Publications

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We present that self-assembly of anisotropic plasmonic nanomaterials into ordered superstructures has become popular in nanoscience because of their unique anisotropic optical and electronic properties. Gold nanorods (GNRs) are a well-defined functional building block for fabrication of these superstructures. They possess important anisotropic plasmonic characteristics that are resulted from strong local electric field and responsive to visible and near infrared light, which attracts extensive attention in various fields, such as biomedical technologies, plasmon-enhanced spectroscopies, and optoelectronic devices. There are recent examples of assembling the GNRs into ordered arrays or superstructures through processes such as solvent evaporation, interfacial assembly. In this review, we describe recent progresses in the development of the self-assembled GNR arrays with focus on the formation of oriented GNR arrays on substrates. We discuss key driving forces such as van der Waals (vdW), electrostatic interactions, steric force, and depletion force. We survey different strategies and self-assembly processes of forming oriented GNR arrays. Lastly, we also overview the applications of the oriented GNR arrays in optoelectronic devices especially surface enhanced Raman scattering (SERS). At the end of this review, we briefly summarize the review and discuss future challenges and perspectives.

We report there has been widespread recent interest in self-assembly and synthesis of porphyrin and its derivatives-based ordered arrays aiming to emulate natural light-harvesting processes and energy storage. However, technologies that leverage the structural advantages of individual porphyrins have not been fully realized and have been limited by available synthesis methods. This article provides general perspectives on porphyrin and derivative chemistry, and discussions on surfactant-assisted cooperative self-assembly using amphiphilic surfactants and functional porphyrins and derivatives. The cooperative self-assembly amplifies the intrinsic advantages of individual porphyrins by engineering them into well-defined one-dimensional–three-dimensional (1D–3D) nanostructures. Surfactant-assisted self-assembly of amphiphilic surfactants and porphyrins has been utilized to form well-defined “micelle-like” nanostructures. Lastly, driven by intermolecular interactions, subsequent nucleation and growth confined within these nanostructures lead to the formation of 1D–3D ordered optically and electrically active nanomaterials with structure and function on multiple length scales.

Due to remarkable electronic property, optical transparency, and mechanical flexibility, monolayer molybdenum disulfide (MoS₂) has been demonstrated to be promising for electronic and optoelectronic devices. To date, the growth of high-quality and large-scale monolayer MoS₂ has been one of the main challenges for practical applications. In this paper, we present a MoS₂–OH bilayer-mediated method that can fabricate inch-sized monolayer MoS₂ on arbitrary substrates. This approach relies on a layer of hydroxide groups (-OH) that are preferentially attached to the (001) surface of MoS₂ to form a MoS₂–OH bilayer structure for growth of large-area monolayer MoS₂ during the growth process. Specifically, the hydroxide layer impedes vertical growth of MoS₂ layers along the [001] zone axis, promoting the monolayer growth of MoS₂, constrains growth of the MoS₂ monolayer only in the lateral direction into larger area, and effectively reduces sulfur vacancies and defects according to density functional theory calculations. Finally, the hydroxide groups advantageously prevent the MoS₂ from interface oxidation in air, rendering high-quality MoS₂ monolayers with carrier mobility up to ~30 cm² V^–1 s^–1. Using this approach, inch-sized uniform monolayer MoS₂ has been fabricated on the sapphire and mica and high-quality monolayer MoS₂ of single-crystalline domains exceeding 200 μm has been grown on various substrates including amorphous SiO₂ and quartz and crystalline Si, SiC, Si₃N₄, and graphene Finally, this method provides a new opportunity for the monolayer growth of other two-dimensional transition metal dichalcogenides such as WS₂ and MoSe₂.

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Nanoparticles; Supramolecular Chemistry; Materials Science Nanoparticles (NPs)of controlled size, shape, and composition are important building blocks for the next generation of devices. There are numerous recent examples of organizing uniformly sized NPs into ordered arrays or superstructures in processes such as solvent evaporation, heterogeneous solution assembly, Langmuir-Blodgett receptor-ligand interactions, and layer-by-layer assembly. This review summarizes recent progress in the development of surfactant-assisted cooperative self-assembly method using amphiphilic surfactants and NPs to synthesize new classes of highly ordered active nanostructures. Driven by cooperative interparticle interactions, surfactant-assisted NP nucleation and growth results in optically and electrically active nanomaterials with hierarchical structure and function. How the approach works with nanoscale materials of different dimensions into active nanostructures is discussed in details. Some applications of these self-assembled nanostructures in the areas of nanoelectronics, photocatalysis, and biomedicine are highlighted. Finally, we conclude with the current research progress and perspectives on the challenges and some future directions.

Porphyrins are vital pigments involved in biological energy transduction processes. Their abilities to absorb light, then convert it to energy, have raised the interest of using porphyrin nanoparticles as photosensitizers in photodynamic therapy. A recent study showed that self- assembled porphyrin-silica composite nanoparticles can selectively destroy tumor cells, but detection of the cellular uptake of porphyrin-silica composite nanoparticles was limited to imaging microscopy. Here we developed a novel method to rapidly identify porphyrin-silica composite nanoparticles using Atmospheric Solids Analysis Probe-Mass Spectrometry (ASAP-MS). ASAP-MS can directly analyze complex mixtures without the need for sample preparation. Porphyrin-silica composite nanoparticles were vaporized using heated nitrogen desolvation gas, and their thermo-profiles were examined to identify distinct mass- to-charge (M/Z) signatures. HeLa cells were incubated in growth media containing the nanoparticles, and after sufficient washing to remove residual nanoparticles, the cell suspension was loaded onto the end of ASAP glass capillary probe. Upon heating, HeLa cells were degraded and porphyrin-silica composite nanoparticles were released. Vaporized nanoparticles were ionized and detected by MS. The cellular uptake of porphyrin-silica composite nanoparticles was identified using this ASAP-MS method.

Controlling microscopic morphology of energetic materials is of significant interest for the improvement of their performance and production consistency. As an important insensitive high explosive material, triaminotrinitrobenzene (TATB) has attracted tremendous research effort for military grade explosives and propellants. In this study, a new, rapid and inexpensive synthesis method for monodispersed TATB microparticles based on micelle-confined precipitation was developed. Surfactant with proper hydrophilic-lipophilic balance value was found to be critical to the success of this synthesis. The morphology of the TATB microparticles can be tuned between quasi-spherical and faceted by controlling the speed of recrystallization.

Here, we present that lead iodide based perovskites are promising optoelectronic materials ideal for solar cells. Recently emerged perovskite nanocrystals (NCs) offer more advantages including improved size-tunable band gap, structural stability, and solvent-based processing. Here we report a simple surfactant-assisted two-step synthesis to produce monodisperse PbI₂ NCs which are then converted to methylammonium lead iodide perovskite NCs. Based on electron microscopy characterization, these NCs showed competitive monodispersity. Additionally, combined results from X-ray diffraction patterns, optical absorption, and photoluminescence confirmed the formation of high quality methylammonium lead iodide perovskite NCs. More importantly, by avoiding the use of hard-to-remove chemicals, the resulted perovskite NCs can be readily integrated in applications, especially solar cells through versatile solution/colloidal-based methods.

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.

Self-assembly of colloidal nanocrystals (NCs) into ordered superlattices (SLs) and supercrystals (SCs) enables new artificial NC solids for nanoelectronic and nanophotonic applications, which requires critical control of nucleation and growth conditions. Herein large SCs of PbS NCs up to ∼100 μm size were synthesized by two controlled self-assembly methods from NC solutions. Both translational symmetry and orientational ordering of the nanocrystals in the SCs were readily tuned by excess oleic acid ligands and antisolvents. Slow evaporation and the counterdiffusion method of solvents resulted in the formation of single SCs with two different SLs from the same PbS NCs: a face-centered cubic SL with weak yet complex orientational order or a body-centered cubic SL with strong and uniform particle orientation, respectively. The translational ordering was mainly determined by the effective shape of the NCs while the difference in orientational order was a result of the balance between ligand-ligand attraction and rotational entropy. The ease of the growth of large SC solids could lead to diverse NC systems and facilitate essential investigation of nanoparticle interactions and coupling based nanoelectronic and nanophotonic properties.

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The performance of energetic materials (EM) varies significantly across production lots due to the inability of current production methods to yield consistent morphology and size. Lot-to-lot variations and the inability to remake the needed characteristics that meet specification is costly, increases uncertainty, and creates additional risk in programs using these materials. There is thus a pressing need to more reliably formulate EMs with greater control of morphology. The goal of this project is to use the surfactant-assisted self-assembly to generate EM particles with welldefined size and external morphologies using triaminotrinitrobenzene (TATB) and hexanitrohexaazaisowurtzitane (CL-20) as these EMs are both prevalent in the stockpile and present interesting/urgent reprocessing challenges. We intend to understand fundamental science on how molecular packing influences EM morphology. We develop scale up fabrication of EM particles with controlled morphology, promising to eliminate inconsistent performance by providing a trusted and reproducible method to improve EMs for NW applications.

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.

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. Here, 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. This method is scalable and cost-effective and could lead to diverse packing structures and functions by variation of guest nanoparticles in the suspensions.

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 paper, 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. 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.

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