A new detection system for extremely small vertically mounted cantilevers

Abstract

Detection techniques currently used in scanning force microscopy impose limitations on the geometrical dimensions of the probes and, as a consequence, on their force sensitivity and temporal response. A new technique, based on scattered evanescent electromagnetic waves (SEW), is presented here that can detect the displacement of the extreme end of a vertically mounted cantilever. The resolution of this method is tested using different cantilever sizes and a theoretical model is developed to maximize the detection sensitivity. The applications presented here clearly show that the SEW detection system enables the use of force sensors with sub-micron size, opening new possibilities in the investigation of biomolecular systems and high speed imaging. Two types of cantilevers were successfully tested: a high force sensitivity lever with a spring constant of 0.17 pN nm(-1) and a resonant frequency of 32 kHz; and a high speed lever with a spring constant of 50 pN nm(-1) and a resonant frequency of 1.8 MHz. Both these force sensors were fabricated by modifying commercial microcantilevers in a focused ion beam system. It is important to emphasize that these modified cantilevers could not be detected by the conventional optical detection system used in commercial atomic force microscopes.

High Sensitivity Deflection Detection of Nanowires

“…4) with its spring constant (0.67 mN/m), we determine its force noise to be 1.9 f N/ √ Hz. Currently, the best custom cantilevers we are aware of have force noises of 23-29f N/ √ Hz in water [1,16,17] and commercial cantilevers have force-noises of 50-200 f N/ √ Hz [18]. We have shown that nanowires directionally scatter sufficient light to provide position detection with high sensitivity and bandwidth, without the need for an interferometric optical pathway.…”

An optimized nano-positioning stage for Bristol’s Transverse Dynamic Force Microscope

“…The interaction with the water molecules results in the cantilever vibration being damped and the amplitude of vibration being decreased. The change in vibration of the probe is measured by a laser optical detection system, which is set beneath the thin quartz plate underneath the specimen (Antognozzi et al, 2008). Thus, when the specimen is moved in the x-y plane, the specimen topography can be measured by the probe at any particular position by keeping the vibration amplitude of the cantilever constant.…”

Harnessing the damping properties of materials for high-speed atomic force microscopy

“…However, the majority of atomic force microscopy experiments are performed in air, and the tapping-mode detection speed of current highspeed cantilevers is an order of magnitude lower in air than in liquids. Traditional approaches to increasing the imaging rate of atomic force microscopy have involved reducing the size of the cantilever 4,5 , but further reductions in size will require a fundamental change in the detection method of the microscope [6][7][8] . Here, we show that high-speed imaging in air can instead be achieved by changing the cantilever material.…”

“…A further size reduction would require fundamentally different detection techniques. Techniques such as near-field scattering and thin-metal-film piezoresistive sensing have been proposed [6][7][8] . However, such small cantilevers suffer from challenges in their implementation and are limited to use on very flat samples because of the minimal chipsample clearance.…”