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被宽的离子束轰击时，图案可以自发产生，包括波长短至人类头发宽度的万分之一的表面波纹。如果形成的图案几乎不总是无序的，这种迷人的自组织类型可能会成为一种广泛使用的方法，来制造特征尺寸太小而不能用传统技术制造的结构。因此，这种合作的理论和实验研究的一个中心目标是开发和改进通过离子轰击产生高度有序的表面波纹的方法。这种表面图案可以用于各种应用，包括必须处理或分析极端紫外光的应用。这个项目涉及研究生、本科生和高中生，他们在一个连接基础研究和应用的领域工作。除了体验和贡献当前材料研究中的一个重要主题外，学生们还在学习使用各种理论、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射线散射设施令人兴奋的多学科环境为学生提供了更广阔的视角。这次合作汇集了两名对离子束纳米粉碎领域做出开创性理论和实验贡献的主要研究人员，产生了一支在该领域处于独特地位的团队。在努力实现产生具有特殊有序程度的表面波纹的目标时，采用了两个关键战略。这些战略还使我们在理解离子轰击期间发生的基本过程方面向前迈出了重要的一步。第一种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图案形成的开始。近阈值行为在三个不同的区域中被研究：1)较高能量区域，在其中溅射是重要的；2)低能量区域，在其中溅射可以忽略，这在很大程度上是该领域的一个未被探索的前沿；以及3)高温区域，在该区域表面保持结晶，从而产生更好质量的电子/光子应用材料。在这些转变附近，运动方程变得更简单，并且可以严格推导出来。此外，它还具有高度的普遍性，即它在很大程度上独立于目标材料和离子物种的选择。研究近阈值行为是相变理论取得根本性进展的关键，对于离子诱导图案的形成也是如此。最后，也是最重要的是，在某些情况下，几乎完全有序的波纹图案预计会形成接近阈值。该项目的第二个核心战略是利用实时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粒子的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提供的不断增加的相干X射线通量来开发相干X射线散射，作为在纳米尺度上研究表面动力学的一种强有力的新方法。相干散射提供了全新的见解，超越了对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变化的波动，使详细的理论预测能够进行新的测试。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.