Direct measurements are presented of the Schottky barrier (SB) heights of carbon nanotube devices contacted with Pd electrodes. The SB barrier heights were determined from the activation energy of the temperature-dependent thermionic emission current in the off-state of the devices. The barrier heights generally decrease with increasing diameter of the nanotubes and they are in agreement with the values expected when assuming little or no influence of Fermi level pinning.

Nanoelectromechanical systems (NEMS) are attracting increasing attention due to their small size, low power consumption, and fast switching speeds. 1 They are regarded as among the most interesting emerging technologies on the ITRS roadmap. 2 Carbon nanotubes are particularly interesting as NEMS components due to their low mobile mass and high Young's modulus, which enables high speed, combined with their high mechanical strength and ability to withstand extreme conditions of temperature and radiation exposure. The predicted possibility of tuning the resonance frequency over a large range by the application of a gate voltage 3 is particularly promising for many applications. Here we present a direct transmission measurement of the resonance frequency of an individual singly clamped carbon nanofiber relay. The experimental results verify the predictions of a small signal model and provide important information for determining the practicality of using carbon-based NEMS as components in real electrical circuits.Some prototype carbon NEMS have been reported in the literature, 4-11 but there have been relatively few experimental studies of the high frequency properties of carbon NEMS based on individual carbon nanotubes. [12][13][14][15][16] The resonance frequencies of doubly clamped suspended singlewalled nanotubes (SWNT) have been determined using an indirect mixing technique with a lock-in amplifier. [12][13][14] The method requires a semiconducting nanotube and can therefore not be applied to the mechanically more rigid multiwalled nanotubes (MWNT) or nanofibers (CNF) that have been studied as a dc prototype NEMS and allow the fabrication of more varied device geometries. [4][5][6][7][8]10,11 The mechanical resonances of singly clamped MWNT have been observed in a field emission microscope, where the broadening of the field emission spot was observed as the nanotube was brought into mechanical resonance by the application of an ac voltage on a side gate. The resonance frequency of the MWNT could be tuned by an order of magnitude by increasing the bias voltage on the nanotube and thus increasing the tension. 15 The same technique has recently been used to demonstrate a so-called nanoradio. 16 Although all of these studies provide interesting information concerning the resonance properties of individual carbon nanotubes, they do not provide muchneeded information on the feasibility of integrating carbonbased NEMS in useful electronic circuits nor on the signal levels that can be expected. In this letter, we report the first direct transmission detection of the resonant behavior of a two-terminal carbon nanofiber relay. We compare the experimental data with the predictions of a small signal model, treating the CNF as a simple series resonator. Good agreement is obtained concerning the resonance amplitude, shape, and phase. We show that it is possible to measure the transmission of a single relay device; however, the signal level is very low and most practical applications will need to be based on arrays of such st...