Thermodynamic and Atomic Vibrational Properties of Metal Nanoparticles: Size, Support, and Adsorbate Effects

技术摘要：该项目旨在深入了解负载金属纳米颗粒的熔化和德拜温度，热膨胀系数，烧结行为和声子态密度的尺寸依赖性演变。随着纳米颗粒尺寸的减小，生产具有明确尺寸分布的样品变得越来越困难。尺寸的微小变化会使自由表面上的原子分数相对于与载体接触的原子分数产生很大的变化。预计小纳米颗粒会受到其局部环境的强烈影响，例如支撑材料和表面吸附物。本研究解决了这些挑战，通过高度控制的样品合成相结合，以产生大量的定义良好的有序金属纳米结构，通过反胶束封装和高度敏感的原位和非原位表征技术在共享的用户设施（布鲁克海文国家实验室-BNL和阿贡国家实验室），以及在PI的研究实验室。所研究的材料系统包括Pt和Fe纳米粒子支持强（Al 2 O3，SrTiO 3）和弱（SiO2）相互作用的基板上。本项目可用的表征技术有：（i）用于结构表征的原子力显微镜（AFM）、透射电子显微镜（TEM）和扫描隧道显微镜（STM），（ii）用于电子和化学表征的X射线光电子能谱（XPS），以及（iii）X射线吸收光谱（EXAFS和XANES）和核共振非弹性X射线散射（NRIXS）用于结构、热力学和晶格振动表征。本申请的智力价值依赖于：（i）我们目前对金属纳米颗粒3 nm的热性能的理解的提高，以及（ii）这些特性如何由于颗粒尺寸效应、纳米颗粒-吸附物和纳米颗粒-载体相互作用而改变。 本研究将有助于深入了解的作用，支持和纳米粒子烧结现象，温度依赖的原子有序-无序转变和结构相变涉及软声子模式的预处理。非技术摘要：纳米结构金属的异常热力学性质，如过热，负热膨胀和超低导热率，已被报道。这些影响的起源有很大的争议。这在一定程度上是由于这些系统的复杂性所带来的科学挑战，以及需要同时考虑许多变量的影响，例如纳米颗粒的几何形状和环境。通过这个项目，洞察的重要特性，如熔化温度，热膨胀系数和烧结行为，随着尺寸的减小，纳米粒子的演变将寻求。对纳米金属系统的热力学性质的更深入了解可能会导致纳米技术和材料科学的未来进步，包括改善纳米颗粒的热稳定性和操作制度，等离子体纳米天线中的热生成和分布，热电应用以及用于太阳能电池应用的纳米结构金属有机复合材料。 该项目将有助于在大学和国家实验室环境中培训博士和本科生，并将为两名K-12学生提供第一次研究经验。PI将与当地科学中心合作，组织一个名为“科学中的艺术，艺术中的科学”的展览，旨在通过视觉上吸引人的海报，包括拟议研究的显微镜图像，吸引公众对纳米科学领域最先进科学研究的关注。

TECHNICAL SUMMARY:This project aims to gain insight into the size-dependent evolution of the melting and Debye temperatures, thermal expansion coefficient, sintering behavior and phonon density of states of supported metallic nanoparticles. As nanoparticle size decreases, it becomes progressively more difficult to produce samples with a well-defined size distribution. Small changes in size produce a large change in the fraction of atoms at free surfaces relative to those in contact with the support. Small nanoparticles are expected to be strongly affected by their local environment, such as the supporting materials and surface adsorbates. This study addresses these challenges through a combination of highly controlled sample synthesis to produce large volumes of well-defined ordered metal nanostructures through inverse micelle encapsulation and highly sensitive in situ and ex situ characterization techniques at shared user facilities (Brookhaven National Laboratory-BNL and Argonne National Laboratory) as well as in the PI's research lab. The material systems under investigation include Pt and Fe nanoparticles supported on strongly (Al2O3, SrTiO3) and weakly (SiO2) interacting substrates. The characterization techniques available for this project are: (i) atomic force microscopy (AFM), transmission electron microscopy (TEM) and scanning tunneling microscopy (STM) for structural characterization, (ii) X-ray photoelectron spectroscopy (XPS) for electronic and chemical characterization, and (iii) X-ray absorption spectroscopy (EXAFS and XANES) and nuclear resonant inelastic X-ray scattering (NRIXS) for structural, thermodynamic and lattice-vibrational characterization. The intellectual merit of this application relies on: (i) the improvement of our current understanding of the thermal properties of metal nanoparticles 3 nm and (ii) how these characteristics change because of particle size-effects, nanoparticle-adsorbate and nanoparticle-support interactions. This study will contribute insight into the role of the support and its pre-treatment on nanoparticle sintering phenomena, temperature-dependent atomic order-disorder transitions and structural phase transitions involving soft phonon modes. NON-TECHNICAL SUMMARY:Anomalous thermodynamic properties, such as superheating, negative thermal expansion and ultra-low thermal conductivities, have been reported for nanostructured metals. The origins of these effects are heavily debated. This is due in part to the scientific challenge posed by the complexity of these systems and the need to consider the influence of a number of variables simultaneously, such as the nanoparticle geometry and environment. Through this project, insight into the evolution of important characteristics, such as the melting temperature, thermal expansion coefficient and sintering behavior, of nanoparticles with decreasing size will be sought. A deeper knowledge of the thermodynamic properties of nano-metallic systems may lead to future advances in nanotechnology and materials science, including improving the thermal stability and operation regime of nanoparticles, heat generation and distribution in plasmonic nano-antennae, thermoelectric applications, and nanostructured metal-organic composites for solar cell applications. This project will contribute to the training of PhD and undergraduate students in university and national laboratory settings and will provide the first research experience to two K-12 students. The PI will work with the local Science Center in the organization of an exhibit entitled "Art in Science, Science in Art," intended to draw the attention of the general public to state-of-the-art scientific research in the nanoscience area through visually appealing posters, including microscopy images from the proposed research.