Biomedical subjects

J Bernholc

Publications and source records attributed to J Bernholc.

At least 19 recordsLinked to original sources

PubMed2006

Interfacial segregation and electro-diffusion of dopants in superlattices.

A first-principles theory of interfacial segregation of dopants and defects in heterostructures is developed and applied to GAN/A1N superlattices. The results indicate that the equilibrium concentrations of a dopant at two sides of an interface may differ by up to a few orders of magnitude, depending on its chemical identity and charge state, and that these cannot be obtained from calculations for bulk constituents alone. In addition, the presence of an internal electric field in polar heterostructures induces electro-migration and accumulation of hydrogen at the appropriate interfaces.

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PubMed2005

Nonequilibrium quantum transport properties of organic molecules on silicon.

Electron transport properties of a Si/organic-molecule/Si junction are investigated by large-scale nonequilibrium Green function calculations. The results provide a qualitative picture and quantitative understanding of the importance of self-consistent screening, broadening of quasimolecular orbitals under large bias, and enhancement of transmission, which occurs when the broadened lowest unoccupied molecular orbital aligns with the conduction band edge of the negative lead. The varying coupling can lead to negative differential resistance for a large class of small molecules.

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PubMed2005

Optical absorption of water: coulomb effects versus hydrogen bonding.

The optical spectrum of water is not well understood. For example, the main absorption peak shifts upwards by 1.3 eV upon condensation, which is contrary to the behavior expected from aggregation-induced broadening of molecular levels. We investigate theoretically the effects of electron-electron and electron-hole correlations, finding that condensation leads to delocalization of the exciton onto nearby hydrogen-bonded molecules. This reduces its binding energy and has a dramatic impact on the line shape. The calculated spectrum is in excellent agreement with experiment.

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PubMed2005

Carbon nanotube-metal cluster composites: a new road to chemical sensors?

Novel carbon nanotube-metal cluster structures are proposed as prototype systems for molecular recognition at the nanoscale. Ab initio calculations show that already the bare nanotube cluster system displays some specificity because the adsorption of ammonia on a carbon nanotube-Al cluster system is easily detected electrically, while diborane adsorption does not provide an electrical signature. Since there are well-established procedures for attaching molecular receptors to metal clusters, these results provide a "proof-of-principle" for the development of novel, high-specificity molecular sensors.

Ammonia↗

PubMed2004

Ab initio studies of polarization and piezoelectricity in vinylidene fluoride and BN-based polymers.

Highly piezoelectric and pyroelectric phases of boron-nitrogen-based polymers have been designed from first principles. They offer excellent electrical and structural properties, with up to 100% improvement in the piezoelectic response and an enhanced thermal stability with respect to polyvinylidene fluoride (PVDF). Since methods for their synthesis are readily available, these polymers are extremely promising for numerous technological applications, rivaling the properties of ferroelectric ceramics and superseding PVDF-based materials in high-performance devices.

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PubMed2004

Mn interstitial diffusion in (ga,mn)as.

We present a combined theoretical and experimental study of the ferromagnetic semiconductor (Ga,Mn)As which explains the remarkably large changes observed on low-temperature annealing. Careful control of the annealing conditions allows us to obtain samples with ferromagnetic transition temperatures up to 159 K. Ab initio calculations, in situ Auger spectroscopy, and resistivity measurements during annealing show that the observed changes are due to out diffusion of Mn interstitials towards the surface, governed by an energy barrier of 0.7-0.8 eV. Electric fields induced by Mn acceptors have a significant effect on the diffusion.

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PubMed2002

Surface segregation of Ge at SiGe(001) by concerted exchange pathways.

The segregation of Ge during growth on SiGe(001) surfaces was investigated by ab initio calculations. Four processes involving adatoms rather than ad-dimers were considered. The two most efficient channels proceed by the concerted exchange mechanism and involve a swap between an incorporated Ge and a Si adatom, or between Si and Ge in the first and the second surface layers, respectively. The calculated activation energies of approximately 1.5 eV explain well the high-temperature experimental data. Segregation mechanisms involving step edges are much less efficient.

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PubMed2000

Optical anisotropy of the SiC((001))- (3x2) surface: evidence for the two-adlayer asymmetric-dimer model

The structure of the ( 3x2) reconstruction of beta-SiC(001) surface has been identified by comparing reflectance anisotropy spectra calculated from first principles with recent measurements. Only the calculations for the two-adlayer asymmetric-dimer model agree with experiment. The two prominent peaks at 3.6 and 5.0 eV found experimentally are assigned to electronic transitions between surface and bulklike electronic states. A further pronounced anisotropy at 2.0 eV, due to transitions between surface states, is predicted.

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PubMed1996