Work function of carbon nanotubes
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Work function of carbon nanotubes Masashi Shiraishi and Masafumi Ata Frontier Science Laboratories, SONY Corporation, Shin-Sakuragaoka 2-1-1, Hodogaya-ku, Yokohama 240-0036, Japan
ABSTRACT The work functions of multi- and single-walled carbon nanotubes are found to be 4.95eV and 5.10eV respectively. The measurements were carried out using the photoelectron emission (PEE) method, which allows easy and precise measurements to be made in air. We have found that the work function of the nanotubes is 0.1–0.2eV larger than that of highly oriented pyrolytic graphite (HOPG) of which the valence state is σ−π orthogonal. This result is ascribable to reflection of the σ−π mixed valence state in the case of carbon nanotubes. The experimental data were well reproduced in ab-initio calculations on planar and cylindrical conjugated states.
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Carbon nanotubes (CNTs) are widely seen as a material with extensive potential for applications based on their electronic structure and one-dimensional geometry. The expected application fields include, for example, single electron transistors (SETs) [1], nanostructure diodes [2], hydrogen-storage [3] and cold emitters [4]. Yao et.al. [2] have recently observed the diode characteristics of a metal/semiconductor heterojunction of CNTs. There is a strong requirement for a precise analysis of the electronic structure of the CNT/metal heterojunction. The work function is one of the basic physical quantities needed to determine the electronic structure and the interaction at CNT/metal interface. There are various reports on the CNT work function. As pointed out by Collins et.al. [5], an estimation of the work function based on the Fowler-Nordheim model is not suitable because of uncertainty in the local geometry of CNTs. A commonly used methodology is ultraviolet photoemission spectroscopy (UPS). Suzuki et.al. [6] measured the work function of single-walled carbon nanotubes (SWNTs), obtaining a value of 4.8eV by UPS. Ago et.al. [7] also measured the work function of multi-walled carbon nanotubes (MWNTs) and reported a value of 4.3eV. In UPS method most of the photoelectrons are inelastically scattered in the case that samples are exposed to the air before the measurements. This fact prevents a precise determination of the work function of purified carbon nanotubes, since atpresent it is very difficult to transfer samples into an ultra high vacuum chamber just after synthesizing and purifying the materials without
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exposing them to air. In the present article, we report the results of work function measurements of SWNT and MWNT by using an open counter photoelectron emission (PEE) technique [8]. In this technique, a D2 lamp with a photon energy of 3.4-6.2eV was used. The surface condition of the samples does not affect the experimental results significantly, the vacuum level is the zero energy level and the valence edge is measured. The valence edge of the CNTs can be regarded as corresponding to the work function. The data obtained has been exa
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