Multibit Memory Using Self‐Assembly of Mixed Ferrocene/Porphyrin Monolayers on Silicon

Abstract

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...

The molecular level modification of surfaces: from self-assembled monolayers to complex molecular assemblies

“…Chemical functionalization of silicon oxide surfaces with silane molecules is an important technique for a variety of device fabrication. [20][21][22] One of the main interests is the formation of silanized glass surfaces and their use in non-linear optical devices, 23 or in sensor applications. 24,25 In 1980 Sagiv published a seminal paper reporting the reaction of chloro-and alkoxy-silanes (RSiCl 3 and RSi(OR 0 ) 3 with R = C 18 H 37 and R 0 = Me or Et) with hydroxyl-terminated surfaces of oxidized substrates to afford a covalent molecular layer.…”

“…For many applications, whether a monolayer, or rather an ill-defined multilayer is formed, is not a significant issue. On the other hand, when using these systems as the base layer upon which truly molecular devices will be fabricated, 22 lack of control in the SAM growth can be a major concern. Another challenge with using organosilanes for modifying SiO 2 surfaces is the lack of stability in basic media.…”

Wet chemical routes to the assembly of organic monolayers on silicon surfaces via the formation of Si–C bonds: surface preparation, passivation and functionalization

“…[23][24][25] As comprehensively reviewed elsewhere, [26][27][28][29][30] and as discussed to an introductory level in a following section of this review (Section 2), formation of molecular layers on crystalline silicon surfaces without an intervening oxide layer is an extremely appealing approach toward robust layers on a surface. [31][32][33][34] Silicon-carbon linked monolayers on silicon substrates are expected to complement and/or extend the applications of the currently most relevant technological material 35 toward the development of atomic scale devices, [36][37][38] diverse molecular devices, [39][40][41][42][43][44][45][46][47][48] and well-defined sensing interfaces. [49][50][51][52][53] The following sections are intended as an up-to-date presentation of the topic of molecular layers on non-oxidized silicon surfaces.…”

Mixed assembly of ferrocene/porphyrin onto carbon nanotube arrays towards multibit information storage

“…For example, nanoscale molecularelectronic circuits consisting of a molecular monolayer of rotaxanes (~1100 molecules) have been successfully fabricated and tested by Yong Chen and his co-workers [2]. Recently Lindsey, Bocian and Mirsa have demonstrated that selfassembled monolayers of electrochemical-active molecules (such as porphyrin and ferrocene) on silicon or gold look to be a promising alternative for next generation memories [3,4]. We have demonstrated surface-mounted ferrocene using covalent immobilisation of an alcohol substituted ferrocene derivative to a pre-assembled single-walled carbon nanotubes directly anchored to a silicon (100) surface (SWCNT-Si) [5], which gave a much higher surface concentration of ferrocenemethanol molecules; about 500-1000 times greater than the experimentally measured coverage of ferrocene directly attached to flat Si(100) surfaces.…”