Recent experiments by Venkatamaran et al. [Nature (London) 442, 904 (2006)] on a series of molecular wires with varying chemical compositions, revealed a linear dependence of the conductance on cos 2 θ, where θ is the angle of twist between neighboring aromatic rings. To investigate whether or not this dependence has a more general applicability, we present a first principles theoretical study of the transport properties of this family of molecules as a function of the chemical composition, conformation and the contact atom and geometry. If the Fermi energy EF lies within the HOMO-LUMO gap, then we reproduce the above experimental results. More generally, however, if EF is located within either the LUMO or HOMO states, the presence of resonances destroys the linear dependence of the conductance on cos 2 θ and gives rise to non-monotonic behaviour associated with the level structure of the different molecules. Our results suggest that the above experiments provide a novel method for extracting spectroscopic information about molecules contacted to electrodes.

We propose a mixed analytical-ab-initio method for the accurate calculation of the conductance in monovalent atomic wires. The method relies on the most general formula for ballistic transport through a monovalent wire, whose parameters can be determined from first-principles calculations. Our central result is the demonstration of the highly non-universal behavior of the conductance, which depends on the fine details of the contacts to the leads. We are therefore able to reconcile a large number of the apparently contradictory results that have recently appeared in the literature. 73.63.Rt,73.40.Cg In the last two decades transport properties of atomic contacts have been the subject of intensive research (for overviews see Ruitenbeek [1] and Agraït, Yeyati and Ruitenbeek [2]). First experimental evidences of the formation of golden atomic chains have been reported by Yanson et al. and Ohnishi et al. [3]. Experiments on chains of Au, Pt and Ir atoms [4] exhibit electrical conductance oscillations as a function of the wire length and similar oscillations as a function of bias voltage and electrode separation [5,6]. Rodrigues et al. [7] investigated the energetically preferred orientation of the crystal planes of the wire by the application of highresolution transmission electron microscopy. Their results show a strong correlation between the atomic arrangement and the conductance.The above experiments were stimulated by early theoretical predictions of conductance quantization [8] and conductance oscillations [9,10]. The latter issue generated a sequence of theoretical papers using a variety of techniques [11,12,13,14,15,16,17,18,19,20,21,22,23]. Density-functional theory predicted that the conductance of Na atom chains is close to the conductance quantum 2e 2 /h for odd numbers of atoms, and smaller than this for even numbers of atoms [10,15,16,20]. In the literature this is called the even-odd effect. A similar effect was found for other monovalent alkali-metal atoms such as Cs, but an opposite behavior, with a conductance bigger for even numbers of atoms than for odd numbers of atoms, was predicted for noble-metals (Cu, Ag and Au) [23]. The even-odd oscillation of the conductance for atomic wires of Na has also been analyzed using a pseudoatom-jellium model [19], where it was found that the sign of the effect is sensitive to the lead cone angle. Applying the first-principle recursion-transfer-matrix method Hirose at al. [22] showed that the bonding nature of the atoms at the contact plays a crucial role in determining transport properties. Lee and Kim[17] and Zeng and Claro [18], studied the effects of symmetries and found that the conductance of atomic chains with mirror symmetry and an odd number of atoms is always equal to the conductance quantum 2e 2 /h.In the literature various heuristic models have been proposed to interpret physically the results mentioned above: the standing wave model proposed by Emberly et al. [12], a simple barrier model suggested by Lee et al. [23], a simplified one-dimension...

We present a first-principles study of the electronic properties of silicon clathrate nanowires intercalated with various types of alkali or alkaline-earth atoms. We find that the band structure of the nanowires can be tailored by varying the impurity atom within the nanowire. The electronic character of the resulting systems can vary from metallic to semiconducting with direct band gaps. These properties make the nanowires specially suitable for electrical and optoelectronic applications. PACS numbers: 73.22.-f,73.21.Hb,81.07.VbThe desire for nanoscale components which integrate gracefully with silicon CMOS technology makes the fabrication and characterization of silicon nanowires particularly attractive. These structures can be grown using a broad range of experimental techniques 1,2,3,4,5,6 and posses novel properties which may make them suitable as interconnects in verylarge-scale integrated devices. Other interesting applications include photoelectronics, since these structures have large direct band-gaps which allow them to work as visible-light emitters with low power consumption. The origin of this change in the band gap is related to quantum confinement 7,17 which increases the band width and produces an indirect-direct transition as the size of the wire shrinks 9 . The band gap generates a low-voltage gap in the I-V characteristic which is considerably enhanced when the surface is passivated 8 . However, upon doping the gap disappears and the resulting I-V curves display ohmic behavior for low voltages 13 .

Supporting information for this article. Synthetic details and characterization data for compounds 2, 11-14, 17 and 18; X-ray crystallographic file for 4 and 11 in CIF format and a discussion of the structures; UV-Vis absorption and fluorescence spectra of 16; details of the theoretical calculations.

Using a tight binding model we calculate the conductance of monovalent atomic chains for different contact geometries. The leads connected to the chains are modelled as semi-infinite fcc lattices with different orientations and couplings. Our aim is twofold: To check the validity of a three-parametric conductance formula for differently oriented leads, and to investigate the geometry dependence of the conductance oscillations. We show that the character of these oscillations depends strongly on the geometry of the chain -lead coupling.