Pranav Agarwal

Real time estimation of equivalent cantilever parameters in tapping mode atomic force microscopy

In this article, a method of imaging is developed, where during tapping-mode operation, equivalent resonant frequency and quality factor can be obtained in real time. It involves exciting the cantilever near its resonant frequency and two other frequencies chosen close to the resonant frequency. It is shown that changes in equivalent cantilever parameters can be registered for topography changes that are less than 1 nm in height and within 400 μs of the change occurring. The estimation time is two orders of magnitude better than current techniques.

Real-time detection of probe loss in atomic force microscopy

In this letter, a real-time methodology is developed to determine regions of dynamic atomic force microscopy based image where the cantilever fails to be an effective probe of the sample. Conventional imaging signals such as the amplitude signal and the vertical piezoactuation signal cannot identify the areas of probe loss. It is experimentally demonstrated that probe-loss affected portion of the image can be unambiguously identified by a real-time signal called reliability index. Reliability index, apart from indicating the probe-loss affected regions, can be used to minimize probe-loss affected regions of the image, thus aiding high speed AFM applications.

Maximum-Likelihood Sequence Detector for Dynamic Mode High Density Probe Storage

There is an increasing need for high density data storage devices driven by the increased demand of consumer electronics. In this work, we consider a data storage system that operates by encoding information as topographic profiles on a polymer medium. A cantilever probe with a sharp tip (few nm radius) is used to create and sense the presence of topographic profiles, resulting in a density of few Tb per in.2. The prevalent mode of using the cantilever probe is the static mode that is harsh on the probe and the media. In this article, the high quality factor dynamic mode operation, that is less harsh on the media and the probe, is analyzed. The read operation is modeled as a communication channel which incorporates system memory due to inter-symbol interference and the cantilever state. We demonstrate an appropriate level of abstraction of this complex nanoscale system that obviates the need for an involved physical model. Next, a solution to the maximum likelihood sequence detection problem based on the Viterbi algorithm is devised. Experimental and simulation results demonstrate that the performance of this detector is several orders of magnitude better than the performance of other existing schemes.

Transient Force Atomic Force Microscopy: A New Nano-Interrogation Method

Atomic force microscopes (AFMs) are the primary investigation systems at the nanoscale. In existing dynamic mode AFM methods steady-state response of microcantilever is monitored for imaging tip-surface interaction forces at the nano-scale. In these methods microcantilevers with high quality factor are employed for high force sensitivity but at the cost of speed due to dependence on steady-state signals. In this paper, a novel methodology for fast interrogation of material that exploits the transient part of the cantilever response is presented. This method effectively addresses the perceived fundamental limitation on bandwidth due to high quality factors. Analysis and experiments show that the method results in significant increase in bandwidth and resolution as compared to the steady-state-based methods. This paper demonstrates the effectiveness of a systems perspective to the field of imaging at the nano-scale and for the first time reports real-time imaging at the nanoscale using the transient method with scan speed 40 times faster than conventional methods.

Real-time probe based quantitative determination of material properties at the nanoscale

Tailoring the properties of a material at the nanoscale holds the promise of achieving hitherto unparalleled specificity of the desired behavior of the material. Key to realizing this potential of tailoring materials at the nanoscale are methods for rapidly estimating physical properties of the material at the nanoscale. In this paper, we report a method for simultaneously determining the topography, stiffness and dissipative properties of materials at the nanoscale in a probe based dynamic mode operation. The method is particularly suited for investigating soft-matter such as polymers and bio-matter. We use perturbation analysis tools for mapping dissipative and stiffness properties of material into parameters of an equivalent linear time-invariant model. Parameters of the equivalent model are adaptively estimated, where, for robust estimation, a multi-frequency excitation of the probe is introduced. We demonstrate that the reported method of simultaneously determining multiple material properties can be implemented in real-time on existing probe based instruments. We further demonstrate the effectiveness of the method by investigating properties of a polymer blend in real-time.

Modeling and identification of the dynamics of electrostatically actuated microcantilever with integrated thermal sensor

Microcantilevers that thermally sense the topography of the sample with the ability of electrostatic actuation enable a highly parallel implementation where multiple cantilevers scan the media. Microcantilevers with integrated sensors are used for a variety of applications viz. calorimetry, thermal dip pen lithography, thermal metrology, room temperature chemical vapor deposition in addition to high density data storage application. The dynamics of these cantilevers is governed by a complex interplay of mechanical, thermal, electrostatic and interatomic forces. Such dynamics are analyzed in this paper for operating conditions that are practical for high density data storage applications (¿ Tb=in2) and imaging. Models for a thermo-mechanical cantilever that are tractable for real-time applications as well as a comprehensive characterization of the relevant physical effects and methods for identifying model parameters are developed. The efficacy of the paradigm developed is proven by a comparison with experimental data.

Immobilization method of yeast cells for intermittent contact mode imaging using the atomic force microscope

The atomic force microscope (AFM) is widely used for studying the surface morphology and growth of live cells. There are relatively fewer reports on the AFM imaging of yeast cells [1] (Kasas and Ikai, 1995), [2] (Gad and Ikai, 1995). Yeasts have thick and mechanically strong cell walls and are therefore difficult to attach to a solid substrate. In this report, a new immobilization technique for the height mode imaging of living yeast cells in solid media using AFM is presented. The proposed technique allows the cell surface to be almost completely exposed to the environment and studied using AFM. Apart from the new immobilization protocol, for the first time, height mode imaging of live yeast cell surface in intermittent contact mode is presented in this report. Stable and reproducible imaging over a 10-h time span is observed. A significant improvement in operational stability will facilitate the investigation of growth patterns and surface patterns of yeast cells.

Real-Time Models of Electrostatically Actuated Cantilever Probes With Integrated Thermal Sensor for Nanoscale Interrogation

Microcantilevers with integrated thermal sensor for topography measurement, which can be electrostatically actuated, are well suited for a highly parallel dynamic-mode operation where multiple cantilevers scan the media. Interpretation of data in dynamic-mode operation utilizing such cantilevers is complex because of diverse forces acting on the cantilever that include electrostatic, interatomic, structural, thermal, and, possibly, magnetic forces. In addition, the thermal sensor introduces new dynamics making interpretation of measured data challenging. In this paper, tractable models that are suited for real-time purposes, which can quantitatively predict the cantilever motion and the thermal-sensor measurement, are presented. Furthermore, it is demonstrated that all parameters of the model can be estimated solely from thermal-sensor data. This paper also provides a comprehensive understanding of the dynamics of the thermal sensor.

Channel modeling and detector design for dynamic mode high density probe storage

Probe based data storage is a promising solution for satisfying the demand for ultra-high capacity storage devices. One of the main problems with probe storage devices is the wear of tip and media over the lifetime of the device. In this paper we present the dynamic mode operation of the cantilever probe that partially addresses the problems of media/tip wear. A communication system model which incorporates modeling of the cantilever interaction with media is proposed for the system. We demonstrate that by using a controllable canonical state space representation, the entire system can be visualized as a channel with a single input which is the tip-media interaction force. A hypothesis testing formulation for bit-by-bit detection is developed. We present three different classes of detectors for this hypothesis test. In particular, we consider two different cases where statistics on the tip-medium interaction are available and not available. Simulation results are presented for all these detectors and their relative merits are explored.

Multimode and multitone analysis of the dynamic mode operation of the Atomic Force Microscope

This article investigates the multimode model of the cantilever beam during probe based imaging. It develops a framework to quantify the effects of different material properties like dissipativity and stiffness in a near tapping mode operation of Atomic Force Microscope (AFM), which is the primary mode of imaging soft matter, when excitation consists of more than one sinusoids. Averaging theory forms an important basis and provides the theoretical foundations. Effect of dissipative and stiffness properties of the sample on the forces experienced by probe is modeled as changes in parameters of an equivalent linear time invariant model, of the cantilever-sample system. It is shown that this model can be extended to the case when multiple modes of the cantilever participate in the nonlinear interaction with the sample forces.