ABSTRACT
A kinetic Monte-Carlo methodology is presented for simulating crystal growth in materials which contain stacking faults. By simulating a large number of potential growth and dissolution events, a representation of the crystal is generated at various stages throughout the crystallisation, allowing the effects of disorder on the evolution of crystal habit and nanoscale surface topography to be explored. As examples, simulations were performed on two intergrown zeolite materials - zeolite T and zeolite beta. In both zeolite T and zeolite beta, simulations demonstrate how an intergrown structure leads to a characteristic roughening of certain crystal facets. In zeolite beta, this is accompanied by the development of internal defects which shows a non-homogeneous distribution. Results of simulations are validated by direct comparison to experimental scanning electron microscopy, atomic force microscopy and X-ray diffraction data. All simulations are performed using the CrystalGrower software package with modifications to account for disorder and should be generally applicable to all classes of crystals.
ABSTRACT
A Monte Carlo crystal growth simulation tool, CrystalGrower, is described which is able to simultaneously model both the crystal habit and nanoscopic surface topography of any crystal structure under conditions of variable supersaturation or at equilibrium. This tool has been developed in order to permit the rapid simulation of crystal surface maps generated by scanning probe microscopies in combination with overall crystal habit. As the simulation is based upon a coarse graining at the nanoscopic level features such as crystal rounding at low supersaturation or undersaturation conditions are also faithfully reproduced. CrystalGrower permits the incorporation of screw dislocations with arbitrary Burgers vectors and also the investigation of internal point defects in crystals. The effect of growth modifiers can be addressed by selective poisoning of specific growth sites. The tool is designed for those interested in understanding and controlling the outcome of crystal growth through a deeper comprehension of the key controlling experimental parameters.
ABSTRACT
The modular structure of metal-organic framework nanosheets (MONs) provides a convenient route to creating two-dimensional materials with readily tuneable surface properties. Here, the liquid exfoliation of two closely related layered metal-organic frameworks functionalised with either methoxy-propyl (1) or pentyl (2) pendent groups intended to bestow either hydrophilic or hydrophobic character to the resulting nanosheets is reported. Exfoliation of the two materials in a range of different solvents highlighted significant differences in their dispersion properties, as well as their molecular and nanoscopic structures. Exchange or loss of solvent was found to occur at the labile axial position of the paddle-wheel based MONs and DFT calculations indicated that intramolecular coordination by the oxygen of the methoxy-propyl pendant groups may take place. The nanoscopic dimensions of the MONs were further tuned by varying the exfoliation conditions and through "liquid cascade centrifugation". Aqueous suspensions of the nanosheets were used as sensors to detect aromatic heterocycles with clear differences in binding behaviour observed and quantified.
