Spin-Polarized Transport in Carbon Nanowires Inside Semiconducting Carbon Nanotubes X. Q. Shi, Z. X. Dai, G. H. Zhong, X. H. Zheng, and Z. Zeng* Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China and Graduate School of the Chinese Academy of Sciences, Beijing 100049 ReceiVed: April 30, 2007 A first-principles approach combined with none-equilibrium Green’s function method is used to investigate the spin-polarized transport properties of a kind of all-carbon composite: carbon nanowires (CNW) in the core of single-walled carbon nanotubes. Our results indicate that if the CNW density is not too high, spin- polarized transmission spectra are obtained resulting from the two majority spin bands induced near the Fermi level. With the bias voltages applied, spin-polarized current and negative differential resistance (NDR) appear. The appearance of NDR can be analyzed from the mismatch of the energy bands in the left and right leads due to external bias voltages. Other factors that influence the degree of spin-polarization are also discussed. 1. Introduction Molecular electronics has attracted increasing interests both for the fundamental reasons and for the potential applications since it represents the ultimate miniaturization of electronic systems. 1 In such an active research field, some important factors for electronics applications have been proven at the molecular level, such as negative differential resistance (NDR) 2 and rectification. 3 Moreover, molecular transistors, memories, and logic gates have also been demonstrated. 4 Since the pioneering work of Tsukagoshi and co-workers, 5 in which spin-polarized electrons are injected into carbon nanotubes, a new field, molecular spintronics, 6 appears. Molecular spintronics is the integration of molecular electronics and spintronics. 7 A growing activity in this burgeoning area has been stimulated and good tunnel magnetoresistance (TMR) characteristics have already been measured in octanethiols 8 and polymers. 9 The carbon nanotubes (CNTs), which are promising materials for future nanoscale electronic devices, 10 have drawn much attention since their discovery. 11 Especially, TMR-like trans- port through carbon nanotubes has been experimentally reported by several groups. 5,12 The rich science of CNTs has been further enriched by two recent experimental observations of a kind of all-carbon composite, which are formed by one- dimensional carbon nanowires (CNWs) inserted into the innermost tube of multiwalled CNTs. 13 Theoretically, several groups have investigated the energetic, electronic, and magnetic properties of CNWs inserted into single-walled carbon nanotubes (referred to as CNW@SWNT in the following). 14-16 Especially, Yang et al. have studied the magnetic properties of the CNW@SWNT composites. They have found that if the CNW density is not too high (i.e., the interatomic distance of CNW is long enough), the flatband ferromagnetism will be found in the composite, which is caused by the weak coupling between the CNW and the SWNT shell. 15 At the same time, the magnetic properties of CNTs have also been probed by other authors. 17 However, to the best of our knowledge, the spin-polarized transport properties of the pure carbon composite CNW@SWNT has not been reported yet. In the present work, we present theoretical investigations on the transport properties of the pure carbon composites, which are assumed to be composed of CNW inserted into the core of SWNTs or a single carbon atom in SWNTs. The calculated results show that as long as the CNW density is not too high spin-polarized transmission spectra can be obtained resulting from two majority spin bands induced near the Fermi level. These results agree well with previous works 15,16 about the electronic structure calculations of this kind of all-carbon composite. This paper is organized in the following way: the compu- tational method and the simulation model are briefly described in section II, the results and discussions are presented in section III, and a short summary is given in section IV. 2. Calculation Method and Simulation Model Our theoretical calculations are performed with the program Atomistix ToolKit, 18 in which the density functional theory is combined with Keldysh none-equilibrium Green’s function method to calculate the electronic and transport properties of nanoscale systems. The whole system, as shown in Figure 1, is divided into three parts from left to right in practical theoretical simulations: the left lead, the central scattering region, and the right lead. The interaction of the semi-infinite left and right leads on the scattering region is taken into account through self- energies. Details of the method and relevant references can be found elsewhere. 18 The bias voltage V b is applied across the nanotube device, which drives a steady-state current to flow along the tube axis. The V b provides the natural electrostatic boundary conditions for the Hartree potential in the scattering region, which is self-consistently solved on a three-dimensional real space grid. 18 The transmission coefficient u v(V) (E,V b ), which is a function of energy E and V b , is calculated from Green’s functions, 18,19 and the electrical current is obtained via the Landauer-Bu ¨ttiker formula where f l,r (E) ) 1/(1 + e (E-μ l,r )/k B T ) is the Fermi distribution function of the left and right lead, respectively, and μ l,r is the * Corresponding author. E-mail: zzeng@theory.issp.ac.cn. I v(V) ) e h ∫ -∞ +∞ u v(V) (E,V b )[f l (E) - f r (E)] dE 10130 J. Phys. Chem. C 2007, 111, 10130-10134 10.1021/jp073320e CCC: $37.00 © 2007 American Chemical Society Published on Web 06/15/2007