Elastic Forces Drive Nonequilibrium Pattern Formation in a Model of Nanocrystal Cation Exchange

Cation exchange is a useful tool for post-synthetic modification of nanocrystals, yet its mechanisms remain poorly understood. Here, we explore an idealized model for ion exchange in which a chemical potential drives compositional defects to accumulate at a crystal's surface. These impurities subsequently diffuse inward. We find that the nature of interactions between sites in a compositionally impure crystal strongly impacts exchange trajectories. In particular, elastic deformations which accompany lattice-mismatched species promote spatially modulated patterns in the composition. These same patterns can be produced at equilibrium in core/shell nanocrystals, whose structure mimics transient motifs observed in nonequilibrium trajectories. Moreover, the core of such nanocrystals undergoes a phase transition - from modulated to unstructured - as the thickness or stiffness of the shell is decreased. Our results help explain the varied patterns observed in heterostructured nanocrystals produced by cation exchange and suggest principles for the rational design of compositionally-patterned nanomaterials

Remembering the work of Phillip L. Geissler: A coda to his scientific trajectory

Phillip L. Geissler made important contributions to the statistical mechanics of biological polymers, heterogeneous materials, and chemical dynamics in aqueous environments. He devised analytical and computational methods that revealed the underlying organization of complex systems at the frontiers of biology, chemistry, and materials science. In this retrospective, we celebrate his work at these frontiers

Unraveling Kinetically-Driven Mechanisms of Gold Nanocrystal Shape Transformations Using Graphene Liquid Cell Electron Microscopy.

Mechanisms of kinetically driven nanocrystal shape transformations were elucidated by monitoring single particle etching of gold nanocrystals using in situ graphene liquid cell transmission electron microscopy (TEM). By systematically changing the chemical potential of the oxidative etching and then quantifying the facets of the nanocrystals, nonequilibrium processes of atom removal could be deduced. Etching at sufficiently high oxidation potentials, both cube and rhombic dodecahedra (RDD)-shaped gold nanocrystals transform into kinetically stable tetrahexahedra (THH)-shaped particles. Whereas {100}-faceted cubes adopt an { hk0}-faceted THH intermediate where h/ k depends on chemical potential, {110}-faceted RDD adopt a {210}-faceted THH intermediate regardless of driving force. For cube reactions, Monte Carlo simulations show that removing 6-coordinate edge atoms immediately reveals 7-coordinate interior atoms. The rate at which these 6- and 7-coordinate atoms are etched is sensitive to the chemical potential, resulting in different THH facet structures with varying driving force. Conversely, when RDD are etched to THH, removal of 6-coordinate edge atoms reveals 6-coordinate interior atoms. Thus, changing the driving force for oxidation does not change the probability of edge atom versus interior atom removal, leading to a negligible effect on the kinetically stabilized intermediate shape. These fundamental insights, facilitated by single-particle liquid-phase TEM imaging, provide important atomic-scale mechanistic details regarding the role of kinetics and chemical driving force in dictating shape transformations at the nanometer length scale

Single-particle mapping of nonequilibrium nanocrystal transformations.

Chemists have developed mechanistic insight into numerous chemical reactions by thoroughly characterizing nonequilibrium species. Although methods to probe these processes are well established for molecules, analogous techniques for understanding intermediate structures in nanomaterials have been lacking. We monitor the shape evolution of individual anisotropic gold nanostructures as they are oxidatively etched in a graphene liquid cell with a controlled redox environment. Short-lived, nonequilibrium nanocrystals are observed, structurally analyzed, and rationalized through Monte Carlo simulations. Understanding these reaction trajectories provides important fundamental insight connecting high-energy nanocrystal morphologies to the development of kinetically stabilized surface features and demonstrates the importance of developing tools capable of probing short-lived nanoscale species at the single-particle level

New model, new strategies: Instructional design for building online wisdom communities.

We discuss the development of an instructional design model, WisCom (Wisdom Communities), based on socio-constructivist and sociocultural learning philosophies and distance education principles for the development of online wisdom communities, and the application and evaluation of the model in an online graduate course in the USA. The WisCom model aims to facilitate transformational learning by fostering the development of a wisdom community, knowledge innovation, and mentoring and learner support in an online learning environment, based on a Cycle of Inquiry module design, and a Spiral of Inquiry program design. Extending beyond current instructional design practice, WisCom provides both a new model for teaching that builds upon the inherent capacity of networked communication to support the growth and intellectual development of communities of practice, and a new model of learning where learners engage in the process of scholarly inquiry that supports individual and collective learning. Evaluation and research data support the WisCom model\u27s ability to design a learning community engaged in the collaborative construction of knowledge

Remembering the Work of Phillip L. Geissler: A Coda to His Scientific Trajectory.

Phillip L. Geissler made important contributions to the statistical mechanics of biological polymers, heterogeneous materials, and chemical dynamics in aqueous environments. He devised analytical and computational methods that revealed the underlying organization of complex systems at the frontiers of biology, chemistry, and materials science. In this retrospective we celebrate his work at these frontiers