Nanomaterials, Mechanics, and MEMS Laboratory
At the Lab at ASU, their research is primarily focused on the synthesis and characterization of nanostructured materials for structural and functional applications. They develop new synthesis techniques for thin films and investigate their synthesis-structure-property relationships. The overarching goal is to engineer the microstructure of thin films to enhance their mechanical properties and functionality. The lab also has an active interest in creating soft, polymer-based micro platforms that can be used as scaffolds for biohybrid devices or for studying cellular mechanics.
Izadi, E.; Darbal, A.; Sarkar, R.; Rajagopalan, J. Grain rotations in ultrafine-grained aluminum films studied using in situ TEM straining with automated crystal orientation mapping. Mater. Des., 113 (Jan. 2017), pp. 186-194; doi: 10.1016/j.matdes (2016)
" In-situ TEM straining allows probing deformation mechanisms of ultrafine-grained and nanocrystalline metals. While obtaining statistically meaningful information about microstructural changes using conventional bright-field/dark-field imaging or diffraction is time-consuming, automated crystal orientation mapping in TEM (ACOM-TEM) enables tracking orientation changes of hundreds of grains simultaneously. We use this technique to uncover extensive grain rotations during in-situ tensile deformation of a freestanding, ultrafine-grained aluminum film (thickness 200 nm, mean grain size 180 nm). During loading, both the fraction of grains that undergo rotations and the magnitude of their rotations increase with strain. The rotations are partially or fully reversible in a significant fraction of grains during unloading, leading to notable inelastic strain recovery. More surprisingly, the direction of rotation remains unchanged for a small fraction of grains during unloading, despite a sharp reduction in the applied stress. The ACOM-TEM measurements also provide evidence of reversible and irreversible grain/twin boundary migrations in the film. These microstructural observations point to a highly inhomogeneous and constantly evolving stress distribution in the film during both loading and unloading."
"A critical assumption underlying in-situ transmission electron microscopy studies is that the electron beam (e-beam) exposure does not fundamentally alter the intrinsic deformation behavior of the materials being probed. Here, we show that e-beam exposure causes increased dislocation activation and marked stress relaxation in aluminum and gold films spanning a range of thicknesses (80 – 400 nm) and grain sizes (50 – 220 nm). Furthermore, the e-beam induces anomalous sample necking, which unusually depends more on the e-beam diameter than intensity. Notably, the stress relaxation in both aluminum and gold occurs at beam energies well below their damage thresholds. More remarkably, the stress relaxation and/or sample necking is significantly more pronounced at lower accelerating voltages (120 kV versus 200 kV) in both the metals. These observations in aluminum and gold, two metals with highly dissimilar atomic weights and properties, indicate that e-beam exposure can cause anomalous behavior in a broad spectrum of nanostructured materials, and simultaneously suggest a strategy to minimize such artifacts."
21. E. Izadi, A. Darbal, R. Sarkar, J. Rajagopalan, “Grain rotations in ultrafine-grained aluminum films studied using in situ TEM straining with automated crystal orientation mapping,” Materials and Design 113, 186–194, 2017.
20. R. Sarkar, C. Ebner, I. Izadi, C. Rentenberger, and J. Rajagopalan, “Revealing anelasticity and structural rearrangements in nanoscale metallic glass films using in situ TEM diffraction,” Materials Research Letters, 2016 DOI:10.1080/21663831.2016.1228709.
19. T. W. Sowers, R. Sarkar, S. E. Prameela, E. Izadi and J. Rajagopalan, “Capillary driven flow of polydimethylsiloxane in open rectangular micro channels,” Soft Matter 12, 5818-5823, 2016.
18. C. Ebner, R. Sarkar, J. Rajagopalan, and C. Rentenberger, “Local, atomic-level elastic strain measurements of metallic glass thin films by electron diffraction,” Ultramicroscopy 165, 51–58, 2016.
17. E. Izadi and J. Rajagopalan, “Texture dependent strain rate sensitivity of ultrafine-grained aluminum films,” Scripta Materialia 114, 65-69, 2016.
16. R. Sarkar, C. Rentenberger and J. Rajagopalan, “Electron beam induced artifacts during in situ TEM deformation of nanostructured metals,” Scientific Reports 5, 16345, 2015.
15. S. Singh, R. Sarkar, H.-X. Xie, C. Mayer, J. Rajagopalan, and N. Chawla, “Tensile behavior of single-crystal tin whiskers,” Journal of Electronic Materials 43, 978-982, 2014.
14. B. J. Williams, S. A. Anand, J. Rajagopalan and M. T. A. Saif, “A self-propelled biohybrid swimmer at low Reynolds number,” Nature Communications 5, 3081, 2014.
13. J. Rajagopalan and M. T. A. Saif, “Fabrication of freestanding 1-D PDMS microstructures using capillary micromolding,” Journal of Microelectromechanical Systems 22, 992-994, 2013.
12. J. Rajagopalan and M. T. A. Saif, “Effect of microstructural heterogeneity on the mechanical behavior of nanocrystalline metal films,” Journal of Materials Research 26, 2826-2832, 2011.
11. J. Rajagopalan and M. T. A. Saif, “MEMS sensors and microsystems for cell mechano-biology,” Journal of Micromechanics and Microengineering 21, 054002, 2011.
10. J. Rajagopalan, A. Tofangchi and M. T. A. Saif, “Linear, high-resolution bioMEMS force sensors with large measurement range,” Journal of Microelectromechanical Systems 19, 1380-1389, 2010.
9. W. Kang, J. Rajagopalan and M. T. A. Saif, “In situ uniaxial mechanical testing of small scale materials – a review,” Nanoscience and Nanotechnology Letters 2, 282-287, 2010.
8. J. Rajagopalan, A. Tofangchi and M. T. A. Saif, “Drosophila neurons actively regulate axonal tension in vivo,” Biophysical Journal 99, 3208-3215, 2010.
7. J. Rajagopalan, C. Rentenberger, P. H. Karnthaler, G. Dehm and M. T. A. Saif, “In situ TEM study of microplasticity and Bauschinger effect in nanocrystalline metals,” Acta Materialia 58, 4772-4782, 2010.
6. J. Rajagopalan, J. H. Han and M. T. A. Saif, “On plastic strain recovery in freestanding nanocrystalline metal thin films,” Scripta Materialia 59, 921-926, 2008.
5. J. Rajagopalan, J. H. Han and M. T. A. Saif, “Bauschinger effect in unpassivated freestanding nanoscale metal films,” Scripta Materialia 59, 734-737, 2008.
4. J. Rajagopalan, J. H. Han, and M. T. A. Saif, “Plastic deformation recovery in freestanding nanocrystalline aluminum and gold thin films,” Science 315, 1831-1834, 2007.
3. J. Rajagopalan and M. T. A. Saif, “A single degree of freedom model for thermoelastic damping,” Journal of Applied Mechanics 74, 461-468, 2007.
2. J. Rajagopalan, K. Balasubramaniam and C. V. Krishnamurthy, “A phase reconstruction algorithm for Lamb wave based structural health monitoring of anisotropic multilayered composite plates,” Journal of the Acoustical Society of America 119, 872-878, 2006.
1. J. Rajagopalan, K. Balasubramaniam and C. V. Krishnamurthy, “A single transmitter multi receiver (STMR) PZT array for guided ultrasonic wave based structural health monitoring of large isotropic plate structures,” Smart Materials and Structures 15, 1190-1196, 2006.