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Fan, Hongyou

Publications and source records attributed to Fan, Hongyou.

At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Mechanics of Gold Nanoparticle Superlattices at High Hydrostatic Pressure

Pressure-driven assembly of ligand-grafted gold nanoparticle superlattices is a promising approach for fabricating gold nanostructures, such as nanowires and nanosheets. However, optimizing this fabrication method requires an understanding of the mechanics of their complex hierarchical assemblies at high pressures. We use molecular dynamics simulations to characterize the response of alkanethiol-grafted gold nanoparticle superlattices to applied hydrostatic pressures up to 15 GPa, and demonstrate that the internal mechanics significantly depend on ligand length. At low pressures, intrinsic voids govern the mechanics of pressure-induced compaction, and the dynamics of collapse of these voids under pressure depend significantly on ligand length. These microstructural observations correlate well with the observed trends in bulk modulus and elastic constants. For the shortest ligands at high pressures, coating failure leads to gold core-core contact, an augur of irreversible response and eventual sintering. This behavior was unexpected under hydrostatic loading, and was only observed for the shortest ligands.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Fabrication of Large-Area Arrays of Vertically Aligned Gold Nanorods

Anisotropic nanoparticles, such as nanorods and nanoprisms, enable packing of complex nanoparticle structures with different symmetry and assembly orientation, which result in unique functions. Despite previous extensive efforts, formation of large areas of oriented or aligned nanoparticle structures still remains a great challenge. In this paper, we report fabrication of large-area arrays of vertically aligned gold nanorods (GNR) through a controlled evaporation deposition process. We began with a homogeneous suspension of GNR and surfactants prepared in water. During drop casting on silicon substrates, evaporation of water progressively enriched the concentrations of the GNR suspension, which induces the balance between electrostatic interactions and entropically driven depletion attraction in the evaporating solution to produce large-area arrays of self-assembled GNR on the substrates. Electron microscopy characterizations revealed the formation of layers of vertically aligned GNR arrays that consisted of hexagonally close-packed GNR in each layer. Benefiting from the close-packed GNR arrays and their smooth topography, the GNR arrays exhibited a surface-enhanced Raman scattering (SERS) signal for molecular detection at a concentration as low as 10 –15 M. Because of the uniformity in large area, the GNR arrays exhibited exceptional detecting reproducibility and operability. Finally, this method is scalable and cost-effective and could lead to diverse packing structures and functions by variation of guest nanoparticles in the suspensions.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Formation of self-assembled gold nanoparticle supercrystals with facet-dependent surface plasmonic coupling

Metallic nanoparticles, such as gold and silver nanoparticles, can self-assemble into highly ordered arrays known as supercrystals for potential applications in areas such as optics, electronics, and sensor platforms. Here we report the formation of self-assembled 3D faceted gold nanoparticle supercrystals with controlled nanoparticle packing and unique facet-dependent optical property by using a binary solvent diffusion method. The nanoparticle packing structures from specific facets of the supercrystals are characterized by small/wide-angle X-ray scattering for detailed reconstruction of nanoparticle translation and shape orientation from mesometric to atomic levels within the supercrystals. We discover that the binary diffusion results in hexagonal close packed supercrystals whose size and quality are determined by initial nanoparticle concentration and diffusion speed. The supercrystal solids display unique facet-dependent surface plasmonic and surface-enhanced Raman characteristics. The ease of the growth of large supercrystal solids facilitates essential correlation between structure and property of nanoparticle solids for practical integrations.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Functionalized Block-Copolymer Templates for Synthesis and Shape Control of Quantum Dots

In this paper, a new quantum dot synthesis method based on metallic-block copolymer precursors was developed. The synthesis produced CdS QDs assembled into chains. This method provides a new model for the study of 1D QD chains to determine its effect on charge transport and optoelectronic coupling. This synthesis method was readily extended to other semiconductor materials including PbS and perovskites producing QDs of various shapes. Finally, it evidenced further promise of this synthesis method to assist in the assembly, shape and size control of various nanomaterials.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Synthesis of Self-Assembled Porphyrin Nanoparticle Photosensitizers

The use of nanoparticles as a potential building block for photosensitizers has recently become a focus of interest in the field of photocatalysis and photodynamic therapy. Porphyrins and their derivatives are effective photosensitizers due to extended π-conjugated electronic structure, high molar absorption from visible to near-infrared spectrum, and high singlet oxygen quantum yields as well as chemical versatility. In this work, we report a synthesis of self-assembled porphyrin nanoparticle photosensitizers using zinc meso-tetra(4-pyridyl)porphyrin (ZnTPyP) through a confined noncovalent self-assembly process. Scanning electron microscopy reveals formation of monodisperse cubic nanoparticles. UV–vis characterizations reveal that optical absorption of the nanoparticles exhibits a red shift due to noncovalent self-assembly of porphyrins, which not only effectively increase intensity of light absorption but also extend light absorption broadly covering visible light for enhanced photodynamic therapy. Electron spin-resonance spectroscopy (ESR) studies show the resultant porphyrin nanoparticles release a high yield of singlet oxygen. Nitric oxide (NO) coordinates to central metal Zn ions to form stabilized ZnTPyP@NO nanoparticles. We show that under light irradiation ZnTPyP@NO nanoparticles release highly reactive peroxynitrite molecules that exhibit enhanced antibacterial photodynamic therapy (APDT) activity. Finally, the ease of the synthesis of self-assembled porphyrin nanoparticles and light-triggered release of highly reactive moieties represent a completely different photosensitizer system for APDT application.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Solution Based Synthesis of Cs 4 PbBr 6 Perovskite Particles with High Luminescence and Stability

Low dimensional lead halide perovskite particles are of tremendous interest due to their size-tunable band gaps, low exciton binding energy, high absorption coefficients, outstanding quantum and photovoltaic efficiencies. In this paper, we report a new solution-based synthesis of stabilized Cs 4 PbBr 6 perovskite particles with high luminescence. This method requires only mild conditions and produces colloidal particles that are ideal for highly efficient solution-based device fabrications. The synthesized microstructures not only display outstanding luminescence quantum yield but also long term stability in atmospheric conditions. Partial halide substitutions were also demonstrated to extend photoluminescence spectra of the perovskite particles. Finally, this convenient synthesis and optical tunability of Cs 4 PbBr 6 perovskite particles will be advantageous for future applications of optoelectronic advices.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Surfactant-Assisted Synthesis of Tetragonal Porphyrin Microparticles

In this paper, in an effort to utilize their unique photoactive properties, porphyrin monomers were assembled into tetragonal microparticles by a surfactant-assisted neutralization method through the cooperative interactions between the porphyrin building blocks including π-π stacking, J-aggregation and metal-ligand coordination. Electron microscopy characterization in combination with x-ray diffraction confirmed the three-dimensional ordered tetragonal microstructures with stable crystalline frameworks and well defined external surface morphology. Optical absorption and fluorescence spectroscopy revealed enhanced absorbance properties as compared with the raw porphyrin material, favourable for chromophore excitation and energy transport. Finally, with active and responsive optical properties, these new porphyrin microparticles look to serve as promising components for a wide range of applications including sensing, diagnostics, solar cells, and optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Exploration of Processing Parameters of Vacuum Assisted Micelle Confinement Synthesis of Spherical CL-20 Microparticles

We recently developed a vacuum assisted micelle confinement synthesis for spherical microparticles of CL-20 with outstanding monodispersity. These microparticles are promising energetic material for explosive devices with enhanced and predictable performances. In this paper, to facilitate further development and application of this synthesis, the particle growth process was monitored by in-situ dynamic light scattering measurements. The result was interpreted by a finite element model to obtain critical parameters. Finally, these parameters were then used to predict the behavior and product quality of batch synthesis under various operation conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Self-Assembled One-Dimensional Porphyrin Nanostructures with Enhanced Photocatalytic Hydrogen Generation

There has been a widespread interest in the preparation of self-assembled porphyrin nanostructures and their ordered arrays, aiming to emulate natural light harvesting processes and energy storage and to develop new nanostructured materials for photocatalytic process. In this paper, we report controlled synthesis of one-dimensional porphyrin nanostructures such as nanorods and nanowires with well-defined self-assembled porphyrin networks that enable efficient energy transfer for enhanced photocatalytic activity in hydrogen generation. Preparation of these one-dimensional nanostructures is conducted through noncovalent self-assembly of porphyrins confined within surfactant micelles. X-ray diffraction and transmission electron microscopy results reveal that these one-dimensional nanostructures contain stable single crystalline structures with controlled interplanar separation distance. Optical absorption characterizations show that the self-assembly enables effective optical coupling of porphyrins, resulting in much more enhanced optical absorption in comparison with the original porphyrin monomers, and the absorption bands red shift to more extensive visible light spectrum. Finally, the self-assembled porphyrin network facilitates efficient energy transfer among porphyrin molecules and the delocalization of excited state electrons for enhanced photocatalytic hydrogen production under visible light.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2017

Synthesis of Carbon Dots with Multiple Color Emission by Controlled Graphitization and Surface Functionalization

Multiple-color-emissive carbon dots (CDots) have potential applications in various fields such as bioimaging, light-emitting devices, and photocatalysis. The majority of the current CDots to date exhibit excitation-wavelength-dependent emissions with their maximum emission limited at the blue-light region. In this paper, a synthesis of multiple-color-emission CDots by controlled graphitization and surface function is reported. The CDots are synthesized through controlled thermal pyrolysis of citric acid and urea. By regulating the thermal-pyrolysis temperature and ratio of reactants, the maximum emission of the resulting CDots gradually shifts from blue to red light, covering the entire light spectrum. Specifically, the emission position of the CDots can be tuned from 430 to 630 nm through controlling the extent of graphitization and the amount of surface functional groups, –COOH. The relative photoluminescence quantum yields of the CDots with blue, green, and red emission reach up to 52.6%, 35.1%, and 12.9%, respectively. Furthermore, it is demonstrated that the CDots can be uniformly dispersed into epoxy resins and be fabricated as transparent CDots/epoxy composites for multiple-color- and white-light-emitting devices. Finally, this research opens a door for developing low-cost CDots as alternative phosphors for light-emitting devices.

36 MATERIALS SCIENCE↗
DOE OSTI2017

pH-Dependent Assembly of Porphyrin–Silica Nanocomposites and Their Application in Targeted Photodynamic Therapy

Structurally controlled nanoparticles, such as core–shell nanocomposite particles by combining two or more compositions, possess enhanced or new functionalities that benefited from the synergistic coupling of the two components. In this paper, we report new nanocomposite particles with self-assembled porphyrin arrays as the core surrounded by amorphous silica as the shell. The synthesis of such nanocomposite nanoparticles was conducted through a combined surfactant micelle confined self-assembly and silicate sol–gel process using optically active porphyrin as a functional building block. Depending on kinetic conditions, these particles exhibit structure and function at multiple length scales and locations. At the molecular scale, the porphyrins as the building blocks provide well-defined macromolecular structures for noncovalent self-assembly and unique chemistry for high-yield generation of singlet oxygen for photodynamic therapy (PDT). On the nanoscale, controlled noncovalent interactions of the porphyrin building block result in an extensive self-assembled porphyrin network that enables efficient energy transfer and impressive fluorescence for cell labeling, evidenced by absorption and photoluminescence spectra. The thin silicate shell on the nanoparticle surface allows easy functionalization, and the resultant targeting porphyrin-silica nanocomposites can selectively destroy tumor cells upon receiving light irradiation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Regulating Multiple Variables To Understand the Nucleation and Growth and Transformation of PbS Nanocrystal Superlattices

Nanocrystals (NCs) can self-assemble into ordered superlattices with collective properties, but the ability for controlling NC assembly remains poorly understandable toward achievement of desired superlattice. This paper regulates several key variables of PbS NC assembly (e.g., NC concentration and solubility, solvent type, evaporation rate, seed mediation and thermal treatment), and thoroughly exploits the nucleation and growth as well as subsequent superlattice transformation of NC assembles and underneath mechanisms. PbS NCs in toluene self-assemble into a single face-centered-cubic (fcc) and body-centered-cubic (bcc) superlattice, respectively, at concentrations ≤17.5 and ≥70 mg/mL, but an intermediate concentration between them causes the coexistence of the two superlattices. Differently, NCs in hexane or chloroform self-assemble into only a single bcc superlattice. Distinct controls of NC assembly in solvent with variable concentrations confirm the NC concentration/solubility mediated nucleation and growth of superlattice, in which an evaporation-induced local gradient of NC concentration causes simultaneous nucleation of the two superlattices. The observation for the dense packing planes of NCs in fast growing fcc rather than bcc reveals the difference of entropic driving forces responsible for the two distinct superlattices. Decelerating the solvent evaporation does not amend the superlattice symmetry, but improves the superlattice crystallinity. In addition to shrinking the superlattice volume, thermal treatment also transforms the bcc to an fcc superlattice at 175 °C. Through a seed-meditated growth, the concentration-dependent superlattice does not change lattice symmetry over the course of continuous growth, whereas the newly nucleated secondary small nuclei through a concentration change have relatively higher surface energy and quickly dissolve in solution, providing additional NC sources for the ripening of the primarily nucleated larger and stable seeds. Finally, the observations under multiple controls of assembly parameters not only provide insights into the nucleation and growth as well as transformation of various superlattice polymorphs but also lay foundation for controlled fabrication of desired superlattice with tailored property.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗