Collaborative Research: Highly Ordered Nanoscale Patterns Produced by Ion Bombardment of Solid Surfaces: Theory and Experiment

当一个平坦的固体表面受到宽离子束的轰击时，图案可以自发地出现，包括波长短至人类头发宽度万分之一的表面波纹。如果形成的图案不总是无序的，这种令人着迷的自组织类型可能会成为一种广泛应用的方法，用于制造特征尺寸太小的结构，而传统技术无法生产。因此，这项理论和实验合作研究的中心目标是开发和改进通过离子轰击产生高度有序表面波纹的方法。这种表面图案可用于各种应用，包括必须操纵或分析极端紫外线的应用。该项目涉及在基础研究和应用领域工作的研究生、本科生和高中生。除了体验和促进当前材料研究的重要主题外，学生还将学习使用广泛的理论，模拟和材料表征工具，这些工具将很好地转化为未来在工业或学术界的职业生涯。除了与本国机构的材料研究团体合作外，该项目还通过参与国家x射线散射设施中令人兴奋的多学科环境，为学生提供了更广阔的视野。此次合作将两位分别在离子束纳米图领域做出开创性理论和实验贡献的主要研究人员聚集在一起，形成了一个独特的团队，以推进该领域的发展。为了达到产生高度有序的表面波纹的目标，研究人员采用了两种关键策略。这些策略也使我们对离子轰击过程中发生的基本过程的理解向前迈出了重要的一步。第一个策略是研究当离子入射角或样品温度通过阈值变化时发生的模式形成的开始。近阈值行为在三种不同的状态下进行了研究：1)溅射是重要的高能量状态；2)溅射可以忽略不计的低能状态，这是该领域尚未开发的前沿领域；3)在高温状态下，表面保持结晶，从而为电子/光子应用提供更高质量的材料。在这些跃迁附近，运动方程变得更简单，可以严格地推导出来。此外，它具有高度的通用性，即在很大程度上不依赖于靶材料和离子种类的选择。研究近阈值行为是相变理论取得根本性进展的关键，离子诱导的模式形成也证明了这一点。最后，也是最重要的，在某些情况下，几乎完美有序的波纹图案被预测形成接近阈值。该项目的第二个核心策略是利用实时x射线散射的力量。掠入射小角x射线散射对表面扰动的极端敏感性使其成为研究纳米图近阈值行为和严格检验理论的理想方法。此外，研究小组正在利用新的和升级的同步加速器源不断增加的相干x射线通量来开发相干x射线散射，作为研究纳米尺度表面动力学的一种强大的新方法。相干散射提供了全新的见解，超越了平均表面演化的研究，测量了平均动力学周围微观结构的随时间波动，从而实现了详细理论预测的新测试。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。

When a flat solid surface is bombarded with a broad ion beam, patterns can emerge spontaneously, including surface ripples with a wavelength as short as one ten-thousandth of the width of a human hair. If the patterns formed were not almost always disordered, this fascinating type of self-organization could become a widely employed method of fabricating structures with feature sizes too small to be produced by conventional techniques. A central goal of this collaborative theoretical and experimental research is therefore to develop and refine methods of producing highly ordered surface ripples by ion bombardment. Surfaces patterns of this kind could be used in a variety of applications, including ones in which extreme ultraviolet light must be manipulated or analyzed. This project involves graduate, undergraduate and high school students who are working in a field that bridges basic research and applications. In addition to experiencing, and contributing to, an important topic in current materials research, the students are learning to use a wide range of theoretical, simulation and materials characterization tools that will translate well to future careers in industry or academia. Beyond working with the materials research communities at their home institutions, the project is providing the students with a broader perspective through participation in the exciting, multi-disciplinary environments at national x-ray scattering facilities.This collaboration brings together two principal investigators who have individually made seminal theoretical and experimental contributions to the field of ion beam nanopatterning, producing a team that is uniquely positioned to advance the field. Two key strategies are being employed in working toward the goal of producing surface ripples with an exceptional degree of order. These strategies are also leading to important steps forward in our understanding of the fundamental processes that take place during ion bombardment. The first strategy is to study the onset of pattern formation that occurs as the angle of ion incidence or the sample temperature is varied through a threshold value. Near-threshold behavior is being examined in three different regimes: 1) the higher energy regime in which sputtering is important; 2) the low-energy regime in which sputtering is negligible, a largely unexplored frontier of the field; and 3) the high-temperature regime in which the surface remains crystalline, resulting in better quality material for electronic/photonic applications. The equation of motion becomes simpler near these transitions and can be rigorously derived. In addition, it has a high degree of universality, i.e., it is to a large extent independent of the choice of target material and ion species. Studying near-threshold behavior was the key to making fundamental advances in the theory of phase transitions, and the same is proving to be true for ion-induced pattern formation. Finally, and most importantly, almost perfectly ordered ripple patterns are in certain circumstances predicted to form close to threshold. The second central strategy of the project is to exploit the power of real-time x-ray scattering. The extreme sensitivity of grazing-incidence small-angle x-ray scattering to surface disturbances makes it an ideal method to study the near-threshold behavior of the nanopatterns and to rigorously test the theory. Moreover, the research team is taking advantage of the increasing coherent x-ray flux available at new and upgraded synchrotron sources to develop coherent x-ray scattering as a powerful new approach to investigating surface dynamics at the nanoscale. Coherent scattering gives entirely new insight, going beyond study of the average surface evolution to measure the time-dependent fluctuations of the microstructure around the average kinetics, enabling novel tests of the detailed theoretical predictions.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.