| Low-dimensional semiconductor nanocrystals are highly desirable for their novel characteristics and versatile functions which depend on the intrinsic atomic environment and morphology.To satisfy the application of different devices and systems,such as intergrate photovoltaic devices,electrically controllable microwave elements,green nanogeneratores and spintronics,we need designing and synthesizing varieties of functional atomic structure or special morphology with controllable properties.In this dissertation,we construct new structures and new morphologies in two low-dimensional semiconductor nanocrystals to meet the requirements of different fuctional devices.Systematic researches have been done on these nanocrystals,which clearly demonstrate the atomic crystallographic structure and explain the origins of some special characteristics.We also find the electrochemical and electromagnetic performances in the nanocrystals.The obtained results are described as follows:1.We successfully fabricate unidirectional aligned microtube(MT)arrays on fluorine-doped tin oxide coated glasses(FTO)in the presence of an upward external electric field of 150 V.The obtained results indicate that the direction of the external electric field determines unidirectional growth of the arrays.We find that the surfaces of the MTs are positively charged.The surface charge density is estimated to be about 5.02840 ×10-4 C/m2 by electric force microscopy(EFM).Because the aligned MT arrays have a denser arrangement and the MTs have no the closed ends,the MT arrays would have a larger specific surface area.At the same time,the MT arrays retain multiple hydrophilic channels which could be expected to increase the capacitance of the MT arrays.Our cyclic voltammetry(CV)and galvanostatic charge-discharge measurements indicate that the capacitance of the MT arrays can reach more than 1000 μF·cm-2.2.Layered ReS2 has both stable centrosymmetric(Tc phase)and none-centrosymmetric structure(Td phase).Ferroelectricity is not expected because the migration of the out-of-plane electric dipoles formed in the Td structure of ReS2 is severely restricted by its high barrier energy of about 4.57 eV.To overcome this restriction,we perturb structural environment of the Td phase at the sub-angstrom scale to construct transitional structure(Tt,between none-centrosymmetry Td and centrosymmetry Tc)by the introduction of Tc structure into pristine Td ReS2,which can effectively lower transition barrier,providing a possibility of out-of-plane ferroelectricity.We fabricate the layered ReS2 with a fairly flat,smooth terrace of about 0.91 nm in thickness.By spherical aberration corrected scanning transmission electron microscopy(Cs-corrected STEM),series of Tt structures are formed between Td and Tc islands.Meantime,some Re vacancies also clearly exist in these Tt structures.3.Because non-centrosymmetry Tt structure is typically ferroelectric,we calculate the out-of-plane electric dipoles and their activiation energy barriers by DFT.The in-plane symmetry breaking occuring at the Tt structure makes the electronic wave functions on the top and bottom surfaces become asymmetrical,leading to out-of-plane electric dipoles(John-Teller effect).Compared to the Td phase,the barrier energy of the Tt phase reduces dramatically,which could improve the possibility of the reversal of electric dipoles.More importantly,beyond 30%of the displacement,the barrier energy drops to the acceptable range below 0.78 eV,showing that electric polarization reversal could be achievable.By piezoresponse force microscopy(PFM),the local piezoelectric responses and ferroelectric domains are successful observed.Moreover,as the PE electric hysteresis at different temperature of the ReS2 films are detected,indicating that the 2-dimensional out-of-plane ferroelctricity in the Tt structure ReS2 is realized.4.The structural transitions and electronic states of the layered ReS2 containing Re vacancies are calculated by DFT.Taking consideration of the Re vacancy,the ground states of Td and Tt structure are found to be magnetic mainly arising from the p orbitals of two most neighboring S.atoms within the Re vacancy region.Based on the theoretical calculations,the magnetic characteristics of the layered ReS2 are conducted using superconducting quantum interference device(SQUID)and vibrating sample magnetometer(VSM).The saturation magnetization tracks well with the change in the Re vacancy concentration and in the layered ReS2 with a Re vacancy concentration of-5.5%,the ferromagnetic order still persists above 350 K.The above findings open a new avenue to construct new multiferroic specially in high-transitional temperature 2D multiferroic materials which are currently a much less developed field.The magnetic-field-dependent ferroelectricity and magnetically induced polarizationfrom of the layered ReS2 sample with 5.5%Re vacancy are measured to determine the intrinsic magnetic-electric magnetoelectric(ME)properties using Physical Property Measurement System(PPMS).The ferroelectric polarization P decreases with increasing applied magnetic fields.When the magnetic field arises from 0 to 2 T,the spontaneous polarization(P0)decreases from 18 to 13.5 μC/cm2 indicating the low-magnetic-field controlled ME coupling effect.The magnetically induced polarization PH obtained by integrating the ME current,which is the direct evidence of a ME coupling,flops with a nonlinear decrease of 3.7 μC/m2 when the magnetic field increases from 0 to less than 0.9 T at 28 K,and the ME coupling effect disappears above 32 K.Thes results clearly show the existence of the H-induced ME coupling phase transition in the layered ReS2.Using the classical illustration of spin-charge-orbital coupling based on DFT calculation,the magnetic moments(Si and Sj)on atoms S1 and S2 are canted from each other,which causes the horizontal mirror symmetry breaking in the various distorted structures(Tt structure)within the region of Re vacancy region.Thus,the ME coupling effect in the 2D layered materials could be formed and controlled by low external magnetic field.A good understanding of the coupling mechanism provides insights into the design of new classes of ferromagnetic ferroelectrics and two-dimensional multiferroic materials with reduced size,weight and energy consumption have large potentials in ultrasensitive sensors,nanoscale electromechnical systems as well as next-generation high-speed and low-power nanoelectronics and nanospintronics. |
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