DOI: 10.1007/s00339-008-4418-1 Appl. Phys. A 91, 429–433 (2008) Materials Science & Processing Applied Physics A s. das 1 s.f. ahmed 1 m.k. mitra 2 k.k. chattopadhyay 1,2, ✉ Morphology and temperature-dependent electron field emission from vertically aligned carbon nanofibers 1 Thin Film and Nanoscience Laboratory, Department of Physics, Jadavpur University, Kolkata 700032, India 2 Nanoscience and Technology Center, Jadavpur University, Kolkata 700032, India Received: 29 August 2007/Accepted: 15 January 2008 Published online: 22 February 2008 • © Springer-Verlag 2008 ABSTRACT Effects of temperature and the aspect ratio on the electron field emission properties of vertically aligned carbon nanofibers in thin-film form were studied in detail. Vertically aligned carbon nanofibers have been synthesized on silicon sub- strates via a direct current plasma enhanced chemical vapor deposition technique. Surface morphologies of the films were studied by an atomic force microscope. It was found that the length of the nanofibers increased and the diameter decreased as the thickness of the Ni catalyst film decreased. The threshold field for the electron field emission was found to be in the range from 4.3 to 5.4V/μ m for carbon nanofibers having different aspect ratios. The threshold field for carbon nanofibers having diameter ∼ 200 nm and aspect ratio ∼ 7.5 was found to decrease from 4.8 to 2.1V/μ m when the temperature was raised from 27 to 350 ◦ C. This dependence was due to the change in work func- tion of the nanofibers with temperature. The field enhancement factor, the current density and the effective work function were calculated and used to explain the emission mechanism. PACS 81.07.De; 61.10.-i; 79.70.+q; 73.30.+y 1 Introduction The first reports on the production of graphite nanofibers date back more than a century [1, 2]. The interest in fibrous carbon has since then been recurrent and a signifi- cant boost in the research in the carbon nanostructure field coincided with the discovery of multi-wall carbon nanotubes (MWCNTs) by Iijima in 1991 [3] and the successive pro- duction of single-wall nanotubes (SWCNTs) [4, 5]. Differ- ent techniques have been reported for synthesizing carbon nanofibers (CNFs), such as arc discharge [3–5], chemical vapor deposition [6], hot-filament-assisted sputtering [7], rf magnetron sputtering [8], laser vaporization [9] and plasma- enhanced chemical vapor deposition (PECVD) [10], by sev- eral research groups. Among them the PECVD method is one of the most promising ways for the synthesis of carbon nanostructures because it has a high yield and is a control- lable method for the production of CNTs and CNFs at lower ✉ Fax: +91-33-2414-6007, E-mail: kalyan_chattopadhyay@yahoo.com substrate temperatures with mass production [11]. Carbon nanotubes and nanofibers can be several micrometers long, and have a diameter of a few nanometers. As a result of this large aspect ratio (h /r , where r is the average radius and h is the length of the tubes, respectively), it is possible to achieve a sufficiently high effective field for electron emission at room temperature [12]. Due to the high physical strength, chemical stability and high electrical and thermal conductivities, CNTs and CNFs have attractive field emission properties, far better than conventional metallic or silicon-tip field emission ar- rays (FEAs) [13, 14]. These have crucial applications in field emission electron guns, flat panel display screens, microwave power-amplifier tubes, X-ray tubes, high-luminescence light sources and micromachined mass spectrometers [15–19]. Although the field emission property of carbon nanotubes has been widely studied during the last decade, most of the field emission experiments for CNTs/CNFs were performed at a fixed temperature. Field emission is a surface-sensitive phenomenon and most of the experimental work on the field emission from CNTs and CNFs focuses on the application in field emission displays [13, 14, 20–22]. There are only a few reports on the studies of temperature-dependent field emis- sion from carbon nanotubes. Chen et al. [23] reported that the electron field emission current increased markedly due to both direct heating and laser irradiation processes. Tan et al. [24] showed from the temperature-dependent field emission of MWCNTs that the work function of MWCNTs changes with temperature when the temperature was above 373 K. Gupta et al. [25] observed that the effect of temperature on the field emission from MWCNTs is relatively enhanced over that from SWCNTs. A study of field emission from materials at higher temperatures is interesting for many reasons. Apart from display applications, the field emission can be applied for other applications, such as for direct thermal to electri- cal energy conversion and design of nanothermometers [24]. High emission current density is also required for many appli- cations, such as in electron microscopes, where field emission at higher temperatures may also be utilized. Apart from the technological aspect, studies of the effect of temperature on the field emission property helps in detailed understanding of the emission mechanism in a better way. In this work, we have reported the effect of aspect ratio and temperature on the field emission property of vertically