Field‐Induced Electron Emission from Nanoporous Carbons

Abstract

Influence of fabrication technology on field electron emission properties of nanoporous carbon (NPC) was investigated. Samples of NPC derived from different carbides via chlorination at different temperatures demonstrated similar low-field emission ability with threshold electric field 2-3 V/μm. This property correlated with presence of nanopores with characteristic size 0.5–1.2 nm, determining high values of specific surface area (>800 m2/g) of the material. In most cases, current characteristics of emission were approximately linear in Fowler-Nordheim coordinates (excluding a low-current part near the emission threshold), but the plots’ slope angles were in notable disagreement with the known material morphology and electronic properties, unexplainable within the frames of the classical emission theory. We suggest that the actual emission mechanism for NPC involves generation of hot electrons at internal boundaries and that emission centers may be associated with relatively large (20–100 nm) onion-like particles observed in many microscopic images. Such particles can serve two functions: to provide additional “internal” enhancement of the electric field and to inhibit relaxation of hot charge carriers due to the “phonon bottleneck” effect.

Field-emission Properties of Ni-C Nanocomposite Films

“…Certainly, the experiment data obtained with only three composite samples cannot give us a statistically significant basis for any general statements. Yet notably, the data are in good agreement with the emission model [10][11][12] that predicted existence of an optimal size of insulated conductive domains at the emitter surface. The mean size of Ni particles of the best emitter (~20 nm) fits well with the expected optimum value, determined as the largest size of a domain where the "phonon bottleneck" phenomenon can efficiently suppress hot electron energy relaxation.…”

“…The threshold field (necessary for extraction of 10 nA) was close to 2 V/m, i.e. had a typical value observed in our previous studies for various forms of nanostructured carbon [10][11][12].…”

“…Field-emission properties of the samples were examined in a special set-up described in more detail in [10][11][12]. The extracting electric field was applied in a planar gap, 0.5-0.6 mm in width, between the sample and a cylindrical anode.…”

“…Moreover, the hot electron energy relaxation in nanocrystals can be less efficient due to so-called "bottleneck effect" suppressing interaction between electrons and phonons. The hypothesis explaining the lowfield emission phenomenon by combination of these factors had been previously proposed [10][11][12] for purely carbon heterogeneous (sp 2 /sp 3 ) structures. Yet, it can be also applied to any composite material or film composed by alternating conductive and dielectric nanosized domains.…”

Electron overheating during field emission from carbon island films due to phonon bottleneck effect

“…Though, the corresponding effects are expected to be "especially efficient owing to the unique properties of graphene: fast carrier-carrier scattering dominates over electron-phonon scattering" [10]. Among sp 2 -carbon nanomaterials capable of low-field electron emission [11], evidence of suppressed electron-phonon interaction were reported for NPC [12,13].…”