Interference effects in phtalocyanine controlled by H-H tautomerization: a potential two-terminal unimolecular electronic switch

We investigate the electrical transport properties of two hydrogen tautomer configurations of phthalocyanine (H2Pc) connected to cumulene and gold leads. Hydrogen tautomerization affects the electronic state of H2Pc by switching the character of molecular orbitals with the same symmetry close to the Fermi level. The near degeneracy between the HOMO and HOMO-1 leads to pronounced interference effects, causing a large change in current for the two tautomer configuratons, especially in the low-bias regime. Two types of planar junctions are considered: cumulene-H2Pc-cumulene and gold-H2Pc-gold. Both demonstrate prominent difference in molecular conductance between ON and OFF states. In addition, junctions with gold leads show pronounced negative differential resistance (NDR) at high bias voltage, as well as weak NDR at intermediate bias.Comment: 10 pages, 7 figures, accepted for publication in Physical Review

Search for alternative magnetic tunnel junctions based on all-Heusler stacks

By imposing the constraints of structural compatibility, stability and a large tunneling magneto-resistance, we have identified the Fe$_3$Al/BiF$_3$/Fe$_3$Al stack as a possible alternative to the well-established FeCoB/MgO/FeCoB in the search for a novel materials platform for high-performance magnetic tunnel junctions. Various geometries of the Fe$_3$Al/BiF$_3$/Fe$_3$Al structure have been analyzed, demonstrating that a barrier of less than 2~nm yields a tunneling magneto-resistance in excess of 25,000~\% at low bias, without the need for the electrodes to be half-metallic. Importantly, the presence of a significant spin gap in Fe$_3$Al for states with $\Delta_1$ symmetry along the stack direction makes the TMR very resilient to high voltages

Topological Line Defects around Graphene Nanopores for DNA Sequencing

Topological line defects in graphene represent an ideal way to produce highly controlled structures with reduced dimensionality that can be used in electronic devices. In this work we propose using extended line defects in graphene to improve nucleobase selectivity in nanopore-based DNA sequencing devices. We use a combination of QM/MM and non-equilibrium Green's functions methods to investigate the conductance modulation, fully accounting for solvent effects. By sampling over a large number of different orientations generated from molecular dynamics simulations, we theoretically demonstrate that distinguishing between the four nucleobases using line defects in a graphene-based electronic device appears possible. The changes in conductance are associated with transport across specific molecular states near the Fermi level and their coupling to the pore. Through the application of a specifically tuned gate voltage, such a device would be able to discriminate the four types of nucleobases more reliably than that of graphene sensors without topological line defects.Comment: 6 figures and 6 page

Механизмы формоизменения клиновидных двойников в локально-деформируемых ионноимплантированных монокристаллах висмута

Изучено влияние имплантации ионов бора, азота, углерода, аргона, циркония и тантала энергией 25 кэВ, дозой 10 17 ион/см 2 на закономерности искривления, ветвления и зарождения вдали от отпечатка индентора клиновидных двойников в монокристаллах висмута. Рассмотрены механизмы формоизменения клиновидных двойниковых ламелей. Предложен механизм зарождения дислокационных стопоров и источников двойникующих дислокаций в ходе ионной имплантации кристаллов. Рассмотрено взаимодействие нанодвойников, сформировавшихся при ионной имплантации, с двойниками, образующимися при локальном деформировании поверхности.The influence of implantation of boron, nitrogen, carbon, argon, zirconium and tantalum ions of energy of 25 keV, dose of 10 17 ion/cm 2 on the mechanism of distortion, branching and origination far from indentation of wedge-shaped twins in monocrystals of bismuth have been studied The mechanisms of lamella wedge-shaped twin deformation are considered. A mechanism is proposed for origination of dislocation stop and the sources of twinning dislocation in the process of crystal ion implantation. The interaction of nano-twins formed at ion implantation with the twins formed at local deformation of the surface is considered

Transverse Electronic Transport through DNA Nucleotides with Functionalized Graphene Electrodes

Graphene nanogaps and nanopores show potential for the purpose of electrical DNA sequencing, in particular because single-base resolution appears to be readily achievable. Here, we evaluated from first principles the advantages of a nanogap setup with functionalized graphene edges. To this end, we employed density functional theory and the non-equilibrium Green's function method to investigate the transverse conductance properties of the four nucleotides occurring in DNA when located between the opposing functionalized graphene electrodes. In particular, we determined the electrical tunneling current variation as a function of the applied bias and the associated differential conductance at a voltage which appears suitable to distinguish between the four nucleotides. Intriguingly, we observe for one of the nucleotides a negative differential resistance effect.Comment: 19 pages, 7 figure

Molecular Electronics : Insight from Ab-Initio Transport Simulations

This thesis presents the theoretical studies of electronic transport in molecular electronic devices. Such devices have been proposed and investigated as a promising new approach that complements conventional silicon-based electronics. To design and fabricate future nanoelectronic devices, it is essential to understand the conduction mechanism at a molecular or atomic level. Our approach is based on the non-equilibrium Green's function method (NEGF) combined with density functional theory (DFT). We apply the method to study the electronic transport properties of two-probe systems consisting of molecules or atomic wires sandwiched between leads. A few molecular electronic devices are characterized; namely, conducting molecular wires, molecular switches and molecular recognition sensors. The considered applications are interconnection of different nanoelectronic units with cumulene molecular wires; adding switching functionality to the molecular connectors by applying stress to the CNT-cumulene-CNT junction or by introducing phthalocyanine unit; sensing of individual nucleotides, e.g., for DNA sequencing applications. The obtained results provide useful insights into the electron transport properties of molecules. Several interesting and significant features are analyzed and explained in particular such as, level pinning, negative differential resistance, interfering of conducting channels etc

Molecular Electronics : Insight from Ab-Initio Transport Simulations

This thesis presents the theoretical studies of electronic transport in molecular electronic devices. Such devices have been proposed and investigated as a promising new approach that complements conventional silicon-based electronics. To design and fabricate future nanoelectronic devices, it is essential to understand the conduction mechanism at a molecular or atomic level. Our approach is based on the non-equilibrium Green's function method (NEGF) combined with density functional theory (DFT). We apply the method to study the electronic transport properties of two-probe systems consisting of molecules or atomic wires sandwiched between leads. A few molecular electronic devices are characterized; namely, conducting molecular wires, molecular switches and molecular recognition sensors. The considered applications are interconnection of different nanoelectronic units with cumulene molecular wires; adding switching functionality to the molecular connectors by applying stress to the CNT-cumulene-CNT junction or by introducing phthalocyanine unit; sensing of individual nucleotides, e.g., for DNA sequencing applications. The obtained results provide useful insights into the electron transport properties of molecules. Several interesting and significant features are analyzed and explained in particular such as, level pinning, negative differential resistance, interfering of conducting channels etc

Transport properties of nucleotides in a graphene nanogap for DNA sequencing

KoF U3ME

Spin injection and magnetoresistance in MoS2-based tunnel junctions using Fe3Si Heusler alloy electrodes

Recently magnetic tunnel junctions using two-dimensional MoS2 as nonmagnetic spacer have been fabricated, although their magnetoresistance has been reported to be quite low. This may be attributed to the use of permalloy electrodes, injecting current with a relatively small spin polarization. Here we evaluate the performance of MoS2-based tunnel junctions using Fe3Si Heusler alloy electrodes. Density functional theory and the non-equilibrium Green?s function method are used to investigate the spin injection efficiency (SIE) and the magnetoresistance (MR) ratio as a function of the MoS2 thickness. We find a maximum MR of ~300% with a SIE of about 80% for spacers comprising between 3 and 5 MoS2 monolayers. Most importantly, both the SIE and the MR remain robust at finite bias, namely MR?>?100% and SIE?>?50% at 0.7?V. Our proposed materials stack thus demonstrates the possibility of developing a new generation of performing magnetic tunnel junctions with layered two-dimensional compounds as spacer

Single-molecule DNA sequencing using two-dimensional Ti2C(OH)(2) MXene nanopores : A first-principles investigation

Nanopore-based devices have provided exciting opportunities to develop affordable label-free DNA sequencing platforms. Over a decade ago, graphene has been proposed as a two-dimensional (2D) nanopore membrane in order to achieve single-base resolution. However, it was experimentally revealed that clogging of the graphene nanopore can occur due to the hydrophobic nature of graphene, thus hindering the translocation of DNA. To overcome this problem, the exploration of alternative 2D materials has gained considerable interest over the last decade. Here we show that a Ti2C-based MXene nanopore functionalized by hydroxyl groups (-OH) exhibits transverse conductance properties that allow for the distinction between all four naturally occurring DNA bases. We have used a combination of density functional theory and non-equilibrium Green's function method to sample over multiple orientations of the nucleotides in the nanopore, as generated from molecular dynamics simulations. The conductance variation resulting from sweeping an applied gate voltage demonstrates that the Ti2C-based MXene nanopore possesses high potential to rapidly and reliably sequence DNA. Our findings open the door to further theoretical and experimental explorations of MXene nanopores as a promising 2D material for nanopore-based DNA sensing