All Publications

  • Diffusion of Anisotropic Colloidal Microparticles Fabricated Using Two-Photon Lithography PARTICLE & PARTICLE SYSTEMS CHARACTERIZATION Doan, D., Kulikowski, J., Gu, X. 2021
  • Ductile Metallic Glass Nanoparticles via Colloidal Synthesis. Nano letters Kiani, M. T., Barr, C. M., Xu, S., Doan, D., Wang, Z., Parakh, A., Hattar, K., Gu, X. W. 2020


    The design of ductile metallic glasses has been a longstanding challenge. Here, we use colloidal synthesis to fabricate nickel-boron metallic glass nanoparticles that exhibit homogeneous deformation at room temperature and moderate strain rates. In situ compression testing is used to characterize the mechanical behavior of 90-260 nm diameter nanoparticles. The force-displacement curves consist of two regimes separated by a slowly propagating shear band in small, 90 nm particles. The propensity for shear banding decreases with increasing particle size, such that large particles are more likely to deform homogeneously through gradual shape change. We relate this behavior to differences in composition and atomic bonding between particles of different size using mass spectroscopy and XPS. We propose that the ductility of the nanoparticles is related to their internal structure, which consists of atomic clusters made of a metalloid core and a metallic shell that are connected to neighboring clusters by metal-metal bonds.

    View details for DOI 10.1021/acs.nanolett.0c02177

    View details for PubMedID 32786936

  • Hardening in Au-Ag nanoboxes from stacking fault-dislocation interactions. Nature communications Patil, R. P., Doan, D., Aitken, Z. H., Chen, S., Kiani, M. T., Barr, C. M., Hattar, K., Zhang, Y., Gu, X. W. 2020; 11 (1): 2923


    Porous, nano-architected metals with dimensions down to ~10nm are predicted to have extraordinarily high strength and stiffness per weight, but have been challenging to fabricate and test experimentally. Here, we use colloidal synthesis to make ~140nm length and ~15nm wall thickness hollow Au-Ag nanoboxes with smooth and rough surfaces. In situ scanning electron microscope and transmission electron microscope testing of the smooth and rough nanoboxes show them to yield at 130±45MPa and 96±31MPa respectively, with significant strain hardening. A higher strain hardening rate is seen in rough nanoboxes than smooth nanoboxes. Finite element modeling is used to show that the structure of the nanoboxes is not responsible for the hardening behavior suggesting that material mechanisms are the source of observed hardening. Molecular dynamics simulations indicate that hardening is a result of interactions between dislocations and the associated increase in dislocation density.

    View details for DOI 10.1038/s41467-020-16760-1

    View details for PubMedID 32522992

  • Stress-Induced Structural Transformations in Au Nanocrystals. Nano letters Parakh, A. n., Lee, S. n., Kiani, M. T., Doan, D. n., Kunz, M. n., Doran, A. n., Ryu, S. n., Gu, X. W. 2020


    Nanocrystals can exist in multiply twinned structures like icosahedron or single crystalline structures like cuboctahedron. Transformations between these structures can proceed through diffusion or displacive motion. Experimental studies on nanocrystal structural transformations have focused on high-temperature diffusion-mediated processes. Limited experimental evidence of displacive motion exists. We report structural transformation of 6 nm Au nanocrystals under nonhydrostatic pressure of 7.7 GPa in a diamond anvil cell that is driven by displacive motion. X-ray diffraction and transmission electron microscopy were used to detect the structural transformation from multiply twinned to single crystalline. Single crystalline nanocrystals were recovered after unloading, then quickly reverted to the multiply twinned state after dispersion in toluene. The dynamics of recovery was captured using TEM which showed surface recrystallization and rapid twin boundary motion. Molecular dynamics simulations showed that twin boundaries are unstable due to defects nucleated from the interior of the nanocrystal.

    View details for DOI 10.1021/acs.nanolett.0c03371

    View details for PubMedID 33016704

  • Nucleation of Dislocations in 3.9 nm Nanocrystals at High Pressure. Physical review letters Parakh, A. n., Lee, S. n., Harkins, K. A., Kiani, M. T., Doan, D. n., Kunz, M. n., Doran, A. n., Hanson, L. A., Ryu, S. n., Gu, X. W. 2020; 124 (10): 106104


    As circuitry approaches single nanometer length scales, it has become important to predict the stability of single nanometer-sized metals. The behavior of metals at larger scales can be predicted based on the behavior of dislocations, but it is unclear if dislocations can form and be sustained at single nanometer dimensions. Here, we report the formation of dislocations within individual 3.9 nm Au nanocrystals under nonhydrostatic pressure in a diamond anvil cell. We used a combination of x-ray diffraction, optical absorbance spectroscopy, and molecular dynamics simulation to characterize the defects that are formed, which were found to be surface-nucleated partial dislocations. These results indicate that dislocations are still active at single nanometer length scales and can lead to permanent plasticity.

    View details for DOI 10.1103/PhysRevLett.124.106104

    View details for PubMedID 32216385