Retrofitting an atomic force microscope with photothermal excitation for a clean cantilever response in low Q environments

Department of Physics, McGill University, Montreal, Quebec H3A 2T8, Canada.
The Review of scientific instruments (Impact Factor: 1.61). 05/2012; 83(5):053703. DOI: 10.1063/1.4712286
Source: PubMed


It is well known that the low-Q regime in dynamic atomic force microscopy is afflicted by instrumental artifacts (known as "the forest of peaks") caused by piezoacoustic excitation of the cantilever. In this article, we unveil additional issues associated with piezoacoustic excitation that become apparent and problematic at low Q values. We present the design of a photothermal excitation system that resolves these issues, and demonstrate its performance on force spectroscopy at the interface of gold and an ionic liquid with an overdamped cantilever (Q < 0.5). Finally, challenges in the interpretation of low-Q dynamic AFM measurements are discussed.

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    • "Several strategies have been used to oscillate cantilevers for tapping-mode operation, but most techniques are not suitable for actuating heated cantilevers. Cantilevers have been actuated using photothermal excitation [13] [14] [15] or integrated piezoelectric [16] [17] [18], thermomechanical [19] [20] [21] [22] and electrostatic [23] [24] [25] elements. However, these actuation techniques complicate the cantilever design and fabrication process, require complex AFM hardware, or the cantilever actuation may be adversely affected by the high operation temperatures of heated cantilevers. "
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    ABSTRACT: This paper reports the development of microcantilevers capable of self-heating and Lorentz-force actuation, and demonstrates applications to thermal topography imaging. Electrical current passing through a U-shaped cantilever in the presence of a magnetic field induces a Lorentz force on the cantilever free end, resulting in cantilever actuation. This same current flowing through a resistive heater induces a controllable temperature increase. We present cantilevers designed for large actuation forces for a given cantilever temperature increase. We analyze the designs of two new cantilevers, along with a legacy cantilever design. The cantilevers are designed to have a spring constant of about 1.5 N m−1, a resonant frequency near 100 kHz, and self-heating capability with temperature controllable over the range 25–600 °C. Compared to previous reports on self-heating cantilevers, the Lorentz–thermal cantilevers generate up to seven times as much Lorentz force and two times as much oscillation amplitude. When used for thermal topography imaging, the Lorentz–thermal cantilevers can measure topography with a vertical resolution of 0.2 nm.
    Nanotechnology 09/2014; 25(39):395501. DOI:10.1088/0957-4484/25/39/395501 · 3.82 Impact Factor
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    • "This type of excitation was first applied to bridge resonators [8] and subsequently to cantilevers in air and liquids [9] [10]. The direct energy transfer avoids spurious resonances, and thus renders photothermal excitation suitable for atomic force microscopy [11] [12], force spectroscopy [13] and sensing applications [14]. "
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    ABSTRACT: Demands to improve the sensitivity and measurement speed of dynamic scanning force microscopy and cantilever sensing applications necessitate the development of smaller cantilever sensors. As a result, methods to directly drive cantilevers, such as photothermal or magnetic excitation, are gaining in importance. Presented is a report on the effect of photothermal excitation of microcantilevers on the increase in steady-state temperature and the dynamics of higher mode vibrations. First, the local temperature increase upon continuous irradiation with laser light at different positions along the cantilever was measured and compared with finite element analysis data. The temperature increase was highest when the heating laser was positioned at the free end of the cantilever. Next, the laser intensity was modulated to drive higher flexural modes to resonance. The dependence of the cantilever dynamics on the excitation laser position was assessed and was in good agreement with the analytical expressions. An optimal position to simultaneously excite all flexural modes of vibration with negligible heating was found at the clamped end of the cantilever. The reports findings are essential for optimisation of the excitation efficiency to minimise the rise in temperature and avoid damaging delicate samples or functionalisation layers.
    Micro & Nano Letters 11/2013; 8(11). DOI:10.1049/mnl.2013.0352 · 0.85 Impact Factor
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    ABSTRACT: Having reached the quantum and thermodynamic limits of detection, atomic force microscopy (AFM) experiments are routinely being performed at the fundamental limit of signal to noise. A critical understanding of the statistical properties of noise leads to more accurate interpretation of data, optimization of experimental protocols, advancements in instrumentation, and new measurement techniques. Furthermore, accurate simulation of cantilever dynamics requires knowledge of stochastic behavior of the system, as stochastic noise may exceed the deterministic signals of interest, and even dominate the outcome of an experiment. In this article, the power spectral density (PSD), used to quantify stationary stochastic processes, is introduced in the context of a thorough noise analysis of the light source used to detect cantilever deflections. The statistical properties of PSDs are then outlined for various stationary, nonstationary, and deterministic noise sources in the context of AFM experiments. Following these developments, a method for integrating PSDs to provide an accurate standard deviation of linear measurements is described. Lastly, a method for simulating stochastic Gaussian noise from any arbitrary power spectral density is presented. The result demonstrates that mechanical vibrations of the AFM can cause a logarithmic velocity dependence of friction and induce multiple slip events in the atomic stick-slip process, as well as predicts an artifactual temperature dependence of friction measured by AFM.
    Physical Review E 09/2012; 86(3-3):1-18. DOI:10.1103/PhysRevE.86.031104 · 2.29 Impact Factor
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