Optical Imaging for Nanowire Mechanics

Abstract: In this work, we present a stroboscopic detection technique for the study of oscillating nanoscale objects. This optical read-out gives a position accuracy better than 1 nm which is a fraction of both the diffraction limit and the pixel size. The advantage of this stroboscopic method compared to conventional time-averaging techniques, is the time-resolution it provides as well as being non-invasive. Another advantage is that it can be readily applied to different experimental situations, the only requirement being that the dynamic behavior is repeated. The versatility of our detection technique allows for different types of actuation of the nanowire oscillation. Moreover, both the steady-state response to a harmonic force and the free ring-down response from a deflected position can be studied. In addition, the nanowire can be mounted with a side view or a top view and the surrounding pressure can be varied. The frequency response, the modal shapes, and the higher modes of epitaxially grown nanowires can be very well modeled using Euler-Bernoulli theory. However, one observed difference between the studied nanowires and macroscopic objects, is the size dependent Young's modulus, which decreases as the diameter is scaled down. Although the trend is quite vague, the Young's modulus of nanowires clearly deviates from that of bulk material. The nanowires are heavily damped at atmospheric pressure, resulting in Q factors of about 1-10. For pressures down to about 1 mbar, the Q factors increase as the surrounding pressure is decreased. For even lower pressure, the Q factors are constant, revealing that the damping is no longer due to the surrounding gas, but instead originates from intrinsic losses. At lower pressures, the damping is low enough to reveal that all the studied nanowires have split resonance frequencies, probably due to an asymmetric cross section. To investigate the direction of the oscillation in detail, the set-up is modified to enable a top view of the nanowire motion. For sensor applications, the spring constant is determined using the dimensions, the resonance frequency and the modal shape. The advantage of this method is that it is valid also for tapered nanowires, as it does not employ the Young's modulus. As a first step towards a mass sensor, a thin layer of gold is evaporated onto the nanowires and the resulting shift in resonance frequency is measured. The nanowires have also been used as force sensors for cellular forces. Nerve cells can be grown on arrays of nanowires. The living cells exert forces on the nanowires, causing the nanowires to bend. To correlate the cellular forces to the deflections of the nanowires, the static spring constant is determined through resonance frequency measurements.