Controlled Synthesis And Property Of Metastable Phase MnSe Nanocrystal

The design of new type of magnetic-optical bifunctional nanomaterials and regulating the crystal structure, composition, size and morphology, is one of the hottest research topic of the modern physics, chemistry and materials science. Mn based chalcogenide semiconductor nanomaterials have excellent magnetic-optical bifunctional. They belong to wide optical bandgap semiconductor, and have multiple kinds of magnetic properties: antiferromagnetic, ferromagnetic and paramagnetic, etc. These features provide potential applications in short wavelength magnetic-light nano-devices, diluted magnetic semiconductors, photochemical material, spintronic devices and antiferromagnetic information storage materials and quantum computing, etc. Modulation of magnetic structure and optical band gap could be obtained by regulating the crystal structure, morphology and size of Mn based chalcogenide semiconductor nanomaterials. Until now, many articles have reported about controlling the crystal structure, morphology and size of Mnx Oy and MnS nanocrystals, many outstanding works, like new phenomena and laws, were achieved. However, there are few results reported on controllable preparation and properties of MnSe nanomaterials. In particular, regulate the preparation of metastable phase MnSe nanocrystal remains a great challenge, due to the strictness of the synthetic conditions and poor stability. In this paper, we designed the preparation of a variety of metastable phase morphology wurtzite MnSe nanocrystals by controlling the chemical reaction and crystal nucleation process via introducing cationic and trace anion doping strategy. We explored the new crystal structure, regulation mechanism, optical and magnetic properties, etcWe successful synthesized high quality of metastable phase wurtzite MnSe nanorods via subtle Sulfur element doping strategy by controlling the nucleation /growth kinetics and thermodynamics. Ultraviolet-visible absorption spectrum results showed that the OLA-Mn and OLA-S solution reacts and generates manganese polysulfide clusters at room temperature. The existence of manganese polysulfide clusters with polymeric S structures makes the system more reactive inducing fast WZ-phase nucleation by shearing dynamics barrier and subsequent growth of high quality wurtzite MnSe nanorods. On the contrary, in the absence of S element doping reaction condition, reaction system is controlled by thermodynamic which generates MnSe nanoparticles of rocksalt phase. It is believed that the subtle element S doping strategy could be widely used in other high quality wurtzite controllable preparation of metal chalcogenide semiconductor nanocrystals. The optical band gap of 6% S doping MnSe nanorods is 3.2 e V, and the magnetic is paramagnetism at 5 k. Based on synthesis of high quality wurtzite MnSe nanorods we successfully prepared core/shell hetero structured MnSe/MnS, MnSe/SnSe and MnSe/PbSe nanorods by ion exchange method.By introducing trace of Ag nanoparticles method, we first obtained straight and kinked MnSe nanowires and sawlike MnSe nanocrystals, these new nanomaterials are of wurtzite structure. Ag2 Se nanocrystals play a crucial role in the synthesis of wurtzite MnSe: the chemistry barrier of MnSe is greatly reduced by the catalysis of Ag2 Se, which allowed the synthesis of MnSe could be taken under a relative low temperature(200 ℃) and set aside the enough temperature range for the formation of metastable-phase crystal nucleus. Under the reaction temperature of 200 ℃, nanowires of wurtzite phase with 8 nm in diameter is generated., The high-resolution transmission electron microscopy(HRTem) results shows the growth direction is {101 0} and the cross section of the nanowires is rectangle. Under the reaction temperature of 250 ℃, kink-structures emerged in nanowires. Moreover, single crystal structures of these kinked nanowires are confirmed by the results of select area electron diffraction and high-resolution transmission electron microscopy. Nanowires grow along nonpolar axis, which is perpendicular to the c axis, is the key factors of kinking behavior. The measurements show that the nanowires grow along the { 2 110} and {101 0} direction. The transformation between these two nonpolar growth axis resulted in 150, 120, 90 and 60 ° kink Angle. We established a statistical model and simulated the results. Under the reaction temperature of 300 ℃, we obtained sawlike MnSe nanocrystals of wurtzite phase. We found that the nanowires undergo a second growth by exploring the growth process. These sawlike MnSe nanocrystals of wurtzite phase showed weak magnetic nanocrystals at low temperature due to the large ratio of surface.By improving the heating rate(> 90 ℃ / min) of the synthesis of wurtzite sawlike MnSe nanocrystals, we successfully prepared rocksalt phase MnSe Nanowires with diameter of 40 nm and 13 nm. The fast heating rate leaves little time for MnSe generating metastable-phase nucleus, as a result, the system generated stationary rocksalt phase nucleus directly instead. Magnetic measurements showed that the MnSe Nanowires with diameter 40 nm is antiferromagnetic; Interestingly, the MnSe Nanowires with diameter 13 nm is ferromagnetic and coercive force is 3000 Oe. This value is the maximum of ferromagnetic Mn base sulfide so far, and almost double of the biggest coercive force(1570 Oe) reported recently. We successfully grow Au nanoparticles on MnSe nanorods, nanowires of wurtzite phase and nanowires of rock salt phase.