No abstract

SummaryFrequency modulated Kelvin probe force microscopy (FM-KFM) is the method of choice for high resolution measurements of local surface potentials, yet on coarse topographic structures most researchers revert to amplitude modulated lift-mode techniques for better stability. This approach inevitably translates into lower lateral resolution and pronounced capacitive averaging of the locally measured contact potential difference. Furthermore, local changes in the strength of the electrostatic interaction between tip and surface easily lead to topography crosstalk seen in the surface potential. To take full advantage of the superior resolution of FM-KFM while maintaining robust topography feedback and minimal crosstalk, we introduce a novel FM-KFM controller based on a Kalman filter and direct demodulation of sidebands. We discuss the origin of sidebands in FM-KFM irrespective of the cantilever quality factor and how direct sideband demodulation enables robust amplitude modulated topography feedback. Finally, we demonstrate our single-scan FM-KFM technique on an active nanoelectronic device consisting of a 70 nm diameter InAs nanowire contacted by a pair of 120 nm thick electrodes.

We report on local measurements of the surface potential and quantum capacitance in single layer graphene as well as multilayers thereof as a function of the carrier density by using frequency-modulated Kelvin probe force microscopy. We find excellent agreement to tight-binding calculations reported for graphene monolayers and extract the minimum quantum capacitance from density sweeps at room temperature. The surface potential of graphene multilayers is found to depend linearly on the carrier density, which suggests treating them as two-dimensional electron gases. In addition, we demonstrate that the simultaneously detected second harmonic of the Kelvin modulation, proportional to |∂2C/∂z2|, is directly sensitive to local changes in the quantum capacitance of graphene.

We have extracted temperature-dependent thermal conductivity values from scanning thermal microscopy measurements of a self-heated multiwalled carbon nanotube supported on a silicon substrate. A deliberately introduced segment of amorphous carbon served as an integrated nanoheater. Kelvin probe force microscopy was used to supplement the thermometry data with values for the nanotube’s electrical resistivity. This way, both the spatially resolved temperature rise and the Joule heating power density were available for further analysis. A one-dimensional heat diffusion model was fitted to the data to extract values for the thermal conductivity along the nanotube axis and the thermal conductance between the nanotube and supporting substrate. We found thermal conductivity values that continuously increase from 200 to 400 W m–1 K–1 in a temperature range of 100 to 400 K above room temperature. The values obtained are about one order of magnitude lower compared to values reported for the freely suspended case. We attribute this observation to the increased phonon scattering and quenching of acoustic phonon modes due to the substrate interaction.

The properties of Bernstein modes were investigated through computer simulations using two-dimensional and two-and-one-half-dimensional (i.e., two spatial and three velocity coordinates) electrostatic models with fixed magnetic field. The measured discrete spectrum was found to agree with the linear dispersion relation for these modes. The quasi-periodic phenomenon of early phase-mixing damping and later recurrence, predicted by Baldwin and Rowlands, was observed. For large wavenumber k⊥, the initial damping rate is the same as that for Landau damping in an unmagnetized plasma; for small k⊥, however, it is much stronger. The recurrence peaks slowly damp in time at a rate proportional to k2⊥D, where D is the measured cross-field particle diffusion coefficient which is dominated by convective transport. Finally, splitting of the main spectral peaks and the appearance of subpeaks at half-integral multiples of the cyclotron frequency are observed and may be explained by nonlinear mode coupling.