Investigation On The Photoluminescence Spectra Of Fe And Mn-doped ZnO Nanowires In High Magnetic Fields

Zinc oxide（ZnO）is a semiconductor with wide band gap（3.7e V）and large exciton binding energy（60me V）,which have a wide range of application in optoelectronic devices.ZnO nanomaterials,such as nanowires exhibit various optical emission bands including band-edge emission（BE）and deep level emission（DLE）.The BE splits into many sharp peaks related to exciton-phonon coupling（EPC）and can be modulated by various parameters such as temperature,pressure and electric/magnetic fields.Searching for new modulation mechanism is useful for expanding the applications.On the other hand,the green emission in DLE is determined by the concentration of the singly ionized oxygen vacancies,and the reliable correlation between the two,as well as the effective modulation of green emission remain open questions.It has been found that doping of transition metal ions into ZnO nanowires would produce ZnO based dilute magnetic semiconductor with both charge and spin properties,which could be deeply investigated at different temperature and magnetic fields.Therefore,the modulation of EPC and the mechanism of green emission can be further studied.Based on the above knowledge,photoluminescence（PL）of ZnO nanowires doped with Fe or Mn has been systematically and experimentally studied at low temperatures and in high magnetic fields,the main content of this thesis are:Ⅰ.Controlled growth and doping of ZnO nanowiresThe synthesis of ZnO nanowires was achieved by the chemical vapor deposition（CVD）method.In the CVD process,different substrates,catalysts,growth pressures,and source materials were modulated,and the morphology and optical properties were investigated.In order to synthesize vertical ZnO nanowire arrays,the above experimental factors were systematically studied.It is showed that the vertical ZnO nanowire arrays can be synthesized on 5 nm gold film modified gallium nitride substrate by using the carbothermal reduction reaction of zinc oxide and graphite powder when the pressure in the tube is stabilized at 1 Torr and the growth temperature is kept at 1000°C.Further,ZnO nanowires doped with Fe or Mn were obtained by adding ion oxides or manganese oxides into the source materials.Finally,scanning electron microscopy,X-ray diffraction and superconducting quantum interference devices were used to characterize the morphology,structure,and magnetic properties of the samples.Ⅱ.Magnetic modulation of low-temperature exciton-phonon coupling（EPC）in the ZnO nanowiresThe modulation of the EPC by the internal and external magnetic fields in ZnO:Fe nanowires were studied by means of PL spectrum at low temperatures and in high magnetic fields.First,the temperature dependent PL spectra showed that the emission associated with the longitudinal optical phonon（LO）replicas in ZnO:Fe is severely suppressed below 50 K,while the emission associated with the LO replicas in ZnO does not change.By analyzing the PL peak energy,PL peak intensity and magnetization data,it can be speculated that both the internal magnetic field(around the Fe³⁺ions)and lattice pressure introduced by local defects may strongly suppress the EPC.High magnetic field PL measuremenet results indicate a similar suppress effect on EPC:it can be seen from the PL spectra of ZnO nanowires in high magnetic fields that the PL related to LO in ZnO nanowires is severely suppressed,while the PL peaks related to LO in ZnO:Fe nanowires are not affected at all by the external magnetic field.The PL spectra of ZnO:Fe nanowire at the highest magnetic field is similar to that of ZnO nanowire in zero field,indicating that there is high internal magnetic field in ZnO:Fe nanowires,and both internal and external magnetic fields can suppress the EPC.According to the attenuation trend of PL intensity in ZnO nanowires with magnetic field,it can be deduced that the internal magnetic field in ZnO:Fe nanowires is the order of 100 T.Ⅲ.Green emission in Fe-and Mn-doped ZnO nanowires studied by magneto-photoluminescenceIn this part,Fe-or Mn-doped ZnO nanowires were synthesized by chemical vapor deposition.During the growth process,Fe or Mn oxides were mixed into the source materials.It is found that the green emission of ZnO nanowires is greatly enhanced by the Mn doping,while it is suppressed by the Fe doping.For ZnO:Fe nanowires,the temperature-and magnetic field dependent green emission show strong EPC and shrinkage of the electron wave function,which indicated the existence of electron-dopant energy transfer and decreasing concentration of oxygen vacancies in ZnO:Fe.While for ZnO:Mn,the PL spectra show weak dependence on temperature and magnetic field,which signified that the highly concentrated oxygen vacancies enhance the green emission.