Nanoparticle assembly enables the synthesis of multiple complex nanoscale structures, but future investigation and application of these materials requires new processing methods to control micro- and macroscale structure as well.

Nanoparticle assembly enables the synthesis of multiple complex nanoscale structures, but future investigation and application of these materials requires new processing methods to control micro- and macroscale structure as well. In the past few decades, nanoparticle (NP) self-assembly has advanced to the point that NP superlattice crystallization can be envisioned as a viable method for the fabrication of functional devices and technologies.(1−3) As with any new material whose synthetic methods are sufficiently developed, multiple questions now arise that must be addressed to progress the field. The two questions for the next phase in NP assembly research most commonly found in the current literature (either implicitly or explicitly) could arguably be stated as “What new properties do these materials have?” and “What are these superlattices good for?”.(4−6) Certainly, these are critical questions to transition NP superlattices from benchtop curiosities to useful materials. However, we run the risk of falling victim to a key blind spot in the standard materials development paradigm if we assume that the identification of properties and applications for these superlattices is the only hurdle remaining in their development. Specifically, advancing a material from its initial synthesis or discovery to its use in a functional application requires us to also answer the question of “How do we process these materials into a useful form?”.(7,8) Future research efforts with NP-based materials are going to rely on the ability to produce them in the right geometric configuration, as structure control beyond just nanoscale ordering is important in enabling both fundamental investigations of structure–property relationships and application in useful devices. Ultimately, the amount of impact NP superlattices will have in advancing both science and engineering depends on how well we can control their hierarchical structures beyond just their initial assembly. Macroscopic structure control (e.g., mm-scale or larger) has obvious implications for the utility of NP superlattices, as the final form factor of the device or technological component dictates both how much material is needed and what its overall shape needs to be. Processing the superlattices in a manner that does not disrupt all of the hard work spent making beautiful crystalline geometries is not necessarily a simple task, though, as most common material processing techniques designed for atomic, molecular, and macromolecular systems are not always compatible with the typically mild conditions used to form well-ordered NP arrays. Nominally 2D films and coatings from particle assembly can already be developed on the centimeter-scale or potentially larger,(5,9−13) but truly 3D macroscopic objects made via NP self-assembly that preserve nanoscale ordering are more rare.(14−17) Moreover, even in assembly methods capable of generating objects with at least some macroscopic dimension, manipulation of material microstructure is still underdeveloped. Microstructural features are key design factors used to control the characteristics and behavior of bulk atomic or molecular materials including mechanical (hardness, toughness, ductility), chemical (corrosion and etching, transport), and optical (light reflection and scattering) properties.(7,18) Understanding how factors like defects, grain sizes, or crystal texture in NP superlattices affect their performance is thus another critical area of investigation that is enabled by innovations in processing science. So how do we establish processing methods for these NP superlattices to manipulate both micro- and macroscopic structure (without sacrificing nanoscale organization), and what are the key aspects of our various NP assembly techniques that we need to improve upon to enable such research? One of the obvious criteria for any processable NP superlattice is the ability to produce materials at an appropriate scale that the given processing technique being examined is physically feasible. This does not necessarily mean that we need to produce superlattice-based materials at the same tonnage as conventional bulk metals, ceramics, and polymers, but the amount of material has to match the form factor and scale of the proposed processing methods and applications. NP superlattice thin film architectures or micron-sized crystallites are valuable for 2D coatings or components of microdevices (and many fantastic and promising examples of devices built from these assemblies are already beginning to emerge(5,11,19)), but other, larger geometries would expand the types of scientific investigations into properties or performance that we could conduct. Molding, pressing, extruding, or even additive manufacturing techniques to make macroscopic 3D objects become much more viable when the NPs can both be synthesized and assembled in large quantities.(14,20) Moreover, an important lesson we can learn from bulk materials processing is that a wider variety of available processing methods significantly improves our ability to control microstructural features like grain sizes or crystal texture in polycrystalline NP superlattices.(14,21,22) It will therefore be advantageous to develop NP assembly methods that are amenable to as many different processing techniques as possible. Progression in this research area requires us to think carefully about the composition of the NPs we choose to synthesize and assemble and to coordinate our efforts with those of chemists, materials scientists, and engineers in related areas that enable large batch fabrication of the NP building blocks we will need. It also requires that we think about methods to integrate our assemblies with other material types, as the use of NP superlattices as additives to traditional bulk materials is a key opportunity to extend their utility. Embedding, mixing, coating, or otherwise incorporating ordered NP arrays as small but active fractions of a much larger and more inherently scalable material would significantly increase our ability to take advantage of the optical, chemical, magnetic, and other properties exhibited by precisely organized particle lattices.(10,11,17,23,24) It is also important to realize that scaling up is more than just “making more” of a material. As noted above, materials that can be shaped or molded into larger objects provide significant opportunities to ask questions about how structural features at those larger length scales influence the properties of a material, such as how grain size affects mechanical deformation or transport behavior (two areas well-known to have major impact on the properties of polycrystalline atomic solids).(18) The ability to hierarchically organize materials across the nano-, micro-, and macroscale also allows for the concept of “systems” development of materials, where factors like macroscopic interfaces, boundary conditions, and the collective behavior of multiple organized units can feed back to influence the behavior of individual building blocks.(4,25,26) This mode of using integrated design features across ∼10⁹ differences in size is a crucial aspect of biological materials and living organisms that enables their hierarchical structures, and assembling NP superlattices at the same macroscopic scale would offer the exciting opportunity to mimic such complexity. The field of NP assembly has expanded significantly in the past few decades to the point that we can make hundreds of different superlattices with control over their composition, crystal symmetry, and lattice parameters.(27−33) Research is now beginning on the next stages of the materials development life cycle, examining the types of properties that we can generate and inventing new functions for these materials in next-generation technologies. However, it is critical that we do not assume that our mastery of NP crystallization means that we are done exploring the area of materials formation and structure control or that the only challenge remaining is figuring out what the materials that we have made are good for. Structure–property relationship development is certainly a key aspect of advancing the field, but there is still much left to investigate in processing and integration of these materials into different micro- and macroscale forms. It is essential to research those aspects of material synthesis with the same fervor that drove the development of new superlattice crystal structures over the past few decades. Indeed, now that we have this level of sophistication in synthesizing different NP-based crystals, the next important steps for this area of nanotechnology research paradoxically lie at size regimes well beyond the 1–100 nm length scale, by establishing methods to hierarchically organize solids with precisely designed and intentionally fabricated microstructural features and macroscopic forms. More simply, the future of NP assembly as a materials development tool is in “thinking bigger”. R.J.M. acknowledges support from the NSF via a CAREER award (CHE-1653289) and the Air Force Office of Scientific Research via a young investigator award (FA9550-17-1-0288). R.J.M. acknowledges Matthew R. Jones for valuable discussions. This article references 33 other publications.

中文翻译：

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从纳米到宏观：在纳米粒子组装中思考更大

纳米粒子组装能够合成多种复杂的纳米结构，但这些材料的未来研究和应用还需要新的加工方法来控制微观和宏观结构。

纳米粒子组装能够合成多种复杂的纳米结构，但这些材料的未来研究和应用需要新的加工方法来控制微观和宏观结构。在过去的几十年中，纳米粒子 (NP) 自组装已经发展到可以将 NP 超晶格结晶视为制造功能器件和技术的可行方法。 (1-3) 与任何合成的新材料一样方法得到充分发展，现在出现了许多问题，必须解决这些问题才能推动该领域的发展。当前文献中最常见的 NP 组装研究下一阶段的两个问题（隐含或明确）可以说是“这些材料有哪些新特性？” 和“这些超晶格有什么用？”。（4-6）当然，这些是将 NP 超晶格从台式好奇心转变为有用材料的关键问题。然而，如果我们假设这些超晶格的特性和应用的识别是它们开发中存在的唯一障碍，我们就有可能成为标准材料开发范式中关键盲点的受害者。具体来说，将材料从最初的合成或发现推进到其在功能应用中的使用，还需要我们回答“我们如何将这些材料加工成有用的形式？”的问题。（7，8) 未来基于 NP 材料的研究工作将依赖于以正确的几何配置生产它们的能力，因为结构控制不仅仅是纳米级排序对于结构-性能关系的基础研究和在有用设备中的应用都很重要. 最终，NP 超晶格在推进科学和工程方面的影响程度取决于我们能在多大程度上控制它们的层次结构，而不仅仅是它们的初始组装。宏观结构控制（例如，毫米级或更大）对 NP 超晶格的效用具有明显的影响，因为设备或技术组件的最终形状因素决定了需要多少材料及其整体形状。然而，以不破坏制造美丽晶体几何结构的所有辛勤工作的方式处理超晶格并不一定是一项简单的任务，因为为原子、分子和大分子系统设计的最常见的材料加工技术并不总是与通常用于形成有序 NP 阵列的温和条件。名义上来自粒子组装的 2D 薄膜和涂层已经可以开发到厘米级或可能更大，(5,9-13) 但通过 NP 自组装制造的真正 3D 宏观物体更罕见。 (14-) 17) 此外，即使在能够产生至少具有一些宏观尺寸的物体的组装方法中，对材料微观结构的操纵仍然不发达。微观结构特征是用于控制大块原子或分子材料的特性和行为的关键设计因素，包括机械（硬度、韧性、延展性）、化学（腐蚀和蚀刻、传输）和光学（光反射和散射）特性。 （7 ,18) 因此，了解 NP 超晶格中的缺陷、晶粒尺寸或晶体结构等因素如何影响其性能是加工科学创新支持的另一个关键研究领域。那么我们如何为这些 NP 超晶格建立处理方法来操纵微观和宏观结构（不牺牲纳米级组织），以及我们需要改进以实现此类研究的各种 NP 组装技术的关键方面是什么？任何可加工的 NP 超晶格的明显标准之一是能够以适当的规模生产材料，即所检查的给定加工技术在物理上是可行的。这并不一定意味着我们需要以与传统大块金属、陶瓷和聚合物相同的吨位生产基于超晶格的材料，但材料的数量必须与所提出的加工方法和应用的形状因子和规模相匹配。NP 超晶格薄膜结构或微米级微晶对于 2D 涂层或微型器件的组件很有价值（并且许多由这些组件构建的器件的奇妙和有前途的例子已经开始出现（5,11,19）），但其他的，更大的几何形状将把科学研究的类型扩展到我们可以进行的属性或性能方面。当 NPs 可以大量合成和组装时，模塑、压制、挤压甚至增材制造技术使宏观 3D 物体变得更加可行。 (14,20) 此外，我们可以从散装材料加工中学到一个重要的教训是，更广泛的可用加工方法显着提高了我们控制多晶 NP 超晶格中的晶粒尺寸或晶体纹理等微观结构特征的能力。 (14,21,22) 因此，开发符合以下条件的 NP 组装方法将是有利的尽可能采用多种不同的处理技术。这一研究领域的进展要求我们仔细考虑我们选择合成和组装的 NPs 的组成，并与相关领域的化学家、材料科学家和工程师协调我们的努力，以实现 NP 构建块的大批量制造我们会需要。它还要求我们考虑将我们的组件与其他材料类型集成的方法，因为使用 NP 超晶格作为传统散装材料的添加剂是扩展其效用的关键机会。嵌入、混合、涂覆或以其他方式将有序 NP 阵列作为更大且更具内在可扩展性的材料的小但活性部分，将显着提高我们利用光学、化学、磁性、以及精确组织的粒子晶格表现出的其他特性。(10,11,17,23,24) 同样重要的是要认识到，按比例放大不仅仅是“制造更多”材料。如上所述，可以塑造或模塑成更大物体的材料提供了重要的机会来询问这些更大长度尺度的结构特征如何影响材料的特性，例如晶粒尺寸如何影响机械变形或传输行为（两个领域众所周知，对多晶原子固体的性质有重大影响。(18) 在纳米、微米和宏观尺度上分层组织材料的能力也允许材料的“系统”开发概念，其中因素如宏观界面、边界条件、⁹大小的差异是生物材料和生物体的一个关键方面，它可以实现它们的层次结构，并且在相同的宏观尺度上组装 NP 超晶格将为模拟这种复杂性提供令人兴奋的机会。在过去的几十年里，NP 组装领域已经显着扩展，以至于我们可以通过控制它们的成分、晶体对称性和晶格参数来制造数百种不同的超晶格。 (27−33) 研究现在开始进入下一阶段材料开发生命周期，检查我们可以生成的属性类型，并在下一代技术中为这些材料发明新功能。然而，至关重要的是，我们不要假设我们对 NP 结晶的掌握意味着我们已经完成了材料形成和结构控制领域的探索，或者剩下的唯一挑战是弄清楚我们制造的材料有什么用处。结构-性能关系的发展无疑是推动该领域发展的一个关键方面，但在将这些材料加工和整合成不同的微观和宏观尺度方面，仍有很多需要研究的地方。以过去几十年推动新超晶格晶体结构发展的热情来研究材料合成的这些方面至关重要。事实上，既然我们在合成不同的基于 NP 的晶体方面已经达到了这种水平，这个纳米技术研究领域的下一个重要步骤矛盾地在于远远超出 1-100 nm 长度尺度的尺寸范围，通过建立方法来分层组织具有精确设计和有意制造的微观结构特征和宏观形式的固体。更简单地说，NP组装作为材料开发工具的未来在于“想得更大”。RJM 通过 CAREER 奖 (CHE-1653289) 和空军科学研究办公室通过年轻研究员奖 (FA9550-17-1-0288) 来感谢 NSF 的支持。RJM 感谢 Matthew R. Jones 的宝贵讨论。本文引用了 33 篇其他出版物。通过建立具有精确设计和有意制造的微观结构特征和宏观形式的分层组织固体的方法。更简单地说，NP组装作为材料开发工具的未来在于“想得更大”。RJM 通过 CAREER 奖 (CHE-1653289) 和空军科学研究办公室通过年轻研究员奖 (FA9550-17-1-0288) 来感谢 NSF 的支持。RJM 感谢 Matthew R. Jones 的宝贵讨论。本文引用了 33 篇其他出版物。通过建立具有精确设计和有意制造的微观结构特征和宏观形式的分层组织固体的方法。更简单地说，NP组装作为材料开发工具的未来在于“想得更大”。RJM 通过 CAREER 奖 (CHE-1653289) 和空军科学研究办公室通过年轻研究员奖 (FA9550-17-1-0288) 来感谢 NSF 的支持。RJM 感谢 Matthew R. Jones 的宝贵讨论。本文引用了 33 篇其他出版物。感谢 Matthew R. Jones 的宝贵讨论。本文引用了 33 篇其他出版物。感谢 Matthew R. Jones 的宝贵讨论。本文引用了 33 篇其他出版物。