Synthesis of and New Functionality in Heteroepitaxial Gallate / Ferrite Core@Shell Nanoparticles

非技术总结：晶体材料中的外延是一种材料在另一种材料上的规律生长，就像一层黄色的乐高积木在几层红砖上的生长一样。当两块砖的长度略有不同时，就会发生应变，从而使黄色层以稍大的间距（拉伸应变）或稍小的间距（压缩应变）生长。在二维薄膜中，外延应变可以产生巨大的物理性质变化，但这种类型的外延应变在球形纳米颗粒和其他纳米结构中尚未得到充分利用。在核/壳结构中生长纳米颗粒会导致不同物理性质的结合，类似于由巧克力和硬糖壳覆盖的花生组成的糖果与由糖壳覆盖的固体巧克力组成的糖果具有不同的味道（物理性质）。在材料研究部固态与材料化学项目的支持下，南佛罗里达大学达里奥·阿雷纳教授和他的团队将探索具有相同晶格结构但物理性质截然不同的核壳材料。核心将是一种氧化物（没食子酸锌），它具有光学特性，对生物医学成像很有用。外壳将是一种叫做磁铁矿的氧化铁，这种材料的某些磁性特征可以用来确认磁铁矿在没食子酸锌核心上的外延生长。实现这种外延光学主动核心和磁敏感外壳的结合，为高频电子、气体传感、环境修复和生物医学应用开辟了新的可能性，这些应用将诊断和治疗能力结合在单个纳米颗粒中。技术摘要：许多矿物和其他化合物的原子晶格中都采用尖晶石结构。在这个由美国国家科学基金会材料研究部固态和材料化学项目支持的项目中，将化学合成结合两种不同类型氧化尖晶石的核/壳纳米颗粒。没食子酸锌（ZnGa2O4）将形成核心，磁铁矿（Fe3O4）将成为外壳材料。没食子酸锌和磁铁矿具有相同的尖晶石晶体结构，这使得磁铁矿外壳能够在没食子酸锌芯上外延生长。没食子酸锌会在磁铁矿外壳上施加0.7%的压应变，这仍然是相对较弱的。在~105 K的温度下，通过检测样品磁矩的突然下降（Verwey跃迁），可以验证磁铁矿外壳中的高度外延。只有结晶度高且铁氧比合适的样品才会出现Verwey转变，磁强计为筛选有前途的合成策略提供了一种有效的方法。在表现出明显Verwey跃迁的样品中，外延将用先进的电子显微镜、x射线光谱学和散射以及中子散射技术进行验证。尖晶石铁氧体和没食子酸盐的这种组合以前从未被开发过，这种组合为高频电子、气体传感、环境修复和生物医学/治疗（诊断+治疗）应用开辟了新的可能性。该项目还将支持两名研究生的博士研究，并将有助于促进与一个或多个少数民族服务机构的研究生项目的合作。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical summary: Epitaxy in crystalline materials is the regular growth of one material on top of another, like the growth of a layer of yellow Lego bricks on top of several layers of red bricks. Strain occurs when the length of the two bricks is slightly different, so that the yellow layer grows with a slightly larger spacing (tensile strain) or slightly smaller (compressive). In two-dimensional thin films, epitaxial strain can produce dramatic variations in physical properties, but this type of epitaxial strain has been under-exploited in spherical nanoparticles and other nanostructures. Growing nanoparticles in a core / shell structure results in the combination of different physical properties, similar to how a candy comprised of a peanut covered with chocolate and hard sugar shell has a different flavor (a physical property) than a candy that is a solid piece of chocolate covered with the sugar shell. With support from the Solid State and Materials Chemistry program in the Division of Materials Research, Prof. Dario Arena and his team at the University of South Florida will explore core and shell materials with the same type of lattice structure, but very different physical properties. The core will be a type of oxide (zinc gallate) that has optical properties which are useful for biomedical imaging. The shell will be an iron oxide called magnetite and certain magnetic signatures of this material can be used to confirm epitaxial growth of the magnetite on the zinc gallate core. Realizing this combination of epitaxial optically-active cores and magnetically-sensitive shells opens up new possibilities for high-frequency electronics, gas sensing, environmental remediation, and biomedical applications that combine diagnostic + therapeutic capabilities in a single nanoparticle. Technical summary: Many minerals and other chemical compounds adopt the spinel structure in their atomic lattice. In this project, supported by the Solid State and Materials Chemistry program in the NSF’s Division of Materials Research, core / shell nanoparticles that combine two different types of oxide spinels will be chemically synthesized. Zinc gallate (ZnGa2O4) will form the core and magnetite (Fe3O4) will be the shell material. Zinc gallate and magnetite share the same spinel crystal structure which will enable the epitaxial growth of magnetite shell on the zinc gallate core. The zinc gallate will impart a compressive strain of 0.7% on the magnetite shell, which is still relatively weak. A high degree of epitaxy in the magnetite shell will be verified with temperature dependent magnetometry by identifying the Verwey transition (an abrupt drop in the sample magnetic moment) at ~105 K. Only samples with excellent crystallinity and which have the proper iron to oxygen ratio will exhibit the Verwey transition, and the magnetometry provides an efficient method of screening promising synthesis strategies. In samples that exhibit a sharp Verwey transition, the epitaxy will be verified with advanced electron microscopy, x-ray spectroscopy and scattering, and neutron scattering techniques. These combinations of spinel ferrites and gallates have not been grown before and the combination opens up new possibilities for high-frequency electronics, gas sensing, environmental remediation, and biomedical / theranostic (diagnostic + therapeutic) applications. The project will also support the PhD study of two graduate students and will help foster collaboration with the graduate program of a one or more Minority Serving Institutions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.