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Full Information Acquisition


Table 2 : G-Mode Limitations Problem or Limitation


Large data volumes: classical 2D SPM image is 1 MB, an equivalent full G-Mode image is 4 GB


Solution/Comment


Compression with minimal information loss reduces the data volume to ~40 MB depending on information content in data.


Due to the emergence of cloud storage and high-density disk arrays, storage has become a much smaller issue, even in comparison to the scenario 5 years ago; this is a trend that is expected to continue in the future.


Information captured in G-Mode is simply inaccessible using classical methods, making the technique worthwhile.


Lossless compression allows full reconstruction of signal for continuous analysis with a wide variety of methods.


Sparse methodology using intrinsic data structure provides a pathway to a much higher degree of compression than currently available.


Information theory methods such as PCA ignore relevant physics and hence can be difficult to interpret


Physics-based compression methods (lossy compression) have been demonstrated in other communities and can be adapted for SPM dynamics.


More robust Bayesian inference, as well as supervised learning methods with built-in physical constraints, are available to analyze G-Mode data.


No current feedback system in G-Mode: precludes realization of frequency-tracking modes


Presently, feedback can be run in parallel to classical SPM processing.


Fast FPGA processing will allow real-time feedback. T e tools are available and are being implemented in next iterations of the G-Mode techniques.


Information overload: full cantilever trajectory as a function of force or voltage excitation often contains many more components than are captured by classical methods, overloading the user


T is information is relevant to material behavior and is rooted in real phenomena that is otherwise lost or ignored.


Broad-based theory support is developing a mathematical framework for extracting material properties from probe trajec- tories, mixed harmonic signals, etc. However, these are generally limited due to the novelty of the technique.


Synergy with the global open-source soſt ware development community (for example, through GitHub distribution) will allow eff ective deployment and a dialogue in the scientifi c community.


Serendipitous phenomena: transients, single events, etc. complicate true material behavior reconstruction


Information without clear interpretation can always be analyzed as new theories are developed.


G-Mode can always be reduced to classical methodology of lock-in, or Band Excitation techniques.


Practically, this new technique can elucidate material behavior coupling, or help characterize the measuring tool itself, decoupling real physical properties from instrumentation bias.


and the corresponding loading map shows characteristics of the transient cantilever response induced by the edges of topographical features, resembling the error signal in the force feedback loop. Appropriate signal de-mixing algorithms can be used to decouple domain contrast from topographic information. T ese G-PFM data can also be analyzed using clustering algorithms, independent component analysis, Bayesian linear unmixing methods, and correlational analysis techniques [ 48 , 81 – 86 ]. T e G-PFM approach can also be extended to study polarization switching in ferroelectric materials, as implemented in G-Mode voltage spectroscopy (G-VS) [ 87 ]. In G-VS, the amplitude of the G-PFM excitation signal is increased beyond


42


the coercive bias of the sample. G-VS uses data-driven adaptive signal fi ltering techniques to reveal strain loops that are indicative of polarization switching in ferroelectric materials, as shown in Figure 5a . T e raw data itself is used to calculate an appropriate noise-fl oor for the signal at each pixel. A band-pass fi lter only retains the signal from the excitation frequency to 10–12 harmonics of the drive frequency, and any signal below the calculated noise fl oor is rejected. T e fi ltered signal reveals numerous bias-induced strain loops. Such extraction of hysteresis loops would not be possible without the complete data or by using the traditional heterodyne detection schemes since the response frequencies are not known a priori . Compared to the current state-of-art


www.microscopy-today.com • 2017 July


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