Robinson group has three journal cover articles in 2018

The Robinson group’s research has been recognized by being awarded journal covers for three publications in 2018. The works are in the area of nanoparticles and x-ray emission spectroscopy characterization methods for nanomaterials, and make use of the Cornell synchrotron (CHESS) for advanced characterization of materials.

A. Nelson, S. Honrao, R.G. Hennig, and R.D. Robinson, “Nanocrystal Symmetry Breaking and Accelerated Solid-State Diffusion in the Lead−Cadmium Sulfide Cation Exchange system,” Chemistry of Materials (accepted 2018). DOI: 10.1021/acs.chemmater.8b04490

In this work, the team found a surprising departure from the expected behaviour of atom diffusion. Understanding diffusion can be a key to stabilizing structures, like metals, against collapse into thermodynamically preferable but less useful forms, like rust. Controlling diffusion is also vitally important to microelectronics and other nanotechnology where the drift of atoms over only a few nanometers can jeopardize the performance of the device. Although thermodynamics explains why such drift occurs, only direct measurements of the speed of diffusive processes can determine how the atoms move on short length scales. Using x-ray diffraction, the Robinson group measured how quickly ions diffuse through cadmium sulfide in the form of a shell only 1-2 nm thick around a nanoparticle core of lead sulfide. Surprisingly, they found that atoms practically flew through the shell, diffusing up to 10,000 times faster than would be expected from all previously reported bulk values. The team surmised that there is a large and unexplained difference between diffusion in bulk crystals and that of nanoparticles. Fast solid-state diffusion at the nanoscale shows that nanoparticles are far more structurally dynamic and imperfect objects than the large-scale properties of their parent materials would suggest.

A. Bhargava, C.Y. Chen, K.D. Finkelstein, M.J. Ward, R.D. Robinson, “X-ray Emission Spectroscopy: An Effective Route to Extract Site Occupation of Cations,” Phys. Chem. Chem. Phys. 20, 28990 (2018). DOI: 10.1039/c8cp04628j  

• Included in themed collection: 2018 PCCP HOT Articles

Understanding the distribution of cations in an atomic lattice is important because it can influence a wide range of material properties. One example is spinel oxides, where, depending on how the transition metal cations are arranged in the lattice, the electronic conductivity can change by orders of magnitude. In this work, the Robinson group demonstrated that x-ray emission spectroscopy (XES) can be used as an accurate and superior method of extracting cation site occupation. They took advantage of spin, ligand geometry, and oxidation state sensitivity of the K-β1,3 (3p - 1s) emission line at the Cornell High Energy Synchrotron to study the geometric structure of a nanoparticle spinel system. By comparing their XES results to the common method of extended x-ray absorption fine structure (EXAFS), they found that while both techniques reach similar findings, XES is superior because it provides not only elemental site occupation, but also oxidation state site occupation of all cation species.

D.R. Nevers* C.B. Williamson*, B.H. Savitzky, I.H. Hadar, U. Banin, L.F. Kourkoutis, Tobias Hanrath†, and R.D. Robinson†, “Mesophase Formation Stabilizes High-purity Magic-sized Clusters,” Journal of the American Chemical Society 140, 3652 (2018) DOI: 10.1021/jacs.7b12175

In this work, the team developed a new nanosynthetic chemical pathway to achieve ultra-pure and highly stable groups of same-sized nanoparticles – known as “magic-sized clusters.” Their magic clusters are two orders of magnitude more pure any cluster previously reported, enabling their use in chemical reactions without the influence of larger nanoparticles that have obscured and complicated all earlier work in this area. The clusters grow within an organic-inorganic ordered mesophase that stabilizes the particles and prevents Ostwald ripening. The discovery builds upon their synthesis methods that demonstrate a crossover in behavior by tuning concentration: ultra-high concentration synthesis promotes a well-defined reaction pathway to produce high purity MSCs (>99.9%) and forms a mesophase assembly that kinetically arrests the magic sized clusters from nanoparticle growth. Using in-situ x-ray analysis at the Cornell High Energy Synchrotron Source (CHESS) they discovered that a highly-ordered mesophase instantly forms around the clusters upon nucleation with very large grains (hundreds of nanometers) and faster than detection limits (within seconds).  This mesophase had not previously been seen in nanosynthetic particle growth, especially with such high structure, and was the key to stabilizing the clusters from further growth.