studied by steady-state and time-resolved PL spectra. It was found that the Mn∶ Zn Se QDs with thick shell( 6. 5 monolayers( MLs)) exhibited better PL thermal stability than the thin shell coated ones( 2. 6 MLs). Because almost no PL quenching occurred for thick shell-coated Mn-doped QDs from 80 to 400 K,their PL quantum yield( QY) could keep 60% even at 400 K. Moreover,based on the change in temperature-dependent PL intensities and lifetimes of Mn ∶ Zn Se QDs,the thermal quenching mechanism was proposed. Finally,the stability of Mn∶ Zn Se QDs with different shell thickness are discussed on the basis of heating-cooling cycling examination( 300-500-300 K). For Mn∶ Zn Se QDs with thick shell,the PL was nearly totally recovered after the cycling examination. Thus,Mn∶ Zn Se QDs are promising for applications in luminescent devices,where strong thermal effect is inevitable.

Tunable Luminescence in CdSe Quantum Dots Doped by Mn Impurities

In this work, we study the effect of a dual luminescence for CdSe quantum dots (QDs) doped by a manganese impurity. In this effect, one line has fast relaxation and corresponds to light emission from CdSe conduction band, whereas the second is very slow in relaxation with the relaxation time of 0.1–1.0 ms. The second line disappears for quantum dots (QDs) with diameters D ≥ 3.3 nm, and therefore, the luminescence becomes tunable by a QD size. The problem is computationally challenging because of large size QDs and a high degeneracy of the energy states. To overcome this problem, we make four assumptions. These assumptions are the following: (1) a QD optical gap is independent of an Mn impurity for small concentrations; (2) we combine electronic structure calculations for medium size CdSe QDs with the effective mass calculations for medium and large QD sizes and match them in the medium size region, assuming that based on assumption 1, the optical spectrum is independent of Mn even for larger QDs; (3) we cut an MnSe4 fragment out of the middle of Cd23Mn1Se24, Cd31Mn1Se32, and Cd81Mn1Se82 QDs and check whether we can quantitatively explain the mechanism of the second, slow relaxation emission in these experiments using the ab initio SAC–CI multiconfiguration method; and (4) the slow luminescence line is independent of a QD size. We have proved theoretically all these assumptions and found that the critical size of a quantum dot, the size when the second luminescence line disappears, is 3.2 nm, and the slow luminescence energy is 2.3 eV in a tetrahedral ligand field. We also study the case for an Mn impurity placed at a QD surface. Then the symmetry of a fragment is C3v, and the results of the calculations reveal that the slow luminescence energy is 2.47 eV with the critical size D = 2.7 nm, instead of 3.2 nm for a tetrahedral ligand field. We also predict how this energy depends on the length of an Mn–Se bond. The dependencies appear to be the opposite in these two cases. For the tetrahedral symmetry, the luminescence energy grows with the bond length, whereas for the pyramid, C3v, symmetry (an Mn is at the surface), it goes down. Furthermore, we study a luminescence energy dependence on a Se–Mn–Se pyramid angle. We find that it is angle independent. These results could be useful for CdSe nanocrystal structures different than Würtzite.

Investigation on the electronic transport properties of MgS nanotube based molecular devices – a first-principles study

The electronic transport properties of pure MgS nanotube based molecular devices, Mn-substituted nanotubes and Se-substituted nanotubes are investigated using density functional theory. The state of the art of this work is to study the transport properties of MgS nanotubes with substitution impurities across electrodes. The electronic transport properties are discussed in terms of device density of states and transmission spectrum of MgS nanotubes. The effects of Mn substitution and Se substitution in nanotubes are studied. The major contribution to density of states arises only from p orbitals in MgS nanotubes. The substitution effect and bias voltages also have influence in the density of states. The transmission spectrum provides information about the transmission of electrons along the nanotube. The information provided in this work gives a clear vision to fine-tune MgS nanostructures with improved transport property in nanoelectronic device fabrication.

Large positive magnetoresistance effects in the dilute magnetic semiconductor (Zn,Mn)Se in the regime of electron hopping

Magnetoresistance in dilute magnetic semiconductors is studied in the hopping transport regime. Measurements performed on Cl-doped Zn1–xMnxSe with x < 8% are compared with simulation results obtained by a hopping transport model. The energy levels of the Cl donors are affected by the magnetization of Mn atoms in their vicinity via the s-d exchange interaction. Compositional disorder, in particular, the random distribution of magnetic atoms, leads to a magnetic-field induced broadening of the donor energy distribution. As the energy distribution broadens, the electron transport is hindered and a large positive contribution to the magnetoresistance arises. This broadening of the donor energy distribution is largely sufficient to account for the experimentally observed magnetoresistance effects in n-type (Zn,Mn)Se with donor concentrations below the metal–insulator transition.

Syntheses, crystal and band structures, and optical properties of a selenidoantimonate and an iron polyselenide

A new selenidoantimonate (CH3NH4)[Mn(phen)2](SbSe4)·phen (1, phen=1,10-phenanthroline) and an iron polyselenide [Fe(phen)2](Se4) (2) were obtained under hydro(solvo)thermal conditions. Compound 1 represents the first example of a selenidoantimonate anion as a ligand to a transition-metal π-conjugated ligand complex cation. Compound 2 containing a κ2Se1,Se4 chelating tetraselenide ligand, represents the only example of a tetraselenide ligand to a Fe complex cation. Compounds 1 and 2 exhibit optical gaps of 1.71 and 1.20eV, respectively and their thermal stabilities have been investigated by thermogravimetric analyses. The electronic band structure along with the density of states calculated by the DFT method indicate that the optical absorptions mainly originate from the charge transitions from the Se 4p and Mn 3d states to the phen p–π⁎ orbital for 1 and the Se 4p and Fe 3d states to the phen p–π⁎ orbital for 2.

Energy relaxation process in exciton transfer between (Zn, Cd, Mn)Se and (Zn, Cd)Se quantum wells

AbstractExcitation transfer in semiconductor nanostructure is an elemental process of next generation optoelectronic devices. Energy relaxation process in exciton transfer from the lowest state of a quantum well to the lowest state of the next well was studied. The energy relaxation process is essential for the completion of the transfer. Longitudinal optical (LO) phonon assisted energy relaxation was found to be most effective for exciton injection into the lowest state of very thin quantum wells on the basis of PLE measurements. A three‐quantum‐well structure including a 0.6‐nm thick (Zn, Cd, Mn)Se well and a 1.2‐nm thick (Zn, Cd)Se well separated by a 15‐nm ZnSe barrier was prepared. Energy difference of exciton in the two wells was close to the LO phonon energy. The exciton transfer efficiency between the two wells was measured as a function of external magnetic field. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Heating of the Mn spin system by photoexcited holes in type‐II (Zn,Mn)Se/(Be,Mn)Te quantum wells

The efficiency of the Mn‐spin system heating under pulsed laser excitation is studied in diluted magnetic semiconductor heterostructures Zn0.99 Mn0.01 Se/Be0.93 Mn0.07 Te with type‐II band alignment by means of time‐resolved photoluminescence and pump‐probe reflectivity. An essential role in the heating is played by multiple spin‐flip scatterings of a hole with localized spins of Mn2+ ions. The efficiency of the spin and energy transfer from photoexcited holes to Mn ions of the Zn0.99 Mn0.01 Se layer considerably depends on the hole lifetime in this layer. This lifetime can be limited by the hole relaxation into the Be0.93 Mn0.07 Te layers and is strongly sensitive to the excitation power and Zn0.99 Mn0.01 Se layer thickness. These dependences allow us to determine a characteristic time of about 20ps for the spin and energy transfer from photoexcited holes to the Mn spin system.

Transport parameters and optical properties of selectively doped Ga(Al)As/Zn(Mn)Se heterovalent structures with a two-dimensional hole channel

The growth of III–V/II–VI:Mn heterostructures with a high hole concentration in the AlGaAs:Be/GaAs/AlGaAs 2D channel situated in the immediate vicinity of the AlGaAs/Zn(Mn)Se heterovalent interface by molecular-beam epitaxy is reported. Despite the decrease in the hole concentration in the GaAs channel upon a decrease in the distance between the channel and the heterovalent interface, the hole concentration reaches a value of 1.5 × 1013 cm−2 at a temperature of 300 K even at the minimum distance of 1.2 nm. Deep profiling by dynamic secondary-ion mass spectrometry confirmed the back diffusion of Mn from ZnMnSe into the III–V part. High hole concentration and the presence of magnetic manganese ions in the GaAs conduction channel determine the interest in the structures as possible objects in which the effect of magnetic ordering in heterogeneous semiconductor systems can be studied.

Molecular beam epitaxy of AlGaAs/Zn(Mn)Se hybrid nanostructures with InAs/AlGaAs quantum dots near the heterovalent interface

Features of the growth of InAs quantum dots in an Al0.35Ga0.65As matrix by molecular beam epitaxy at different substrate temperatures, deposition rates, and amounts of deposited InAs are studied. The optimum conditions for growing an array of low-density (≤2 × 1010 cm−2) small (height of no more than 4 nm) self-organized quantum dots are determined. The possibility of the formation of optically active InAs quantum dots emitting in the energy range 1.3–1.4 eV at a distance of no more than 10 nm from the coherent heterovalent GaAs/ZnSe interface is demonstrated. It is established that inserting an optically inactive 5-nm GaAs quantum well resonantly coupled with InAs quantum dots into the upper AlGaAs barrier layer enhances the photoluminescence efficiency of the quantum-dot array in hybrid heterostructures.

The syntheses and structures of mixed-metal dichalcogen complexes [CpMn(CO)2]2(E2)Pt(PPh3)2

The oxidative substitution of diphenylacetylene from the Pt(0) complex (PPh3)2Pt(PhCCPh) by the dimanganese-dichalcogen fragment [CpMn(CO)2]2(E)2 (E=S, Cp=C5H5, C5H4Et (1); E=Se, Cp=C5H5) occurs without E–E bond rupture and allows the formation of the new mixed-metal complexes [CpMn(CO)2]2(E2)Pt(PPh3)2 (2–4 respectively). The analogous Te-containing cluster [CpMn(CO)2]2(Te2)Pt(PPh3)2 (5) was prepared in the transmetallation of [CpMn(CO)2]3(Te2) by (PPh3)2Pt(PhCCPh). The Mn–E and E–E distances in the structures of 1–5 are considerably shortened (Mn–S 2.25, Mn–Se 2.37, Mn–Te 2.53Å). The IR spectra, cyclic voltammetry data and DFT calculations indicate increasing of electron density on the Mn atoms in 2–5.

Nanocrystal Diffusion Doping

A diffusion-based synthesis of doped colloidal semiconductor nanocrystals is demonstrated. This approach involves thermodynamically controlled addition of both impurity cations and host anions to preformed seed nanocrystals under equilibrium conditions, rather than kinetically controlled doping during growth. This chemistry allows thermodynamic crystal compositions to be prepared without sacrificing other kinetically trapped properties such as shape, size, or crystallographic phase. This doping chemistry thus shares some similarities with cation-exchange reactions, but proceeds without the loss of host cations and excels at the introduction of relatively unreactive impurity ions that have not been previously accessible using cation exchange. Specifically, we demonstrate the preparation of Cd(1-x)Mn(x)Se (0 ≤ x ≤ ∼0.2) nanocrystals with narrow size distribution, unprecedentedly high Mn(2+) content, and very large magneto-optical effects by diffusion of Mn(2+) into seed CdSe nanocrystals grown by hot injection. Controlling the solution and lattice chemical potentials of Cd(2+) and Mn(2+) allows Mn(2+) diffusion into the internal volumes of the CdSe nanocrystals with negligible Ostwald ripening, while retaining the crystallographic phase (wurtzite or zinc blende), shape anisotropy, and ensemble size uniformity of the seed nanocrystals. Experimental results for diffusion doping of other nanocrystals with other cations are also presented that indicate this method may be generalized, providing access to a variety of new doped semiconductor nanostructures not previously attainable by kinetic routes or cation exchange.

Magneto-optical studies of ensembles of semimagnetic self-organized Cd(Mn)Se/Zn(Mn)Se quantum dots

Ensembles of Cd(Mn)Se/Zn(Mn)Se semimagnetic self-organized quantum dots with different Mn content have been studied by photoluminescence and Raman scattering under strong magnetic fields in Faraday and Voigt geometries and with selective excitation. Electron spin-flip transitions have been observed in Voigt geometry in the structures with large Mn content, characterized by a large magnetic shift of photoluminescence bands. Narrow exciton peaks completely ${\ensuremath{\sigma}}^{\ensuremath{-}}{\ensuremath{\sigma}}^{+}$ polarized have been observed under selective excitation in the Faraday geometry in the structures with medium and small Mn content. A number of effects due to s,p-d exchange interaction and exciton magnetic polaron formation manifested themselves in the structures with the smallest Mn content, where no Zeeman shift of the photoluminescence bands was observed.

Mn–Se interactions at the cathode interface during the electrolytic-manganese process

The Mn–Se interaction at the cathode interface during the electrolytic-manganese process was characterized by square-wave voltammetry and linear-sweep voltammetry. The mechanism of the effects of trace-amount SeO2 additive was analyzed. When SeO2 was added, α-MnSe was the only Mn–Se species that formed on the cathode surface. This species is critical to the activities of the cathode. During electrolysis, an electrochemical equilibrium exists among α-MnSe, Mn2+, and Se(IV). This equilibrium can be controlled by varying the concentrations of Mn2+ and SeO2, through which the electrolytic-manganese process can be improved.

Spin injection in a heterovalent structure with coupled quantum wells GaAs/(Al,Ga)As/(Zn,Mn)Se/ZnSe

AbstractIn this report we present the study, by means of low‐temperature magneto‐optical measurements, of the spin transport dynamics in a heterovalent MBE‐grown III‐V/II‐VI structure under optical pumping. The structure consists of two coupled GaAs quantum wells (QWs) of 100 Å and 50 Å surrounded by (Al,Ga)As barriers, a heterovalent (Al,Ga)As/ZnSe interface, a 10 nm (Zn,Mn)Se spin aligner, and a 70 nm thick ZnSe cap layer. We obtain values of about 30% at 4.5 T for the degree of circular polarization. These results give some hints on the design improvements needed for higher spin‐filtering efficiency. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Suppression of antiferromagnetic interactions through Cu vacancies in Mn-substituted CuInSe2 chalcopyrites

Stoichiometric and Cu-poor Cu0.95−xMn0.05InSe2 (x = 0–0.20) compounds were synthesized by high-temperature, solid-state reactions. The presence of copper vacancies is revealed by Rietveld refinements of combined neutron and x-ray powder diffraction data. The antiferromagnetic interaction is depressed by the copper deficiency, which may be explained as the competition between the antiferromagnetic Mn–Se–Mn superexchange interaction and the hole-mediated ferromagnetic exchange induced by the copper vacancy. The introduction of copper vacancies is proposed to be a viable route to impart carrier-mediated ferromagnetic exchange in the chalcopyrite-based dilute magnetic semiconductors.

Growth kinetic on the optical properties of the Pb1−xMnxSe nanocrystals embedded in a glass matrix: thermal annealing and Mn2+ concentration

Semimagnetic Pb(1-x)Mn(x)Se nanocrystals were synthesized by a fusion method in a glass matrix and characterized by optical absorption (OA), atomic/magnetic force microscopy (AFM/MFM), and photoluminescence techniques. MFM images strongly indicated the formation of Pb(1-x)Mn(x)Se magnetic phases in the glass system. Quantum dot size was manipulated by tuning annealing time. It was shown that Mn(2+) impurity affects nucleation, where Mn(2+)-doped samples present a redshift of the OA peak after a short annealing time and a blueshift after long annealing time compared to undoped PbSe NCs. This behavior was linked to the dependence of band-gap energy and the absorption selection rule on Mn(2+) concentration. Photoluminescence in the Pb(1-x)Mn(x)Se nanocrystals increases as the temperature rises up to a point and then decreases at higher temperatures. Anomalous increases in emission efficiency were analyzed by considering temperature induced carrier-transfer in semimagnetic Pb(1-x)Mn(x)Se quantum dots nanocrystals of different sizes.

Diluted magnetic layered semiconductor InSe:Mn with high Curie temperature

We present a detailed study of layered semiconductor InSe doped with Mn. X- ray and neutron diffraction analyses of (In,Mn)Se single crystals show the presence of a main phase as solid solution, the second antiferromagnetic MnSe phase, and traces of In Se Mn In 1 x x − 4Se3. Magnetic measurements reveal ferromagnetic behavior of (In,Mn)Se with the Curie temperature about 800 K. The ferromagnetic cluster model and exchange interaction via 2D electron gas, as the reasons of spontaneous magnetization, are discussed. The dramatic transformation of (In,Mn)Se electron spin resonance (ESR) spectra as a function of temperature is revealed. At the magnetic field perpendicular to crystallographic c axis, a low-field line within the temperature range 70 down to 4.7 K is observed. It shifts to smaller magnetic fields with temperature decrease. Neutron diffraction studies reveal the strong rise for one of the reflection peaks with temperature decrease in the same temperature region where ESR spectra transformation occurs. This peak corresponds to double MnSe interplanar distance in the (111) direction what is a period of its magnetic lattice. Magnetic structure of (In,Mn)Se single crystal is discussed.

Optical and Structural Properties of Zn–Cd–Mn–Se Double Quantum Well Systems

Double quantum well (DQW) structures consisting of a ZnCdSe well and a ZnCdMnSe well separated by a ZnSe barrier are grown with molecular beam epitaxy (MBE). The DQW structures are characterized by using X-ray diffraction measurement and simulation. Thickness of each well layer is designed so that the lowest energy level of ZnCdMnSe well is close to the excited level of the ZnCdSe well. Optical properties of the DQWs are studied with photoluminescence (PL) and reflection spectra in external magnetic fields up to 8 T in the Faraday geometry. Exciton transfer from ZnCdMnSe well to ZnCdSe well is observed in magneto PL with energy selective photoexcitation. Exciton energies in ground and excited states are estimated from PL excitation spectra and reflection spectra.

Optical and Structural Properties of Zn–Cd–Mn–Se Double Quantum Well Systems

Optical and Structural Properties of Zn–Cd–Mn–Se Double Quantum Well Systems