Advances and challenges in single-molecule electron transport

Abstract

Electronic transport properties of single-molecule junctions have been widely measured by several techniques, including mechanically controllable break junctions, electromigration break junctions, and by means of scanning tunneling microscopes. In parallel, many theoretical tools have been developed and refined for describing such transport properties and for obtaining numerical predictions. Most prominent among these theoretical tools are those based upon density functional theory. In this review, theory and experiment are critically compared, and this confrontation leads to several important conclusions. The theoretically predicted trends nowadays reproduce the experimental findings well for series of molecules with a single well-defined control parameter, such as the length of the molecules. The quantitative agreement between theory and experiment usually is less convincing, however. Two main sources for the quantitative discrepancies can be identified. Experimentally, the atomic structure of the junction typically realized in the measurement is not well known, so simulations rely on plausible scenarios. In theory, correlation effects can be included only in approximations that are difficult to control for experimentally relevant situations. Therefore, one typically expects qualitative agreement with present modeling tools; in exceptional cases a quantitative agreement has already been achieved. For further progress, benchmark systems are required that are sufficiently well defined by experiment to allow quantitative testing of the approximation schemes underlying the theoretical modeling. Several key experiments can be identified suggesting that the present description may even be qualitatively incomplete in some cases. Such key experimental observations and their current models are also discussed here, leading to several suggestions for extensions of the models toward including dynamic image charges, electron correlations, and polaron formation.

Electronic conductance and thermopower of single-molecule junctions of oligo(phenyleneethynylene) derivatives

“…In this context, we mention a recent review that highlights challenges in theory and experiment as well as the currently achievable agreement between the two. 54…”

Single-molecule functionality in electronic components based on orbital resonances

“…In addition, the four LMOs were computed using eqn (11), based on the wave functions of the HOMOÀ3 till HOMO, and the fitted four-site model parameters obtained in Fig. 11a.…”

“…charge and spin degree of freedom, [2][3][4][5][6][7][8][9][10][11] such as switching, 12,13 thermoelectricity, 14,15 spin-crossover, 16,17 Kondo effect, [18][19][20][21] and mechanical tunability 20,[22][23][24][25][26] with as central theme the link between chemical structure and device functionality. 6,27,28 The ultimate goal is the on-demand tunability of the orbital structure of a molecule, to enable the engineering of singlemolecule device functionalities.…”

Design Considerations for Oligo(p-Phenyleneethynylene) Organic Radicals in Molecular Junctions

“…This implies that the information contained in a single measured conductance value is close to zero, but it is rather the statistical analysis of a large set of measured conductance values for a given junction that leads to meaningful and reproducible results (in particular the most likely conductance value). This is one of several factors which makes the first-principles prediction of absolute conductance values challenging (another being level alignment) [144]. However, qualitative trends in conductance and in transmission shapes when comparing different molecules tend to be independent of the details of electrode-molecule contacts [145][146][147], as long as the same contact configuration is used for each different molecule.…”