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Full Information Acquisition in Scanning Probe Microscopy


S. Jesse , S. Somnath , L. Collins , and S.V. Kalinin*


T e Institute for Functional Imaging of Materials and T e Center for Nanophase Materials Sciences , Oak Ridge National Laboratory , Oak Ridge , TN 37831 * sergei2@ornl.gov


Abstract: Scanning Probe Microscopy (SPM) has unlocked the nanoworld for exploration and control. While substantial effort has been dedicated toward the development of better instrumental platforms and probes, opportunities related to signal processing and data acquisition in SPM are often overlooked. Here, we discuss opportunities offered by capturing the full information of the data stream from the detector, referred to as General Mode (G-Mode) SPM. This approach allows exploration of the complex tip-surface interactions, spatial mapping of multidimensional variability of material properties and their mutual interactions, and imaging at the information channel capacity limit, providing a new paradigm for SPM detection.


Introduction Since the invention of atomic force microscopy (AFM) thirty


years ago [ 1 ], scanning probe microscopies (SPM) have become an enabling technology for nanoscience and technology [ 2 ]. T e collective eff ort of commercial, academic, and government institutions have created a fl eet of SPM platforms over 50,000 units strong, enabling a broad range of studies from quantum transport imaging in low dimensional systems [ 3 ], functional magnetic [ 4 , 5 ] and ferroelectric studies [ 6 , 7 ], atomically resolved imaging in ultra-high vacuum (UHV) [ 8 – 11 ] and liquid environ- ments [ 12 ], imaging active device structures [ 13 , 14 ], single molecule reactions [ 15 , 16 ] , biological recognition imaging [ 17 , 18 ], and many others. Without exaggeration, SPM has become the key that unlocked the nanoworld for exploration and control. T e original SPMs, including scanning tunneling microscopy


(STM) [ 19 , 20 ] and contact mode atomic force microscopy (AFM) [ 21 ], were based on detection of static force or current signals, which imposed severe limitations on the detection limits. Modern dynamic SPMs typically use heterodyne signal processing to amplify weak periodic signals [ 22 ]. In this process, they compress the information stream from 10 MHz at the photodetector to ~1–10 kHz, as limited by the rate of the feedback operation and pixel acquisition. In this process the information on transients, non-linear interactions, etc., not captured by the excitations of harmonics, is essentially lost. Correspondingly, a number of groups have suggested approaches based on multiple excitations [ 23 – 27 ], detection of intermodulation signals [ 26 , 28 ], etc. [ 29 – 31 ]. However, the decoding of this information and its transformation to material-specifi c properties remains compli- cated. Furthermore, these complex detection schemes do not provide information on the fundamental question of whether all available information is collected. Here, we discuss opportunities off ered by capturing the full data stream from the detector, referred to as General Mode (G-Mode) SPM. T is approach allows exploration of the complex tip-surface interactions, spatial mapping of multidimensional variability of material properties and their mutual interactions, and imaging at the information channel capacity limit—providing


34


a new paradigm for SPM detection. T is approach circumvents limitations of heterodyne detection, and as a result unlocks capabilities such as simultaneous multi-resolution imaging at multiple frequencies, smart data compression, noise analysis, and novel spectroscopic methods. In this article, we explore the opportunities for comprehensive materials characterization that are now opened by G-Mode SPM, along with associated instru- mental and mathematical challenges. T e opportunities enabled by the G-Mode SPM as applied to structural and functional imaging are summarized in Table 1 .


Materials and Methods Signal detection methods . In conventional SPM, a probe with a sharp tip is raster-scanned over the surface while the topography and material properties of the sample are measured by tracking changes in the tip-sample interaction. T e vast majority of force-based SPM techniques use a laser-based photodetector system to track changes in the defl ection of a cantilever as it interacts with the surface of a sample [ 32 , 33 ]. Correspondingly, much of the SPM development traditionally targeted the instrument and probe functionality [ 34 ]. At the same time, the continuous development of the excitation and signal processing methods is less recognized. In order to achieve a high signal-to-noise ratio, traditional SPM techniques use the


Figure 1 : The information in any imaging method can be represented as a combination of the spatial resolution, time resolution, and chemical/functional resolution. Correspondingly, increasing data fl ow in microscopy can be translated into increased spatial, time, or functional resolution, with the associated conversion factors dependent on technique.


doi: 10.1017/S1551929517000633 www.microscopy-today.com • 2017 July


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