Showing papers by "Arvind Raman published in 2013"

Sub-surface imaging of carbon nanotube-polymer composites using dynamic AFM methods

Abstract: High-resolution sub-surface imaging of carbon nanotube (CNT) networks within polymer nanocomposites is demonstrated through electrical characterization techniques based on dynamic atomic force microscopy (AFM). We compare three techniques implemented in the single-pass configuration: DC-biased amplitude modulated AFM (AM-AFM), electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM) in terms of the physics of sub-surface image formation and experimental robustness. The methods were applied to study the dispersion of sub-surface networks of single-walled nanotubes (SWNTs) in a polyimide (PI) matrix. We conclude that among these methods, the KPFM channel, which measures the capacitance gradient (∂C/∂d) at the second harmonic of electrical excitation, is the best channel to obtain high-contrast images of the CNT network embedded in the polymer matrix, without the influence of surface conditions. Additionally, we propose an analysis of the ∂C/∂d images as a tool to characterize the dispersion and connectivity of the CNTs. Through the analysis we demonstrate that these AFM-based sub-surface methods probe sufficiently deep within the SWNT composites, to resolve clustered networks that likely play a role in conductivity percolation. This opens up the possibility of dynamic AFM-based characterization of sub-surface dispersion and connectivity in nanostructured composites, two critical parameters for nanocomposite applications in sensors and energy storage devices.

Multiple regimes of operation in bimodal AFM: understanding the energy of cantilever eigenmodes.

Abstract: One of the key goals in atomic force microscopy (AFM) imaging is to enhance material property contrast with high resolution. Bimodal AFM, where two eigenmodes are simultaneously excited, confers significant advantages over conventional single-frequency tapping mode AFM due to its ability to provide contrast between regions with different material properties under gentle imaging conditions. Bimodal AFM traditionally uses the first two eigenmodes of the AFM cantilever. In this work, the authors explore the use of higher eigenmodes in bimodal AFM (e.g., exciting the first and fourth eigenmodes). It is found that such operation leads to interesting contrast reversals compared to traditional bimodal AFM. A series of experiments and numerical simulations shows that the primary cause of the contrast reversals is not the choice of eigenmode itself (e.g., second versus fourth), but rather the relative kinetic energy between the higher eigenmode and the first eigenmode. This leads to the identification of three distinct imaging regimes in bimodal AFM. This result, which is applicable even to traditional bimodal AFM, should allow researchers to choose cantilever and operating parameters in a more rational manner in order to optimize resolution and contrast during nanoscale imaging of materials.

Mapping in vitro local material properties of intact and disrupted virions at high resolution using multi-harmonic atomic force microscopy

Abstract: Understanding the relationships between viral material properties (stiffness, strength, charge density, adhesion, hydration, viscosity, etc.), structure (protein sub-units, genome, surface receptors, appendages), and functions (self-assembly, stability, disassembly, infection) is of significant importance in physical virology and nanomedicine. Conventional Atomic Force Microscopy (AFM) methods have measured a single physical property such as the stiffness of the entire virus from nano-indentation at a few points which severely limits the study of structure–property–function relationships. We present an in vitro dynamic AFM technique operating in the intermittent contact regime which synthesizes anharmonic Lorentz-force excited AFM cantilevers to map quantitatively at nanometer resolution the local electro-mechanical force gradient, adhesion, and hydration layer viscosity within individual ϕ29 virions. Furthermore, the changes in material properties over the entire ϕ29 virion provoked by the local disruption of its shell are studied, providing evidence of bacteriophage depressurization. The technique significantly generalizes recent multi-harmonic theory (A. Raman, et al., Nat. Nanotechnol., 2011, 6, 809–814) and enables high-resolution in vitro quantitative mapping of multiple material properties within weakly bonded viruses and nanoparticles with complex structure that otherwise cannot be observed using standard AFM techniques.

Subsurface imaging of carbon nanotube networks in polymers with DC-biased multifrequency dynamic atomic force microscopy.

Abstract: The characterization of dispersion and connectivity of carbon nanotube (CNT) networks inside polymers is of great interest in polymer nanocomposites in new material systems, organic photovoltaics, and in electrodes for batteries and supercapacitors. We focus on a technique using amplitude modulation atomic force microscopy (AM-AFM) in the attractive regime of operation, using both single and dual mode excitation, which upon the application of a DC tip bias voltage allows, via the phase channel, the in situ, nanoscale, subsurface imaging of CNT networks dispersed in a polymer matrix at depths of 10–100 nm. We present an in-depth study of the origins of phase contrast in this technique and demonstrate that an electrical energy dissipation mechanism in the Coulomb attractive regime is key to the formation of the phase contrast which maps the spatial variations in the local capacitance and resistance due to the CNT network. We also note that dual frequency excitation can, under some conditions, improve the contrast for such samples. These methods open up the possibility for DC-biased amplitude modulation AFM to be used for mapping the variations in local capacitance and resistance in nanocomposites with conducting networks.

Estimating residual stress, curvature and boundary compliance of doubly clamped MEMS from their vibration response

Abstract: Structural parameters of doubly clamped microfabricated beams such as initial curvature, boundary compliance, thickness and mean residual stress are often critical to the performance of microelectromechanical systems (MEMS) and need to be estimated as a part of quality control of the microfabrication process. However, these parameters couple and influence many metrics of device response and thus are very difficult to disentangle and estimate using conventional methods such as the M-test, static mechanical tests, pull-in measurements or dynamic mechanical tests. Here we present a simple, non-destructive experimental method to extract these parameters based on the non-contact measurement of the natural frequencies of the lowest few eigenmodes of the microfabricated beam, and knowledge of Young's modulus and plan dimensions of the beam alone. The method exploits the fact that certain eigenmodes are insensitive to some of these structural parameters which enable a convenient decoupling and estimation of the parameters. As a result, the method does not require complicated finite element analysis, is insensitive to the gap height and introduces no contact wear or dielectric charging effects. Experiments are performed using laser Doppler vibrometry to measure the natural frequencies of doubly clamped, nickel, RF-MEMS capacitive switches and the method is applied to extract the residual stress, beam thickness, boundary compliance and post-release curvature.

High-resolution dynamic atomic force microscopy in liquids with different feedback architectures

Abstract: The recent achievement of atomic resolution with dynamic atomic force microscopy (dAFM) [Fukuma et al., Appl. Phys. Lett. 2005, 87, 034101], where quality factors of the oscillating probe are inherently low, challenges some accepted beliefs concerning sensitivity and resolution in dAFM imaging modes. Through analysis and experiment we study the performance metrics for high-resolution imaging with dAFM in liquid media with amplitude modulation (AM), frequency modulation (FM) and drive-amplitude modulation (DAM) imaging modes. We find that while the quality factors of dAFM probes may deviate by several orders of magnitude between vacuum and liquid media, their sensitivity to tip-sample forces can be remarkable similar. Furthermore, the reduction in noncontact forces and quality factors in liquids diminishes the role of feedback control in achieving high-resolution images. The theoretical findings are supported by atomic-resolution images of mica in water acquired with AM, FM and DAM under similar operating conditions.

Multiscale contact mechanics model for RF–MEMS switches with quantified uncertainties

Abstract: We introduce a multiscale model for contact mechanics between rough surfaces and apply it to characterize the force–displacement relationship for a metal-dielectric contact relevant for radio frequency micro-electromechanicl system (MEMS) switches. We propose a mesoscale model to describe the history-dependent force–displacement relationships in terms of the surface roughness, the long-range attractive interaction between the two surfaces, and the repulsive interaction between contacting asperities (including elastic and plastic deformation). The inputs to this model are the experimentally determined surface topography and the Hamaker constant as well as the mechanical response of individual asperities obtained from density functional theory calculations and large-scale molecular dynamics simulations. The model captures non-trivial processes including the hysteresis during loading and unloading due to plastic deformation, yet it is computationally efficient enough to enable extensive uncertainty quantification and sensitivity analysis. We quantify how uncertainties and variability in the input parameters, both experimental and theoretical, affect the force–displacement curves during approach and retraction. In addition, a sensitivity analysis quantifies the relative importance of the various input quantities for the prediction of force–displacement during contact closing and opening. The resulting force–displacement curves with quantified uncertainties can be directly used in device-level simulations of micro-switches and enable the incorporation of atomic and mesoscale phenomena in predictive device-scale simulations.

Spatial spectrograms of vibrating atomic force microscopy cantilevers coupled to sample surfaces

Abstract: Many advanced dynamic Atomic Force Microscopy (AFM) techniques such as contact resonance, force modulation, piezoresponse force microscopy, electrochemical strain microscopy, and AFM infrared spectroscopy exploit the dynamic response of a cantilever in contact with a sample to extract local material properties Achieving quantitative results in these techniques usually requires the assumption of a certain shape of cantilever vibration We present a technique that allows in-situ measurements of the vibrational shape of AFM cantilevers coupled to surfaces This technique opens up unique approaches to nanoscale material property mapping, which are not possible with single point measurements alone

In situ monitoring of dynamic bounce phenomena in RF MEMS switches

Abstract: This paper presents the first ultra-low-power complementary metal?oxide?semiconductor (CMOS)-based measurement technique for monitoring the cold-switched dynamic behavior of ohmic radiofrequency microelectromechanical systems (RF MEMS) switches in real time. The circuit is capable of providing precise information about contact timing and ohmic contact events. Sampling of dynamic events at frequencies of 1 and 5?MHz shows contact timing accuracy of 99% when compared with real-time true-height information obtained from laser Doppler vibration data. The technique is validated for an ohmic RF MEMS switch with multiple bounces. The actuation voltage has also been designed to enhance bouncing behavior to more clearly study the performance and limits of the presented technique. More than 13?bounces are successfully captured by the electronic measurement technique. The weakest bounces exhibit vertical displacements of less than 20?nm as recorded by a laser Doppler vibrometer. This demonstrates the ability to capture precise timing information even for weak contacting events. A detailed discussion of how parasitics influence this technique is also presented for the first time.

nanoHUB-U: A science gateway ventures into structured online education

Abstract: nanoHUB.org is arguably the largest online nanotechnology user facility in the world. From an initial user base of about 1,000 users, nanoHUB has grown to support over 250,000 users annually. nanoHUB supports users in 172 countries with materials for research and education, along with a wide variety of simulation tools covering many nano-related areas. Preliminary assessments of user behavior patterns have shown that nanoHUB's open access approach enables published resources to be integrated directly into classrooms. However, there is an increasing demand for pedagogically sound, workforce-ready, advanced courses that allow users to gain depth in topical areas related to nanotechnology. This paper explores an initial case study where an evolving cyber environment, based on the powerful HUBzero platform, begins to offer structured online courses to its massive audience through an experiment known as nanoHUB-U. This paper describes the impetus for this new offering and discusses how new and cutting-edge content formats are being combined with online simulations in significant ways. Further, it explores in-depth the outcomes related to one of the most popular courses offered to date.

Nonlinear dynamics of atomic force microscope microcantilevers in liquid environments - a review

Abstract: Surface forces at the liquid-solid interface play a fundamental role in a wide range of scientific disciplines including electrochemistry, energy storage, wetting, catalysis, environmental science, biochemistry, biophysics, and physical biology. Dynamic Atomic Force Microscopy (dAFM) is being increasingly used to quantify and map these forces with nanometer resolution. In dynamic AFM surface forces are sensed from the changes they cause in the nonlinear dynamics of the oscillating AFM cantilever. However, the dynamics of the AFM cantilever at liquid-solid interface differs significantly compared to air or vacuum environments due to both the low Q-factor of the cantilever in liquids as well as the unique nature of intermolecular forces at solid-liquid interface. In this article we review the state-of-art of understanding of nonlinear dynamics of AFM microcantilevers in liquid environments and outline the many open areas of research both in mathematical modeling and in experiments that remain to be explored.