Leszek Adamowicz

Charging and discharging processes of carbon nanotubes probed by electrostatic force microscopy

Electrostatic properties of individually separated single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multiwalled carbon nanotubes (MWCNTs) deposited on insulating layers have been investigated by charge injection and electric force microscopy (EFM) experiments. Delocalized charge patterns are observed along the CNTs upon local injection from the EFM tip, corresponding to (i) charge storage in the nanotubes and to (ii) charge trapping in the oxide layer along the nanotubes. The two effects are dissociated easily for CNTs showing abrupt discharge processes in which the charge stored in the CNT are field emitted back to the EFM tip, while trapped oxide charge can subsequently be imaged by EFM, clearly revealing field-enhancement patterns at the CNT caps. The case of continuous discharge processes of SWCNTs, DWCNTs, and MWCNTs is discussed, as well as the evolution of the discharge time constants with respect to the nanotube diameter.

Charging and emission effects of multiwalled carbon nanotubes probed by electric force microscopy

Electrostatic properties of single-separated multiwalled carbon nanotubes (MWCNTs) deposited on a dielectric layer have been investigated by charge injection and electric force microscopy (EFM) experiments. We found that upon local injection from the biased EFM tip, charges delocalize over the whole nanotube length (i.e., 1–10μm), consistent with a capacitive charging of the MWCNT-substrate capacitance. In addition, the insulating layer supporting the nanotubes is shown to act as a charge-sensitive plate for electrons emitted from the MWCNTs at low electric fields, thus allowing the spatial mapping of MWCNT field-emission patterns.

Monte Carlo based microscopic description of electron transport in GaAs/Al0.45Ga0.55As quantum-cascade laser structure

Results of multiparticle Monte Carlo simulations of midinfrared quantum cascade lasers structure initially fabricated by Page et al. are presented. The main aim of this paper is to discuss in details how electric current flows through the structure and which subbands are involved in this process. Monte Carlo method allows to predict the electron population inversion between the lasing levels and gives microscopic insight into processes leading to such behavior. Importance of a subband belonging to the laser injector region, with energy slightly below the upper lasing level, is demonstrated. The electron–electron Coulomb interactions influence the shapes of electron distribution functions; the values of average electron energies and effective subbands’ temperatures are calculated.

Monte Carlo study of electron transport in monolayer silicene

Electron mobility and diffusion coefficients in monolayer silicene are calculated by Monte Carlo simulations using simplified band structure with linear energy bands. Results demonstrate reasonable agreement with the full-band Monte Carlo method in low applied electric field conditions. Negative differential resistivity is observed and an explanation of the origin of this effect is proposed. Electron mobility and diffusion coefficients are studied in low applied electric field conditions. We demonstrate that a comparison of these parameter values can provide a good check that the calculation is correct. Low-field mobility in silicene exhibits temperature dependence for nondegenerate electron gas conditions and for higher electron concentrations, when degenerate conditions are imposed. It is demonstrated that to explain the relation between mobility and temperature in nondegenerate electron gas the linearity of the band structure has to be taken into account. It is also found that electron–electron scattering only slightly modifies low-field electron mobility in degenerate electron gas conditions.

Modified Monte Carlo method for study of electron transport in degenerate electron gas in the presence of electron–electron interactions, application to graphene

Standard computational methods used to take account of the Pauli Exclusion Principle into Monte Carlo (MC) simulations of electron transport in semiconductors may give unphysical results in low field regime, where obtained electron distribution function takes values exceeding unity. Modified algorithms were already proposed and allow to correctly account for electron scattering on phonons or impurities. Present paper extends this approach and proposes improved simulation scheme allowing including Pauli exclusion principle for electronelectron (ee) scattering into MC simulations. Simulations with significantly reduced computational cost recreate correct values of the electron distribution function. Proposed algorithm is applied to study transport properties of degenerate electrons in graphene with ee interactions. This required adapting the treatment of ee scattering in the case of linear band dispersion relation. Hence, this part of the simulation algorithm is described in details.

Raman scattering study of gallium nitride heavily doped with manganese

Abstract Small MnxGa1−xN crystals grown by the resublimation method have been characterised by Raman scattering technique. It has been shown, that for small concentration of manganese the concentration of free electrons in the material is high as in typical free standing GaN crystals, but for higher contents of manganese the concentration of free carriers in the alloy has been substantially reduced. The measured Raman spectra have been compared with the calculated phonon density of states, and it has been shown that the Raman peaks correspond to the peaks of the phonon density of states. In consequence the new Raman peaks found in the material are ascribed to disorder activated phonon modes.

Frequency dependent rotation and translation of nanowires in liquid environment

In this paper, an approach of aligning and handling silicon nanowires in liquid environment on the large scale is presented. Traveling dielectrophoresis was used to simultaneously pump a weakly ionic nanowire suspension and to rotate nanowires in a plane perpendicular to the electrodes. The pumping force on the solution was maximized by monitoring the cell impedance using impedance spectroscopy and by matching the frequency of the supply voltage to the impedance crossover. At frequencies above or below impedance crossover, trapping or rotation of nanowires was observed which is explained by means of a competition between stationary and drag forces.

Combined rate equation and Monte Carlo studies of electron transport in a GaAs/Al0.45Ga0.55As quantum-cascade laser

Comparison of the Monte Carlo and rate equation methods as applied to the study of electron transport in a mid-infrared quantum cascade laser structure initially proposed by Page et al (2001 Appl. Phys. Lett. 78 3529) is presented for a range of realistic injector doping levels. An analysis of the difference between these two methods is given. It is suggested that justified approximations of the rate equation method, originated by imposing Fermi–Dirac statistics and the same electron effective temperature for each of the energy sub-bands, can be interpreted as partial inclusion of electron–electron interactions. Results of the rate equation method may be used as good initial conditions for a more precise Monte Carlo simulation. An algorithm combining rate equation and Monte Carlo simulations is examined. A reasonable agreement between the introduced method and a fully self-consistent resolution of Monte Carlo and Schr¨ odinger coupled with Poisson equations is demonstrated. The computation time may be reduced when the combined algorithm is used. (Some figures may appear in colour only in the online journal)

Solution of the differential equation of statistical kinematics: Planar diffusion of hydrogen in a palladium monocrystal

Abstract A method of solving the differential equation of statistical kinematics for solids is presented. The planar diffusion of hydrogen in a palladium monocrystal is taken into account as an example. It is shown that there exist such particular solutions which have the form of standing waves of concentration. In the range of times which are used in experiments the difference between the flux densities in the [1 0 0] and [1 1 0] directions is small and changes its sign three times. This may explain the common but not carefully justified opinion on the isotropy of diffusion in cubic lattices. As opposed to the Fick law the flux density remains finite while the concentration gradient tends to infinity. For long diffusion times, when the concentration harmonics of high orders vanish, the obtained solutions become similar to those of the classical diffusion equation.

Modeling of light propagation in photonic crystal fibres

The full vectorial finite difference beam propagation method is used for modelling light propagation in photonic crystal fibre. We have simulated light propagation in 10 mm long fibre with the structure consisting of an air-filled silica and characterised by triangular lattice of air holes with a pitch Λ = 2μm and with the central high refractive index defect acting as a core. Both the index guiding and photonic band gap effect guiding are considered. The computed wave field is discussed and compared with the results of other authors.