Synthesis And Transport Properties Of Bi-based Topological Insulator And Cd₃As₂ Topological Dirac Semimetal Nanostructures

Dirac materials,including topological insulators（TIs）and topological Dirac semimetal,have gradually been a hot research topic in the fields of electronic materials science and condensed-matter physics.Bi₂Se₃,as a representative of the three dimension（3D）topological insulator,has a Dirac-type conducting surface state which is protected by the time-reversal symmetry.Cd₃As₂,as a representative of the Dirac semimetal,has a liner dispersion relation along all three directions in momentum space,which is known as the 3D analogue of graphene.In the field of fundamental science,Dirac materials have been a sound platform to study many novel phenomena,such as quantum spin Hall effect,quantum anomalous Hall effect,Majorana Fermion physics and so on.Dirac materials’ dissipationless and high mobility caused by unique band structure leads to the potential applications in dissipationless devices and quantum computation.In this thesis,Bi-based second-generation topological insulators and topological Dirac semimetal Cd₃As₂ are studied.The unique properties of Dirac materials come from the topological surface states.However,the signal of surface states in topological insulators is often overshadowed by the signal from bulk states in the experiment.So in order to study the surface states of topological insulators,we prepared mesoscopic samples through chemical vapor deposition（CVD）.Unlike macroscopic samples of Dirac semimetal materials,low dimensional materials have not been fully studied.We used a facile CVD method to synthesize Cd₃As₂ nanostructures with different morphologies,and we analyzed the growth mechanism and studied the transport properties.The main results are summarized and listed as follows:（1）Bi-based topological insulators with different composition were synthesized successfully.By the CVD method,we synthesized Bi₂Se₃,Bi₂(Te_xSe_1-x)₃,Bi_2-xSb_xTe₃,Bi_2-xSb_xTe_ySe_3-y nanowires and nanobelts and studied their growth mechanism.The good crystalline qualities of the as-grown nanostructures were verified by scanning electron microscope（SEM）and transmission electron microscope（TEM）.Besides we tried to introduce magnetic impurities to the Bi₂(Te_xSe_1-x)₃ nanowires and Bi_2-xSb_xTe₃ nanoplates.And the results of component analysis demonstrated that a small amount of Fe and Cr was successfully doped in our samples.（2）We have measured electronic transport of several topological insulator nanowires.The observed weak weak antilocalization effect（WAL）is derived from both the surface states and bulk states.We also observed Aharonov-Bohm（AB）oscillations in our nanowire devices,and the AB oscillations are derived only from topological surface states,which provides the direct evidence of the existence of surface states.The magnetoresistance of the wide ribbon with a width over 300 nm does not show obvious periodicity,and the characteristics of its Fourier spectrum suggest that AB oscillations are no longer dominant.Thus,the phase-coherent length in our nanowires is likely to be around 300 nm at 1.8 K,consistent with the estimate from WAL analysis of 250 nm at 2K.In this situation,the phase-coherent length can be comparable with the diameter to realize 1D quantum transport.（3）Cd₃As₂ nanostructures with different morphologies have been synthesized successfully by a facile CVD method.And the morphologies can be regulated by changing the pressure and argon flow rate.We analyzed the detailed growth mechanism of nanostructures with different morphologies.By TEM,Raman spectroscopy and X-ray diffraction,we confirmed the growth direction and crystal structure of the nanostructures.And the results demonstrate the[112]growth direction and I41 cd symmetry structure of Cd₃As₂ nanowires and nanobelts.（4）We have measured electronic transport of several Cd₃As₂ nanowires and nanobelts.The as-grown Cd₃As₂ nanostructures display unsaturated magnetoresistance and linear magnetoresistance,and there are Shubnikov-de Haas（SdH）oscillations superposed on the linear field dependence background magnetoresistance.All the samples host low carrier density and the Fermi level is closer to the Dirac point compared with its bulk materials,which may lead to the insulating behaviour.The estimated carrier density is about 5.8×1017cm-3,which is much smaller than the carrier density of the Cd₃As₂ bulk material.The linear magnetoresistances were observed in the nanowire and nanobelt devices,and there is Shubnikov-de Haas（SdH）oscillation superposed on the linear magnetoresistance in nanowire device.Besides,the observed negative magnetoresistance in Cd₃As₂ nanobelt give the evidence of chiral anomaly of Weyl fermions.