First-Principles Calculation of Transport Properties of a Molecular Device

M. Di Ventra, S. T. Pantelides, N. D. Lang
2000 Physical Review Letters  
We report first-principles calculations of the current-voltage (I-V ) characteristics of a molecular device and compare with experiment. We find that the shape of the I-V curve is largely determined by the electronic structure of the molecule, while the presence of single atoms at the molecule-electrode interface play a key role in determining the absolute value of the current. The results show that such simulations would be useful for the design of future microelectronic devices for which the
more » ... ices for which the Boltzmann-equation approach is no longer applicable. PACS numbers: 73.40.Jn, 73.40.Cg, 73.40.Gk, 85.65. + h Conventional Si-based microelectronics is likely to reach its limit of miniaturization in the next 10-15 years when feature lengths shrink below 100 nm. The main problem is the onset of quantum phenomena, e.g., tunneling, that would make scaled-down conventional devices inoperable. Successor technologies currently under development, such as tunneling field-effect transistors and single-electron transistors, are in fact based on quantum phenomena. For the ultimate miniaturization below nm, devices made from single molecules are currently attracting attention. Prototypes have already been fabricated. Reed et al. [1] reported I-V characteristics of single benzene-1,4-dithiolate molecules. Alivisatos and coworkers [2] reported similar I-V characteristics of semiconductor and metal nanoclusters between gold electrodes. Dekker and co-workers [3] reported transistorlike behavior in carbon nanotubes. Similar devices have been demonstrated by Avouris and co-workers using single-walled and multiwalled carbon nanotubes [4] . Theoretical modeling played a key role in the invention of the transistor and in the subsequent development of integrated circuits. Device modeling continues to provide indispensable input to circuit modeling for designing logic and memory chips and microprocessors. It is based on a "semiclassical approximation" that treats electrons and holes as classical particles, except that their kinetic energies are determined by the semiconductor energy bands, most commonly in the effective-mass approximation. In this scheme, transport is governed by Boltzmann's equation. Most industrial modeling is done in the drift-diffusion approximation, to a lesser extent in the higher-order hydrodynamic approximation, and, at times, by solving Boltzmann's equation directly by Monte Carlo techniques [5] . For nanodevices, however, when quantum phenomena are dominant, the semiclassical Boltzmann's equation does not apply. Quantum mechanical simulations are needed. So far, only semiempirical approaches have been employed to investigate transport in molecular systems, providing useful insights into the fundamental mechanisms [6] [7] [8] [9] . In this Letter we report first-principles calculations of the I-V characteristics of a molecule. In this method, the electrostatic potentials through the molecule and at the contacts are calculated self-consistently without empirical adjustments. We report calculations for the benzene-1,4-dithiolate molecule for which experimental data are available [1] . We find that, when the molecule is placed between two electrodes made of an ideal metal (homogeneous electron gas or jellium model [10]), the shape of the I-V characteristic is determined by the electronic structure of the molecule in contact with the electrodes and in the presence of the external electric field. The shape is essentially the same as that of the experimental curve. The absolute magnitude of the current, however, is more than 2 orders of magnitude larger than the experimental values. We investigated the origins of this discrepancy and found that insertion of a single gold atom at each metal-molecule contact, as suggested by the experimental setup, leaves the shape of the I-V characteristic globally unchanged, but reduces the absolute value of the current by more than an order of magnitude, reflecting reduced coupling between the s states of the gold and the p orbitals of the molecule, in the directions parallel to the electrode surfaces. Replacement of the single gold atoms by aluminum atoms, which have p z orbitals in the relevant energy region, raises the value of the current by about 1 order of magnitude. Other factors that can affect the absolute value of the current are discussed later in this paper. These results show that such calculations can provide valuable quantitative information to help design molecular devices. We begin with the study of the I-V characteristic of a single benzene-1,4-dithiol molecule between two ideal metallic contacts. It is well known that when the benzene-1,4-dithiol molecule is adsorbed on gold surfaces the H of the thiol terminations desorbs and the sulfur atoms at each end bond strongly to the Au(111) surfaces [11] . The remaining molecule (benzene-1,4-dithiolate) is then simply the one represented in Fig. 1 , where we show the contour plot of the electronic density in the benzene ring plane. We 0031-9007͞00͞84(5)͞979(4)$15.00
doi:10.1103/physrevlett.84.979 pmid:11017420 fatcat:ndatubcozbgjzlbryn6wro7ayu