Interaction force microscopy based on quartz tuning fork force sensor

Yexian Qin, Purdue University

Abstract

The ability to sense small changes in the interaction force between a scanning probe microscope (SPM) tip and a substrate requires cantilevers with a sharp mechanical resonance. A typical commercially available cantilever in air is characterized by a resonance with a Q factor of 100 ∼ 300. The low Q factor can be attributed to imperfections in the cantilever itself as well as damping effects of the surrounding air. To substantially increase the Q factor, novel concepts are required. For this reason, we have performed a systematic study of quartz tuning fork resonators for possible use with SPMs. We find that tuning fork resonators operating in air are characterized by Q factors in the order of 104, thereby greatly improving the SPM's ability to measure small shifts in the interaction force. By carefully attaching commercially available SPM tips to the tuning fork, it is possible to obtain SPM images using non-contact imaging techniques and analyze the tip-sample interactions. The assembly of uniform molecular monolayers on atomically flat substrates for molecular electronics applications has received widespread attention during the past ten years. Scanning probe techniques are often used to assess substrate topography, molecular ordering and electronic properties, yet little is known about the fundamental tip-molecule interaction. To address this issue we have built an Interaction Force Microscope using a quartz tuning fork to probe tip-molecular monolayer interactions using scanning probe microscopy. The high quality factor and stable resonant frequency of a quartz tuning fork allows accurate measurement of small shifts in the resonant frequency as the tip interacts with the substrate. To permit an accurate measure of surface interaction forces, the electrical and piezomechanical properties of a tuning fork have been calibrated using a fiber optical interferometer. In prior work [1], we have studied molecular layers formed from either 4-Trifluoro-methyl-benzenediazonium; tetrafluoroborate or 4-Methyl-benzenediazonium; tetrafluoroborate on H-passivated Si(111). Both these molecules are interesting for future molecular electronics applications. UHV STM imaging shows well-developed Si(111) step edges and terraces both on Si(111):H and Si(111) substrates covered with a molecular layer. UHV STM I(V) data acquired at different tip-substrate separations reveal an asymmetric I(V) curve that is consistent with the expected orientation of the dipole moment of the molecules. XPS data independently confirm that H-passivated Si(111) remains oxygen free for short exposures to ambient conditions and provides evidence that the molecules chemically react with the silicon surface, forming a Si-C bond. Interaction force vs. distance has been measured for an etched tungsten tip approaching a number of different hydrogen-passivated Si(111) substrates covered with self-assembled molecular layers under ambient conditions. The interaction force data will be discussed as well as numerical fits to the data using analytical functions.

Degree

Ph.D.

Advisors

Reifenberger, Purdue University.

Subject Area

Condensed matter physics

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