Better Films for Electro-optics

MATERIALS CHEMISTRY

Using a vapor-deposition process, an international research team has demonstrated that transparent organic films with very high electro-optical responses can self-assemble through hydrogen-bonding interactions onto almost any type of substrate. The method could lead to a low-cost method for making electro-optical modulators and other high-speed switching devices to convert radio-frequency signals to optical signals for broadband telecommunications systems.

The research team that developed the new technique includes Antonio Facchetti and Tobin J. Marks of Northwestern University; Giorgio A. Pagani of the University of Milano-Bicocca, in Italy; and their coworkers [Nat. Mater., 3, 910 (2004)].

Self-organization achieved through hydrogen bonding is widely used in chemistry and materials science, but it has rarely been used to fabricate electro-optically active thin films, the researchers note. Using vapor deposition to grow electro-optic films is also still in its infancy, Marks says, but it offers the advantages of precise film thickness, smoothness, and high chemical purity. These processes are challenging to carry out because they require tuning the templating steps to generate films having a net polarization in one direction, which is necessary to achieve nonlinear optical properties, he adds.

Efficient electro-optic films can be made using layer-by-layer self-assembly or by film-transfer techniques. Another process, called poling, can reorient organic molecules in a deposited film by using an electric field to induce a dipole. Organic films having electro-optical properties exceeding those of typically used inorganic crystals, such as lithium niobate (LiNbO₃), have been made this way.

Poling, however, doesn't take full advantage of the large nonlinear optical properties that are possible using asymmetric organic molecules, notes Bernard Kippelen of Georgia Institute of Technology in an accompanying commentary. A more elegant approach is to produce noncentrosymmetric films (those without a center of symmetry) by self-assembly techniques, he says. That's what the Northwestern-Milano team has done.

The researchers designed and synthesized three different compounds containing heteroaromatic rings as precursors for the films. Each contains a pyridine ring on one end as a hydrogen-bond acceptor. Two of the compounds have a hydrazobenzoic acid group on the opposite end, which functions as a hydrogen-bond donor. The third molecule has a hydrazophenyl group on the end as the hydrogen-bond donor. The hydrogen-bonding end groups are linked together by either a pyrrole or a thiophene ring to form conjugated asymmetric molecules.

Several types of electro-optically active films were fabricated using the new molecules. In general, noncentrosymmetric films were grown by subliming the molecules and allowing them to self-assemble in successive layers on a substrate by head-to-tail alignment, as dictated by the hydrogen-bonding interactions of their end groups.

The electro-optical responses of the films at the 1.30- and 1.55-m telecommunications wavelengths are sufficient for many applications, the researchers note, with the pyrrole-based pyridine-benzoic acid film deposited on a siloxane-modified substrate showing the greatest promise. Its electro-optical response is more than two orders of magnitude above that reported for other hydrogen-bonded electro-optic films and approaches that of LiNbO₃ films.

The results reported by Facchetti, Marks, Pagani, and their coworkers "illustrate the strength and potential of self-assembly techniques using noncovalent inter-

molecular forces for the implementation of materials with complex architectures," Kippelen comments. "Further refinements of the technique are likely to generate films with even more impressive optical and electronic properties."-