Direction-specific interaction forces underlying zinc oxide crystal growth by oriented attachment

Crystallization by particle attachment is impacting our understanding of natural mineralization processes and holds promise for novel materials design. When particles assemble in crystallographic alignment, expulsion of the intervening solvent and particle coalescence are enabled by near-perfect co-alignment via interparticle forces that remain poorly quantified. Here we report measurement and simulation of these nanoscale aligning forces for the ZnO (0001)-ZnO(000 (1) over bar) system in aqueous solution. Dynamic force spectroscopy using nanoengineered single crystal probes reveals an attractive force with 60 degrees rotational periodicity. Calculated distance and orientation-dependent potentials of mean force show several attractive free energy wells distinguished by numbers of intervening water layers, which reach a minimum when aligned. The calculated activation energy to separate the attractively bound solvated interfaces perfectly reproduces the measured 60 degrees periodicity, revealing the key role of intervening water structuring as a basis to generate the interparticle torque that completes alignment and enables coalescence.