Screening of Complex Nanomaterials Through Combinatorial Libraries

Kluender, Edward James

doi:https://doi.org/10.21985/n2-60qd-6d13

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Screening of Complex Nanomaterials Through Combinatorial Libraries

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As humans have evolved over time, so have the materials that they rely on. From the Stone Age through the Iron Age, entire eras have been defined by specific materials, their corresponding properties, and the applications they have enabled. This is still true to this day, as the Silicon Age has led way to the Nano Age. The discovery of new materials has traditionally been a slow process. For nanoscale materials, this has been especially true due to two significant hurdles: synthetic complexity and a massive parameter space. The synthesis of nanoparticles with precise control over size, composition, and shape is extremely challenging because when working on the nanoscale small variations can produce large differences in material properties. Additionally, when one includes size as a variable, in addition to composition and phase structure, the parameter space that needs to be investigated increases exponentially. In this thesis, these challenges are addressed through a combinatorial nanomaterial synthesis method paired with equally high throughput characterization methods. By using large area nanolithography techniques, millions of nanoparticles were deposited onto a single substrate, with their sizes and compositions spatially encoded. Using these “Megalibraries” of nanomaterials, each nanoparticle’s chemical and physical properties were investigated. Specifically, three properties were explored: catalyzed carbon nanotube growth, heterogeneous catalysis (for both hydrogenation of organic molecules and electrochemical reduction of oxygen), and localized surface plasmon tuning. Chapter one provides an introduction to complex nanomaterial synthesis and characterization methods, establishing literature precedent for the importance of discovering new multi-component nanostructures. Included in this literature review, the need for new high throughput synthesis and screening methods is highlighted. While there have been many advances in the field of multicomponent nanomaterial screening, current methods have been limited in throughput and flexibility, preventing full exploration of the massive parameter space. Chapter two describes a method of synthesizing the nanoparticle Megalibraries. Using polymer pen lithography, attoliter volumes of a block copolymer solution coordinated with metal ions can be deposited onto a single substrate over cm-scale areas. To ensure that the deposition occurred in a uniform manner across the entire substrate, new polymer pen arrays were fabricated to overcome any defects in the pen array that would result in variations in the deposited material. With a reliable deposition method, the composition and quantity of the block copolymer ink on the pen array was systematically varied. Continuous gradients were explored using the overlapping linear regimes of two Gaussian spray systems. These novel spray coating methods produced dual gradients of size and composition allowing for the synthesis of 15,876 unique nanoparticle structures on a single substrate with size and composition spatially encoded. This method shrinks the massive material landscape to a single manageable sample. Chapter three explores these continuous gradients and each nanoparticle’s ability to catalyze the growth of single-walled carbon nanotubes. For screening platforms to be used at their maximum capacity, screening methods with a throughput comparable to the synthesis throughput are required. This was achieved by depositing the nanoparticle Megalibraries on top of thermally isolated micropillars for the laser-induced heating required to characterize their ability to grow single-walled carbon nanotubes in a high throughput manner. Using the particle synthesis methods described in chapter one and these screening methods, the highest resolution screen, with respect to nanoparticle composition, was performed, resulting in the discovery of Au3Cu as a new catalyst composition not previously known for this application. Chapter four describes the use of customized well plate reactors to isolate sections of the combinatorial gradient for heterogeneous catalysis. By using the proper materials and geometries, these reactors were used to study the catalytic activity of nanoparticles on a surface under extreme reaction conditions. This allowed for the study of hydrogenation, a reaction necessary for producing many pharmaceutical reagents as well as electrochemical oxygen reduction. For electrochemical reactions, a conductive glassy carbon substrate was developed to provide a uniform surface for the nanoparticles to be deposited on and analyzed. In chapter five, the localized surface plasmons of these multimetallic nanoparticles were studied. Due to the isolated nature of this synthesis technique (nanoparticles are microns away from one another) single particle studies are possible for optical characterization. To create desirable multi-plasmon interactions with isolated particles, a thermally stable nanoparticle on mirror substrate was developed to increase the intensity of the plasmons by an order of magnitude. Using multiple adhesion layers, a gold thin film was fabricated to retain its structure when heated above 500 ºC, a temperature that would traditionally dewet the film. Using this nanoparticle on mirror geometry, the first study of a multimetallic nanoparticle on mirror was performed, allowing for the surface enhanced Raman spectroscopy measurement of benzene dithiol using a single gold-silver alloy nanoparticle. Taken together, this thesis presents methodology that has significantly increased the rate of nanomaterial synthesis and characterization. In addition to the specific applications described above, the work can potentially impact almost every field of science that is dependent on new materials. While only bimetallic systems were studied for these three applications, up to septenary compositions are possible, and screening methods for other applications are actively being developed. Ultimately, this platform will increase the number of nanomaterials accessible to humankind and lead to the discovery of impactful nanomaterials over an accelerated time-frame.

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