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Speaker Spotlight - Article:
Convenient Approaches to Molecular Electronics
Maria A. Rampi

Professor M. A. Rampi
Dipartimento di Chimica, Via Borsari 46,
Università di Ferrara, 44100 Ferrara, Italy
E-mail: rmp (at) unife.it

Whether or not molecular species will be used in practical microelectronic devices is still a matter of discussion. Certainly organic and organometallic molecules deserve consideration as components of devices, both for their electronic structure tunable with great precision by synthesis, and for self assembling in structures that provide a convenient route toward fabrication of functional systems.

The most accurate information about electron transfer processes through organic molecules has been provided by a large number of studies performed by transient spectroscopy in supramolecular systems, D-B-A, where D and A are respectively electron donor and electron acceptor units covalently linked by a molecular bridge, B.¹

In the last decades, the exponential implementation of nanofabrication technology and of scanning probe microscopies performances made it possible to study electron transfer processes by measuring the current flowing through molecules between metal electrodes in metal-molecule(s)-metal junctions and open the door to the field of "molecular electronics".^2,3 The interest of the scientific community for this research topic is well represented by the exponential increase of studies correlating the electronic structure of organic molecules with the current-voltage characteristics.

Still, after a decade of intense activity in this field, the "conductivity" of organic molecules seems to be elusive.⁴ Two main, still open problems are possibly responsible for this deficiency. One is related to the experimental challenge in fabricating metal-molecule(s)-metal junctions able to provide a large number of reproducible data. The chemical and mechanical fragility of the organic compounds, either as single molecules or organized in self assembled monolayers (SAMs) leads often to short circuits between the metal electrodes. The second problem is related to the unknown role played by the metal-molecule interaction in contributing to the "resistivity" of a metal-molecule(s)-metal system.⁵

In the attempt to answer these questions, a large number of different kind of molecular junctions have been assembled by using a combination of sophisticated and expensive techniques. Significantly, regardless their geometry, the behavior of the resulting function seems to be always related to the structure of the incorporated molecules. Each type of junction – break^{2, 6} and planar junctions,^7,8 those based on crossed gold wires⁹ - incorporates specific advantages and disadvantages with respect to fabrication, reproducibility, and application. It is interesting to note that while a great effort was addressed to the measurement of current flowing through single or few molecules - by using break junctions, Scanning Tunneling Microscopy and modified Atomic Force Microscopy - only a few studies were devoted to measuring electrical properties of molecules in large assemblies. Certainly ultrathin layers or monolayers of organic molecules (SAMs) are better accessible systems than single molecules for application in organic electronics and can be incorporated more easily between two metal electrodes.

In the attempt to fabricate junctions M1-SAM//M2, able to characterize electrical properties of organic SAMs organized on metal surface M1, a number of different strategies have been employed to fabricate the top electrode M2: deposition of cold gold,¹⁰ fabrication of such as lift-off float-on Au pads,¹¹ nano-transfer printing technique of gold electrodes,¹² deposition of functionalized single walled carbon nanotubes,¹³ intercalation of ultrathin film of conjugated polymer to prevent shortages.^14,15

A convenient approach for fabricating metal-SAM-metal junctions has been adopted by several groups: these junctions are based on electrodes formed by hanging mercury drops. Mercury as a liquid, atomically flat metal performs soft contact with organic molecules organized in SAMs on solid surfaces, then preventing damages to delicate organic structures, and can bind organic molecules via thiol anchoring group to forms well ordered SAMs. On the basis of these characteristics, a variety of junctions based on Hg electrodes have been assembled: 1) junctions based on two Hg electrodes, each one covered by SAMs,^16,17 Hg-SAM//SAM-Hg, 2) junctions based on one Hg electrode and one solid surface consisting of metals M (M= Au, Ag, Pt, Pd), Hg-SAM1//SAM2-M, ^18-20 doped silicon surface, Hg-SAM//Si, ²¹ or reconstructed graphite, Hg//SAM-graphite.²² These junctions are characterized by the several advantages: i) they are very easy to be assemble and relatively un-expensive, ii) they are mechanically stable and provide reproducible results, iii) able to incorporate molecular systems of increasing complexity iv) suitable to perform in electrical and electrochemical mode. They have been used for studies correlating electron transfer rate with the electronic structure of organic molecules,^16-19 to test NDR effects,²⁰ and to study the effect of metal-molecules interfaces.^21,22 These junctions represent indeed convenient test-beds able to provide fundamental information in the field of molecular electronics, in spite of the main disadvantage of incorporating an environmental unfriendly metal as Hg.

The second open question in molecular electronics is related to the nature of the contact between molecules and the metal surface as demonstrated theoretically ²³ and experimentally. ²¹ A barrier to the charge injection could lead to drops in electrostatic potential at the interface and affect the mechanism of charge transfer and the values of the transported current. In order to gain fundamental information about the ability of organic molecules to transport electrons in spite of such an unsolved problem, a convenient approach relays on the relation holding for electron transfer tunneling mechanism,

that describes the dependence of the measured current I on the length of the molecule, d, through a decay factor β. In a first approximation, the β parameter, is related to the molecular electronic structure by the equation

   (2)
where E is the energy of the incident electron, equal, at zero bias, to the Fermi energy of the metal contacts. For a metal-contact system, β is related to the barrier φo= (E_LUMO-E) for tunneling through the molecular LUMO, or φo= (E-E_HOMO) for tunneling through the molecular HOMO. Equation 2 shows the direct relationship between the β factor and the electronic structure of the organic molecules, represented by the energy of the molecular orbitals. The β parameter provides then an important correlation between the electrical property and the electronic structure of the molecules.

Measurements of current flowing through molecules of different length and same electronic structure allow to calculate the decay factor, β according to equation 1. Since the metal-molecules distance represents a constant contribution when the molecular length is varied, the unknown role of the interface does not enter into the calculation of β value. Unknown but constant parameters - as the molecule-metal interactions, the undefined interface between the two facing electrode-SAMs, or between the SAM and the top electrode - represents a contribution to the total resistance of the molecular system but do not affect the information related to the electrical properties of the backbone of organic molecular wires.

Equation 1, allows also to calculate the "resistance" of the metal-molecule contact, for different metals and different anchoring groups.^19,24 These values can be obtained by extrapolating the linear plot Ln I/Io= -βd of current values measured at different molecular length, to the length corresponding to the metal-molecule distance.

In conclusion, this approach allows for a direct correlation between the electronic structure and the electrical properties of the backbone of the organic molecule and to estimate the contribution of the interface metal-molecule to the total resistance of the metal-molecules-metal system. In spite of the mentioned problems, junctions able to supply reproducible measurements and therefore allowing for comparisons of current values flowing through different molecules and different metal-molecule interactions are able to provide fundamental information. A meaningful example is represented by the studies performed by conductive AFM ²⁴or by junctions²⁵ where each electrode has been covered by SAMs incorporating molecules with different functional head groups, then providing different types of interactions at the interface Metal-SAM1~ SAMs-Metal. The results show the efficiency of the electronic coupling provided by covalent, H bond, Van der Waals interactions in electron transfer processes. Comparison of current values flowing through junctions have recently indicated highly conjugated units, as haxabenzacoronene, to be "transparent" to electrons respect to aliphatic chains ²⁶ and redox centers incorporated in organic backbone as units for facilitating electron transfer processes. ²⁷

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