2022 |
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9. | M. G. Ruppert; D. Martin-Jimenez; Y. K. Yong; A. Ihle; A. Schirmeisen; A. J. Fleming; D. Ebeling Experimental Analysis of Tip Vibrations at Higher Eigenmodes of QPlus Sensors for Atomic Force Microscopy Journal Article In: Nanotechnology, vol. 33, iss. 18, pp. 185503, 2022, ISSN: 1361-6528. Abstract | Links | BibTeX | Tags: Actuator, AFM, Cantilever, Multifrequency AFM, piezoelectric @article{Ruppert2022, QPlus sensors are non-contact atomic force microscope probes constructed from a quartz tuning fork and a tungsten wire with an electrochemically etched tip. These probes are self-sensing and offer an atomic-scale spatial resolution. Therefore, qPlus sensors are routinely used to visualize the chemical structure of adsorbed organic molecules via the so-called bond imaging technique. This is achieved by functionalizing the AFM tip with a single CO molecule and exciting the sensor at the first vertical cantilever resonance mode. Recent work using higher-order resonance modes has also resolved the chemical structure of single organic molecules. However, in these experiments, the image contrast can differ significantly from the conventional bond imaging contrast, which was suspected to be caused by unknown vibrations of the tip. This work investigates the source of these artefacts by using a combination of mechanical simulation and laser vibrometry to characterize a range of sensors with different tip wire geometries. The results show that increased tip mass and length cause increased torsional rotation of the tuning fork beam due to the off-center mounting of the tip wire, and increased flexural vibration of the tip. These undesirable motions cause lateral deflection of the probe tip as it approaches the sample, which is rationalized to be the cause of the different image contrast. The results also provide a guide for future probe development to reduce these issues. | |
2020 |
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8. | M. G. Ruppert; D. M. Harcombe; A. J. Fleming Traditional and Novel Demodulators for Multifrequency Atomic Force Microscopy Conference 8th Multifrequency AFM Conference, Madrid, Spain, 2020. Abstract | BibTeX | Tags: AFM, Demodulation, Multifrequency AFM @conference{Ruppert2020b, A number of multifrequency atomic force microscopy (MF-AFM) methods make use of the excitation and detection of higher harmonics of the fundamental frequency, higher flexural eigenmodes or intermodulation products generated by the non-linear tip-sample force [1]. Schematically, these methods are depicted in Figure 1(a) where the main difference is the resulting spacing and amplitude of the frequency components in the generated spectrum shown in Figure 1(b). Regardless of which particular MF-AFM method is employed, each requires a demodulator to obtain amplitude and phase to form observables for the characterization of nanomechanical sample information. Since high-speed non-synchronous demodulators such as the peak-hold method, peak detector and RMS-to-DC converter are incompatible with MF-AFM [2], there is a need for high-bandwidth demodulation techniques capable of estimating multiple frequencies at once while maintaining robustness against unwanted frequency components [3]. In this talk, the performance of traditional and recently proposed demodulators for multifrequency atomic force microscopy is assessed experimentally. The compared methods include the lock-in amplifier, coherent demodulator, Kalman filter, Lyapunov filter, and direct-design demodulator. Each method is implemented on a field-programmable gate array (FPGA) with a sampling rate of 1.5 MHz. The metrics for comparison include implementation complexity, the sensitivity to other frequency components and the magnitude of demodulation artifacts for a range of demodulator bandwidths. Performance differences are demonstrated through higher harmonic atomic force microscopy imaging. | |
7. | A. J. Fleming; M. G. Ruppert; B. S. Routley; L. McCourt Overcoming the Limitations of Tip Enhanced Raman Spectroscopy with Intermittent Contact AFM Conference 8th Multifrequency AFM Conference, Madrid, Spain, 2020. Abstract | BibTeX | Tags: AFM, Multifrequency AFM, Optics, SPM @conference{Fleming2020, Tip enhanced Raman spectroscopy (TERS) is a promising technique for mapping the chemical composition of surfaces with molecular scale. However, current TERS methods are limited by a number of issues including high tip-sample forces, high laser power, low topographical resolution, and short probe lifetime. As a result, TERS methods are best suited to robust samples that can tolerate high optical intensity. To overcome these issues and extend the application of TERS to delicate samples, a number of new probes andimaging modes are in development at the University of Newcastle. This talk will provide an overview of these methods and present preliminary results, including new methods for optical probe optimization and fabrication, and a new dynamic-mode AFM method to reduce contact forces and applied laser power. | |
6. | D. M. Harcombe; M. G. Ruppert; A. J. Fleming A review of demodulation techniques for multifrequency atomic force microscopy Journal Article In: Beilstein Journal of Nanotechnology, vol. 11, pp. 76-97, 2020, ISSN: 21904286. Abstract | Links | BibTeX | Tags: AFM, Demodulation, Multifrequency AFM, SPM @article{Harcombe2020, This article compares the performance of traditional and recently proposed demodulators for multifrequency atomic force microscopy. The compared methods include the lock-in amplifier, coherent demodulator, Kalman filter, Lyapunov filter, and direct-design demodulator. Each method is implemented on a field-programmable gate array (FPGA) with a sampling rate of 1.5 MHz. The metrics for comparison include the sensitivity to other frequency components and the magnitude of demodulation artifacts for a range of demodulator bandwidths. Performance differences are demonstrated through higher harmonic atomic force microscopy imaging. | |
2019 |
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5. | S. I. Moore; M. G. Ruppert; D. M. Harcombe; A. J. Fleming; Y. K. Yong Design and Analysis of Low-Distortion Demodulators for Modulated Sensors Journal Article In: IEEE/ASME Transactions on Mechatronics, vol. 24, no. 4, pp. 1861-1870, 2019, ISSN: 10834435. Abstract | Links | BibTeX | Tags: Demodulation, Multifrequency AFM, Nanopositioning, SPM @article{Moore2019, System-based demodulators in the form of a Kalman and Lyapunov filter have been demonstrated to significantly outperform traditional demodulators, such as the lock-in amplifier, in bandwidth sensitive applications, for example high-speed atomic force microscopy. Building on their closed loop architecture, this article describes a broader class of high-speed closed-loop demodulators. The generic structure provides greater flexibility to independently control the bandwidth and sensitivity to out-of-band frequencies. A linear time-invariant description is derived which allows the utilization of linear control theory to design the demodulator. Experimental results on a nanopositioner with capacitive sensors demonstrate the realization of arbitrary demodulator dynamics while achieving excellent noise rejection. | |
4. | M. G. Ruppert; B. S. Routley; A. J. Fleming; Y. K. Yong; G. E. Fantner Model-based Q Factor Control for Photothermally Excited Microcantilevers Proceedings Article In: Int. Conference on Manipulation, Automation and Robotics at Small Scales (MARSS), Helsinki, Finland, 2019, ISSN: 978-1-7281-0948-0. Abstract | Links | BibTeX | Tags: AFM, Multifrequency AFM, Sensors, Smart Structures, SPM, Vibration Control @inproceedings{Ruppert2019, Photothermal excitation of the cantilever for dynamic atomic force microscopy (AFM) modes is an attractive actuation method as it provides clean cantilever actuation leading to well-defined frequency responses. Unlike conventional piezo-acoustic excitation of the cantilever, it allows for model-based quality (Q) factor control in order to increase the cantilever tracking bandwidth for tapping-mode AFM or to reduce resonant ringing for high-speed photothermal offresonance tapping (PORT) in ambient conditions. In this work, we present system identification, controller design and experimental results on controlling the Q factor of a photothermally driven cantilever. The work is expected to lay the groundwork for future implementations for high-speed PORT imaging in ambient conditions. | |
3. | M. G. Ruppert; S. I. Moore; M. Zawierta; A. J. Fleming; G. Putrino; Y. K. Yong Multimodal atomic force microscopy with optimized higher eigenmode sensitivity using on-chip piezoelectric actuation and sensing Journal Article In: Nanotechnology, vol. 30, no. 8, pp. 085503, 2019. Abstract | Links | BibTeX | Tags: AFM, Cantilever, MEMS, Multifrequency AFM, Piezoelectric Transducers and Drives, Sensors, Smart Structures, SPM @article{Ruppert2018b, Atomic force microscope (AFM) cantilevers with integrated actuation and sensing provide several distinct advantages over conventional cantilever instrumentation. These include clean frequency responses, the possibility of down-scaling and parallelization to cantilever arrays as well as the absence of optical interference. While cantilever microfabrication technology has continuously advanced over the years, the overall design has remained largely unchanged; a passive rectangular shaped cantilever design has been adopted as the industry wide standard. In this article, we demonstrate multimode AFM imaging on higher eigenmodes as well as bimodal AFM imaging with cantilevers using fully integrated piezoelectric actuation and sensing. The cantilever design maximizes the higher eigenmode deflection sensitivity by optimizing the transducer layout according to the strain mode shape. Without the need for feedthrough cancellation, the read-out method achieves close to zero actuator/sensor feedthrough and the sensitivity is sufficient to resolve the cantilever Brownian motion. | |
2018 |
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2. | D. M. Harcombe; M. G. Ruppert; M. R. P. Ragazzon; A. J. Fleming Lyapunov Estimation for High-Speed Demodulation in Multifrequency Atomic Force Microscopy Journal Article In: Beilstein Journal of Nanotechnology, vol. 9, pp. 490-498, 2018, ISSN: 21904286. Abstract | Links | BibTeX | Tags: AFM, Multifrequency AFM @article{J18c, An important issue in the emerging field of multifrequency atomic force microscopy (MF-AFM) is the accurate and fast demodulation of the cantilever-tip deflection signal. As this signal consists of multiple frequency components and noise processes, a lock-in amplifier is typically employed for its narrowband response. However, this demodulator suffers inherent bandwidth limitations as high frequency mixing products must be filtered out and several must be operated in parallel. Many MF-AFM methods require amplitude and phase demodulation at multiple frequencies of interest, enabling both z-axis feedback and phase contrast imaging to be achieved. This article proposes a model-based multifrequency Lyapunov filter implemented on a Field Programmable Gate Array (FPGA) for high-speed MF-AFM demodulation. System descriptions and simulations are verified by experimental results demonstrating high tracking bandwidths, strong off-mode rejection and minor sensitivity to cross-coupling effects. Additionally, a five-frequency system operating at 3.5MHz is implemented for higher harmonic amplitude and phase imaging up to 1MHz. | |
2017 |
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1. | D. M. Harcombe; M. G. Ruppert; A. J. Fleming Higher-harmonic AFM Imaging with a High-Bandwidth Multifrequency Lyapunov Filter Proceedings Article In: IEEE/ASME Advanced Intelligent Mechatronics (AIM), Munich, Germany, 2017. Abstract | BibTeX | Tags: Multifrequency AFM, SPM @inproceedings{C17e, A major difficulty in multifrequency atomic force microscopy (MF-AFM) is the accurate estimation of amplitude and phase at multiple frequencies for both z-axis feedback and material contrast imaging. Typically a lock-in amplifier is chosen as it is both narrowband and simple to implement. However, it inherently suffers drawbacks including a limited bandwidth due to post mixing low-pass filters and the necessity for multiple to be operated in parallel for MF-AFM. This paper proposes a multifrequency demodulator in the form of a modelbased Lyapunov filter implemented on a Field Programmable Gate Array (FPGA). System modelling and simulations are verified by experimental results demonstrating high tracking bandwidth and off-mode rejection at modelled frequencies. Additionally, AFM scans with a five-frequency-based system are presented wherein higher harmonic imaging is performed up to 1 MHz. |
2022 |
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9. | Experimental Analysis of Tip Vibrations at Higher Eigenmodes of QPlus Sensors for Atomic Force Microscopy Journal Article In: Nanotechnology, vol. 33, iss. 18, pp. 185503, 2022, ISSN: 1361-6528. | |
2020 |
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8. | Traditional and Novel Demodulators for Multifrequency Atomic Force Microscopy Conference 8th Multifrequency AFM Conference, Madrid, Spain, 2020. | |
7. | Overcoming the Limitations of Tip Enhanced Raman Spectroscopy with Intermittent Contact AFM Conference 8th Multifrequency AFM Conference, Madrid, Spain, 2020. | |
6. | A review of demodulation techniques for multifrequency atomic force microscopy Journal Article In: Beilstein Journal of Nanotechnology, vol. 11, pp. 76-97, 2020, ISSN: 21904286. | |
2019 |
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5. | Design and Analysis of Low-Distortion Demodulators for Modulated Sensors Journal Article In: IEEE/ASME Transactions on Mechatronics, vol. 24, no. 4, pp. 1861-1870, 2019, ISSN: 10834435. | |
4. | Model-based Q Factor Control for Photothermally Excited Microcantilevers Proceedings Article In: Int. Conference on Manipulation, Automation and Robotics at Small Scales (MARSS), Helsinki, Finland, 2019, ISSN: 978-1-7281-0948-0. | |
3. | Multimodal atomic force microscopy with optimized higher eigenmode sensitivity using on-chip piezoelectric actuation and sensing Journal Article In: Nanotechnology, vol. 30, no. 8, pp. 085503, 2019. | |
2018 |
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2. | Lyapunov Estimation for High-Speed Demodulation in Multifrequency Atomic Force Microscopy Journal Article In: Beilstein Journal of Nanotechnology, vol. 9, pp. 490-498, 2018, ISSN: 21904286. | |
2017 |
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1. | Higher-harmonic AFM Imaging with a High-Bandwidth Multifrequency Lyapunov Filter Proceedings Article In: IEEE/ASME Advanced Intelligent Mechatronics (AIM), Munich, Germany, 2017. |