Self-assembly and self-organization are widely believed to be central issues to the utilization of nanoscaled materials in real-world fabrication and application of devices based technologies and/or functions exploiting this scale. Conceptually, self-assembly is used to design and form structures wherein the function allows little tolerance for defects and generally uses specific intermolecular interactions to form specific, discrete, supramolecular structures albeit with varying yields. Self-organization, on the other hand, is used when there is more tolerance for defects and generally relies on a combination of non-specific interactions. The inherent redundancies in the latter mitigate the defects, but high quality crystals needed for some applications are a notable exception. In these regards one may look upon the organization and structure of mater into functional nanoscaled materials as falling into four broad motifs.
The primary structure is molecular structure -- of which we have exquisite control by the highly developed principles of organic chemistry.
The secondary structure is supramolecular structure -- again a well-developed field based on well-understood principles.
The tertiary structure describes how the supermolecules interact to form a higher ordered material that can be aggregate or crystalline -- though significant progress has been made to design and predict hierarchical structure, this area is still developing.
The quaternary structure describes how the self-assembled and/or self-organized material self-incorporates into the device or devices and may include the interconnects to the macroscopic world. The quaternary structure is the least developed and the least studied aspect of nanoscaled materials.
We report a model system wherein all of these self-processed are exploited to make functional phototransistors. Beyond simple substituent and environmental factors that affect any chromophore, further modulation of the chemistry is accomplished by the relative order imposed in supramolecular arrays.
The synthesis of zinc porphyrin molecules appended with H-bonding groups or exocyclic ligands represent the primary structure. The self-assembly of these into linear supramolecular arrays is the secondary structure. These self-assembled tapes can be placed in an 8 nm thick by ~1 mm2 lipid bilayer separating two compartments with 0.1 M salt solutions where they orient perpendicular to the bilayer-water interface and can adjust their length to the width of the bilayer. The orientation and length adjustment in the bilayer is an example of tertiary organization of matter into a material. The quaternary structure, in terms of a functional device, results when an electron donor (source) is placed on one side of the bilayer, an acceptor on the other side (drain), and calomel electrodes connected to an operational amplifier are placed in the salt solutions on each side of the membrane. In this device white light (or monochromatic light that corresponds to the porphyrin absorption bands) serves as the gate. Thus, when the membrane device is illuminated a photocurrent is observed.[1, 2]
Drain CM. PNAS 2002; 99: 5178-5182.
Drain CM, Batteas JD, Flynn GW, Milic T, Chi N, Yablon DG, Sommers H. PNAS 2002; 99: 6498-6502.