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Molecules for Charge-Based Information
Storage
, †
JONATHAN S. LINDSEY* AND DAVID F. BOCIAN* , ‡
†
Department of Chemistry, North Carolina State University, Raleigh, North
‡
Carolina 27695-8204, United States, and Department of Chemistry, University
of California, Riverside, California 92521-0403, United States
RECEIVED ON APRIL 5, 2011
CONSPECTUS
he inexorable drive to miniaturize information storage and processing devices has fueled the dreams of scientists pursuing
T molecular electronics: researchers in the field envisage exquisitely tailored molecular materials fulfilling the functions now
carried out by semiconductors. A bottom-up assembly of such all-molecular devices would complement, if not supplant, the present
top-down lithographic procedures of modern semiconductor fabrication. Short of these grand aspirations, a more near-term
objective is to construct hybrid architectures wherein molecules are incorporated in semiconductor-based devices. Such a combined
approach exploits the advantages of molecules for selected device functions while retaining the well-developed lithographic
approaches for fabrication of the overall chip.
In this Account, we survey more than a decade of results from our research programs to employ porphyrin molecules as
charge-storage elements in hybrid semiconductormolecular dynamic random access memory. Porphyrins are attractive for a
variety of reasons: they meet the stability criteria for use in real-world applications, they are readily prepared and tailored
synthetically, they undergo readwrite processes at low potential, and they store charge for extended periods (up to minutes) in
the absence of applied potential. Porphyrins typically exhibit two cationic redox states. Molecular architectures with greater than
two cationic redox states are achieved by combinations of porphyrins in a variety of structures (for example, dyads, wherein the
porphyrins have distinct potentials, triple deckers, and dyads of triple deckers). The incorporation of porphyrins in hybrid
architectures has also required diverse tethers (alkyl, alkenyl, alkynyl, aryl, and combinations thereof) and attachment groups
(alcohol, thiol, selenol, phosphonate, and hydrocarbon) for linkage to a variety of surfaces (Au, Si, SiO 2 , TiN, Ge, and so forth).
The porphyrins as monolayers exhibit high charge density and are robust to high-temperature excursions (400 °C for 30 min)
under inert atmosphere conditions. Even higher charge densities, which are invaluable for device applications, were achieved by in
situ formation of porphyrin polymers or by stepwise growth of porphyrinimide oligomers. The various molecular architectures
have been investigated by diverse surface characterization methods, including ellipsometry, atomic force microscopy, FTIR
spectroscopy, and X-ray photoelectron spectroscopy, as well as a variety of electrochemical methods. These studies have further
revealed that the porphyrin layers are robust under conditions of deposition of a top metal contact.
The results to date indicate the superior features of selected molecular architectures for molecular electronics applications. The
near-term utilization of such materials depends on further work for appropriate integration in semiconductor-based devices,
whereas ultimate adoption may depend on advances that remain far afield, such as the development of fully bottom-up assembly
processes.
Introduction will fail to retain their characteristic properties as sizes
The development of molecular-based materials for electro- decrease to nanoscale dimensions. The use of molecules
nics applications has been stimulated by the prospect that in electronic devices is attractive owing to the intrinsic
devices relying on the bulk properties of semiconductors scalability of molecular properties and the ability to tune
638 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 638–650 ’ 2011 ’ Vol. 44, No. 8 Published on the Web 05/31/2011 www.pubs.acs.org/accounts
10.1021/ar200107x & 2011 American Chemical Society