My research interests have been focusing on how individual building blocks come together resulting in complex functions which are hard to predict, if possible, from the individual identities. Similar to a digital screen displaying a movie, the complicated pattern and story can hardly be interpreted from the dynamic traces of a single pixel. Specifically, I have been studying the general topic of self-assembly and non-equilibrium behaviors in soft matter systems, using both experimental and simulation tools.

I obtained my B.S. degree in physics from University of Science and Technology of China (USTC) in 2015. In my undergraduate research, I tried to use computer simulation to study multiple systems in Prof. Zhonghuai Hou’s group, such as the Viscek model for self-propelled particles. In 2014, I visited Oxford University to study the phase behaviors of active nematics using Lattice-Boltzmann method in Prof. Julia M. Yeomans' group. In 2020, I obtained my Ph.D. degree in Materials Science and Engineering at University of Illinois at Urbana-Champaign (UIUC) under the supervision of Prof. Qian Chen. During my Ph.D. research, we illustrated the nonclassical crystallization pathway of nanoparticles (Nat. Mater., 19, 450–455, 2020) and supracrystal growth kinetics (Nat. Commun., 11, 4555, 2020) using liquid-phase TEM. I also studied other nonequilibrium behaviors in novel colloidal systems, such as shape transformation of metal-organic framework crystals during chemical etching (ACS Appl. Mater. Interfaces, 10, 48, 40990–40995, 2018), application of ferromagnetic colloids in inductor design (Science Adv., 6, 3, eaay4508, 2020) and electron transport in redox-active colloids.

In August 2020, I joined Prof. Guosong Hong’s group at the materials science and engineering department at Stanford University to develop novel nanomaterials that can interact with neurons at the subcellular level. Armed with the knowledge of nanotechnology and theoretical modeling, we are extending the tools that can be used to investigate the challenging questions in neuroscience.

Honors & Awards

  • Wu Tsai Neurosciences Institute Interdisciplinary Scholar Award, Stanford University (2022)
  • Racheff-Intel Award, UIUC (2020)
  • Conference Travel Awards for Grdaduate Students, UIUC (2019)
  • Dow Best Presentation Award for Soft Materials Seminar, UIUC (2019)
  • Outstanding Teaching Assistant Award, USTC (2015)

Professional Education

  • Ph.D., University of Illinois at Urbana-Champaign, Materials Science and Engineering (2020)
  • B.S., University of Science and Technology of China, Physics (2015)

Stanford Advisors


  • Xiuling Li,Wen Huang,Zhendong Yang,Mark D. Kraman,Jimmy Ni,Zihao Ou,Qian Chen,J. Gary Eden. "United States Patent 16,434,524 Rolled-up electromagnetic component for on-chip applications and method of making a rolled-up electromagnetic component", Jun 8, 2021

All Publications

  • Palette of Rechargeable Mechanoluminescent Fluids Produced by a Biomineral-Inspired Suppressed Dissolution Approach. Journal of the American Chemical Society Yang, F., Wu, X., Cui, H., Jiang, S., Ou, Z., Cai, S., Hong, G. 2022


    Mechanoluminescent materials, which emit light in response to mechanical stimuli, have recently been explored as promising candidates for photonic skins, remote optogenetics, and stress sensing. All mechanoluminescent materials reported thus far are bulk solids with micron-sized grains, and their light emission is only produced when fractured or deformed in bulk form. In contrast, mechanoluminescence has never been observed in liquids and colloidal solutions, thus limiting its biological application in living organisms. Here, we report the synthesis of mechanoluminescent fluids via a suppressed dissolution approach. We demonstrate that this approach yields stable colloidal solutions comprising mechanoluminescent nanocrystals with bright emissions in the range of 470-610 nm and diameters down to 20 nm. These colloidal solutions can be recharged and discharged repeatedly under photoexcitation and hydrodynamically focused ultrasound, respectively, thus yielding rechargeable mechanoluminescent fluids that can store photon energy in a reversible manner. This rechargeable fluid can facilitate a systemically delivered light source gated by tissue-penetrant ultrasound for biological applications that require light in the tissue, such as optogenetic stimulation in the brain.

    View details for DOI 10.1021/jacs.2c06724

    View details for PubMedID 36190898

  • Shedding light on neurons: optical approaches for neuromodulation. National science review Jiang, S., Wu, X., Rommelfanger, N. J., Ou, Z., Hong, G. 2022; 9 (10): nwac007


    Today's optical neuromodulation techniques are rapidly evolving, benefiting from advances in photonics, genetics and materials science. In this review, we provide an up-to-date overview of the latest optical approaches for neuromodulation. We begin with the physical principles and constraints underlying the interaction between light and neural tissue. We then present advances in optical neurotechnologies in seven modules: conventional optical fibers, multifunctional fibers, optical waveguides, light-emitting diodes, upconversion nanoparticles, optical neuromodulation based on the secondary effects of light, and unconventional light sources facilitated by ultrasound and magnetic fields. We conclude our review with an outlook on new methods and mechanisms that afford optical neuromodulation with minimal invasiveness and footprint.

    View details for DOI 10.1093/nsr/nwac007

    View details for PubMedID 36196122

    View details for PubMedCentralID PMC9522429

  • High-Resolution Electron Tomography of Ultrathin Boerdijk-Coxeter-Bernal Nanowire Enabled by Superthin Metal Surface Coating. Small (Weinheim an der Bergstrasse, Germany) Song, X., Zhang, X., Chang, Q., Yao, X., Li, M., Zhang, R., Liu, X., Song, C., Ng, Y. X., Ang, E. H., Ou, Z. 2022: e2203310


    The rapid advancement of transmission electron microscopy has resulted in revolutions in a variety of fields, including physics, chemistry, and materials science. With single-atom resolution, 3D information of each atom in nanoparticles is revealed, while 4D electron tomography is shown to capture the atomic structural kinetics in metal nanoparticles after phase transformation. Quantitative measurements of physical and chemical properties such as chemical coordination, defects, dislocation, and local strain have been made. However, due to the incompatibility of high dose rate with other ultrathin morphologies, such as nanowires, atomic electron tomography has been primarily limited to quasi-spherical nanoparticles. Herein, the 3D atomic structure of a complex core-shell nanowire composed of an ultrathin Boerdijk-Coxeter-Bernal (BCB) core nanowire and a noble metal thin layer shell deposited on the BCB nanowire surface is discovered. Furthermore, it is demonstrated that a new superthin noble metal layer deposition on an ultrathin BCB nanowire could mitigate electron beam damage using an in situ transmission electron microscope and atomic resolution electron tomography. The colloidal coating method developed for electron tomography can be broadly applied to protect the ultrathin nanomaterials from electron beam damage, benefiting both the advanced material characterizations and enabling fundamental in situ mechanistic studies.

    View details for DOI 10.1002/smll.202203310

    View details for PubMedID 36084232

  • A biomineral-inspired approach of synthesizing colloidal persistent phosphors as a multicolor, intravital light source. Science advances Yang, F., Wu, X., Cui, H., Ou, Z., Jiang, S., Cai, S., Zhou, Q., Wong, B. G., Huang, H., Hong, G. 2022; 8 (30): eabo6743


    Many in vivo biological techniques, such as fluorescence imaging, photodynamic therapy, and optogenetics, require light delivery into biological tissues. The limited tissue penetration of visible light discourages the use of external light sources and calls for the development of light sources that can be delivered in vivo. A promising material for internal light delivery is persistent phosphors; however, there is a scarcity of materials with strong persistent luminescence of visible light in a stable colloid to facilitate systemic delivery in vivo. Here, we used a bioinspired demineralization (BID) strategy to synthesize stable colloidal solutions of solid-state phosphors in the range of 470 to 650 nm and diameters down to 20 nm. The exceptional brightness of BID-produced colloids enables their utility as multicolor luminescent tags in vivo with favorable biocompatibility. Because of their stable dispersion in water, BID-produced nanophosphors can be delivered systemically, acting as an intravascular colloidal light source to internally excite genetically encoded fluorescent reporters within the mouse brain.

    View details for DOI 10.1126/sciadv.abo6743

    View details for PubMedID 35905189

  • Pristine carbon nanotubes are efficient absorbers at radio frequencies. Nanotechnology Rommelfanger, N. J., Brinson, K., Bailey, J. E., Bancroft, A. M., Ou, Z., Hong, G. 2022


    Radio frequency ablation and microwave hyperthermia are powerful tools for destroying dysfunctional biological tissues, but wireless application of these techniques is hindered by the inability to focus the electromagnetic energy to small targets. The use of locally injected radio frequency- or microwave-absorbing nanomaterials can help to overcome this challenge by confining heat production to the injected region. Previous theoretical work suggests that high-aspect-ratio conducting nanomaterials, such as carbon nanotubes, offer powerful radio frequency and microwave absorption. While carbon nanotubes have been previously studied for radio frequency and microwave hyperthermia enhancement, these studies have employed sonication for sample preparation, reducing the volume fraction and average length within the carbon nanotube suspensions. In this manuscript, we use a sonication-free preparation technique to preserve both the length of carbon nanotubes and the high volume fraction of their bundled state. We measure the heating of these samples at 2 GHz compared to the heating of a biological tissue reference using infrared thermography. We report an increase in heating by 4.5 fold compared to the tissue reference, with localized heating clearly observable within a three-dimensional biological tissue phantom. Numerical simulations further aid in producing a temperature map within the phantom and demonstrating localized heating. Due to their significant differential heating ratio, we believe that sonication-free carbon nanotube samples may bring unforeseen opportunities to the fields of radio frequency ablation and microwave hyperthermia.

    View details for DOI 10.1088/1361-6528/ac6cf8

    View details for PubMedID 35512668

  • Mechanism and performance relevance of nanomorphogenesis in polyamide films revealed by quantitative 3D imaging and machine learning. Science advances An, H., Smith, J. W., Ji, B., Cotty, S., Zhou, S., Yao, L., Kalutantirige, F. C., Chen, W., Ou, Z., Su, X., Feng, J., Chen, Q. 2022; 8 (8): eabk1888


    Biological morphogenesis has inspired many efficient strategies to diversify material structure and functionality using a fixed set of components. However, implementation of morphogenesis concepts to design soft nanomaterials is underexplored. Here, we study nanomorphogenesis in the form of the three-dimensional (3D) crumpling of polyamide membranes used for commercial molecular separation, through an unprecedented integration of electron tomography, reaction-diffusion theory, machine learning (ML), and liquid-phase atomic force microscopy. 3D tomograms show that the spatial arrangement of crumples scales with monomer concentrations in a form quantitatively consistent with a Turing instability. Membrane microenvironments quantified from the nanomorphologies of crumples are combined with the Spiegler-Kedem model to accurately predict methanol permeance. ML classifies vastly heterogeneous crumples into just four morphology groups, exhibiting distinct mechanical properties. Our work forges quantitative links between synthesis and performance in polymer thin films, which can be applicable to diverse soft nanomaterials.

    View details for DOI 10.1126/sciadv.abk1888

    View details for PubMedID 35196079

  • Shedding light on neurons: optical approaches for neuromodulation NATIONAL SCIENCE REVIEW Jiang, S., Wu, X., Rommelfanger, N. J., Ou, Z., Hong, G. 2022
  • Differential Heating of Metal Nanostructures at Radio Frequencies PHYSICAL REVIEW APPLIED Rommelfanger, N. J., Ou, Z., Keck, C. C., Hong, G. 2021; 15 (5)
  • Differential heating of metal nanostructures at radio frequencies. Physical review applied Rommelfanger, N. J., Ou, Z., Keck, C. H., Hong, G. 2021; 15 (5)


    Nanoparticles with strong absorption of incident radio frequency (RF) or microwave irradiation are desirable for remote hyperthermia treatments. While controversy has surrounded the absorption properties of spherical metallic nanoparticles, other geometries such as prolate and oblate spheroids have not received sufficient attention for application in hyperthermia therapies. Here, we use the electrostatic approximation to calculate the relative absorption ratio of metallic nanoparticles in various biological tissues. We consider a broad parameter space, sweeping across frequencies from 1 MHz to 10 GHz, while also tuning the nanoparticle dimensions from spheres to high-aspect-ratio spheroids approximating nanowires and nanodiscs. We find that while spherical metallic nanoparticles do not offer differential heating in tissue, large absorption cross sections can be obtained from long prolate spheroids, while thin oblate spheroids offer minor potential for absorption. Our results suggest that metallic nanowires should be considered for RF- and microwave-based wireless hyperthermia treatments in many tissues going forward.

    View details for DOI 10.1103/physrevapplied.15.054007

    View details for PubMedID 36268260

    View details for PubMedCentralID PMC9581340

  • Nanoscopic morphological effect on the optical properties of polymer-grafted gold polyhedra. Nanoscale advances Lee, J., Bae, C., Ou, Z., Park, S., Kim, J., Kim, J. 2021; 3 (7): 1927-1933


    Plasmonic nanoparticles show highly sensitive optical properties upon local dielectric environment changes. Hybridisation of plasmonic nanoparticles with active polymeric materials can allow stimuli-responsive and multiplex sensing over conventional monotonic sensing capacity. Such heterogeneous adlayers around the plasmonic core component, however, are likely to perturb the local refractive index in the nanometre regime and lead to uncertainty in its intrinsic sensitivity. Herein we prepare a series of polystyrene-grafted polyhedral gold nanoparticles, cubic and concave cubic cores, with different edge lengths and polymer thicknesses with precise synthesis control. Their localised surface plasmon resonance (LSPR) spectral changes are monitored to understand the effect of core morphological details in the interplay of nanoscale polymeric shells. Quantitative image analysis of changes in the core and shell shape contours and finite-difference time-domain simulations of the corresponding LSPR spectra and electric field distributions reveal that the magnitude of the LSPR spectral shift is closely dependent on the core morphology, polymer shell thickness and electric field intensity. We also demonstrate that the polystyrene-grafted gold concave cube displays higher sensitivity for nanoscale refractive index change in the polymer shell than the polystyrene-grafted gold cube at different temperatures. Our systematic investigation will help design polymer-composited plasmonic nanosensors for desirable applications.

    View details for DOI 10.1039/d1na00035g

    View details for PubMedID 36133089

    View details for PubMedCentralID PMC9419197

  • Revealing Structure Properties of ZIF-8 Particles Prepared by Wet Chemical Etching via 3D Electron Tomography ACS Materials Letters Song, X., Ou, Z., Hu, X., Zhang, X., Li, M., Wen, L., Li, M. 2021; 3: 171–178
  • Direct imaging on the deformation and sintering of polymeric particles at the nanoscale by liquid-phase TEM Microscopy and Microanalysis Liu, C., Ou, Z., Chen, Q. 2021; 27 (S1): 2630-2632
  • Bioinspired nanoantennas for opsin sensitization in optogenetic applications: a theoretical investigation Multifunctional Materials Keck, C., Rommelfanger, N., Ou, Z., Hong, G. 2021

    View details for DOI 10.1088/2399-7532/abf0f9

  • Controlling the anisotropic surface wetting of metal nanoparticles by a competitive ligand packing strategy: Implications for encapsulation ACS Applied Nano Materials Song, X., Zhang, X., Ou, Z., Zhang, Y., Li, M. 2021

    View details for DOI 10.1021/acsanm.1c02276

  • Nanoscopic morphological effect on the optical properties of polymer-grafted gold polyhedra Nanoscale Advances Li, J., Bae, C., Ou, Z., Park, S., Kim, J., Kim, J. 2021

    View details for DOI 10.1039/D1NA00035G

  • Machine Learning to Reveal Nanoparticle Dynamics from Liquid-Phase TEM Videos. ACS central science Yao, L., Ou, Z., Luo, B., Xu, C., Chen, Q. 2020; 6 (8): 1421-1430


    Liquid-phase transmission electron microscopy (TEM) has been recently applied to materials chemistry to gain fundamental understanding of various reaction and phase transition dynamics at nanometer resolution. However, quantitative extraction of physical and chemical parameters from the liquid-phase TEM videos remains bottlenecked by the lack of automated analysis methods compatible with the videos' high noisiness and spatial heterogeneity. Here, we integrate, for the first time, liquid-phase TEM imaging with our customized analysis framework based on a machine learning model called U-Net neural network. This combination is made possible by our workflow to generate simulated TEM images as the training data with well-defined ground truth. We apply this framework to three typical systems of colloidal nanoparticles, concerning their diffusion and interaction, reaction kinetics, and assembly dynamics, all resolved in real-time and real-space by liquid-phase TEM. A diversity of properties for differently shaped anisotropic nanoparticles are mapped, including the anisotropic interaction landscape of nanoprisms, curvature-dependent and staged etching profiles of nanorods, and an unexpected kinetic law of first-order chaining assembly of concave nanocubes. These systems representing properties at the nanoscale are otherwise experimentally inaccessible. Compared to the prevalent image segmentation methods, U-Net shows a superior capability to predict the position and shape boundary of nanoparticles from highly noisy and fluctuating background-a challenge common and sometimes inevitable in liquid-phase TEM videos. We expect our framework to push the potency of liquid-phase TEM to its full quantitative level and to shed insights, in high-throughput and statistically significant fashion, on the nanoscale dynamics of synthetic and biological nanomaterials.

    View details for DOI 10.1021/acscentsci.0c00430

    View details for PubMedID 32875083

    View details for PubMedCentralID PMC7453571

  • "Colloid-Atom Duality" in the Assembly Dynamics of Concave Gold Nanoarrows. Journal of the American Chemical Society Liu, C., Ou, Z., Guo, F., Luo, B., Chen, W., Qi, L., Chen, Q. 2020; 142 (27): 11669-11673


    We use liquid-phase transmission electron microscopy (TEM) to study self-assembly dynamics of charged gold nanoarrows (GNAs), which reveal an unexpected "colloid-atom duality". On one hand, they assemble following the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory for colloids when van der Waals attraction overruns slightly screened electrostatic repulsion. Due to concaveness in shape, GNAs adopt zipper motifs with lateral offset in their assembly matching with our modeling of inter-GNA interaction, which form into unconventional structures resembling degenerate crystals. On the other hand, further screening of electrostatic repulsion leads to merging of clusters assembled from GNAs, reminiscent of the coalescence growth mode in atomic crystals driven by minimization of surface energy, as we measure from the surface fluctuation of clusters. Liquid-phase TEM captures the initial formation of highly curved necks bridging the two clusters. Analysis of the real-time evolution of neck width illustrates the first-time observation of coalescence in colloidal assemblies facilitated by rapid surface diffusion of GNAs. We attribute the duality to the confluence of factors (e.g., nanoscale colloidal interaction, diffusional dynamics) that we access by liquid-phase TEM, taking turns to dominate at different conditions, which is potentially generic to the nanoscale. The atom aspect, in particular, can inspire utilization of atomic crystal synthesis strategies to encode structure and dynamics in nanoscale assembly.

    View details for DOI 10.1021/jacs.0c04444

    View details for PubMedID 32543864

  • Kinetic pathways of crystallization at the nanoscale NATURE MATERIALS Ou, Z., Wang, Z., Luo, B., Luijten, E., Chen, Q. 2020; 19 (4): 450-+


    Nucleation and growth are universally important in systems from the atomic to the micrometre scale as they dictate structural and functional attributes of crystals. However, at the nanoscale, the pathways towards crystallization have been largely unexplored owing to the challenge of resolving the motion of individual building blocks in a liquid medium. Here we address this gap by directly imaging the full transition of dispersed gold nanoprisms to a superlattice at the single-particle level. We utilize liquid-phase transmission electron microscopy at low dose rates to control nanoparticle interactions without affecting their motions. Combining particle tracking with Monte Carlo simulations, we reveal that positional ordering of the superlattice emerges from orientational disorder. This method allows us to measure parameters such as line tension and phase coordinates, charting the nonclassical nucleation pathway involving a dense, amorphous intermediate. We demonstrate the versatility of our approach via crystallization of different nanoparticles, pointing the way to more general applications.

    View details for DOI 10.1038/s41563-019-0514-1

    View details for Web of Science ID 000521751700020

    View details for PubMedID 31659295

  • Monolithic mtesla-level magnetic induction by self-rolled-up membrane technology. Science advances Huang, W., Yang, Z., Kraman, M. D., Wang, Q., Ou, Z., Rojo, M. M., Yalamarthy, A. S., Chen, V., Lian, F., Ni, J. H., Liu, S., Yu, H., Sang, L., Michaels, J., Sievers, D. J., Eden, J. G., Braun, P. V., Chen, Q., Gong, S., Senesky, D. G., Pop, E., Li, X. 2020; 6 (3): eaay4508


    Monolithic strong magnetic induction at the mtesla to tesla level provides essential functionalities to physical, chemical, and medical systems. Current design options are constrained by existing capabilities in three-dimensional (3D) structure construction, current handling, and magnetic material integration. We report here geometric transformation of large-area and relatively thick (~100 to 250 nm) 2D nanomembranes into multiturn 3D air-core microtubes by a vapor-phase self-rolled-up membrane (S-RuM) nanotechnology, combined with postrolling integration of ferrofluid magnetic materials by capillary force. Hundreds of S-RuM power inductors on sapphire are designed and tested, with maximum operating frequency exceeding 500 MHz. An inductance of 1.24 muH at 10 kHz has been achieved for a single microtube inductor, with corresponding areal and volumetric inductance densities of 3 muH/mm2 and 23 muH/mm3, respectively. The simulated intensity of the magnetic induction reaches tens of mtesla in fabricated devices at 10 MHz.

    View details for DOI 10.1126/sciadv.aay4508

    View details for PubMedID 32010770

  • Nonclassical Crystallization Observed by Liquid-Phase Transmission Electron Microscopy Crystallization via Nonclassical Pathways Liu, C., Ou, Z., Zhou, S., Chen, Q. American Chemical Society. 2020: 115–146
  • Imaging how thermal capillary waves and anisotropic interfacial stiffness shape nanoparticle supracrystals. Nature communications Ou, Z. n., Yao, L. n., An, H. n., Shen, B. n., Chen, Q. n. 2020; 11 (1): 4555


    Development of the surface morphology and shape of crystalline nanostructures governs the functionality of various materials, ranging from phonon transport to biocompatibility. However, the kinetic pathways, following which such development occurs, have been largely unexplored due to the lack of real-space imaging at single particle resolution. Here, we use colloidal nanoparticles assembling into supracrystals as a model system, and pinpoint the key role of surface fluctuation in shaping supracrystals. Utilizing liquid-phase transmission electron microscopy, we map the spatiotemporal surface profiles of supracrystals, which follow a capillary wave theory. Based on this theory, we measure otherwise elusive interfacial properties such as interfacial stiffness and mobility, the former of which demonstrates a remarkable dependence on the exposed facet of the supracrystal. The facet of lower surface energy is favored, consistent with the Wulff construction rule. Our imaging-analysis framework can be applicable to other phenomena, such as electrodeposition, nucleation, and membrane deformation.

    View details for DOI 10.1038/s41467-020-18363-2

    View details for PubMedID 32917872

  • Charting the quantitative relationship between two-dimensional morphology parameters of polyamide membranes and synthesis conditions MOLECULAR SYSTEMS DESIGN & ENGINEERING An, H., Smith, J. W., Chen, W., Ou, Z., Chen, Q. 2020; 5 (1): 102–9

    View details for DOI 10.1039/c9me00132h

    View details for Web of Science ID 000508398900035

  • Nanoscale Cinematography of Soft Matter System under Liquid-Phase TEM Accounts of Materials Research Ou, Z., Liu, C., Yao, L., Chen, Q. 2020
  • Effects of Particle Size on Mg2+ Ion Intercalation into lambda-MnO2 Cathode Materials NANO LETTERS Chen, W., Zhan, X., Luo, B., Ou, Z., Shih, P., Yao, L., Pidaparthy, S., Patra, A., An, H., Braun, P. V., Stephens, R. M., Yang, H., Zuo, J., Chen, Q. 2019; 19 (7): 4712–20


    An emergent theme in mono- and multivalent ion batteries is to utilize nanoparticles (NPs) as electrode materials based on the phenomenological observations that their short ion diffusion length and large electrode-electrolyte interface can lead to improved ion insertion kinetics compared to their bulk counterparts. However, the understanding of how the NP size fundamentally relates to their electrochemical behaviors (e.g., charge storage mechanism, phase transition associated with ion insertion) is still primitive. Here, we employ spinel λ-MnO2 particles as a model cathode material, which have effective Mg2+ ion intercalation but with their size effect poorly understood to investigate their operating mechanism via a suite of electrochemical and structural characterizations. We prepare two differently sized samples, the small nanoscopic λ-MnO2 particles (81 ± 25 nm) and big micron-sized ones (814 ± 207 nm) via postsynthesis size-selection. Analysis of the charge storage mechanisms shows that the stored charge from Mg2+ ion intercalation dominates in both systems and is ∼10 times higher in small particles than that in the big ones. From both X-ray diffraction and atomic-resolution scanning transmission electron microscopy imaging, we reveal a fundamental difference in phase transition of the differently sized particles during Mg2+ ion intercalation: the small NPs undergo a solid-solution-like phase transition which minimizes lattice mismatch and energy penalty for accommodating new phases, whereas the big particles follow conventional multiphase transformation. We show that this pathway difference is related to the improved electrochemical performance (e.g., rate capability, cycling performance) of small particles over the big ones which provides important insights in encoding within the particle dimension, that is, the single-phase transition pathway in high-performance electrode materials for multivalent ion batteries.

    View details for DOI 10.1021/acs.nanolett.9b01780

    View details for Web of Science ID 000475533900064

    View details for PubMedID 31251071

  • Hierarchical self-assembly of 3D lattices from polydisperse anisometric colloids NATURE COMMUNICATIONS Luo, B., Kim, A., Smith, J. W., Ou, Z., Wu, Z., Kim, J., Chen, Q. 2019; 10: 1815


    Colloids are mainly divided into two types defined by size. Micron-scale colloids are widely used as model systems to study phase transitions, while nanoparticles have physicochemical properties unique to their size. Here we study a promising yet underexplored third type: anisometric colloids, which integrate micrometer and nanometer dimensions into the same particle. We show that our prototypical system of anisometric silver plates with a high polydispersity assemble, unexpectedly, into an ordered, three-dimensional lattice. Real-time imaging and interaction modeling elucidate the crucial role of anisometry, which directs hierarchical assembly into secondary building blocks-columns-which are sufficiently monodisperse for further ordering. Ionic strength and plate tip morphology control the shape of the columns, and therefore the final lattice structures (hexagonal versus honeycomb). Our joint experiment-modeling study demonstrates potentials of encoding unconventional assembly in anisometric colloids, which can likely introduce properties and phase behaviors inaccessible to micron- or nanometer-scale colloids.

    View details for DOI 10.1038/s41467-019-09787-6

    View details for Web of Science ID 000464979600009

    View details for PubMedID 31000717

    View details for PubMedCentralID PMC6472373

  • Reconfigurable nanoscale soft materials CURRENT OPINION IN SOLID STATE & MATERIALS SCIENCE Ou, Z., Kim, A., Huang, W., Braun, P. V., Li, X., Chen, Q. 2019; 23 (1): 41–49
  • Liquid-phase TEM imaging of self-assembly pathways of anisotropic nanoparticles Microscopy and Microanalysis Ou, Z., Luo, B., Liu, C., Chen, Q. 2019; 25 (S2): 1414-1415
  • Synthesis and Self-Assembly of Janus and Triblock Patchy Particles Self-Assembly of Nano- and Micro-structured Materials using Colloidal Engineering Ou, Z., Luo, B., Neophytou, A., Chakrabarti, D., Chen, Q. Elsevier . 2019: 61–85
  • Colloidal Metal-Organic Framework Hexapods Prepared from Postsynthesis Etching with Enhanced Catalytic Activity and Rollable Packing ACS APPLIED MATERIALS & INTERFACES Ou, Z., Song, X., Huang, W., Jiang, X., Qu, S., Wang, Q., Braun, P., Moore, J. S., Li, X., Chen, Q. 2018; 10 (48): 40990–95


    Recent studies on the effect of particle shapes have led to extensive applications of anisotropic colloids as complex materials building blocks. Although much research has been devoted to colloids of convex polyhedral shapes, branched colloids remain largely underexplored because of limited synthesis strategies. Here we achieved the preparation of metal-organic framework (MOF) colloids in a hexapod shape, not directly from growth but from postsynthesis etching of truncated rhombic dodecahedron (TRD) parent particles. To understand the branch development, we used in situ optical microscopy to track the local surface curvature evolution of the colloids as well as facet-dependent etching rate. The hexapods show unique properties, such as improved catalytic activity in a model Knoevenagel reaction likely due to enhanced access to active sites, and the assembly into open structures which can be easily integrated with a self-rolled-up nanomembrane structure. Both the postsynthesis etching and the hexapod colloids demonstrated here show a new route of engineering micrometer-sized building blocks with exotic shapes and intrinsic functionalities originated from the molecular structure of materials.

    View details for DOI 10.1021/acsami.8b17477

    View details for Web of Science ID 000452694100005

    View details for PubMedID 30398328

  • Reconfigurable Polymer Shells on Shape-Anisotropic Gold Nanoparticle Cores MACROMOLECULAR RAPID COMMUNICATIONS Kim, J., Song, X., Kim, A., Luo, B., Smith, J. W., Ou, Z., Wu, Z., Chen, Q. 2018; 39 (14): e1800101


    Reconfigurable hybrid nanoparticles made by decorating flexible polymer shells on rigid inorganic nanoparticle cores can provide a unique means to build stimuli-responsive functional materials. The polymer shell reconfiguration has been expected to depend on the local core shape details, but limited systematic investigations have been undertaken. Here, two literature methods are adapted to coat either thiol-terminated polystyrene (PS) or polystyrene-poly(acrylic acid) (PS-b-PAA) shells onto a series of anisotropic gold nanoparticles of shapes not studied previously, including octahedron, concave cube, and bipyramid. These core shapes are complex, rendering shell contours with nanoscale details (e.g., local surface curvature, shell thickness) that are imaged and analyzed quantitatively using the authors' customized analysis codes. It is found that the hybrid nanoparticles based on the chosen core shapes, when coated with the above two polymer shells, exhibit distinct shell segregations upon a variation in solvent polarity or temperature. It is demonstrated for the PS-b-PAA-coated hybrid nanoparticles, the shell segregation is maintained even after a further decoration of the shell periphery with gold seeds; these seeds can potentially facilitate subsequent deposition of other nanostructures to enrich structural and functional diversity. These synthesis, imaging, and analysis methods for the hybrid nanoparticles of anisotropically shaped cores can potentially aid in their predictive design for materials reconfigurable from the bottom up.

    View details for DOI 10.1002/marc.201800101

    View details for Web of Science ID 000439816900022

    View details for PubMedID 29722094

  • Imaging the polymerization of multivalent nanoparticles in solution NATURE COMMUNICATIONS Kim, J., Ou, Z., Jones, M. R., Song, X., Chen, Q. 2017; 8: 761


    Numerous mechanisms have been studied for chemical reactions to provide quantitative predictions on how atoms spatially arrange into molecules. In nanoscale colloidal systems, however, less is known about the physical rules governing their spatial organization, i.e., self-assembly, into functional materials. Here, we monitor real-time self-assembly dynamics at the single nanoparticle level, which reveal marked similarities to foundational principles of polymerization. Specifically, using the prototypical system of gold triangular nanoprisms, we show that colloidal self-assembly is analogous to polymerization in three aspects: ensemble growth statistics following models for step-growth polymerization, with nanoparticles as linkable "monomers"; bond angles determined by directional internanoparticle interactions; and product topology determined by the valency of monomeric units. Liquid-phase transmission electron microscopy imaging and theoretical modeling elucidate the nanometer-scale mechanisms for these polymer-like phenomena in nanoparticle systems. The results establish a quantitative conceptual framework for self-assembly dynamics that can aid in designing future nanoparticle-based materials.Few models exist that describe the spontaneous organization of colloids into materials. Here, the authors combine liquid-phase TEM and single particle tracking to observe the dynamics of gold nanoprisms, finding that nanoscale self-assembly can be understood within the framework of atomic polymerization.

    View details for DOI 10.1038/s41467-017-00857-1

    View details for Web of Science ID 000412053100007

    View details for PubMedID 28970557

    View details for PubMedCentralID PMC5624893

  • Polymerization-Like Co-Assembly of Silver Nanoplates and Patchy Spheres ACS NANO Lao, B., Smith, J. W., Wu, Z., Kim, J., Ou, Z., Chen, Q. 2017; 11 (8): 7626–33


    Highly anisometric nanoparticles have distinctive mechanical, electrical, and thermal properties and are therefore appealing candidates for use as self-assembly building blocks. Here, we demonstrate that ultra-anisometric nanoplates, which have a nanoscale thickness but a micrometer-scale edge length, offer many material design capabilities. In particular, we show that these nanoplates "copolymerize" in a predictable way with patchy spheres (Janus and triblock particles) into one- and two-dimensional structures with tunable architectural properties. We find that, on the pathway to these structures, nanoplates assemble into chains following the kinetics of molecular step-growth polymerization. In the same mechanistic framework, patchy spheres control the size distribution and morphology of assembled structures, by behaving as monofunctional chain stoppers or multifunctional branch points during nanoplate polymerization. In addition, both the lattice constant and the stiffness of the nanoplate assemblies can be manipulated after assembly. We see highly anisometric nanoplates as one representative of a broader class of dual length-scale nanoparticles, with the potential to enrich the library of structures and properties available to the nanoparticle self-assembly toolbox.

    View details for DOI 10.1021/acsnano.7b02059

    View details for Web of Science ID 000408520900009

    View details for PubMedID 28715193

  • Quantifying the Self-Assembly Behavior of Anisotropic Nanoparticles Using Liquid-Phase Transmission Electron Microscopy ACCOUNTS OF CHEMICAL RESEARCH Luo, B., Smith, J. W., Ou, Z., Chen, Q. 2017; 50 (5): 1125–33


    For decades, one of the overarching objectives of self-assembly science has been to define the rules necessary to build functional, artificial materials with rich and adaptive phase behavior from the bottom-up. To this end, the computational and experimental efforts of chemists, physicists, materials scientists, and biologists alike have built a body of knowledge that spans both disciplines and length scales. Indeed, today control of self-assembly is extending even to supramolecular and molecular levels, where crystal engineering and design of porous materials are becoming exciting areas of exploration. Nevertheless, at least at the nanoscale, there are many stones yet to be turned. While recent breakthroughs in nanoparticle (NP) synthesis have amassed a vast library of nanoscale building blocks, NP-NP interactions in situ remain poorly quantified, in large part due to technical and theoretical impediments. While increasingly many applications for self-assembled architectures are being demonstrated, it remains difficult to predict-and therefore engineer-the pathways by which these structures form. Here, we describe how investigations using liquid-phase transmission electron microscopy (TEM) have begun to play a role in pursuing some of these long-standing questions of fundamental and far-reaching interest. Liquid-phase TEM is unique in its ability to resolve the motions and trajectories of single NPs in solution, making it a powerful tool for studying the dynamics of NP self-assembly. Since 2012, liquid-phase TEM has been used to investigate the self-assembly behavior of a variety of simple, metallic NPs. In this Account, however, we focus on our work with anisotropic NPs, which we show to have very different self-assembly behavior, and especially on how analysis methods we and others in the field are developing can be used to convert their motions and trajectories revealed by liquid-phase TEM into quantitative understanding of underlying interactions and dynamics. In general, liquid-phase TEM studies may help bridge enduring gaps in the understanding and control of self-assembly at the nanoscale. For one, quantification of NP-NP interactions and self-assembly dynamics will inform both computational and statistical mechanical models used to describe nanoscale phenomena. Such understanding will also lay the groundwork for establishing new and generalizable thermodynamic and kinetic design rules for NP self-assembly. Synergies with NP synthesis will enable investigations of building blocks with novel, perhaps even evolving or active behavior. Moreover, in the long run, we foresee the possibility of applying the guidelines and models of fundamental nanoscale interactions which are uncovered under liquid-phase TEM to biological and biomimetic systems at similar dimensions.

    View details for DOI 10.1021/acs.accounts.7b00048

    View details for Web of Science ID 000401674600001

    View details for PubMedID 28443654

  • In Situ Electron Microscopy Imaging and Quantitative Structural Modulation of Nanoparticle Superlattices ACS NANO Kim, J., Jones, M. R., Ou, Z., Chen, Q. 2016; 10 (11): 9801–8


    We use liquid-phase transmission electron microscopy (LP-TEM) to characterize the structure and dynamics of a solution-phase superlattice assembled from gold nanoprisms at the single particle level. The lamellar structure of the superlattice, determined by a balance of interprism interactions, is maintained and resolved under low-dose imaging conditions typically reserved for biomolecular imaging. In this dose range, we capture dynamic structural changes in the superlattice in real time, where contraction and smaller steady-state lattice constants are observed at higher electron dose rates. Quantitative analysis of the contraction mechanism based on a combination of direct LP-TEM imaging, ensemble small-angle X-ray scattering, and theoretical modeling allows us to elucidate: (1) the superlattice contraction in LP-TEM results from the screening of electrostatic repulsion due to as much as a 6-fold increase in the effective ionic strength in the solution upon electron beam illumination; and (2) the lattice constant serves as a means to understand the mechanism of the in situ interaction modulation and precisely calibrate electron dose rates with the effective ionic strength of the system. These results demonstrate that low-dose LP-TEM is a powerful tool for obtaining structural and kinetic properties of nanoassemblies in liquid conditions that closely resemble real experiments. We anticipate that this technique will be especially advantageous for those structures with heterogeneity or disorder that cannot be easily probed by ensemble methods and will provide important insight that will aid in the rational design of sophisticated reconfigurable nanomaterials.

    View details for DOI 10.1021/acsnano.6b05270

    View details for Web of Science ID 000388913100007

    View details for PubMedID 27723304