Sergei F. Lyuksyutov

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.

Robust reduction of graphene fluoride using an electrostatically biased scanning probe

AbstractWe report a novel and easily accessible method to chemically reduce graphene fluoride (GF) sheets with nanoscopic precision using high electrostatic fields generated between an atomic force microscope (AFM) tip and the GF substrate. Reduction of fluorine by the electric field produces graphene nanoribbons (GNR) with a width of 105-1,800 nm with sheet resistivity drastically decreased from >1 TΩ·sq.−1 (GF) down to 46 kΩ·sq.−1 (GNR). Fluorine reduction also changes the topography, friction, and work function of the GF. Kelvin probe force microscopy measurements indicate that the work function of GF is 180–280 meV greater than that of graphene. The reduction process was optimized by varying the AFM probe velocity between 1.2 μm·s−1 and 12 μm·s−1 and the bias voltage applied to the sample between −8 and −12 V. The electrostatic field required to remove fluorine from carbon is ∼1.6 V·nm−1. Reduction of the fluorine may be due to the softening of the C-F bond in this intense field or to the accumulation and hydrolysis of adventitious water into a meniscus.

Electric charging and nanostructure formation in polymeric films using combined amplitude-modulated atomic force microscopy-assisted electrostatic nanolithography and electric force microscopy

A hybrid technique, combining lithography which exploits atomic force microscope tip manipulation with modified electric force microscopy was used to study surface electric charging (deposition and evolution) of polymethyl methacryalate and polystyrene films. Upon charging the films past a threshold voltage, two distinct regimes were observed: (1) stable feature formation related to electric breakdown and mass transport resulting in stable film deformation due to the negative surface charging (negative tip bias) and (2) no stable feature formation regime attributed to viscoelastic deformation of polymer surface followed by the surface relaxation in the case of positive surface charging (positive tip bias).

Dynamic spatial structure of spontaneous beams in photorefractive bismuth silicon oxide

We report the domain structure of spontaneously occurring beams (subharmonics) in photorefractive bismuth silicon oxide with an applied electric field from 1 to 6 kV/cm and a running grating. The subharmonic beams are generated in a pattern of domains that evolve dynamically as they move through the crystal. We find that the domains move as a whole with a speed approximately equal to that of the primary grating, but in the opposite direction. The domains are separated by narrow boundary regions, where the phase of the subharmonic waves changes by π. The domain motion is consistent with the group velocity for running space-charge waves.

Running gratings in photoconductive materials

Starting from the three-dimensional version of a standard photorefractive model (STPM), we obtain a reduced compact set of equations for an electric field based on the assumption of a quasi-steady-state fast recombination. The equations are suitable for evaluation of a current induced by running gratings at small-contrast approximation and also are applicable for the description of space-charge wave domains. We discuss spatial domain and subharmonic beam formation in bismuth silicon oxide (BSO) crystals in the framework of the small-contrast approximation of STPM. The experimental results confirming holographic current existence in BSO crystal are reported.

Smart photogalvanic running-grating interferometer

Photogalvanic effect produces actuation of periodic motion of macroscopic LiNbO3 crystal. This effect was applied to the development of an all-optical moving-grating interferometer usable for optical trapping and transport of algae chlorella microorganisms diluted in water with a concentration of 27×104ml−1.

Relations between spontaneously occurring beams in bismuth silicon oxide with two frequency-detuned pump beams.

We report strong competition of intensities between spontaneously emerging beams (subharmonics) and other diffracted beams in two-wave mixing experiments with two frequency-detuned pump beams in photorefractive bismuth silicon oxide. The measurements show that the onset of subharmonics strongly affects the strength of the fundamental component of the primary grating, which caused the subharmonics in the first place. Suppression of subharmonics by vibration of the crystal increased the diffraction efficiency of the fundamental grating component by more than an order of magnitude.