arXiv subjects

Rajeev Ahuja

Publications and source records attributed to Rajeev Ahuja.

At least 37 records · Page 2Linked to original sources

arXiv2013

Mechano-switching devices from carbon wire-carbon nanotube junctions

Well-known conductive molecular wires, like cumulene or polyyne, provide a model for interconnecting molecular electronics circuit. In the recent experiment, the appearance of carbon wire bridging two-dimensional electrodes - graphene sheets - was observed [PRL 102, 205501 (2009)], thus demonstrating a mechanical way of producing the cumulene. In this work, we study the structure and conductance properties of the carbon wire suspended between carbon nanotubes (CNTs) of different chiralities (zigzag and armchair), and corresponding conductance variation upon stretching. We find the geometrical structure of the carbon wire bridging CNTs similar to the experimentally observed structures in the carbon wire obtained between graphene electrodes. We show a capability to modulate the conductance by changing bridging sites between the carbon wire and CNTs without breaking the wire. Observed current modulation via cumulene wire stretching/elongation together with CNT junction stability makes it a promising candidate for mechano-switching device for molecular nanoelectronics.

cond-mat.mes-hall↗

arXiv2013

Single-layer MoS2 as efficient photocatalyst

The potential application of the single-layer MoS2 as photocatalyst was revealed in this work based on first-principles calculations. It is found that the pristine single-layer MoS2 is a good candidate for photocatalyst, and its catalyzing ability can be tuned by the applied mechanical strain. Furthermore, the p-type doping could improve the catalyzing ability more efficiently: first, the p-type doping makes the direct transition of electron from valence band maximum (VBM) to conduction band minimum (CBM) viable; second, it separates the water splitting reaction sites by having reduction in Mo sites and oxidation in P dopant sites; third, it improves the efficiency by equalizing the reducing power Δ1 and oxidizing power Δ2, as well as by enhancing the light absorption in long wavelength end of the visible light. Thus enables the single-layer MoS2 to be a efficient visible-light photocatalyst.

cond-mat.mtrl-sci↗

arXiv2012

Strain-induced stabilization of Al functionalization in graphene oxide nanosheet for enhanced NH3 storage

Strain effects on the stabilization of Al ad-atom on graphene oxide(GO)nanosheet as well as its implications for NH3 storage have been investigated using first-principles calculations.The binding energy of Al ad-atom on GO is found to be a false indicator of its stability.Tensile strain is found to be very effective in stabilizing the Al ad-atom on GO.It strengthens the C-O bonds through an enhanced charge transfer from C to O atoms. Interestingly,C-O bond strength is found to be the correct index for Al's stability.Optimally strained Al-functionalized GO binds up to 6 NH3 molecules,while it binds no NH3 molecule in unstrained condition.

cond-mat.mtrl-sci↗

arXiv2012

Polylithiated (OLi2) functionalized graphane as a potential hydrogen storage material

Hydrogen storage capacity, stability, bonding mechanism and the electronic structure of polylithiated molecules (OLi2) functionalized graphane (CH) has been studied by means of first principle density functional theory (DFT). Molecular dynamics (MD) have confirmed the stability, while Bader charge analysis describe the bonding mechanism of OLi2 with CH. The binding energy of OLi2 on CH sheet has been found to be large enough to ensure its uniform distribution without any clustering. It has been found that each OLi2 unit can adsorb up to six H2 molecules resulting into a storage capacity of 12.90 wt% with adsorption energies within the range of practical H2 storage application.

cond-mat.mtrl-sci↗

arXiv2012

Strain induced lithium functionalized graphane as a high capacity hydrogen storage material

Strain effects on the stability, electronic structure, and hydrogen storage capacity of lithium-doped graphane (CHLi) have been investigated by stateof-the art first principle density functional theory (DFT). Molecular dynamics MD) simulations have confirmed the stability of Li on graphane sheet when it is subject to 10% of tensile strain. Under biaxial asymmetric strain, the binding energy of Li of graphane (CH) sheet increases by 52% with respect to its bulk's cohesive energy. With 25% doping concentration of Li on CH sheet,the gravimetric density of hydrogen storage is found to reach up to 12.12wt%. The adsorption energies of H2 are found to be within the range of practical H2 storage applications.

cond-mat.mtrl-sci↗

arXiv2012

Hybrid density functional study of electronic and optical properties of phase change memory material: $\mathrm{Ge_{2}Sb_{2}Te_{5}}$

In this article, we use hybrid density functional (HSE06) to study the crystal and electronic structures and optical properties of well known phase change memory material $\mathrm{Ge_{2}Sb_{2}Te_{5}}$. We calculate the structural parameters, band gaps and dielectric functions of three stable structures of this material. We also analyze the electron charge distribution using the Bader's theory of charge analysis. We find that hybrid density functional slightly overestimate the value of 'C' parameter. However, overall, our results calculated with the use of hybrid density functional (HSE06) are very close to available experimental values than calculated with the use of PBE functional. Specifically, the electronic band gap values of this material calculated with HSE06 are in good agreement with the available experimental data in the literature. Furthermore, we perform the charge analysis and find that naive ionic model fails to explain the charge distribution between the constituent atoms, showing the complex nature of this compound.

cond-mat.mtrl-sci↗

arXiv2012

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.

cond-mat.mes-hall↗

arXiv2011

Anisotropy in the electronic structure of V2GeC investigated by soft x-ray emission spectroscopy and first-principles theory

The anisotropy of the electronic structure of ternary nanolaminate V2GeC is investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured polarization-dependent emission spectra of V L2,3, C K, Ge M1 and Ge M2,3 in V2GeC are compared to those from monocarbide VC and pure Ge. The experimental emission spectra are interpreted with calculated spectra using ab initio density-functional theory including dipole transition matrix elements. Different types of covalent chemical bond regions are revealed; V 3d - C 2p bonding at -3.8 eV, Ge 4p - C 2p bonding at -6 eV and Ge 4p - C 2s interaction mediated via the V 3d orbitals at -11 eV below the Fermi level. We find that the anisotropic effects are high for the 4p valence states and the shallow 3d core levels of Ge, while relatively small anisotropy is detected for the V 3d states. The macroscopic properties of the V2GeC nanolaminate result from the chemical bonds with the anisotropic pattern as shown in this work.

cond-mat.mtrl-sci↗

arXiv2011

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.

cond-mat.mes-hall↗

arXiv2011

Theoretical study of C60 as catalyst for dehydrogenation in LiBH4

Complex light metal hydrides possess many properties which make them attractive as a storage medium for hydrogen, but typically, catalysts are required to lower the hydrogen desorption temperature and to facilitate hydrogen uptake in the form of a reversible reaction. The overwhelming focus in the search for catalysing agents has been on compounds containing titanium, but the precise mechanism of their actions remains somewhat obscure. A recent experiment has now shown that fullerenes (C$_{60}$) can also act as catalyst for both hydrogen uptake and release in lithium borohydride (LiBH$_4$). In an effort to understand the involved mechanism, we have employed density functional theory to carry out a detailed study of the interaction between this complex metal hydride and the carbon nanomaterial. Considering a stepwise reduction of the hydrogen content in LiBH$_4$, we find that the presence of C$_{60}$ can lead to a substantial reduction of the involved H-removal energies. This effect is explained as a consequence of the interaction between the BH$_x^-$ complex and the C$_{60}$ entity.

cond-mat.mtrl-sci↗

arXiv2011

Borane Derivatives: A New Class as Super and Hyperhalogens

Super and hyperhalogens are a class of highly electronegative species whose electron affinities far exceed those of halogen atoms and are important to chemical industry as oxidizing agents, bio-catalysts, and building blocks of salts. Using Wade-Mingos rule well known for describing the stability of closo-boranes (BnHn2-) and state of the art theoretical method we show that a new class of super and hyperhalogens, guided by this rule, can be formed by tailoring the size and composition of borane derivatives. Unlike conventional superhalogens which have a metal atom at the core surrounded by halogen atoms, the superhalogens formed using the Wade-Mingos rule do not have to have either halogen or metal atoms. We demonstrate this by using B12H13 and its isoelectronic cluster, CB11H12 as examples. We also show that while conventional superhalogens containing alkali atoms require at least two halogen atoms, only one borane-like moiety is sufficient to render M(B12H12) (M=Li, Na, K, Rb, Cs) clusters superhalogen properties. In addition, hyperhalogens can be formed by using the above superhalogens as building blocks. Examples include M(B12H13)2 and M(C-B11H12)2 (M=Li - Cs). This finding opens the door to an untapped source of superhalogens and weakly coordinating anions with potential applications.

physics.chem-ph↗

arXiv2011

Transverse conductance of DNA nucleotides in a graphene nanogap from first principles

The fabrication of nanopores in atomically-thin graphene has recently been achieved and translocation of DNA has been demonstrated. Taken together with an earlier proposal to use graphene nanogaps for the purpose of DNA sequencing, this approach can resolve the technical problem of achieving single-base resolution in electronic nucleobase detection. We have theoretically evaluated the performance of a graphene nanogap setup for the purpose of whole-genome sequencing, by employing density functional theory and the non-equilibrium Green's function method to investigate the transverse conductance properties of nucleotides inside the gap. In particular, we determined the electrical tunneling current variation at finite bias due to changes in the nucleotides orientation and lateral position. Although the resulting tunneling current is found to fluctuate over several orders of magnitudes, a distinction between the four DNA bases appears possible, thus ranking the approach promising for rapid whole-genome sequencing applications.

physics.ins-det↗

arXiv2010

Enhanced DNA sequencing performance through edge-hydrogenation of graphene electrodes

We propose using graphene electrodes with hydrogenated edges for solid-state nanopore-based DNA sequencing, and perform molecular dynamics simulations in conjunction with electronic transport calculations to explore the potential merits of this idea. The results of our investigation show that, compared to the unhydrogenated system, edge-hydrogenated graphene electrodes facilitate the temporary formation of H-bonds with suitable atomic sites in the translocating DNA molecule. As a consequence, the average conductivity is drastically raised by about 3 orders of magnitude while exhibiting significantly reduced statistical variance. We have furthermore investigated how these results are affected when the distance between opposing electrodes is varied and have identified two regimes: for narrow electrode separation, the mere hindrance due to the presence of protruding hydrogen atoms in the nanopore is deemed more important, while for wider electrode separation, the formation of H-bonds becomes the dominant effect. Based on these findings, we conclude that hydrogenation of graphene electrode edges represents a promising approach to reduce the translocation speed of DNA through the nanopore and substantially improve the accuracy of the measurement process for whole-genome sequencing.

physics.bio-ph↗

arXiv2010

Relativity and the lead-acid battery

The energies of the solid reactants in the lead-acid battery are calculated ab initio using two different basis sets at non-relativistic, scalar relativistic, and fully relativistic levels, and using several exchange-correlation potentials. The average calculated standard voltage is 2.13 V, compared with the experimental value of 2.11 V. All calculations agree in that 1.7-1.8 V of this standard voltage arise from relativistic effects, mainly from PbO2 but also from PbSO4.

cond-mat.mtrl-sci↗

arXiv2010

Cumulene Molecular Wire Conductance from First Principles

We present first principles calculations of current-voltage characteristics (IVC) and conductance of Au(111):S2-cumulene-S2:Au(111) molecular wire junctions with realistic contacts. The transport properties are calculated using full self-consistent ab initio NEGF-DFT methods under external bias. The conductance of the cumulene wires shows oscillatory behavior depending on the number of carbon atoms (double bonds). Among all conjugated oligomers, we find that cumulene wires with odd number of carbon atoms yield the highest conductance with metallic-like ballistic transport behavior. The reason is the high density of states in broad LUMO levels spanning the Fermi level of the electrodes. The transmission spectrum and the conductance depend only weakly on applied bias, and the IVC is nearly linear over a bias region from +1 to -1 V. Cumulene wires are therefore potential candidates for metallic connections in nanoelectronic applications.

cond-mat.mes-hall↗

arXiv2007

First-principles Study of Physisorption of Nucleic Acid Bases on Small-Diameter Carbon Nanotube

We report the results of our first-principles study based on density functional theory on the interaction of the nucleic acid base molecules adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U), with a single-walled carbon nanotube (CNT). Specifically, the focus is on the physisorption of base molecules on the outer wall of a (5,0) metallic CNT possessing one of the smallest diameters possible. Compared to CNTs with large diameters, the physisorption energy is found to be reduced in the high-curvature case. The base molecules exhibit significantly different interaction strengths, and the calculated binding energies follow the hierarchy G > A > T > C > U, which appears to be independent of the tube curvature. The stabilizing factor in the interaction between the base molecule and CNT is dominated by the molecular polarizability that allows a weakly attractive dispersion force to be induced between them. The present study provides an improved understanding of the role of the base sequence in deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) on their interactions with carbon nanotubes of varying diameters.

cond-mat.soft↗

arXiv2007

Functionalized nanopore-embedded electrodes for rapid DNA sequencing

The determination of a patient's DNA sequence can, in principle, reveal an increased risk to fall ill with particular diseases [1,2] and help to design "personalized medicine" [3]. Moreover, statistical studies and comparison of genomes [4] of a large number of individuals are crucial for the analysis of mutations [5] and hereditary diseases, paving the way to preventive medicine [6]. DNA sequencing is, however, currently still a vastly time-consuming and very expensive task [4], consisting of pre-processing steps, the actual sequencing using the Sanger method, and post-processing in the form of data analysis [7]. Here we propose a new approach that relies on functionalized nanopore-embedded electrodes to achieve an unambiguous distinction of the four nucleic acid bases in the DNA sequencing process. This represents a significant improvement over previously studied designs [8,9] which cannot reliably distinguish all four bases of DNA. The transport properties of the setup investigated by us, employing state-of-the-art density functional theory together with the non-equilibrium Green's Function method, leads to current responses that differ by at least one order of magnitude for different bases and can thus provide a much more robust read-out of the base sequence. The implementation of our proposed setup could thus lead to a viable protocol for rapid DNA sequencing with significant consequences for the future of genome related research in particular and health care in general.

physics.bio-ph↗

arXiv2007

Physisorption of Nucleobases on Graphene

We report the results of our first-principles investigation on the interaction of the nucleobases adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U) with graphene, carried out within the density functional theory framework, with additional calculations utilizing Hartree--Fock plus second-order Moeller-Plesset perturbation theory. The calculated binding energy of the nucleobases shows the following hierarchy: G > T ~ C ~ A > U, with the equilibrium configuration being very similar for all five of them. Our results clearly demonstrate that the nucleobases exhibit significantly different interaction strengths when physisorbed on graphene. The stabilizing factor in the interaction between the base molecule and graphene sheet is dominated by the molecular polarizability that allows a weakly attractive dispersion force to be induced between them. The present study represents a significant step towards a first-principles understanding of how the base sequence of DNA can affect its interaction with carbon nanotubes, as observed experimentally.

cond-mat.mtrl-sci↗