Preparation And Electromagnetic Properties Of Doped And Compostied Topological Insulator Bi₂Se₃

Topological insulator (TI) has attracted considerable interest in the field of material science and condensed matter physics due to its peculiar surface state (SS). TI is a new quantum state of matter which is totally different from the conventional metal and insulator. Namely, the bulk of such material is insulating and it also possesses conducting SS in the boundaries which is protected by time-reversal symmetry. Therefore, TI is robust against disorders and contaminations. In this case, the electron can go through the boundaries freely without any scatters and energy consuming. As a result, TI has exotic physical properties and potential technological applications in both spintronics and quantum computers.The3D TI properties have been relaized in Bi2Se3, Bi2Te3, and Sb2Te3by theoretical calculation and experiments. In this thesis, Bi2Se3was choosed as the research object. Firstly, Bi2Se3is a binary compound and it’s easy to obtain without any poison starting material; secondly, there is only one Dirac point on SS which is the simplest TI; thirdly, Bi2Se3has a large bulk bandgap around0.3eV providing great potential for room-temperature applications. Furthermore, Bi2Se3is a thermoelectric material has been researched for many years. However, all the obtained Bi2Se3bulk crystal is not intrinsic "TI" because Se vacancies and anti-site defects Bise induced electron doping effect which leads the Fermi level locate at the bottom of the conducting band.Doping methods are used to improve the electron doping effect motivated by the suggestion that Fermi level can be lowered into the bulk band gap and even at the Dirac point. There is a theoretical prediction that the SS is immune to the non-magnetic impurities for the time-reversal symmetry protection, while the magnetic impurities can induce magnetic ordering, result in gap opening at the Dirac point, and lead to other exotic phenomena. In this thesis, we mainly studied the structure and properties of non-magnetic element (Ag, Ga, Zn, K, Mg, I, Nb, Cu) and magnetic element (Fe and Mn) doping Bi2Se3bulk crystal. At last, we put forward and studied the properties and modification effect of the composite system of Bi2Se3and bonded perovskite manganite oxide. The main research work and obtained conclusions are listed as follows:single crystals was prepared though melt-grown reaction. The sintering parameters of holding time and cooling rate obviously affected the phase structure and electrical properties which provided a reference for the doping systems. Long holding time led to Se reacted with Si, while too fast or too low cooling rate both increased carrier concentration n. The best sintering parameter was holding for5h and cooling rate was～3.8℃/h (5-3.8). The temperature dependence of resistivity (p-T) curve of5-3.8showed metallic; its carrier concentration was～1019cm-3; the temperature dependence of magnetization (M-T) indicated it was diamagnetic and its susceptibility was about-8.2x10-5emu/mol.It is found that Ag was able to dope or intercalate into Bi2Se3lattice either as a substitution defect to replace Bi or as an intercalant in the van der Waals gaps. The lattice parameters of doping groups decreased as Ag content increased while the intercalating group showed an opposite tendency because of Ag located between the quintuple layers (QL). After Ag was introduced into Bi2Se3systems, the p was larger than pure Bi2Se3. However, the p-T curves still showed typical metallic in AgxBi2-xSe3, while the metal-insulator (M-T) transition in low temperature region was obvious in AgxBi2Se3where the p increased as T decreased. Under magnetic field, the temperature dependence of Magnetoresisitivity (MR-7) curves of AgxBi2-xSe3decreased in low T region which may attributed to the Ag located between the QL-QL can directly affect the electron of SS.In GaxBi2-xSe3crystals, the terraces of Ga content x=0.05samples were smooth and no defect was observed, while some Ga enriched nanorods and nano-clusters were detected in Ga content x>0.05samples. The Hall resistivity and mobility mechanism were the same as Bi2Se3in Ga content x=0.05samples, while the mobility fluctuated in Ga content x>0.05samples. The resistivity increased monotonously as Ga dopant increases. All samples exhibited approximately similar p-T curves under applied field. The MR changed from positive to negative and this abnormal MR-T behavior may origin from the nanorods and nano-clusters defects.The properties of a series of other non-magnetic element doping Bi2Se3were studied. K and Zn was easy to react with Se leading to the failure of grow Bi2Se3bulk crystals; Nb and Ir was difficult to enter into Bi2Se3lattice and it precipitated out on the surface of bulk crystals. I2doping Bi2Se3led to complicated polycrystalline compounds.The properties of magnetic atoms Fe and Mn doped Bi2Se3were studied. The magnetic atoms doping led to different electro-magnetic properties attributed to the magnetic atoms are able to break the time-reversal symmetry protection.Fe doped samples FexBi2-xSe3had the same lattice structure as pure Bi2Se3, the lattice parameter c firstly increased and then decreased since some Fe atoms can located at QL-QL. There were some nano-scaled defects on the surface of high Fe content samples. The Fe concentration in defect area was much higher than the normal area which indicated Fe-Se compound were dominated in the defect area. Long-time annealing treatment can suppress the defects; n was decreased by nearly one order; p-T curve was deviated from linear tendency. All samples showed weakly metallic resistivity which increased monotonously as Fe dopant increased. The electron-eletron scatters were dominated below30K. For highly doped samples (x=0.1and0.15), a typical Mott law was added and simultaneous crossover in p-T and MR-T characteristics around30K was observed which indicated that contribution of surface state had been significantly improved. The change of MR-T was related to three mechanisms, i.e. phonon scattering effect, magnetic ordering and spin disorder scattering. According to the M-T curve, non-annealing sample showed ferromagnetism which might arise from Fe-Se alloy. After annealing, all samples showed paramagnetic behavior. Curie-Weiss law was used to fit the M-T curves and calculated spin is close to2.5which suggested that the valence state of iron in the FexBi2-xSe3crystal was Fe3+. The SdH oscillations were obvious in the Fe0.15Bi1.85Se3sample indicated Fe doping enhanced the magnetic quantum effect.The morphologies of Mn doped samples MnxBi2-xSe3were largely changed and may have great influence on its transport properties. The layer structure and crystal boundaries were increased as Mn content increasing. Mn atoms weakened the cleavage and decreased c-axis lattice parameter. From the p-T curves, obvious M-I transformation in x=0.07and0.1samples were observed. The samples contained low concentration of Mn (x=0.05) show metallic the same as Bi2Se3. While the sample (x=0.07and0.1) becomes p-type semiconductor with the transition temperature is about153and177K, respectively. The hole-dominated behavior may origin from the complicated scatter mechanism between Mn atoms and the defects or the crystal boundaries. The manganese-based perovskite oxide La0.7Sr0.3MnO3(LSMO) is an electron strong correlation system and the spin-polarized electron played an important role in its MR effect. Combine TI and LSMO may lead to a great many exotic properties. In our study, there were peaks in both p-T and MR-T curves in the high T region in the composite of Bi2Se3/(7%,10%,12%) silicone rubber bonded LSMO, which were came from intrinsic LSMO. The resistivity of such composite came from both LSMO and Bi2Se3. The LSMO was dominated in the high T region while the Bi2Se3played a import role in the low T region. The silicone rubber may break the conducting abilities and form new boundaries which increased the disorders and led complicated transporties.The composite of Bi2Se3/2%epoxy resin bonded LSMO remained metallic, but the magnetic field induced complicated p-T behavior which may have relation with rearrangement of magnetic domains. In high content of epoxy resin bonded LSMO composites, we only observed the Bi2Se3properties under both zero field and applied magentic field. The modification of LSMO on Bi2Se3was largely weakened indicated that the spin-polarization and double-exchange mechanism were suppressed by macromolecule encapsulation.