Research
My research lies at the intersection between nanomaterials, soft matter, and optical microscopy. The diverse family of nano and soft materials (nanowires, quantum dots, polymers, etc.) offer a rich catalog of building blocks with unique optical, electrical, and catalytic properties. Furthermore, arrays of individual blocks can be linked together using DNA molecules forming an ordered macroscopic structure. The resulting long-range order is necessary for collective wave-matter interactions, giving rise to photonic crystals and metamaterials in the visible, infrared, and terahertz regime. My research aims to understand DNA-coordinated interactions in self-assembled structures, and synthesize hybrid assemblies comprising nanomaterials and colloidal soft matters. My work involves chemical synthesis, optical measurements, and numerical modeling.
Soft matter & DNA-directed assembly
Soft materials such as colloids, polymers, liquids, and bio-tissues are important materials systems in both technological applications and biological processes. Many successful applications hinge on self-assembly of individual material units and nanoscale engineering of interfacial interactions. Introduction of chemical functionalities can create soft materials that interact with or mimic biological systems. However, on-demand structural control of synthetic colloids remains a longstanding challenge. In my works, synthetic chemistry has been developed to functionalize micro/nano-scale organic/inorganic particles with selective DNA linkers, guiding the system towards stable self-assembly.
Image credit: Sophie Marbach
Interactions between DNA-coated colloids
Notably, in colloidal assemblies, the interaction between DNA strands is analogous to the inter-atomic potential that governs solid-state crystallization. DNA has emerged as a most versatile tool to drive targeted structure formation due to its programmability, which enables specific attractive binding with controllable interaction strength and length scale. However, control of disorder, defects, melting, and crystal growth is hindered by the lack of a microscopic understanding of DNA-mediated colloidal interactions. We aim to understand the nanoscale interactions between DNA-coated colloids and relate the microscopic interactions to their macroscopic behaviors like melting and diffusion. We explore how these interactions are affected by material structures such as DNA sequence, polymer length, grafting density, and complementary fraction.
Self-assembly of shaped metal/dielectric particles
The self-assembly of colloidal particles into highly-ordered structures offers great promise for functional materials. The assembly structure is determined by a combination of particle properties, such as size, shape, interactions, and diffusion rate. We explore new strategies that assemble metal/dielectric nanoparticles into complex structures. We utilize shape-control and DNA ligands to guide particles into specific attachments. We use in situ beamline X-ray diffractions to characterize and monitor the formation of superlattices (in collaboration with BNL Beamline).
Advanced optical microscopy
Optical microscopy offers a rich toolbox for versatile, non-invasive, in-situ characterizations in materials science and biochemistry. However, in many systems of interest (such as particle assembly, nanocrystal growth, and nanofluidics) the critical interactions occur at a nanometer length scale that is far below the optical diffraction-limit. I am interested in developing super-resolution techniques that can realize in-situ dynamics studies with nanometer-scale spatial resolution. In previous works, we have developed total internal reflection microscopes (TIRM) that take advantages of the out-of-plane evanescent field, and revealed nanoscale dynamics of DNA-linked particles along the vertical direction. Exploitation of horizontal evanescent fields and/or photon localization techniques will further empower us to study lateral dynamics. Additionally, the super-resolution microscopy capabilities will naturally extend to other areas such as cell motion, virus-antibody binding, drug deliveries, and heterogeneous catalysis.
in situ nano dynamics - total internal reflection microscopy (TIRM)
Total internal reflection microscopy (TIRM) is a powerful method for measuring the microscopic interactions of colloidal particles in a liquid suspension. Using evanescent waves as the illumination source, TIRM can resolve nanometer-scaled changes in the distance and the potential energy between a colloidal particle and substrate. I designed and built a customized TIRM with precise environment-control capabilities. I am interested in understanding single-colloid motions and interactions on a nanometer scale and their relationship with surface structures and environmental stimuli.
Effect of photon counting shot noise on total internal reflection microscopy
The spatial resolution of TIRM can be as small as 1 nm for distance measurement. However, the measured potential energy profile is subject to distortion from multiple factors. We use a combination of experimental and theoretical (Brownian dynamic simulations) studies to evaluate how various noise sources affect a TIRM measurement's resolution. We find that photon-counting shot noise has a critical role in determining the measured potential curve, and close-range interactions are particularly prone to this corruption. We establish a theoretical model to quantify the shot noise effect on a measured potential curve. We also explore approaches to mitigate the shot-noise effect and improve the TIRM resolution.
Nanomaterials have a massive impact on many modern technological applications. Due to the reduced size, they often exhibit exotic optical, electrical, mechanical, and catalytic properties. The extraordinary versatility of nanomaterial applications lies in the capability to control their physicochemical structures. I am interested in developing functional inorganic nanomaterials for energy, electronics, and environmental applications. My research emphasizes the targeted syntheses of function-directed structures that optimize specific performance. I aim to achieve synthetic routes that precisely control the material shape, size, composition, and surface types. Moreover, I focus on solution-based synthetic strategies to obtain high-throughput, low-cost, and sustainable material processes.
Functional nanomaterials
Nanowire transparent conductors
Metal nanowire mesh is considered the best candidate for replacing indium-tin-oxide as the next-generation transparent electrode material. Compared to conventional oxide-based conductors, nanowire mesh can be made with sustainable indium-free elements, e.g., copper, and has the advantage of superior mechanical flexibility and low processing cost. However, the significant challenges of nanowire electrodes are the strong light scattering (haze) and low oxidation/moisture resistance. We developed radical-based chemistries that achieve ultrathin diameters in metal nanowire syntheses. We also explore ways to engineer core-shell structures through various surface modifications to make air-resistive nanowires.
Syntheses of 0, 1, & 2 D nanomaterials
Depending on the number of dimensions that are not confined to the nanometer region, nanomaterials can be classified as 0-D (nanocrystals), 1-D (nanowires), or 2-D (nanosheet). The control of the shape and dimensions of the nanomaterials is a complex process that involves nucleation, surface atomic arrangement, chemical diffusion, ligand attachment, solvent environment, etc. We explore synthetic routes that precisely control the size, shape, and composition of inorganic nanomaterials.
Nano-engineering of Cu catalysts towards CO2 conversion
Electrocatalytic CO2 conversion to value-added products is an attractive means for mitigating the unsustainable rise in CO2 emissions. It provides simultaneous carbon fixation and renewable energy storage. Copper is uniquely active in the electrocatalytic reduction of CO2 to products beyond CO, such as CH4 and C2H4. We explore the selectivity trends for CO2 electroreduction on different copper nanostructures, including nanowires, nanocubes, and nanospheres. Particularly, we find Cu nanowires with 5-fold twinned structures exhibit high CH4 selectivity over other carbon products. This morphology-defined selectivity can be protected through surface engineering that maintains the shape of the nanowires while allowing the diffusion of CO2 molecules.