Ultrasonic force microscopy for nanometer resolution subsurface imaging

Abstract

We present a novel method for nanometer resolution subsurface imaging. When a sample of atomic force microscope (AFM) is vertically vibrated at ultrasonic frequencies much higher than the cantilever resonance, the tip cannot vibrate but it is cyclically indented into the sample. By modulating the amplitude of ultrasonic vibration, subsurface features are imaged from the cantilever deflection vibration at the modulation frequency. By adding low-frequency lateral vibration to the ultrasonic vibration, subsurface features with different shear rigidity are imaged from the torsional vibration of cantilever. Thus controlling the direction of vibration forces, we can discriminate subsurface features of different elastic properties.For the development of nanometer scale electronic and mechanical devices, there is an increasing need for nanometer resolution imaging method of subsurface features (groups of ions, clusters, lattice defects, crystal grains, etc.). Some relating methods have been proposed in scanning force microscopy (SFM) where the tipl'2 or the sample3,a is vibrated to modulate the force. The response to the force modulation is measured to image ion implanted layers,l embedded wires,2 carbon fiber and epoxy composites,3 and Langmuir-Blodgett films.a These methods are characterized, by a tip mounting spring with a spring constant comparable to that of the sample. It is sometimes different from the usual AFM requirement for the spring constant to be as small as possible.s In this letter we propose an alternative imaging method, ultrasonic force microscopy (UFM) that employs a tip mounting cantilever much softer than the tip-sample contact rigidity. We vibrate the sample at frequencies much higher than the resonant frequency of the cantilever6 and measure the deflection and/or torsional vibration of the cantilever. It gives nanometer resolution elastic or subsurface images, and moreoveq discriminates features of different elastic properties, by controlling the direction of vibration forces. We present a general imaging scheme extending our preliminary work,7 and an analysis to compare the elastic contrast of the force modulation mode3'a and the UFM. Then, it is verified by imaging two different subsurface features in a highly oriented pyrolytic graphite (HOPG) sample.We model the AFM with springs and the mass of tip cantilever rn as illustrated in Fig. 1. First, the cantilever is displaced by z" from its free position due to a static repulsive force. When the sample is vibrated at a frequency F lower than the cantilever resonant frequency Fs, the cantilgver is also vibrated following the sample vibration. The tip-sample contact rigidity is expressed as a spring constant s, as a slope u)Also at of the force-displacement relation.5 If s is approximated by a linear spring, the peak{o-peak cantilever vibration amplitude is given by a/zV :2211fu, where a is the sample vibration amplitude and /r is the cantilever spring constant. The amplitude V does not significantly depend upon the spring constant ratio K:t/s repre...

Revealing Subsurface Vibrational Modes by Atom-Resolved Damping Force Spectroscopy

“…Reported observations of molecular vibrations by Inelastic Scanning Tunnelling Spectroscopy (IESTS) require an electrically conducting substrate [1]. Atomic Force Microscopy (AFM) experiments involving ultrasonic oscillations of elastically indented samples [2,3] can be performed on electrically insulating systems, but yield subsurface images with nanoscale resolution at best. Building on the high spatial resolution and sensitivity of dynamic non-contact AFM [4,5], we introduce Damping Force Spectroscopy (DFS) as a non-invasive tool to study subsurface structure and vibrational modes in complex molecular systems at the atomic scale.…”

“…We elucidate the physical origin of damping in a microscopic model and provide quantitative interpretation of the observations by calculating the vibrational spectrum and damping of Dy@C82 inside nanotubes with different diameters using ab initio total energy and molecular dynamics calculations. [2,3] can be performed on electrically insulating systems, but yield subsurface images with nanoscale resolution at best. Building on the high spatial resolution and sensitivity of dynamic non-contact AFM [4, 5], we introduce Damping Force Spectroscopy (DFS) as a non-invasive tool to study subsurface structure and vibrational modes in complex molecular systems at the atomic scale.…”

Nanoscale resolution scanning thermal microscopy using carbon nanotube tipped thermal probes

“…The reference AFM tapping mode and ultrasonic force imaging 53 of the graphene steps of sample shown in Fig. 5 verified their topography.…”

Quantitative imaging of nanoscale mechanical properties using hybrid nanoindentation and force modulation

“…Furthermore, the inherent problems associated with vibrating cantilevers ͑e.g., bistabilities 21 and multiple resonances͒, unknown electronics corrections, and feedback issues make quantitative interpretation of image data extremely difficult. 22 Like normal modulation techniques ͑e.g., perpendicular to sample normal͒, lateral modulation of the sample in the AFM can be used to map sample inhomogeneities like defects 12 or friction contrast, 24 as well as glass transition temperatures. 25 If great care is taken, quantitative stiffness images can be obtained; 26 however, potential frictional contributions due to slip, as well as cantilever issues ͑e.g., matching cantilever and sample stiffnesses, and lateral force calibration difficulties͒ limit the technique.…”

“…Most of these techniques utilize displacement modulation, where either the tip holder or sample is oscillated, and the tip-sample response ͑amplitude or phase shift͒ used to provide an image. [11][12][13][14][15][16][17] In these techniques, the tip can be either in continuous or intermittent contact with the sample.…”