合成温度和N2/O2流量比对碳纤维衬底上生长的SnO2纳米线形貌及场发射性能影响

采用化学气相沉积法系统研究了合成温度和N₂/O₂流量对生长在碳纤维衬底上的SnO₂纳米线形貌及场发射性能的影响规律. 利用扫描电镜(SEM)、透射电镜(TEM), X射线衍射(XRD)及能谱仪(EDS)对产物细致表征, 结果表明, SnO₂纳米线长径比随反应温度的升高而增大; 随N₂/O₂流量比值的增大先增大后变小, 场发射测试表明, 合成温度780 ℃, N₂/O₂流量比为300 : 3 时SnO₂纳米线阵列具有最佳的场发射性能, 开启电场为1.03 V/μm, 场强增加到1.68 V/μm时, 发射电流密度达0.66 mA/cm², 亮度约2300 cd/m².

Effect of synthesis temperature and N2/O2 flow on morphology and field emission property of SnO2 nanowires

A large amount of tin oxide (SnO₂) nanowire arrays were synthesized on the flexible conductive carbon fiber substrate by thermal evaporation of tin powders in a tube furnace. The temperature, as well as the flow rate of the carrier N₂ gas and the reaction O₂ gas, plays an important role in defining the morphology of the SnO₂ nanowires. Morphology and structure of the as-grown SnO₂ samples are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Results show that all the samples possess a typical rutile structure, and no other impurity phases are observed. The morphology changes from rod to wire with the increase of reaction temperature. Ratio of length to diameter of the nanowires increases first and then decreases with the flow ratio of N₂/O₂ gas. The optimum synthesis conditions of SnO₂ nanowire are: reaction temperature 780 ℃, N₂ and O₂ flow rates being 300 sccm and 3 sccm respectively. In our growth process, the nanowire grows mainly due to the vapor-liquid-solid (VLS) growth process, but both the VLS process and surface diffusion combined with a preferential growth mechanism play the important role in morphology evolution of the SnO₂.Field emission measurements for Samples 1-6 are carried out in a vacuum chamber and a diode plate configuration is used. Relationship between the growth orientation, aspect ratio, density and uniformity of the arrays and field emission performances will be investigated first. Results reveal that the field emission performance of SnO₂ nanostructures depends on their morphologies and array density. The turn-on electric field (at the current density of 10 μupA/cm²) decreases and the emission site density increases with tin oxide array density, and the turn-on electric field of Sample 5 (synthesized at 780 ℃, nitrogen and oxygen flow rates being 300 sccm and 3 sccm respectively) is about 1.03 V/μm at a working distance of 500 μm. By comparison, for the turn-on electric fields of the not well-aligned SnO₂ nanowire arrays we have 1.58, 2.13, 2.42, 1.82, and 1.97 V/μm at 500 μm. These behaviors indicate that such an ultralow turn-on field emission and marked enhancement in β (～ 4670) can be attributed to the better orientation, the good electric contact with the conducting fiber substrate where they grow, and the weaker field-screening effect. Our results demonstrate that well-aligned nanowire arrays, with excellent field-emission performance, grown on fiber substrate can provide the possibility of application in flexible vacuum electron sources.