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The crystallization of amorphous solids impacts fields ranging from inorganic crystal growth to biophysics. Promoting or inhibiting nanoscale epitaxial crystallization and selecting its final products underpin applications in cryopreservation, semiconductor devices, oxide electronics, quantum electronics, structural and functional ceramics, and advanced glasses. As precursors for crystallization, amorphous solids are distinguished from liquids and gases by the comparatively long relaxation times for perturbations of the mechanical stress and for variations in composition or bonding. These factors allow experimentally controllable parameters to influence crystallization processes and to drive materials toward specific outcomes. For example, amorphous precursors can be employed to form crystalline phases, such as polymorphs of Al 2 O 3 , VO 2 , and other complex oxides, that are not readily accessible via crystallization from a liquid or through vapor-phase epitaxy. Crystallization of amorphous solids can further be guided to produce a desired polymorph, nanoscale shape, microstructure, or orientation of the resulting crystals. These effects enable advances in applications in electronics, magnetic devices, optics, and catalysis. Directions for the future development of the chemical physics of crystallization from amorphous solids can be drawn from the structurally complex and nonequilibrium atomic arrangements in liquids and the atomic-scale structure of liquid–solid interfaces. 

Surface diffusion has been measured in the glass of an organic semiconductor, MTDATA, using the method of surface grating decay. The decay rate was measured as a function of temperature and grating wavelength, and the results indicate that the decay mechanism is viscous flow at high temperatures and surface diffusion at low temperatures. Surface diffusion in MTDATA is enhanced by 4 orders of magnitude relative to bulk diffusion when compared at the glass transition temperature T g . The result on MTDATA has been analyzed along with the results on other molecular glasses without extensive hydrogen bonds. In total, these systems cover a wide range of molecular geometries from rod-like to quasi-spherical to discotic and their surface diffusion coefficients vary by 9 orders of magnitude. We find that the variation is well explained by the existence of a steep surface mobility gradient and the anchoring of surface molecules at different depths. Quantitative analysis of these results supports a recently proposed double-exponential form for the mobility gradient: log  D( T, z) = log  D v ( T) + [log  D 0 − log  D v ( T)]exp(− z/ξ), where D( T, z) is the depth-dependent diffusion coefficient, D v ( T) is the bulk diffusion coefficient, D 0 ≈ 10 −8  m 2 /s, and ξ ≈ 1.5 nm. Assuming representative bulk diffusion coefficients for these fragile glass formers, the model reproduces the presently known surface diffusion rates within 0.6 decade. Our result provides a general way to predict the surface diffusion rates in molecular glasses. 

Improved energy levels for singly ionized and neutral hafnium of both even and odd parity are determined from Fourier transform spectrometer data using a least-squares optimization procedure. Data from interferometric spectrometers provide much tighter control of systematic uncertainties of line position measurements than can be achieved using dispersive spectrometers. The strong optical and near-UV lines connecting these levels are most likely to be used in the determination of isotopic abundance patterns. Comparisons of new results to published ones strongly suggest that our energy levels have systematic uncertainties in the mK (1 mK = 0.001 cm⁻¹) range or smaller, and that widely used tables of energy levels for ionized Hf have systematic errors of approximately 70 mK.

 

Electrochemical synthesis of hydrogen peroxide (H 2 O 2 ) in acidic solution can enable the electro-Fenton process for decentralized environmental remediation, but robust and inexpensive electrocatalysts for the selective two-electron oxygen reduction reaction (2e − ORR) are lacking. Here, we present a joint computational/experimental study that shows both structural polymorphs of earth-abundant cobalt diselenide (orthorhombic o -CoSe 2 and cubic c -CoSe 2 ) are stable against surface oxidation and catalyst leaching due to the weak O* binding to Se sites, are highly active and selective for the 2e − ORR, and deliver higher kinetic current densities for H 2 O 2 production than the state-of-the-art noble metal or single-atom catalysts in acidic solution. o -CoSe 2 nanowires directly grown on carbon paper electrodes allow for the steady bulk electrosynthesis of H 2 O 2 in 0.05 M H 2 SO 4 with a practically useful accumulated concentration of 547 ppm, the highest among the reported 2e − ORR catalysts in acidic solution. Such efficient and stable H 2 O 2 electrogeneration further enables the effective electro-Fenton process for model organic pollutant degradation.