Scanning probe microscopy and field emission schemes for studying electron emission from polycrystalline diamond

Abstract

The letter introduces a diagram that rationalizes tunneling atomic force microscopy (TUNA) observations of electron emission from polycrystalline diamonds as described in recent publications [1,2]. The direct observations of electron emission from grain boundary sites by TUNA could indeed be evidence of electrons originating from grain boundaries under external electric fields. At the same time, from the diagram it follows that TUNA and field emission schemes are complimentary rather than equivalent for results interpretation. It is further proposed that TUNA could provide better insights into emission mechanisms by measuring the detailed structure of the potential barrier on the surface of polycrystalline diamonds.The question is yet to be answered of why synthetic polycrystalline diamonds (micro-, nano-, or ultra-nanocrystalline diamond, abbreviated MCD, NCD, UNCD) containing a large amount of the carbon sp 2 phase are such excellent electron field emitters. These diamonds have a low threshold (turn-on) electric field ∼10 MV/m and yield significant current densities. A powerful approach, called the graphitic patch model, to explain this behavior was attempted by Cui, Ristein and Ley [3]. It first originated to plausibly explain sub-bandgap photoelectric emission in single-crystal diamond. The main idea was that the surface always has small carbonic (graphitic) phase patches (electron emitters). The work function (4.6 eV) is reduced when the property of negative electron affinity (NEA, which can be as low as −1.3 eV) is induced on the surrounding diamond host surface. Experimentally, the potential barrier [4] of a patch can be as low as 3.0 eV. The model showed excellent agreement with experiments. While the existence of carbon patches on the surface of a single-crystal diamond could be questioned (signatures seen from indirect spectroscopic measurements), in polycrystalline diamonds the carbon sp 2 phase is a separation interlayer between diamond micro-or nano-crystallites (or grains), and can be directly imaged by transmission electron microscopy in large amounts. The carbon interlayer is also called the grain boundary (GB). Recent observations [1,2] have demonstrated that tunneling electron emission originates from GBs. The observations were made by a specialty atomic force microscope equipped with tunneling current measurement capability, abbreviated as TUNA. The authors hypothesized that TUNA measurements should be representative in the conventional field emission case, meaning that GBs emitting in the TUNA scheme should be emitting sites in a field emitter based on a polycrystalline diamond. Nevertheless, they did not provide a straightforward explanation how exactly TUNA represents the field emission mechanism. In this letter, we present a diagram bridging the TUNA scheme results and the conventional field emission scheme. The diagram is based on the graphitic patch model and the usual electrostatics.Panel (a) in the figure illustrates a case in which a GB of a lateral size of 1 nm is brought in contact ...

Evolution of Granular Structure and the Enhancement of Electron Field Emission Properties of Nanocrystalline and Ultrananocrystalline Diamond Films Due to Plasma Treatment Process

“…Moreover, it should be mentioned that many different groups − have worked on the local EFE behavior of UNCD films and observed that electrons were primarily emitted from the grain boundaries amidst the asperities of diamond surfaces. This occurs because the electrons, which were supplied by the bottom external electrodes (through the substrates), were transported along the path with the smallest resistance in the diamond materials upward to the top surface of the UNCD films and were then emitted there.…”

Boron-Doped Nanocrystalline Diamond–Carbon Nanospike Hybrid Electron Emission Source

“…Furthermore, the localized FEE properties of the BNCD–CNSs hybrids were also investigated by the AFM-based PF-TUNA technique so as to directly locate the precise electron emission sites locally to understand the role of the BNCD and the CNSs materials on improving the FEE characteristics. Figure a shows the AFM surface morphology along with the representing PF-TUNA current mapping image (Figure b) of the BNCD films.…”