S. Belikov, J. Alexander, M. Surtchev, I. Malovichko, and S. Magonov

“Automatic Probe Landing in Atomic Force Microscopy Resonance Modes”
2017 American Control Conference, May 24-26, 2017, Seattle, USA, pp. 2894-2899
Abstract Landing of the probe is one of the most critical elements of Atomic Force Microscopy (AFM). However, it remains inadequately studied. Practical implementations of landing usually rely of operator experience, ad hoc semi-automation, or both. In this paper, a systematic model-based approach to automatic soft landing of the probe is applied to resonance AFM modes. Analysis shows that landing in frequency modulation is the least invasive, which is the main focus of the paper. Control of the landing process and reliable detection of signal patterns indicating landing are discussed and illustrated. Automatic Landing Procedures are implemented and verified on commercial AFMs.

S. Belikov, J. Alexander, M. Surtchev, and S. Magonov
“Digital Q-Control and Automatic Probe Landing in Amplitude Modulation Phase Imaging AFM Mode”
Preprints of the 20th World Congress The International Federation of Automatic Control, Toulouse, France, July 2-14, 2017
Abstract – We present a new digital design of Q-control, i.e. controllable change of cantilever quality factor, targeted for digital implementation at an FPGA. The designed Q-controller changes the amplitude and phase of cantilever excitation based on amplitude and phase of the deflection signal measured by a digital lock-in amplifier. This significantly differs from the conventional design, where effective Q-factor is modified by adding a self-excitation force proportional to the cantilever deflection of an earlier time. Qour new digital Q-control algorithm is justified by flexible beam asymptotic models based on the Euler-Bernoulli equation and the Krylov-Bogoliubov-Mitropolsky averaging technique. Many controversial features of Q-control can now be verified by models. By contrast with conventional implementation that requires additional analog electronics, our design is implemented at an FPGA in parallel with many other control and signal processing algorithms. The ability of Q-control to decrease tip-sample interaction with a higher effective Q and increase it with a lower one is illustrated on automatic landing (soft approach) of the tip with minimal indenting of the sample surface, or damage to the tip.  

S. Belikov and S. Magonov
“Modeling of Cantilever Beam under Dissipative and Nonlinear Forces with Application to Multi-Resonant Atomic Force Microscopy”
IFAC PapersOnLine 50-1 (2017) 8654-8661
Abstract – Dynamic modes of an Atomic Force Microscope (AFM) can be modeled as a cantilever beam excited near one or more resonant frequencies described by the Euler-Bernoulli PDE with additional damping and controlled excitation. The PDE is equivalent to a series of coupled nonlinear ODEs; each ODE defines its resonant frequency. Using the Krylov-Bogoliubov-Mitropolsky (KBM) averaging techniques, these ODEs can be approximated by asymptotic amplitude-phase multi-resonant dynamics. We present the PDE with parameters estimated by AFM measurements, derive the asymptotic multi-resonant dynamics and discuss perspective nomenclature of AFM multi-resonant modes, their control, and implementation that extends single-resonant modes.

J. Alexander, S. Belikov, and S. Magonov
“AFM-Based Characterization of Electric Properties of Materials”
Chapter in "Nanoscale Imaging" (Ed. Yu. Lyubchenko), Springer Protocols, 2018 (in press)
Abstract - Capabilities of atomic force microscopy (AFM) for characterization of local electrical properties of materials are presented in this chapter. At the beginning the probe–sample force interactions, which are employed for detection of surface topography and materials properties, are described theoretically in their application in different AFM modes and electrical techniques. The electrical techniques, which are based on detection of electrostatic probe–sample forces, are outlined in AFM contact and oscillatory resonant modes. The basic features of the detection of surface potential and capacitance gradients are explained. The applications of these techniques are illustrated on metals, surfactant compounds, semiconductors, and different polymers. Practical recommendations on use of the AFM-based electrical methods and the related challenges are given in the last section.

J-Y. Kim, M-G. Han,, M-B. Lien, S. Magonov, Y. Zhu,,  H. George, T. B. Norris, and N. A. Kotov
“Dipole-like electrostatic asymmetry of gold nanorods”
Science Advances, 09 Feb 2018: Vol. 4, no. 2, e1700682 DOI: 10.1126/sciadv.1700682
Abstract - The symmetry of metallic nanocolloids, typically envisaged as simple geometrical shapes, is rarely questioned. However, the symmetry considerations are so essential for understanding their electronic structure, optical properties, and biological effects that it is important to reexamine these foundational assumptions for nanocolloids. Gold nanorods (AuNRs) are generally presumed to have nearly perfect geometry of a cylinder and therefore are centrosymmetric. We show that AuNRs, in fact, have a built-in electrostatic potential gradient on their surface and behave as noncentrosymmetric particles. The electrostatic potential gradient of 0.11 to 0.07 V/nm along the long axes of nanorods is observed by off-axis electron holography. Kelvin probe microscopy, secondary electron imaging, energy-filtered transmission electron microscopy, and plasmon mapping reveal that the axial asymmetry is associated with a consistently unequal number of cetyltrimethylammonium bromide moieties capping the two ends of the AuNRs. Electrostatic field maps simulated for the AuNR surface reproduce the holography images. The dipole-like surface potential gradient explains previously puzzling discrepancies in nonlinear optical effects originating from the noncentrosymmetric nature of AuNRs. Similar considerations of symmetry breaking are applicable to other nanoscale structures for which the property-governing symmetry of the organic shell may differ from the apparent symmetry of inorganic core observed in standard electron microscopy images.

M. Vatankhah-Varnosfaderani, A. N. Keith, Y. Cong, H. Liang, M. Rosenthal, M. Sztucki, C. Clair, S. Magonov, D. A. Ivanov, A. V. Dobrynin, and S. S. Sheiko
“Chameleon-like elastomers with molecularly encoded strain-adaptive stiffening and coloration”
Science 359, 1509–1513 (2018)
Abstract - Active camouflage is widely recognized as a soft-tissue feature, and yet the ability to integrate adaptive coloration and tissue-like mechanical properties into synthetic materials remains elusive. We provide a solution to this problem by uniting these functions in moldable elastomers through the self-assembly of linear-bottlebrush-linear triblock copolymers. Microphase separation of the architecturally distinct blocks results in physically cross-linked networks that display vibrant color, extreme softness, and intense strain stiffening on par with that of skin tissue. Each of these functional properties is regulated by the structure of one macromolecule, without the need for chemical cross-linking or additives. These materials remain stable under conditions characteristic of internal bodily environments and under ambient conditions, neither swelling in bodily fluids nor drying when exposed to air.

J. Alexander, S. Belikov, S. Magonov, and M. Smith
“Evaluation of Atomic Force Microscopy Probes and Instruments with Dynamic Cantilever Calibrator”
MRS Advances 1-7 (2018). doi: 10.1557/adv.2018.77
Abstract - The main functions of the dynamic cantilever calibrator (DCC), which are related to characterization of AFM probes and instruments, are demonstrated on a variety of probes. The resonant frequency, Q-factor and spring constant of the rectangular and V-shaped probes were evaluated by thermal tune method. The inverse optical sensitivity and optical beam deflection noise, which define performance of AFM microscopes, were extracted from DCC data. Peculiarities of thermal tune studies and the use of DCC for advanced applications are discussed.

S. Magonov, and S. Wu
“Expanding Functionality of Atomic Force Microscopy with Environmental Studies”
MRS Advances 3(11), 587-593 (2018). doi: 10.1557/adv.2018.77
Abstract - Environmental atomic force microscopy (AFM) study of brush macromolecules, polymer blends and bitumen was performed with regular and Quick Scan imaging. Condensation of different vapors on sample surface has induced swelling of hydrophilic domains that helps recognizing the components of heterogeneous compounds. High-resolution imaging of brush macromolecules was achieved in ethyl acetate vapor. Fast monitoring of aggregation/spreading of brush macromolecules revealed dynamics of conformational changes and molecular motion.   

S. Magonov and S. Wu
“Combining Fast Imaging and Variable Temperature Studies in Atomic Force Microscopy”
MRS Advances, 2018, submitted
Abstract - Fast imaging in Atomic Force Microscopy enhances the capability of studying phase transitions and surface properties of materials at variable temperatures. This is demonstrated by measurements of several polymers [poly(diethylsiloxane), low-density polyethylene and ethylene-octene copolymer] and bitumen at low (down to -20˚C) and high (up to +150˚C) temperatures. Monitoring of structural transitions was performed at small and large (up to 40 um) areas with 1-5˚C/min cooling/heating rates. Novel data about dynamics and structural transitions of mesomorphic transitions and crystallization were obtained.

S. Belikov, J. Alexander, and S. Magonov
“Dynamic Probe Calibration for Quantitative Measurements with Atomic Force Microscopy”
American Control Conference, 2018, in press.
Abstract—Atomic Force Microscopy (AFM) quantitative measurements are based on optimal instrumentation and control design, as well as diagnostics, calibration of AFM probes, and extraction of sample material properties from experimental data. In this paper three interrelated topics are discussed. (a) We analyze probe thermal noise to extract AFM probe dynamic parameters (resonant frequency, Q-factor, spring constant), verify instrumental capabilities for multi-frequency measurements, and obtain optical beam deflection sensitivity and noise level. This analysis is performed on experimental data acquired by Dynamic Cantilever Calibrator (DCC), designed to boost a performance of AFM electronic controllers. (b) We characterize tip shape and tip-sample force interaction with parametric models and apply them for evaluation of the probe geometry and shapes of their tips. These are important factors for quantitative AFM studies of local electrical and mechanical properties. (c) We analyze the accuracy of elastic modulus calculations by AFM nanoindentation based on error propagation that leads to practical recommendations on choice of the probe's spring constant.  

S. Belikov, M. Surtchev, I. Malovichko, and S. Magonov
“Quantitative Viscoelasticity Studies with Atomic Force Microscopy”
American Control Conference, 2018, in press
Abstract - Linear theory of viscoelasticity has many parallels with LTI control systems, and identification methods for LTI can be applied to study viscoelastic materials. However, conducting such a study with atomic force microscopy (AFM) requires overcoming many challenges, including: (a) accurate piezo control of the tip trajectory; (b) calibration of the probe (spring constant and tip shape) and opto-mechanical system (inverse optical sensitivity); (c) converting z-position to indentation depth and limit error propagation of the conversion; (d) identification of the point of contact. Accounting for the tip shape also makes the viscoelastic models nonlinear. We describe the development of an AFM experiment and analysis that overcomes these challenges and demonstrate practical viscoelastic measurements at the nanometer scale.





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