Promising anchoring groups for single-molecule conductance measurements

Abstract

The understanding of the charge transport through single molecule junctions is a prerequisite for the design and building of electronic circuits based on single molecule junctions. However, reliable and robust formation of such junctions is a challenging task to achieve. In this topical review, we present a systematic investigation of the anchoring group effect on single molecule junction conductance by employing two complementary techniques, namely scanning tunneling microscopy break junction (STM-BJ) and mechanically controllable break junction (MCBJ) techniques, based on the studies published in the literature and important results from our own work. We compared conductance studies for conventional anchoring groups described earlier with the molecular junctions formed through π-interactions with the electrode surface (Au, Pt, Ag) and we also summarized recent developments in the formation of highly conducting covalent Au-C σ-bonds using oligophenyleneethynylene (OPE) and an alkane molecular backbone. Specifically, we focus on the electron transport properties of diaryloligoyne, oligophenyleneethynylene (OPE) and/or alkane molecular junctions composed of several traditional anchoring groups, (dihydrobenzo[b]thiophene (BT), 5-benzothienyl analogue (BTh), thiol (SH), pyridyl (PY), amine (NH2), cyano (CN), methyl sulphide (SMe), nitro (NO2)) and other anchoring groups at the solid/liquid interface. The qualitative and quantitative comparison of the results obtained with different anchoring groups reveals structural and mechanistic details of the different types of single molecular junctions. The results reported in this prospective may serve as a guideline for the design and synthesis of molecular systems to be used in molecule-based electronic devices.

Conductance in a bis-terpyridine based single molecular breadboard circuit

“…4–8 In STM-BJ/MCBJ, single molecules are contacted by two electrodes to form a metal–molecule–metal junction. Under the application of a bias voltage, the current flowing through the molecule, from one electrode to the other is sensitive to the molecular structure (length, conjugation pattern and conformation), 9,10 the chemical nature of the linker which connects the molecule to the electrode, 6,11–13 the relative position of linkers attached to the molecular core, 14,15 the binding geometry of molecule between the electrodes, 16–18 and the effect of external triggers on molecular structure and environment. 2,6,19–22 Thus, these experimental techniques when combined with charge transport theory present a unique opportunity to create dynamic circuits with a library of molecules, which can act like wires, 9,23–25 switches 26,27 and diodes.…”

“…Experimentally, the conductance of organic molecules utilizing a number of anchoring groups have been compared including benzothiophene (–BT), pyridyl (–PY), amino (–NH 2 ), thiol (–SH), isothiocyanide (–SCN), cyanide (–CN), nitro (–NO 2 ), carboxylic acid (–COOH), dimethyl phosphine (–PMe 2 ), methyl sulphide (–SMe), hydroxyl (–OH), and carbodithiolate based linkers. 6,11–13,30 These studies have helped deconvolute the effect of the linkers from that of the molecular core on the single molecular junction conductance. Several experimental and theoretical attempts have been made to understand and/or predict the possible quantum interference patterns which govern the conductance properties of conductance pathways across the molecular core.…”

Correlation of breaking forces, conductances and geometries of molecular junctions

“…In particular, conductance studies with small organic molecules revealed unique correlations between molecular structure and junction conductances. These studies explored the influence of molecular length and conjugation789101112, torsion angle131415, electrode material1617, and anchoring group181920212223, on junction conductance. To date, generic force spectroscopy experiments primarily address supramolecular bonds2425262728, and almost no studies of mechanical properties of covalently-bound junctions were reported4.…”

Insulated molecular wires: inhibiting orthogonal contacts in metal complex based molecular junctions

“…54,55 Recently, the molecular conductances of Pt(II) complexes trans- Through numerous studies of molecular junctions, the nature of the molecule-electrode contact has proven important, 57 and recognition of the role that anchor groups play in the electrical performance of the junction has led to the development of a wide range of contacting groups and electrode materials. [58][59][60][61][62][63] However, even well-known contacting groups such as thiolates bind to a range of surface sites, including terraces or near to adatoms as well as idealised pristine atomically flat terraces, giving rise to a range of molecular conductance signatures from a given molecular backbone. 64 Consequently, interest has increasingly focussed on molecular junctions in which the molecule is contacted to the electrode by strong electrode-carbon covalent bonds.…”