Delivered by Ingenta to: Purdue University Libraries IP: 95.85.80.168 On: Thu, 19 May 2016 20:26:01 Copyright: American Scientific Publishers Copyright © 2008 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 8, 469–478, 2008 Functionality of C(4,4) Carbon Nanotube as Molecular Detector Osman Barı¸ s Malcıoˇ glu * and ¸ Sakir Erkoç Department of Physics, Middle East Technical University, 06531 Ankara, Turkey Alterations in the electronic transport properties of C(4,4) single walled carbon nanotube when an agent is introduced to the outer surface are investigated theoretically. Several chemical agents in this context are investigated. The calculations are performed in two steps: First an optimized geometry for the functionalized carbon nanotube is obtained using semi-empirical calculations at the PM3 level, and then the transport relations are obtained using non equilibrium green-function approach. Gaussian and Transiesta-C simulation packages are used in the calculations correspondingly. The “electrodes” are chosen to be ideal geometry of the particular carbon nanotube, eliminating current quantization effects due to contact region. By varying chemical potential in the electrode regions, an I –V curve is traced for each particular functionalisation. Conductance in carbon nanotubes show a strong dependence on the geometry and aromaticity, both are which altered when the suitable agent is introduced. This dependence results in rather dramatic response in the I –V trace, the current is reduced significantly, and quantization effects are observed, even for a single molecule. However due to chemically stable nature, not all agents form a chemical bond to the surface. Overall, the material is a promising candidate for detector equipment. Keywords: Carbon Nanotubes, Molecular Detector. 1. INTRODUCTION Carbon nanotube is one of the most promising struc- tures in nanotechnology. Owing to its structural stability, flexibility and extraordinary electronic properties, several applications can be devised. 1 One such application is functionalisation as a high precision detector for chem- icals at the molecular level. 2 Detectors of this kind are highly anticipated in industry and commercial applica- tions, mostly in security. Due to cylindrical geometry, the carbon nanotube has a large surface area, furthermore elec- tronic properties are drastically altered with disruption of the surface configuration. In this respect, it is an ideal receptor for a detector. However, there are a couple of problems in realization of such a device. The electronic and structural properties of carbon nano- tube originate from the unique hybridization capabilities of carbon atom. Carbon nanotube can be modeled as a bent graphene sheet, thus the ideal structure consist of sp 2 hybridized carbon atoms strained due to surface curva- ture. Aromatic nature of this hybrid leads to geometry- dependent conductance of the carbon nanotube, similar to the conductance mechanism in a graphene sheet. In fact, the graphene sheet is called a “0-band gap semiconductor” due to particular arrangement of bands through the two * Author to whom correspondence should be addressed. dimensional crystal. Geometry and hybridization depen- dence translate into alterations in electronic conductance with the disruption of the surface. However, due to chemi- cally inert nature, not all elements can interact with a car- bon nanotube. This drawback is most commonly overcome by using intermediate receptors, organic or otherwise. 3 Although the number of chemicals that can be detected is drastically increased, the effective surface area is reduced, and additional problem of interaction of intermediate mate- rial with nanotube is introduced. In some of the rare cases, especially in highly reactive chemicals, the intermediate material can be bypassed. 4–7 High surface area and detection even at single molecular level features may be exploited in a potential application in this case. Increasing the surface curvature increases the tension on the sp 2 hybridized carbon, thus a carbon nano- tube with smaller diameter should be more susceptible to surface intrusion. Another problem in realization of a detector is the ever- present environmental fluctuations. Carbon nanotubes are quite flexible, thus in a typical exposed environment, sig- nificant vibrational modes are present, thermal or other- wise. These geometrical vibrations show as fluctuations in the conductance. Any detectable alteration in conductance thus should be significant enough to be observable among these, or alter the structural behavior itself. Use of smaller J. Nanosci. Nanotechnol. 2008, Vol. 8, No. 2 1533-4880/2008/8/469/010 doi:10.1166/jnn.2008.D020 469