arXiv subjects

J. Schuster

Publications and source records attributed to J. Schuster.

9 recordsLinked to original sources

arXiv2017

Interaction between carbon nanotubes and metals: Electronic properties, stability, and sensing

The interactions between carbon nanotubes (CNTs) and metal adatoms as well as metal contacts are studied by means of ab initio electronic structure calculations. We show that the electronic properties of a semiconducting (8,4) CNT can be modified by small amounts of Pd adatoms. Such a decoration conserves the piezoelectric properties of the CNT. Besides the electronic influence, the stability of a single adatom, which is of big importance for future technology applications, is investigated as well. We find only small energy barriers for the diffusion of a Pd adatom on the CNT surface. Thus, single Pd adatoms will be mobile at room temperature. Finally we present results for the interaction between a metallic (6,0) CNT and metal surfaces. Binding energies and distances for Al, Cu, Pd, Ag, Pt, and Au are discussed and compared, showing remarkable agreement between the interaction of single metal atoms and metal surfaces with CNTs.

cond-mat.mes-hall↗

arXiv2017

Nanomechanics of CNTs for Sensor Application

A nanoscopic simulation for an acceleration sensor is aimed based on the piezoresistive effect of carbon nanotubes (CNTs). Therefore, a compact model is built from density functional theory (DFT), compared with results of molecular dynamics (MD) that describes the mechanics of carbon nanotubes in a parameterized way. The results for the interesting kind of CNTs [(6,3) and (7,4)] within the two approaches agree in a satisfying way, when DFT-calculations are performed with atomic configurations obtained by MD geometry optimization. Geometry optimization yields the Poisson ratio for CNTs. Thus, values from MD and DFT are compared. The simulation finally aims the modeling of the conductive behavior of CNTs when strain is applied, but this needs further verification. Here, we present the prediction of the tight binding model for suitable CNTs.

cond-mat.mes-hall↗

arXiv2017

Theoretical investigation of in situ k-restore processes for damaged ultra-low-k dielectrics

Ultra-low-k (ULK) materials are essential for today's production of integrated circuits (ICs). However, during the manufacturing process the ULK's low dielectric constant (k-value) increases due to the replacement of hydrophobic species with hydrophilic groups. We investigate the use of plasma enhanced fragmented silylation precursors to repair this damage. The fragmentation of the silylation precursors octamethylcyclotetrasiloxane (OMCTS) and bis(dimethylamino)-dimethylsilane (DMADMS) and their possible repair reactions are studied using density functional theory (DFT) and molecular dynamics (MD) simulations.

physics.chem-ph↗

arXiv2017

Theoretical investigation of an in situ k-restore process for damaged ultra-low-k materials based on plasma enhanced fragmentation

We present theoretical investigations of a k-restore process for damaged pourous ultra-low-k (ULK) materials. The process is based on plasma enhanced fragmented silylation precursors to replace k-value damaging, polar Si-OH and Si-H bonds by k-value lowering Si-CH$_{3}$ bonds. We employ density functional theory (DFT) to determine the favored fragments of silylation precursors and show the successful repair of damaged bonds on our model system. This model system consists of a small set of ULK-fragments which represent various damaged states of ULK materials. Our approach provides a fast scanning method for a wide variety of possible repair reactions. Further, we show that oxygen containing fragments are required to repair Si-H bonds and fragments with dangling Si-bonds are most effective to repair polar Si-OH bonds.

cond-mat.mes-hall↗

arXiv2017

Ab-initio Study of the Trimethylaluminum Atomic Layer Deposition Process on Carbon Nanotubes - An Alternative Initial Step

Electronic applications of carbon nanotubes (CNTs) require the deposition of dielectric films on the tubes while conserving their excellent electronic properties. In our density functional theory study we use the trimethylaluminum (TMA) atomic layer deposition (ALD) as a model process for a CNT-functionalization. Since this functionalization relies on the presence of OH-groups, the CNTs are exposed to a water or oxygen pre-treatment. We show that only CNTs with a single-vacancy defect are able to react with H$_{2}$O or O$_{2}$. Further, the defect is preferably saturated by oxygen. This leaves the CNT without the necessary hydroxyl groups for the first TMA addition. Therefore, we propose an alternative initial step after which a classical TMA ALD process can be performed on the CNT.

cond-mat.mes-hall↗

arXiv2017

DFT investigations of the piezoresistive effect of carbon nanotubes for sensor application

We investigate the piezoresistive effect of carbon nanotubes (CNTs) within density functional theory (DFT) aiming at application-relevant CNTs. CNTs are excellent candidates for the usage in nano-electromechanical sensors (NEMS) due to their small band gap at zero strain leading to a finite resistivity at room temperature. The application of strain induces a band gap-opening leading to a tremendous change in the resistivity. DFT with the LDA approximation yields reasonable results for pure carbon systems like CNTs and is applied to calculate the electronic structure of experimentally relevant CNTs. For the transport part, a simple ballistic transport model based on the band gap is used. We compare our DFT results for the band gaps of strained CNTs to results of tight binding (TB) models. By introducing a scaling factor of $\sqrt{2}$, an excellent agreement of the the DFT data with TB model published by Yang et al. [1] is obtained.

cond-mat.mes-hall↗

arXiv2017

Empirical transport model of strained CNT transistors used for sensor applications

We present an empirical model for the nearballistic transport in carbon nanotube (CNT) transistors used as strain sensors. This model describes the intrinsic effect of strain on the transport in CNTs by taking into account phonon scattering and thermally activated charge carriers. As this model relies on a semiempirical description of the electronic bands, different levels of electronic structure calculations can be used as input. The results show that the electronic structure of strained single-walled CNTs with a radius larger than 0.7nm can be described by a fully analytical model in the sensing regime. For CNTs with smaller diameter, parameterized data from electronic structure calculations can be used for the model. Depending on the type of CNTs, the conductance can vary by several orders of magnitude when strain is applied, which is consistent with the current literature. Further, we demonstrate the tuning of the sensor by an external gate which allows shifting the signal amplitude and the strain sensitivity. These parameters have to be balanced to get good sensing properties. Due to its basically analytical nature, the transport model can be formulated as a compact model for circuit simulations.

cond-mat.mes-hall↗

arXiv2003

Avalanches in a Bose-Einstein condensate

Collisional avalanches are identified to be responsible for an 8-fold increase of the initial loss rate of a large 87-Rb condensate. We show that the collisional opacity of an ultra-cold gas exhibits a critical value. When exceeded, losses due to inelastic collisions are substantially enhanced. Under these circumstances, reaching the hydrodynamic regime in conventional BEC experiments is highly questionable.

cond-mat↗

arXiv2002

Feshbach resonances in rubidium 87: Precision measurement and analysis

More than 40 Feshbach resonances in rubidium 87 are observed in the magnetic field range between 0.5 and 1260 G for various spin mixtures in the lower hyperfine ground state. The Feshbach resonances are observed by monitoring the atom loss, and their positions are determined with an accuracy of 30 mG. In a detailed analysis, the resonances are identified and an improved set of model parameters for the rubidium interatomic potential is deduced. The elastic width of the broadest resonance at 1007 G is predicted to be significantly larger than the magnetic field resolution of the apparatus. This demonstrates the potential for applications based on tuning the scattering length.

cond-mat.stat-mech↗