Magnetic Properties of 4-nm Cd1-yMnyS Nanoparticles Differing by Their Compositions, y

The aggregation of Mn2+, its d-d transition in CdS:Mn(II) nanobelts and bound magnetic polaron formation at room temperature

Tuning the photoluminescence (PL) and magnetic properties of 1D semiconductor nanostructures is extremely important in processing light, improving the speed and storage capacity for optoelectronic and spintronic applications. Here, we have reported the 1D Cd_1-x Mn _x S (x = 0-0.102) nanobelts (NBs) and investigated their optical and magnetic properties. These NBs were synthesized by chemical vapor deposition method. The successful incorporation of Mn ions into an individual CdS NB has been confirmed through several characterization tools: SEM-EDX analysis, significant higher angle and phonon mode shifts were observed in the XRD and Raman spectra. Room-temperature PL showed two emission peaks at the near band edge. The first peak is related to exciton magnetic polaron (EMP) and the second one appeared on the low-energy side of the band edge emission and showed very large red-shift (∼33 nm) compared to EMP, which is attributed to bound magnetic polaron (BMP). BMP emission was detected for the first time in CdS low-dimensional nanostructures. Our study showed that Mn ions tuned CdS emission more than 400 nm (from 512 to 929 nm) covering the whole visible spectral region up to the near infrared region for the first time, and significantly boosted the room-temperature ferromagnetism, which shows promise for optoelectronic and spintronic applications.

Benefitting from Dopant Loss and Ostwald Ripening in Mn Doping of II-VI Semiconductor Nanocrystals

Annealing or growth at high temperatures for an extended period of time is considered detrimental for most synthetic strategies for high-quality Mn-doped II-VI semiconductor nanocrystals. It can lead to the broadening of size distribution and, more importantly, to the loss of dopants. Here, we examine how ripening can be beneficial to doping in a simple "heat-up" approach, where high dopant concentrations can be achieved. We discuss the interplay of the loss of dopants, Ostwald ripening, and the clustering of Mn near the surface during nanocrystal growth. Smaller nanocrystals in a reaction batch, on average, exhibit higher undesirable band-edge photoluminescence (PL) and lower desirable dopant PL. The optimization of dopant loss and the removal of such smaller undesirable nanocrystals through Ostwald ripening along with surface exchange/passivation to remove Mn clustering lead to high Mn PL quantum yields (45 to 55 %) for ZnSxSe1-x, ZnS, CdS, and CdSxSe1-x host nanocrystals. These results provide an improved understanding of the doping process in a simple and potentially scalable synthetic strategy for achieving "pure" and bright dopant emission.

The two-step thermochemical growth of ZnS:Mn nanocrystals and a study of luminescence evolution

In this work we report a new thermochemical method for the synthesis of ZnS:Mn nanocrystals. Zn(NO(3))(2) and Na(2)S(2)O(3) were used as the precursors and Mn(NO(3))(2) was the source of impurity. Thioglycerol (TG,C(3)H(8)O(2)S) was also used as the capping agent and the catalyst of the reaction. Na(2)S(2)O(3) is a heat sensitive material which releases S species upon heating. Consequently, the reaction proceeds in temperatures higher than room temperature. The reaction was done in two steps. In the first step, the precursors were heated at 96 degrees C for an hour without TG. In the second step, TG was injected to the solution and the heating process was continued for longer heating durations. A fast growth occurred in the first 10 min after the addition of TG, resulting in a sample with a band edge located at 4.0 eV. The growth was followed by elimination of the sample's scattering and emergence and increase of the luminescence during the heating process. Transmission electron microscopy and x-ray diffraction analyses demonstrated round shaped cubic phase ZnS:Mn nanocrystals with an average size around 3.0 nm. The luminescence emerged from about 15 min after the addition of TG. The emission was located at around 585 nm, demonstrating the Mn incorporation inside the particles. The luminescence intensity increased with time and saturated during the second step of the heating process. Finally, we propose a model explaining the formation and changes in the scattering and luminescence characteristic of the ZnS:Mn nanoparticles. The model is based on the separation of the nucleation and growth steps of the synthesis in the first and second steps of the reaction. This separation directly affects the achievement of the luminescent ZnS:Mn nanocrystals.

Oscillation of absorption bands of Zn(1-x)Mn(x)S clusters: an experimental and theoretical study

Electroporation of synthetic vesicles is utilized for the preparation of molecular size uncapped Zn(1-x)Mn(x)S clusters. The absence of caps permits (i) continued growth of the Zn(1-x)Mn(x)S clusters formed, (ii) the assessment of their true absorption spectra unaltered by stabilizing ligands, and (iii) the previously inaccessible live observation of the growth of the clusters in the molecular size regime. Upon cluster growth, the UV spectra exhibit novel, time-dependent, oscillation of red and blue shifts of the characteristic absorption band. The structure and electronic properties of Zn(N-1)MnS(N) clusters with N = 1-9 are calculated using the first-principles DMol(3) package. On the basis of similarities between the oscillating trend of the experimentally observed absorption spectra and that of the calculated highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap of Zn(N-1)MnS(N) clusters with N = 1-9, the wavelengths of the sequential spectral peaks can be assigned to Zn(2)MnS(3), Zn(3)MnS(4), Zn(4)MnS(5), Zn(6)MnS(7), and Zn(8)MnS(9), respectively. Our results demonstrate that both the cluster size and the composition can be used to tune the optical properties.

Switching-on superparamagnetism in Mn/CdSe quantum dots

Mn ion doping of CdSe and other semimagnetic quantum dot (QDs) alloys has been an area of active speculation for over a decade. We report evidence of Mn(II) doping of CdSe grown from a cubic single source precursor that is superparamagnetic (SPM) with a blocking temperature of 40 K following thermal annealing. Prior to thermal annealing the 4 nm Mn/CdSe (1% Mn) QDs exhibit mainly paramagnetic behavior between 300 and 2 K, with a weak antiferromagnetic exchange. Following thermal annealing of the sample, high-temperature ferromagnetic exchange is observed in the magnetization data with the onset of an SPM phase at 40 K that exhibits a coercivity of 0.1 T at 2 K. The switching-on of SPM behavior is believed to be linked to ion migration with formation of (Se-Mn-Se-Mn-Se-Mn)n centers within the nanocrystal that exhibit coupled magnetic moments. Electron paramagnetic resonance (EPR) provides evidence of two distorted T(d) Mn core sites, a clustered site (dipolar broadened), and a localized Mn site (hyperfine-split). The ratio of the EPR signature for the dipolar broadened site increases following annealing and shows a hysteretic response around the blocking temperature. These observations suggest that thermal annealing results in enhanced cluster formation explaining the onset of the SPM phase in these nanoscale materials. Evidence of SPM behavior is evident in the field-dependent non-Langevin magnetization with a tangential loss in the ac-magnetic susceptibility and the Mydosh parameter (phi = 0.16).

Photoluminescence of Cd1-xMnxS (x ≤ 0.3) Nanowires

Cd1-xMnxS (x = 0.1-0.3) nanowires were synthesized by using the chemical vapor deposition method. They all consist of a single-crystalline wurtzite CdS structure with a [010] or [011] growth direction. The X-ray diffraction pattern reveals the contraction of the lattice constants due to the incorporation of Mn. The Mn2+ emission at approximately 2.15 eV, originating from the d-d (4T1 --> 6A1) transition, appears below 50-80 K. Its decay time is in the range of 0.55-1 ms, showing a decrease with increasing Mn content. The Mn doping reduces significantly the decay time of band-edge emission from 590 ps to 20-30 ps. Upon applying magnetic field (up to 7 T), the Mn2+ emission is suppressed and donor-acceptor pair emission becomes dominant, suggesting the energy transfer from the band electrons to the Mn2+ ions.

Emission Properties of Manganese-Doped ZnS Nanocrystals

We have performed steady-state and time-resolved fluorescence studies on undoped and Mn-doped ZnS nanocrystals with approximately 16 A diameter. While there is no band-edge emission, the intensity of the steady-state blue fluorescence from ZnS surface states decreases upon Mn incorporation, which gives rise to an orange emission. These results show that Mn incorporation competes very effectively with the donor-acceptor surface states for the energy transfer from the electron-hole pair excited across the band gap. In both undoped and doped samples, the time-resolved fluorescence studies establish the presence of a distribution of decay lifetimes possibly due to a number of emission centers in the nanocrystals. A faster short-time decay of the blue emission in the Mn-doped samples compared to that in the undoped sample suggests an additional decay channel for the surface states via an energy transfer from these states to the dopant levels.

The first synthesis of Pb(1-x)Mn(x)Se nanocrystals

In this work, Mn-doped PbSe nanocrystals (NCs) have been, for the first time, prepared through a high-temperature organic solution approach. To ensure that all the Mn2+ ions are indeed incorporated into the NCs and not only physically presented at the surface, Mn-Se prebonded precursor was selected, and a ligand-exchange process was also conducted before and after the synthesis, respectively. Various analyses including EDS, ICP, XRD, SQUID, and EPR confirm that the Mn2+ ions have been successfully doped into PbSe NCs.

Hybrid composites of monodisperse pi-conjugated rodlike organic compounds and semiconductor quantum particles

Composite materials of quantum particles (Q-particles) arranged in layers within crystalline powders of pi-conjugated, rodlike dicarboxylic acids are reported. The synthesis of the composites, either as three-dimensional crystals or as thin films at the air-water interface, comprises a two-step process: 1) The preparation of the Cd salts 6 (Cd), 8 (Cd) or Pb salts 6 (Pb), 8 (Pb) of the oligo(p-phenyleneethynylene)dicarboxylic acids 6 (H), 8 (H), in which the metal ions are arranged in ribbons and are separated by the long axis of the organic molecules, as demonstrated by X-ray powder diffraction analysis of the solids and grazing incidence X-ray diffraction analysis of the films on water. 2) Topotactic solid/gas reaction of these salts with H(2)S to convert the metal ions into Q-particles of CdS or PbS embedded in the organic matrix that consists of the acids 6 (H) and 8 (H). These hybrid materials have been characterized by X-ray photoelectron spectroscopy and transmission electron microscopy.

Architectural control of magnetic semiconductor nanocrystals

Shape- and dopant-controlled magnetic semiconductor nanocrystals have been achieved by the thermolysis of nonpyrophoric and less reactive single molecular precursors under a monosurfactant system. Reaction parameters governing both the intrinsic crystalline phase and the growth regime (kinetic vs thermodynamic) are found to be important for the synthesis of various shapes of MnS nanocrystals that include cubes, spheres, 1-dimensional (1-D) monowires, and branched wires (bipods, tripods, and tetrapods). Obtained nanowires exhibit enhanced optical and magnetic properties compared to those of 0-D nanospheres. Proper choice of molecular precursors and kinetically driven low-temperature growth afford dopant controlled 1-D Cd1-xMn(x)S nanorods at high levels (up to approximately 12%) of Mn, which is supported by repeated surface exchange experiments and X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) analyses.

Electronic absorption spectroscopy of cobalt ions in diluted magnetic semiconductor quantum dots: demonstration of an isocrystalline core/shell synthetic method

This paper reports the application of ligand-field electronic absorption spectroscopy to probe Co(2+) dopant ions in diluted magnetic semiconductor quantum dots. It is found that standard inverted micelle coprecipitation methods for preparing Co(2+)-doped CdS (Co(2+):CdS) quantum dots yield dopant ions predominantly bound to the nanocrystal surfaces. These Co(2+):CdS nanocrystals are unstable with respect to solvation of surface-bound Co(2+), and time-dependent absorption measurements allow identification of two transient surface-bound intermediates involving solvent-cobalt coordination. Comparison with Co(2+):ZnS quantum dots prepared by the same methods, which show nearly isotropic dopant distribution, indicates that the large mismatch between the ionic radii of Co(2+) (0.74 A) and Cd(2+) (0.97 A) is responsible for exclusion of Co(2+) ions during CdS nanocrystal growth. An isocrystalline core/shell preparative method is developed that allows synthesis of internally doped Co(2+):CdS quantum dots through encapsulation of surface-bound ions beneath additional layers of CdS.