Biomedical subjects

Oded Hod

Publications and source records attributed to Oded Hod.

7 recordsLinked to original sources

PubMed2006

First-principles electronic transport calculations in finite elongated systems: a divide and conquer approach.

We present a first-principles method for the evaluation of the transmittance probability and the coherent conductance through elongated systems composed of a repeating molecular unit and terminated at both ends. Our method is based on a divide and conquer approach in which the Hamiltonian of the elongated system can be represented by a block tridiagonal matrix, and therefore can be readily inverted. This allows us to evaluate the transmittance and the conductance using first-principles electronic structure methods without explicitly performing calculations involving the entire system. A proof of concept model based on a trans-polyacetylene chain bridging two aluminum leads indicates that our divide and conquer approach is able to capture all the features appearing in the transmittance probability curves obtained by a full scale calculation.

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PubMed2006

Magnetoresistance of nanoscale molecular devices.

Affecting the current through a molecular or nanoscale junction is usually done by a combination of bias and gate voltages. Magnetic fields are less studied because nanodevices can capture only low values of the magnetic flux. We review recent work done with the aim of finding the conditions for magnetic fields to significantly affect the conductance of such junctions. The basic idea is to create narrow tunneling resonances through a molecular ring-like structure that are highly sensitive to the magnetic field. We describe a computational method that allows us to examine atomistic models of such systems and discuss several specific examples of plausible systems, such as the quantum corral, carbon nanotubes, and polycyclic aromatic hydrocarbon molecules. A unique property of the magnetic field, namely, its ability to split degenerate levels on the ring, is shown to allow prototypes of interesting new nanoscale devices, such as the three-terminal parallel logic gate.

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PubMed2006

Electronic structure and stability of semiconducting graphene nanoribbons.

We present a systematic density functional theory study of the electronic properties, optical spectra, and relative thermodynamic stability of semiconducting graphene nanoribbons. We consider ribbons with different edge nature including bare and hydrogen-terminated ribbons, several crystallographic orientations, and widths up to 3 nm. Our results can be extrapolated to wider ribbons providing a qualitative way of determining the electronic properties of ribbons with widths of practical significance. We predict that in order to produce materials with band gaps similar to Ge or InN, the width of the ribbons must be between 2 and 3 nm. If larger bang gap ribbons are needed (like Si, InP, or GaAs), their width must be reduced to 1-2 nm. According to the extrapolated inverse power law obtained in this work, armchair carbon nanoribbons of widths larger than 8 nm will present a maximum band gap of 0.3 eV, while for ribbons with a width of 80 nm the maximum possible band gap is 0.05 eV. For chiral nanoribbons the band gap oscillations rapidly vanish as a function of the chiral angle indicating that a careful design of their crystallographic nature is an essential ingredient for controlling their electronic properties. Optical excitations show important differences between ribbons with and without hydrogen termination and are found to be sensitive to the carbon nanoribbon width. This should provide a practical way of revealing information on their size and the nature of their edges.

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PubMed2005

Magnetoresistance devices based on single-walled carbon nanotubes.

We demonstrate the physical principles for the construction of a nanometer-sized magnetoresistance device based on the Aharonov-Bohm effect [Phys. Rev. 115, 485 (1959)]. The proposed device is made of a short single-walled carbon nanotube (SWCNT) placed on a substrate and coupled to a tip/contacts. We consider conductance due to the motion of electrons along the circumference of the tube (as opposed to the motion parallel to its axis). We find that the circumference conductance is sensitive to magnetic fields threading the SWCNT due to the Aharonov-Bohm effect, and show that by retracting the tip/contacts, so that the coupling to the SWCNT is reduced, very high sensitivity to the threading magnetic field develops. This is due to the formation of a narrow resonance through which the tunneling current flows. Using a bias potential the resonance can be shifted to low magnetic fields, allowing the control of conductance with magnetic fields of the order of 1 T.

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PubMed2005

Self-assembly of nanoparticles in three-dimensions: formation of stalagmites.

We develop a coarse-grained lattice gas model to describe drying-mediated self-assembly of nanoparticles in three dimensions. Our model is an extension of the model developed by Rabani et al. [Nature 2003, 426, 271] in two dimensions. We show that when liquid evaporation occurs layer by layer and solvation forces are strong, the resulting morphologies agree well with the predictions of the 2D model. We discuss scenarios in which the full 3D treatment is necessary and predict the formation of nanostalagmites.

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PubMed2005

A parallel electromagnetic molecular logic gate.

A theoretical study of transport through a three-terminal molecular ring in the presence of a magnetic field is presented. The physical principles necessary to achieve logic operations based on the Aharonov-Bohm effect are discussed. We show that a proper combination of a gate potential and a realistic magnetic field can be used to obtain parallel logic operations such as AND and AND+NOT.

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PubMed2003

A coarse-grained model for a nanometer-scale molecular pump.

A theory for a nanometer-scale pump based on the ratchet concept is presented. A lattice gas model with a set of moves that satisfy hydrodynamic equations is used to describe an asymmetric nanometer channel connecting two reservoirs of fluid. The channel, which is coupled to an external oscillatory (or stochastic) driving force, pumps fluid from one reservoir to the other. The frequency of the external driving force, the fluid density, and the channel dimensions are used to control the fluid flow. We observe a nonmonotonic behavior of the flow with respect to some model parameters and discuss the efficiency of the device.

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