AVS 47th International Symposium
    Processing at the Nanoscale/NANO 6 Tuesday Sessions
       Session NS+NANO6+MM-TuM

Paper NS+NANO6+MM-TuM1
Nanomechanical Properties of Molecular Organic Thin Films

Tuesday, October 3, 2000, 8:20 am, Room 302

Session: Nanomechanics
Presenter: J. Fraxedas, Institut de Ciencia de Materials de Barcelona (CSIC), Spain
Authors: J. Caro, Institut de Ciencia de Materials de Barcelona (CSIC), Spain
P. Gorostiza, Universitat de Barcelona, Spain
F. Sanz, Universitat de Barcelona, Spain
J. Fraxedas, Institut de Ciencia de Materials de Barcelona (CSIC), Spain
Correspondent: Click to Email
Using Atomic Force Microscopy we have studied the nanomechanical response to nanoindentations of surfaces of highly-oriented molecular organic (MO) thin films (thickness < 1000 nm). Fundamental parameters as the Young' s modulus E, unknown for most MO materials, can be estimated from the elastic deformation using Hertzian mechanics. In the case of the quasi-one-dimensional MO conductor tetrathiafulvalene tetracyanoquinodimethane (TTF-TCNQ) we obtain E ~ 22 GPa, in excellent agreement with previous reported values obtained on single crystals using neutron scattering (E ~ 20 GPa).@footnote 1@ Above ~ 200 nN the surface deforms plastically as evidenced by discrete discontinuities in the indentation curves (~ 1 nm) associated to molecular layers being expelled by the penetrating tip. The estimated critical shear stress @tau@ is ~ 2 GPa. Nanoindentation permits the determination of nanomechanical parameters of MO metastable polymorphs. This is illustrated with the MO radical p-nitrophenyl nitronyl nitroxide (p-NPNN). The @alpha@-phase of p-NPNN, stabilized as thin film,@footnote 2@ exhibits values of E and @tau@ two times smaller than the corresponding values of the thermodynamically most stable @beta@-phase. Measurements were performed with the same tip under the same experimental conditions, thus eliminating the uncertainty associated to the cantilever constant and tip radius. @FootnoteText@@footnote 1@J. P. Pouget et al. Phys. Rev. B 19 (1979) 1792. @footnote 2@J. Fraxedas et al. Europhys. Lett. 48 (1999) 461.