Force sensing with nanowires

‘Bottom-up’ fabricated nano-resonators have emerged as particularly promising mechanical transducers, over the last decade. In fact, the exceptional force sensitivity exhibited by such nearly defect-free structures, is ensured by their small motional mass, low dissipation and high resonant frequency.
In this dissertation, we aim to explore the potential of as-grown nanowires (NWs) as scanning force sensors. Their singly clamped structure makes them suitable to scan over a sample in the pendulum geometry, enabling the measurement of very weak lateral force gradients. Furthermore, by virtue of slight cross-sectional asymmetries, the flexural modes of a NW are split into doublets oscillating along two orthogonal directions. These characteristics enable the peculiar vectorial sensing nature of such devices.
We developed a custom built scanning probe microscope, operating at cryogenic temperatures in a liquid helium bath cryostat. The microscope features an integrated fiber-based interferometer setup for the optical detection of individual NWs' motion. To demonstrate their vectorial scanning capabilities, we scan them over a sample with gold patterned electrodes. By monitoring the frequency shift and direction of oscillation of both fundamental modes, we construct a map of all spatial tip–sample force derivatives in the plane. Moreover, using an electric field to resonantly drive the mechanical modes, we are able to spatially probe forces of distinct origins, arising from the NW's residual charge and its polarizability, respectively. In addition, we show quantitative control over the coupling between two orthogonal mechanical modes, obtained by measuring avoided crossings as a function of position and applied electric field, which allowed to record Rabi oscillations between the two modes in the strong-coupling regime.
In general, such universally applicable scanning technique enables a form of atomic force microscopy particularly suited to mapping the size and direction of weak tip-sample forces.
NWs produced by molecular beam epitaxy also offer the possibility of ‘in-situ’ functionalization of the mechanical resonator during the growth process. In particular, we studied a scanning magnetic force sensor based on an individual magnet-tipped GaAs NW. Its magnetic tip consists of a final segment of single-crystal MnAs formed by sequential crystallization of the liquid Ga catalyst droplet. We characterize the mechanical and magnetic properties of such NWs by measuring their flexural mechanical response in an applied magnetic field. Taking advantage of the excellent force sensitivity, the magnetic properties of such tips are studied via dynamic torque magnetometry and precisely fitted by micro-magnetic simulations, showing vortex and dipole-like configurations. To determine a NW’s performance as a magnetic scanning probe, we measure its response to the field profile produced by a current-carrying micro-wire, characterizing its behavior as current sensor and its high sensitivity.
The ability of a NW sensor to map all in-plane spatial force derivatives can provide fine detail of stray field profiles above magnetic and current carrying samples, in turn revealing information on the underlying phenomena and anisotropies. Directional measurements of dissipation may also prove useful for visualizing domain walls and other regions of inhomogeneous magnetization.