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  • Near the emission threshold the disagreement between the mea

    2018-10-24

    Near the emission threshold, the disagreement between the measured current characteristics and theoretical predictions was often even more notable. Fig. 5 represents a few typical emission plots with the intervals of current saturation and even reduction. Such features were most typical for the emission current values below 3–5 μA, but sometimes they were seen at much higher currents (Fig. 5b). High probability of their appearance does not allow explaining their existence through fluctuations of the current. The features were reproduced in multiple measurements in increasing and decreasing field, which also excludes a number of possible explanations, such as thermal effects or removal of contaminant layers. The contradiction between the observed presence of the “fine structure” at low-current parts of the emission plots and smooth exponential growth at higher currents can be naturally explained by statistical averaging of the emission characteristics of local centers, individually having complicated shapes. ionophore Near the emission threshold, only a limited number of the most efficient centers contribute to the full current [58]. As the field increases, this number grows and individual features are averaged out.
    Discussion Some authors [58–60] associate the phenomenon of low-field emission from all the forms of nanocarbon with the enhancement of electric field at high-aspect-ratio elements (nanotubes, fibers, etc.), even when these elements are not introduced intentionally and are present at the emitter surface in relatively low numbers as a technological contaminant. Even though this explanation may be correct for some of the discussed emitters, we think that in the NPC materials investigated in this work, the emission mechanism is different. Besides the fact of low-field emission itself, the actual emission model for NPC has also to explain other experimentally observed features of the emission behavior, including:
    We assume that all these features of NPC emission may be explained in terms of the two-stage model of the emission mechanism [2,18–20,41–47] with a few important modifications discussed below. According to this model, electrons are transferred from the emitter bulk ionophore states near the Fermi level (EF) to vacuum not directly, but through two successive steps via some intermediate states localized near the emitter surface, with energies substantially higher than EF. If electrons are elevated onto such non-equilibrium states, they can be effectively emitted to vacuum because the surface potential barrier is lower in this case and more transparent for them than for the electrons at Fermi level. Yet, for the realization of this emission mechanism, taking into account the phenomena mentioned above, the following conditions must be satisfied:
    According to [50–55] and our own microscopic data, the NPC is a porous conglomerate of small (1–2 nm) graphene sheets mixed with larger onion-like or graphitic-shelled particles, up to 50–200 nm in size. Electronic properties of the material are those of a p-type semiconductor [51], which means a non-zero band gap and Fermi level position near the valence band top. The excessive holes appear due to electron trapping at interface boundary states, and thus all the crystalline volumes are charged positively relative to their boundaries. Due to strong band bending, the nanodomains are separated from each other by tunnel junctions allowing external field to penetrate into the material. Polarization of the domains leads to enhancement of the applied field at the junctions (Fig. 6). The enhancement factor can be roughly estimated via the solution of an electrostatic problem considering a conductive sphere (the nanodomain) of radius R placed at a small distance h (junction) from a conductive plane (the rest of the emitter). In this system, the electric field E applied normally to the plane is locally enhanced by the factor β0 = AR/h, where A ≈ 0.5. For example, if the particle size is 80 nm and the junction width is 0.4 nm, the additional internal field enhancement can reach 100, which may be sufficient for our model. A similar problem was investigated in [64] for ellipsoidal sp2 particles in an amorphous layer on a conductive substrate, and the enhancement factors up to 300 were considered. Potential difference between the adjacent domains can reach 1 V in the electric field with moderate magnitude of less than 10 V/μm, and the electrons injected through the interface junctions will have the energies much higher than the local Fermi level (Fig. 6).