Ab Initio Transport Research Articles

Precision control of single-molecule electrical junctions

There is much discussion of molecules as components for future electronic devices. However, the contacts, the local environment and the temperature can all affect their electrical properties. This sensitivity, particularly at the single-molecule level, may limit the use of molecules as active electrical components, and therefore it is important to design and evaluate molecular junctions with a robust and stable electrical response over a wide range of junction configurations and temperatures. Here we report an approach to monitor the electrical properties of single-molecule junctions, which involves precise control of the contact spacing and tilt angle of the molecule. Comparison with ab initio transport calculations shows that the tilt-angle dependence of the electrical conductance is a sensitive spectroscopic probe, providing information about the position of the Fermi energy. It is also shown that the electrical properties of flexible molecules are dependent on temperature, whereas those of molecules designed for their rigidity are not.

Bistability in a H-terminated Si(1 0 0)2 × 1 surface obtained by ab initio transport calculations

We have analyzed electron tunneling due to the electric field from a hydrogen-terminated Si(1 0 0)2 × 1 ultrathin film on a metal substrate by density functional transport calculations. We have obtained a hysteresis loop in the tunneling current, which comes from the existence of two electronic structures. Furthermore, we have clarified that, as a condition of bistable electron transport, a double-barrier potential structure is not necessarily required for zero field, because it can be induced by the electric field.

Atomic chains of group-IV elements and III-V and II-VI binary compounds studied by a first-principles pseudopotential method

Using the first-principles plane wave pseudopotential method we have studied structural, electronic, and transport properties of atomic chains of group-IV elements and group III-V and group II-VI binary compounds. Several materials which are insulating or semiconducting in bulk are found to be metallic in nanowire structures. Our calculations reveal that monatomic chains of Si, Ge, and Sn elements, and of binary compounds such as InP, GaAs, and AlSb, are stable and metallic. On the other hand, compound wires of BN, SiC, GaN, ZnSe, and several others have semiconducting or insulating properties. Ideal mechanical strength calculations show that some of these atomic chains can sustain strains of up to $\ensuremath{\epsilon}=0.3$. We have presented ab initio electron transport calculations for Si and AlP linear chain segments in between Al electrodes. Conductance of Si monatomic chains displays some nontrivial features as the number of atoms in the chain is varied or as the chain is strained. In addition to single atomic chain structures, junctions and grid structures of Si are investigated.

Thermally driven hopping and electron transport in amorphous materials from densityfunctional calculations

In this paper we study electron dynamics and transport in models of amorphous silicon andamorphous silicon hydride. By integrating the time-dependent Kohn–Sham equation, wecompute the time evolution of electron states near the gap, and study the spatialand spectral diffusion of these states due to lattice motion. We perform thesecalculations with a view to developing ab initio hopping transport methods. Thetechniques are implemented with the ab initio local basis code SIESTA, and maybe applicable to molecular, biomolecular and other condensed matter systems.

First-principles phase-coherent transport in metallic nanotubes with realistic contacts.

We present first-principles calculations of phase coherent electron transport in a carbon nanotube (CNT) with realistic contacts. We focus on the zero-bias response of open metallic CNT's considering two archetypal contact geometries (end and side) and three commonly used metals as electrodes (Al, Au, and Ti). Our ab initio electrical transport calculations make, for the first time, quantitative predictions on the contact transparency and the transport properties of finite metallic CNT's. Al and Au turn out to make poor contacts while Ti is the best option of the three.

Ab Intio Simulations of Quatum Transport: Si Clusters and Fullerene Chains

AbstractThe I-V characteristics of small Sin clusters and short C20 chains between atomistic Al leads were calculated using a new ab initio transport code based on a nonequilibrium Greens function formalism. All of the Si diaplay metallic I-V characteristics with typical conductances ranging between tow and three (units Go = 2e2/h). The transport properties of these molecular devices may be understood in terms of both the bandstructure ofelectrodes, and the molecular levels of the clusters as modified by the lead environment. Turning to the short C20 chains, these behave as short C20 chains, these behave as short nanowires whose tranmission depends sensitively on the orientation and distance between the individual C20 molecules. Transport through the molecular chains is accompained by a significant amount of charge transfer, which however remains localized at the electrode/molecular interface.

Ab initio transport theory for digital ferromagnetic heterostructures.

We present a theoretical density functional study of the electronic, magnetic, and transport properties of digital ferromagnetic heterostructures, obtained by delta doping GaAs with Mn. In the absence of intrinsic donors these systems have a half metallic density of states, with an exchange interaction much stronger than that of a random alloy with the same Mn concentration. Our ab initio ballistic transport calculations show that the carriers at the Fermi energy are strongly confined within a few monolayers around the MnAs plane. This strong confinement is responsible for the large exchange coupling and the two-dimensional half metallic behavior.

Electron irradiation damage in silicon containing carbon and oxygen

Ab initio electronic structure and transport calculations of 2D hexagonal germanium with four possible structural defects were performed. The considered defects were Stone–Wales (SW), single vacancy (5–9) and two divacancies (5–8–5 and 555–777). We showed that these defects present a local reconstruction that can be clearly identified by STM images. Among the investigated defects, we verified that the SW defect has the lowest formation energy. We showed that in the presence of structural defects the 2D hexagonal germanium maintains its Dirac cone feature only for the single vacancy. The divacancies and the SW defect destroy the linear dispersion relation of the electrons, near the Fermi level, in this system. Moreover, we verified that these defects create scattering centers, which can lead to diminishing of the current by roughly 42% for the Stone–Wales and single vacancy, 55% for the divacancy 5–8–5 and 68% for the 555–777 divacancy.