Oriented crystal growth of 4-lodo-4'-nitrobiphenyl on polar self-assembled monolayer templates: A case for "Chemical epitaxy"

Self-assembled monolayers of 4-X'-mercaptobiphenyl and 4-X-benzenethiol (X = NO2, L and H) were used as substrates to direct the nucleation and subsequent oriented growth of 4'-iodo-4'-nitrobiphenyl (INBP) polar crystals. INBP exhibits a solvent-dependent morphology, developing either as prisms bounded by {011}, {111}, and {010} faces or as needles elongated along the crystallographic c axis, from growth in 3:1 EtOH/CH2Cl2 and benzene solutions, respectively. The introduction of SAM templates into the different crystal growth solutions resulted in prism formation on all SAMs, and needle formation on SAMs of 4'nitro- and 4'-iodo-4-mercaptobiphenyl. The contacting planes of INBP observed at the crystal interface were {001} and {011}, surfaces which have 2D lattice periodicities that are related by coincident epitaxy to the monolayer. In all cases, the absolute direction of the crystal's polar axis, as determined by the pyroelectric effect and copper iodide chemistry, was determined by the directionality of nitro... iodo interactions across the interface. Nitro-terminated SAM surfaces promoted growth along the -c crystallographic axis, while iodo-terminated SAM surfaces promoted growth along +c. This study demonstrates that the oriented growth of polar molecular crystal phases can be rationally achieved by using geometrically and chemically compatible SAM templates.