Pavel B. Paramonov

Interaction of Charge Carriers with Lattice Vibrations in Oligoacene Crystals from Naphthalene to Pentacene

A key feature of organic π-conjugated materials is the strong connection between their electronic and geometric structures. In particular, it has been recently demonstrated that nonlocal electron-vibration (electron-phonon) interactions, which are related to the modulation of the electronic couplings (transfer integrals) between adjacent molecules by lattice vibrations, play an important role in the charge-transport properties of organic semiconductors. Here, we use density functional theory calculations and molecular mechanics simulations to estimate the strength of these nonlocal electron-vibration couplings in oligoacene crystals as a function of molecular size from naphthalene through pentacene. The effect of each optical vibrational mode on the electronic couplings is evaluated quantitatively. The results point to a very strong coupling to both intermolecular vibrational modes and intramolecular (including high-frequency) modes in all studied systems. Importantly, our results underline that the amount of relaxation energy associated with nonlocal electron-phonon coupling decreases as the size of the molecule increases. This work establishes an original relationship between chemical structure and nonlocal vibrational coupling in the description of charge transport in organic semiconductor crystals.

Peculiarities of an anomalous electronic current during atomic force microscopy assisted nanolithography on n-type silicon

We report the observation of anomalously high currents of up to 500µA during direct oxide nanolithography on the surface of n-type silicon {100}. Conventional nanolithography on silicon with an atomic force microscope (AFM) normally involves currents of the order of 10−10 –10−7 A and is associated with ionic conduction within a water meniscus surrounding the tip. The anomalous current we observe is related to an electrical breakdown resulting in conduction dominated by electrons rather than ions. We discuss the electron source during the AFM-assisted nanolithography process, and the possibility of using this breakdown current for nanoscale parallel writing.

Amplitude-modulated electrostatic nanolithography in polymers based on atomic force microscopy

Amplitude modulated electrostatic lithography using atomic force microscopy (AFM) on 20–50 nm thin polymer films is discussed. Electric bias of AFM tip increases the distance over which the surface influences the oscillation amplitude of an AFM cantilever, providing a process window to control tip-film separation. Arrays of nanodots, as small as 10–50 nm wide by 1–10 nm high are created via a localized Joule heating of a small fraction of polymer above the glass transition temperature, followed by electrostatic attraction of the polarized viscoelastic polymer melt toward the AFM tip in the strong (108–109 V/m) nonuniform electric field.

Precise formation of nanoscopic dots on polystyrene film using z-lift electrostatic lithography

Z-lift electrostatic lithography on thin (10–50nm) polystyrene (PS) films is discussed. The height of nanostructures can be controlled via mechanically drawing or depressing the cantilever height (z-lift) during the application of a voltage. Since polymer is not removed or crosslinked during structure formation, the features are erasable. Various aspects such as voltage doses, film thickness, z-lift height, and rate are explored. Structure height formation relies mainly on, and is proportional, to the z-lift magnitude; however, only a narrow range of voltages yields structures for any given film thickness. Structures ranging from 0–10nm are produced on a 40nm thick PS film using −36V by varying the z-lift on a 0.1–0.9N∕m cantilever from −20nm to +400nm.

Density-functional description of water condensation in proximity of nanoscale asperity

We apply nonlocal density-functional formalism to describe an equilibrium distribution of the waterlike fluid in the asymmetric nanoscale junction presenting an atomic force microscope tip dwelling above an arbitrary surface. The hydrogen bonding dominating in intermolecular attraction is modeled as a square-well potential with two adjustable parameters (energy and length) characterizing wells depth and width. A liquid meniscus formed inside the nanoscale junction is explicitly described for different humidity. Furthermore, we suggest a simple approach using polymolecular adsorption isotherms for the evaluation of an energetic parameter characterizing fluid (water) attraction to substrate. This model can be easily generalized for more complex geometries and effective intermolecular potentials. Our study establishes a framework for the density-functional description of fluid with orientational anisotropy induced by nonuniform external electric field.