Ming-Siao Hsiao

Redox Exfoliation of Layered Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDs) have attracted considerable attention in a diverse array of applications due to the breadth of possible property suites relative to other low-dimensional nanomaterials (e.g., graphene, aluminosilicates). Here, we demonstrate an alternative methodology for the exfoliation of bulk crystallites of group V-VII layered TMDs under quiescent, benchtop conditions using mild redox chemistry. Anionic polyoxometalate species generated from edge sites adsorb to the TMD surface and create Coulombic repulsion that drives layer separation without the use of shear forces. This method is generalizable (MS2, MSe2, and MTe2) and effective in preparing high-concentration (>1 mg/mL) dispersions with narrow layer thickness distributions more rapidly and with safer reagents than alternative solution-based approaches. Finally, exfoliation of these TMDs is demonstrated in a range of solvent systems that were previously inaccessible due to large surface energy differences. These characteristics could be beneficial in the preparation of high-quality films and monoliths.

Highly Concentrated Seed-Mediated Synthesis of Monodispersed Gold Nanorods

The extremely large optical extinction coefficient of gold nanorods (Au-NRs) enables their use in a diverse array of technologies, rnging from plasmonic imaging, therapeutics and sensors, to large area coatings, filters, and optical attenuators. Development of the latter technologies has been hindered by the lack of cost-effective, large volume production. This is due in part to the low reactant concentration required for symmetry breaking in conventional seed-mediated synthesis. Direct scale up of laboratory procedures has limited viability because of excessive solvent volume, exhaustive postsynthesis purification processes, and the generation of large amounts of waste (e.g., hexadecyltrimethylammonium bromide(CTAB)). Following recent insights into the growth mechanism of Au-NRs and the role of seed development, we modify the classic seed-mediated synthesis via temporal control of seed and reactant concentration to demonstrate production of Au-NRs at more than 100-times the conventional concentration, while maintaining independent control and narrow distribution of nanoparticle dimensions, aspect ratio, and volume. Thus, gram scale synthesis of Au-NRs with prescribed aspect ratio and volume is feasible in a 100 mL reactor with 1/100th of organic waste relative to conventional approaches. Such scale-up techniques are crucial to cost-effectively meet the increased demand for large quantities of Au-NRs in emerging applications.

Microscopic Characterization of Fracture Mechanisms in Polystyrene Grafted Nanoparticle Assemblies: The Role of Film Thickness and Grafting Density

Assemblies of polymer-grafted “hairy” nanoparticles (HNPs) are of current interest for a wide array of mechanical, photonic and electrical applications. In contrast to nanoparticles dispersed in a free polymer matrix, the grafted polymer determines particle spacing and circumvents nanoparticle agglomeration. HNPs are prepared by surface-initiated ATRP/RAFT polymerization. The thickness (monolayer or multilayers), and the order of nanoparticles assembly in HNPs films are controlled by the solution concentration and preparation methods (drop-casting and flow coating assembly at the air-water interface). The extent to which these grafted polymers are entangled determines the robustness and strength of the HNP assembly. As these polymer-grafted “hairy” nanoparticles (HNPs) are a relatively new class of materials, investigation of fundamental failure mechanisms have been limited [1,2]. Here in, we discuss the craze (nucleation and growth) and crack (fracture) process in thin film assemblies of polystyrene-grafted HNPs under strain using static (bright field, HAADF-STEM, tomography) and insitu TEM and AFM techniques. Results show that crack processes dominant for HNPs assemblies with low grafting density, but crazing process occurs for assemblies with high grafting density. As with linear polystyrene, molecular weight and strain rate also impact the transition from crack to craze. These correlations between HNP architecture and assembly deformation and failure modes refine the HNP design space for the synthesis and fabrication of assemblies with excellent mechanical properties.

Model-based Iterative Reconstruction for Low-dose Electron Tomography

Bright-field TEM tomography of biological specimens typically requires low electron dosages in order to prevent sample degradation [1]. Reconstruction of low-dose tomography is challenging due to the inability of standard tomographic reconstruction algorithms like filtered back projection (FBP) to handle low signal-to-noise ratio data sets. Furthermore, the limited tilt angles associated with the data result in significant missing wedge artifacts with the conventional methods. Model-based iterative reconstruction (MBIR) algorithms have been shown to significantly improve the quality of reconstructions for High Angle Annular Dark-Field STEM tomography [2], and they have also enabled significant dosage reductions in medical tomography [3] while preserving the quality of reconstructions. MBIR includes an explicit model for the physics of image formation with a noise model for the detector, and a model for the unknown sample, while formulating the reconstruction as minimizing a cost function.

Aqueous Assembly of Oxide and Fluoride Nanoparticles into 3D Microassemblies

We demonstrate rapid [∼mm3/(h·L)] organic ligand-free self-assembly of three-dimensional, >50 μm single-domain microassemblies containing up to 107 individual aligned nanoparticles through a scalable aqueous process. Organization and alignment of aqueous solution-dispersed nanoparticles are induced by decreasing their pH-dependent surface charge without organic ligands, which could be temperature-sensitive or infrared light absorbing. This process is exhibited by transforming both dispersed iron oxide hydroxide nanorods and lithium yttrium fluoride nanoparticles into high packing density microassemblies. The approach is generalizable to nanomaterials with pH-dependent surface charge (e.g., oxides, fluorides, and sulfides) for applications requiring long-range alignment of nanostructures as well as high packing density.

In Situ Study of Mechanical Testing and Fracture Process of Glassy Polystyrene Grafted Nanoparticle Assembly: Impact of Film Thickness and Strain Rate

Three dimensional assemblies of polymer-grafted “hairy” nanoparticles (HNPs) are of current interest for a wide array of mechanical and electrical applications. The areal grafting density of the polymer chains on the nanoparticle surfaces, and the molecular weight of the grafted polymers, determine the resulting interparticle spacing and volume fraction of the polymer and nanoparticle constituents. Because the polymer is chemically grafted to the particle surface particle-particle contacts are eliminated, as opposed to a physical blending process in which nanoparticle agglomeration is typically observed. The mechanical properties of HNPs films are therefore dominated by the polymer-polymer entanglements and the ordering in the nanoparticle assembly. Previous studies of HNP thin films have largely relied on nanoindentation, due to the challenges of preparing sub-micron testing specimens [1]. The emergence of microelectromechanical systems (MEMS) techniques for fabricating devices and microscale specimen preparation using focus ion beam (FIB) allows for in situ study of failure process of HNP assemblies. Previous results in ex-situ tests [2] have shown that crack processes dominate deformation in HNP assemblies with low grafting density, but crazing occurs for assemblies with high grafting density. As with linear polystyrene, molecular weight and strain rate also impact the transition from crack to craze. These correlations between HNP architecture and subsequent failure mode refines the HNP design space for the synthesis and fabrication of assemblies with defined mechanical properties. In this study fundamental processes of craze initiation, the effect of film thickness, and the role of silica nanoparticles are examined in polystyrene-silica HNP assemblies synthesized using the grafting-from method.

Visualization of peptide-peptide interactions in FET biosensors with liquid-cell TEM

Graphene-based field effect transistor (g-FET) sensors have been broadly applied in detection of biological macromolecules, such as RNA, DNA, peptides, and small toxic compounds [1]. The specificity and selectivity rely heavily on graphene-functionalization with biological recognizing elements (BREs) and the characterization of BRE engagement with target molecule is therefore one of the critical steps in sensor development. Transmission electron microscopy (TEM), because of its ability to offer high spatial resolution in compared to other image-based approaches, is a useful tool for examining the dynamic process involved in sensor detection. Here we present our TEM studies on a Neuropeptide Y specific g-FET biosensor in a liquid environment.

Atomic level cleaning of poly-methyl-methacrylate residues from the graphene surface using radiolized water at high temperatures

Large-scale application of graphene requires its clean transfer from thin metal films, where it is grown via chemical vapor deposition (CVD), to any other substrates of interest. All the existing transfer methodologies, however, leave residues at different degrees on graphene surfaces and can only provide atomically clean graphene surfaces in areas as large as ∼10−4 μm2. Here, we transfer CVD-grown graphene using Poly-methyl-methacrylate (PMMA) and present a method that can atomically clean the PMMA residues from a larger surface area of graphene using radiolized water obtained via electron-water interaction at high temperatures. The cleaning process was monitored in-situ using an environmental-mode transmission electron microscopy and electron energy loss spectroscopy. These showed the effectiveness of PMMA removal over areas as large as ∼0.02 μm2, whose size was only limited by the size of the electron beam and the presence of inorganic residues after the transfer process. By overcoming these limitation...

In Situ TEM Characterization of Nanostructured Dielectrics

Polymer nanocomposites are being considered as potential materials for high energy capacitor due to the possibility of obtaining high dielectric breakdown strength characteristic of the organic matrix and large dielectric permittivity from inorganic filler [1,2]. However in most case the increase in the content of inorganic ceramic filler in polymer nanocomposite inevitably causes a decrease in the dielectric breakdown strength due to the agglomeration of fillers, the existence of defects, and inorganic-organic interfacial effects. Many studies have attempted to improve the compatibility of organic and inorganic, however in the large majority the dielectric breakdown strength is poor and the dielectric loss is high. Additionally, as nanostructured dielectrics are a relatively new class of materials for this application, reports of investigating fundamental mechanisms of dielectric breakdown for the dielectric nanocomposite on the nanoscale have been limited.