Rectifying Self-Assembled Ultrathin Films
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Self-Assembled Ultrathin Films
T.P. Cassagneau, B. Sweryda-Krawiec, and J.H. Fendler Introduction “Soft solution processing” is rapidly becoming a viable approach for the fabrication of advanced nanostructured materials. It involves the use of environmentally friendly chemicals and preparation methods with minimum energy input. Construction of ultrathin films by the room-temperature, layer-by-layer self-assembly of dilute aqueous solutions (or dispersions) of polyelectrolytes (or polymers), nanoparticles, and nanoplatelets is clearly soft solution processing (see the articles by Yoshimura et al. in this issue). Biomineralization,1 the natural process of oriented crystallization within and/or on the cell membrane, represents the most efficient method of self-assembly. It is energyefficient, proceeds at ambient temperatures, is hierarchical, and generally accomplishes synthesis and topographic organization in a single step. The nacre of an abalone shell represents but one example where highly oriented self-assembly of alternative layers of aragonite (CaCO3) and biopolymers results in a laminated structure that is twice as hard and a thousand times tougher than either of its constituents.2 Little wonder that materials scientists have been and continue to be inspired by biomineralization and have used a membrane-mimetic approach to advanced materials syntheses.3 Rectifying self-assembled ultrathin films will be surveyed in this article. Following a brief description of the methodology and characterization of self-assembled films, construction of diode heterojunctions will be highlighted. Emphasis will be placed on work developed in our own laboratories.
Methodology of Self-Assembly The layer-by-layer adsorption of oppositely charged colloids was reported in a seminal paper in 1966.4 Self-assembly was subsequently “rediscovered” and extended
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to the preparation of multilayers of polycations and phosphonate anions,5 as well as to the layering of polyelectrolytes.6,7 Construction of electrodes coated with polyelectrolytes, clays, and other materials often involved self-assembly,8 although the method had not been called as such. Selfassembly is now routinely employed for the fabrication of ultrathin films from charged nanoparticles (e.g., metallic, semiconducting, magnetic, ferroelectric, and insulating materials), nanoplatelets (e.g., clays or graphite platelets), proteins, pigments, and other supramolecular species.9–11 Self-assembly of alternating layers of oppositely charged polyelectrolytes and nanoparticles (or nanoplatelets) is deceptively simple (see Figure 1). A well-cleaned substrate is immersed into a dilute aqueous solution of a cationic polyelectrolyte for a period of time optimized for adsorption of a monolayer (2 nm thick), then it is rinsed and dried. The next step is the immersion of the polyelectrolyte-monolayercovered substrate into a dilute dispersion of negatively charged nanoparticles (or nanoplatelets, or any other species with appropriate sizes and charge distributions), also for a period of
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