BRIGE: Thermal Transport in Single-Domain Three-Dimensional Colloidal Nanocrystal Superlattices

胶体纳米晶是一种表面结合了有机配体分子的无机纳米颗粒。由于配体分子之间的范德华相互作用，这些纳米晶体可以自组装成周期阵列。类似于晶体的原子晶格，这些纳米晶体组合被称为纳米晶体超晶格。这些超晶格以其光学和电学输运性质而闻名，但它们的热输运性质仍未被探索。我假设这些超晶格应该具有极低的热导率，因为：(I)较大的界面密度，(Ii)无机纳米颗粒核心与有机配体基质之间较大的声阻抗失配，以及(Iii)过滤由其微观结构引起的晶格振动(即声子)。声子是晶体振动能的量子，类似于光子，光子是电磁能量的量子。在非金属固体中，声子是主要的热传导方式。由于这些自组装纳米晶体的精细周期性，这些超晶格应该表现为声子晶体。声子晶体是一种具有周期性声阻抗变化的人工结构材料。这种周期性导致声子带隙，这阻止了声子在特定能量范围内的传播。声子晶体类似于众所周知的光子晶体，它利用折射率的周期性变化来产生光子带隙。声子晶体可以用于声子滤光器、波导、谐振器和超透镜。为了研究这些热输运假说，我将(1)合成单分散纳米晶体并将它们组装成单域三维纳米晶体超晶格，(2)实验测量它们的热导率随纳米晶体尺寸的变化，(3)通过数值确定它们的声子能带结构，从而识别它们作为声子晶体的潜力，以及(4)建立一种声子光谱装置，用于实验测量这些超晶格的声子带隙作为纳米晶体尺寸的函数。智能优点：这个提议通过研究纳米晶体中的热输运向前迈出了重要的一步，它们是一类未被探索的材料。它还从数值上探索了这些超晶格的声子晶体性质，并激发了未来对这一主题进行实验研究的建议。由于其超晶格周期较小，其声子禁带宽度应在100 GHz？1 THz范围内，这是目前报道最好的三维声子晶体的100倍。由于它们具有频率相关的声子输运性质，这些材料应该具有与传统固体截然不同的热传导性质。此外，这种类型的声子传输工程可能会导致热电和电子热管理方面的重大进步。广泛的影响：为了增强多样性以及整合研究和教育，我制定了一个全面的外展计划，针对所有教育级别的教学。该计划包括：(1)通过在课堂上以热能和相关主题为重点的互动演讲，吸引K-12学生的参与。本报告将包括将热传输和热电的研究主题整合在一起的动手演示。(2)设计胶体纳米晶体合成和表征的实验室单元，纳入社区学院的课程。包括研究工具的集成，如透射电子显微镜。(3)从代表性不足的群体中招募个人加入我的研究，并指导他们实现各自的职业目标。我对这些举措的计划包括衡量其有效性的指标。例如，将使用AAAS科学评估数据库中的概念知识问题对K-12倡议进行评估。这个数据库中的问题符合严格的心理测量学标准，并有大量数据集可用来比较服务接受者的答案。

Abstract#1227979Wang, RobertColloidal nanocrystals are inorganic nanoparticles with organic ligand molecules bonded to their surface. These nanocrystals can self-assemble into periodic arrays due to the van der Waals interactions between their ligand molecules. In analogy to the atomic lattice of a crystal, these nanocrystal assemblies are termed nanocrystal superlattices. These superlattices are best known for their optical and electronic transport properties, however their thermal transport properties remain unexplored. I hypothesize that these superlattices should have an extremely low thermal conductivity because of: (i) a large interfacial density, (ii) a large acoustic impedance mismatch between the inorganic nanoparticle cores and organic ligand matrix, and (iii) filtering of lattice vibrations (i.e. phonons) resulting from its microstructure. A phonon is a quantum of crystal vibrational energy and is analogous to the photon, which is a quantum of electromagnetic energy. In non-metallic solids, phonons are the dominant mode of heat conduction. Do to the exquisite periodicity of these self-assembled nanocrystals, these superlattices should behave as phononic crystals. Phononic crystals are artificially structured materials with periodic variations in acoustic impedance. This periodicity results in a phononic band gap, which forbids the propagation of phonons in a particular energy range. The phononic crystal is analogous to the well-known photonic crystal, which uses periodic variations in refractive index to create a photonic band gap. Phononic crystals can be used for phonon filters, waveguides, resonators, and superlenses.To investigate these thermal transport hypotheses, I will (1) Synthesize monodisperse nanocrystals and assemble them into single-domain three-dimensional nanocrystal superlattices, (2) Experimentally measure their thermal conductivity as a function of nanocrystal size, (3) Numerically determine their phonon band structure, and thereby identify their potential as phononic crystals, and (4) Create a phonon spectroscopy apparatus to be used in future proposals to experimentally measure the phonon band gap of these superlattices as a function of nanocrystal size.Intellectual Merit: This proposal takes significant steps forward by investigating thermal transport in nanocrystal superlattices, which are an unexplored class of materials. It also numerically explores the phononic crystal properties of these superlattices and motivates future proposals to experimentally study this topic. Due to the small ~10 nm superlattice periodicity, their phononic band gap should be in the 100 GHz ? 1 THz range, which is 100x higher than the best-reported 3-dimensional phononic crystal. Due to their frequency-dependent phonon transport properties, these materials should have drastically different heat conduction properties than conventional solids. Furthermore, this type of phonon transport engineering could lead to significant advances in thermoelectricity and electronics thermal management.Broader Impacts: To enhance diversity as well as integrate research and education, I have developed a comprehensive outreach plan that addresses instruction at all education levels. This plan includes: (1) Engaging K-12 students via interactive presentations on-site in their classrooms that focus on thermal energy and related topics. Included in this presentation will be hands-on demonstrations that integrate research topics on thermal transport and thermoelectricity. (2) Designing a laboratory module on colloidal nanocrystal synthesis and characterization for incorporation into curriculum at the community college level. The integration of research tools such as transmission electron microscopy is included. (3) Recruiting individuals from underrepresented groups into my research and mentoring them toward their respective career goals. My plan for these initiatives includes metrics to gauge their effectiveness. For example, the K-12 initiative will be assessed using concept knowledge questions from the AAAS Science Assessment database. The questions in this database meet rigorous psychometric standards and have a large data set with which to compare the answers of the service recipients.