Evidence of phonon-assisted tunnelling in electrical conduction through DNA molecules

We propose a phonon-assisted tunnelling model for explanation of conductivity dependence on temperature and temperature-dependent I-V characteristics in deoxyribonucleic acid (DNA) molecules. The capability of this model for explanation of conductivity peculiarities in DNA is illustrated by comparison of the temperature dependent I-V data extracted from some articles with tunnelling rate dependences on temperature and field strength computed according to the phonon-assisted tunnelling theory. PACS Codes: 87.15.-v, 71.38.-k, 73.40.Gk Introduction Conductance properties of DNA have recently attracted a lively interest for theoreticians as well as experimenters [1]. Understanding the charge carriers transfer mechanism along DNA double helix is important for possible applications of DNA molecules in nanoelectronic circuit technology [1-6]. Direct conductivity measurements have shown a very wide range of conducting properties ranging from no conduction [5,7,8] to a good linear conductor [2,9,10], while in other experiments semiconducting conductivity behaviour emerges [3,6,11-14]. The wide range of charge transport behaviour seems to arise from different experimental conditions in which the measurements are carried out. These include the nature of the devices used to measure the conductivity, the sequence and length of the DNA, the type of contacts, the environment in different experiments, etc., all can greatly effects the conductivity of the DNA molecules. For instance, Kasumov et al. [4] have shown that strongly deformed DNA molecules Published: 19 February 2008 PMC Physics B 2008, 1:6 doi:10.1186/1754-0429-1-6 Received: 26 November 2007 Accepted: 19 February 2008 This article is available from: http://www.physmathcentral.com/1754-0429/1/6 © 2008 Pipinys and Kiveris This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/ licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Page 1 of 6 (page number not for citation purposes) PMC Physics B 2008, 1:6 http://www.physmathcentral.com/1754-0429/1/6 deposited on a substrate, whose thickness is less than half the native thickness of the molecule, are insulating, whereas molecules keeping their native thickness are conducting down to very low temperature with a non-ohmic behaviour characteristics of a one dimensional (1D) conductor. Extensive experimental and theoretical work over the past decade has led to substantial clarification of charge transport mechanisms in DNA. The dominant mechanisms appear to be shortrange quantum mechanical tunnelling [14-18] or long-range thermally activated hopping [10,13,19-24]. But these mechanisms are not capable to explain all the fieldand temperaturebehaviour of experimental data associated with conduction of the DNA molecules. Indeed, the hopping models confronted with difficulties in explaining the observed strong conductivity dependence on the temperature along λ-DNA double helix at high temperatures and a very week dependence at low temperatures [9]. The tunnelling mechanism was excluded in the case of temperature-dependent results [9,11]. We affirm that in many cases the temperature-dependent conductivity of DNA molecules could be explained by the quantum mechanical tunnelling theories in which the impact of phonons on tunnelling rate is included. In the event, "pure" tunnelling can be observed at low temperatures when the vibrations modes of the molecule are frozen. At moderate temperatures the input of phonon energy to the process of tunnelling must be taken into account and contemporary phonon-assisted tunnelling theories (PhAT) [25-27] realise this. Recently, it has been shown that the PhAT describes well not only the nonlinear I-V curves, but also the temperature-dependent conductivity in conducting polymers [28,29]. Therefore, we invoke the PhAT theory to describe some the temperature-dependent experimental data on electrical transport through DNA molecules presented by other authors. Model and comparison with experimental data We suggest that the thermoactivated current through the DNA molecules is caused by the charge carriers released from localised states located between HOMO and LUMO levels of DNA ones [11]. In the dc case, the said levels are continuously filled from the electrode. Assuming that the electrons are released from these states due to phonon-assisted tunnelling, we will compare the current (the same as the conductance) dependence on the temperature and field strength with the tunnelling rate dependence on these parameters, computed using the PhAT theory. For this purpose we explore the equation (18) in [27] derived by Makram-Ebeid and Lannoo for the phonon-assisted tunnelling of the electrons from the impurity centre. Taking into consideration the fact that this theory has been evaluated using the Condon approximation, it is more suitable for the molecular structures than other ones. For the tunnelling rate dependence on field strength E and temperature T this theory gives: Page 2 of 6 (page number not for citation purposes) PMC Physics B 2008, 1:6 http://www.physmathcentral.com/1754-0429/1/6