Interaction-induced negative differential resistance in asymmetric molecular junctions

Combining insights from quantum chemistry calculations with master equations, we discuss a mechanism for negative differential resistance (NDR) in molecular junctions, operated in the regime of weak tunnel coupling. The NDR originates from an interplay of orbital spatial asymmetry and strong electron-electron interaction, which causes the molecule to become trapped in a non-conducting state above a voltage threshold. We show how the desired asymmetry can be selectively introduced in individual orbitals in e.g., OPE-type molecules by functionalization with a suitable side group, which is in linear conjugation to one end of the molecule and cross-conjugated to the other end.Comment: 8 page

Designing all-graphene nanojunctions by covalent functionalization

We investigated theoretically the effect of covalent edge functionalization, with organic functional groups, on the electronic properties of graphene nanostructures and nano-junctions. Our analysis shows that functionalization can be designed to tune electron affinities and ionization potentials of graphene flakes, and to control the energy alignment of frontier orbitals in nanometer-wide graphene junctions. The stability of the proposed mechanism is discussed with respect to the functional groups, their number as well as the width of graphene nanostructures. The results of our work indicate that different level alignments can be obtained and engineered in order to realize stable all-graphene nanodevices

A new route towards uniformly functionalized single-layer graphene

It is shown, by DFT calculations, that the uniform functionalization of upper layer of graphite by hydrogen or fluorine does not change essentially its bonding energy with the underlying layers, whereas the functionalization by phenyl groups decreases the bonding energy by a factor of approximately ten. This means that the functionalized monolayer in the latter case can be easily separated by mild sonication. According to our computational results, such layers can be cleaned up to pure graphene, as well as functionalized further up to 25% coverage, without essential difficulties. The energy gap within the interval from 0.5 to 3 eV can be obtained by such one-side funtionalization using different chemical species.Comment: 15 pages, 3 figures, to appear in J. Phys. D: Applied Physic

Functionalization of carbon nanotubes with -CHn, -NHn fragments, -COOH and -OH groups

We present results of extensive theoretical studies concerning stability, morphology, and band structure of single wall carbon nanotubes (CNTs) covalently functionalized by -CHn(for n=2,3,4),-NHn(for n=1,2,3,4),-COOH and -OH groups. Our studies are based on ab initio calculations in the framework of the density functional theory. We determine the dependence of the binding energies on the concentration of the adsorbed molecules, critical densities of adsorbed molecules, global and local changes in the morphology, and electronic structure paying particular attention to the functionalization induced changes of the band gaps. These studies reveal physical mechanisms that determine stability and electronic structure of those systems and also provide valuable theoretical predictions relevant for application. Functionalization of CNTs causes generally their elongation and locally sp2 -> sp3 rehybridization in the neighborhood of chemisorbed groups. For adsorbants making particularly strong covalent bonds with the CNTs(-CH2), we observe formation of the 5/7 defects. In CNTs functionalized with -CH2,-NH4, and -OH, we determine critical density of molecules that could be covalently bound to CNTs. Functionalization of CNTs can be utilized for band gap engineering and also lead to changes in their metallic/semiconductor character. In semiconducting CNTs, adsorbants such as -CH3,-NH2,-OH and -COOH, introduce 'impurity' bands in the band gap of pristine CNTs. In the case of -CH3,-NH2, the induced band gaps are typically smaller than in the pure CNT and depend strongly on the concentration of adsorbants. However, functionalization of semiconducting CNTs with -OH leads to the metallization of CNTs. On the other hand, the functionalization of semi-metallic (9,0)CNT with -CH2 causes the increase of the band gap and induces semi-metal to semiconductor transition.Comment: accepted in Journal of Chemical Physic

Electronic properties and applications of MXenes: a theoretical review

Recent chemical exfoliation of layered MAX phase compounds to novel two-dimensional transition metal carbides and nitrides, so called MXenes, has brought new opportunity to materials science and technology. This review highlights the computational attempts that have been made to understand the physics and chemistry of this very promising family of advanced two-dimensional materials, and to exploit their novel and exceptional properties for electronic and energy harvesting applications.Comment: 12 figure

Do Balance Demands Induce Shifts in Visual Proprioception in Crawling Infants?

The onset of hands-and-knees crawling during the latter half of the first year of life heralds pervasive changes in a range of psychological functions. Chief among these changes is a clear shift in visual proprioception, evident in the way infants use patterns of optic flow in the peripheral field of view to regulate their postural sway. This shift is thought to result from consistent exposure in the newly crawling infant to different patterns of optic flow in the central field of view and the periphery and the need to concurrently process information about self-movement, particularly postural sway, and the environmental layout during crawling. Researchers have hypothesized that the demands on the infant's visual system to concurrently process information about self-movement and the environment press the infant to differentiate and functionalize peripheral optic flow for the control of balance during locomotion so that the central field of view is freed to engage in steering and monitoring the surface and potentially other tasks. In the current experiment, we tested whether belly crawling, a mode of locomotion that places negligible demands on the control of balance, leads to the same changes in the functional utilization of peripheral optic flow for the control of postural sway as hands-and-knees crawling. We hypothesized that hands-and-knees crawlers (n = 15) would show significantly higher postural responsiveness to movements of the side walls and ceiling of a moving room than same-aged pre-crawlers (n = 19) and belly crawlers (n = 15) with an equivalent amount of crawling experience. Planned comparisons confirmed the hypothesis. Visual-postural coupling in the hands-and-knees crawlers was significantly higher than in the belly crawlers and pre-crawlers. These findings suggest that the balance demands associated with hands-and-knees crawling may be an important contributor to the changes in visual proprioception that have been demonstrated in several experiments to follow hands-and-knees crawling experience. However, we also consider that belly crawling may have less potent effects on visual proprioception because it is an effortful and attention-demanding mode of locomotion, thus leaving less attentional capacity available to notice changing relations between the self and the environment

Exploring the Charge Localization and Band Gap Opening of Borophene: A First-Principles Study

Recently synthesized two-dimensional (2D) boron, borophene, exhibits a novel metallic behavior rooted in the s-p orbital hybridization, distinctively different from other 2D materials such as sulfides/selenides and semi-metallic graphene. This unique feature of borophene implies new routes for charge delocalization and band gap opening. Herein, using first-principles calculations, we explore the routes to localize the carriers and open the band gap of borophene via chemical functionalization, ribbon construction, and defect engineering. The metallicity of borophene is found to be remarkably robust against H- and F-functionalization and the presence of vacancies. Interestingly, a strong odd-even oscillation of the electronic structure with width is revealed for H-functionalized borophene nanoribbons, while an ultra-high work function (~ 7.83 eV) is found for the F-functionalized borophene due to its strong charge transfer to the atomic adsorbates