Nanotube Carbon Structure Tips - A Source of High Field Emission of Electrons
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rbon tubelite films htave bpen obtained on substrates by electron beam evaporation of graphite in 10i5 - 10" Torr vacuum: A-tubelites have been placed on f lass and quartz by directed carbon particle flows several times greater than those orming B-tubelites on Si substrates.. The films produced were 0.1-03 m thick. Texture of all tubelite films is orientel along the normal to the substrate surface. Diode structures usually worked in 10" Torr vafuum. Their emission ability remained at the same level while vacuum varied from 10i to 10" Torr. Fig.1 shows characteristic structures of film surfaces (ab - STM pictures): (a) Atubelite consists of single layer tubes about 1 nm in diameter forming the filaments 1030 nm in diameter (see also HREM pictures [5]), having tips 2-10 nm in height [4]; Btubelite surface is formed by conoid and dome-like tips of nanotubes about 10 nm in diameter and height forming the bundles [5] - (b) STM image, (c) HREM image (scale bar is 10 nm). 2. RESULTS AND DISCUSSION Usually the dependence of emission current density J (A/cm2 ) on microscopic field F and work function (P (eV) is well described by Fowler-Nordheim (F-N) low: j
=
(1.5 x 10-6 F2 /)) x exp{-6.8x10 7 (p3/ 2 /F}
(1)
99 Mat. Res. Soc. Symp. Proc. Vol. 359 01995 Materials Research Society
Fig-1
a)
b)
c)
100
here the field F = b x E is enhanced due to a tip by a factor b over the average field E (V/cm), where d is a gap between electrodes one of which is a microtips field emitter. Typical F-N characteristics of tubelite field emission are presented in Fig2. It is clear that they are fairly close to siwiysr charaqristics of autoemission of new diamond-like films for which factor G = d/-'/b (eV /-)is in the range 0.05-0.06 [8]. If the formula (1) is used, then the work function value 3.5-4.5 eV for graphite gives an estimate of the coefficient of electric field growth b about or more than 100, which proves a high amount of nanodimension tips at the surface of analyzed films, i.e., it corresponds to their surface morphology in Fig.1. The emission current of A-t structure exceeds the value of B-t structure by nearly an order of magnitude under the small (E < 40 V/I m) field; the maximum value of pre-breakdown current of investigated structures was up to 1-3 A/cm' for A-tubelite, it was smaller for B-tibelite. This fact is connected probably with a great curvature of A-t bundle tips and high interconnection of singlelayer tubelenes due to covalent sp3 bonds between their high curvature cylinder surfaces similar to those in C60-barrelite [9]. On the contrary, B-tubelenes in a film should be bound by molecular forces through small parts of their surfaces and can be "torn out" by an electric field Factor G value distribution and deviation from a straight line of some F-N characteristics - Fig.2 can be explained by "switching on" and "off" of tips of various types and various heights from the average level of a nanotube structure while working voltage is changed. Thus, for example, the parameter G for an A-tubelite is almost three ti
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