Urs Gysin

Dynamics of damped cantilevers

In atomic force microscopy cantilevers are used to detect forces caused by interactions between probing tip and sample. The minimum forces which can be detected with commercial sensors are typically in the range of 10−12 N. In the future, the aim will be to construct sensors with improved sensitivities to detect forces in the range of 10−18 N. These sensors could be used for mass spectroscopy or magnetic resonance force microscopy. Achieving this goal requires smaller sensors and increased quality factor Q. In this article we describe a model to characterize the dynamics of cantilevers of each eigenmode. In contrast to previous models, the damping is treated rigorously in the calculations.

Suppression of electronic friction on Nb films in the superconducting state

Investigations on the origins of friction are still scarce and controversial. In particular, the contributions of electronic and phononic excitations are poorly known. A direct way to distinguish between them is to work across the superconducting phase transition. Here, non-contact friction on a Nb film is studied across the critical temperature TC using a highly sensitive cantilever oscillating in the pendulum geometry in ultrahigh vacuum. The friction coefficient Γ is reduced by a factor of three when the sample enters the superconducting state. The temperature decay of Γ is found to be in good agreement with the Bardeen-Cooper-Schrieffer theory, meaning that friction has an electronic nature in the metallic state, whereas phononic friction dominates in the superconducting state. This is supported by the dependence of friction on the probe-sample distance d and on the bias voltage V. Γ is found to be proportional to d-1 and V2 in the metallic state, whereas Γ∼d-4 and Γ∼V4 in the superconducting state. Therefore, phononic friction becomes the main dissipation channel below the critical temperature.

The noise of cantilevers

Micromechanical cantilevers used in atomic force microscopy are characterized by the geometry, the elastic modulus E and the quality factor Q. The sensor can be regarded as a rectangular bar clamped on one side and free on the other. In contrast to a simple harmonic oscillator a cantilever has different eigenfrequencies ωn and a mode-dependent spring constant Dn. Using the fluctuation-dissipation theorem we developed a simple model to calculate the thermal noise on each eigenmode for a free cantilever. With this result we can decide whether measuring on higher eigenmodes increases the force sensitivity.

Using higher flexural modes in non-contact force microscopy

The oscillation characteristics of higher flexural modes of a rectangular microfabricated silicon cantilever have been studied in ultra-high vacuum (UHV) for a free cantilever and for a typical situation in non-contact force microscopy. The results are discussed with respect to the use of such modes in dynamic force microscopy (DFM) and local dissipation measurements.

Friction force microscopy studies on SiO2 supported pristine and hydrogenated graphene

A graphene sample supported on SiO2 with pristine and plasma-hydrogenated parts is investigated by friction force microscopy. An initial contrast in friction is apparent between the two regions. A tip induced cleaning of the surface in the course of continuous scanning results in a very clean surface accompanied with a reduction of the friction force by a factor of up to 4. The contamination is adhering stronger to hydrogenated regions, but once cleaned, the frictional behavior is the same on pristine and hydrogenated graphene. Raman imaging demonstrates that the hydrogenation remains intact under the mechanical treatment.

Magnetic properties of nanomagnetic and biomagnetic systems analyzed using cantilever magnetometry

Magnetic properties of nanomagnetic and biomagnetic systems are investigated using cantilever magnetometry. In the presence of a magnetic field, magnetic films or particles deposited at the free end of a cantilever give rise to a torque on the mechanical sensor, which leads to frequency shifts depending on the applied magnetic field. From the frequency response, the magnetic properties of a magnetic sample are obtained. The magnetic field dependences of paramagnetic and ferromagnetic thin films and particles are measured in a temperature range of 5-320 K at a pressure below 10(-6) mbar. We present magnetic properties of the ferromagnetic materials Fe, Co and Ni at room temperature and also for the rare earth elements Gd, Dy and Tb at various temperatures. In addition, the magnetic moments of magnetotactic bacteria are measured under vacuum conditions at room temperature. Cantilever magnetometry is a highly sensitive tool for characterizing systems with small magnetic moments. By reducing the cantilever dimensions the sensitivity can be increased by an order of magnitude.

Low temperature ultrahigh vacuum noncontact atomic force microscope in the pendulum geometry

A noncontact atomic force microscope (nc-AFM) operating in magnetic fields up to ±7 T and liquid helium temperatures is presented in this article. In many common AFM experiments the cantilever is mounted parallel to the sample surface, while in our system the cantilever is assembled perpendicular to it; the so called pendulum mode of AFM operation. In this mode measurements employing very soft and, therefore, ultrasensitive cantilevers can be performed. The ultrahigh vacuum conditions allow to prepare and transfer cantilevers and samples in a requested manner avoiding surface contamination. We demonstrate the possibility of nc-AFM and Kelvin force probe microscopy imaging in the pendulum mode. Ultrasensitive experiments on small spin ensembles are presented as well.

Hydrogen plasma microlithography of graphene supported on a Si/SiO2 substrate

In this work, a silicon stencil mask with a periodic pattern is used for hydrogen plasma microlithography of single layer graphene supported on a Si/SiO2 substrate. Obtained patterns are imaged with Raman microscopy and Kelvin probe force microscopy, thanks to the changes in the vibrational modes and the contact potential difference (CPD) of graphene after treatment. A decrease of 60 meV in CPD as well as a significant change of the D/G ratio in the Raman spectra can be associated with a local hydrogenation of graphene, while the topography remains invariant to the plasma exposure.

Fabrication and evaluation of single-crystal silicon cantilevers with ultra-low spring constants

We successfully fabricated single-crystal silicon cantilevers with spring constants as low as 10−5 N m−1 for use in magnetic resonance force microscopy applications. The fabricated ultra-thin silicon cantilevers had thicknesses ranging from 200 to 400 nm, lengths ranging from 340 to 450 µm and a width of 5 µm. We characterized their force sensitivity in the vacuum range from ambient pressure to 10−3 Pa and the temperature range from 15 to 300 K. A minimum value is observed for the internal friction, Q−1, at 160 K, which corresponds to an activation peak due to phonon scattering by atomic-scale defects. The best force sensitivity was achieved at 20 K, where it was increased by a factor of 10 as compared to that observed at room temperature.

Two-Dimensional Carrier Profiling on Lightly Doped n-Type 4H-SiC Epitaxially Grown Layers

Electronically active dopant profiles of epitaxially grown n-type 4H-SiC calibration layer structures with concentrations ranging from 3.1015 cm-3 to 1·1019 cm-3 have been investigated by non-contact Scanning Probe Microscopy (SPM) methods. We have shown that Kelvin Probe Force Microscopy (KPFM) and Electrostatic Force Microscopy (EFM) are capable of resolving two-dimensional carrier maps in the low doping concentration regime with nanoscale spatial resolution. Furthermore, different information depths of this wide band gap semiconductor material could be assessed due to the inherent properties of each profiling method. We additionally observed a resolution enhancement under laser illumination which we explain by reduced band-bending conditions. To gauge our SPM signals, we utilized epitaxially grown layers which were calibrated, in terms of dopant concentration, by C-V measurements.