(3t) Nobel Materials Created from Nanocrystals through Self-Assembly and High-Pressure Chemistry

From energy materials to healthcare products, nanoscience has made a revolutionary impact on state-of-art materials creation both in industry and fundamental research. Well-designed nanomaterials possess superior properties compared to conventional materials, pushing forward the frontline of modern technologies. Nanomaterials are created either chemically or non-chemically. Non-chemical methods include lithography and 3d printing, which enable one to create nanometer-scale architectures. Compared to these technologies, chemical methods are indispensable when nanomaterials need to be designed at the molecular level. Particularly, inorganic nanochemistry offers abundant functionalities, thus, burgeoning possibilities of further state-of-art materials would be open up by smartly bridging inorganic chemistry and materials science.

In this poster presentation, I am going to talk about some of my research achievements at the intersection between materials science and nanochemistry. My research concentrates on nanocrystal/nanomaterials synthesis, high-pressure chemistry, and self-assembly. In these fields, I have published 21 papers and 2 patents, including recent ones in Science (2018, 362, 1396-1400) and Nature (2018, 561, 378-382), and Adv. Mater. (2017, 29, 1606666). My research has been featured in various media such as Science masthead, Department of Energy (DOE) News, National Science Foundation (NSF) News, and I am the awardee of the Material Research Society postdoc award 2019 fall.

We have developed a number of nanocrystals including CdSe-CdS core-shell truncated-tetrahedral quantum dots (TTQDs), perovskite nanocrystals, surface-engineered nanocrystals, and multicomponent nanocrystals. Among these nanocrystals we developed, I will especially discuss TTQDs which exhibited very interesting self-assemble property. We employed a core-shell synthesis method to achieve high uniformity in size and adjusted the reaction solution condition to transform nanocrystals into a truncated-tetrahedral shape. TEM measurements confirmed the size (~ c.a. 8 nm in the edge length) and the shape. Furthermore, the synthesized TTQDs possessed 4 main faces; 3 faces were covered with oleic acid and the other one was tethered with octadecylphosphonic acid (ODPA).

Using TTQDs, we have made a series of superlattices with distinguished structures from conventional nanocrystal superlattices. Specifically, four types of nanocrystal superlattices were discovered i.e. tetrahelix, 12-fold quasicrystal approximant, bcc-cluster single supercrystal, 10-fold quasicrystal superlattice. It is worth mentioning that our 10-fold quasicrystal superlattice is the first example of single-component quasicrystals. We thoroughly characterized these nanocrystal superlattices both in its nanocrystals’ spatial positions and orientations. Among structural- characterization techniques we employed, the “supercrystallography” technique is featured here. The “supercrystallography” is a newly developed synchrotron-based structural analysis technique and constitutes rotational X-ray scattering measurements (see Figure). The setup is similar to single-crystal X-ray diffraction technique, and by correlatively analyzing both 2d WAXS and SAXS (wide- and small-angle X-ray scattering) the whole structure of the nanocrystals superlattices both in the structure and orientation can be determined (see Figure). This technique makes measurements possible for samples with a large size (millimeter scale) for conventional techniques (for example, TEM can only apply for samples with a thickness of several hundred nanometers or less). Also, the supercrystallography technique enables one to decode too intricate superstructure for conventional techniques to decode before, thus, we expect that this techniques will facilitate further exploration of unveiled nanocrystal-based materials.

The detailed analysis of the 10-fold quasicrystal superlattice from TTQD led to a discovery of a new mathematical rule related to aperiodic tilings. Quasicrystals possess aperiodic structures that can be explained using geometrical “tiling rules” or tessellation. All the experimentally generated quasicrystals were explained using existing tiling rules, however, we found our 10-fold quasicrystals exhibited a totally new structure that was not even predicted mathematically or theoretically. Thus, we proposed a new tiling rule to generate our quasicrystal structure, and we named it “flexible polygon tiling rule”. This new tiling rule is mathematically robust and can produce quasicrystal patterns with an arbitrary symmetry (i.e. 8-, 10-, 12- and 14- fold).

As another research direction, we also use high pressure (1-50 GPa) to process nanocrystal-based materials and characterize them under extreme conditions. Correlative understanding of structure-property relationships of nanomaterials by using pressure as a tunable parameter allows us to have unique insights into the nanomaterials. Furthermore, unique phenomena happen to nanocrystal superlattices under high pressure, such as interparticle sintering and pressure-driven phase purification, which can be used as novel processing to improve nanomaterials’ properties. In the paper published in Adv. Mater. 2017, we demonstrated that a pressure process could enhance the optical properties of CsPbBr3 perovskite nanocrystals through the improvement of its crystallinity and surface reconstruction. We also investigated how CsPbBr3 perovskite nanocrystals behave under pressurizing process up to 15 GPa in its atomic crystal and superlattices structures.

Recently, we have been working on nanostructured materials where nanosized crystal domains exist in the materials. Grain-boundary conditions are a decisive factor for the materials’ properties, such as hardness, phonon conductivity, and catalytic property. However, the top-down methodologies cannot control the grain-boundary conditions precisely, which prevents one from designing materials that informed with targeted structure-properties relationships. In my current research project entitled “bulk grain boundary materials from nanocrystals,” I addressed this problem. The bottom-up method creates bulk grain-boundary materials from metal nanocrystals by employing surface treatment with inorganic ligands and pressure sintering process. Created bulk grain-boundary materials exhibited metallic features in its appearance, conductivity, and high hardness value originated from the Hall-Petch effect. This methodology allows one to create materials with tunable grain-boundary conditions at will. Taking the advantages, we are thrived to produce new materials such as superhard materials and thermoelectric materials.

In conclusion, we showed a series of my research achievements intervening in materials science and nanochemistry. We have created novel nanomaterials by employing various methods including nanocrystal synthesis, high-pressure chemistry, and self-assembly. We assessed the properties and structures of the produced materials precisely. We believe that nanochemistry would further help one creating novel nanomaterials, and address some of the issues demanded in various fields such as energy materials, biotechnology, and photonics materials.

Teaching Interests

I view teaching as an opportunity to transform lives. I have been continuously involved in teaching since I was a college student in Japan. In my Ph.D. program, I have four semesters of TA experiences, and as a mentor in my research group I have been continuously teaching subjects related to my research to my students. I also completed the Teaching Certificate I Program 2019 fall that Brown University Sheridan teaching center offered where I learned organization and communication skills in class. I have attracted positive feedback from my students.

My educational background is chemistry, thus I can teach subjects related to basic-level chemistry such as inorganic chemistry and analytical chemistry. Although I do not have educational background in chemical engineering or materials science, I consulted with textbooks of chemical engineering and materials science through my research activity, thus I am also familiar with these topics. My research intervene between materials science and chemistry, and I believe I am knowledgeable enough to teach subjects such as X-ray diffraction, electron microscopy, crystallography, and electrochemistry. As advanced topics possibly for graduate programs, I can develop classes like nanomaterials, nanochemistry, quasicrystals, and photochemistry.

In my class, I will be a communicative lecturer. In order to accurately know students’ need and progress, I will frequently conduct survey from students. Small quizzes in class will also be used to monitor students’ progress and keep them up. I will be crystal clear about my requirements, which is important to raise students’ studying motivation and keep the standard at an adequate level. I also will be descriptive about subjects that I teach. Chemistry is often recognized as an arcane subject, however, luckily, the subjects listed above can be related to daily phenomena (for example, although students may not know plasmon physics, they know metal has characteristic rusty appearance. This is great introduction when I teach surface plasmon resonance in nanochemistry class). I will also patiently work with students when they have a problem with the subject.

As a final note, equality, diversity, and inclusive environment is of my uttermost importance. As an international students, I truly appreciate these values and believe in the positive impact on society. I will provide supportive learning-environments for any sexual, racial, international, and physically disable minority groups. Therefore, the primary questions I, as a teacher, continue to ask myself are "What teaching philosophies and strategies should I employ to maximize student learning best?" and "How can my teaching help students'?" I provide a flexible environment for myself and students where students can communicate with me when they need help, advice, or guidance, so that I can try various pedagogy and approaches.