Fabrication Of CsPbBr₃ Microarrays Using Self-Assembled Polystyrene Template And Its Application In Field Emission Devices

Field Electron Emission is a form of quantum tunneling in which the electron emission occurs from the emitter material to the anode in the presence of a high electric field.Therefore,electrons inside the solid can be emitted through the potential barrier without additional energy,that is,without excitation.Field electron emission has no time delay,low power consumption,and the efficiency for electrons emission is high.Field emission（FE）devices based on FEE have the advantages of integration,low power consumption,radiation resistance,and wide operating temperature range.It has been applied in many fields including field emission displays,scanning electron microscopes,pressure sensors,and microwave devices.The fabrication of FE devices requires both the construction of the field emission cathode and the selection of emitter materials.On the one hand,the density,aspect ratio,and tip structure of field emission cathode have critical impact on the turn-on electric field,emission current density and stability of FE devices.On the other hand,the work function,electron affinity,thermal conductivity and stability of emitter materials determine the performance and application range of FE devices.In the case of CsPbBr₃,the work function is relatively low（～3.98 e V）,which facilitates electron emission from the surface through the potential barrier.Moreover,the simultaneously ionic and semiconducting nature of perovskite materials allows easy tuning of electronic properties of cesium lead halide perovskites（including band gap width,work function,etc.）by simply varying the halide ions（I,Br,Cl）.In addition,various methods have been developed for the synthesis of CsPbBr₃ nanostructures,i.e.nanoplates,nanowires,nanolasers,nanobelts,and nanorods.Although cesium lead halide perovskites should be promising emitter materials as we have discussed above,little research interests have been focused on them beyond their applications in optoelectronic devices.Therefore,the study of CsPbBr₃ microarrays fabrication and their applications in FE devices is highly desired.In this thesis,CsPbBr₃ microarrays with tunable periodicity are prepared by self-assembled polystyrene nanosphere（PS）template and co-evaporation method,and the relationship between the array periodicity and its field emission properties is systematically studied.PS microspheres with sizes of 1.0μm,350 nm,and 150 nm are selected to construct templates through interfacial self-assembly.The results show that CsPbBr₃ microarrays with tunable periodicities inherit the periodicity of the self-assembled PS nanospheres,and hence the periodicities of CsPbBr₃ microarrays are 1.0μm,350 nm and 150 nm,respectively.It is worth noting that long-range ordered CsPbBr₃ is much easier to be observed when the periodicities microarray 1.0μm,whereas the long-range ordered structure becomes less distinct with the decreased periodicities of CsPbBr₃ microarrays.Because the fabrication of CsPbBr₃ microarrays on PS template belongs to space-confined growth method,it is beneficial for the growth of CsPbBr₃ microarrays with high crystallinity.Compared to the compact CsPbBr₃films on Si substrates,the better crystallinity of CsPbBr₃ microarrays is confirmed by our XRD results.Moreover,random orientation of crystals has been suppressed for CsPbBr₃ microarrays which is evidenced by the decreased intensity of（010）diffraction peaks.Better crystallinity leads to the larger grain size and lower density of shallow traps,which results in the prolonged carrier lifetime of CsPbBr₃ microarrays.The longest carrier lifetime of 58.3 ns is obtained for CsPbBr₃ microarrays with the periodicity of 1.0μm.It decreases to 45.1 and 31.7 ns for CsPbBr₃ microarrays with the periodicity of 350 and 150 nm,respectively.The shortest carrier lifetime of 14.9 ns is estimated in the case of compact CsPbBr₃ films.In addition,we investigate the impact of the periodicity of CsPbBr₃ microarrays on their FE properties.Compared to compact CsPbBr₃ films,the turn-on electric field decreases from 9.58 Vμm^-1 to 7.35,4.76 and 3.05 Vμm^-1 for CsPbBr₃ microarrays with the periodicity of 1.0μm,350 nm and 150 nm,respectively.The field enhancement factor increases from 967.7 to 714.9,2164.7 and 3346.6,respectively.Moreover,a good emission current stability for CsPbBr₃ microarrays based FE devices has also been observed.The enhanced field emission properties of CsPbBr₃ microarrays is attributed to the higher density of emitter tips which leads to larger local field enhancement,and hence device performance.