Research Interests

The core research interest of my group is experimental mechanics and physics of small scale materials and interfaces. We enjoy miniaturizing experimental setups that allow us to investigate thermo-physical properties with distinct advantages of in-situ microscopy (virtually all forms). Our expertise in design and fabrication of high resolution force (up to pico-Newton) and displacement (up to nanometers) sensors with very small overall footprint allows us to perform all our experiments inside the scanning or transmission electron microscope. Our current focus is integrating mechanical testing devices with electrical (conductivity, band gap) and thermal (thermal conductivity) transport characterization capabilities. This page is intended to be a picture gallery, so feel free to contact me for more information.

At the fundamental level, we are interested in the multi-physics of nanoscale (nanotubes and nanowires) and biological (cellular, extra and subcellular) materials. The dimensional and micro-structural constraints of these materials impose pronounced inter-play of multi-fields. Therefore it is more relevant to study them in coupled domain (for example electro-mechanical or chemo-mechanical) rather than traditional individual domains. Perhaps the best example comes from the cells - they are so sensitive to electrical, mechanical, thermal and bio-chemical stimuli, that single domain studies are less appealing. The theme of our research group is "smearing all the boundaries". To justify it, we often spend time on diverse application areas, such as man-machine dynamics (human vigilance monitoring).

Illustrative Activity 1: Quantitative In-situ TEM Studies

One unique advantage from the miniaturization is the possibility of performing quantitative studies inside the Transmission Electron Microscope. TEM shows the specimen microstructures with up to atomic resolution, but its chamber is also very small to accommodate off-the-shelf experimental tools. The impact is due to the ability of the tool to bridge the gap between experimental and theoretical/computational/modeling studies.

While our objective is to characterize all physical properties of materials, surfaces and interfaces, the simplest example is on tensile testing. One needs to shrink a tensile testing tool (zoomed view at right) into the tiny dot on the conventional specimen (left)

At the end, the challenge is worth taking, as one can obtain the numbers (stress-strain) and pictures (dislocations, grain boundaries, voids, cracks); all at the same time!

Visual evidence can be obtained to support the fundamental mechanisms for deformation at the smaller scales. This crack, for example, has no plastic zone ahead (dislocation starved).

Illustrative Activity 2: Multi-Physics at the Nanoscale

The striking overlap in the various characteristic length-scales and the pre-dominance of surface atoms suggest unprecedented multi-domain coupling. The state of the art, however, still remains to be single domain characterization.

We are currently developing a lab-on-a-chip device that will measure stress, strain, electrical and thermal conductivity, band gap of materials and interfaces. Additional (chemical, optical, magnetic) etc features also possible depending upon the microscopy used.

Work-in Progress: A multi-physics chip (with results to come)

Figures below show a few of our devices for mechanical testing of nanoscale materials.

Thermal Actuator

Electrostatic Actuator

Micro-mechanical Actuator (Linear)

Micro-mechanical Actuator (Non-linear)

We can cofabricate these devices with thin film specimens. For other structures such as nanowires or thin sectioning of bulk materials, we can perform microscale pick, place and glue operations. This is shown in the figures below. Specimens are synthesized by the Foley/Rajagopalan and the Wolf Groups at Penn State.

3 nm thick Glassy carbon film specimen patterned and released from substrate. Transparent even in SEM

Integrating nanowire specimens on our devices. 150 nm diameter Glassy carbon nanowire (left) before and (right) after the pick-place-glue operation inside the FIB-SEM

Integrating nanoscale specimens sculpted from bulk materials, as seen on (left) Ti-TiN multi-layers and (right) the specimen after the pick-place-glue-notch operation in the FIB-SEM

Sample Projects:

In-situ Monitoring of Opto-electro-mechanical Responses of Single Cells to External Stimuli using MEMS

The objective of this research is to investigate how individual biological cells react to mechanical (coupled normal and shear forces) and bio-chemical (pH level and epsilon toxin) stimuli. The approach is to monitor their mechanical, electrical and bio-chemical responses, in-situ, by combining fluorescence/confocal microscopy with novel micro-electro-mechanical testing devices. The simultaneously qualitative (direct visualization) and quantitative (real-time measurement of force, displacement and bio-impedance) data on the mechanical and chemical assays will enable research in two unexplored areas: (i) cellular structural and adhesion mechanics under more realistic, combined normal and shear loading and (ii) relationship between cell health and its electrical impedance.

This proposal will provide fundamental understanding of multi-physics of cellular stimuli coupling and transduction. Relating cell health to its electrical impedance will result in automated and label-less electrical detection (electrical pathology) that may replace conventional manual labeling and microscopy-based schemes for faster diagnostics and drug discovery applications. The proposed research will impact the society by integrating multi-disciplinary research with education at all levels while promoting diversity. The accompanying educational plan is to (a) provide research experience to undergraduates in collaboration with the Penn State Multicultural Engineering Program (b) bring electron microscopy in local elementary, middle and high-school students through an outreach program . Microscopy for K-12 and (c) conduct summer workshop for local school K-12 teachers. Schools throughout the nation will be benefited from the outreach website with the streaming videos of these outreach sessions and associated multi-media materials.

Thermo-Mechanical Effects on Electrical Transport

Carbon nanotubes (mono/multi-layers of carbon atoms rolled into seamless tubes) are known to have superior electrical, mechanical, thermal and chemical properties. Interestingly, the same reasons behind their superiority also make them very sensitive to electrical, mechanical, thermal and chemical fields. Practical applications, as in electronics, sensors and actuators, composites and bio-medical, are likely to involve nanotubes that are mechanically strained and chemically treated during fabrication - and not the pristine ones. The very high power density of such ultra-miniaturized devices will cause higher operating temperatures (observed even in the existing computer chips), which will drastically alter the transport properties of the highly confined nanotube electrons. Studies involving the sensitivity of electrical properties under mechanical or thermal fields (such as tuning electrical properties with mechanical displacement or vice versa) have been mostly theoretical. Experimental studies focusing on the simultaneous effects of temperature (>300 K) and mechanical force-displacement on the electrical properties of the nanotubes are rare in the literature, an observation that motivates this research proposal.

The specific aims of this research are, (i) Development of a micro-electro-mechanical characterization instrument with co-fabricated freestanding single carbon nanotube specimens. The 1mm x 1mm size device will be compatible with any type of microscopy (Optical/SEM/TEM/STM). It will measure force and displacement with 20 pico-Newton and 5 nm resolutions respectively, using an optical microscope. The mechanical sensors (electrically isolated and metallized silicon micro-beams) will also work as electrical connectors to measure current-voltage signals. (ii) Study the effects of elevated temperature on the mechanical properties (Young’s modulus, fracture stress and strain) of individual carbon nanotubes, in-situ inside the transmission electron microscope. (iii) Study the electrical properties of carbon nanotubes for a wide range of temperature (300-500K) and mechanical strain (up to 30%), for which data is not yet available in the literature.

Nano-mechanics of Carbon Nanotube-Polymer Interfaces

The objectives of this research are to experimentally study and model deformation and failure of nanoscale interfaces using nanowires and nanotubes as reinforcing agents. To meet these objectives, nanoscale 'pull-out' specimens and high resolution micro-electro-mechanical (MEMS) force and displacement sensors will be fabricated. Single nanowire or nanotube pull-out experiments will be performed in-situ inside the Scanning Electron Microscope (SEM) at up to 100,000x magnification while interfacial forces and displacements are measured. The simultaneously qualitative and quantitative information on the nanoscale interfaces will be used to model the load bearing, deformation and failure mechanics after exploring the effects of surface functionalization and accounting for adhesion and friction, which are known to be dominant at the nanoscale. The advances in the fundamental understanding in the mechanics of nanoscale interfaces will help develop nano-composites with novel properties.

Nano-mechanical Properties of Grain Boundaries

More than 50 years of research on grain boundaries has established their impact on the overall strength of materials, yet experimental studies on their yield strength or Young's modulus are rare in the literature. This is because grain boundaries are random networks of interfaces that are only a few nanometers wide and cannot be isolated and characterized by conventional tensile, bending, indentation tools. This project aims to address this challenge by developing an experimental setup using micro-electro-mechanical (MEMS) force and displacement sensors and in-situ transmission electron microscopy (TEM). Since TEM renders the microstructures visible, the experimental setup will allow simultaneous characterization of crystallographic orientations, crack propagation and plastic deformation mechanisms with atomic resolution. Models based on interfacial fracture mechanics will be developed to extract the grain boundary properties from the experimental data. The fundamental understanding on the interfaces in materials will impact grain boundary engineering, an evolving research direction towards optimized materials design.

Honda Initiation Grant: Driver Vigilance Monitoring

Alertness of individuals operating vehicles, aircrafts and machinery is a pre-requisite for safety of the individual and for avoiding economic losses. Exploiting the interactions at the operator-machine interface, we are developing a technique that is (i) non-intrusive (ii) non image processing based (iii) small and simple to install, (iv) accurate and mainly (v) cheap. Our hypothesis is that the operator-machine interface data can be processed so that it becomes a fingerprint of the operator. This means, it will not only be possible to discern different levels of alertness in an individual, but also to distinguish among different individuals. Our preliminary results corroborate this hypothesis. We have developed a model to quantify the alertness during car driving. By feeding an artificial neural network with training data, we envision intelligent vigilance monitoring.  The low cost, ruggedness and low-volume data processing requirements of the proposed technique give it a competitive edge over existing predominantly image processing based vigilance monitoring systems.

Nanoscale Materials Behavior

Extreme dimensional (thickness) and micro-structural (grain size) constraints and the large proportion of interfaces at the nano-scale cause deviation in the materials behavior. Fundamental understanding in nano-scale materials behavior will impact nano-technology, MEMS and micro-electronics. In this research, we study the effect of all these factors in failure of nano-grained materials. One outcome of this research is probably the world's smallest nanotensilometer. This chip integrates the specimen with force and displacement sensors. We performed, for the first time, tensile testing of nano-scale thin films in-situ in TEM and SEM. The advantage of using TEM is the ability to directly observe deformation mechanisms. This picture shows crack propagation in a 100 nm thick aluminum specimen with no dislocation activities ahead of the tip. This video shows crack nucleation and failure in a 200 nm thick freestanding aluminum film. Here is an example of electrostatic comb drive actuators with nano-N force resolution we designed and fabricated for materials studies. We performed bending experiments on both end clamped and cantilever 100 nm thick aluminum films to study strain gradient effects on plasticity at the nano-scale.

My group is interested in development of MEMS/NEMS sensors and actuator to study length scale effects in typical mechanical engineering disciplines. Specifically, we aim to measure grain boundary properties of materials at different scales. The motivation comes from the observation that even after decades of research, mechanical properties of grain boundaries (alone) have been rarely studied quantitatively. Our in-situ TEM testing tool allows us to measure forces and displacement in electron transparent specimens with a interfacial fracture running through the grain boundaries. Another technique would be to apply elastic theories of dislocations in estimating the strength of grain boundaries as we observe dislocation pileups in TEM and measure the corresponding stress and strain values.