- Mesoscale studies benefit from the experimental, theoretical, and simulation tools developed to explore nanoscale phenomena and build the field of nanotechnology. These studies also benefit from the expertise and knowledge base acquired over decades of semiconductor manufacturing, from a century or more of advances in synthetic chemistry, and from the biotechnology and molecular biology revolution.
- Following the dynamics of these systems and understanding how they evolve at the mesoscale is important, not at the level of each and every atom or molecule, but at a level that will provide useful insights.
- It is also important—and challenging—to understand the role of heterogeneity at the mesoscale. The ability to control chemical functionality is a key to mesoscale studies because that will in turn control and tune the interactions and interfaces at the mesoscale in ways that can be used to couple materials together to develop new capabilities.
- Chemists have used their tools to design building blocks that can be put together to create larger structures in ways that provide specific control over molecular interactions and interfaces. Thus they can study and investigate these phenomena at the mesoscale and develop a theoretical understanding of how mesoscale properties emerge.
In a plenary session to start the workshop, Paul Weiss, Distinguished Professor of Chemistry and Biochemistry and of Materials Science and Engineering, and the Fred Kavli Chair in NanoSystems Sciences at the University of California, Los Angeles, described the defining chemical, physical, optical, mechanical, and electronic properties of surfaces and supramolecular assemblies and discussed the cooperative behavior observed between functional molecules. He explained that there are two ways to approach the mesoscale, just as there is the nanoscale—from the bottom up or from the top down—though in the mesoscale, the requirements for precision in measurement and synthesis are more relaxed than they are when operating at the nanoscale. For example, when looking at the statistical and ensemble properties at the mesoscale or at cooperative effects across different regions or materials in a mesoscale structure, there are some details that can be ignored or minimized without sacrificing precision in describing the phenomena that are occurring in a given system. The two approaches are complementary, however, and Weiss noted that mesoscale studies are benefiting from the experimental, theoretical, and simulation tools developed to explore nanoscale phenomena and build the field of nanotechnology, as well as from the expertise and knowledge base acquired over decades of semiconductor manufacturing, from a century or more of advances in synthetic chemistry, and from the biotechnology and molecular biology revolution.
Weiss explored many key components of mesoscale phenomena: granularity, defects, energy quantization, collective behavior, fluctuations and variations, interacting degrees of freedom, and structural and dynamical heterogeneity. At the
mesoscale, these components play important roles in determining the band structure of conductors and semiconductors and are important to understand for creating photonic, plasmonic, spintronic, and metamaterials. Collective effects, such as superconductivity, ferroelectricity, and piezoelectricity, also result from interactions occurring at the mesoscale.
One important challenge in studying mesoscale collective effects is accounting for heterogeneity and fluctuations. “We want to understand these, and we do not want to say that there’s an ensemble distribution,” said Weiss, because in studying these systems, “heterogeneity turns out to be critically important.” Using the human immunodeficiency virus as an example of a structure that fits between the nanoscale and microscale, he explained that there are many repeated patterns in the structure of a virus, but there is also a great complexity of extreme importance found in the details of the encoded genome of the virus, in the variations and fluctuations in that genome, and in their effects on billions of people worldwide.
When working at the nanoscale, said Weiss, it is possible to exercise exquisite control over the position of every atom, but working at the mesoscale requires giving up that kind of control and instead embracing the granularity of structures at this scale. He noted that this is where techniques developed in the 1990s by the semiconductor industry could prove valuable for developing and studying mesoscale structures and that there are many unused semiconductor foundries built to produce earlier-generation computer chips that could be turned to this work. At a time when research in chip design and development was pushing the boundaries of Moore’s Law to ever smaller scales, there were advantages, Weiss explained, in backing away from those boundaries and working on a broader range of materials. As he put it, nobody will complain about the exact placement of impurities in a semiconductor when working at the mesoscale. Techniques such as soft lithography, microcontact printing (Dameron et al. 2005, Love et al. 2002), dip-pen lithography (Hong, Zhu, and Mirkin 1999), and chemical lift-off lithography (Liao et al. 2012), which work well at the 100-nanometer scale but can also work at the centimeter scale, are proving useful in creating structures at the mesoscale. However, they are not applicable for creating the latest semiconductor structures. He stressed that the ability to control chemical functionality is the key to this work because that will in turn control and tune the interactions and interfaces at the mesoscale in ways that can be used to couple materials together to develop new capabilities.
Working at the mesoscale and the averaging that comes with that does mean giving up some details. For example, x-ray data from crystals of DNA led to deducing the double-helix structure of DNA, but it would not provide any information on the sequence of bases. One of the surprises from the Human Genome Project, Weiss noted, was that though there are a million different proteins that are present in human beings, there are only about 21,000 genes to provide that coding. Sequencing alone does not reveal the mechanisms by which this complexity is achieved, and there are lessons to be learned by studying this system about how “mix-and-match” systems can create such a variety of results. As an aside, he reminded the workshop that the U.S. government’s investment in the Human Genome Project has generated outsized returns in terms of economic activity and gains in personal income, and that mesoscale science has the same potential for return on investment.
Returning to the subject at hand, Weiss also reminded the workshop participants that it is important when making materials at the mesoscale to keep in mind the desired function of those materials and the phenomena that need to be measured. “There’s this beautiful interplay taking place now in nanoscience and nanotechnology where we build devices in order to understand properties. I think the same is going to be extremely true of the mesoscale,” he said. “What can we build that will give us feedback about the key components in terms of structure, function, spectra, and so forth?”
Working from a chemistry-based perspective in both the nano- and mesoscale provides useful insights, especially when coupled with tools and knowledge drawn from other fields. In nanotechnology, researchers have already learned to bridge the communication gaps among chemists, physicists, material scientists, biologists, clinicians, mathematicians, computer scientists, and others to exchange ideas and benefit from each other’s expertise. Chemists, meanwhile, have used their tools to design building blocks that can be put together to create larger structures in ways that provide specific control over molecular interactions and interfaces to study and investigate these
phenomena. The construction of colloidal fullerene crystals with precise structures (Claridge et al. 2009) supports that idea, said Weiss, as does the work that his group did using various inorganic building blocks and linkers to create zero-, one-, two-, and three-dimensional materials with well-defined band gaps. Using tools and understanding from other fields, his group could predict the values of these band gaps from an understanding of the properties of the linker’s electronic structure. “From very simple rules of thumb, we could predict the properties of the materials,” said Weiss. The reason for that predictability, he explained, is that the interactions between the different components in these systems are relatively weak because of the way the linkers hold the different components apart from one another. Using chemistry, it is possible to create additional clusters and materials that can in turn inform the theoretical understanding of how mesoscale properties emerge and to test those theories. Weiss built on this example of the benefit of chemistry as part of an interdisciplinary approach to research at the mesoscale with a few more examples.
Turning to the subject of defects, Weiss explained that they can be exploited for patterning but that they can also lead to pattern dissolution. In early studies of mesoscale defects, he and his colleagues showed that mesoscale systems using self-assembled monolayers are never at equilibrium but that the defects can be guided to a specific location and with a specific density and then frozen at the place (Weiss 2008). “We could annihilate things such as substrate vacancy islands and voids and step edges by adding more molecules to the matrix, by allowing flow to get rid of those defects, and by filling in the kinds of defects we didn’t want,” he explained. The defects that remained were ones that could be used to insert functional components into the material, and these functional components could be observed and measured using either scanning tunneling microscopy or spectroscopic methods (Zheng et al. 2013).
Studies of carboranes—molecules in which the carbon atoms are hexacoordinate rather than tetracoordinate and are therefore electron deficient—demonstrate one aspect of the kind of control that can be achieved over mesoscale structures. Placement of the carbon atoms in these materials determines the direction of the dipole in the carborane molecule. When these cage molecules self-assemble in monolayers, they can have the dipole oriented parallel or perpendicular to the material surface (Figure 2-1), which creates either polar or nonpolar structures (Hohman et al. 2009) that can in turn be mixed to create organoelectronics with controlled interfacial energy (Kim, J., et al. 2014). Weiss and his colleagues have learned how to control the average surface charge of these self-assembled monolayers to create “essentially perfect band alignment while preserving an important property of the polymer—how it wets.” In contrast, the typical approach to modify surface charge is to use partial fluorination, but this changes the wetting property of the polymer and the morphology of the active layer.
Weiss noted that it is possible to measure buried dipoles in these systems. In one set of experiments, his group wanted to know if aligned dipoles are responsible for the competitive advantage of the nonpolar surface and, in fact, aligned dipoles do create a defect-tolerant system. Spectroscopic measurements and mathematical analysis showed that the dipole interaction occurs over a long enough range to create a two-dimensional ferroelectric system. “If we only had neighbor–neighbor interactions, that would be classically forbidden,” explained Weiss. Instead, these mesoscale interactions allow the molecules to cross step edges that are a couple of angstroms high and cross domain boundaries that are offset by a couple of angstroms. The result is that these self-assembled monolayers have large regions where the dipoles are aligned even when there are defects that would normally prevent such alignment from occurring.
In his final remarks, Weiss drew some lessons from nanotechnology to comment on the type of tools that need to be developed and applied to understand mesoscale phenomena. In nanotechnology, he said, there were structural tools that let researchers observe and control the placement of individual atoms and molecules to develop some predictive rules regarding structure and function at the nanoscale. “What we learned, and what we are going to need to learn in the mesoscale, is structural control at multiple scales all at the same time. We’re going to need to associate those and use those as input for theory and simulations, and that’s going to ultimately give us control over chemical, physical, electronic, optical, and biological properties,” he said.
Figure 2-1 The alignment of dipoles on the surface of a self-assembled monolayer has an important impact on the chemical properties of the resulting surface. SOURCE: Hohman et al. (2009). Reprinted with permission from ACS Nano.
He also noted that while he did not talk about dynamics, it is going to be important to follow dynamics and understand how these systems evolve at the mesoscale, not at the level of each and every atom or molecule but at a level that will provide useful insights. He predicted that mathematical tools will be useful in this regard. The ability to measure and detect rare events in mesoscale structures will also be important, particularly for developing novel catalysts, where rare structures are often the ones that have the largest catalytic activity. “One of the keys there is going to be figuring out what those are and guiding the system into those areas,” said Weiss. He concluded his talk with the prediction that the tools needed to understand the mesoscale are going to be complicated and it will be necessary to piece together information in a way that does not require knowing the position and function of every atom in these structures.
In response to a question about the field’s ability to create well-defined mesoscale systems at a commercial scale, Weiss said roll printing of organic electronics is an example of mesoscale structures being produced at scale. However, he noted that much of the work with organic electronics has been empirical rather than driven by predictions based on well-formulated theory, and this is an area where mesoscale research could produce significant advances in terms of understanding how to intentionally control chemical and polymer properties to produce specific properties.
Weiss then addressed the issue of metastability at the mesoscale. He stated that when researchers were first performing chemical conversions on self-assembled monolayers, they would use the most vigorous chemical reactions they could find so that these conversions would go to completion. That, however, turned out to be a bad idea because the monolayers would anneal. What was needed, he explained, was to develop chemistry that preserves metastable structures and yet goes to completion or to develop methods of only carrying out chemistry at defects. These systems, he said, are metastable systems, not equilibrium systems, and it is necessary to preserve that metastability to produce materials with novel and useful properties.