NIRT: Artificially Engineered Nanoscale Ferroelectrics

我们的NIRT的技术目标是了解人工工程纳米铁电体的结构，介电和光学响应的基础科学，这可能与传统的均匀铁电体有很大的不同。利用“第一性原理有效哈密顿”方法（基于晶格万尼尔函数）和landau - ginzburg型现象学方法，我们将预测一维组成和应变梯度、机械和电边界条件对这些系统中自发极化的外观和稳定性以及铁电畴结构的修饰的影响。这些预测将与钙钛矿铁电体相应的纳米结构（由反应性MBE制成）的观察结果进行比较，其中钙钛矿铁电体的组成和应变在一个方向上变化。所得薄膜将通过TEM、x射线衍射（包括同步加速器研究）、拉曼光谱、二次谐波产生、作为电场和温度函数的介电特性测量、压电和热释电技术进行表征，并与相应的理论预测进行比较，以完善我们对纳米级铁电体的理解。铁电薄膜的组成和应变梯度将作为一种纳入新功能的手段进行研究：增强的介电和热释电响应，以及各种新的光学性质。30多年来，分子束外延（MBE）一直被用于原子一个原子地构建层状半导体纳米结构，以研究和提高我们对半导体物理的理解，并创造新的器件。这些设备（包括激光二极管、高性能晶体管和磁场传感器）促进了医疗保健、国家安全、通信、娱乐和交通——显著提高了所有美国人的生活质量。最近的研究进展表明，同样的原子-原子合成技术可以用于构建氧化物的纳米结构，包括铁电体，具有类似的纳米级分层控制。由于铁电材料表现出各种各样的电学、光学和机电特性，它们被广泛应用于医疗保健（例如，医用超声）、国防（例如，夜视和声纳系统）和通信（例如，用于手机和计算机的微型电容器）。在原子层水平上定制铁电材料分层的能力为创造新的功能材料开辟了令人兴奋的可能性，我们相信这些功能材料可以被设计（在充分理解的情况下）具有卓越的性能。通过这项研究获得的更好的理解将应用于改进的（增强的性能和更小的尺寸）电容器、夜视设备和光学元件的开发。这个NIRT项目还将在一个高度跨学科的研究环境中培养和教育未来的科学家，在一个具有国家重要性的技术重要领域。该提案是响应“纳米尺度科学与工程”（NSF 00-119）的征求而提交的。该奖项由外部资源和MPS材料研究部的NSF陶瓷和电子材料项目共同支持，并得到了该倡议的帮助。

0103354SchlomThe technical objective of our NIRT is to understand the fundamental science underlying the structural, dielectric, and optical response of artificially-engineered nanoscale ferroelectrics, which can be drastically different from that of conventional homogeneous ferroelectrics. Using "first-principles effective Hamiltonian" approaches (based on lattice Wannier functions) and Landau-Ginzburg-type phenomenological methods, we will predict the effect of one-dimensional composition and strain gradients, and mechanical and electrical boundary conditions on the appearance and stability of the spontaneous polarization in these systems and on the modifications of ferroelectric domain structures. These predictions will be compared against observations on corresponding nanostructures (made by reactive MBE) of perovskite ferroelectrics in which composition and strain are varied in one direction. The resulting films will be characterized via a combination of TEM, x-ray diffraction (including synchrotron studies), Raman spectroscopy, second harmonic generation, dielectric property measurements as a function of electric field and temperature, and piezoelectric and pyroelectric techniques and compared with corresponding theoretical predictions in order to refine our understanding of nanoscale ferroelectrics. Composition and strain gradients in ferroelectric films will be investigated as a means to incorporate new functionalities: enhanced dielectric and pyroelectric responses, as well as a variety of novel optical properties.%%%For over 30 years molecular beam epitaxy (MBE) has been used to build up layered semiconductor nanostructures atom-by-atom to investigate and improve our understanding of semiconductor physics and create new devices. These devices (which include laser diodes, high-performance transistors, and magnetic field sensors) have advanced healthcare, national security, communications, entertainment, and transportation-resulting in significant improvements in the quality of life for all Americans. Recent progress in research has demonstrated that this same atom-by-atom synthesis technique can be used to build up nanostructures of oxides, including ferroelectrics, with comparable nanometer-scale layering control. Since ferroelectric materials exhibit a wide variety of electrical, optical, and electromechanical properties, they are extensively used in healthcare (e.g., medical ultrasound), national defense (e.g., night vision and sonar systems), and communications (e.g., miniature capacitors for cell phones and computers). The ability to customize the layering of ferroelectric materials at the atomic-layer level opens exciting possibilities in terms of creating new functional materials that we believe can be designed (with sufficient understanding) to have exceptional properties. The improved understanding gained via this research will be applied to the development of improved (enhanced performance and smaller size) capacitors, night vision devices, and optical components. This NIRT program will also train and educate future scientists in a highly interdisciplinary research environment in a technologically-significant area of national importance. This proposal was submitted in response to the solicitation "Nanoscale Science and Engineering" (NSF 00-119). The award is jointly supported through outside sources and the NSF Ceramics and Electronic Materials programs of the Division of Materials Research in MPS with the assistance of the initiative.