Predrag S. Krstic

Translocation of Single-Stranded DNA through Single-Walled Carbon Nanotubes

Carbon Nanotube Bridge for DNA Transport The nanoporosity of carbon nanotubes has been exploited in the control of molecular transport—for example, in creating membranes. Liu et al. (p. 64) fabricated devices in which one single-walled carbon nanotube connects two fluid reservoirs. In some of these devices, apparently those in which the nanotube is metallic, the ionic conductivity is anomalously higher than that expected from the bulk resistivity of the electrolyte. This high conductivity was exploited for the transport of single-stranded DNA, which was accompanied by large but transient increases in the ion current. Transfer of DNA by electrophoresis through some carbon nanotubes is accompanied by giant current pulses. We report the fabrication of devices in which one single-walled carbon nanotube spans a barrier between two fluid reservoirs, enabling direct electrical measurement of ion transport through the tube. A fraction of the tubes pass anomalously high ionic currents. Electrophoretic transport of small single-stranded DNA oligomers through these tubes is marked by large transient increases in ion current and was confirmed by polymerase chain reaction analysis. Each current pulse contains about 107 charges, an enormous amplification of the translocated charge. Carbon nanotubes simplify the construction of nanopores, permit new types of electrical measurements, and may open avenues for control of DNA translocation.

Elastic scattering and charge transfer in slow collisions: isotopes of H and H+ colliding with isotopes of H and with He

Differential and integral cross sections are calculated in the centre of mass energy range 0.1-100 eV for elastic scattering of protons (H+, D+, T+) by helium and of symmetric combinations of protons (H+, D+, T+) and hydrogen atoms (H, D, T) by hydrogen atoms (H, D, T). In addition, we derive from the elastic differential cross sections the momentum transfer and viscosity cross sections of use in the study of plasma transport, and compute the charge transfer cross section. Fully quantal and semiclassical approaches are utilized in these calculations, as are very accurate electronic potential energy curves. The results are compared with available data.

Origin of Giant Ionic Currents in Carbon Nanotube Channels

Fluid flow inside carbon nanotubes is remarkable: transport of water and gases is nearly frictionless, and the small channel size results in selective transport of ions. Very recently, devices have been fabricated in which one narrow single-walled carbon nanotube spans a barrier separating electrolyte reservoirs. Ion current through these devices is about 2 orders of magnitude larger than predicted from the bulk resistivity of the electrolyte. Electroosmosis can drive these large excess currents if the tube both is charged and transports anions or cations preferentially. By building a nanofluidic field-effect transistor with a gate electrode embedded in the fluid barrier, we show that the tube carries a negative charge and the excess current is carried by cations. The magnitude of the excess current and its control by a gate electrode are correctly predicted by the Poisson-Nernst-Planck-Stokes equations.

Enhancement of the transverse conductance in DNA nucleotides

We theoretically study the electron transport properties of DNA nucleotides placed in the gap between two single-wall carbon nanotubes capped or terminated with H or N. We show that in the case of C-cap and H-termination the current at low electric bias is dominated by nonresonant tunneling, similarly to the cases of gold electrodes. In nitrogen-terminated nanotube electrodes, the nature of current is primarily quasiresonant tunneling and is increased by several orders of magnitude. We discuss the consequence of our result on the possibility of recognition at the level of single molecule.

Chemical sputtering from amorphous carbon under bombardment by deuterium atoms and molecules

We perform classical molecular dynamics simulations of the chemical sputtering of deuterated amorphous carbon surfaces by D and D2, at energies of 7.5–30 eV D−1. Particular attention is paid to the preparation of the target surfaces for varying impact projectile fluence, energy and species, to the vibrational state of D2 projectiles, as well as to the variation in sputtering yields with target surface and impact projectile. The methane and acetylene sputtering yields per deuteron, obtained with atomic and molecular projectiles, agree quantitatively with recent experimental values. We study the distribution of sputtered species, as well as their kinetic energy and angular spectra.

Rate Coefficient for H+ + H2(X1Σg+, ν = 0, J = 0) → H(1s) + H2+ Charge Transfer and Some Cosmological Implications

Krstic has carried out the first quantum mechanical calculations near threshold for the charge transfer (CT) process H+ + H2(X 1Σ, ν = 0, J = 0) → H(1s) + H. These results are relevant for models of primordial galaxy and first star formation that require reliable atomic and molecular data for obtaining the hydrogen chemistry of the early universe. Using the results of Krstic, we calculate the relevant CT rate coefficient for temperatures between 100 and 30,000 K. We also present a simple fit that can be readily implemented into early universe chemical models. Additionally, we explore how the range of previously published data for this reaction translates into uncertainties in the predicted gas temperature and H2 relative abundance in a collapsing primordial gas cloud. Our new data significantly reduce these cosmological uncertainties that are due to the uncertainties in the previously published CT rate coefficients.

Charge Transfer in Collisions of C+ with H and H+ with C

Charge transfer rate coefficients for collisions of C+ with H and H+ with C are presented for temperatures from 30,000 to 107 K and from 10 to 107 K, respectively. The rate coefficients were calculated from recommended cross sections deduced in a recent theoretical and experimental investigation that took into account previous measurements. Nonadiabatic radial coupling is the dominant mechanism for both reactions above ~50,000 K, but for lower temperatures the reaction of H+ with C proceeds primarily by radiative charge transfer. Implications, due to the magnitude of the rate coefficients, for various astrophysical environments are discussed.

Elastic and vibrationally inelastic slow collisions: H + H2, H+ + H2

We report on a comprehensive study of the scattering of hydrogen atoms on the ground electronic surface of hydrogen molecules in the range of centre of mass energies 0.1-100 eV. Differential and integral elastic cross sections, the related transport cross sections, and vibrationally inelastic cross sections starting from both ground and excited vibrational states, are calculated using a fully quantal, coupled-channel approach in a truncated vibrational basis set, while the rotational dynamics of H2 is treated with the infinite order sudden approximation prescription. For comparison and to highlight the major physical mechanisms revealed in these collisions, a parallel study is carried out for scattering of protons on hydrogen molecules.

H+ + H Scattering and Ambipolar Diffusion Heating

We report new and highly accurate quantum mechanical calculations of the astrophysically interesting scattering of H+ by H atoms. The effects of the quantum indistinguishability of the two protons are treated consistently throughout, including the calculation of the momentum transfer cross section where elastic scattering and charge transfer cannot be separated at low energies. We are able to resolve the numerous oscillations in the energy variation of the angle-integrated cross sections. With decreasing energy below 1 eV, the oscillations grow in amplitude until a smooth, approximately 1/v2 dependence on velocity is reached below 10-5 eV. The 1/v behavior, traditionally associated with a constant Langevin rate coefficient, is never realized down to the lowest energy calculated (10-10 eV). We use the momentum transfer cross section to calculate accurately the transport rate coefficient that characterizes the drag force between ionic and neutral fluids and the strength of ambipolar diffusion heating for temperatures and ion-neutral drift speeds relevant for astrophysical applications. An early fit of this rate coefficient by Draine is sustained except at low energies. The application of these results to the role of ambipolar diffusion in heating jets from young stellar objects is discussed.

Collisions, magnetization, and transport coefficients in the lower solar atmosphere

Context. The lower solar atmosphere is an intrinsically multi-component and collisional environment with electron and proton collision frequencies in the range 10 8 10 10 Hz, which may be considerably higher than the gyro-frequencies for both species. Collisions between di erent species are altitude dependent because of the variation in density and temperature of all species. Aims. We aim to provide a reliable quantitative set of data for collision frequencies, magnetization, viscosity, and thermal conductivity for the most important species in the lower solar atmosphere. Having such data at hand is essential for any modeling that is aimed at describing realistic properties of the considered environment. Methods. The relevant elastic and charge transfer cross sections in the considered range of collision energies are now accepted by the scientific community as known with unprecedented accuracy for the most important species that may be found in the lower solar atmosphere. These were previously calculated using a quantum-mechanical approach and were validated by laboratory measurements. Only with reliable collision data one can obtain accurate values for collision frequencies and coe cients of viscosity and thermal conductivity. Results. We describe the altitude dependence of the parameters and the di erent physics of collisions between charged species, and between charged and neutral species. Regions of dominance of each type of collisions are clearly identified. We determine the layers within which either electrons or ions or both are unmagnetized. Protons are shown to be unmagnetized in the lower atmosphere in a layer that is at least 1000 km thick even for a kilo-Gauss magnetic field that decreases exponentially with altitude. In these layers the dynamics of charged species cannot be a ected by the magnetic field, and this fact is used in our modeling. Viscosity and thermal conductivity coe cients are calculated for layers where ions are unmagnetized. We compare viscosity and friction and determine the regions of dominance of each of the phenomena. Conclusions. We provide the most reliable quantitative values for most important parameters in the lower solar atmosphere to be used in analytical modeling and numerical simulations of various phenomena such as waves, transport and magnetization of particles, and the triggering mechanism of coronal mass ejections.