Amplitude and frequency modulation torsional resonance mode atomic force microscopy of a mineral surface
Introduction
Scanning shear force measurements with an atomic force microscope (AFM) provide access to in-plane surface properties such as friction, shear stiffness, and other tribological surface properties with nanometer resolution. In a torsional resonance mode AFM [1], the fundamental torsion mode of the force sensor is used. Provided small oscillation amplitudes, the motion of the tip is parallel to the surface [2]. Thus, the nonlinear dynamics of a flexurally vibrating cantilever with a periodic impact of the tip on the sample can be avoided. This implies that the torsional resonance mode is well suited to investigate tribological surface properties.
Several technical concepts were demonstrated, which help to achieve and analyze such a pendulum-like oscillation of the tip. Excitation and analysis of the torsional resonances of AFM cantilevers during contact mode experiments allow characterizing in-plane properties of metals or semiconductors [3], [4]. In an inverted scheme, shear waves induced by a piezoelectric transducer and traveling through the specimen may be used as a source of excitation [5], [6]. Furthermore, high-resolution imaging in shear force mode has been achieved with STM tips [7], [8] or AFM sensors [9] mounted to the prong of a tuning fork. In this case, the tuning fork serves as a detector for the tip–sample interaction. For scanning near-field optical microscopy using optical fibers, the shear-force distance control is a very common method [10]. Based on a similar concept, transverse dynamic force microscopy provides high sensitivity and high resolution [11].
Most techniques for shear force imaging require a modified force sensor, special substrates, or tailored sample shakers. To alleviate this, a special cantilever holder was introduced recently, which allows the direct excitation of torsional cantilever resonances in standard AFM setups [1]. This special type of probe excitation improves the signal level while reducing possible cross talk between flexural and torsional vibrations. In particular, two piezoelectric elements mounted near the cantilever chip drive the tip to a small torsional oscillation. Typically, the tip oscillates close to parallel with the surface at amplitudes smaller than 1 nm and high Q-factors (Q>1000). For measurements in water magnetostrictive actuators can be used to achieve torsional resonance mode imaging [12].
Torsional cantilever resonances allow for topographic imaging [1], [13] and a quantitative characterization of in-plane sample properties such as friction or shear stiffness [14]. Using the fundamental torsional resonance, Pfeiffer et al. [15] could measure friction on the Cu (1 0 0) surface in UHV. Kunstmann et al. [16] demonstrated a combined normal and torsional measurement mode working in frequency modulation (FM). In general, frequency modulation techniques in atomic force microscopy [17] have proven to be powerful tools to achieve atomic resolution [18], [19], [20]. In order to explain the contrast mechanisms in detail, the imaging dynamics of frequency-modulated AFM was investigated extensively, both in theory [21], [22], [23] and in experiment [24], [25], [26]. In spectroscopic applications, FM techniques open the door to study the local energy dissipation, which can be related to mechanical and electrical surface properties [27], [28]. Frequency modulation in shear force imaging was shown to enhance the performance of topographic surface profiling [29], [30]. The relation between frequency shift, distance, and amplitude has been determined for optical fibers as force sensors [31].
In the following, we discuss shear force imaging of a mineral surface with a torsional resonance mode AFM operated in amplitude and frequency modulation feedback. Using the FM technique, both frequency shift and energy dissipation can be mapped to topographic surface features under ambient conditions. In order to clarify the fundamental imaging process in torsional resonance AFM, detuning vs. distance curves help to identify the operation regime non-contact vs. contact.
Section snippets
Materials and methods
A Dimension 3100 atomic force microscope with a NanoScope IV controller was used for the experiments. The instrument was equipped with a cantilever holder for torsional resonance mode (Veeco Metrology Inc., Santa Barbara, CA). Such a cantilever holder contains two dither piezos driven out of phase in order to induce torsional oscillations of the cantilever. Silicon cantilevers with a nominal spring constant of 20 N/m were used for the measurements (ZEIHR Nanosensors, Neuchatel, Switzerland).
Results and discussion
We imaged freshly cleaved chlorite to demonstrate the capabilities of the torsional resonance mode to investigate delicate specimen. Clinochlore exhibits heterogeneous surface properties due to alternating mineral layers. On such a cleaved sample, both surface areas can be observed within a few square micrometers. Due to its heterogeneous surface properties after cleavage, chlorite minerals recently gained attention as substrates for the preparation of DNA samples [36], [37]. In the following,
Acknowledgments
We gratefully acknowledge financial support by the German Federal Ministry of Education and Research (BMBF) Nanofutur Grant 03N8706 and the DFG cluster of excellence “Nanosystems Initiative Munich”. A.Y. is supported by the scholarship of the Elite Network of Bavaria, International Doctorate Program Nano-Bio-Technology (IDK-NBT).
References (37)
- et al.
Ultramicroscopy
(2004) - et al.
Appl. Surf. Sci.
(1999) - et al.
Appl. Surf. Sci.
(2000) - et al.
Ultramicroscopy
(1998) - et al.
Appl. Phys. Lett.
(2007) - et al.
New J. Phys.
(1999) - et al.
Phys. Rev. B
(2001) - et al.
Appl. Phys. Lett.
(2000) - et al.
Appl. Phys. Lett.
(2003) - et al.
Appl. Phys. Lett.
(1995)
Phys. Rev. B
Rev. Sci. Instrum.
J. Appl. Phys.
Single Mol.
Appl. Phys. Lett.
J. Phys. D Appl. Phys.
Phys. Rev. B
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