Hi Das,

sample having large number of NPs and lest number of nanorods is the better field emitter than other one due to low turn on field and high current density. in this sample at high field Screening effect suppress the electrons to comes out from field emitting sites that why you are obtaining low current at high field region. 

Hi!

You asked a very interesting question. However, the problem is described only in general terms. It is clear that numerous details could affect the final result significantly. These settings include the the electrophysical properties, the geometry of nanoparticles and nanorods, the patterns of their mutual disposition. It is desirable to know the results of the experiments, not only in qualitative but also in quantitative terms.

Since both, the nanoparticles and nanorods, are made of the same material, then their work function is the same; however their size must be significantly different. The nanoparticles must be significantly smaller than the nanorods, leading to higher local field enhancement at the surface of the nanoparticles and, thus, lower turn-on field. However, being smaller, the nanoparticles have a wider bandgap and, therefore, less number of electrons in the conduction band, which leads to lower current density. There must be an optimum mixing ratio at which you can obtain low turn-on field and high current density by taking advantage of the best properties of each type of nanoparticle.