The use of redox-active molecules as the active storage elements in memory chips requires the ability to attach the molecules to an electroactive surface in a reliable and robust manner. To explore the use of porphyrins tethered to silicon via carbosilane linkages, 17 porphyrins have been synthesized. Fourteen porphyrins bear a tether at a single meso site, and three porphyrins bear functional groups at two beta sites for possible two-point attachment. Two high-temperature processing methods (400 degrees C under inert atmosphere) have been developed for rapid (minutes), facile covalent attachment to Si platforms. The high-temperature processing conditions afford attachment either by direct deposition of a dilute solution (1 microM-1 mM) of the porphyrin sample onto the Si substrate or sublimation of a neat sample onto the Si substrate. The availability of this diverse collection of porphyrins enables an in-depth examination of the effects of the tether (length, composition, terminal functional group, number of tethers) and steric bulk of nonlinking substituents on the information-storage properties of the porphyrin monolayers obtained upon attachment to silicon. Attachment proceeds readily with a wide variety of hydrocarbon tethers, including 2-(trimethylsilyl)ethynyl, vinyl, allyl, or 3-butenyl directly appended to the porphyrin and iodo, bromomethyl, 2-(trimethylsilyl)ethynyl, ethynyl, vinyl, or allyl appended to the 4-position of a meso-phenyl ring. No attachment occurs with substituents such as phenyl, p-tolyl, mesityl, or ethyl. Collectively, the studies show that the high-temperature attachment procedure (1) has broad scope encompassing diverse functional groups, (2) tolerates a variety of arene substituents, and (3) does not afford indiscriminate attachment. The high-temperature processing conditions are ideally suited for use in fabrication of hybrid molecular/semiconductor circuitry.

During the past decade there has been intense interest in developing molecular-based devices for applications in electronics, including memory.[1±4] Among the various approaches towards building devices that incorporate molecules, the hybrid silicon/molecular approach is attractive as a transition technology because it leverages certain advantages afforded by a molecule-based active medium with the vast infrastructure of traditional metal±oxide±semiconductor (MOS) technology. Recently, we demonstrated that self-assembled monolayers (SAMs) of redox-active molecules on silicon are excellent candidates for hybrid memory devices. The SAMs were prepared using either a benzyl alcohol-tethered ferrocene (4-ferrocenylbenzyl alcohol, Fc-BzOH) or a benzyl alcohol-tethered porphyrin (5-(4-hydroxymethylphenyl)-10,15,20-trimesitylporphinatozinc(II), Por-BzOH). In addition to the neutral state, the Fc-BzOH provides one state (monopositive) while the Por-BzOH provides two states (monopositive, dipositive). The availability of charged states at distinct voltages is highly advantageous for applications in charge-storage memory devices such as dynamic random access memory (DRAM) and FLASH memory. In addition, the molecularbased devices exhibit very low write and erase voltages [5,6] and long charge-retention times. [7,8] One strategy to increase memory density entails a multibit approach wherein the charge-storage element contains molecules with multiple redox states. We have previously demonstrated this approach using a variety of porphyrinic molecules, including a ferrocene±porphyrin conjugate bearing a single thiol tether. [9] In this design, the characteristic oxidation potentials of the ferrocene and the porphyrin components are maintained, thereby affording four states (neutral and the three cationic oxidation states from the ferrocene and porphyrin). This approach is limited only by the requirements for synthesis of the covalently linked multiredox molecule. An alternative and perhaps simpler strategy for achieving multibit functionality is afforded by mixing different redoxactive molecules whose potentials are well separated. In this paper, we demonstrate this approach using mixed SAMs of Fc-BzOH and Por-BzOH on silicon surfaces to achieve a fourstate (2-bit) memory element. The four states include the neutral state and three distinct cationic states obtained upon oxidation of the Fc-BzOH (monopositive) and the Por-BzOH (monopositive, dipositive) molecules. Although mixed SAMs have been previously investigated, these studies typically involved mixtures of redox-active and non-redox-active molecules.[10±14] In our study, cyclic voltammetry (CV) has been used to measure the coverage and redox potentials of the mixed SAMs. Since each redox state represents the transfer of a single electron per molecule, the total measured charge per unit area can be directly converted to molecules per unit area. Conventional capacitance±voltage and conductance±voltage (C±V and G±V) methods have been used to further characterize the mix...

Self-assembled monolayers of 4-ferrocenylbenzyl alcohol attached to silicon provided the basis for electrolyte-molecule-silicon capacitors. Characterization by conventional capacitance and conductance techniques showed very high capacitance and conductance peaks near ∼0.6 V associated with charging and discharging of electrons into and from discrete levels in the monolayer owing to the presence of the redox-active ferrocenes. The reversible charge trapping of these molecules suggest their potential application in memory devices. Due to the molecular scalability and low-power operation, molecular-silicon hybrid devices may be strong candidates for next-generation electronic devices.

Hybrid silicon capacitors have been successfully fabricated by attaching monolayers of redox-active molecules via self-assembly to ultrathin silicon dioxide layers. Capacitance, conductance, and cyclic voltammetric measurements have been used to characterize these capacitors. The presence of distinct capacitance and conductance peaks associated with oxidation and reduction of the monolayers at low gate voltages indicates discrete electron storage states for these capacitors, suggesting their feasibility in memory devices. The inherent molecular scalability and low-power operation coupled with existing silicon technology support the approach of hybrid molecule-silicon devices as a strong candidate for next generation electronic devices.