Novel Hybrid Covalent / Ionic Self-Assembly Technique for Improved Second-Order Nonlinear Optical Films
- PDF / 85,507 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 68 Downloads / 194 Views
Novel Hybrid Covalent / Ionic Self-Assembly Technique for Improved Second-Order Nonlinear Optical Films P. J. Neymana, M. Guzyb, S. M. Shahb, R. M. Davisb, K. E. Van Cottb, H. Wangc, H. W. Gibsonc, C. Brandsd, J. R. Heflina,d a Department of Materials Science and Engineering, Virginia Tech, Blacksburg, VA 24061 b Department of Chemical Engineering, Virginia Tech, Blacksburg, VA 24061 c Department of Chemistry, Virginia Tech, Blacksburg, VA 24061 d Department of Physics, Virginia Tech, Blacksburg, VA 24061 ABSTRACT Ionically self-assembled monolayer (ISAM) films have been shown to spontaneously produce noncentrosymmetric ordering that gives rise to a substantial second order nonlinear optical (NLO) response. Typically, the ISAM films for NLO response are an assemblage of bilayers of oppositely charged polymers whose thickness can be controlled through variation of pH and ionic strength of the immersion solutions. Here, we investigate the effects of replacing the NLO-active polymer layers with layers of monomeric chromophores containing ionic and covalent bonding sites. Films fabricated exclusively using polyelectrolytes contain some fraction of both randomly oriented and anti-parallel oriented chromophores. We have examined the incorporation of monomeric chromophores into ISAM films in order to increase the net polar orientation of the chromophores and reduce bilayer thickness. INTRODUCTION The formation of ionically self-assembled monolayer (ISAM) films, grown monolayer by monolayer by alternately immersing a charged substrate in anionic and cationic polymer solutions, has proven to be an easy, fast, and inexpensive method for creating laterally homogeneous nanostructured thin films[1,2]. Upon each immersion, an exceptionally homogeneous layer is rapidly (less than one minute) adsorbed and the thickness of each monolayer can be controlled to be 0.3 to >5.0 nm by varying the pH and ionic strength of the solutions. A few groups, including ours, have been examining the use of the ISAM technique for the production of films with a second order nonlinear optical (NLO) response because of the potential for dramatically improved electro-optic devices[3-10]. These films show substantial χ(2) values with excellent temporal and thermal stability. In addition, the study of the structure of the ISAM films via second harmonic generation (SHG) affords valuable insight for other applications. Films fabricated with an NLO polymer result in significant cancellation of the chromophore orientations. This cancellation occurs by two mechanisms: competitive orientation due to the ionic bonding of the polymer chromophore with the subsequent polycation layer, and random orientation of the chromophores within the bulk of each polyanion layer. A reduction in film thickness accompanied by an increase in net polar ordering is one possible avenue to obtain the χ(2) necessary for efficient electro-optic devices. In this paper, we introduce a hybrid covalent / ionic self-assembly technique that utilizes monomer chromophores with ionic and covalent
Data Loading...
