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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
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