doi:10.1017/S1551929518001050 12 www.microscopy-today.com • 2018 November Tapping Mode AFM Imaging in Liquids with blueDrive Photothermal Excitation Aleksander Labuda, 1 Sophia Hohlbauch, 1 Marta Kocun, 1 F. Ted Limpoco, 1 Nathan Kirchhofer, 1 Ben Ohler, 1 * and Donna Hurley 2 1 Oxford Instruments Asylum Research, 6310 Hollister Ave., Santa Barbara, CA 93117 2 Lark Scientifc LLC, 325 20th St., Boulder, CO 80302 *Ben.Ohler@oxinst.com Abstract: Photothermal excitation represents a signifcant instrumentation advance for imaging with dynamic modes of the atomic force microscope (AFM). Using a power-modulated laser to directly drive the cantilever oscillation provides substantial benefts over conventional piezoelectric excitation. Here we discuss photothermal excitation and its implementation as blueDrive by Oxford Instruments Asylum Research. We explain its operating principles and discuss its use for a range of practical applications. The results show that blueDrive sets new standards for imaging in liquid, visualizing dynamic events, and mapping nanomechanical properties. Keywords: atomic force microscopy, photothermal excitation, tapping mode, liquid imaging, nanoscale surface characterization Introduction Since its invention over 30 years ago, the atomic force microscope (AFM) has become frmly established as a high-performance tool for applications throughout biology, materials science, chemistry, and physics. Te AFM’s core capability, namely nanoscale structural characterization by imaging topography (height), has only improved with time (Figure 1). In addition, a number of other modes have been developed over the years that interrogate functional behavior by sensing electrostatic, capacitive, magnetic, and related tip-sample interactions. Many of these modes are dynamic methods, in which the cantilever is driven to oscillate at a specifc frequency or frequencies. Probably the most familiar dynamic mode is tapping mode for imaging topography. However, other examples are Kelvin probe force microscopy (KPFM) for electrical characterization, magnetic force microscopy (MFM) for magnetic interrogation, and bimodal AFM for evaluating mechanical properties, where the cantilever is oscillated at two frequencies simultaneously. Dynamic approaches are widely used because they ofer distinct advantages in terms of performance, as discussed below. Terefore, improving AFM performance when operating in dynamic modes is an important aspect of instrument development. Tis is especially true for new applications with challenging requirements such as ever-higher spatial resolution, operation in non-ambient environments, and improved quantitative accuracy. Here, we describe a recent instrumentation innovation for dynamic modes, blueDrive photothermal excitation. Implemented on the Cypher family of AFMs by Oxford Instruments Asylum Research, blueDrive provides improved performance for imaging in tapping mode and other dynamic modes. Tis article describes blueDrive, explaining its operating principles, comparing it to conventional approaches, and showing results that illustrate its value in a range of research areas. Practical Considerations for Dynamic Modes Tapping mode. For discussion purposes we focus on a specific dynamic mode: tapping mode, also called intermittent contact (AC) or amplitude-modulated (AM) mode. In tapping mode, the cantilever is driven at a constant frequency near or at its lowest flexural resonance (typically tens to hundreds of kilohertz for conventional cantilevers). An early AFM imaging innovation [1], tapping mode is the most widely used AFM imaging mode today. Tapping mode quickly gained popularity because of greatly reduced sample and tip damage compared to contact mode imaging. Later, the resonance behavior of the cantilever was recognized as a rich source of information about the tip-sample interaction. In the simplest implementation, the phase response of cantilever oscillation can provide useful material contrast. More advanced analyses also have been developed to extract Figure 1: Topography image of crystalline β-DBDCS, an organic semiconductor, demonstrating lattice-scale resolution. Acquired in water with tapping mode on the Cypher S with blueDrive photothermal excitation. Sample courtesy S. Y. Park, Seoul National Univ., J. Gierschner, IMDEA Nanociencia, and E. Gnecco, Univ. Jena. Downloaded from https://www.cambridge.org/core. IP address: 172.245.146.91, on 17 Nov 2018 at 16:29:47, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1551929518001050