The Role of Adsorbates on the Field Emission Properties of Single-Walled Carbon Nanotubes
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The role of adsorbates on the field emission properties of single-walled carbon nanotubes
R. Collazo, M. Liang, R. Schlesser, and Z. Sitar Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695-7919
ABSTRACT Two field-emission states of single-walled carbon nanotubes were identified according to their respective emission current levels. The state yielding increased emission current was attributed to the presence of adsorbates on the nanotubes as confirmed by electron emission measurements at different background pressures. Application of high electric fields induced large emission currents and a transition between the two states. During this transition, a current drop to 10% of the original value was observed. Under a constant applied electric field, the current took around 1000 s to recover its original level at a background pressure of 10-10 Torr, while it took half that time at 10-6 Torr. For the high current state, field-emitted electrons originated from states located up to 1 eV below the Fermi level, as was determined by field-emission energy distribution measurements. This suggested that adsorbates introduced a resonant state on the surface that enhanced the tunneling probability of electrons. The adsorbed states are removed at high applied electric fields, presumably due to ohmic heating caused by large emission currents. This adsorption/desorption process is completely reversible.
INTRODUCTION The geometrical shape of carbon nanotubes immediately suggests applications for field emission based devices. [1] Interest in the development of these devices led to exploration and understanding of the nature of field emission from carbon nanotubes. Several research groups studied the field emission properties of single- and multi- walled nanotubes using current-voltage (I-V) characterization and field-emission energy distribution (FEED). [2] Although a simple Fowler-Nordheim analysis of the I-V characteristics suggests emission similar to that from a high aspect ratio metallic cathode, FEED, along with temperature dependence data, suggests that the emission mechanism is more complicated. [3] Three field-emission states were observed to occur in single-walled carbon nanotubes [3]. One state is distinguished by the presence of adsorbates that enhance the emission current and two additional states where the emission occurs from clean nanotubes. A reversible emission current drop was found to be characteristic for these last two states. The transition between the adsorbate-enhanced emission state and the clean nanotube emission states can be achieved in two ways: (1) by externally heating the sample and causing desorption of the adsorbates, and (2) by applying high electric fields that induce large emission currents. [4] Z5.7.1
In this report, we present evidence for explaining the role of adsorbates in the enhancement of field emission current. Also, field-emission energy distribution data at room temperature that supports the suggested adsorbate model is pr
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