H. Diesinger

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.

Note: Quantitative (artifact-free) surface potential measurements using Kelvin force microscopy

The measurement of local surface potentials by Kelvin force microscopy (KFM) can be sensitive to external perturbations which lead to artifacts such as strong dependences of experimental results (typically in a ∼1 V range) with KFM internal parameters (cantilever excitation frequency and/or the projection phase of the KFM feedback-loop). We analyze and demonstrate a correction of such effects on a KFM implementation in ambient air. Artifact-free KFM measurements, i.e., truly quantitative surface potential measurements, are obtained with a ∼30 mV accuracy.

Kelvin force microscopy at the second cantilever resonance: An out-of-vacuum crosstalk compensation setup

We investigate the gap-voltage control loop in a Kelvin force microscopy setup with simultaneous non-contact topography imaging. The Kelvin controller electrostatically excites the second resonance of the cantilever at about 6.3 times the first resonance frequency and adjusts the DC component of the gap voltage to cancel the oscillation amplitude at this frequency, while the non-contact topography imaging is based on a frequency control loop that maintains a constant frequency of the mechanically excited first resonance of the cantilever by adjusting the tip-sample separation. Due to the self-excitation of the first resonance in our setup, it has to be considered that the electrostatic excitation at the second resonance frequency is applied to a closed feedback loop and cannot be considered as a simple superposition to the oscillation at the first resonance frequency. In particular, special care has to be taken about internal capacitive crosstalk between the tip bias and the cantilever deflection output signal. It is shown that such a coupling cannot be corrected by subtraction of a constant offset at the demodulator output since the crosstalk is sent into the self-excitation loop and is multiplied by the closed loop transfer function. We present a circuit that actively compensates, outside the vacuum environment, the internal crosstalk by adding to the deflection output a dephased fraction of the electrostatic excitation signal.

Cross-talk artefacts in Kelvin probe force microscopy imaging: A comprehensive study

We provide in this article a comprehensive study of the role of ac cross-talk effects in Kelvin Probe Force Microscopy (KPFM), and their consequences onto KPFM imaging. The dependence of KPFM signals upon internal parameters such as the cantilever excitation frequency and the projection angle of the KPFM feedback loop is reviewed, and compared with an analytical model. We show that ac cross-talks affect the measured KPFM signals as a function of the tip-substrate distance, and thus hamper the measurement of three-dimensional KPFM signals. The influence of ac cross-talks is also demonstrated onto KPFM images, in the form of topography footprints onto KPFM images, especially in the constant distance (lift) imaging mode. Our analysis is applied to unambiguously probe charging effects in tobacco mosaic viruses (TMVs) in ambient air. TMVs are demonstrated to be electrically neutral when deposited on silicon dioxide surfaces, but inhomogeneously negatively charged when deposited on a gold surface.

Hysteretic behavior of the charge injection in single silicon nanoparticles

Charge injection in individual silicon nanoparticles has been investigated by electric force microscopy (EFM). Stored charges injected from the EFM tip have been counted using a quantitative method. Injection kinetics reveals the setting-up of an equilibrium regime. Equilibrium charge–voltage characteristics are analyzed, and display an overall linear behavior corresponding to successive tunneling through the nonequivalent tip–nanoparticle and nanoparticle–substrate oxide barriers. A hysteretic behavior is observed in the equilibrium charge–voltage characteristics, and attributed to a secondary charge injection process associated with the nanoparticle oxide surface.

Submicron nickel deposition on silicon from an electrolytic solution controlled by near-field optics

The application of a near-field optical device to the electrochemical deposition of submicron nickel dots on silicon is demonstrated. The silicon–electrolyte junction behaves like a Schottky diode where the electrolyte plays the role of the metal. The junction is reverse biased so that only a negligible dark current is flowing across the junction. The optical tip of the near-field device is used as a local lightsource to control a photocurrent on a submicron scale, which allows one to create submicron objects of nickel by locally triggering the electrochemical reduction of nickel ions. The effect of the lateral diffusion of the photogenerated carriers on the form of the deposited nickel dots is described by a two-dimensional carrier diffusion model.

Traveling wave dielectrophoresis micropump based on the dispersion of a capacitive electrode layer

A traveling wave dielectrophoresis microfluid pump based on structural dispersion is demonstrated. The phase shift between medium polarization and applied propagating field, necessary to generate asynchronous propagative forces in dielectrophoresis, is generated by an RC circuit consisting of the electrode insulator and the liquid conductivity. Since the device characteristics involve only bulk properties, the micropump does not require conductivity gradient or double layers, unlike existing micropumps using electro-osmosis and electrohydrodynamic shear forces. Its frequency of maximum pumping force can be made considerably lower than the dielectric relaxation frequency of the liquid. By decomposing the traveling wave electrode array into a rudimentary RC model, coincidence is found between optimized pumping conditions and crossover of the impedance measured between electrode combs. By using impedance spectroscopy alternately with pumping, the frequency of the applied signal can be matched in real-time to...

Capacitive Crosstalk in AM-Mode KPFM

In Kelvin probe force microscopes based on electrostatic tip excitation, a nonnegligible capacitive crosstalk occurs between the electrostatic probe excitation signal and the probe deflection output signal. In atomic force microscopy setups where a self-oscillation force feedback loop is used, the parasitic coupling may also superpose onto the piezomechanical tip excitation signal which provides the oscillation for topography imaging. As a result, the crosstalk cannot be described as a constant coupling to the deflection signal output, but rather has the effect of a spurious excitation signal, which makes it more difficult to quantify and compensate the effect. In this chapter, the phenomenon of capacitive crosstalk is studied in two frequently used AM-KPFM setups, operating in ultrahigh vacuum and in air. Different methods of reducing or eliminating the effect on the measured surface potential are described and compared.

Electromagnetic modeling and optimization of photoconductive switches for terahertz generation and photocurrent transient spectroscopy

The frequency response of photoconductive switches critically determines the performance of terahertz generating photoconductive antennas and of fast photocurrent measurements. Width and shape of the electrical transient depend on both the photoconducting material and on the geometry of the switch. In this work, the response of the photoconductive switch is described by a hybrid model based on the intrinsic response of the active material and the electromagnetic model of the switch geometry. The transmission line with its gap is expressed in terms of transconductance. Hence, its response can be optimized with respect to bandwidth, and moreover be taken into account by deconvolving the transconductance transient from the measured transient to determine more accurately the intrinsic properties of the active material. The method is illustrated for photoconductive switches with organic semiconductors as the active material. Incompatible with standard lithographic techniques, photoconductive switches based on organic semiconductors are typically embedded in microstrip lines and have feature size limitations imposed by the electrode deposition technique. An alternative structure with coplanar access is shown to have a bandwidth greater than 70 GHz, which is almost one order of magnitude higher than the state of the art.

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.