The Gu Group studies the mechanical behavior of nanomaterials. We work at the intersection of solid mechanics, materials science and nano-chemistry. We research the unique properties of nanoscale metals, ceramics and nano-architected composites in order to design strong, tough and lightweight structural materials, materials for extreme environments, and mechanically-actuated sensors. Our experimental tools include nanoindentation, electron microscopy, and colloidal synthesis.
Honors & Awards
ARO Young Investigator Program Award, Army Research Office (2020)
DOE Early Career Award, Department of Energy (2020)
Hellman Fellow, Hellman Foundation (2019)
Terman Faculty Fellow, Stanford Engineering (2017)
National Defense Science and Engineering Graduate Fellowship, DoD (2011)
Fulbright Award, Fulbright (2009)
Boards, Advisory Committees, Professional Organizations
Symposium organizer, Society of Engineering Sciences (2018 - Present)
Member, Metals, Minerals and Materials Society (2017 - Present)
Member, Materials Research Society (2017 - Present)
BS, University of California, Berkeley (2009)
MS/PhD, California Institute of Technology (2014)
Mehrdad T. Kiani, X. Wendy Gu. "United States Patent 62/914089 Solution processed metallic nano-glass films", Leland Stanford Junior University,, Oct 11, 0019
- Intro to Solid Mechanics
ENGR 14 (Win)
- Mechanics - Elasticity and Inelasticity
ME 340 (Aut)
Independent Studies (6)
- Engineering Problems
ME 391 (Aut, Win, Spr)
- Engineering Problems and Experimental Investigation
ME 191 (Aut, Win, Spr)
- Experimental Investigation of Engineering Problems
ME 392 (Aut, Win, Spr)
- Ph.D. Research
MATSCI 300 (Aut, Win, Spr)
- Ph.D. Research Rotation
ME 398 (Aut, Win, Spr, Sum)
- Undergraduate Research
MATSCI 150 (Aut, Win, Spr)
- Engineering Problems
Prior Year Courses
- Mechanics - Elasticity and Inelasticity
ME 340 (Aut)
- Imperfections in Crystalline Solids
ME 209 (Win)
- Mechanical Behavior of Nanomaterials
MATSCI 241, ME 241 (Aut)
- Mechanical Measurements
ME 149 (Spr)
- Seminar in Solid Mechanics
ME 395 (Aut, Win, Spr)
- Mechanics - Elasticity and Inelasticity
Doctoral Dissertation Reader (AC)
Postdoctoral Faculty Sponsor
Doctoral Dissertation Advisor (AC)
David Doan, Mehrdad Kiani, John Kulikowski, Andrew Lee, Abhinav Parakh, Radhika Pramod Patil, Melody Wang
Doctoral Dissertation Co-Advisor (AC)
Master's Program Advisor
Zhengqiu Lou, Danny Pugh
Bright NIR-II Photoluminescence in Rod-Shaped Icosahedral Gold Nanoclusters.
Small (Weinheim an der Bergstrasse, Germany)
Fluorophores with high quantum yields, extended maximum emission wavelengths, and long photoluminescence (PL) lifetimes are still lacking for sensing and imaging applications in the second near-infrared window (NIR-II). In this work, a series of rod-shaped icosahedral nanoclusters with bright NIR-II PL, quantum yields up to 8%, and a peak emission wavelength of 1520nm are reported. It is found that the bright NIR-II emission arises from a previously ignored state with near-zero oscillator strength in the ground-state geometry and the central Au atom in the nanoclusters suppresses the non-radiative transitions and enhances the overall PL efficiency. In addition, a framework is developed to analyze and relate the underlying transitions for the absorptions and the NIR-II emissions in the Au nanoclusters based on the experimentally defined absorption coefficient. Overall, this work not only shows good performance of the rod-shaped icosahedral series of Au nanoclusters as NIR-II fluorophores, but also unravels the fundamental electronic transitions and atomic-level structure-property relations for the enhancement of the NIR-II PL in gold nanoclusters. The framework developed here also provides a simple method to analyze the underlying electronic transitions in metal nanoclusters.
View details for DOI 10.1002/smll.202007992
View details for PubMedID 33620777
In Situ TEM Study of Radiation Resistance of Metallic Glass-Metal Core-Shell Nanocubes.
ACS applied materials & interfaces
Radiation damage can cause significantly more surface damage in metallic nanostructures than bulk materials. Structural changes from displacement damage compromise the performance of nanostructures in radiation environments such as nuclear reactors and outer space, or used in radiation therapy for biomedical treatments. As such, it is important to develop strategies to prevent this from occurring if nanostructures are to be incorporated into these applications. Here, in situ transmission electron microscope ion irradiation was used to investigate whether a metallic glass (MG) coating mitigates sputtering and morphological changes in metallic nanostructures. Dislocation-free Au nanocubes and Au nanocubes coated with a Ni-B MG were bombarded with 2.8 MeV Au4+ ions. The formation of internal defects in bare Au nanocubes was observed at a fluence of 7.5 * 1011 ions/cm2 (0.008 dpa), and morphological changes such as surface roughening, rounding of corners, and formation of nanofilaments began at 4 * 1012 ions/cm2 (0.04 dpa). In contrast, the Ni-B MG-coated Au nanocubes (Au@NiB) showed minimal morphological changes at a fluence of 1.9 * 1013 ions/cm2 (0.2 dpa). The MG coating maintains its amorphous nature under all irradiation conditions investigated.
View details for DOI 10.1021/acsami.0c10664
View details for PubMedID 32805810
Ductile Metallic Glass Nanoparticles via Colloidal Synthesis.
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.
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
Structural distortion and electron redistribution in dual-emitting gold nanoclusters.
2020; 11 (1): 2897
Deciphering the complicated excited-state process is critical for the development of luminescent materials with controllable emissions in different applications. Here we report the emergence of a photo-induced structural distortion accompanied by an electron redistribution in a series of gold nanoclusters. Such unexpected slow process of excited-state transformation results in near-infrared dual emission with extended photoluminescent lifetime. We demonstrate that this dual emission exhibits highly sensitive and ratiometric response to solvent polarity, viscosity, temperature and pressure. Thus, a versatile luminescent nano-sensor for multiple environmental parameters is developed based on this strategy. Furthermore, we fully unravel the atomic-scale structural origin of this unexpected excited-state transformation, and demonstrate control over the transition dynamics by tailoring the bi-tetrahedral core structures of gold nanoclusters. Overall, this work provides a substantial advance in the excited-state physical chemistry of luminescent nanoclusters and a general strategy for the rational design of next-generation nano-probes, sensors and switches.
View details for DOI 10.1038/s41467-020-16686-8
View details for PubMedID 32518297
Stress-Induced Structural Transformations in Au Nanocrystals.
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
Design and synthesis of multigrain nanocrystals via geometric misfit strain.
2020; 577 (7790): 359–63
The impact of topological defects associated with grain boundaries (GB defects) on the electrical, optical, magnetic, mechanical and chemical properties of nanocrystalline materials1,2 is well known. However, elucidating this influence experimentally is difficult because grains typically exhibit a large range of sizes, shapes and random relative orientations3-5. Here we demonstrate that precise control of the heteroepitaxy of colloidal polyhedral nanocrystals enables ordered grain growth and can thereby produce material samples with uniform GB defects. We illustrate our approach with a multigrain nanocrystal comprising a Co3O4 nanocube core that carries a Mn3O4 shell on each facet. The individual shells are symmetry-related interconnected grains6, and the large geometric misfit between adjacent tetragonal Mn3O4 grains results in tilt boundaries at the sharp edges of the Co3O4 nanocube core that join via disclinations. We identify four design principles that govern the production of these highly ordered multigrain nanostructures. First, the shape of the substrate nanocrystal must guide the crystallographic orientation of the overgrowth phase7. Second, the size of the substrate must be smaller than the characteristic distance between the dislocations. Third, the incompatible symmetry between the overgrowth phase and the substrate increases the geometric misfit strain between the grains. Fourth, for GB formation under near-equilibrium conditions, the surface energy of the shell needs to be balanced by the increasing elastic energy through ligand passivation8-10. With these principles, we can produce a range of multigrain nanocrystals containing distinct GB defects.
View details for DOI 10.1038/s41586-019-1899-3
View details for PubMedID 31942056
Pressure-Induced Optical Transitions in Metal Nanoclusters.
Currently, a comprehensive understanding of the relationship between atomic structures and optical properties of ultrasmall metal nanoclusters with diameters between 1 and 3 nm is lacking. To address this challenge, it is necessary to develop tools for perturbing the atomic structure and modulating the optical properties of metal nanoclusters beyond what can be achieved using synthetic chemistry. Here, we present a systematic high-pressure study on a series of atomically precise ligand-protected metal nanoclusters. A diamond anvil cell is used as a high-pressure chamber to gradually compress the metal nanoclusters, while their optical properties are monitored in situ. Our experimental results show that the photoluminescence (PL) of these nanoclusters is enhanced by up to 2 orders of magnitude at pressures up to 7 GPa. The absorption onset red-shifts with increasing pressure up to ∼12 GPa. Density functional theory calculations reveal that the red-shift arises because of narrowing of the spacing between discrete energy levels of the cluster due to delocalization of the core electrons to the carbon ligands. The pressure-induced PL enhancement is ascribed to (i) the enhancement of the near-band-edge transition strength, (ii) suppression of the nonradiative vibrations, and (iii) hindrance of the excited-state structural distortions. Overall, our results demonstrate that high pressure is an effective tool for modulating the optical properties and improving the luminescence brightness of metal nanoclusters. The insights into structure-property relations obtained here also contribute to the rational design of metal nanoclusters for various optical applications.
View details for DOI 10.1021/acsnano.0c04813
View details for PubMedID 32790326
Nucleation of Dislocations in 3.9 nm Nanocrystals at High Pressure.
Physical review letters
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
- Dislocation surface nucleation in surfactant-passivated metallic nanocubes MRS COMMUNICATIONS 2019; 9 (3): 1029–33
Strengthening Mechanism of a Single Precipitate in a Metallic Nanocube
2019; 19 (1): 255–60
Nano-precipitates play a significant role in the strength, ductility and damage tolerance of metallic alloys through their interaction with crystalline defects, especially dislocations. However, the difficulty of observing the action of individual precipitates during plastic deformation has made it challenging to conclusively determine the mechanisms of the precipitate-defect interaction for a given alloy system, and presents a major bottleneck in the rational design of nanostructured alloys. Here we demonstrate the in situ compression of core-shell nanocubes as a promising platform to determine the precise role of individual precipitates. Each nanocube with a dimension of ~85 nm contains a single spherical precipitate of ~25 nm diameter. The Au-core/Ag-shell nanocubes show a yield strength of 495 MPa with no strain hardening. The deformation mechanism is determined to be surface nucleation of dislocations which easily traverses through the coherent Au-Ag interface. On the other hand, the Au-core/Cu-shell nanocubes show a yield strength of 829 MPa with a pronounced strain hardening rate. Molecular dynamics and dislocation dynamics simulations, in conjunction with TEM analysis, have demonstrated the yield mechanism to be the motion of threading dislocations extending from the semi-coherent Au-Cu interface to the surface, and strain hardening to be caused by a single-armed Orowan looping mechanism. Nanocube compression offers an exciting opportunity to directly compare computational models of defect dynamics with in situ deformation measurements to elucidate the precise mechanisms of precipitate hardening.
View details for DOI 10.1021/acs.nanolett.8b03857
View details for Web of Science ID 000455561300032
View details for PubMedID 30525680
Mechanical Properties of Architected Nanomaterials Made from Organic-Inorganic Nanocrystals
Mechanical Properties of Architected Nanomaterials Made from Organic-Inorganic Nanocrystals
View details for DOI 10.1007/s11837-018-3094-7
Pseudoelasticity at Large Strains in Au Nanocrystals.
Physical review letters
2018; 121 (5): 056102
Pseudoelasticity in metals is typically associated with phase transformations (e.g., shape memory alloys) but has recently been observed in sub-10 nm Ag nanocrystals that rapidly recovered their original shape after deformation to large strains. The discovery of pseudoelasticity in nanoscale metals dramatically changes the current understanding of the properties of solids at the smallest length scales, and the motion of atoms at surfaces. Yet, it remains unclear whether pseudoelasticity exists in different metals and nanocrystal sizes. The challenge of observing deformation at atomistic to nanometer length scales has prevented a clear mechanistic understanding of nanoscale pseudoelasticity, although surface diffusion and dislocation-mediated processes have been proposed. We further the understanding of pseudoelasticity in nanoscale metals by using a diamond anvil cell to compress colloidal Au nanocrystals under quasihydrostatic and nonhydrostatic pressure conditions. Nanocrystal structural changes are measured using optical spectroscopy and transmission electron microscopy and modeled using electrodynamic theory. We find that 3.9 nm Au nanocrystals exhibit pseudoelastic shape recovery after deformation to large uniaxial strains of up to 20%, which is equivalent to an ellipsoid with an aspect ratio of 2. Nanocrystal absorbance efficiency does not recover after deformation, which indicates that crystalline defects may be trapped in the nanocrystals after deformation.
View details for PubMedID 30118265
Tolerance to structural disorder and tunable mechanical behavior in self-assembled superlattices of polymer-grafted nanocrystals.
Proceedings of the National Academy of Sciences of the United States of America
Large, freestanding membranes with remarkably high elastic modulus (>10 GPa) have been fabricated through the self-assembly of ligand-stabilized inorganic nanocrystals, even though these nanocrystals are connected only by soft organic ligands (e.g., dodecanethiol or DNA) that are not cross-linked or entangled. Recent developments in the synthesis of polymer-grafted nanocrystals have greatly expanded the library of accessible superlattice architectures, which allows superlattice mechanical behavior to be linked to specific structural features. Here, colloidal self-assembly is used to organize polystyrene-grafted Au nanocrystals at a fluid interface to form ordered solids with sub-10-nm periodic features. Thin-film buckling and nanoindentation are used to evaluate the mechanical behavior of polymer-grafted nanocrystal superlattices while exploring the role of polymer structural conformation, nanocrystal packing, and superlattice dimensions. Superlattices containing 3-20 vol % Au are found to have an elastic modulus of ∼6-19 GPa, and hardness of ∼120-170 MPa. We find that rapidly self-assembled superlattices have the highest elastic modulus, despite containing significant structural defects. Polymer extension, interdigitation, and grafting density are determined to be critical parameters that govern superlattice elastic and plastic deformation.
View details for DOI 10.1073/pnas.1618508114
View details for PubMedID 28242704
View details for PubMedCentralID PMC5358368
- In Situ Lithiation-Delithiation of Mechanically Robust Cu-Si Core-Shell Nanolattices in a Scanning Electron Microscope ACS ENERGY LETTERS 2016; 1 (3): 492-499
Tailoring of Interfacial Mechanical Shear Strength by Surface Chemical Modification of Silicon Microwires Embedded in Nafion Membranes
2015; 9 (5): 5143–53
The interfacial shear strength between Si microwires and a Nafion membrane has been tailored through surface functionalization of the Si. Acidic (-COOH-terminated) or basic (-NH2-terminated) surface-bound functionality was introduced by hydrosilylation reactions to probe the interactions between the functionalized Si microwires and hydrophilic ionically charged sites in the Nafion polymeric side chains. Surfaces functionalized with SiOx, Si-H, or Si-CH3 were also synthesized and investigated. The interfacial shear strength between the functionalized Si microwire surfaces and the Nafion matrix was quantified by uniaxial wire pull-out experiments in an in situ nanomechanical instrument that allowed simultaneous collection of mechanical data and visualization of the deformation process. In this process, an axial load was applied to the custom-shaped top portions of individual wires until debonding occurred from the Nafion matrix. The shear strength obtained from the nanomechanical measurements correlated with the chemical bond strength and the functionalization density of the molecular layer, with values ranging from 7 MPa for Si-CH3 surfaces to ∼16-20 MPa for oxygen-containing surface functionalities. Hence surface chemical control can be used to influence the mechanical adhesion forces at a Si-Nafion interface.
View details for DOI 10.1021/acsnano.5b00468
View details for Web of Science ID 000355383000050
View details for PubMedID 25872455
- Ductility and work hardening in nano-sized metallic glasses APPLIED PHYSICS LETTERS 2015; 106 (6)
Ultra-strong Architected Cu Meso-lattices
Extreme Mechanics Letters
View details for DOI 10.1016/j.eml.2015.01.006
Mechanisms of Failure in Nanoscale Metallic Glass
2014; 14 (10): 5858-5864
The emergence of size-dependent mechanical strength in nanosized materials is now well-established, but no fundamental understanding of fracture toughness or flaw sensitivity in nanostructures exists. We report the fabrication and in situ fracture testing of ∼70 nm diameter Ni-P metallic glass samples with a structural flaw. Failure occurs at the structural flaw in all cases, and the failure strength of flawed samples was reduced by 40% compared to unflawed samples. We explore deformation and failure mechanisms in a similar nanometallic glass via molecular dynamics simulations, which corroborate sensitivity to flaws and reveal that the structural flaw shifts the failure mechanism from shear banding to cavitation. We find that failure strength and deformation in amorphous nanosolids depend critically on the presence of flaws.
View details for DOI 10.1021/nl5027869
View details for Web of Science ID 000343016400060
View details for PubMedID 25198652
Effects of Helium Implantation on the Tensile Properties and Microstructure of Ni73P27 Metallic Glass Nanostructures
2014; 14 (9): 5176–83
We report fabrication and nanomechanical tension experiments on as-fabricated and helium-implanted ∼130 nm diameter Ni73P27 metallic glass nanocylinders. The nanocylinders were fabricated by a templated electroplating process and implanted with He(+) at energies of 50, 100, 150, and 200 keV to create a uniform helium concentration of ∼3 atom % throughout the nanocylinders. Transmission electron microscopy imaging and through-focus analysis reveal that the specimens contained ∼2 nm helium bubbles distributed uniformly throughout the nanocylinder volume. In situ tensile experiments indicate that helium-implanted specimens exhibit enhanced ductility as evidenced by a 2-fold increase in plastic strain over as-fabricated specimens with no sacrifice in yield and ultimate tensile strengths. This improvement in mechanical properties suggests that metallic glasses may actually exhibit a favorable response to high levels of helium implantation.
View details for DOI 10.1021/nl502074d
View details for Web of Science ID 000341544500039
View details for PubMedID 25084487
Microstructure versus Flaw: Mechanisms of Failure and Strength in Nanostructures
2013; 13 (11): 5703-5709
Understanding failure in nanomaterials is critical for the design of reliable structural materials and small-scale devices with nanoscale components. No consensus exists on the effect of flaws on fracture at the nanoscale, but proposed theories include nanoscale flaw tolerance and maintaining macroscopic fracture relationships at the nanoscale with scarce experimental support. We explore fracture in nanomaterials using nanocrystalline Pt nanocylinders with prefabricated surface notches created using a "paused" electroplating method. In situ scanning electron microscopy (SEM) tension tests demonstrate that the majority of these samples failed at the notches, but that tensile failure strength is independent of whether failure occurred at or away from the flaw. Molecular dynamics simulations verify these findings and show that local plasticity is able to reduce stress concentration ahead of the notch to levels comparable with the strengths of microstructural features (e.g., grain boundaries). Thus, failure occurs at the stress concentration with the highest local stress whether this is at the notch or a microstructural feature.
View details for DOI 10.1021/nl403453h
View details for Web of Science ID 000327111700111
View details for PubMedID 24168654
Size-Dependent Deformation of Nanocrystalline Pt Nanopillars
2012; 12 (12): 6385-6392
We report the synthesis, mechanical properties, and deformation mechanisms of polycrystalline, platinum nanocylinders of grain size d = 12 nm. The number of grains across the diameter, D/d, was varied from 5 to 80 and 1.5 to 5 in the experiments and molecular dynamics simulations, respectively. An abrupt weakening is observed at a small D/d, while the strengths of large nanopillars are similar to bulk. This "smaller is weaker" trend is opposite to the "smaller is stronger" size effect in single crystalline nanostructures. The simulations demonstrate that the size-dependent behavior is associated with the distinct deformation mechanisms operative in interior versus surface grains.
View details for DOI 10.1021/nl3036993
View details for Web of Science ID 000312122100057
View details for PubMedID 23148764
- Exploring Deformation Mechanisms in Nanostructured Materials JOM 2012; 64 (10): 1241–52
- Photoconductive CdSe Nanowire Arrays, Serpentines, and Loops Formed by Electrodeposition on Self-Organized Carbon Nanotubes JOURNAL OF PHYSICAL CHEMISTRY C 2012; 116 (37): 20121–26
- Suppression of Catastrophic Failure in Metallic Glass-Polyisoprene Nanolaminate Containing Nanopillars ADVANCED FUNCTIONAL MATERIALS 2012; 22 (9): 1972–80
- Liquid Crystalline Orientation of Rod Blocks within Lamellar Nanostructures from Rod Coil Diblock Copolymers MACROMOLECULES 2010; 43 (16): 6531–34
- A Universal and Solution-Processable Precursor to Bismuth Chalcogenide Thermoelectrics Chemistry of Materials 2010: 1943-1945